ANS and neurotransmitters Flashcards
How big is the hypothalamus? What shape?
4cm squared wallnut shaped grey matter centre
What are the relations of the hypothalamus
◦ Mammilary bodies, periaqueductal grey matter and tegmentum posteriorly
◦ Lamina terminalis anteriorly / optic chiasm
◦ Inferior to the 3rd ventricle
◦ Surrounded by basal ganglia
◦ Extends from mammillary bodies posteriorly to the lamina terminalis anteriorly
◦ Connected to the pituitary by the infundibulum inferiorly
◦ Separated from the thalamus by the sulcus of Monro
Blood supply of the hypothalamus
◦ Anterior cerebral artery
◦ Perforating branches of the PCA
◦ Posteriormedial branches of PComm
Veinous drainage of the hypothalamus
into cavernous sinus via inferior hypophyseal veins and into the hypothalamo
* - hypophyseal portal system
What is contained in the anteiror hypothalamus
Supraoptic nuclei
Paracentricular nuclei
Controllin
- PSNS
- Heat loss
- Sleep
- Posterior pituitary ormones -osmoreceptors in supraoptic nuclei, oxytocin in paraventricular nuclei
What does the medial hypothalamus do
Energy balance
Sexual behaviour
Satiety
Lateral hypothalamic function
Efferent pathways ot the brainstem
Emotion and defence
Thirst
Desire to seek food
Posterior hypothalamus does what
SNS - vasomotor centres of the brain
Wakefullness
Main functions of the hypothalamus
Water balance and tonicity
Temperature
Autonomic nervous system
Pituitary endocrine function
Appetite and satirety
Behaviour and emotion
Circadian rhtyhm
How is the autonomic nervous system related to the hypothalamus?
◦ PSNS from anterior pituitary
◦ SNS from posterior pituitary - vasomotor centres of the brain
What are the 6 main hormones secreted from the hypothalamus triggering anterior pituitary release
◦ Thyrotropin-releasing hormone (TRH) - release cycle time 2-4 hours, T4 negative feedback
◦ Gonadotropin-releasing hormone (GnRH) - Pulsatile cycle time 90 mins, emotion, circadian, sexual stimuli cause release
◦ Growth hormone-releasing hormone (GHRH) - 3 isoforms, circadian release in sleep, physiological stress increases release
◦ Corticotropin-releasing hormone (CRH) - GC main regulatory feature over release
◦ Somatostatin
◦ Dopamine - suppressing prolactin
Outline the anatomy of the sympathetic chain
‣ Cervical part - extending superiorly from thoracic origin to head, neck and thorax
‣ Thoracic part T1-5 extends to aortic plexus, pulmonary plexus, cardiac plexus and thoracic splanchnic nerves
‣ Lumbar ganglia - coeliac plexus
‣ Pelvic plexus
thoracolumbar (T1 - L2) paraspinal columns from which arise post-ganglionic nerves which travel to peripheral locations for effect
Preganglionic SNS nerve cells arise from where? Describe their path
‣ Cell bodies arise from grey matter of lateral horns T1-L2–> leave through ventral route of spinal nerve/ primary anterior rami –> rami communicates (myelinated B fibres) –> sympathetic chain (ganglia of sympathetic trunk) synapse with post ganglionic neurones in ganglion
* Preganglionic fibre length short
* release ACh to stimulate post ganglionic cells (nicotinic receptor) - in the sympathetic chain at the same level,
* a different level of leaving through splanchnic nerves to a prevertebral ganglion
What is the path of post ganglionic sympathetic nerve fibres?
‣ unmyelinated pass into adjacent spinal nerve via grey rami communicates and travel with spinal nerves to target organs
‣ Post ganglionic nerve cell body in the sympathetic chain pre/para vertebral
‣ Long fibre length
‣ Adrenergic –> release norepinephrine stimulating alpha or beta G protein coupled receptors.
* Special circumstance –> adrenal medulla which has preganglionic nerves directly synapsing with chromaffin cells which secrete adrenaline (80%) into the blood stream in response to ACh stimulation
‣ Cholinergic –> sweat glands, vasodilator blood vessels in skeletal muscle
How is adrenal outflow different for SNS supply
‣ unmyelinated pass into adjacent spinal nerve via grey rami communicates and travel with spinal nerves to target organs
‣ Post ganglionic nerve cell body in the sympathetic chain pre/para vertebral
‣ Long fibre length
‣ Adrenergic –> release norepinephrine stimulating alpha or beta G protein coupled receptors.
* Special circumstance –> adrenal medulla which has preganglionic nerves directly synapsing with chromaffin cells which secrete adrenaline (80%) into the blood stream in response to ACh stimulation
‣ Cholinergic –> sweat glands, vasodilator blood vessels in skeletal muscle
What is the effect of the SNS
- Fight or flight response as a diffuse physiological accelerator
- Cardiovascular
◦ Increased chronograph, inotropy, lusitrophy and dromotrophy
◦ Increased afterload due to increased vascular constriction with increased venous return from increased venous tone - Pulmonary - bronchial dilation
- MSK - sweating, constriction, lipolysios
- Pupillary dilation
- GI/GUT - decreased secretions, increased sphincter tone, gluconeogenesis
◦ Saliva production decreases
Describe the path and type of fibre fo PSNS
◦ Myelinated B fibres
◦ Site of ganglia for synapse with post ganglionic cells near or in effector organ
◦ Therefore long pre-ganglionic fibres
‣ Cell body within the brain stem for cranial nerves or sacral grey matter (hypo gastric plexus)
◦ Acetylcholine released from preganglionic cell activates post ganglionic neuron via nicotinic receptors
Describe the path and characteristics of post ganglionic PSNS
◦ Short in length, unmyelinated C fibres
◦ In smooth muscle, heart, glands
◦ Acetylcholine via muscarinic receptors (GPCR) to modulate target organ activity
What are the PSNS CN
3 7 9 10
CN 3 acts as a PSNS via?
‣ CN 3 occulomotor nucleus —> ciliary ganglion —> ciliary muscle, iris spincter
CN7 acts as a PSNS
‣ CN 7 - Superior salivary nucleus —> submaxillary ganglion —> submaxillary and sublingual salivary glands
CN9 acts as a PSNS
‣ CN 9 - Inferior salivary nucleus —> optic ganglion —> parotid gland
How is CN10 implicated in PSNS function? How far down does it stretch
‣ CN10 Vagal nuclei in medulla from —> dorsal nucleus of vagus (visceral), nucleus ambiguus (PSNS to heart, motor to larynx), NTS (visceral afferents and taste) and spinal tract of trigeminal nucleus
* vagus is the major parasympathetic nerve innervating
◦ Cardiac plexus - SA node by R vagus, AV node by L vagus, and ventricles sparsely by L vagus
◦ Lungs via pulmonary plexus
◦ Stomach, liver, spleen, pancreas and gut proximal to the splenic flexure by the gastric plexus
Describe the overall effects of the parasympathetic nervous system
- Rest and digest - a physiological brake on cellular function
- CN3 - pupillary constriction (M3)
- CN 7 - lacrimation
- CN 9 - salivation
- CN10
◦ Cardiac (M2) - reduced chronotropy, reduced dromotropy, minor reduction in inotropy and lusitropy (affects atria more than ventricles)
◦ Respiratory (M3) - bronchoconstriction, increased mucous production
◦ GIT (M3 motility, Beta 2 and alpha glycogen)- increased secretions, increased motility, decreased sphincter tone - Sacral plexus - GU - (M3) detrusor contraction and erection,anus relaxation with rectal contraction, uterine contraction
What is acetylcholinesterase
- Acetylcholinesterase (AChE) is an enzyme that hydrolyse acetylcholine (ACh) into choline & acetate AChE is found in synaptic clefts and is responsible for the termination of synaptic transmission
- Common action of anti-cholinesterases = allow build up of Ach and prevent it from being destroyed.
Two types of cholinesterase
- Achesterase - nerve endings & in RBCs
- Non-specific or pseudocholinesterases - destroy other esters - tissues & plasma
Acetylcholinesterase binding sites include
anionic and esteratic
◦ ANionic site binds quaternary amine group of ACh
◦ Esteratic site binds ester group of ACh
◦ Binding –> hydrolysis and breakdown into choline and Acetyl CoA
How does Acetylcholinesterase break down ACh
anionic and esteratic
◦ ANionic site binds quaternary amine group of ACh
◦ Esteratic site binds ester group of ACh
◦ Binding –> hydrolysis and breakdown into choline and Acetyl CoA
How does Acetylcholinesterase break down ACh
anionic and esteratic
◦ ANionic site binds quaternary amine group of ACh
◦ Esteratic site binds ester group of ACh
◦ Binding –> hydrolysis and breakdown into choline and Acetyl CoA
How do you classify the acetylcholinesterase drugs
- Reversible antongist via electrostatic binding - edrophonium
- Reversible antagnoiist via covalent binding - neostigmine
- Irreverrsible antagonist via covalent binding - organophosphates
How does edrophonium act
Reversible antagonist via electrostatic binding – e.g. edrophonium
‣ causes electrostatic attachment to the anionic site of the enzyme -> stabilising the H+ bond at the esteratic site -> edrophonium-AchE complex prevents Ach from binding
How does neostigmine act
Reversible antagonist via covalent bonding – e.g. neostigmine
‣ [Formation of carbamyl esters (carbamates)] - ie. neostigmine, physostigmine & pyridostigmine
‣ antagonise AchE enzyme by being competitive substrate for Ach -> forms a carbamyl-ester complex at the esteratic site of enzyme. - longer lasting bond (15-30min)
How do organophosphates act?
Irreversible antagonist via covalent bonding – e.g. organophosphates
‣ combine with Ach at the esteraic site to form a stable covalent bond -> does not undergo hydrolysis. - synthesis of a new AchE is required.
What is a nicotinic effect
◦ Reversal of non-depolarising neuromuscular blockers
◦ Prolongs effect of suxamethonium (depolarising NMB)
◦ Anticholinesterase overdose → excess synaptic ACh → depolarisation block ± fasciculation
What is a muscurinic effect
Wh
Which occurs first a muscurinic effect or nicontinnic effect with acetylcholinesterase inhibitors
Muscurinic at lower doses
What is a muscurinic effect
◦ CVS – bradycardia ± hypotension
◦ RESP – bronchoconstriction ± bronchospasm
◦ CNS – miosis, cholinergic syndrome – confusion, agitation, nausea/vomiting
◦ GIT – hypersalivation, ↑GIT motility, nausea/vomiting, diarrhoea
◦ GUT – urination, incontinence
◦ OTHER – lacrimation, diaphoresis
‣ Mnemonic SLUDGE-BM: Salivation/Sweating, Lacrimation, Urination, Diaphoresis/Diarrhoea, GI upset, Emesis, Bradycardia/bronchospasm, Miosis
Which occurs first a muscurinic effect or nicontinnic effect with acetylcholinesterase inhibitors
Muscurinic at lower doses
What is a central cholinergic syndromev
◦ CVS – bradycardia ± hypotension
◦ RESP – bronchoconstriction ± bronchospasm
◦ CNS – miosis, cholinergic syndrome – confusion, agitation, nausea/vomiting
◦ GIT – hypersalivation, ↑GIT motility, nausea/vomiting, diarrhoea
◦ GUT – urination, incontinence
◦ OTHER – lacrimation, diaphoresis
‣ Mnemonic SLUDGE-BM: Salivation/Sweating, Lacrimation, Urination, Diaphoresis/Diarrhoea, GI upset, Emesis, Bradycardia/bronchospasm, Miosis
What are the clinical uses of acetylcholinesterase inhibitors
Reversal of non depolarising block
Diagnosis and treatment of myasthenia gravis
Treatment of cognitive impairmeent
Glaucoma
Anticholinergic syndrome
How is duration of effect different between acetylcholinesterase drugs?
◦ Edrophonium short
◦ Neostigmine medium
◦ Organophosphate long
Wy
Which acetylcholinesterase drugs are reversible? Which are not?
◦ Edorphonium and neostigmine
◦ Irreversible organophosphate
Edrophonium structure
Quaternal armine
O
Onset of edrophonium
1-2 minutes
Offset of edrophonium
10minutes
Does edrophonium cross BBB
no
How is edrophonium cleared
Liver glucoronidation
How is edrophonium cleared
Liver glucoronidation
◦ 35% as biliary metabolites and 65% renally unchanged
Pyridostigmine as structurally
An analogue of neostigmine
How does pyridostigmine compare to neostigmine
Structural analogue with 1/4 potency
Why is pyridostigmine useful
Longer mechanism of action - 6 hours
Slower onset
What are some pharmacokinetic factors important to organophosphates
◦ Lipid soluble, transcutaneous absorption
◦ Large Vd
◦ Long excretion time
How does G protein behaviour translate to effects in GPCR
◦ Alpha - subunit can bind GDP and GTP; inactive GDP is bound to the alpha subunit ; when activated by extracellular ligand GDP exchanged for GTP and alpha unit dissociates from Beta/Gamma chains enabling it to affect ion channels or intracelluilar messengers
‣ Signal amplification occurs via multuple secondary mesengers and ion channels being affected by one GPCR
‣ Inactivated when alpa unit hydrolyses GTP to GDP and rejoins complex
‣ Intrinisc GTPase of the unit mens self limiting step
Gs protein coupled receptors have what intermediaries
‣ Activated by epinephrine, NA, histamine, glucagon and others
‣ Stimulates adenylate cyclase –> ^cAMP (from ATP)
Gi protein coupled receptors act how
‣ Activated by NA, PG, opiates and many peptides
‣ Decreases cAMP
Gq protein coupled receptors act how
‣ Acetycholine
‣ Catalyses phosphatidylinositol conversion via phosphodiesterases to –> Increase IP3 + DAG –> increased Ca intracellularly via endoplasmic reticulum and calcium membrane channels
What are the neurotransmitter vesicular transport proteins
nside the cell, there are two vesicular monoamine transporters: VMAT1 and VMAT2
‣ They transfer the reabsorbed neurotransmitter back into the vesicles
‣ They have little specificity- they will just take anything and drag it back into the vesicles
4 excitatory neurotransmitters
- Glutamate
- Dopamine
- Noradrenaline
- Acetylcholine (nicotinic receptors
What is the enzyme implicated in ACh creation
- Ester of choline (acetylated)
- Synthesized from choline (quaternary saturated amine water soluble nutrient complexed with B vitamins) and acetyl-CoA by choline acetyltransferase; cholinergic neurons actively suck choline up through a transporter.
How are substrates for ACh sourced or made
cholinergic neurons actively suck choline up through a transporter.
◦ Acetate from acetyl CoA formed from pyruvate and CoA by pyruvate dehydrogenase
What is the rate limiting step in ACh creation
Reuptake
How is ACh reuptaken
‣ One is Na/Cl dependent hihg affinity transporter
‣ The other is a lower affinity transport
How does suxamethonium cause hyperkalaemia
Depolarises nicotonic receptors at NMJ which causes K flow out, Na flow in
In anyone with heaps of nicotonic receptors this is more likely to occur in large amounts e.g. burns, spinal cord injury etc
Synaptic plasticity occurs via
Phosphorylation
endocytosis
To remove activity of surface membrane receptors
Muscurinic receptors - subtypes and activity
M1, 3, M5 GPCR Gq
M2, M4 Gi protein coupled receptors
How do M1 , 3 , 5 recepetors act? Where?
‣ Phospholipase C –> IP3, DAG system to increase IC Ca and open external calcium gated K+ channels (via phospholipase C activation of K+ channels) to hyperpolarise the cell
‣ M1 receptors in sympathetic ganglia and high affinity for antagonist pirenzepine; neural effect on memory, gastric secretio and GIT mobility
Where do M2 receptors act? How?
Gi protein (decrease cAMP by inhibiting adenylyl cyclase) —> increased K+ channels
‣ M2 = heart; depressing autorhythmicity by opening inward rectifying K+ channels
How are adrenaline and noradrenaline made?
What is the amino acid from which adrenaline is produced
Tyrosine
What is the intermediary molecule between tyrosine and dopamine?
DOPA
How is tyrosine converted into dopamine
How are catecholamines broken down?
MAO
COMT
Where is MAO
OXIDATION MAO into inactive metabolites inside the cell (after reuptake)- this is the main way of getting rid of intracellular catecholamines. It is in high concentration in the liver and kidney but is everywhere
◦ MAO is located on the outer surface of the mitochondria, and comes in two flavours:
‣ MAO-A - peripherally located in the syncytioblast in the term placenta and the liver
* Centrally in noradrenergic neurons mainly the locus Coeruleus
‣ MAO-B - Peripehrally in platelets, lymphocytes and the liver, entrally in serotonergic neurons
What structurally must be present for MAO to work
Amine group without a too large substituent, and nothing on the alpha carbon
What is the end product of catecholamine metabolsim
Vanillyl Mandelic acid VMA
COMT degrades catecholamine by?
- Some of the metabolism (methylation by exchnaging hydroxyl group at the 3 position on the catechol ring for methyl group) is also performed by catechol-O-methyltransferase (COMT) and this is the main way of getting rid of extracellular catecholamines
◦ Concentrated in the liver and kideny but present everywhere
◦ In cytoplasm
◦ Mostly responisble for IV catecholamine metabolism
◦ For it to work the catechiolamine ring must be intact with 2 hydroxyl groups
What does COMT need to wrok on catecholamines
- Some of the metabolism (methylation by exchnaging hydroxyl group at the 3 position on the catechol ring for methyl group) is also performed by catechol-O-methyltransferase (COMT) and this is the main way of getting rid of extracellular catecholamines
◦ Concentrated in the liver and kideny but present everywhere
◦ In cytoplasm
◦ Mostly responisble for IV catecholamine metabolism
◦ For it to work the catechiolamine ring must be intact with 2 hydroxyl groups
What is a catecholamine ring?
A benzene ring (6 carbon phenyl ring) with two hydroxyl groups in the 3 and 4 position which allow it to be effective at catecholamine receptors
◦ 3, 4 dihydroxybenzene
◦ Losing one hydroxyl group
‣ Increases lipid solubility and decreases the potency 10-fold
‣ Prevents metabolism by COMT, prolonging duration of action
◦ Losing both hydroxyl groups decreases the potency 100-fold - maximum potency occurs when separation of the hydroxy groups and ethyl amine groups is maximal.
Draw the structure of a catecholamine including labellling the important components often altered
What is the action of dobutamine at alpha and beta receptors
- Dobutamine is a potent (full) beta-1 agonist
- Dobutamine is a potent (partial) alpha agonist, which means it acts as an antagonist in situations where there is massive sympathetic overdrive (or co-administration of alpha agonist)
- Dobutamine is a weak (partial) beta-2 agonist, which means it acts as an antagonist in presence of full beta-2 agonists like adrenaline or isoprenaline.
How is noradrenaline created in the synapse
- Noradrenaline from the cytoplasm (not in vesicles) is concentrated by VMAT 2 protein (vesicular amine transporter) into vesicles
◦ VMAT2 has equal affinity for dopamine, adrenaline, NA, seratonin
◦ Gets 90% into vesicles - the rest float about in the cytoplasm and get metabolised by mitochondrial MAO
◦ Once in vesicles they are trapped by low pH - existing in ionised water soluble form unable to diffuse out
◦ Dopamine is converted to NA by dopamine beta hydroxylase within the vesicles
What calcium channels mediate exocytosis of neurotransmitters
◦ Mediated by Calcium influx through voltage gated N type calcium channels which open when the action potential reaches the synapse
How is noradrenaline release modulated at the presynpatic membrane
◦ Enhanced release - presynaptic Beta 2 receptors –> increased cAMP
◦ Inhibited release - reduced cAMP
‣ Presynaptic alpha 2 AND adenosine A1 receptors presynaptically
presynaptic alpha 2 receptors do what
Inhibits the release of catecholamines
presynaptic beta 2 receptors do what
Enhance release of catecholamines
Presynaptic adenosine receptors do what
Inhibit the release of catecholamines
Caffeine is an antagonist of this therefore enhance the release of catecholamines
Reuptake of noradrenaline occurs via
DAT and NET transporters
What is the rate limiting step of adrenaline syntheiss
tyrosine transformation to DOPA by tyrosine hydroxylase
Alpha 1 action acts via which GPCR? What effect odes this have intracellularly?
◦ post synaptic, stimulation = vasoconstriction + contractility (different biochemical action to Beta1 receptors)
‣ MOA –> Gprotein (Gq) –> Phospholipase C –> IP3 + DAG –> Ca2+ release from sarcoplasmic reticulum and increased calcium sensitivity –> activation of calmodulin sensitive enzymes including myosin light chain kinase, calmodulin dependent protein kinase 1 and 2, phosphodiesterases
‣ Most pronounced inotropic effect an low frequencies of myocardial contraction (hypothermia) and does not affect HR
‣ Vasoconstriction in vascular smooth muscle
What actions does alpha 1 have
‣ Vasoconstriction - arterioles of heart, brain, kidneys, lungs, skeletal muscle, skin
‣ Mydriasis - radial muscle of the iris (dilating pupil)
‣ Contrcats gut sphincters
‣ Inhibition of insulin release
How does an alpha 2 receptor cause intracellular changes
◦ Pre and post synaptic
◦ Release of NA from presynaptic terminal activates alpha 2 receptor to inhibit the further release of noradrenaline (i.e. negative feedback), post junctional alpha 2 receptors are also located on reistsnace and capacitance vessels which mediate vasoconstriction
◦ Inhibit adenylate cyclase via Gi protein –> decreased cAMP and opening of K+ channels –> reduced phosphorylation of proteins and hyperpolarisatino of the membranes
What are the observed effects of alpha 2 agonism
◦ Action
‣ relaxes the walls of the gut wall smooth muscles
‣ Central effects are sympatholytic - presynaptic inhibition takes place
How does alpha 1 compare with alpha 2
Alpha 2 Difference to alpha 1
‣ Slower in onset
‣ Longer lastic
‣ More sensitive to pH and temperature change
‣ Can also be affected by AT2
‣ Central affects lower sympathetic outflow (postulated mechanism for clonidine)
Beta 1 mechanism of action
- Stimulatory Gs protein –> adenylyl cyclase –> (ATP –> cAMP) increased cAMP —> protein kinases —> phosphorylation and inhibition of K+ channels and blocks afterdepolarisation increasing ability of pyramidal cells to generate action potentials
Beat 1 effects
‣ Heart - increased contractility, tachycardia (SA node rate/ectopic pacemakers), increased AV conduction velocity, reduced refractor period
‣ Increased glycogenlysis and adipose tissue lipolysis
‣ Increased renin release by the kidney
‣ Platelets –> aggregation
Beta 2 effects
‣ Bronchial smooth muscle relaxation
‣ Relaxes gut wall smooth muscle and the baldder
‣ Uterine relaxation (if pregnant)
- Gluconeogenesis and glycogenlysis
How does Beta 2 cause effects
(GPCR –> increased cAMP –> increased Na/K activity and hyper polarisation)
DOpamine receptors react to what?
Dopamine
Catecholamines
How many dopamine receptors
5 GPCRs
Dopamine 1 receptor is what type of receptor? Intracellukar action
GPCR
◦ Gproteins stimulating adenylate cyclase --> increased cAMP ‣ Inhibition of Na/K ATPase via secondary mesenger depolarising RMP ◦ D5 is essentially the same receptor ◦ post synaptic on sympathetic nerve --> stimulation leads to vasodilation of renal, mesenteric, coronary and cerebral vessels + sodium excretion
D2 receptor is what type of receptor?
Reduced adenylate cyclase, reduced cAMP
Likely Gi PCR
Effects of D1 receptors
Vasodilation of renal, mesenteric, coronary and cerebral vessels
Action of D2 receptors
Reduced pituitary hormone release
Produces nausea and vomiting
Inhibits release of noradrenaline
What neurotransmitters are amino acids
GABA
Glutamate
Glycine
GABA stands for
- Gamma aminobutyric acid - formed from the decarboxylation of glutamate (glutamate decarboxylase)
How is GABA made
- Gamma aminobutyric acid - formed from the decarboxylation of glutamate (glutamate decarboxylase)
How is GABA metabolised
◦ Catabolised into succinate and plugged into the citric acid cycle by GABA transaminase
2 GABA receptors , how do they differ
GABA A ligand gated chloride channel
GABA B metabotropic receptor
GABA A receptor structure
pentamic structure ligand gated chloride channel hyperpolarising the membrane (5 protein subunits form a pore – 2 alpha, 2beta, gamma - multiple variants)
◦ 2x Alpha subunits - GABA binding site
‣ GABA binding causes increased opening frequency, augmenting Cl- conductance and hyperpolarising the membrane
◦ Modulatory units - Alpha/Gamma subunit - Benzodiazepines, flumazenil
‣ 2 different types Benzodiazepine binding sites - one more anxiolytic, on more sedative
◦ 2x Beta subunit - etomidate, barbiturates, propofol, volatile anaesthetics
‣ Stereospecificity evidenced by etomidate - as an enantiopure preparation the R+ isomer is the only clinically active isomer, the S- is not active
‣ Action: incease opening time –> hyperpolarisation
‣ Etomidate is GABA specific
◦ Both sites produce positive allosteric modulation – increase channel opening time so allowing for increased chloride entry resulting in hyperpolarisation
What are the binding sites of allosteric modulators of GABA recpeotrs
pentamic structure ligand gated chloride channel hyperpolarising the membrane (5 protein subunits form a pore – 2 alpha, 2beta, gamma - multiple variants)
◦ 2x Alpha subunits - GABA binding site
‣ GABA binding causes increased opening frequency, augmenting Cl- conductance and hyperpolarising the membrane
◦ Modulatory units - Alpha/Gamma subunit - Benzodiazepines, flumazenil
‣ 2 different types Benzodiazepine binding sites - one more anxiolytic, on more sedative
◦ 2x Beta subunit - etomidate, barbiturates, propofol, volatile anaesthetics
‣ Stereospecificity evidenced by etomidate - as an enantiopure preparation the R+ isomer is the only clinically active isomer, the S- is not active
‣ Action: incease opening time –> hyperpolarisation
‣ Etomidate is GABA specific
◦ Both sites produce positive allosteric modulation – increase channel opening time so allowing for increased chloride entry resulting in hyperpolarisation
What binds at the alph gamma subunit of GABA A recepeotrs
Benzodiazepines
What binds to beta subunits of GABA A receptors for action
Propofol, barbituates, etomidate, volatile
What binds to the alpha subunit of the GABA A recepotr
GABA
Draw a GABA A channel and label the locations of bidning sites
GABA B receptor type
Metabotropic
COupled to G protein and increases K conductance and reduced Ca influx
Generally inhibits neurotransmitter release via hyperpolarisation
What drug acts on GABA B? Where are its receptors found
POst synaptically in the brain but also in the spinal cord
Involved in nociception and muscle tone
Baclofen
Glycine is invovled in?
Sensitising NMDA recepeotrs
Inhibitory transmitter in the spinal cord via pentameric chloride chanel
Glutamate produced from
- Made from alpha-ketoglutarate (the krebs cycle intermediate) by reductive amination
◦ GABA shunt a reaction in the neuron bypassing the alpha ketogluarate dehydrogenase step of the Krebs cycle accounting for 10-40% of enuronal TCA cycle activity
Function of glutamnate
75% of excitatory inputs in the CNS - fast synaptic transmission in the spinal cord, memory and learning, central sensitisation and plasticity
Why is there glutamate build up in ischaemic areas?
◦ Therefore is removed from the synapse by Na mediated transport via 2 mechanisms (its own and a shared)
◦ If you can’t maintain a Na gradient however you can’t pump glutamate and the cells begin dying - this happens in ischaemic regions
What are the receptors for glutamate?
NMDA
AMPA
Kainate
Draw a NMDA receptor
‣ It is a heterotrtamer transmembrane protein, consisting of four subunits
* 2 x NR1 pore forming
* 1x NR2A subunit binding glutamate (orthosteric site)
* 1x NR 2B binding glycine (coactivating siter)
◦ The ligands are the neurotransmitters glycine and glutamate, both of which have to bind in order to activate the receptor
‣ Pore is blocked by Magnesium
‣ Phencyclidine binding site for Ketamine and PCP
‣ N20 and Xe antagonists at unknown site
◦ Activation
‣ Priming – activation of adjacent AMPA or NK1 receptor
‣ Partial depolarisation removes Mg or Zn plug
* Interestingly in the spinal cord with sufficient pain stimulus the magnesium ion is displaced by substance P and other cotransmitters
‣ Coactivation – binding of glycine
‣ Activation - Binding of glutamate –> opening –> ion flux
◦ Opening of the receptor channel permits the flow of:
‣ Potassium (out of the cell)
‣ sodium ( into the cell)
‣ calcium (into the cell)
Describe the activation sequence of an NMDA receptor
‣ It is a heterotrtamer transmembrane protein, consisting of four subunits
* 2 x NR1 pore forming
* 1x NR2A subunit binding glutamate (orthosteric site)
* 1x NR 2B binding glycine (coactivating siter)
◦ The ligands are the neurotransmitters glycine and glutamate, both of which have to bind in order to activate the receptor
‣ Pore is blocked by Magnesium
‣ Phencyclidine binding site for Ketamine and PCP
‣ N20 and Xe antagonists at unknown site
◦ Activation
‣ Priming – activation of adjacent AMPA or NK1 receptor
‣ Partial depolarisation removes Mg or Zn plug
* Interestingly in the spinal cord with sufficient pain stimulus the magnesium ion is displaced by substance P and other cotransmitters
‣ Coactivation – binding of glycine
‣ Activation - Binding of glutamate –> opening –> ion flux
◦ Opening of the receptor channel permits the flow of:
‣ Potassium (out of the cell)
‣ sodium ( into the cell)
‣ calcium (into the cell)
Describe the movement of electrolytes with NDMA receptor activation
‣ It is a heterotrtamer transmembrane protein, consisting of four subunits
* 2 x NR1 pore forming
* 1x NR2A subunit binding glutamate (orthosteric site)
* 1x NR 2B binding glycine (coactivating siter)
◦ The ligands are the neurotransmitters glycine and glutamate, both of which have to bind in order to activate the receptor
‣ Pore is blocked by Magnesium
‣ Phencyclidine binding site for Ketamine and PCP
‣ N20 and Xe antagonists at unknown site
◦ Activation
‣ Priming – activation of adjacent AMPA or NK1 receptor
‣ Partial depolarisation removes Mg or Zn plug
* Interestingly in the spinal cord with sufficient pain stimulus the magnesium ion is displaced by substance P and other cotransmitters
‣ Coactivation – binding of glycine
‣ Activation - Binding of glutamate –> opening –> ion flux
◦ Opening of the receptor channel permits the flow of:
‣ Potassium (out of the cell)
‣ sodium ( into the cell)
‣ calcium (into the cell)
Describe AMPA receptors - agonist, action
◦ AMPA - 2 types, one is only for Na and the other allows Ca also
‣ Agonist: glutamate, AMPA
‣ Antagonist: CNQX
‣ Action: Na and Ca influx; K efflux
Kainate receptors are found where? What type of receptor? Cause
◦ Kainate - ionotropic, presynaptic on GABA nerve endings and post synaptically elsewhere.
‣ Agonist: Kainate, glutamate
‣ Antagonist: CNQX
‣ Action: Na influx and K efflux
Is there a GPCR for glutamate?
Yes
◦ G-protein coupled ◦ Increase intracellular IP3 and DAG, or decrease cAMP ◦ 11 varieties; they can be post or presynaptic ◦ Seem to be involved in synaptic plasticity and learning
What are imidazoline receptors
I1 - hypotension - GPCR decreased cAMP and other signal pthways. Hyperpolarise adrenergic neurons
I2 - analgesic - found on mitochondria and potentiate opioid effects
I3 - insulin secretion effects
How does adenosine act
Blocks reuptake of noradrenaline and adrenaline
- ATP is released from synaptic vesicles with acetylcholine
- In smooth muscle ATP produces excitation by opening Na and Ca channels
How does substance P act
◦ Receptors are G-proten-coupled
◦ Activation of its receptor leads to activation of phospolipase C, as well as IP3 and DAG
Histamine is a peptide, catecholamine, monoamine or other?
Monoamine
imidazole ring connected to an amine group by an etheylene group
3 types of histamine receptors? How does each act
◦ H1 receptors activate phospholipase C via Gq –> increased IP3 –> increased calcium
‣ Excitatory by reducing potassium conductance and inhibiting hyperpolarisation
◦ H2 receptors Gs GPCR and increase intracellular cAMP concentration
‣ Prevent long afterdepolarisation in cortical and thalamic neurons, reducing refractory period
◦ Most of the H3 receptors are presynaptic and G-protein-coupled – inhibit the release of histamine and other neurotransmitters
Seratonin derived from
Tryptophan hydroxylation and decarboxylation
Seratonin belongs to what class of transmitters
Monoamines
What is the role of seratonin
◦ Regulation of mood - 5HT2A
◦ Role in nausea - 5HT3 at the CTZ a dorsal medullarys tructure
◦ Intestinal motility
◦ Thermoregulation especially 5HT7 in hypothalamic nuclei
◦ Migraines
What metabolises seratonin
MAO
Describe the seratonin receptor family
- Receptors - 7 receptor suptypes
◦ 5HT1, 5HT2, 5HT4 and 5HT7 are GPCR
‣ 5HT1 - is in the CNS adn vascular smooth muscles - sumatriptan acts
* 5HT1A - cAMP activation increasing K+ ion conductance and IPSP
‣ 5HT2 - orbitofrontal cortex in visual processing - Mirtazepine is an antagonist (LSD an agonist)
‣ 5HT 4 - involved in the enteric nervous system
‣ 5HT7 is found in the hypothalamic nuclei and involved in thermoregulation
◦ 5HT3 is a ligand gated ion channel inhibited by ondansetron
‣ 5HT3 receptor activation increased Na and K+ conductance leading to excitatory post synaptic potential (EPSP) - non selective
‣ It is present in the enteric nervous system (motility) and the dorsal medullary chemoreceptor trigger zone involved in vomiting
How does 5HT3 work
- Receptors - 7 receptor suptypes
◦ 5HT1, 5HT2, 5HT4 and 5HT7 are GPCR
‣ 5HT1 - is in the CNS adn vascular smooth muscles - sumatriptan acts
* 5HT1A - cAMP activation increasing K+ ion conductance and IPSP
‣ 5HT2 - orbitofrontal cortex in visual processing - Mirtazepine is an antagonist (LSD an agonist)
‣ 5HT 4 - involved in the enteric nervous system
‣ 5HT7 is found in the hypothalamic nuclei and involved in thermoregulation
◦ 5HT3 is a ligand gated ion channel inhibited by ondansetron
‣ 5HT3 receptor activation increased Na and K+ conductance leading to excitatory post synaptic potential (EPSP) - non selective
‣ It is present in the enteric nervous system (motility) and the dorsal medullary chemoreceptor trigger zone involved in vomiting
What i the only ion channel of the seratonin receptors
- Receptors - 7 receptor suptypes
◦ 5HT1, 5HT2, 5HT4 and 5HT7 are GPCR
‣ 5HT1 - is in the CNS adn vascular smooth muscles - sumatriptan acts
* 5HT1A - cAMP activation increasing K+ ion conductance and IPSP
‣ 5HT2 - orbitofrontal cortex in visual processing - Mirtazepine is an antagonist (LSD an agonist)
‣ 5HT 4 - involved in the enteric nervous system
‣ 5HT7 is found in the hypothalamic nuclei and involved in thermoregulation
◦ 5HT3 is a ligand gated ion channel inhibited by ondansetron
‣ 5HT3 receptor activation increased Na and K+ conductance leading to excitatory post synaptic potential (EPSP) - non selective
‣ It is present in the enteric nervous system (motility) and the dorsal medullary chemoreceptor trigger zone involved in vomiting
What are the GPCR seratonin receptors
- Receptors - 7 receptor suptypes
◦ 5HT1, 5HT2, 5HT4 and 5HT7 are GPCR
‣ 5HT1 - is in the CNS adn vascular smooth muscles - sumatriptan acts
* 5HT1A - cAMP activation increasing K+ ion conductance and IPSP
‣ 5HT2 - orbitofrontal cortex in visual processing - Mirtazepine is an antagonist (LSD an agonist)
‣ 5HT 4 - involved in the enteric nervous system
‣ 5HT7 is found in the hypothalamic nuclei and involved in thermoregulation
◦ 5HT3 is a ligand gated ion channel inhibited by ondansetron
‣ 5HT3 receptor activation increased Na and K+ conductance leading to excitatory post synaptic potential (EPSP) - non selective
‣ It is present in the enteric nervous system (motility) and the dorsal medullary chemoreceptor trigger zone involved in vomiting
What agents are implicated in seratonin syndrome
SSRI
SNRI
MAO
TCAs
Pethidine, tramadol
MDMA
Sumatriptan, fentanyl, LSD
What receptors are most implicated in seratonin syndrome?
- Specific serotonin receptors are involved:
◦ 5-HT1A and 5-HT2A receptors are the most responsible - Stimulation of these receptors exceeds safe thresholds when:
◦ Serotonin synthesis is increased (eg. dietary tryptophan, soft cheese etc.)
◦ Serotonin release is enhanced (pethidine)
◦ Serotonin reuptake is inhibited (SSRIs, SNRIs, MDMA, tramadol)
◦ Serotonin metabolism is impaired (MAOIs, linezolid, methylene blue)
◦ Serotonin receptor agonists are present (eg. sumatriptan, LSD, fentanyl)
◦ St Johns Wart
◦ Or any combination of the above.
How do agents causing seratonin syndrome act
- Specific serotonin receptors are involved:
◦ 5-HT1A and 5-HT2A receptors are the most responsible - Stimulation of these receptors exceeds safe thresholds when:
◦ Serotonin synthesis is increased (eg. dietary tryptophan, soft cheese etc.)
◦ Serotonin release is enhanced (pethidine)
◦ Serotonin reuptake is inhibited (SSRIs, SNRIs, MDMA, tramadol)
◦ Serotonin metabolism is impaired (MAOIs, linezolid, methylene blue)
◦ Serotonin receptor agonists are present (eg. sumatriptan, LSD, fentanyl)
◦ St Johns Wart
◦ Or any combination of the above.
How does seratonin increases result in the manifestations of seratonin syndrome?
◦ Altered function of other neurotransmitter systems for which serotonin is a neuromodulator (usually, an inhibitory influence)
◦ For the majority of these, serotonin excess leads to the disinhibition of presynaptic release of these mediators
◦ They include noradrenaline, acetylcholine, dopamine and glutamate
manifestations of srratonin syndrome
Seratonin syndrome (RASCAL) - earlier onset (12 hours) than NMS
* Rhabdomyolysis
* Agitation
* Seizures
* Clonus (spontaneous) - hyperreflexia (differentiates from NMS where they are depressed, and NMS has rigidity without clonus)
* Autonomic - overdrive state with tachycardia, hypertension, hyperthermia, diarrhoea
* Large pupils - mydriasis (differentiated from NMS where there is no change to pupils)
How do you manage seratonin syndrome
Supportively
Management - cyproheptadine, haloperidol/olanzapine/chlorpromazine
NMS is characterised by what symptoms
NMS (FEVER LAD) - over days
* Fever
* Encephalopathy - stupour, coma, mutism
* Vitals unstable - hyper or hypotension, brady or tachycardia
* Elevated enzymes - CK
* Rigidity of the muscles, hypertonia
* Leucocytosis
* Acidosis and aspiration (increased secretions)
* Diaphoresis, dysphagia
Management
* Amantadine
* Bromocryptne
* Dantrolene has been used
Describe the structural elements of a catecholamine?
What is the modifications that can occur that alter function of the Beta carbon of a catecholamine (2)
What alterations to the alpha carbon can be done on a catecholamien? What happens as a result? (2)
What positions are the hydroxyl groups of a catecholamine on?
3 and 4
What is the relevance of the hydroxyl groups on a catecholamine? How can they be modified to change function?
- Change to 3 + 5 Beta 2 selective
- Both at usual locations 3 + 4 necessary for COMT action
- Losing one OH group increased lipid soliubility but reduces potency by 10x; losing 2 decreases potency by 100x
What relevance does the terminal amine group have on catecholamine function?
Smaller = alpha > beta
Bigger = Beta > alpha
Methyl groups confer beta selectivity
What is the name of the beta and alpha carbon along with the terminal amine of a catecholamine?
Ethylamine tail
