Regulation of chronic stress-induced changes in hypothalamic-pituitary-adrenal activity by the basolateral amygdala.

Little is known about the role of the basolateral amygdala (BLA) in regulating hypothalamic-pituitary-adrenal (HPA) activity, particularly chronic stress-induced HPA activity. In the current studies, we examined the effects of manipulations of the BLA on HPA responses to the eighth restraint, to novel restraint after repeated cold, or to acute novel restraint alone. Excitotoxic lesions of the BLA, in general, inhibited HPA activity in both acute and chronically stressed animals. To examine the role of the BLA in chronic stress without affecting the response to the first stress, we injected the GABA agonist muscimol to temporarily inactivate the BLA prior to restraint in the same three groups of animals. In contrast to the lesion data, muscimol enhanced the HPA response to acute restraint and to novel restraint after repeated cold, but it did not affect responses to the eighth restraint. These data suggest that the BLA inhibits HPA responses to novel stress but is not important in animals repeatedly exposed to the same stressor. Future studies will focus on the neuro-anatomical substrates of BLA's effects on HPA activity including whether inputs from the pPVTh are important.