Effect of the CB1 receptor antagonists rimonabant and AM251 on the firing rate of dorsal raphe nucleus neurons in rat brain slices

Background and purpose: Previous studies have suggested a regulation of 5-hydroxytryptamine (5-HT) neurons by the endocannabinoid system. The aim of our work was to examine the effect of two CB1 receptor antagonists, SR141716A (rimonabant, Sanofi-Synthelabo Recherche, Montpellier, France) and N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251, Tocris Cookson, Bristol, UK), on the firing rate of dorsal raphe nucleus (DRN) neurons. Experimental approach: Single-unit extracellular recordings were performed to study the effect of CB1 receptor antagonists in slices of the DRN from rat brain. Key results: Rimonabant (1 mu M) and AM251 (1 mu M) decreased the firing rate of about 50% of all the recorded DRN 5-HT cells. The GABA(A)receptor antagonist picrotoxin (20 mu M) (Sigma) prevented and also reversed the inhibitory effect of rimonabant (1 mu M) and AM251 (1 mu M), suggesting that CB1 receptors regulate 5-HT neurons through the GABAergic system. However, the CB1/CB2 receptor agonist R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)-methyl]pyrrolol[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate salt (10 mu M) (WIN55212-2, Sigma, St. Louis, MO, USA) failed to change the firing activity of non-5-HT (presumably GABAergic) neurons in the DRN. The endocannabinoid N-(2-hydroxyethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (anandamide, Tocris Cookson) (10 mu M) also inhibited the firing activity of a number of 5-HT neurons, but this inhibition was not blocked by rimonabant (1 mu M) or AM251 (1 mu M), and the stable analogue R-(+) N-(2-hydroxy-1methylethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (methanandamide, Tocris Cookson) (10 mu M) did not mimic this effect. The selective CB1 receptor agonist arachidonoyl-2-chloroethylamide (ACEA) (1 mu M) only slightly increased the firing rate of DRN 5-HT cells. Conclusions and implications: These results suggest a tonic/constitutive regulation of DRN 5-HT neurons by the endocannabinoid system, which may occur through a CB1 receptor-mediated inhibition of the GABAergic system. The inhibitory effect of anandamide may be mediated through a CB1 receptor-independent mechanism.