Local enhancement of cannabinoid CB₁ receptor signalling in the dorsal hippocampus elicits an antidepressant-like effect

Abstract

Systemic administration of direct cannabinoid CB₁ receptor agonists and inhibitors of the hydrolytic enzyme fatty acid amide hydrolase have been shown to elicit antidepressant effects. Moreover, the endocannabinoid system in the hippocampus is sensitive to both chronic stress and antidepressant administration, suggesting a potential role of this system in emotional changes associated with these regimens. The aim of this study was to determine if cannabinoid CB₁ receptors in the hippocampus modulate emotionality in rats as assessed via the forced swim test. Male Sprague–Dawley rats were bilaterally implanted with cannulae directed at the dentate gyrus of the dorsal hippocampus and subsequently received three infusions of either the cannabinoid CB₁ receptor agonist HU-210 (1 and 2.5 μg), the fatty acid amide hydrolase inhibitor URB597 (0.5 and 1 μg), the cannabinoid CB₁ receptor antagonist AM251 (1 and 2.5 μg), or vehicle (dimethyl sulfoxide) and were assessed in the forced swim test. Infusion of both doses of HU-210 resulted in a dramatic reduction in immobility and increase in swimming behaviour, indicative of an antidepressant response, which was partially reversed by coadministration of AM251. No effect of URB597 administration or any effect following the administration of AM251 alone was, however, observed. These data indicate that activation of CB₁ receptors in the dentate gyrus of the hippocampus results in an antidepressant-like response. Collectively, these data highlight the potential importance of changes in the hippocampal endocannabinoid system following stress or antidepressant treatment with respect to the manifestation and/or treatment of depression.