Responsiveness of vomeronasal cells to a newt peptide pheromone, sodefrin as monitored by changes of intracellular calcium concentrations

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.

Quantitative comparative analysis of the nasal chemosensory organs of anurans during larval development and metamorphosis highlights the relative importance of chemosensory subsystems in the group

The anuran peripheral olfactory system is composed of a number of subsystems, represented by distinct neuroepithelia. These include the main olfactory epithelium and vomeronasal organ (found in most tetrapods) and three specialized epithelia of anurans: the buccal-exposed olfactory epithelium of larvae, and the olfactory recess and middle chamber epithelium of postmetamorphic animals. To better characterize the developmental changes in these subsystems across the life cycle, morphometric changes of the nasal chemosensory organs during larval development and metamorphosis were analyzed in three different anuran species (Rhinella arenarum, Hypsiboas pulchellus, and Xenopus laevis). We calculated the volume of the nasal chemosensory organs by measuring the neuroepithelial area from serial histological sections at four different stages. In larvae, the vomeronasal organ was relatively reduced in R. arenarum compared with the other two species; the buccal-exposed olfactory epithelium was absent in X. laevis, and best developed in H. pulchellus. In postmetamorphic animals, the olfactory epithelium (air-sensitive organ) was relatively bigger in terrestrial species (R. arenarum and H. pulchellus), whereas the vomeronasal and the middle chamber epithelia (water-sensitive organs) was best developed in X. laevis. A small olfactory recess (likely homologous with the middle chamber epithelium) was found in R. arenarum juveniles, but not in H. pulchellus. These results support the association of the vomeronasal and middle chamber epithelia with aquatic olfaction, as seen by their enhanced development in the secondarily aquatic juveniles of X. laevis. They also support a role for the larval buccal-exposed olfactory epithelium in assessment of oral contents: it was absent in X. laevis, an obligate suspension feeder, while present in the two grazing species. These initial quantitative results give, for the first time, insight into the functional importance of the peripheral olfactory subsystems across the anuran life cycle.

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.

Beyond sodefrin: evidence for a multi-component pheromone system in the model newt Cynops pyrrhogaster (Salamandridae)

Sodefrin, a decapeptide isolated from the male dorsal gland of the Japanese fire belly newt Cynops pyrrhogaster, was the first peptide pheromone identified from a vertebrate. The fire belly salamander and sodefrin have become a model for sex pheromone investigation in aquatically courting salamanders ever since. Subsequent studies in other salamanders identified SPF protein courtship pheromones of around 20 kDa belonging to the same gene-family. Although transcripts of these proteins could be PCR-amplified in Cynops, it is currently unknown whether they effectively use full-length SPF pheromones next to sodefrin. Here we combined transcriptomics, proteomics and phylogenetics to investigate SPF pheromone use in Cynops pyrrhogaster. Our data show that not sodefrin transcripts, but multiple SPF transcripts make up the majority of the expression profile in the dorsal gland of this newt. Proteome analyses of water in which a male has been courting confirm that this protein blend is effectively secreted and tail-fanned to the female. By combining phylogenetics and expression data, we show that independent evolutionary lineages of these SPF’s were already expressed in ancestral Cynops species before the origin of sodefrin. Extant Cynops species continue to use this multi-component pheromone system, consisting of various proteins in addition to a lineage-specific peptide.

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.

Expression of G proteins in the olfactory receptor neurons of the newt Cynops pyrrhogaster: their unique projection into the olfactory bulbs

We analyzed the expression of G protein α subunits and the axonal projection into the brain in the olfactory system of the semiaquatic newt Cynops pyrrhogaster by immunostaining with antibodies against Gαolf and Gαo , by in situ hybridization using probes for Gαolf , Gαo , and Gαi2 , and by neuronal tracing with DiI and DiA. The main olfactory epithelium (OE) consists of two parts, the ventral OE and dorsal OE. In the ventral OE, the Gαolf - and Gαo -expressing neurons are located in the apical and basal zone of the OE, respectively. This zonal expression was similar to that of the OE in the middle cavity of the fully aquatic toad Xenopus laevis. However, the Gαolf - and Gαo -expressing neurons in the newt ventral OE project their axons toward the main olfactory bulb (MOB) and the accessory olfactory bulb (AOB), respectively, whereas in Xenopus, the axons of both neurons project solely toward the MOB. In the dorsal OE of the newt, as in the principal cavity of Xenopus, the majority of the neurons express Gαolf and extend their axons into the MOB. In the vomeronasal organ (VNO), the neurons mostly express Gαo . These neurons and quite a few Gαolf -expressing neurons project their axons toward the AOB. This feature is similar to that in the terrestrial toad Bufo japonicus and is different from that in Xenopus, in which VNO neurons express solely Gαo , although their axons invariably project toward the AOB. We discuss the findings in the light of diversification and evolution of the vertebrate olfactory system.