Pituitary adenylate cyclase-activating polypeptide and its receptors in amphibians

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.

Potential Therapeutic Role of Pituitary Adenylate Cyclase-Activating Polypeptide for Dry Eye Disease

Dry eye disease (DED) is caused by a reduction in the volume or quality of tears. The prevalence of DED is estimated to be 100 million in the developed world. As aging is a risk factor for DED, the prevalence of DED is expected to grow at a rapid pace in aging populations, thus creating an increased need for new therapies. This review summarizes DED medications currently in clinical use. Most current medications for DED focus on stimulating tear secretion, mucin secretion, or suppressing inflammation, rather than simply replenishing the ocular surface with moisture to improve symptoms. We recently reported that the neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) induces tear secretion and suppresses corneal injury caused by a reduction in tears. Moreover, it has been reported that a PACAP in water and a 0.9% saline solution at +4 °C showed high stability and achieved 80-90% effectiveness after 2 weeks of treatment. These results reveal PACAP as a candidate DED medication. Further research on the clinical applications of PACAP in DED is necessary.

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.

Distribution and molecular evolution of the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors in the lizard Podarcis sicula (Squamata, Lacertidae)

The presence of the pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors PAC(1), VPAC(1), and VPAC(2) was studied in the lizard Podarcis sicula gastrointestinal and respiratory tissues. The expression and distribution of this neuropeptide was investigated using RT-PCR, immunohistochemistry, and in situ hybridization techniques. RT-PCR showed that several tissues of this reptile synthesize an mRNA encoding for PACAP. Performing in situ hybridization and immunohistochemistry, we found a wide distribution of PACAP and its mRNA in intestine, stomach, liver, and lung. PACAP receptors possess a specific distribution in both gastrointestinal and respiratory system. Further, we analyzed the conservation of PACAP amino acid sequence demonstrating that this peptide in the lizard is very similar to that of other vertebrates. Our findings suggest that also in reptiles an effective PACAP system is present and that it could be implicated in some essential physiological functions as a result of its high conservation amongst vertebrates.

PAC1 receptor localization in a model nervous system: light and electron microscopic immunocytochemistry on the earthworm ventral nerve cord ganglia

The presence and pattern of pituitary adenylate cyclase activating polypeptide (PACAP) type I (PAC1) receptors were identified by means of pre- and post-embedding immunocytochemical methods in the ventral nerve cord ganglia (VNC) of the earthworm Eisenia fetida. Light and electron microscopic observations revealed the exact anatomical positions of labeled structures suggesting that PACAP mediates the activity of some interneurons, a few small motoneurons and certain sensory fibers that are located in ventrolateral, ventromedial and intermediomedial sensory longitudinal axon bundles of the VNC ganglia. No labeling was located on large interneuronal systems such as dorsal medial and lateral giant axon systems and ventral giant axons. At the ultrastructural level labeling was mainly restricted to endo- and plasma membranes showing characteristic unequal distribution in various neuron parts. An increasing abundance of PAC1 receptors located on both rough endoplasmic reticulum and plasma membranes was seen from perikarya to neural processes, indicating that intracellular membrane traffic might play a crucial role in the transportation of PAC1 receptors. High number of PAC1 receptors was found in both pre- and postsynaptic membranes in addition to extrasynaptic sites suggesting that PACAP acts as neurotransmitter and neuromodulator in the earthworm nervous system.

Actions of PACAP and VIP on melanotrope cells of Xenopus laevis

The neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are implicated in the regulation of gene expression and hormone secretion in mammalian melanotrope cells and a mammalian pro-opiomelanocortin (POMC)-producing tumor cell line, but the physiological relevance of this regulation is elusive. The purpose of the present study was to establish if these peptides affect biosynthetic and secretory processes in a well-established physiological model for endocrine cell functioning, the pituitary melanotrope cells of the amphibian Xenopus laevis, which hormonally control the process of skin color adaptation to background illumination. We show that both PACAP and VIP are capable of stimulating the secretory process of the Xenopus melanotrope cell. As the peptides are equipotent, they may exert their actions via a VPAC receptor. Moreover, PACAP stimulated POMC biosynthesis and POMC gene expression. Strong anti-PACAP immunoreactivity was found in the pituitary pars nervosa (PN), suggesting that this neurohemal organ is a source of neurohormonal PACAP action on the melanotropes in the intermediate pituitary. We propose that the PACAP/VIP family of peptides has a physiological function in regulating Xenopus melanotrope cell activity during the process of skin color adaptation.

Molecular characterization and gene expression of the pituitary adenylate cyclase-activating polypeptide (PACAP) in the lizard brain

The pituitary adenylate cyclase-activating polypeptide (PACAP) is considered a pleiotropic neuropeptide in vertebrate physiology. The nucleotide sequence, the expression and the distribution of PACAP were determined in the brain of the lizard Podarcis sicula. RT-PCR showed that the brain of this reptile synthesizes an mRNA coding for PACAP. By performing in situ hybridization and immunohistochemistry techniques, a wide distribution of PACAP and its mRNA in neurons, nervous fibers and other cells was found. Phylogenetic sequence analysis indicates that lizard PACAP is highly conserved, resembling the vertebrate PACAP. Our data demonstrate that PACAP is not only highly preserved during vertebrate evolution but also suggest that PACAP could be implicated in a wide number of functions in the physiology of the reptile brain.

Localization and characterization of pituitary adenylate cyclase-activating polypeptide receptors in the human cerebellum during development

Pituitary adenylate cyclase-activating polypeptide (PACAP) receptors are actively expressed in the cortical layers of the cerebellum of rodents and contribute to cerebellar development. The present report provides the first anatomical localization and characterization of PACAP receptors in the developing human cerebellum. RT-PCR analysis from 15-week-old fetuses to 22-year-old subject showed that PAC1-R and VPAC1-R are expressed in the cerebellum at all stages, whereas VPAC2-R mRNA was barely detectable. In situ hybridization labeling indicated that, in human fetuses, PAC1-R mRNA is associated with the external granule cell layer (EGL), a germinative neuroepithelium, and with the internal granule cell layer (IGL). The distribution pattern of VPAC1-R mRNA was very similar to that of PAC1-R mRNA, whereas VPAC2-R mRNA was visualized only in 7-22-year-old subjects. The localization of [(125)I]PACAP27 binding sites was fully consistent with the distribution of PAC1-R and VPAC1-R mRNA. Pharmacological characterization revealed that, in the EGL and IGL from 15-24-week-old fetuses and in the granule cell layer from 7-22-year-old patients, binding sites exhibit a PAC1-R profile. In contrast, PACAP binding sites observed in the molecular layer and medulla of the adult cerebellum consisted of a heterogeneous population of PAC1-R and VPAC(1/2)-R. Altogether, these data provide the first evidence that PACAP receptors are expressed in the human cerebellar cortex. PAC1-R is the predominant PACAP receptor found in fetuses, and both PAC1-R and VPAC1-R are expressed in the mature cerebellum. These observations suggest that PACAP has neurodevelopmental functions in the human cerebellum.

Phosphoproteomic analysis of the effect of cyclo-[His-Pro] dipeptide on PC12 cells

The effects of dipeptide cyclo-[His-Pro] (CHP), known to participate in the appetite behavior and food intake control, have been investigated using PC12 cells in culture as model system. We found that only in the presence of experimental conditions that cause cellular stress the cyclic dipeptide affect cellular proliferation and protects from apoptosis. It greatly enhances the phosphorylation of hsp27, alpha-B-crystallin, Cdc2, and p-38 MAPK, whereas it decreases the phosphorylation of MEK1, Cav 2, GSK3a, PKB/Akt, PKCdelta, PKCgamma, and Erk2. PKA and PKG are involved in ERK1/2 deactivation via a receptor that appears to be dually coupled to Gs and Gq protein subfamilies.

Pituitary adenylate cyclase-activating polypeptide (PACAP) as a growth hormone (GH)-releasing factor in grass carp. I. Functional coupling of cyclic adenosine 3',5'-monophosphate and Ca2+/calmodulin-dependent signaling pathways in PACAP-induced GH secretion and GH gene expression in grass carp pituitary cells

Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the glucagon/secretin peptide family, has been recently proposed to be the ancestral GH-releasing factor. Using grass carp as a model for bony fish, we examined the mechanisms for PACAP regulation of GH synthesis and secretion at the pituitary level. Nerve fibers with PACAP immunoreactivity were identified in the grass carp pituitary overlapping with the distribution of somatotrophs. At the somatotroph level, PACAP was shown to induce cAMP synthesis and Ca(2+) entry through voltage-sensitive Ca(2+) channels (VSCC). In carp pituitary cells, PACAP but not vasoactive intestinal polypeptide increased GH release, GH content, total GH production, and steady-state GH mRNA levels. PACAP also enhanced GH mRNA stability, GH promoter activity, and nuclear expression of GH primary transcripts. Increasing cAMP levels, induction of Ca(2+) entry, and activation of VSCC were all effective in elevating GH secretion and GH mRNA levels. PACAP-induced GH secretion and GH mRNA expression, however, were abolished by inhibiting adenylate cyclase and protein kinase A, removing extracellular Ca(2+) or VSCC blockade, or inactivating calmodulin (CaM)-dependent protein kinase II (CaM kinase II). Similar sensitivity to VSCC and CaM kinase II blockade was also observed by activating cAMP production as a trigger for GH release and GH gene expression. These results suggest that PACAP stimulates GH synthesis and secretion in grass carp pituitary cells through PAC(1) receptors. These stimulatory actions probably are mediated by the adenylate cyclase/cAMP/protein kinase A pathway coupled to Ca(2+) entry via VSCC and subsequent activation of CaM/CaM kinase II cascades.

Pituitary adenylate cyclase-activating polypeptide (PACAP)-like immunoreactivity in the brain of a teleost, Uranoscopus japonicus: immunohistochemical relationship between PACAP and adenohypophysial hormones

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) could play a role in stimulating pituitary hormone release in fish brain. In this study, we used immunochemical techniques to examine the histological and quantitative distribution of PACAP in the central nervous system (CNS) of a teleost, the stargazer, Uranoscopus japonicus. In addition, high performance liquid chromatographic (HPLC) analysis was performed to characterize the form of PACAP present, while the relationship between PACAP and adenohypophysial hormones was also determined immunohistochemically. PACAP-like immunoreactive (LI) neuronal cell bodies and fibers were found not only in the hypothalamo-pituitary region but also in the midbrain and hindbrain regions. PACAP-LI fibers were identified in the neurohypophysis in close proximity to pituitary cells containing immunoreactive hormones such as somatolactin, the N-terminal peptide of proopiomelanocortin, and N-acetyl endorphin. The concentration of immunoreactive PACAP in whole brain tissue was approximately 300 pmol/g wet weight. The average concentrations of immunoreactive PACAP in regions of the telencephalon, diencephalon, tectum, cerebellum, and rhombencephalon were 217.53, 510.26, 83.30, 148.64, and 364.62 pmol/g, respectively. In reverse-phase HPLC experiments, the predominant form of immunoreactive PACAP eluted closely with synthetic stargazer PACAP38, while PACAP27-like immunoreactivity was negligible. These results suggest that PACAP38 is the predominant PACAP form in the stargazer CNS, and that PACAP acts not only as a hypophysiotropic factor for adenohypophysial hormone release but also as a neurotransmitter and neuromodulator in the CNS.

Regulation of growth hormone release in common carp pituitary cells by pituitary adenylate cyclase-activating polypeptide: signal transduction involves cAMP- and calcium-dependent mechanisms

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the glucagon/secretin peptide family and its molecular structure is highly conserved among vertebrates. In this study, the role of PACAP in regulating growth hormone (GH) secretion in fish was examined in vitro using common carp pituitary cells under column perifusion. A dose-dependent increase in GH release was observed after exposing pituitary cells to increasing doses of ovine PACAP38 (oPACAP38) and PACAP27 (oPACAP27), but not vasoactive intestinal polypeptide (VIP). A lack of GH response to VIP stimulation is consistent with the pharmacological properties of PAC-1 receptors, suggesting that this receptor subtype may be involved in PACAP-induced GH secretion in carp species. Although the maximal GH responses induced by oPACAP38 and oPACAP27 were similar, the minimal effective dose and ED50 value for oPACAP38 were significantly lower than that for oPACAP27. These results may indicate that common carp PAC-1 receptors are more sensitive to stimulation by oPACAP38 than by oPACAP27. In parallel studies, oPACAP38 and oPACAP27 were also effective in increasing cAMP release, cellular cAMP content, total cAMP production, and intracellular Ca(2+) ([Ca(2+)](i)) levels in common carp pituitary cells. Besides, the rise in [Ca(2+)](i) induced by oPACAP38 was blocked by removing extracellular Ca(2+) ([Ca(2+)](e)) or by treatment with nifedipine, an inhibitor of voltage-sensitive Ca(2+) channels (VSCC). The dose dependence of PACAP-stimulated GH release in common carp pituitary cells was mimicked by activating adenylate cyclase using forskolin, inhibiting cAMP degradation using IBMX, increasing functional levels of intracellular cAMP using CPT-cAMP, or inducing [Ca(2+)](e) entry using the Ca(2+) ionophore A23187. In contrast, the GH-releasing effect of oPACAP38 was suppressed by treatment with the adenylate cyclase inhibitor MDL12330A, protein kinase A inhibitor H89, and VSCC blocker nifedipine, or by perifusion with a Ca(2+)-free culture medium. These results, as a whole, suggest that PACAP functions as a GH-releasing factor in common carp by activating pituitary receptors resembling mammalian PAC-1 receptors. Apparently, the GH-releasing action of PACAP is mediated through the adenylate cyclase/cAMP/protein kinase A pathway and [Ca(2+)](e) influx through VSCC.

Novel splice variants of type I pituitary adenylate cyclase-activating polypeptide receptor in frog exhibit altered adenylate cyclase stimulation and differential relative abundance

Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts its various effects through activation of two types of G protein-coupled receptors, a receptor with high affinity for PACAP named PAC1-R and two receptors exhibiting similar affinity for both PACAP and vasoactive intestinal polypeptide named VPAC1-R and VPAC2-R. Here, we report the characterization of PAC1-R and novel splice variants in the frog Rana ridibunda. The frog PAC1-R has 78% homology with human PAC1-R and is highly expressed in the central nervous system. Two splice variants of the frog receptor that display additional amino acid cassettes in the third intracellular loop were characterized. PAC1-R25 carries a 25-amino acid insertion that matches the hop cassette of the mammalian receptor, whereas PAC1-R41 carries a cassette with no homology to any mammalian PAC1-R variant. A third splice variant of PAC1-R, exhibiting a completely different intracellular C-terminal domain, named PAC1-Rmc has also been identified. Determination of cAMP formation in cells transfected with the cloned receptors showed that PACAP activated PAC1-R, PAC1-R25, and PAC1-R41 with similar potency. In contrast, PACAP failed to stimulate adenylate cyclase in cells transfected with PAC1-Rmc. Fusion of PAC1-R or PAC1-Rmc with the green fluorescent protein revealed that both receptors are expressed and targeted to the plasma membrane in transfected cells. The different PAC1-R variants are highly expressed in the frog brain and spinal cord and to a lesser extent in peripheral tissues, where only certain isoforms could be detected. The present data indicate that in frog, PACAP may act through different PAC1-R splice variants that differ in their G(s) protein coupling and their abundance in various tissues.

Multiple control and dynamic response of the Xenopus melanotrope cell

Some amphibian brain-melanotrope cell systems are used to study how neuronal and (neuro)endocrine mechanisms convert environmental signals into physiological responses. Pituitary melanotropes release alpha-melanophore-stimulating hormone (alpha-MSH), which controls skin color in response to background light stimuli. Xenopus laevis suprachiasmatic neurons receive optic input and inhibit melanotrope activity by releasing neuropeptide Y (NPY), dopamine (DA) and gamma-aminobutyric acid (GABA) when animals are placed on a light background. Under this condition, they strengthen their synaptic contacts with the melanotropes and enhance their secretory machinery by upregulating exocytosis-related proteins (e.g. SNAP-25). The inhibitory transmitters converge on the adenylyl cyclase system, regulating Ca(2+) channel activity. Other messengers like thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH, from the magnocellular nucleus), noradrenalin (from the locus coeruleus), serotonin (from the raphe nucleus) and acetylcholine (from the melanotropes themselves) stimulate melanotrope activity. Ca(2+) enters the cell and the resulting Ca(2+) oscillations trigger alpha-MSH secretion. These intracellular Ca(2+) dynamics can be described by a mathematical model. The oscillations travel as a wave through the cytoplasm and enter the nucleus where they may induce the expression of genes involved in biosynthesis and processing (7B2, PC2) of pro-opiomelanocortin (POMC) and release (SNAP-25, munc18) of its end-products. We propose that various environmental factors (e.g. light and temperature) act via distinct brain centers in order to release various neuronal messengers that act on the melanotrope to control distinct subcellular events (e.g. hormone biosynthesis, processing and release) by specifically shaping the pattern of melanotrope Ca(2+) oscillations.