pccol3022 Flashcards
Where does morphine bind
preferentially bind to mu opioid receptor
Where are mu opioid receptors found
dorsal horn of spinal cord, VTA, nucleus accumbens, locus cereuleus
Give 2 examples of lower potency mu agonists
Codeine and oxycodone
What is the mechanism of action for aspirin and non-steroidal anti-inflammatory drugs
They produce analgesia by decreasing sensitization. They inhibit the enzyme cyclo-oxygenase 2. COX2 is an important mediator which releases an inflammatory molecule called prostaglanding E2
What are 3 ways that an opioid limits pain?
1) Opioids inhibit adenylyl cyclase
2) Opioids increase postsynaptic potassium efflux
3) They reduce presynaptic calcium influx
How does reducing calcium influx cause analgesia
Calcium causes the release of excitatory neurotransmitters such as glutamate in response to an action potential.
This reduces excitatory signals from reaching the post synaptic neurons.
How does increasing potassium efflux cause analgesia
Potassium efflux causes the post-synaptic neuron to hyperpolarise and therefore less likely to depolarise and cause a symptom
How does inhibiting adenylyl cyclase cause analgesia
So, basically beta and gamma subunits are important for inhibiting VGCCs or GIRKs.
Moreover, inhibition of voltage dependent pacemaker Ih-cation non-selective current activated at hyperpolarised potentials to depolarise membrane.
What are the pharmacokinetics of morphine?
It rapidly enters all body tissues, however only a smal percentage crosses the blood brain barrier
How does naloxone work
Naloxone binds to the mu opioid receptor with a much higher affinity than opioids. It acts as a competitive inhibitor that blocks opioids from binding. Naloxone’s higher affinity means that it can rapidly displace all receptor bound opioids
Can we separate the positive and negative side-effects of opioids?
No because both the positive and negative side-effects are mediated by the same receptor
Which sensory fibres do opioids primarily act on
C fibres-
the slow and non-myelinated opioids.
This is why opioids inhibit slow pain, but not fast pain
Where are c fibres primarily found
C fibres are afferent neurons which terminate in the dorsal horn of the spine.
Mu opioid receptors are also very concentrated in the dorsal horn of the spine
What role does norepinephrine and serotonin play in controlling pain?
Norepinephrine and serotonin are released from the medulla. They excite enkephalin and inhibit projection neurons and therefore inhibit sensory synpases.
What are endogenous opioids
These are the enkephalins and endorphins that are primarily produced in the brain. Enkephalin: noradrenaline and sertonin from the medulla excite enkephalin neurons.
What are the main kinds of opioid drugs
1) opiates-drugs made from opium poppy
agonists
morphine related opioids
opioid agonists
Opioid antagonists
Explain mechanism of tolerance
(find out more)
As tolerance is likely to depend upon the level of receptor occupancy, the degree of tolerance observed may reflect the response being assessed (e.g. analgesia versus respiratory depression), the intrinsic efficacy of the drug and the dose being administered
Tolerance results in part from desensitisation of the µ receptors (i.e. at the level of the drug target) as well as from long-term adaptive changes at the cellular, synaptic and network levels (see Williams et al., 2013 ). Tolerance is a general phenomenon of opioid receptor ligands, irrespective of which type of receptor they act on. Cross-tolerance occurs between drugs acting at the same receptor, but not between opioids that act on different receptors.
Explain synergistic actions with non-opioids to better relieve pain
(find out more)
Why is heroin so powerful
Heroin is a prodrug. It is basically 3,6 acetyl morphine.
Esterification of Heroin means that it can go into the brain extremely fast.
There is a lot of esterases in the brain which forms 6 acetyl morphine and normal morphine
Break down CYP2D6
What are the 2 main types of chronic pain
1) inflammatory pain or neuropathic pain
What is inflammatory pain
pain often characterised by tissue injury and inflammatory processes. There is often a nociceptive activity
What is neuropathic pain
Neuropathic pain is a lesion or disease affecting the somatosensory system. It is caused by lesions to the nerve.
What are the symptoms of chronic pain
1) spontaneous pain
2) allodynia
3) hyperalgesia
What is allodynia
normal stimuli perceived as painful
What is hyperalgesia
painful stimuli that becomes more painful. Reduced threshold to pain
How do we model neuropathic pain in mice
do a peripheral nerve injury-like sciatic nerve ligation.
Give mice chemotherapy drugs like paclitaxel to damage the neurons
Literally stimulate a disease that mimics human chronic pain
How do we mimic inflammatory pain in mice
Use intraplantar inflammation: complete Freund’s adjuvant is a solution containing mycobacterium in paraffin oil. Inject it into the paws and it causes necrosis and ulceration.
How do we measure mechanical allodynia
Use von frey’s filaments to push into paws. The force you use to push into the paw varies.
In the von frey’s test, we are looking for a defensive response
How do we measure mechanical hyperalgesia
Randall-Sellito device measures the threshold force
How do we measure heat
Hotplate or Hargreaves test
Are opioids good for treating chronic pain?
Not very good in chronic pain models.
It causes some reduction in activity in mu opioid receptors but mu opioid receptors not on non-noxious afferents?
No effect on pain transmission from non-noxious afferents
Some reduction in the activation of ascending pain tract neurons
We can’t use drugs like opioids over time, because it can lead to addiction issues
Ziconomide
Ziconotide is an N type channel blocker.
It mediates neurotransmitter disease
Generally not approved because these channels expressed in pretty much every afferent pathway which means that it can have severe systemic side effects
Ziconomide vs Opioid
Ziconotide vs Opioid:
Better than opioids because it can target allodynia
Channel blocker- so it doesn’t cause as much abuse or tolerance
It is not recommended because of its poor blood brain barrier penetration and its severe side effects
What is the current best neuropathic pain medication?
GABA analogues
Where do GABA analogues bind
They can pass through the blood-brain barrier and therefore can get into the brain or spinal cord
They bind to the a2delta subunit of L-,N-, P/Q type VGCCs
How do GABA analogues work
Decrease VGCC membrane expression in central terminals of afferents AND they are less active
This causes less neurotransmitter release and reduced excitation
This also reduces pain transmission
They alter the trafficking of VGCCs but not the acute inhibition of channel opening
Side-effects of GABA analogues
Side-effects: dose dependent dizziness, sedation, incoordination, memory
TCA
Block NA and 5 HT transporters to prevent reuptake from the synapse so increase endogenous NA and 5HT and prolong signalling
They are more subtle than globally acting agonists, with fewer side-effects
Can have a few off target effects
TCAs are also channel blockers which can cause alternative analgesic mechanism to target 5HT/NA systems
SNRI
Dual noradrenaline and serotonin reuptake inhibitors
capsaicin-TRPV1 agonist
TRPV1-senses pain signals above 40C
Located on a major group of nociceptive afferents- in their terminals within the skin
It can cause localised neuropathic pain. Which causes dysfunction of nociceptives
Depletion of nociceptives
Where are voltage gated ion channels found
Pretty much everywhere on a neuron; they are found on excitable cells such as neuronal and muscle cells to allow a rapid and coordinated depolarisation.
On a neuron, they are found along the axon and the synapse
General structure of a voltage gated ion channel
Each voltage gated ion channel are made out of alpha subunits and beta subunits.
There are 4 main alpha subunits for the sodium voltage gated ion channel which are identical.
Each of these subunits are made out of 6 transmembrane domain called:s1,s2,s3,s4,s5,s6
What are the roles of beta subunits
The beta subunits are more diverse accessory units which help change the function of the alpha subunits:
Expression levels
Location
Trafficking
Alter voltage dependence of activation/inactivation
How do voltage gated ion channels open?
S4 is positively charged; it is made out of amino acids such as asginine. They play an important role in helping open the pore region which would be explained later. We call them voltage sensors
S5-S6 help form the regions where the pores are. One really important thing to consider is that the pore loop forms between s5 and s6
What are the differences between sodium and potassium channels
Difference between potassium and sodium channels is that instead of having 4 alpha subunits, we instead have 4 separate proteins. This is because there is no joining loop between different subunits in the potassium ion channel compared with the sodium ion channel
Different voltage gated ion channels have a pore forming region which have different selectivities for different ions.
Presumably, they have different selectivity filters
Describe the different functional states of voltage dependent ion channels
And how they function:
There are three different states of voltage gated ion channels:
1: open state
The pores are open ions can move down the selectivity filter.
After there is a change in membrane potential, the voltage sensor moves in s4 (it is a 25 degree tilt).
The voltage sensor is connected to a linker protein and the linker protein moves outwards and allows the channel to open up
2: closed state:
Opposite of open state
3: inactivated state
This is like the intermediate between a closed and open state. On one hand, the voltage sensor is still in it’s 25 degree tilt and the linker protein is still at an outward location which allows the channel to remain open. However, unlike the open or closed state, there is a ball and chain model for inactivation. This “ball” (inactivation gate) blocks up the open channel. This ball is also an intracellular domain. Helps prevent prolonged open state. Pore is blocked so no cation could move. Only repolarisation could
How is ion selectivity between different ion channels achieved
Different amino acids on the pore loops, so different amino acids for S1, S2, S3 and S4 on the S5-S6 pore loop
What is the function of a TRPV channel
These are members which are mainly involved in pain perception and also nociception
They are sensitive to
Heat
Acidic pH
Mechanical stimuli
They are relatively non-selective to cations
What is the function of a calcium channel?
Basically let calcium from extracellular places in.
Useful statistic to know:
Extracellular Calcium is about 1-5mM whereas intracellular calcium is 0.1-0.2 micromole before opening calcium channels but may rise to 100 micromoles after the opening of calcium channels
Calcium entry can affect many intracellular processes: Muscle contraction Neurotransmitter Activation of second messenger systems Alteration in gene expression Apoptosis depolarisation
Explain the differences in the mechanism of action of benzodiazepines and pentobarbitol on GABA receptors
Phenobarbital increases the channel open time which could mean that a very large amount of sodium could make it into the neuron. That makes it very dangerous
On the other hand, benzodiazepines open the frequency of channel opening, which also allows the chemicals in, but not necessarily that much that it could be lethal
(If it is a GABA receptor, why does it allow sodium ions and stuff in?)
Where do benzodiazepine bind?
BZ binding site contains a crucial Histidine residue, which is present on α1, α2, α3, α5
Why is pentobarbitone so dangerous
Benzodiazepines are unable to activate GABA receptors themselves whereas Pentobarbitone are able to enhance the actions of GABA or activate GABA receptors themselves.
Phenobarbital increases the channel open time which could mean that a very large amount of sodium could make it into the neuron. That makes it very dangerous
How does ethanol modulate the activity of GABA?
Ethanol, enhance the actions of GABA, prolongs the open time of the channel and binds with the transmembrane domain
How does general anaesthesia modulate GABA
General anaesthetics(e.g propofol) Enhance the actions of GABA, may directly stimulate receptors at high concentration. Bind at the interface between alpha and beta subunits within the transmembrane region of the channel. Propofol binds between M1,M2,M3 and m4, presumably keeping it more open?
