Cannabinoid type 1 receptors located on single-minded 1-expressing neurons control emotional behaviors

This study has investigated the role of hypothalamic and amygdalar type-1 cannabinoid (CB1) receptors in the emotional and neuroendocrine responses to stress. To do so, we used the Cre/loxP system to generate conditional mutant mice lacking the CB1 gene in neurons expressing the transcription factor single-minded 1 (Sim1). This choice was dictated by former evidence for Sim1-Cre transgenic mice bearing Cre activity in all areas expressing Sim1, which chiefly includes the hypothalamus (especially the paraventricular nucleus, the supraoptic nucleus, and the posterior hypothalamus) and the mediobasal amygdala. Genomic DNA analyses in Sim1-CB1(-/-) mice indicated that the CB1 allele was excised from the hypothalamus and the amygdala, but not from the cortex, the striatum, the thalamus, the nucleus accumbens, the brainstem, the hippocampus, the pituitary gland, and the spinal cord. Double-fluorescent in situ hybridization experiments further indicated that Sim1-CB1(-/-) mice displayed a weaker CB1 receptor mRNA expression in the paraventricular nucleus of the hypothalamus and the mediobasal part of the amygdala, compared to wild-type animals. Individually housed Sim1-CB1(-/-) mice and their Sim1-CB1(+/+) littermates were exposed to anxiety and fear memory tests under basal conditions as well as after acute/repeated social stress. A principal component analysis of the behaviors of Sim1-CB1(-/-) and Sim1-CB1(+/+) mice in anxiety tests (open field, elevated plus-maze, and light/dark box) revealed that CB1 receptors from Sim1-expressing neurons exert tonic, albeit opposite, controls of locomotor and anxiety reactivity to novel environments. No difference between genotypes was observed during the recall of contextual fear conditioning or during active avoidance learning. Sim1-CB1(-/-), but not Sim1-CB1(+/+), mice proved sensitive to an acute social stress as this procedure reverted the increased ambulation in the center of the open field. The stimulatory influence of repeated social stress on body and adrenal weights, water intake, and sucrose preference was similar in the two genotypes. On the other hand, repeated social stress abolished the decrease in cued-fear conditioned expression that was observed in Sim1-CB1(-/-) mice, compared to Sim1-CB1(+/+) mice. This study suggests that CB1 receptors located on Sim1-expressing neurons exert a tonic control on locomotor reactivity, unconditioned anxiety, and cued-fear expression under basal conditions as well as after acute or repeated stress.