Quantitative relationship between light intensity and luteinizing hormone surges in ovariectomized rats treated with...

The purpose of this study was to determine if patterns of gonadotropin secretion during estrogen-induced luteinizing hormone (LH) surges are altered by abrupt changes in light exposure. When light intensity was increased approximately threefold, before the "critical period" or during the rising phase of the LH surge, serum LH increased significantly (P less than 0.01) compared to control values. Furthermore, LH surges were prolonged when exposure to light of high intensity was continued beyond the normal extent of the photophase. In contrast to the effects of increased light intensity, acute darkness did not alter the magnitude of LH surges compared with controls exposed to light of low intensity. Thus, phasic LH secretion is enhanced by abrupt increases in light intensity, and the duration of LH secretion can be extended by prolonging the photophase. Although LH surges occur in total darkness, this study suggests that light modulates the intensity of the signal for LH secretion by a stimulatory action.