Expression of the leptin receptor gene has been examined in mouse hypothalamns and other brain regions by in situ hybridization. With a probe recognizing all the known splice ~ariants, receptor mRNA was evident in several brain regions (cortex, hippocampus, thalamus), with strong expression in the hypothalamus (arcuate, ventromedial, paraventricniar and ventral premammillary nuclei), choroid plexus and leptomeninges. A probe specific to the long splice variant of the leptin receptor (Ob-Rb), containing the putative intracellniar signaling domain, again revealed strong expression in the hypothalamus; there was, however, minimal hybridization to choroid plexus and leptomeninges. These results indicate that the hypothalamus is a key site of leptin action, although other brain regions are also targeted.

Leptin is a 167-aa protein that is secreted from adipose tissue and is important in the regulation of energy balance. It also functions in hematopoiesis and reproduction. To assess whether leptin is involved in fetal growth and development we have examined the distribution of mRNAs encoding leptin and the leptin receptor (which has at least six splice variants) in the 14.5-day postcoitus mouse fetus and in the placenta using reverse transcription-PCR and in situ hybridization. High levels of gene expression for leptin, the leptin receptor, and the long splice variant of the leptin receptor with an intracellular signaling domain were observed in the placenta, fetal cartilage͞bone, and hair follicles. Receptor expression also was detected in the lung, as well as the leptomeninges and choroid plexus of the fetal brain. Western blotting and immunocytochemistry, using specific antibodies, demonstrated the presence of leptin and leptin receptor protein in these tissues. These results suggest that leptin may play a role in the growth and development of the fetus, both through placental and fetal expression of the leptin and leptin receptor genes. In the fetus, leptin may be multifunctional and have both paracrine and endocrine effects.

Leptin, the protein product of the adipose tissue-specific ob (obese) gene (1), reduces the body weight, adiposity and food intake of obese ob/ob mice on peripheral or central injection (2, 3, 4). [125I]leptin binding has been detected in mouse choroid plexus (5), from which a leptin receptor gene was expression cloned (5). The gene has at least 6 splice variants (6, 7). Leptin receptor mRNA was localized in the hypothalamus by in situ hybridization being particularly abundantly expressed in the arcuate nucleus (8). There is evidence linking the physiological effects of injected leptin with hypothalamic neuropeptide Y (9, 10) (NPY), which has potent central effects on food intake and energy balance (11), and is also expressed in the arcuate nucleus. Here we report dual in situ hybridization studies for leptin receptor and NPY gene expression in the mouse arcuate nucleus, where the majority of cells examined expressed both genes. This provides the first direct evidence that leptin acts on cells that express NPY mRNA.

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Siberian hamsters decreased body weight by 30% during 18 wk in short day (SD) vs. long day (LD) controls. Subsequent imposed food deprivation (FD; 24 h) caused a further 10% decrease. In the hypothalamic arcuate nucleus (ARC), SDs reduced proopiomelanocortin (POMC) gene expression and agouti-related protein (AGRP) mRNA was elevated, changes that summate to reduced catabolic drive through the melanocortin receptors. There was no effect of photoperiod on neuropeptide Y (NPY), melanin concentrating hormone, orexin, or corticotropin-releasing factor mRNAs. Superimposed FD increased AGRP gene expression and caused a localized elevation of NPY mRNA in the ARC. Both adipose tissue leptin and ARC leptin receptor (OB-Rb) mRNAs were downregulated in SDs, whereas FD increased OB-Rb gene expression. Thus OB-Rb mRNA is differentially regulated by acute and chronic changes in plasma leptin in this species. In a separate experiment in LDs, AGRP gene expression was increased by 24 or 48 h FD, whereas POMC mRNA was downregulated in the caudal ARC. AGRP and NPY mRNAs were extensively coexpressed in the ARC, and their differential regulation by photoperiod and FD is suggestive of transcript-specific regulation at the level of individual neurons.