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The crustacean eyestalk CHH/MIH/GIH neurohormone gene family represents a unique group of neuropeptides identified mainly in crustaceans. In this study, we report the cloning and characterization of the cDNA and the gene encoding the hyperglycemic hormone (MeCHH-B) of the shrimp Metapenaeus ensis.

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The influence of GABA on pituitary gonadotrophin (GTH) release in the goldfish was studied by means of in vivo and in vitro techniques. It was found that GABA injected intraperitoneally caused an increase of serum GTH levels in regressed or early maturing fish, but not in late maturing animals. Moreover, injection of a GABA transaminase inhibitor caused a significant increase of GABA within the hypothalamus and pituitary, and a dose-dependent increase in serum GTH levels. To determine if this effect could be exerted directly at the level of the pituitary, dispersed pituitary cells in static incubation or in perifusion were exposed to increasing concentrations of GABA or its agonists muscimol and baclofen. None of these drugs was able to modify the spontaneous or GnRH-induced secretion of GTH, indicating that the in vivo effect of GABA was most likely mediated via another hypothalamic factor. Using in vitro incubation of pituitary slices, it was found that GABA caused a dose-related stimulation of GnRH release at the level of the pituitary, providing a possible explanation for the observed in vivo stimulatory effect of GABA on GTH release. Since the seasonal effect of GABA in vivo indicated a possible interaction of GABA with sexual steroids, GABA was given intraperitoneally to female goldfish implanted with either testosterone or estradiol. We found that the stimulatory effect of GABA on GTH release was abolished in estradiol-treated females but was still observed in testosterone-implanted fish. Moreover, estradiol but not testosterone caused a decrease of the GABA concentration within the telencephalon. Taken together, these data indicate that GABA is involved in the regulation of GTH secretion in the goldfish, possibly by stimulating the release of GnRH from the pituitary, an effect that appeared to be modulated by estrogens. The inhibitory effects of estrogens on GABA actions may be part of the mechanism of estrogen negative feedback on the brain-pituitary axis.

The teleostean adenohypophysis has direct innervation by neurosecretory fibers; in effect, the median eminence is located in the adenohypophysis, and neurohormones are released directly in the proximity of the endocrine cells. Immunocytochemical studies demonstrate that neurohormonal fibers and terminals are specifically localized in the pituitary in association with the target endocrine cells. Because of this special anatomical arrangement, teleosts are a unique experimental model for determining the origin(s) in the brain of peptides and monoamines involved in regulation of the endocrine cells of the pituitary. To illustrate, the terminals of a specific group of dopaminergic neurons in the preoptic region of goldfish have been shown to be associated with gonadotropes, and likely serve as the source of dopamine for inhibiting gonadotrope activity. Functional studies on the regulation of activity of the adenohypophysis and release of neurohormones can be integrated. To illustrate, norepinephrine stimulates whereas dopamine inhibits gonadotropin release by acting directly on the gonadotropes. Catecholamines also modulate the release of gonadotropin-releasing hormone (GnRH); norepinephrine and epinephrine stimulate GnRH neurons centrally, whereas dopamine inhibits GnRH neurons centrally as well as at the level of the terminals in the pituitary. In contrast to the actions on gonadotropes, norepinephrine and epinephrine inhibit whereas dopamine stimulates growth hormone release. The overall role of catecholamines in the regulation of release of the neuropeptides involved in regulating the activity of somatotropes remains to be investigated. As demonstrated by these studies, teleosts are very useful models for neuroendocrine studies.The adenohypophysis in teleost fishes (Infraclass Teleostei; Subclass Actinopterygii, rayfinned fishes; Class Osteichthyes, bony fishes) is unique amongst vertebrates in that it is directly innervated by neurosecretory fibers. In effect, the median eminence has moved into the adenohypophysis. Neurohormones are released directly in the adenohypophysis and thereby reach target endocrine cells without the intervening transportation by the hypothalamo-hypophyseal blood portal system. Below we review the evolution of this special anatomical arrangement, and discuss the results of some studies on teleosts to demonstrate the usefulness of this unique model in neuroendocrinology . MORPHOLOGY AND VASCULARITY OFTHE PITUITARY A number of reviews of the structure-function relationships of the teleostean pituitary gland have appeared (e.g., Ball and Baker, '69; Holmes and Ball, '74; Schreibman, '86; Batten and Ingleton, '87). We will present only a n overview of the 0 1990 WILEY-LISS. INC. morphology and vascularity of the teleostean pituitary, rather than repeat the details in these reviews.The teleostean adenohypophysis has three distinct lobes, the rostra1 pars distalis (RPD), the proximal or caudal pars distalis (PPD), and the neurointermediate lobe (NL), consisting of the pars intermedia (...