PACAP controls adrenomedullary catecholamine secretion and expression of catecholamine biosynthetic enzymes at high splanchnic nerve firing rates characteristic of stress transduction in male mice

Impact of PACAP and PAC1 receptor deficiency on the neurochemical and behavioral effects of acute and chronic restraint stress in male C57BL/6 mice

Acute restraint stress (ARS) for 3 h causes corticosterone (CORT) elevation in venous blood, which is accompanied by Fos up-regulation in the paraventricular nucleus (PVN) of male C57BL/6 mice. CORT elevation by ARS is attenuated in PACAP-deficient mice, but unaffected in PAC1-deficient mice. Correspondingly, Fos up-regulation by ARS is greatly attenuated in PACAP-deficient mice, but much less so in PAC1-deficient animals. We noted that both PACAP- and PAC1-deficiency greatly attenuate CORT elevation after ARS when CORT measurements are performed on trunk blood following euthanasia by abrupt cervical separation: this latter observation is of critical importance in assessing the role of PACAP neurotransmission in ARS, based on previous reports in which serum CORT was sampled from trunk blood. Seven days of chronic restraint stress (CRS) induces non-habituating CORT elevation, and weight loss consequent to hypophagia, in wild-type male C57BL/6 mice. Both CORT elevation and weight loss following 7-day CRS are severely blunted in PACAP-deficient mice, but only slightly in PAC1-deficient mice. However, longer periods of daily restraint (14-21 days) resulted in sustained weight loss and elevated CORT in wild-type mice, and these effects of long-term chronic stress were attenuated or abolished in both PACAP- and PAC1-deficient mice. We conclude that while a PACAP receptor in addition to PAC1 may mediate some of the PACAP-dependent central effects of ARS and short-term (<7 days) CRS on the hypothalamo-pituitary-adrenal (HPA) axis, the PAC1 receptor plays a prominent role in mediating PACAP-dependent HPA axis activation, and hypophagia, during long-term (>7 days) CRS.

Partial blockade of nicotinic acetylcholine receptors improves the counterregulatory response to hypoglycemia in recurrently hypoglycemic rats

Recurrent exposure to hypoglycemia can impair the normal counterregulatory hormonal responses that guard against hypoglycemia, leading to hypoglycemia unawareness. This pathological condition known as hypoglycemia-associated autonomic failure (HAAF) is the main adverse consequence that prevents individuals with type 1 diabetes mellitus from attaining the long-term health benefits of tight glycemic control. The underlying molecular mechanisms responsible for the progressive loss of the epinephrine response to subsequent bouts of hypoglycemia, a hallmark sign of HAAF, are largely unknown. Normally, hypoglycemia triggers both the release and biosynthesis of epinephrine through activation of nicotinic acetylcholine receptors (nAChR) on the adrenal glands. We hypothesize that excessive cholinergic stimulation may contribute to impaired counterregulation. Here, we tested whether administration of the nAChR partial agonist cytisine to reduce postganglionic synaptic activity can preserve the counterregulatory hormone responses in an animal model of HAAF. Compared with nicotine, cytisine has limited efficacy to activate nAChRs and stimulate epinephrine release and synthesis. We evaluated adrenal catecholamine production and secretion in nondiabetic rats subjected to two daily episodes of hypoglycemia for 3 days, followed by a hyperinsulinemic hypoglycemic clamp on day 4. Recurrent hypoglycemia decreased epinephrine responses, and this was associated with suppressed TH mRNA induction (a measure of adrenal catecholamine synthetic capacity). Treatment with cytisine improved glucagon responses as well as epinephrine release and production in recurrently hypoglycemic animals. These data suggest that pharmacological manipulation of ganglionic nAChRs may be promising as a translational adjunctive therapy to avoid HAAF in type 1 diabetes mellitus.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.

Pituitary adenylate cyclase activating polypeptide modulates catecholamine storage and exocytosis in PC12 cells

A number of efforts have been made to understand how pituitary adenylate cyclase activating polypeptide (PACAP) functions as a neurotrophic and neuroprotective factor in Parkinson's disease (PD). Recently its effects on neurotransmission and underlying mechanisms have generated interest. In the present study, we investigate the effects of PACAP on catecholamine storage and secretion in PC12 cells with amperometry and transmission electron microscopy (TEM). PACAP increases quantal release induced by high K+ without significantly regulating the frequency of vesicle fusion events. TEM data indicate that the increased volume of the vesicle is mainly the result of enlargement of the fluidic space around the dense core. Moreover, the number of docked vesicles isn't modulated by PACAP. When cells are acutely treated with L-DOPA, the vesicular volume and quantal release both increase dramatically. It is likely that the characteristics of amperometric spikes from L-DOPA treated cells are associated with increased volume of individual vesicles rather than a direct effect on the mechanics of exocytosis. Treatment with PACAP versus L-DOPA results in different profiles of the dynamics of exocytosis. Release via the fusion pore prior to full exocytosis was observed with the same frequency following treatment with PACAP and L-DOPA. However, release events have a shorter duration and higher average current after PACAP treatment compared to L-DOPA. Furthermore, PACAP reduced the proportion of spikes having rapid decay time and shortened the decay time of both fast and slow spikes. In contrast, the distributions of the amperometric spike decay for both fast and slow spikes were shifted to longer time following L-DOPA treatment. Compared to L-DOPA, PACAP may produce multiple favorable effects on dopaminergic neurons, including protecting dopaminergic neurons against neurodegeneration and potentially regulating dopamine storage and release, making it a promising therapeutic agent for the treatment of PD.

Discrete signal transduction pathway utilization by a neuropeptide (PACAP) and a cytokine (TNF-alpha) first messenger in chromaffin cells, inferred from coupled transcriptome-promoter analysis of regulated gene cohorts

Cultured bovine adrenal chromaffin cells (BCCs) are employed to study first messenger-specific signaling by cytokines and neurotransmitters occurring in the adrenal medulla following immune-related stress responses. Here, we show that the cytokine TNF-alpha, and the neuropeptide transmitter PACAP, acting through the TNFR2 and PAC1 receptors, activate distinct signaling pathways, with correspondingly distinct transcriptomic signatures in chromaffin cells. We have carried out a comprehensive integrated transcriptome analysis of TNF-alpha and PACAP gene regulation in BCCs using two microarray platforms to maximize transcript identification. Microarray data were validated using qRT-PCR. More than 90% of the transcripts up-regulated either by TNF-alpha or PACAP were specific to a single first messenger. The final list of transcripts induced by each first messenger was subjected to multiple algorithms to identify promoter/enhancer response elements for trans-acting factors whose activation could account for gene expression by either TNF-alpha or PACAP. Distinct groups of transcription factors potentially controlling the expression of TNF-alpha or PACAP-responsive genes were found: most of the genes up-regulated by TNF-alpha contained transcription factor binding sites for members of the Rel transcription factor family, suggesting TNF-alpha-TNFR2 signaling occurs mainly through the NF-KB signaling pathway. Surprisingly, EGR1 was predicted to be the primary transcription factor controlling PACAP-modulated genes, suggesting PACAP signaling to the nucleus occurs predominantly through ERK, rather than CREB activation. Comparison of TNFR2-dependent versus TNFR1-dependent gene induction, and EGR1-mediated transcriptional activation, may provide a pharmacological avenue to the unique pathways activated by the first messengers TNF-alpha and PACAP in neuronal and endocrine cells.

Rapgef2 connects GPCR-mediated cAMP signals to ERK activation in neuronal and endocrine cells

G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR)-mediated increases in the second messenger cyclic adenosine monophosphate (cAMP) activate the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase (ERK), and in neuroendocrine cells, this pathway leads to cAMP-dependent neuritogenesis mediated through Rap1 and B-Raf. We found that the Rap guanine nucleotide exchange factor Rapgef2 was enriched from primary bovine neuroendocrine cells by cAMP-agarose affinity chromatography and that it was specifically eluted by cAMP. With loss-of-function experiments in the rat neuronal cell line Neuroscreen-1 (NS-1) and gain-of-function experiments in human embryonic kidney 293T cells, we demonstrated that Rapgef2 connected GPCR-dependent activation of adenylate cyclase and increased cAMP concentration with the activation of ERK in neurons and endocrine cells. Furthermore, knockdown of Rapgef2 blocked cAMP- and ERK-dependent neuritogenesis. Our data are consistent with a pathway involving the cAMP-mediated activation of Rapgef2, which then stimulates Rap1, leading to increases in B-Raf, MEK, and ERK activity.

Pituitary adenylate cyclase-activating polypeptide (PACAP): a master regulator in central and peripheral stress responses

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is a master regulator of central and peripheral stress responses required to restore and maintain homeostasis. PACAP modulates the hypothalamic-pituitary-adrenal (HPA) axis in response to acute psychogenic but not systemic stressors, through activation of corticotropin-releasing hormone (CRH) release to drive adrenal corticosterone (CORT) output. During direct high-frequency stimulation of the splanchnic nerve that is designed to mimic stress, PACAP regulates adrenomedullary catecholamine secretion. In addition to transmission, PACAP simultaneously facilitates the biosynthesis of adrenomedullary catecholamines through stimulus-secretion-synthesis coupling. During periods of chronic psychogenic stress, PACAP-mediated CORT elevation fails to desensitize and contributes to the development of maladaptive behaviors such as anxiety and depression. Based on these findings, PACAP regulates not only adaptive responses to stress but also maladaptive responses to sustained psychological stress. PACAP receptor antagonists could have therapeutic relevance in preventing hyperactivity of the HPA axis and offering protection against chronic stress-associated anxiety and depression.