Metabolic factors affecting the reproductive axis in male sheep

Changes in food intake affect the reproductive axis in both sexes, and the nutritional signals involved and the sites that receive those signals are n ow beginning to be unravelled. Our studies have focussed on the mature male sheep, a model in which high food intake stimulates GnRH-LH pulse frequenc y for only 10-20 days but continues to promote testicular growth over sever al months. Different signals and different target organs seem to be respons ible for these short- and long-term responses. Short-term dietary treatment s lead to changes in blood concentrations of glucose, fatty acids, insulin and leptin, and concentrations of glucose, insulin, leptin and some amino a cids in cerebrospinal fluid. It seems unlikely that amino acids affect GnRH -LH secretion directly in sheep. Intracerebroventricular infusions of insul in specifically increase LH pulse frequency, but intravenous, intra-abomasa l or intracerebroventricular infusions of glucose have no effect, despite t heir effects on cerebrospinal fluid insulin concentrations. The addition of fatty acids to the diet also increases LH pulse frequency, but does not af fect the concentrations of insulin or leptin in the cerebrospinal fluid. It appears that acute responses to changes in nutrition involve a range of al ternative pathways, possibly including interactions among insulin, leptin a nd energy substrates. Effects of long-term dietary treatments on testicular size are only partly dependent on the GnRH-LH system (that is, on brain co ntrol) and so must also depend on other, as yet unknown pathways. Concepts of 'metabolic sensing and integration' are being developed from the basis o f existing knowledge of the central control of appetite and reproduction.