To investigate the existence of a dopaminergic preoptico-hypophyseal pathway in the goldfish, electrolytic lesions were placed in the rostral preoptic area and their effects on gonadotrophin levels and pituitary innervation examined. In a first experiment, the fish were sacrificed 2 days after surgery and the pituitary studied by electron microscopy. Numerous exocytosis profiles were observed in the gonadotrophs, confirming the large increase in serum gonadotrophin levels measured in the animals. In addition, type A and B degenerating fibers were detected in the neurohypophysis and the pars distalis, in particular at the level of the gonadotrophs. In the second experiment, the distribution of tyrosine hydroxylase-immunopositive fibers was studied in the pituitary of controls and lesioned animals. It was found that lesioning the anterior ventral preoptic region resulted in the disappearance of all positive fibers in the pars distalis, while those in the neurointermediate lobe appeared unaffected. The presence of a large group of catecholaminergic perikarya in the destroyed area was confirmed in control animals. These results and other data strongly support the existence of a dopaminergic preoptico-hypophyseal pathway, providing a morphological support for the inhibitory effect of dopamine on the release of anterior pituitary hormones in teleosts, in particular gonadotrophin.
In fish as in other vertebrates, the brain is actively involved in the control of reproduction, first by participating, under the influence of external factors, in the establishment of an appropriate endocrine status, but also by allowing synchronization of the partners by the time of spawning. It is now well established that the pituitary gonadotropic function is controlled by multiple stimulatory and inhibitory factors, originating mainly from the preoptic region and the mediobasal hypothalamus, both target regions for sexual steroids. Little is known about the mechanisms involved in the mediation of external and internal factors, however there is indication that internal factors, such as androgens and melatonin, known to trigger particular behavioural and endocrine responses, act both at the level of neuroendocrine territories, but also on sensorial systems, which are the actual sites of action for external factors. This paper represents an attempt to summarize and integrate the recent literature devoted to the different aspects of the brain as a major participant in the complex endocrine and behavioural mechanisms of reproduction in fish.
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.
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