Fourth-ventricle leptin infusions dose-dependently activate hypothalamic signal transducer and activator of transcription 3

Abstract

Previous studies have shown that very low-dose infusions of leptin into the third or the fourth ventricle alone have little effect on energy balance, but simultaneous low-dose infusions cause rapid weight loss and increased phosphorylation of STAT3 (p-STAT3) in hypothalamic sites that express leptin receptors. Other studies show that injecting high doses of leptin into the fourth ventricle inhibits food intake and weight gain. Therefore, we tested whether fourth-ventricle leptin infusions that cause weight loss are associated with increased leptin signaling in the hypothalamus. In a dose response study 14-day infusions of increasing doses of leptin showed significant hypophagia, weight loss, and increased hypothalamic p-STAT3 in rats receiving at least 0.9 μg leptin/day. In a second study 0.6 μg leptin/day transiently inhibited food intake and reduced carcass fat, but had no significant effect on energy expenditure. In a final study, we identified the localization of STAT3 activation in the hypothalamus of rats receiving 0, 0.3, or 1.2 μg leptin/day. The high dose of leptin, which caused weight loss in the first experiment, increased p-STAT3 in the ventromedial, dorsomedial, and arcuate nuclei of the hypothalamus. The low dose that increased brown fat UCP1 but did not affect body composition in the first experiment had little effect on hypothalamic p-STAT3. We propose that hindbrain leptin increases the precision of control of energy balance by lowering the threshold for leptin signaling in the forebrain. Further studies are needed to directly test this hypothesis.

Low-dose leptin infusion in the fourth ventricle of rats enhances the response to third-ventricle leptin injection

“…This suggests that either leptin is cleared more slowly from the cerebrospinal fluid (CSF) during the postinjection period or that fourth-ventricle leptin infusion allows the forebrain to continue to respond to leptin as the concentration gradually declines in the postinjection period. The second option would imply that fourth-ventricle leptin infusions lower the threshold of response to leptin in the third ventricle and would be consistent with previous reports that leptin injections into the fourth ventricle or directly into the NTS lead to activation of p-STAT3 in the hypothalamus (13,24). A third alternative is that leptin injected into the third ventricle diffuses into the fourth ventricle and the resultant rise in fourth-ventricle leptin concentration is responsible for the enhanced response to leptin injection, but this would be expected to exaggerate, not extend, the changes that follow leptin injection.…”

“…It is clear from the experiments described here in addition to many in the literature (3,13,20,21,26) that there is a large difference in hypothalamic vs. hindbrain leptin sensitivity. The 10-fold lower sensitivity in the hindbrain could be interpreted as it having a less important role in mediating the effects on energy balance; however, it is possible that the difference in sensitivity is associated with the different functions of the brain sites that express leptin receptors.…”

“…This is consistent with previous results (4, 5), and it is possible that leptin is activating a different signaling pathway in the hindbrain. ERK activation has been measured in other studies, but there was no measurable response to fourth-ventricle leptin infusions (4,13). The Western blots did show that infusing 0.01 g (or more) of leptin per day into the third ventricle doubled hypothalamic p-STAT3.…”

“…The weight loss is associated with increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) in hypothalamic nuclei that are known to express leptin receptors (5). In addition, we have shown that weight loss in rats receiving fourth-ventricle infusions of progressively increasing doses of leptin correlates with phosphorylation of STAT3 (p-STAT3) in these hypothalamic nuclei (13). We have interpreted these data as evidence that activation of receptors in both the forebrain and hindbrain is required for a leptin-induced loss of weight and that when leptin concen-tration is increased in the hindbrain, this results in a lowering of the threshold for activation of forebrain leptin receptors.…”

“…We and others have reported that single injections (20,26) or continuous infusions of leptin into either the third (27) or the fourth ventricle (13) inhibit food intake and body weight of rats; however, activation of leptin receptors distant from the site of injection or infusion was not evaluated. Leptin injected into the third ventricle has the potential to diffuse through the brain ventricles and activate receptors in the fourth ventricle (9,27), and it has been reported that leptin injection (24) or infusion (13) into the fourth ventricle increases p-STAT3 in hypothalamic nuclei. Thus leptin receptors in both the hypothalamus and hindbrain are activated when leptin induces a state of negative energy balance.…”