PAC1 receptor antagonism in the bed nucleus of the stria terminalis (BNST) attenuates the endocrine and behavioral consequences of chronic stress

Summary

Chronic or repeated stressor exposure can induce a number of maladaptive behavioral and physiological consequences and among limbic structures, the bed nucleus of the stria terminalis (BNST) has been implicated in the integration and interpretation of stress responses. Previous work has demonstrated that chronic variate stress (CVS) exposure in rodents increases BNST pituitary adenylate cyclase activating polypeptide (PACAP, Adcyap1) and PAC1 receptor (Adcyap1r1) transcript expression, and that acute BNST PACAP injections can stimulate anxiety-like behavior. Here we show that chronic stress increases PACAP expression selectively in the oval nucleus of the dorsolateral BNST in patterns distinct from those for corticotropin releasing hormone (CRH). Among receptor subtypes, BNST PACAP signaling through PAC1 receptors not only heightened anxiety responses as measured by different behavioral parameters but also induced anorexic-like behavior to mimic the consequences of stress. Conversely, chronic inhibition of BNST PACAP signaling by continuous infusion with the PAC1 receptor antagonist PACAP(6–38) during the week of CVS attenuated these stress-induced behavioral responses and changes in weight gain. BNST PACAP signaling stimulated the hypothalamic–pituitary–adrenal (HPA) axis and heightened corticosterone release; further, BNST PACAP(6–38) administration blocked corticosterone release in a sensitized stress model. In aggregate with recent associations of PACAP/PAC1 receptor dysregulation with altered stress responses including post-traumatic stress disorder, these data suggest that BNST PACAP/PAC1 receptor signaling mechanisms may coordinate the behavioral and endocrine consequences of stress.

Introduction

Severe or repeated stress places undue challenges on physiological and psychological systems, which typically results in the mobilization of adaptive response systems designed to reestablish homeostatic states. Multiple response systems are engaged following stressor exposure, including the hypothalamic–pituitary–adrenal (HPA) axis, the sympathetic nervous system, and central nervous system (CNS) circuits mediating fear and anxiety-like behavior. The bed nucleus of the stria terminalis (BNST), a region of the extended amygdala, has been argued to mediate all three of these responses to sustained threats (Walker et al., 2003, Walker et al., 2009, Herman et al., 2005, Walker and Davis, 2008, Radley and Sawchenko, 2011). For example, hypothalamic paraventricular nucleus (PVN) activity, which mediates both the endocrine and autonomic response to stressor exposure, is tightly regulated by both anterior and posterior BNST subregions (Choi et al., 2007, Choi et al., 2008a, Radley and Sawchenko, 2011). Moreover, Waddell et al. (2006) and Walker et al., 2003, Walker et al., 2009 have argued that the BNST mediates responding to long duration threats, and that the BNST may mediate anxiety-like emotional states (Walker et al., 2003, Waddell et al., 2006, Choi et al., 2007, Choi et al., 2008a, Walker and Davis, 2008, Radley and Sawchenko, 2011). Consistent with these observations, the BNST has been implicated in regulating anxious temperament in nonhuman primates (Kalin et al., 2005, Kalin et al., 2008, Fox et al., 2008, Oler et al., 2009) and humans (Straube et al., 2007, Somerville et al., 2010).

Accordingly, altered BNST functioning is likely critical for the etiology of many anxiety disorders associated with stressor exposure (Schulkin et al., 1998, Vyas et al., 2003, Pego et al., 2008, Hammack et al., 2009, Hammack et al., 2010). In support, repeated exposure to stressors or drugs of abuse has been correlated with enhanced BNST neuroplasticity, including increased neuronal peptide expression (Stout et al., 2000, Choi et al., 2006, Hammack et al., 2009), dendritic length and branching (Vyas et al., 2003, Pego et al., 2008), volume (Pego et al., 2008), and synaptic activity (Dumont et al., 2005). These observations suggest that repeated or chronic stressor exposure may enhance BNST function, which may lead to pathological anxiety.

Increasingly, the neuropeptides that mediate BNST signaling during stressor exposure have generated significant interest. In addition to the roles of extra-hypothalamic corticotropin-releasing hormone (CRH) signaling in fear- and anxiety-like behavior (Lee and Davis, 1997, Koob and Heinrichs, 1999, Hammack et al., 2002), pituitary adenylate cyclase activating polypeptide (PACAP) and its cognate PAC1 receptor have been implicated in mediating responses associated with stress and anxiety. PACAP and PAC1 receptors are selectively upregulated in the BNST following chronic variate stress, and BNST PACAP activation is anxiogenic (Hammack et al., 2009). PACAP and PAC1 receptor null mice also demonstrate reduced anxiety-like behavior (Hashimoto et al., 2001, Otto et al., 2001, Girard et al., 2006, Gaszner et al., 2012, Hattori et al., 2012), and the glucocorticoid response in PACAP null animals is altered after stressor exposure (Morita et al., 2006, Hatanaka et al., 2008, Stroth and Eiden, 2010, Tsukiyama et al., 2011, Lehmann et al., 2013). In humans, PACAP dysregulation has recently been associated with a number of psychopathologies including post-traumatic stress disorder (PTSD). Elevated circulating PACAP levels, and PAC1 receptor gene polymorphism and methylation each predicted PTSD symptoms and diagnosis (Ressler et al., 2011, Jovanovic et al., 2013).

However, while accumulating evidence implicate PACAP in the consequences of stressor exposure, the mechanisms and functions of PACAP activities in the BNST are still unclear. In the current work, we examined the intersection of PACAP and CRH in the BNST, the PACAP/VIP receptor subtypes involved in BNST function, and the roles of endogenous BNST PACAP in mediating the stress-induced HPA and behavioral responses. Notably, we demonstrate that blocking BNST PACAP/PAC1 receptor signaling can attenuate the physiological and behavioral consequences of repeated stress exposure which may offer insights for therapeutics.