Pulsatile leptin secretion is independent of luteinizing hormone secretion in prepubertal sheep.

Abstract

Many studies have suggested that leptin modulates the gonadal axis. A synchronicity of luteinizing hormone (LH) and leptin has been described in humans, suggesting that leptin may modulate the episodic secretion of LH. The objective of this study was to establish whether episodic leptin secretion depends on the episodic LH secretion in prepubertal sheep. We used two different approaches. The first consisted of blocking the release of LH using a long-acting LH-releasing hormone (LHRH) agonist and analyzing the episodic LH and leptin secretions. The second method stimulated the pituitary gland with pulses of LHRH and again LH and leptin secretions were analyzed. Spring-born 20-wk-old Suffolk ewe lambs (n = 5) received intramuscularly a long-acting LHRH agonist (Decapeptyl). Treatment was repeated at 24 and 28 wk of age. Control lambs (n = 6) received the vehicle of Decapeptyl. Diurnal and nocturnal pulsatilities of LH and leptin were studied at 20 (before Decapeptyl injection), 26, and 30 wk. Blood samples were taken at 10-min intervals for 6 h, beginning at 10:00 AM (diurnal sampling) and at 10:00 PM (nocturnal sampling). In all samples, LH and leptin were measured by radioimmunoassay, and pulsatile hormone secretion characteristics were assessed by the CLUSTER program. To characterize further the synchronicity between LH and leptin pulses, LHRH (10 ng/kg body wt) was injected at 60-min intervals, six times, to another five 30-wk-old ewe lambs, for the same time period as the pulsatility study. In the control group, LH secretion did not change between lambs of 20 and 30 wk of age. In LHRH agonist-treated lambs, LH secretion diminished from 20 to 30 wk of age and was lower than in control lambs at 26 and 30 wk of age (p < 0.05). The transversal mean (ng/[mL x 6 h]) of leptin concentrations was different between control lambs of 20 wk of age and 26 and 30 wk of age (p < 0.01). Contrary to the findings in LH secretion, in LHRH agonist-treated lambs, mean plasma leptin concentrations did not decrease. Furthermore, the mean diurnal and nocturnal leptin concentrations and the pulse amplitude were higher at 26 and 30 wk than at 20 wk in LHRH agonist-treated lambs (p < 0.05). There were no differences between diurnal and nocturnal parameters of leptin secretion in both groups. There was no synchronicity between LH and leptin pulses. LHRH pulses significantly increased plasma LH concentrations, producing discernible LH pulses; however, leptin amplitude and leptin pulse frequency were not modified by the exogenous LHRH pulses, exhibiting no coincidence with LH pulses. The data suggest that pulsatile leptin secretion is independent of LH secretion in ewe lambs.