Saccus Vasculosus Research Articles

The saccus vasculosus of the neotropical cichlid fish Cichlasoma dimerus: characterization, developmental studies and its response to photoperiod.

The saccus vasculosus is an organ present in gnathostome fishes, located ventral to the hypothalamus and posterior to the pituitary gland, whose structure is highly variable among species. In some fishes, this organ is well-developed; however, its physiological function is still under debate. Recently, it has been proposed that this organ is a seasonal regulator of reproduction. In the present work, we examined the histology, ultrastructure, and development of the saccus vasculosus in Cichlasoma dimerus. In addition, immunohistochemical studies of proteins related to reproductive function were performed. Finally, the potential response of this organ to different photoperiods was explored. C. dimerus presented a well-developed saccus vasculosus consisting of a highly folded epithelium, composed of coronet and supporting cells, closely associated with blood vessels, and a highly branched lumen connected to the third ventricle. Coronet cells showed all the major characteristics described in other fish species. In addition, some of the vesicles of the globules were positive for thyrotropin beta subunit, while luteinizing hormone beta subunit immunostaining was observed at the edge of the apical processes of some coronet cells. Furthermore, neuropeptide Y and gonadotropin inhibitory hormone innervation in the saccus vasculosus of C. dimerus were shown. Finally, animals exposed to the long photoperiod showed lower levels of thyrotropin beta and common alpha subunits expression in the saccus compared to those of animals exposed to short photoperiod. All these results support the hypothesis that the saccus vasculosus is involved in the regulation of reproductive function in fish.

Distribution of gamma-aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes.

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. Immunohistochemical techniques with specific antibodies against GABA or against its synthesizing enzyme, glutamic acid decarboxylase (GAD) allowed characterizing GABAergic neurons and fibers in the CNS. However, studies on the CNS distribution of GABAergic neurons and fibers of bony fishes are scant and were done in teleost species. With the aim of understanding the early evolution of this system in bony vertebrates, we analyzed the distribution of GABA-immunoreactive (-ir) and GAD-ir neurons and fibers in the CNS of a basal ray-finned fish, the Siberian sturgeon (Chondrostei, Acipenseriformes), using immunohistochemical techniques. Our results revealed the presence and distribution of GABA/GAD-ir cells in different regions of the CNS such as olfactory bulbs, pallium and subpallium, hypothalamus, thalamus, pretectum, optic tectum, tegmentum, cerebellum, central grey, octavolateralis area, vagal lobe, rhombencephalic reticular areas, and the spinal cord. Abundant GABAergic innervation was observed in most brain regions, and GABAergic fibers were very abundant in the hypothalamic floor along the hypothalamo-hypophyseal tract and neurohypophysis. In addition, GABA-ir cerebrospinal fluid-contacting cells were observed in the alar and basal hypothalamus, saccus vasculosus, and spinal cord central canal. The distribution of GABAergic systems in the sturgeon brain shows numerous similarities to that observed in lampreys, but also to those of teleosts and tetrapods.

Cloning and expression of two isoforms of deiodinase gene DIO3 in spotted sea bass (Lateolabrax maculatus)

Type 3 iodothyronine deiodinase (DIO3) is a key enzyme for thyroid hormones, which plays a role in the growth and development of animals. In this study, we identified and cloned two isoforms of DIO3 (LmDIO3a and LmDIO3b) from spotted sea bass (Lateolabrax maculatus). The nucleotide sequences of both LmDIO3a and LmDIO3b had a SECIS (SElenoCysteine Insertion Sequence) element and a T4 domain, and the deduced amino acid sequence similarity was 64.42 %. The mRNAs of both LmDIO3 isoforms were widely distributed in the brain, with high expression levels in the hypothalamus, pituitary, telencephalon and saccus vasculosus (SV). In the non-brain tissues, the expression of LmDIO3a was higher in the retina, heart and spleen, while LmDIO3b was highly expressed in the head kidney and skin. The expression levels of LmDIO3a and LmDIO3b in the hypothalamus and SV were lower in the early stage of gonadal development. However, the expression levels of both in the hypothalamus increased significantly in the stage IV of gonadal development, but in the SV were up-regulated significantly in the stage V. The photoperiod experiment showed that the expressions of them were promoted by short photoperiod (8L:16D) and inhibited by long photoperiod (16L:8D) in the hypothalamus. In the SV, their expression trends were consistent (They firstly decreased, and then increased). In the pituitary, the expression of LmDIO3a presented a trend of up and down, while the expression of LmDIO3b showed an opposite trend. In the gonad, long photoperiod promoted the expression of LmDIO3a, but inhibited the expression of LmDIO3b (P < 0.05). There was no significant change in the expression of LmDIO3a in the retina, but the expression of LmDIO3b was significantly inhibited by short photoperiod and promoted by long photoperiod (P < 0.05). That different tissues have different responses to photoperiod, indicating photoperiod induction may have tissue specificity. Altogether, this study provides a reference for further research on the photoperiod regulation mechanism of DIO3 gene in fish.

Temporal expression of thyroid hormone regulating genes (tsh-b, tsh-r, dio2 and dio3) and their correlation with annual reproductive cycle of the Indian freshwater catfish, Heteropneustes fossilis (Bloch).

Photoperiod and temperature are well-established environmental cues for gonadal growth in seasonally reproducing organisms. The photoperiod is known to regulate seasonal reproduction via induction of thyroid hormone regulating genes in the saccus vasculosus (SV) of fishes. However, SV is absent in many seasonally breeding fishes, including Heteropneustes fossilis. H. fossilis is a long-day breeding catfish in which gonadal recrudescence begins six months earlier than spawning. The present study attempted to analyse the expression of thyroid hormone-regulating genes (tsh-b, tsh-r, dio2 and dio3) in the brain, liver and ovary. In the brain, upregulated expression of thyrotropin-beta subunit (tsh-b), deiodinase2 (dio2) and deiodinase3 (dio3) genes is concomitant with the increasing photoperiod and temperature in nature, which may appear to regulate seasonal reproduction. Both deiodinases, dio2 and dio3, were also upregulated in the liver and ovarian tissue during the gonadal growth phase. The upregulation of deiodinases may enhance the metabolism and activity of tissues, thereby facilitating their respective roles. The expression of these genes was also assessed in the brain, liver, ovary, kidney, skin, spleen and gill tissues during the spawning period. The ubiquitous expression of deiodinases may point to enhanced activity in their organ-specific role. The present study speculates that expression of tsh-b, tsh-r, dio2 and dio3 genes during the reproductive phase of H. fossilis might be involved in the regulation of seasonal reproduction.

Effects of different photoperiod conditions on survival, growth, and gonadal development of Takifugu rubripes adults

Photoperiod has a significant influence on fish. In this study, 2.5-year-old Takifugu rubripes (tiger puffer) adults were selected to evaluate the photoperiod impacts (8 h light (L):16 h dark (D), 12L:12D, 16L:8D) from September to January at 15–17 °C on their growth, gonadal development, and survival for a 100-day. The mortality rate of males and females tiger puffer differed among the different photoperiods, and the 8L:16D photoperiod was not suitable for survival of males. At the end of the experiment, the body length, weight gain rate, specific growth rate and condition factor of both males and females tiger puffer were similar among the three photoperiods (P > 0.05). Higher percentage of females had developed to later developing phase and males had developed to spawning capable phase in the 12L:12D treatment, respectively. In particular, 100% of males had testis at the spawning capable phase. The gonadosomatic index values of males and females in the 12L:12D group were significantly higher than those in the 16L:8D group (P < 0.05), but no significant differences between the 8L:16D and 12L:12D groups (P > 0.05). The serum testosterone (T) content of males and the serum estradiol-17β (E2) content of females were highest in the 12L:12D group. The expression of aralkylamine N-acetyltransferase 1a (aanat1a) in the diencephalon/midbrain (D/M) was elevated under 12L:12D treatment in both sex (P < 0.05). The expression level of iodothyronine deiodinase type 2 (dio2) in the saccus vasculosus in males was elevated under the 12L:12D photoperiod (P < 0.05). The levels of gonadotropin-releasing hormone 1 (gnrh1) and gnrh3 mRNA in the D/M of females at later developing stages in the 12L:12D treatment were significantly higher than those in of other treatment groups, and the level of gnrh3 in males under this photoperiod was significantly elevated (P < 0.05). The levels of gonadotropin-inhibiting hormone (gnih) mRNA under 12L:12D were significantly elevated in both males and females (P < 0.05). Under this photoperiod, the levels of follicle stimulating hormone b-subunit (fshb) and luteinizing hormone b-subunit (lhb) in the pituitary gland of females at later developing stages were significantly higher than those of the other two groups (P < 0.05). The expression levels of lhb in males in the 12L:12D and 8L:16D photoperiods were significantly higher than that at 16L:8D (P < 0.05). These results indicated that culture under the 12L:12D photoperiod from September prior to the spawning season could result in accelerated gonadal development of both male and female tiger puffer.

Characterization of CYP11A1 and its potential role in sex asynchronous gonadal development of viviparous black rockfish Sebastes schlegelii (Sebastidae)

Mitochondrial cytochrome P450 side-chain cleavage (P450scc), encoded by the cyp11a1 gene, initiates the first step of steroid biosynthesis. In this study, a 1554-bp open reading frame (ORF) of black rockfish (Sebastes schlegelii) cyp11a1 was cloned. The cyp11a1 gene is located on chromosome 5 and has 9 exons. The ORF encodes a putative precursor protein of 517 amino acids, and the predicted cleavable mitochondrial targeting peptide is located at amino acids 1–39. P450scc shares homology with other teleosts and tetrapods, which have relatively conserved binding regions with heme, cholesterol and adrenodoxin. Tissue distribution analysis revealed that the highest expression levels of cyp11a1 were detected in mature gonads and head kidney but that low levels were detected in gestational/regressed ovaries, regressed testes and other tissues. Immunostaining of P450scc was observed in testicular Leydig cells, ovarian theca cells, interrenal glands of head kidney, pituitary and multiple regions of brain. Particularly, two kinds of fish-specific P450scc-positive cells, including coronet cells of brain saccus vasculosus and hypophyseal somatolactin cells, were identified in black rockfish. Our results provide novel evidence for the potential role played by P450scc in reproduction behavior by mediating steroidogenesis in viviparous teleost.

Photoperiodic regulation of pituitary thyroid-stimulating hormone and brain deiodinase in Atlantic salmon

Seasonal timing is important for many critical life history events of vertebrates, and photoperiod is often used as a reliable seasonal cue. In mammals and birds, it has been established that a photoperiod-driven seasonal clock resides in the brain and pituitary, and is driven by increased levels of pituitary thyroid stimulating hormone (TSH) and brain type 2 iodothyronine deiodinase (DIO2), which leads to local increases in triiodothyronine (T3). In order to determine if a similar mechanism occurs in fish, we conducted photoperiod manipulations in anadromous (migratory) Atlantic salmon (Salmo salar) that use photoperiod to time the preparatory development of salinity tolerance which accompanies downstream migration in spring. Changing daylength from short days (light:dark (LD) 10:14) to long days (LD 16:8) for 20 days increased gill Na+/K+-ATPase (NKA) activity, gill NKAα1b abundance and plasma growth hormone (GH) levels that normally accompany increased salinity tolerance of salmon in spring. Long-day exposure resulted in five-fold increases in pituitary tshβb mRNA levels after 10 days and were sustained for at least 20 days. tshβb mRNA levels in the saccus vasculosus were low and not influenced by photoperiod. Increased daylength resulted in significant increases in dio2b mRNA levels in the hypothalamus and midbrain/optic tectum regions of the brain. The results are consistent with the presence of a photoperiod-driven seasonal clock in fish which involves pituitary TSH, brain DIO2 and the subsequent production of T3, supporting the hypothesis that this is a common feature of photoperiodic regulation of seasonality in vertebrates.

Gonadotropin-inhibitory hormone (GnIH) distribution in the brain of the ancient fish Atractosteus tropicus (Holostei, Lepisosteiformes)

The Holostei group occupies a critical phylogenetic position as the sister group of the Teleostei. However, little is known about holostean pituitary anatomy or brain distribution of important reproductive neuropeptides, such as the gonadotropin-inhibitory hormone (GnIH). Thus, the present study set out to characterize the structure of the pituitary and to localize GnIH-immunoreactive cells in the brain of Atractosteus tropicus from the viewpoint of comparative neuroanatomy. Juveniles of both sexes were processed for general histology and immunohistochemistry. Based on the differences in cell organization, morphology, and staining properties, the neurohypophysis and three regions in the adenohypophysis were identified: the rostral and proximal pars distalis (PPD) and the pars intermedia. This last region was found to be innervated by the neurohypophysis. This organization, together with the presence of a saccus vasculosus, resembles the general teleost pituitary organization. A vast number of blood vessels were also recognized between the infundibulum floor of the hypothalamus and the PPD, evidencing the characteristic presence of a median eminence and a portal system. However, this well-developed pituitary portal system resembles that of tetrapods. As regards the immunohistochemical localization of GnIH, we found four GnIH-immunoreactive (GnIH-ir) populations in three hypothalamic nuclei (suprachiasmatic, retrotuberal, and tuberal nuclei) and one in the diencephalon (prethalamic nucleus), as well as a few scattered neurons throughout the olfactory bulbs, the telencephalon, and the intersection between them. GnIH-ir fibers showed a widespread distribution over almost all brain regions, suggesting that GnIH function is not restricted to reproduction only. In conclusion, the present study describes, for the first time, the pituitary of A. tropicus and the neuroanatomical localization of GnIH in a holostean fish that exhibits a similar distribution pattern to that of teleosts and other vertebrates, suggesting a high degree of phylogenetic conservation of this system.

The Melanocortin System in Atlantic Salmon (Salmo salar L.) and Its Role in Appetite Control.

The melanocortin system is a key neuroendocrine network involved in the control of food intake and energy homeostasis in vertebrates. Within the hypothalamus, the system comprises two main distinct neuronal cell populations that express the neuropeptides proopiomelanocortin (POMC; anorexigenic) or agouti-related protein (AGRP; orexigenic). Both bind to the melanocortin-4 receptor (MC4R) in higher order neurons that control both food intake and energy expenditure. This system is relatively well-conserved among vertebrates. However, in Atlantic salmon (Salmo salar L.), the salmonid-specific fourth round whole-genome duplication led to the presence of several paralog genes which might result in divergent functions of the duplicated genes. In the current study, we report the first comprehensive comparative identification and characterization of Mc4r and extend the knowledge of Pomc and Agrp in appetite control in Atlantic salmon. In silico analysis revealed multiple paralogs for mc4r (a1, a2, b1, and b2) in the Atlantic salmon genome and confirmed the paralogs previously described for pomc (a1, a2, and b) and agrp (1 and 2). All Mc4r paralogs are relatively well-conserved with the human homolog, sharing at least 63% amino acid sequence identity. We analyzed the mRNA expression of mc4r, pomc, and agrp genes in eight brain regions of Atlantic salmon post-smolt under two feeding states: normally fed and fasted for 4 days. The mc4ra2 and b1 mRNAs were predominantly and equally abundant in the hypothalamus and telencephalon, the mc4rb2 in the hypothalamus, and a1 in the telencephalon. All pomc genes were highly expressed in the pituitary, followed by the hypothalamus and saccus vasculosus. The agrp genes showed a completely different expression pattern from each other, with prevalent expression of the agrp1 in the hypothalamus and agrp2 in the telencephalon. Fasting did not induce any significant changes in the mRNA level of mc4r, agrp, or pomc paralogs in the hypothalamus or in other highly expressed regions between fed and fasted states. The identification and wide distribution of multiple paralogs of mc4r, pomc, and agrp in Atlantic salmon brain provide new insights and give rise to new questions of the melanocortin system in the appetite regulation in Atlantic salmon.

The underlying mechanisms of vertebrate seasonal reproduction.

Animals make use of changes in photoperiod to adapt their physiology to the forthcoming breeding season. Comparative studies have contributed to our understanding of the mechanisms of seasonal reproduction in vertebrates. Birds are excellent models for studying these phenomena because of their rapid and dramatic responses to changes in photoperiod. Deep brain photoreceptors in birds perceive and transmit light information to the pars tuberalis (PT) in the pituitary gland, where the thyroid-stimulating hormone (TSH) is produced. This PT-TSH locally increases the level of the bioactive thyroid hormone T3 via the induction of type 2 deiodinase production in the mediobasal hypothalamus, and an increased T3 level, in turn, controls seasonal gonadotropin-releasing hormone secretion. In mammals, the eyes are the only photoreceptive structure, and nocturnal melatonin secretion encodes day-length information and regulates the PT-TSH signaling cascade. In Salmonidae, the saccus vasculosus plays a pivotal role as a photoperiodic sensor. Together, these studies have uncovered the universality and diversity of fundamental traits in vertebrates.

Localization, distribution and expression of growth hormone in the brain of Asian Catfish, Clarias batrachus.

Intense immunoreactivity was observed in several neurons of the nucleus preopticus (NPO) located in the preoptic area (POA), in addition to several GH cells in the proximal pars distalis (PPD) of the pituitary gland of Clarias batrachus. The immunoreactive cells were located in the paraventricular as well as supraoptic subdivisions of the NPO. GH immunoreactive fibers projecting from the neurons were traced caudally to the pituitary gland via the conspicuous preoptico-hypophysial tract (PHT) in the ventral tuberal area to the neurohypophysis of the pituitary. Apart from these immunoreactive fibers in the preoptico-hypophysial tract, some fine caliber fiber probably arising from the neurons located dorsally in the NPO also showed GH immunoreactivity, and these fibers constituted an independent tract. Bilaterally, it extended caudally through the dorsal hypothalamus almost as far as the saccus vasculosus where it curved sharply to descend into the caudal tuberal region and finally entered into the pituitary gland. The fibers of this tract were mainly distributed in the rostral pars distalis (RPD). This tract is quite distinct from the preoptico-hypophysial tract and herein called as the accessory preoptico-hypophysial tract (APHT). Expression of GH mRNA in the NPO was found 65-fold more than that of the control region, rostral cerebellum. These results altogether suggest that GH secreted by NPO neurons might serve as a neuro-modulatory role in the brain of C. batrachus. Transportation of GH to the pituitary via two independent tracts may represent the duality of neuroendocrine function. The present study underscores the possibility that GH in the brain of vertebrates may be a phylogenetically conserved phenomenon and provide clues to our understanding of the evolutionary course taken by the hormone.

Photoperiod may regulate growth via leptin receptor A1 in the hypothalamus and saccus vasculosus of Atlantic salmon (Salmo salar)

ABSTRACTPhotoperiod is believed to regulate growth in fish, although the mechanism involved is still unclear. In this paper, we report a relationship between leptin-receptor A1 (AsLRa1), melatonin-receptor (AsMR) and photoperiod in Atlantic salmon. Atlantic salmon (mean weight 1071.70 ± 155.54 g) were reared under six photoperiod regimes, four constant light regimes 24L:0D, 18L:6D, 12L:12D and 8L:16D, hours of light (L) and dark (D) and two varying light regimes, LL-SL = 24L:0D-8L:16D, and SL-LL = 8L:16D-24L:0D over a period of seven months. The results showed that AsLRa1 transcripts were mainly existed in the hypothalamus and saccus vasculosus (SV), AsMR was mainly expressed in the hypothalamus. Long photoperiod inhibited the expression of AsLRa1 and AsMR transcripts in the Atlantic salmon brain. The expression pattern of AsLRa1 was similar to the expression pattern of AsMR in the hypothalamus. Food intake was higher in fish with lower AsLRa1 transcript levels. This demonstrated that photoperiod influenced somatic growth by changing expression of AsLRa1 in the hypothalamus and SV to affect appetite. In addition, we found that the SV appears to act as a seasonal sensor regulating reproduction in a similar way to the hypothalamus.

Brain Distribution of Key Neuropeptides Involved in Appetite Control in Atlantic Salmon, Salmo Salar (L.)

The physiological mechanisms that control food intake in the brain of mammals consist of orexigenic and anorexigenic neuronal populations that integrate both central and peripheral signals and play a pivotal role on the control of appetite. The neuropeptide Y (NPY) and agouti‐related proteins (AgRP) are neuropeptides expressed and secreted by the orexigenic neurons, while anorexigenic neurons synthesizes and releases proopiomelanocortin (POMC) and Cocaine‐ and amphetamine‐regulated transcript (CART) neuropeptides. These central signals controlling the appetite system are integrated in complex neuronal nucleus within the hypothalamus, but other brain regions are also involved.Several studies indicate that analogue control systems are conserved among vertebrates, although limited information is available for teleost species. The expression of these neuropeptides has been correlated with food intake, hunger and satiety, in several fish species. However, the majority of these studies analyzed the whole brain, thereby limiting our understanding on which brain areas are involved in teleost appetite control. This is particularly important, because these neuropeptides are involved in other physiological and behavioral functions. For e.g., NPY expressed in the neural circuits in amygdala and hippocampus of mammals has been shown to be involved in fear and anxiety behaviours. However, in teleosts, very few attempts have been made to specific locate in which brain areas the neuropeptides related to appetite control are expressed.In this study, we analyzed the mRNA distribution level of the key neuropeptides npy, agrp‐1 and agrp‐2, cart, pomc‐a1, ‐a2, a2s and ‐b in six brain regions of Atlantic salmon (Salmo salar), a commercially key species in the Norwegian aquaculture. In addition, we analyzed the response of these neuropeptides after 3 days of fasting in a pilot study.Our data show that both anorexigenic and orexigenic neuropeptides genes are expressed in the six brain regions analyzed at very different levels. npy, cart and agrp‐2 showed a high mRNA expression in the forebrain. In contrast to this, agrp‐1 had high expression in the hypothalamus, pituitary and saccus vasculosus. For all pomc genes, the pituitary had considerable much higher expression compared to the other brain regions. With the exception of the pituitary, the hypothalamus was the brain region with the highest expression in pomc‐a genes, followed by the saccus vasculosus. A preliminary comparison between fed and fasted fish did not indicate significant differences in the mRNA expression of the key neuropeptides in the hypothalamus.These results indicate that to understand the teleosts neuropeptides involvement in appetite control their spatial and temporal expression in the brain needs to be fully characterized.Support or Funding InformationFunded by Regional Research Fund Vest (RFFV 259183; Greenbag) and Research Council of Norway (NFR. 267626 LeuSENSE).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Photoperiodic Regulation of Reproduction in Vertebrates.

Organisms use changes in photoperiod for seasonal reproduction to maximize the survival of their offspring. Birds have sophisticated seasonal mechanisms and are therefore excellent models for studying these phenomena. Birds perceive light via deep-brain photoreceptors and long day-induced thyroid-stimulating hormone (TSH, thyrotropin) in the pars tuberalis of the pituitary gland (PT), which cause local thyroid hormone activation within the mediobasal hypothalamus. The local bioactive thyroid hormone controls seasonal gonadotropin-releasing hormone secretion and subsequent gonadotropin secretion. In mammals, the eyes are believed to be the only photoreceptor organ, and nocturnal melatonin secretion triggers an endocrine signal that communicates information about the photoperiod to the PT to regulate TSH. In contrast, in Salmonidae fish the input pathway to the neuroendocrine output pathway appears to be localized in the saccus vasculosus. Thus, comparative analysis is an effective way to uncover the universality and diversity of fundamental traits in various organisms.

Ontogeny of Gonadotropin-Inhibitory Hormone (GnIH) in the cichlid fish Cichlasoma dimerus.

RFamide peptides are expressed in the early stages of development in most vertebrates. Gonadotropin-inhibitory hormone (GnIH) belongs to the RFamide family, and its role in reproduction has been widely studied in adult vertebrates, ranging from fish to mammals. As only three reports evaluated GnIH during development, the aim of this study was to characterise the ontogeny of GnIH in a fish model, Cichlasoma dimerus. We detected the presence of two GnIH-immunoreactive (GnIH-ir) cell clusters with spatial and temporal differences. One cluster was observed by 3 days post-hatching (dph) in the nucleus olfacto-retinalis (NOR) and the other in the nucleus posterioris periventricularis by 14 dph. The number of GnIH-ir neurons increased in both nuclei, whereas their size increased only in the NOR from hatchling to juvenile stages. These changes occurred from the moment larvae started feeding exogenously and during development and differentiation of gonadal primordia. We showed by double-label immunofluorescence that only GnIH-ir neurons in the NOR co-expressed GnRH3 associated peptide. In addition, GnIH-ir fibre density increased in all brain regions from 5 dph. GnIH-ir fibres were also detected in the retina, optic tract and optic tectum, suggesting that GnIH acts as a neuromodulator of photoreception and the integration of different sensory modalities. Also, there were GnIH-ir fibres in the pituitary from 14 dph, which were in close association with somatotropes. Moreover, GnIH-ir fibres were observed in the saccus vasculosus from 30 dph, suggesting a potential role of GnIH in the modulation of its function. Finally, we found that gnih was expressed from 1 dph, and that the pattern of variation of its transcript levels was in accordance with that of cell number. Present results are the starting point for the study of new GnIH roles during development. This article is protected by copyright. All rights reserved.

Photoperiod regulate gonad development via kisspeptin/kissr in hypothalamus and saccus vasculosus of Atlantic salmon (Salmo salar)

Atlantic salmon exhibit seasonal reproduction. However, the mechanisms governing this are still unclear. Generally speaking, kisspeptin has been recognized as a regulator of reproduction. Here, we report a relationship between kisspeptin, GnRH and photoperiod in Atlantic salmon. The results demonstrated that the expression of the Atlantic salmon kisspeptin-receptor (skissr) was not always consistent with the expression pattern of Atlantic salmon GnRH3 (sGnRH3) during all developmental processes. Kisspeptin may exert its influence primarily in the early and later stages of gonad development by promoting the secretion of sGnRH3. Meanwhile, the expression levels of kissr were higher in fish with gonads at stage II and stage V under the long-day photoperiod regime than under the short-day regime. In addition, both skissr and sGnRH3 were also expressed in the saccus vasculosus (SV), an organ only found in fish. The SV might be a seasonal sensor regulating reproduction in addition to the hypothalamus (Hyp).

Histophysiological and surface ultrastructural studies of the saccus vasculosus of Notopterus chitala (Hamilton)

We investigated the cytoarchitecture and surface ultrastructural characteristics of different cells lining the epithelium of the saccus vasculosus of Notopterus chitala (Hamilton, 1822). The saccus vasculosus, the richly vascularized reddish, sac like organ is situated on the ventral aspect of the diencephalon just behind the pituitary gland. Histologically, the saccus vasculosus consisted of several loculi lined with a heterogeneous population of a large number of specialized coronet cells and a smaller number of alternately arranged supporting cells. The loculi were densely surrounded by blood sinusoids. The loculi opened into several collecting channels that united and ultimately drained into the third ventricle of the brain. The coronet cells possessed central or basal nuclei and an apical globular protrusion. Under scanning electron microscopy, the coronet cells were variable in shape with prominent nuclei. The supporting cells were triangular in shape and placed in between the coronet cells. The apical parts of the coronet cells had different shapes of globular protrusions with distinct stalks. Intense silver stain was noted in the terminal end of the coronet cells and nerves were attached with blood vessels. Under scanning electron microscopy observation, nerve fibers of different caliber were attached to the coronet cells and blood vessels. The various cells of the saccus vasculosus in N. chitala were correlated with their functional significance.

Molecular basis for regulating seasonal reproduction in vertebrates.

Animals that inhabit mid- to high-latitude regions exhibit various adaptive behaviors, such as migration, reproduction, molting and hibernation in response to seasonal cues. These adaptive behaviors are tightly regulated by seasonal changes in photoperiod, the relative day length vs night length. Recently, the regulatory pathway of seasonal reproduction has been elucidated using quail. In birds, deep brain photoreceptors receive and transmit light information to the pars tuberalis in the pituitary gland, which induces the secretion of thyroid-stimulating hormone. Thyroid-stimulating hormone locally activates thyroid hormone via induction of type 2 deiodinase in the mediobasal hypothalamus. Thyroid hormone then induces morphological changes in the terminals of neurons that express gonadotropin-releasing hormone and facilitates gonadotropin secretion from the pituitary gland. In mammals, light information is received by photoreceptors in the retina and neurally transmitted to the pineal gland, where it inhibits the synthesis and secretion of melatonin, which is crucial for seasonal reproduction. Importantly, the signaling pathway downstream of light detection and signaling is fully conserved between mammals and birds. In fish, the regulatory components of seasonal reproduction are integrated, from light detection to neuroendocrine output, in a fish-specific organ called the saccus vasculosus. Various physiological processes in humans are also influenced by seasonal environmental changes. The findings discussed herein may provide clues to addressing human diseases, such as seasonal affective disorder.

Ontogeny of the Saccus Vasculosus, a Seasonal Sensor in Fish.

TSH secreted from the pars distalis (PD) of the pituitary gland stimulates the thyroid gland. In contrast, TSH secreted from the pars tuberalis (PT) of the pituitary gland regulates seasonal reproduction. The ontogeny of thyrotrophs and the regulatory mechanisms of TSH are apparently different between the PD and the PT. Interestingly, fish do not have an anatomically distinct PT, and the saccus vasculosus (SV) of fish is suggested to act as a seasonal sensor. Thus, it is possible that the SV is analogous to the PT. Here we examined the ontogeny of the pituitary gland and SV using rainbow trout. A histological analysis demonstrated the development of the pituitary anlage followed by that of the SV. Lhx3 and Pit-1, which are required for the development of PD thyrotrophs, clearly labeled the pituitary anlage. The common glycoprotein α-subunit (CGA) and TSH β-subunit (TSHB) genes were also detected in the pituitary anlage. In contrast, none of these genes were detected in the SV anlage. We then performed a microarray analysis and identified parvalbumin (Pvalb) as a marker for SV development. Because Pvalb expression was not detected in the pituitary anlage, no relationship was observed between the development of the SV and the pituitary gland. In contrast to embryos, Lhx3, Pit-1, CGA, and TSHB were all expressed in the adult SV. These results suggest that the morphological differentiation of SV occurs during the embryonic stage but that the functional differentiation into a seasonal sensor occurs in a later developmental stage.

Comparative analysis reveals the underlying mechanism of vertebrate seasonal reproduction

Animals utilize photoperiodic changes as a calendar to regulate seasonal reproduction. Birds have highly sophisticated photoperiodic mechanisms and functional genomics analysis in quail uncovered the signal transduction pathway regulating avian seasonal reproduction. Birds detect light with deep brain photoreceptors. Long day (LD) stimulus induces secretion of thyroid-stimulating hormone (TSH) from the pars tuberalis (PT) of the pituitary gland. PT-derived TSH locally activates thyroid hormone (TH) in the hypothalamus, which induces gonadotropin-releasing hormone (GnRH) and hence gonadotropin secretion. However, during winter, low temperatures increase serum TH for adaptive thermogenesis, which accelerates germ cell apoptosis by activating the genes involved in metamorphosis. Therefore, TH has a dual role in the regulation of seasonal reproduction. Studies using TSH receptor knockout mice confirmed the involvement of PT-derived TSH in mammalian seasonal reproduction. In addition, studies in mice revealed that the tissue-specific glycosylation of TSH diversifies its function in the circulation to avoid crosstalk. In contrast to birds and mammals, one of the molecular machineries necessary for the seasonal reproduction of fish are localized in the saccus vasculosus from the photoreceptor to the neuroendocrine output. Thus, comparative analysis is a powerful tool to uncover the universality and diversity of fundamental properties in various organisms.