Relationship between nutritional stimulation of gonadotrophin secretion and the peripheral and cerebrospinal fluid (CSF) concentrations of glucose and insulin in rams

Abstract

In numerous studies it has been shown that supplementing diets of rams with lupin grain (Lupinus angustifolius), a highly digestible energy and protein source, increases gonadotrophin secretion, perhaps by providing metabolic signals that stimulate gonadotrophin-releasing hormone (GnRH) neurons. In the first experiment, we tested the hypothesis that the increase in gonadotrophin secretion caused by lupin supplementation is accompanied by an increase in cerebrospinal fluid (CSF) concentrations of glucose and insulin. In the second experiment, we tested the hypothesis that supplying peripheral glucose to rams, in quantities approximating those associated with digestion of a lupin supplement, would increase gonadotrophin secretion. After 10 days of dietary treatment in the first study, rams (n = 6) given a lupin supplement had increased luteinising hormone (LH) pulse frequencies (P < 0.05) and mean follicle stimulating hormone (FSH) concentrations (P < 0.05) compared with rams (n = 6) not given a lupin supplement. Compared with control values, glucose concentrations in CSF, but not plasma, were higher (P < 0.05) in the lupin-supplemented group by Day 10, and insulin concentrations in both plasma and CSF were higher (P < 0.05) in the lupin supplemented group on Days 3 and 10. In the second study, rams (n = 6) infused intravenously for 10 days with glucose (1.6 mol in l l of saline infused over 4 h) had increased plasma concentrations of glucose and insulin (P < 0.05) on Days 2 and 10, compared with values in saline-infused controls (n = 6). The secretion of LH and FSH was not affected by infusion with either saline or glucose. These experiments demonstrate that nutritional supplementation increases gonadotrophin secretion and peripheral and CSF concentrations of glucose and insulin, but short-term glucose infusion, which increases peripheral glucose and insulin concentrations, does not increase gonadotrophin concentrations. The absence of an increase in gonadotrophin secretion in glucose-infused rams suggests that glucose and insulin in the peripheral circulation do not influence gonadotrophin secretion. However, the glucose and insulin in CSF are not necessarily derived from the peripheral circulation, so further studies are needed to determine whether central glucose or insulin are involved directly in the control of gonadotrophin secretion.