LEPTIN SIGNALING IN THE HYPOTHALAMUS OF NORMAL RATS IN-VIVO

Leptin has been shown to activate multiple signaling molecules in cult ured cells, including Janus kinase-2, STAT (signal transducer and acti vator of transcription) proteins, and mitogen-activated protein kinase , and to stimulate the DNA-binding activity of STAT3 in mouse hypothal amus. In this study, the activation of candidate leptin signaling mole cules in the hypothalamus of normal rats in vivo was investigated. Fas ted male Sprague-Dawley rats were injected iv with recombinant murine leptin or vehicle. Plasma leptin concentrations were determined at def ined time points, and the phosphorylation of signaling proteins was as sessed in hypothalamic lysates. There was a marked increase in plasma leptin concentration at 2 min and a gradual decline by 45 min after le ptin injection. Immunoblotting analysis of hypothalamic lysates with a phosphospecific STAT3 antibody demonstrated a time-dependent stimulat ion of STAT3 tyrosine phosphorylation. STAT3 phosphorylation was first evident at 5 min and was maximal at 30 min after leptin injection. By contrast, leptin did not increase the phosphorylation of Janus kinase proteins, mitogen-activated protein kinase, or STAT1 and -5 despite a bundant expression of these signaling molecules in the hypothalamus. T hese results differ fi om findings in cultured cells and in vitro syst ems. It remains unclear how signaling is propagated downstream from th e leptin receptor to STAT3, but this may involve novel signaling inter mediates.