Developmental changes of leptin receptors in cerebral microvessels: unexpected relation to leptin transport.

The adipokine leptin participates not only in the regulation of feeding and obesity in adults but also in neonatal development. It crosses the blood-brain barrier (BBB) by receptor-mediated transport. Leptin concentrations in blood differ between neonates and adults. We determined the developmental changes of leptin receptor subtypes in the cerebral microvessels composing the BBB and examined their expected correlation with leptin transport across the BBB. Total RNA was extracted from enriched cerebral microvessels of mice 1, 7, 14, and 60 d of age for real-time RT-PCR analysis of leptin receptor subtypes. In cerebral microvessels from neonates, ObRa, ObRb, ObRc, and ObRe mRNA were all higher than in adults, but ObRd was not detectable. Hypothalamus showed similar age-related changes except for ObRb, which was higher in adults. The homologous receptor gp130 did not show significant age-related changes in either region. Despite the increase of leptin receptors, leptin permeation across the BBB after iv injection was less in the neonates. In situ brain perfusion with blood-free buffer showed no significant difference in the brain uptake of leptin between neonates and adults, indicating an antagonistic role of leptin-binding proteins in the circulation, especially the soluble receptor ObRe. The results are consistent with our previous finding that ObRe antagonizes leptin endocytosis in cultured endothelia and transport from blood to brain in mice. Overall, the developmental changes observed for leptin receptors unexpectedly failed to correlate with the entry of leptin into brain, and this may indicate different functions of the receptors in neonates and adults.