FEEDING TRYPTOPHAN TO SEASONALLY ANESTROUS MARES - OVARIAN CHARACTERISTICS AND SECRETION OF LUTEINIZING-HORMONE, FOLLICLE-STIMULATING-HORMONE AND PROLACTIN

Stimulation of serotonergic pathways via administration of serotonin p recursors stimulates prolactin (PRL) secretion in various species. Thi s experiment was designed to determine 1) if tryptophan feeding in win ter would alter basal and (or) secretagogue-induced PRL secretion in m ares, 2) if time of feeding of tryptophan (0800 vs 1600) would produce different effects on PRL secretion and 3) if gonadotropin secretion a nd (or) ovarian activity would be stimulated by either feeding regimen . Twenty anestrous, light horse mares were randomly allotted to four g roups of five on December 31. The experiment was performed as a comple tely randomized design with a 2 x 2 factorial arrangement of treatment s. Factor 1 was tryptophan feeding (15 g daily mixed in 0.45 kg of gro und corn vs corn only) and factor 2 was time of feeding (0800 vs 1600) . Feeding was for 45 days beginning January 1 (day 1). Concentrations of PRL, luteinizing hormone (LH) and follicle stimulating hormone (FSH ) in blood samples collected every 3 days were not affected (p>.1) by tryptophan feeding, although there was an effect of day (p<0.05) on PR L concentrations. The PRL, LH and FSH responses to secretagogue (gonad otropin releasing hormone and thyrotropin releasing hormone) after 31 days of treatment were not affected (p>.1) by tryptophan feeding. On d ay 30 of treatment, PRL concentrations were greater (p<0.05) at 1200 i n mares fed tryptophan at 0800 than in mares fed com only; however, th ere was no similar effect in mares fed tryptophan at 1600. At the end of treatment, ovarian and follicular characteristics were similar (p>. 1) among groups. We conclude that tryptophan feeding of mares in this manner has little effect on secretion of PRL, LH or FSH and thus is no t a viable means of reversing the anestrous state in winter.