Feeding tryptophan to seasonally anestrous mares: ovarian characteristics and secretion of luteinizing hormone, follicle stimulating hormone and prolactin

Abstract

Abstract Stimulation of serotonergic pathways via administration of serotonin precursors stimulates prolactin (PRL) secretion in various species. This experiment was designed to determine 1) if tryptophan feeding in winter would alter basal and (or) secretagogue-induced PRL secretion in mares, 2) if time of feeding of tryptophan (0800 vs 1600) would produce different effects on PRL secretion and 3) if gonadotropin secretion and (or) ovarian activity would be stimulated by either feeding regimen. Twenty anestrous, light horse mares were randomly allotted to four groups of five on December 31. The experiment was performed as a completely randomized design with a 2 x 2 factorial arrangement of treatments. Factor 1 was tryptophan feeding (15 g daily mixed in 0.45 kg of ground 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>.l) by tryptophan feeding, although there was an effect of day (p .l) by tryptophan feeding. On day 30 of treatment, PRLconcentrations were greater (p .l) 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 not a viable means of reversing the anestrous state in winter.