Editorial: Central & Peripheral Nervous Systems: Cannabinoid receptor ligands: clinical and neuropharmacological considerations relevant to future drug discovery and development

Cannabinoid receptor ligands: clinical and neuropharmacological considerations, relevant to future drug discovery and development

This review highlights some important advances that have taken place in cannabinoid research over the last four years. It focuses on novel ligands that are of interest either as experimental tools or as lead compounds for therapeutic agents and possible clinical applications for some of these ligands. The molecular targets for these compounds are various components of the system of endogenous cannabinoids (endocannabinoids) and receptors that together constitute the 'endocannabinoid system'. These are CB(1) cannabinoid receptors that are present mainly on central and peripheral neurones, CB(2) cannabinoid receptors that are expressed predominantly by immune cells, the biochemical mechanisms responsible for the tissue uptake or metabolism of endocannabinoids and vanilloid receptors. Other cannabinoid receptor types may also exist. Recently developed ligands include potent and selective agonists for CB(1) and CB(2) receptors, a potent CB(2)-selective antagonist/inverse agonist and inhibitors of endocannabinoid uptake or metabolism. Future research should be directed at characterising the endocannabinoid system more completely and at obtaining more conclusive clinical data about the possible beneficial effects of cannabinoids as well as their adverse effects. There is also a need for improved cannabinoid formulations/modes of administration in the clinic and advances in this area should be facilitated by the recent development of a potent water-soluble CB(1)/CB(2) receptor agonist. A growing number of strategies for separating sought-after therapeutic effects of cannabinoid receptor agonists from the unwanted consequences of CB(1) receptor activation are now emerging and these are discussed at the end of this review.

Evidence for the presence of CB1 cannabinoid receptors on peripheral neurones and for the existence of neuronal non-CB1 cannabinoid receptors

The discovery of CB1 and CB2 receptors and of endogenous agonists for these receptors has sparked renewed interest in the therapeutic potential of cannabinoids. This has led to a need for strategies that will provide a better separation of wanted from unwanted effects, particularly for CB1 receptor agonists. Possible strategies are to target CB1 receptors present on neurones outside the central nervous system or novel types or subtypes of neuronal cannabinoid receptor. This paper reviews evidence for the presence of CB1 receptors on peripheral neurones and for the existence of neuronal non-CB1 cannabinoid receptors.

Evidence for inverse agonism of SR141716A at human recombinant cannabinoid CB1 and CB2 receptors

The cannabinoid receptor antagonist SR141716A has been suggested to be an inverse agonist at CB1 receptors in some isolated intact tissues. We found that the basal incorporation of [35S]-GTPgammaS in Chinese hamster ovary cells expressing human recombinant CB1 and CB2 receptors was inhibited by SR141716A (mean pEC50s 8.26 and 6.00, respectively), whereas cannabinol (10 microM) had no significant effect at hCB1 receptors but inhibited the binding at hCB2 receptors. As cannabinol had no effect on basal [35S]-GTPmicroS binding at hCB1 at a concentration 100 fold higher than its binding affinity (K = 0.1 microM), we conclude that endogenous cannabinoid receptor agonists are not a confounding factor and suggest the actions of SR141716A at the hCB1 receptor, and the actions of SR141716A and cannabinol at the hCB2 receptor, are due to inverse agonism.

Evidence that methyl arachidonyl fluorophosphonate is an irreversible cannabinoid receptor antagonist

1. Methyl arachidonyl fluorophosphonate (MAFP) (1 microM) significantly attenuated the ability of WIN 55,212-2, CP 55,940, (-)-delta 9-tetrahydrocannabinol (THC), nabilone and (R)-(+)-arachidonoyl-1'-hydroxy-2'-propylamide (methanandamide) to inhibit electrically-evoked isometric contractions of the myenteric plexus-longitudinal muscle preparation of guinea-pig small intestine. 2. The sizes of the maximal responses to WIN 55,212-2 and CP 55,940 decreased significantly in the presence of 1 microM MAFP. 3. MAFP (1 microM) essentially abolished the inhibitory effects on the twitch response of the highest concentration of methanandamide used (3.162 microM). The dextral shift it induced in the log concentration-response curve of nabilone was non-parallel. In contrast, the dextral shift in the log concentration-response curve of THC produced by MAFP did not deviate significantly from parallelism and was relatively small with a mean value of 3.45 and 95% confidence limits of 1.19 and 13.08. 4. MAFP (1 microM) did not attenuate the effects of normorphine or clonidine on the twitch response of the myenteric plexus-longitudinal muscle preparation or affect the contractile response of this preparation to acetylcholine. 5. When administered by itself at concentrations of 1 to 1000 nM, MAFP had no detectable effect on the twitch response of the myenteric plexus-longitudinal muscle preparation. 6. These results support the hypothesis that MAFP is an irreversible cannabinoid CB1 receptor antagonist that possesses some degree of selectivity.