Background. Melatonin regulates various physiological and neuroendocrinological processes that occur rhythmically, and stimulates or inhibits endocrine activity of various body glands. This acts on the hypothalamic-pituitary-gonadal axis by synchronizing animals with their reproductive cycles. The proximity of melatonin receptors and dopamine and gonadoliberin production sites has led to a hypothesis that dopamine may be a link between melatonin and hypothalamic LHRH. Melatonin may have an indirect influence on animal reproduction through dopaminergic structures of the hypothalamus, but the mechanism involved remains unknown, also in fish, for this reason, the present experiment was conducted. The aim of the study was to determine the effect of melatonin on dopamine release from hypothalamic cells of mature female carp in vitro. Materials and Methods. Hypothalami were perifused with a mineral medium containing melatonin (group 1), in the presence of implanted pineal glands (group 2), and with a pure mineral medium (control). Perifusion was 180 min long and samples of the effluent perifusate were collected at 15-minute intervals. Dopamine concentration in the medium was analysed radioenzymatically. The experiment was carried out in the summer during spawning and in the winter during regression. Results. The results indicate that melatonin inhibits the release of dopamine from hypothalamic cells. This effect was only noticeable in the experiment conducted during the spawning period. Conclusion. The present findings show that melatonin may have a role in the hypothalamic control of hypophyseal activity during the spawning period of carp.
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