Intracerebroventricular Infusion of Leptin into Mature Merino Rams of Different Metabolic Status: Effects on Blood Concentrations of Glucose and Reproductive and Metabolic Hormones P Celi, D Blache, MA Blackberry and GB Martin Faculty of Natural and Agricultural Sciences, School of Animal Biology, The University of Western Australia, Crawley, WA, Australia Contents In mature Merino rams, nutrition is one of the external cues that most strongly affects the reproductive centres of the preoptic-hypothalamic continuum. The signalling pathways that link dietary status and the activity of the neurones that produce gonadotrophin-releasing hormone signals are thought to be partly hormonal in nature to reflect the amount of body reserves. Among the hormones thought to be involved are insulin and leptin. This study tested whether recombinant bovine leptin infused (0.4 lg/h) into the third cerebral ventricle would stimulate pulsatile luteinizing hormone (LH) secretion in mature Merino rams when their energy status was low or decreasing, during both chronic (fasting) and acute reductions of energy balance. Leptin may interact with other hormones that depend on energy availability, so we also monitored changes in circulating concentrations of insulin, thyroid hor- mones, growth hormone, prolactin and adrenocorticotrophin. Overall, our data do not support this hypothesis. The dietary regimes induced clear responses in the metabolic profiles of the animals but there was no clear effect of central leptin administration on LH pulse frequency. The relationships between the hormonal systems measured in the present study add weight to the contention that leptin plays only a permissive role in the nutritional control of the reproductive axis and that other hormonal signals (particularly insulin) or pathways are acting in concert with leptin to stimulate the reproductive axis. Introduction In mature Merino rams, nutrition is one of the external cues that most strongly affects the reproductive centres of the preoptic-hypothalamic continuum (Blache et al. 2002, 2003). For example, the frequency of pulses of luteinizing hormone (LH) and, thus the pulses of gonadotrophin-releasing hormone (GnRH), increases by at least 50% within a few hours after the intake of energy and protein is increased to double the require- ments for maintenance (Zhang et al. 2004). This response stabilizes 5–7 days after the start of dietary treatment (Blache et al. 2002). Surprisingly, this stimu- latory effect of dietary supplementation on the gonado- trophic axis diminishes and disappears by 3–4 weeks, around the time that body weight and fat deposition have started to increase. This led us to question the nature of the signalling pathways that link dietary status and the activity of the neurones that produce the GnRH signal. The link is thought to be partly hormonal in nature (Woods et al. 2000) and may reflect the amount of body reserves (Blache et al. 2002, 2003), but it is clearly poorly understood. Among the hormones thought to link metabolism and reproduction in ruminants are insulin and leptin (Barb and Kraeling 2004). However, central administration of leptin or insulin does not always induce consistent responses in the pulsatile secretion of GnRH/LH (Blache et al. 2002). In some studies, central leptin administration reduced feed intake but failed to affect pulsatile LH secretion in ovariectomized ewes that were fed ad libitum (Henry et al. 1999; Morrison et al. 2001). Leptin also failed to affect LH pulse frequency in diet- restricted ewe lambs (Morrison et al. 2001). In one study, central leptin administration actually decreased LH pulse frequency, apparently as a flow-on from the reduction in feed intake, intact rams that were fed ad libitum (Blache et al. 2000a). In a study with castrated male sheep, leptin treatment prevented the suppression of LH secretion that was induced by short-term fasting (Nagatani et al. 2000). Finally, in castrated male sheep fed to maintenance, and in long-term food-restricted ovariectomized ewes, intracerebroventricular (icv) leptin was found to increase LH pulse frequency (Henry et al. 2001; Miller et al. 2002). Comparison of these studies suggests that, as has been proposed for postpartum dairy cattle, the hormonal signals need to reflect not only the nutrient intake at a given time but also the overall status of the energy balance. In turn, this balance depends on both the level of intake, the level of body reserves and the rate of energy expenditure (Woods et al. 1998, 2000). Recently, in male Merino sheep, the short-term stimulatory effect of a high-protein and high-energy supplement has been shown to depend on the level of energy reserves, as measured by body fatness (Zhang et al. 2005). The results of this study also suggest that (1) leptin, and perhaps insulin, would be relevant signals only if the body reserves are low, and (2) the concen- tration of both leptin and insulin need to be low if an increase in plasma leptin is to stimulate the pulsatile secretion of GnRH. In female sheep, leptin and its receptor are expressed in the hypothalamus and the expression of both are increased in hypothalamic and pituitary tissues after underfeeding (Dyer et al. 1997a,b). This suggests that there should be an increase in leptin sensitivity after feed restriction so we hypothesized that, in underfed rams, exogenous leptin would stimulate LH pulse frequency when endogenous levels of leptin are low because of low energy availability. However, metabolic status and the concentration of leptin might both be reduced when either body reserves are low (chronic reduction) or when there has been a short period of fasting (acute reduc- tion). We therefore tested the efficacy of an icv infusion Reprod Dom Anim 41, 79–90 (2006) ISSN 0936-6768 Ó 2006 Blackwell Verlag, Berlin