INS and obesity due to melanocortin 4 receptor deficiency: In this study, insulin signaling and Tau phosphorylation were followed in the hippocampi of 12- (young) and 33-week-old (old) obese Zucker fa/fa rats and their lean controls to verify the hypothesis that peripheral insulin resistance resulting from obesity and/or old age represents a risk factor for central insulin resistance and that such possible central IR is linked to the pathologic phosphorylation of Tau protein.