The mechanisms of marked increase in plasma leptin soon after ventromedial hypothalamus (VMH) lesions were investigated. Although rats did not gain body weight or parametrial fat-pad mass 24 h after the operation, the acute VMH-lesioned rats exhibited substantial five- and fourfold increases in plasma leptin levels compared with sham-operated control rats in fed (22.6 +/- 3.2 vs. 5.8 +/- 1.2 ng/ml) and fasted (8.8 +/- 2.0 vs. 2.3 +/- 0.3 ng/ml) states, respectively. Plasma insulin concentration was doubled in VMH-lesioned rats compared with sham-operated controls in both fed and fasting states. Northern blot analysis revealed that mRNA of ob gene was not increased in parametrial fat pad of animals 24 h after the creation of VMH lesions. However, leptin content in the fat pad was significantly increased in VMH-lesioned rats compared with sham-operated controls (32.2 +/- 4.7 vs. 17.4 +/- 2.3 ng/g wet tissue). The leptin content in parametrial fat pad was highly correlated with plasma leptin concentrations (r = 0.898, P < 0.001). To define the effect of hyperinsulinemia on their hyperleptinemia, a small dose of streptozotocin (STZ) (25 mg/kg body wt) was intravenously administered into rats 5 days before the creation of VMH lesions. Plasma insulin levels were not increased after VMH lesions in STZ-pretreated rats. Plasma leptin levels were halved in the absence of hyperinsulinemia, but still remained twofold higher than those in their sham-operated counterparts (9.9 +/- 1.3 vs. 4.8 +/- 0.7 ng/ml). These results indicate that the destruction of VMH rapidly promotes leptin production before obesity develops through an enhanced translational process in which hyperinsulinemia occurring after VMH lesioning plays an important role. The present study also suggests that there are other mechanisms that rapidly upregulate leptin production in adipocytes in VMH-lesioned rats in which the target organ of this hormone has been destroyed.

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