Cannabinoids and pain

Cannabinoids have been used to treat pain for many centuries. However, only during the past several decades have rigorous scientific methods been applied to understand the mechanisms of cannabinoid action. Cannabinoid receptors were discovered in the late 1980s and have been found to mediate the effects of cannabinoids on the nervous system. Several endocannabinoids were subsequently identified. Many studies of cannabinoid analgesia in animals during the past century showed that cannabinoids block all types of pain studied. These effects were found to be due to the suppression of spinal and thalamic nociceptive neurons, independent of any actions on the motor systems. Spinal, supraspinal and peripheral sites of cannabinoid analgesia have been identified. Endocannabinoids are released upon electrical stimulation of the periaqueductal gray, and in response to inflammation in the extremities. These observations and others thus suggest that a natural function of cannabinoid receptors and their endogenous ligands is to regulate pain sensitivity. The therapeutic potential of cannabinoids remains an important topic for future investigations, with previous work suggesting utility in clinical studies of cancer and surgical pain. New modes of delivery and/or new compounds lacking the psychotropic properties of the standard cannabinoid ligands offer promise for cannabinoid therapeutics for pain.

Liquid chromatographic-mass spectrometric measurement of the endogenous cannabinoid 2-arachidonylglycerol in the spinal cord and peripheral nervous system

AIM

To develop a sensitive method for measuring the putative endocannabinoid 2-arachidonylglycerol (2-AG) in the peripheral and central nervous system.

METHODS

A method using atmospheric pressure chemical ionization (APCI) liquid chromatography/mass spectrometry (LC/MS) was developed to determine the levels of 2-AG in methanol extracts of the rat lumbar spinal cord, dorsal root ganglion (DRG), and sciatic nerve.

RESULTS

2-AG was detected with high sensitivity and minimal sample preparation. The levels in the tissues analyzed were < or = pmol/mg wet weight. Similar levels were found in the spinal cord and the DRG, whereas approximately 7-fold lower levels were observed in the sciatic nerve.

CONCLUSION

2-AG is present in the peripheral nervous system, and the levels are markedly higher in cell bodies than those in axons.

CB1 receptor localization in rat spinal cord and roots, dorsal root ganglion, and peripheral nerve

AIM

The localization of CB1 receptors in the spinal cord, spinal roots, dorsal root ganglion (DRG), and peripheral nerve of the rat was determined.

METHODS

We studied the distribution of CB1 cannabinoid receptors by immunohistochemistry using an antibody raised against the N-terminal of the receptor.

RESULTS

The spinal cord showed numerous transverse fibers labelled for CB1 receptors throughout and concentrated in the dorsal horn. Lightly-stained cells were observed throughout the spinal cord gray matter. The DRG also showed cells and fibers labelled for CB1 receptors. Labelled fibers were observed in both dorsal and ventral roots as well as in peripheral nerves.

CONCLUSION

The presence of CB1 receptors in the DRG, the dorsal root, and the dorsal horn is in accordance with the analgesic effects of cannabinoids. The presence of labelled cells and fibers in the ventral horn and ventral root provides a substrate for cannabinoid-induced muscle relaxant and antispastic effects.

Pain modulation by release of the endogenous cannabinoid anandamide

Synthetic cannabinoids produce behavioral analgesia and suppress pain neurotransmission, raising the possibility that endogenous cannabinoids serve naturally to modulate pain. Here, the development of a sensitive method for measuring cannabinoids by atmospheric pressure-chemical ionization mass spectrometry permitted measurement of the release of the endogenous cannabinoid anandamide in the periaqueductal gray (PAG) by in vivo microdialysis in the rat. Electrical stimulation of the dorsal and lateral PAG produced CB1 cannabinoid receptor-mediated analgesia accompanied by a marked increase in the release of anandamide in the PAG, suggesting that endogenous anandamide mediates the behavioral analgesia. Furthermore, pain triggered by subcutaneous injections of the chemical irritant formalin substantially increased the release of anandamide in the PAG. These findings indicate that the endogenous cannabinoid anandamide plays an important role in a cannabinergic pain-suppression system existing within the dorsal and lateral PAG. The existence of a cannabinergic pain-modulatory system may have relevance for the treatment of pain, particularly in instances where opiates are ineffective.

The neurobiology of cannabinoid analgesia

The discovery of cannabinoid receptors and their putative endogenous ligands raises questions as to the nature of the effects produced by cannabinoids on neural circuits that mediate pain and whether endogenous cannabinoids produced by the brain or in the periphery serve naturally to modulate pain. A sizable body of previous work showed that cannabinoid agonists suppress pain behavior in a variety of models of acute and chronic pain. However, at appropriate doses, cannabinoids also profoundly suppress motor behavior (see Sañudo-Peña et al., this volume), which complicates the interpretation of behavioral analgesia since a motor response is the endpoint of virtually all such studies. Studies conducted in this laboratory used biochemical and neurophysiological measures to determine whether cannabinoids suppress nociceptive neurotransmission. The results showed that cannabinoids suppress nociceptive neurotransmission at the level of the spinal cord and the thalamus. These effects are reversible, receptor mediated, selective for painful as opposed to nonpainful somatic stimuli, and track the behavioral analgesia both in time course and potency.

Cannabinoid WIN 55,212-2 inhibits the activity-dependent facilitation of spinal nociceptive responses

Cannabinoids suppress nociceptive processing of acute stimuli, but little is known about their effects on processes that lead to hyperexcitability of nociceptive neurons following prolonged noxious stimulation. Wind-up, the increasingly strong response of spinal nociceptive neurons to repetitive noxious electrical stimuli, results from a fast-rising cumulative depolarization and increase in intracellular calcium concentration. These processes produce central sensitization, the increased excitability of spinal nociceptive neurons that contributes to the hyperalgesia and allodynia associated with chronic pain. Intravenous injection of the potent, synthetic cannabinoid agonist WIN 55, 212-2, but not the inactive enantiomer, WIN 55,212-3, dose-dependently decreased the wind-up of spinal wide dynamic range and nociceptive-specific neurons independent of acute responses to activation of low- and high-threshold primary afferents. This is the first direct evidence that cannabinoids inhibit the activity-dependent facilitation of spinal nociceptive responses.

Evidence for a role of endogenous cannabinoids in the modulation of acute and tonic pain sensitivity

The competitive CB1 receptor antagonist SR141716A was used to test the hypothesis that endogenous cannabinoids modulate tonic pain sensitivity. Pretreatment with the antagonist significantly enhanced the response to a chemical nociceptive stimulus in the formalin test. Postreatment with the antagonist 5 min following the induction of tonic pain produced hyperalgesia during the tonic phase only. These findings suggest that endogenous cannabinoids serve naturally to modulate the maintenance of pain following repeated noxious stimulation.