Systemic leptin dose-dependently increases STAT3 phosphorylation within hypothalamic and hindbrain nuclei.

Leptin released peripherally acts within the central nervous system (CNS) to modulate numerous physiological and behavioral functions. Histochemical identification of leptin-responsive CNS cells can reveal the specific cellular phenotypes and neural circuits through which leptin signaling modulates these functions. Leptin signaling elicits phosphorylation of signal transducer and activator of transcription 3 (pSTAT3), making pSTAT3-immunoreactivity (ir) a useful proxy for identifying leptin-responsive cells. Relatively low systemic doses of leptin (i.e., 10-130 μg/kg body wt) are sufficient to decrease food intake, inhibit gastric emptying, and increase sympathetic activity, but there are no histological reports of central pSTAT3-ir following leptin doses within this range. Considering this, we quantified central pSTAT3-ir in rats after intraperitoneal injections of leptin at doses ranging from 50 to 800 μg/kg body wt. Tissue sections were processed to identify pSTAT3-ir alone or in combination with immunolabeling for cocaine- and amphetamine-regulated transcript (CART), glucagon-like peptide-1 (GLP-1), prolactin-releasing peptide (PrRP), or dopamine-β-hydroxylase (DβH). Leptin doses as low as 50, 100, and 200 μg/kg body wt significantly increased the number of pSTAT3-ir cells in the arcuate nucleus of the hypothalamus (ARC), nucleus of the solitary tract (NTS), and ventromedial nucleus of the hypothalamus, respectively, and also led to robust pSTAT3 labeling in neural processes. The differential dose-dependent increases in pSTAT3-ir across brain regions provide new information regarding central leptin sensitivity. Within the ARC, CART-ir and pSTAT3-ir were often colocalized, consistent with evidence of leptin sensitivity in this neural population. Conversely, within the NTS, pSTAT3 only rarely colocalized with PrRP and/or DβH, and never with GLP-1.