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

The functional heterogeneity of PACAP: Stress, learning, and pathology

Pituitary adenylate cyclase-activating peptide (PACAP) is a highly conserved and widely expressed neuropeptide that has emerged as a key regulator of multiple neural and behavioral processes. PACAP systems, including the various PACAP receptor subtypes, have been implicated in neural circuits of learning and memory, stress, emotion, feeding, and pain. Dysregulation within these PACAP systems may play key roles in the etiology of pathological states associated with these circuits, and PACAP function has been implicated in stress-related psychopathology, feeding and metabolic disorders, and migraine. Accordingly, central PACAP systems may represent important therapeutic targets; however, substantial heterogeneity in PACAP systems related to the distribution of multiple PACAP isoforms across multiple brain regions, as well as multiple receptor subtypes with several isoforms, signaling pathways, and brain distributions, provides both challenges and opportunities for the development of new clinically-relevant strategies to target the PACAP system in health and disease. Here we review the heterogeneity of central PACAP systems, as well as the data implicating PACAP systems in clinically-relevant behavioral processes, with a particular focus on the considerable evidence implicating a role of PACAP in stress responding and learning and memory. We also review data suggesting that there are sex differences in PACAP function and its interactions with sex hormones. Finally, we discuss both the challenges and promise of harnessing the PACAP system in the development of new therapeutic avenues and highlight PACAP systems for their critical role in health and disease.

Female reproductive functions of the neuropeptide PACAP

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide originally isolated as a hypothalamic peptide. It has a widespread distribution in the body and has a diverse spectrum of actions. Among other processes, PACAP has been shown to be involved in reproduction. In this review we summarize findings related to the entire spectrum of female reproduction. PACAP is a regulatory factor in gonadal hormone production, influences follicular development and plays a role in fertilization and embryonic/placental development. Furthermore, PACAP is involved in hormonal changes during and after birth and affects maternal behavior. Although most data come from cell cultures and animal experiments, increasing number of evidence suggests that similar effects of PACAP can be found in humans. Among other instances, PACAP levels show changes in the serum during pregnancy and birth. PACAP is also present in the human follicular and amniotic fluids and in the milk. Levels of PACAP in follicular fluid correlate with the number of retrieved oocytes in hyperstimulated women. Human milk contains very high levels of PACAP compared to plasma levels, with colostrum showing the highest concentration, remaining steady thereafter for the first 7 months of lactation. All these data imply that PACAP has important functions in reproduction both under physiological and pathological conditions.

Stereology of gonadotropin-releasing hormone and kisspeptin neurons in PACAP gene-deficient female mice

The hypothalamic gonadotropin-releasing hormone (GnRH)-kisspeptin neuronal network regulates fertility in all mammals. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide isolated from the hypothalamus that is involved in the regulation of several releasing hormones and trop hormones. It is well-known that PACAP influences fertility at central and peripheral levels. However, the effects of PACAP on GnRH and kisspeptin neurons are not well understood. The present study investigated the integrity of the estrous cycle in PACAP-knockout (KO) mice. The number and immunoreactivity of GnRH (GnRH-ir) neurons in wild-type (WT) and PACAP KO female mice were determined using immunohistochemistry. In addition, the number of kisspeptin neurons was measured by counting kisspeptin mRNA-positive cells in the rostral periventricular region of the third ventricle (RP3V) and arcuate nucleus (ARC) using the RNAscope technique. Finally, the mRNA and protein expression of estrogen receptor alpha (ERα) was also examined. Our data showed that the number of complete cycles decreased, and the length of each cycle was longer in PACAP KO mice. Furthermore, the PACAP KO mice experienced longer periods of diestrus and spent significantly less time in estrus. There was no difference in GnRH-ir or number of GnRH neurons. In contrast, the number of kisspeptin neurons was decreased in the ARC, but not in the R3PV, in PACAP KO mice compared to WT littermates. Furthermore, ERα mRNA and protein expression was decreased in the ARC, whereas in the R3PV region, ERα mRNA levels were elevated. Our results demonstrate that embryonic deletion of PACAP significantly changes the structure and presumably the function of the GnRH-kisspeptin neuronal network, influencing fertility.

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.

Investigation of pituitary adenylate cyclase activating polypeptide (PACAP) in human amniotic fluid samples

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide acting as a hormone, a neuromodulator, a neurotransmitter, a trophic factor and is involved in a variety of developmental and regenerative processes. PACAP is present in several human tissues and biological fluids. In many pathological conditions, changes in PACAP levels have been described to reflect disease progression, therefore PACAP has diagnostic value as a potential biomarker. Since PACAP has been shown to play an important role in reproductive physiology and development, it was of interest to examine whether this neuropeptide occurs in the human amniotic fluid. Amniotic fluid samples were collected between the 15-19^th weeks of gestation from volunteering pregnant women undergoing amniocentesis as a prenatal diagnostic tool due to maternal age. Pathological cases were excluded after prenatal karyotype analysis. PACAP-like immunoreactivity was measured by radioimmunoassay and could be detected in all samples. The present study provides evidence for the presence of PACAP in human amniotic fluid, but determination of the exact physiological or pathological significance awaits further investigation.

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.

Possible effects of pituitary adenylate cyclase activating polypeptide (PACAP) on early embryo implantation marker HB-EGF in mouse

Pituitary adenylate cyclase activating polypeptide (PACAP) was originally isolated as a hypothalamic neuropeptide stimulating adenylate cyclase activity. Besides its neuroprotective effects, numerous data proved its role in reproductive processes. However, there are limited data on its role in preimplantation embryo development and implantation. Our aim was to analyse the mRNA expression of Adcyap1 (coding region of PACAP) and Hbegf [coding region of HB-EGF (Heparin-binding EGF-like growth factor)] in embryos and pregnant uterus to investigate the possible correlation between them. Eight-week-old BDF1 mice were superovulated and subsequently mated overnight or left in their cage after hCG treatment. Day4 embryos were flushed from mated females. After morphological analysis, Adcyap1 and Hbegf gene expression of embryos and uterine tissues was assessed with qPCR. Our results showed significantly higher Adcyap1 and Hbegf mRNA levels in females producing embryos compared to non-mated ones. Robust elevation of Adcyap1 and slight elevation of Hbegf were detected in females with blastocyst embryos compared with non-blastocysts. We found low rate of Hbegf mRNA expression in uncompacted embryos, whereas morulae and blastocysts expressed high amounts of Hbegf. However, we did not find detectable Adcyap1 mRNA in embryos. Strong correlation was found between uterine tissue and embryonic Hbegf levels, slight correlation between uterine Adcyap1 and Hbegf levels. Uterine tissue Adcyap1 and embryonic Hbegf showed no correlation. In summary, our present data show, for the first time, the correlation between PACAP and HB-EGF mRNA expression suggesting that PACAP might play a role during the peri-implantation period of early mouse embryo development.

Protective Effects of PACAP in Peripheral Organs

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide widely distributed in the nervous system, where it exerts strong neuroprotective effects. PACAP is also expressed in peripheral organs but its peripheral protective effects have not been summarized so far. Therefore, the aim of the present paper is to review the existing literature regarding the cytoprotective effects of PACAP in non-neuronal cell types, peripheral tissues, and organs. Among others, PACAP has widespread expression in the digestive system, where it shows protective effects in various intestinal pathologies, such as duodenal ulcer, small bowel ischemia, and intestinal inflammation. PACAP is present in both the exocrine and endocrine pancreas as well as liver where it reduces inflammation and steatosis by interfering with hepatic pathology related to obesity. It is found in several exocrine glands and also in urinary organs, where, with its protective effects being mainly published regarding renal pathologies, PACAP is protective in numerous conditions. PACAP displays anti-inflammatory effects in upper and lower airways of the respiratory system. In the skin, it is involved in the development of inflammatory pathology such as psoriasis and also has anti-allergic effects in a model of contact dermatitis. In the non-neuronal part of the visual system, PACAP showed protective effects in pathological conditions of the cornea and retinal pigment epithelial cells. The positive role of PACAP has been demonstrated on the formation and healing processes of cartilage and bone where it also prevents osteoarthritis and rheumatoid arthritis development. The protective role of PACAP was also demonstrated in the cardiovascular system in different pathological processes including hyperglycaemia-induced endothelial dysfunction and age-related vascular changes. In the heart, PACAP protects against ischemia, oxidative stress, and cardiomyopathies. PACAP is also involved in the protection against the development of pre-senile systemic amyloidosis, which is presented in various peripheral organs in PACAP-deficient mice. The studies summarized here provide strong evidence for the cytoprotective effects of the peptide. The survival-promoting effects of PACAP depend on a number of factors which are also shortly discussed in the present review.

Phenotypic characterization of testicular immune cells expressing immune checkpoint molecules in wild-type and pituitary adenylate cyclase-activating polypeptide-deficient mice

PROBLEM

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide having several regulatory functions in the nervous system and in peripheral organs including those of the reproductive system. PACAP-deficient male mice have several morphological, biochemical, behavioral defects and show disturbed signaling in spermatogenesis affecting fertility in PACAP KO mice. Reproductive functions such as fertility, mating, and maternal behaviors have been widely investigated, but no immune analyses are available regarding the testicular immune-privileged environment in male PACAP-deficient mice.

METHOD OF STUDY

We performed detailed immunophenotyping of testicular immune cells and investigated the expression of TIM-3 and PD-1 Immune checkpoint molecules of immune cells together with the detection of galectin-9 and perforin. We investigated the percentage of numerous immune cell populations in the testis of wild-type and PACAP-deficient mice.

RESULTS

We demonstrated a significant increase in the frequency of testicular CD8+ T cells together with the decrease in Treg cell number obtained from PACAP KO mice compared with wild-type mice. Investigating Immune checkpoint receptors, only PD-1 showed a significantly decreased expression in CD8+ T cells in PACAP KO mice compared with wild-type suggesting an impaired PD-1/PD-L1 pathway. Regarding TIM-3 expression, we did not find any significant difference between the investigated groups.

CONCLUSION

We hypothesize that these local changes may result in an immune activation with disturbed testicular immunoregulation in PACAP KO mice; however, determining the exact function requires further investigations. Our data further support the view that besides a systemic immune tolerance, localized active immunosuppression is involved in the regulation of testicular immune privilege.

Hearing impairment and associated morphological changes in pituitary adenylate cyclase activating polypeptide (PACAP)-deficient mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is a regulatory and cytoprotective neuropeptide, its deficiency implies accelerated aging in mice. It is present in the auditory system having antiapoptotic effects. Expression of Ca²⁺-binding proteins and its PAC1 receptor differs in the inner ear of PACAP-deficient (KO) and wild-type (WT) mice. Our aim was to elucidate the functional role of PACAP in the auditory system. Auditory brainstem response (ABR) tests found higher hearing thresholds in KO mice at click and low frequency burst stimuli. Hearing impairment at higher frequencies showed as reduced ABR wave amplitudes and latencies in KO animals. Increase in neuronal activity, demonstrated by c-Fos immunolabeling, was lower in KO mice after noise exposure in the ventral and dorsal cochlear nuclei. Noise induced neuronal activation was similar in further relay nuclei of the auditory pathway of WT and KO mice. Based on the similar inflammatory and angiogenic protein profile data from cochlear duct lysates, neither inflammation nor disturbed angiogenesis, as potential pathological components in sensorineural hearing losses, seem to be involved in the pathomechanism of the presented functional and morphological changes in PACAP KO mice. The hearing impairment is probably concomitant with the markedly accelerated aging processes in these animals.

Intranasal application of PACAP and β-cyclodextrin before the "critical period of proestrous stage" can block ovulation

INTRODUCTION

It was previously shown that intracerebroventricular administration of pituitary adenylate cyclase-activating polypeptide (PACAP) prior to GnRH mobilization in proestrus prevents ovulation in rats. In this study, we examined whether PACAP given intranasally could influence luteinizing hormone (LH) and prolactin (PRL) surges and ovulation.

METHODS

On the day of proestrus PACAP, p-cyclodextrin (modifier of blood-brain barrier) or PACAP + p-cyclodextrin was applied intranasally between 12:30 and 13:00. Blood samples were taken at 16:00, 18:00, and 20:00 for measuring plasma hormone levels. In the next morning, the expelled ova were counted. p-Cyclodextrin was also administered to male and diestrous female rats between 12:30 and 13:00 and blood was taken at 18:00.

RESULTS

PACAP prevented LH and PRL surges and ovulation in about half of the rats, p-cyclodextrin alone more effectively prevented ovulation. When PACAP and p-cyclodextrin were administered together, more rats ovulated like when PACAP given alone. p-Cyclodextrin did not influence LH and PRL levels in diestrous females; however, in males, it significantly enhanced PRL level.

DISCUSSION

Not only the intracerebroventricular, but the intranasal application of PACAP prevented ovulation. p-Cyclodextrin alone is more effective than PACAP and enhances PRL levels in male rats. PACAP and p-cyclodextrin given together weaken each other's effect. p-Cyclodextrin, as excipient of various drugs, has to be used carefully in human medications.

Immunomodulatory Roles of PACAP and VIP: Lessons from Knockout Mice

A bidirectional cross-talk is established between the nervous and immune systems through common mediators including neuropeptides, neurotransmitters, and cytokines. Among these, PACAP and VIP are two highly related neuropeptides widely distributed in the organism with purported immunomodulatory actions. Due to their well-known anti-inflammatory properties, administration of these peptides has proven to be beneficial in models of acute and chronic inflammatory diseases. Nevertheless, the relevance of the endogenous source of these peptides in the modulation of immune responses remains to be elucidated. The development of transgenic mice with specific deletions in the genes coding for these neuropeptides (Vip and Adcyap1) or for their G-protein-coupled receptors VPAC1, VPAC2, and PAC1 (Vipr1, Vipr2, Adcyap1r1) has allowed to address this question, underscoring the complexity of the immunoregulatory properties of PACAP and VIP. The goal of this review is to integrate the existing information on the immune phenotypes of mice deficient for PACAP, VIP, or their receptors, to provide a global view on the roles of these endogenous neuropeptides during immunological health and disease.

Comparative analysis of decidual and peripheral immune cells and immune-checkpoint molecules during pregnancy in wild-type and PACAP-deficient mice

PROBLEM

PACAP is a neuropeptide having a major relevance in the nervous system and in several peripheral organs including those of the reproductive system. PACAP-deficient mice have several morphological, biochemical, behavioral defects, and show reduced fertility. Female reproductive functions such as fertility, mating behavior, maternal behaviors, and implantation alterations have been widely investigated, but no comparative immune analyses are available in pregnant wild-type (WT) and PACAP knockout (KO) mice.

METHODS OF STUDY

Therefore, we performed a detailed immunophenotyping of decidual and peripheral immune cells and investigated the expression of two immune-checkpoint molecules by immune cells together with immunohistochemistry detecting Galectin-9 in placental tissues. We investigated the percentage of numerous immune cell populations in the periphery and in the decidua of pregnant mice.

RESULTS

We demonstrated a significant increase in the frequency of decidual Gal-9+ Th cells obtained from PACAP KO mice compared to the decidua of WT mice. We could not determine statistical differences in TIM-3 and programmed cell death-1 expression by different immune cells in the decidua and in the periphery between WT and KO mice. In conclusion, we could not find any significant alteration either in the distribution or in the cytotoxicity of the investigated decidual immune cells which could elucidate any reproductive alterations in PACAP KO mice.

CONCLUSION

The only remarkable finding is the recruitment of Gal-9+ Th cells to the decidua promoting local immune homeostasis in PACAP KO mice, which nevertheless cannot explain the reduced fertility observed in these mice.

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.

Disturbed spermatogenic signaling in pituitary adenylate cyclase activating polypeptide-deficient mice

PACAP is a neuropeptide with diverse functions in various organs, including reproductive system. It is present in the testis in high concentrations, and in addition to the stage-specific expression within the seminiferous tubules, PACAP affects spermatogenesis and the functions of Leydig and Sertoli cells. Mice lacking endogenous PACAP show reduced fertility, but the possibility of abnormalities in spermatogenic signaling has not yet been investigated. Therefore, we performed a detailed morphological analysis of spermatozoa, sperm motility and investigated signaling pathways that play a role during spermatogenesis in knockout mice. No significant alterations were found in testicular morphology or motility of sperm in homozygous and heterozygous PACAP-deficient mice in spite of the moderately increased number of severely damaged sperms. However, we found robust changes in mRNA and/or protein expression of several factors that play an important role in spermatogenesis. Protein kinase A expression was markedly reduced, while downstream phospho-ERK and p38 were elevated in knockout animals. Expression of major transcription factors, such as Sox9 and phospho-Sox9, was decreased, while that of Sox10, as a redundant factor, was increased in PACAP-deficient mice. The reduced phospho-Sox9 expression was partly due to increased expression and activity of phosphatase PP2A in knockout mice. Targets of Sox transcription factors, such as collagen type IV, were reduced in knockout mice. In summary, our results show that lack of PACAP leads to disturbed signaling in spermatogenesis, which could be a factor responsible for reduced fertility in PACAP knockout mice, and further support the role of PACAP in reproduction.

Early Neurobehavioral Development of Mice Lacking Endogenous PACAP

Pituitary adenylate cyclase activating polypeptide (PACAP) is a multifunctional neuropeptide. In addition to its diverse physiological roles, PACAP has important functions in the embryonic development of various tissues, and it is also considered as a trophic factor during development and in the case of neuronal injuries. Data suggest that the development of the nervous system is severely affected by the lack of endogenous PACAP. Short-term neurofunctional outcome correlates with long-term functional deficits; however, the early neurobehavioral development of PACAP-deficient mice has not yet been evaluated. Therefore, the aim of the present study was to describe the postnatal development of physical signs and neurological reflexes in mice partially or completely lacking PACAP. We examined developmental hallmarks during the first 3 weeks of the postnatal period, during which period most neurological reflexes and motor coordination show most intensive development, and we describe the neurobehavioral development using a complex battery of tests. In the present study, we found that PACAP-deficient mice had slower weight gain throughout the observation period. Interestingly, mice partially lacking PACAP weighed significantly less than homozygous mice. There was no difference between male and female mice during the first 3 weeks. Some other signs were also more severely affected in the heterozygous mice than in the homozygous mice, such as air righting, grasp, and gait initiation reflexes. Interestingly, incisor teeth erupted earlier in mice lacking PACAP. Motor coordination, shown by the number of foot-faults on an elevated grid, was also less developed in PACAP-deficient mice. In summary, our results show that mice lacking endogenous PACAP have slower weight gain during the first weeks of development and slower neurobehavioral development regarding a few developmental hallmarks.

PACAP suppresses dry eye signs by stimulating tear secretion

Dry eye syndrome is caused by a reduction in the volume or quality of tears. Here, we show that pituitary adenylate cyclase-activating polypeptide (PACAP)-null mice develop dry eye-like symptoms such as corneal keratinization and tear reduction. PACAP immunoreactivity is co-localized with a neuronal marker, and PACAP receptor (PAC1-R) immunoreactivity is observed in mouse infraorbital lacrimal gland acinar cells. PACAP eye drops stimulate tear secretion and increase cAMP and phosphorylated (p)-protein kinase A levels in the infraorbital lacrimal glands that could be inhibited by pre-treatment with a PAC1-R antagonist or an adenylate cyclase inhibitor. Moreover, these eye drops suppress corneal keratinization in PACAP-null mice. PACAP eye drops increase aquaporin 5 (AQP5) levels in the membrane and pAQP5 levels in the infraorbital lacrimal glands. AQP5 siRNA treatment of the infraorbital lacrimal gland attenuates PACAP-induced tear secretion. Based on these results, PACAP might be clinically useful to treat dry eye disorder.

Expression and Regulation of Pituitary Adenylate Cyclase-Activating Polypeptide in Rat Placental Cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) was first identified as a hypophysiotropic factor that regulates pituitary cell functions and has been subsequently shown to be widely distributed and have multiple functions. The PACAP is known to be expressed in placental tissues and is suggested to have a critical role in physiological function of the placenta. In addition to PACAP, the hypothalamic peptides kisspeptin and gonadotropin-releasing hormone (GnRH) are also expressed in placental cells. In this study, we used primary cultures of placental tissues from rats of 16 to 18 days gestation and examined the regulation and function of PACAP. The PACAP messenger RNA (mRNA) expression and PACAP-immunoreactive cells were detected in primary cultures of rat placental cells. The PACAP mRNA expression in placental cells was upregulated in the presence of the sex steroids estradiol and progesterone; however, their combined treatment failed to enhance their individual effects. When the cells were stimulated with kisspeptin, PACAP mRNA expression was increased. Similarly, GnRH had a stimulatory effect on PACAP expression. Conversely, kisspeptin expression in placental cells was increased by PACAP stimulation, whereas PACAP failed to stimulate GnRH mRNA expression in these cells. Finally, we found that PACAP had a stimulatory effect on human chorionic gonadotropin expression in placental cells. Our current observations suggest that the hypothalamic peptides PACAP, kisspeptin, and GnRH are interrelated and maintain placental functions.

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.

Investigation of the possible functions of PACAP in human trophoblast cells

Pituitary adenylate cyclase activating polypeptide (PACAP) is an endogenous neuropeptide having a widespread distribution both in the nervous system and peripheral organs including the female reproductive system. Both the peptide and its receptors have been shown in the placenta but its role in placental growth, especially its human aspects, remains unknown. The aim of the present study was to investigate the effects of PACAP on invasion, proliferation, cell survival, and angiogenesis of trophoblast cells. Furthermore, cytokine production was investigated in human decidual and peripheral blood mononuclear cells. For in vitro studies, human invasive proliferative extravillous cytotrophoblast (HIPEC) cells and HTR-8/SVneo human trophoblast cells were used. Both cell types were used for testing the effects of PACAP on invasion and cell survival in order to investigate whether the effects of PACAP in trophoblasts depend on the examined cell type. Invasion was studied by standardized invasion assay. PACAP increased proliferation in HIPEC cells, but not in HTR-8 cells. Cell viability was examined using MTT test, WST-1 assay, and annexin V/propidium iodide flow cytometry assay. Survival of HTR-8/SVneo cells was studied under oxidative stress conditions induced by hydrogen peroxide. PACAP as pretreatment, but not as co-treatment, significantly increased the number of surviving HTR-8 cells. Viability of HIPEC cells was investigated using methotrexate (MTX) toxicity, but PACAP1-38 could not counteract its toxic effect. Angiogenic molecules were determined both in the supernatant and the cell lysate by angiogenesis array. In the supernatant, we found that PACAP decreased the secretion of various angiogenic markers, such as angiopoietin, angiogenin, activin, endoglin, ADAMTS-1, and VEGF. For the cytokine assay, human decidual and peripheral blood lymphocytes were separated and treated with PACAP1-38. Th1 and Th2 cytokines were analyzed with CBA assay and the results showed that there were no significant differences in control and PACAP-treated cells. In summary, PACAP seems to play various roles in human trophoblast cells, depending on the cell type and microenvironmental influences.

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.

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.

Neuropeptide PACAP in mouse liver ischemia and reperfusion injury: immunomodulation by the cAMP-PKA pathway

Hepatic ischemia and reperfusion injury (IRI), an exogenous antigen-independent local inflammation response, occurs in multiple clinical settings, including liver transplantation, hepatic resection, trauma, and shock. The immune system and the nervous system maintain extensive communication and mount a variety of integrated responses to danger signals through intricate chemical messengers. This study examined the function and potential therapeutic potential of neuropeptide pituitary adenylate cyclase-activating polypeptides (PACAP) in a murine model of partial liver "warm" ischemia (90 minutes) followed by reperfusion. Liver IRI readily triggered the expression of intrinsic PACAP and its receptors, whereas the hepatocellular damage was exacerbated in PACAP-deficient mice. Conversely, PACAP27, or PACAP38 peptide monotherapy, which elevates intracellular cyclic adenosine monophosphate/protein kinase A (cAMP-PKA) signaling, protected livers from IRI, as evidenced by diminished serum alanine aminotransferase levels and well-preserved tissue architecture. The liver protection rendered by PACAP peptides was accompanied by diminished neutrophil/macrophage infiltration and activation, reduced hepatocyte necrosis/apoptosis, and selectively augmented hepatic interleukin (IL)-10 expression. Strikingly, PKA inhibition readily restored liver damage in otherwise IR-resistant, PACAP-conditioned mice. In vitro, PACAP treatment not only diminished macrophage tumor necrosis factor alpha/IL-6/IL-12 levels in a PKA-dependent manner, but also prevented necrosis and apoptosis in primary mouse hepatocyte cultures.

CONCLUSION

Our novel findings document the importance of PACAP-mediated cAMP-PKA signaling in hepatic homeostasis and cytoprotection in vivo. Because the enhancement of neural modulation differentially regulates local inflammation and prevents hepatocyte death, these results provide the rationale for novel approaches to manage liver inflammation and IRI in transplant patients.

Role of PACAP in Female Fertility and Reproduction at Gonadal Level - Recent Advances

Pituitary adenylate cyclase activating polypeptide (PACAP) is a pleiotropic neuropeptide, first isolated from hypothalamic extracts, but later shown in peripheral organs, such as endocrine glands, gastrointestinal system, cardiovascular system, and reproductive organs. PACAP plays a role in fertility and reproduction. Numerous studies report on the gonadal regulatory effects of PACAP at hypothalamo-hypophyseal levels. However, the local effects of PACAP at gonadal levels are also important. The present review summarizes the effects of PACAP in the ovary. PACAP and its receptors are present in the ovary, and PACAP plays a role in germ cell migration, meiotic division, follicular development, and atresia. The autocrine-paracrine hormonal effects seem to play a regulatory role in ovulation, luteinization, and follicular atrophy. Altogether, PACAP belongs to the ovarian regulatory peptides.

Mice deficient in pituitary adenylate cyclase activating polypeptide (PACAP) are more susceptible to retinal ischemic injury in vivo

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuroprotective peptide exerting protective effects in neuronal injuries. We have provided evidence that PACAP is neuroprotective in several models of retinal degeneration in vivo. Our previous studies showed that PACAP treatment ameliorated the damaging effects of chronic hypoperfusion modeled by permanent bilateral carotid artery occlusion. We have also demonstrated in earlier studies that treatment with PACAP antagonists further aggravates retinal lesions. It has been shown that PACAP deficient mice have larger infarct size in cerebral ischemia. The aim of this study was to compare the degree of retinal damage in wild type and PACAP deficient mice in ischemic retinal insult. Mice underwent 10 min of bilateral carotid artery occlusion followed by 2-week reperfusion period. Retinas were then processed for histological analysis. It was found that PACAP deficient mice had significantly greater retinal damage, as shown by the thickness of the whole retina, the morphometric analysis of the individual retinal layers, and the cell numbers in the inner nuclear and ganglion cell layers. Exogenous PACAP administration could partially protect against retinal degeneration in PACAP deficient mice. These results clearly show that endogenous PACAP reacts as a stress-response peptide that is necessary for endogenous protection against different retinal insults.

Stimulatory effect of pituitary adenylate-cyclase activating polypeptide (PACAP) and its PACAP type I receptor (PAC1R) on prolactin synthesis in rat pituitary somatolactotroph GH3 cells

In this present study, we investigated the role of pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor, PACAP type I receptor (PAC1R) on prolactin synthesis in pituitary somatolactotroph GH3 cells. PACAP increased prolactin promoter activity up to 1.3 ± 0.1-fold. This increase, while significant, was less than the increase resulting from thyrotropin-releasing hormone (TRH) stimulation. By transfection of a PAC1R expression vector to the cells, the response to PACAP on prolactin promoter activity was dramatically potentiated to a degree proportional to the amount of PAC1R transfected. In the PAC1R expressing GH3 cells, TRH and PACAP alone increased prolactin promoter up to 3.3 ± 0.3-fold and 4.9 ± 0.2-fold, respectively, and combined treatment with TRH and PACAP further increased prolactin promoters up to 6.8 ± 0.6-fold. PACAP binds both Gs- and Gq-coupled receptors and stimulates adenylate cyclase/cAMP and protein kinase C/extracellular signal-regulated kinase (ERK) signaling pathways. PACAP increased ERK phosphorylation in PAC1R expressing cells to the same degree as TRH. Combined treatment with TRH and PACAP had a synergistic effect on ERK activation. GH3 cells produce both prolactin and growth hormone. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin and attenuated growth hormone mRNA expression. PACAP increased both prolactin and growth hormone mRNA levels, particularly in PAC1R expressing cells. In addition, increasing amount of PAC1R in GH3 cells potentiated the action of TRH on prolactin promoter activity, as well as on ERK phosphorylation. PAC1R was induced by PACAP itself, but not by TRH. Our current study demonstrates that PACAP and its PAC1R, functions as a stimulator of prolactin alone or with TRH in prolactin producing cells.

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.

Alterations of pituitary adenylate cyclase-activating polypeptide-like immunoreactivity in the human plasma during pregnancy and after birth

BACKGROUND

Since its discovery, several distinct effects of pituitary adenylate cyclase activating polypeptide (PACAP) have been established - predominantly in animal studies - in the nervous system, various peripheral organs as well as in the endocrine regulation. It is unknown whether PACAP has any effect on human pregnancy regarding either utero-maternal or perinatal aspects of the gestation.

AIM

We investigated alterations of PACAP38-like immunoreactivity (PACAP38-LI) in the human plasma throughout normal pregnancy, during and after delivery, and its level in the umbilical vessels, as well as in the peripheral blood of term healthy newborns.

MATERIALS AND METHODS

A 2 ml blood sample was used for each test, PACAP38-LI was determined by radioimmunoassay.

RESULTS

In the 2nd and 3rd trimester significant elevation was observed in the PACAP38-LI compared to the earlier gestation and non-pregnant conditions. During delivery its level significantly decreased and returned to the original values 3 days after birth. In the neonates PACAP38-LI level of the peripheral blood was similar to that of healthy adults, but umbilical arteries and veins contained significantly lower concentrations of PACAP38-LI. Besides, the levels were lower in the umbilical vein compared to the artery.

CONCLUSIONS

PACAP38-LI levels show sensitive change during normal pregnancy and delivery. Our findings suggest that the fetal organs actively synthesize PACAP. Further investigations are required to elucidate the physiological importance of the alterations observed.

Changes in the expression of pituitary adenylate cyclase-activating polypeptide in the human placenta during pregnancy and its effects on the survival of JAR choriocarcinoma cells

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide with survival-promoting actions, has been observed in endocrine organs and is thought to play a role in reproductive functions, including pregnancy. PACAP occurs in two forms, 27 and 38 amino acid residues, with PACAP38 being the predominant form in human tissues. In the present study, we determined the concentrations of PACAP38 and PACAP27 in first-trimester and full-term human placentas using radioimmunoassay. We found high levels of PACAP38 and lower levels of PACAP27 in different parts of the full-term human placenta. PACAP38 content increased in the placenta during pregnancy, both on the maternal side and on the fetal side. The effects of PACAP on the survival of JAR human choriocarcinoma cells were investigated using flow cytometry and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) cell viability assay in cells exposed to the widely used chemotherapeutic agent methotrexate (MTX). It was found that PACAP neither influenced the survival of JAR cytotrophoblast cells nor affected cellular response to the death-inducing effect of the chemotherapeutic agent MTX. The present observations further support the significance of PACAP in the human placenta. The observation that PACAP did not influence the effects of MTX may have future clinical importance, showing that PACAP does not decrease the effects of certain chemotherapeutic agents.

Control of oocyte release by progesterone receptor-regulated gene expression

The progesterone receptor (PGR) is a nuclear receptor transcription factor that is essential for female fertility, in part due to its control of oocyte release from the ovary, or ovulation. In all mammals studied to date, ovarian expression of PGR is restricted primarily to granulosa cells of follicles destined to ovulate. Granulosa cell expression of PGR is induced by the pituitary Luteinizing Hormone (LH) surge via mechanisms that are not entirely understood, but which involve activation of Protein Kinase A and modification of Sp1/Sp3 transcription factors on the PGR promoter. Null mutations for PGR or treatment with PGR antagonists block ovulation in all species analyzed, including humans. The cellular mechanisms by which PGR regulates ovulation are currently under investigation, with several downstream pathways having been identified as PGR-regulated and potentially involved in follicular rupture. Interestingly, none of these PGR-regulated genes has been demonstrated to be a direct transcriptional target of PGR. Rather, in ovarian granulosa cells, PGR may act as an inducible coregulator for constitutively bound Sp1/Sp3 transcription factors, which are key regulators for a discrete cohort of ovulatory genes.

Effects of pituitary adenylate cyclase activating polypeptide on the survival and signal transduction pathways in human choriocarcinoma cells

The pituitary adenylate cyclase activating polypeptide (PACAP) has several effects in endocrine and reproductive organs, including the placenta. PACAP is generally known as a survival-promoting peptide acting on divergent signal transduction pathways. However, its effects on the survival and signaling mechanisms of trophoblast cells are not known. In the present study we found that 1-h pretreatment with PACAP38 did not significantly influence the survival of JAR cytotrophoblast cells. However, the survival rate of cells exposed to oxidative stress or CoCl(2)-induced in vitro hypoxia showed a significant further decrease in PACAP-treated cells, implying that PACAP sensitizes the cells to these stressors. This was not observed in the case of lipopolysaccharide or ethanol treatment. Western blot data revealed that, in cells exposed to oxidative stress, PACAP treatment decreased phosphorylation of all extracellular signal-regulated kinase (ERK), phospho-jun N-terminal kinase (JNK), protein kinase B, p38 mitogen-activated protein kinase (MAPK), and phospho-glycogen synthase kinase (GSK) and the expression of bax. The overall effect seems to be a sensitizing effect in almost all examined pathways when oxidative stress was applied, which may explain the enhancing effect of PACAP on cell death in contrast to most other cell types examined so far. Our data show that the signaling mechanism of PACAP may be different in trophoblast cells to that observed in other cell lines.

Hormones and receptors in fish: do duplicates matter?

Modern fish are the result of major changes in evolution including three possible duplications of the whole genome. Retained duplicate genes are often involved with metabolism, transcription, neurogenic processes and development. Here we examine the consequences of the most recent (350 mya) teleost-specific duplication in five fishes (zebrafish, fugu, medaka, stickleback and rainbow trout) in regard to duplicate copies of hormones and receptors in the secretin superfamily. This subset of genes was selected as the superfamily is limited to ten hormones and their receptors and includes some important members: glucagon, growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP). We used reports from the literature and an extensive database search of the fish genomes to evaluate the status of the superfamily and its duplicate genes. We found that all five fish species have an almost complete set of orthologs with the human superfamily of hormones, although they lack secretin and its receptor. Receptor orthologs are present in zebrafish, fugu, medaka, stickleback and to a lesser extent in salmonids. Zebrafish retain duplicate copies for seven hormones and five receptors. Duplicated genes in fugu, medaka, stickleback and salmonids are also present, based mainly on genome annotation or mRNA transcription. Separate chromosome locations and synteny support zebrafish duplicates as the result of large-scale duplications. Novel changes in fish include the modification of a duplicate glucagon receptor to a GLP-1 receptor and, unlike humans, the presence of bioactive and specific PHI and GHRH-like peptide receptors. We conclude that fish duplicates in the secretin superfamily are a rich, mostly unexplored area for endocrine research.