Pituitary adenylate cyclase-activating polypeptide increases [Ca2]i in rat gonadotrophs through an inositol trisphosphate-dependent mechanism

Ion Channels of Pituitary Gonadotrophs and Their Roles in Signaling and Secretion

Gonadotrophs are basophilic cells of the anterior pituitary gland specialized to secrete gonadotropins in response to elevation in intracellular calcium concentration. These cells fire action potentials (APs) spontaneously, coupled with voltage-gated calcium influx of insufficient amplitude to trigger gonadotropin release. The spontaneous excitability of gonadotrophs reflects the expression of voltage-gated sodium, calcium, potassium, non-selective cation-conducting, and chloride channels at their plasma membrane (PM). These cells also express the hyperpolarization-activated and cyclic nucleotide-gated cation channels at the PM, as well as GABA_A, nicotinic, and purinergic P2X channels gated by γ-aminobutyric acid (GABA), acetylcholine (ACh), and ATP, respectively. Activation of these channels leads to initiation or amplification of the pacemaking activity, facilitation of calcium influx, and activation of the exocytic pathway. Gonadotrophs also express calcium-conducting channels at the endoplasmic reticulum membranes gated by inositol trisphosphate and intracellular calcium. These channels are activated potently by hypothalamic gonadotropin-releasing hormone (GnRH) and less potently by several paracrine calcium-mobilizing agonists, including pituitary adenylate cyclase-activating peptides, endothelins, ACh, vasopressin, and oxytocin. Activation of these channels causes oscillatory calcium release and a rapid gonadotropin release, accompanied with a shift from tonic firing of single APs to periodic bursting type of electrical activity, which accounts for a sustained calcium signaling and gonadotropin secretion. This review summarizes our current understanding of ion channels as signaling molecules in gonadotrophs, the role of GnRH and paracrine agonists in their gating, and the cross talk among channels.

Intrinsic and Regulated Gonadotropin-Releasing Hormone Receptor Gene Transcription in Mammalian Pituitary Gonadotrophs

The hypothalamic decapeptide gonadotropin-releasing hormone (GnRH), acting via its receptors (GnRHRs) expressed in pituitary gonadotrophs, represents a critical molecule in control of reproductive functions in all vertebrate species. GnRH-activated receptors regulate synthesis of gonadotropins in a frequency-dependent manner. The number of GnRHRs on the plasma membrane determines the responsiveness of gonadotrophs to GnRH and varies in relation to age, sex, and physiological status. This is achieved by a complex control that operates at transcriptional, translational, and posttranslational levels. This review aims to overview the mechanisms of GnRHR gene (Gnrhr) transcription in mammalian gonadotrophs. In general, Gnrhr exhibits basal and regulated transcription activities. Basal Gnrhr transcription appears to be an intrinsic property of native and immortalized gonadotrophs that secures the presence of a sufficient number GnRHRs to preserve their functionality independently of the status of regulated transcription. On the other hand, regulated transcription modulates GnRHR expression during development, reproductive cycle, and aging. GnRH is crucial for regulated Gnrhr transcription in native gonadotrophs but is ineffective in immortalized gonadotrophs. In rat and mouse, both basal and GnRH-induced Gnrhr transcription rely primarily on the protein kinase C signaling pathway, with subsequent activation of mitogen-activated protein kinases. Continuous GnRH application, after a transient stimulation, shuts off regulated but not basal transcription, suggesting that different branches of this signaling pathway control transcription. Pituitary adenylate cyclase-activating polypeptide, but not activins, contributes to the regulated transcription utilizing the protein kinase A signaling pathway, whereas a mechanisms by which steroid hormones modulate Gnrhr transcription has not been well characterized.

Role of pituitary adenylate cyclase-activating polypeptide in modulating hypothalamus-pituitary neuroendocrine functions in mouse cell models

Pituitary adenylate cyclase-activating polypeptide (PACAP) was originally identified as a hypothalamic activator of cyclic adenosine monophosphate production in pituitary cells. PACAP and its receptor are expressed not only in the central nervous system, but also in peripheral organs, and function to stimulate pituitary hormone synthesis and secretion as both a hypothalamic-pituitary-releasing factor and an autocrine-paracrine factor within the pituitary. PACAP stimulates the expression of the gonadotrophin α, luteinising hormone (LH) β and follicle-stimulating hormone (FSH) β subunits, as well as the gonadotrophin-releasing hormone (GnRH) receptor and its own PACAP type I receptor (PAC1R) in gonadotrophin-secreting pituitary cells. In turn, GnRH, which is known to be a crucial component of gonadotrophin secretion, stimulates the expression of PACAP and PAC1R in gonadotrophs. In addition, PAC1R and PACAP modulate the functions of GnRH-producing neurones in the hypothalamus. This review summarises the current understanding of the possible roles of PACAP and PAC1R in modulating hypothalamus and pituitary neuroendocrine cells in the mouse models.

PACAP modulates GnRH signaling in gonadotropes

Hypothalamic gonadotropin-releasing hormone is known to be critical for normal gonadotropin biosynthesis and secretion by the gonadotrope cells of the anterior pituitary gland. Additional regulation is provided by gonadal steroid feedback as well as by intrapituitary factors, such as activin and follistatin. Less well-appreciated is the role of pituitary adenylate-cyclase activating polypeptide (PACAP) as both a hypothalamic-pituitary releasing factor as well as an autocrine-paracrine factor within the pituitary. PACAP regulates gonadotropin expression alone and through modulation of GnRH responsiveness achieved by increases in GnRH receptor expression and interactions at the level of intracellular signaling pathways. In addition to direct effects on the gonadotrope, PACAP stimulates follistatin secretion by the folliculostellate cells and thereby contributes to differential expression of the gonadotropin subunits. Conversely, GnRH augments the ability of PACAP to regulate gonadotrope function by increasing pituitary PACAP and PACAP receptor expression. This review will summarize the current understanding of the mechanisms by which PACAP modulates gonadotrope function, with a focus on interactions with GnRH.

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.

Regulation of spinal dynorphin 1-17 release by endogenous pituitary adenylyl cyclase-activating polypeptide in the male rat: relevance of excitation via disinhibition

Opioids inhibit release of primary afferent transmitters but it is unclear whether the converse occurs. To test the hypothesis that primary afferent transmitters influence opioid-ergic tone, we studied the functional and anatomical relationships between pituitary adenylyl cyclase-activating polypeptide (PACAP) and dynorphin 1-17 (Dyn) in spinal cord. We found that activation of the PACAP-specific receptor PAC(1) (PAC(1)R) inhibited, whereas PAC(1)R blockade augmented, spinal release of Dyn. It is noteworthy that in the formalin-induced pain model PAC(1)R blockade (via PACAP6-38) also resulted in antinociception that was abolished by spinal κ-opioid receptor blockade. These findings indicate that Dyn release is tonically inhibited by PACAP and that blocking this inhibition, which increases the spinal release of Dyn, results in antinociception. Consistent with this conclusion, we found in the spinal dorsal horn that Dyn-immunoreactive neurons 1) expressed PAC(1)R and 2) were apposed by PACAP terminals. Present results, in combination with the previous demonstration that the release of spinal Dyn is tonically inhibited by opioid- and nociceptin/orphanin FQ-coupled pathways (J Pharmacol Exp Ther 298:1213-1220, 2001), indicate that spinal Dyn-ergic neurons integrate multiple inhibitory inputs, the interruption of any one of which (i.e., disinhibition) is sufficient to enhance spinal Dyn release and generate antinociception. Gaining a better understanding of the role of primary afferent neurotransmitters in negatively modulating the spinal release of Dyn and the physiological use of disinhibition to increase spinal Dyn activity could suggest novel clinically useful approaches for harnessing endogenous Dyn for pain control.

What is the role of PACAP in gonadotrope function?

Strong evidence in favor of a direct action of hypothalamic PACAP at the pituitary to modulate gonadotrope function has been acquired mainly by in vitro studies using cultured pituitary cells or gonadotrope cell lines. In particular, PACAP has been shown to cooperate with GnRH, the primary regulator of gonadotropes, to regulate/modulate gonadotropin subunit gene expression, gonadotropin release as well as gonadotrope responsiveness. These effects of PACAP appear to be due essentially to its high potent stimulatory action on the cAMP/protein kinase pathway. Ensuing mechanisms include signaling cross-talk and/or enhanced gene expression within gonadotropes. PACAP may also indirectly operate on these cells through paracrine mechanisms. While PACAP has long been viewed as a hypophysiotropic factor, a locally produced PACAP has also been described. Interestingly, both appear similarly up-regulated at proestrus of the reproductive cycle in female rats. Further in vivo investigation is now necessary to ascertain the physiological relevance of the observed pituitary PACAP effects and especially to evaluate the respective contribution of hypothalamic and pituitary PACAP in the dynamic control of gonadotrope function.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates the oxygen sensing type I (glomus) cells of rat carotid bodies via reduction of a background TASK-like K+current

Pituitary adenylate cyclase-activating polypeptide (PACAP)-deficient mice are prone to sudden neonatal death and have reduced respiratory response to hypoxia. Here we found that PACAP-38 elevated cytosolic [Ca(2+)] ([Ca(2+)](i)) in the oxygen sensing type I cells but not the glial-like type II (sustentacular) cells of the rat carotid body. This action of PACAP could not be mimicked by vasoactive intestinal peptide but was abolished by PACAP 6-38, implicating the involvement of PAC(1) receptors. H89, a protein kinase A (PKA) inhibitor attenuated the PACAP response. Simultaneous measurement of membrane potential and [Ca(2+)](i) showed that the PACAP-mediated [Ca(2+)](i) rise was accompanied by depolarization and action potential firing. Ni(2+), a blocker of voltage-gated Ca(2+) channels (VGCC) or the removal of extracellular Ca(2+) reversibly inhibited the PACAP-mediated [Ca(2+)](i) rise. In the presence of tetraethylammonium (TEA) and 4-aminopyridine (4-AP), PACAP reduced a background K(+) current. Anandamide, a blocker of TWIK-related acid-sensitive K(+) (TASK)-like K(+) channel, occluded the inhibitory action of PACAP on K(+) current. We conclude that PACAP, acting via the PAC(1) receptors coupled PKA pathway inhibits a TASK-like K(+) current and causes depolarization and VGCC activation. This stimulatory action of PACAP in carotid type I cells can partly account for the role of PACAP in respiratory disorders.

Dopamine D(2) receptor expression and regulation of gonadotropin alpha-subunit gene in clonal gonadotroph LbetaT2 cells

This study investigated the role of dopamine on the regulation of gonadotropin secretion at the gonadotroph cell line. We examined the function of the dopamine D(2) receptor in the regulation of pituitary gonadotropin gene expression using LbetaT2 cells, a mature, well differentiated clonal gonadotroph cell line. The presence of the dopamine D(2) receptor in the LbetaT2 cells was confirmed by both RT-PCR and Western blot. Gonadotropin releasing hormone (GnRH) stimulation resulted in gonadotropin LHbeta, FSHbeta and alpha-subunit promoter activation, and none were inhibited by quinpirol, a specific dopamine D(2) receptor agonist. Pituitary adenylate cyclase-activating polypeptide (PACAP) increased gonadotropin alpha-subunit promoter activity, but not LHbeta and FSHbeta promoter activity. The activity of PACAP was significantly inhibited in the presence of quinpirol. The protein kinase A inhibitor, H89, also inhibited PACAP-induced alpha-subunit gene expression. PACAP increased intracellular cAMP more than GnRH did in LbetaT2 cells, and the elevation of cAMP was strongly inhibited in the presence of various dopamine D(2) agonists. These results suggest that in pituitary gonadotrophs, the dopamine D(2) receptor is a negative regulator of gonadotropin alpha-subunit gene expression which is induced by cAMP-elevating factors in a cAMP-dependent pathway.

Pituitary Adenylate Cyclase Activating Polypeptide Messenger RNA in the Paraventricular Nucleus and Anterior Pituitary During the Rat Estrous Cycle1

The neuropeptide pituitary adenylate cyclase activating polypeptide (ADCYAP 1, or PACAP) has been demonstrated to enhance gonadotropin-releasing hormone (GnRH)-induced gonadotropin secretion and regulate gonadotropin subunit gene expression in cultures of anterior pituitary cells. In the present study, we used in situ hybridization and real-time polymerase chain reaction to examine the expression of Pacap mRNA within the paraventricular nucleus (PVN) and anterior pituitary throughout the estrous cycle of the rat. Levels of luteinizing hormone in serum and pituitary gonadotropin subunit mRNAs were evaluated and displayed cyclic fluctuations similar to those reported previously. Pacap mRNA expression in the PVN and pituitary varied significantly during the estrous cycle, with the greatest changes occurring on the day of proestrus. Pacap mRNA levels in the PVN declined significantly on the morning of diestrus. During proestrus, PVN Pacap mRNA levels significantly increased 3 h before the gonadotropin surge and then declined. Pituitary expression of Pacap mRNA also varied on the afternoon of proestrus with a moderate decline at the time of the gonadotropin surge and a significant increase later in the evening. Expression of the mRNA species encoding the 288 amino acid form of follistatin increased significantly following the rise in pituitary Pacap mRNA, at the termination of the secondary surge in follicle-stimulating hormone beta (Fshb) gene expression. These results suggest that PACAP is involved in events before and following the gonadotropin surge, perhaps through increased gonadotroph sensitivity to GnRH and suppression of Fshb subunit expression through increased follistatin, as previously observed in vitro.

Effect of PACAP on LH release studied by cell immunoblot assay depends on the gender, on the time of day and in female rats on the day of the estrous cycle

We have previously demonstrated that pituitary adenylate cyclase activating polypeptide (PACAP) can be released from cultured rat anterior pituitary cells and when added to the medium in physiological concentration it releases LH from individual gonadotropes. In the present work, we studied whether the release of PACAP and the responsiveness of LH cells to PACAP depend on the gender, on the time of day when the animals were sacrificed, and in females on the stage of the estrous cycle. Anterior pituitary cells were cultured on nitrocellulose membrane. We found that the number of PACAP releasing cells was higher in proestrous than in diestrous female or in male rats and their number was always higher in the evening than at the other times. The effect of PACAP on LH cells was stimulatory in the morning of proestrus and diestrus. In proestrous rats, PACAP did not influence LH release in the afternoon or the evening, but in diestrous rats it decreased it in the afternoon and the evening. In males, there was a decrease of LH due to PACAP treatment at 10 and 20 h; however, PACAP did not influence LH at 16 h. It was concluded that in vivo PACAP might be involved in the circadian and episodic release of LH at pituitary level.

Phosphatidylinositol 3'-OH kinase and protein kinase A pathways mediate the anti-apoptotic effect of pituitary adenylyl cyclase-activating polypeptide in cultured cerebellar granule neurons: modulation by ethanol

Cerebellar granule neurons cultured in the presence of 5 mm KCl undergo spontaneous apoptosis, which is reduced by exposure to pituitary adenylyl cyclase-activating polypeptide (PACAP). Previous work has suggested roles for the cyclic AMP/PKA and MAP kinase signaling pathways in the anti-apoptotic effect of PACAP. In the present study, the use of specific inhibitors confirmed the role of the cyclic AMP/PKA pathway, and also demonstrated a role for the phosphatidylinositol 3'-OH kinase (PI 3-kinase) neuroprotective pathway in the action of PACAP. Ethanol exposure accelerates the anti-apoptotic effect of PACAP by a mechanism that involves the PKA and PI-3 kinase pathways. The results demonstrate that ethanol can increase neuroprotection induced by PACAP. As previous work has shown that ethanol can increase apoptosis of cerebellar granule neurons by inhibiting the protective effect of agents such as NMDA or IGF-1, the overall effect of ethanol on cerebellar neuron apoptosis during development may reflect the balance between inhibition and enhancement of the actions of various endogenous neuroprotective agents.

Tilapia glycoprotein hormone alpha subunit: cDNA cloning and hypothalamic regulation

The cDNA encoding the glycoprotein alpha (GPalpha) subunit of tilapia (Oreochromis mossambicus) was partially cloned using RACE-polymerase chain reaction (PCR) technique. The amplified cDNA was found to be 583 bases long, and to consist of a portion of the signal peptide, the full sequence encoding the mature peptide (94 amino acids) and the 3' untranslated region. Northern blot analysis revealed a single band of approximately 600 bp. Alignment of the deduced amino acids of the mature protein showed that the tilapia GPalpha subunit shares more than 80% identity with that of other perciform fish (i.e. striped bass, sea bream and yellowfin porgy) and less than 70% with that of more taxonomically remote fish and other vertebrates. Exposure of dispersed tilapia pituitary cells to salmon gonadotropin-releasing hormone (sGnRH) elevated GPalpha mRNA levels via both PKC and cAMP-protein kinase A (PKA) pathways. The transcript levels were also regulated by pituitary adenylate cyclase activating polypeptide (PACAP) and neuropeptide Y (NPY), both acting through PKC and PKA pathways. Moreover, a combined treatment of PACAP or NPY with GnRH seems to have an additive effect on the GPalpha subunit gene transcription. These results suggest that in tilapia the expression of GPalpha subunit is regulated by GnRH mainly via PKC and PKA pathways. Furthermore, PACAP and NPY can elevate the GnRH-stimulated GPalpha subunit transcription and can directly affect the subunit mRNA levels, via the same transduction pathways.

The differential regulation of CART gene expression in a pituitary cell line and primary cell cultures of ovine pars tuberalis cells

The cocaine-amphetamine regulated transcript (CART) encodes for a protein which has an important role in the regulation of appetite and body weight. To date, no details of the molecular events and signal transduction pathways which regulate this gene are available. We report the identification of CART gene expression in the GH3 pituitary cell line. We have used activators of the cAMP or protein kinase C (PKC) signal transduction pathways to show that, in GH3 cells, CART is transcriptionally up-regulated by activators of the cAMP signal transduction pathway. We also identify CART gene expression in ovine pars tuberalis (PT) tissue and primary cell cultures. In PT cells in contrast to GH3 cells, CART gene expression is upregulated by activators of the PKC signal transduction pathway. Cultured cells have provided a valuable resource for the detailed analysis of specific regulatory mechanisms underlying transcriptional or translational regulation of genes, signal transduction events and many other cellular processes. GH3 and PT cells may therefore provide a resource for the further detailed molecular analysis of the events regulating CART gene expression and processing.

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.

Pituitary adenylate cyclase-activating polypeptide may function as a neuromodulator in guinea-pig adrenal medulla

The role of pituitary adenylate cyclase-activating polypeptide (PACAP) in catecholamine secretion from dissociated adrenal chromaffin cells of the guinea-pig was investigated using amperometry, the patch clamp technique and immunochemistry. Pretreatment of adrenal chromaffin cells with 0.3-10 nM PACAP for 2 min resulted in enhancement of nicotine- and muscarine-induced secretions in either the presence of external Ca2+ ions or nominally Ca2+-free solution, with no change in basal secretion or the holding current at -60 mV in most of the cells tested. Pretreatment with PACAP augmented the muscarine-induced non-selective cation current, but did not affect the muscarine-induced outward current or nicotine-induced current. PACAP-induced enhancement of nicotine- and muscarine-induced secretions was suppressed by the simultaneous application of PACAP and the protein kinase inhibitors 100 microM HA1004 or 2 microM H89. Application of forskolin enhanced both muscarine- and nicotine-induced secretions, whereas application of a phorbol ester augmented the nicotine-induced secretion, but suppressed the muscarine-induced secretion in a reversible manner. Immunohistochemical analysis of adrenal medullae revealed that PACAP-like immunoreactivity was present in nerve fibres surrounding putative chromaffin cells. PAC1R-like immunoreactivity was distributed diffusely in the plasma membrane, whereas nicotinic ACh receptor-like immunoreactivity was concentrated at the plasma membrane near the nucleus, where the synapses were mainly localized. These observations suggest that PACAP in the guinea-pig adrenal medulla functions as a neuromodulator to facilitate ACh-induced secretion through a cAMP-protein kinase A-dependent pathway.

Pituitary adenylyl cyclase-activating peptide activates multiple intracellular signaling pathways to regulate ion channels in PC12 cells

Pituitary adenylyl cyclase-activating peptide (PACAP) stimulates calcium transients and catecholamine secretion in adrenal chromaffin and PC12 cells. The PACAP type 1 receptor in these cells couples to both adenylyl cyclase and phospolipase C pathways, but although phospolipase C has been implicated in the response to PACAP, the role of adenylyl cyclase is unclear. In this study, we show that PACAP38 stimulates Ca(2+) influx in PC12 cells by activating a cation current that depends upon the dual activation of both the PLC and adenylyl cyclase signaling pathways but does not involve protein kinase C. In activating the current, PACAP38 has to overcome an inhibitory effect of Ras. Thus, in cells expressing a dominant negative form of Ras (PC12asn17-W7), PACAP38 induced larger, more rapidly activating currents. This effect of Ras could be overidden by intracellular guanosine-5'-O-3-(thio)triphosphate (GTPgammaS), suggesting that it was mediated by inhibition of downstream G proteins. Ras may also inhibit the current through a G protein-independent mechanism, because cAMP analogues activated the current in PC12asn17-W7 cells, provided GTPgammaS was present, but not in PC12 cells expressing wild type Ras. We conclude that coupling of PACAP to both adenylyl cyclase and phospholipase C is required to activate Ca(2+) influx in PC12 cells and that tonic inhibition by Ras delays and limits the response.

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.

Melatonin inhibits pituitary adenylyl cyclase-activating polypeptide-induced increase of cyclic AMP accumulation and [Ca2+]i in cultured cells of neonatal rat pituitary

The effects of melatonin on pituitary adenylyl cyclase-activating polypeptide-induced increase of cyclic AMP and [Ca2+]i were studied in neonatal rat pituitary cells. The polypeptide increased cyclic AMP accumulation. In the presence of melatonin the increase of cyclic AMP was inhibited in a dose-dependent manner, the maximal inhibition was achieved with 1-10 nM melatonin. Pituitary adenylyl cyclase-activating polypeptide also increased [Ca2+]i in 30% of the pituitary cells and melatonin inhibited the effect. Most of the cells sensitive to adenylyl cyclase-activating polypeptide (77%) were also sensitive to GnRH, suggesting they are gonadotrophs. The remaining cells were not identified. The polypeptide-induced [Ca2+]i increase was inhibited in Ca2+-free medium in 2/3 of the cells indicating that Ca2+ influx was involved. To examine causal relationship between cyclic AMP and [Ca2+]i increase, we have studied the effect of adenylyl cyclase activation by forskolin on intracellular Ca2+ concentration. Forskolin had similar effects as adenylyl cyclase-activating polypeptide: it increased [Ca2+]i in the pituitary cells and the increase was dependent on presence of Ca2+ in the medium. Melatonin inhibited the forskolin induced [Ca2+]i increase. Our observations indicate that increase of cyclic AMP stimulates Ca2+ influx in the pituitary cells of neonatal rat and that this mechanism is involved in [Ca2+]i increase induced by the pituitary adenylyl cyclase-activating polypeptide. Because melatonin inhibits increase of cyclic AMP induced by pituitary adenylyl cyclase-activating polypeptide or forskolin, the inhibitory effect of melatonin on Ca2+-influx may be mediated by the decrease of cyclic AMP concentration. This mechanism of melatonin action has not been described previously. Because melatonin inhibits the polypeptide- or forskolin-induced [Ca2+]i also in the cells not sensitive to GnRH, melatonin receptors seem to be present on both gonadotrophs and non-gonadotrophic pituitary cells.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

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.

Pituitary adenylate cyclase-activating polypeptides 38 and 27 increase cytosolic free Ca2+ concentration in porcine somatotropes through common and distinct mechanisms

Ca2+ plays an essential role in pituitary adenylate cyclase-activating polypeptide (PACAP)-stimulated growth hormone (GH) secretion from porcine somatotropes. Here, Indo-1 microfluorimetry was used to investigate the dynamics of free cytosolic Ca2+ concentration ([Ca2+]i) in single porcine somatotropes in response to PACAP38 and PACAP27. We also evaluated the relative contributions of extra- and intracellular Ca2+ sources and of cAMP-dependent protein kinase (PKA) and phospholipase C (PLC). A high proportion of somatotropes responded to PACAP38 (79.4%) and PACAP27 (68.4%) with [Ca2+]i rises that could be followed by a refractory plateau (type 1 response), or by a decrease in [Ca2+]i during which somatotropes were responsive to a subsequent PACAP pulse (type II response). Although Ca2+ profiles in response to both peptides were similar, PACAP38-induced [Ca2+]i rises were higher. Somatotrope response to PACAP38 or PACAP27 was markedly reduced by removing extracellular Ca2+, blocking Ca2+ entry through L-type voltage sensitive Ca2+ channels (VSCC), or inhibiting PKA. Conversely, Ca2+ depletion from intracellular stores or PLC inactivation did not affect the response to PACAP27 but considerably reduced maximal [Ca2+]i induced by PACAP38. We conclude that both peptides stimulate extracellular Ca2+ influx through L-type VSCC by a PKA-dependent mechanism. However, PACAP38 also triggers a PLC-mediated Ca2+ mobilization from intracellular stores, thereby indicating that the two molecular forms of PACAP activate common and distinct second messenger pathways within porcine somatotropes.

Modulatory action of PACAP27 on NMDA receptor channel activity in cultured chick cortical neurons

The modulatory effect of PACAP27 on NMDA receptor channel activity in cultured chick cortical neurons was investigated using the outside-out recording mode of the patch clamp technique. Channel opening frequency elicited by 20 microM NMDA, or 20 microM NMDA plus 1 microM glycine, was potentiated in the presence of 100 nM PACAP27 and inhibited with 1000 nM PACAP27. These effects were reversible on washout and reduced when glycine concentration was increased to 10 microM, but were not affected by the PACAP antagonist PACAP6-27 (1 microM) or the GTP inhibitor GDP-beta-S (100 microM). It is suggested that PACAP27 may exert its modulatory action on NMDA receptor channel activity through the glycine site(s).

PACAP increases the cytosolic Ca2+ concentration and stimulates somatodendritic vasopressin release in rat supraoptic neurons

Pituitary adenylate cyclase activating polypeptide (PACAP)-like immunoreactivity and its receptor mRNA have been reported in the supraoptic and the paraventricular nucleus (SON and PVN, respectively) and PACAP has been implicated in the regulation of magnocellular neurosecretory cell function. To examine the site and the mechanism of the action of PACAP in the neurosecretory cells, we measured AVP release from SON slice preparations and the cytosolic Ca2+ concentration ([Ca2+]i) from single dissociated SON neurons. PACAP at concentrations from 10(-12) to 10(-7) M increased [Ca2+]i in dissociated SON neurons in a dose-dependent manner. The patterns of the PACAP-induced [Ca2+]i increase were either sustained increase or cytosolic Ca2+ oscillations. PACAP (10[-7] M) increased [Ca2+]i in 27 of 27 neurons and glutamate (10[-4] M) increased [Ca2+]i in 19 of 19 SON neurons examined, whereas angiotensin II (10[-7] M) increased [Ca2+]i in only 15 of 60 SON neurons examined. PACAP at lower concentrations (10[-10] to 10[-8] M) increased [Ca2+]i in 70-80% of neurons examined. Although the onset and recovery of the PACAP-induced [Ca2+]i increase were slower than those observed with glutamate, the spatial distribution of the [Ca2+]i increases in response to the two ligands were similar: [Ca2+]i increase at the proximal dendrites was larger and faster and that at the center of the soma was smaller and slower. The PACAP-induced [Ca2+]i responses were abolished by extracellular Ca2+ removal, the L-type Ca2+-channel blocker, nicardipine, or by replacement of extracellular Na+ with N-methyl D-glucamine, and were partially inhibited by the Na+-channel blocker, tetrodotoxin. The N-type Ca2+-channel blocker, omega-conotoxin GVIA did not significantly inhibit the PACAP-induced [Ca2+]i responses. Furthermore, PACAP (10[-7] M) as well as glutamate (10[-4] M) increased AVP release from SON slice preparations, and extracellular Ca2+ removal or nicardipine inhibited the AVP release in response to PACAP. These results indicate that PACAP enhances Ca2+ entry via voltage-gated Ca2+ channels and increases [Ca2+]i, which, in turn, stimulates somatodendritic vasopressin release by directly activating PACAP receptors on SON neurons. The results also suggest that PACAP in the SON may play a pivotal role in the control of the neurohypophyseal function at the level of the soma or the dendrites.

Functional characterization and mRNA expression of pituitary adenylate cyclase activating polypeptide (PACAP) type I receptors in rat peritoneal macrophages

The present work characterizes the mRNA expression of PACAP type I receptors in rat peritoneal macrophages but not in peritoneal lymphocytes by both retrotranscriptase and polymerase chain reaction (RT-PCR) and homologous Southern hybridization and the stimulation by PACAP27, PACAP38 and vasoactive intestinal peptide (VIP) of sn-1,2-diacylglycerol production in rat peritoneal macrophage membranes. The binding of [125I]PACAP27 was time and cell concentration dependent. Scatchard analysis of displacement of the bound tracer by unlabeled PACAP27 indicates the existence of two classes of binding sites. The dissociation constant (Kd) was 0.64 +/- 0.08 nM and the maximal binding capacity (Bmax) was 8.85 +/- 1.45 fmol/10(6) cells for the high affinity binding site. The low affinity binding site had a Kd of 0.10 +/- 0.06 microM with a Bmax of 300 +/- 21.9 fmol/10(6) cells. Scatchard analysis of VIP displacement data indicated the presence of two classes of binding sites with a Kd and Bmax different to those of PACAP27. These results suggest that PACAP binds to two binding sites, PACAP type I receptors and PACAP type II receptors. The PACAP27-stimulated diacylglycerol production was not affected by treatment with pertussis toxin. However, the presence of GTP partially inhibited this PACAP27 stimulation of 1,2-diacylglycerol in a dose dependent manner, although GTP alone stimulates diacylglycerol accumulation. In conclusion, for the first time we demonstrate by biochemical and molecular biology criteria the existence of PACAP type I receptors on rat peritoneal macrophages and the evidence for coupling with a pertussis toxin-insensitive G regulatory protein.

Stimulation of Ca2+ influx in alpha T3-1 gonadotrophs via the cAMP/PKA signaling system

To investigate the regulation of free cytosolic calcium concentration ([Ca2+]i) by the adenosine 3',5'-cyclic monophosphate (cAMP) signaling system in clonal gonadotrophs, microfluorimetric recordings were made in single indo 1-loaded alpha T3-1 cells. Forskolin, 8-bromoadenosine 3',5'-cyclic monophosphate, or a low concentration (100 pM) of the hypothalamic factor pituitary adenylate cyclase-activating polypeptide (PACAP) stimulated Ca2+ step responses or repetitive Ca2+ transients, which were blocked by the removal of extracellular Ca2+ by the dihydropyridine (DHP) (+)PN 200-110 or by preincubation with the protein kinase A (PKA) antagonist H-89 (10 microM). Thus activation of the cAMP/PKA system in alpha T3-1 gonadotrophs stimulates Ca2+ influx through DHP-sensitive (L-type) Ca2+ channels. In contrast, high PACAP concentrations (100 nM) stimulated biphasic Ca2+ spike-plateau responses. The Ca2+ spike was independent of extracellular Ca2+, and similar responses were observed by microperfusion of individual cells with D-myo-inositol 1,4,5-trisphosphate, suggesting the involvement of the phospholipase C (PLC) signaling pathway. The Ca2+ plateau depended on Ca2+ influx, was blocked by (+)PN 200-110, but was only partially blocked by H-89 pretreatment. In conclusion, PACAP stimulates [Ca2+]i increases in alpha T3-1 gonadotrophs through both the PLC and adenylate cyclase signaling pathways. Furthermore, this is the first clear demonstration that the cAMP/PKA system can mediate changes in [Ca2+]i in gonadotroph-like cells.

PACAP38 modulates activity of NMDA receptors in cultured chick cortical neurons

The outside-out recording mode of the patch-clamp technique was used to study modulatory effects of pituitary adenylate cyclase-activating polypeptide (PACAP38) on N-methyl--aspartate (NMDA) receptor activity in cultured chick cortical neurons. Biphasic concentration-dependent effects of PACAP38 on channel opening frequency induced by NMDA (20 microM) and glycine (1 microM) were found, with low concentrations (0.5-2 nM) of PACAP38 increasing activity and higher concentrations (10-1,000 nM) causing inhibition. These effects were reversible, reduced with higher concentrations of glycine (2-10 microM) but not by 200 microM NMDA, and inhibited by 10 microM 7-chlorokynurenic acid. In addition, 1 microM PACAP6-38 (a PACAP antagonist) inhibited channel activity due to 20 microM NMDA and 1 microM glycine by 66%, and this inhibition was reduced to 13% in the additional presence of 2 nM PACAP38. These observations suggest that PACAP38 has a direct modulatory effect on the NMDA receptor that is independent of intracellular second messengers and probably mediated through the glycine coagonist site(s).

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.

Pituitary adenylate cyclase activating polypeptide stimulates rat Leydig cell steroidogenesis through a novel transduction pathway

The aim of the present study was to evaluate the effects of pituitary adenylate cyclase activating polypeptide (PACAP) on testosterone production in isolated adult rat Leydig cells and its possible mechanisms of action. PACAP-38 stimulated testosterone secretion in a dose-dependent manner with a minimal and a maximal efficacious dose of 1.0 nM and 100 nM, respectively. PACAP-27 was without effect on testosterone secretion at any dose tested. Similarly, vasoactive intestinal peptide did not stimulate steroidogenesis nor interfere with PACAP-38 activity, as well as preincubation of Leydig cells with the vasoactive intestinal peptide-antagonist [Lys(1), Pro(2,5), Arg(3,4), Tyr(6)]-vasoactive intestinal peptide. Removal of extracellular Ca2+ did not inhibit the stimulatory effects of PACAP-38 on Leydig cell testosterone production. Neither PACAP-38 nor PACAP-27 modified intracellular free Ca2+ and cAMP levels at any dose tested thus excluding a role for Ca2+ and cAMP in the stimulatory effects of PACAP. PACAP-38 was able to induce a plasma membrane depolarization that was dependent on an influx of Na+ from the extracellular medium as confirmed by the monitoring of intracellular Na+ with the Na+-sensitive fluorescent dye sodium benzofuran isophtalate. When Na+ was removed from the extracellular medium, PACAP-38 did not stimulate testosterone production, demonstrating that Na+ influx through the plasma membrane is strictly related to the stimulatory effects of this peptide. In addition, preincubation of Leydig cells in the presence of pertussis-toxin (500 ng/ml for 5 h) significantly reduced PACAP-38-stimulated effects both on plasma membrane depolarization and testosterone secretion. These results demonstrate that PACAP-38 stimulates testosterone secretion in isolated adult rat Leydig cells through the interaction with a novel PACAP receptor subtype coupled to a pertussis toxin sensitive G protein whose activation induces a Na+-dependent depolarization of the plasma membrane and testosterone production.

Pituitary adenylate cyclase-activating polypeptide regulates rat Leydig cell function in vitro

The present study was designed to evaluate the effects of both pituitary adenylate cyclase-activating polypeptide (PACAP)-27 and PACAP-38 on testosterone, cAMP and prostaglandin E2 (PGE2) production by purified rat Leyding cells. Because PACAP-38 shares homology with vasoactive intestinal peptide (VIP), the effects of VIP and both PACAP and VIP receptor antagonists on testicular steroidogenesis were also examined. PACAP-38 potentiated testosterone response to a low effective dose of human chorionic gonadotropin (hCG), while PACAP-27 was without effect. Furthermore, PACAP-38 amplified testosterone response to a wide concentration range of hCG until the submaximal dose. VIP evoked a dose-dependent increase of both basal and hCG-induced testosterone production. PACAP potentiation of steroidogenesis was nullified in the presence of a PACAP antagonist, but was not modified by a VIP antagonist. Moreover, while VIP antagonist blunted testosterone response to VIP, PACAP antagonist was without effect. Increasing concentrations of PACAP-38 evoked a dose-response enhancement of both cAMP and PGE2 production. However, this fatty acid is not involved in PACAP activity, as a prostaglandin blocker indomethacin did not modify the effect of PACAP on steroidogenesis. Taken together these findings: (i) demonstrate that PACAP-38 is able to activate both cAMP- and phosphatidylinositol-dependent mechanisms in Leydig cells; (ii) indicate that the peptide exerts an amplificatory action on testicular steroidogenesis stimulated by hCG and that this activity is receptor-mediated, as it is prevented by a PACAP receptor antagonist; (iii) predict the existence of specific PACAP receptors (type 1 binding sites) on Leydig cells.

Genomic organization of the rat pituitary adenylate cyclase-activating polypeptide receptor gene. Alternative splicing within the 5'-untranslated region

Pituitary adenylate cyclase-activating polypeptide (PACAP) elicits its diverse biological actions by interacting with both PACAP-selective type I PACAP receptors (PACAPRs) and type II PACAPRs that do not distinguish between PACAP and vasoactive intestinal polypeptide. Using long distance polymerase chain reaction, we amplified and characterized the entire coding region of the rat type I PACAPR (rPACAPR) gene, which spans 40 kilobases and contains 15 exons. Mapping of the exons and sequencing of all intron-exon boundaries revealed a structural organization of the rPACAPR gene that is very similar to those encoding other members of the calcitonin/secretin/parathyroid hormone receptor family. Southern blot analysis demonstrated a single copy of the rPACAPR gene. A combination of rapid amplification of cDNA ends and reverse transcriptase polymerase chain reaction revealed an unexpected diversity in the rPACAPR mRNA in the 5'-untranslated (5'-UTR) region. Four rPACAPR cDNAs were identified with 5'-UTR sequences that all diverged from the genomic sequence at a site 76 bp upstream of the ATG start codon, where a consensus 3' slice acceptor sequence was located. Sequence analysis of these amplified transcripts demonstrated that they arise by tissue-specific differential usage of four exons in the 5' noncoding region of the rPACAPR gene. This study is the first to elucidate the structural organization of a PACAPR gene and to demonstrate that alternative splicing generates rPACAPR transcripts with unique 5'-UTRs.

Activation of Na+ channels in GH3 cells and human pituitary adenoma cells by PACAP

The effects of pituitary adenylate cyclase activating polypeptide (PACAP) on ion channels were examined in GH3 cells human pituitary adenoma cells. In GH3 cells, PACAP-38 (10-9 M) reversibly activated tetrodotoxin-sensitive NA+ channels but had little effect on nicardipine-sensitive Ca2+ channels. PACAP-induced increase in Na+ currents was inhibited by PACAP (6-38), a specific PACAP receptor antagonist, and Rp-cAMPs, an inhibitor for protein kinase A, and mimicked by 8-bromo-cAMP. In human pituitary adenoma cells, PACAP also activated tetrodotoxin-sensitive Na+ channels and growth hormone secretion. These results suggest the possibility that PACAP can activate voltage-gated Na+ channels via adenylate cyclase-protein kinase A pathway in the pituitary.

PACAP/VIP receptor subtypes, signal transducers, and effectors in pituitary cells

Rat anterior pituitary tissue expresses mRNA for PVR1 and PVR3, as well as a low level of PVR2. The PVR1 appears to be highly expressed in gonadotroph-like cells, while somatotroph-like cells apparently express the PVR3. We have recently demonstrated the expression of mRNA for both PVR2 and PVR3 in corticotroph-like AtT20 cells (FIG.3). If normal corticotrophs express the same mRNA as AtT20 cells, this may partly explain the low levels of PVR2 seen in normal pituitary tissue. Significant levels of at least two PVR1 splice variants mRNAs (PVR1s and PVR1hop) were expressed in clonal gonadotroph-like alpha T3-1 cells and normal rat anterior pituitary tissue. However, these splice variants are reported to have almost identical pharmacological characteristics in terms of binding, and the activation of AC and PLC. Further experiments are necessary to determine the functional consequences of differential splice variant expression in such cells. Interestingly, all three pituitary-cell lines studied expressed mRNA for the PVR3 (FIG.3), whereas earlier binding studies demonstrate a predominance of PACAP-preferring binding sites on normal anterior pituitary-cell membranes. In addition, it is clear that the different PVR subtypes can couple to different intracellular messenger systems. Thus it will be important to determine the expression of the different PVR subtypes in normal anterior pituitary-cell types if we are to begin to understand the regulation of pituitary-cell regulation by PACAP. Such questions form the basis of some of the ongoing studies in our laboratory.

Molecular cloning of a novel variant of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor that stimulates calcium influx by activation of L-type calcium channels

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a novel neuropeptide that produces its biological effects by interacting with G protein-coupled receptors. Molecular cloning of the PACAP receptor revealed the existence of five splice variant receptor forms differing in the third intracellular loop region, with four variants activating both adenylyl cyclase and phosphoinositide phospholipase C and one variant activating only adenylyl cyclase (Spengler, D., Waeber, C., Pantaloni, C., Holsboer, F., Bockaert, J., Seeburg, P. H., and Journot, L. (1993) Nature 365, 170-175). Here, we report cloning of a novel PACAP receptor variant, designated PACAPR TM4 (transmembrane domain IV), that differs from the previously cloned short form of the PACAP receptor (PACAPR) primarily by discrete sequences located in transmembrane domains II and IV. Reverse transcriptase-polymerase chain reaction and primer extension analyses demonstrated tissue-specific differential expression of mRNAs encoding PACAPR TM4 and splice variant forms of the PACAP receptor. PACAPR TM4 and PACAPR possess identical intracellular domains, implicated as primary determinants of G protein recognition by rhodopsin-like receptors. However, unlike the PACAPR, PACAPR TM4 does not activate either adenylyl cyclase or phosphoinositide phospholipase C in response to PACAP in either transient or stable expression systems. However, PACAP stimulates increases in [Ca2+]i in cells expressing PACAPR TM4 by activating L-type Ca2+ channels, a response not elicited by stimulation with vasoactive intestinal polypeptide. The signaling phenotype of PACAPR TM4 is characteristic of the PACAP receptor involved in regulation of insulin secretion from pancreatic beta islets, a tissue expressing transcripts for PACAPR TM4 but not for PACAPR or its longer splice variant forms. These findings are consistent with a role of PACAPR TM4 in the physiological control of insulin release by PACAP in beta-islet cells. The finding that PACAPR TM4 has a unique signaling phenotype, although it possesses intracellular domains identical to those of the PACAPR, suggests that receptor-G protein recognition by rhodopsin-like receptors can be determined by sequences other than those located in intracellular receptor domains.

Pituitary adenylate cyclase-activating polypeptide (PACAP38) modulates lymphocyte and macrophage functions: stimulation of adherence and opposite effect on mobility

The effects of pituitary adenylate cyclase-activating polypeptide (PACAP38) in a concentration range from 10(-13) to 10(-6) M were studied, in vitro, on two functions of peritoneal rat lymphocytes and macrophages: adherence and mobility (spontaneous and chemotaxis). The results show that PACAP38 raised the adherence of the two cell types, increased the mobility of macrophages and decreased the mobility of lymphocytes. The maximal effects were observed at 10(-10) M in macrophages and at 10(-9) M in lymphocytes. Moreover, incubation with increasing concentrations of phorbol myristate acetate (PMA), a protein kinase C (PKC) activator, resulted in a progressive enhancement of adherence and chemotaxis of both macrophages and lymphocytes. In contrast, retinal, a PKC inhibitor, significantly decreased these capacities. Incubation of macrophages with both PMA and PACAP38 did not have a synergistic effect on chemotaxis and adherence whereas, with lymphocytes, adherence was increased and chemotaxis was partially decreased. On the other hand, incubation with forskolin (an enhancer of intracellular cyclic AMP [cAMP] levels) caused inhibition and stimulation of chemotaxis and adherence, respectively, in both cell types. PACAP38 prevented the inhibitory effect of forskolin on chemotaxis of macrophages but not of lymphocytes, whereas the simultaneous presence of PACAP38 and forskolin was synergistic for adherence of both peritoneal cells. In addition, PACAP38 was chemoattractant for macrophages but not for lymphocytes. Furthermore, a VIP receptor antagonist was able to partially reverse the modulatory effects of PACAP38 on lymphocytes, but not on macrophages. These data suggest that PACAP38 exerts its action through the binding to type I PACAP receptors and PKC activation in macrophages and through the elevation of intracellular cAMP levels by binding to type II PACAP receptors in lymphocytes. The present work reveals an additional link between neuropeptides and the immune system and suggests that the peptide PACAP modulates the immunological function of macrophages and lymphocytes.

Pituitary adenylate cyclase-activating polypeptide regulates [Ca(2+)](i) and electrical activity in pituitary cells through cell type-specific mechanisms

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a recently identified hypothalamic factor that acts on a variety of anterior pituitary cell types. It is clear, however, that its actions are not mediated by the same intracellular signaling mechanisms in each cell type. The signaling pathways by which PACAP regulates changes in [Ca(2+)], and electrical activity in rat somatotrophs and gonadotrophs is described in the present article. Finally, the possibility that the differences in PACAP-regulated signaling in anterior pituitary cells is due to the differential expression and coupling of PACAP receptor subtypes is discussed.

Functional heterogeneity of pituitary gonadotropes in response to a variety of neuromodulators

Agents previously implicated in control of the hypothalamo-pituitary-gonadal axis were screened for their ability to regulate male rat gonadotropes directly. GnRH-evoked gonadotropin release is accompanied by oscillations of intracellular Ca2+ concentration ([Ca2+]i) and of an outward K+ current that is activated by Ca2+. Substances that caused current responses similar to those with GnRH were hypothesized to evoke secretion. Endothelin-1, oxytocin, neurotensin, pituitary adenylate cyclase-activating polypeptide, and serotonin raised [Ca2+]i and evoked LH release as assayed by the reverse hemolytic plaque assay. These agents affected only subpopulations of gonadotropes establishing functional heterogeneity of pituitary gonadotropes. One neuromodulator (ATP) evoked ionic current in all gonadotropes but the current was different than that evoked by GnRH. Many other substances, including galanin and neuropeptide Y, caused no changes in currents and were considered not to affect [Ca2+]i and not to be secretagogues for gonadotropes.

Alternative splicing in the N-terminal extracellular domain of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor modulates receptor selectivity and relative potencies of PACAP-27 and PACAP-38 in phospholipase C activation

Pituitary adenylate cyclase-activating polypeptide (PACAP)-27 and PACAP-38 are neuropeptides of the vasoactive intestinal peptide/secretin/glucagon family. We previously described alternative splicing of the region encoding the third intracellular loop of the PACAP receptor generating six isoforms with differential signal transduction properties (Spengler, D., Waeber, C., Pantaloni, C., Holsboer, F., Bockaert, J., Seeburg, P. H., and Journot, L. (1993) Nature 365, 170-175). In addition, we demonstrated that the potencies of the two forms of PACAP are similar for adenylate cyclase stimulation, whereas PACAP-38 is more potent than PACAP-27 in phospholipase C activation. In the present work, we document the existence of a new splice variant of the PACAP receptor that was characterized by a 21-amino-acid deletion in the N-terminal extracellular domain. We demonstrate that this domain modulates receptor selectivity with respect to PACAP-27 and -38 binding and controls the relative potencies of the two agonists in phospholipase C stimulation.

Facilitatory effects of pituitary adenylate cyclase activating polypeptide (PACAP) on neurons in the magnocellular portion of the rat hypothalamic paraventricular nucleus (PVN) in vitro

To establish the role of pituitary adenylate cyclase activating polypeptide (PACAP), a member of vasoactive intestinal polypeptide (VIP) family, as a neurotransmitter/neuromodulator in the central nervous system, the effects of PACAP38, PACAP27 and VIP on the single neuron activity in the magnocellular portion of the hypothalamic paraventricular nucleus (mg.PVN) were examined in rat brain slice preparations. Extracellular recordings were made from 111 neurons in the mg.PVN, which fired spontaneously at an average rate of 1.85 +/- 0.2 spikes/s (mean +/- SEM). PACAP38 and PACAP27 were applied to 78 and 33 of the 111 neurons, respectively. Perfusion with PACAP38 in doses between 10 nM and 1 microM increased the firing rate of 56 (71.8%) of the 78 neurons in a dose-dependent manner. The threshold dose of PACAP38 to excite the neurons seemed to lie below 10 nM. The application of PACAP27 (1 microM) also increased the firing rate of 19 (57.6%) of the 33 neurons tested. Eleven (52.4%) of 21 neurons which were excited by PACAP38 also showed excitation following perfusion with VIP (1 microM). The responses to PACAP38 in 12 of 20 neurons and those to VIP in 6 of 9 neurons tested were still observed in a low Ca2+ and high Mg2+ medium. Although there was no difference in the mean latency between the responses to PACAP38 (1 microM) and VIP (1 microM) (2.1 +/- 0.1 min and 2.4 +/- 0.4 min, respectively), the duration of the PACAP38-induced excitation (59.0 +/- 5.0 min) was much longer than that of the VIP-induced one (18.8 +/- 3.1 min). The PACAP38 (30 nM)-induced excitation was reversibly blocked by a concurrent application of PACAP5-38 (300 nM), a PACAP receptor antagonist. While a selective VIP receptor antagonist, [Lys1, Pro2,5, Arg3,4, Tyr6]-VIP (1 microM), did not affect the excitatory responses to PACAP38 (300 nM), it completely blocked the VIP (1 microM)-induced excitation. These findings suggest that PACAP may therefore modulate the secretion of the pituitary hormones at least partly by its action on the neurons in the mg.PVN through the activation of specific receptors for PACAP.

Pituitary adenylate cyclase-activating polypeptide (PACAP-38) stimulates rat peritoneal macrophage functions

The present study shows that PACAP-38, in a dose-dependent manner, increased in vitro two steps of the phagocytic process in rat peritoneal macrophages: ingestion of inert particles (latex beads) and production of superoxide anion as measured by nitroblue tetrazolium reduction. The most effective concentration of PACAP-38 was 10(-10) M. Similarly, PMA, an activator of protein kinase C (PKC), increased the phagocytic activity in a dose-dependent manner, whereas retinal, a PKC inhibitor, decreased the activity. Macrophages incubated with forskolin, an enhancer of intracellular cAMP levels, produced an inhibitory effect on both phagocytic functions. The maximum stimulation of the phagocytic activity by PACAP-38 was not further enhanced by addition of PMA, suggesting that the enhancement of the phagocytic activity by PACAP-38 and PMA is mediated by a common signaling pathway. In addition, retinal as well as forskolin inhibited partially the stimulatory effect that PACAP-38 produced in the macrophage functions studied. Furthermore, this study showed that a VIP receptor antagonist was unable to suppress the stimulatory effect of PACAP-38. These results could prove that PACAP-38 stimulates the phagocytosis and production of superoxide anion in macrophages through PKC activation by binding to type I PACAP receptor.

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)

Possible involvement of vasoactive intestinal polypeptide in the central stimulating action of pituitary adenylate cyclase-activating polypeptide on prolactin secretion in the rat

Effect of pituitary adenylate cyclase-activating polypeptide (PACAP) on prolactin (PRL) secretion was studied in Wistar strain male rats in vivo. Intravenous injection of PACAP-38 and PACAP-27 resulted in a dose-related increase in plasma PRL levels both in conscious rats and in urethane-anesthetized rats. In anesthetized animals, i.c.v. injection of PACAP-38 and PACAP-27 also elicited a dose-related increase in plasma PRL. The PRL secretion induced by i.c.v. injection of PACAP-38 was attenuated by pretreatment with rabbit antiserum to vasoactive intestinal polypeptide (VIP). These findings suggest that PACAPs stimulate PRL secretion in the rat by acting not only directly on the pituitary but also indirectly through the central nervous system, at least partly, via hypothalamic VIP.

Pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal peptide stimulate two signaling pathways in CHO cells stably transfected with the selective type I PACAP receptor

The properties of the pituitary adenylate cyclase activating polypeptide (PACAP) type I receptor were studied on a clone of Chinese hamster ovary cells (CHO) stably transfected with the recombinant receptor. PACAP(1-27), PACAP(1-38) and VIP inhibited [125I-acetyl-His1]PACAP (1-27) binding, stimulated cyclic AMP and inositol phosphates production and induced [Ca2+]i increase with the same order of potency: PACAP(1-27) = PACAP(1-38) > VIP. The concentrations required for half maximal receptor occupancy, IP3- and [Ca2+]i increase were not different for both PACAPs (1 nM) and 100-fold higher than those required for cyclic AMP increase (0.010 nM). These data suggest that the occupancy of a portion of the total receptors available was sufficient for maximal cyclic AMP production but not for maximal IP3 production. It is concluded that the possibility of the type I PACAP receptor being coupled to a transduction pathway is not located at the level of the ligand but rather at the level of the G-proteins.