| Summary: | <p xmlns:etd="http://www.ouls.ox.ac.uk/ora/modsextensions">The effects of constant light (LL) exposure on different patterns of gonadotrophin release have been studied in the female rat. Exposure of animals with regular oestrous cycles to LL during the 3 days preceding pro-oestrous, results in a 3 h delay and a 40% reduction in the height of the pro-oestrous LH surge. This delay and reduction may involve a reduction in the sensitivity of the hypothalamo- hypophysial complex to oestradiol and progesterone. Diurnal and pulsatile patterns of LH release (seen in ovariectomized rats in the presence and absence of oestradiol) are driven by different neural mechanisms; LL inhibits diurnal but not pulsatile LH release. Analysis of tissue using HPLC suggests that in the suprachiasmatic nucleus alterations in the release and reuptake mechanisms of 5-HT may contribute to the loss of diurnal LH release. The pulsatile LH release found in rats ovariectomized after 60 days in LL, exhibits a lower mean plasma LH concentration, pulse amplitude, and pituitary responsiveness to LHRH, than rats ovariectomized in LD. Injection of oestradiol into gonadectomized animals shows that the centres upon which oestradiol acts to inhibit pulsatile LH release, are neither sexually differentiated nor affected by neonatal inhibitory action of oestradiol. LH release after progesterone treatment in LL exposed female rats, is accompanied by increases in the pituitary responsiveness to LHRH and the magnitude of the priming effect of LHRH. Analysis of plasma steroid concentrations after mating in females exposed to LL, suggests that progesterone may potentiate matinginduced ovulation. Studies using female rats with spontaneous degeneration of the retina, show that the effects of LL on gonadotrophin secretion are due to effects upon the brain, and are not a consequence of retinal degeneration.</p>
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