Richard S. Nishioka

Neurohormones from fish tails: the caudal neurosecretory system. I. "Urophysiology" and the caudal neurosecretory system of fishes.

Publisher Summary This chapter discusses the urophysiology and the caudal neurosecretory system of fishes, chemistry of urotensin II, immunocytochemistry of the caudal neurosecretory system, biological activities of urotensin II in teleosts, and effects of urotensin II in mammals. The caudal system in teleosts is heavily innervated, consistently by aminergic input and occasionally by cholinergic and peptidergic pathways from the brain. The complexity and intensity of the innervation shows some important physiological role for this system; yet, it has been difficult to ascribe a definite functional contribution to the caudal system and its hormones—the urotensins. The urophysis contains and presumably releases a variety of substances: (1) urotensin I—a 41-amino acid peptide which is homologous and analogous to mammalian corticotropin-releasing factors (CRFs); (2) urotensin II—a 12-amino acid peptide which is partially homologous and partially analogous to somatostatin-14; (3) urotensin IV—indistinguishable from arginine vasotocin and present in only a few teleost species; (4) urophysins—putative carrier proteins, cysteine-free, analogous to the neurophysins and with unknown biogenetic relationships to the urotensins; and (5) acetylcholine—the highest concentrations of this substance reported for any tissue but of unknown significance in this locale.

Control of prolactin secretion in teleosts, with special reference to Gillichthys mirabilis and Tilapia mossambica ☆

Two euryhaline teleosts, Gillichthys mirabilis (seawater) and Tilapia mossambica (fresh-water), were used to study the control of prolactin secretion. Cytological changes of prolactin cells, plasma sodium measurements, and densitometry of disk electrophoretograms of pituitary extracts and incubation media provided the data for the study. When seawater Gillichthys are transferred to a hypotonic environment, both morphological and physiological data indicate that prolactin secretion is activated dramatically, confirming the physiological role of prolactin in adaptation to hyposmotic conditions. Prolactin cells are cytologically activated by pituitary transplantation as well as by injections of reserpine and 6-hydroxydopamine with a significant elevation of plasma sodium concentration. In contrast, the cells were inactivated by the injection of l-DOPA. When seawater fish bearing an autografted pituitary were transferred to fresh water, a significant decrease in prolactin cell granules was observed. In addition, the injection of estradiol-17β caused cytological activation of both prolactin cells and ACTH cells. Pituitary glands of Tilapia were incubated with 3H-leucine for 5–6 hr, and the radioactive prolactin present in the pituitary gland and that released in the incubation medium were measured. Prolactin release is directly stimulated by low osmotic pressure of the incubation medium. Addition of dopamine caused a significant decrease in the amount of radioactive prolactin released into the hyposmotic medium. The results indicate that at least four mechanisms may be involved in the control of prolactin secretion in teleosts: (1) inhibitory control from the hypothalamus presumably mediated by aminergic fibers, (2) direct stimulation due to decreasing plasma osmotic pressure, (3) negative feedback by either prolactin itself or increased plasma sodium level, and (4) stimulation by circulating estrogen.

Stimulation of coho salmon growth by insulin-like growth factor I ☆

The effect of insulin-like growth factor I on growth rate of coho salmon (Oncorhynchus kisutch) was examined. Juvenile coho salmon received implants of osmotic minipumps containing recombinant bovine insulin-like growth factor I (rbIGF-I) or saline for a period of 3 to 4 weeks. High doses of rbIGF-I (greater than 0.13 microgram.g-1.d-1) resulted in hypoglycemia and death. In 2-year-old coho salmon, 0.09 microgram.g-1.d-1 rbIGF-I administered for 25 days doubled linear growth rate and increased growth rate in weight by 40%. In rapidly growing, 1-year-old coho salmon, growth rate was not altered by rbIGF-I at 0.01 or 0.05 micrograms.g-1.d-1 for 31 days. In ration-limited fish exhibiting slow growth in the control group, rbIGF-I (0.02 microgram.g-1.d-1) increased linear growth rate by up to threefold and growth rate in weight by up to fourfold. The results indicate that exogenous treatment with mammalian IGF-I can stimulate coho salmon growth under some conditions, and that endogenous IGF-I may be an important factor in regulating growth of teleosts.

Factors affecting in vitro activity of prolactin cells in the euryhaline teleost Sarotherodon mossambicus (Tilapia mossambica)

The rostral pars distalis (RPD) of the euryhaline teleost Sarotherodon mossambicus (Tilapia mossambica) was incubated in vitro to study the activity of the prolactin cells. Total prolactin release during 18 hr was measured by disc gel electrophoresis; release of newly synthesised prolactin was followed using a 3H-leucine tracer. Synthetic activity was estimated by calculating the specific activity of the prolactin bands. Prolactin release from the control tissues was always greater in hyposmotic than in hyperosmotic medium, indicating a direct effect of osmotic pressure upon the cells. Dopamine (1 μg/ml) inhibited release, but not synthesis, of prolactin in hyposmotic medium. Octopamine (5 μg/ml and 10 ng/ml) in hyposmotic medium and γ-amino-n-butyric acid (GABA) (100 ng/ml) in hyperosmotic medium had no effect on prolactin secretion. Cortisol (1 μg/ml) inhibited prolactin release in hyposmotic medium and, to a lesser extent, in hyperosmotic medium. Prolactin specific activity was increased only in the latter medium. Estradiol-17β (100 ng/ml) did not alter prolactin release in hyperosmotic medium, but did appear to enhance hormone synthesis. Thyrotropin-releasing hormone (TRH) (100 ng/ml) inhibited prolactin release, but not synthesis, in hyposmotic medium, and did not affect prolactin secretion in hyperosmotic medium. TRH at 100 pg/ml was ineffective in hyperosmotic medium. Somatostatin (300 ng/ml) inhibited prolactin synthesis and release in hyposmotic medium, but inhibited only release in hyperosmotic medium. These results indicate that there may be a complex regulation of the prolactin cells in this teleost. The in vitro responses of teleost prolactin cells to various potential mediators resemble those seen in other vertebrates; however, the differences in detail may have special adaptive significance for the teleost.

Response of “prolactin-secreting” cells of Tilapia mossambica to environmental salinity

Abstract The cytological changes of the eta (“prolactin”) cells in the pituitary of Tilapia mossambica due to changes in environmental salinity have been described at both the light- and electron-microscope levels. In fish maintained in fresh water, the eta cells in the well-developed rostral lobe are large and contain many large secretory granules. A prominent Golgi system in addition to a multilayered endoplasmic reticulum suggests a high level of secretory activity. On the other hand the rostral lobe is smaller in seawater fish and the eta cells appear condensed, the organelles as well as granule size are reduced suggesting a diminished synthetic activity. The possible role of the secretion of eta cells in the osmoregulation of fishes is discussed.

Factors determining the occurrence of the surge in thyroid activity in salmon during smoltification

Abstract Environmental cues: photoperiod, temperature, and the lunar cycle among others, acting through several endocrine systems including the thyroid, set the timing of smoltification. The thyroxine surge associated with smoltification is precisely timed to coincide with the new moon phase of the lunar cycle. The careful timing of the thyroxine surge suggests that a highly organized sequential pattern of hormone release and interaction leads directly to the orderly progression of smoltification-related developmental events. The elucidation of this pattern and its morphological, physiological, and behavioral consequences is essential to the complete understanding of parr—smolt transformation.

Effects of hypophysectomy and subsequent hormonal replacement therapy on hormonal and osmoregulatory status of coho salmon, Oncorhynchus kisutch ☆

This study investigates the effects of hypophysectomy and subsequent hormone replacement therapy upon the hormonal and osmoregulatory status of coho salmon, Oncorhynchus kisutch, in 7% seawater (SW) and SW. Following hypophysectomy, coho salmon were injected every 2 days for 8 days with thyroxine, growth hormone, and cortisol, alone or in combinations, and sampled 2 days after the final injection. Increased environmental salinity raises plasma sodium, calcium, and magnesium levels, as well as plasma osmolality. Cortisol is hypercalcemic and thyroxine is hypocalcemic in hypophysectomized salmon, but it is unclear whether these effects are due directly to calcium regulation or are the consequence of general effects on the plasma osmotic/ionic balance. Growth hormone and thyroxine together, but not separately, decrease and increase magnesium levels, at low and high environmental salinities, respectively, indicating a complex endocrine control of plasma magnesium. Gill Na+, K+-ATPase activity in hypophysectomized salmon is stimulated by growth hormone and cortisol, but inhibited by thyroxine and raised environmental salinity. This implies a complex endocrine control and indicates that hormonal support is needed to sustain or raise gill Na+, K+-ATPase activity in seawater. Increased environmental salinity induces elevation of plasma cortisol levels in apparent absence of pituitary control, indicating that the interrenals may respond to changes in external and/or internal environment, either directly or indirectly through extrapituitary hormonal or nervous control. Cortisol is a potent inhibitor of calcitonin secretion, as seen by the large decrease in plasma calcitonin levels in cortisol-treated hypophysectomized fish. The study was carried out at a time when thyroxine plasma levels were low. These basal levels were not affected by hypophysectomy, possibly indicating a basal release of thyroxine from the thyroid without stimulatory support of the pituitary gland.

The hypothalamo-hypophyseal neurosecretory system of the parakeet, Melopsittacus undulatus☆

Abstract The hypothalamo-hypophyseal neurosecretory system of the parakeet, Melopsittacus undulatus, was studied with both light and electron microscopes. The neurosecretory centers consist of paired supraoptic nuclei and paraventricular nuclei. Each nucleus is divisible into medial and lateral cell groups. The neurosecretory axonal tracts arising in the nuclei proceed first toward the median eminence. At the hilar region of the median eminence, they roughly branch into two tracts. One tract proceeds directly to the pars nervosa and terminates in it, whereas the other one proceeds to the lower surface of the median eminence and terminates there. The anterior median eminence alone is neurosecretory, and fibers from this region do not proceed caudad to rejoin the tract to the pars nervosa. No loops of neurosecretory axons were evident in our material. Neurosecretory axons enjoy a close association with glial cell bodies in both neurohemal regions of the neurohypophysis. Terminal processes of glia and/or ependyma are generally interposed between axon endings and capillary basement membranes. Four types of granules or vesicles were found in the axon endings adjacent to the blood capillaries of the median eminence primary plexus and of the pars nervosa: (1) synaptic vesicle-like structures (390 A), (2) ovoid granules (490 A), (3) electrondense neurosecretory granules (600–1750 A), and (4) electron-lucent “neurosecretory” vesicles. The distribution of these various granules and vesicles is different in different endings. There are more typical neurosecretory granules in the tractus supraopticohypophyseus and the pars nervosa than in the neurohemal region of the median eminence. The possible significance of the spectrum of axonal inclusions is diseussed.

Association of elementary neurosecretory granules with the Golgi complex

Electron microscope observations have been made on neurosecretory cells from the supraesophageal ganglion of the leech Theromyzon rude, from the pars intercerebralis of the cockroach Periplaneta americana, and from the preoptic nucleus of the hypothalamus of the frog Rana pipiens. A consistent association was noted between Golgi membranes and elementary neurosecretory granules. The Golgi apparatus was generally more prominent in the invertebrate neurons than in the amphibian neurons.

Physiology of seawater acclimation in the striped bass, Morone saxatilis (Walbaum).

Several experiments were performed to investigate the physiology of seawater acclimation in the striped bass, Morone saxatilis. Transfer of fish from fresh water (FW) to seawater (SW; 31–32 ppt) induced only a minimal disturbance of osmotic homeostasis. Ambient salinity did not affect plasma thyroxine, but plasma cortisol remained elevated for 24h after SW transfer. Gill and opercular membrane chloride cell density and Na+,K+-ATPase activity were relatively high and unaffected by salinity. Average chloride cell size, however, was slightly increased (16%) in SW-acclimated fish. Gill succinate dehydrogenase activity was higher in SW-acclimated fish than in FW fish. Kidney Na+, K+-ATPase activity was slightly lower (16%) in SW fish than in FW fish. Posterior intestinal Na+,K+-ATPase activity and water transport capacity (Jv) did not change upon SW transfer, whereas middle intestinal Na+,K+-ATPase activity increased 35% after transfer and was correlated with an increase in Jv (110%). As salinity induced only minor changes in the osmoregulatory organs examined, it is proposed that the intrinsic euryhalinity of the striped bass may be related to a high degree of “preparedness” for hypoosmoregulation that is uncommon among teleosts studied to data.