PHOTOPERIOD-INDUCED CHANGES IN LH, FSH, PROLACTIN AND TESTOSTERONE SECRETION IN THE RAM

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Abstract
Two "annual" reproductive–endocrine cycles were produced in rams during a 12-mo period by photoperiodic manipulation. Two Finnish Landrace rams were housed in a "light-proof" room between 21 Dec. 1974 and 21 June 1976; they were exposed to three consecutive photoperiod cycles in which the sinusoidal fluctuation in the annual daylight rhythm characteristic of Southern Manitoba was reproduced in 6 mo. Daylength started to decline on the 21st day of December and June. Light-controlled rams and two rams penned outside (controls) were bled at 3- to 5-wk intervals between mid-June 1975 and mid-June 1976. Jugular venous blood was collected at 20-min intervals for 8 h on each occasion. Sera were assayed for luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin (PRL) and testosterone. The onset of decreasing daylength was followed by increases in serum FSH and testosterone concentrations and a decrease in PRL concentration in both pairs of rams. However, photoperiod-related changes in serum LH concentration in rams were not comparable during 6- and 12-mo "annual" photoperiod cycles. In all rams, periods of elevated testosterone concentration were characterized by increases in LH- and testosterone-peak frequencies and testosterone-peak height, and a decrease in LH-peak height.