Involvement of the cannabinoid receptors of the basolateral amygdala in the orexin A-mediated modulation of the stress-induced analgesia.

The amygdala receives extensive projections from the dorsomedial and ventromedial hypothalamus (DMH and VMH), which modulate innate fear responses. The basolateral amygdala (BLA) receives GABAergic inputs from the DMH/VMH and orexinergic inputs from the lateral hypothalamus. Therefore, this study investigated the potential interaction between the orexinergic and cannabinoid systems within the BLA during stress-induced analgesia. First, the DMH/VMH and BLA were stereotaxically cannulated. Innate fear, as a defensive response, was induced by intra-DMH/VMH injection of bicuculline. Orexin-A (OrxA), SB334867, and AM251, antagonists of orexin receptor type 1 (Orx1R) and cannabinoid receptor type 1 (Cb1R), respectively, were then microinjected into the BLA. Following formalin injection, the frequency of flinches was recorded for 60 min. The mRNA expression levels of Orx1R and Cb1R were subsequently quantified using real-time PCR. Our results showed that induction of innate fear decreased flinch frequency only during the first phase of the formalin test. Intra-BLA administration of OrxA counteracted stress-induced analgesia. Pretreatment of the BLA with AM251 abolished OrxA-induced hyperalgesia. Neither the co-administration of SB334867 and OrxA nor the co-injection of SB334867, AM251, and OrxA altered the responses. Cb1R mRNA expression in the BLA was higher than Orx1R mRNA expression. Stress induction markedly reduced Orx1R expression in the BLA. In contrast, intra-BLA administration of OrxA increased Cb1R expression in the BLA, an effect that was reversed by co-treatment with SB334867 or AM251 and OrxA, as well as by the combined administration of SB334867, AM251, and OrxA. These findings suggest that intra-BLA administration of orexin-A may retrogradely affect Cb1R-containing interneurons and increase Cb1R expression, thereby counteracting stress-induced analgesia.