PROLACTIN CONCENTRATIONS ARE NOT SUPPRESSED IN MARES ADMINISTERED CONSTANT EXOGENOUS MELATONIN

On the summer solstice (June 21, 1996), six of 12 intact light horse m ares randomly chosen from a larger herd mere subcutaneously implanted with ALZET(R)(a) osmotic minipumps containing a melatonin solution (16 mg/ml) designed to release approximately 960 mu g of melatonin/day. A n additional two mares received implants containing only the saline-DM SO vehicle and four remained untreated. Blood samples were collected o n days 5, 26, and 59 of treatment to monitor melatonin concentrations and to verify pump function. Prolactin concentrations were determined from blood samples collected via jugular cannulae every 12 min for 8 h ours on days 25, 46, and 89 after initial implantation. On day 89, sam ples were collected hourly for 16 hours following the initial 8-hour s ampling period. Melatonin and prolactin concentrations were determined in the blood samples by radioimmunoassay. Mean circulating concentrat ions of melatonin in treated mares (n=6) were found to be significantl y elevated when compared to controls (n=6); however, there was no sign ificant difference in prolactin concentrations between the groups. The se studies demonstrate that longterm treatment with melatonin is unacc ompanied by a change in prolactin secretion.