Level of nutrition affects leptin concentrations in plasma and cerebrospinal fluid in sheep

In mature male sheep, the level of nutrition acutely influences the secreti on of reproductive hormones. The mechanism involved is not fully understood but findings in humans and laboratory rodents would suggest a major role f or leptin that is secreted from adipose tissue and then travels via the cir culation to the central nervous system. Before we can begin to test this hy pothesis, we need to be able to measure leptin concentrations in blood plas ma and cerebrospinal fluid. We have therefore developed a radioimmunoassay using antibodies raised against biologically active recombinant bovine-ovin e leptin. Using this assay, we found that plasma concentrations of leptin w ere highly correlated to back-fat thickness and to the ratio of back-fat th ickness to liveweight, in female and castrated male sheep. Plasma concentra tions of leptin were higher in female sheep than in castrated or intact mal e sheep. Serial samples (every 5 min) suggested that the secretion of lepti n in male sheep is episodic but it does not appear to show clear pulsatilit y, increases post-prandially, or a diurnal rhythm. Leptin concentrations in both plasma and cerebrospinal fluid increased within 5 days in male sheep fed a diet with a high content of energy and protein that also stimulates t he secretion of LH pulses. These data suggest that in sheep, as in other sp ecies, leptin production is correlated with the mass of adipose tissue and that the hormone passes from the circulation to the cerebrospinal fluid and then to hypothalamic sites. There, it may affect appetite and perhaps GnRH secretion. The role of leptin in the Link between nutrition and reproducti on needs further investigation.