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J.Y. Lee, P. Mayo, C. Robinson, S.B. Koevary; Topically Applied Leptin Accumulates in the Hypothalamus of Lewis Rats . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5091.
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It has been suggested that blood–brain–barrier (BBB) resistance to leptin, a peptide that plays an important role in the hypothalamic regulation of appetite, is responsible, in part, for diet–induced obesity. We previously reported that topically applied leptin could accumulate in the CSF, suggesting that such an approach might be efficacious in bypassing the BBB. In this study, we specifically quantified leptin levels in the hypothalamus following topical leptin administration.
A 10 microliter drop of a 1.25 mg/ml solution of leptin in 1% saponin was applied to the eyes of fasted, Lewis rats; untreated fasted rats served as controls. The hypothalamus from these animals was harvested 20 minutes later, homogenized, and their supernatants assessed for the presence of leptin using an ELISA assay; results (mean+SE) are expressed as pg of leptin/mg protein (the protein content of the tissue pellets was determined using the Lowry method). To control for the effects of systemically absorbed leptin, a separate cohort of rats was subcutaneously injected with leptin (19 micrograms/rat) and their hypothalamic leptin levels determined after 30 minutes.
Animals treated with leptin eye drops displayed significantly elevated levels of this peptide in the hypothalamus. While baseline values were 22.5+2.9, levels rose to 33.8+3.1 after 20 minutes (p<0.03) in treated animals; serum leptin levels (mean+SE), measured in pg/ml, in these animals were 1698+392 and 2360+794, respectively (not significant, p=0.5). Though serum leptin levels in leptin–injected rats rose 7 fold over controls in leptin–injected rats (p<0.005), their hypothalamic levels were not significantly elevated (42.3+6.8 vs. 30+4.6, p=0.18).
Our results showed that leptin accumulated in the hypothalamus following topical delivery, supporting the contention that this approach should be investigated further to determine its effectiveness for the treatment of diet–induced obesity.
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