Elsevier

Neuroscience

Volume 146, Issue 3, 25 May 2007, Pages 1212-1219
Neuroscience

Neuroanatomy
Identification of the cannabinoid receptor type 1 in serotonergic cells of raphe nuclei in mice

https://doi.org/10.1016/j.neuroscience.2007.02.021Get rights and content

Abstract

The endocannabinoid system (ECS) possesses neuromodulatory functions by influencing the release of various neurotransmitters, including GABA, noradrenaline, dopamine, glutamate and acetylcholine. Even though there are studies indicating similar interactions between the ECS and the serotonergic system, there are no results showing clear evidence for type 1 cannabinoid receptor (CB1) location on serotonergic neurons. In this study, we show by in situ hybridization that a low but significant fraction of serotonergic neurons in the raphe nuclei of mice contains CB1 mRNA as illustrated by the coexpression with the serotonergic marker gene tryptophane hydroxylase 2, the rate limiting enzyme for the serotonin synthesis. Furthermore, by double immunohistochemistry and confocal microscopy, we were able to detect CB1 protein on serotonergic fibers and synapses expressing the serotonin uptake transporter in the hippocampus and the amygdala. Our findings indicate that the CB1-mediated regulation of serotonin release can depend in part on a direct cross-talk between the two systems at single cell level, which might lead to functional implications in the modulation of emotional states.

Section snippets

Animals

This study was performed on adult (3–5 months old) C57BL/6N female mice. Animals were housed in a temperature- and humidity-controlled room with a 12-h light/dark cycle and had access to food and water ad libitum. As the amount of TPH2 mRNA was found to be under circadian control with the highest level 1–2 h before the ending of the light cycle (Malek et al., 2005), the animals were killed in this time point. The experimental protocols were carried out in accordance with the European

Single ISH

TPH2 transcripts were detected in mid- and hindbrain areas identified as the raphe nuclei in agreement with previously published data (Patel et al., 2004), thus confirming TPH2 as a marker gene for serotonergic neurons (Fig. 1). Following the classification of Dahlström and Fuxe (1964), all nine raphe nuclei, nucleus raphe pallidus (B1), nucleus raphe obscurus (B2), nucleus raphe magnus (B3), nucleus paragigantocellularis (B4), nucleus raphe pontis resp. medianus (B5), nucleus dorsalis (caudal

Discussion

In this study, we show that CB1 receptors are present in a subset of serotonergic neurons in the brain. These results support the concept that a direct cross-talk between the ECS and serotonergic neurons exists and that the observed effects of exogenous and/or endogenous activation of CB1 receptors on serotonin release (Nakazi et al 2000, Egashira et al 2002, Darmani et al 2003, Tzavara et al 2003) can be at least in part due to a direct action of CB1 receptors on serotonergic terminals.

Acknowledgments

We would like to thank Dr. Ken Mackie (University of Washington, Department of Anesthesiology, Seattle, WA, USA) for providing the rabbit L15 antiserum against CB1. We also thank Dr. Paresh D. Patel (University of Michigan, Medical Center, Ann Arbor, MI, USA) for providing the mouse TPH2 probe for the in situ experiments and Christof Rickert (Department of Genetics, Johannes Gutenberg University, Mainz, Germany) for technical assistance with the confocal analysis.

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    Present address: AVENIR INSERM, Institute Francois Magendie,146 rue Leo Saignat, 33077 Bordeaux, France.

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