Conditional cannabinoid receptor type 1 mutants reveal neuron subpopulation-specific effects on behavioral and neuroendocrine stress responses

Summary

The complete genetic loss or pharmacological blockade of cannabinoid receptor type 1 (CB1) in mice results in both altered behavioral performance and increased stress hormone secretion in response to stressful encounters such as forced swim test (FST) exposure. CB1 is expressed on nerve terminals belonging to different neurotransmitter systems, including the glutamatergic and GABAergic system, where it is able to suppress excitatory and inhibitory neurotransmission, respectively. In the current study, we used the conditional mutagenesis approach in mice to investigate the neurotransmitter systems involved in these behavioral and neuroendocrine phenotypes in regard to CB1 signaling. Mice lacking CB1 in cortical glutamatergic neurons (Glu-CB1^−/−) showed decreased passive stress coping (decreased immobility) in the FST, whereas mice lacking CB1 in principal forebrain neurons (CaMK-CB1^−/−) and GABAergic neurons (GABA-CB1^−/−), respectively, behaved as littermate controls. However, we found increased FST-induced corticosterone secretion only in CaMK-CB1^−/− mice, whereas Glu-CB1^−/− and GABA-CB1^−/− mice exhibited normal corticosterone release as compared to controls. Thus, behavioral and neuroendocrine acute stress coping in response to the FST is mainly influenced by CB1 signaling on different glutamatergic neuronal subpopulations, but not by CB1 on GABAergic neurons.

Introduction

Endocannabinoids are fatty acid derivatives that can act as retrograde messengers in the nervous system by activation of presynaptically located cannabinoid receptor type 1 (CB1) (Piomelli, 2003). CB1 is one of the most abundant and widely expressed G-protein coupled neuronal receptors (Piomelli, 2003). It is located on both GABAergic and glutamatergic synapses where it suppresses inhibitory and excitatory neurotransmission (Monory et al., 2006). Therefore, the endocannabinoid system forms an intricate and complex neuromodulatory network that controls neuronal circuits of different nature at multiple sites (Piomelli, 2003, Monory et al., 2006).

CB1 expression is present in many brain regions involved in stress processing (Marsicano and Lutz, 1999, Piomelli, 2003), and significant interactions between endocannabinoid signaling and stress responses have been reported (Viveros et al., 2005). CB1 signaling is not only involved in the neuroendocrine (Patel et al., 2004, Steiner et al., 2008a), but also in the behavioral response to stress (Viveros et al., 2005, Steiner et al., 2008a). Although the use of complete CB1 knockout mice together with pharmacological approaches allowed the clear identification of endocannabinoid-mediated stress effects (Steiner et al., 2008a), the cellular substrates of these effects with regard to brain areas and neuronal subpopulations involved (e.g., GABAergic versus glutamatergic neurons) has remained unexplored.

Conditional CB1 knockout animals generated by using the Cre/loxP system, leading to a lack of CB1 in certain neuronal subpopulations (Marsicano et al., 2003, Monory et al., 2006, Monory et al., 2007), constitute an important tool to answer these questions. We have used three conditional CB1-mutant lines, lacking CB1 either in cortical glutamatergic neurons, in principal forebrain neurons or in GABAergic neurons, respectively, to investigate the differential roles of CB1 signaling in these neuronal subpopulations on neuroendocrine and behavioral stress coping in response to the forced swim test (FST), one of the most widely used paradigms in rodents to detect antidepressant-like effects of novel drugs and depression-like behavior of mutant mice (Cryan and Holmes, 2005).