Elevation of gene expression for salmon gonadotropin-releasing hormone in discrete brain loci of prespawning chum salmon during upstream migration

Does global change increase the risk of maladaptation of Atlantic salmon migration through joint modifications of river temperature and discharge?

In freshwater ecosystems, water temperature and discharge are two intrinsically associated triggers of key events in the life cycle of aquatic organisms such as the migration of diadromous fishes. However, global changes have already profoundly altered the thermal and hydrological regimes of rivers, affecting the timing of fish migration as well as the environmental conditions under which it occurs. In this study, we focused on Atlantic salmon (Salmo salar), an iconic diadromous species whose individuals migrate between marine nursery areas and continental spawning grounds. An innovative multivariate method was developed to analyse long-term datasets of daily water temperature, discharge and both salmon juvenile downstream and adult upstream migrations in three French rivers (the Bresle, Oir and Nivelle rivers). While all three rivers have gradually warmed over the last 35 years, changes in discharge have been very heterogeneous. Juveniles more frequently used warmer temperatures to migrate. Adults migrating a few weeks before spawning more frequently used warm temperatures associated with high discharges. This has already led to modifications in preferential niches of both life stages and suggests a potential mismatch between these populations' ecological preference and changes in their local environment due to global change.

Expression dynamics of the genes for the hypothalamo-pituitary-gonadal axis in tiger puffer (Takifugu rubripes) at different reproductive stages

Tiger puffer, Takifugu rubripes, a commercially important long-distance migratory fish, return to specific spawning grounds for reproduction. To clarify reproductive neuroendocrine system of the tiger puffer, the changes in the expression levels of the genes encoding three gonadotropin-releasing hormones (GnRHs), gonadotropin-inhibitory hormone (GnIH), GnIH receptor (GnIH-R), kisspeptin and kisspeptin receptor in the brain and gonadotropin (GTH) subunits, growth hormone (GH) and prolactin (PRL) in the pituitary were examined in the tiger puffer captured in the wild at different reproductive stages, namely immature and mature fish of both sexes, and post-ovulatory females that were obtained by hormonal treatment. The amounts of three gnrh mRNAs, gnih, gnih-r, fshb and lhb were substantially increased in the mature fish compared to the immature fish, especially in the females, and these augmented expressions were drastically decreased in the post-ovulatory females. gh expression showed a slight increase in the mature males. In contrast, kiss2, kiss2r and prl did not show significant changes in the males but significantly decreased in the post-ovulatory females. The present results demonstrate the expression dynamics of the hypothalamo-pituitary-gonadal axis genes associated with the reproductive conditions and the possible involvement of the GnRH/GnIH/GTH system in the regulation of the sexual maturation and spawning in the wild tiger puffer.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

Zebrafish Olfacto-Retinal Centrifugal Axon Projection and Distribution: Effects of Gonadotropin-Releasing Hormone and Dopaminergic Signaling

The terminalis neurons (TNs) have been described in teleost species. In zebrafish, the TNs are located in the olfactory bulb. The TNs synthesize and release gonadotropin-releasing hormone (GnRH) as one of the major neurotransmitters. The TNs project axons to many brain areas, which include the neural retina. In the retina, the TN axons synapse with dopaminergic interplexiform cells (DA-IPCs) and retinal ganglion cells (RGCs). In this research, we examine the role of GnRH and dopaminergic signaling in TN axon projection to the retina using the transgenic zebrafish Tg(GnRH-3::GFP). While the TNs developed at 34 h postfertilization (hpf), the first TN axons were not detected in the retina until 48-50 hpf, when the first DA-IPCs were differentiated. In developing embryos, inhibition of retinal GnRH signaling pathways severely interrupted the projection of TN axons to the retina. However, inhibition of retinal dopaminergic signaling produced little effect on TN axon projection. In adult retinas, inactivation of GnRH receptors disrupted the patterns of TN axon distribution, and depletion of DA-IPCs abolished the TN axons. When DA-IPCs regenerated, the TN axons reappeared. Together, the data suggest that in developing zebrafish retinas GnRH signaling is required for TN axon projection, whereas in adult retinas activation of GnRH and dopaminergic signaling transduction is required for normal distribution of the TN axons.

Expression changes of mRNAs encoding kisspeptins and their receptors and gonadotropin-releasing hormones during early development and gonadal sex differentiation periods in the brain of chub mackerel (Scomber japonicus)

In recent years, brain kisspeptin system has been shown to be involved in diverse reproductive function, including sexual differentiation in vertebrates. Our previous reports demonstrated that the chub mackerel (Scomber japonicus) brain expresses two kisspeptin (kiss1, kiss2), two kisspeptin receptor (kissr1, kissr2) and three gonadotropin-releasing hormone (gnrh1, gnrh2, gnrh3) genes. In the present study, using quantitative real-time PCR (qRT-PCR) assays, we analysed expression changes of these genes during early development (0-30dphs) and gonadal sex differentiation periods (37-60dphs). Absolute expression level of kiss-kissr-gnrh in the whole head was higher between 0 and 15dphs, in comparison to later developmental periods. Histological analyses revealed presence of sexually differentiated males and females with testicular and ovarian features at 37, 45, and 60dphs. In both males and females, kiss2, kissr1, and kissr2 levels were higher at 37dph, in comparison to 45 and 60dphs, with kiss1 showing no significant differences. Levels of all three gnrh mRNAs were higher at 45dph, in comparison to 60dph. Changes in the expression level of kiss-kissr-gnrh mRNAs in different brain regions of sexually differentiated males and females indicated differences in their regional distribution. These results suggest possible involvement of Kiss-KissR-GnRH systems during early development and gonadal sex differentiation in the chub mackerel.

Olfactory navigation during spawning migrations: a review and introduction of the Hierarchical Navigation Hypothesis

Migrations are characterized by periods of movement that typically rely on orientation towards directional cues. Anadromous fish undergo several different forms of oriented movement during their spawning migration and provide some of the most well-studied examples of migratory behaviour. During the freshwater phase of the migration, fish locate their spawning grounds via olfactory cues. In this review, we synthesize research that explores the role of olfaction during the spawning migration of anadromous fish, most of which focuses on two families: Salmonidae (salmonids) and Petromyzontidae (lampreys). We draw attention to limitations in this research, and highlight potential areas of investigation that will help fill in current knowledge gaps. We also use the information assembled from our review to formulate a new hypothesis for natal homing in salmonids. Our hypothesis posits that migrating adults rely on three types of cues in a hierarchical fashion: imprinted cues (primary), conspecific cues (secondary), and non-olfactory environmental cues (tertiary). We provide evidence from previous studies that support this hypothesis. We also discuss future directions of research that can test the hypothesis and further our understanding of the spawning migration.

The Expression of Leptin, Estrogen Receptors, and Vitellogenin mRNAs in Migrating Female Chum Salmon, Oncorhynchus keta: The Effects of Hypo-osmotic Environmental Changes

Leptin plays an important role in energy homeostasis and reproductive function in fish, especially in reproduction. Migrating fish, such as salmonoids, are affected by external environmental factors, and salinity changes are a particularly important influence on spawning migrations. The aim of this study was to test whether changes in salinity affect the expression of leptin, estrogen receptors (ERs), and vitellogenin (VTG) in chum salmon (Oncorhynchus keta). The expression and activity of leptin, the expression of ERs and VTG, and the levels of estradiol-17β and cortisol increased after the fish were transferred to FW, demonstrating that changes in salinity stimulate the HPG axis in migrating female chum salmon. These findings reveal details about the role of elevated leptin levels and sex steroid hormones in stimulating sexual maturation and reproduction in response to salinity changes in chum salmon.

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.

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.

Diversity of the hypothalamo-neurohypophysial system and its hormonal genes

The hypothalamic neurosecretory cells (NSCs) which produce and release neurohypophysial hormones are involved in controls of diverse physiological phenomena including homeostatic controls of unconscious functions and reproduction. The far and wide distribution of neurosecretory processes in the discrete brain loci and the neurohypophysis is appropriate for coordination of neural and endocrine events that are required for the functions of NSCs. The presence of dye couplings and intimate contacts among NSCs supports harmonious production and release of hormone to maintain the plasma level within a certain range which is adequate for a particular physiological condition. Neurosecretory cells integrate diverse input signals from internal and external sources that define this particular physiological condition, although reactions of NSCs vary among different species, and among different cell types. An input signal to NSC is received by specific receptors and transduced as unique intracellular signals, important for the various functions of neurohypophysial hormones. Orchestration of multiple intracellular signaling systems, activities of which are individually modulated by input signals, determines the rates of synthesis and release of hormone through regulation of gene expression. The first step of gene expression, i.e., transcription, is amenable for diverse reaction of NSCs, because the 5' upstream regions of genes encoding neurohypophysial hormones are highly variable.

Neuromodulatory Effect of GnRH on the Synaptic Transmission of the Olfactory Bulbar Neural Circuit in Goldfish, Carassius auratus

Gonadotropin-releasing hormone (GnRH) is well known as a hypophysiotropic hormone that is produced in the hypothalamus and facilitates the release of gonadotropins from the pituitary gonadotropes. On the other hand, the functions of extrahypothalamic GnRH systems still remain elusive. Here we examined whether the activity of the olfactory bulbar neural circuits is modulated by GnRH that originates mainly from the terminal nerve (TN) GnRH system in goldfish ( Carassius auratus). As the morphological basis, we first observed that goldfish TNs mainly express salmon GnRH (sGnRH) mRNA and that sGnRH-immunoreactive fibers are distributed in both the mitral and the granule cell layers. We then examined by extracellular recordings the effect of GnRH on the electrically evoked in vitro field potentials that arise from synaptic activities from mitral to granule cells. We found that GnRH enhances the amplitude of the field potentials. Furthermore, these effects were observed in both cases when the field potentials were evoked by stimulating either the lateral or the medial olfactory tract, conveying functionally different sensory information, separately, and suggesting that GnRH may modulate the responsiveness to wide categories of odorants in the olfactory bulb. Because GnRH also changed the paired-pulse ratio, it is suggested that the increased amplitude of the field potential results from changes in the presynaptic glutamate release of mitral cells rather than the increase in the glutamate receptor sensitivity of granule cells. These results suggest that TN regulates the olfactory responsiveness of animals appropriately by releasing sGnRH peptides in the olfactory bulbar neural circuits.

Expression of GnRH genes is elevated in discrete brain loci of chum salmon before initiation of homing behavior and during spawning migration

Our previous studies suggested the importance of gonadotropin-releasing hormones (GnRHs) for initiation of spawning migration of chum salmon, although supporting evidence had been not available from oceanic fish. In farmed masu salmon, the amounts of salmon GnRH (sGnRH) mRNAs in the forebrain increased in the pre-pubertal stage from winter through spring, followed by a decrease toward summer. We thus hypothesized that gene expression for GnRHs in oceanic chum salmon changes similarly, and examined this hypothesis using brain samples from winter chum salmon in the Gulf of Alaska and summer fish in the Bering Sea. They were classified into sexually immature and maturing adults, which had maturing gonads and left the Bering Sea for the natal river by the end of summer. The absolute amounts of GnRH mRNAs were determined by real-time PCRs. The amounts of sGnRH mRNA in the maturing winter adults were significantly larger than those in the maturing summer adults. The amounts of sGnRH and chicken GnRH mRNAs then peaked during upstream migration from the coast to the natal hatchery. Such changes were observed in various brain loci including the olfactory bulb, terminal nerve, ventral telencephalon, nucleus preopticus parvocellularis anterioris, nucleus preopticus magnocellularis and midbrain tegmentum. These results suggest that sGnRH neurons change their activity for gonadal maturation prior to initiation of homing behavior from the Bering Sea. The present study provides the first evidence to support a possible involvement of neuropeptides in the onset of spawning migration.

Reproductive status regulates expression of sex steroid and GnRH receptors in the olfactory bulb

Neuromodulators including gonadotropin-releasing hormone (GnRH) and sex steroids help integrate an animal's internal physiological state with incoming external cues, and can have profound effects on the processing of behaviorally relevant information, particularly from the olfactory system. While GnRH and steroid receptors are present in olfactory processing regions across vertebrates, little is known about whether their expression levels change with internal physiological state or external social cues. We used qRT-PCR to measure mRNA levels of two GnRH receptors (GnRH-R1, GnRH-R2), five sex steroid receptors (estrogen receptors: ERalpha, ERbetaa, ERbetab; androgen receptors: ARalpha, ARbeta), and aromatase in the olfactory bulb of the highly social African cichlid fish Astatotilapia burtoni. We asked whether these receptor levels changed with reproductive condition in females, or with social status, which regulates reproductive capacity in males. Our results reveal that mRNA levels of multiple sex steroid, GnRH receptor subtypes, and aromatase in the olfactory bulb vary with sex, social status in males, and reproductive condition in females, which highlights the potential importance of changing receptor levels in fine-tuning the olfactory system during the reproductive cycle. Further, steroid receptor mRNA levels were positively correlated with circulating steroid levels in males, but negatively correlated in females, suggesting different regulatory control between sexes. These results provide support for the hypothesis that the first-order olfactory relay station is a substrate for both GnRH and sex steroid modulation, and suggest that changes in receptor levels could be an important mechanism for regulating reproductive, social, and seasonal plasticity in olfactory perception observed across vertebrates.

Changes in gene expression for GH/PRL/SL family hormones in the pituitaries of homing chum salmon during ocean migration through upstream migration

Gene expression for growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family hormones in the pituitaries of homing chum salmon were examined, because gene expression for these hormones during ocean-migrating phases remains unclear. Fish were collected in the winter Gulf of Alaska, the summer Bering Sea and along homing pathway in the Ishikari River-Ishikari Bay water system in Hokkaido, Japan in autumn. The oceanic fish included maturing adults, which had developing gonads and left the Bering Sea for the natal river by the end of summer. The absolute amounts of GH, PRL and SL mRNAs in the pituitaries of the maturing adults in the summer Bering Sea were 5- to 20-fold those in the winter Gulf of Alaska. The amount of GH mRNA in the homing adults at the coastal seawater (SW) areas was smaller than that in the Bering fish, while the amount of PRL mRNA remained at the higher level until fish arrived at the Ishikari River. The gill Na(+),K(+)-ATPase activity in the coastal SW fish and the plasma Na(+) levels in the brackish water fish at the estuary were lowered to the levels that were comparable to those in the fresh water (FW) fish. In conclusion, gene expression for GH, PRL and SL was elevated in the pituitaries of chum salmon before initiation of homing behavior from the summer Bering Sea. Gene expression for GH is thereafter lowered coincidently with malfunction of SW adaptability in the breeding season, while gene expression for PRL is maintained high until forthcoming FW adaptation.

The role of the terminal nerve and GnRH in olfactory system neuromodulation

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Changes in the plasma levels of insulin-like growth factor-I from the onset of spawning migration through upstream migration in chum salmon

An increase in activity of the pituitary-gonadal axis (PG-axis) and gonadal development are essential for the onset of spawning migration of chum salmon from the Bering Sea. In the Bering Sea, fish with larger body sizes initiated gonadal development and commenced spawning migration to the natal river by the end of summer. We thus hypothesized that insulin-like growth factor-I (IGF-I), a somatotropic signal that interacts with the PG-axis, can be one of such factors responsible for the onset of migration, and examined changes in plasma levels and hepatic expression of IGF-I gene in oceanic and homing chum salmon in 2001-2003. The plasma IGF-I levels and corresponding body sizes in maturing adults, which had developing gonads, were significantly higher than those in immature fish in all years examined. Such increase in the plasma IGF-I levels in maturing fish was observed even in the Gulf of Alaska during February 2006, while coincident increase was not observed in the hepatic amounts of IGF-I mRNA. In autumn, the plasma IGF-I levels in homing adults decreased during upstream migration in the Ishikari River-Ishikari bay water system in Hokkaido, Japan. In conclusion, the plasma IGF-I levels increased with gonadal development when chum salmon migrated from the winter Gulf of Alaska to the summer Bering Sea. Circulating IGF-I may interact with the PG-axis and promote gonadal development that is inseparable from the onset of spawning migration. Circulating IGF-I levels were thereafter lowered in accordance with final maturation during upstream migration in the breeding season.

Social cues from conspecifics alter electrical activity of gonadotropin-releasing hormone neurons in the terminal nerve via visual signals

There are multiple populations of gonadotropin-releasing hormone (GnRH) neurons in the brains of vertebrates. The population located in the hypothalamus/preoptic area is the best studied and is known to ultimately control reproduction. Teleost fish have an additional population of GnRH neurons in the terminal nerve (TN) associated with the olfactory bulbs, the physiological function of which is still unclear. Anatomical and physiological studies provide evidence that TN-GnRH neurons have extensive projections in the brain and modulate neuronal activity. Although there is anatomical evidence that the TN receives olfactory and optic sensory inputs, it is not known if sensory information is transmitted to TN-GnRH neurons to modulate their activity. In the present study, we tested the hypothesis that social cues from conspecifics modulate electrical activity of TN-GnRH neurons from the intact brain of female medaka fish ( Oryzias latipes). We further investigated the potential roles of chemosensory and visual signals in mediating the social cue response. We used a transgenic line of medaka with TN-GnRH neurons genetically tagged with green fluorescent protein, allowing visualization of specific neurons for whole-cell current clamp electrophysiology. We demonstrated that 24-h exposure to male visual and chemosensory cues suppressed the electrical activity of female TN-GnRH neurons compared with exposure to other females. Chemosensory cues alone were insufficient to induce this social cue response. However, visual cues alone replicated the “combined” social cue response. These findings support our hypothesis that sensory signals—and specifically, visual social cues—modulate electrical activity of TN-GnRH neurons.

Activity of the pituitary-gonadal axis is increased prior to the onset of spawning migration of chum salmon

The activity of the pituitary-gonadal axis (PG axis) in pre-migratory and homing chum salmon was examined because endocrine mechanisms underlying the onset of spawning migration remain unknown. Pre-migratory fish were caught in the central Bering Sea in June, July and September 2001, 2002 and 2003, and in the Gulf of Alaska in February 2006. They were classified into immature and maturing adults on the basis of gonadal development. The maturing adults commenced spawning migration to coastal areas by the end of summer, because almost all fish in the Bering Sea were immature in September. In the pituitaries of maturing adults, the copy numbers of FSHbeta mRNA and the FSH content were 2.5- to 100-fold those of the immature fish. Similarly, the amounts of LHbeta mRNA and LH content in the maturing adults were 100- to 1000-fold those of immature fish. The plasma levels of testosterone, 11-ketotestosterone and estradiol were higher than 10 nmol l(-1) in maturing adults, but lower than 1.0 nmol l(-1) in immature fish. The increase in the activity of the PG-axis components had already initiated in the maturing adults while they were still in the Gulf of Alaska in winter. In the homing adults, the pituitary contents and the plasma levels of gonadotropins and plasma sex steroid hormones peaked during upstream migration from the coast to the natal hatchery. The present results thus indicate that the seasonal increase in the activity of the PG axis is an important endocrine event that is inseparable from initiation of spawning migration of chum salmon.

Integrative and genomics approaches to uncover the mechanistic bases of fish behavior and its diversity

Understanding the molecular mechanisms underlying fish behavior is of fundamental importance to further our understanding of the proximate and ultimate causes of variation in this trait and informs us on issues of animal husbandry, conservation, and welfare. One way to approach this question is to study variation in gene expression in individuals exhibiting different behaviors and relating it to variations at other phenotypic levels in an organismic, ecological and evolutionary context. Here we review studies that have shown that the use of such an integrative and genomics approach is greatly useful for shedding new light on the mechanisms of behaviors as diverse as social dominance, mate choice, reproduction and migration. We present studies that use functional genomics tools and integrate several biological levels of organization, including transcription variation, which are important in the context of integrative biology and genomics of fish behavior. We review studies of phenotype-level variation in transcription but also studies that focus on variation at the individual-level. Dissecting the molecular bases of among-individual variation in behavior, including the study of variation in temperament (behavioral syndrome/coping style) within and among populations, will gain importance in the field in the years to come.

Expression of hormone genes and osmoregulation in homing chum salmon: a minireview

Pacific salmon migrate from ocean through the natal river for spawning. Information on expression of genes encoding osmoregulatory hormones and migratory behavior is important for understanding of molecular events that underlie osmoregulation of homing salmon. In the present article, regulation of gene expression for osmoregulatory hormones in pre-spawning salmon was briefly reviewed with special reference to neurohypophysial hormone, vasotocin (VT), and pituitary hormones, growth hormone (GH) and prolactin (PRL). Thereafter, we introduced recent data on migratory behavior from SW to FW environment. In pre-spawning chum salmon, the hypothalamic VT mRNA levels increased in the males, while decreased in the females with loss of salinity tolerance when they were kept in SW. The amounts of GH mRNA in the pituitary decreased during ocean migration prior to entrance into FW. Hypo-osmotic stimulation by SW-to-FW transfer did not significantly affect the amount of PRL mRNA, but it was elevated in both SW and FW environments along with progress in final maturation. Behaviorally, homing chum salmon continued vertical movement between SW and FW layers in the mouth of the natal river for about 12h prior to upstream migration. Pre-spawning chum salmon in an aquarium, which allowed fish free access to SW and FW, showed that individuals with the lower plasma testosterone (T) and higher estradiol-17beta (E2) levels spent longer time in FW when compared with the SW fish. Taken together, neuroendocrine mechanisms that underlie salt and water homeostasis and migratory behavior from SW to FW may be under the control of the hypothalamus-pituitary-gonadal axis in pre-spawning salmon.

Reproductive stage-related effects of salmon GnRH and sex steroid hormones on expression of genes encoding fushi tarazu factor 1 homolog and estrogen receptor alpha in masu salmon pituitary cells in vitro

Expression of genes encoding gonadotropin (GTH) subunits in the salmon pituitary was regulated by salmon gonadotropin-releasing hormone (sGnRH) and sex steroid hormones in a reproductive stage-dependent manner, probably through DNA-binding transcription factors. Direct effects of these hormones on expression of genes encoding salmon fushi tarazu factor 1 homolog (sFF1-I) and estrogen receptor alpha (ERalpha) were therefore examined by use of primary pituitary cell cultures of masu salmon at different reproductive stages. Pituitaries were collected in March (before initiation of gonadal maturation), in May (early maturing), in July (late maturing), and in September (spawning period). Amounts of sFF1-I and ERalpha mRNAs in the pituitary cells were determined by real-time polymerase chain reactions after a treatment with sGnRH, estradiol-17beta (E2), testosterone (T) or 11-ketotestosterone (11KT). The amounts of sFF1-I mRNA were elevated by E2 in the males, and by sGnRH and T in the females before initiation of gonadal maturation and at the early maturing stage. The amounts of ERalpha mRNA in the early maturing females were elevated by sGnRH. Effects of sGnRH were not significant at the late maturing and spawning stages. The amounts of ERalpha mRNA in the spawning males were halved by 11KT and E2, and those of sFF1-I and ERalpha mRNAs in the late maturing females were decreased by T and 11KT. These results indicated that responsiveness of sFF1-I and ERalpha genes to sGnRH and sex steroid hormones is seasonally variable in relation to reproductive stages. Expression of sFF1 and ERalpha genes should be stimulated at the early stages of gonadal maturation prior to increases in the amounts of GTH subunit mRNAs, while attenuated after the late maturing period when stored amounts of GTH subunit mRNAs reached near the maximum.

Roles of periventricular neurons in retinotectal transmission in the optic tectum

The midbrain roof is a retinorecipient region referred to as the optic tectum in lower vertebrates, and the superior colliculus in mammals. The retinal fibers projecting to the tectum transmit visual information to tectal retinorecipient neurons. Periventricular neurons are a subtype of these neurons that have their somata in the deepest layer of the teleostean tectum and apical dendrites ramifying at more superficial layers consisting of retinal fibers. The retinotectal synapses between the retinal fibers and periventricular neurons are glutamatergic, and ionotropic glutamate receptors mediate the transmission in these synapses. This transmission involves long-term potentiation, and is modulated by hormone action. Visual information processed in the periventricular neurons is transmitted to adjacent tectal cells and target nuclei of periventricular neuron axonal branches, some of which relay the visual information to other brain areas controlling behavior. We demonstrated that periventricular neurons play a principal role in visual information processing in the teleostean optic tectum; the effects of tectal output on behavior is discussed also in the present review.

Regulation of salmon gonadotrophin-releasing hormone gene expression by sex steroids in rainbow trout brain

Salmon gonadotrophin-releasing hormone (sGnRH) is the major form of gonadotrophin-releasing hormone in the brain of Salmonids and is encoded by two different genes: sGnRH1 and sGnRH2. In the present study, we examined the expression patterns of these two genes during development and throughout the reproductive cycle of the female rainbow trout (Oncorhynchus mykiss), and also investigated the feedback action of sex steroids on brain mRNA levels. Both genes are expressed as early as 14 days postfertilisation and show a similar expression pattern during early life stages. In the adult female, sGnRH1 and sGnRH2 mRNAs are both present in neurones located in the ventral forebrain. This gene expression in the brain appears to be low during early vitellogenesis, and increases during oocyte maturation to reach a maximum after ovulation. The expression of sGnRH1 was not modified by in vivo steroid treatments in any experiment; however, testosterone and 5alpha-dihydrotestosterone down-regulate brain sGnRH2 gene in immature and adult ovariectomised females. Oestradiol treatment decreases sGnRH2 mRNA levels in the brain of adult ovariectomised females only. In the triploid fish brain, none of the steroids affect brain sGnRH mRNA levels. Our results suggest that, unlike sGnRH1, the sGnRH2 gene is under a strongly androgenic inhibitory control in the immature and adult female rainbow trout.