Nicotine self-administration differentially modulates glutamate and GABA transmission in hypothalamic paraventricular nucleus to enhance the hypothalamic-pituitary-adrenal response to stress.

ABSTRACT

The mechanisms by which chronic nicotine self-administration augments hypothalamo-pituitary-adrenal (HPA) responses to stress are only partially understood. Nicotine self-administration alters neuropeptide expression in corticotropin-releasing factor (CRF) neurons within paraventricular nucleus (PVN) and increases PVN responsiveness to norepinephrine during mild footshock stress. Glutamate and GABA also modulate CRF neurons, but their roles in enhanced HPA responsiveness to footshock during chronic self-administration are unknown. We show that nicotine self-administration augmented footshock-induced PVN glutamate release, but further decreased GABA release. In these rats, intra-PVN kynurenic acid, a glutamate receptor antagonist, blocked enhanced adrenocorticotropic hormone and corticosterone responses to footshock. In contrast, peri-PVN kynurenic acid, which decreases activity of GABA afferents to PVN, enhanced footshock-induced corticosterone secretion only in control rats self-administering saline. Additionally, in rats self-administering nicotine, footshock-induced elevation of corticosterone was significantly less than in controls after intra-PVN saclofen (GABA-B receptor antagonist). Therefore, the exaggerated reduction in GABA release by footshock during nicotine self-administration disinhibits CRF neurons. This disinhibition combined with enhanced glutamate input provides a new mechanism for HPA sensitization to stress by chronic nicotine self-administration. This mechanism, which does not preserve homeostatic plasticity, supports the concept that smoking functions as a chronic stressor that sensitizes the HPA to stress.