Mechanisms of pituitary adenylate cyclase activating polypeptide (PACAP)-induced depolarization of sympathetic superior cervical ganglion (SCG) neurons

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

Intrabladder PAC1 Receptor Antagonist, PACAP(6-38), Reduces Urinary Bladder Frequency and Pelvic Sensitivity in Mice Exposed to Repeated Variate Stress (RVS)

Stress causes symptom exacerbation in functional disorders of the urinary bladder. However, the potential mediators and underlying mechanisms of stress effects on micturition reflex function are unknown. We have characterized PACAP (Adcyap1) and PAC1 receptor (Adcyap1r1) signaling in stress-induced urinary bladder dysfunction in mice. We determined PACAP and PAC1 transcripts and protein expressions in the urinary bladder and lumbosacral dorsal root ganglia (DRG) and spinal cord in repeated variate stress (RVS) or control mouse (handling only) groups. RVS in mice significantly (p ≤ 0.01) increased serum corticosterone and urinary bladder NGF content and decreased weight gain. PACAP and PAC1 mRNA and protein were differentially regulated in lower urinary tract tissues with changes observed in lumbosacral DRG and spinal cord but not in urinary bladder. RVS exposure in mice significantly (p ≤ 0.01) increased (2.5-fold) voiding frequency as determined using conscious cystometry. Intrabladder administration of the PAC1 receptor antagonist, PACAP(6-38) (300 nM), significantly (p ≤ 0.01) increased infused volume (1.5-2.7-fold) to elicit a micturition event and increased the intercontraction interval (i.e., decreased voiding frequency) in mice exposed to RVS and in control mice, but changes were smaller in magnitude in control mice. We also evaluated the effect of PAC1 blockade at the level of the urinary bladder on pelvic sensitivity in RVS or control mouse groups using von Frey filament testing. Intrabladder administration of PACAP(6-38) (300 nM) significantly (p ≤ 0.01) reduced pelvic sensitivity following RVS. PACAP/receptor signaling in the CNS and PNS contributes to increased voiding frequency and pelvic sensitivity following RVS and may represent a potential target for therapeutic intervention.

PACAP/PAC1 Expression and Function in Micturition Pathways

Neural injury, inflammation, or diseases commonly and adversely affect micturition reflex function that is organized by neural circuits in the CNS and PNS. One neuropeptide receptor system, pituitary adenylate cyclase-activating polypeptide (PACAP; Adcyap1), and its cognate receptor, PAC1 (Adcyap1r1), have tissue-specific distributions in the lower urinary tract. PACAP and associated receptors are expressed in the LUT and exhibit changes in expression, distribution, and function in preclinical animal models of bladder pain syndrome (BPS)/interstitial cystitis (IC), a chronic, visceral pain syndrome characterized by pain, and LUT dysfunction. Blockade of the PACAP/PAC1 receptor system reduces voiding frequency and somatic (e.g., hindpaw, pelvic) sensitivity in preclinical animal models and a transgenic mouse model that mirrors some clinical symptoms of BPS/IC. The PACAP/receptor system in micturition pathways may represent a potential target for therapeutic intervention to reduce LUT dysfunction following urinary bladder inflammation.

PACAP38-Mediated Bladder Afferent Nerve Activity Hyperexcitability and Ca2+ Activity in Urothelial Cells from Mice

Pituitary adenylate cyclase-activating polypeptide (PACAP; Adcyap1) and its cognate PAC1 receptor (Adcyap1r1) have tissue-specific distributions in the lower urinary tract (LUT). The afferent limb of the micturition reflex is often compromised following bladder injury, disease, and inflammatory conditions. We have previously demonstrated that PACAP signaling contributes to increased voiding frequency and decreased bladder capacity with cystitis. Thus, the present studies investigated the sensory components (e.g., urothelial cells, bladder afferent nerves) of the urinary bladder that may underlie the pathophysiology of aberrant PACAP activation. We utilized bladder-pelvic nerve preparations and urothelial sheet preparations to characterize PACAP-induced bladder afferent nerve discharge with distention and PACAP-induced Ca²⁺ activity, respectively. We determined that PACAP38 (100 nM) significantly (p ≤ 0.01) increased bladder afferent nerve activity with distention that was blocked with a PAC1/VPAC2 receptor antagonist PACAP6-38 (300 nM). PACAP38 (100 nM) also increased Ca²⁺ activity in urothelial cells over that observed in control preparations. Taken together, these results establish a role for PACAP signaling in bladder sensory components (e.g., urothelial cells, bladder afferent nerves) that may ultimately facilitate increased voiding frequency.

What's New in Endocrinology: The Chromaffin Cell

Recent advances in understanding the intracellular and intercellular features of adrenal chromatin cells as stress transducers are reviewed here, along with their implications for endocrine function in other tissues and organs participating in endocrine regulation in the mammalian organism.

PACAP/Receptor System in Urinary Bladder Dysfunction and Pelvic Pain Following Urinary Bladder Inflammation or Stress

Complex organization of CNS and PNS pathways is necessary for the coordinated and reciprocal functions of the urinary bladder, urethra and urethral sphincters. Injury, inflammation, psychogenic stress or diseases that affect these nerve pathways and target organs can produce lower urinary tract (LUT) dysfunction. Numerous neuropeptide/receptor systems are expressed in the neural pathways of the LUT and non-neural components of the LUT (e.g., urothelium) also express peptides. One such neuropeptide receptor system, pituitary adenylate cyclase-activating polypeptide (PACAP; Adcyap1) and its cognate receptor, PAC1 (Adcyap1r1), have tissue-specific distributions in the LUT. Mice with a genetic deletion of PACAP exhibit bladder dysfunction and altered somatic sensation. PACAP and associated receptors are expressed in the LUT and exhibit neuroplastic changes with neural injury, inflammation, and diseases of the LUT as well as psychogenic stress. Blockade of the PACAP/PAC1 receptor system reduces voiding frequency in preclinical animal models and transgenic mouse models that mirror some clinical symptoms of bladder dysfunction. A change in the balance of the expression and resulting function of the PACAP/receptor system in CNS and PNS bladder reflex pathways may underlie LUT dysfunction including symptoms of urinary urgency, increased voiding frequency, and visceral pain. The PACAP/receptor system in micturition pathways may represent a potential target for therapeutic intervention to reduce LUT dysfunction.

Intravesical PAC1 Receptor Antagonist, PACAP(6-38), Reduces Urinary Bladder Frequency and Pelvic Sensitivity in NGF-OE Mice

Chronic NGF overexpression (OE) in the urothelium, achieved through the use of a highly urothelium-specific uroplakin II promoter, stimulates neuronal sprouting in the urinary bladder, produces increased voiding frequency and non-voiding contractions, and referred somatic sensitivity. Additional NGF-mediated pleiotropic changes might contribute to increased voiding frequency and pelvic hypersensitivity in NGF-OE mice such as neuropeptide/receptor systems including PACAP(Adcyap1) and PAC1 receptor (Adcyap1r1). Given the presence of PAC1-immunoreactive fibers and the expression of PAC1 receptor expression in bladder tissues, and PACAP-facilitated detrusor contraction, whether PACAP/receptor signaling contributes to increased voiding frequency and somatic sensitivity was evaluated in NGF-OE mice. Intravesical administration of the PAC1 receptor antagonist, PACAP(6-38) (300 nM), significantly (p ≤ 0.01) increased intercontraction interval (2.0-fold) and void volume (2.5-fold) in NGF-OE mice. Intravesical instillation of PACAP(6-38) also decreased baseline bladder pressure in NGF-OE mice. PACAP(6-38) had no effects on bladder function in WT mice. Intravesical administration of PACAP(6-38) (300 nM) significantly (p ≤ 0.01) reduced pelvic sensitivity in NGF-OE mice but was without effect in WT mice. PACAP/receptor signaling contributes to the increased voiding frequency and pelvic sensitivity observed in NGF-OE mice.

Stress peptide PACAP engages multiple signaling pathways within the carotid body to initiate excitatory responses in respiratory and sympathetic chemosensory afferents

Consistent with a critical role in respiratory and autonomic stress responses, the carotid bodies are strongly excited by pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide implicated in stress responses throughout the sympathetic nervous system. PACAP excites isolated carotid body glomus cells via activation of PAC1 receptors, with one study suggesting PAC1-induced excitation is due entirely to protein kinase A (PKA)-mediated inhibition of TASK channels. However, in other systems, PAC1 is known to be coupled to multiple intracellular signaling pathways, including PKA, phospholipase C (PLC), phospholipase D (PLD), and protein kinase C (PKC), that trigger multiple downstream effectors including increased Ca²⁺ mobilization, inhibition of various K⁺ channels, and activation of nonselective cation channels. This study tests if non-PKA/TASK channel signaling helps mediate the stimulatory effects of PACAP on the carotid body. Using an ex vivo arterially perfused rat carotid body preparation, we show that PACAP-38 stimulates carotid sinus nerve activity in a biphasic manner (peak response, falling to plateau). PKA blocker H-89 only reduced the plateau response (~41%), whereas the TASK-1-like K⁺ channel blocker/transient receptor potential vanilloid 1 channel agonist anandamide only inhibited the peak response (~48%), suggesting involvement of additional pathways. The PLD blocker CAY10594 significantly inhibited both peak and plateau responses. The PLC blocker U73122 decimated both peak and plateau responses. Brefeldin A, a blocker of Epac (cAMP-activated guanine exchange factor, reported to link Gs-coupled receptors with PLC/PLD), also reduced both phases of the response, as did blocking signaling downstream of PLC/PLD with the PKC inhibitors chelerythrine chloride and GF109203X. Suggesting the involvement of non-TASK ion channels in the effects of PACAP, the A-type K⁺ channel blocker 4-aminopyridine, and the putative transient receptor potential channel (TRPC)/T-type calcium channel blocker SKF96365 each significantly inhibited the peak and steady-state responses. These data suggest the stimulatory effect of PACAP-38 on carotid body sensory activity is mediated through multiple signaling pathways: the PLC-PKC pathways predominates, with TRPC and/or T-type channel activation and Kv channel inactivation; only partial involvement is attributable to PKA and PLD activation.

Is PACAP the major neurotransmitter for stress transduction at the adrenomedullary synapse?

It has been known for more than a decade that the neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) is co-stored with acetylcholine in the splanchnic nerve terminals innervating the adrenal medulla. Both transmitters are robust secretagogues for catecholamine release from chromaffin cells. Here, we review the unique contribution of PACAP to the functioning of the splanchnic-adrenal synapse in stress. While acetylcholine is released across a wide range of firing frequencies, PACAP is released only at high frequencies of stimulation, and its role in the regulation of epinephrine secretion and biosynthesis is highly specialized. PACAP is responsible for long-term catecholamine secretion using secretory mechanisms different from the rapidly desensitizing depolarization evoked by acetylcholine through nicotinic receptor activation. PACAP signaling also maintains catecholamine synthesis required for sustained secretion during prolonged stress via induction of the enzymes TH and PNMT, and enhances transcription of additional secreted molecules found in chromaffin cells that alter further secretion through both autocrine and paracrine mechanisms. PACAP thus mediates chromaffin cell plasticity via functional encoding of cellular experience. These features of PACAP action at the splanchnic-adrenal synapse may be paradigmatic for the general actions of neuropeptides as effectors of stimulus-secretion-synthesis coupling in stress.

The hop cassette of the PAC1 receptor confers coupling to Ca2+ elevation required for pituitary adenylate cyclase-activating polypeptide-evoked neurosecretion

We have identified the single PAC1 receptor variant responsible for Ca2+ mobilization from intracellular stores and influx through voltage-gated Ca2+ channels in bovine chromaffin cells and the domain of this receptor variant that confers coupling to [Ca2+]i elevation. This receptor (bPAC1hop) contains a 28-amino acid "hop" insertion in the third intracellular loop, with a full-length 171-amino acid N terminus. Expression of the bPAC1hop receptor in NG108-15 cells, which lack endogenous PAC1 receptors, reconstituted high affinity PACAP binding and PACAP-dependent elevation of both cAMP and intracellular Ca2+ concentrations ([Ca2+]i). Removal of the hop domain and expression of this receptor (bPAC1null) in NG108-15 cells reconstituted high affinity PACAP binding and PACAP-dependent cAMP generation but without a corresponding [Ca2+]i elevation. PC12-G cells express sufficient levels of PAC1 receptors to provide PACAP-saturable coupling to adenylate cyclase and to drive PACAP-dependent differentiation but do not express PAC1 receptors at levels found in postmitotic neuronal and endocrine cells and do not support PACAP-mediated neurosecretion. Expression of bPAC1hop, but not bPAC1(null), at levels comparable with those of bPAC1hop in bovine chromaffin cells resulted in acquisition by PC12-G cells of PACAP-dependent [Ca2+]i increase and extracellular Ca2+ influx. In addition, PC12-G cells expressing bPAC1hop acquired the ability to release [3H]norepinephrine in a Ca2+ influx-dependent manner in response to PACAP. Expression of PACAP receptors in neuroendocrine rather than nonneuroendocrine cells reveals key differences between PAC1hop and PAC1null coupling, indicating an important and previously unrecognized role of the hop cassette in PAC1-mediated Ca2+ signaling in neuroendocrine cells.

Pituitary adenylyl cyclase-activating polypeptide (PACAP) and its receptor (PAC1-R) are positioned to modulate afferent signaling in the cochlea

Pituitary adenylyl cyclase-activating polypeptide (PACAP), via its specific receptor pituitary adenylyl cyclase-activating polypeptide receptor 1 (PAC1-R), is known to have roles in neuromodulation and neuroprotection associated with glutamatergic and cholinergic neurotransmission, which, respectively, are believed to form the primary basis for afferent and efferent signaling in the organ of Corti. Previously, we identified transcripts for PACAP preprotein and multiple splice variants of its receptor, PAC1-R, in microdissected cochlear subfractions. In the present work, neural localizations of PACAP and PAC1-R within the organ of Corti and spiral ganglion were examined, defining sites of PACAP action. Immunolocalization of PACAP and PAC1-R in the organ of Corti and spiral ganglion was compared with immunolocalization of choline acetyltransferase (ChAT) and synaptophysin as efferent neuronal markers, and glutamate receptor 2/3 (GluR2/3) and neurofilament 200 as afferent neuronal markers, for each of the three cochlear turns. Brightfield microscopy giving morphological detail for individual immunolocalizations was followed by immunofluorescence detection of co-localizations. PACAP was found to be co-localized with ChAT in nerve fibers of the intraganglionic spiral bundle and beneath the inner and outer hair cells within the organ of Corti. Further, evidence was obtained that PACAP is expressed in type I afferent axons leaving the spiral ganglion en route to the auditory nerve, potentially serving as a neuromodulator in axonal terminals. In contrast to the efferent localization of PACAP within the organ of Corti, PAC1-R immunoreactivity was co-localized with afferent dendritic neuronal marker GluR2/3 in nerve fibers passing beneath and lateral to the inner hair cell and in fibers at supranuclear and basal sites on outer hair cells. Given the known association of PACAP with catecholaminergic neurotransmission in sympathoadrenal function, we also re-examined the issue of whether the organ of Corti receives adrenergic innervation. We now demonstrate the existence of nerve fibers within the organ of Corti which are immunoreactive for the adrenergic marker dopamine beta-hydroxylase (DBH). DBH immunoreactivity was particularly prominent in nerve fibers both at the base and near the cuticular plate of outer hair cells of the apical turn, extending to the non-sensory Hensen's cell region. Evidence was obtained for limited co-localization of DBH with PAC1-R and PACAP. In the process of this investigation, we obtained evidence that efferent and afferent nerve fibers, in addition to adrenergic nerve fibers, are present at supranuclear sites on outer hair cells and distributed within the non-sensory epithelium of the apical cochlear turn for rat, based upon immunoreactivity for the corresponding neuronal markers. Overall, PACAP is hypothesized to act within the organ of Corti as an efferent neuromodulator of afferent signaling via PAC1-R that is present on type I afferent dendrites, in position to afford protection from excitotoxicity. Additionally, PACAP/PAC1-R may modulate secretion of catecholamines from adrenergic terminals within the organ of Corti.

Neurochemical plasticity and the role of neurotrophic factors in bladder reflex pathways after spinal cord injury

Transection of the spinal cord that interrupts the spinobulbospinal micturition reflex pathway, abolishes voluntary voiding and initially produces an areflexic bladder with complete urinary retention. However, depending upon the species, reflex bladder activity slowly recovers over the course of weeks or months. In chronic spinal animals, reflex mechanisms in the lumbosacral spinal cord are capable of duplicating many of the functions performed by reflex pathways in animals with an intact spinal cord and can induce bladder hyperreflexia. However, the bladder does not empty efficiently due to a loss of bladder-sphincter coordination (bladder-sphincter dyssynergia). In contrast to normal animals in which the sphincter relaxes during voiding, animals with a spinal cord injury exhibit sphincter contractions during voiding, an increase in urethral outlet resistance, urinary retention, bladder hyperreflexia, bladder overdistension, and an increase in bladder afferent cell size. Changes in electrophysiological or neurochemical properties of bladder afferent cells in the dorsal root ganglia and of spinal pathways could contribute to the emergence of the spinal micturition reflex, bladder hyperreflexia and changes in the pharmacologic responses of reflex pathways in the lumbosacral spinal cord after spinal cord injury. Urinary bladder hyperreflexia after spinal cord injury may reflect a change in the balance of neuroactive compounds in bladder reflex pathways. This review will detail: (1) changes in the neurochemical phenotype of bladder afferent neurons and of spinal neurons mediating micturition reflexes after spinal cord injury, with an emphasis on three neuroactive compounds, neuronal nitric oxide synthase (nNOS), galanin, and pituitary adenylate cyclase activating polypeptide (PACAP); (2) possible functional consequences on bladder reflexes of changes in spinal cord neurochemistry after spinal cord injury, and (3) the potential role of neurotrophic factors expressed in the urinary bladder or spinal cord after spinal cord injury in mediating these neurochemical changes.

PKA-catalyzed phosphorylation of tomosyn and its implication in Ca2+-dependent exocytosis of neurotransmitter

Neurotransmitter is released from nerve terminals by Ca2+-dependent exocytosis through many steps. SNARE proteins are key components at the priming and fusion steps, and the priming step is modulated by cAMP-dependent protein kinase (PKA), which causes synaptic plasticity. We show that the SNARE regulatory protein tomosyn is directly phosphorylated by PKA, which reduces its interaction with syntaxin-1 (a component of SNAREs) and enhances the formation of the SNARE complex. Electrophysiological studies using cultured superior cervical ganglion (SCG) neurons revealed that this enhanced formation of the SNARE complex by the PKA-catalyzed phosphorylation of tomosyn increased the fusion-competent readily releasable pool of synaptic vesicles and, thereby, enhanced neurotransmitter release. This mechanism was indeed involved in the facilitation of neurotransmitter release that was induced by a potent biological mediator, the pituitary adenylate cyclase-activating polypeptide, in SCG neurons. We describe the roles and modes of action of PKA and tomosyn in Ca2+-dependent neurotransmitter release.

Pituitary adenylate cyclase-activating polypeptide enhances the hyperpolarization-activated nonselective cationic conductance, Ih, in dissociated guinea pig intracardiac neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) peptides, which are co-localized with acetylcholine in preganglionic parasympathetic fibers innervating guinea pig intracardiac ganglia, depolarize and increase excitability of intracardiac neurons. Perforated patch whole cell recordings were used to test whether PACAP27-enhanced activation of Ih contributed to the increase in excitability. In current clamp, 100 nM PACAP27 increased rectification during 500-ms hyperpolarizations and increased the number of anodal break action potentials (APs). PACAP27 also increased the number of APs produced by 500-ms depolarizing currents. In voltage clamp, the effects of 100 nM PACAP27 were determined during hyperpolarizing steps from -50 mV to voltages between -60 and -120 mV. PACAP27 increased the amplitude and rate of activation of Ih. PACAP27 shifted the voltage dependence of activation of Ih by 6.6 mV. The effect of PACAP27 was eliminated by pretreatment with the Ih inhibitor ZD7288 (100 microM). The adenylyl cyclase activator forskolin (10 microM) produced a similar shift in the voltage dependence of Ih activation. We conclude that PACAP27 enhances Ih by shifting the voltage dependence of activation and propose that this effect is mediated primarily by PAC1 receptor activation of adenylyl cyclase and generation of cAMP. Furthermore, we propose that the peptide-enhanced Ih contributes to the PACAP27-induced increase in membrane excitability.

PACAP modulation of the colon-inferior mesenteric ganglion reflex in the guinea pig

We investigated the effect of pituitary adenylate cyclase activating peptide (PACAP) on the colon-inferior mesenteric ganglion (IMG) reflex loop in vitro. PACAP27 and PACAP38 applied to the IMG caused a prolonged depolarization and intense generation of fast EPSPs and action potentials in IMG neurones. Activation of PACAP-preferring receptors (PAC1-Rs) with the selective agonist maxadilan or vasoactive intestinal peptide (VIP)/PACAP (VPAC) receptors with VIP produced similar effects whereas prior incubation of the IMG with selective PAC1-R antagonists PACAP6-38 and M65 inhibited the effects of PACAP. Colonic distension evoked a slow EPSP in IMG neurones that was reduced in amplitude by prolonged superfusion of the IMG with either PACAP27, maxidilan, PACAP6-38, M65 or VIP. Activation of IMG neurones by PACAP27 or maxadilan resulted in an inhibition of ongoing spontaneous colonic contractions. PACAP-LI was detected in nerve trunks attached to the IMG and in varicosities surrounding IMG neurones. Cell bodies with PACAP-LI were present in lumbar 2-3 dorsal root ganglia and in colonic myenteric ganglia. Colonic distension evoked release of PACAP peptides in the IMG as measured by radioimmunoassay. Volume reconstructed images showed that a majority of PACAP-LI, VIP-LI and VAChT-LI nerve endings making putative synaptic contact onto IMG neurones and a majority of putative receptor sites containing PAC1-R-LI and nAChR-LI on the neurones were distributed along secondary and tertiary dendrites. These results suggest involvement of a PACAP-ergic pathway, operated through PAC1-Rs, in controlling the colon-IMG reflex.

Evidence for the involvement of VPAC1 and VPAC2 receptors in pressure-induced vasodilatation in rodents

A transient increase in skin blood flow in response to an innocuous local pressure application, defined as pressure-induced vasodilatation (PIV), delays the occurrence of ischaemia, suggesting a protective feature against applied pressure. The PIV response depends on capsaicin-sensitive nerve fibres and calcitonin gene-related peptide (CGRP) has been shown to be involved. In these fibres, CGRP coexists with pituitary adenylate cyclase-activating polypeptide (PACAP). Three distinct receptors mediate the biological effects of PACAP: VPAC1 and VPAC2 receptors binding with the same affinity for PACAP and vasoactive intestinal peptide and PAC1 receptors showing high selectivity for PACAP. Because the receptors are widely expressed in the nervous system and in the skin, we hypothesized that at least one of them is involved in PIV development. To verify this hypothesis, we used [D-p-Cl-Phe(6),Leu(17)]-VIP (nonspecific antagonist of VPAC1/VPAC2 receptors), PG 97-269 (antagonist of VPAC1 receptors), PACAP(6-38) (antagonist of VPAC2/PAC1 receptors) and Max.d.4 (antagonist of PAC1 receptors) in anaesthetized rodents. The blockade of VPAC1/VPAC2, VPAC1 or VPAC2/PAC1 receptors eliminated the PIV response, whereas PAC1 blockade had no effect, demonstrating an involvement of VPAC1/VPAC2 receptors in PIV development. Moreover, endothelium-independent and -dependent vasodilator responses were unchanged by the VPAC1/VPAC2 antagonist. Thus, the absence of a PIV response following VPAC1/VPAC2 blockade cannot be explained by any dysfunction of the vascular smooth muscle or endothelial vasodilator capacity. The involvement of VPAC1/VPAC2 receptors in the development of PIV seems to imply a series relationship in which each receptor type (CGRP, VPAC1, VPAC2) is necessary for the full transmission of the response.

Pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide inhibit dendritic growth in cultured sympathetic neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are related neuropeptides that are released by the preganglionic sympathetic axons. These peptides have previously been implicated in the regulation of sympathetic neurotransmitter metabolism and cell survival in postganglionic sympathetic neurons. In this study we consider the possibility that PACAP and VIP also affect the morphological development of these neurons. Postganglionic rat sympathetic neurons formed extensive dendritic arbors after exposure to bone morphogenetic protein-7 (BMP-7) in vitro. PACAP and VIP reduced BMP-7-induced dendritic growth by approximately 70-90%, and this suppression was maintained for 3 weeks. However, neither PACAP nor VIP affected axonal growth or cell survival. The actions of PACAP and VIP appear to be mediated by PAC1 receptors because their effects were suppressed by an antagonist that binds to PAC1 and VPAC2 receptors (PACAP6-38), but not by an antagonist that binds to the VPAC1 and VPAC2 receptors. Moreover, exposure to PACAP and VIP caused phosphorylation and nuclear translocation of cAMP response element-binding protein, and agents that increase the intracellular concentration of cAMP mimicked the PACAP-induced inhibition of dendritic growth. These data suggest that peptides released by preganglionic nerves modulate dendritic growth in sympathetic neurons by a cAMP-dependent mechanism.

Expression of VPAC2 receptor and PAC1 receptor splice variants in the trigeminal ganglion of the adult rat

PACAP and VIP are members of the VIP/secretin/glucagon family of peptides with neurotransmitter, neuroprotective, and neurotrophic functions. PACAP and VIP are known to be upregulated in primary sensory neurons following nerve injury, implying that these neuropeptides could be mediators of sensory transmission in neuropathic pain states. Nerve injury at the level of the trigeminal root is thought to be the prime cause of trigeminal neuralgia. Since cross-excitation (a chemically-mediated form of nonsynaptic transmission) within the TG is postulated to play a central role in trigeminal neuralgia, we studied the expression of PACAP and VIP receptors in the TG by RT PCR and immunocytochemistry. Of the three known receptors (PAC1, VPAC1 and VPAC2), RT PCR revealed the presence of mRNA for VPAC2 and several splice variants of the PAC1 receptor. Immunocytochemistry showed PAC1 and VPAC2 to be present in small-diameter TG neurons. Thus, PACAP and VIP are potential mediators of cross-excitation in the TG.

Herpesvirus quiescence in neuronal cells IV: virus activation induced by pituitary adenylate cyclase-activating polypeptide (PACAP) involves the protein kinase A pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a naturally occurring peptide found in the central nervous system that plays a role in somatosensory processing and activation of protein kinase A (PKA) and protein kinase C (PKC). Because activation of PKA or PKC results in reactivation of HSV-1 from latently infected embryonic neuronal cells, PACAP was used to evaluate HSV-1 activation from quiescently infected (QIF)-PC12 cells. Our studies demonstrate that physiologically relevant concentrations of PACAP38 and PACAP27 induce HSV-1 activation from QIF-PC12 cell cultures in a dose-dependent fashion. PACAP-induced activation of virus was significantly impaired by the PKA-inhibitor, H-89 (20 microM), whereas treatment with the PKC-inhibitor, GF109203X (1 microM), was without affect. Additionally, direct activation of PKA with cAMP analogs, 8-(4-chlorophenylthio)- and dibutyryl-cAMP, only partially mimicked the effect of PACAP on virus activation. Taken together, PACAP induced HSV-1 activation from QIF-PC12 cells involves the PKA and possibly cAMP-independent pathways. This report is the first to demonstrate that PACAP induces HSV-1 activation from a quiescent state and that this in vitro cell model is useful for studying early inductive events that lead to virus production from quiescence.

Mechanisms mediating pituitary adenylate cyclase-activating polypeptide depolarization of rat sympathetic neurons

The direct effects of pituitary adenylate cyclase-activating polypeptides (PACAP) on sympathetic neurons were investigated using rat superior cervical ganglion neurons. Electrophysiological and pharmacological analyses were used to evaluate PACAP modulation of sympathetic neuron membrane potentials and to investigate potential ionic and intracellular signaling mechanisms mediating the responses. More than 90% of the sympathetic neurons were depolarized by the PACAP peptides even when stimulated release was blocked, indicating that the PACAP peptides elicited primary responses in the postganglionic neurons. The response profile was consistent for activation of PACAP-selective PAC(1) receptors: nanomolar concentrations of PACAP27 and PACAP38 were required to stimulate depolarization, whereas vasoactive intestinal peptide failed to evoke any response. Furthermore, depolarizations elicited by PACAP27 were reduced by the PAC(1) receptor antagonist PACAP(6-38). Both sodium influx and inhibition of a potassium current contributed to the peptide-induced depolarizations. Activation of neither pertussis toxin- nor cholera toxin-sensitive G-proteins was required for generation of the depolarizations. cAMP and diacylglycerol production and activation of protein kinase A or protein kinase C also were not requisite for the responses. By contrast, phospholipase C (PLC)-dependent inositol 1,4,5-triphosphate (IP(3)) synthesis was crucial to the PACAP-mediated depolarizations. Although calcium release from IP(3)-sensitive stores was not required for the PACAP-induced responses, inhibition of IP(3) receptors reduced the depolarizations. Thus, among the many signal transduction pathways coupled to the PAC(1) receptor, the PACAP-induced depolarization of sympathetic neurons appears to require activation of PLC and subsequent generation of IP(3).

Differences in the ways sympathetic neurons and endocrine cells process, store, and secrete exogenous neuropeptides and peptide-processing enzymes

Most neurons store peptides in large dense core vesicles (LDCVs) and release the neuropeptides in a regulated manner. Although LDCVs have been studied in endocrine cells, less is known about these storage organelles in neurons. In this study we use the endogenous peptide NPY (neuropeptide Y) and the endogenous peptide-processing enzyme PAM (peptidylglycine alpha-amidating monooxygenase) as tools to study the peptidergic system in cultured neurons from the superior cervical ganglion (SCG). Once mature, SCG neurons devote as much of their biosynthetic capabilities to neurotransmitter production as endocrine cells devote to hormone production. Unlike pituitary and atrium, SCG neurons cleave almost all of the bifunctional PAM protein they produce into soluble monofunctional enzymes. Very little PAM or NPY is secreted under basal conditions, and the addition of secretagogue dramatically stimulates the secretion of PAM and NPY to a similar extent. Although endocrine cells typically package "foreign" secretory products together with endogenous products, pro-opiomelanocortin- and PAM-derived products encoded by adenovirus in large part were excluded from the LDCVs of SCG neurons. When expressed in corticotrope tumor cells and primary anterior pituitary cultures, the same virally encoded products were metabolized normally. The differences that were observed could reflect differences in the properties of neuronal and endocrine peptidergic systems or differences in the ability of neurons and endocrine cells to express viral transcripts.

Pituitary adenylate cyclase-activating polypeptides directly stimulate sympathetic neuron neuropeptide Y release through PAC(1) receptor isoform activation of specific intracellular signaling pathways

Pituitary adenylate cyclase-activating polypeptides (PACAP) have potent regulatory and neurotrophic activities on superior cervical ganglion (SCG) sympathetic neurons with pharmacological profiles consistent for the PACAP-selective PAC(1) receptor. Multiple PAC(1) receptor isoforms are suggested to determine differential peptide potency and receptor coupling to multiple intracellular signaling pathways. The current studies examined rat SCG PAC(1) receptor splice variant expression and coupling to intracellular signaling pathways mediating PACAP-stimulated peptide release. PAC(1) receptor mRNA was localized in over 90% of SCG neurons, which correlated with the cells expressing receptor protein. The neurons expressed the PAC(1)(short)HOP1 receptor but not VIP/PACAP-nonselective VPAC(1) receptors; low VPAC(2) receptor mRNA levels were restricted to ganglionic nonneuronal cells. PACAP27 and PACAP38 potently and efficaciously stimulated both cAMP and inositol phosphate production; inhibition of phospholipase C augmented PACAP-stimulated cAMP production, but inhibition of adenylyl cyclase did not alter stimulated inositol phosphate production. Phospholipase C inhibition blunted neuron peptide release, suggesting that the phosphatidylinositol pathway was a prominent component of the secretory response. These studies demonstrate preferential sympathetic neuron expression of PACAP-selective receptor variants contributing to regulation of autonomic function.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

The PACAP ligand/receptor system regulates cerebral cortical neurogenesis

The PACAP ligand/type I receptor system is expressed throughout the embryonic nervous system, suggesting roles in regulating neural patterning and neurogenesis. In the forebrain, precursors of the six-layered cerebral cortex cease dividing in a highly reproducible spatiotemporal sequence. The time of cell cycle exit in fact determines neuron laminar fate. Our studies indicate that PACAP signaling may elicit cortical precursor withdrawal from the cell cycle, antagonizing mitogenic stimulators. PACAP inhibited embryonic day 13.5 rat cortical precursor [3H]thymidine incorporation, decreasing the proportion of mitotic cells. PACAP promoted morphological and biochemical differentiation, indicating that PACAP-induced cell cycle withdrawal was accompanied by neuronal differentiation. In vivo, embryonic cortex contains PACAP. In culture, 85% of cells expressed PACAP while 64% exhibited receptor. Co-localization studies indicated that PACAP ligand and receptor were expressed by the mitotic precursors that divided in response to bFGF, suggesting that precursors integrate mitogenic and anti-mitogenic signals to determine the timing of cell cycle exit. The expression of PACAP ligand and receptor in precursors raised the possibility of autocrine function. Indeed, peptide antagonists increased proliferation, suggesting that the PACAP system is expressed to elicit cell cycle exit. During ontogeny, an inhibitory signal, such as PACAP, may be required to counter the stimulatory activity of mitogenic bFGF and IGFI whose expression during cortical neurogenesis is sustained. The dynamic interplay of positive and negative regulators would regulate the timing of cell cycle withdrawal, and thus neuronal phenotype and laminar position.