Reproductive biology of freshwater fishes from the Venezuelan floodplains.

This review describes the endocrine changes that occur during the annual reproductive cycle of Pygocentrus cariba, Pimelodus blochii, and Oxydoras sifontesi and their relationships with the environmental characteristics of Venezuelan floodplains. Most reproductive studies of teleosts have focused on changes that occur during annual cycles in temperate species but, in tropical fish, this has been examined less frequently. P. cariba, P. blochii, and O. sifontesi are seasonal breeders widely distributed along the Orinoco River. Under natural conditions they have an annual gonadal cycle closely related to changes in the annual hydrology cycle of the Orinoco River which defines two seasons on the floodplain: inundation and isolation. The reproductive cycle of these species seems to be controlled by cues from the external environment. Relevant data about gonadal maturation, for example gonadosomatic index and sexual hormones secretion, are contrasted. The role of catecholamines in neuroendocrine control of the reproductive axis is also considered in this work.

Gonadotrophin-releasing hormone immunoreactivity in the brain of the tropical freshwater fish, Pygocentrus notatus (Teleostei-Characidae).

The distribution of GnRH in the brain of the teleost Pygocentrus notatus was demonstrated with the avidin-biotin peroxidase immunocytochemical method using highly specific antibody against synthetic mammalian GnRH. Optimal immunoreaction was obtained using: 1) Bouin's fluid for fixation; 2) repeated incubation with primary antiserum; 3) the use of a detergent in the dilution buffer; 4) the high sensitivity of the avidin-biotin immunoperoxidase method with the cobalt intensification of 3-3' diaminobenzidine tetrahydrochloride; and 5) the use of primary antibody with high specificity. GnRH-immunoreactive (GnRH-ir) in cells and/or axons was observed in all main brain regions. In the forebrain, GnRH-ir was located in a network extending from the caudal part of the olfactory bulb to the telencephalon. GnRH-ir fibres were also observed in the optic tectum, cerebellum and hypothalamus. Two groups of neuronal cell bodies were identified. One group was located in the antero-ventral telencephalon corresponding to the nucleus olfactoretinalis. The second group was found in the rostrodorsal hypothalamus. No GnRH-ir material was detected in the pituitary gland, thus confirming the results of previous studies on brain GnRH-ir distribution obtained by radioimmunoanalysis in this species. These results demonstrate a high degree of similarity between the GnRH systems of P. notatus and other teleost species.

Hypothalamic and telencephalic catecholamine content in the brain of the teleost fish, Pygocentrus notatus, during the annual reproductive cycle.

The catecholamines noradrenaline (NA), dopamine (DA), and adrenaline (A) were measured in hypothalamic and telencephalic extracts of the Venezuelan freshwater fish "caribe colorado," Pygocentrus notatus, at different stages of the reproductive cycle. The concentration of NA was found to be significantly higher in the telencephalon than in the hypothalamus, but that of DA was higher in the hypothalamus than in the telencephalon. Fluctuations depending upon the reproductive stage and environmental conditions occurred in both hypothalamus and telencephalon. In the hypothalamus, DA content was highest during the prespawning period (June) as compared to other periods of the cycle. Although the NA concentration was reduced during spawning there was no significant variation during any other period. DA concentrations in both telencephalon and hypothalamus showed a similar pattern of changes. In the telencephalon, NA levels increased between preparatory and prespawning periods but decreased sharply during spawning. No sex differences were observed in either area at any stage of reproduction.

Changes in plasma levels of 17 beta-estradiol after administration of an analogue of luteinizing hormone releasing hormone (LHRH-A) and pimozide in Diplectrum formosum (Teleostei: Serranidae).

Gonadotropin secretion in teleosts is known to be stimulated by gonadotropin hormone releasing hormone (GnRH) and inhibited by dopamine. This study has investigated the effects of administration of pimozide (PIM) alone or in combination with an analogue of GnRH on plasma 17 beta-estradiol (17 beta-E) levels and the gonadosomatic index. Adult Diplectrum formosum were treated with a gonadotropin-releasing hormone analogue [des-Gly10 D-Ala6 Pro9 N-ethylamide (LHRH-A); 50, 70, and 100 ng/g body wt] alone or in combination with pimozide (PIM), a dopamine antagonist (0.5, 1.0, and 5.0 micrograms/g body wt). LHRH-A alone caused a dose-dependent increase in plasma 17 beta-E levels. A single injection of PIM (0.5 or 5.0 micrograms/g) 12 hr previous to a single injection of 100 ng/g body wt of LHRH-A increased 17 beta-E concentrations compared with fish receiving PIM alone; however, 1 microgram/g PIM did not change 17 beta-E plasma levels. Simultaneous administration of PIM (0.5, 1.0, and 5.0 micrograms/g body wt) and 100 ng/g body wt of LHRH-A at an interval of 12 hr (30% of the total dose of both PIM and LHRH-A was administered in the first injection and 70% in the second injection) significantly increased plasma 17 beta-E concentrations. The magnitude of this increase was at least threefold higher than those of fish receiving a single injection of PIM, 12 hr before a single injection of LHRH-A. LHRH-A or PIM alone or combined was ineffective in causing ovulation.

Photic regulation of mt(1) melatonin receptors and 2-iodomelatonin binding in the rat and Siberian hamster.

We have investigated the photic regulation of melatonin receptors both at the level of binding capacity and mt(1) mRNA expression in the suprachiasmatic nucleus (SCN) and the pars tuberalis (PT) of the pituitary of two species: a highly photoperiodic one, the Siberian hamster, and a nonphotoperiodic one, the Wistar rat. This study has been performed by looking at the effect of a light pulse applied during the night on the two receptor parameters. The results show that the photic regulations of mt(1) mRNA expression and receptor density are distinct from each other in both the SCN and PT of the two species studied. They also show that, depending on the species and the structure, this regulation may implicate either the circadian clock or melatonin.

Melatonin regulates the mRNA expression of the mt(1) melatonin receptor in the rat Pars tuberalis.

The pars tuberalis (PT) of the pituitary is a major neuroendocrine target site for melatonin as it contains a large number of high-affinity melatonin receptors. We have previously shown that melatonin autoregulates the density of its own receptors in the PT. However, whether melatonin regulation includes mRNA expression in vivo is unclear. In the present study we have used quantitative in situ hybridization to (1) follow the daily profile of mt(1) mRNA expression in the rat PT and (2) investigate whether mt(1) mRNA expression could be regulated in vivo by melatonin. We found clear diurnal variations of mt(1) mRNA expression that persist in constant darkness. We also showed, on pinealectomized animals, that the rhythmic pineal melatonin secretion is necessary for the expression of these daily variations. In a second step, we studied the effect of an acute suppression of endogenous melatonin synthesis on mt(1) melatonin receptors by applying a 1-hour light pulse during the night. We found that light induced a dramatic increase in mt(1) mRNA which was totally prevented by a melatonin injection showing that the acute effect of melatonin on the receptor mRNA is strongly inhibitory. A light pulse applied to animals with a chronic absence of melatonin was ineffective showing that light only affects melatonin receptors via the light-induced plasma melatonin suppression. Altogether our results show that melatonin regulates mt(1) melatonin receptor mRNA expression. However, this regulation seems to be complex: acute changes in plasma melatonin concentration regulate negatively the gene transcription, even if the daily endogenous nocturnal melatonin peak seems a prerequisite for variations in its receptor expression.

Effects of two melatonin analogues, S-20098 and S-20928, on melatonin receptors in the pars tuberalis of the rat.

By using quantitative autoradiography, we studied the effects of two drugs related to melatonin on the 2-(125)I-melatonin binding in the pars tuberalis (PT) of rats. The drugs tested were two naphthalenic analogues of melatonin, S-20098 (N-[2-(7-methoxy-1-naphthyl) ethyl] acetamide), an agonist, and S-20928 (N-[2-(1-naphthyl) ethyl] cyclobutyl carboxamide), a putative antagonist. Melatonin (s.c. and i.p.), S-20098 (s.c.), and S-20928 (i.p.) were injected 4 hr before sacrifice. Acute administration of both melatonin and S-20098 decreased melatonin receptor density. In contrast, the putative antagonist S-20928, at a low dose (1 mg/kg), was ineffective on melatonin receptors. It neither affected the 2-(125)I-melatonin specific binding observed in the control group nor did it prevent the decrease in binding induced by melatonin when injected 5 min before the hormone. At a high dose (10 mg/kg), S-20928 totally blocked the effect of melatonin on melatonin receptor density and induced a decrease in binding capacity as melatonin did when injected alone. These results indicate that in the rat pars tuberalis, the melatonin agonist, S-20098, is able to down-regulate melatonin receptors, whereas S-20928 seems to behave as a partial agonist.