Sexually Different Expression of Neurohypophysial Hormone Genes in the Preoptic Nucleus of Pre-Spawning Chum Salmon

Reproductive roles of the vasopressin/oxytocin neuropeptide family in teleost fishes

The vertebrate nonapeptide families arginine vasopressin (AVP) and oxytocin (OXT) are considered to have evolved from a single vasopressin-like peptide present in invertebrates and termed arginine vasotocin in early vertebrate evolution. Unprecedented genome sequence availability has more recently allowed new insight into the evolution of nonapeptides and especially their receptor families in the context of whole genome duplications. In bony fish, nonapeptide homologues of AVP termed arginine vasotocin (Avp) and an OXT family peptide (Oxt) originally termed isotocin have been characterized. While reproductive roles of both nonapeptide families have historically been studied in several vertebrates, their roles in teleost reproduction remain much less understood. Taking advantage of novel genome resources and associated technological advances such as genetic modifications in fish models, we here critically review the current state of knowledge regarding the roles of nonapeptide systems in teleost reproduction. We further discuss sources of plasticity of the conserved nonapeptide systems in the context of diverse reproductive phenotypes observed in teleost fishes. Given the dual roles of preoptic area (POA) synthesized Avp and Oxt as neuromodulators and endocrine/paracrine factors, we focus on known roles of both peptides on reproductive behaviour and the regulation of the hypothalamic-pituitary-gonadal axis. Emphasis is placed on the identification of a gonadal nonapeptide system that plays critical roles in both steroidogenesis and gamete maturation. We conclude by highlighting key research gaps including a call for translational studies linking new mechanistic understanding of nonapeptide regulated physiology in the context of aquaculture, conservation biology and ecotoxicology.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Arginine Vasotocin Preprohormone Is Expressed in Surprising Regions of the Teleost Forebrain

Nonapeptides play a fundamental role in the regulation of social behavior, among numerous other functions. In particular, arginine vasopressin and its non-mammalian homolog, arginine vasotocin (AVT), have been implicated in regulating affiliative, reproductive, and aggressive behavior in many vertebrate species. Where these nonapeptides are synthesized in the brain has been studied extensively in most vertebrate lineages. While several hypothalamic and forebrain populations of vasopressinergic neurons have been described in amniotes, the consensus suggests that the expression of AVT in the brain of teleost fish is limited to the hypothalamus, specifically the preoptic area (POA) and the anterior tuberal nucleus (putative homolog of the mammalian ventromedial hypothalamus). However, as most studies in teleosts have focused on the POA, there may be an ascertainment bias. Here, we revisit the distribution of AVT preprohormone mRNA across the dorsal and ventral telencephalon of a highly social African cichlid fish. We first use in situ hybridization to map the distribution of AVT preprohormone mRNA across the telencephalon. We then use quantitative real-time polymerase chain reaction to assay AVT expression in the dorsomedial telencephalon, the putative homolog of the mammalian basolateral amygdala. We find evidence for AVT preprohormone mRNA in regions previously not associated with the expression of this nonapeptide, including the putative homologs of the mammalian extended amygdala, hippocampus, striatum, and septum. In addition, AVT preprohormone mRNA expression within the basolateral amygdala homolog differs across social contexts, suggesting a possible role in behavioral regulation. We conclude that the surprising presence of AVT preprohormone mRNA within dorsal and medial telencephalic regions warrants a closer examination of possible AVT synthesis locations in teleost fish, and that these may be more similar to what is observed in mammals and birds.

Vasotocin--A new player in the control of oocyte maturation and ovulation in fish

In this article, the physiological role of ovarian vasotocin (VT) on fish final oocyte maturation (FOM) and ovulation is reviewed based on the studies mainly available in the catfish Heteropneustes fossilis. The VT system is characterized in the follicular layer of the oocytes by both immunocytochemical and in situ hybridization techniques. The distribution was confirmed in isolated follicular layer preparations by HPLC characterization and quantification. Three VT receptor subtype genes are identified: V1a1 and V1a2 subtypes are distributed in the follicular layer and V2 subtype is present along the granulosa-oocyte membrane junction. The expression of peptide, VT precursor gene and VT receptor genes shows seasonal and periovulatory changes in the ovary. VT secretion is modulated by E2 differentially in a season-specific manner, and by progestin steroids positively. VT modulates E2 in a biphasic manner in early recrudescent phase and induces a steroidogenic shift inhibiting E2 and stimulating progestin steroid (P4, 17P4 and 17,20β-DP) pathways in the late recrudescent phase. VT stimulates prostaglandin secretion, germinal vesicle breakdown (GVBD), oocyte hydration and ovulation. VT acts through different receptors to stimulate these processes. It uses the V1 type receptor to stimulate GVBD and ovulation, and the V2 type to stimulate oocyte hydration. VT acts as an important link in the cascade of gonadotropin control of FOM and ovulation. More research is required in other species.

Vasotocinergic and isotocinergic systems in the gilthead sea bream (Sparus aurata): an osmoregulatory story

To investigate the physiological roles of arginine vasotocin (AVT) and isotocin (IT) in osmoregulatory process in gilthead sea bream (Sparus aurata), a time course study (0, 12h, and 1, 3, 7 and 14 days) has been performed in specimens submitted to hypoosmotic (from 40‰ salinity to 5‰ salinity) or hyperosmotic (from 40‰ salinity to 55‰ salinity) challenges. Plasma and liver osmoregulatory and metabolic parameters, as well as AVT and IT pituitary contents were determined concomitantly with hypothalamic pro-vasotocin (pro-VT) and pro-isotocin (pro-IT) mRNA expression levels. Previously, sequences coding for pro-VT and pro-IT cDNAs were cloned. Two osmoregulatory periods related to plasma osmolality and metabolic parameter variations could be distinguished: i) an adaptative period, from 12h to 3 days after transfer, and ii) a chronic regulatory period, starting at day 3 after transfer. Higher values in hypothalamic pro-VT and pro-IT mRNA expression as well as in pituitary AVT and IT storage levels in both hypo- and/or hyper-osmotic transfers have been distinguished. These increase correlated with changes in plasma cortisol levels, suggesting an interaction between this hormone and pro-VT expression. Furthermore, pro-IT expression enhancement also suggests a role of the isotocinergic system as a modulator in the acute stress response induced by hyper-osmotic challenge in S. aurata.

Estrogen regulation of brain vasotocin secretion in the catfish Heteropneustes fossilis: an interaction with catecholaminergic system

Vasotocin (VT) is a basic neurohypophysial nonapeptide in non-mammalian vertebrates and is involved in diverse functions like osmoregulation, reproduction, metabolism and behavior. In this study, we report that estradiol-17β (E(2)) regulates brain and plasma VT secretion through the involvement of the catecholaminergic (CA) system. To demonstrate this, E(2) level was altered through ovariectomy (OVX, 3 weeks) and replacement study with low and high E(2) doses (0.1 and 0.5 μg/g body weight). CA activity was inhibited by treatment with α-methylparatyrosine (α-MPT; 250 μg/g body weight), a competitive inhibitor of tyrosine hydroxylase. VT was assayed by an enzyme immunoassay method. In the sham group, the low E(2) dose produced 82% and 104% increase, respectively, in brain and plasma VT levels. The high E(2) dose decreased the VT levels significantly. The low E(2) dose decreased brain E(2) but elevated plasma E(2). In the high E(2) group, the E(2) level increased further in both brain and plasma. OVX resulted in a significant inhibition (69% and 25%, respectively) of both brain and plasma VT, which was correlated with low E(2) levels. The low E(2) dose not only reversed the inhibition, but increased the VT level in both brain and plasma in comparison to the sham groups. The high E(2) replacement inhibited VT levels further low in both brain and plasma. The α-MPT treatment inhibited VT levels significantly in both sham and OVX groups. The drug treatment abolished partially the restorative effect of the low E(2) dose in the ovariectomized fish. In the high E(2) dose group, α-MPT decreased brain and plasma VT levels further low compared to the sham + 0. 5 μg E(2) group or OVX + 0.5 μg E(2) group except the brain VT level, which increased in the OVX+0.5 μg E(2) group. It is inferred that E(2) may exert biphasic effects on VT through the mediation of the CA system.

Dopamine and mesotocin neurotransmission during the transition from incubation to brooding in the turkey

We investigated the neuroendocrine changes involved in the transition from incubating eggs to brooding of the young in turkeys. Numbers of mesotocin (MT; the avian analog of mammalian oxytocin) immunoreactive (ir) neurons were higher in the nucleus paraventricularis magnocellularis (PVN) and nucleus supraopticus, pars ventralis (SOv) of late stage incubating hens compared to the layers. When incubating and laying hens were presented with poults, all incubating hens displayed brooding behavior. c-fos mRNA expression was found in several brain areas in brooding hens. The majority of c-fos mRNA expression by MT-ir neurons was observed in the PVN and SOv while the majority of c-fos mRNA expression in dopaminergic (DAergic) neurons was observed in the ventral part of the nucleus preopticus medialis (POM). Following intracerebroventricular injection of DA or oxytocin (OT) receptor antagonists, hens incubating eggs were introduced to poults. Over 80% of those injected with vehicle or the D1 DA receptor antagonist brooded poults, while over 80% of those receiving the D2 DA receptor antagonist or the OT receptor antagonist failed to brood the poults. The D2 DA/OT antagonist groups also displayed less c-fos mRNA in the dorsal part of POM and the medial part of the bed nucleus of the stria terminalis (BSTM) areas than did the D1 DA/vehicle groups. These data indicate that numerous brain areas are activated when incubating hens initially transition to poult brooding behavior. They also indicate that DAergic, through its D2 receptor, and MTergic systems may play a role in regulating brooding behaviors in birds.

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.

Effects of hCG and ovarian steroid hormones on vasotocin levels in the female catfish Heteropneustes fossilis

Effects of hCG, ovariectomy and estradiol replacement on brain, plasma and/or ovarian vasotocin in vivo, and estradiol, progesterone, 17alpha, 20beta-hydroxy-4-pregnen-3-one and hCG on ovarian vasotocin in vitro were investigated in the catfish. A 100IU/fish of hCG induced ovulation and elicited both periovulatory and post-ovulatory changes in vasotocin concentrations with a significant increase up to 8h in the brain and up to 16h in both plasma and ovary. After stripping the fish at 16h, the peptide concentration decreased significantly with time, up to 4 days. Ovariectomy in early pre-spawning phase resulted in a duration-dependent significant reduction of both brain and plasma vasotocin. Estradiol replacement in 3-week ovariectomized fish produced dosage-dependent biphasic effects: the lower dosage (0.1microg/g) restored the vasotocin level while the higher dosage (0.5microg/g) decreased it significantly below the control level. In vitro incubation of ovarian tissues with estradiol produced season-dependent effects on vasotocin. The incubation of pre-vitellogenic ovarian pieces with estradiol (1, 10, and 100ng/ml) elevated vasotocin level in a dose- and duration-dependent manner while that of post-vitellogenic follicles resulted in a significant decrease. The incubation of intact post-vitellogenic follicles or follicular envelope with progesterone and 17alpha, 20beta-hydroxy-4-pregnen-3-one (1microg/ml) or hCG (20IU/ml) for 8 and 16h significantly increased vasotocin in a duration-dependent manner. The results show that both gonadotropin and ovarian steroids modulate vasotocin titer, which may influence follicular growth, ovulation and spawning in the catfish.

Immunocytochemical localization, HPLC characterization, and seasonal dynamics of vasotocin in the brain, blood plasma and gonads of the catfish Heteropneustes fossilis

Immunocytochemical distribution and dynamics of vasotocin (VT) were studied in the air-breathing catfish Heteropneustes fossilis in relation to the reproductive cycle. Vasotocin was localized in the brain and ovary by streptavidin-biotin immunocytochemistry. The immunoreactivity was found throughout the hypothalamo-hypophysial neurosecretory system consisting of the magnocellular and parvocellular neurons of the nucleus preopticus, neurosecretory axonal tract and neurohypophysis (NH). The VT neurons showed seasonal changes; they were numerically less in resting phase but increased during the recrudescent phase. The neurons were hypertrophied and degranulated in pre-spawning phase and heavily degranulated and vacuolated in spawning phase. In the NH, the density of VT fibers increased up to the pre-spawning phase and decreased thereafter. In the ovary, VT immunoreactivity was noticed in the follicular layer and varied with the growth of the follicles. Vasotocin was characterized and quantified by a high performance liquid chromatography with UV detection method in the brain, plasma and ovary. Brain and plasma VT concentrations were also assayed with an EIA method, which was more sensitive than the HPLC method with values about 2-fold higher. Vasotocin levels showed significant seasonal and sexual differences with higher concentrations in females in the recrudescent (preparative, pre-spawning and spawning) phase. Brain VT recorded the highest concentration in the preparative phase (both sexes) while plasma (both sexes) and ovarian VT in the spawning phase. The ovarian concentration of VT was 15- and 25-fold higher in the pre-spawning and spawning phases (when expressed per mg protein), respectively, than plasma but lower than brain levels. In testis, VT concentration was relatively low and apparently did not show any significant seasonal variation. The seasonal activity patterns and gonadal distribution of VT indicate a reproductive function of the peptide.

Structure of neurohypophysial hormone genes and changes in the levels of expression during spawning season in grass puffer (Takifugu niphobles)

Vasotocin (VT) has been shown to influence various aspects of social and sexual behaviors in a broad range of vertebrate species, but less is known about the mechanisms through which this peptide modulates behavior. Additionally, much less is known about roles of isotocin (IT) in regulation of behavior. Grass puffer, Takifugu niphobles, has unique spawning behavior; spawning occurs on beach only for several days around the spring tide and is conducted by a group of 10-60 individuals, of which one is female. As a first step toward investigating the roles of VT and IT in this species' spawning behavior, we determined the structures of the VT and IT genes from grass puffer using the genome resources of the closely related tiger puffer and green puffer. We then used these sequences to develop real-time PCR assays and examined changes in expression of the VT and IT genes over the spawning season. The structures of VT and IT genes are well conserved among three puffer species. Particularly, the sequence similarities between grass and tiger puffers were very high not only in the coding region (85-99%), but also in the non-coding regions (92-98%) that include the 5'-upstream regions. The levels of expression of VT gene increased in the brain of pre-spawning females. The levels of VT mRNA in the spawning females tended to be higher than that in the spawning males. In contrast, the levels of IT mRNA did not show such variation. The present results suggest that VT gene expression augments in the brain of females during the spawning period. The unique spawning behavior of grass puffer provides a useful model for studying the molecular mechanism of sexual behavior utilizing the genome resources of tiger puffer.

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.

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.

Arginine vasotocin (AVT) and isotocin (IT) in fish brain: Diurnal and seasonal variations

An HPLC assay with solid-phase extraction and fluorescence derivatization was developed for measurement of arginine vasotocin (AVT) and isotocin (IT) in the neural tissues of fish. The efficiency and usefulness of the method have been verified in experiments by examination of peptides concentrations in brains of three fish species. The day-night changes in neuropeptides levels have been studied in brains of adult sea bream (Sparus aurata) and juvenile Atlantic salmon (Salmo salar). Seasonal fluctuations have been investigated in brains of three-spined sticklebacks (Gasterosteus aculeatus). The AVT and IT biosynthesis in brain seems to be controlled independently and probably each neuropeptide plays a different role in a circadian time-keeping system and an endocrine calendar in fish.

Plasticity of nonapeptidergic neurosecretory cells in fish hypothalamus and neurohypophysis

The structure and function of nonapeptidergic neurosecretory cells (NP-NSC) are considered in terms of comparative morphology. Among NSC of different ergicity for NP-NSC the most characteristic involve massive accumulation and storage of neurohormonal products. Only in NP-NSC are the secretory cycles of functioning clearly expressed. Their highest reactivity is established during experimental and physiological stresses. In contrast, liberinergic, statinergic, and monoaminergic NSC, unlike NP-NSC, are characterized even in the "norm" by a constantly high level of extrusion processes. As signs of maximum NP-NSC plasticity, we consider the largest size of elementary neurosecretory granules, the diversity of secretion forms, and the maximum development of Herring bodies-clear manifestations of secretory cycles of functioning. In particular, phases of massive storage of neurosecretory granules in the extrusion cycle of NP-NSC neurosecretory terminals express accumulation of neurosecretory products. It is concluded that a particularly high degree of plasticity of NP-NSC is provided by their capability for functional reversion. This reversion is manifested first in the form of the restoration of the initial moderate level of functioning and especially in the accumulation of neurosecretory products. The reversion is considered an important mechanism providing a high degree of NSC plasticity. This degree turns out to be sufficient for participation of NP-NSC in the integration of fish reproduction. It is shown that NP-NSC are organized by the principle of a triad of the balanced system. This system consists of two alternative states: accumulation and release of neurosecretory products and the center of control of dynamics of their interrelations, the self-regulating center. In the latter, the key role is probably played by the Golgi complex.

Vasotocin/Isotocin-immunoreactive Neurons in the Medaka Fish Brain Are Sexually Dimorphic and Their Numbers Decrease After Spawning in the Female

In teleosts, the distribution of neurons in the preoptic-hypothalamic region and their associated neurohypophysial hormones, such as vasotocin (VT), appears to be different among species. This differential distribution is thought to reflect the social and/or sexual status of individuals within a species. In the present study, we analyzed the number, size and the distribution of vasotocin/isotocin (VT/IT) neurons in the brains of both male and female medaka (Oryzias latipes) using immunohistochemistry. VT/IT neurons were similarly located in an inverted L-shape in the nucleus preopticus in both gender, as has been already reported in salmonids. However, computer-assisted image analysis revealed sexual dimorphism in the number of VT/IT-immunoreactive (ir) neurons, with greater numbers found in males as compared to females. Further, in the female brain, the number of VT/IT-ir neurons decreased significantly after spawning. In pre-spawning compared to post-spawning females, the small-sized VT/IT-ir neurons dominated. Sexual differentiation of the medaka is fully dependent upon the steroid status during the early developmental stages and steroids are also known to trigger gender-specific behavior in the adult medaka. Our findings strongly suggest that VT and/or IT neurons may be functionally related to ovulation and/or the reproductive axes through connections to their steroidal status.

Localization of mRNAs encoding α and β subunits of soluble guanylyl cyclase in the brain of rainbow trout: comparison with the distribution of neuronal nitric oxide synthase

Detailed distribution of mRNAs encoding alpha and beta subunits of soluble guanylyl cyclase (sGC) was examined in the brain of rainbow trout by in situ hybridization. In addition, distribution of nitric oxide synthase (NOS) was mapped in adjacent parallel sections by neuronal NOS (nNOS) immunocytochemistry and NADPH-diaphorase (NADPHd) histochemistry. Following application of digoxigenin-labeled riboprobes for sGC alpha and beta subunit mRNAs, we found comparatively intense hybridization signals in the telencephalon, preoptic area, thalamus, hypothalamus, pretectum and tegmentum. Both nNOS immunocytochemistry and NADPHd histochemistry showed extensive distribution of nitroxergic neurons in various brain areas, although various degrees of dissociation of nNOS immunoreactivity (ir) and NADPHd staining were detected. In comparison with sGC subunit mRNAs, nNOS signals were more widely distributed in many neurons, including parvocellular neurons in the preoptic area, nucleus anterior tuberis in the hypothalamus, periventricular neurons in the optic tectum, most of the rhombencephalic neurons and pituitary cells. However, wide overlaps of sGC mRNA-containing neurons and nNOS-positive neurons were observed in the olfactory bulb, telencephalon, preoptic area, thalamus, hypothalamus, pretectum, optic tectum, tegmentum and cerebellum. The widespread overlapping in sGC subunit mRNAs and nNOS distribution suggests a role for sGC in various neuronal functions, such as processing of olfactory and visual signals and neuroendocrine function, possibly via NO/cGMP signaling in the brain of rainbow trout.

Development of galanin-like immunoreactivity in the brain of the brown trout (Salmo trutta fario), with some observations on sexual dimorphism

The development of galanin-like immunoreactive (GAL-ir) cells and fibers was investigated in the brain of brown trout embryos, alevins, juveniles, and adults (some spontaneously releasing their gametes). The earliest GAL-ir neurons appeared in the preoptic region and the primordial hypothalamic lobe of 12-mm embryos. After hatching, new GAL-ir neurons appeared in the lateral, anterior, and posterior tuberal nuclei, and in late alevins, GAL-ir neurons appeared in the area postrema. In juveniles, further GAL-ir populations appeared in the nucleus subglomerulosus and magnocellular preoptic nucleus. The GAL-ir neuronal groups present in juveniles were also observed in sexually mature adults, although the area postrema of males lacked immunoreactive neurons. Moreover, spawning males exhibited GAL-ir somata in the olfactory bulb and habenula, which were never observed in adult females or in developing stages. In adults, numerous GAL-ir fibers were observed in the ventral telencephalon, preoptic area, hypothalamus, neurohypophysis, mesencephalic tegmentum, ventral rhombencephalon, and area postrema. Moderate to low GAL-ir innervation was seen in the olfactory bulbs, dorsomedial telencephalon, epithalamus, medial thalamus, optic tectum, cerebellum, and rhombencephalic alar plate. There were large differences among regions in the GAL-ir innervation establishment time. In embryos, GAL-ir fibers appeared in the preoptic area and hypothalamus, indicating early expression of galanin in hypophysiotrophic centers. The presence of galanin immunoreactivity in the olfactory, reproductive, visual, and sensory-motor centers of the brain suggest that galanin is involved in many other brain functions. Furthermore, the distribution of GAL-ir elements observed throughout trout development indicates that galaninergic system maturation continues until sexual maturity.

Social behavior functions and related anatomical characteristics of vasotocin/vasopressin systems in vertebrates

The neuropeptide arginine vasotocin (AVT; non-mammals) and its mammalian homologue, arginine vasopressin (AVP) influence a variety of sex-typical and species-specific behaviors, and provide an integrational neural substrate for the dynamic modulation of those behaviors by endocrine and sensory stimuli. Although AVT/AVP behavioral functions and related anatomical features are increasingly well-known for individual species, ubiquitous species-specificity presents ever increasing challenges for identifying consistent structure-function patterns that are broadly meaningful. Towards this end, we provide a comprehensive review of the available literature on social behavior functions of AVT/AVP and related anatomical characteristics, inclusive of seasonal plasticity, sexual dimorphism, and steroid sensitivity. Based on this foundation, we then advance three major questions which are fundamental to a broad conceptualization of AVT/AVP social behavior functions: (1) Are there sufficient data to suggest that certain peptide functions or anatomical characteristics (neuron, fiber, and receptor distributions) are conserved across the vertebrate classes? (2) Are independently-evolved but similar behavior patterns (e.g. similar social structures) supported by convergent modifications of neuropeptide mechanisms, and if so, what mechanisms? (3) How does AVT/AVP influence behavior - by modulation of sensorimotor processes, motivational processes, or both? Hypotheses based upon these questions, rather than those based on individual organisms, should generate comparative data that will foster cross-class comparisons which are at present underrepresented in the available literature.

Differences in seasonal expression of neurohypophysial hormone genes in ordinary and precocious male masu salmon

Our previous study showed the seasonal variations in expression of vasotocin (VT) and isotocin (IT) genes in preoptic magnocellular neurons of female masu salmon (Oncorhynchus masou). The changes in the level of VT mRNA were coincident with those in plasma testosterone and estradiol levels. In the present study, generality of this phenomenon in salmonid was verified in males. We examined changes in expression of VT and IT genes by an in situ hybridization technique and an immunohistochemical avidin-biotin complex method in the preoptic nuclei of ordinary and precocious male masu salmon. Plasma levels of testosterone and estradiol were measured by enzyme immunoassay. Fish were sampled in March, May, August, and November 1994 and January 1995. The intensities of hybridization signals for VT and IT mRNAs, as well as immunoreactivity of VT and IT, showed seasonal variations, although the profiles were different between the ordinary and precocious males. In the ordinary males, the intensities of hybridization signals for VT and IT mRNAs were high in January. These strong hybridization signals, representing elevation of VT and IT gene expression, were accompanied by increases in plasma levels of testosterone and estradiol. However, in precocious males, changes in VT and IT mRNA levels were not coincident with variation of plasma levels of sex steroid hormones. The sensitivity to sex steroid hormones of VT and IT gene expression may be different between the ordinary and precocious male masu salmon.

Seasonal changes in expression of neurohypophysial hormone genes in the preoptic nucleus of immature female masu salmon

In relevance to osmoregulatory and reproductive functions, activity of the hypothalamic magnocellular neurosecretory system may vary seasonally in teleosts. The changes in the expression of vasotocin (VT) and isotocin (IT) genes were thus studied by an in situ hybridization technique and an immunohistochemical avidin-biotin complex method in immature female masu salmon (Oncorhynchus masou). The plasma levels of testosterone and estradiol were also measured by enzyme immunoassay. Fish were sampled in March, May, August, and November 1994 and January 1995. The intensity of autoradiographic hybridization signals and immunoreactivity were determined in individual neurosecretory cells (NSC) in the rostroventral, middle, and dorsocaudal regions of the magnocellular part of the preoptic nucleus (PM). The VT hybridization signals and immunoreactivity were high in November, along with the elevation of plasma levels of testosterone and estradiol. These results suggest that sex steroid hormones are involved in seasonal regulation of VT gene expression. The hybridization signals for IT mRNA were increased in May and decreased in November, whereas IT immunoreactivity was low in March and high in November. NSCs thus showed seasonal variations in the intensity of hybridization signals for VT and IT mRNAs and immunoreactivity of VT and IT, although the patterns of changes were different between VT and IT. VT and IT genes may be seasonally expressed under different regulatory mechanisms.