The effects of intracerebroventricular infusion of prolactin on luteinizing hormone, testosterone and growth hormone secretion in male sheep

Abstract

This study tested a hypothesis that the enhancement of the prolactin (PRL) concentration within the central nervous system (CNS) disturbs pulsatile luteinizing hormone (LH) and growth hormone (GH) secretion in rams that are in the natural breeding season. A 3 h long intracerebroventricular (icv.) infusion of ovine PRL (50 μg/100 μl/h) was made in six rams during the daily period characterized by low PRL secretion in this species (from 12:00 to 15:00 h); the other six animals received control infusions during the same time. Blood samples were collected from 9:00 to 18:00 h at 10 min intervals. A clear daily pattern of LH secretion was shown in control animals, with the lowest concentration at noon and an increasing basal level around the time of sunset ( P<0.001). No significant changes in LH concentration occurred in PRL-infused animals and the concentration noted after infusion of PRL was significantly ( P<0.05) lower than after the control infusion. The frequency of LH pulses tended to decrease in rams after PRL treatment. The changes in LH secretion clearly carried over to the secretion of testosterone in the rams of both groups. The GH concentrations changed throughout the experiment in both groups of rams, being higher after the infusions ( P<0.001). However, the mean GH concentration and GH pulse amplitude noted after PRL infusion were significantly lower ( P<0.001 and P<0.05, respectively) from those recorded in the control. The continued fall in PRL secretion observed in rams following PRL infusion ( P<0.05 to P<0.001) indicates a high degree of effectiveness of exogenous PRL at the level of the CNS. In conclusion, maintenance of an elevated PRL concentration within the CNS leads to disturbances in the neuroendocrine mechanisms responsible for pulsatile LH and GH secretion in sexually active rams.