Anandamide inhibits excitatory transmission to rat substantia gelatinosa neurones in a manner different from that of capsaicin

Actions of anandamide (10 muM) were examined on monosynaptic glutamatergic transmission from the periphery to substantia gelatinosa (SG) neurones in adult rat spinal cord slices using the whole-cell patch-clamp technique. In 64% of neurones examined, Adelta-fibre-evoked excitatory postsynaptic currents (EPSCs) were depressed (in amplitude; by 32 +/- 4%, n = 21) by anandamide. On the contrary, an inhibitory action on C-fibre-evoked EPSCs was observed in only 31% of neurones tested; this magnitude (17 +/- 3%, n = 4) was less than that of Adelta-fibre EPSCs (P < 0.05). A cannabinoid-receptor agonist, WIN 55,212-2 (5 muM), exhibited similar actions on the EPSCs. In a neurone with minimal effects of anandamide on C-fibre EPSCs, capsaicin (1 muM) largely depressed the EPSCs (n 3); Adelta-fibre EPSCs were little affected by capsaicin. It is concluded that unlike capsaicin, anandamide inhibits more effectively Adelta-fibre than C-fibre-mediated excitatory transmission in the SG, possibly through the activation of the cannabinoid receptor. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.