Effects of the pineal gland and melatonin on the metabolism of oocytes in vitro and on ovulation in Bufo arenarum.

The pineal gland and the hormone melatonin appear to be responsible to some extent for the metabolic behaviour of Bufo arenarum oocytes during the winter. This assumption is supported by the observation that, in isolated mitochondria, both elements stimulate the rate of oxidation of citrate and inhibit that of fumarate, thus raising the oxidizing ratio that relates these parameters (C/F) to values close to or above 1, which is characteristic of these animals in winter. The sensitivity of oocytes to melatonin, in terms of the C/F ratio, varies throughout the year. It is maximal in fall and early winter at a dose of 1.8 micrograms/ml after 30 min treatment, minimal or nonexistent in late winter, and it increases gradually during the spring and summer. The response of oocytes to melatonin is more significant if the animals have been previously injected with a homogenate of homologous hypophysis. A direct metabolic effect of melatonin on oocytes was observed when coelomic oocytes, without follicle cells, responded to hormonal treatment. Both an extract of pineal gland and melatonin inhibit in vitro ovulation, the inhibitory effect of the extract being greater than that of the hormone.