Abstract

The obese gene (ob) product, leptin, has recently been shown to be produced by adipocytes and to circulate in the plasma acting as a hormone to modulate appetite and metabolism. Intriguingly, the ob/ob mutant female mouse, which does not produce an active form of leptin due to a mutation of the ob gene, has been shown to be acyclic and sterile. This sterility can be reversed by treatment with recombinant leptin, but not by diet restriction - suggesting that leptin is required for normal reproductive function. The mechanism(s) whereby leptin modulates reproductive function are unknown; however, it is possible that leptin could directly regulate reproductive tissues. To determine whether endocrine and neuroendocrine tissues could be targets for leptin action, we examined whether these tissues express the leptin receptor mRNA by utilizing reverse-transcription polymerase chain reaction (RT-PCR) analysis in selected tissues from the male and female rat. The results revealed that the leptin receptor mRNA transcript is highly expressed in the ovary, uterus and testis, moderately expressed in the hypothalamus and anterior pituitary, with low to no expression in the adrenal. The RT-PCR results were confirmed by Northern analysis. Furthermore, immortalized GnRH (GT1-7 and NLT) neurons and ovarian granulosa cells were also demonstrated by RT-PCR analysis to express the leptin receptor, suggesting that GnRH neurons and steroid-producing cells of the ovary could be targets for leptin action. Immunohistochemical studies revealed dense immunolocalization of the leptin receptor in the choroid plexus, and interestingly, in the arcuate nucleus/median eminence of the female rat - a key sit in the control of feeding and reproduction. Finally, treatment of the ob/ob mouse with recombinant leptin (0.15 mg/kg/day x 2 weeks) was found to markedly upregulate side chain cleavage and 17 alpha-hydroxylase mRNA levels in the ovary, demonstrating that leptin, acting either through a direct or indirect mechanism, can regulate gene expression in reproductive tissues.