Seasonal changes in peptide, receptor and ion channel mRNA expression in the caudal neurosecretory system of the European flounder (Platichthys flesus)

The excitatory effect of 5-HT1A and 5-HT2B receptors on the caudal neurosecretory system Dahlgren cells in olive flounder, Paralichthys olivaceus

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) plays an essential role in the regulation of neural activity via multiple receptors. Here, we investigated the functional role of serotoninergic input on the Dahlgren cell population in the caudal neurosecretory system (CNSS) of olive flounder. In this study, the effect of 5-HT on the firing activity of Dahlgren cells was explored in terms of changes in firing frequency and firing pattern using multicellular recording electrophysiology ex vivo, and the role of several 5-HT receptor subtypes in the regulation was determined. The results revealed that 5-HT increased the firing frequency in a concentration-dependent manner and altered the firing pattern of Dahlgren cells. The effect of 5-HT on the firing activity of Dahlgren cells was mediated through the 5-HT1A and 5-HT2B receptors, selective agonists of both receptors effectively increased the firing frequency of Dahlgren cells, and selective receptor antagonists could also effectively inhibit the increase in firing frequency caused by 5-HT. In addition, the mRNA levels of major signaling pathway-related genes, ion channels, and major secretion hormone genes were significantly upregulated in CNSS after treatment with 5-HT. These findings demonstrate that 5-HT acts as an excitatory neuromodulator on Dahlgren cells and enhances neuroendocrine activity in CNSS.

[The caudal neurosecretory system, the other "neurohypophysial" system in fish]

The caudal neurosecretory system (CNSS) is a neuroendocrine complex whose existence is specific to fishes. Structurally, it has many similarities with the hypothalamic-neurohypophyseal complex of other vertebrates. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most important being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is totally unknown and its role is still poorly understood. Paradoxically, it is almost no longer studied today. Recent developments in imaging and genome editing could make it possible to resume investigations on CNSS in order to solve the mysteries that still surround it.

Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue-adrenocorticotropic hormone (ACTH)-as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Neuromodulatory effects of GnRH on the caudal neurosecretory Dahlgren cells in female olive flounder

Gonadotropin-releasing hormone (GnRH) is considered a key player in reproduction. The caudal neurosecretory system (CNSS) is a unique neurosecretory structure of fish that may be involved in osmoregulation, nutrition, reproduction, and stress-related responses. However, a direct effect of GnRH on Dahlgren cells remains underexplored. Here, we examined the electrophysiological response of Dahlgren cell population of the CNSS to GnRH analog LHRH-A₂ and the transcription of related key genes of CNSS. We found that GnRH increased overall firing frequency and may be changed the firing pattern from silent to burst or phasic firing in a subpopulation of Dahlgren cells. The effect of GnRH on a subpopulation of Dahlgren cells firing activity was blocked by the GnRH receptor (GnRH-R) antagonist cetrorelix. A positive correlation was observed between the UII and GnRH-R mRNA levels in CNSS or gonadosomatic index (GSI) during the breeding season. These findings are the first demonstration of the ability of GnRH acts as a modulator within the CNSS and add to our understanding of the physiological role of the CNSS in reproduction and seasonal adaptation.

GABAA receptor-mediated inhibition of Dahlgren cells electrical activity in the olive flounder, Paralichthys olivaceus

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system. We investigated its potential role as a neurotransmitter in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the flounder Paralichthys olivaceus. The application of GABA in vitro resulted in a decrease in electrical activity of Dahlgren cells, followed by an increase of the number of silent cells, together with a decreased firing frequency of all three activity patterns (tonic, phasic, bursting). GABA_A receptor agonist etomidate decreased Dahlgren cell firing activity, in a similar way to GABA. The response to GABA was blocked by the GABA_A receptor antagonist bicuculline. GABA_A receptor gamma2 subunit (Gabrg2) and chloride channel (Clcn2) mRNA expression were significantly upregulated in the CNSS after GABA superfusion. These data suggest that GABA may modulate CNSS activity in vivo mediated by GABA_A receptors.

The caudal neurosecretory system: A novel thermosensitive tissue and its signal pathway in olive flounder (Paralichthys olivaceus)

Ectotherm animals, such as fish, are vulnerable when facing an extreme temperature fluctuation as a result of their inability to maintain body temperature. The caudal neurosecretory system (CNSS) is unique to fish and has been shown to maintain homeostasis in response to seasonal changes. However, its temperature sensitivity is unknown. Here, we used in vitro electrophysiological and anatomical approaches to investigate a thermosensory pathway in the CNSS. We showed that the CNSS responds directly to local hypothermal challenge via the TRP channel, and transmits this signal using the neurotransmitter, GABA, to the neurosecretory Dahlgren cells of the CNSS. These findings are the first demonstration of the thermal perception of the CNSS and add to our understanding of the physiological role of the CNSS in thermoregulation and seasonal adaptation.

Dynamic Responses of the Caudal Neurosecretory System (CNSS) Under Thermal Stress in Olive Flounder (Paralichthys olivaceus)

Temperature is a critical environmental factor that affect most biological and physiological processes in fish. The caudal neurosecretory system (CNSS) is unique to fish and is proved to maintain homeostasis during seasonal alterations. However, the dynamic expression and secretion pattern of its major hormones, corticotrophin-releasing hormone (CRH), urotensin I (UI), and urotensin II (UII), and their response to thermal stress has not been studied. CRH, UII and cortisol in plasma, gene expression levels of CRH, UI, and UII in the CNSS of olive flounder (Paralichthys olivaceus) were therefore characterized. UI- and UII-positive Dahlgren cells, as well as cell proliferation in the CNSS, were also quantified. The results showed that plasma cortisol and CRH were increased in both low temperature (LT) and high temperature (HT) groups. However, there was no difference in plasma UI and UII during thermal stress. In CNSS, CRH, UI, and UII mRNA levels were all significantly elevated in response to acute hypothermal stress and recovered back to the control (normal) level after 8 days of adaptation. During hyperthermal challenge, gene expression of CRH and UI only significantly increased after 8-days of transfer but no change in UII was observed. We also demonstrated an increasing percent of UI-positive Dahlgren cells in the CNSS of 8-days hyperthermal stressed fish. However, no BrdU-labeled Dahlgren cells were found among the three treatment groups. Collectively, our results demonstrate that the CNSS is subjected to dynamic responses under thermal stress and expands upon the role of the CNSS in thermoregulation. The dynamic responses of hormone levels and the gene expression of CRH, UI and UII in CNSS are all involved in the process of hyper- or hypo-thermal stress and adaptation.

Changes in PRL Gene Expression During Upstream Movement of the Japanese Eel, Anguilla japonica

Changes in mRNA levels of prolactin (PRL) during upstream movement were examined in juvenile Japanese eels, Anguilla japonica. Glass eels and elvers were collected from 2007 to 2009 near the entrance of Hamana Lake, and in a small inflowing stream, the Egawa River. Quantification of mRNA was performed by real-time PCR and expressed as whole-body content. PRL mRNA levels of glass eels caught in the coastal zone and tidal area were low. Eels that moved downward in the tidal zone and migrated upstream to enter into freshwater showed increased levels of PRL mRNA. These changes suggest the importance of up-regulation of PRL gene expression in juvenile eels during their upstream movement from seawater to fresh water, particularly in relation to hyperosmoregulation.

Dynamic Expression and Regulation of Urotensin I and Corticotropin-Releasing Hormone Receptors in Ovary of Olive Flounder Paralichthys olivaceus

Urotensin I (UI), a fish corticotropin-releasing hormone (CRH) like peptide, has been found throughout vertebrate species that has great effects on adaptive physiology comprising stress-related responses, and osmotic regulation by binding with CRH type I receptor (CRHR1) and CRH type II receptor (CRHR2) in fish. Dynamic expression and regulation of UI and CRH receptors in the olive flounder ovarian follicle were studied so as to make further efforts to understand the role of UI in the development of teleost ovary. The results showed that stage-specific change in UI mRNA levels in ovarian follicles of olive flounder. UI and CRHR1 mRNA levels were higher in stage III follicles (300∼500 μm diameter) compared to stage II (90∼300 μm diameter) and IV (500∼800 μm diameter) follicles, however, the levels of CRHR2 mRNA were decreased in line with the ovarian development from stage II to stage IV. A strong signal of UI protein was observed in the follicular cells and oocyte in stage III and IV follicles by immunohistochemistry. In vitro treatment of olive flounder ovarian follicles with human chorionic gonadotropin (hCG) showed that the mRNA expression of UI increased significantly at low concentration and decreased at high concentration at 6 h, but the CRHR1 and CRHR2 mRNA did not change obviously. In addition, the results of incubation with 17α, 20β-dihydroxy-4-oregnen-3-one (DHP) show that UI and CRHR1 mRNA expression were elevated with increasing concentrations at 9 h. All above results indicated that UI and CRH receptors may have a vital effect on olive flounder ovarian development.

Gene expression and hormone secretion profile of urotensin I associated with osmotic challenge in caudal neurosecretory system of the euryhaline flounder, Platichthys flesus

The caudal neurosecretory system (CNSS) is a part of stress response system, a neuroendocrine structure unique to fish. To gain a better understanding of the physiological roles of CNSS in fluid homeostasis, we characterized the tissue distribution of urotensin I (UI) expression in European flounder (Platichthys flesus), analyzed the effect chronic exposure to seawater (SW) or freshwater (FW), transfer from SW to FW, and reverse transfer on mRNA levels of UI, L-type Ca²⁺ channels and Ca-activated K⁺ channels transcripts in CNSS. The tissue distribution demonstrated that the CNSS is dominant sites of UI expression, and UI mRNA level in fore brain appeared greater than other non-CNSS tissues. There were no consistent differences in CNSS UI expression or urophysis UI content between SW- and FW-adapted fish in July and September. After transfer from SW to FW, at 8 h CNSS UI expression was significantly increased, but urophysis UI content was no significantly changes. At 24 h transfer from SW to FW, expression of CNSS UI was no apparent change and urophysis UI content was reduced. At 8 h and 24 h after transfer from FW to SW UI expression and urophysis UI content was no significantly effect. The expression of bursting dependent L-type Ca²⁺ channels and Ca-activated K⁺ channels in SW-adapted fish significantly decreased compared to those in FW-adapted. However, there were no differences in transfer from SW to FW or from FW to SW at 8 h and 24 h. Thus, these results suggest CNSS UI acts as a modulator in response to osmotic stress and plays important roles in the body fluid homeostasis.

Modulatory effect of glutamate GluR2 receptor on the caudal neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

A neuromodulatory role for glutamate has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined the potential role of glutamate as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) in the euryhaline flounder Paralichthys olivaceus. In pharmacological experiments in vitro, glutamate (Glu) caused an increase in electrical activity of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The glutamate substrate, glutamine (Gln), led to increased firing frequency, cell recruitment and enhanced bursting activity. The glutamate effect was not blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, or the GluR1/GluR3 (AMPA) receptor antagonist IEm1795-2HBr, but was blocked by the broad-spectrum α-amino-3-hydroxy- 5- methyl-4-isoxazo-lepropionic acid (AMPA) receptor antagonist ZK200775. Our transcriptome sequencing study revealed three AMPA receptor (GluR1, GluR2 and GluR3) in the olive flounder CNSS. Quantitative RT-PCR revealed that GluR2 receptor mRNA expression was significant increased following dose-dependent superfusion with glutamate in the CNSS. GluR1 and GluR3 receptor mRNA expression were decreased following superfusion with glutamate. L-type Ca²⁺ channel mRNA expression had a significant dose-dependent decrease following superfusion with glutamate, compared to the control. In the salinity challenge experiment, acute transfer from SW to FW, GluR2 receptor mRNA expression was significantly higher than the control at 2 h. These findings suggest that GluR2 is one of the mechanisms which can medicate glutamate action within the CNSS, enhancing electrical activity and hence secretory output.

Influence of long (16L:8D) and short (8L:16D) photoperiods on blood metabolites and hepatic metabolism in Olive flounder, Paralichthys olivaceus

In the present study the influence of long photoperiod (LP, 16L:8D) and short photoperiod (SP, 8L:16D) on hepatic energy metabolism in the olive flounder (Paralichthys olivaceus) was investigated. Flounders were maintained under LP or SP conditions for 2 weeks then plasmatic and hepatic parameters were assessed. At the plasmatic level, the concentration of cortisol was enhanced in flounder maintained under LP compared to SP. Alkaline phosphatase (ALP), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) enzyme activities in plasma also increased in LP flounder. There was no significant difference in plasma glucose levels between the two experimental groups. Plasma osmotic pressure, Na and Cl levels were significantly higher in LP compared to the SP group. In liver, a significant decrease of triglycerides together with an increase in glycogen was observed in the LP group. Hepatic hsl and pepck and muscle hsl mRNA expression in LP was significantly higher in the SP group. Overall the results indicate that the LP treatment caused a mild stress response and increased hepatic energy metabolism in the flounder, which in turn could affect osmoregulation. In conclusion, it would appear that LP treatment can adversely influence hepatic energy metabolism in adult olive flounder under fasting condition.

Role of serotonin in fish reproduction

The neuroendocrine mechanism regulates reproduction through the hypothalamo-pituitary-gonadal (HPG) axis which is evolutionarily conserved in vertebrates. The HPG axis is regulated by a variety of internal as well as external factors. Serotonin, a monoamine neurotransmitter, is involved in a wide range of reproductive functions. In mammals, serotonin regulates sexual behaviors, gonadotropin release and gonadotropin-release hormone (GnRH) secretion. However, the serotonin system in teleost may also play unique role in the control of reproduction as the mechanism of reproductive control in teleosts is not always the same as in the mammalian models. In fish, the serotonin system is also regulated by natural environmental factors as well as chemical substances. In particular, selective serotonin reuptake inhibitors (SSRIs) are commonly detected as pharmaceutical contaminants in the natural environment. Those factors may influence fish reproductive functions via the serotonin system. This review summarizes the functional significance of serotonin in the teleosts reproduction.

Dynamic expression pattern of corticotropin-releasing hormone, urotensin I and II genes under acute salinity and temperature challenge during early development of zebrafish

Corticotropin-releasing hormone (CRH), urotensin I (UI) and urotensin II (UII) are found throughout vertebrate species from fish to human. To further understand the role of crh, uI and uII in teleosts during development, we investigated the expression pattern of crh, uI, uIIα and uIIβ genes, and their response to acute salinity and temperature challenge during early development of zebrafish, Danio rerio. The results reveal that crh, uI, uIIα and uIIβ mRNA are detected from 0hpf, and the expression levels increase to a maximum at 6 days post fertilization (dpf), with the exception of uIIα that peak at 5dpf. Exposure of zebrafish embryos and larvae to acute osmotic (30ppt) stress for 15 min failed to modify expression levels of crh, uI, uIIα and uIIβ mRNA from levels in control fish except at 6dpf when uIIα and uIIβ were significantly (P < 0.05) modified. Exposure of embryos and larvae to a cold (18 °C) or hot stress (38 °C) generally down-regulated mRNA levels of crh, uI, uIIα and uIIβ apart from at 3dpf. The results indicate that the contribution of crh, uI, uIIα and uIIβ genes to the stress response in zebrafish may be stressor-specific during early development. Overall, the results from this study provide a basis for further research into the developmental and stressor-specific function of crh, uI, uIIα and uIIβ in zebrafish.

Daily rhythms of urotensin I and II gene expression and hormone secretion in the caudal neurosecretory system of the euryhaline flounder (Platichthys flesus)

The caudal neurosecretory system (CNSS) is a unique neuroendocrine structure for environmental adaptation in fish, and is the major site of expression and secretion of urotensin I (UI) and II (UII). This study examined daily changes in mRNA expression and the secretion profile of UI and UII in the CNSS. Daily rhythms were observed in mRNA level of CNSS UI, urophysis UI, plasma UII, glucose, potassium and sodium. No statistically significant (Cosinor, P>0.05) diel rhythmicity in mRNA level of CNSS UII, urophysis UII, cortisol, lactate, osmolality and chloride were detected. The calculated acrophase of sodium, cortisol, plasma UII, urophysis UII, urophysis UI and mRNA level of CNSS UI rhythms were recorded at 13:04 h, 13:39 h, 14:45 h, 15:27 h, 14:41 h and 14:39 h, respectively and a positive relationship was evident among them. The acrophase of glucose and potassium rhythms were recorded at 18:57 h and 22:35 h, respectively. The glucose levels increased progressively at the onset of the UII surge at 15:00 h and reached peak values at dusk. The results support the hypothesis that the CNSS may play a role in the control of co-ordinated daily changes in energy mobilization, nutritional behavior and osmoregulatory systems in euryhaline flounder. Our findings described for the first time the existence of daily rhythms of CNSS hormone expression and secretion in Platichthys flesus. These results reveal the importance of taking into account the time of day when assessing stress responses and evaluating UI and UII as physiological indicators of stress in this species.

Localized expression of urocortin genes in the developing zebrafish brain

The corticotropin-releasing hormone (CRH) family consists of four paralogous genes, CRH and urocortins (UCNs) 1, 2, and 3. In a previous study, we analyzed CRH in the teleost model organism zebrafish and its transcript distribution in the embryonic brain. Here, we describe full-length cDNAs encoding urotensin 1 (UTS1), the teleost UCN1 ortholog, and UCN3 of zebrafish. Major expression sites of uts1 in adult zebrafish are the caudal neurosecretory system and brain. By using RT-PCR analysis, we show that uts1 mRNA is also present in ovary, maternally contributed to the embryo, and expressed throughout embryonic development. Expression of ucn3 mRNA was detected in a range of adult tissues and during developmental stages from 24 hours post fertilization onward. Analysis of spatial transcript distributions by whole-mount in situ hybridization revealed limited forebrain expression of uts1 and ucn3 during early development. Small numbers of uts1-synthesizing neurons were found in subpallium, hypothalamus, and posterior diencephalon, whereas ucn3-positive cells were restricted to telencephalon and retina. The brainstem was the main site of uts1 and ucn3 synthesis in the embryonic brain. uts1 Expression was confined to the midbrain tegmentum; distinct hindbrain cell groups, including locus coeruleus and Mauthner neurons; and the spinal cord. ucn3 Expression was localized to the optic tectum, serotonergic raphe, and distinct rhombomeric cell clusters. The prominent expression of uts1 and ucn3 in brainstem is consistent with proposed roles of CRH-related peptides in stress-induced modulation of locomotor activity through monoaminergic brainstem neuromodulatory systems.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.