Effect of pinealectomy on prolactin, testosterone and luteinizing hormone concentration in plasma of bull calves exposed to 8 or 16 hours of light per day

Pineal tissue was removed from eight 6-wk-old bull calves (PX), whereas eight similar calves received sham pinealectomies (SPX). Before and after surgery, calves received 8 h of light (L):16 h of darkness (D) daily until 20 wk of age (wk 0 of experiment), at which time eight calves (four PX and four SPX) were maintained under 8L:16D for 12 additional wk, whereas the remaining eight calves received 16L:8D. At 0, 4, 8 and 12 wk of experiment, blood was collected from each animal for 26 h at 30-min intervals. Melatonin in SPX calves at wk 12 increased from 16.2 pg/ml of plasma when lights were on to 81.6 pg/ml during lights off, whereas in PX calves the nocturnal increase was absent. However, the nocturnal surge did occur in three PX calves on 8L:16D treatment, and those animals were excluded from calculations and analysis. At wk 0, prolactin (PRL) averaged 47 ng/ml among all calves. By wk 4 PRL increased (P less than .01) to 80 and 96 ng/ml in PX and SPX calves receiving 16L:8D, respectively; PRL averaged 53 and 48 ng/ml, respectively in PX and SPX calves maintained on 8L:16D. Prolactin was greater (P less than .05) through wk 12 in PX (64 ng/ml) and SPX (64 ng/ml) calves receiving 16L:8D than in calves exposed to 8L:16D, which remained unchanged (54 and 48 ng/ml). Testosterone was unaffected by photoperiod, but tended to be less (P = .13) in plasma of PX than of SPX calves at wk 0 (.90 vs 1.45 ng/ml of plasma).