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


Joseba Pineda, Department of Pharmacology, Faculty of Medicine, University of the Basque Country, Barrio Sarriena s/n, E-48940 Leioa, Bizkaia, Spain. E-mail: joseba.pineda@ehu.es


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-Synthélabo 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 µM) and AM251 (1 µM) decreased the firing rate of about 50% of all the recorded DRN 5-HT cells. The GABAAreceptor antagonist picrotoxin (20 µM) (Sigma) prevented and also reversed the inhibitory effect of rimonabant (1 µM) and AM251 (1 µM), 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 µM) (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 µM) also inhibited the firing activity of a number of 5-HT neurons, but this inhibition was not blocked by rimonabant (1 µM) or AM251 (1 µM), and the stable analogue R-(+) N-(2-hydroxy-1methylethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (methanandamide, Tocris Cookson) (10 µM) did not mimic this effect. The selective CB1 receptor agonist arachidonoyl-2-chloroethylamide (ACEA) (1 µM) 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.