CENTRAL METABOLIC MESSENGERS AND THE EFFECTS OF NUTRITION ON GONADOTROPIN-SECRETION IN SHEEP

Nutrition influences the reproductive axis via alteration of gonadotro phin secretion. However, a link between nutrition and the secretion of GnRH, which drives the axis, has yet to be established. The aim of th e present study was to measure the change in the concentrations of met abolic substances in the cerebrospinal fluid of adult male sheep offer ed a diet designed to maintain constant gonadotrophin secretion (Group M; n = 6), or a diet known to increase gonadotrophin secretion (Group M + L; n = 6). On days 1, 3 and 10 of the dietary treatments, cerebro spinal fluid and jugular blood were sampled and analysed for metabolic fuels (glucose, amino acids and free fatty acids) and metabolic hormo nes (insulin, insulin-like growth factor I, GH, prolactin, cortisol an d the thyroid hormones). On day II of the dietary treatment, LH pulse frequency and mean FSH concentrations in Group M + L had increased rel ative to Group M and to day 0. Plasma concentrations of prolactin and insulin on days 3 and 10, and glucose and insulin-like growth factor 1 on day 10, were higher in Group M + L than in Group M, but only cereb rospinal fluid concentrations of insulin, glucose and certain amino ac ids were affected by the dietary treatments on days 3 and 10. Cerebros pinal fluid, but not plasma, concentrations of aspartate, tyrosine, cy stine, phenylalanine and arginine on day 3, and glutamine, gamma-amino butyric acid, threonine, alanine on days 3 and 10, were higher in Grou p M + L relative to Group M. On day 10, plasma and cerebrospinal fluid concentrations of arginine, phenylalanine, proline, tyrosine, methion ine and phosphoserine, but only the plasma concentrations of linoleic acid, aspartate and serine, were higher in Group M + L than in Group M . Concentrations of triiodothyronine, thyroxine, and cortisol in plasm a and cerebrospinal fluid were not affected. These results show that t he nutritional stimulation of gonadotrophin secretion is accompanied p rimarily by fluctuations in plasma and cerebrospinal fluid concentrati ons of insulin and certain amino acids, which suggests that, when nutr itional status is improved, insulin, amino acids and possibly glucose interact to modulate GnRH secretion.