Gonadal Function in Underfed Rats: II. Effect of Estrogen on Plasma Gonadotropins After Pinealectomy or Constant Light Exposure

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Abstract
The effect of underfeeding on the control of plasma levels of FSH and LH was investigated in female rats after long term food deprivation. Concentrations of these hormones were measured in ovariectomized animals by radioimmunoassay, before and after injection of a single dose of estradiol. Similar measurements were made in animals which had been previously pinealectomized or exposed to continuous illumination, since such treatment partially restores activity to the quiescent reproductive tissues of rats which are maintained on a low caloric diet. While levels of plasma FSH were comparable in fed and underfed ovariectomized animals. LH was significantly lowered in the latter group. It is suggested, therefore, that the major factor contributing to reproductive dormancy in the starved animal is reduced plasma LH. This reduction was not apparently due totally to enhanced feedback sensitivity by estrogen, since it was observed in ovariectomized rats, but occurred in part through some mechanism which is sensitive to constant light exposure or pineal gland removal. Such treatment resulted in elevations of blood levels of LH, but had little effect on plasma FSH levels in underfed rats. It is therefore suggested, that energy conservation during periods of food shortage is facilitated by the establishment of control mechanisms leading to gonadal quiescence by inhibition of LH secretion. This mechanism is in part dependent upon the pineal gland for optimal function and it is potentiated by a relatively short photoperiod.