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

Abstract

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 mM) and AM251 (1 mM) decreased the firing rate of about 50% of all the recorded DRN 5-HT cells. The GABAA receptor antagonist picrotoxin (20 mM) (Sigma) prevented and also reversed the inhibitory effect of rimonabant (1 mM) and AM251 (1 mM), 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 mM) (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 mM) also inhibited the firing activity of a number of 5-HT neurons, but this inhibition was not blocked by rimonabant (1 mM) or AM251 (1 mM), and the stable analogue R-(+) N-(2-hydroxy1methylethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (methanandamide, Tocris Cookson) (10 mM) did not mimic this effect. The selective CB1 receptor agonist arachidonoyl-2-chloroethylamide (ACEA) (1 mM) 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.

Drug discovery strategies that focus on the endocannabinoid signaling system in psychiatric disease

“…In vitro studies provide a more direct role for eCB regulation of the DR-5-HT system. Several studies have shown that CB1r stimulation and administration of WIN 55,212-2 decrease glutamatergic signaling in DR-5-HT neurons [44, 45]; however, other findings indicate that eCB tone regulates DR-5-HT signaling by modulating GABAergic inputs to serotonergic neurons [46]. Neuroanatomical studies examining the synaptic relationship of CB1r with GABAergic and glutamatergic neurons are needed to better define sites of action of eCBs in the modulation of DR-5-HT signaling [46].…”

Modulation of the serotonin system by endocannabinoid signaling

“…The outcome of these studies also suggests that CB1 receptors are most likely located on glutamatergic terminals impinging on DR 5-HT neurons. In addition to the inhibition of glutamate release, a recent in vitro extracellular recording study reports that eCB tone in the DR can regulate the firing activity of 5-HT neurons by regulating GABAergic transmission (Mendiguren and Pineda, 2009). However, the detailed cellular mechanism of such regulation remains to be determined.…”

A new designer drug 5F-ADB activates midbrain dopaminergic neurons but not serotonergic neurons

“…This result suggests that local application of 5F-ADB had no effect on serotonergic systems via either CB 1 receptors or serotonin receptors. Recent evidence shows that CB 1 receptor antagonists decreased serotonergic firing rates (Mendiguren and Pineda, 2009), suggesting that endocannabinoid levels are high even in acute brain slices. Therefore, we tested the effect of the CB 1 receptor antagonist AM251 (1 μM) on serotonergic firing rates.…”