Leptin actions on the reproductive neuroendocrine axis in sheep.

There is a growing literature on the role of leptin in appetite and neuroendocrine regulation in domestic ruminants. Circulating leptin concentration is higher in fat than in thin sheep, is reduced by chronic underfeeding and is higher in sheep subjected to long-day rather than short-day photoperiods. Leptin is reduced acutely by fasting and increases after meals so that there are long- and short-term components to the systemic leptin signal. Nutritional stimulation of reproductive neuroendocrine output is associated with increased circulating concentrations of leptin; peripheral leptin administration restores LH secretion in fasted sheep, and leptin is permissive (although not a trigger) for puberty. Intracerebroventricular (i.c.v.) pharmacological leptin infusion stimulates LH in underfed but not in well-fed sheep, and reduces food intake in well-fed sheep. A single i.c.v. pharmacological injection or physiological infusion of leptin stimulates LH in well-fed sheep, with or without a concomitant decrease in appetite. Furthermore, these appetite and LH responses are differentially affected by photoperiod, indicating that different neuronal pathways may mediate the two responses. Hypothalamic leptin receptors co-localize with orexigenic and anorexigenic neurones, some of which contact GnRH cells, but the confluence of leptin signalling with photoperiod (melatonin) signalling remains unresolved. Photoperiod-entrained sheep provide potential models of altered central leptin sensitivity, in which downstream mechanisms regulating appetite and GnRH may be dissociated.