Caudal Neurosecretory Cells Research Articles

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.

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.

Ontogenetic Development of the Caudal Neurosecretory System in the Chum Salmon, Oncorhynchus keta, with Regard to Its Ultrastructural Changes and with Relation to Neuropeptide Y-immunoreactive Fibers

The ontogeny of the caudal neurosecretory cells (Dahlgren cells) in the caudal spinal cord of the chum salmon, Oncorhynchus keta, was examined by conventional electron microscopy and with immunohistochemistry for urotensins (U) and neuropeptide Y (NPY). The precursors of the Dahlgren cells first appeared as agranular ovoid cells in the caudal region of the neural tube of 40-day-old embryos about one week before hatching. The occurrence of cytoplasmic granules in the immature Dahlgren cells became evident by the 14th day after hatching. At this moment, the U-positive reaction was merely demonstrated in some of the granules. Close association of NPY-positive fibers with the caudal neurosecretory structures was recognizable in 1-month-old larvae. Thus, it is apparent that the salmon Dahlgren cells start their secretory activity (production of the secretory granules) in early larval stages and that, thereafter, NPYergic afferent innervation of the caudal neurosecretory system becomes evident.

The caudal spinal cord of coho salmon (Oncorhynchus kisutch): Immunocytochemical evidence of a “caudal serotoninergic system”

The caudal spinal cord of the coho salmon was investigated by means of immunocytochemistry using antisera against serotonin, urotensin I, urotensin II, somatostatin and a urea-extract of bovine Reissner's fiber (AFRU). Populations of serotonin-immunoreactive (IR) neurons were found rostral and dorsal to the urophysis in close spatial association with caudal secretory neurons. Thick, smooth serotonin-IR processes extended toward the external surface of the spinal cord where they displayed conspicuous terminal dilatations. Thin, beaded serotonin-IR fibers appeared to innervate populations of caudal secretory and somatostatin-IR cerebrospinal fluid-contacting neurons. Most caudal neurosecretory cells displayed both urotensin I and urotensin II immunoreactivities; only a minority reacted exclusively with either urotensin I or urotensin II antisera. Urotensin II-IR and somatostatin-IR cerebrospinal fluid (CSF)-contacting neurons were found as an integral component of the central canal wall in the caudal spinal cord and filum terminale; their dendritic processes appeared to contact Reissner's fiber, which displayed a weak AFRU-immunoreactivity while inside the central canal, but became strongly reactive in the interior of the terminal ventricle as it formed the massa caudalis. The distribution of serotoninergic processes points to a regulatory role in the function of caudal secretory and CSF-contacting neurons and to a putative serotonin release into the subarachnoid space and/or meningeal vasculature. It is also suggested that the CSF-contacting neurons of the central canal may participate in a feedback mechanism controlling the secretory activity of the subcommissural organ.

Differential effects of low pH and aluminium on the caudal neurosecretory system of the brook trout, Salvelinus fontinalis

Brook trout were subjected to soft water at pH 6·5, 5·5 or 5·0 without aluminium added, or to water at pH 5·5 with 200,300 or 500 μg Al I‐1 added. The response of the caudal neurosecretory system to low pH or aluminium was evaluated after one week by measuring the urotensin I and urotensin II concentrations in the urophysis by radioimmunoassay, and by morphometric analysis of the caudal neurosecretory cells. A positive correlation was found between urotensin I concentrations and acidity, and a negative correlation was found between urotensin II concentrations and total aluminium in the water. Morphometric indices (cell size and proportion of lobed nuclei in the caudal neurosecretory cells) suggested increased synthetic activity in the caudal neurosecretory cells of fish at pH 5·5 compared to pH 6·5.

A double sequential immunofluorescence method demonstrating the co-localization of urotensins I and II in the caudal neurosecretory system of the teleost, Gillichthys mirabilis.

A double immunofluorescence method was devised to localize simultaneously urotensin-I (UI) and -II (UII) immunoreactivities in the caudal neurosecretory system of the goby, Gillichthys mirabilis. In a sequential fashion, sections of the posterior spinal cord and urophysis were treated with antiserum to corticotropin-releasing factor (CRF) that cross-reacts with UI, fluorescein-conjugated sheep anti-rabbit IgG, biotinylated anti-UII and rhodamine-conjugated avidin. UI and UII immunoreactivities appeared to coexist in some neurons and in most fibers and urophysial tissue; the remainder of the fibers and urophysis and the majority of neurons were immunoreactive for CRF/UI only. No convincing evidence of immunoreactivity for UII only was found. A few nonreactive cells were seen, but these may not be neurosecretory neurons. The two immunoreactive cell types were not segregated topographically, and the intensity of perikaryal immunofluorescence for CRF/UI was variable. To explain these results a hypothesis that all caudal neurosecretory cells may synthesize both UI and UII and that immunoreactive differences may reflect different states of cellular activity, is suggested. This sequential double immunofluorescence method offers several advantages over other techniques and is especially useful for co-localization studies when primary antisera from different species are not available.

Peptidergic innervation of caudal neurosecretory neurons

Luteinizing hormone releasing hormone (LHRH)-like immunoreactivity was observed in neurosecretory neurons at mesencephalic levels of the poecilid brain. This nucleus of peptide producing neurons has been shown to project to the spinal cord neurosecretory complex in this species. LHRH-like immunoreactivity was localized in fibers and terminals surrounding caudal neurosecretory cells. This investigation suggests that there is a mesencephalic descending LHRH innervation of caudal neurosecretory neurons.

Coexistence of urotensin I/corticotropin-releasing factor and urotensin II immunoreactivities in cells of the caudal neurosecretory system of a teleost and an elasmobranch fish

By use of the indirect immunofluorescence method in conjunction with an elution-restaining technique, cells in the caudal neurosecretory system of the big skate ( Raja binoculata) and the plainfin midshipman ( Porichthys notatus) were shown to contain both urotensin I (UI)/corticotropin-releasing factor (CRF) and urotensin II (UII) immunoreactivities. In the skate coexistence was complete, while in the midshipman some cells demonstrated immunoreactivity for only one or the other neuropeptide. The present results do not support previous suggestions that, like vasopressin and oxytocin in the hypothalamic magnocellular system, UI and UII exist in separate cellular populations in the caudal neurosecretory system. Results of these and other experiments indicate that there exists a wide range of species variation in the degree of coexistence of UI/CRF and UII immunoreactivities within caudal neurosecretory cells.

Response of caudal neurosecretory cells of Salvelinus fontinalis to variations in the ionic composition of the environment

The response of caudal neurosecretory cells in Salvelinus fontinalis to exposure to media in which one ion (Na, K, Mg, Ca and Cl) was either selectively enriched or depleted was evaluated by morphometric criteria. Morphological changes indicating stimulation of synthetic activity were observed: exposure to sea water with low potassium concentration was the most efficacious in inducing the increase in the average diameter of caudal cells and the number of nucleoli for both cells with lobed and with nonlobed nuclei. The proportion of cells with lobed nuclei was also increased. To a lesser degree and in decreasing order, experimental milieu with modified magnesium contents, either fresh water enriched in this ion or seawater depleted of the same ion, and fresh water enriched in calcium also resulted in significant increases in cell diameter and the number of nucleoli.

Régulations aminergique et cholinergique du système caudal neurosécréteur de l'omble de fontaine, Salvelinus fontinalis, en relation avec l'osmo-iono-regulation

The innervation of the caudal neurosecretory system of the brook trout, Salvelinus fontinalis, was studied under light and electron microscopy in order to characterize its nature, distribution, and regulatory function over the activity of the caudal neurosecretory cells. A dual innervation of the cell bodies and axons of neurosecretory cells was disclosed. One type of axosomatic connection exhibited small lucent vesicles and large dense-cored granules. These boutons were identified as monoaminergic since they appeared depleted after reserpine treatment and they were selectively labeled with 5-OH-dopamine. In fish exposed to demineralized water, reserpine induced a condition that stimulated the synthetic activity of caudal neurosecretory cells, a clear reduction of this activity, according to morphometric (cell and nucleus diameters) and ultrastructural criteria (dimensions of the Golgi complex). By comparison, no significant variation of the synthetic activity was noted in freshwater-adapted trout treated with reserpine. A second type of innervation was also identified as cholinergic by histochemical localization of acetylcholinesterase. Electron microscopic analysis also revealed axosomatic and axoaxonic cholinergic synaptic connections with characteristic small 500-Å diameter lucent vesicles. The injection of fenitrothion, an anticholinesterase agent, enhanced discharge of neurosecretory material from axonal endings of caudal cells while the synthetic activity did not appear to be modified. Our findings suggest an important role of aminergic and cholinergic controls over the response of the caudal neurosecretory system of Salvelinus fontinalis during hyperosmotic adaptation.

Maturation of the goldfish preoptic area: Ultrastructural correlates of position and maturation of neurosecretory cells

A gradation of small rostral to larger caudal neurosecretory cells is found in the preoptic area of fish. Regional variations in ultrastructural features were assessed in this region of fish ranging widely in size (1.7–283.1 g), some of which had received intraperitoneal injections of horseradish peroxidase in order to label neurons projecting beyond the blood-brain barrier (i.e. neurosecretory cells). In small (≤6.6. g) fish, neurosecretory cells of the caudal preoptic area were 10–15 μm in diameter and contained dense core vesicles. More rostral smaller cells had few or no dense core vesicles, but were labeled following intraperitoneal horseradish peroxidase injections. Cells in both areas were progressively larger, with larger nuclei and more granular reticulum in larger fish. In large fish, neurons that contained dense core vesicles ranged from 10 μm in diameter rostrally to 70 μm in diameter caudally. Some intermediate and large cells were extensively vacuolated and contained extremely convoluted and possibly multiple nuclei. Degenerating material was seen in apparently normal large fish. A few neurosecretory cells were extensively surrounded by perineuronal electron-dense glial cells. Most neurosecretory cells were involved in extensive soma-somatic apposition, which was especially pronounced in fish which were removed from cold water in late fall. Gap junctions were also clearly present on the somata of preoptic neurosecretory cells of these fall fish. These data imply that increased capacity for hormone secretion in large fish may be accomplished in part by neuronal hypertrophy. Maturational and seasonal variation in structure suggest that physiological characteristics are also variable in this nucleus.

Monoaminergic innervation of the caudal neurosecretory system of the brook trout Salvelinus fontinalis in relation to osmotic stimulation

The distribution of monoamine fluorescence reaction (Falck and Hillarp technique) was investigated in the posterior spinal cord of the brook trout, Salvelinus fontinalis, in relation with the caudal neurosecretory system. A semiquantitative analysis of fluorescence was made following variations of external salinity, injections of salt solutions, and drugs aimed at modifying hydromineral balance. Under all experimental conditions, an extensive network of beaded fibers innervate a number of caudal neurosecretory cells whereas some others do not appear to be contacted by fluorescent varicosities. Three types of aminergic cells different from nonfluorescent caudal neurosecretory neurons were observed in the posterior spinal cord: (i) green fluorescent CSF-contacting neurons interspersed between ordinary ependymal cells; (ii) small perikarya with discrete spots of intense yellow fluorescence; (iii) small somata with diffuse green fluorescence. No fluorescence was observed in neurohemal region or urophysis. The action of nialamide and reserpine indicated that histofluorescent reaction was due to monoamines. In comparison to freshwater exposure to demineralized water for 3 days, but not 7 days, increased markedly the fluorescence of beaded fibers and varicosities contiguous to caudal neurosecretory cells while transfer to seawater for 1 and 7 days reduced them significantly. Injections of isotonic NaCl, hypertonic Na 2SO 4, and hypertonic choline chloride in freshwater fish, were less effective in producing fluorescence changes; however, these treatments showed a common tendency to enhance frequency of caudal neurosecretory cells with abutting varicosities in the most anterior part of the population. Acetazolamide and aldactone treatments also induced significant variations of aminergic innervations. These findings indicate a monoaminergic participation in the control of the function of caudal neurosecretory cells in relation to iono- and osmoregulation.

In vivo incorporation of [3H]leucine and [3H]tyrosine by caudal neurosecretory cells of the trout Salvelinus fontinalis in relation to osmotic manipulations. A radioautographic study.

The in vivo incorporation of tritiated leucine and tyrosine during various time intervals by caudal neurosecretory cells of trout Salvelinus fontinalis was studied quantitatively with light microscopic radioautography in relation to osmotic manipulations. The [ 3 H]leucine labeling in fish exposed to deionized water for 3 days was much higher than in fish kept in fresh water, indicating enhanced synthetic activity. Under both conditions, the maximum of incorporation was observed 12 hr after injection of the tracer. The reduced radioautographic reaction noted at 24 hr suggested a rapid transit of newly synthesized proteinaceous material from cell body cytoplasm toward the axonal compartment. [ 3 H]Tyrosine was incorporated to a greater degree than [ 3 H]leucine in cells of fish in fresh water. The varied synthetic activities exhibited by caudal neurosecretory cells in response to osmotic stimuli indicate possible participation by these cells in osmotic and ionic regulation.

Ultrastructural changes in the caudal neurosecretory cells of the trout Salvelinus fontinalis in relation to external salinity

Ultrastructural changes in the caudal neurosecretory cells of the teleost Salvelinus fontinalis were studied when fishes were exposed to fresh water, deionized water, and seawater. In freshwater-adapted fishes, most cells presented a pattern of reduced synthetic activity of proteinaceous secretory material. The rough endoplasmic reticulum and the Golgi complex were moderately elaborated whereas secretory material was rare in the cell bodies. The nucleus appeared slightly irregular, with masses of heterochromatin. Exposure to deionized water for 3 days led to doubling of the mean size of the cell perikarya. The formation of neurosecretory material was strikingly stimulated. The rough endoplasmic reticulum was highly developed, and the Golgi complex showed intense vesiculation of elementary secretory granules which were abundant in the cells. Numerous and large globules of secretory material were observed in the cytoplasm. Also, the enlarged nucleus was lobulated and contained homogeneous euchromatin. After a 7- or 10-day exposure, no sign of activation of cell secretory activity was found. Transfer from fresh water to 25% seawater for 1 day enhanced synthetic activity in certain caudal neurosecretory cells but the majority remained inactive. Progressive exposure to 100% seawater was accompanied by a decreased level of activity in all cells. These findings appear to provide further evidence for the involvement of the caudal neurosecretory system in osmo- (iono)-regulation in the fish.

Aminergic Innervation of the Cranial and Caudal Neurosecretory Systems in the Teleost Gillichthys mirabilis

Abstract Numerous fluorescent varicosities surround most of the caudal neurosecretory neurons and also regularly occur among pars intermedia cells of the adenohypophysis in the teleost, Gillichthys mirabilis. The color of the varicosities, as well as their responses to pharmacological treatments, is diagnostic of catecholaminergic neurons and processes. No fluorescence characteristic of monamines is found in the rostral pars distalis, in the proximal pars distalis or in the cells of the nucleus lateralis tuberis (NLT), although fluorescent varicosities are found within the ventral hypothalamus in the vicinity of the NLT. Bilateral clusters of fluorescent cell bodies are located in the ventral hypothalamus (posterior to the NLT); some of these cells border the neurohypophysis. Fluorescent tracts from these cell clusters extend to a pair of elongate nuclei of nonfluorescent neurons which are surrounded by fluorescent varicosities. Alteration of osmotic conditions did not effect the fluorescence, except for the caudal neurosecretory cells of fish exposed to fresh water for long periods. Adrenergic nervous input thus seems to be an important component of both the cranial and caudal neurosecretory systems.

The karyometric response of caudal neurosecretory cells in Clarias batrachus to sex steroids

The effect of ovariectomy and administration of sex steroids has been studied in the teleost Clarias batrachus for its secretory activity in the caudal neurosecretory cells. Spaying caused an enlargement of the caudal neurosecretory cells. Estrogen-treated ovariectomized Clarias show granulation of their cytoplasm. The results are compared with those of nucleus lateralis tuberis.

The caudal neurosecretory system of the isospondylous teleost, Albula vulpes, from different habitats

The caudal neurosecretory system of the isospondylous teleost, Albula vulpes , was studied by light and electron microscopy. The fish were taken in Kaneohe Bay, Hawaii, from two habitats: oceanic ponds with wide variations in salinity, and the open sea. The caudal system of Albula shows some primitive characteristics, including extensive extraurophysial neurohemal regions resembling those in elasmobranchs, and some advanced characteristics including a well-developed lobate urophysis. Two types of caudal neurosecretory cells were distinguished, based on staining reactions and on the presence of elementary neurosecretory granules of two different sizes in axon terminals. The elementary granules seem to be formed not only in the perikaryon by the Golgi complex but also by transformation of Golgi vesicles into granules. In the neurosecretory terminals, vesicles and granules may originate locally from a tubular system. Depletion of neurosecretory material from the urophysis appears to involve the loss of electron-dense material from the granules and their transformation into vesicles. Some neurosecretory neurons send a process into the central canal of the spinal cord, as well as to the urophysis. Fish from the two habitats could be distinguished on the following bases: (1) The amount of secretory material in the neurohemal areas is much greater in the pond fish, as indicated by the opalescence of the freshly-dissected urophysis, the elaborately-developed stainable areas in the urophysis, and the presence in terminals of many more electron-dense granules. (2) A much larger proportion of caudal neurosecretory cells in the pond fish show prominent intranuclear cytoplasmic invaginations, which appear to be active centers for elementary granule production. In general, the observations are interpreted in support of an osmo(iono)-regulatory role for the urophysial system.

SECRETION MASSES IN THE NUCLEI OF THE CAUDAL NEUROSECRETORY CELLS OF THE TELEOST ALBULA VULPES.

‘NUCLEAR secretion’ has been reported by a number of students of neurosecretory cells, especially in fish hypothalamic neurones1–7. Neurones exhibiting similar characteristics have been described in the caudal neurosecretory system of two isospondylous teleosts: Albula vulpes and Elops hawaiensis8,9. The caudal neurones of many teleosts may possess various kinds of nuclear inclusions, generally less prominent than those of Albula and Elops, and not necessarily considered as ‘secretion’10–12. Inasmuch as the actual production of proteinaceous secretory product within the nucleus is not consonant with the present conception of the protein-secretory cell, we undertook a re-examination of the caudal neurosecretory neurones in Albula vulpes, the Hawaiian ‘oio’ or bonefish, with the electron microscope.

Electrical activity of the caudal neurosecretory cells in the eel Anguilla japonica with special reference to synaptic transmission

The caudal neurosecretory cells of the eel, Anguilla japonica, synapse with numerous nerve fibers from the more anterior spinal cord. After stimulation of the spinal cord anterior to the location of the caudal neurosecretory cells, a postsynaptic potential can be recorded intracellularly from the cell bodies of the neurosecretory cells. Spatial or temporal summation of the postsynaptic potential is necessary to initiate the spike potentials in the neurosecretory cells. The caudal neurosecretory cells also respond with spike potentials to direct stimuli applied through intracellular microelectrodes. The characteristics of the intracellular action potential recorded from the caudal neurosecretory cell are similar to those obtained from the spinal ganglion cell of the frog. Antidromic stimulation of the axons, that is, the stimulation of the urohypophysis, also elicits spike potentials in the cell body, thus indicating that the axon functioning as a neurosecretory pathway has the ability of impulse conduction.

Electrophysiology of caudal neurosecretory cells in the skate and fluke

The caudal neurosecretory cells of two species of skate ( Raja erinacea and R. ocellata) and of the fluke ( Paralichthys dentatus) were studied by intracellular microelectrode techniques. The cells were capable of producing spikes and of being synaptically activated. The neurosecretory cells of the skate were intermingled with other neurons within the spinal cord and could not be penetrated under visual control. however, a class of responses was recorded that were readily distinguishable from those of motoneurons and myelinated fibers and which very likely came from the much larger neurosecretory cells. The spikes were longer in duration and had pronounced undershoots. Nearby stimulation on the surface of the cord activated the cell bodies directly rather than evoking antidromic spikes. Brief stimuli, intra- or extracellular, could produce direct spikes of very long latency, thus indicating a low degree of accommodation. The presynaptic pathway was slowly conducting and of high threshold. The neurosecretory cells of the fluke were anatomically isolated from other cell bodies, but recordings from axons and somata had to be distinguished electrophysiologically. Recordings from cell bodies were characterized by the occurrence of postsynaptic potentials, which could be adequate to initiate spikes. The spikes were rather long in duration and usually followed by an undershoot. Those in response to antidromic stimulation had an inflection on the rising phase caused by delay of propagation at the axon hillock. Invasion of the soma could be blocked by hyperpolarization or refractoriness, leaving the axonal component of the spike. The threshold in the initial part of the axon was not significantly lower than in the soma. There was little accommodation, as indicated by the long latencies of responses to brief threshold stimuli. The antidromic responses in a cell often had several distinct latencies when stimulation was over the urohypophysis. The properties of the longer latency responses suggested that they were initiated in the terminals of the axons and that the delay resulted from very slow propagation in these structures. Antidromic conduction could be blocked in the axon at points some distance from the soma as well as at the axon hillock. Spikes recorded from the neurosecretory axons were similar in duration to those from the cell bodies. They were characterized by the occurrence either of synaptically evoked spikes without recording of p.s.p.'s adequate to iniate them or of direct responses to stimulation on each side of the recording site. Hyperpolarization could cause block of propagation along the axon which was associated with block of a later component of the recorded spike. The presynaptic pathway ran anteriorly to the neurosecretory cells. Its conduction velocity suggested that it consisted of small myelinated fibers. The individual presynaptic fibers appeared to innervate both rostral and caudal cells. Intravenous injections of hypotonic solutions could cause sufficient activity in the presynaptic fibers to initiate impulses in the neurosecretory cells.