Cannabinoid-based medications for pain

Against the backdrop of an increasing opioid use epidemic and a marked acceleration of prescription opioid–related deaths,^1,2 there has been an impetus to explore the usefulness of alternative and co-analgesic agents to assist patients with chronic pain. Preclinical studies employing animal-based models of human pain syndromes have demonstrated that cannabis and chemicals derived from cannabis extracts may mitigate several pain conditions.³

Because there are significant comorbidities between psychiatric disorders and chronic pain, psychiatrists are likely to care for patients with chronic pain. As the availability of and interest in cannabinoid-based medications (CBM) increases, psychiatrists will need to be apprised of the utility, adverse effects, and potential drug interactions of these agents.

The endocannabinoid system and cannabis receptors

The endogenous cannabinoid (endocannabinoid) system is abundantly present within the peripheral and central nervous systems. The first identified, and best studied, endocannabinoids are N-arachidonoyl-ethanolamine (AEA; anandamide) and 2-arachidonoylglycerol (2-AG).⁴ Unlike typical neurotransmitters, AEA and 2-AG are not stored within vesicles within presynaptic neuron axons. Instead, they are lipophilic molecules produced on demand, synthesized from phospholipids (ie, arachidonic acid derivatives) at the membranes of post-synaptic neurons, and released into the synapse directly.⁵

Acting as retrograde messengers, the endocannabinoids traverse the synapse, binding to receptors located on the axons of the presynaptic neuron. Two receptors—CB1 and CB2—have been most extensively studied and characterized.^6,7 These receptors couple to Gi/o-proteins to inhibit adenylate cyclase, decreasing Ca2+ conductance and increasing K+ conductance.⁸ Once activated, cannabinoid receptors modulate neurotransmitter release from presynaptic axon terminals. Evidence points to a similar retrograde signaling between neurons and glial cells. Shortly after receptor activation, the endocannabinoids are deactivated by the actions of a transporter mechanism and enzyme degradation.^9,10

The endocannabinoid system and pain transmission

Cannabinoid receptors are present in pain transmission circuits spanning from the peripheral sensory nerve endings (from which pain signals originate) to the spinal cord and supraspinal regions within the brain.^11-14 CB1 receptors are abundantly present within the CNS, including regions involved in pain transmission. Binding to CB1 receptors, endocannabinoids modulate neurotransmission that impacts pain transmission centrally. Endocannabinoids can also indirectly modulate opiate and N-methyl-d-aspartate (NMDA) receptors involved in pain relay and transmission.¹⁵

By contrast, CB2 receptors are predominantly localized to peripheral tissues and immune cells, although there has been some discovery of their presence within the CNS (eg, on microglia). Endocannabinoid activation of CB2 receptors is thought to modulate the activity of peripheral afferent pain fibers and immune-mediated neuro­inflammatory processes—such as inhibition of prostaglandin synthesis and mast cell degranulation—that can precipitate and maintain chronic pain states.^16-18

Evidence garnered from preclinical (animal) studies points to the role of the endocannabinoid system in modulating normal pain transmission (see Manzanares et al³ for details). These studies offer a putative basis for understanding how exogenous cannabinoid congeners might serve to ameliorate pain transmission in pathophysiologic states, including chronic pain.

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