Abstract
Endogenous cannabinoids (endocannabinoids, eCB) are expressed throughout the body and contribute to regulation of the hypothalamo-pituitary-adrenal (HPA) axis and general stress reactivity. This study assessed the contributions of CB1 receptors (CB1R) in the modulation of basal and stress-induced neural and HPA axis activities. Catheterized adult male rats were placed in chambers to acclimate overnight, with their catheters connected and exteriorized from the chambers for relatively stress-free remote injections. The next morning, the CB1R antagonist AM251 (1 or 2 mg/kg) or vehicle was administered, and 30 min later, rats were exposed to loud noise stress (30 min) or no noise (basal condition). Blood, brains, pituitary and adrenal glands were collected immediately after the procedures for analysis of c-fos and CB1R mRNAs, corticosterone (CORT) and adrenocorticotropin hormone (ACTH) plasma levels. Basally, CB1R antagonism induced c-fos mRNA in the basolateral amygdala (BLA) and auditory cortex (AUD) and elevated plasma CORT, indicating disruption of eCB-mediated constitutive inhibition of activity. CB1R blockade also potentiated stress-induced hormone levels and c-fos mRNA in several regions such as the bed nucleus of the stria terminalis (BST), lateral septum (LS), and basolateral amygdala (BLA) and the paraventricular nucleus of the hypothalamus (PVN). CB1R mRNA was detected in all central tissues investigated, and the adrenal cortex, but at very low levels in the anterior pituitary gland. Interestingly, CB1R mRNA was rapidly and bidirectionally regulated in response to stress and/or antagonist treatment in some regions. eCBs therefore modulate the HPA axis by regulating both constitutive and activity-dependent inhibition at multiple levels.