Electrical activity of caudal neurosecretory neurons in seawater and freshwater-adapted Platichthys flesus, in vivo

[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.

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.

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.

Nitric Oxide and the Neuroendocrine Control of the Osmotic Stress Response in Teleosts

The involvement of nitric oxide (NO) in the modulation of teleost osmoresponsive circuits is suggested by the facts that NO synthase enzymes are expressed in the neurosecretory systems and may be regulated by osmotic stimuli. The present paper is an overview on the research suggesting a role for NO in the central modulation of hormone release in the hypothalamo-neurohypophysial and the caudal neurosecretory systems of teleosts during the osmotic stress response. Active NOS enzymes are constitutively expressed by the magnocellular and parvocellular hypophysiotropic neurons and the caudal neurosecretory neurons of teleosts. Moreover, their expression may be regulated in response to the osmotic challenge. Available data suggests that the regulatory role of NO appeared early during vertebrate phylogeny and the neuroendocrine modulation by NO is conservative. Nonetheless, NO seems to have opposite effects in fish compared to mammals. Indeed, NO exerts excitatory effects on the electrical activity of the caudal neurosecretory neurons, influencing the amount of peptides released from the urophysis, while it inhibits hormone release from the magnocellular neurons in mammals.

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.

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.

Precise and fuzzy coding by olfactory sensory neurons

The exact nature of the olfactory signals that arrive in the brain from the periphery, and their reproducibility, remain essentially unknown. In most organisms, the sheer number of olfactory sensory neurons (OSNs) makes it impossible to measure the individual responses of the entire population. We measured the individual in situ electrophysiological activity of OSNs in Drosophila larvae, in response to stimulation with 10 aliphatic odors (alcohols and esters). We studied control larvae (a total of 296 OSNs) and larvae with a single functional OSN, created using the Gal4-upstream activator sequence system. Most OSNs showed consistent, precise responses (either excitation or inhibition) in response to a given odor. Some OSNs also showed qualitatively variable responses ("fuzzy coding"). This robust variability was an intrinsic property of these neurons: it was not attributable to odor type, concentration, stimulus duration, genotype, or interindividual differences, and was seen in control larvae and in larvae with one and two functional OSNs. We conclude that in Drosophila larvae the peripheral code combines precise coding with fuzzy, stochastic responses in which neurons show qualitative variability in their responses to a given odor. We hypothesize that fuzzy coding occurs in other organisms, is translated into differing degrees of activation of the glomeruli, and forms a key component of response variability in the first stages of olfactory processing.

Comparative distribution of the mRNAs encoding urotensin I and urotensin II in zebrafish

The neural neurosecretory system of fishes produces two biologically active neuropeptides, i.e. the corticotropin-releasing hormone paralog urotensin I (UI) and the somatostatin-related peptide urotensin II (UII). In zebrafish, we have recently characterized two UII variants termed UIIalpha and UIIbeta. In the present study, we have investigated the distribution of UI, UIIalpha and UIIbeta mRNAs in different organs by quantitative RT-PCR analysis and the cellular localization of the three mRNAs in the spinal cord by in situ hybridization (ISH) histochemistry. The data show that the UI gene is mainly expressed in the caudal portion of the spinal cord and, to a lesser extent, in the brain, while the UIIalpha and the UIIbeta genes are exclusively expressed throughout the spinal cord. Single-ISH labeling revealed that UI, UIIalpha and UIIbeta mRNAs occur in large cells, called Dahlgren cells, located in the ventral part of the caudal spinal cord. Double-ISH staining showed that UI, UIIalpha and UIIbeta mRNAs occur mainly in distinct cells, even though a few cells were found to co-express the UI and UII genes. The differential expression of UI, UIIalpha and UIIbeta genes may contribute to the adaptation of Dahlgren cell activity during development and/or in various physiological conditions.

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

The caudal neurosecretory system (CNSS) of the euryhaline flounder Platichthys flesus has suggested roles in osmoregulatory, reproductive and nutritional adaptation, as fish migrate between seawater (winter) and brackish/freshwater (summer) environments. This study examined seasonal changes in mRNA expression profile of functionally important genes in the CNSS. cDNAs encoding neuropeptides, receptors and ion channels were cloned by reverse transcriptase polymerase chain reaction (RT-PCR) and screening of a flounder CNSS cDNA library. The expression profile of cloned genes was determined by real-time RT-PCR at 2-month intervals throughout the year in CNSS from seawater-adapted fish. Plasma cortisol (measured by radioimmunoassay) showed a peak in April, the time of spawning. Expression levels of mRNA for peptides urotensins I and II (UI, UII) and corticotropin releasing factor (CRF) all showed a seasonal cycle, with lowest expression in April and highest in August-October. The expression of CRF2(UI), UT(UII) and CRF1 receptors was not correlated with corresponding peptide expression. Receptors for potential neuromodulators of CNSS activity also displayed a seasonal mRNA expression profile. Glucocorticoid, 5-hydroxytryptamine, kappa-opioid and glutamate receptor expression peaked around April, suggesting that modulation of electrical activity of the neurosecretory Dahlgren cells is of particular importance at this time. Expression of mRNA for L-type Ca(2+) and Ca-activated K(+) channels was lower during the summer months. These channels underlie electrical bursting activity in Dahlgren cells. Ion channel mRNA expression was also lower in CNSS from flounder fully adapted to freshwater as opposed to seawater, consistent with previously reported observations of reduced bursting activity in Dahlgren cells from freshwater-adapted CNSS. These findings support the hypothesis that the CNSS is functionally reprogrammed to cope with changes in physiological challenge as fish migrate between sea and estuaries in winter and spring.

Evidence for nitric oxide role in the caudal neurosecretory system of the European flounder, Platichthys flesus

A neuromodulatory role for nitric oxide has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined its potential as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the euryhaline flounder. Immunocytochemistry using an antibody raised against human neuronal nitric oxide synthase (NOS) indicated the presence of NOS in the Dahlgren cells. Quantitative RT-PCR, using a flounder-specific probe, revealed NOS mRNA expression in the CNSS. In July, though not in September, NOS mRNA expression was significantly higher in fish fully adapted to seawater, compared to freshwater-adapted fish. Following acute transfer of fish from freshwater to seawater, NOS mRNA expression was elevated at 8h and then recovered by 24h. In pharmacological experiments in vitro, application of NO donors (SNAP, SNP) caused an increase in electrical activity (firing frequency) of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The NOS substrate, l-arginine, led to increased firing frequency, cell recruitment and enhanced bursting activity. However, this effect was not blocked by the NOS inhibitor L-NAME. These findings suggest that NO acts as a modulator within the CNSS, potentially enhancing electrical activity and hence secretory output. A role in supporting adaptation to hyperosmotic conditions is also indicated.

Fish caudal neurosecretory system: a model for the study of neuroendocrine secretion

The caudal neurosecretory system (CNSS) is unique to fish and has suggested homeostatic roles in osmoregulation and reproduction. Magnocellular neuroendocrine Dahlgren cells, located in the terminal segments of the spinal cord, project to a neurohaemal organ, the urophysis, from which neuropeptides are released. In the euryhaline flounder Platichthys flesus Dahlgren cells synthesise at least four peptides, including urotensins I and II and CRF. These peptides are differentially expressed with co-localisation of up to three in a single cell. Dahlgren cells display a range of electrical firing patterns, including characteristic bursting activity, which is dependent on L-type Ca(2+) and Ca-activated K(+)channels. Activity is modulated by a range of extrinsic and intrinsic neuromodulators. This includes autoregulation by the secreted peptides themselves, leading to enhanced bursting. Electrophysiological and mRNA expression studies have examined changes in response to altered physiological demands. Bursting activity is more robust and more Dahlgren cells are recruited in seawater compared to freshwater adapted fish and this is mirrored by a reduction in mRNA expression for L-type Ca(2+) and Ca-activated K(+) channels. Acute seawater/freshwater transfer experiments support a role for UII in adaptation to hyperosmotic conditions. Responses to stress suggest a shared role for CRF and UI, released from the CNSS. We hypothesise that the Dahlgren cell population is reprogrammed, both in anticipation of and in response to changed physiological demands, and this is seen as changes in gene expression profile and electrical activity. The CNSS shows striking parallels with the hypothalamic-neurohypophysial system, providing a highly accessible system for studies of neuroendocrine mechanisms. Furthermore, the presence of homologues of urotensins throughout the vertebrates has sparked new interest in these peptides and their functional evolution.

Molecular characterization and expression of urotensin II and its receptor in the flounder (Platichthys flesus): a hormone system supporting body fluid homeostasis in euryhaline fish

Urotensin II (UII) is a potent vasoconstrictor in mammals, but the source of circulating UII remains unclear. Investigations of the caudal neurosecretory system (CNSS), considered the major source of UII in fish, alongside target tissue expression of UII receptor (UT), can provide valuable insights into this highly conserved regulatory system. We report UII gene characterization, expression of the first fish UT, and responses to salinity challenge in flounder. The 12-aa UII peptide shares 73% sequence identity with pig and human UII. Flounder UT receptor shares 56.7% identity with rat. Although the CNSS is the major site of UII expression, RT-PCR revealed expression of UII and UT in all tissues tested. Around 30-40% of large CNSS Dahlgren cells expressed UII, alone or in combination with urotensin I and/or corticotrophin releasing hormone. Immunolocalization of UT in osmoregulatory tissues (gill, kidney) was associated with vascular elements. There were no consistent differences in CNSS UII expression or plasma UII between seawater (SW)- and freshwater (FW)-adapted fish, although gill and kidney UT expression was lower in FW animals. After acute transfer from SW to FW, plasma UII and kidney and gill UT expression were reduced, whereas UT expression in kidney was increased after reverse transfer. UII appears to be more important to combat dehydration and salt-loading in SW than the hemodilution faced in FW. Potentially, altered target tissue sensitivity through changes in UT expression, is an important physiological controlling mechanism, not only relevant for migratory fish but also likely conserved in mammals.