Hippocampal CB1 Receptors Mediate the Memory Impairing Effects of Δ9-Tetrahydrocannabinol

Abstract

It is firmly established that the hippocampus, a brain region implicated in spatial learning, episodic memory, and consolidation, contains a high concentration of CB₁ receptors. Moreover, systemic and intrahippocampal administration of cannabinoid agonists have been shown to impair hippocampal-dependent memory tasks. However, the degree to which CB₁ receptors in the hippocampus play a specific functional role in the memory disruptive effects of marijuana or its primary psychoactive constituent Δ⁹-tetrahydrocannabinol (Δ⁹-THC) is unknown. This study was designed to determine whether hippocampal CB₁ receptors play a functional role in the memory disruptive effects of systemically administered cannabinoids, using the radial arm maze, a well characterized rodent model of working memory. Male Sprague–Dawley rats were implanted with bilateral cannulae aimed at the CA1 region of the dorsal hippocampus. The CB₁ receptor antagonist, rimonabant, was delivered into the hippocampus before to a systemic injection of either Δ⁹-THC or the potent cannabinoid analog, CP-55,940. Strikingly, intrahippocampal administration of rimonabant completely attenuated the memory disruptive effects of both cannabinoids in the radial arm maze task, but did not affect other pharmacological properties of cannabinoids, as assessed in the tetrad assay (that is, hypomotility, analgesia, catalepsy, and hypothermia). Infusions of rimonabant just dorsal or ventral to the hippocampus did not prevent Δ⁹-THC-induced memory impairment, indicating that its effects on mnemonic function were regionally selective. These findings provide compelling evidence in support of the view that hippocampal CB₁ receptors play a necessary role in the memory disruptive effects of marijuana.