Cytochemical characterization of anterior pituitary target cells for the neuropeptide, pituitary adenylate cyclase activating polypeptide (PACAP), using biotinylated ligands

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.

Current State of Understanding of the Role of PACAP in the Hypothalamo-Hypophyseal Gonadotropin Functions of Mammals

PACAP was discovered 30 years ago in Dr. Akira Arimura's laboratory. In the past three decades since then, it has become evident that this peptide plays numerous crucial roles in mammalian organisms. The most important functions of PACAP are the following: 1. neurotransmitter, 2. neuromodulator, 3. hypophysiotropic hormone, 4. neuroprotector. This paper reviews the accumulated data regarding the distribution of PACAP and its receptors in the mammalian hypothalamus and pituitary gland, the role of PACAP in the gonadotropin hormone secretion of females and males. The review also summarizes the interaction between PACAP, GnRH, and sex steroids as well as hypothalamic peptides including kisspeptin. The possible role of PACAP in reproductive functions through the biological clock is also discussed. Finally, the significance of PACAP in the hypothalamo-hypophysial system is considered and the facts missing, that would help better understand the function of PACAP in this system, are also highlighted.

Role of pituitary adenylate cyclase-activating polypeptide in modulating hypothalamic-pituitary system

BACKGROUND

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a multifunctional peptide that is isolated and identified from the ovine hypothalamus, whose effects and mechanisms have been elucidated in numerous studies. The PACAP and its receptor are widely expressed, not only in the hypothalamus but also in peripheral organs.

METHODS

The studies on the role of PACAP in the hypothalamic-pituitary system, including those by the authors, were summarized.

RESULTS

In the pituitary gonadotrophs, PACAP increases the gonadotrophin α-, luteinizing hormoneβ-, and follicle-stimulating hormone β-subunit expression and the expression of gonadotropin-releasing hormone (GnRH) receptor and its own receptor, PAC1R. Moreover, a low-frequency GnRH pulse increases the expression of PACAP and PAC1R more than a high-frequency GnRH pulse in the gonadotrophs. The PACAP stimulates prolactin synthesis and secretion and increases PAC1R in the lactotrophs. In the hypothalamus, PACAP increases the expression of the GnRH receptors, although it is unable to increase the expression of GnRH in the GnRH-producing neurons.

CONCLUSION

The PACAP not only acts directly in each hormone-producing cell, it possibly might regulate hormone synthesis via the expression of its own receptors or those of other hormones.

Interactions between Two Different G Protein-Coupled Receptors in Reproductive Hormone-Producing Cells: The Role of PACAP and Its Receptor PAC1R

Gonadotropin-releasing hormone (GnRH) and gonadotropins are indispensable hormones for maintaining female reproductive functions. In a similar manner to other endocrine hormones, GnRH and gonadotropins are controlled by their principle regulators. Although it has been previously established that GnRH regulates the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—both gonadotropins—from pituitary gonadotrophs, it has recently become clear that hypothalamic GnRH is under the control of hypothalamic kisspeptin. Prolactin, which is also known as luteotropic hormone and is released from pituitary lactotrophs, stimulates milk production in mammals. Prolactin is also regulated by hypothalamic factors, and it is thought that prolactin synthesis and release are principally under inhibitory control by dopamine through the dopamine D2 receptor. In addition, although it remains unknown whether it is a physiological regulator, thyrotropin-releasing hormone (TRH) is a strong secretagogue for prolactin. Thus, GnRH, LH and FSH, and prolactin are mainly regulated by hypothalamic kisspeptin, GnRH, and TRH, respectively. However, the synthesis and release of these hormones is also modulated by other neuropeptides in the hypothalamus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic peptide that was first isolated from sheep hypothalamic extracts based on its ability to stimulate cAMP production in anterior pituitary cells. PACAP acts on GnRH neurons and pituitary gonadotrophs and lactotrophs, resulting in the modulation of their hormone producing/secreting functions. Furthermore, the presence of the PACAP type 1 receptor (PAC1R) has been demonstrated in these cells. We have examined how PACAP and PAC1R affect GnRH- and pituitary hormone-secreting cells and interact with their principle regulators. In this review, we describe our understanding of the role of PACAP and PAC1R in the regulation of GnRH neurons, gonadotrophs, and lactotrophs, which are regulated mainly by kisspeptin, GnRH, and TRH, respectively.

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) Regulates the Hypothalamo-Pituitary-Thyroid (HPT) Axis via Type 2 Deiodinase in Male Mice

The hypothalamic activation of thyroid hormones by type 2 deiodinase (D2), catalyzing the conversion of thyroxine to T3, is critical for the proper function of the hypothalamo-pituitary-thyroid (HPT) axis. Regulation of D2 expression in tanycytes alters the activity of the HPT axis. However, signals that regulate D2 expression in tanycytes are poorly understood. The pituitary adenylate cyclase-activating polypeptide (PACAP) increases intracellular cAMP level, a second messenger known to stimulate the DIO2 gene; however, its importance in tanycytes is not completely characterized. Therefore, we tested whether this ubiquitously expressed neuropeptide regulates the HPT axis through stimulation of D2 in tanycytes. PACAP increased the activity of human DIO2 promoter in luciferase reporter assay that was abolished by mutation of cAMP-response element. Furthermore, PAC1R receptor immunoreactivity was identified in hypothalamic tanycytes, suggesting that these D2-expressing cells could be regulated by PACAP. Intracerebroventricular PACAP administration resulted in increased D2 activity in the mediobasal hypothalamus, suppressed Trh expression in the hypothalamic paraventricular nucleus, and decreased Tshb expression in the pituitary demonstrating that PACAP affects the D2-mediated control of the HPT axis. To understand the role of endogenous PACAP in the regulation of HPT axis, the effect of decreased PACAP expression was studied in heterozygous Adcyap1 (PACAP) knockout mice. These animals were hypothyroid that may be the consequence of altered hypothalamic T3 degradation during set-point formation of the HPT axis. In conclusion, PACAP is an endogenous regulator of the HPT axis by affecting T3-mediated negative feedback via cAMP-induced D2 expression of tanycytes.

Regulatory System for Stem/Progenitor Cell Niches in the Adult Rodent Pituitary

The anterior lobe of the pituitary gland is a master endocrine tissue composed of five types of endocrine cells. Although the turnover rate of pituitary endocrine cells is as low as about 1.6% per day, recent studies have demonstrated that Sex-determining region Y-box 2 (SOX2)⁺-cells exist as pituitary stem/progenitor cells in the adult anterior lobe and contribute to cell regeneration. Notably, SOX2⁺-pituitary stem/progenitor cells form two types of niches in this tissue: the marginal cell layer (MCL-niche) and the dense cell clusters scattering in the parenchyma (parenchymal-niche). However, little is known about the mechanisms and factors for regulating the pituitary stem/progenitor cell niches, as well as the functional differences between the two types of niches. Elucidation of the regulatory mechanisms in the niches might enable us to understand the cell regeneration system that acts in accordance with physiological demands in the adult pituitary. In this review, so as to reveal the regulatory mechanisms of the two types of niche, we summarize the regulatory factors and their roles in the adult rodent pituitary niches by focusing on three components: soluble factors, cell surface proteins and extracellular matrixes.

cAMP signalling in the normal and tumorigenic pituitary gland

cAMP signalling plays a key role in the normal physiology of the pituitary gland, regulating cellular growth and proliferation, hormone production and release. Deregulation of the cAMP signalling pathway has been reported to be a common occurrence in pituitary tumorigenesis. Several mechanisms have been implicated including somatic mutations, gene-gene interactions and gene-environmental interactions. Somatic mutations in G-proteins and protein kinases directly alter cAMP signalling, while malfunctioning of other signalling pathways such as the Raf/MAPK/ERK, PI3K/Akt/mTOR and Wnt pathways which normally interact with the cAMP pathway may mediate indirect effects on cAMP and varying downstream effectors. The aryl hydrocarbon receptor signalling pathway has been implicated in pituitary tumorigenesis and we review its role in general and specifically in relation to cAMP de-regulation.

Dopamine-2 receptor activation suppresses PACAP expression in gonadotrophs

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed at a high level in the fetal pituitary and decreases profoundly between embryonic day 19 and postnatal day 1 (PN1), with a further decrease from PN1 to PN4. In this series of experiments, we investigated the hypothesis that dopamine 2 receptor (Drd2) activation interrupts a cAMP-dependent feed-forward loop that maintains PACAP expression at a high level in the fetal pituitary. Using single-cell RT-PCR of pituitary cell cultures from newborn rats, Drd2 mRNA was identified in gonadotrophs that were also positive for PACAP mRNA. PACAP expression in pituitary cultures from embryonic day 19 rats was suppressed by the PACAP6-38 antagonist and by the Drd2 agonist bromocriptine. Increasing concentrations of bromocriptine inhibited cAMP production as well as cAMP signaling based on cAMP response element-luciferase activity, decreased PACAP promoter activity, and decreased PACAP mRNA levels in αT3-1 gonadotroph cells. Furthermore, blockade of dopamine receptors by injecting haloperidol into newborn rat pups partially reversed the developmental decline in pituitary PACAP mRNA that occurs between PN1 and PN4. These results provide evidence that dopamine receptor signaling regulates PACAP expression under physiological conditions and lend support to the hypothesis that a rise in hypothalamic dopamine at birth abrogates cAMP signaling in fetal gonadotrophs to interrupt a feed-forward mechanism that maintains PACAP expression at a high level in the fetal pituitary. We propose that this perinatal decline in pituitary PACAP reduces pituitary follistatin which permits GnRH receptors and FSH-β to increase to facilitate activation of the neonatal gonad.

Advent and recent advances in research on the role of pituitary adenylate cyclase-activating polypeptide (PACAP) in the regulation of gonadotropic hormone secretion of female rats

PACAP (ADCYAP1) was isolated from ovine hypothalami. PACAP activates three distinct receptor types: G-protein coupled PAC1, VPAC1, and VPAC2 with seven transmembrane domains. Eight splice variants of PAC1 receptor are described. A part of the hypothalamic PACAP is released into the hypophyseal portal circulation. Both hypothalamic and pituitary PACAP are involved in the dynamic control of gonadotropic hormone secretion. In female rats, PACAP in the paraventricular nucleus is upregulated in the morning and pituitary PACAP is upregulated in the late evening of the proestrus stage of the reproductive cycle. PACAP mRNA peak in the hypothalamic PVN precedes the LHRH release into the portal circulation. It is supposed that PACAP peak is evoked by the elevated estrogen on proestrous morning. At the beginning of the so-called critical period of the same day, PACAP level starts to decline allowing LHRH release into the portal circulation, resulting in the LH surge that evokes ovulation. Just before the critical period, icv-administered exogenous PACAP blocks the LH surge and ovulation. The blocking effect of PACAP is mediated through CRF and endogenous opioids. The effect of the pituitary-born PACAP depends on the intracellular cross-talk between PACAP and LHRH.

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.

Vascular endothelial growth factor secretion from pituitary folliculostellate cells: role of KATP channels

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen responsible for physiological and pathological angiogenesis. Abnormal regulation of VEGF expression in anterior pituitary folliculostellate (FS) cells has been implicated in pituitary tumour progression. FS and endocrine cells express VEGF, which is considered to be secreted by the constitutive pathway. The present study investigated the mechanism of VEGF secretion in TtT/GF cells, a mouse FS cell line. TtT/GF cells were shown to express VEGF(164), the most potent and bioavailable isoform of VEGF. Immunofluorescence and immunogold electron microscopy localised VEGF to the cytoplasm and small electron-lucent vesicles. Pituitary adenylate cyclase-activating polypeptide (PACAP), a well-documented stimulant of VEGF secretion, caused a robust increase in VEGF secretion over 24 h. Glyburide, an ABCA1 and K(ATP) channel blocker, also caused an increase in VEGF secretion when applied alone, and amplified the response to PACAP. Other ABCA1 transport blockers did not affect VEGF secretion. Exposure of TtT/GF cells to cycloheximide with PACAP or glyburide inhibited the increased secretion of VEGF, consistent with control of secretion at the transcription level. The SUR2B/Kir6.1 form of K(ATP) channels was shown to be expressed by TtT/GF cells. Diazoxide, a K(ATP) activator, inhibited PACAP- and PACAP + glyburide-stimulated VEGF secretion but not that of glyburide alone. These data suggest that K(ATP) channels are expressed by FS cells and play a significant role in the control of VEGF secretion, and also that activation of K(ATP) channels inhibits the secretion of VEGF at the level of transcription.

Interaction of gonadal steroids and gonadotropin-releasing hormone on pituitary adenylate cyclase-activating polypeptide (PACAP) and PACAP receptor expression in cultured rat anterior pituitary cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are expressed in the hypothalamus, the gonadotrope cells of the anterior pituitary gland, and the gonads, forming an autocrine-paracrine system in these tissues. Within the pituitary, PACAP functions either alone or synergistically with gonadotropin-releasing hormone (GnRH) to stimulate gonadotropin gene expression and secretion. Our goal was to define the hormonal regulation of pituitary PACAP and PACAP receptor (PAC1) gene expression by dihydrotestosterone (DHT), estradiol, and progesterone alone or in conjunction with GnRH. Treatment of adult male rat pituitary cell cultures with DHT or progesterone augmented GnRH-mediated increase in PACAP messenger RNA (mRNA) levels, but neither had an effect when present alone. Conversely, estradiol treatment blunted PACAP gene expression but did not alter GnRH effects on PACAP expression. Expression of PACAP receptor mRNA was decreased by GnRH treatment, minimally increased by DHT treatment, but not altered by the addition of estradiol or progesterone. DHT and GnRH together blunted PACAP receptor gene expression. Taken together, these results suggest that the activity of the intrapituitary PACAP-PAC1 system is regulated via the complex interaction of gonadal steroids and hypothalamic GnRH.

Prolonged stimulation with thyrotropin-releasing hormone and pituitary adenylate cyclase-activating polypeptide desensitize their receptor functions in prolactin-producing GH3 cells

We used somatolactotroph GH3 cells to examine changes in response to stimulation with thyrotropin-releasing hormone (TRH) and pituitary adenylate cyclase-activating polypeptide (PACAP) after sustained treatment with these peptides. TRH and PACAP increased prolactin promoter activity in mock- and PACAP type 1 receptor (PAC1R)-transfected cells. When the cells were pretreated with TRH for 48 h, the response of the prolactin promoter to both TRH and PACAP was diminished. Similarly, in PAC1R-transfected GH3 cells pretreated with PACAP, the effects of TRH and PACAP on the prolactin promoter were eliminated. The stimulation of prolactin mRNA expression by TRH and PACAP was eliminated by prolonged pretreatment with these peptides in PAC1R-transfected cells. Both the serum response element (SRE) promoters and cAMP response element (CRE) promoters were activated by TRH and PACAP in either mock- or PAC1R-transfected cells. Pretreatment for 48 h with TRH also eliminated the effects of TRH and PACAP on the SRE and CRE promoters, and pretreatment of PAC1R-transfected cells with PACAP for 48 h reduced the responses of the SRE and CRE promoters to TRH and PACAP. These observations demonstrated that sustained stimulation with TRH and PACAP desensitizes their own and each other's receptors.

Intercellular communication within the rat anterior pituitary gland. XV. Properties of spontaneous and LHRH-induced Ca2+ transients in the transitional zone of the rat anterior pituitary in situ

In the transitional zone of the rat anterior pituitary, spontaneous and LHRH-induced Ca(2+) dynamics were visualized using fluo-4 fluorescence Ca(2+) imaging. A majority of cells exhibited spontaneous Ca(2+) transients, while small populations of cells remained quiescent. Approximately 70% of spontaneously active cells generated fast, oscillatory Ca(2+) transients that were inhibited by cyclopiazonic acid (10 μm) but not nicardipine (1 μm), suggesting that Ca(2+) handling by endoplasmic reticulum, but not Ca(2+) influx through voltage-dependent L-type Ca(2+) channels, plays a fundamental role in their generation. In the adult rat anterior pituitary, LHRH (100 μg/ml) caused a transient increase in the Ca(2+) level in a majority of preparations taken from the morning group rats killed between 0930 h and 1030 h. However, the second application of LHRH invariably failed to elevate Ca(2+) levels, suggesting that the long-lasting refractoriness to LHRH stimulation was developed upon the first challenge of LHRH. In contrast, LHRH had no effect in most preparations taken from the afternoon group rats euthanized between 1200 h and 1400 h. In the neonatal rat anterior pituitary, LHRH caused a suppression of spontaneous Ca(2+) transients. Strikingly, the second application of LHRH was capable of reproducing the suppression of Ca(2+) signals, indicating that the refractoriness to LHRH had not been established in neonatal rats. These results suggest that responsiveness to LHRH has a long-term refractoriness in adult rats, and that the physiological LHRH surge may be clocked in the morning. Moreover, LHRH-induced excitation and associated refractoriness appear to be incomplete in neonatal rats and may be acquired during development.

PACAP, an autocrine/paracrine regulator of gonadotrophs

Hypothalamic-hypophysiotropic peptides are the proximate regulators of pituitary cells, but they cannot fully account for the complex functioning of these cells. Accordingly, awareness is growing that an array of peptides produced in the pituitary exert paracrine/autocrine functions. One such peptide, pituitary adenylate cyclase-activating polypeptide (PACAP), was originally identified as a hypothalamic activator of cAMP production in pituitary cells. Gonadotrophs and folliculostellate cells are the main source of pituitary PACAP, and each pituitary cell type expresses a PACAP receptor. PACAP increases alpha-subunit (Cga) and Lhb mRNAs, and it stimulates the transcription of follistatin (Fst) that, in turn, restrains activin signaling to repress Fshb and gonadotropin-releasing hormone-receptor (Gnrhr) expression as well as other activin-responsive genes. The PACAP (Adcyap1) promoter is activated by cAMP, and pituitary cells may communicate by a feed-forward, cAMP-dependent mechanism to maintain a high level of PACAP in the fetal pituitary. At birth, pituitary PACAP declines and pituitary follistatin levels decrease, which together with increased gonadotropin-releasing hormone secretion allow Gnrhr and Fshb to increase and facilitate activation of the newborn gonads. Changes in Adcyap1 expression levels in the adult pituitary may contribute to the selective rise in follicle-stimulating hormone (FSH) from age 20-30 days to the midcycle surge and to the secondary increase in FSH that occurs before estrus. These results provide further support for the notion that PACAP is a key player in reproduction through its actions as a pituitary autocrine/paracrine hormone.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is important for embryo implantation in mice

Mice lacking pituitary adenylate cyclase-activating polypeptide (PACAP) show high mortality during the postnatal period, as well as impaired reproduction in females. This study characterizes the reproductive phenotype in female mice lacking PACAP due to targeted disruption (knockout) of the single copy pacap gene (Adcyap1) to determine the site(s) of action of PACAP in the cascade of reproductive events. PACAP null females showed normal puberty onset, estrous cycles, and seminal plugs when paired with a male of proven fertility. However, significantly fewer PACAP null females (21%) than wild-type females (100%) gave birth following mating. Although a defect was not detected in ovulation, ovarian histology or fertilization of released eggs in PACAP null females, only 13% had implanted embryos 6.5 days after mating. Associated with the decrease in implantation, prolactin and progesterone levels were significantly lower in females lacking PACAP than in wild types on day 6.5 after mating. Our evidence suggests that impaired implantation is the defect responsible for decreased fertility in PACAP null female mice.

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.

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.

Pituitary adenylate cyclase-activating polypeptide in the brain, spinal cord and sensory organs of the zebrafish, Danio rerio, during development

The distribution of Pituitary adenylate cyclase-activating polypeptide (PACAP) was investigated in the brain, pituitary and sensory organs of the zebrafish, Danio rerio, during development, in juvenile and adult specimens, using the immunofluorescence method. In 24 h post fertilization (hpf) embryos, PACAP immunoreactive cells appeared in the rostral telencephalon, dorsal diencephalon, caudal and medial rhombencephalon, spinal cord and retina. At 48 hpf stage, positive cells were present in the dorsal diencephalon, medial rhombencephalon, spinal cord, retina and olfactory placode (Op). At 72 hpf stage, additional immunoreactive elements appeared in the medial telencephalon, hypothalamus, mesencephalic tegmentum, retina and otic sensory epithelium (Ose). At day 5, new immunoreactive cells were found in the anterior rhombencephalon and pituitary pars distalis. At day 13, positive cells were mainly concentrated in the mesencephalic tegmentum and spinal cord. In the telencephalon, diencephalon, rhombencephalon and pituitary, the distribution of positive cells was similar to that previously reported. At 1 month stage, positive cells were detected in the hypothalamus, nucleus of the medial longitudinal fascicle (nMlf), rhombencephalic griseum centrale (Gc) and pituitary pars distalis. At 2-3 month stages, immunoreactive elements were found in several hypothalamic nuclei, in the mesencephalic nucleus isthmi, cerebellum and pituitary. In adults, PACAP immunoreactivity was confined to a few brain regions and the pituitary. PACAP immunoreactivity was transiently expressed in several regions suggesting that the peptide may have a role in the control of cells differentiation and proliferation during zebrafish ontogeny. The finding of positive fibers in the pituitary from day 5 onward indicates that PACAP may function from this stage as a hypophysiotropic peptide.

Differential expression of the pituitary gonadotropin subunit genes during male rat sexual maturation: reciprocal relationship between hypothalamic pituitary adenylate cyclase-activating polypeptide and follicle-stimulating hormone beta expression

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to differentially regulate the expression of the gonadotropin subunit genes in cultures of rat pituitary cells. PACAP is expressed within the hypothalamus, and concentrations of PACAP are 2- to 4-fold higher in the portal circulation than in the general circulation. Therefore, PACAP is a candidate regulator of pituitary function. In the present study, we examined the expression of PACAP mRNA within the paraventricular nucleus (PVN) during maturation (Days 20-60) in the male rat and compared this expression to the levels of the gonadotropin subunits, follistatin, and GnRH-receptor mRNAs within the anterior pituitary. Serum concentrations of FSH and LH confirm the established maturational pattern of divergent secretion of LH and FSH. Northern analysis of the gonadotropin subunit mRNAs revealed that FSHbeta expression parallels FSH secretion whereas LHbeta mRNA concentrations do not change during development. Expression of the GnRH receptor in the pituitary parallels that of FSHbeta. In situ hybridization revealed a developmental pattern of PACAP mRNA within the PVN that is reciprocal to that of FSHbeta. Competitive reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of total pituitary follistatin mRNA revealed no significant changes; however, semiquantitative RT-PCR analyses revealed the presence of two follistatin mRNA species, one of which, corresponding to follistatin-288, was developmentally regulated. These studies identified a reciprocal relationship between PVN PACAP and FSHbeta gene expression in maturing rats. We propose that PACAP contributes to the selective regulation of FSHbeta expression during maturation in the male rat, perhaps via regulation of follistatin.

Cell immunoblot assay study demonstrating the release of PACAP from individual anterior pituitary cells of rats and the effect of PACAP on LH release

The presence of pituitary adenylate cyclase activating polypeptide (PACAP) was previously demonstrated in the anterior pituitary by radioimmunoassay, immunohistochemistry, and reverse transcript-polymerase chain reaction (RT-PCR). With the use of cell immunoblot assay (CIBA), when the pituitary cells were cultured on nitrocellulose membrane, the release of PACAP by individual anterior pituitary cells was observed. The released peptide, trapped by the nitrocellulose membrane forming a blot around the cells, was demonstrated by immunocytochemistry. Double labeling revealed that a part of PACAP-immunoreactive cells can release LH as well. With the use of sandwich enzyme immunoassay (S-EIA), it was found that the concentration of PACAP in the anterior pituitaries is 10(-10) M. In cell culture in a similar concentration, PACAP stimulated the LH release from female gonadotropes, but did not influence it from male ones. The stimulated release of LH was indicated by the enhancement in the diameter of LH blots compared to the untreated control cultures. We concluded that PACAP may be released from the anterior pituitary cells in a concentration which would be able to influence LH release not only in vitro but under in vivo conditions as well. The effect of PACAP on LH release was different in female and male pituitary cultures.

A newly developed enzyme-immunoassay for measuring the tissue contents of PACAP in fish

We have developed a novel and easy enzyme-immunoassay (EIA) for pituitary adenylate cyclase-activating polypeptide (PACAP). We used it to determine immunoreactive PACAP levels in the central nervous system (CNS) and peripheral tissues of two fishes, a teleost (the stargazer) and an elasmobranch (a stingray). An antiserum was raised in a white rabbit immunized with a conjugate of synthetic stargazer PACAP27 plus keyhole limpet hemocyanin. The EIA system used an antiserum/biotin-labeled PACAP/avidin/biotin-conjugated enzyme complex, and a double antibody method was used to precipitate the immune complexes. We call the system the avidin-biotin complex detectable EIA (ABCDEIA) for PACAP. ABCDEIA with biotin-labeled PACAP27 detected only PACAP27, recognizing neither the longer forms of PACAP nor any other peptides. PACAPs with 27, 38, and 44 residues cross-reacted in another ABCDEIA with biotin-labeled PACAP38 or PACAP44. Whole brains of both fishes contained much higher levels of PACAP, 6-30 times as high as the levels in the mammalian brain, but unexpectedly, no immunoreactive PACAP27 was found in any CNS or peripheral tissue in either fish. The gastrointestinal tracts of fish also contained lower, but significant amounts of PACAP.

Evidence that PACAP and GnRH down-regulate follicle-stimulating hormone-beta mRNA levels by stimulating follistatin gene expression: effects on folliculostellate cells, gonadotrophs and LbetaT2 gonadotroph cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates alpha-subunit transcription and lengthens LH-beta mRNA transcripts, but reduces FSH-beta mRNA levels in rat pituitary cell cultures. PACAP also stimulates follistatin transcription, an effect which may explain the decrease in FSH-beta mRNA. To begin to investigate the cells in which PACAP activates the follistatin gene, quantitative in situ hybridization for follistatin mRNA combined with immunostaining for LHbeta and S100 protein was performed. In control cultures, follistatin mRNA was expressed in 70% of gonadotrophs and in 47% of folliculostellate cells (S-100+). PACAP increased (P<0.001) both the number of follistatin-expressing cells as well as the number of grains per cell in both gonadotrophs and folliculostellate cells, while GnRH only affected (P=0.01) gonadotrophs. Follistatin and FSH-beta gene expression in rat pituitary cultures were also measured by competitive quantitative RT-PCR and northern analysis, respectively. Both PACAP and GnRH increased (P<0.05) follistatin gene expression and suppressed (P<0.05) FSH-beta mRNA, and the effect of PACAP together with GnRH on follistatin exceeded that of GnRH alone. PACAP regulation of follistatin and FSH-beta gene expression was studied further in LbetaT2 cells that were found to express receptors for the specific PACAP receptor, PAC(1). Follistatin mRNA was undetectable in cultures exposed to control media, or stimulated with PACAP, GnRH or rh-activin-A. In contrast to the results in primary pituitary cultures, PACAP increased FSH-beta mRNA in these follistatin-deficient cells. Moreover, using transient transfection, PACAP stimulated transcription of ovine-FSH-beta-luciferase. GnRH likewise increased FSH-beta mRNA and stimulated FSH-beta gene transcription in LbetaT2 cells. Activin-A increased FSH-beta gene expression dose-dependently, and activin induction of FSH-beta mRNA was blocked completely by 3-fold excess follistatin. These results indicate that PACAP stimulates follistatin gene expression in both gonadotrophs and folliculostellate cells, and provide further evidence that follistatin is required for PACAP or continuous GnRH to down-regulate FSH-beta mRNA. These experiments suggest a mechanism by which PACAP influences FSH production selectively by an autocrine effect on gonadotrophs and by a paracrine mechanism through folliculostellate cells that involves follistatin.

Embryonic expression of pituitary adenylyl cyclase-activating polypeptide and its selective type I receptor gene in the frog Xenopus laevis neural tube

The genes encoding pituitary adenylyl cyclase-activating peptide (PACAP) and its selective type I receptor (PAC1) are expressed in the embryonic mouse neural tube, where they may be involved in neurogenesis and neural tube development. We examined here the early expression and potential actions of PACAP and PAC1 in the vertebrate developmental model Xenopus laevis. PACAP and PAC1 mRNAs were first detected by RT-PCR in stage 16-18 embryos (18 hours after fertilization). Two distinct PACAP precursor mRNAs were identified. One encoded both growth hormone-releasing hormone and PACAP, whereas the other encoded only full-length PACAP. Unlike that in the adult, the latter represented the predominant embryonic PACAP mRNA species. In situ hybridization revealed that PACAP and PAC1 mRNAs were restricted to neural cells. PAC1 gene expression was observed mainly in the ventricular zone in the ventral parts of the prosencephalon, mensencephalon, rhombencephalon, and anterior spinal cord. In contrast, PACAP mRNA was localized exclusively in postmitotic cells in the dorsolateral parts of the rhombencephalon and entire spinal cord. Most PACAP mRNA-containing cells were characterized as Rohon-Beard neurons. Exposure of early embryos to UV irradiation, which ventralizes embryos and inhibits neural induction, reduced the expression of PACAP and PAC1 genes. These results suggest that PACAP may be involved in the early development of the embryonic Xenopus neural tube.

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.

PACAP38 and PACAP27 administered intracerebroventricularly have an opposite effect on LH secretion

The effect of PACAP38 on the LH surge and ovulation was compared with that of PACAP27 and VIP in the same model. The peptides were administered intracerebroventricularly before the critical period of the proestrous stage. PACAP38 was able to inhibit ovulation and to prevent the preovulatory LH surge; however, PACAP27 did not inhibit the ovulation and VIP inhibited the ovulation in 2/11 animals. In those animals of the last two groups in which ovulation occurred, the preovulatory LH surge was higher than in control rats. It is speculated that the opposite effect of PACAP38 and PACAP27 on the preovulatory LH surge and ovulation is possibly mediated through different receptors.

Inhibition of Zac1, a new gene differentially expressed in the anterior pituitary, increases cell proliferation

Zac1 is a new zinc finger protein that concomitantly controls apoptosis and cell cycle arrest through separate pathways. The mouse Zac1 gene is mainly expressed in the pituitary gland and in different brain areas. In this study regional and cellular expression of Zac1 in the pituitary gland was determined by in situ hybridization. Zac1 messenger RNA was abundantly expressed in the anterior pituitary lobe compared with that in the intermediate and posterior lobes. Zac1 transcripts were found in all hormone-secreting cell types, with the highest levels in GH- and PRL-producing cells. To investigate the impact of Zac1 in pituitary cell proliferation, we ablated the endogenous Zac1 gene by antisense treatment in two murine cell types, AtT-20 and TtT/GF, that are representative of granular and agranular cell lineages, respectively. The decline in Zac1 protein levels under antisense treatment was accompanied by increased DNA synthesis in clonal corticotroph and folliculo-stellate cells, as demonstrated by enhanced [3H]thymidine incorporation (36% and 50%, respectively). Antisense oligonucleotides against Zac1 controlled cell proliferation in a dose-dependent way, and mutagenized antisense oligonucleotides were inert. Conclusively, our data provide the first evidence of a role for Zac1 in pituitary growth control.

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.

Is Ps4 (prepro-TRH [160-169]) more than an enhancer for thyrotropin-releasing hormone?

Ps4 (thyrotropin-releasing hormone [TRH]-enhancing peptide), one of the cryptic peptides resulting from the proteolytic processing of prepro-TRH to produce TRH, has a growing list of functions in addition to its well-established ability to enhance the TRH-induced release of thyrotropin (TSH) and prolactin from the pituitary. Intramedullary coadministration of Ps4 and TRH increased gastric acid secretion above the level produced by TRH alone and intracisternal infusion of Ps4 resulted in a substantial reduction in the levels of prepro-TRH-derived peptide levels in the rat pituitary, including Ps4. High-affinity receptors for Ps4 are widely distributed. In addition to the very high Ps4 binding capacity of the folliculo-stellate cells of the anterior pituitary, abundant Ps4 receptors are found in the urinary bladder, vas deferens, central nervous system, reproductive tissues, and pancreas. Targeted prepro-TRH gene disruption results in hyperglycemia as well as the expected hypothyroidism. The observed disregulation of thyroid and glucose homeostasis in the TRH "knockout" mouse clearly demonstrates that prepro-TRH-derived peptides and their cognate receptors within the pituitary, pancreas, and other neural and endocrine systems are of fundamental importance to a variety of physiological systems and merit structural and functional characterization.

Hypophysiotropic action of pituitary adenylate cyclase-activating polypeptide (PACAP) in the goldfish: immunohistochemical demonstration of PACAP in the pituitary, PACAP stimulation of growth hormone release from pituitary cells, and molecular cloning of pituitary type I PACAP receptor

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the glucagon/secretin peptide family, and its molecular structure is highly conserved in vertebrates. In this study, the functional role of PACAP in regulating GH release in the goldfish was investigated. Using immunohistochemical staining, nerve fibers with PACAP immunoreactivity were identified in the vicinity of goldfish somatotrophs, suggesting that this neuropeptide may influence GH release in the goldfish. The direct regulatory action of PACAP on GH secretion was demonstrated in vitro in perifused goldfish pituitary cells. PACAPs (0.01 nM to 1 microM) from different species, including ovine PACAP27, ovine PACAP38, frog PACAP38, zebra fish PACAP27, and zebra fish PACAP38, were all effective in stimulating GH release with ED50 values of 8.9 +/- 3.5, 3.3 +/- 1.6, 14.4 +/- 3.5, 15.4 +/- 4.1, and 1.4 +/- 0.2 nM, respectively. Similar concentrations of vasoactive intestinal polypeptide (VIP), a peptide related to PACAP, was not effective in this respect. In addition, the GH-releasing action of ovine PACAP38 (10 nM) was inhibited by the PACAP antagonist PACAP(6-38) (10 microM), but not by the VIP antagonist [4-Cl-D-Phe6,Leu17]VIP (10 microM). The pharmacology of these GH responses is consistent with the mammalian type I PACAP receptors, suggesting that a similar receptor subtype is present in the goldfish pituitary and mediates the GH-releasing action of PACAP. To establish the structural identity of this goldfish PACAP receptor, a complementary DNA (cDNA) clone sharing a high degree of sequence homology with mammalian type I PACAP receptors was isolated from a goldfish pituitary cDNA library. This cDNA was 5.2 kb in size with a 1.4-kb open reading frame and encoded a 465-amino acid protein with the typical structure of a 7-transmembrane domain-containing, G protein-coupled receptor. Functional expression of this cDNA in COS-7 cells revealed that this fish type I PACAP receptor could be activated by ovine PACAP27 and PACAP38 to increase cAMP synthesis with ED50 values of 2.4 +/- 0.8 and 4.2 +/- 1.2 nM, respectively. Other structurally related peptides, including VIP (100 nM), GH-releasing hormone (100 nM), glucagon (100 nM), secretin (100 nM), gastric inhibitory polypeptide (100 nM), and PTH (100 nM), were not effective in altering cAMP production. Using Northern blot and RT-PCR, messenger RNA transcripts of this PACAP receptor were identified in the brain, heart, and pituitary of the goldfish. These results, taken together, support the hypothesis that PACAP functions as a novel GH-releasing factor in the goldfish through activation of type I PACAP receptors.

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.

Effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on corticotropin-releasing hormone (CRH) gene expression in the rat hypothalamic paraventricular nucleus

Pituitary adenylate cyclase-activating peptide (PACAP) is a 38-amino-acid polypeptide, first isolated from hypothalamus, which directly stimulates in vitro the production of cAMP as well as the release of several pituitary hormones, such as growth hormone and luteinizing hormone. In vivo, PACAP has been shown to stimulate ACTH release. The presence of PACAP receptors in several brain areas, including the hypothalamus, suggests that this peptide might play a role as a neurotransmitter/neuromodulator and might be involved in the regulation of hypophysiotropic neurohormones. In order to study the role of PACAP on corticotropin-releasing hormone (CRH) neuron, we have investigated the effects of intracerebroventricular (i.c.v.) and intravenous (i.v.) injections of PACAP and the potent PACAP antagonist PACAP(6-38) on CRH gene expression in the hypothalamic paraventricular nucleus (PVN) in the male rat. The levels of CRH mRNA were evaluated by quantitative in situ hybridization. The i.c.v. injection of PACAP (4 microg/kg b.wt.) produced a 22% increase in the hybridization signal, an effect which was completely prevented by the concomitant injection of the PACAP antagonist (4 microg/kg b.wt.). On the other hand, the administration of the PACAP antagonist induced by itself a 40% decrease in the amounts of CRH mRNA. The i.v. injection of the same peptides (100 microg/kg. b.wt.) produced very similar results. These data strongly suggest that PACAP is involved in the positive regulation of CRH gene expression via specific central receptors and then can play a role as a neurotransmitter/neuromodulator. The effect observed after i.v. injection of PACAP also suggests that the circulating levels of PACAP can play a role in the modulation of CRH gene expression. PACAP might then be involved in the regulation of the HPA axis by a double mechanism: stimulation of CRH gene expression at the central level and direct effect on pituitary corticotrophs.

Gonadotropin-releasing hormone and pituitary adenylate cyclase-activating polypeptide affect levels of cyclic adenosine 3',5'-monophosphate-dependent protein kinase A (PKA) subunits in the clonal gonadotrope alphaT3-1 cells: evidence for cross-talk between PKA and protein kinase C pathways

We have shown previously that protein kinase A (PKA) subunit levels are regulated by activation of PKA or protein kinase C (PKC) in anterior pituitary cells. GnRH also influenced PKA subunit levels, suggesting that hormonal regulation occurs in gonadotrophs, and therefore, we have reexamined this question using the clonal gonadotrope-derived cell line (alphaT3-1 cells). Western blot analysis, using specific immunoaffinity purified immunoglobulins, revealed expression of catalytic (Cat) and regulatory type I (RI) and type II (RII) subunits of PKA in these cells. Activation of adenylyl cyclase (AC) with forskolin, or of PKC with tetradecanoyl phorbol acetate (TPA), caused a rapid (detectable at 0.5-1 h) and concentration-dependent loss of all PKA subunits. Forskolin (10-100 microM) reduced Cat and RI by 60% and RII by 30%, whereas TPA (0.1-1 microM) reduced Cat and RII by 50% and RI by 40%. Simultaneous activation of PKA and PKC caused the expected dose-dependent reductions in Cat, and the effects of forskolin or TPA were nearly additive. RI and RII were reduced similarly by 10 nM TPA, whereas 100 nM TPA tended to prevent the reduction of RI or RII caused by forskolin. GnRH, which activates phosphoinositidase C and not AC in these cells, caused a clear loss of Cat or RII at all concentrations tested and of RI at 0.1 nM. Pituitary adenylate cyclase-activating polypeptide 38, which acts via PVR-1 receptors to stimulate both phosphoinositidase C and AC in these cells, also caused a clear dose-dependent decrease in Cat, RI, and RII, although higher concentrations were needed for the latter effects. Together, the data demonstrate that catalytic and regulatory subunits of PKA are subject to both hormonal and receptor-independent regulation in alphaT3-1 cells, reinforcing the possibility that such effects occur in nonimmortalized gonadotropes. Whereas the effects of PKA activators very likely involve proteolytic degradation of the dissociated PKA holoenzyme, the effects of TPA and GnRH occur in the absence of cAMP elevation by unknown mechanisms. Whatever the mechanisms involved, the data reveal a mechanism for cross-talk between phosphoinositidase C and AC-mediated hormonal signals, in which PKC activation seems to play a pivotal role.

Functional interaction between gonadotropin-releasing hormone and PACAP in gonadotropes and alpha T3-1 cells

Gonadotropes, like other cells, receive informational input from multiple receptor types, acting through multiple intracellular signaling pathways, and are therefore faced with the task of integrating this input in order to respond appropriately to their environment. In recent years an increasing number of examples of functional interactions occurring between the PIC and adenylyl cyclase signaling pathways in gonadotropes have been described, and the discovery that these cells are targets for PACAP has provided a physiological context for earlier work on gonadotrope regulation by cyclic AMP. The development of the alpha T3-1 cell line has greatly facilitated investigation of the interaction between these signaling systems. In these cells we have obtained no evidence for interaction between the GnRH and PACAP receptor-effector systems at the level of receptor occupancy or expression, but these systems clearly do have reciprocal modulatory effects on second messenger generation and/or mobilization. We are now faced with the challenge of determining the physiological and/or pathophysiological relevance of such interactions.

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.

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.