Immunohistochemical localization of corticotropin-releasing factor- and arginine vasotocin-like immunoreactivities in the brain and pituitary of the American bullfrog (Rana catesbeiana) during development and metamorphosis

Ontogenetic Expression of Aquaporins in the Kidney and Urinary Bladder of the Japanese Tree Frog, Dryophytes japonicus

For adult anuran amphibians, the kidney and urinary bladder play important osmoregulatory roles through water reabsorption. In the present study, we have examined ontogenetic expression of aquaporins, i.e., AQP2, AQPamU (AQP6ub, AQPa2U), and AQP3, in these organs using the Japanese tree frog, Dryophytes japonicus. Immunohistochemistry using the metamorphosing larvae at stages 40-43 localized AQP2 protein to the collecting ducts in the dorsal zone of the mesonephric kidney. At prometamorphic stages 40 and 41, labelling of AQP2 protein was observed in the apical/ subapical regions of the collecting duct cells. At climax stages 42 and 43, labels for AQP2 and AQP3 became observed in the apical/subapical regions and basolateral membrane of the collecting duct cells, respectively, as seen in the adults. As for the urinary bladder, immuno-positive labels for AQPamU were localized to the apical/subapical regions of granular cells in the mucosal epithelium at stages 40-43. On the other hand, AQP3 immunoreactivity was hardly observed in the urinary bladder at stage 40, and weakly appeared in many granular cells at stage 41. Thereafter, labels for AQP3 became evident along the basolateral membrane of granular cells at stages 42 and 43, together with AQPamU in the apical/subapical regions. These results suggest that the kidney and urinary bladder might be capable of water reabsorption, via AQP2, AQPamU, and AQP3, at stage 42, contributing to the acclimation of the tree frogs to terrestrial environments.

An integrated physiological perspective on anthropogenic stressors in the Gulf coast toad (Incilius nebulifer)

Anthropogenic environmental change, including climate change and urbanization, results in warmer temperatures in both terrestrial and aquatic habitats and changes in community assemblages including invasive species introductions, among many other alterations. Anurans are particularly susceptible to these changes because generally they have a biphasic lifecycle and rely on aquatic and terrestrial habitats for survival. Changes such as warmer water temperature can result in direct and carryover effects, after metamorphosis that decrease fitness. However, Gulf Coast toads (Incilius (Bufo) nebulifer) are expanding their range, including into anthropogenically disturbed areas. We hypothesize that I. nebulifer copes with warmer water, reduced water levels, and invasive species by altering their physiology and/or behavior. Corticosterone is the primary glucocorticoid in amphibians, and it modulates many aspects of physiology and behavior, potentially including lipid storage and hop performance, during unpredictable (stressful) events. As a true toad, I. nebulifer also produces bufadienolide toxins that aid in its antipredator defense and may have tradeoffs with corticosterone. In a fully factorial design, we measured baseline corticosterone levels in tadpoles in response to two treatments: decreased water levels and increased water temperatures. After metamorphosis, we measured the corticosterone profile and other associated responses to exposure to the predatory red imported fire ant (Solenopsis invicta; RIFA). We found that tadpoles had elevated baseline corticosterone release rates when reared in warmer water and reduced water levels. Toadlets also had elevated baseline corticosterone release rates when exposed to any combination of two of the three treatments but when exposed to all three treatments toadlets instead showed elevated magnitude of their stress response. Predator avoidance (as measured by hop performance) was reduced after exposure to RIFA. Tadpoles from warmer water developed more quickly and were smaller in mass after metamorphosis. Toadlets had reduced production of two of the three detected bufadienolides and increased energy storage (lipids) after exposure to warmer water and reduced growth after exposure to reduced water levels. We found direct and carryover effects of common anthropogenic changes in I. nebulifer that may aid in their ability to persist despite these changes.

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

Arginine vasotocin is the major adrenocorticotropic hormone-releasing factor in the bullfrog Rana catesbeiana

In a previous study, we showed that corticotropin-releasing factor (CRF) is the major thyroid-stimulating hormone (TSH)-releasing factor in the bullfrog (Rana catesbeiana) hypothalamus. Our findings prompted us to ascertain whether CRF or arginine vasotocin (AVT), a known adrenocorticotropic hormone (ACTH) secretagogue in several vertebrates, is the main stimulator of the release of ACTH from the bullfrog pituitary. Both the frog CRF and AVT stimulated the release of immunoassayable ACTH from dispersed anterior pituitary cells in vitro in a concentration-dependent manner. AVT, however, exhibited far more potent ACTH-releasing activity than CRF. Although CRF by itself weakly stimulated ACTH release, it acted synergistically with AVT to enhance the release of ACTH markedly. Mesotocin and AVT-related peptides such as hydrin 1 and hydrin 2 showed relatively weak ACTH-releasing activity. Subsequently, cDNAs encoding the bullfrog AVT V1a-type and V1b-type receptors were molecularly cloned. Reverse transcriptase-PCR using specific primers revealed that the anterior lobe of the pituitary predominantly expressed AVT V1b-type receptor mRNA but scarcely expressed AVT V1a-type receptor mRNA. Abundant signals for V1b-type receptor mRNA in the corticotropes were also detected by in situ hybridization. The results obtained by the experiments with the bullfrog pituitary indicate that AVT acts as the main ACTH-releasing factor through the AVT V1b-type receptor and that CRF acts synergistically with AVT to enhance the release of ACTH.

Views on the co-evolution of the melanocortin-2 receptor, MRAPs, and the hypothalamus/pituitary/adrenal-interrenal axis

A critical regulatory component of the hypothalamus/pituitary/adrenal axis (HPA) in mammals, reptiles and birds, and in the hypothalamus/pituitary/interrenal (HPI) axis of amphibians and teleosts (modern bony fishes) is the strict ligand selectivity of the melanocortin-2 receptor (MC2R). Tetrapod and teleost MC2R orthologs can only be activated by the anterior pituitary hormone, ACTH, but not by any of the MSH-sized ligands coded in POMC. In addition, both tetrapod and teleost MC2R orthologs require co-expression with the accessory protein, MRAP. However, the MC2R ortholog of the elephant shark, a cartilaginous fish, can be activated by either ACTH or the MSH-sized ligands, and the elephant shark MC2R ortholog does not require co-expression with an MRAP for activation. Given these observations, this review will provide a scenario for the co-evolution of MC2R and MRAP, based on the assumption that the obligate interaction between MC2R and MRAP evolved during the early radiation of the ancestral bony fishes.

Development of food intake controls: neuroendocrine and environmental regulation of food intake during early life

This article is part of a Special Issue "Energy Balance". The development of neuroendocrine regulation of food intake during early life has been shaped by natural selection to allow for optimal growth and development rates needed for survival. In vertebrates, neonates or early larval forms typically exhibit "feeding drive," characterized by a developmental delay in 1) responsiveness of the hypothalamus to satiety signals (e.g., leptin, melanocortins) and 2) sensitivity to environmental cues that suppress food intake. Homeostatic regulation of food intake develops once offspring transition to later life history stages when growth is slower, neuroendocrine systems are more mature, and appetite becomes more sensitive to environmental or social cues. Across vertebrate groups, there is a tremendous amount of developmental plasticity in both food intake regulation and stress responsiveness depending on the environmental conditions experienced during early life history stages or by pregnant/brooding mothers. This plasticity is mediated through the organizing effects of hormones acting on the food intake centers of the hypothalamus during development, which alter epigenetic expression of genes associated with ingestive behaviors. Research is still needed to reveal the mechanisms through which environmental conditions during development generate and maintain these epigenetic modifications within the lifespan or across generations. Furthermore, more research is needed to determine whether observed patterns of plasticity are adaptive or pathological. It is clear, however, that developmental programming of food intake has important effects on fitness, and therefore, has ecological and evolutionary implications.

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.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Neuroanatomical distribution of vasotocin and mesotocin in two urodele amphibians (Plethodon shermani and Taricha granulosa) based on in situ hybridization histochemistry

Previous research suggests that considerable species-specific variation exists in the neuroanatomical distributions of arginine vasotocin (AVT) and mesotocin (MST), non-mammalian homologues of vasopressin and oxytocin. An earlier study in rough-skinned newts (Taricha granulosa) indicated that the neuroanatomical distribution of cells labeled for AVT-immunoreactivity (ir) was greater in this urodele amphibian than in any other species. It was unknown whether the widespread distribution of AVT-ir is unique to T. granulosa or a feature common among salamanders. Using in situ hybridization (ISH) histochemistry and gene-specific riboprobes, the current study labeled AVT and MST mRNA in T. granulosa and the red-legged salamander (Plethodon shermani). In T. granulosa, AVT ISH-labeled cells were found to be widespread and localized in brain areas including the dorsal and medial pallium, lateral and medial septum, bed nucleus of the stria terminalis, amygdala, preoptic area, ventral hypothalamus, nucleus isthmus, tectum mesencephali, inferior colliculus, and hindbrain. In P. shermani, the distribution of AVT ISH-labeled neurons matched that of T. granulosa, except in the lateral septum, ventral hypothalamus, and inferior colliculus, but did however include labeled cell bodies in the lateral pallium. The distribution of MST ISH-labeled cells was more restricted than AVT ISH labeling and was limited to regions of the preoptic area and ventral thalamus, which is consistent with the limited distribution of MST/OXY in other vertebrates. These findings support the conclusion that urodele amphibians possess a well-developed vasotocin system, perhaps more extensive than other vertebrate taxa.

Roles of stress hormones in food intake regulation in anuran amphibians throughout the life cycle

Towards understanding the ontogeny of energy balance regulation in vertebrates we analyzed the responses of corticotropin-releasing factor (CRF) and corticosterone to food deprivation in the Western spadefoot toad (Spea hammondii) at three developmental stages: premetamorphic tadpole, prometamorphic tadpole, and juvenile. Corticosterone responses to 5 days of food deprivation varied among developmental stages. Both pre- and prometamorphic tadpoles increased whole-body corticosterone content with food deprivation, but the magnitude of the response of premetamorphic tadpoles was significantly greater. By contrast, juvenile toads decreased plasma corticosterone concentration. Similarly, brain CRF peptide content tended to increase in food-deprived tadpoles but did not change in food-deprived juveniles. Therefore, there is an ontogenetic difference in the way the hypothalamic-pituitary-interrenal (HPI) axis responds to food deprivation in amphibians. In tadpoles, the HPI axis is activated in response to fasting as is seen in birds and mammals, and may be associated with mobilization of stored fuels and increased foraging. Juvenile toads do not respond to food deprivation by activating the HPI axis, but instead pursue a strategy of energy conservation that involves a reduction in plasma corticosterone concentration.

Distribution and acute stressor-induced activation of corticotrophin-releasing hormone neurones in the central nervous system of Xenopus laevis

In mammals, corticotrophin-releasing hormone (CRH) and related peptides are known to play essential roles in the regulation of neuroendocrine, autonomic and behavioural responses to physical and emotional stress. In nonmammalian species, CRH-like peptides are hypothesized to play similar neuroendocrine and neurocrine roles. However, there is relatively little detailed information on the distribution of CRH neurones in the central nervous system (CNS) of nonmammalian vertebrates, and there are currently no comparative data on stress-induced changes in CRH neuronal physiology. We used a specific, affinity-purified antibody raised against synthetic Xenopus laevis CRH to map the distribution of CRH in the CNS of juvenile South African clawed frogs. We then analysed stress-induced changes in CRH immunoreactivity (CRH-ir) throughout the CNS. We found that CRH-positive cell bodies and fibres are widely distributed throughout the brain and rostral spinal cord of juvenile X. laevis. Strong CRH-immunoreactivity (ir) was found in cell bodies and fibres in the anterior preoptic area (POA, an area homologous to the mammalian paraventricular nucleus) and the external zone of the median eminence. Specific CRH-ir cell bodies and fibres were also identified in the septum, pallium and striatum in the telencephalon; the amygdala, bed nucleus of the stria terminalis and various hypothalamic and thalamic nuclei in the diencephalon; the tectum, torus semicircularis and tegmental nuclei of the mesencephalon; the cerebellum and locus coeruleus in the rhombencephalon; and the ventral horn of the rostral spinal cord. To determine if exposure to an acute physical stressor alters CRH neuronal physiology, we exposed juvenile frogs to shaking/handling and conducted morphometric analysis. Plasma corticosterone was significantly elevated by 30 min after exposure to the stressor and continued to increase up to 6 h. Morphometric analysis of CRH-ir after 4 h of stress showed a significant increase in CRH-ir in parvocellular neurones of the anterior preoptic area, the medial amygdala and the bed nucleus of the stria terminalis, but not in other brain regions. The stress-induced increase in CRH-ir in the POA was associated with increased Fos-like immunoreactivity (Fos-LI), and confocal microscopy showed that CRH-ir colocalized with Fos-LI in a subset of Fos-LI-positive neurones. Our results support the view that the basic pattern of CNS CRH expression arose early in vertebrate evolution and lend further support to earlier studies suggesting that amphibians may be a transitional species for descending CRH-ergic pathways. Furthermore, CRH neurones in the frog brain exhibit changes in response to a physical stressor that parallel those seen in mammals, and thus are likely to play an active role in mediating neuroendocrine, behavioural and autonomic stress responses.

Ontogeny of corticotropin-releasing factor effects on locomotion and foraging in the Western spadefoot toad (Spea hammondii)

We investigated the effects of corticotropin-releasing factor (CRF) and corticosterone (CORT) on foraging and locomotion in Western spadefoot toad (Spea hammondii) tadpoles and juveniles to assess the behavioral functions of these hormones throughout development. We administered intracerebroventricular injections of ovine CRF or CRF receptor antagonist alphahelical CRF((9-41)) to tadpoles and juveniles, and observed behavior within 1.5 h after injection. In both premetamorphic (Gosner stage 33) and prometamorphic (Gosner stages 35-37) tadpoles, CRF injections increased locomotion and decreased foraging. Injections of alphahelical CRF((9-41)) reduced locomotion but did not affect foraging in premetamorphic tadpoles, but dramatically increased foraging in prometamorphic tadpoles compared to both placebo and uninjected controls. Similarly, alphahelical CRF((9-41)) injections stimulated food intake and prey-catching behavior in juveniles. These results suggest that in later-staged amphibians, endogenous CRF secretion modulates feeding by exerting a suppressive effect on appetite. By contrast to the inhibitory effect of CRF, 3-h exposure to CORT (500 nM added to the aquarium water) stimulated foraging in prometamorphic tadpoles. These tadpoles also exhibited a CORT-mediated increase in foraging 6 h after CRF injection, which was associated with elevated whole-body CORT content and blocked by glucocorticoid receptor (GR) antagonist (RU486) injections. Thus, exogenous CRF influences locomotion and foraging in both pre- and prometamorphic tadpoles, but endogenous CRF secretion in relatively unstressed animals does not affect foraging until prometamorphic stages. Furthermore, the opposing actions of CRF and CORT on foraging suggest that they are important regulators of energy balance and food intake in amphibians throughout development.

Expression and hypophysiotropic actions of corticotropin-releasing factor in Xenopus laevis

Members of the corticotropin-releasing factor (CRF) family of peptides play pivotal roles in the regulation of neuroendocrine, autonomic, and behavioral responses to physical and emotional stress. In amphibian tadpoles, CRF-like peptides stimulate both thyroid and interrenal (adrenal) hormone secretion, and can thereby modulate the rate of metamorphosis. To better understand the regulation of expression and actions of CRF in amphibians we developed a homologous radioimmunoassay (RIA) for Xenopus laevis CRF (xCRF). We validated this RIA and tissue extraction procedure for the measurement of brain CRF content in tadpoles and juveniles. We show that the CRF-binding protein, which is highly expressed in X. laevis brain, is largely removed by acid extraction and does not interfere in the RIA. We analyzed CRF peptide content in five microdissected brain regions in prometamorphic tadpoles and juveniles. CRF was detected throughout the brain, consistent with its role as both a hypophysiotropin and a neurotransmitter/neuromodulator. CRF content was highest in the region of the preoptic area (POa) and increased in all brain regions after metamorphosis. Exposure to 4h of handling/shaking stress resulted in increased CRF peptide content in the POa in juvenile frogs. Injections of xCRF into prometamorphic tadpoles increased whole body corticosterone and thyroxine content, thus supporting findings in other anuran species that this peptide functions as both a corticotropin- and a thyrotropin (TSH)-releasing factor. Furthermore, treatment of cultured tadpole pituitaries with xCRF (100nM for 24h) resulted in increased medium content, but decreased pituitary content of TSHbeta-immunoreactivity. Our results support the view that CRF functions as a stress neuropeptide in X. laevis as in other vertebrates. Furthermore, we provide evidence for a dual hypophysiotropic action of CRF on the thyroid and interrenal axes in X. laevis as has been shown previously in other amphibian species.

Involvement of arginine vasotocin in reproductive events in the male newt Cynops pyrrhogaster

Effects of arginine vasotocin (AVT) on reproductive events such as courtship behavior, pheromone release, and spermatophore discharge were investigated in the male newt Cynops pyrrhogaster. AVT enhanced the incidence and frequency of androgen-induced courtship behavior. In this case, AVT was likely to act centrally because the behavior was evoked with a much smaller amount of AVT when the hormone was administered intracerebroventricularly than when given intraperitoneally. Involvement of endogenous AVT in spontaneously occurring courtship behavior was also evidenced by the fact that administration of a V1 (vasopressor) receptor antagonist, [d(CH2)5(1), Tyr(Me)2, Arg8-vasopressin] suppressed the expression of the courtship behavior. The water in which AVT-treated males had been kept showed considerable female-attracting activity as compared with the water in which saline-injected males had been kept. Moreover, the content of sodefrin, a female-attracting pheromone in the abdominal gland, was decreased by the intraperitoneal injection of AVT, suggesting that the neurohypophyseal hormone stimulated the release of sodefrin from the abdominal gland into the water. AVT induced contraction of the excised abdominal gland concentration-dependently, and, again, the V1 receptor antagonist suppressed the AVT-induced contraction. Thus, we concluded that AVT induces the pheromone discharge, acting peripherally on a contractile structure of the abdominal gland. AVT was also found to induce spermatophore deposition in the male kept in the absence of the female. Administration of the V1 receptor blocker to the sexually developed males suppressed the spermatophore deposition. All these results indicate the involvement of AVT in reproductive events acting centrally and peripherally.

Immunoidentification of CRF-like material in the intermaxillary glands during Bufo arenarum development

The presence of corticotropin-releasing factor-like material in the intermaxillary glands was studied by immunocytochemical techniques during the metamorphosis of Bufo arenarum. The intermaxillary glands appeared at stage XV (midprometamorphosis) with CRF-like material slightly immunoreactive. These glands are located posterior to the premaxillae and between the nasal capsules in the roof of the mouth and are formed of alveoli or tubules. During metamorphic climax, corticotropin-releasing factor-like material was identified strongly immunostained at the apices of the secretory cells. It was observed that collecting ducts of the gland open to the anterior palatal surface suggesting that the secretion could be ingested by tadpoles. Our results clearly showed that ir-CRF-like material present in the intermaxillary glands is ingested by tadpoles during metamorphosis and could play an important role during amphibian development.

Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates

Corticotropin-releasing factor (CRF), urotensin-I, urocortin and sauvagine belong to a family of related neuropeptides found throughout chordate taxa and likely stem from an ancestral peptide precursor early in metazoan ancestry. In vertebrates, current evidence suggests that CRF on one hand, and urotensin-I, urocortin and sauvagine, on the other, form paralogous lineages. Urocortin and sauvagine appear to represent tetrapod orthologues of fish urotensin-I. Sauvagine's unique structure may reflect the distinctly derived evolutionary history of the anura and the amphibia in general. The physiological actions of these peptides are mediated by at least two receptor subtypes and a soluble binding protein. Although the earliest functions of these peptides may have been associated with osmoregulation and diuresis, a constellation of physiological effects associated with stress and anxiety, vasoregulation, thermoregulation, growth and metabolism, metamorphosis and reproduction have been identified in various vertebrate species. The elaboration of neural circuitry for each of the two paralogous neuropeptide systems appears to have followed distinct pathways in the actinopterygian and sarcopterygian lineages of vertebrates. A comparision of the functional differences between these two lineages predicts additional functions of these peptides.

Comparative neuroanatomy of vasotocin and vasopressin in amphibians and other vertebrates

This review focuses on the neuroanatomical distribution of vasotocin (VT) and vasopressin (VP) and presents a comparative analysis of brain areas in which VT and VP cell bodies have been reported in fish, amphibians, reptiles, birds and mammals. A comparison of information from previous neuroanatomical studies of VT and VP with findings from a recent study of VT in an amphibian (Taricha granulosa) supports the conclusions that the VT/VP system can be subdivided into identifiable groups of cell bodies, based on neuroanatomical and cell morphology characteristics, and that these cell groups are not necessarily delimited by classical neuroanatomical boundaries. The comparative neuroanatomy of the distribution of VT and VP cell bodies also indicates that the neuroanatomy of the VT/VP system is fairly conserved among vertebrates. The review uses comparative data to present a series of tentative hypotheses about the homology of the VT cell groups and VP cell groups in the different vertebrate taxa.

Immunocytochemical and histological differences in the interrenal axis of feral brown trout, Salmo trutta, in metal-contaminated waters

There are more CRH-like immunoreactive neurons in the preoptic nucleus and nucleus lateralis tuberis in the brain of feral brown trout, Salmo trutta, living in cadmium- and zinc-contaminated regions of the Eagle River than in fish from an uncontaminated control site. Histological analyses revealed that interrenal cells are more stimulated (exhibiting both hypertrophy and hyperplasia) in fish living in contaminated sites than interrenal cells of fish at the control site. These results suggest that the hypothalamo-pituitary-interrenal (HPI) axis of fish living in the metal-contaminated water shows evidence of chronic stimulation. We suggest that assessment of these parameters of the HPI axis may be useful indices of chronic environmental stress in trout.

Immunocytochemical distribution of corticotropin-releasing factor in the brain and hypophysis of larval Bufo arenarum; effect of KClO4 during early development

The maturation of the corticotropin-releasing factor (CRF) neuronal system was evaluated by immunocytochemistry and morphometry in Bufo arenarum, during spontaneous metamorphosis and in tadpoles with inhibited thyroid function. The first appearance of CRF immunoreactive fibers was at the end of premetamorphosis (stage VIII). These fibers were found in small numbers and weakly stained in the median eminence and infundibular stalk. With the advance of larval development, CRF-like material stained intensely and tended to aggregate in the external zone of the median eminence. At climax stages, immunoreactive fibers and perikarya (weakly stained) were identified in the interpeduncular nucleus and in the dorsal infundibular nucleus. Morphometric and immunocytochemical results indicate that the maturation of the CRF neuronal system in Bufo arenarum occurs just before metamorphic climax, coinciding with high levels of thyroid and steroid hormones. We have also found that in larvae with inhibited thyroid function, the CRF neuronal system is able to develop, and that thyroid hormone could exert a negative feedback control on the synthesis of CRF.

Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia

This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.

Neuroanatomical distribution of vasotocin in a urodele amphibian (Taricha granulosa) revealed by immunohistochemical and in situ hybridization techniques

Immunohistochemical and in situ hybridization techniques were used to investigate the neuroanatomical distribution of arginine vasotocin-like systems in the roughskin newt (Taricha granulosa). Vasotocin-like-immunoreactive neuronal cell bodies were identified that, based on topographical position, most likely, are homologous to groups of vasopressin-immunoreactive neuronal cell bodies described in mammals, including those in the bed nucleus of the stria terminalis, medial amygdala, basal septal region, magnocellular basal forebrain-including the horizontal limb of the diagonal band of Broca, paraventricular and supraoptic nuclei, suprachiasmatic nucleus, and dorsomedial hypothalamic nucleus. Several additional vasotocin-like-immunoreactive cell groups were observed in the forebrain and brainstem regions; these observations are compared with previous studies of vasotocin- and vasopressin-like systems in vertebrates. Arginine vasotocin-like-immunoreactive fibers and presumed terminals also were widely distributed with high densities in the basal limbic forebrain, the ventral preoptic and hypothalamic regions, and the brainstem ventromedial tegmentum. Based on in situ hybridization studies with synthetic oligonucleotide probes for vasotocin and the related neuropeptide mesotocin, as well as double-labeling studies with combined immunohistochemistry and in situ hybridization, we conclude that the vasotocin immunohistochemical procedures used identify vasotocin-like, but not mesotocin-like, elements in the brain of T. granulosa. The distribution of arginine vasotocin-like systems in T. granulosa is greater than the distribution previously reported for any other single vertebrate species; however, it is consistent with an emerging pattern of distribution of vasotocin- and vasopressin-like peptides in vertebrates. Complexity in the vasotocinergic system adds further support to the conclusion that this peptide regulates multiple neurophysiological and neuroendocrinological functions.

Environmental stress as a developmental cue: corticotropin-releasing hormone is a proximate mediator of adaptive phenotypic plasticity in amphibian metamorphosis

Environmentally induced phenotypic plasticity allows developing organisms to respond adaptively to changes in their habitat. Desert amphibians have evolved traits which allow successful development in unpredictable environments. Tadpoles of these species can accelerate metamorphosis as their pond dries, thus escaping mortality in the larval habitat. This developmental response can be replicated in the laboratory, which allows elucidation of the underlying physiological mechanisms. Here I demonstrate a link between a classical neurohormonal stress pathway (involving corticotropin-releasing hormone, CRH) and the developmental response to habitat desiccation. Injections of CRH-like peptides accelerated metamorphosis in western spadefoot toad tadpoles. Conversely, treatment with two CRH antagonists, the CRH receptor antagonist alpha-helical CRH(9-41) and anti-CRH serum, attenuated the developmental acceleration induced by habitat desiccation. Tadpoles subjected to habitat desiccation exhibited elevated hypothalamic CRH content at the time when they responded developmentally to the declining water level. CRH injections elevated whole-body thyroxine, triiodothyronine, and corticosterone content, the primary hormonal regulators of metamorphosis. In contrast, alpha-helical CRH(9-41) reduced thyroid activity. These results support a central role for CRH as a neurohormonal transducer of environmental stimuli into the endocrine response which modulates the rate of metamorphosis. Because in mammals, increased fetal/placental CRH production may initiate parturition, and CRH has been implicated in precipitating preterm birth arising from fetal stress, this neurohormonal pathway may represent a phylogenetically ancient developmental regulatory system that allows the organism to escape an unfavorable larval/fetal habitat.

Development of arginine vasotocin innervation in two species of anuran amphibian: Rana catesbeiana and Rana sylvatica

Arginine vasotocin (AVT) is a neurotransmitter in the amphibian central nervous system and is released from the neurohypophysis in the regulation of hydromineral balance and other homeostatic functions. Many amphibians experience drastic changes in habitat with respect to water availability during their transformation from aquatic larvae to terrestrial adults. To examine whether metamorphosis is accompanied by a reorganization of central vasotocinergic neurons, the developmental organization of vasotocin neurons and nerve fibers was studied with immunocytochemistry in the brains of bullfrogs (Rana catesbeiana) and woodfrogs (R. sylvatica). In bullfrogs, early limb-bud-stage tadpoles had AVT-immunoreactive neurons and nerve fibers in the lateral septal nucleus, amygdala, preoptic hypothalamus, suprachiasmatic nucleus, and posterodorsal tegmentum. Woodfrog larvae showed similar patterns of hypothalamic AVT immunoreactivity, although neuronal staining in the amygdala did not appear until metamorphic climax, and never appeared in septal neurons or in the posterodorsal tegmentum. Whereas the highly terrestrial R. sylvatica adults must adapt to an adult habitat with prolonged periods of dehydration, R. catesbeiana adults remain semiaquatic and, as such, need not develop extreme mechanisms for water retention. Nonetheless, vasotocinergic pathways showed developmental similarities in the two species. The early appearance of AVT innervation in both Rana suggests that AVT has neuroregulatory functions well before metamorphosis.

Ontogeny of vasotocinergic and mesotocinergic systems in the brain of the South African clawed frog Xenopus laevis

For a better understanding of the development of neurotransmitter systems and of their putative functional significance during ontogenesis, the development of the vasotocin (AVT) and mesotocin (MST) systems in the brain of Xenopus laevis was studied by means of immunohistochemical techniques. Weakly immunoreactive fibers were already present at late embryonic stage 38 in the caudoventral part of the telencephalon and in the ventral part of the diencephalon. The earliest immunodetectable AVT and MST immunoreactive cell bodies were found in the developing preoptic area at late embryonic stage 43. At the end of the embryonic period (stage 45), AVT immunoreactive fibers have reached the future medial amygdala, the midbrain tegmentum, the median eminence and the neural lobe of the pituitary. When compared with AVT immunoreactive fibers, the development of MST fibers shows some temporal delay. During the premetamorphosis (stages 45-52), AVT immunoreactive cell bodies appear in the medial part of the suprachiasmatic nucleus, the dorsal infundibular region, and the midbrain tegmentum, whereas fibers can now be traced to the nucleus accumbens, the septum and the medial amygdala in the forebrain, to the midbrain tegmentum, the reticular formation, the raphe nuclei, and the solitary tract nucleus in the brainstem, and to the spinal cord. Further maturation of the AVT system during prometamorphosis (stages 53-58) includes the appearance of immunoreactive cell bodies in the lateral part of the suprachiasmatic nucleus, the ventral preoptic area, and the dorsal infundibular region. By the end of the metamorphosis (stage 65), the maturation of the AVT/MST systems reaches an almost adult-like pattern. It should be noted that in amphibians, in contrast to mammals, the early appearance of the AVT/MST systems, including their extensive extrahypothalamic component, suggests that the two neuropeptidergic systems may play a significant role during development.

Immunohistochemical studies on the development of the hypothalamo-hypophysial system in Xenopus laevis

BACKGROUND

Few attempts have been made to clarify the relational development of the hypothalamo-adenohypophysial and -neurohypophysial systems in species higher than amphibians.

METHODS

The appearance and topographical distribution of endocrine and neuroendocrine cells and fibers in these systems were immunohistochemically examined in the larvae of Xenopus laevis from immediately before hatching (stage 32, Nieuwkoop and Faber's classification) to the end of metamorphosis (stage 66).

RESULTS

(1) Each endocrine cell differentiated until the middle premetamorphic period. MSH cells initially appeared in the posterior half of the pituitary anlage at stage 35/36, followed by the differentiation of GH cells at stage 39 in the middle part, PRL cells at stage 46 in the anterior half of the pituitary anlage, and LH cells at stage 50 in the posterior two thirds of the pars distalis. With the progression of development, the cells which differentiated at early stages shifted from their initial positions; MSH cells, to the pars intermedia; and GH cells, to the posterior half of the pars distalis. 2) Oxytocin and vasopressin fibers were observed at stage 47/48 in the median eminence, and converged to the pars nervosa at later stages. 3) Neuroendocrine fibers innervated the median eminence during the middle premetamorphic to prometamorphic period: SOM fibers, at stage 45; CRH, 47/48; GRH, 48; dopamine, 58; and LHRH, 60. The cells containing these hormones were observed in the (presumptive) preoptic and/or infundibular nuclei.

CONCLUSION

These results suggest the following three chronological steps in the development of hypothalamo-hypophysial systems and their target organs: independent development of target organs at early developmental stages; appearance of hypophysial hormones to control the development of target organs at middle developmental stages; appearance of hypothalamic hormones to control the function or maturation of the hypophysis at late developmental stages.

Involvement of retinohypothalamic input, suprachiasmatic nucleus, magnocellular nucleus and locus coeruleus in control of melanotrope cells of Xenopus laevis: a retrograde and anterograde tracing study

The amphibian Xenopus laevis is able to adapt the colour of its skin to the light intensity of the background, by releasing alpha-melanophore-stimulating hormone from the pars intermedia of the hypophysis. In this control various inhibitory (dopamine, gamma-aminobutyric acid, neuropeptide Y, noradrenaline) and stimulatory (thyrotropin-releasing hormone and corticotropin-releasing hormone) neural factors are involved. Dopamine, gamma-aminobutyric acid and neuropeptide Y are present in suprachiasmatic neurons and co-exist in synaptic contacts on the melanotrope cells in the pars intermedia, whereas noradrenaline occurs in the locus coeruleus and noradrenaline-containing fibres innervate the pars intermedia. Thyrotropin-releasing hormone and corticotropin-releasing hormone occur in axon terminals in the pars nervosa. In the present study, the neuronal origins of these factors have been identified using axonal tract tracing. Application of the tracers 1,1'dioctadecyl-3,3,3',3' tetramethyl indocarbocyanine and horseradish peroxidase into the pars intermedia resulted in labelled neurons in two brain areas, which were immunocytochemically identified as the suprachiasmatic nucleus and the locus coeruleus, indicating that these areas are involved in neural inhibition of the melanotrope cells. Thyrotropin-releasing hormone and corticotropin-releasing hormone were demonstrated immunocytochemically in the magnocellular nucleus. This area appeared to be labelled upon tracer application into the pars nervosa. This finding is in line with the idea that corticotropin-releasing hormone and thyrotropin-releasing hormone stimulate melanotrope cell activity after diffusion from the neural lobe to the pars intermedia. After anterograde filling of the optic nerve with horseradish peroxidase, labelled axons were traced up to the suprachiasmatic area where they showed to be in contact with suprachiasmatic neurons. These neurons showed a positive reaction with anti-neuropeptide Y and the same held for staining with anti-tyrosine hydroxylase. It is suggested that a retino-suprachiasmatic pathway is involved in the control of the melanotrope cells during the process of background adaptation.

Development of vasotocin pathways in the bullfrog brain

The brain of adult bullfrogs (Rana catesbeiana) contains six populations of cells which are immunoreactive for the neurohypophysial peptide arginine vasotocin (AVT). It is unknown when some of these cell populations first appear during development and when the sexual differences in AVT distribution first become apparent. We therefore used immunocytochemistry to examine development of AVT pathways in developing bullfrog tadpoles and in newly metamorphosed froglets of both sexes. AVT-immunoreactive (AVT-ir) cells were already present in the three diencephalic areas (magnocellular preoptic nucleus, suprachiasmatic nucleus and hypothalamus) at stage III (Taylor and Kollros stages), the earliest stage examined. Cell size in the magnocellular nucleus was not bimodally distributed in either tadpoles or froglets. AVT-ir cells in the telencephalic septal nucleus and amygdala did not appear until stage VI. There was no sexual difference in the density of AVT-ir cells or fibers in the amygdala of tadpoles or froglets. Finally, cells in the hindbrain pretrigeminal nucleus appeared much later--after stage XX. Thus, different populations of neurons begin to express AVT at unique times during development. The sexual dimorphism in AVT content observed in the amygdala of adult bullfrogs must appear during juvenile development or at adulthood.

Distribution of corticotropin-releasing factor immunoreactive neurons in the brain of the tigerfrog, Rana tigrina

The distribution of immunoreactive (ir) neurons containing corticotropin-releasing factor (CRF) is described in the brain of the tigerfrog, Rana tigrina. The olfactory bulb, medial pallium, nucleus of the diagonal band of Broca and medial area of the amygdala of the telencephalon showed ir-CRF perikarya. The anterior and ventromedial thalamic nuclei, and the magnocellular nucleus preopticus (NPO) revealed several ir cells; a few NPO neurons were cerebrospinal fluid contacting in nature. The nucleus hypothalamicus ventromedialis contained a few cells, but the nucleus infundibularis ventralis of the infundibulum revealed several diffusely distributed perikarya. Individual ir-CRF perikarya were visualized in the optic tectum and interpeduncular nucleus. Extensive fiber terminals were present in the median eminence, but no fibers were discerned either in the neural lobe or in the pituitary gland.

Immunocytochemical localization of atrial natriuretic factor (ANF)-like peptides in the brain and heart of the treefrog Hyla japonica: effect of weightlessness on the distribution of immunoreactive neurons and cardiocytes

The localization of atrial-natriuretic factor (ANF)-like immunoreactivity was investigated in the brain and heart of the treefrog Hyla japonica by the indirect immunofluorescence technique. Concurrently, the effect of weightlessness on the distribution of ANF-containing neurons and cardiocytes was studied in frogs that were sent into space for 9 days on the space station "MIR." In control animals, the amygdala contained the most prominent group of ANF-immunoreactive cells and fibers. ANF-positive neurons and nerve processes were also detected in other areas of the telencephalon such as the nucleus olfactorius, the pallium mediale, and the striatum. In "space frogs," the intensity of labeling of the amygdala and nucleus olfactorius was similar to that seen in control animals. In contrast, the pallium and the striatum of "space frogs" were totally devoid of positive cell bodies. In the diencephalon, of all animals, numerous ANF-immunoreactive perikarya and fibers were seen in the hypothalamus, the anterior thalamus, the infundibulum, and the median eminence. ANF-positive cell bodies were also noted in the lateral forebrain bundle of control frogs but were absent in "space frogs." The major difference between control and "space frogs" was observed in the posterior nuclei of the thalamus. In "space frogs," the nucleus posterocentralis thalami and the nucleus posterolateralis thalami exhibited large ANF-immunoreactive perikarya, while, in control frogs, these nuclei only contained scarce positive nerve fibers. In the mesencephalon, ANF-positive cell bodies and nerve processes were seen in the nucleus tegmenti mesencephali, the interpeduncular nucleus, and the nucleus cerebelli of all animals. However, stained perikarya were only observed in the nucleus reticularis isthmi of control frogs. In the heart, atrial cardiocytes exhibited intense ANF-like immunoreactivity. ANF-positive myocytes were also detected in the subpericardial region of the ventricle. The density and distribution of the staining were identical in the heart of control and "space frogs." These data support the concept that prolonged exposure to microgravity affects biosynthesis and/or release of ANF-related peptides in discrete regions of the amphibian brain.

Monoamines and their metabolites in the amphibian (Ambystoma tigrinum) brain: Quantitative changes during metamorphosis and captivity

1. Monoamine neurotransmitters (epinephrine, norepinephrine, dopamine, serotonin and some of their metabolites (DOPEG, MHPG, DOPAC, 5-HIAA) were measured by HPLC in extracts from telencephalon (TEL) and diencephalon-midbrain (DM) before, during at the end of metamorphosis. 2. During metamorphosis MHPG increased and 5-HIAA decreased in TEL and DM while DOPEG decreased only in DM. 3. Monoamine levels were greater in the TEL and a larger increase in MHPG occurred there. 4. Captivity without metamorphosis also caused a significant depression of 5-HIAA in TEL and depression of DOPEG, MHPG and DOPAC in DM.

Corticotropin-releasing factor stimulates metamorphosis and increases thyroid hormone concentration in prometamorphic Rana perezi larvae

Attempts to identify a hypothalamic molecule that stimulates thyrotropin (TSH) secretion from amphibian pituitary have been unsuccessful to date. The effects of mammalian (ovine and human) corticotropin-releasing factor (CRF) on the thyroid function of prometamorphic (Taylor & Kollros stages XI-XVII) (Taylor and Kollros, 1946) Rana perezi larvae were studied. Chronic treatments with both ovine and human CRF (oCRF, hCRF) stimulated metamorphosis while delaying larval growth. Chronic hCRF (1 microgram) administration induced 3.2- and 5.3-fold increases in whole body concentration of thyroxine (T4) and triiodothyronine (T3), respectively. In contrast, the 0.5-microgram dose of hCRF stimulated a significant (3.4-fold) increase in whole body concentration of T4 but not of T3. Histological studies of the thyroid gland revealed a 22% increase in the number of follicles per section as a result of the chronic treatment with oCRF (1 microgram). Acute oCRF (2 micrograms) treatment induced a significant increase in T4 concentration at 4 hr (1.3-fold) and 8 hr (2.3-fold) postinjection. T3 concentration was not altered. These results support previous reports and lead us to conclude that a CRF-like peptide, and not TRH, is involved in the regulation of thyroid activity in anuran amphibians during metamorphosis.

Comparative analysis of the vasotocinergic and mesotocinergic cells and fibers in the brain of two amphibians, the anuran Rana ridibunda and the urodele Pleurodeles waltlii

To obtain more insight into the vasotocinergic and mesotocinergic systems of amphibians and the evolution of these neuropeptidergic systems in vertebrates in general, the distribution of vasotocin (AVT) and mesotocin (MST) was studied immunohistochemically in the brains of the anuran Rana ridibunda and the urodele Pleurodeles waltlii. In Rana, AVT-immunoreactive cell bodies are located in the nucleus accumbens, the dorsal striatum, the lateral and medial part of the amygdala, an area adjacent to the anterior commissure, the magnocellular preoptic nucleus, the hypothalamus, the mesencephalic tegmentum, and in an area adjacent to the solitary tract. In Pleurodeles, AVT-immunoreactive somata are confined to the medial amygdala, the preoptic area, and an area lateral to the presumed locus coeruleus. In both species, the distribution of MST-immunoreactive cell bodies is more restricted: in the frog, MST-immunoreactive somata are present in the medial amygdala and the preoptic area, whereas, in the urodele, cell bodies are found only in the preoptic area. Both in Rana and Pleurodeles, AVT- and MST-immunoreactive fibers are distributed throughout the brain and spinal cord. A major difference is that in Rana the number of MST-immunoreactive fibers is evidently higher than that of AVT-immunoreactive fibers, whereas the opposite is found in Pleurodeles. This holds, in particular, for the forebrain and the brainstem. The presence of several extrahypothalamic AVT-immunoreactive cell groups and the existence of well-developed extrahypothalamic networks of AVT- and MST-immunoreactive fibers are features that amphibians share with amniotes. However, this study has revealed that major differences exist not only between species of different classes of vertebrates, but also within a single class. In order to determine whether features of these neuropeptidergic systems are primitive or derived, a broad selection of species of each class of vertebrates is needed.

Effect of vasotocin on locomotor activity in bullfrogs varies with developmental stage and sex

Although neurohypophysial peptides are present in many regions of the developing and adult bullfrog (Rana catesbeiana) brain, the function of these peptides remains unclear. To investigate possible behavioral actions, we examined locomotor activity following peptide injection in bullfrogs at various developmental stages. An intraperitoneal (ip) injection of arginine vasotocin (AVT) in tadpoles (stages V, X, or XVII) produced an immediate and dose-dependent inhibition of locomotor activity. On the other hand, AVT stimulated activity when administered ip to juvenile or adult female bullfrogs, but did not influence activity in juvenile or adult males. The minimum effective dose of AVT, when injected directly into the brain of tadpoles, was 100-fold less than that observed when injected ip, suggesting a central nervous system site of action for this peptide. A vasopressin receptor antagonist (d(CH2)5[Tyr(Me)2]AVP administered ip or icv) significantly increased locomotor activity in tadpoles, compared to controls. Oxytocin, vasopressin, and AVP4-9 inhibited activity in tadpoles while mesotocin, des Gly(NH2)AVP, and pressinoic acid had no significant effect. Injection of PGF2 alpha also significantly decreased activity levels in tadpoles. However, pretreatment of tadpoles with indomethacin, a prostaglandin synthesis inhibitor, did not prevent the behavioral effects of AVT, suggesting that prostaglandin synthesis is not required for this response. In summary, AVT influenced locomotor activity in bullfrog tadpoles and female frogs. This effect shifted during development from an inhibitory action in tadpoles to a stimulatory effect in metamorphosed female frogs. The effect of AVT on juvenile and adult frog locomotion was sexually dimorphic, as this peptide altered female behavior but not male behavior.