The CB1 receptor antagonists sr 141716a and am 251 inhibit the firing activity of a subpopulation of dorsal raphe 5-HT neurons through gabaergic system

Aitziber Mendiguren, Joseba Pineda. University of the Basque Country, Leioa, Bizkaia, Spain.

Several studies have suggested that the dorsal raphe nucleus (DRN) is a pharmacological target of cannabinoid compounds (Martin et al., 1995; Gobbi et al., 2005). The aim of this work was to examine, by single-unit extracellular recordings, the role of the endogenous cannabinoid system in regulating the firing activity of DRN neurons in brain slices from male Sprague-Dawley (250-300 g) rats. For this purpose, we evaluated the effect of different CB1 receptor antagonists (SR 141716A and AM 251) and agonists (anandamide and WIN 55212-2) on the firing rate of DRN neurons.

In 9 out of 21 5-HT cells recorded in the DRN, perfusion with SR 141716A (1 μM) inhibited the firing rate of these neurons to values below the 25th percentile (-0.5%) of the vehicle group. The inhibitory effect achieved in these responsive cells was significantly stronger than that in the vehicle group (53 ± 13% vs 3 ± 0.9%; n=9 and n=18, respectively; p<0.05 by a Student´s t test). Perfusion with AM 251 (1 μM) also inhibited the firing activity in 6 out of 12 recorded 5-HT cells and the inhibition reached in this responsive cells was higher than in the vehicle group (62 ± 15%, n=6, p<0.05 by a Student´s t test). In responsive cells, blockade of GABAA receptors with picrotoxin (20 μM) strongly attenuated the inhibitory effect of SR 141716A (1 μM) (10 ± 5%; n=5; p<0.05 by a Student´s t test) and AM 251 (1 μM) (12 ± 11%; n=3; p<0.05 by a Student´s t test). In addition, perfusion with WIN 55212-2 (10 μM) failed to change the firing activity of DRN non-5-HT cells (presumably GABAergic) (n=7). On the other hand, perfusion with anandamide (10 μM) inhibited the firing activity of a subpopulation of DRN 5-HT neurons to values below the 25th percentile (-1.5%) of the vehicle group (n=24/47 vs n=9/34, p<0.05 by a Fischer´s exact test). The inhibitory effect achieved in responsive cells was 43 ± 7% vs 11 ± 2% of the vehicle group (n=24, p<0.0005 by a Student´s t test). Unexpectedly, perfusion with SR 141716A (1 μM) or AM 251 (1 μM) failed to modify the inhibitory effect of anandamide (10 μM) or the number of responsive cells to the endocannabinoid. Moreover, methanandamide (10 μM) did not significantly change the firing activity of DRN 5-HT cells. In our experiments, application of anandamide (10 μM) did not change the parameters of the concentration-effect curves for 5-HT (1-200 μM), which is mediated by the 5-HT1A autoreceptor.

These results suggest the presence of a tonically active endocannabinoid system in the DRN, which seems to positively regulate the firing activity of a subpopulation of 5-HT neurons probably through a CB1 receptor-mediated presynaptic inhibition of the GABAergic system. The inhibitory effect of anandamide may be mediated through an undetermined mechanism which is independent of CB1 receptors and not related to 5-HT1A receptors.

Gobbi G. et al. (2005) Proc. Natl. Acad. Sci. 102, 18620-5

Martin W.J. et al. (1995) Life Sci. 56, 2103-9.