Cannabinoids

Question 1

The endocannabinoid system

Answer 1

The endocannabinoid system describes the actions of the endogenous cannabinoids anandamide and 2AG on the cannabinoid receptors CB1/CB2. These receptors are primarily expressed pre-synaptically, which owes to their role as neuromodulators. They occur throughout the brain, as well as on immune cells. Their expression in the cortex, hippocampus, cerebellum, and basal ganglia informs the action of the endocannabinoid system on thought processes, mood, memory, appetite, and immune regulation.

Question 2

Ligand synthesis

Answer 2

Endocannabinoids are naturally occurring agonists of the cannabinoid receptors CB1/CB2. There are two known endocannabinoids- anandamide and 2AG- both of which are lipophilic thus are secreted on demand following calcium influx and subsequent activation of calcium-dependent enzymes.

Anandamide is sythesised through the transfer of archidonic acid to phospholipids through calcium-dependent enzymes such as N-acetyltransferase. The phospholipid are then cleaved by NAPE-PLD.

2AG is synthesised in a similar phospholipid-dependent manner, and involves cleavage via DAG lipase.

Following synthesis the endocannabinoids are released into the synapse where they undergo retrograde transport to act on pre-synaptic and astrocytic CB1/CB2 receptors.

The mechanism of how these lipophilic molecules leave the plasma membrane and move through the extracellular environment is not well understood. The current proposed mechanisms involve the existence of an endocannabinoid transport protein or intracellular and extracellular endocannabinoid carrier proteins such as fatty acids.

Question 3

Receptor binding/activation

Answer 3

Endocannabinoids diffuse into the plasma membrane, where they bind to the transmembrane domain of CB1/CB2 receptors. This produces a conformational changes during which salt bridges are broken to create a space for functional interaction with G proteins

Question 4

Receptor mechanism

Answer 4

CB1 and CB2 are both Gai/Go coupled GPCRs. Their activation results in a decrease in cAMP production, stimulation of GIRKs (inwardly rectifying K+ channels) and inhibition of Ca2+ channels. This generally results in decreased neuronal excitability, although the specific effects are dependent on the type of neuron where the receptors are expressed. In astrocytes, these actions result in decreased gliotransmitter release.

Question 5

Degradation

Answer 5

Following receptor binding and activation endocannabinoids diffuse into the cell where they undergo degradation. Anandamide is primarily degraded by FAAH to produce arachidonic acid and ethanolamine. 2AG is primarily degraded by MAG lipase into arachidonic acid and glycerol. Both endocannabinoids can also undergo degradation through COX/LOX/p450 enzymes which results in prostaglandin production and other molecules.

Question 6

Targeting of endocannabinoid receptors

Answer 6

Inhibitors of synthesis (NAPE-PDL or DAG lipase)
- reduced endocannabinoid levels
- reduced receptor activation
- potentially increased neuronal excitability + increased gliotransmitter release

Agonists/antagonists/inverse agonists of CB1 or CB2
- neurons
- astrocytes
- mitochondrial membrane

Inhibitors of degradation enzymes (FAAH/MAG lipase)
- increase concentrations of endocannabinoids
- increase likelihood of receptor activation

Inhibitors of endocannabinoid transporters/carrier proteins
- reduced movement
- reduced action on receptors

Question 7

Current regulations

Answer 7

Covered under 2019 amendment to the Misuse of Drugs Act to enable the research, cultivation, manufacturing, and supply of medical cannabis
- licensing regimes
- quality standards (low levels of heavy metals, pesticides, alfatoxins, and microbiological contamination) + (shelf life of ≥6 months)
- labelling of active ingredients, dose, excipients
- prescription must be recommended by doctors and must be collected from pharmacy or doctor
- large variation in CBD:THC ratio as well as different administrations makes obtaining data difficult
- unapproved medicines must be recommended by a specialist and approved on a case-by-case basis by MoH

Technically unapproved medicines
- no safety/efficacy testing
- lack of pharmacokinetic data which is important as different administration routes result in different effects

Question 8

Medical cannabis/cannabinoids for chronic pain: a clinical practise guideline

Answer 8

• Aimed to determine role of non-inhaled medical cannabis for those with chronic pain
• Four systematic reviews including 27 RCTs w 3939 participants
• Examined proportion of patients that experienced a minimally important distance = a movement of 1cm on the 10cm visual analog scale for pain
• found a 0.5cm effect size, indicating a 10% increased in the proportion of patients that experienced minimally important change
• moderate to high confidence that medical cannabis provides a small/very small benefit
• side effects include cognitive impairment, attention impairment, dizziness, and nausea
• only recommended medical cannabis be used if standard care fails

Question 9

International association for the study of psin

Answer 9

• Took into account pre-clinical and clinical data on efficacy and harm, as well as societal impacts
• looked at 57 systematic reviews, and conducted their own meta-analysis of RCTs with >30 participants
• found a very small effect size
• many studies had high/uncertain risk of bias due to lack of randomisation, blinding, or incomplete data
• data did not refute/support use of medical cannabis but more robust and rigorous research is needed
• did not recommend medical cannabis for chronic pain unless in a clinical trial