Howard A. Bern

Insulin-like growth factor signaling in fish.

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

The comparative endocrinology of prolactin.

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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.

Smoltification and seawater adaptation in coho salmon (Oncorhynchus kisutch): plasma prolactin, growth hormone, thyroid hormones, and cortisol.

The status of circulating growth hormone and prolactin during the parr-smolt transformation and during seawater adaptation of coho salmon (Oncorhynchus kisutch) was investigated in relation to changes in plasma levels of thyroxine, triiodothyronine, and cortisol, and in hypoosmoregulatory ability. Sampling (biweekly or monthly) occurred between early February and October. When peak hypoosmoregulatory ability was achieved (mid-April), one group of fish was acclimated to seawater over a period of 18 hr and was sampled 1, 3, and 7 days after the introduction of fish to seawater and biweekly thereafter. Plasma prolactin levels rose steadily from the first sampling date to a peak of 15 ng/ml in early April, declined rapidly, and remained low until June when a second increase occurred. Prolactin declined to 2 ng/ml within 1 day of the beginning of seawater adaptation. Growth hormone increased twofold from February to late March, and achieved plateau levels of 20 ng/ml in the period from mid-April to July and then gradually declined to 10 ng/ml in September and October. Plasma levels of growth hormone in seawater-acclimated fish were similar to those of freshwater coho, but with larger fluctuations; no increase was apparent during the first week of seawater acclimation. Plasma cortisol and plasma triiodothyronine increased at the same time as plasma growth hormone; increases in plasma thyroxine occurred later. In general, both growth hormone and cortisol levels were elevated when hypoosmoregulatory ability was high. Conversely, prolactin levels generally showed a negative relationship with hypoosmoregulatory ability.

Prolactin and Osmoregulation in Vertebrates

Studies on the roles of prolactin in vertebrate osmoregulation have expanded in recent years to include examination of ionic and osmotic factors contributing to the regulation of secretion by paraneuronal prolactin-secreting cells and the effects of prolactin on a number of ion- and water-transporting epithelial tissues. Coupled with information on whole-animal homeostatic effects, the application of surgical, tissue-culture, biophysical and biochemical techniques to individual components will facilitate broader and more integrated functional analyses of the important osmoregulatory effects of this hormone in all vertebrate classes.

Long-term effects of perinatal exposure to sex steroids and diethylstilbestrol on the reproductive system of male mammals.

Publisher Summary This chapter reviews the experimental data concerning the effects of perinatal sex steroid and the diethylstilbestrol (DES) treatment on the central and peripheral aspects of genital structural and functional development in male mammals, in particular male rodents. The pathology of the testis of perinatally estrogenized animals is of complex origin, with several critical factors directly or indirectly involved, whose relative importance is possibly dependent on the dose, time, and duration of perinatal estrogen treatment. The chapter discusses the direct and indirect effects of sex hormones given perinatally on the male reproductive tract. The experimental data suggest the desirability of follow-up studies on the possibly higher risk of genital cancer development and gonadal dysfunction in the prenatally DES-exposed human male population.

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.

Prolactin and tadpole growth.

Summary Mammalian prolactin inhibits thyroxin-induced resorption of the tail in Rana catesbeiana tadpoles. Mammalian growth hormone does not show this effect. The possible importance of prolactin as a larval growth hormone in amphibians is emphasized.

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.