Inverse agonism and neutral antagonism at cannabinoid CBI receptors

There are at least two types of cannabinoid receptor, CB1 and CB2, both G protein coupled. CB1 receptors are expressed predominantly at nerve terminals and mediate inhibition of transmitter release whereas CB2 receptors are found mainly on immune cells, one of their roles being to modulate cytokine release. Endogenous cannabinoid receptor agonists also exist and these endocannabinoids together with their receptors constitute the endocannabinoid system. These discoveries were followed by the development of a number of CB1- and CB2-selective antagonists that in some CB, or CB2 receptor-containing systems also produce inverse cannabimimetic effects, effects opposite in direction from those produced by cannabinoid receptor agonists. This review focuses on the CBI-selective antagonists, SR141716A, AM251, AM281 and LY320135, and discusses possible mechanisms by which these ligands produce their inverse effects: (1) competitive surmountable antagonism at CB, receptors of endogenously released endocannabinoids, (2) inverse agonism resulting from negative, possibly allosteric, modulation of the constitutive activity of CB, receptors in which CB, receptors are shifted from a constitutively active on state to one or more constitutively inactive off states and (3) CB, receptor-independent mechanisms, for example antagonism of endogenously released adenosine at A, receptors. Recently developed neutral competitive CB, receptor antagonists, which are expected to produce inverse effects through antagonism of endogenously released endocannabinoids but not by modulating CB, receptor constitutive activity, are also discussed. So too are possible clinical consequences of the production of inverse cannabimimetic effects, there being convincing evidence that released endocannabinoids can have autoprotective roles. (C) 2004 Elsevier Inc. All rights reserved.