Unit 4 Flashcards
What are adrenoceptors?
Membrane-bound receptors located throughout the body
- Mediates a diverse range of physiological responses to noradrenaline (norepinephrine) and adrenaline (epinephrine)
- Noradrenaline - transmitter released by sympathetic nerve terminals
- Adrenaline - a hormone secreted by the adrenal medulla
G protein-coupled receptors
Responsible for the “fight or flight” response
How are adrenoceptors classified?
Alpha and beta receptors
A total of 9 distinct subtypes
2 Main alpha-adrenoceptor subtypes, alpha1 and alpha2, each divided into 3 further subtypes (A, B and C)
3 Main beta-adrenoceptor subtypes, ß1, ß2, ß3
What are the second messengers of adrenoceptors?
Alpha 1:
- Activate phospholipase C, producing inositol triphosphate (IP3) and diacylglycerol (DAG) as second messengers to activate protein kinase C, resulting in phosphorylation of other enzymes
Alpha 2:
- Inhibit adenylyl cyclase, decreasing cAMP formation
ß-adrenoceptors (all subtypes):
- Stimulate adenylyl cyclase, increasing cAMP formation
What are the main effects of alpha-adrenoceptors?
Alpha 1 receptors:
- Vasoconstriction
- Relaxation of gastrointestinal smooth muscle
- Urethral smooth muscle contraction
- Salivary secretion
- Hepatic glycogenolysis
Alpha 2 receptors:
- Inhibition of transmitter release (inc. noradrenaline and acetylcholine release from autonomic nerves) [presynaptic receptors on nerve endings]
- Vascular smooth muscle contraction
- Platelet aggregation
- Insulin release
What are the main effects of ß-adrenoceptors?
ß1 Receptors:
- Found mainly in the heart
- Positive inotropic (increase heart rate) and chronotropic effects (increase contractility); positive lusitropic effect (speeds rate of relaxation)
ß2 Receptors:
- Smooth muscle relaxation
- Hepatic glycogenolysis
ß3 Receptors:
- Lipolysis and thermogenesis
- Bladder detrusor muscle relaxation
What do we know about drugs acting on adrenoceptors?
Overall activity to these drugs is governed by their affinity, efficacy and selectivity with respect to different types of receptor. In general:
- ß2-adrenoceptor selective agonists are useful as smooth muscle relaxants (especially in the airways - bronchodilator)
- ß1-adrenoceptor selective antagonists (often called ß blockers) are used mainly for their cardio-depressant effects
- Alpha1-adrenoceptor selective antagonists are used mainly for their vasodilator effects in cardiovascular indications
- Adrenaline (alpha/beta agonist) is uniquely important in cardiac arrest with its mixture of cardiac stimulant, vasodilator and vasoconstrictor actions
What are adrenoceptor agonists?
Noradrenaline and adrenaline show relatively little receptor selectivity
- Cardiac arrest
Selective alpha-adrenoceptor agonists have relatively few clinical uses
- Selective alpha1 agonists include phenylephrine and oxymetazoline
- Selective alpha2 agonists include clonidine, UK14304 (brimonidine), B-HT920 (Talipexole) and alpha-methylnoradrenaline
Selective ß1 agonists include dobutamine. Increased cardiac contractility may be useful clinically, but all ß1 agonists can cause cardiac dysrhythmias
Selective ß2 agonists include salbutamol and terbutaline; used mainly for their bronchodilator action in asthma
What are adrenoceptor antagonists?
Alpha-adrenoceptor antagonists:
- Non-selective: Phenoxybenzamine
- Alpha1-selective: Prazosin, Doxazosin
- Alpha2-selective: Yohimbine, Idazoxan (alpha2 antagonists are not used therapeutically)
Is Phenoxybenzamine a selective adrenoceptor antagonist?
Phenoxybenzamine is NOT selective for alpha receptors and also antagonises the actions of acetylcholine, histamine and serotonin
- Decreases blood pressure (blockade of alpha-receptor-mediated vasoconstriction)
- Reflex tachycardia (due to increase of noradrenaline release through blockage of alpha 2 receptors)
- Long lasting as it binds covalently to the receptor (irreversible antagonist)
- Not widely used clinically, except in the preparation for patients with phaeochromocytoma for surgery.
What do the alpha-adrenoceptor antagonists prazosin, doxazosin and tamsulosin do? What about yohimbine?
PRAZOSIN/DOXAZOSIN/ TAMULOSIN:
- Alpha1-antagonists
- Cause vasodilation and decrease arterial pressure
- Less reflex tachycardia compared to phenoxybenzamine
- Relaxation of the smooth muscle of the bladder neck and prostate capsule –> reduce urinary sphincter tone and inhibit prostatic hypertrophy in humans.
YOHIMBINE:
- Alpha2-antagonist
- Not used clinically
What are ß-adrenoceptor antagonists?
Important effects on the cardiovascular system and on bronchial smooth muscle
Non-selective between ß1 and ß2
- Propanolol, alprenolol, oxprenolol
ß1-antagonists (less effect on smooth muscle)
- Atenolol, metoprolol
CLINICAL USES:
- Angina, myocardial infarction, prevention of recurrent dysrhythmias
- Heart failure (only in well-compensated patients)
UNWANTED EFFECTS:
- Bronchoconstriction, hypoglycaemia etc
- Lower risk with ß1-selective antagonists
Most ß-adrenoceptor antagonists are inactive on ß3 receptors so do not affect lipolysis
What are metabolism and pharmacokinetic studies?
ISOLATED TISSUE (IN VITRO):
- Dose-response curve
- Evaluating drug metabolism, transporter function and drug-induced organ toxicity
- Optimise choice of doses for in vivo studies
PERFUSED ORGAN (EX VIVO):
- Greater control of the drug concentration applied to the organ - better prediction of drug metabolism and pharmacokinetics
- Mimic diseased models with altered metabolism (different expression of certain transporters)
WHOLE ANIMAL (IN VIVO):
- Bioavailability: Determine gastrointestinal absorption and elimination rates
- Multifactorial: metabolism, distribution, interaction of drugs
- Compare pharmacokinetics across species
- Determine the no-observed-adverse-effect-level (NOAEL)
What type of receptors are adrenoceptors?
All adrenoceptors are G protein-coupled receptors (GPCRs)
Why is an ion channel necessary?
Ions are charged molecules and unable to cross the plasma membrane due to the arrangement of the hydrophilic heads and hydrophobic tails in the lipid bilayer
To cross the plasma membrane, they must interact with membrane-spanning receptors = ion channels
What are 2 distinctive features of ion channels?
- The rate of ion transport through the channel is very high
- Ions pass through channels down their electrochemical gradient
What is diversity of ion channels?
Selectivity for a particular ion species:
- Determined by the size of the pore and the nature of the channel lining
Their gating properties:
- The mechanism that controls the transition between opened and closed states of the channel
Their molecular architecture/ structure:
- Physical make up sequence similarity
Their localisation
What is ion channel selectivity?
Ion channels are usually either cation (+) or anion (-) selective
CATION CHANNELS:
- May be selective for Na+, K+, Ca2+
- May be non-selective and permeable to all three ions
ANION CHANNELS:
- Mainly permeable to Cl- but other types do exist
What is selectivity filter?
The narrowest region of an ion channel pore
EXAMPLE:
The selectivity filter in KcsA is composed of a sequence of 5 amino acids: TVGYG. Four K+ binding sites inside the selectivity filter.
In each of these binding sites, the K+ ion is located at the centre of a cage formed by 8 oxygen atoms
The archetypal channel pore is just 1 or 2 atoms wide at its narrowest point and is selective for specific species of ion, such as sodium or potassium. However, some channels may be permeable to the passage of more than one type of ion, typically sharing a common charge. Ions often move through the segments of the channel pore in single file nearly as quickly as the ions move through free solution
What is ion channel gating?
Gating involves some type of conformational changes in the protein.
Conformational change occurs in a discrete area of the channel, leading to its opening
The entire channel changes conformation
Ball and chain type mechanism
Ion channel gating, what opens the channel?
- VOLTAGE-GATED CHANNELS (VOCs):
- Underlie the mechanism of membrane excitability
- Are opened when the cell is depolarised
- Most important ones are selective Na+, K+ and Ca2+ channels - LIGAND-GATED CHANNELS:
- Have a ligand binding site and are activated following the binding of a ligand to the channel protein (all are non-selective cation channels)
- Typically these are receptors on which fast neurotransmitters act:
–> Nicotinic acetylcholine receptors (nAChR), Glutamate, GABA - RECEPTOR-OPERATED CHANNELS (ROCs):
- Open in response to hormones (or drugs) acting on other cell membrane receptors. Second messenger
Where are ion channels found?
In ALL cells - focussed on excitable cells: neurons, muscle cells, and secretory cells
What are excitable cells?
Excitability is a property of cell membrane
Excitable cells can generate changes in their membrane potential, and produce an action potential. The electric current in neurons is used to rapidly transmit signals through the animal. In muscles, it is used to initiate contraction.
What happens in activation of excitable cells and ion channels?
Activation of excitable cells (depolarisation) may involve:
- An increase in membrane permeability to Na+
- An increase in intracellular Ca2+ due to:
–> Release from intracellular Ca2+ stores (endoplasmic reticulum/ sarcoplasmic reticulum (ER/SR))
–> Increased membrane permeability to Ca2+
What are ion gradients maintained by in nerves? And in which direction do ions move?
Ion Pumps - i.e. ATPases such as the Na+/K+ pump
The relative impermeability of the cell membrane
Ions always move down the concentration gradient from a high to low conc.
What happens in a typical cell during DEPOLARISATION
1) Stimulus
2) Membrane depolarisation
3) Change in membrane potential
4) Na+ enters via ion channel
5) Then Na+ is slowly removed from the cell and the cell returns to rest
What is an ion channel?
- Transmembrane pore forming proteins - allow the passage of ions (charged particles) in/out of a cell
- Distinguished based on ion selectivity, gating mechanism, and sequence similarity
- Can be voltage-gated, ligand-gated, pH-gated, or mechanically-gated
What is ion channel superfamily; diversity?
- More than 300 types of ion channels
- More than 500 genes encoding ion channels subunits
Can be classified by molecular structure ex. 2,4,6TM
How are ion channels classified?
- Classification by molecular structure (number of gates)
- Classification by the nature of gating
- Classification by type of ions passing through the gates
- Classification by cellular localisation
Other classification: Duration of response to stimuli
What is classification by gating?
In general terms, there are 3 main groups of gated ion channels:
- Voltage-gated ion channels (VOC)
- Ligand-gated ion channels (neurotransmitter)
- Receptor-operated ion channels (ROCs) or ion channels activated by second messengers
In what state do voltage-gated ion channels exist?
Exist in 3 different states:
- Resting - channel closed, this prevails at the normal resting potential
- Activated - open state favoured by a brief depolarisation
- Inactivated - blocked state resulting from a trap-door like occlusion of the channel by an intracellular “floppy” part of the channel protein
Open in response to changes in membrane potential
Provide rapid changes in the ion content of the cell
What are the types of voltage-gated ion channels?
Within this group there are several examples:
POTASSIUM (K+) CHANNELS (Ky):
- Primarily, stabilisation of negative membrane potential
- Depolarisation/ hyperpolarisation (at least 40 members)
SODIUM (NA+) CHANNELS:
- Critical for action potential initiation and propagation (at least 9 members)
CALCIUM (Ca2+) CHANNELS:
- Mediate calcium influx required for muscle contraction, secretion and neurotransmitter release (at least 10 members)
SOME TRANSIENT RECEPTOR POTENTIAL CHANNELS (TRP):
- The family is very diverse (at least 28 members)
What is the structure of Voltage gated Na+ Channels?
4 alpha subunits
The alpha subunit is repeated 4 times (I-V) to form the Na+ channel
Each alpha subunit contains 6 membrane-spanning regions
24 transmembrane segments
What are some examples of drugs acting on voltage-gated sodium channels?
CHANNEL BLOCK:
Neurotoxins:
- Tetrodotoxin
- Saxitoxin
- Conotoxins
Other:
- Local anaesthetics
BLOCK OF INACTIVATION:
- Veratridine
- Scorpio toxins
- DTT
Persistent activation of nerves
- Inhibit contraction of myocytes
- Treatment for some cardiac arrhythmias
- Treatment of pain
What is the structure of ligand-gated ion channels (inotropic receptors)?
Have a ligand binding site and are activated following binding of a ligand to the channel protein (no 2nd messengers required!)
E.g. Nicotinic acetylcholine receptors (nAChR)
- These gate Na+ and K+
Assembled from 4 different subunits alpha x2, ß, γ, δ
- Each subunit comprises four transmembrane domains
2ACh binding sites - Ash binding to both sites activates channel opening
What are some examples of drugs acting on nicotinic receptors?
Agonists:
- Nicotine, Lobeline
Antagonists:
- Non-depolarising neuromuscular blockers:
–> Curare, tubocurarine, pancuronium
- Depolarising neuromuscular blockers:
–> Suxamethonium
How are ion channels activated by second messengers?
Open in response to hormones (or drugs) acting on other cell membrane receptors. Are METABOTROPIC - use 2nd messengers
Examples of these types of ion channel are Transient Receptor Potential Channels (TRPs) which are a type of non-selective cation channels that gate both Na+ and Ca2+
EXAMPLES OF DRUGS:
- Agonist = capsaicin
- Antagonists = Capsazepine (novartis), NGX-4010 (Qutenza; Gruenthal Ltd)
What are some drugs acting at potassium channels (Agonists and antagonists)?
AGONISTS:
- Potassium channel openers (activate ATP-sensitive K+ channels in vascular smooth muscle). This leads to relaxation and vasodilation
- Antihypertensives = Pinacidil, minoxidil
- Anti-angina, antiarrhythmic
ANTAGONISTS/BLOCKERS:
- Sulphonylureas (anti-diabetic) block inwardly rectifying potassium channels, (depolarisation that opens voltage-gated calcium channels), this then stimulates insulin secretion
- Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) (multiple sclerosis) block voltage-gated potassium channels (Kv channels). Prolong action potentials and increase neurotransmitter release.
- Dendrotoxins are a class of presynaptic neurotoxins produced by mamba snakes that block particular subtypes of voltage-gate potassium channels (enhance release of acetylcholine)
What are some diseases related to potassium channels?
Channelopathies
- 90 genes coding for the main subunits of potassium channels
- Mutations in genes coding for potassium channels lead to dysfunctions in:
–> Neuronal system (alzheimer’s, Parkinson’s)
–> Cardiac system (arrhythmias)
–> Neonatal diabetes mellitus (KATP)
What are some examples of calcium channels?
Functionally there are three types;
1) Voltage-operated calcium channels (VOCCs):
- Found in the membrane of excitable cells (neuron, muscle)
- For example, L-type calcium channel. But also N-type, P/Q and R-type and T-type (pacemaker potential)
2) Store-operated calcium channels (SOCs) (ligand gated):
- Found on the plasma membrane of both excitable and non-excitable cells
- The major Ca2+ entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+ stores activated Ca2+ influx (store-operated Ca2+ entry)
–> Major source of intracellular calcium in non-excitable cells (myocytes, endocrine cells)
- IP2R/RyR (Intracellular calcium channels):
–> Are located on the endoplasmic reticulum/ sarcoplasmic reticulum (ER/SR) and activated by second messengers (IP3, Ca)
What are calcium channel blockers?
Clinically used - calcium channel blocking drugs
- Calcium channel blockers prevent calcium from entering cells of the heart and blood vessel walls, resulting in lower blood pressure
- L-type calcium channel blockers are used to treat hypertension
- EXAMPLES OF DRUGS: Verapamil, Amlodipine
How are ion channels classified by location?
PLASMA MEMBRANE CHANNELS:
- Ex. voltage-gated potassium channels (Kv), Sodium channels (NaV), calcium channels (CaV) and chloride channels (ClC)
ENDOPLASMIC RETICULUM CHANNELS:
-Example: RyR
MITOCHONDRIAL CHANNELS:
- Example: KATP
What is nociception?
= Pain awareness
Is mediated by nerve endings receptors in peripheral tissues and transmitted to the CNS
Transmission can be disrupted by drugs acting on neurotransmitter receptors or by blocking the sodium channels
What are local anaesthetics?
Drugs which reversibly block the generation and propagation of electrical impulses in the excitable tissues
Have local analgesic effects. Reversible loss of sensation in a portion of the body.
How do local anaesthetics work?
Local anaesthetics disrupt voltage-dependent Na+ ion channel (Navs) function within the neuronal membrane preventing the transmission of the neuronal action potential (in sensory, motor and sympathetic nerve fibres)
It does this by physically plugging the pore of the channel
What are the structures of local anaesthetics?
Clinical local anaesthetics belong to 2 classes (related to cocaine)
1) Aminoamide (-NH-CO-) most commonly used clinically
2) Aminoester (-O-CO-) more likely to cause an allergic reaction
What are some examples of local anaesthetics?
Ester Agents:
- Cocaine, procaine, amethocaine and chloroprocaine
- Cocaine, derived from the leaves of the coca plant Erythroxylon coca, is historically important as the forerunner of all local anaesthetic compounds. Although its use in Vet Med was popularised by Sir Frederick Hobday, it is now rarely used.
- Procaine was the first synthetic agent and is now no longer used clinically. Short half-life, poor penetration
- Tetracaine (=amethocaine) and cinchocaine are older agents. Tetracaine is used for spinal anaesthesia.
Amides:
- Lignocaine, Buvivacaine
- Lidocaine (=lignocaine) and mepivacaine. Good penetration, medium half-life. Lidocaine is used by IV injection as an anti-dysrhythmic
- Bupivacaine, ropivacaine and levobupivacaine. Longest duration of action (200 mins) widely used.
What is pKa?
Acid dissociation constant.
HCl –> H+ + Cl-
The larger the value of pKa, the smaller the extent to which an acid dissociates.
pKa is a log of Ka
A weak acid has a pKa value in the approximate range 2-12 in water
A strong acid has a pKa value around 2 in water
pKa determines the proportion of molecules that exist in lipid-soluble rather than a water-soluble state:
- The time for onset of local anaesthesia is determined by the proportion of molecules that convert to the tertiary lipid structure when exposed to physiological pH.
What are properties of local anaesthetics?
Ester-containing compounds are inactivated in the plasma and tissue by esterase enzymes.
Amides are more stable and these have a longer plasma half life.
All LAs are weak bases with a pKa between 8 and 9
Thus, at physiological pH, they are mainly ionised but not completely
This is essential for the penetration of the nerve sheath and axon membrane since compounds that are totally ionised cannot penetrate
How are local anaesthetics applied?
Topical surface anaesthesia:
- Applied by spray or ointment. Usually applies to the nose, mouth, bronchial tree, cornea, and urinary tract
- Risks: systemic toxicity at high concentration
Infiltration:
- Applied subcutaneously directly into tissue or around the nerves
- Used in minor surgery
- Risks: Only suitable for small areas or risk of systemic toxicity
Intravenous regional anaesthesia:
- Injected intravenously distal to pressure cuff to arrest blood flow
- Used for limb surgery
- Risks: Systemic toxicity when cuff is removed
Nerve block:
- Injection close to nerve trunk to cause loss of sensations
- Used for dental surgery
- Risks: Accurate placement of needle is essential
Spinal anaesthesia:
- Applied into subcarotid space to act on spinal roots.
- Used when general anaesthesia can’t be used
- Risks: Bradycardia and hypotension due to sympathetic block
Epidural anaesthesia:
- Applied into epidural spaces blocking spinal roots
- Used when general anaesthesia cannot be used (childbirth)
- Risks: Similar to spinal but less probable
Why is local anaesthesia used?
- Acute pain (trauma, surgery, infection)
- Chronic pain
- Surgery and dentistry
- To enable painless venipuncture
- Surface anaesthesia for endoscopic procedures
Often minor procedures:
- Epidural anaesthesia, c-sections and orthopaedic procedures
- Local anaesthesia of the eye for examination
What are pharmacokinetic properties of local anaesthesia?
ABSORPTION (UPTAKE):
- Determined by dosage, site of injection, physical properties of the drug, local tissue blood flow
DISTRIBUTION (LIPID SOLUBILITY, PROTEIN BINDING):
- Amides are widely distributed after intravenous bolus administration
- Rapid distribution: Brain, liver, kidney, heart
- Slower distribution in less perfused tissues: muscle, GIT
- Sequestration in fat
METABOLISM AND EXCRETION:
- Converted into the liver (amide) or plasma (esters) to more water-soluble metabolites which are excreted in urine
WHAT IS BUPIVACAINE?
- Half time distribution (min) 28
- T1/2 elimination (h) 3.5
- Volume distribution at steady state (L) 72
- Clearance (L/min) 0.47
What is the duration of action of Local Anaesthetics?
Depends on their affinity for proteins:
- Plasma proteins
- Sodium channels
Depends on the time a local anaesthetic remains in close proximity to neural fibre
- Sequestration
- Constriction of neighbouring vasculature
What is sensitivity to local anaesthetics?
Small nerve fibres are more sensitive than large nerve fibres:
- Thus, motor axons being large in diameter are relatively resistant to the actions of local anaesthetics.
- Myelinated fibres are blocked before non-myelinated fibres of the same diameter
- Thus, the sequence of loss of nerve function proceeds as loss of pain, temperature, touch, proprioception, and then skeletal muscle tone
- This is why people may still feel touch but not pain when using local anaesthesia (nociceptive impulses are carried by A and C fibres)
Nociceptive and sympathetic transmission are blocked first!!!!
How do local anaesthetics interact with Sodium ion channels?
Local anaesthetics block the initiation and propagation of action potentials by preventing the voltage-dependent increase in Na+ conductance
This is achieved by physically plugging the pore of voltage-gated Na+ conductance
This is achieved by physically plugging the pore of voltage-gated Na+ channels (depends on the state)
Local anaesthetics must reach their site of action by penetrating the nerve sheath and axonal membrane as unionised species
So they have to be weak bases!!!
What are ionic/non-ionic forms of local anaesthetics?
Local anaesthetic activity is strongly pH dependent and increased at alkaline pH and vice versa
This is because at an alkaline pH the proportion of ionised molecules is low
Local anaesthetics must penetrate the nerve sheath and axon membrane (as their non-ionised form) to reach their site of action on the inner side of the Na+ channel
Because the ionised form is not membrane permeant - penetration is very poor at acidic pH
Clinical implication: inflamed tissue is acidic and resistant to Local anaesthetics
Once inside the axon, it is the ionised form of Local anaesthetics that interact with the Na” channel
How do local anaesthetics interact with Na+ channels?
Use-dependent block of Na+ channels
The more the channels are opened the greater the block becomes
This is typical of antidysrhythmic and anti epileptic drugs
Many local anaesthetics enter the channel more readily when it is open than when it is closed
The blocking site of the channel can be reached via the open channel on the inner surface of the membrane by the “ion” version of the local anaesthetic.
= HYDROPHILIC PATHWAY (USE-DEPENDENT)
The blocking site of the channel in the closed state can be reached through the outer pore of the membrane by the non ‘ion’ version of the local anaesthetic (more lipid soluble drugs)
= HYDROPHOBIC PATHWAY (NON-USE DEPENDENT)
What do membrane lipid-interacting local anaesthetics do?
Interact with membrane lipids to change fluidity, microviscosity and permeability of membranes.
Also influence the electric potential across lipid bilayers.
What are some unwanted side effects of local anaesthetics for ester agents and amides?
Ester agents can produce allergic reactions:
- Due to the sensitivity to their metabolite-produced para-aminobenzoic acid (PABA)
- They are metabolised by pseudocholine esterase
Amides are metabolised in the liver:
- Problem with patients with liver failure
- May induce liver injuries
What are some unwanted side effects of local anaesthetics involving the CNS and CVS?
Result mainly from leakage into circulation (but local anaesthetic plasma half-life is generally short)
Most local anaesthetics produce a mixture of both stimulant and depressive
CNS - agitation, confusion, tremors, convulsions and respiratory depression (main threat to life)
CVS - myocardial depression (bradycardia) and vasodilation leading to hypotension (rapid drop in BP can be life threatening
Hypersensitivity reactions (rare), usually in the form of allergic dermatitis
As local anaesthetics are absorbed from the injection site, their concentration rises in the bloodstream and the PNS and CNS are depressed in a dose-dependent manner
What do we know about new local anaesthetics?
Tens of millions of patients have operations requiring local anaesthesia each year. Current local anaesthetics act for less than 8hrs
Neosaxitoxin (neoSTX):
- Provides local anaesthesia for more than 24 hrs. It is a site 1 sodium-channel blocker, part of a larger class of emerging anaesthetics based on molecules derived from aquatic organisms (tetrodoxin)
Articaine (amide):
- Used in dentistry
- Amide type but contains a thiophene ring that increases lipid solubility
- Articaine is exceptional because it contains an additional ester group that is metabolised by esterases in blood and tissue
- Since articaine is hydrolised very quickly in the blood, the risk of systemic intoxication seems to be lower than with the other anaesthetics.
What is inflammation and how is inflammation initiated?
Protective process to mobilise defence mechanisms against infectious and non-infectious agents that cause tissue damage.
Skin and mucosal surfaces have cells beneath the epithelia/dermis which possess pattern-recognition receptors (e.g. Toll-like receptors):
- Pathogen associated molecular patterns (PAMPs)
- Damage associated molecular patterns (DAMPs) [released by damaged cells]
Dendritic cells, tissue macrophages, mast cells when activated by PAMPs/DAMPs
- Release cytokines (tumour necrosis factor-alpha and interleukin-1)
- Eicosanoids (lipid mediators - prostaglandins, leukotrienes) produced from damaged cells
- Mediators release from stored secretory granules (e.g. histamine)
What do acute phases of inflammation increase?
Blood flow to the area (vasodilation of arterioles) [redness and heat]
Post-capillary venule permeability - exudation of plasma into tissues [swelling]
- Activated complement, coagulation, fibrinolytic and kinin systems
Leucocyte recruitment into the tissues (chemotaxis) - primarily neutrophils first
Activation of primary afferent nerve endings; axon reflex (flare) [pain]
Redirection of tissue fluid flow towards lymph nodes (adaptive immune response)
Core body temperature [heat-fever]
What are the 4 cardinal signs of inflammation noted centuries ago?
- Rubor (redness)
- Calor (heat)
- Tumor (swelling)
- Dolor (pain)
Why inhibit an acute inflammatory response?
Swelling and fluid accumulation inhibits organ function/causes further damage
- E.g. lung function reduced with fulminant pneumonia due to tissue oedema
- Pressure on the brain with meningoencephalitis could lead to permanent damage
Pain becomes a welfare issue
Tissue irritation leads to the animal chewing/licking the lesion - further tissue damage
Drugs can be used to damp-down the acute inflammatory response
Complex nature of cell and mediator interactions - no one target will stop inflammatory response
Important inflammation allowed to resolve - leads to tissue healing
- Unless inflammatory stimulus persists, becomes chronic (e.g. auto-immune disease)
- Management of chronic inflammation is a whole other topic - need to suppress adaptive immune system
What are mediators of acute inflammation?
Understanding what mediates;
- Increased blood flow, swelling (plasma exudation), leucocyte chemotaxis, fever, pain. Helps to explain the properties of drugs targeting these mediators.
Lipid mediators - formed from cell membrane fatty acids under action of phospholipase A2 (PLA2)
- Activated by cell damage, C5a (complement) on neutrophils, Bradykinin on fibroblasts
- Acts on membrane phospholipids to release arachidonic acid (5,8,11,14-eicoatetraenoic acid)
- Arachidonic acid is a substrate for:
–> Cyclo-oxygenase enzymes (induced in the tissues by IL-1) generates prostaglandins
–> Lipoxygenase enzymes - generates leukotrienes and lipoxins - PLA2 also leads to the formation of platelet-activating factor (PAF)
What are Non-steroidal anti-inflammatory drugs (NSAIDs)?
Generally NSAIDs is to mean COX-inhibitors (aspirin-like drugs)
Aspirin (acetyl salicylic acid) has been in medical use since the 1890s
- Differs from all others - enters active site acetylates ser530, irreversibly inactivating COX
What are some Veterinary authorised NSAIDs?
Non-selective COX inhibitors: Flunixin, phenylbutazone, ketoprofen, tolfenamic acid, salicylate
COX-2 preferential: Carprofen, meloxicam
COX-2 selective: Robenacoxib, Firocoxib, cimicoxib, enflicoxib, mavacoxib
Dual COX and lipoxygenase inhibitor: Tepoxalin (no longer available)
EP4 receptor inhibitor: Grapiprant
Widely used in veterinary medicine to manage acute pain and inflammation in the peri-operative period and following musculoskeletal injury.
Chronic use is in the management of pain in osteoarthritis (not licensed for chronic use)
How selective do COX-2 inhibitors need to be?
Not straight forward as looking at relative potency vs. the two enzymes
- Need to be below IC20 for COX-1 and above IC80 for COX-2
- Need to look at drug kinetics
Drugs sequestered at sites of inflammation due to protein binding.
- Plasma kinetics do not determine duration of action.
Is inhibition of COX the only mechanism of anti-inflammatory action of NSAIDs?
Depends on the drug!
Some have free-radical scavenging activity (sulindac)
Aspirin inhibits NFkB - transcription factor for inflammatory mediator genes:
- Long-acting inhibition of platelet function - used to prevent thrombosis
What do we know about paracetamol action?
Paracetamol is an analgesic/antipyretic but much weaker anti-inflammatory
Selective for COX enzymes in the CNS - postulated different isoform (COX-3)
Analgesic/antipyretic of choice in infants (aspirin - risk of Reyes syndrome)
TOXIC TO CATS
What is GRAPIPRANT?
A selective antagonist at the EP4 receptor
EP4 receptor is a key receptor mediating the effects of PGE2
Primary receptor involved in PGE2 mediated effects on hyperalgesia (pain)
EP4 receptor also involved in vasodilatory and increase vascular permeability effects
When is the EP4 receptor Grapiprant used?
Authorised for the treatments of pain associated with mild to moderate osteoarthritis in dogs.
Very commonly causes vomiting in dogs (PGE2 has gastroprotective effects)
Shares the same potential to cause adverse effects on the kidney
Is a methylbenzenesulfonamide - not known whether sulphonamide hypersensitivity is a problem
What do we know about histamine as an inflammatory mediator?
Histamine is stored in mast cells found in sub-mucosal and dermal layers and in circulating basophils
Formed by decarboxylation of histidine; stored in granules.
Released by exocytosis during inflammatory or allergic reactions; stimuli include:
- Complement components (C5a and C3a)
- Cell fixed IgE; antigen binds - type 1 hypersensitivity reaction
- Various basic drugs (morphine, d-turbocurare)
- Beta-adrenoceptor agonists inhibit histamine release (cAMP-mediated effect)
What are the different types of histamine receptors?
There are 4 types of histamine receptors (H1, H2, H3, and H4)
Antagonists at the H1 receptor are used as anti-inflammatory agents
Used for type 1 hypersensitivity prior to/immediately after exposure to antigen (for hay fever and insect bites; does not work well for canine atopy)
Sedating vs. non-sedating drugs (cyclizine; chlorphenamine vs acrivastine; fexofenadine)
In anaphylactic shock (emergency) give adrenaline to functionally antagonise histamine
How are histamine receptors used in veterinary medicine?
Inhibit gastric acid secretion (although protons pump inhibitors are used more) - H2 receptor antagonists (e.g. famotidine)
To prevent motion sickness (e.g. acepromazine - has many other actions)
Over the counter drugs (NB - terfenadine/erythromycin interaction)
Mast cell stabilisers - sodium cromoglycate - mechanism poorly understood, used less now in humans.
What do you know about glucocorticoids for acute inflammation?
Inhibit both the innate and adaptive immune response
Highly effective - broad range of anti-inflammatory actions
In acute inflammation inhibition of:
- PLA2, prevents formation of prostaglandins and leukotrienes
- Inhibition of cytokine release:
–> Prevents induction of many processes in the acute inflammatory response including activation of leukocytes
Endogenous glucocorticoids - act as the break on inflammation
Why is it best not to use glucocorticoids in acute inflammation associated with bacterial infection?
Risk of inhibiting defence mechanism - if invading pathogen (use with bactericidal antibiotics)
Inhibit adaptive immune response to the pathogen - detrimental
Suppressing inflammation as much as steroids do prevents resolution of the initial damage
- If used - short term where acute inflammation is life-threatening (e.g. fulminant pneumonia)
- May need to be used for chronic inflammation but side effects are an issue
What is the anti-inflammatory action of corticosteroids?
Anti-inflammatory effects are broad and complex - may vary between tissues
Effects on inflammatory mediators:
Decreased production of eicosanoids
–> Inhibition of PLA2 (via annexing-1) and prevention of induction of COX-2
Decreased generation of cytokines secondary to inhibition of gene transcription
- IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, TNFa, cell adhesion factors and granulocyte-macrophage colony stimulating factor.
Reduced plasma conc. of complement components in plasma
Decreased iNOS expression
Reduced histamine release from mast cells
Increased production of anti-inflammatory factors (IL-10, soluble IL-1 receptor, annexing-1)
Why are macrophages important in their anti-inflammatory action?
During the tissue repair process, macrophages are known to produce various growth factors, cytokines, and other signaling molecules that help stimulate and coordinate the growth and regeneration of new tissues.
What is chronic inflammation?
Inflammation that occurs over a very long period of time.
What was the basic understanding of COX inhibitors in the 1990s?
COX-2 is an inducible enzyme
COX-1 is a constitutive enzyme with protective roles:
- Gastro-protection
- Haemostasis (platelet activation)
- Reno-protection
What are the gastro-protective roles of PGE2 and PGI2?
Vasodilation to maintain effective blood flow to GI mucosa
Increase mucous secretion (unstirred layer)
Inhibit Hal secretion by parietal cells
Can co-formulate non-steroidal anti-inflammatory drugs with misoprostol or NO-releasing compounds.
Are NSAIDs acids or alkalis?
They are Weak Acids
What happens in the platelet with COX-1?
COX-1 metabolite, Thromboxane A2, is produced by activated platelets.
Thromboxane A2 stimulates:
- The release of mediators from platelet granules
- Aggregation in most species
What happens in the endothelium with COX-1 and COX-2?
Both COX-1 and COX-2 are responsible for producing prostacyclin (PGI2)
This is a vasodilatory mediator which inhibits platelet aggregation
Balance between TxA2 (prothrombotic) and PGI2 (antithrombotic) is important for thrombotic risk
Inducible COX-2 is important in endothelium at sites of high sheer (where thrombi tend to form)
In large trials, selective COX-2 inhibitors increased the risk of heart attacks when compared to non-selective COX inhibitors at equi-effective doses.
What is the traditional view of NSAID toxicity and the kidney?
House-keeping prostaglandin production by the COX-1 enzymes within the kidney are vasodilatory
When renal blood flow is compromised, the RAAS is activated, PGI2 protects against excessive vasoconstriction
Non-selective COX-inhibitors in hypovolaemic/hypotensive patients:
- Risk of precipitating ischaemic renal injury
- Acute kidney injury can result
Anti-inflammatory drugs that are COX-2 selective should be less nephrotoxic
How do COX metabolites and renal function link?
Prostaglandins (PGs) are not required for normal function. Protect against stress:
PGE2 and PGI2 are major COX metabolites needed for:
- Maintaining Glomerular filtration rate in hypotensive/hypovolaemic state
–> Signal for renin secretion (produced by macula densa)
–> Vasodilation of pre-glomerular microvasculature and relaxation of mesangial cells
Full expression of the pressure-natriuresis response when faced with an overload of salt
Ability to maintain water and electrolyte balance in the face of water deprivation.
What does Prostaglandin E2 (PGE2) do in the kidney?
Decrease sodium ion (Na+) reabsorption at the thick ascending limb of the loop of Henle
Natriuretic
What does Prostaglandin I2 (PGI2) do in the kidney?
Increase renin release causing increased aldosterone, increasing potassium ion (K+) excretion
Vasodilation increased renal blood flow and glomerular filtrate rate in stressed states
Autoregulation and renoprotection
What are the renal side effects of COX inhibitors?
NOT seen in healthy individuals but…
- Lead to an acute decrease in glomerular filtration rate in hypotensive/shocked/ dehydrated patients
- Damage the interstitium and papillary areas of the kidney in dehydrated patients
- Cause sodium retention and inhibit the action of diuretic drugs in human heart failure and hypertensive patients.
What do we need to know to determine if COX-2 inhibitors are safer than non-selective COX inhibitors?
Physiological role of COX-2:
- Development of COX-2 selective inhibitors has facilitated study of this
- Clues to function can be obtained from localisation of enzymes and manipulating physiology experimentally
Where is COX-1 and COX-2 expression localised in the kidney?
COX-1:
- Renal vasculature (incl. afferent arteriole)
- Glomerular mesangial cells
- Collecting duct epithelial cells (cortex and medulla)
COX-2:
- Macula densa and adjacent afferent arteriole
- Cortical thick ascending limb loop of Henle
- Medullary interstitial cells
N.b. Varies with species; most published data based on rodents or humans
What are the COX metabolites prostaglandins E2 and I2 needed for?
Maintaining glomerular filtration rate in hypotensive/hypovolaemic state
- Signal for renin secretion (produced by the macula densa)
- Vasodilation of pre-glomerular microvasculature and relaxation of mesangial cells
Full expression of the pressure-natriuresis response when faced with an overload of salt
Ability to maintain water and electrolyte balance in the face of water deprivation
How is glomerular filtration rate maintained in volume depletion?
When renal blood flow decreases renin secretion occurs.
This is important for:
- Autoregulation and tubuloglomerular feedback
Stimuli for renin secretion:
- Sympathetic nervous system activation
- Low pressure in the afferent arteriole
- Low chloride detected by the macula densa (COX-2)
How is COX-2 regulated in the Macula densa?
Macula densa is key to detection of low filtration and low tubular flow
Constitutive expression of COX-2 found in:
- Mouse, rate, dog and rabbit macula densa
Increased expression stimulated by:
- Feeding a low salt diet
- Treating with diuretics (increased NaCl excretion)
- Treating with ACE-inhibitors
What is the role of COX in the macula densa?
Evidence for role in renin secretion:
- Non-selective COX-inhibitors or COX-2 selective inhibitors prevent renin secretion in response to the low Cl- transport by macula densa
Mineralocorticoid hormones:
- Tonically inhibit COX-2 expression in the macula densa (negative feedback mechanism)
- Spironolactone (mineralocorticoid receptor antagonist) treatment leads to up-regulation of COX-2
How is water balance regulated?
Medulla is the site where this occurs and is a MAJOR site of prostaglandin synthesis:
- COX-1 is found in the epithelial cells of the collecting ducts
- COX-2 is found in the medullary interstitial cells and in the endothelial cells of the vasa recta
What is the role of COX in response to water deprivation?
Renal medullary COX-2 increases, COX-1 is unaffected
Increased expression of COX-2 allows medullary interstitial cells to survive hypertonic stress
COX-2 required for interstitial cells to adapt to hyper-osmotic environment - accumulate organic osmolytes (prevent shrinkage)
COX-2 selective inhibition and water deprivation:
- Renal medullary interstitial cells undergo apoptosis
- No apoptosis if deprived of water without COX-2 inhibition
What is the role of COX in sodium regulation by the kidney?
Prostaglandins are an important component of the renal medullary natriuretic system:
In rate there is a region-specific regulation of COX-2 by sodium chloride intake;
- Sodium chloride depletion induces COX-2 in cortex
–> Macula densa cells - signal for renin secretion - Sodium chloride loading induces 5-fold increase in COX-2 expression in the inner medulla
- Natriuretic effect of prostaglandins in the loop of Henle
Regulation of renal medullary blood flow and loop of Henle sodium chloride reabsorption by prostaglandins has:
- Significant implications for regulation of salt excretion and systolic arterial blood pressure regulation:
- COX-2 selective inhibitors reduce renal medullary blood flow
- Interstitial cell prostaglandin production by COX-2 modulate epithelial cell solute reabsorption
- Loss of tonic inhibitory effect of COX-3 derived prostaglandins on NaCl absorption contributes to the NaCl retention seen with NSAIDs
- In human patients, COX-2 selective inhibitors predispose to hypertension and oedema formation (heart failure)
Which of the renotoxic effects of NSAIDs would still be evident if COX-2 selective inhibitors were introduced into clinical practice?
Acute decrease in glomerular filtration rate in hypotensive/shocked/ dehydrated patients
Damage to the interstitial and papillary areas of the kidney in dehydrated patients
Sodium retention and inhibition of the action of diuretic drugs in congestive heart failure patient and hypertensive patients
What is the definition of pain in humans?
An unpleasant sensory and emotional experience associated with, or resembling that associated with actual or potential tissue damage
What are some key things to remember about pain?
Always subjective
- No gold standard measurement for pain in others
- Problematic in absence of self-report
Pain cannot be inferred solely from activity in sensory neurones
Inability to communicate does not negate the possibility of pain in humans - or animals!
What is animal pain?
An aversive sensory and emotional experience representing an awareness by the animal of damage or threat to the integrity of its tissues. It changes the animal’s physiology and behaviours to reduce the likelihood of recurrence, and to promote recovery
Why is pain so important?
Hippocratic oath: First, do no harm
Pain has a protective function, but also impairs patient wellbeing and welfare
Altered behaviour
Early pain experiences affect later life
Un(der)treated pain induces a stress response that negatively affects recovery
What are the clinical consequences of acute pain in animals?
Welfare compromise!!
Reduced immune function (cortisol, blood flow redistribution)
- Higher risk of surgical site infections
- Impaired wound healing
Poorer quality recovery from general anaesthesia
Longer hospitalisation time
Increased weight loss/muscle wasting after major surgery
More ancillary drug use
Higher chronic pain risk
What is Nociception vs pain?
NOCICEPTION:
- Neurophysiological encoding of noxious stimuli
PAIN:
- Sensory and emotional experience in response to these stimuli
Pain is not just sensory-discriminative (where it hurts, how much it hurts), but also
Affective-motivational (how it makes you feel and behave)
Pain = conscious, subjective experience, not just the neuronal connections
Under general anaesthesia, nociception can continue while pain will by definition be absent
What are the 4 stages of nociception?
- Transduction
- Transmission
- Modulation
- Perception
What is transduction in nociception?
PROCESS:
Noxious stimulus becomes an electrical impulse (action potential)
SITE:
Nociceptors - Specialised peripheral sensory nerve endings
- Detect nociceptive (i.e. potentially painful) stimuli
- Distinct from non-nociceptive receptors
What are nociceptor subtypes?
Mechanoreceptors (pressure, touch)
Thermoreceptors (heat, cold)
Chemoreceptors (H+, K+, inflammatory mediators)
Nociceptors carry multiple receptors/ion channels –> firing threshold can change
What is transmission in nociception?
PROCESS:
Action potential travels via Na+ channel activation
SITE:
Afferent sensory nerve axon, travelling
- Via dorsal root ganglion (DRG): Cell bodies and glial cells
- To dorsal horn: Synapse
Two different types of nerve fibres, Aδ fibres and C Fibres
What are Aδ fibres and C fibres?
Aδ fibres:
Fast, sharp pain
Slightly larger, myelinated
C fibres:
Slow, dull pain
Small, unmyelinated
What happens at the dorsal horn in nociception?
Afferent impulse enters the spinal cord via the dorsal horn
Connection with efferent motor neurone at the same level –> Reflex Withdrawal
This does NOT equate with PAIN (that requires conscious awareness, experience)
What is modulation in nociception?
PROCESS:
Alteration of incoming sensory input
SITE:
Neural synapses at dorsal horn of spinal cord and supra spinal sites
Relay is not 1:1 - multiple inputs from spinal/brain centers
- Facilitating or inhibiting
- Incoming signal is ‘modulated’
The end result (onwards signalling) depends on the total + and - inputs
What happens in modulation processing of pain in the spinal cord?
The dorsal horn is like a relay station:
- Descending inhibition, facilitation, interneurons
Multiple receptors, ion channels and mediators involved:
- Inhibitory: NA, 5HT, Glycine, GABA, Endorphins…
- Excitatory: Glutamate, Neuropeptides (substance P, CGRP), prostaglandins
Prime targets for analgesics
What is “Gate control theory” in Modulation?
Intrinsic method of controlling the amount of painful stimuli that reach the brain
Touch inhibiting nociceptive input via inhibitory neurones in the spinal cord
E.g. rubbing reduces bruise pain, TENS therapy
What is perception in nociception?
PROCESS:
Awareness of noxious inputs –> Emotional and motivational response, behaviour change
SITE:
Higher brain centres, Somatosensory Cortex
The actual site of pain perception is still contentious!
BUT, Pain = In the brain
What are the different kinds of pain?
Nociceptive pain
Inflammatory pain
Neuropathic pain
Nociplastic pain
What is nociceptive pain?
E.g. Burn, incision, electrical shock
Direct stimulation of nerve endings by noxious stimulus
Can be dampened by local anaesthetics, NSAIDs, Opioids, Alpha-2 agonists
What is inflammatory pain?
Result of local release of inflammatory mediators
- From injured tissue
- From the stimulated nerve: Neurogenic inflammation
Peripheral sensitisation –> Firing threshold of nerve endings is reduced
Can be effectively targeted with NSAIDs, Opioids, (locoregional) corticosteroids
What is neuropathic pain?
Result of direct injury or lesion to the peripheral or central nervous system
- Major surgery or amputations
- Trigeminal neuralgia (face pain, head shaking)
- Some osteoarthritis and laminitis patients
In humans: Spontaneous ‘shooting’ pain, tingles, itch…
- Limited response to anti-inflammatories
- Responds better to drugs affecting neuronal ion channels (e.g. gabapentinoids), NMDA, antagonists, NA re-uptake inhibitors)
What is mixed pain?
Aspect of nociceptive/ inflammatory and neuropathic pain
E.g.
- Acute exacerbation of chronic laminitis
- Osteoarthritis pain
- Cancer pain
Patient may need both anti-inflammatory analgesic and drugs acting on neuronal ion channels
What is nociplastic pain?
Covers both nervous system legions and functional pain due to altered sensory processing (without lesion)
- Maladaptive (does not protect nor support recovery)
- Changes to the CNS pathways that lead to enhanced pain
Central sensitisation
What is sensitisation?
Peripheral: ‘sensitising mediator soup’, neurogenic inflammation –> Nociceptors –> Transductor sensitivity increases
Central: Altered gene expression and synaptic activity –> Increased responsiveness in CNS –> Pain hypersensitivity
What is hyperalgesia?
An exaggerated pain response to a normally painful stimulus
What is allodynia?
A painful response to a normally non-painful (innocuous) stimulus
When is pain chronic?
> 3 months duration
Pain persisting beyond the time required for tissue healing
Chronic pain doesn’t equal acute pain that lasts longer!!
What happens in chronic pain that doesn’t occur in acute pain?
Brain changes
Nerve and spinal cord changes
Other mediators and receptors become involved
Drugs that worked before now no longer do the trick
CHRONIC PAIN IS A DISEASE IN AND OF ITSELF
What are the consequences of chronic pain?
In humans, highly correlated with depression and reduced Quality of Live (QOL)
Coping mechanism –> differs between individuals
In animals, how may it show?
- Social ranking, wasting
- Demeanour: Withdrawal, aggression
- Reduction in previous spontaneous behaviour + specific signs (e.g. dental/ ocular/osteoarthritis)
- Differentiation vs normal ageing can be hard!!!!!!
What do we know about pain assessment and monitoring?
Pain has been recognised as the 5th vital sign after body temp, pulse rate, respiration rate, blood pressure
How could we NOT monitor it in veterinary patients?
- Clinical patients
- Laboratory animals
–> Ethics
–> Welfare
–> Validity of study outcomes
How can we measure pain in animals?
Subjective (qualitative) assessment
Biochemical parameters
Semi-quantitative measures:
- VAS (visual analogue scale)
- NRS (numerical rating scale)
- SDS (simple descriptive scale)
- CPS (composite pain scale)
- Time budget analysis
- Facial expression based pain scales
- Owner assessment tools
What are simple pain scales (‘rulers’)?
VAS scores are useful in human verbal subjects
Not so much so in absence of self-report!! Assumptions, anthropomorphism
Observational VAS scores have high bias and low inter-observer agreement
- Observer’s own attitudes, experience, upbringing, societal views
What are objective and quantifiable signs of pain?
Physiological and biochemical markers
What are some clinical signs of pain?
Tachycardia, tachypnoea, temperature, blood pressure changes –> go up with pain but many confounders
Changes in pupil size, sweating, poor body or coat condition –> likewise
What are some biochemical parameters for pain?
Changes in ‘stress hormones’ (cortisol, glucose, insulin, adrenaline, endorphins)
Not very specific either: increased by disease, fear/ anxiety/excitation, stress, handling, anaesthesia etc.
Pain mediators (prostaglandins, substance P) –> Pain is more complex than this!!
What are multidimensional/Composite pain scales?
Scales that include multiple variables, scored individually using clear descriptors, which are then combined to give an overall pain score
Scales based on different classes/types of parameters
Incorporate spontaneous and interactive behaviour –> emotional-affective component of pain
Species and breed effects
What makes a pain scale reliable?
The scale captures what you hope to measure
It is sensitive and specific
Scores are reproducible
There are no redundant items
What makes a pain scale practically useful?
Easy to perform, instant results
Repeatable and reproducible
Minimal time (<2 minutes)
Does not require extra observers/help
Effective to monitor treatment
What are the drawbacks of multidimensional pain scales?
Not yet ideal
Some not (fully) validated
Redundant items
Omit physiological variables?
Include temperament score?
Species, breed and age effects
Composite scales may not always be more reliable than unidimensional ones, but they appear more sensitive
What is the practical problem of pain scales?
SPECIES DIFFERENCES!
Anatomy
Physiology
Behavior
Human-animal bond
What is another practical problem of pain scales, specifically facial expressions as pain indicators?
BREEDS!!
Breed influences:
- Facial features
- Behaviour/temperament
- Human-animal bond
What are facial pain scales?
Developed for cats (FGS)
Not yet for dogs, may need to be validated per dog breed
What are the more clinical challenges when deciding “is it really pain”?
Effects of lingering anaesthetics?
Excessive sedation?
Dysphoria?
Full bladder?
Elizabethan collar?
Big bandage?
Agitation during recovery?
Nausea?
Hyperthermia?
How are pain scales validated (type of pain matters too!)?
Remember: No “gold standard” measurement…
One pain scale may not capture all clinical pain states
Majority of scales have been developed for acute post-surgical pain in dogs
Same in horses (HGS, FAP), farm and lab animals
Visceral vs somatic pain (e.g. colic vs fracture)
Medical pain? Ear disease? Pneumonia?
Chronic pain? Neuropathic pain? Cancer pain?
How do we measure chronic pain in animals?
Pain scales for acute pain less useful here!
Humans: Validated questionnaires, quality of life impact
Owner/Caretaker questionnaires:
- Psychometric testing and validation
- Influence of language and human-animal bond
What are the stages for selecting a pain assessment tool?
- Species
- Breed, age - e.g. neonate vs adult - if scale available
- Specific painful condition
- Feasibility in practical setting:
- Clinic vs lab vs at home
- Owner/caretaker vs vet assessment
- Time!
- Training, experience, motivation
- Other resources (video..)
What is the systematic approach to clinical analgesia?
- Understanding patient’s pain pathophysiology
- Adequate pain recognition
- Adequate and prompt intervention
- Preventative, multimodal
- Step-up, step-down - Monitor, evaluate response
- Adjust treatment plan as needed
What is preventative analgesia?
Analgesic drugs given before stimulus will prevent sensitisation of the nervous system after tissue injury
- More effective pain relief
- Effect can last beyond duration of action of the drug
What is multi-modal analgesia?
The combined use of drugs from different classes via one or more routes of administration to achieve better pain relief at lower dosages of each drug –> fewer adverse effects
- Block sequential steps in nociceptive pathway
- Target different mediators (e.g. PGE2, Na channels, NMDA-receptor)
Add another, vs giving more of the same drug –> Let each drug do what it is good at!
What is non-pharmacological pain management?
Nursing care
Physical support
Cooling, warming
Nutritional support
Mental support - contact with other animals, distraction(!), at times: Sedation –> anxiety worsens pain!
Stretching, massage, physiotherapy
Acupuncture, chiropractics
Owner - Vet - Therapists
Teamwork
What is in our analgesic toolkit?
- NSAIDs
- Paracetamol
- Opioids
- A2-agonists
- Local anaesthetics
- Others/adjuvant analgesics
- E.g. ketamine, amantadine, gabapentin, antidepressants
What are the routes of administration of analgesics?
Topical: Mucous membranes (OTM), Skin (transdermal)
Systemic: PO (by mouth), IM (intramuscular), IV (intravenous), SC (subcutaneous)
Locoregional: Perineural (nerve blocks), Epidural, Intra-articular
What is our medicine cabinet from a mechanistic perspective?
Targets and analgesics
TARGET:
- Prostaglandin production
- Cannabinoid receptors, serotonin
- Neuronal CaV channels
- Opioid receptor (mu, kappa)
- Glutamate signals via NMDA receptor
- Noradrenaline, serotonin
ANALGESIC:
- NSAIDs (paracetamol)
- CBD, paracetamol
- Gabapentin
- Opioids (tramadol)
- Ketamine (methadone)…
- A2-agonists, TCAs/SSRIs
What do NSAIDs do?
Produce analgesia through anti-inflammatory effects –> Arachidonic acid cascade
Central as well as peripheral effects
Heterogenous class: Aspirin, carprofen, phenylbutazone (‘Bute’), firocoxib
Reduce peripheral sensitisation
Do NOT give concurrently with Steroids (PLA2 inhibition on top –> Toxicity)
What are the roles of prostaglandins and leukotrienes?
Inflammatory, nociceptive and vasoactive mediators
Also involved in:
- Gastro-protection (prevent stomach ulcers)
- Renal protection (preserve renal blood flow)
- Blood clotting
- Airway constriction
- Nerve transmission
COX-1 constitutive, COX-2 inducible
Not as clear-cut as thought
What is the current thinking around Cyclo-oxygenase (COX)?
Overlap in functions of COX-1 and COX-2
- COX-1 is up-regulated at sites of injury (e.g. COX-1 is involved in ~30% of prostaglandin production in osteoarthritis)
- COX-2 also has physiological functions in the body, e.g. at sites of gastroduodenal erosion (e.g. COX-2 is involved in healing of GI ulcers)
- Selective COX-2 inhibitors will not eliminate all side effects!!
Can use gastroprotectants, trial different NSAIDs
What does paracetamol do?
Thought to act on COX-3 iso-enzyme within the CNS, may be a splice variant of COX-1 centrally
Paracetamol: Not classified as a traditional NSAID
- Limited inhibition of COX in periphery –> not a good anti-inflammatory
- Good anti-pyretic (fever), analgesia:
–> Serotonergic pathway activation
–> Acts via AM404 metabolite on central cannabinoid (CB1) and TRPV receptors
CATS: TOXICITY!!! (DEFICIENT GLUCURONIDATION)
Useful in dogs and horses –> PK/PD, concurrent use with NSAIDs, owner self-dosing? CAUTION!!
What are opioids?
Endogenous opioids:
- Produced by body
- Endorphins, dynorphins, enkephalins
Exogenous opioids:
- Can be agonists, partial agonists, antagonists, or agonist-antagonists at opioid receptors
Opium poppy: Papava somniferum;
- Contains ~20 pharmacologically active drugs
- Main component is morphine
What are the opioid receptors?
µ (mu); MOP; OP3
- Main receptor for analgesia, also: side effects
k (kappa); KOP; OP2
- Some supra-spinal analgesia, dysphoria
δ (delta); DOP; OP1
- May modify action of other receptors
Nociceptin (orphaning FQ) receptor; NOP
- May have anti-opioid effects (pro-nociceptive!)
Many receptor subtypes eg. µ1, µ2, µ3
What do we know about opioid receptor binding?
Receptors found in brain, spinal cord and periphery (e.g. joints, GI tract, cornea…)
Inhibitory G-protein coupled receptors
Agonist binding causes:
- Opening of K+ channels, cell hyperpolarisation
- Inhibition of adenylate cyclase –> cAMP decreases
- Closing of voltage gated Ca2+ channels
Decreased neuronal excitability and neurotransmitter release
Can also act via beta1 arresting - involved in receptor internalisation - tolerance
What are opioid agonists?
Gold standard analgesics for moderate to severe pain
Full µ agonists:
- Morphine (“gold standard” opioid)
- Methadone
- Fentanyl, remifentanil etc.
Euphoria, tolerance, dependence and addiction in humans –> Opioid crisis!!!
Controlled drugs: Misuse of drugs act 1971
- Schedule 1: Illegal (heroin…)
- Schedule 2: Full agonists, must be kept in locked cupboard, records of individual animal use, disposal, ordering… no at home use!!
What are the effects of opioids?
(Profound) Analgesia
Altered mentation: Species dependent
- Sedation, Euphoria or dysphoria (mood altering)
- Excitation
- Receptor distribution CNS (Cat/horse vs dog)
Miosis/mydriasis:
- Dosage
- Pain state
Respiratory depression:
- Mainly a problem in humans, not so much in healthy animals except w/potent opioids e.g. fentanyl
Bradycardia (slow heart rate)
Nausea and vomiting - mainly with morphine (less lipophilic)
What are agonist-antagonists? (opioids)
Agonist at one type of opioid receptor while antagonist at another opioid receptor
E.g. Butophanol:
- µ antagonist
- k agonist
- Limited analgesia
- Good sedation
- Not controlled in the UK at the moment
What are partial agonists? (opioids)
Agonists at opioid receptor, but maximum effect is less than that of full agonist
E.g. Buprenorphine
- Partial µ agonist
- Decent analgesia, but not used for very painful procedures (see later)
- Schedule 3: Fewer controls than schedule 2 but must be kept in locked cupboard
Does potency matter clinically?
Potency describes the amount of drug required to obtain a specified effect
- Less potent drug –> Need higher dose (mg/kg vs µg/kg)
- Buprenorphine analgesic dose = 0.02 mg/kg
- Methadone analgesic dose = 0.2 mg/kg
Efficacy describes the clinical effect attributed to a drug
- Methadone (a full µ agonist) is more efficacious than buprenorphine (a partial µ agonist) in dogs
- Can treat more severe pain (max effect is bigger)
Potency is clinically relevant when the required dose physically cannot be given (i.e. due to solubility issues)
After a surgical procedure, how is opioid reversed?
Could antagonist an opioid overdose with naloxone (pure opioid antagonist)
- Will antagonise all exogenous opioids
- But: Can leave the animal very painful!!!
Buprenorphine: Partial µ agonist:
- Able to displace the full agonist from the receptor
- Hence antagonises some of the effects of full agonists
- Provides some analgesia of its own :)
How does buprenorphine act as a partial µ agonist by displacing the full agonist?
Slow receptor kinetics:
- Takes a long time to bind to the receptor
- Once bound, remains there for a long time
- Long onset and duration of action
High affinity for µ receptor
- Binds very tightly to the receptor
- Hard to displace from receptor
- :( If you give buprenorphine and analgesia is insufficient, you will need a high dose of a full agonist (e.g. fentanyl) to “out-compete” buprenorphine at receptor
How does buprenorphine behave in the body?
Bell shaped curve in effect
Despite partial receptor agonism, its analgesic effect may be strong
Not as addictive in humans - less prone to abuse
Ceiling in respiratory depression
Used for weaning in anti-addiction programmes
Lipophilic - transmucosal administration route
PK similar to intravenous administration (saliva pH)
Oral admin (swallowed): High first-pass metabolism in liver
What are the species specific opioid adverse effects?
CATS, DOGS:
- Nausea
- Vomiting
- Sedation/euphoria/ dysphoria
FEARED IN HERBIVORES:
- Reduced GI motility
- Stomach emptying decreases
- Borborygmi decreases
–> Risk of colic, impaction
BUT SEVERE PAIN ALSO REDUCES GI MOTILITY!!
- Multimodal: Locoregional whenever possible
- Monitor faecal output, laxatives, taper asap
How can we administer opioids?
Locoregional (morphine; e.g. epidural, in a joint)
Systemic:
- Oral (animals; low bioavailability)
- Transdermal (patch)
- Transmucosal: Buccal/ sublingual buprenorphine, nasal (fentanyl spray)
- IV/IM/SC injection (bolus)
- Continuous rate infusion (CRI; IV) avoids “peaks and troughs”
What do we know about opioid administration via transdermal route?
Fentanyl/Buprenorphine patch
Long duration analgesia without injection
Lipophilic drugs: Absorbed across intact skin
What do we know about opioid administration via the epidural or spinal/intrathecal routes?
Epidural
- E.g. morphine, more hydrophilic, slower to penetrate CNS, long-acting
Spinal/intrathecal
- Shorter acting
What do we know about opioid administration via the intra-articular route?
Opioid receptors up-regulated in inflamed joints
Drug stays within joint space
- Excellent analgesia
- Long duration (q12-24hrs)
- Minimal systemic side effects
What are alpha-2 adrenoceptor agonists examples in human and veterinary use?
Human use:
- Clonidine
- Dexmedetomidine
Veterinary use:
- Xylazine
- Detomidine (also sublingual gel form
- Romifidine
- Dexmedetomidine
Different cardiovascular effects per drug (e.g. xylazine) and per species (e.g. ruminants vs dogs/cats
Can be given systemically (sedation, analgesia) or locoregionally (nerve blocks, epidural): Receptors co-localise with opioid receptors!!
What do alpha 2 adrenoceptor agonists do on a cellular level?
Act on alpha 2 adrenoceptors (G-protein coupled receptors) to inhibit adenylate cyclase and decrease cAMP
Presynaptic receptors (CNS)
- Reduced NA release
—> Symatholysis
—> Sedation
—> Analgesia
- Postsynaptic (periphery): eg. vessel bed: Vasoconstriction
The pharmacology of individual drugs varies because of alpha 2 receptor subtype selectivity (A, B and C)
What do alpha 2 adrenoceptor agonists do as a more overall effect?
Analgesia
- Central sites of action: Spinal and supra-spinal
(Profound) sedation, ataxia
Marked cardiovascular effects:
- Vasoconstriction
- Bradycardia and Brady-arrhythmias
- CO drop
What are alpha 2 adrenoceptor agonist examples and what do they do?
Atipamezole - also reverses analgesic effect!
Vatinoxan - only reverses peripheral effect
What are some Alpha 2 adrenoceptor agonist side effects?
Sedation, ataxia, recumbency
Cardiovascular:
- Hypertension followed by normo-hypotension
- Bradycardia
Low dosages/CRI:
- Haemodynamic stability
Reduced GI motility (prolonged/high doses)
Increased uterine tone (avoid in pregnancy if possible)
Reduced ADH and insulin release: Urine output and blood glucose increases
What are local anaesthetics?
Drugs that block voltage-gated Na+ channels
Inhibit transmission of nerve impulses
- Especially nociceptive fibres (Aδ and C fibres)
- Sympathetic fibres also affected (vasodilation)
- Motor blockade can occur (first to un-block)
- Block nociceptive input into the CNS from the periphery –> Prevent sensitisation
Reduce the amount of other analgesics and anaesthetics required
-Caine: Lidocaine (lignocaine), bupivacaine, Etc…
Ester vs Amide, pH/pKa (weak bases), lipophilicity:
- Determine onset, duration, practical application!!
What are some examples of other analgesic drugs?
Lots of drugs have been found to have analgesic efficacy in addition to their main/other clinical usage:
- Ketamine - anaesthetic
- Amantadine - anti-viral agent
- Gabapentinoids - anti-convulsive
- Tricyclic antidepressants
- Maropitant? Anti-emetic
What is glutamate receptor signalling?
Glutamate: Key excitatory neurotransmitter in the spinal cord
Normally acts on AMPA and kainate receptors
Repeated release of glutamate and substance P (repeated pain stimulation)
- Primes NMDA (N-methyl-d-aspartate) receptor
- Removes Mg+ plug on NMDA receptor
- Allows Ca2+ influx and activation of 2nd messengers
What are NMDA receptor agonists?
E.g. Ketamine, amantadine, methadone, nitrous oxide
- Receptor is key in development of chronic pain states
- Neuropathic pain: “wind up”
- NMDA antagonists are useful to manage chronic pain and hyperalgesic states
What are gabapentinoids?
Ex. Gabapentin, pregabalin
Anti-epileptic drugs
Block neuronal a2δ calcium channels, reducing firing
Niche: Neuropathic and nociplastic pain (“wind up”)
Common usage in veterinary species (cats, dogs, (horses))
- Oral bioavailability
- When NSAID precluded? –> Not the best indication!!!
- Sedative effect (~cage rest/calming!) very useful
- Concerns over human diversion
