Prolactin replacement fails to inhibit reactivation of gonadotropin secretion in rams treated with melatonin under long days

This study tested the hypothesis that prolactin (PRL) inhibits gonadotropin secretion in rams maintained under long days and that treatment with melat onin (s.c. continuous-release implant; MEL-IMP) reactivates the reproductiv e axis by suppressing PRL secretion. Adult Soay rams were maintained under long days (16L:8D) and received 1) no further treatment (control, C); 2) ME L-IMP for 16 wk and injections of saline/vehicle for the first 8 wk (M); 3) MEL-IMP for 16 wk and exogenous PRL (s.c, 5 mg ovine PRL 3x daily) for the first 8 wk (M+P), The treatment with melatonin induced a rapid increase in the blood concentrations of FSH and testosterone, rapid growth of the test es, an increase in the frequency of LH pulses, and a decrease in the LH res ponse to N-methyl-D,L-aspartic acid. The concomitant treatment with exogeno us PRL had no effect on these reproductive responses but caused a significa nt delay in the timing of the sexual skin color and growth of the winter pe lage. These results do not support the hypothesis and suggest that PRL at p hysiological long-day concentrations, while being totally ineffective as an inhibitor of gonadotropin secretion, acts in the peripheral tissues and sk in to maintain summer characteristics.