INFLUENCE OF LIGHT-DARK CYCLES ON ESTRADIOL-17-BETA INDUCED LUTEINIZING-HORMONE PATTERNS OF THE PREPUBERAL GILT

Two groups of Yorkshire gilts (110 d of age) were maintained in two li ght regimens. Both light regimens consisted of 14 h of light and 10 h of darkness, but were 180 degrees out of phase. Gilts in Group 1 recei ved light from 1200 to 0200 and gilts in Group 2 from 2400 to 1400. At approximately 140 d of age each group was divided into four subgroups of eight gilts each (1A, 1B, 1C, 1D or 2A, 2B, 2C, 2D). All gilts wer e blood sampled at 2-h intervals for 5 d commencing on d 142. The four subgroups received a single injection of estradiol (15 mu g/kg body w eight) on d 143 at either 2400 (A), 0600(B), 1200 (C), or 1800 (D). Fo r pigs in Groups 1A and 1D, the injection of estradiol coincided with the animals' ''subjective day'' and the injections given to Groups 1B and 1C with their ''subjective night.'' When estradiol-17 beta (E(2)) was administered to the gilts during their subjective day the LH profi le showed one peak, whereas when Ea was administered during dark hours the profile exhibited two peaks (P <.0001). In Group 2 for which the light cycle was reversed, the well-defined spikes of LH were found to coincide with the injections of estradiol administered during the dark hours. Smaller biphasic peaks of LH occurred when injections of estra diol coincided with the light hours. This dramatic reversal of the cha racteristic of the LH peaks by the reversal of the light dark cycle su ggests an interaction between the light dark (LD) cycle and estrogen o n release of LH in the pig.