Hormonal induction of male courtship behavior in the Japanese newt, Cynops pyrrhogaster

Central regulation of reproduction in amphibians

This review article includes a literature review of synteny analysis of the amphibian gonadotropin-releasing hormone (GnRH) genes, the distribution of GnRH 1 and GnRH2 neurons in the central nervous system of amphibians, the function and regulation of hypophysiotropic GnRH1, and the function of GnRH1 in amphibian reproductive behaviors. It is generally accepted that GnRH is the key regulator of the hypothalamic-pituitary-gonadal axis. Three independent GnRH genes, GnRH1, GnRH2, and GnRH3, have been identified in vertebrates. Previous genome synteny analyses suggest that there are likely just two genes, gnrh1 and gnrh2, in amphibians. In three groups of amphibians: Anura, Urodela, and Gymnophiona, the distributions of GnRH1 and GnRH2 neurons in the central nervous system have also been previously reported. Moreover, these neuronal networks were determined to be structurally independent in all species examined. The somata of GnRH1 neurons are located in the terminal nerve, medial septum (MS), and preoptic area (POA), and some GnRH1 neurons in the MS and POA project into the median eminence. In contrast, the somata of GnRH2 neurons are located in the midbrain tegmentum. In amphibians, GnRH1 neurons originate from the embryonic olfactory placode, while GnRH2 neurons originate from the midbrain. The characterization and feedback regulation mechanisms of hypophysiotropic GnRH1 neurons in amphibians, the involvement of GnRH1 in amphibian reproductive behavior, and its possible mechanism of action should be elucidated in future.

Hormonal and pheromonal studies on amphibians with special reference to metamorphosis and reproductive behavior

In this article, we review studies which have been conducted to investigate the hormonal influence on metamorphosis in bullfrog (Rana catesbeiana) and Japanese toad (Bufo japonicus) larvae, in addition to studies conducted on the hormonal and pheromonal control of reproductive behavior in red-bellied newts (Cynops pyrrhogaster). Metamorphosis was studied with an emphasis on the roles of prolactin (PRL) and thyrotropin (TSH). The release of PRL was shown to be regulated by thyrotropin-releasing hormone (TRH) and that of TSH was evidenced to be regulated by corticotropin-releasing factor. The significance of the fact that the neuropeptide that controls the secretion of TSH is different from those encountered in mammals is discussed in consideration of the observation that the release of TRH, which stimulates the release of PRL, is enhanced when the animals are subjected to a cold temperature. Findings that were made by using melanin-rich cells of Bufo embryos and larvae, such as the determination of the origin of the adenohypophyseal primordium, identification of the pancreatic chitinase, and involvement of the rostral preoptic recess organ as the hypothalamic inhibitory center of α-melanocyte-stimulating hormone (α-MSH) secretion, are mentioned in this article. In addition, the involvement of hormones in eliciting courtship behavior in male red-bellied newts and the discovery of the peptide sex pheromones and hormonal control of their secretion are also discussed in the present article.

Pheromonal communication in urodelan amphibians

Pheromonal communication is an ancient and pervasive sensory modality in urodelan amphibians. One family of salamander pheromones (the sodefrin precursor-like factor (SPF) family) originated 300 million years ago, at the origin of amphibians. Although salamanders are often thought of as relatively simple animals especially when compared to mammals, the pheromonal systems are varied and complex with nuanced effects on behavior. Here, we review the function and evolution of pheromonal signals involved in male-female reproductive interactions. After describing common themes of salamander pheromonal communication, we describe what is known about the rich diversity of pheromonal communication in each salamander family. Several pheromones have been described, ranging from simple, invariant molecules to complex, variable blends of pheromones. While some pheromones elicit overt behavioral responses, others have more nuanced effects. Pheromonal signals have diversified within salamander lineages and have experienced rapid evolution. Once receptors have been matched to pheromonal ligands, rapid advance can be made to better understand the olfactory detection and processing of salamander pheromones. In particular, a large number of salamander species deliver pheromones across the skin of females, perhaps reflecting a novel mode of pheromonal communication. At the end of our review, we list some of the many intriguing unanswered questions. We hope that this review will inspire a new generation of scientists to pursue work in this rewarding field.

Some aspects of the hypothalamic and pituitary development, metamorphosis, and reproductive behavior as studied in amphibians

In this review article, information about the development of the hypothalamo-hypophyseal axis, endocrine control of metamorphosis, and hormonal and pheromonal involvements in reproductive behavior in some amphibian species is assembled from the works conducted mainly by our research group. The hypothalamic and pituitary development was studied using Bufo embryos and larvae. The primordium of the epithelial hypophysis originates at the anterior neural ridge and migrates underneath the brain to form a Rathke's pouch-like structure. The hypothalamo-hypophyseal axis develops under the influence of thyroid hormone (TH). For the morphological and functional development of the median eminence, which is a key structure in the transport of regulatory hormones to the pituitary, contact of the adenohypophysis with the undeveloped median eminence is necessary. For the development of proopiomelanocortin-producing cells, contact of the pituitary primordium with the infundibulum is required. The significance of avascularization in terms of the function of the intermediate lobe of the pituitary was evidenced with transgenic Xenopus frogs expressing a vascular endothelial growth factor in melanotropes. Metamorphosis progresses via the interaction of TH, adrenal corticosteroids, and prolactin (PRL). We emphasize that PRL has a dual role: modulation of the speed of metamorphic changes and functional development of organs for adult life. A brief description about a novel type of PRL (1B) that was detected was made. A possible reason why the main hypothalamic factor that stimulates the release of thyrotropin is not thyrotropin-releasing hormone, but corticotropin-releasing factor is considered in light of the fact that amphibians are poikilotherms. As regards the reproductive behavior in amphibians, studies were focused on the courtship behavior of the newt, Cynops pyrrhogaster. Male newts exhibit a unique courtship behavior toward sexually developed conspecific females. Hormonal interactions eliciting this behavior and hormonal control of the courtship pheromone secretion are discussed on the basis of our experimental results.

Imorin: a sexual attractiveness pheromone in female red-bellied newts (Cynops pyrrhogaster)

The male red-bellied newt (Cynops pyrrhogaster) approaches the female’s cloaca prior to performing any courtship behaviour, as if he is using some released substance to gauge whether she is sexually receptive. Therefore, we investigated whether such a female sexual attractiveness pheromone exists. We found that a tripeptide with amino acid sequence Ala-Glu-Phe is secreted by the ciliary cells in the epithelium of the proximal portion of the oviduct of sexually developed newts and confirmed that this is the major active substance in water in which sexually developed female newts have been kept. This substance only attracted sexually developed male newts and acted by stimulating the vomeronasal epithelial cells. This is the first female sexual attractiveness peptide pheromone to be identified in a vertebrate.

Neuroendocrine control of spawning in amphibians and its practical applications

Across vertebrates, ovulation and sperm release are primarily triggered by the timed surge of luteinizing hormone (LH). These key reproductive events are governed by the action of several brain neuropeptides, pituitary hormones and gonadal steroids which operate to synchronize physiology with behaviour. In amphibians, it has long been recognized that the neuropeptide gonadotropin-releasing hormone (GnRH) has stimulatory effects to induce spawning. Extensive work in teleosts reveals an inhibitory role of dopamine in the GnRH-regulated release of LH. Preliminary evidence suggests that this may be a conserved function in amphibians. Emerging studies are proposing a growing list of modulators beyond GnRH that are involved in the control of spawning including prolactin, kisspeptins, pituitary adenylate cyclase-activating polypeptide, gonadotropin-inhibitory hormone and endocannabinoids. Based on these physiological data, spawning induction methods have been developed to test on selective amphibian species. However, several limitations remain to be investigated to strengthen the evidence for future applications. The current state of knowledge regarding the neuroendocrine control of spawning in amphibians will be reviewed in detail, the elements of which will have wide implications towards the captive breeding of endangered amphibian species for conservation.

Possible hormonal interaction for eliciting courtship behavior in the male newt, Cynops pyrrhogaster

Reproductive behavior in amphibians, as in other vertebrate animals, is under the control of multiple hormonal substances. Prolactin (PRL), arginine vasotocin (AVT), androgen, and 7α-hydroxypregnenolone (7α-OH PREG), four such substances with hormonal activity, are known to be involved in the expression of the tail vibration behavior which is the initial step of courtship performed by the male newt, Cynops pyrrhogaster. As current information on the interaction(s) between these hormones in terms of eliciting tail vibration behavior is limited, we have investigated whether the decline of expression of tail vibration behavior due to suppression of the activity of any one of these hormones can be restored by supplying any one of the other three hormones exogenously. Expression of the behavior was determined in terms of incidence (% of test animals exhibiting the behavior) and frequency (number of times that the behavior was repeated during the test period). Neither PRL nor androgen restored the decline in the incidence and frequency of the tail vibration behavior caused by the suppression of the activity of any one of other three hormones. AVT completely restored both the anti-PRL antibody-induced and flutamide (an androgen receptor antagonist)-induced, but not ketoconazole (an inhibitor of the steroidogenic CYP enzymes)-induced decline in the incidence and frequency of the tail vibration behavior. The neurosteroid, 7α-OH PREG, failed to restore flutamide-induced decline in the incidence and frequency of the behavior. However, it was able to restore both anti-PRL antibody-induced and AVT receptor antagonist-induced decline in the incidence, but not in the frequency of the behavior. In another experiment designed to see the activity of hormones enhancing the frequency of the tail vibration behavior, AVT was revealed to be more potent than 7α-OH PREG. The role of each hormonal substance in determining the expression of the tail vibration behavior was discussed based on the results.

Chemosignals, hormones, and amphibian reproduction

This article is part of a Special Issue "Chemosignals and Reproduction". Amphibians are often thought of as relatively simple animals especially when compared to mammals. Yet the chemosignaling systems used by amphibians are varied and complex. Amphibian chemosignals are particularly important in reproduction, in both aquatic and terrestrial environments. Chemosignaling is most evident in salamanders and newts, but increasing evidence indicates that chemical communication facilitates reproduction in frogs and toads as well. Reproductive hormones shape the production, dissemination, detection, and responsiveness to chemosignals. A large variety of chemosignals have been identified, ranging from simple, invariant chemosignals to complex, variable blends of chemosignals. Although some chemosignals elicit straightforward responses, others have relatively subtle effects. Review of amphibian chemosignaling reveals a number of issues to be resolved, including: 1) the significance of the complex, individually variable blends of courtship chemosignals found in some salamanders, 2) the behavioral and/or physiological functions of chemosignals found in anuran "breeding glands", 3) the ligands for amphibian V2Rs, especially V2Rs expressed in the main olfactory epithelium, and 4) the mechanism whereby transdermal delivery of chemosignals influences behavior. To date, only a handful of the more than 7000 species of amphibians has been examined. Further study of amphibians should provide additional insight to the role of chemosignals in reproduction.

Breakthrough in neuroendocrinology by discovering novel neuropeptides and neurosteroids: 2. Discovery of neurosteroids and pineal neurosteroids

Bargmann-Scharrer's discovery of "neurosecretion" in the first half of the 20th century has since matured into the scientific discipline of neuroendocrinology. Identification of novel neurohormones, such as neuropeptides and neurosteroids, is essential for the progress of neuroendocrinology. Our studies over the past two decades have significantly broadened the horizons of this field of research by identifying novel neuropeptides and neurosteroids in vertebrates that have opened new lines of scientific investigation in neuroendocrinology. We have established de novo synthesis and functions of neurosteroids in the brain of various vertebrates. Recently, we discovered 7α-hydroxypregnenolone (7α-OH PREG), a novel bioactive neurosteroid that acts as a key regulator for inducing locomotor behavior by means of the dopaminergic system. We further discovered that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol (CHOL). The pineal gland secretes 7α-OH PREG and 3α,5α-tetrahydroprogesterone (3α,5α-THP; allopregnanolone) that are involved in locomotor rhythms and neuronal survival, respectively. Subsequently, we have demonstrated their mode of action and functional significance. This review summarizes the discovery of these novel neurosteroids and its contribution to the progress of neuroendocrinology.

Brain and pineal 7α-hydroxypregnenolone stimulating locomotor activity: identification, mode of action and regulation of biosynthesis

Biologically active steroids synthesized in the central and peripheral nervous systems are termed neurosteroids. However, the biosynthetic pathways leading to the formation of neurosteroids are still incompletely elucidated. 7α-Hydroxypregnenolone, a novel bioactive neurosteroid stimulating locomotor activity, has been recently identified in the brain of newts and quail. Subsequently, the mode of action and regulation of biosynthesis of 7α-hydroxypregnenolone have been determined. Moreover, recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity in juvenile chickens, connecting light-induced gene expression with locomotion. This review summarizes the advances in our understanding of the identification, mode of action and regulation of biosynthesis of brain and pineal 7α-hydroxypregnenolone, a potent stimulator of locomotor activity.

Involvement of the neurosteroid 7α-hydroxypregnenolone in the courtship behavior of the male newt Cynops pyrrhogaster

Reproductive behavior in amphibians, as in other vertebrate animals, is controlled by multiple hormones. A neurosteroid, 7α-hydroxypregnenolone, has recently been found to enhance locomotor activity in the male newt, Cynops pyrrhogaster. Here, we show that this neurosteroid is also involved in enhancing the expression of courtship behavior. Intracerebroventricular (ICV) injection of 7α-hydroxypregnenolone enhanced courtship behavior dose-dependently in the sexually undeveloped males that had been pretreated with prolactin and gonadotropin, which is known to bring the males to a sexually developed state. But, unlike the case in the locomotion activity, 7α-hydroxypregnenolone did not elicit the behavior in males receiving no prior injections of these hormones. ICV administration of ketoconazole, an inhibitor of the steroidogenic enzyme cytochrome P450s, suppressed the spontaneously occurring courtship behavior in sexually active males. Supplementation with 7α-hydroxypregnenolone reversed the effect of ketoconazole in these animals. It was also demonstrated that the effect of the neurosteroid on the courtship behavior was blocked by a dopamine D2-like, but not by a D1-like, receptor antagonist. These results indicate that endogenous 7α-hydroxypregnenolone enhances the expression of the male courtship behavior through a dopaminergic system mediated by a D2-like receptor in the brain.

7α-Hydroxypregnenolone, a new key regulator of amphibian locomotion: discovery, progress and prospect

Seasonally-breeding amphibians have served as excellent animal models to investigate the biosynthesis and biological actions of neurosteroids. Previous studies have demonstrated that the brain of amphibians possesses key steroidogenic enzymes and produces pregnenolone, a precursor of steroid hormones, and other various neurosteroids. We recently found that the brain of seasonally-breeding newts actively produces 7α-hydroxypregnenolone, a previously undescribed amphibian neurosteroid. This novel amphibian neurosteroid acts as a neuronal modulator to stimulate locomotor activity in newts. Subsequently, the mode of action of 7α-hydroxypregnenolone has been demonstrated in the newt brain. 7α-Hydroxypregnenolone stimulates locomotor activity through activation of the dopaminergic system. To understand the functional significance of 7α-hydroxypregnenolone in the regulation of locomotor activity, diurnal and seasonal changes in synthesis of 7α-hydroxypregnenolone have also been demonstrated in the newt brain. Melatonin derived from the pineal gland and eyes regulates 7α-hydroxypregnenolone synthesis in the brain, thus inducing diurnal locomotor changes. Prolactin, an adenohypophyseal hormone, regulates 7α-hydroxypregnenolone synthesis in the brain, and also induces seasonal locomotor changes. In addition, 7α-hydroxypregnenolone mediates corticosterone action to increase locomotor activity under stress. This review summarizes the discovery, progress and prospect of 7α-hydroxypregnenolone, a new key regulator of amphibian locomotion.

Regulation of neurosteroid biosynthesis by neurotransmitters and neuropeptides

The enzymatic pathways leading to the synthesis of bioactive steroids in the brain are now almost completely elucidated in various groups of vertebrates and, during the last decade, the neuronal mechanisms involved in the regulation of neurosteroid production have received increasing attention. This report reviews the current knowledge concerning the effects of neurotransmitters, peptide hormones, and neuropeptides on the biosynthesis of neurosteroids. Anatomical studies have been carried out to visualize the neurotransmitter- or neuropeptide-containing fibers contacting steroid-synthesizing neurons as well as the neurotransmitter, peptide hormones, or neuropeptide receptors expressed in these neurons. Biochemical experiments have been conducted to investigate the effects of neurotransmitters, peptide hormones, or neuropeptides on neurosteroid biosynthesis, and to characterize the type of receptors involved. Thus, it has been found that glutamate, acting through kainate and/or AMPA receptors, rapidly inactivates P450arom, and that melatonin produced by the pineal gland and eye inhibits the biosynthesis of 7α-hydroxypregnenolone (7α-OH-Δ(5)P), while prolactin produced by the adenohypophysis enhances the formation of 7α-OH-Δ(5)P. It has also been demonstrated that the biosynthesis of neurosteroids is inhibited by GABA, acting through GABA(A) receptors, and neuropeptide Y, acting through Y1 receptors. In contrast, it has been shown that the octadecaneuropetide ODN, acting through central-type benzodiazepine receptors, the triakontatetraneuropeptide TTN, acting though peripheral-type benzodiazepine receptors, and vasotocin, acting through V1a-like receptors, stimulate the production of neurosteroids. Since neurosteroids are implicated in the control of various neurophysiological and behavioral processes, these data suggest that some of the neurophysiological effects exerted by neurotransmitters and neuropeptides may be mediated via the regulation of neurosteroid production.

Historical view of development of comparative endocrinology in Japan

This article describing a brief history of development of comparative endocrinology in Japan is contributed to the journal General and Comparative Endocrinology, in commemoration of the 50th anniversary of its publication. It covers significant works in the field of comparative endocrinology that have been done by Japanese endocrinologists, focusing those achieved during the past 70 years. The contents were arranged according to the taxonomical order of the experimental animals with which individual researchers or research groups have contributed to the acquisition of important knowledge in comparative endocrinology.

Mode of action and functional significance of 7α-hydroxypregnenolone stimulating locomotor activity

Previous studies over the past two decades have demonstrated that the brain and other nervous systems possess key steroidogenic enzymes and produces pregnenolone and other various neurosteroids in vertebrates in general. Recently, 7α-hydroxypregnenolone, a novel bioactive neurosteroid, was identified in the brain of newts and quail. Importantly, this novel neurosteroid is produced from pregnenolone through the enzymatic activity of cytochrome P450(7α) and acts on brain tissue as a neuronal modulator to stimulate locomotor activity in these vertebrates. Subsequently, the mode of action of 7α-hydroxypregnenolone was demonstrated. 7α-Hydroxypregnenolone stimulates locomotor activity through activation of the dopaminergic system. To understand the functional significance of 7α-hydroxypregnenolone in the regulation of locomotor activity, diurnal, and seasonal changes in 7α-hydroxypregnenolone synthesis were further characterized. Melatonin derived from the pineal gland and eyes regulates 7α-hydroxypregnenolone synthesis in the brain, thus inducing diurnal locomotor changes. Prolactin, an adenohypophyseal hormone, regulates 7α-hydroxypregnenolone synthesis in the brain, and also induces seasonal locomotor changes. In addition, 7α-hydroxypregnenolone mediates corticosterone action to modulate locomotor activity under stress. This review summarizes the current knowledge regarding the mode of action and functional significance of 7α-hydroxypregnenolone, a newly identified bioactive neurosteroid stimulating locomotor activity.

Prolactin increases the synthesis of 7alpha-hydroxypregnenolone, a key factor for induction of locomotor activity, in breeding male Newts

We recently found that the Japanese red-bellied newt, Cynops pyrrhogaster, actively produces 7alpha-hydroxypregnenolone, a previously undescribed amphibian neurosteroid. 7alpha-Hydroxypregnenolone stimulates locomotor activity of male newts. Locomotor activity of male newts increases during the breeding period as in other wild animals, but the molecular mechanism for such a change in locomotor activity is poorly understood. Here we show that the adenohypophyseal hormone prolactin (PRL) stimulates 7alpha-hydroxypregnenolone synthesis in the brain, thus increasing locomotor activity of breeding male newts. In this study, cytochrome P450(7alpha) (CYP7B), a steroidogenic enzyme catalyzing the formation of 7alpha-hydroxypregnenolone, was first identified to analyze seasonal changes in 7alpha-hydroxypregnenolone synthesis. Only males exhibited marked seasonal changes in 7alpha-hydroxypregnenolone synthesis and CYP7B expression in the brain, with a maximum level in the spring breeding period when locomotor activity of males increases. Subsequently we identified PRL as a key component of the mechanism regulating 7alpha-hydroxypregnenolone synthesis. Hypophysectomy decreased 7alpha-hydroxypregnenolone synthesis in the male brain, whereas administration of PRL but not gonadotropins to hypophysectomized males caused a dose-dependent increase in 7alpha-hydroxypregnenolone synthesis. To analyze the mode of PRL action, CYP7B and the receptor for PRL were localized in the male brain. PRL receptor was expressed in the neurons expressing CYP7B in the magnocellular preoptic nucleus. Thus, PRL appears to act directly on neurosteroidogenic magnocellular preoptic nucleus neurons to regulate 7alpha-hydroxypregnenolone synthesis, thus inducing seasonal locomotor changes in male newts. This is the first report describing the regulation of neurosteroidogenesis in the brain by an adenohypophyseal hormone in any vertebrate.

7α-hydroxypregnenolone, a new key regulator of locomotor activity of vertebrates: identification, mode of action, and functional significance

Steroids synthesized de novo by the central and peripheral nervous systems are called neurosteroids. The formation of neurosteroids from cholesterol in the brain was originally demonstrated in mammals by Baulieu and colleagues. Our studies over the past two decades have also shown that, in birds and amphibians as in mammals, the brain expresses several kinds of steroidogenic enzymes and produces a variety of neurosteroids. Thus, de novo neurosteroidogenesis from cholesterol is a conserved property that occurs throughout vertebrates. However, the biosynthetic pathways of neurosteroids in the brain of vertebrates was considered to be still incompletely elucidated. Recently, 7α-hydroxypregnenolone was identified as a novel bioactive neurosteroid stimulating locomotor activity in the brain of newts and quail through activation of the dopaminergic system. Subsequently, diurnal and seasonal changes in synthesis of 7α-hydroxypregnenolone in the brain were demonstrated. Interestingly, melatonin derived from the pineal gland and eyes regulates 7α-hydroxypregnenolone synthesis in the brain, thus inducing diurnal locomotor changes. Prolactin, an adenohypophyseal hormone, regulates 7α-hydroxypregnenolone synthesis in the brain, and may also induce seasonal locomotor changes. This review highlights the identification, mode of action, and functional significance of 7α-hydroxypregnenolone, a new key regulator of locomotor activity of vertebrates, in terms of diurnal and seasonal changes in 7α-hydroxypregnenolone synthesis, and describes some of their regulatory mechanisms.

The reproductive pattern of male dusky salamanders (genus Desmognathus) is neither associated nor dissociated

Many seasonally breeding vertebrate species have an associated reproductive pattern: mating behavior, gonadal activity, and peak circulating androgen levels occur simultaneously. In these species, androgens influence the expression of male mating behavior. Other species have a dissociated reproductive pattern: mating behavior occurs at a different time than peak gonadal activity. In such species, it is hypothesized that mating behavior is not dependent on androgen levels [Crews, D., 1984. Gamete production, sex hormone secretion, and mating behavior uncoupled. Horm. Behav. 18, 22-28]. The salamander Desmognathus ochrophaeus mates in the spring and fall while spermatogenesis occurs during the summer, suggesting that it has a dissociated reproductive pattern and that androgens do not mediate mating behavior. To assess whether mating behavior is regulated by gonadal androgens, we castrated males to reduce endogenous androgens and implanted testosterone propionate (TP) to restore androgen levels. Castrated males mated significantly less than did control males. Castrated males given TP mated as much as control males. Compared to controls, circulating androgen levels (both testosterone (T) and dihydrotestosterone (DHT)) were reduced in castrated males and elevated in castrated males given TP implants. We also found that plasma corticosterone (CORT) levels were strongly and positively correlated with T levels. Together, these data indicate that, although spermatogenesis is dissociated in time from mating behavior, androgens are associated with the expression of mating. Thus, the associated-dissociated dichotomy does not adequately describe the reproductive pattern of D. ochrophaeus. We discuss the limitations of the associated-dissociated framework in clarifying hormone-behavior relationships in reptiles and amphibians.

Historical perspective: Hormonal regulation of behaviors in amphibians

This review focuses on research into the hormonal control of behaviors in amphibians that was conducted prior to the 21st century. Most advances in this field come from studies of a limited number of species and investigations into the hormonal mechanisms that regulate reproductive behaviors in male frogs and salamanders. From this earlier research, we highlight five main generalizations or conclusions. (1) Based on studies of vocalization behaviors in anurans, testicular androgens induce developmental changes in cartilage and muscles fibers in the larynx and thereby masculinize peripheral structures that influence the properties of advertisement calls by males. (2) Gonadal steroid hormones act to enhance reproductive behaviors in adult amphibians, but causal relationships are not as well established in amphibians as in birds and mammals. Research into the relationships between testicular androgens and male behaviors, mainly using castration/steroid treatment studies, generally supports the conclusion that androgens are necessary but not sufficient to enhance male behaviors. (3) Prolactin acts synergistically with androgens and induces reproductive development, sexual behaviors, and pheromone production. This interaction between prolactin and gonadal steroids helps to explain why androgens alone sometimes fail to stimulate amphibian behaviors. (4) Vasotocin also plays an important role and enhances specific types of behaviors in amphibians (frog calling, receptivity in female frogs, amplectic clasping in newts, and non-clasping courtship behaviors). Gonadal steroids typically act to maintain behavioral responses to vasotocin. Vasotocin modulates behavioral responses, at least in part, by acting within the brain on sensory pathways that detect sexual stimuli and on motor pathways that control behavioral responses. (5) Corticosterone acts as a potent and rapid suppressor of reproductive behaviors during periods of acute stress. These rapid stress-induced changes in behaviors use non-genomic mechanisms and membrane-associated corticosterone receptors.

Localization of prolactin receptor in the newt brain

In the male newt Cynops pyrrhogaster, prolactin (PRL) acts directly on the central nervous system and induces courtship behavior. As a step to elucidate the localization of neurons on which PRL acts, we developed a polyclonal antibody against an oligopeptide having a sequence completely identical with a part of the sequence of PRL receptors (PRLRs) of two species of newts, C. pyrrhogaster and C. ensicauda, and performed an immunohistochemical study with this antibody. PRLR-immunoreactive cells were observed in the medial amygdala, anterior preoptic area, magnocellular preoptic nucleus, suprachiasmatic nucleus, nucleus of the periventricular organ, ventral hypothalamic nucleus, and choroid plexus. We also performed in situ hybridization with a (35)S-labeled newt PRLR antisense RNA probe and detected signals in the preoptic area and choroid plexus. Colocalization of both PRLR-like immunoreactivity and arginine vasotocin-like or mesotocin-like immunoreactivity was demonstrated in the magnocellular preoptic nucleus. This is the first report of PRLR localization in the amphibian brain.

Prolactin acts centrally to enhance newt courtship behavior

The effects of intracerebroventricular (ICV) and intraperitoneal (IP) injections of ovine prolactin (PRL), antiserum against newt PRL, and antibody against the newt PRL receptor on the expression of courtship behavior of male newts, Cynops pyrrhogaster, were studied to see whether PRL acts centrally or peripherally to induce the behavior. Injections of PRL by either route into gonadotropin-primed males enhanced the expression of the behavior dose-dependently. The minimum effective amount of ovine PRL administered intracerebroventricularly was 0.1 microg, whereas it was 100 microg when injected intraperitoneally. ICV injection of antiserum against newt PRL blocked the spontaneously occurring male courtship behavior when the anti-newt PRL serum was given either intracerebroventricularly or intraperitoneally. The minimum effective dose of the antiserum administered intracerebroventricularly was 0.05 microl, whereas it was 20 microl when injected intraperitoneally. Neither ICV nor IP injection of preimmune serum affected the expression of the behavior. Furthermore, ICV, but not IP, administration of 0.3 microg of anti-newt PRL receptor antibody, purified from antiserum against newt PRL receptor by use of an antigen-conjugated affinity column, blocked the spontaneously occurring courtship behavior in sexually developed males. Neither ICV nor IP injection of the same amount of normal rabbit IgG affected the expression of the behavior. The results strongly suggest that endogenous PRL enhances the behavior by acting centrally through the PRL receptors localized in the brain area.

Evolutionary insights into the regulation of courtship behavior in male amphibians and reptiles

Comparative studies of species differences and similarities in the regulation of courtship behavior afford an understanding of evolutionary pressures and constraints shaping reproductive processes and the relative contributions of hormonal, genetic, and ecological factors. Here, we review species differences and similarities in the control of courtship and copulatory behaviors in male amphibians and reptiles, focusing on the role of sex steroid hormones, the neurohormone arginine vasotocin (AVT), and catecholamines. We discuss species differences in the sensory modalities used during courtship and in the neural correlates of these differences, as well as the value of particular model systems for neural evolution studies with regard to reproductive processes. For example, in some genera of amphibians (e.g., Ambystoma) and reptiles (e.g., Cnemidophorus), interspecific hybridizations occur, making it possible to compare the ancestral with the descendant species, and these systems provide a window into the process of behavioral and neural evolution as well as the effect of genome size. Though our understanding of the hormonal and neural correlates of mating behavior in a variety of amphibian and reptilian species has advanced substantially, more studies that manipulate hormone or neurotransmitter systems are required to assess the functions of these systems.

Peptide pheromones in newts

This article reviews the current state of understanding of reproductive pheromones in amphibians, focusing mainly on the purification and characterization of peptide pheromones in newts of the genus Cynops, molecular cloning of cDNAs encoding the pheromone molecules, and hormonal control of secretion of these pheromones. Pheromones that attract sexually developed female Cynops pyrrhogaster and C. ensicauda newts were isolated from the male abdominal glands. The C. pyrrhogaster and C. ensicauda pheromones are peptides, designated sodefrin and silefrin, with the amino acid sequences SIPSKDALLK and SILSKDAQLK, respectively. Each pheromone attracts only conspecific females. Molecular cloning of cDNAs encoding sodefrin and silefrin revealed the presence of precursor proteins that are considered to generate these pheromone peptides. Pheromone precursor mRNA levels and radioimmunoassayable pheromone concentrations in the abdominal glands were elevated by prolactin and androgen. Sexual dimorphism and hormone dependency of the responsiveness of vomeronasal epithelium to sodefrin were noted. Significance of pheromones in the form of peptide for those performing reproductive behavior in an aquatic environment was also discussed.

Processing of multiple forms of preprosodefrin in the abdominal gland of the red-bellied newt Cynops pyrrhogaster: regional and individual differences in preprosodefrin gene expression

Peptides derived from the post-translational processing of preprosodefrin were isolated from an extract of the abdominal glands of male red-bellied newts Cynops pyrrhogaster obtained 5 months prior to the onset of the breeding season. Structural characterization of the peptides showed that the pheromone sodefrin (SIPSKDALLK) is stored in a biologically inactive COOH-terminally extended form (SIPSKDALLKISA). It follows, therefore, that the activation of a protease that cleaves at a Lys-Ile bond to generate the active pheromone must occur by the time of onset of reproductive behavior. Additional peptides (representing preprosodefrin-(146-175)-peptide and preprosodefrin-(159-173)-peptide), that are derived from the precursor by cleavage at monobasic and dibasic processing sites, were also purified from the extract. The isolation of paralogs of these peptides, including an inactive COOH-terminally extended form of [Asn10]sodefrin, provides evidence for the expression of multiple genes encoding preprosodefrin. PCR products derived from total RNAs from the abdominal gland of individual newts collected from three different regions of Japan were analyzed. The data confirm the existence of multiple genes encoding sodefrin and its variants whose expression varied according to the individuals and the regions. However, genes encoding sodefrin were found to be expressed in all the specimens sampled.

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.

Peptide and protein pheromones in amphibians

Purification, characterization and biological activity of urodele and anuran sex-pheromones were reviewed. Female-attracting pheromones obtained from the abdominal gland of Cynops pyrrhogaster and C. ensicauda males are peptides consisting of 10 amino acid residues being designated sodefrin and silefrin, respectively. Each pheromone attracted only conspecific females. Molecular cloning of cDNAs encoding sodefrin and silefrin revealed that both are generated from precursor proteins. Synthesis of these pheromones is regulated by prolactin (PRL) and androgen. Responsiveness of the female vomeronasal epithelium to sodefrin is enhanced by PRL and estrogen. The submandibular gland of the male terrestrial salamander, Plethodon jardani secretes a 22-kD proteinaceous pheromone that enhances female receptivity. It was revealed that every salamander synthesizes multiple isoforms of this pheromone, Plethodontid receptivity factor. The magnificent tree frog, Litoria splendida breed in an aquatic environment. The skin glands of the male secrete a female-attracting peptide pheromone, splendipherin, comprising 25 amino acid residues. The significance of the structure of the amphibian sex-pheromone as peptide and protein is discussed in terms of their species specificity.

Temperature-dependent prolactin secretion and reproductive biology of the newt Triturus carnifex Laur

The effects of temperature on pituitary prolactin (PRL) gene expression and peripheral levels were studied in both male and female newts obtained from wild conditions during reproductive (Experiment I) and nonreproductive (Experiment II) periods; moreover, changes in parameters related to reproductive function are also described. Male and female newts were taken from a pond in February (Experiment I, reproductive period) and maintained for 1 month in tanks at 4 and 18 degrees C. In male newts kept at 4 degrees C, increase of PRL mRNA in the pituitary and plasma PRL was found compared with that measured in those kept at 18 degrees C. The increase in PRL secretion was parallel to that of plasma androgens and related secondary sexual characteristics (SSC) in males and of plasma estradiol-17beta and vitellogenin in females. On the contrary, in nonreproductive newts (Experiment II), taken from the field in May, no significant changes in plasma PRL, androgens, and SSC were found in those maintained at low temperature (4 degrees C), whereas low temperature significantly increased PRL mRNA expression in the male pituitary and PRL mRNA plus plasma PRL levels in females. These findings suggest that low temperature regulates PRL secretion in this urodele species, showing a sex- and season-related control mechanism; moreover, low temperature failed to influence the reproductive biology of newts taken from the field in May, after naturally occurring reproduction during winter months.

Molecular cloning of newt prolactin (PRL) cDNA: effect of temperature on PRL mRNA expression

A partial prolactin (PRL) cDNA was specifically PCR amplified from a cDNA library constructed from pituitary mRNAs of the newt (Cynops pyrrhogaster) and cloned into plasmid vectors. One clone thus obtained contained a 739-bp insert encoding the C-terminal amino acid sequence of the mature hormone molecule. Using this clone as a probe, the full-length newt PRL cDNA was screened from the cDNA library. The PRL cDNA clone thus obtained consisted of 1024 bp encoding the entire sequence of the mature PRL molecule in addition to its signal peptide. The amino acid sequence of newt PRL deduced from its nucleotide sequence showed higher homologies with those PRL sequences of tetrapod animals than with those of teleosts. Northern blot analysis revealed the newt PRL mRNA size to be approximately 1 kb. In situ hybridization using the newt PRL cDNA as a probe revealed that the pituitary region expressing PRL mRNA corresponded to that immunoreactive with antiserum against PRL. PRL mRNA levels in the pituitary of newts subjected to room and low temperatures were determined by Northern analysis employing the PRL cDNA as a probe. PRL mRNA levels were significantly higher in the pituitaries of newts subjected to 10 degrees than in those of newts kept at 23 degrees. Likewise, immunoassayable plasma PRL levels were higher in animals subjected to 10 degrees than in those kept at 23 degrees.

Effect of prolactin and androgen on the expression of the female-attracting pheromone silefrin in the abdominal gland of the newt, Cynops ensicauda

Silefrin is a sodefrin-like, female-attracting pheromone comprising 10 amino acids that was isolated from the abdominal gland of the sword-tailed newt, Cynops ensicauda. Hormonal effects on the silefrin precursor mRNA expression and silefrin content in the abdominal gland were investigated in the present study by using Northern blot analysis and radioimmunoassay, respectively. In the abdominal gland of newts treated with prolactin (PRL) plus testosterone propionate (TP), silefrin precursor mRNA expression was markedly enhanced as compared with that in the newts injected with saline, PRL, or TP. Values for radioimmunoassayable silefrin content in the abdominal gland paralleled those for the silefrin precursor mRNA levels. Moreover, silefrin precursor mRNA signals, as revealed by in situ hybridization, as well as stainability of immunoreactive silefrin were much more intense in the epithelial cells of the abdominal gland of the PRL-plus-TP-treated animals than in those of controls. We thus conclude that PRL and androgen are important factors for enhancing silefrin synthesis.

Newt prolactin and its involvement in reproduction

The amino acid sequence of newt (Cynops pyrrhogaster) prolactin deduced from the nucleotide sequence of its cDNA showed a relatively high homology with sequences of chicken and sea turtle prolactins as well as with those of anuran prolactins. Cynops prolactin receptor transcripts were detected in various tissues and organs, suggesting that prolactin plays multiple roles in urodeles. Urodele prolactin was purified from the pituitaries of C. pyrrhogaster. Antiserum against this prolactin was used for radioimmunoassay of plasma prolactin and immunoneutralization experiments. Endogenous prolactin was shown to induce migration to water, courtship behavior, and cessation of spermatocytogenesis in the Cynops newt. The hormone was found to be involved in the development of cloacal glands such as the lateral and abdominal glands, growth of the tail and Mauthner neurons, secretion of oviducal jelly, and enhanced synthesis of a female attracting pheromone (sodefrin), and responsiveness of the olfactory epithelium to sodefrin. In most of these cases, prolactin was found to act synergistically or antagonistically with sex steroids. We also discovered that hypersecretion of prolactin in the newts subjected to cold temperature was induced by hypothalamic stimulation rather than release from hypothalamic inhibition.Key words: prolactin, newts, reproduction.

Hormonal control of urodele reproductive behavior

Hormonal control of expression of courtship behavior and of development of structures related to the reproductive behavior in two species of Japanese newts, Cynops pyrrhogaster and Cynops ensicauda, was described. Prolactin (PRL) and androgen were essential factors for eliciting courtship behavior. In addition, arginine vasotocin markedly enhanced the expression of courtship behavior. PRL induced migration to water, in which courtship and oviposition take place, and converted the integument from the terrestrial type to the aquatic one. PRL also stimulated the growth of the tail fin, which was blocked by estrogen. Cellular and nuclear size and number of synapses on the somata of Mauthner cells, which are involved in tail movement, were also increased by PRL and androgen. Synthesis of sodefrin, a female-attracting pheromone, in the abdominal gland as well as that of mucopolysaccharides constituting the sac of sperm in the lateral gland was enhanced by PRL and androgen. Structural development of oviducts was elicited by estrogen or PRL to a certain extent, and full oviducal development by the combination of these two hormones, PRL being indispensable for the oviducal jelly secretion.

Elevation of plasma prolactin concentrations by low temperature is the cause of spermatogonial cell death in the newt, Cynops pyrrhogaster

Temperature plays an important role in reproduction of urodeles. Spermatogenesis in newts is arrested when the environmental temperature lowers. We found that transfer of newts, Cynops pyrrhogaster, to low temperature (8 and 12 degrees C) caused cell death of spermatogonia just before meiosis and elevation of prolactin concentration in the newt plasma. Injection of a dopamine antagonist (pimozide), which is known to increase the plasma prolactin concentration, to the newt caused significant increase of spermatogonial degeneration, whereas treatment with an agonist (bromocryptin), which is known to decrease the prolactin concentration, suppressed the cell death. Finally, injection of anti-prolactin serum into the newts which had been transferred to low temperature almost completely inhibited the spermatogonial degeneration for as long as 3 days. These results demonstrate that low temperature caused elevation of prolactin concentration in the newt blood, which induced cell death of spermatogonia just before meiosis.

Female-attracting pheromone in newt cloacal glands

It has been postulated that male newts emit pheromones that attract females of the same species. Female newts of the species Cynops pyrrhogaster were attracted to water in which sexually active conspecific males had been kept, but not to water in which abdominal gland-ablated males had been kept, indicating that the attracting pheromone was secreted by or through the abdominal gland of the cloaca. An attempt has been made to isolate and characterize the female-attracting pheromone in the abdominal glands of male newts. Female-attracting activity was monitored using a preference test. The active substance was isolated by two steps of purification using reverse-phase high-performance liquid chromatography. Direct sequencing of the final product revealed that it is a decapeptide with the amino acid sequence Ser-Ile-Pro-Ser-Lys-Asp-Ala-Leu-Leu-Lys. Its minimum effective concentration was estimated to be between 0.1 pM and 1.0 pM. The synthetic peptide showed a female-attracting activity similar to that of the native peptide. It seems to act through the olfactory organ of female newts, because the effect of the peptide was blocked by bilateral nostril plugging with cotton balls soaked in melted vaseline. An antiserum against sodefrin was generated in a rabbit. An immunoelectron microscopic study using this antiserum revealed that sodefrin exists predominantly within secretory granules in the epithelial cells of the abdominal glands. A radioimmunoassay for sodefrin was developed in which the antiserum was used along with sodefrin which was N-terminally extended with a tyrosine residue as a radioligand. The immunoassayable sodefrin content in C. pyrrhogaster males was diminished by castration and hypophysectomy. The sodefrin content was increased markedly in the castrated and hypophysectomized newts after treatment with both testosterone and prolactin. Testosterone but not prolactin increased the sodefrin content to a lesser extent. Aqueous extract of the abdominal glands of C. ensicauda showed no inhibition of binding in this assay. Moreover, C. ensicauda females were insensitive to sodefrin, although they were attracted to a water extract of abdominal glands from males of their own species. On the other hand, C. pyrrhogaster females responding to sodefrin were not attracted to the water extract of the abdominal glands from C. ensicauda males. Sequence analyses of sodefrin cDNA clones obtained from a C. ensicauda abdominal gland cDNA library revealed that the cDNA encoded a variant type of sodefrin peptide with substitutions of Leu3 and Gln8. The synthetic [Leu3, Gln8]-sodefrin attracted C. ensicauda females but not C. pyrrhogaster females. These results indicate that the female-attracting pheromone differs between these two species of genus Cynops.

Enhancement by prolactin of the GnRH-induced release of LH from dispersed anterior pituitary cells of the bullfrog (Rana catesbeiana)

The response of enzymatically dispersed anterior pituitary cells of the bullfrog (Rana catesbeiana) to gonadotropin-releasing hormone (GnRH) was studied by monitoring the release of luteinizing hormone (LH) into the culture medium. The cells responded to GnRH by releasing LH according to the incubation time and to the GnRH concentration. The responsiveness to GnRH became less conspicuous as the cell density was reduced. Addition of prolactin (PRL) to the medium enhanced the responsiveness to the secretagogue, and addition of antiserum against PRL lowered the responsiveness to a certain extent. Immunohistochemical studies of sectioned pituitaries revealed that PRL cells most frequently located in contact with LH cells. The possibility that PRL acts directly on gonadotrophs to enhance their responsiveness to GnRH was suggested.

Comparison of Mauthner cell size in sexually developed and undeveloped male red-bellied newts

Differences in Mauthner (M) cell size were examined in sexually developed and undeveloped male red-bellied newts, Cynops pyrrhogaster. The mean areas of nuclei and cell bodies of M cells and mean maximum and minimum diameters of the cell bodies in the sexually developed males were significantly larger than those in the sexually undeveloped ones. In the hypophysectomized male newts, all these parameters were not significantly different from those in the sexually undeveloped ones. These values were significantly increased by treatment with both bovine prolactin and human chorionic gonadotropin every other day for 3 weeks after hypophysectomy, and these measures were comparable to those in the sexually developed males. These findings suggest that differences in M cell size between sexually developed and undeveloped male newts are due to alteration in hormonal milieu.

Hormonal control of response to and secretion of sex attractants in Japanese newts

Hormonal control of sexual chemoattraction in the newt, Cynops pyrrhogaster, was investigated by a water preference test. Sexually inert newts injected with prolactin (PRL) and human chorionic gonadotropin (HCG) in combination preferred the water in which newts of the opposite sex had been kept, whereas saline-injected specimens did not. This indicates that PRL and HCG enhance the response of the newts to attractant(s) secreted by the opposite sex. The water in which PRL plus HCG-treated newts had been kept attracted the opposite sex more intensely than the water in which PRL-, HCG-, or saline-injected newts had been kept. In castrated males, PRL plus testosterone propionate (TP), and in ovariectomized females, PRL plus estradiol induced the secretion of substance(s) that attracted newts of the opposite sex. Removal of the abdominal gland of the cloaca in the male or the oviduct in the female attenuated the attracting activity of the water in which they were kept. Involvement of PRL and gonadal steroids in the secretion of sex attractants by the male abdominal gland and female oviduct was suggested.