Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates cyclic AMP formation as well as peptide output of cultured pituitary melanotrophs and AtT-20 corticotrophs

PACAP: A regulator of mammalian reproductive function

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an ancestral molecule that was isolated from sheep hypothalamic extracts based on its action to stimulate cAMP production by pituitary cell cultures. PACAP is one of a number of ligands that coordinate with GnRH to control reproduction. While initially viewed as a hypothalamic releasing factor, PACAP and its receptors are widely distributed, and there is growing evidence that PACAP functions as a paracrine/autocrine regulator in the CNS, pituitary, gonads and placenta, among other tissues. This review will summarize current knowledge concerning the expression and function of PACAP in the hypothalamic-pituitary-gonadal axis with special emphasis on its role in pituitary function in the fetus and newborn.

The Role of Kiss1 Neurons As Integrators of Endocrine, Metabolic, and Environmental Factors in the Hypothalamic-Pituitary-Gonadal Axis

Kisspeptin-GPR54 signaling in the hypothalamus is required for reproduction and fertility in mammals. Kiss1 neurons are key regulators of gonadotropin-releasing hormone (GnRH) release and modulation of the hypothalamic-pituitary-gonadal (HPG) axis. Arcuate Kiss1 neurons project to GnRH nerve terminals in the median eminence, orchestrating the pulsatile secretion of luteinizing hormone (LH) through the intricate interaction between GnRH pulse frequency and the pituitary gonadotrophs. Arcuate Kiss1 neurons, also known as KNDy neurons in rodents and ruminants because of their co-expression of neurokinin B and dynorphin represent an ideal hub to receive afferent inputs from other brain regions in response to physiological and environmental changes, which can regulate the HPG axis. This review will focus on studies performed primarily in rodent and ruminant species to explore potential afferent inputs to Kiss1 neurons with emphasis on the arcuate region but also considering the rostral periventricular region of the third ventricle (RP3V). Specifically, we will discuss how these inputs can be modulated by hormonal, metabolic, and environmental factors to control gonadotropin secretion and fertility. We also summarize the methods and techniques that can be used to study functional inputs into Kiss1 neurons.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Cloning and characterization of a PAC1 receptor hop-1 splice variant in goldfish (Carassius auratus)

In several vertebrates, it has been demonstrated that alternative splicing of PAC1 receptor (PAC1-R) transcripts can generate a number of functional receptor variants which utilize different signal transduction pathways to mediate their activities. As PACAP is a physiological growth hormone-releasing factor in fish, and PACAP and the PAC1-R are highly conserved in vertebrate evolution, it would be of interest to investigate the structure and cellular distribution, particularly in the pituitary, of PAC1-R splice variants in a fish model. Our laboratory has previously cloned a receptor cDNA corresponding to the goldfish PAC1-R-s (goldfish PAC1-R-short). In the present study, a goldfish PAC1-R-hop1 variant was characterized. Functional expression of goldfish PAC1-R-s and PAC1-R-hop1 in Chinese Hamster Ovary cells revealed that, upon stimulation by ovine PACAP38, these receptor variants exhibited similar EC50 values (8.7+/-1.5 and 8.8+/-1.9 nM, respectively) and maximal responses in activating intracellular cAMP production. The presence and expression levels of these transcripts were measured by quantitative real-time PCR in the brain, heart, pituitary and male gonad, and goldfish PAC1-R-s were found to be the predominant form. In situ hybridization of goldfish PAC1-R in the pituitary revealed its prevalent presence in the pars distalis. In summary, the present study provides information to confirm the role of PACAP in the pituitary and to elucidate the pleiotropic effects of PACAP in fish.

Expression of novel neurotrophin-1/B-cell stimulating factor-3 (NNT-1/BSF-3) in murine pituitary folliculostellate TtT/GF cells: pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide-induced stimulation of NNT-1/BSF-3 is mediated by protein kinase A, protein kinase C, and extracellular-signal-regulated kinase1/2 pathways

Novel neurotrophin-1/B cell stimulating factor-3 (NNT-1/BSF-3) is a gp130 cytokine potently stimulating corticotroph proopiomelanocortin gene expression and ACTH secretion by a Janus kinase-signal transducer and activator of transcription (JAK-STAT)-dependent mechanism. In the current study, we examined the regulation of NNT-1/BSF-3 mRNA expression in murine pituitary folliculostellate TtT/GF cells using Northern blot technique. A 5- to 9-fold and a 4- to 7-fold induction in NNT-1/BSF-3 mRNA expression was observed between 2 and 6 h stimulation with the protein kinase C (PKC) stimulus phorbol-12-myristate-13-acetate (100 nm) and the protein kinase A (PKA) stimulus Bu(2)cAMP (5 mm), respectively. Pituitary adenylate cyclase-activating polypeptide (PACAP-38, 50 nm) and vasoactive intestinal peptide (VIP, 50 nm) also stimulated NNT-1/BSF-3 mRNA expression 5- to 9-fold between 2 and 6 h. Preincubation with PKC and PKA inhibitors such as H-7 (20 microm), GF109203X (50 microm), and H-89 (50 microm) decreased the stimulatory effects of PACAP and VIP. Both PACAP-38 and VIP also rapidly induced ERK1/2 phosphorylation and their stimulatory effect on NNT-1/BSF-3 mRNA expression was reduced by the MAPK kinase/ERK kinase (MEK) inhibitor U0126 (10 microm). Dexamethasone (10(-7) m) was a potent inhibitor of phorbol-12-myristate-13-acetate-induced NNT-1/BSF-3 expression. RT-PCR analysis demonstrated TtT/GF cells to express the short and the hop variant but not the hip variant of the PACAP-1 receptor (PAC1-R). In addition, TtT/GF cells express the VIP/PACAP-2 receptor (VPAC2-R). In summary, NNT-1/BSF-3 is expressed in pituitary folliculostellate TtT/GF cells and induced by PKC-, PKA-, and ERK1/2-dependent mechanisms. The novel gp130 cytokine NNT-1/BSF-3 derived from folliculostellate cells might act as a paracrine neuroimmunoendocrine modulator of pituitary corticotroph function.

Pituitary adenylate cyclase-activating polypeptide induces testicular testosterone synthesis through PGE(2) mediation in crested newt, Triturus carnifex

The aim of the present work was to study the possible role of adenylate cyclase-activating polypeptide (PACAP) 38 in the testicular intracellular mechanism regulating steroidogenesis of crested newt, Triturus carnifex. Gonads were incubated in vitro with PACAP 38 and prostaglandin (PG) E(2) alone or with inhibitors of cyclooxygenase (COX), adenylate cyclase (AC), and phospholipase C (PLC) for 30 min and 60 min. PGE(2), PGF(2 alpha), testosterone, and estradiol-17 beta were measured in the culture medium; aromatase (AR) activity and cAMP were assessed in the tissue. PACAP 38 increased PGE(2) (30 min and 60 min), estradiol-17 beta (60 min), cAMP (60 min), and AR (60 min) but decreased testosterone (60 min). PGE(2) increased estradiol-17 beta, cAMP, and AR and decreased testosterone at 30 and 60 min.PLC inhibitor counteracted the effects of PACAP 38, while AC inhibitor counteracted these effects except for PGE(2) increase. AC inhibitor counteracted the effects of PGE(2), while PLC did not. COX inhibitor decreased PGF(2 alpha) (30 min and 60 min), PGE(2) (30 min and 60 min), estradiol-17 beta (60 min), cAMP (60 min), and AR (60 min), but increased testosterone (60 min). These in vitro results suggest that, in newt testis, PACAP 38 acts on PLC, inducing the increase of PGE(2) which, in turn, acting on AC, increases AR activity with the consequent estradiol-17 beta increase and testosterone decrease.

C2-ceramide and reactive oxygen species inhibit pituitary adenylate cyclase activating polypeptide (PACAP)-induced cyclic-AMP-dependent signalling pathway

The pituitary adenylate cyclase activating polypeptide (PACAP) type I receptor, a seven-domain transmembrane receptor, is positively coupled to both adenylate cyclase and phospholipase C. PACAP exerts neurotrophic effects which are mainly mediated through the cAMP/protein kinase A pathway. Here we show that the cell-permeable C2-ceramide selectively blocks PACAP-activated cAMP production, without affecting phosphoinositide breakdown. Thus by blocking the neuroprotective cAMP signalling pathway, C2-ceramide will reinforce its direct death-inducing signalling. We found that a reactive oxygen species scavenger reversed the C2-ceramide effect and that H2O2 mimicked it. Together these data indicate that reactive oxygen species (ROS) mediates C2-ceramide-induced cAMP pathway uncoupling. This uncoupling did not involve ATP supply or Galphas protein function but rather adenylate cyclase function per se. Further, the tyrosine phosphatase inhibitors, but not the serine/threonine phosphatase inhibitors, prevent inhibition of cAMP production by ROS. This suggests that H2O2 requires a functional tyrosine phosphatase(s) to block PACAP-dependent cAMP production.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Novel action of pituitary adenylate cyclase-activating polypeptide. Stimulation of extracellular acidification in rat pituitary GH4C1 cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the vasoactive intestinal peptide/secretin family. Using microphysiometry, we have found that PACAP acutely (1 min) increased the extracellular acidification rate (ECAR) in GH4C1 cells approximately 40% above basal in a concentration-dependent manner. ECAR, maximally induced by PACAP, can be increased further by thyrotropin-releasing hormone (TRH), indicating that the signalling pathways for these two neuropeptides are not identical. In studies on the mechanism of PACAP-enhanced ECAR, we found that maximum stimulation of the cAMP/PKA pathway by treatment with FSK, or the PKC pathway with PMA, did not inhibit the ECAR response to PACAP. The PKC inhibitor calphostin C and the MAP kinase inhibitor PD98059 had no effect on the ECAR response to PACAP. Furthermore, PACAP induced little or no change in cytosolic Ca(2+) ([Ca(2+)](i)), while TRH induced a large increase in [Ca(2+)](i). However, the tyrosine kinase inhibitor genistein completely blocked PACAP-induced ECAR, suggesting involvement of tyrosine kinase(s). We conclude that PACAP causes an increase in ECAR in GH4C1 rat pituitary cells, which is not dependent on the PKA, PKC, MAP kinase or Ca(2+) signalling pathways, but does require tyrosine kinase activity.

Perspectives on pituitary adenylate cyclase activating polypeptide (PACAP) in the neuroendocrine, endocrine, and nervous systems

PACAP is a pleiotropic neuropeptide that belongs to the secretin/glucagon/VIP family. PACAP functions as a hypothalamic hormone, neurotransmitter, neuromodulator, vasodilator, and neurotrophic factor. Its structure has been remarkably conserved during evolution. The PACAP receptor is G protein-coupled with seven transmembrane domains and also belongs to the VIP receptor family. PACAP, but not VIP, binds to PAC1-R, whereas PACAP and VIP bind to VPAC1-R and VPAC2-R with a similar affinity. Despite the sizable homology of the structures of PACAP and VIP and their receptors, the distribution of these peptides and receptors is quite different. At least eight subtypes of PACAP specific, or PAC1-R, result from alternate splicing. Each subtype is coupled with specific signaling pathways, and its expression is tissue or cell specific. Although PACAP fulfills most requirements for a physiological hypothalamic hypophysiotropic hormone, it does not consistently stimulate secretion of the adenohypophysial hormones, except for stimulation of IL-6 release from the FS cells of the pituitary. The major regulatory role of PACAP in pituitary cells appears to be the regulation of gene expression of pituitary hormones and/or regulatory proteins that control growth and differentiation of the pituitary glandular cells. These effects appear to be exhibited directly and indirectly through a paracrine or autocrine action. Although PACAP stimulates the release of AVP, the physiological role of neurohypophysial PACAP remains unknown. One important action of PACAP in the endocrine system is its role as a potent secretagogue for adrenaline from the adrenal medulla through activation of TH. PACAP also stimulates the release of insulin and increases [Ca2+]i from pancreatic beta-cells at an extremely small concentration. The stage-specific expression of PACAP in testicular germ cells during spermatogenesis suggests its regulatory role in the maturation of germ cells. In the ovary, PACAP is transiently expressed in the granulosa cells of the preovulatory follicles and appears to be involved in the LH-induced cellular events in the ovary, including prevention of follicular apoptosis. In the central nervous system, PACAP acts as a neurotransmitter or neuromodulator, which has been supported by IHC and electrophysiological methods. More important, PACAP is a neurotrophic factor that may play an important role during the development of the brain. In the adult brain, PACAP appears to function as a neuroprotective factor that attenuates the neuronal damage resulting from various insults.

Anorectic effect of pituitary adenylate cyclase activating polypeptide (PACAP) in rats: lack of evidence for involvement of hypothalamic neuropeptide gene expression

We investigated the effect of centrally administered pituitary adenylate cyclase activating polypeptide (PACAP) on feeding in rats, and the involvement of hypothalamic neuropeptide gene expression using in situ hybridization. Intracerebroventricular injection of PACAP (1000 pmol/rat) significantly decreased food intake in a dose-dependent manner. In PACAP-treated rats, neuropeptide Y (NPY) mRNA levels in the arcuate nucleus and galanin mRNA levels in the paraventricular nucleus increased, and corticotropin-releasing hormone (CRH) mRNA levels in the paraventricular nucleus decreased. In rats fasted for 72 h, NPY mRNA levels increased, and CRH mRNA levels decreased, but galanin mRNA levels were unchanged. These results indicate that the anorectic function of PACAP is not mediated by NPY or CRH, and that PACAP increases galanin synthesis.

Role of VIP, PACAP, and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are members of a family of regulatory peptides that are widely distributed in the body and share numerous biologic actions. The two peptides display a remarkable amino acid-sequence homology, and bind to a class of G protein-coupled receptors, named PACAP/VIP receptors (PVRs), whose signaling mechanism mainly involves the activation of adenylate-cyclase and phospholipase-C cascades. A large body of evidence suggests that VIP and PACAP play a role in the control of the hypothalamo--pituitary-adrenal (HPA) axis, almost exclusively acting in a paracrine manner, since their blood concentration is very low. VIP and PACAP are contained in both nerve fibers and neurons of the hypothalamus, and VIP, but not PACAP, is also synthesized in the pituitary gland. Both peptides are expressed in the adrenal gland, and especially in medullary chromaffin cells. All the components of the HPA axis are provided with PVRs. VIP and PACAP enhance pituitary ACTH secretion, VIP by eliciting the hypothalamic release of CRH and potentiating its secretagogue action, and PACAP by directly stimulating pituitary corticotropes. Through this central mechanism, VIP and PACAP may increase mineralo- and glucocorticoid secretion of the adrenal cortex. VIP but not PACAP also exerts a weak direct secretagogue action on adrenocortical cells by activating both PVRs and probably a subtype of ACTH receptors. VIP and PACAP raise aldosterone production via a paracrine indirect mechanism involving the stimulation of medullary chromaffin cells to release catecholamines, which in turn enhance the secretion of zona glomerulosa cells via a beta-adrenoceptor-mediated mechanism. PACAP appears to be able to evoke a glucocorticoid response through the activation, at least in the rat, of the intramedullary CRH/ACTH system. The relevance of these effects of VIP and PACAP under basal conditions is questionable, although there are indications that endogenous VIP is involved in the maintenance of the normal growth and steroidogenic capacity of rat adrenal cortex. However, indirect evidence suggests that these peptides might play a relevant role under paraphysiological conditions (e.g., in the mediation of HPA axis responses to cold and inflammatory stresses) or may be somehow involved in the pathogenesis of Cushing disease or some case of hyperaldosteronism associated with secreting pheochromocytomas.

PACAP colocalizes with luteinizing and follicle-stimulating hormone immunoreactivities in the anterior lobe of the pituitary gland

Pituitary adenylate cyclase activating polypeptide (PACAP) and its close relative vasoactive intestinal polypeptide (VIP) were demonstrated in the anterior pituitary gland. The cells which exhibited PACAP immunoreactivity were oval or round shaped. Their distribution was similar to that of gonadotropes but the number of PACAP immunoreactive cells was less. Double labeling revealed that PACAP immunoreactivity partially colocalized with luteinizing and follicle-stimulating hormone; however, colocalization with other pituitary hormone immunoreactivities was not demonstrated. Our results suggest an autocrine or paracrine role of PACAP in the regulation of pituitary functions.

Pituitary adenylate cyclase-activating polypeptide potentiation of Ca2+ entry via protein kinase C and A pathways in melanotrophs of the pituitary pars intermedia of rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been reported to stimulate melanotroph secretion, and PACAP-like immunoreactivity and expression of PACAP type I receptor messenger RNA have been identified in the pituitary pars intermedia (PI). The present study showed that PACAP messenger RNA is also expressed in the PI. To examine the mechanism of PACAP action in the PI, cytosolic Ca2+ concentrations ([Ca2+]i) and ionic currents were measured in acutely dissociated rat melanotrophs. In about 40% of the melanotrophs studied, PACAP induced an increase in [Ca2+]i, which was suppressed by extracellular Ca2+ removal; extracellular Na+ replacement; the blocker of L-type Ca2+ channels, nicardipine; or the secreto-inhibitory neurotransmitter, dopamine. The PACAP-induced [Ca2+]i increase was mimicked by activators of protein kinase A (PKA) and protein kinase C (PKC), Sp-diastereomer of cAMP and 1-oleoyl-2-acetyl-sn-glycerol, and was reduced by inhibitors of PKA and PKC, Rp-diastereomer of cAMP and staurosporine. Patch-clamp analysis revealed that PACAP caused inward currents with a reversal potential of -0.8 mV and facilitated voltage-dependent Ba2+ currents. It further revealed that PACAP-induced inward currents were mimicked by 1-oleoyl-2-acetyl-sn-glycerol and inhibited by staurosporine, and that Sp-diastereomer of cAMP facilitated Ba2+ currents. These results suggest that PACAP potentiates Ca2+ entry mechanisms of rat melanotrophs by activation of nonselective cation channels via PKC and facilitation of voltage-dependent Ca2+ channels via PKA.

Differential signaling and immediate-early gene activation by four splice variants of the human pituitary adenylate cyclase-activating polypeptide receptor (hPACAP-R)

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide belonging to the VIP/secretin/glucagon family, is present in the hypothalamus, anterior pituitary, and adrenal gland where it regulates hormone release, in the GI tract where it modulates motility, and in human tumoral cell lines where it shows a growth-promoting effect. It is now appreciated that alternative splicing of two exons of the rat PACAP-R gene generate four major rPACAP-R splice variants that are differentially expressed in tissues and variably coupled to intracellular second messengers. Because of the potential implications of these findings in human physiology, we cloned the hPACAP-R gene. Similar to the rat, two exons (SV-1 and SV-2) are alternatively spliced to account for four major hPACAP-R receptor splice variants. These splice variants (hPACAP-R-null, hPACAP-R-SV1, hPACAP-R-SV2, hPACAP-R-SV-3) were cloned from a human frontal cortex cDNA library, stably transfected in NIH/ 3T3 cells and each characterized for ligand affinity, stimulation of adenylate cyclase (AC) and phospholipase C (PLC), and ligand-induced expression of the proto-oncogenes, c-fos, and c-myc. Stably transfected NIH/3T3 cells expressing similar numbers of receptors of the four splice variants showed nearly identical responses for ligand affinity and potency for P-38- and P-27-stimulated increases in cAMP and total inositol phosphates. However, each receptor splice variant differed in their ligand-stimulated efficacy for total inositol phosphate stimulation. The hPACAP-R-SV2 showed the greatest efficacy for stimulating phospholipase C that was approximately seven-fold greater than the hPACAP-R-SV1, twofold greater than the hPACAP-R-Null, and 1.5-fold greater than the hPACAP-R-SV-3 splice variants. To determine whether the splice variants also differ in their ability to stimulate immediate early gene expression, c-fos and c-myc transcripts were assayed by Northern blot and quantified by densitometry. PACAP-38 increased c-fos and c-myc expression for all four of the receptor splice variants that paralleled the efficacy for PLC stimulation, with the the SV-2 splice variant showing the greatest stimulation. These results show that the hPACAP-R-SV2 exhibits enhanced efficacy for coupling to both PLC and activation of the protooncogenes, c-fos and c-myc suggesting a novel and potentially important mechanism for differentially activating signal transduction pathways that influence cellular growth and differentiation.

New aspects of the neuroendocrine role of PACAP

The presence of PACAP was revealed in the anterior pituitary with RIA, HPLC, and with the demonstration of its mRNA. The level of PACAP mRNA in the anterior pituitary is the highest during the proestrous LH surge. In our immunohistochemical studies we were able to demonstrate PACAP immunoreactive cells in the anterior pituitary. The shape and the distribution of PACAP immunoreactive cells were very similar to that of the gonadotropes; however, the number of PACAP cells was less than that of LH cells. Additionally, another PACAP-positive cell population with small diameter appeared in the proestrous stage, during pregnancy and lactation. Double labeling revealed that the major part of large PACAP cells exhibited LH immunoreactivity and those with a small diameter contained PRL. It is not clear whether the pituitary- or the hypothalamic-born PACAP, or both, influence pituitary LH and PRL secretion. I.c.v. administration of PACAP just prior to the critical period in the proestrous stage inhibited the expected ovulation and blocked the proestrus LH and PRL surge, although i.v. administration of PACAP had no effect. PACAP antiserum did not interfere with ovulation when i.c.v. or i.v. injection was used. Our results support the view that PACAP has a role in the control of LH and PRL secretion during the estrous cycle, pregnancy, and lactation. The inhibitory effect of PACAP on ovulation is mediated through the hypothalamus.

Cloning and characterization of the signal transduction of four splice variants of the human pituitary adenylate cyclase activating polypeptide receptor. Evidence for dual coupling to adenylate cyclase and phospholipase C

Alternative splicing of two exons of the rat pituitary adenylate cyclase activating polypeptide (PACAP) receptor gene generates four major splice variants that are differentially expressed in specific tissues and variably coupled to intracellular second messengers. To evaluate the potential implications of these findings in human physiology, the human PACAP receptor gene was cloned. Alternative splicing about two exons of the gene allowed for four major splice variants that were subsequently identified on cDNA cloning. Each of the four splice variant cDNAs (null, SV-1, SV-2, and SV-3) was stably expressed in NIH/3T3 cells at similar receptor densities. For each splice variant, PACAP (both PACAP-38 and PACAP-27) had similar affinity and potency for stimulating either adenylate cyclase or phospholipase C. However, each receptor splice variant differed in their ligand-stimulated maximal response (efficacy) for total inositol phosphate accumulation with the SV-2 showing the greatest efficacy, followed by the null, SV-1, and SV-3 splice variants. Therefore, unlike the rat, PACAP binds and stimulates signal transduction with nearly equal affinity and potency for each of the receptor splice variants although with varying efficacy for the stimulation of phospholipase C. These results suggest a novel and potentially important mechanism for a single hormone to not only couple to dual signal transduction cascades but also elicit tissue-specific differential activation of phospholipase C in humans.

Vascular effects and cyclic AMP production produced by VIP, PHM, PHV, PACAP-27, PACAP-38, and NPY on rabbit ovarian artery

The relationship between vessel tone and cAMP production induced by vasoactive intestinal polypeptide (VIP), peptide histidine methionine (PHM), peptide histidine valine (PHV), pituitary adenylate cyclase activating polypeptide (PACAP-27 and PACAP-38), and neuropeptide Y (NPY) was investigated in rabbit ovarian arteries in vitro. VIP, PHM, PHV, PACAP-27, and PACAP-38 added in single-dose experiments (10(-9), 10(-8), 10(-7), and 10(-6) M) induced all a significant dose-related relaxation of noradrenaline (NA)-precontracted vessels and displayed similar potencies. VIP, PHM, PHV, PACAP-27, and PACAP-38 all increased cyclic adenosine monophosphate (cAMP) accumulation. The cAMP accumulation induced by PACAP-27 and PACAP-38 was five times higher than the cAMP content induced by the other three peptides. The peptide-induced smooth muscle relaxation did not correlate to the cAMP accumulation. NPY (10(-7) M) markedly reversed the relaxations induced by VIP, PHM, PHV, PACAP-27, and PACAP-38, but did not influence the cAMP production induced by these peptides. In conclusion, the relaxation induced by VIP, PHM, PHV, PACAP-27, and PACAP-38 and the contraction induced by NPY are not solely related to the changes of cAMP contents. These findings indicate that in addition to cAMP, another intracellular signal transduction pathway may be involved in the relaxation and contraction induced by these peptides in rabbit ovarian artery.

Differential alternative splicing of PACAP receptor in pituitary cell subpopulations

The capability of rat pituitary cells to express receptors for pituitary adenylate cyclase activating polypeptide (PACAP) and VIP was evaluated by binding studies and measurement of adenylate cyclase activity on whole gland preparations and by reverse transcriptase-polymerase chain reaction (TR-PCR) using specific primers on preparations from isolated cell populations enriched in PRL- and GH-producing cells. Data obtained on whole gland preparations indicated that selective PACAP receptors (PACAP Type I) predominated. The mRNA coding for PACAP Type I and for the non-selective PACAP receptors Type II VIP2 (but not VIP1) were identified. The mRNA coding for four different spliced variants of the PACAP Type I receptor were detected. In PRL producing cells, three variants and the VIP2 mRNA were detected, whereas in GH-producing cells the mRNA coding for the variant having a 28-amino acid insert (termed HOP) in the third intracellular loop was the only present.

Multifactorial regulation of pituitary adenylate cyclase-activating polypeptide (PACAP)-induced production of cyclic AMP in ATT-20 corticotrophs: major involvement of Rolipram-sensitive and insensitive phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) appear to play a major role in the modulation of cellular accumulations of cAMP/cGMP and hence the magnitude of the cell response to a hormone signal. These enzymes are present in cells as multiple isoforms and lie under control of various protein kinases. Because PACAP, unlike corticotropin-releasing factor (CRF), may stimulate a dual signalling pathway in pituitary cells (activating both adenylyl cyclase and phospholipase C), we used AtT-20 corticotrophs and primary cultures of rat pituitary cells to study the effect and possible differential influence of these peptides on cAMP formation. Time-course analysis indicated that, both in the absence and the presence of Rolipram (a selective type IV PDE inhibitor), PACAP stimulated a rapid and short-lived accumulation of cAMP in tumor corticotrophs, while in the presence of the non-selective inhibitor IBMX, the peptide produced a sustained high plateau level of second messenger (10 times the level generated with Rolipram at 20 min). On the contrary, when exposed to CRF, cAMP production augmented in parallel, irrespective of whether Rolipram or IBMX were present. The differential effects of the PDE inhibitors were seen with PACAP concentrations ranging from 0.1 to 100 nM, and could also be demonstrated in primary cultures of pituitary cells. Co-incubation of AtT-20 cells with Rolipram along with inhibitors of type I (but not of type III) PDEs, enhanced cAMP formation elicited by PACAP to a level significantly higher than that induced by CRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Anorectic and neurochemical effects of pituitary adenylate cyclase activating polypeptide in rats

Pretreatment of rats with intrahypothalamic injections of pituitary adenylate cyclase activating peptide (PACAP) 10 min prior to the injection of neuropeptide Y (NPY) significantly reduced food and water intake during the 4-h measurement period. Intrahypothalamic injection of PACAP in schedule-fed rats also reduced food and water intake for 2 h. A smaller 1-h reduction of water intake was observed in water-deprived rats, suggesting that the anticonsummatory effects of PACAP were primarily against food intake. PACAP treatment did not alter hypothalamic concentration of NPY, nor were neurotransmitters, precursors, or metabolites altered substantially in corpus striatum or nucleus accumbens regions. These results demonstrate primary anorectic effects of intrahypothalamic injection of PACAP. The demonstration of these anorectic effects may suggest a role of cyclic AMP activation and inhibition in the control of satiety and hunger.

Immunohistochemical localization of pituitary adenylate cyclase-activating polypeptide (PACAP) in the adrenal medulla of the rat

Localization of PACAP in rat adrenal glands was examined by light and electron microscopic immunohistochemistry using a specific antiserum to PACAP 38, R0831. In the light microscopic study, PACAP immunoreactivity was observed in some cell groups in the medulla, but not in the cortex. In comparison with adjacent sections stained with antisera to catecholamine synthesizing enzymes, PACAP-positive cells were immunoreactive to tyrosine hydroxylase and dopamine beta-hydroxylase, but not to phenylethanolamine-N-methyltransferase, suggesting that they were coincident with noradrenaline secreting cells. In the electron microscopic study using the ABC method, DAB reaction products were diffusely distributed in the cytoplasmic matrix of PACAP-positive cells, without intense accumulation on the secretory granules. The splanchnic nerve terminals were PACAP negative. In postembedding immunohistochemistry, gold particles were localized diffusely in the cytoplasma, but not aggregated on the secretory granules. It was suggested that PACAP would localize in the cytoplasmic matrix of noradrenaline cells and stimulate the catecholamine synthesis and release in the adrenal medulla.

Localization of pituitary adenylate cyclase-activating polypeptide (PACAP) in the hypothalamus-pituitary system in rats: light and electron microscopic immunocytochemical studies

The localization of pituitary adenylate cyclase-activating polypeptide (PACAP) in the hypothalamus-pituitary system in rats was examined in light and electron microscopic immunocytochemistry using a specific antiserum to synthetic PACAP 1-38 (R0831). In light microscopic study, intensely PACAP-immunostained perikarya were observed in the supraoptic and paraventricular magnocellular nucleus in the hypothalamus. In the median eminence, many immunoreactive nerve fibers were observed in the internal layer, but a few immunoreactive terminals were noticed in the external layer. In the pituitary gland, numerous immunoreactive nerve fibers were observed in the posterior lobe. In the intermediate lobe, moderately immunostained cells were observed, but in the anterior lobe no immunostained cells were noticed. In electron microscopic study, PACAP-immunoreactivity was examined by avidin-biotin peroxidase complex method. In the perikarya of the supraoptic and paraventricular magnocellular nucleus, DAB-reaction products were distributed diffusely in the cytoplasmic matrix, frequently attaching to the rough-surfaced endoplasmic reticulum. In the nerve terminals of the posterior lobe, reaction products were observed among the secretory granules, but sometimes upon them. In the cells of the intermediate lobe, reaction products were also distributed in the cytoplasmic matrix.

Inhibition of protein phosphatases by okadaic acid and calyculin-A differentially modulates hormonal- and forskolin-stimulated formation of cyclic AMP in AtT-20 corticotrophs: effect of pituitary adenylate activating polypeptide and corticotropin-releasing factor

The effect of phosphatase inhibitors okadaic acid and calyculin-A on cAMP formation and adrenocorticotropic hormone (ACTH) secretion in AtT-20 corticotrophs was investigated. Both okadaic acid and calyculin-A inhibited dose-dependently the accumulation of cAMP in cells stimulated with pituitary adenylate cyclase activating factor (PACAP) and corticotropin-relating hormone (CRF). While in the case of okadaic acid the half-maximum inhibiting concentration was similar for both peptides (IC50 = 4 x 10(-7) M), it appeared that calyculin-A was about one order of magnitude more efficient in inhibiting the effect of PACAP than that of CRF (IC50 = 3.8 x 10(-9) M vs 2.0 x 10(-8) M, respectively). Importantly, the inhibitors blocked the activation by cholera toxin (which acts on Gs-like proteins) of cAMP formation, but failed to alter the effect of forskolin (which bypasses the receptor-G protein complex and activates adenylyl cyclase directly). Treatment of cells with calyculin-A significantly dampened adenylyl cyclase activity in cell membrane fraction, though to a lesser extent than it blocked cAMP formation in the whole cell. Both okadaic acid and calyculin-A inhibited CRF- and PACAP-induced secretion of ACTH. Our data hint that in AtT-20 corticotrophs, inhibition of phosphatases by modulating the state of phosphorylation of the receptor-G proteins complexes for CRF and PACAP, regulates cAMP formation and ACTH secretion.

Type I receptors for PACAP (a neuropeptide even more important than VIP?)

Among vertebrates, there is an extreme conservation in amino acid sequence for the neuropeptide PACAP-38 and its C-terminal shortened derivative PACAP-27. The PACAP gene is assigned to chromosome 18 in man and its organization has been characterized. PACAP-38 and its minor derivative PACAP-27 are widely distributed in the central nervous system. PACAP-38 is particularly abundant in hypothalamus. The mapping of the afferentation and efferentation of PACAP systems are progressively delineated, including a search for the colocalization with other neurotransmitters. In several peripheral organs positive neuronal perikarya and fibers are also seen. PACAP acts through two types of receptors: (1) the highly selective type I that displays a 500 to 2000 selectivity for PACAP-38 and PACAP-27 as compared to VIP; (2) type II is the so-called VIP receptor showing similar high affinity for PACAP-38, PACAP-27 and VIP. It is less selective, therefore, than previously thought. This is why this second receptor, qualifying as an unspecific VIP-PACAP receptor, is hardly considered here. Type I receptors can stimulate two enzymes: the adenylate cyclase and phospholipase C (whose activation leads to the inositol phosphate-cytosolic Ca2+ cascade). This dual coupling may have several distal consequences including on gene expression, cell growth and differentiation. Although a relatively comprehensive spectrum of pharmacological activities has already been established we still need to limit the physiological roles of PACAP as neurotransmitter and/or neuromodulator. Concerning the hypothalamo-pituitary axis, PACAP reduces food intake in mice and raises plasma arginine vasopressin in rat, probably through PACAP-ir neurons in paraventricular and supraoptic nuclei projecting to the neurohypophysis. PACAP originating in the hypothalamus may also be transported to the anterior pituitary through portal vessels. Data on the antehypophysis suggest a role on i.a. reproduction and growth. PACAP stimulates adenylate cyclase and increases [Ca2+] in gonadotropes, somatotropes, and folliculo-stellate cells. It elevates the secretion of alpha-MSH from melanotropes, and that of interleukin-6 from pituitary folliculo-stellate cells. PACAP potentiates the effects of LHRH on LH and FSH secretion. More clearly perhaps, PACAP increases the synthesis of LH, GH, PRL and ACTH after 1-2 days. In human pathology, PACAP-27 and PACAP-38 stimulate adenylate cyclase activity in membranes from 'null'-, gonadotropin-, GH-, and ACTH-producing pituitary adenomas but are inactive in prolactinomas.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential signal transduction by five splice variants of the PACAP receptor

The two forms of pituitary adenylyl cyclase-activating polypeptide (PACAP-27 and -38) are neuropeptides of the secretin/glucagon/vasoactive intestinal polypeptide/growth-hormone-releasing hormone family and regulate hormone release from the pituitary and adrenal gland. They may also be involved in spermatogenesis, and PACAP-38 potently stimulates neuritogenesis and survival of cultured rat sympathetic neuroblast and promotes neurite outgrowth of PC-12 cells. The PACAP type-I receptor (found in hypothalamus, brain stem, pituitary, adrenal gland and testes), specific for PACAP, is positively coupled to adenylyl cyclase and phospholipase C. The recently cloned type II receptor does not discriminate between PACAP and vasoactive intestinal polypeptide and is coupled to only adenylyl cyclase. Here we have used a new expression cloning strategy, based on the induction of a reporter gene by cyclic AMP, to isolate a complementary DNA encoding the type-I PACAP receptor. On transfection of this cDNA, both PACAP-27 and -38 stimulate adenylyl cyclase with similar EC50 values (50% effective concentration, 0.1-0.4 nM), whereas only PACAP-38 stimulates phospholipase C with high potency (EC50 = 15 nM). Four other splice variants were isolated with insertions at the C-terminal end of the third intracellular loop. Expression of these cDNAs revealed altered patterns of adenylyl cyclase and phospholipase C stimulation, suggesting a novel mechanism for fine tuning of signal transduction.

Pituitary adenylate cyclase activating peptide and vasoactive intestinal polypeptide: differentiation effects on human neuroblastoma NB-OK-1 cells

Pituitary adenylate cyclase activating peptide-38 (PACAP-38), PACAP-27 and vasoactive intestinal polypeptide (VIP) increased intracellular cAMP content in human neuroblastoma NB-OK-1 cells transiently. PACAP and VIP also arrested cell growth and induced morphological differentiation, which lasted for 24 h in spite of removal of PACAP-38 and PACAP-27. The order of potencies for the neurite outgrowth and the arrest of cell growth is PACAP-38 > PACAP-27 > VIP. The results suggest the possibility that these neuropeptides are new candidates for differentiation activity.

Molecular cloning and functional expression of the pituitary adenylate cyclase-activating polypeptide type I receptor

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide belonging to the vasoactive intestinal peptide (VIP)/secretion/glucagon family of peptides, interacts with a distinct high-affinity receptor (type I receptor) on a number of tissues. These PACAP type I receptors have a high affinity for PACAP and a low affinity for VIP and are present in the hypothalamus and anterior pituitary, where they regulate the release of adrenocorticotropin, luteinizing hormone, growth hormone, and prolactin, and in the adrenal medulla, where they regulate the release of epinephrine. Type I PACAP receptors are also present in high concentrations in testicular germ cells, where they may regulate spermatogenesis, and some transformed cell lines, such as the rat pancreatic acinar carcinoma cell AR4-2J. Here we report the molecular cloning and functional expression of the PACAP type I receptor isolated from an AR4-2J cell cDNA library by cross-hybridization screening with a rat VIP receptor cDNA. The cDNA sequence encodes a unique 495-amino acid protein with seven transmembrane domains characteristic of guanine nucleotide-binding regulatory protein-coupled receptors. A high degree of sequence homology with the VIP, secretin, glucagon-like peptide 1, parathyroid, and calcitonin receptors suggests its membership in this subfamily of Gs-coupled receptors. Results of binding studies and stimulation of cellular cAMP accumulation in COS-7 cells transfected with this cDNA are characteristic of a PACAP type I receptor. Cloning of the PACAP type I receptor will enhance our understanding of its distribution, structure, and functional properties and ultimately increase our understanding of its physiological role.

Neuroanatomical and physiological evidence for the involvement of pituitary adenylate cyclase-activating polypeptide in the regulation of the distal lobe of the frog pituitary

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 38 amino-acid peptide which belongs to the glucagon/secretin/vasoactive intestinal peptide superfamily. The sequence of PACAP is identical in all mammalian species studied so far but frog PACAP differs by one amino-acid from mammalian PACAP. The aim of the present study was to investigate the presence of PACAP in the hypothalamo-pituitary complex of the frog Rana ribibunda and to determine the biological activity of frog PACAP on homologous pituitary cells. The distribution of PACAP-containing neurons and fibers was examined by the indirect immunofluorescence method using an antiserum raised against the N-terminal region of the peptide. In the hypothalamus, PACAP-immunoreactive perikarya were localized in the preoptic nucleus and the dorsal and ventral infundibular nuclei. Beaded nerve fibers were observed coursing from the ventral infundibular nucleus to the external vascular layer of the median eminence. A dense network of immunoreactive axons terminated in the vicinity of the capillaries of the hypophysial portal system. The neurointermediate lobe and the distal lobe of the pituitary were devoid of immunoreactive elements. The amount of PACAP-like immunoreactive material in hypothalamus extracts was measured by radioimmunoassay; the apparent concentration of PACAP was 4.5 ng/mg protein. Synthetic frog PACAP38 and PACAP27 induced a similar dose-dependent stimulation of cAMP production in isolated frog distal lobe pituitary fragments (ED50 = 2 x 10(-8) M). At the maximum dose tested (5 x 10(-6) M), both frog PACAP38 and PACAP27 produced a 4-fold increase in cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin, unlike phorbol ester, fails to synergistically interact with pituitary adenylate cyclase activating polypeptide (PACAP) in stimulating cyclic AMP formation and ACTH secretion in cultured anterior pituitary cells

In an attempt to determine if PACAP synergistically interacts with vasopressin (VP) and protein kinase C (PKC) to enhance cyclic AMP formation and adrenocorticotrophic hormone (ACTH) secretion, the effects of PACAP, either alone or together with VP and the phorbol ester phorbol 12-myristate 13-acetate (PMA) were examined in primary cultures of rat anterior pituitary cells. VP failed to potentiate the stimulatory effect of PACAP on cyclic AMP formation, while it dramatically enhanced the effect of corticotropin-releasing factor (CRF). However, activation of PKC upon exposure of cells to PMA amplified cyclic AMP production induced by both peptides, though in the case of PACAP, contrary to that of CRF, potentiation was markedly dependent on the blockade of phosphodiesterase (PDE) activity, for it was undetectable in the absence of the inhibitor Rolipram. Depletion of PKC by long-term treatment of pituitary cells with PMA abolished the synergistic influence of PMA. There was no significant effect of PACAP, either alone or together with PMA, on ACTH secretion, while PMA enhanced peptide secretion elicited by CRF. The data show that in anterior pituitary cells cyclic AMP accumulation induced by PACAP and CRF was differentially modulated by PKC and PDE activities and that the potentiation of PACAP-stimulated cyclic AMP accumulation by PMA was not reflected by parallel increment of ACTH secretion.

Immunohistochemical distribution and biological activity of pituitary adenylate cyclase-activating polypeptide (PACAP) in the central nervous system of the frog Rana ridibunda

The primary structure of frog pituitary adenylate cyclase-activating polypeptide (PACAP) has recently been determined and the results show that the sequence of PACAP has been highly conserved during evolution. In particular, the structure of the 1-27 fragment of PACAP is identical in frog and mammals. Using an antiserum raised against PACAP27, we have investigated the distribution of PACAP-containing neurons in the central nervous system of the frog Rana ridibunda by the immunofluorescence technique. The main populations of immunoreactive perikarya were located in the medial and ventral diencephalon, i.e., the preoptic nucleus, the ventral and dorsal infundibular nuclei, the nucleus posterocentralis thalami, and the ventral and ventrolateral areas of the thalamus. In the telencephalon, sparse cell bodies were found in the nucleus accumbens septi, the amygdala, the pallial commissure, and the bed nucleus of the pallial commissure. In the hindbrain, the torus semicircularis, the nucleus profundus and the nucleus anteroventralis tegmenti of the mesencephalon also contained populations of PACAP-immunoreactive perikarya. Beaded nerve fibers were observed throughout the brain. Occasionally they formed bundles, e.g., from the ventral infundibulum to the external vascular layer of the median eminence, from the central thalamus to the optic tectum, and rostrocaudally, from the nucleus accumbens septi to the nucleus entopeduncularis. Other areas, such as the interpeduncular nucleus, the nucleus isthmi and the roots of cranial nerves V and VIII in the medulla oblongata, were also densely innervated. The adenylate cyclase-stimulating activity of PACAP was tested by using a static incubation technique for hypothalamic slices.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary adenylate cyclase polypeptide (PACAP) stimulates cyclic AMP formation in pituitary fibroblasts and 3T3 tumor fibroblasts: lack of enhancement by protein kinase C activation

A number of neuropeptides were shown to produce potent mitogenic effects on Swiss 3T3 fibroblasts by activating the phospholipase C pathway. Here we provide evidence for the activation by PACAP of the adenylate cyclase pathway in 3T3, as well as in non-tumoral pituitary fibroblasts, similarly to what was seen in pituitary endocrine cells. In these cells, PACAP triggered elevation of both intracellular and extracellular contents of cAMP and the effect was time- and dose-dependent, with half-maximal stimulations being induced with about 0.1 nM. Following activation of protein kinase C (PKC) by the phorbol ester phorbol 12-myristate 13-acetate (PMA), PACAP-induced cAMP production was amplified in pituitary endocrine cells, but was either unchanged or dampened in 3T3 and pituitary fibroblasts, respectively. Pretreatment of cells with pertussis toxin (PT) failed to change the effect of PMA on PACAP-stimulated adenylate cyclase activity, irrespective of the cell type being used. However, PT dramatically reduced the potentiation by PMA of cAMP production enhanced by forskolin in 3T3 cells. These results provide new evidence pointing to the presence in fibroblasts of receptors for PACAP, coupled to cAMP production, which may play a role in the modulation of the mitogenic signal. They also indicate that, compared with pituitary endocrine cells, PKC activation in fibroblasts differentially affected PACAP-induced cAMP formation and that these effects were unaltered upon inhibition by PT of Gi-like proteins.