Leptin, the peptide hormone product of the ob gene, regulates food intake and energy expenditure at the hypothalamic level via the long-form of the leptin receptor (Ob-Rb). Leptin also plays a key role in determining the onset of puberty, but there is controversy as to whether leptin provides a trigger for puberty or is a permissive signal. Thus, although leptin administration can advance puberty onset in rodents, circulating leptin appears stable across puberty. While these data suggest a permissive role for leptin in rat puberty, it is possible that a change in hypothalamic response to leptin (e.g. via increased Ob-Rb expression) could enhance leptin action and thus trigger puberty without a rise in circulating leptin. In the present study we assessed developmental changes in hypothalamic Ob-Rb mRNA and protein expression in female and male rats from late fetal to postpubertal life. Quantitative RT-PCR showed that Ob-Rb mRNA increased (P<0.05) by around fivefold from fetal to postpubertal life in both females and males. These increases in Ob-Rb mRNA expression were gradual, but did not increase significantly between postnatal day 30 (pre-puberty) and day 51 (post-puberty). By day 51, hypothalamic Ob-Rb mRNA expression was higher (P<0.05) in females relative to males. Hypothalamic Ob-Rb protein showed a comparable developmental pattern (approximate threefold increase from fetal to postpubertal life), although a significant increase (15%; P<0.05) was observed between days 30 and 51 in females. Plasma leptin levels exhibited a dynamic pattern in both male and female rats during the prepubertal period, characterised by a precipitous fall after birth, relative stability to day 5, then a rapid increase to a transient peak on day 12. Plasma leptin then remained unchanged from day 15 in female rats but increased in males after puberty, thus confirming the well-recognised sex difference in adult rat leptin levels. In conclusion, this study shows that developmental increases occur not only in plasma leptin but also in hypothalamic Ob-Rb expression, suggesting that both are likely to influence the timing of puberty onset. Moreover, our data show that sex differences in both hypothalamic Ob-Rb and plasma leptin emerge only after puberty.

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