Opiates II

Question 1

What are the mechanisms of opioid action?

Answer 1

The primary site of action of all opioid drugs are opioid receptors in the CNS and periphery. All of their effects are initiated by binding to these receptors, and different effects are due to actions at different receptor subtypes located on different cells in different tissues. Just because a drug is having an effect doesn’t mean it is acting on a receptor.

Question 2

What endogenous ligands (NTs) bind to opioid receptors?

Answer 2

Earliest evidence for the existence of a receptor came from Pert and Snyder. 3 part argument – 1st experiment looked at binding of antagonist (naloxone). As you increase the dose of naloxone, they saw increased binding, and at some point saw saturable binding (at some point, there was no increase in binding – law of mass action). Tentative evidence of the existence of a receptor. Second experiment asked if this binding correlated with some physiological response. Compared codeine, methadone, morphine etc. Does amount of drug (and binding) correlate with amount of affect? Found greater binding was correlating with bigger inhibitory effect. The third experiment does this type of physiological effect that correlates with binding effects, does it correlate with well known effect in humans to cause analgesic effect? Found more binding created more potent analgesic effects. Predicts how opioids produce their typical effect (analgesia).

Question 3

What is the 1st of 4 major known types of opioid receptors?

Answer 3

1. MOR - morphine opiate receptor. Endogenous ligand is endorphins and endorphins. Location in the thalamus, etc. Functions are analgesia, reinforcement, respiratory depression, vomiting, sensorimotor integration

Question 4

What is the 2nd of 4 major known types of opioid receptors?

Answer 4

DOR - delta receptor. Endogenous ligands are enkephalin and endorphins (POMC). Found in the neocortex, striatum, olfactory areas, substantia nigra, NAcc, spinal cord. Functions are analgesia, reinforcement, cognitive function, olfaction, motor integration.

Question 5

What is the 3rd of 4 major known types of opioid receptors?

Answer 5

KOR - kappa opiate receptor. Endogenous ligand is dynorphins. Found in the pituitary, hypothalamus, amygdala, striatum, nucleus accumebns. Functions are water balance, feeding, temp control, analgesia.

Question 6

What is the 4th of 4 major known types of opioid receptors?

Answer 6

NOR - nociceptin orphanin receptor. Found in periaqueductal grey, spinal cord etc. Functions are analgesia etc.
Thought must be at least three different receptors due to the different effects of the drugs. Drugs will bind to these receptors with selectivity, but most opiates will bind to all of these types of receptors.

Question 7

How are opioid receptors distributed in the brain?

Answer 7

Can develop ligands that will selectively bind to the different receptors. Looking at the top of the rat brain. Olfactory bulb – in rats they’re larger than humans. Receptors have different distributions – expressed in different areas, tells us they are involved in different functions.
Found in many areas, including olfactory areas, striatum, cortex, and thalamus.

Question 8

What are opioid receptors?

Answer 8

All opioid receptors are GPCRs (g-coupled receptors), and all Gi (inhibitory, inhibit adenylyl cyclase and cAMP activity). Their initial effect is they shut things down.

Question 9

What are Gi effects?

Answer 9

1. Inhibition of adenylate cyclase
2. Shutdown of voltage-gated calcium channels. Binding can result in decreases of calcium flux, and this results in exocytosis (process by which NTs are released, so decrease NT).
3. Open potassium channels via action on GIRKs. Get less excitability as they hyper polarise the cell.

Question 10

What are the three possibilities that opioid receptors can do?

Answer 10

Can be pre- or post-synaptic:

(a) postsynaptic inhibition - they open potassium channels.
(b) axoaxonic inhibition - they close calcium channels.
(c) presynaptic autoreceptors - they reduce transmitter release. Neuron is co-released with other neurotransmitter. Alters neurotransmitter y decreasing calcium flux (for example).

Question 11

What are variations in opioid receptor sequences?

Answer 11

Small changes in amino acid sequence exist and alter the function of these receptors. There is also naturally occurring differences in the population in the nucleotide, that can dramatically alter the function of these receptors. Suggests this may alter vulnerability of becoming addicted.

Question 12

What endogenous ligands (neurotransmitters) bind to opioid receptors?

Answer 12

After discovering receptors, next question was what endogenous ligand binds to it? First discovery of this was in 1974. Need lots of brain tissue to analyse this. Took pig and camel brain tissue. Found first endogenous ligand was peptides. First was endorphin. Large list of peptides we now know are agonists to opiod receptors, e.g. endorphins, enkephalins, and dynorphins.

Question 13

What are peptide transmitters?

Answer 13

Requires gene transcription, mRNA translation into longer hormones which are translated to the terminals where they are cleaved. Neuropeptides are not typically the only neurotransmitter at a synapse. Rather, they are co-release together with a classical neurotransmitter.

Question 14

All opioid peptides are products of 4 gene families - what are these gene families?

Answer 14

1. Pro-opiomelanocortin (POMC) - codes and produces the endorphins. Beta endorphin was the first to be discovered.
2. Proenkephalin (PENK) - gave rise to enkerhaines.
3. Prodynorphin (PDYN) - give rise to dynorphines.
4. Pronociceptin
   All produced through protein post translation (cleaved into it in terminals).

Question 15

What are POMC (endorphins)?

Answer 15

Not only produces endorphin, but also ACTH – another product of post-translation process. This is the hormone that is released from the pituitary and acts in your adrenal gland after stressful response. Also gives rise to MSH – allow chameleons to change the colour of their skin.
Released by the same neuron.

Question 16

Where will endogenous ligands and exogenous opioids bind?

Answer 16

Endogenous ligands will selectively bind like the exogenous opioids to certain receptors. Can bind to many, some with a lesser affinity.

Question 17

How are endogenous opioid peptides distributed?

Answer 17

The location of the cells that produce these peptides. POMC is restrictive of where the cell bodies are – mostly in the pituitary and the hypothalamus. Different for Proenkephalin (PENK) – widely distributed throughout cortex and sub-cortical regions (striatal regions for example). A bit less with Prodynorphin PDYN. PENK co-localised with D2 receptors and modulate the activity of cells that produce the D2 receptors. In terms of receptors, we see different distribution patterns and where these cell bodies are produced.

Question 18

What are the effects of opioid drugs?

Answer 18

Analgesia, reward, gastrointestinal, respiratory depression, cough suppression, nausea and vomiting, motor effects.

Question 19

What mediates pain perception?

Answer 19

System in your brain and spinal cord that mediates pain perception. Pain info comes in through spinal cord and synapses through motor neurons. More importantly for pain perception, have ascending pathways where they synapse and have higher order activity. There are also big descending perceptions from brain regions and mid-brain that project down to modulate pain perception. Opioids produce their analgesic effects in two places – directly in the spinal cord (spinal analgesia) and higher up (supra-spinal analgesia). Higher up is involved in emotional interpretation of pain.

Question 20

What is spinal analgesia?

Answer 20

Have sensory neuron coming in through dorsal horn, synapses with projection neurons (also motor neurons) that project up to the hind brain. Opioid receptors are located on those projection neurons. What opioids do is they bind to the postsynaptic projection neuron and hyperpolarise, and reduce the transmission of pain information into the brain. This is why it is possible to inject into the spine and get analgesic effect. Do this so it doesn’t affect the fetus.

Question 21

What is supra-spinal analgesia?

Answer 21

Very strong/dense neuronal projections that go into the spinal cords. Involves a number of different brain regions. PVG = para-ventricular gray, which downward changes into the periaqueductal gray (PAG), which is the tube in which CSF flows. Connects with the ventricles. Once get to ventricles (go up) changes to PVG to indicate it is around the ventricles. Send projections down to the raphe nuclei – this is the straddle seam of the brain stem. They send projections down to the dorsal horn of the spinal cord. Evidence of this pathway came from accidents. Found stimulation of PAG produced profound analgesia in rats, without opioids. No pain perception. First evidence PAG was part of more complex circulatory involved in analgesic effects of endogenous peptides and endogenous opioids. Lesions of the raphe nuclei block analgesic to systemic or local morphine injections into the PAG. Some evidence that the PAG and PVG are involved – if you stimulate you get analgesic effect, if you lesion you block this effect.

Question 22

What are supra-spinal actions for analgesia (involving GABA)?

Answer 22

At the level of the PAG there are GABAergic neurons – these become active when you have painful stimuli (“on to pain” neurons). Release GABA (inhibitory) to inhibit neurons in the raphe that are serotonergic – release serotonin at the level of the dorsal horn. These neurons turn off, meaning they inhibit serotonin release at the level of the spinal cord, and facilitates pain perception. Endogenous opioids inhibit the disinhibition. There are opioid receptors of GABAergic neurons, so they inhibit the inhibitory process of serotonergic neurons. Stimulus comes in, GABA neurons turn on, but if have opioid you inhibit this facilitating effect.

Question 23

What are sensory responses vs. affective responses in analgesia?

Answer 23

Looking at midbrain brain stem mechanisms e.g. raphe. Know there are even higher systems that activate the previous systems. Which parts of the brain are activated when people report their experience pain and their emotional state (emotional level). See activation in NAcc, amygdala, and thalamic regions. All brain regions will be activated, but depends on whether it is a sensory or affective response. Can also see these areas activated under the influence of placebo effects. Given placebo and told it was analgesic. Not just physical pain stimuli – it is also emotional pain stimuli (imagined).

Question 24

What is the different between analgesia in morphine and heroin actions?

Answer 24

Is the difference because heroin gets to the brain fast (chemically produced in such a way that it is more lipid soluble), but then it is converted back to morphine in the brain, so the effects experienced are of morphine? Studies with mice, suggesting that the difference is not what’s suggested above, but there is more going on. Mice insensitive to the analgesic effects of morphine but not heroin. Also studies with MOR opiate knockout mice – DAMGO is an experimental drug that is highly selective to MOR opiate receptors, more than morphine. Those with the MOR receptor – more than 80% analgesic effect. Those without – morphine, but not heroin, analgesia is abolished in MOR knockout mice. Suggests there is something else going on with heroin that makes it different from morphine.

Question 25

What is the effect of morphine in MOR opioid receptor knockout mice?

Answer 25

Spinal analgesia - abolished.
Supra-spinal analgesia - abolished in morphine but not in heroin. 
Reward - abolished.
Withdrawal - abolished.
Respiratory depression - abolished.
Inhibition GI motility - abolished.
Psychoactivivating effect - abolished. 
Effects are dependent on the ability to bind to MOR opiate receptor, as all effects are maintained without DOR and KOR.

Question 26

What are different dopaminergic populations?

Answer 26

The rewarding effects are heavily dependent on the dopamine system. Lots of evidence it activates this system and are reliant on the dopamine system, especially in the NAcc. If change dopamine levels in NAcc, you change reward levels e.g. in food. Substantia nigra – in the midbrain, project to dorsal striatal regions to the VTA, project to NAcc and less dense projections into cortical regions. If present food, see activation of cell bodies in VTA. As a consequence of this, get dopamine release in the NAcc. All drugs seem to do exactly this. Opioids are inhibitory – opioid receptors in the VTA and NAcc, how would that increase dopamine levels? Similar to pain system. Dopamine neurons from VTA send axons to NAcc, cause release in dopamine. Opioid receptors are located on GABA receptors that now inhibit the inhibition of dopamine neurons (disinhibition). This is how you get increases in dopamine in opioids, through a process of disinhibition.
¥ Morphine self-administration is blocked by dopamine lesions or dopamine antagonists in NAccs - doesn’t seem to work with heroin in quite the same way.

Question 27

What are opioid/dopamine interactions?

Answer 27

Morphine-conditined place preference is abolished in mice lacking dopamine D2 receptors. But food-conditioned place preference is normal in these mice. This evidence suggests its dopamine systems intact that produce rewarding effects of opioid drugs. Suggests D2 receptor is needed for opioid reward. Necessity for dopamine in opioid reward through disinhibition effect of GABA.

Question 28

What is the relationship between opioids and subjective pleasure?

Answer 28

One of functions of opioid are regulation of reward effects of foods. Measuring responses to pleasurable or aversive liquids being squirted into the mouth. Measure of positive affect of liquids. Babies born without a cortex show this response – show it is a basic response.
If inject opioids into NAcc you see changes in hedonic responses – either no change or decreases in aversive responses. Cannabis is another drug that causes changes in hedonic responses.