Ontogeny of pituitary adenylate cyclase-activating polypeptide (PACAP) in the frog (Rana ridibunda) tadpole brain: immunohistochemical localization and biochemical characterization

Organization of serotonergic system in Sphaerotheca breviceps (Dicroglossidae) tadpole brain

The monoaminergic neurotransmitter 5-hydroxytryptamine (5-HT) is known to be involved in several physiological, behavioural and neuroendocrine functions in vertebrates. In this study, we investigated the distribution of 5-HT neuronal system in the central nervous system (CNS) of Sphaerotheca breviceps tadpoles at metamorphic climax stage. In the telencephalon, there was no 5-HT-immunoreactive (5-HT-ir) perikarya, but conspicuous fibres were observed in the olfactory bulb, pallium, subpallium and amygdala complexes. The preoptic area showed dense 5-HT-ir somata and cerebrospinal fluid contacting fibres, whereas a few varicose 5-HT-ir fibres were noticed in the suprachiasmatic nucleus. 5-HT-ir cells and fibres were found in the ventral, lateral dorsal subdivisions of the hypothalamus and in the nucleus tuberculi posterioris, but only 5-HT-ir fibres were localised in the periventricular area and pituitary gland. Numerous 5-HT-ir cells and/or fibres were detected in the thalamus, entopeduncular area and mesencephalic subdivisions. In the rhombencephalon, although 5-HT-ir cells and fibres were noticed in the subdivisions of the raphe nucleus and reticular formation, a moderate plexus of fibres was observed in the cerebellum, parabrachial nucleus and solitary tract. Distinct 5-HT-ir fibres, but no perikarya, were observed in the rostral spinal cord. Overall, extensively labelled 5-HT-ir cells and fibres in the CNS of the metamorphic tadpole suggest possible roles for the involvement of 5-HT in various somatosensory, behavioural and neuroendocrine functions during final stages of development.

NPY up-regulation in the tadpole brain of Euphlyctis cyanophlyctis during osmotic stress

Most of the amphibians breed in temporary ponds vulnerable to occasional desiccation, thus, leaving their larvae exposed to stressful fluctuations in various environmental parameters including salinity. These animals possess a well suited central adaptive mechanism to adapt to these alterations. Neuropeptide Y (NPY), a 36 amino acid neurotransmitter, has been reported to antagonize various neuropsychological consequences of stress within the mammalian brain. Osmotic regulation of NPY in the hypothalamo-neurohypophysial pathway of mammalian brain is also known. Although the molecule possesses an extensive distribution in the brain of amphibians, its functional association is not well understood. We have investigated the endogenous response of NPY-ergic system to osmotically stressful conditions in the brain of Indian skipper frog-Euphlyctis cyanophlyctis tadpoles. Using Immunohistochemistry, we observed an up-regulation of NPY immunoreactivity (NPY-ir) in the brain of tadpoles exposed to stressful salt concentrations. A significant increase of NPY-ir occurred in the pallium and septum regions of telencephalon; preoptic area, epithalamic, thalamic and hypothalamic parts of diencephalon. Most of the regions are implicated in the modulation of stress and anxiety related brain functions and have also been shown to respond to the salinity stress in mammals. In addition, NPY producing neurons in pre-optic and hypothalamic parts show a close co-existence with the vasopressin-ergic neurons. Thus, our study suggests a possible role of NPY in stabilizing the neuro-endocrinological consequences of osmotic stress in an amphibian brain.

Temperature dependent changes in cocaine- and amphetamine regulated transcript (CART) peptide in the brain of tadpole, Sylvirana temporalis

Cocaine- and amphetamine-regulated transcript peptide (CARTp) has emerged as a novel neurotransmitter in the brain. Although the physiological role of the peptide has been intensely investigated in mammals, its role in amphibians has not been investigated. In the present study, an attempt has been undertaken to study the expression of CART in the tadpole brain of frog Sylvirana temporalis, subjected to thermal stress. Cells with strong CART-immunoreactivity were observed in the nucleus preoptic area (NPO) of tadpoles exposed to high temperature (37±2°C) as compared to those in the tadpoles exposed to low (12±2°C) and normal (24±2°C) temperatures. In the ventromedial thalamic nucleus (VM) and nucleus posterocentralis thalami (NPC), moderate CART-ir cells were observed in the control groups while number of cells and intensity of immunoreactivity was increased in tadpoles at low and high temperatures. In the nucleus infundibularis ventralis (NIV) and raphe nucleus (RA), CART immunoreactivity increased in the low as well as high temperature treated groups. Intensely stained CART cells were observed in the pituitary of tadpoles exposed to high temperature as compared to low temperature and control groups. We suggest that CART system in the brain and pituitary of tadpole may play a very important role in mediating responses to temperature variations in the environment.

Ontogeny of cocaine- and amphetamine-regulated transcript peptide in brain of frog, Microhyla ornata

The cocaine- and amphetamine-regulated transcript (CART) peptide is widely distributed in the brains of adult vertebrates including amphibians. Several physiological roles of CART have been intensely investigated in mammals. Despite these studies, the expression of CART during development of brain has not been studied in amphibians. In the present study, distribution of CART was investigated during development in the post hatched stage 23 to premetamorphic stage 30 of frog Microhyla ornata. CART is expressed as early as in stage 23 in ventral thalamus and rhombencephalon. As development progressed, CART immunoreactivity was observed in the olfactory bulb, telencephalon, rhombencephalon and spinal cord in stage 24. At stage 25, the CART immunoreactivity was observed in the ventromedial thalamic nucleus, posterocentral thalamic nucleus, torus nucleus, central gray and inferior reticular nucleus. In stage 26, CART reactivity was seen in the medial septum, preoptic area, nucleus entopeduncularis, magnocellular nucleus, median eminence, optic tectum, hypophysis and cerebellum. Additionally, CART immunoreactivity was observed in the medial pallium, anterior commissure, nucleus infundibularis dorsalis, ventralis and raphe nucleus at stage 30. The occurrences of CART immunoreactivity at early stage of development suggest that the peptide may have a functional significance during development. The wider appearance of CART in the brain of tadpoles, M. ornata suggests that the peptide may act as a neurohormone during the ontogeny.

Changes in the distribution of corticotropin-releasing factor (CRF)-like immunoreactivity in the larval bullfrog brain and the involvement of CRF in the cessation of food intake during metamorphosis

In submammalian vertebrates, corticotropin-releasing factor (CRF) acts as an anorexigenic neuropeptide as well as a potent stimulator of corticotropin and thyrotropin release from the pituitary. As a step for demonstrating the involvement of CRF in the feeding regulation of anuran larvae, which are known to stop feeding toward the metamorphic climax, we studied firstly the changes in the distribution of CRF-like immunoreactivity (CRF-LI) in the brain of metamorphosing bullfrog larvae. Neuronal cell bodies showing CRF-LI were invariably present in the thalamic regions throughout larval development. Cells with CRF-LI were also found in the hypothalamus. The number of cells with CRF-LI in the hypothalamus, but not in the thalamus, showed a significant increase as metamorphosis progressed. Immunoreactive nerve fibers were observed mainly in the median eminence, and became abundant as metamorphosis proceeded. The number of cells showing CRF-LI in the hypothalamus as well as the density of immunoreactive fibers in the median eminence decreased at the end of metamorphosis. Secondly, we examined the effect of intracerebroventricular (ICV) injection of CRF on the food intake in the premetamorphic larvae. ICV injection of CRF at 10 pmol/g body weight (BW) induced a significant decrease of food intake during 15 min. The CRF-induced anorexigenic action was blocked by the treatment with a CRF receptor antagonist [alpha-helical CRF(9-41)] at 100 pmol/g BW. The results suggest the involvement of CRF in the accomplishment of metamorphosis through the pituitary and in the feeding restriction that occurs during the later stages of metamorphosis through the central nervous system.

Ontogeny of PAC1-R and VPAC1-R in the frog, Rana esculenta

The distribution of pituitary adenylate cyclase-activating polypeptide (PACAP) and PACAP receptors in the brain of amphibians has been previously described. In the present study, we have investigated the ontogeny of the selective PACAP receptor, PAC1-R, and the PACAP-vasoactive intestinal polypeptide (VIP) mutual receptor, VPAC1-R, in frog embryos by whole-mount in situ hybridization histochemistry. At stage 20, expression of PAC1-R and/or VPAC1-R mRNAs was detected in the brain, the auditory vesicles, the external gills, the buds of the lateral lines and the coelomatic cavity. At stage 25, PAC1-R and/or VPAC1-R mRNAs were observed in the buds of the orbital lateral line, the pancreas and heart. At stage 30, PAC1-R and VPAC1-R mRNAs were widely distributed in the telencephalon and diencephalon as well as in the bud of the lateral line, the heart and the pancreas. The anatomical distribution of PAC1-R and VPAC1-R mRNAs, although similar, did not totally overlap, indicating that PACAP and VIP may exert differential effects in frog during development.

VIP and PACAP stimulate TSH release from the bullfrog pituitary

We have recently shown that corticotropin-releasing hormone (CRH) is a major thyrotropin (TSH)-releasing factor in amphibians, but we have also found that, besides CRH, other hypothalamic substances stimulate TSH secretion in frog. In order to characterize novel TSH secretagogues, we have investigated the effect of frog (Rana ridibunda) vasoactive intestinal polypeptide (VIP) (fVIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) (fPACAP38 and PACAP27) on TSH release from bullfrog (Rana catesbeiana) pituitary cells in primary culture. Incubation of pituitary cells for 24h with graded concentrations of fVIP, fPACAP38 and PACAP27 (10(-9) to 10(-6)M) induced a dose-dependent stimulation of TSH release with minimum effective doses of 10(-9)M for fVIP and 10(-8)M for fPACAP38 and PACAP27. The PAC1-R/VPAC2-R antagonist PACAP(6-38) (10(-7) and 10(-6)M) dose-dependently suppressed the stimulatory effects of fVIP and fPACAP38 (10(-7)M each). Likewise, this antagonist (10(-6) and 10(-5)M) dose-dependently attenuated the stimulatory effect of PACAP27 (10(-7)M). On the other hand, the VPAC1-R/VPAC2-R antagonist [d-pCl-Phe(6), Leu(17)]VIP (10(-6) and 10(-5)M) dose-dependently inhibited the stimulatory effect of fVIP (10(-9)M) and PACAP27 (10(-8)M), but did not affect the response to fPACAP38 (10(-8)M). These data indicate that, in amphibians, the activity of thyrotrophs can be regulated by VIP and PACAP acting likely through VPAC2-R and PAC1-R.

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.

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.

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.

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.

Pituitary adenylate cyclase-activating polypeptide and its receptors in amphibians

Pituitary adenylate cyclase-activating polypeptide (PACAP), a novel peptide of the secretin/glucagon/vasoactive intestinal polypeptide superfamily, has been initially characterized in mammals in 1989 and, only 2 years later, its counterpart has been isolated in amphibians. A number of studies conducted in the frog Rana ridibunda have demonstrated that PACAP is widely distributed in the central nervous system (particularly in the hypothalamus and the median eminence) and in peripheral organs including the adrenal gland. The cDNAs encoding the PACAP precursor and 3 types of PACAP receptors have been cloned in amphibians and their distribution has been determined by in situ hybridization histochemistry. Ontogenetic studies have revealed that PACAP is expressed early in the brain of tadpoles, soon after hatching. In the frog Rana ridibunda, PACAP exerts a large array of biological effects in the brain, pituitary, adrenal gland, and ovary, suggesting that, in amphibians as in mammals, PACAP may act as neurotrophic factor, a neurotransmitter and a neurohormone.