Pituitary adenylate cyclase-activating polypeptide (PACAP) is a regulator of astrocytes: PACAP stimulates proliferation and production of interleukin 6 (IL-6), but not nerve growth factor (NGF), in cultured rat astrocyte

Increased Expression of the Neuropeptides PACAP/VIP in the Brain of Mice with CNS Targeted Production of IL-6 Is Mediated in Part by Trans-Signalling

Inflammation with expression of interleukin 6 (IL-6) in the central nervous system (CNS) occurs in several neurodegenerative/neuroinflammatory conditions and may cause neurochemical changes to endogenous neuroprotective systems. Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are two neuropeptides with well-established protective and anti-inflammatory properties. Yet, whether PACAP and VIP levels are altered in mice with CNS-restricted, astrocyte-targeted production of IL-6 (GFAP-IL6) remains unknown. In this study, PACAP/VIP levels were assessed in the brain of GFAP-IL6 mice. In addition, we utilised bi-genic GFAP-IL6 mice carrying the human sgp130-Fc transgene (termed GFAP-IL6/sgp130Fc mice) to determine whether trans-signalling inhibition rescued PACAP/VIP changes in the CNS. Transcripts and protein levels of PACAP and VIP, as well as their receptors PAC1, VPAC1 and VPAC2, were significantly increased in the cerebrum and cerebellum of GFAP-IL6 mice vs. wild type (WT) littermates. These results were paralleled by a robust activation of the JAK/STAT3, NF-κB and ERK1/2MAPK pathways in GFAP-IL6 mice. In contrast, co-expression of sgp130Fc in GFAP-IL6/sgp130Fc mice reduced VIP expression and activation of STAT3 and NF-κB pathways, but it failed to rescue PACAP, PACAP/VIP receptors and Erk1/2MAPK phosphorylation. We conclude that forced expression of IL-6 in astrocytes induces the activation of the PACAP/VIP neuropeptide system in the brain, which is only partly modulated upon IL-6 trans-signalling inhibition. Increased expression of PACAP/VIP neuropeptides and receptors may represent a homeostatic response of the CNS to an uncontrolled IL-6 synthesis and its neuroinflammatory consequences.

Pituitary Adenylate Cyclase-Activating Polypeptide: A Potent Therapeutic Agent in Oxidative Stress

Stroke is a life-threatening condition that is characterized by secondary cell death processes that occur after the initial disruption of blood flow to the brain. The inability of endogenous repair mechanisms to sufficiently support functional recovery in stroke patients and the inadequate treatment options available are cause for concern. The pathology behind oxidative stress in stroke is of particular interest due to its detrimental effects on the brain. The oxidative stress caused by ischemic stroke overwhelms the neutralization capacity of the body's endogenous antioxidant system, which leads to an overproduction of reactive oxygen species (ROS) and reactive nitrogen species (RNS) and eventually results in cell death. The overproduction of ROS compromises the functional and structural integrity of brain tissue. Therefore, it is essential to investigate the mechanisms involved in oxidative stress to help obtain adequate treatment options for stroke. Here, we focus on the latest preclinical research that details the mechanisms behind secondary cell death processes that cause many central nervous system (CNS) disorders, as well as research that relates to how the neuroprotective molecular mechanisms of pituitary adenylate cyclase-activating polypeptides (PACAPs) could make these molecules an ideal candidate for the treatment of stroke.

Targeting the PAC1 Receptor for Neurological and Metabolic Disorders

The pituitary adenylate cyclase-activating polypeptide (PACAP)-selective PAC1 receptor (PAC1R, ADCYAP1R1) is a member of the vasoactive intestinal peptide (VIP)/secretin/glucagon family of G protein-coupled receptors (GPCRs). PAC1R has been shown to play crucial roles in the central and peripheral nervous systems. The activation of PAC1R initiates diverse downstream signal transduction pathways, including adenylyl cyclase, phospholipase C, MEK/ERK, and Akt pathways that regulate a number of physiological systems to maintain functional homeostasis. Accordingly, at times of tissue injury or insult, PACAP/PAC1R activation of these pathways can be trophic to blunt or delay apoptotic events and enhance cell survival. Enhancing PAC1R signaling under these conditions has the potential to mitigate cellular damages associated with cerebrovascular trauma (including stroke), neurodegeneration (such as Parkinson's and Alzheimer's disease), or peripheral organ insults. Conversely, maladaptive PACAP/PAC1R signaling has been implicated in a number of disorders, including stressrelated psychopathologies (i.e., depression, posttraumatic stress disorder, and related abnormalities), chronic pain and migraine, and metabolic diseases; abrogating PAC1R signaling under these pathological conditions represent opportunities for therapeutic intervention. Given the diverse PAC1R-mediated biological activities, the receptor has emerged as a relevant pharmaceutical target. In this review, we first describe the current knowledge regarding the molecular structure, dynamics, and function of PAC1R. Then, we discuss the roles of PACAP and PAC1R in the activation of a variety of signaling cascades related to the physiology and diseases of the nervous system. Lastly, we examine current drug design and development of peptides and small molecules targeting PAC1R based on a number of structure- activity relationship studies and key pharmacophore elements. At present, the rational design of PAC1R-selective peptide or small-molecule therapeutics is largely hindered by the lack of structural information regarding PAC1R activation mechanisms, the PACAP-PAC1R interface, and the core segments involved in receptor activation. Understanding the molecular basis governing the PACAP interactions with its different cognate receptors will undoubtedly provide a basis for the development and/or refinement of receptor-selective therapeutics.

IL-6 knockout mice are protected from cocaine-induced kindling behaviors; possible involvement of JAK2/STAT3 and PACAP signalings

IL-6 has been recognized as an anticonvulsant against certain neuroexcitotoxicities. We aimed to investigate on the interactive role between IL-6 and PACAP in cocaine-induced kindling behaviors. Although we found that cocaine (45 mg/kg, i.p./day x 5) significantly increased IL-6 and TNF-α expression, it resulted in a decrease in IFN-γ expression. We observed that the cocaine-induced increase in IL-6 expression was more pronounced than that in TNF-α expression. Genetic depletion of IL-6 significantly activated cocaine kindling behaviors. This phenomenon was also consistently observed in WT mice that received a neutralizing IL-6 receptor antibody. Cocaine-treated IL-6 knockout mice exhibited significantly decreased PACAP and PACAP receptor (PAC1R) mRNA levels and significantly increased TNF-α gene expression. TNF-α knockout mice were protected from cocaine kindling via an up-regulation of IL-6, phospho-JAK2/STAT3, PACAP, and PAC1R levels, which produced anti-apoptotic effects. Recombinant IL-6 protein (rIL-6, 2 μg, i.v./mouse/day x 5) also up-regulated phospho-JAK2/STAT3, PACAP, and PAC1R mRNA levels, leading to anti-apoptotic effects in IL-6 knockout mice. Consistently, AG490, a JAK2/STAT3 inhibitor, and PACAP 6-38, a PAC1 receptor antagonist, counteracted rIL-6-mediated protection. Combined, our results suggest that IL-6 gene requires up-regulation of phospho-JAK2/STAT3, PACAP, and PAC1R and down-regulation of the TNF-α gene to modulate its anticonvulsive/neuroprotective potential.

PACAP as a neuroprotective factor in ischemic neuronal injuries

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 27- or 38-amino acid neuropeptide, which belongs to the vasoactive intestinal polypeptide/glucagon/secretin family. PACAP and its three receptor subtypes are expressed in neural tissues, with PACAP known to exert pleiotropic effects on the nervous system. This review provides an overview of current knowledge regarding the neuroprotective effects, mechanisms of action, and therapeutic potential of PACAP in response to ischemic brain injuries.

Augmented cystine-glutamate exchange by pituitary adenylate cyclase-activating polypeptide signaling via the VPAC1 receptor

In the central nervous system, cystine import in exchange for glutamate through system xc- is critical for the production of the antioxidant glutathione by astrocytes, as well as the maintenance of extracellular glutamate. Therefore, regulation of system xc- activity affects multiple aspects of cellular physiology and may contribute to disease states. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuronally derived peptide that has already been demonstrated to modulate multiple aspects of glutamate signaling suggesting PACAP may also target activity of cystine-glutamate exchange via system xc-. In this study, 24-h treatment of primary cortical cultures containing neurons and glia with PACAP concentration-dependently increased system xc- function as measured by radiolabeled cystine uptake. Furthermore, the increase in cystine uptake was completely abolished by the system xc- inhibitor, (S)-4-carboxyphenylglycine (CPG), attributing increases in cystine uptake specifically to system xc- activity. Time course and quantitative PCR results indicate that PACAP signaling may increase cystine-glutamate exchange by increasing expression of xCT, the catalytic subunit of system xc-. Furthermore, the potentiation of system xc- activity by PACAP occurs via a PKA-dependent pathway that is not mediated by the PAC1R, but rather the shared vasoactive intestinal polypeptide receptor VPAC1R. Finally, assessment of neuronal, astrocytic, and microglial-enriched cultures demonstrated that only astrocyte-enriched cultures exhibit enhanced cystine uptake following both PACAP and VIP treatment. These data introduce a novel mechanism by which both PACAP and VIP regulate system xc- activity. Synapse 68:604-612, 2014. © 2014 Wiley Periodicals, Inc.

Inhibition of food intake by PACAP in the hypothalamic ventromedial nuclei is mediated by NMDA receptors

Central injections of pituitary adenylate cyclase-activating polypeptide (PACAP) into the ventromedial nuclei (VMN) of the hypothalamus produce hypophagia that is dependent upon the PAC1 receptor; however, the signaling downstream of this receptor in the VMN is unknown. Though PACAP signaling has many targets, this neuropeptide has been shown to influence glutamate signaling in several brain regions through mechanisms involving NMDA receptor potentiation via activation of the Src family of protein tyrosine kinases. With this in mind, we examined the Src-NMDA receptor signaling pathway as a target for PACAP signaling in the VMN that may mediate its effects on feeding behavior. Under nocturnal feeding conditions, NMDA receptor antagonism prior to PACAP administration into the VMN attenuated PACAP-mediated decreases in feeding suggesting that glutamatergic signaling via NMDA receptors is necessary for PACAP-induced hypophagia. Furthermore, PACAP administration into the VMN resulted in increased tyrosine phosphorylation of the GluN2B subunit of the NMDA receptor, and inhibition of Src kinase activity also blocked the effects of PACAP administration into the VMN on feeding behavior. These results indicate that PACAP neurotransmission in the VMN likely augments glutamate signaling by potentiating NMDA receptors activity through the tyrosine phosphorylation events mediated by the Src kinase family, and modulation of NMDA receptor activity by PACAP in the hypothalamus may be a primary mechanism for its regulation of food intake.

Involvement of the Notch pathway in terminal astrocytic differentiation: role of PKA

The Notch pathway is a highly conserved signaling system essential for modulating neurogenesis and promoting astrogenesis. Similarly, the cAMP signaling cascade can promote astrocytic commitment in several cell culture models, such as the C6 glioma cell line. These cells have the capacity to differentiate into oligodendrocytes or astrocytes, characteristics that allow their use as a glial progenitor model. In this context, we explore here the plausible involvement of cAMP in Notch-dependent signal transactions. The exposure of C6 cells to a non-hydrolysable cAMP analogue resulted in a sustained augmentation of Notch activity, as detected by nuclear translocation of its intracellular domain portion (NICD) and transcriptional activity. The cAMP effect is mediated through the activation of the γ-secretase complex, responsible for Notch cleavage and is sensitive to inhibitors of the cAMP-dependent protein kinase, PKA. As expected, Notch cleavage and nuclear translocation resulted in the up-regulation of the mRNA levels of one of its target genes, the transcription factor Hair and enhancer of split 5. Moreover, the glutamate uptake activity, as well as the expression of astrocytic markers such as glial fibrillary acidic protein, S100β protein and GLAST was also enhanced in cAMP-exposed cells. Our results clearly suggest that during the process of C6 astrocytic differentiation, cAMP activates the PKA/γ-secretase/NICD/RBPJκ pathway and Notch1 expression, leading to transcriptional activation of the genes responsible for glial progenitor cell fate decision.

STC1 induction by PACAP is mediated through cAMP and ERK1/2 but not PKA in cultured cortical neurons

The neuroprotective actions of PACAP (pituitary adenylate cyclase-activating polypeptide) in vitro and in vivo suggest that activation of its cognate G protein coupled receptor PAC1 or downstream signaling molecules,and thus activation of PACAP target genes, could be of therapeutic benefit. Here, we show that cultured rat cortical neurons predominantly expressed the PAC1hop and null variants. PACAP receptor activation resulted in the elevation of the two second messengers cAMP and Ca(2+) and expression of the putative neuroprotectant stanniocalcin 1(STC1). PACAP signaling to the STC1 gene proceeded through the extracellular signal-regulated kinases 1 and 2(ERK1/2), but not through the cAMP-dependent protein kinase (PKA), and was mimicked by the adenylate cyclase activator forskolin. PACAP- and forskolin-mediated activation of ERK1/2 occurred through cAMP, but not PKA.These results suggest that STC1 gene induction proceeds through cAMP and ERK1/2, independently of PKA, the canonical cAMP effector. In contrast, PACAP signaling to the BDNF gene proceeded through PKA, suggesting that two different neuroprotective cAMP pathways co-exist in differentiated cortical neurons. The selective activation of a potentially neuroprotective cAMP-dependent pathway different from the canonical cAMP pathway used in many physiological processes, such as memory storage, has implications for pharmacological activation of neuroprotection in vivo.

Expression and distribution of pituitary adenylate cyclase-activating polypeptide receptor in reactive astrocytes induced by global brain ischemia in mice

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide acting as a neuroprotectant. We previously showed that PACAP receptor (PAC1R) immunoreactivity was elevated in reactive astrocytes after stab wound injury. However, the pattern of PAC1R expression in astrocytes after brain injury is still unknown. In this study, PAC1R expression was evaluated in mouse hippocampal astrocytes after bilateral common carotid artery occlusion. PAC1R mRNA levels in the hippocampus peaked on day 7, and glial fibrillary acidic protein (GFAP) mRNA levels increased from day 3 to day 7 after ischemia. We then observed co-localization of PAC1R and GFAP by double immunostaining. GFAP-immunopositive cells showed signs of hypertrophy 3 days after the ischemia, and by day 7 had fine processes, were hypertrophied, and are known as reactive astrocytes. A low number of PAC1R-immunopositive astrocytes were detectable in the hippocampal area until 3 days after ischemia. PAC1R-positive astrocytes were widely distributed in the hippocampus between day 7 and day 14 after ischemia, and they were converging around the damaged CA1 pyramidal cell layer by day 28. These results suggest that PAC1R might be expressed in the middle to late stage of reactive astrocytes and PACAP plays an important role in the reactive astrocytes after brain injury.

Distribution and protective function of pituitary adenylate cyclase-activating polypeptide in the retina

Pituitary adenylate cyclase-activating polypeptide (PACAP), which is found in 27- or 38-amino acid forms, belongs to the VIP/glucagon/secretin family. PACAP and its three receptor subtypes are expressed in neural tissues, with PACAP known to exert a protective effect against several types of neural damage. The retina is considered to be part of the central nervous system, and retinopathy is a common cause of profound and intractable loss of vision. This review will examine the expression and morphological distribution of PACAP and its receptors in the retina, and will summarize the current state of knowledge regarding the protective effect of PACAP against different kinds of retinal damage, such as that identified in association with diabetes, ultraviolet light, hypoxia, optic nerve transection, and toxins. This article will also address PACAP-mediated protective pathways involving retinal glial cells.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates proliferation of reactive astrocytes in vitro

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide originally isolated from ovine hypothalamus. Recently, we have shown that the PACAP receptor (PAC1-R) is expressed in reactive astrocytes following an in vivo stub wound brain injury. However, the functional role of PACAP has not yet been clarified. In order to investigate the effect of PACAP on the proliferation of reactive astrocytes, a scratch wound paradigm was applied to astrocytic monolayers. Following injury, there was an increase in PAC1-R and glial fibrillary acidic protein (GFAP) immunoreactivity in the astrocytes surrounding the scratch line. PACAP at concentrations of 10(-15) to 10(-7) M was applied immediately after scratching, and the proliferating astrocytes were visualized by multiple immunofluorescence labeling. The percentage of cells that colabeled for Ki67 (a marker of proliferating cells) and GFAP increased in the 10(-11)- and 10(-13)-M PACAP-treated groups. The proliferating astrocytes induced by PACAP treatment mainly occurred in the proximal wound area where many reactive astrocytes were observed. Pretreatment with the PACAP receptor antagonist PACAP6-38 significantly suppressed the PACAP-induced effects. These results strongly suggest that PACAP plays an important role in the proliferation of reactive astrocytes following nerve injury.

cAMP activation by PACAP/VIP stimulates IL-6 release and inhibits osteoblastic differentiation through VPAC2 receptor in osteoblastic MC3T3 cells

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the glucagon/vasoactive intestinal peptide (VIP) superfamily, stimulates cyclic AMP accumulation initiating a variety of biological processes such as: neurotropic actions, immune and pituitary function, learning and memory, catecholamine biosynthesis and regulation of cardiopulmonary function. Both osteoclasts and osteoblasts have been shown to express receptors for PACAP/VIP implicated in their role in bone metabolism. To further understand the role of PACAP/VIP family in controlling bone metabolism, we investigated differentiation model of MC3T3-E1 cells, an osteoblastic cell line derived from mouse calvaria. Quantitative RT-PCR analysis demonstrated that MC3T3-E1 cells expressed only VPAC2 receptor and its expression was upregulated during osteoblastic differentiation, whereas VPAC1 and PAC1 receptors were not expressed. Consistent with expression of receptor subtype, both PACAP and VIP stimulate cAMP accumulation in a time- and dose-dependent manner with the similar potency in undifferentiated and differentiated cells, while Maxadilan, a specific agonist for PAC1-R, did not. Furthermore, downregulation of VPAC2-R by siRNA completely blocked cAMP response mediated by PACAP and VIP. Importantly, PACAP/VIP as well as forskolin markedly suppressed the induction of alkaline phosphatase mRNA upon differentiation and the pretreatment with 2',5'-dideoxyadenosine, a cAMP inhibitor, restored its inhibitory effect of PACAP. We also found that PACAP and VIP stimulated IL-6 release, a stimulator of bone resorption, and VPAC2-R silencing inhibited IL-6 production. Thus, PACAP/VIP can activate adenylate cyclase response and regulate IL-6 release through VPAC2 receptor with profound functional consequences for the inhibition of osteoblastic differentiation in MC3T3-E1 cells.

Pituitary adenylate cyclase-activating peptide (PACAP) stimulates production of interleukin-6 in rat Müller cells

Pituitary adenylate cyclase-activating peptide (PACAP) is known to regulate not only neurons but also astrocytes. Here, we investigated, both in vitro and in vivo, the effects of PACAP38 on rat Müller cells, which are the predominant glial element in the retina. Müller cells isolated from juvenile Wistar rats were treated with PACAP38 or PACAP6-38, a PACAP selective antagonist. Cell proliferation was determined by measuring the incorporation of bromodeoxyuridine with ELISA. Interleukin-6 (IL-6) levels in the culture medium were determined by a bioassay using B9 cells, IL-6 dependent hybridoma. In adult Wistar rats, the expression of IL-6 in the retina after intravitreal injection of PACAP38 (10 pmol) was assessed by immunohistochemistry. PACAP38 stimulated IL-6 production in Müller cells at a concentration as low as 10(-12) M, which did not induce cell proliferation. This elevation of IL-6 production was inhibited by PACAP6-38. Radial IL-6 expression was observed throughout the retina at 2 and 3 days after PACAP38 injection. These data demonstrate that Müller cells are one of the target cells for PACAP. IL-6, which is released from Müller cells with stimulation by PACAP, may play a significant role in the retina.

Ischaemic preconditioning: therapeutic implications for stroke?

Ischaemic preconditioning (IPC), also known as ischaemic tolerance (IT), is a phenomenon whereby tissue is exposed to a brief, sublethal period of ischaemia, which activates endogenous protective mechanisms, thereby reducing cellular injury that may be caused by subsequent lethal ischaemic events. The first description of this phenomenon was in the heart, which was reported by Murry and co-workers in 1986. Subsequent studies demonstrated IPC in lung, kidney and liver tissue, whereas more recent studies have concentrated on the brain. The cellular mechanisms underlying the beneficial effects of IPC remain largely unknown. This phenomenon, which has been demonstrated by using various injury paradigms in both cultured neurons and animal brain tissue, may be utilised to identify and characterise therapeutic targets for small-molecule, antibody, or protein intervention. This review will examine the experimental evidence demonstrating the phenomenon termed IPC in models of cerebral ischaemia, the cellular mechanisms that may be involved and the therapeutic implications of these findings.

Treatments for stroke

Stroke is the third leading cause of death and the leading cause of disability in developed countries, yet remains a poorly treated condition. Treatments for stroke can be aimed at acutely improving blood flow or protecting brain tissue against ischaemia, enhancing stroke recovery or reducing the risk of stroke recurrence. This paper reviews each of these approaches, particularly focusing on mechanisms for which there are agents in clinical trials. There are a number of appealing neuroprotective agents in Phase II and III clinical trials. However, the majority of acute treatments are likely to suffer from a narrow therapeutic time window and hence limited patient access. Combinations of acute approaches are likely to offer the greatest benefit, but present challenges in development. Promotion of recovery following stroke offers enormous potential for successful therapeutic intervention. Excitingly, new developments in preclinical research have identified possible ways in which this may be achieved.

Possible involvement of a cyclic AMP-dependent mechanism in PACAP-induced proliferation and ERK activation in astrocytes

In cultured astrocytes, PACAP activates extracellular signal-regulated kinase (ERK) and induces cell proliferation at picomolar concentrations. Here, we examined the role of cyclic AMP signaling underlying the effects of PACAP. PACAP38 induced accumulation of cyclic AMP in astrocytes at concentrations as low as 10(-12)M. PACAP38 (10(-12)-10(-9)M)-stimulated cell proliferation was completely abolished by the cyclic AMP antagonist Rp-cAMP, whereas the protein kinase A (PKA) inhibitor H89 had no effect. This PACAP38-mediated effect was also abolished by the ERK kinase inhibitor PD98059, suggesting the involvement of ERK in PACAP-induced proliferation. PACAP38 (10(-12)M)-stimulated phosphorylation of ERK lasted for at least 60 min. This effect was completely abolished by Rp-cAMP but not by H89. Dibutyryl cyclic AMP maximally stimulated the incorporation of thymidine and activation of ERK at 10(-10)M. These results suggest that PACAP-mediated stimulation of ERK activity and proliferation of astrocytes may involve a cyclic AMP-dependent, but PKA-independent, pathway.

Functional retinal input stimulates expression of astroglial elements in the suprachiasmatic nucleus of postnatal developing rat

Astrocytes are abundant in the hypothalamic suprachiasmatic nucleus (SCN), particularly in the retinorecipient region. Using glial fibrillary acidic protein (GFAP) immunocytochemistry, we investigated the effect of light on the development of astrocytes in the SCN housing under light-dark (LD) or constant dark (DD) conditions after birth. GFAP immunoreactivity in the DD group showed lower levels than those in the LD group at P50. However, there was no difference in density of retinohypothalamic tract (RHT) terminals in the SCN between the DD and LD groups. After the adult pattern of GFAP immunoreactivity was established at P30, transferring rats to different LD conditions produced changes in GFAP immunoreactivity evident when rats were sacrificed at P50. We next examined, using a primary culture of hypothalamic astrocytes, whether neurotransmitters of RHT such as glutamate and pituitary adenylate cyclase activating polypeptide (PACAP) can stimulate GFAP expression directly. PACAP-38 increased the length and number of astrocytic processes but glutamate did not. These findings indicate that the functional aspects of RHT such as the light stimulated release of neurotransmitters is important for the development of astrocytes in rat SCN. Dynamic plasticity of astroglial elements in the SCN occurs even after GFAP shows an adult pattern.

Effects of pituitary adenylate cyclase-activating polypeptide on facial nerve recovery in the Guinea pig

OBJECTIVES/HYPOTHESIS

Pituitary adenylate cyclase-activating polypeptide (PACAP) has neurotrophic effects of neural regeneration and gives protection to the nervous system. We investigated whether PACAP had a neurotrophic effect on peripheral motoneurons and whether PACAP could facilitate glial cell line-derived neurotrophic factor (GDNF), a neurotrophin, in nerve regeneration. The presence and distribution of PACAP receptors following facial nerve transection were also investigated.

STUDY DESIGN

Animal experiment.

METHODS

Unilateral transection of the facial nerve was performed in male Hartley guinea pigs, and PACAP was injected at the site. Saline was substituted as a control. Compound muscle action potentials were recorded to measure the changes of latency. Glial cell line-derived neurotrophic factor (GDNF) content in facial target muscle was measured using enzyme-linked immunosorbent assay. The regenerating site was taken for histological studies.

RESULTS

Pituitary adenylate cyclase-activating polypeptide hastened the appearance of compound muscle action potentials and shortened the latency. Pituitary adenylate cyclase-activating polypeptide increased and prolonged the nerve transection-induced GDNF increase in the facial muscles. The number of myelinated fibers at 1 to 4 weeks after the transection was increased. PAC1 receptor or VPAC1 receptor or both were identified in the injury area at 2 to 4 weeks.

CONCLUSIONS

Pituitary adenylate cyclase-activating polypeptide facilitated the recovery of latency of compound muscle action potentials or the number of myelinated axons, or both. Pituitary adenylate cyclase-activating polypeptide prolonged the GDNF levels in target organs. These data indicated that PACAP promoted regeneration of the facial nerve.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates endozepine release from cultured rat astrocytes via a PKA-dependent mechanism

Astroglial cells synthesize and release endozepines, neuropeptides that are related to the octadecaneuropeptide ODN. Glial cells also express PACAP/VIP receptors. We have investigated the possible effect of PACAP on the release of ODN-like immunoreactivity (ODN-LI) by cultured rat astrocytes. Administration of PACAP27 and PACAP38 induced a concentration-dependent increase in secretion of ODN-LI whereas VIP was approximately 1000-fold less potent. The maximum effect of PACAP38 occurred after 5 min, then gradually declined during the next 10 min. The stimulatory effects of PACAP and VIP were abrogated by the PACAP antagonist PACAP6-38. PACAP38 stimulated cAMP formation, activated polyphosphoinositide turnover, and provoked calcium mobilization from IP3-sensitive pools. The PKA inhibitor H89 suppressed PACAP-induced secretion of ODN-LI, whereas PLC inhibitor U73122 and the PKC inhibitor chelerythrine had no effect. In contrast, U73122 restored the stimulatory action of PACAP on ODN-LI release and cAMP formation during prolonged (15 min) incubation with the peptide, and this effect was prevented by PMA. The present results demonstrate that PACAP stimulates endozepine release through activation of PAC1 receptors coupled to the AC/PKA pathway. Our data also show that activation of the PLC/PKC pathway down-regulates the effect of PACAP on endozepine release.

Fibroblast growth factor-2 converts PACAP growth action on embryonic hindbrain precursors from stimulation to inhibition

Pituitary adenylyl cyclase activating peptide (PACAP) has been shown either to stimulate or to inhibit neural cell proliferation depending on the origin of the cell population. We show here that, depending on the presence or absence of fibroblast growth factor-2 (FGF-2, also called basic FGF), PACAP may either stimulate or inhibit DNA synthesis in neural precursors isolated from embryonic day 10.5 mouse hindbrain. In the absence of FGF-2, PACAP stimulated 3H-thymidine incorporation in a dose-dependent manner. This stimulatory action was unaffected by antagonists of protein kinases A and C but was abolished in the presence of the MEK1/2 antagonist PD98059. In contrast, when FGF-2 was present, PACAP inhibited DNA synthesis. This inhibitory action was insensitive to PD98059 but was fully blocked by the protein kinase A (PKA) inhibitor H89. The differential blockades by MEK1/2 and PKA inhibitors indicate that the FGF-2-induced switch in PACAP action on DNA synthesis was accomplished by a change in PACAP signaling pathways. We hypothesize that the actions of PACAP in the specific parts of the developing nervous system are determined in part by the presence or absence of FGFs and other growth factors.

PACAP mediates the neural proliferative pathway of Mastomys enterochromaffin-like cell transformation

BACKGROUND AND AIM

Pituitary adenylate-cyclase activating peptide (PACAP) is a more potent proliferative agent than gastrin for rat enterochromaffin-like (ECL) cell proliferation in vitro. The role of this neurotransmitter during gastrin-mediated ECL cell tumor formation and gastrin-autonomous ECL cell neoplasia is unknown.

METHODS AND RESULTS

ECL cell transformation was induced in the Mastomys using 16 wk H2 receptor blockade of acid inhibition. Examination of the epithelial fundic mucosa demonstrated that PACAP-immunoreactivity significantly increased in the tumor mucosa compared to the naïve stomach, and was associated with ECL cells. Naïve and tumor ECL cells were then purified (approximately 95%) from Mastomys and the presence of all three PACAP/VPAC receptor subtypes was demonstrated by polymerase chain-reaction amplification. Thereafter, cells were maintained in short-term (48 h) primary cultures. PACAP significantly (p<0.05) increased 24 h bromo-deoxyuridine uptake (approximately 4-fold) in both cell types with estimated EC(50) values of approximately 4x10(-16) M and approximately 2x10(-16) M, respectively. Specific receptor antagonists (PAC1/VPAC1) of PACAP competitively inhibited these proliferative effects in naïve cells. Oligonucleotide antisense directed against PAC1 significantly inhibited PACAP-stimulated DNA synthesis by approximately 85% (p<0.05) in tumor cells.

CONCLUSION

PACAP is a potent and effective modulator of ECL cell proliferation. The expression of this neuropeptide and its receptors, particularly PAC1, suggest the existence of a neural regulatory pathway of ECL cell proliferation and transformation.

Pituitary adenylate cyclase-activating polypeptide enhances Ca(2+)-dependent neurotransmitter release from PC12 cells and cultured cerebellar granule cells without affecting intracellular Ca(2+) mobilization

The effects of pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide isolated from mammalian hypothalamus, was investigated on neurotransmitter release from clonal rat pheochromocytoma PC12 cells and cultured rat cerebellar granule cells. We found that PACAP38 stimulates the neurotransmitter release from PC12 cells by two distinct mechanisms in different concentration ranges. In the lower concentration range (<1 nM), PACAP38 enhanced depolarization- and ionomycin-dependent dopamine release without mobilizing intracellular Ca(2+), while in the higher concentration range (>1 nM), PACAP38 induced profound Ca(2+) influx and concomitant dopamine release from PC12 cells. In cultured rat cerebellar granule cells, PACAP38 failed to increase intracellular Ca(2+); however, it enhanced depolarization-dependent glutamate release remarkably. These results indicate that PACAP38 enhances Ca(2+)-dependent neurotransmitter release by modulating step(s) subsequent to Ca(2+) entry.

Development of astroglial elements in the suprachiasmatic nucleus of the rat: with special reference to the involvement of the optic nerve

The development of astroglial cells and the effect of the retinohypothalamic tract on it were studied by vimentin and glial fibrillary acidic protein (GFAP) immunocytochemistry in the suprachiasmatic nucleus (SCN) of the rat. At the embryonic stage, vimentin-immunoreactive (VIM-IR) radial glia, precursors of astrocytes, were dominant. However, their filaments vanished in the first few postnatal days. Instead of VIM-IR glial filaments, GFAP-immunoreactive (GFAP-IR) astrocytes appeared at E20 and grew rapidly from the P3 stage. GFAP immunoreactivity in the ventrolateral portion of the SCN (VLSCN) was measured using a computer-assisted image analyzing system. In normal rats, GFAP immunoreactivity showed a stepwise pattern with two slopes at P3-P4 and P20-P25. Bilaterally eye-enucleated rats operated on the day of birth showed lower GFAP immunoreactivity than normal rats and the GFAP immunoreactivity did not increase between P20 and P25 when GFAP-IR glial processes rapidly expand. Electron microscopic investigation at P50 (adult stage) revealed that neurons in the VLSCN had often direct apposition without astroglial processes and the frequency of this finding was significantly higher in eye-enucleated rats than in the control rats. These findings strongly suggest that the postnatal development of astroglial elements, particularly the expansion of GFAP-IR processes in the SCN, is regulated by retinohypothalamic projection.

Effect of middle cerebral artery occlusion on the passage of pituitary adenylate cyclase activating polypeptide across the blood-brain barrier in the rat

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to be a potent neuroprotective agent in global and focal ischemia. We demonstrated that PACAP could cross the blood-brain barrier (BBB) by a saturable transport system, and a systemic administration of PACAP reduced the infarct induced by unilateral middle cerebral artery occlusion (MCAO). Therefore, we studied whether this transport system is affected by MCAO in the rat. The entry of PACAP38 into the brain was compared in five groups: control, 4, 6, 24, and 48 h after MCAO. [(125)I]PACAP38 was injected intravenously and serum and various brain regions were collected 3 min later. The rate of entry into the brain of PACAP38 was also determined. We showed that PACAP entered the rat brain via a rapid transport system when the BBB is intact. After transient (2 h) unilateral MCAO, all regions of the brain, showed a selective increase in the passage of PACAP38 across the BBB after 4 h after the occlusion, which was not related to any generalized change in the permeability of the BBB, as measured with albumin. A significant decrease in the amount of PACAP38 entering the brain was observed in the 6- and 24-h groups, but it returned to the baseline level in the 48-h group. These results suggest that focal cerebral ischemia can selectively modify the passage of PACAP38 across the BBB, in both damaged and undamaged sides of the brain, and that these changes in influx are not solely due to the disruption of BBB. These findings imply the necessity of adjusting the dose of intravenously administered PACAP38 in order to maximize its therapeutic effect on the brain damage resulting from focal ischemia

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuron-derived peptide regulating glial glutamate transport and metabolism

In the brain, glutamatergic neurotransmission is terminated predominantly by the rapid uptake of synaptically released glutamate into astrocytes through the Na(+)-dependent glutamate transporters GLT-1 and GLAST and its subsequent conversion into glutamine by the enzyme glutamine synthetase (GS). To date, several factors have been identified that rapidly alter glial glutamate uptake by post-translational modification of glutamate transporters. The only condition known to affect the expression of glial glutamate transporters and GS is the coculturing of glia with neurons. We now demonstrate that neurons regulate glial glutamate turnover via pituitary adenylate cyclase-activating polypeptide (PACAP). In the cerebral cortex PACAP is synthesized by neurons and acts on the subpopulation of astroglia involved in glutamate turnover. Exposure of astroglia to PACAP increased the maximal velocity of [(3)H]glutamate uptake by promoting the expression of GLT-1, GLAST, and GS. Moreover, the stimulatory effects of neuron-conditioned medium on glial glutamate transporter expression were attenuated in the presence of PACAP-inactivating antibodies or the PACAP receptor antagonist PACAP 6-38. In contrast to PACAP, vasoactive intestinal peptide promoted glutamate transporter expression only at distinctly higher concentrations, suggesting that PACAP exerts its effects on glial glutamate turnover via PAC1 receptors. Although PAC1 receptor-dependent activation of protein kinase A (PKA) was sufficient to promote the expression of GLAST, the activation of both PKA and protein kinase C (PKC) was required to promote GLT-1 expression optimally. Given the existence of various PAC1 receptor isoforms that activate PKA and PKC to different levels, these findings point to a complex mechanism by which PACAP regulates glial glutamate transport and metabolism. Disturbances of these regulatory mechanisms could represent a major cause for glutamate-associated neurological and psychiatric disorders.

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.

Functional and molecular diversity of PACAP/VIP receptors in cortical neurons and type I astrocytes

In the present study we determined the mRNA-expression of pituitary adenylate cyclase activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) receptors in primary cultures of rat cortical neurons and type I astrocytes, and investigated the effects of PACAP38 on adenylyl cyclase, inositol phospholipid hydrolysis and intracellular calcium homeostasis. PACAP38 elicited a concentration-dependent (1 nM-100 nM) increase in inositol phosphate levels and [Ca2+]i in neurons but not in type I astrocytes. The PACAP-induced increase of intracellular calcium concentration, [Ca2+]i, was characterized by a spike, compatible with inositol trisphosphate (IP3) -induced calcium mobilization from intracellular stores, and a plateau phase, sustained by activation of capacitative calcium entry triggered by depletion of IP3-sensitive calcium stores. In the absence of extracellular calcium, only the spike phase was present while the plateau phase was clearly reduced. In addition, thapsigargin pretreatment abolished the PACAP38-induced [Ca2+]i rise. Treatment with 1 microM VIP did not affect [Ca2+]i in either neurons or type I astrocytes, clearly indicating the coupling of PAC1-HOP subtype to phospholipase-C in neurons. In addition, as previously reported, PACAP38 stimulated cAMP formation in both neurons and type I astrocytes. Using the reverse transcription polymerase chain reaction, we found mRNA-expression of PAC1 (PACAP - HOP variant) and VPAC2 in neurons, PAC1 (PACAP - R variant), VPAC1 and VPAC2 in astrocytes. These data indicate both a functional and molecular diversity of PACAP and VIP receptors in these cell types and support the view that the PAC1-HOP variant may be responsible for phospholipase-C activation and [Ca2+]i elevation in cortical neurons.

Pituitary adenylate cyclase-activating polypeptide modulates gastric enterochromaffin-like cell proliferation in rats

BACKGROUND & AIMS

Gastric carcinoids (types I and II) involve the transformation of naive enterochromaffin-like (ECL) cells to the neoplastic state and are associated primarily with hypergastrinemia. In this study, we evaluated the effects of two related neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP), on ECL cell proliferation and characterized the receptor subtype(s) and signal transduction pathways that mediate this effect.

METHODS

Purified rat ECL cells were analyzed in culture for DNA synthesis as measured by 24-hour 5-bromo-2-deoxyuridine (BrdU) uptake. Reverse-transcription polymerase chain reaction (RT-PCR) with gene-specific oligonucleotide primers was performed to characterize the PACAP/VIP receptor subtype(s).

RESULTS

PACAP/VIP neuropeptide-stimulated BrdU uptake was significantly greater (3.4-3.8-fold greater than control) than that at the maximal dose of gastrin (2.2-fold greater than control). PACAP-stimulated ECL cell proliferation (EC50, approximately 3 x 10(-)14 mol/L) was approximately 100-fold more potent than VIP (EC50, approximately 3x 10(-)12 mol/L). The stimulated BrdU uptake by both PACAP and VIP was competitively inhibited by PACAP-receptor antagonist (IC50, 10(-)9 mol/L, 3 x 10(-)9 mol/L, respectively) and VIP-receptor antagonist (IC50, 3 x 10(-)7 mol/L, 5 x 10(-)7 mol/L, respectively). RT-PCR identified the presence of the PACAP-specific but not PACAP/VIP receptor subtypes. The PACAP-stimulated BrdU uptake was inhibited (70%-80%) by inhibitors of adenosine 3',5'-cyclic monophosphate, phosphatidylinositol 3 kinase, and protein tyrosine kinase as well as mitogen-activated protein kinase.

CONCLUSIONS

PACAP/VIP-related peptides are more potent modulators of ECL cell proliferation than gastrin, and their effect is mediated by a PACAP-specific receptor whose activation is transduced by multiple intracellular messenger systems.

Distribution of pituitary adenylate cyclase activating polypeptide mRNA in the developing rat brain

PACAP is a member of the secretin/vasoactive intestinal peptide (VIP) family, isolated from hypothalamus. Recent studies have shown that PACAP is expressed in many parts of adult brain. We have studied the precise distribution of PACAP mRNA in developing rat brain, employing in situ hybridisation. PACAP mRNA is expressed in distinct parts of the embryonic rat brain from embryonic day 13, with a robust expression in developing cortex, hippocampus, amygdala and hypothalamus as well as in spinal cord and dorsal root ganglia. The expression in hippocampus and cortex diminishes towards adulthood, compared to new-born rat brain. In the mature brain, PACAP mRNA is located in alternating layers of cerebral cortex (layers I, III and V), in the dentate gyrus, in CA4 and CA1 regions, but not in CA2 or CA3 of the hippocampus. The presence of PACAP mRNA in different structures of developing rat brain suggests an important function for this peptide during brain development.

Neurotrophic and neuroprotective effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on mesencephalic dopaminergic neurons

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is present in many regions of the adult and developing brain as are receptors for PACAP. PACAP stimulates different signalling cascades in neurons, involving cAMP, MAP kinase, and calcium. These characteristics suggest that PACAP may influence neuronal development. Here we have studied the effects of PACAP on mesencephalic dopaminergic neurons using primary cultures from embryonic rats. PACAP increased the number of tyrosine hydroxylase (TH)-immunoreactive neurons, elevated TH protein, and enhanced tritiated dopamine uptake in these cultures. Moreover, PACAP counteracted the effects of 6-hydroxydopamine treatments, which induce cell death of dopaminergic neurons. In situ hybridisation showed that both PACAP and PACAP receptor type 1 are present in developing and adult rat mesencephalon. These results show that PACAP has a neurotrophic action on dopaminergic neurons and partially protects them against 6-OHDA induced neurotoxicity.

Expression of PACAP, and PACAP type 1 (PAC1) receptor mRNA during development of the mouse embryo

Pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) have been reported to have a number of neurotrophic effects. We have examined the expression of mRNA for PACAP and PACAP type 1 (PAC1) receptor in the mouse embryo by in situ hybridization and the effects of PACAP and VIP on the growth of mouse embryos in vitro. Although we were unable to detect gross effects of either peptide on the growth rates of embryos maintained in culture, mRNAs for both PAC1 receptor and PACAP peptide were present in the nervous system from day 9.5 of embryonic development. PAC1 receptor mRNA was most abundant in the neural tube and the rhombencephalon and was present also in the dorsal root and trigeminal ganglia and the sympathetic chain. The distribution of mRNA for the PACAP peptide overlapped in part with that of the receptor, but was more extensively distributed in the rhombencephalon and in the developing hypothalamus. Within the neural tube, PAC1 receptor mRNA was located in the roof and floor plates, while the distribution of PACAP peptide mRNA was more complex, being located in two columns of cells in the ventromedial neural tube (consistent with the position of developing autonomic motor neurons) and in cells in the dorsolateral neural tube. These data are concordant with a role for PACAP or a related peptide in neural development.

Pituitary adenylate cyclase activating polypeptide (PACAP) stimulates mitogen-activated protein kinase (MAPK) in cultured rat astrocytes

Astrocytes, a subtype of glial cells, have been demonstrated to have an abundant number of receptors for pituitary adenylate cyclase activating polypeptide (PACAP), a neuropeptide of the VIP/secretin family which stimulates cAMP accumulation 1000 times more potent than VIP in astrocytes. PACAP is reported to stimulate the proliferation of astrocytes at low concentrations at which it does not yet stimulate the cAMP accumulation. In the present study, we examined the effect of PACAP on the activation of mitogen-activated protein kinase (MAPK), one of the important intracellular signals for the proliferation, and compared it with that of epidermal growth factor (EGF). To investigate the activation of MAPK, we focused on ERK2, one of MAPK, in cultured rat astrocytes. The activation of ERK2 was determined by immunoblotting and measurement of the activity in terms of the phosphorylating activity of immunoprecipitates with MAPK antibody on myelin basic protein. One pM of PACAP38 temporarily activated ERK2 at 10 min. In contrast, EGF activated ERK2 from 10 min to 60 min continuously. As for the dose-response effect, PACAP stimulated ERK2 at as low a concentration as 10-14 M and peaked at 10-12 M. Thereafter, its activating effect gradually decreased at 10-10 M and returned to the basal level at 10-8 M, forming a bell-shaped dose-dependency. Neither an inhibitor of PKA (H89) nor inhibitors of PKC (staurosporine and calphostin C) had any effect on the ERK2 activation induced by 1 pM PACAP38. Dibutyryl cAMP suppressed ERK2 activity in a dose-dependent manner. These data clearly demonstrated that PACAP stimulates MAPK in both a PKA- and a PKC-independent manner in cultured rat astrocytes.