We present that nicotine self-administration augmented footshock-induced PVN glutamate release, but decreased GABA release further. handles after intra-PVN saclofen (GABA-B receptor antagonist). As a result, the exaggerated decrease in GABA discharge by footshock during nicotine self-administration disinhibits CRF neurons. This disinhibition coupled with improved glutamate input offers a brand-new system for HPA sensitization to tension by chronic nicotine self-administration. This system, which will not protect homeostatic plasticity, supports the concept that smoking functions as a chronic stressor that sensitizes the HPA to stress. Keywords:nicotine self-administration, adrenocorticotropic hormone, corticosterone, footshock stress, homeostatic plasticity, rat == Introduction == Nicotine, the principal psychoactive component Rabbit polyclonal to AMDHD1 of tobacco, acutely stimulates secretion of the stress-responsive hypothalamo-pituitary-adrenal (HPA) axis hormones, adrenocorticotropic hormone (ACTH) and corticosterone (Mattaet al. 1987). In addition, chronic nicotine self-administration (Chenet al. 2008) and other stressors (Aguilera 1994) augment ACTH and corticosterone responses to a novel stressor, such as mild footshock stress. However, the neuroplastic changes underlying this effect of chronic nicotine exposure on the stress response are only partially understood. In the hypothalamic paraventricular nucleus (PVN), the critical output nucleus controlling the HPA axis, corticotropin-releasing factor (CRF) neurons in the parvocellular division (pcPVN), integrate the HPA responsiveness to diverse stressors and stimulate plasma ACTH secretion (Hermanet al. 2005). We have shown that acutely injected nicotine and chronically self-administered nicotine are stressors that activate CRF neurons and elevate plasma ACTH levels (Valentineet al. 1996,Yuet al. 2008,Chen et al. 2008). Chronic nicotine self-administration also alters the phenotype of pcPVN CRF neurons by inducing the co-expression of arginine vasopressin (AVP). Stressor stimulation of these CRF+/AVP+neurons would potentiate CRF-dependent ACTH secretion by co-releasing AVP, another modulator of pituitary corticotrophs (Rivier and Vale 1983,Yu et al. 2008). We have reported that PVN responsiveness to norepinephrine, a primary regulator of CRF neurons, is selectively enhanced in rats self-administering nicotine, but only during stress (Yu and Sharp 2010). Neuroplasticity in the phenotype of CRF neurons and their responsiveness to norepinephrine during stress would both contribute to CADD522 the enhanced secretion of ACTH and corticosterone during mild footshock stress. However, the contribution of other neurotransmitters to this interaction between nicotine self-administration and a stressor is unknown. From a neuroanatomical perspective, pcPVN CRF neurons receive both glutamatergic and GABAergic afferents (van den Polet al. 1990,Decavel and Van CADD522 den Pol 1990), with CRF neurons juxtaposed to glutamatergic or GABAergic terminals (Ziegler and Herman 2000,Miklos and Kovacs 2002). Additionally, ionotropic glutamate receptor and GABA receptor subunits are highly expressed on pcPVN CRF neurons (Cullinan 2000,Hermanet al. 2000). Pharmacological CADD522 studies have shown that an intra-PVN injection of either glutamate or a GABA receptor antagonist elicited ACTH and corticosterone release (Feldman and Weidenfeld 1997,Cole and Sawchenko 2002). Blockade of PVN glutamate receptors inhibited the corticosterone response to restraint stress (Ziegler and Herman 2000), whereas GABA receptor blockade enhanced restraint stress-induced corticosterone release (Cullinanet al. 2008). Therefore, both glutamate and GABA inputs to PVN CRF neurons modulate basal activity and stress responsiveness, and could potentially affect responsiveness to norepinephrine during stress. We have proposed that nicotine self-administration is a chronic stressor, sensitizing the HPA to novel stressors (Chen et al. 2008,Yu et al. 2008). We have shown that increased c-Fos expression in CRF+/AVP+neurons after stress is a distal component of the HPA sensitization induced by chronic nicotine self-administration. However, apart from norepinephrine (Yu and Sharp 2010), the effect of nicotine self-administration on neurotransmitters modulating these critical neurons is unknown. We hypothesized that nicotine self-administration would alter the balance of glutamatergic stimulation and GABAergic inhibition during mlid footshock stress, thereby augmenting the response of CRF neurons to the stressor. We determined the effects of nicotine self-administration on footshock-induced release of (1) PVN glutamate and GABA, and (2) plasma ACTH and corticosterone after blockade of glutamate or GABA receptors.