Caudal Neurosecretory Neurons Research Articles

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.

The caudal neurosecretory system and its afferent synapses in the goldfish, Carassius auratus: Morphology, immunohistochemistry, and fine structure

Morphological features of the goldfish caudal neurosecretory system were investigated by means of immunohistochemical localization of urotensins I and II (UI and UII) and electron microscopic examination of the caudal neurosecretory neurons, the urophysis, and the synaptic neuropil. The aim of the work is to provide a detailed morphological description of the afferent synapses to the caudal neurons and to analyze their distribution through the rostrocaudal extension of the caudal neurosecretory system. Three morphologically different types of neurosecretory cells have been identified according to size and shape: large, medium, and small Dahlgren cells. The three different-sized cells share similar patterns of immunoreactivity with the UI (or oCRF) and the UII antisera. Electron microscopic examination of the synaptic neuropil throughout the caudal system revealed the presence of four types of terminals: dense-cored-vesicle end bulbs (DC), spherical-vesicle end bulbs (S), flattened-vesicle end bulbs (F), and granular-vesicle end bulbs (G). The present study demonstrates that the small Dahlgren cells receive different synaptic inputs from the large and the medium neurosecretory cells. Indeed, G terminals are only found on the small Dahlgren cells, whereas DC, S, and F terminals are distributed on the large, medium, and small Dahlgren cell bodies and proximal processes. J. Morphol. 235:59–76, 1998. © 1998 Wiley-Liss, Inc.

The caudal neurosecretory system and its afferent synapses in the goldfish, Carassius auratus: morphology, immunohistochemistry, and fine structure.

Morphological features of the goldfish caudal neurosecretory system were investigated by means of immunohistochemical localization of urotensins I and II (UI and UII) and electron microscopic examination of the caudal neurosecretory neurons, the urophysis, and the synaptic neuropil. The aim of the work is to provide a detailed morphological description of the afferent synapses to the caudal neurons and to analyze their distribution through the rostrocaudal extension of the caudal neurosecretory system. Three morphologically different types of neurosecretory cells have been identified according to size and shape: large, medium, and small Dahlgren cells. The three different-sized cells share similar patterns of immunoreactivity with the UI (or oCRF) and the UII antisera. Electron microscopic examination of the synaptic neuropil throughout the caudal system revealed the presence of four types of terminals: dense-cored-vesicle end bulbs (DC), spherical-vesicle end bulbs (S), flattened-vesicle end bulbs (F), and granular-vesicle end bulbs (G). The present study demonstrates that the small Dahlgren cells receive different synaptic inputs from the large and the medium neurosecretory cells. Indeed, G terminals are only found on the small Dahlgren cells, whereas DC, S, and F terminals are distributed on the large, medium, and small Dahlgren cell bodies and proximal processes.

Structures Immunoreactive with Porcine NPY in the Caudal Neurosecretory System of Several Fishes and Cyclostomes

Abstract Using a double immunostaining technique, we examined the relationships between neuropeptide Y (NPY)- and urotensin I (UI)- and/or urotensin II (UII)-positive structures in the caudal spinal cord of various species of fishes, particularly in the teleost Oncorhynchus masou and in elasmobranchs Scyliorhinus torazame, Triakis scyllium, and Raja kenojei. Primitive actinopterygians, i.e., Acipenser transmontanus, Polypterus senegalus, and Lepisosteus productus, and cyclostomes Lethenteron japonica and Eptatretus atami were also studied from the viewpoint of comparative endocrinology. In the teleost and elasmobranchs, NPY-positive fibers were demonstrated to be often in contact with UI- and/or UII-positive cells and fibers, suggesting a control of the caudal neurosecretory neurons by NPY or a related substance. Similar association was seen in the primitive actinopterygians. As to the cyclostomes, immunoreactivities for NPY and UII were found in the lamprey, but none of the immunoreactivity for these pep...

In situ hybridization demonstrating coexpression of urotensins I, II-α, and II-γ in the caudal neurosecretory neurons of the carp, Cyprinus carpio

In order to examine the coexpression of different urotensins in the same caudal neurosecretory neurons, in situ hybridization with synthetic oligonucleotide probes specific for mRNAs for urotensins I, II-α, and II-γ (UI, UII-α, and UII-γ) was applied to the caudal neurosecretory system of the carp, Cyprinus carpio, together with immunohistochemistry. Almost all identifiable caudal neurosecretory neurons were labeled with the oligonucleotide and cDNA probes for UI mRNA. Further, any two of the probes for UI, UII-α, and UII-γ mRNAs labeled the same neurons in serial sections. These results suggest that essentially all caudal neurosecretory neurons of the carp coexpress UI, UII-α, and UII-γ. Although most neurons were densely labeled with oligonucleotide probes, only a small portion of the neurons were intensely immunoreactive. This suggests that most caudal neurosecretory neurons actively synthesize all three hormones, but that in some neurons, all or some of the hormones are rapidly transported to the urophysis, resulting in low or undetectable immunoreactivity in the perikarya.

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.

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.

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.

Caudal Neurosecretory System: A Physiologist's View

Several active principles (urotensins) have been isolated from teleost caudal neurosecretory systems. The original classification of these substances has been recently modified: There are at least two mammalian hypotensive factors (with no effects yet known in fish), possibly two smooth muscle contractants, arginine vasotocin, and factors affecting Na+ movement. A factor affecting lymph heart frequency has not yet been compared with other peptides. Evidence for the hormonal nature of some of these substances includes subcellular localization in granules and calcium-dependent release by depolarization. The physiology of the system is still not clear. The best evidence for an osmoregulatory role includes effects of extracts on renal diuresis and Na+ movement and changes in firing rates of caudal neurosecretory neurons by altered ionic composition of the blood. Most other evidence is subject to alternative explanation. Several observations suggest a role for this system in reproduction.

Regeneration of the caudal neurosecretory system in the cichlid teleost Tilapia mossambica

AbstractAfter total extirpation of the caudal neurosecretory system in the teleost Tilapia mossambica by ligature and removal of the caudal peduncle, a new system regenerates rapidly. The new neurosecretory cells differentiate from ependyma. A series of transitional stages between slightly modified ependymal cells and fully developed caudal neurosecretory neurons can be readily constructed. Immediately after the operation, the distal part of the remaining spinal cord undergoes degeneration, during which no traces of the caudal system can be found. Eleven days after the operation the first signs of the new system appear, and after three weeks a potentially functional, although much reduced, system has developed, the elements of which are detectable with the electron microscope. After 5–6 months, an extensive system has regenerated; however, the neurohemal area equivalent to the normal urophysis is contained within the spinal cord. There is strong cytologic evidence for a considerable release of elementary neurosecretory granules into the cerebrospinal fluid in the regenerating and regenerated system, possibly by several different mechanisms. Prominent tubular reticula occur in the distal (preterminal) parts of the neurosecretory axons, and the possibility of local axonal production of vesicles and granules is strongly supported. Signs of secretion into the central canal and of distal production of neurosecretory material are also present in normal Tilapia, but both phenomena are much more strikingly demonstrated in regenerating systems.

Secretion into the cerebrospinal fluid by caudal neurosecretory neurons.

Neurons of the regenerating caudal neurosecretory system of Tilapia mossambica and of the normal system of Albula vulpes send processes into the central canal of the spinal cord. Evidence for appreciable release of secretion into the cerebrospinal fluid from these processes has been obtainied with both light and electron microscopes.

Electrophysiologic analysis of the response of the caudal neurosecretory system of Tilapia mossambica to osmotic manipulations

Microelectrode recordings (extracellular) from caudal neurosecretory units have been utilized to determine the reactivity of the caudal (urophysial) system to various osmotic and ionic changes. Detection of the neurosecretory units for recording of electrical activity was based on their long-duration action potentials. Results of intravenous injection of hypotonic (Na +-free) and hypertonic (NaCl, NaNO 3, Na 2SO 4, sucrose, choline chloride, tetramethylammonium chloride) solutions indicate the presence of two kinds of differentially responsive neurosecretory units: those activated by high Na + concentration and those activated by Na +-free solutions. A small group of neurosecretory fibers and all non-neurosecretory fibers tested failed to respond to any osmotic manipulation. Our results indicate that it is not change in osmotic pressure per se which alters the electrical activity of caudal neurosecretory units, but rather changes in Na + concentration regardless of the accompanying anion. Transection of the spinal cord demonstrates that the caudal neurosecretory neurons are synaptically controlled by a center located probably in the brain. A feedback loop is proposed in an attempt to delineate the possible role of the caudal neurosecretory system in ionoregulation.

The caudal neurosecretory system of fishes

The comparative microanatomy, cytology, and ultrastructure of the caudal neurosecretory system of teleost fishes are surveyed, and some of the variations in morphologic features of the urohypophysis are indicated, based on the study of 125 teleost species. The neurohemal organ, the urohypophysis, was present in every teleost specimen studied. The general morphology of the teleost caudal system parallels the cranial (hypothalamo-neurohypophyseal) system in the tetrapods. The ultrastructural characteristics are also strictly comparable; the “Gomori-negative” neurosecretion of the caudal system is composed largely of elementary neurosecretory granules in the 1000 to 3000 A range. These granules appear to be organized in the perikaryon by the Golgi apparatus. No urohypophysis has been found in any nonteleost fish. The caudal neurosecretory neurons are present in elasmobranchs; here the neurochemal area is a diffuse capillary bed on the ventral surface of the spinal cord. Some primitive isospondylous teleosts, as well as some highly specialized forms, have a diffuse and extended urohypophyseal region. The possible evolution of this system is considered. Experimental studies on urohypophysectomized Tilapia mossambica exposed to hypertonic saline, with and without attempted replacement therapy, support an osmoregulatory role for the urohypophysis. Tilapia without their urohypophyses, subjected to a hypertonic environment, showed increased mortality, increased loss of body weight, increased serum chloride levels, and increased prominence of the preoptic nucleus, as compared with sham-operated or intact controls.