Receptor-independent effects of endocannabinoids on ion channels

Endogenous cannabinoids (endocannabinoids), produced from membrane-bound precursors via calcium and/or G-protein dependent processes, mimic the effects of cannabinoids by activating cannabinoid CB1 and/or CB2 receptors. Several reports however, also indicate that endocannabinoids can produce effects that are independent of cannabinoid receptors. Thus, in pharmacologically relevant concentrations, endocannabinoids have been demonstrated to modulate the functional properties of voltage-gated ion channels including Ca2+ channels, Na+ channels and various types of K+ channels, and ligand-gated ion channels such as 5-HT3, and nicotinic ACh receptors. In addition, the functional modulations by endocannabinoids of other ion-transporting membrane proteins such as transient potential receptor-class channels, gap junctions, and neurotransmitter transporters have also been reported. These findings indicate that additional molecular targets for endocannabinoids exist and that these targets may represent important sites for cannabinoids to alter either the excitability of the neurons or the response of the neuronal systems. This review focuses on the results of recent studies indicating that beyond their receptor-mediated effects, endocannabinoids alter the function of ion channels directly.