Daily patterns of plasma leptin in sheep: effects of photoperiod and food intake

Circulating concentrations of leptin in sheep correlate with body fatness a nd are affected by level of food intake and photoperiod. The present object ive was to elucidate the short-term dynamics of leptin secretion. Frequent blood samples were taken over 48 h from 12 Soay rams after 16 weeks in shor t-day photoperiod (SD, 16 h darkness:8 h light) with freely available food, and then after 16 weeks in long days (16 h light:8 h darkness) with food f reely available (LD) or restricted to 90% maintenance (LDR) (n=6/ group). D uring the second 24 h of sampling, half were food deprived (,n=6, SD and LD ) and half had their meal times shifted (n=6, SD and LDR). A homologous RIA was developed, using antibodies raised in chicken against recombinant ovin e leptin, to measure plasma concentrations. Simultaneous 24 h profiles of p lasma insulin, glucose and non-esterified fatty acids (NEFA) were measured. Plasma leptin was higher in LD than SD, and in LD than LDR, associated wit h higher food intake, liveweight and body condition score (adiposity), but tended to be lower in LDR than SD, associated with lower food intake, Livew eight and body condition score. There was no evidence for a circadian rhyth m of plasma leptin, but clear evidence for post-prandial peaks of low ampli tude (15-36%) 2-8 h after meals given at normal and shifted times. Complete food deprivation caused a dramatic fail in plasma leptin to basal levels w ithin 24 h. There was a positive association of plasma leptin with plasma i nsulin, and negative association with NEFA, both between meals and during f asting. Thus, plasma leptin concentrations in sheep are sensitive to short- term changes in energy balance, as well as to long-term photoperiod-driven changes in food intake and adiposity.