Differential distribution of gonadotropin-releasing hormone-immunoreactive neurons in the stingray brain: functional and evolutionary considerations

Ontogenetic changes in the tyrosine hydroxylase immunoreactive preoptic area in the small-spotted catshark Scyliorhinus canicula (L., 1758) females: catecholaminergic involvement in sexual maturation

Introduction

The catecholaminergic component of the brain-pituitary-gonadal axis, which mediates the influence of external and internal stimuli on the central nervous system and gonad development in vertebrates, is largely unexplored in Chondrichthyes. We considered Scyliorhinus canicula (L., 1758) females as a model for this vertebrate's class, to assess the involvement of the catecholaminergic system of the brain in its reproduction. Along the S. canicula reproductive cycle, we characterized and evaluated differences in somata morphometry and the number of putative catecholaminergic neurons in two brain nuclei: the periventricular preoptic nucleus, hypothesized to be a positive control for ovarian development, and the suprachiasmatic nucleus, examined as a negative control.

Materials and methods

16 S. canicula wild females were sampled and grouped in maturity stages (immature, maturing, mature, and mature egg-laying). The ovary was histologically processed for the qualitative description of maturity stages. Anti-tyrosine hydroxylase immunofluorescence was performed on the diencephalic brain sections. The immunoreactive somata were investigated for morphometry and counted using the optical fractionator method, throughout the confocal microscopy.

Results and discussions

Qualitative and quantitative research confirmed two separate populations of immunoreactive neurons. The modifications detected in the preoptic nucleus revealed that somata were more numerous, significantly smaller in size, and more excitable during the maturing phase but decreased, becoming slightly bigger and less excitable in the egg-laying stage. This may indicate that the catecholaminergic preoptic nucleus is involved in the control of reproduction, regulating both the onset of puberty and the imminent spawning. In contrast, somata in the suprachiasmatic nucleus grew in size and underwent turnover in morphometry, increasing the total number from the immature-virgin to maturing stage, with similar values in the more advanced maturity stages. These changes were not linked to a reproductive role. These findings provide new valuable information on Chondrichthyes, suggesting the existence of an additional brain system implicated in the integration of internal and environmental cues for reproduction.

Multiple gonadotropin-releasing hormone systems in non-mammalian vertebrates: Ontogeny, anatomy, and physiology

Three paralogous genes for gonadotropin-releasing hormone (GnRH; gnrh1, gnrh2, and gnrh3) and GnRH receptors exist in non-mammalian vertebrates. However, there are some vertebrate species in which one or two of these paralogous genes have become non-functional during evolution. The developmental migration of GnRH neurons in the brain is evolutionarily conserved in mammals, reptiles, birds, amphibians, and jawed teleost fish. The three GnRH paralogs have specific expression patterns in the brain and originate from multiple sites. In acanthopterygian teleosts (medaka, cichlid, etc.), the preoptic area (POA)-GnRH1 and terminal nerve (TN)-GnRH3 neuronal types originate from the olfactory regions. In other fish species (zebrafish, goldfish and salmon) with only two GnRH paralogs (GnRH2 and GnRH3), the TN- and POA-GnRH3 neuronal types share the same olfactory origin. However, the developmental origin of midbrain (MB)-GnRH2 neurons is debatable between mesencephalic or neural crest site. Each GnRH system has distinctive anatomical and physiological characteristics, and functions differently. The POA-GnRH1 neurons are hypophysiotropic in nature and function in the neuroendocrine control of reproduction. The non-hypophysiotropic GnRH2/GnRH3 neurons probably play neuromodulatory roles in metabolism (MB-GnRH2) and the control of motivational state for sexual behavior (TN-GnRH3).

Contraception within an elasmobranch captive breeding programme: a case report on the eagle ray Aetobatus ocellatus using gonadotrophin-releasing hormone (GnRH) agonist deslorelin (Suprelorin)

This study describes the first case of contraception within a captive elasmobranch breeding programme where an implant of Suprelorin, containing a long-acting gonadotrophin-releasing hormone (GnRH) analogue Deslorelin, was implanted in an eagle ray Aetobatus ocellatus, which successfully halted breeding.

Role of elasmobranchs and holocephalans in understanding peptide evolution in the vertebrates: Lessons learned from gonadotropin releasing hormone (GnRH) and corticotropin releasing factor (CRF) phylogenies

The cartilaginous fishes (Class Chondrichthyes) comprise two morphologically distinct subclasses; Elasmobranchii and Holocephali. Evidence indicates early divergence of these subclasses, suggesting monophyly of their lineage. However, such a phylogenetic understanding is not yet developed within two highly conserved peptide lineages, GnRH and CRF. Various GnRH forms exist across the Chondrichthyes. Although 4-7 immunoreactive forms have been described in Elasmobranchii, only one has been elucidated in Holocephali. In contrast, Chondrichthyan CRF phylogeny follows a pattern more consistent with vertebrate evolution. For example, three forms are expressed within the lamprey, with similar peptides present within the genome of the Callorhinchus milii, a holocephalan. Although these findings are consistent with recent evidence regarding the phylogenetic age of Chondrichthyan lineages, CRF evolution in vertebrates remains elusive. Assuming that the Elasmobranchii and Holocephali are part of a monocladistic clade within the Chondrichthyes, we interpret the findings of GnRH and CRF to be products of their respective lineages.

A Type IIb, but Not Type IIa, GnRH Receptor Mediates GnRH-Induced Release of Growth Hormone in the Ricefield Eel

Multiple gonadotropin-releasing hormone receptors (GnRHRs) are present in vertebrates, but their differential physiological relevances remain to be clarified. In the present study, we identified three GnRH ligands GnRH1 (pjGnRH), GnRH2 (cGnRH-II), and GnRH3 (sGnRH) from the brain, and two GnRH receptors GnRHR1 (GnRHR IIa) and GnRHR2 (GnRHR IIb) from the pituitary of the ricefield eel Monopterus albus. GnRH1 and GnRH3 but not GnRH2 immunoreactive neurons were detected in the pre-optic area, hypothalamus, and pituitary, suggesting that GnRH1 and GnRH3 may exert hypophysiotropic roles in ricefield eels. gnrhr1 mRNA was mainly detected in the pituitary, whereas gnrhr2 mRNA broadly in tissues of both females and males. In the pituitary, GnRHR1 and GnRHR2 immunoreactive cells were differentially distributed, with GnRHR1 immunoreactive cells mainly in peripheral areas of the adenohypophysis whereas GnRHR2 immunoreactive cells in the multicellular layers of adenohypophysis adjacent to the neurohypophysis. Dual-label fluorescent immunostaining showed that GnRHR2 but not GnRHR1 was localized to somatotropes, and all somatotropes are GnRHR2-positive cells and vice versa at all stages examined. GnRH1 and GnRH3 were shown to stimulate growth hormone (Gh) release from primary culture of pituitary cells, and to decrease Gh contents in the pituitary of ricefield eels 12 h post injection. GnRH1 and GnRH3 stimulated Gh release probably via PLC/IP₃/PKC and Ca²⁺ pathways. These results, as a whole, suggested that GnRHs may bind to GnRHR2 but not GnRHR1 to trigger Gh release in ricefield eels, and provided novel information on differential roles of multiple GnRH receptors in vertebrates.

Sequence analysis, tissue distribution and molecular physiology of the GnRH preprogonadotrophin in the South American plains vizcacha (Lagostomus maximus)

Gonadotropin-releasing hormone (GnRH) is the regulator of the hypothalamic-hypophyseal-gonadal (HHG) axis. GnRH and GAP (GnRH-associated protein) are both encoded by a single preprohormone. Different variants of GnRH have been described. In most mammals, GnRH is secreted in a pulsatile manner that stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The South-American plains vizcacha, Lagostomus maximus, is a rodent with peculiar reproductive features including natural poly-ovulation up to 800 oocytes per estrous cycle, pre-ovulatory follicle formation throughout pregnancy and an ovulatory process which takes place at mid-gestation and adds a considerable number of secondary corpora lutea. Such features should occur under a special modulation of the HHG axis, guided by GnRH. The aim of this study was to sequence hypothalamic GnRH preprogonadotrophin mRNA in the vizcacha, to compare it with evolutionarily related species and to identify its expression, distribution and pulsatile pattern of secretion. The GnRH1variant was detected and showed the highest homology with that of chinchilla, its closest evolutionarily related species. Two isoforms of transcripts were identified, carrying the same coding sequence, but different 5' untranslated regions. This suggests a sensitive equilibrium between RNA stability and translational efficiency. A predominant hypothalamic localization and a pulsatile secretion pattern of one pulse of GnRH every hour were found. The lower homology found for GAP, also among evolutionarily related species, depicts a potentially different bioactivity.

Endosulfan affects GnRH cells in sexually differentiated juveniles of the perciform Cichlasoma dimerus

Endosulfan (ES) is an organochlorine pesticide widely used in agriculture despite its high toxicity towards non-target organisms such as fish. It has been demonstrated that ES can cause negative effects on aquatic animals, including disruption of hormonal systems. However, the alterations produced by this pesticide on the reproductive axis of fish prior to sexual maturity, as well as possible modes of action have hardly been studied. This study aimed at assessing the effect of waterborne exposure to the pesticide ES on the reproductive axis during sexual differentiation of juveniles of the South American freshwater cichlid fish Cichlasoma dimerus. No mortality was observed due to ES subchronic exposure (90 days post-fertilization). Exposure to ES did not affect body weight nor morphometric parameters, indicating that larvae nutritional state was not affected. Timing of sexual differentiation, gonadal morphology and sex ratio were likewise not altered by ES. However, ES acted as an endocrine disrupting chemical in this species as the morphometry of gonadotropin-releasing hormones (GnRH) producing cells was altered. Exposure to ES altered nuclear area, cell area and nucleus/cytoplasm ratio of GnRH II neurons, and cell and nuclear area and diameter of GnRH III neurons. Interestingly, in our previous study, exposure before sex differentiation (30 day exposure) caused no alteration to GnRH II and III, and did alter GnRH I and FSH cells. These alterations could lead to changes in circulating hormone levels, especially when fish are exposed for prolonged periods, ultimately impairing reproductive fitness. C. dimerus juveniles can be an interesting biological model to perform toxicological studies with the intent to assess early disruption endpoints in the reproductive axis during development.

Development of the terminal nerve system in the shark Scyliorhinus canicula

The nervus terminalis (or terminal nerve) system was discovered in an elasmobranch species more than a century ago. Over the past century, it has also been recognized in other vertebrate groups, from agnathans to mammals. However, its origin, functions or relationship with the olfactory system are still under debate. Despite the abundant literature about the nervus terminalis system in adult elasmobranchs, its development has been overlooked. Studies in other vertebrates have reported newly differentiated neurons of the terminal nerve system migrating from the olfactory epithelium to the telencephalon as part of a 'migratory mass' of cells associated with the olfactory nerve. Whether the same occurs in developing elasmobranchs (adults showing anatomically separated nervus terminalis and olfactory systems) has not yet been determined. In this work we characterized for the first time the development of the terminal nerve and ganglia in an elasmobranch, the lesser spotted dogfish (Scyliorhinus canicula), by means of tract-tracing techniques combined with immunohistochemical markers for the terminal nerve (such as FMRF-amide peptide), for the developing components of the olfactory system (Gα0 protein, GFAP, Pax6), and markers for early postmitotic neurons (HuC/D) and migrating immature neurons (DCX). We discriminated between embryonic olfactory and terminal nerve systems and determined that both components may share a common origin in the migratory mass. We also localized the exact point where they split off near the olfactory nerve-olfactory bulb junction. The study of the development of the terminal nerve system in a basal gnathostome contributes to the knowledge of the ancestral features of this system in vertebrates, shedding light on its evolution and highlighting the importance of elasmobranchs for developmental and evolutionary studies.

Reproductive endocrinology in chondrichthyans: the present and the future

The class Chondrichthyes, that includes Elasmobranchii and Holocephali, is a diverse group of fish occupying a key position at the base of vertebrate evolution. Their evolutionary success is greatly attributed to their wide range of reproductive strategies controlled by different endocrine mechanics. As in other vertebrates, hormonal control of reproduction in chondrichthyans is mediated by the neuropeptide gonadotropin-releasing hormone (GnRH) that regulates the brain control of gonadal activity via a hypothalamus-pituitary-gonadal (HPG) axis. Chondrichthyans lack of a direct vascular supply from the hypothalamus to the zone of the pituitary where the gonadotropic activity resides, thus transport between these two zones likely occurs via the general circulation. In the brain of elasmobranchs, two groups of GnRH, GnRH-I and GnRH-II were identified, and the presence of two immunoreactive gonadotropins similar to the luteinising (LH) and follicle stimulating (FSH) hormones was identified in the pituitary. In holocephalans, only GnRH-II has been confirmed, and while gonadotropin activity has been found in the buccal pituitary lobe, the presence of gonadotropin receptors in the gonads remains unknowns. The diversity of reproductive strategies display by chondrichthyans makes it difficult to generalize the control of gametogenesis and steroidogenesis; however, some general patterns emerge. In both sexes, androgens and estrogens are the main steroids during gonadal growth; while progestins have maturational activity. Androgens also form the precursors for estrogen steroid production. Estrogens stimulate the hepatic synthesis of yolk and stimulate the development of different part of the reproductive tract in females. The role of other gonadal steroids may play in chondrichthyan reproduction remains largely unknown. Future work should concentrate in filling the gaps into the current knowledge of the HPG axis regulation, and the use of reproductive endocrinology as a non-lethal technique for management of chondrichthyan populations.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

Connections of the terminal nerve and the olfactory system in two galeomorph sharks: an experimental study using a carbocyanine dye

In elasmobranchs the terminal nerve courses separately from the olfactory nerve. This characteristic makes elasmobranchs excellent models to study the anatomy and function of these two systems. Here we study the neural connections of the terminal nerve and olfactory system in two sharks by experimental tracing methods using carbocyanine dyes. The main projections from the terminal nerve system (consisting of three ganglia in Scyliorhinus canicula) course ipsilaterally to the medial septal nucleus and bilaterally to the ventromedial telencephalic pallial region. Minor terminal nerve projections were also traced ipsilaterally to diencephalic and mesencephalic levels. With regard to the olfactory connections, our results show that in sharks, unlike ray-finned fishes, the primary olfactory projections are mainly restricted to the olfactory bulb. We also performed tracer application to the olfactory bulb in order to analyze the possible central neuroanatomical relationship between the projections of the terminal nerve and the olfactory bulb. In these experiments labeled neurons and fibers were observed from telencephalic to caudal mesencephalic regions. However, we observe almost no overlap between the two systems at central levels. The afferent and the putatively efferent connections of the dogfish olfactory bulb are compared with those previously reported in other elasmobranchs. The significance of the extratelencephalic secondary olfactory projections is also discussed in a comparative context.

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Gonadotropin-releasing hormone (GnRH) modulates auditory processing in the fish brain

Gonadotropin-releasing hormone 1 (GnRH1) neurons control reproductive activity, but GnRH2 and GnRH3 neurons have widespread projections and function as neuromodulators in the vertebrate brain. While these extra-hypothalamic GnRH forms function as olfactory and visual neuromodulators, their potential effect on processing of auditory information is unknown. To test the hypothesis that GnRH modulates the processing of auditory information in the brain, we used immunohistochemistry to determine seasonal variations in these neuropeptide systems, and in vivo single-neuron recordings to identify neuromodulation in the midbrain torus semicircularis of the soniferous damselfish Abudefduf abdominalis. Our results show abundant GnRH-immunoreactive (-ir) axons in auditory processing regions of the midbrain and hindbrain. The number of extra-hypothalamic GnRH somata and the density of GnRH-ir axons within the auditory torus semicircularis also varied across the year, suggesting seasonal changes in GnRH influence of auditory processing. Exogenous application of GnRH (sGnRH and cGnRHII) caused a primarily inhibitory effect on auditory-evoked single neuron responses in the torus semicircularis. In the majority of neurons, GnRH caused a long-lasting decrease in spike rate in response to both tone bursts and playbacks of complex natural sounds. GnRH also decreased response latency and increased auditory thresholds in a frequency and stimulus type-dependent manner. To our knowledge, these results show for the first time in any vertebrate that GnRH can influence context-specific auditory processing in vivo in the brain, and may function to modulate seasonal auditory-mediated social behaviors.

Differential co-localization with choline acetyltransferase in nervus terminalis suggests functional differences for GnRH isoforms in bonnethead sharks (Sphyrna tiburo)

The nervus terminalis (NT) is a vertebrate cranial nerve whose function in adults is unknown. In bonnethead sharks, the nerve is anatomically independent of the olfactory system, with two major cell populations within one or more ganglia along its exposed length. Most cells are immunoreactive for either gonadotropin-releasing hormone (GnRH) or RF-amide-like peptides. To define further the cell populations and connectivity, we used double-label immunocytochemistry with antisera to different isoforms of GnRH and to choline acetyltransferase (ChAT). The labeling patterns of two GnRH antisera revealed different populations of GnRH-immunoreactive (ir) cell profiles in the NT ganglion. One antiserum labeled a large group of cells and fibers, which likely contain mammalian GnRH (GnRH-I) as described in previous studies and which were ChAT immunoreactive. The other antiserum labeled large club-like structures, which were anuclear, and a sparse number of fibers, but with no clear labeling of cell bodies in the ganglion. These club structures were choline acetyltrasferase (ChAT)-negative, and preabsorption control tests suggest they may contain chicken-GnRH-II (GnRH-II) or dogfish GnRH. The second major NT ganglion cell-type was immunoreactive for RF-amides, which regulate GnRH release in other vertebrates, and may provide an intraganglionic influence on GnRH release. The immunocytochemical and anatomical differences between the two GnRH-immunoreactive profile types indicate possible functional differences for these isoforms in the NT. The club-like structures may be sites of GnRH release into the general circulation since these structures were observed near blood vessels and resembled structures seen in the median eminence of rats.

Dynamic limbic networks and social diversity in vertebrates: from neural context to neuromodulatory patterning

Vertebrate animals exhibit a spectacular diversity of social behaviors, yet a variety of basic social behavior processes are essential to all species. These include social signaling; discrimination of conspecifics and sexual partners; appetitive and consummatory sexual behaviors; aggression and dominance behaviors; and parental behaviors (the latter with rare exceptions). These behaviors are of fundamental importance and are regulated by an evolutionarily conserved, core social behavior network (SBN) of the limbic forebrain and midbrain. The SBN encodes social information in a highly dynamic, distributed manner, such that behavior is most strongly linked to the pattern of neural activity across the SBN, not the activity of single loci. Thus, shifts in the relative weighting of activity across SBN nodes can conceivably produce almost limitless variation in behavior, including diversity across species (as weighting is modified through evolution), across behavioral contexts (as weights change temporally) and across behavioral phenotypes (as weighting is specified through heritable and developmental processes). Individual neural loci may also express diverse relationships to behavior, depending upon temporal variations in their functional connectivity to other brain regions ("neural context"). We here review the basic properties of the SBN and show how behavioral variation relates to functional connectivity of the network, and discuss ways in which neuroendocrine factors adjust network activity to produce behavioral diversity. In addition to the actions of steroid hormones on SBN state, we examine the temporally plastic and evolutionarily labile properties of the nonapeptides (the vasopressin- and oxytocin-like neuropeptides), and show how variations in nonapeptide signaling within the SBN serve to promote behavioral diversity across social contexts, seasons, phenotypes and species. Although this diversity is daunting in its complexity, the search for common "organizing principles" has become increasingly fruitful. We focus on multiple aspects of behavior, including sexual behavior, aggression and affiliation, and in each of these areas, we show how broadly relevant insights have been obtained through the examination of behavioral diversity in a wide range of vertebrate taxa.

GnRH and GnRH receptors in metazoa: a historical, comparative, and evolutive perspective

About 50years after Harris's first demonstration of its existence, GnRH has strongly stimulated the interest and imagination of scientists, resulting in a high number of studies in an increasing number of species. For the endocrinologist, GnRH, via its actions on the synthesis and release of pituitary gonadotrophins, is first an essential hormone for the initiation and maintenance of the reproductive axis, but recent data suggest that GnRH emerged in animals lacking a pituitary. In this context, this review intends to explore the current status of knowledge on GnRH and GnRH receptors in metazoa in order to see if it is possible to draw an evolutive scenario according to which GnRH actions progressively evolved from the control of simple basic functions in early metazoa to an indirect mean of controlling gonadal activity in vertebrates through a sophisticated network of finely tuned neurons developing in a rather fascinating way. This review also intends to provide an evolutive scenario based on the recent advances of whole genome sequencing possibly explaining the number of GnRH and GnRH receptor variants according to the 2R and 3R theories accompanied by gene losses.

Gonadotropin-Releasing Hormone and Receptor Distributions in the Visual Processing Regions of Four Coral Reef Fishes

Gonadotropin-releasing hormone (GnRH) is widely distributed in the brain of fishes where it may function as a neuromodulator of sensory processing and behavior. Immunocytochemical and neuronal label experiments were conducted on species from four families of coral reef fishes (Chaetodontidae, butterflyfish; Pomacentridae, damselfish; Gobiidae, goby; and Labridae, wrasse) to assess conservation of GnRH targets in the visual processing retina and brain. In all species, GnRH-immunoreactive (-ir) axons from the terminal nerve project principally to the boundary between the inner plexiform (IPL) and inner nuclear (INL) layers of the retina, and are less prominent in the optic nerve, ganglion cell, IPL and INL. However, the density of GnRH innervation within the retina differed among fish species with highest concentrations in the damselfish and butterflyfish and lowest in the goby and wrasse. Experiments also show that GnRH receptors are associated with GnRH-ir axons within the fish retina primarily at the IPL-INL boundary, the region of light-dark adaptation and image processing of contrast, motion or color. GnRH-ir axons overlapped central projections of retinal ganglion cell axons primarily within the stratum album centrale and stratum griseum centrale of the tectum in all species, and were concentrated in several diencephalic visual processing centers. GnRH receptors are also localized to diencephalic visual centers and the stratum griseum periventriculare of the tectum, where motion perception and coordination of motor behavioral responses in three-dimensional space occur. This work demonstrates that the basic neural substrates for peptide-sensory convergence are conserved at multiple processing levels in the visual system of several reef fishes. Species differences in GnRH innervation to the retina and GnRH receptor distributions may be related to phylogeny, their use of vision in natural behaviors, or possibly binding properties of the antibodies. Future studies are needed to characterize the exact GnRH variants and receptor types found in these species so that possible functional consequences of GnRH influence on vision can be defined.

Sex and seasonal co-variation of arginine vasotocin (AVT) and gonadotropin-releasing hormone (GnRH) neurons in the brain of the halfspotted goby

Gonadotropin-releasing hormone (GnRH) and arginine vasotocin (AVT) are critical regulators of reproductive behaviors that exhibit tremendous plasticity, but co-variation in discrete GnRH and AVT neuron populations among sex and season are only partially described in fishes. We used immunocytochemistry to examine sexual and temporal variations in neuron number and size in three GnRH and AVT cell groups in relation to reproductive activities in the halfspotted goby (Asterropteryx semipunctata). GnRH-immunoreactive (-ir) somata occur in the terminal nerve, preoptic area, and midbrain tegmentum, and AVT-ir somata within parvocellular, magnocellular, and gigantocellular regions of the preoptic area. Sex differences were found among all GnRH and AVT cell groups, but were time-period dependent. Seasonal variations also occurred in all GnRH and AVT cell groups, with coincident elevations most prominent in females during the peak- and non-spawning periods. Sex and temporal variability in neuropeptide-containing neurons are correlated with the goby's seasonally-transient reproductive physiology, social interactions, territoriality and parental care. Morphological examination of GnRH and AVT neuron subgroups within a single time period provides detailed information on their activities among sexes, whereas seasonal comparisons provide a fine temporal sequence to interpret the proximate control of reproduction and the evolution of social behavior.

Role of neuropeptide Y in the regulation of gonadotropin releasing hormone system in the forebrain of Clarias batrachus (Linn.): Immunocytochemistry and high performance liquid chromatography-electrospray ionization-mass spectrometric analysis

Although the importance of neuropeptide Y (NPY) in the regulation of gonadotropin releasing hormone (GnRH) and reproduction has been highlighted in recent years, the neuroanatomical substrate within which these substances might interact has not been fully elucidated. Present work was undertaken with a view to define the anatomical-physiological correlates underlying the role exercised by NPY in the regulation of GnRH in the forebrain of the teleost Clarias batrachus. Application of double immunocytochemistry revealed close associations as well as colocalizations of the two peptides in the olfactory receptor neurons (ORNs), olfactory nerve fibers and their terminals in the glomeruli, ganglion cells of nervus terminalis, medial olfactory tract, fibers in the area ventralis telencephali/pars supracommissuralis and cells as well as fibers in the pituitary. NPY containing axons were found to terminate in the vicinity of GnRH cells in the pituitary with light as well as electron microscopy. Double immunoelectron microscopy demonstrated gold particles for NPY and GnRH colocalized on the membrane and in dense core of the secretory granules in the cells distributed in all components of the pituitary gland. To assess the physiological implication of these observations, NPY was injected via the intracranial route and the response of GnRH immunoreactive system was evaluated by relative quantitative morphometry as well as high performance liquid chromatography (HPLC) analysis. Two hours following NPY (20 ng/g body weight) administration, a dramatic increase was observed in the GnRH immunoreactivity in the ORNs, in the fibers of the olfactory bulb (163%) and medial olfactory tract (351%). High performance liquid chromatography-electrospray ionization-mass spectrometric analysis confirmed the immunocytochemical data. Significant rise in the salmon GnRH (sGnRH)-like peptide content was observed in the olfactory organ (194.23%), olfactory bulb (146.64%), telencephalon+preoptic area (214.10%) and the pituitary (136.72%) of the NPY-treated fish. However, GnRH in the hypothalamus was below detection limit in the control as well as NPY-treated fish. Present results suggest the involvement of NPY in the up-regulation of sGnRH containing system at different level of neuraxis extending from the olfactory epithelium to the pituitary in the forebrain of C. batrachus.

Extrabulbar olfactory system and nervus terminalis FMRFamide immunoreactive components in Xenopus laevis ontogenesis

The extrabulbar olfactory system (EBOS) is a collection of nerve fibers which originate from primary olfactory receptor-like neurons and penetrate into the brain bypassing the olfactory bulbs. Our description is based upon the application of two neuronal tracers (biocytin, carbocyanine DiI) in the olfactory sac, at the cut end of the olfactory nerve and in the telencephalon of the developing clawed frog. The extrabulbar olfactory system was observed already at stage 45, which is the first developmental stage compatible with our techniques; at this stage, the extrabulbar olfactory system fibers terminated diffusely in the preoptic area. A little later in development, i.e. at stage 50, the extrabulbar olfactory system was maximally developed, extending as far caudally as the rhombencephalon. In the metamorphosing specimens, the extrabulbar olfactory system appeared reduced in extension; caudally, the fiber terminals did not extend beyond the diencephalon. While a substantial overlapping of biocytin/FMRFamide immunoreactivity was observed along the olfactory pathways as well as in the telencephalon, FMRFamide immunoreactivity was never observed to be colocalized in the same cellular or fiber components visualized by tracer molecules. The question whether the extrabulbar olfactory system and the nervus terminalis (NT) are separate anatomical entities or represent an integrated system is discussed.

Sheep exhibit novel variations in the organization of the mammalian type II gonadotropin-releasing hormone receptor gene

Species-specific differences in genes encoding type II GnRH receptor indicate that a functional hepta-helical receptor is produced in monkeys but not in rodents, cows, chimpanzees, or humans. To further investigate the extent of evolutionary differences, we sequenced the type II GnRH receptor gene from wild-type Soay sheep. The gene was isolated by long-distance PCR using primers to PEX11beta and RBM8A genes known to flank type II GnRH receptor gene homologues. The gene spans 5.7-kb DNA and was sequenced after shot-gun subcloning. Its novel features include absence of a Pit-1 transcription factor binding site, a premature stop codon (TAG) in exon 1, an in-frame deletion of 51 bp (17 codons) in exon 2, and several nonconservative codon changes. Sheep breed variation in the gene was assessed using genomic DNA in PCR-restriction digest assays for the premature stop codon and in a PCR assay for the deletion. Both characteristics were present in all 15 breeds tested. Receptor gene expression was investigated using poly-A(+) RNA Northern analysis, RT-PCR, and in situ hybridization. An oligonucleotide probe to exon 1 revealed an alternative transcript in testis but not in pituitary gland. No transcripts in testis or pituitary were detectable using an exon 2-3 probe. All tissues examined including multiple brain areas and gonadotrope-enriched cell cultures were negative for type II GnRH receptor in RT-PCR. Testis and pituitary sections were negative with exon 1 riboprobes and exon 1 or 2-3 oligonucleotide probes in in situ hybridization. A hepta-helical type II GnRH receptor is therefore not expressed from this sheep gene.

Evidence for a Novel Gonadotropin-Releasing Hormone in Hypothalamic and Forebrain Areas in Songbirds

One or more of three distinct forms of gonadotropin-releasing hormones (GnRHs), has been found in all vertebrates studied. Birds are known to contain two types of GnRH, chicken GnRH-I and -II (cGnRH-I and -II), but to date nobody has investigated the possibility of the presence of lamprey GnRH-III in birds. Thus, our objective was to investigate the presence and distribution of immunoreactive lamprey (ir-lamprey) GnRH-III in hypothalamic and forebrain areas in house sparrows (Passer domesticus) and Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii). In addition to its distribution in the CNS, we investigated whether or not exogenous lamprey GnRH-III peptide could elicit luteinizing hormone (LH) release in vivo. This study presents two novel findings: 1) The widespread presence in the central nervous system of an immunoreactive third GnRH in birds that is clearly hypophysiotropic and has gonadotropin-releasing capabilities. Intravenous injection of 100 ng lamprey-GnRH-III peptide elicited a rapid rise in circulating luteinizing hormone as compared to controls. This third GnRH, ir-lamprey GnRH-III, likely has multiple functions, as suggested by its widespread distribution. 2) Ir-lamprey GnRH-III is present in abundance in telencephalic areas, including the hippocampal formation and the song control system. In no vertebrate has a GnRH been localized in these 'higher' control regions before. The fact that ir-lamprey-GnRH-III is present in auditory processing areas (higher vocal center, or HVc, area X, caudomedial neostriatum) and song-producing areas (HVc, robust nucleus of the neostriatum, or RA) suggests a direct way in which auditory processing and song production can affect circulating gonadotropins.

Evidence that gonadotropin-releasing hormone II is not a physiological regulator of gonadotropin secretion in mammals

Gonadotropin-releasing hormone (GnRH)-II stimulates luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion when administered at high doses in mammals, and this effect has been assumed to be mediated through the GnRH-II receptor expressed on gonadotropes. This study used two selective GnRH-I receptor antagonists to test the alternative hypothesis that GnRH-II acts through the GnRH-I receptor to elicit gonadotropin secretion. The antagonist, antide, was used to characterize the receptor-relay because it was a pure antagonist in vitro based on inositol phosphate responses in COS-7 cells transfected with either mammalian GnRH-I and GnRH-II receptors and, in vivo, potently antagonized the gonadotropin-releasing effect of a single injection of 250 ng GnRH-I in our sexually inactive sheep model. In a series of studies in sheep, antide (i). blocked the acute LH response to a single injection of GnRH-II (20 microg antide: 10 microg GnRH-II); (ii). blocked both the acute, pulsatile LH response and the FSH priming response to 2-hourly injections of GnRH-II over 36 h (100 microg antide/8 h: 4 microg GnRH-II/2 h); and (iii). chronically blocked both the pulsatile LH response and the marked FSH priming response to 4-hourly injections of GnRH-II over 10 days (75 microg antide/8 h: 4 microg GnRH-II/4 h). In two final experiments, the GnRH-I antagonist 135-18, shown previously to agonize the mammalian GnRH-II receptor, blocked the gonadotropin-releasing effects of GnRH-I (250 ng) but failed to elicit an LH response when given alone, and simultaneous administration of GnRH-II (250 ng) failed to alter the LH-releasing effect of GnRH-I (50-500 ng). These data thus support our hypothesis. Based on additional literature, it is unlikely that the GnRH-II decapeptide is a native regulator of the gonadotrope in mammals.

Gonadotropin-releasing hormone (GnRH): from fish to mammalian brains

This work deals with a family of neuropeptides, gonadotropin-releasing hormone (GnRH), that play a key role in the development and maintenance of reproductive function in vertebrates. 2. Until now, a total of 16 GnRH structural variants have been isolated and characterized from vertebrate and protochordate nervous tissue. All vertebrate species already investigated have at least two GnRH forms coexisting in the central nervous system. However, it is now well accepted that three forms of GnRH in early and late evolved bony fishes are present. 3. In these cases, cGnRH-II is expressed by midbrain neurons, a species-specific GnRH is present mainly in the preoptic area and the hypothalamus, and sGnRH is localized in the terminal nerve ganglion (TNG). In this context it is possible to think that three GnRH forms and three GnRH receptor (GnRH-R) subtypes are expressed in the central nervous system of a given species. 4. Then it is possible to propose three different GnRH lineages expressed by distinct brain areas in vertebrates: (1) the conserved cGnRH-II or mesencephalic lineage; or (2) the hypothalamic or "releasing" lineage whose primary structure has diverged by point mutations (mGnRH and its orthologous forms: hrGnRH, wfGnRH, cfGnRH, sbGnRH, and pjGnRH); and (3) the telencephalic sGnRH form. Also different GnRH nomenclatures are discussed.

The terminal nerve ganglion cells project to the olfactory mucosa in the dwarf gourami

Single- and double-label immunocytochemical studies were conducted using antisera to salmon gonadotropin-releasing hormone (sGnRH) and molluscan cardioexcitatory peptide (FMRFamide) to determine whether terminal nerve ganglion cells project to the olfactory mucosa in the dwarf gourami, Colisa lalia. Both peptides were present in terminal nerve ganglion perikarya and fibers in brain and nasal cavity. Labeled fibers were present in the olfactory nerve and could be traced to the olfactory mucosa. All terminal nerve ganglion cells contained both sGnRH and FMRFamide-like peptides. This study suggests that the terminal nerve ganglion cells can influence both brain and chemoreceptive structures.

Evolutionary development of three gonadotropin-releasing hormone (GnRH) systems in vertebrates

Gonadotropin-releasing hormone (GnRH) is the neuropeptide that links the brain to the reproductive system. Most vertebrate species express two forms of GnRH, which differ in amino acid sequence, localization, distribution, and embryological origin. The GnRH system in the ventral forebrain produces a species-specific GnRH form and projects toward the gonadotropic cell in the pituitary. The GnRH neurons of this system originate from the olfactory placode and migrate into the brain during early development. The other GnRH system is localized in a nucleus in the midbrain, where large cells express chicken-GnRH-II, of which the function is still unclear. In modern teleosts, a third GnRH system is present in the terminal nerve, which contains salmon GnRH. The three GnRH systems appear at different times during fish evolution. Besides the two accepted lineages in GnRH evolution (of conserved chicken GnRH-II in the midbrain and of mammalian GnRH or species-specific GnRH in the hypophysiotropic system), we propose a third lineage: of salmon GnRH in the terminal nerve.

Development of three distinct GnRH neuron populations expressing two different GnRH forms in the brain of the African catfish (Clarias gariepinus)

The early development of both the catfish gonadotropin-releasing hormone (cfGnRH)- and the chicken GnRH-II (cGnRH-II) system was investigated in African catfish by immunocytochemistry by using antibodies against the GnRH-associated peptide (GAP) of the respective preprohormones. Weakly cfGnRH-immunoreactive (ir) neurons and fibers were present at 2 weeks after hatching (ph) but only in the ventral telencephalon and pituitary. Two weeks later, cfGnRH fibers and neurons were also observed in more rostral and in more caudal brain areas, mainly in the preoptic area and hypothalamus. Based on differences in temporal, spatial, and morphologic appearance, two distinct cfGnRH populations were identified in the ventral forebrain: a population innervating the pituitary (ventral forebrain system) and a so-called terminal nerve (TN) population. DiI tracing studies revealed that the TN population has no neuronal connections with the pituitary. The cGnRH-II system is present from 2 weeks ph onward in the midbrain tegmentum and only their size and staining intensity increased during development. Based on the comparison of GnRH systems amongst vertebrates, we hypothesize that during fish evolution, three different GnRH systems evolved, each expressing their own molecular form: the cGnRH-II system in the midbrain, a hypophysiotropic GnRH system in the hypothalamus with a species-specific GnRH form, and a salmon GnRH-expressing TN population. This hypothesis is supported by phylogenetic analysis of known GnRH precursor amino acid sequences. We hypothesize, because the African catfish is a less advanced teleost species, that it contains the cfGnRH form both in the ventral forebrain system and in the TN population.

Annual cycles of steroid hormone production, gonad development, and reproductive behavior in the Atlantic stingray

The mating season of the Atlantic stingray (Dasyatis sabina), which begins in August and continues through April, is the longest documented for any elasmobranch fish. Despite this protracted mating period, female stingrays ovulate synchronously at the end of the mating season and there is no evidence for sperm storage by females. Thus, the proximate causal factors and ultimate function of this extended preovulatory mating are unknown. Annual cycles of the gonadal steroids testosterone (T), dihydrotestosterone (DHT), 17beta-estradiol (E2), and progesterone (P4) were measured for 26 months in a wild estuarine population of Atlantic stingrays to test for associations with their reproductive biology, gametogenesis, and sexual behavior. Serum androgen levels in males showed four phases within an annual cycle: (1) androgen suppression between reproductive seasons (April-July), (2) primary androgen increase during the onset of spermatocyte development (August-October), (3) androgen decrease following maximum testis growth and spermatocyte development (November-December), and (4) secondary androgen increase during the peak of sperm maturation (January-March). Increases in male E2 and P4 were correlated with spermatocyte/spermatocyst formation, maximum testis weight, and the primary (but not secondary) androgen surge. We propose that the production of male androgens across the full seven-month preovulatory mating period promotes their aggressive reproductive behavior and drives the protracted mating season of this species. In females, serum T and DHT showed relatively brief increases near ovulation, whereas E2 and P4 showed brief increases near both ovulation and parturition. The increase in female androgens near ovulation may increase female aggression when they are impregnable by courting males and enhance their choice of mates. This estuary sample population shows higher absolute steroid levels and distinct differences in temporal cycles compared to another Florida fresh water lake population, but the cause and significance of these differences are unknown. Experiments are needed to confirm that the aggressive and protracted mating behavior is the result of prolonged male androgen production and to determine whether the sustained preovulatory mating serves some function related to female reproduction.