Appearance of a nocturnal peak of leptin secretion in the pubertal rat.

Whether leptin is involved in the timing of puberty remains highly controversial in the rat. Daytime leptin secretion shows little change during the transition into adulthood. Because leptin exhibits a diurnal variation in the adult, it is possible that the ontogeny of such a rhythm provides important information for the timing of puberty. To begin to evaluate this hypothesis, we determined the development of the diurnal leptin secretion in the rat. The young females were raised in a light-controlled environment (12L, 0700 h light on). A cannula was placed in the right atrium on the previous day, and blood samples were collected every 4 h on Days 21, 24, 28, 32, 36 (1 day after vaginal opening), and 48 (adult, diestrus of estrous cycle). In addition to vaginal opening, plasma prolactin levels were determined as an endocrine index of puberty. Changes in food intake were monitored because nocturnal food intake has been considered to be a synchronizer for the leptin rhythm. This pattern of food intake was clearly evident throughout the ages studied. By contrast, there was no leptin rhythm at 21 and 24 days of age. Beginning at 28 days, leptin secretion exhibited a significant nocturnal peak (2300 h); this nocturnal peak increased in amplitude at 32 and 36 days and was still apparent in the cycling adult at Day 48. Plasma prolactin did not exhibit a diurnal rhythm but it increased from Days 32 to 48. The present findings indicate that in the rat, both the appearance of the nocturnal leptin rhythm and the nocturnal increase in circulating leptin levels during development carry information for timing the onset of puberty.