Stress hormone synthesis in mouse hypothalamus and adrenal gland triggered by restraint is dependent on pituitary adenylate cyclase-activating polypeptide signaling

Abstract

Stress responses are elicited by a variety of stimuli and are aimed at counteracting direct or perceived threats to the well-being of an organism. In the mammalian central and peripheral nervous systems, specific cell groups constitute signaling circuits that indicate the presence of a stressor and elaborate an adequate response, ultimately restoring homeostasis. Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in central and peripheral parts of these circuits and has recently been identified as a candidate for regulation of the stress axis. In the present experiments, we tested the involvement of PACAP in the response to a psychological stressor in vivo. We used a restraint paradigm and compared PACAP-deficient mice (PACAP−/−) to wild-type controls (PACAP+/+). Acute secretion of corticosterone elicited by 1h of restraint was found to be identical between genotypes, whereas sustained secretion provoked by 6h of unrelieved restraint was 48% lower in PACAP−/− mice. Within the latter time frame, expression of mRNA encoding corticotropinreleasing hormone (CRH) was increased in the hypothalamus of wild-type, but not PACAP-deficient mice. Expression of hypothalamic activity-regulated transcription factors (Egr1, Fos) was rapidly and transiently induced by restraint in a PACAP-dependent fashion, a pattern that was also found in the adrenal glands. Here, abundance of transcripts encoding enzymes required for adrenomedullary catecholamine biosynthesis (TH, PNMT) was higher in PACAP+/+ mice after 6h of unrelieved restraint. Our results suggest that sustained corticosterone secretion, synthesis of the hypophysiotropic hormone CRH in the hypothalamus, as well as enzymes producing the hormone adrenaline in the adrenal medulla, are controlled by PACAP signaling in the mouse. These findings identify PACAP as a major potential contributor to the stimulus-secretion-synthesis coupling that supports stress responses in vivo.

CRF Mediates the Anxiogenic and Anti-Rewarding, But Not the Anorectic Effects of PACAP

“…Our finding that PACAP activates the HPA axis confirms previous ones (Agarwal et al, 2005;Norrholm et al, 2005) and is consistent with the very important observation that mice lacking the gene coding for PACAP do not show HPA axis activation following restraint stress (Lehmann et al, 2012;Stroth and Eiden, 2010). Interestingly, we show that the ability of PACAP to activate the HPA axis does not require central CRF receptors, as the CRF receptor antagonist did not block the elevation in circulating corticosterone levels.…”

“…PACAP also induces c-Fos expression and CREB phosphorylation in CRF neurons in the PVN, raising plasma corticosterone levels (Agarwal et al, 2005;Norrholm et al, 2005). These findings, together with the observation that CRF mRNA and corticosterone levels are reduced in PACAPdeficient mice after restraint stress (Stroth and Eiden, 2010), suggest that PACAP might have a physiological role in the regulation of CRF synthesis and activity.…”

“…Numerous pieces of evidence suggest a close relationship between the CRF and the PACAP systems: centrally administered PACAP increases the CRF mRNA expression in the PVN and the PAC1 receptor antagonist PACAP (6-38) decreases it (Grinevich et al, 1997); PACAP induces c-Fos expression and CREB phosphorylation in CRF neurons in the PVN, also causing an increase in plasma corticosterone levels (Agarwal et al, 2005;Norrholm et al, 2005). Interestingly, PACAP knockout mice show reduced CRF mRNA and corticosterone levels in response to restraint stress (Stroth and Eiden, 2010). Our finding that the anxiogenic effects of PACAP are mediated by CRF is an important one because it implicates that the PACAP system is located upstream of CRF, and therefore providing a potentially novel site to modulate the activity of the CRF system, which is dysregulated in stress-related disorders (Koob and Zorrilla, 2012).…”

Pituitary adenylate cyclase-activating peptide enhances electrical coupling in the mouse adrenal medulla

“…Another possibility is that under long-term PACAP exposure connexin gene expression increases via a PKA-or PKC-dependent mechanism, effectively remodeling the adrenal medulla to support sustained secretion. Indeed, PACAP has been shown to activate a number of stress-related genes on the hour timescale (38,41,49). Although a significant enhancement of connexin gene transcription is unlikely to account for the substantial and rapid enhancement in electrical coupling observed in our study, it seems likely that PACAP stimulates gene expression of connexins under a longer time scale to electrically remodel the adrenal medulla.…”

“…In the longer term (Ͼ10 min), PACAP-mediated adrenal excitation further depolarizes the membrane and recruits an L-type Ca 2ϩ influx (35). Ultimately, long-term PACAP excitation elicits a broader modulation of adrenal exocytic function through the activation of secretion-associated genes (38,41,49). More recent studies (9) have suggested a wider role for PACAP in the sympathoadrenal stress response that may include increased intercellular electrical coupling within the medulla.…”