Comparing the hypothalamic and extrahypothalamic actions of endogenous hyperleptinemia

To determine whether the depletion of body fat caused by adenovirus-induced hyperleptinemia is mediated via the hypothalamus, we used as a "bioassay" for hypothalamic leptin activity the hypothalamic expression of a leptin-re gulated peptide, cocaine- and amphetamine-regulated transcript (CART), The validation of this strategy was supported by the demonstration that CART mR NA was profoundly reduced in obese rats with impaired leptin action, whethe r because of ablation of the ventromedial hypothalamus (VMH) or a loss-of-f unction mutation in the leptin receptor, as in Zucker diabetic fatty rats, We compared leptin activity in normal rats made hyperleptinemic by adenovir us-leptin treatment (43 +/- 9 ng/ml, cerebrospinal fluid leptin 100 pg/ml) with normal rats made hyperleptinemic by a 60% fat intake (19 +/- 4 ng/ml, cerebrospinal fluid leptin 69 +/- 22 pg/ml), CART was increased 5-fold in t he former and 2-fold in the latter, yet in adenovirus-induced hyperleptinem ia, body fat had disappeared, whereas in high-fat-fed rats, body fat was ab undant. Treatment of the high-fat-fed rats with adenovirus-leptin further i ncreased their hyperleptinemia to 56 +/- 6 ng/ml without changing CART mRNA or food intake, indicating that leptin action on hypothalamus had not been increased. Nevertheless, their body fat declined 36%, suggesting that an e xtrahypothalamic mechanism was responsible. We conclude that in diet-induce d obesity body-fat depletion by leptin requires supraphysiologic plasma con centrations that exceed the leptin-transport capacity across the blood-brai n barrier.