Roles for pituitary adenylate cyclase-activating peptide (PACAP) expression and signaling in the bed nucleus of the stria terminalis (BNST) in mediating the behavioral consequences of chronic stress

Abstract

Anxiety disorders are frequently long-lasting and debilitating for more than 40 million American adults. Although stressor exposure plays an important role in the etiology of some anxiety disorders, the mechanisms by which exposure to stressful stimuli alters central circuits that mediate anxiety-like emotional behavior are still unknown. Substantial evidence has implicated regions of the central extended amygdala, including the bed nucleus of the stria terminalis (BNST) and the central nucleus of the amygdala as critical structures mediating fear- and anxiety-like behavior in both humans and animals. These areas organize coordinated fear- and anxiety-like behavioral responses as well as peripheral stress responding to threats via direct and indirect projections to the paraventricular nucleus of the hypothalamus and brainstem regions (Walker et al. Eur J Pharmacol 463:199-216, 2003, Prog Neuropsychopharmacol Biol Psychiatry 33(8):1291-1308, 2009; Ulrich-Lai and Herman Nat Rev Neurosci 10:397-409, 2009). In particular, the BNST has been argued to mediate these central and peripheral responses when the perceived threat is of long duration (Waddell et al. Behav Neurosci 120:324-336, 2006) and/or when the anxiety-like response is sustained (Walker and Davis Brain Struct Funct 213:29-42, 2008); hence, the BNST may mediate pathological anxiety-like states that result from exposure to chronic stress. Indeed, chronic stress paradigms result in enhanced BNST neuroplasticity that has been associated with pathological anxiety-like states (Vyas et al. Brain Res 965:290-294, 2003; Pego et al. Eur J Neurosci 27:1503-1516, 2008). Here we review evidence that suggests that pituitary adenylate cyclase-activating polypeptide (PACAP) and corticotropin-releasing hormone (CRH) work together to modulate BNST function and increase anxiety-like behavior. Moreover, we have shown that BNST PACAP as well as its cognate PAC1 receptor is substantially upregulated following chronic stress, particularly in the BNST oval nucleus where PACAP-containing neurons closely interact with CRH-containing neurons (Kozicz et al. Brain Res 767:109-119, 1997; Hammack et al. Psychoneuroendocrinology 34:833-843, 2009). We describe how interactions between PACAP and CRH in the BNST may mediate stress-associated behaviors, including anorexia and anxiety-like behavior. These studies have the potential to define specific mechanisms underlying anxiety disorders, and may provide important therapeutic strategies for stress and anxiety management.

Figure 1. Stress pathway schematic

Stress activates both limbic and nonlimbic pathways, which can converge on the paraventricular nucleus (PVN) of the hypothalamus for endocrine and autonomic responses. Stress-mediated activation limbic BNST either directly, or indirectly via the PVN, contribute to anxiety behavior. Chronic stress insults can result in long term neuroplasticity changes leading to anxiety and somatic disorders

Figure 2. Schematic of extended amygdala areas and interactive pathways

A) Simplified rat coronal sections illustrating areas within the bed nucleus of stria terminalis (BNST, bregma −0.26) and the amygdala (bregma −2.80). The BNST oval nucleus (OV) in the dorsolateral BNST (BNSTdl) is illustrated; the anterolateral BNST encompasses the BNSTdl and may include the dorsomedial BNST (BNSTdm). Similarly, the two largest components of the central nucleus of the amygdala (CeA) are the medial (CeM) and lateral (CeL) divisions. CPu, caudate putamen; GP, globus pallidus; IC, internal capsule; BNSTv, ventral BNST; Fu, fusiform nucleus; BLA, basolateral amygdala. B) The BNST and CeA have reciprocal projections. The BLA projects not only to the CeA but have en passant fibers that can reach the BNST. The reciprocal hypothalamic (PVN) and CeA projections are best described although direct PVN fibers can be identified in the BNST. The BNST can influence hypothalamic function via direct or indirect neural pathways (dashed arrow).

Figure 3. BNST CRH and PACAP immunoreactivity after chronic stress

(A) Cryosections from chronically stressed rats were processed and CRH immunoreactivity was detected using Cy3-conjugated secondary antibody fluorescence. In the photomontage, CRH immunoreactivity n the oval nucleus of the BNST (BNSTov) is dense compared to staining levels in the ventral zone (BNSTv); from the plane of section CRH staining is compact in the fundus striatum (FS). (B) Adjacent cryosection from the same chronically stressed brain was processed for PACAP immunoreactivity using tyramide amplification and diaminobenzidine as substrate. Though the distribution of PACAP staining in the BNST can be broad, the highest density of PACAP is found in the BNSTov. Even in chronically stressed animals, PACAP staining in the BNSTv appears relatively diffuse compared to PACAP immunoreactivity in the oval nucleus.