The fat cell hormone leptin plays a central role in regulation of energy reserves (total body fat content). Changes in fat content produce corresponding changes in leptin signaling to the brain to affect energy intake and expenditure in order to regulate energy reserves. However, in a growing number of people chronically exposed to large serving sizes of highly palatable, energy dense foods, the stimulus to eat hedonically attractive food appears to offset leptin signaling to reduce food intake. The chronic hyperleptinemia that occurs with weight gain in these individuals is thought to produce leptin insensitivity, further weight gain, and obesity. Current anti-obesity therapies - dieting, exercise, medications - are woefully ineffective in producing long-term weight loss. Recent studies suggest that weight loss in obese humans produces a relative leptin deficiency, which activates potent feeding stimulatory and energy conservation mechanisms in the brain to restrict further weight loss and promote weight regain. It remains to be determined whether leptin replacement increases the efficacy of caloric restriction or pharmacotherapy to produce weight loss in obese individuals. Since chronic hyperleptinemia can cause leptin insensitivity, defining an optimal dosing regimen for leptin replacement is critical for this combination therapy to work. Experiments outlined for Specific Objective 1 will determine whether leptin replacement after a caloric restriction-induced weight loss of 10% in diet-induced obese (DIO) rats, attenuates post-restriction hyperphagia and weight regain. Several different doses of leptin will be administered for 3 weeks either by intermittent intraperitoneal (IP) infusion or continuous subcutaneous (SC) infusion; measurements will include hourly and daily food intake, weekly body weight, and body fat content, plasma leptin, and leptin sensitivity at end of treatment period. Leptin sensitivity will be assessed by immunostaining leptin signaling protein phosphorylated signal transducer and activator of transcription 3 (pSTAT3) in cells of the arcuate nucleus of the hypothalamus. Information gained from these experiments will be crucial for determining whether leptin replacement attenuates hyperphagia and weight regain in the DIO rats, whether this effect is dependent on dose or method of leptin delivery, or correlated with change in plasma leptin, or state of leptin sensitivity. Experiments outlined for Specific Objective 2 will determine whether leptin replacement increases the efficacy of chronic intermittent administration of potent anorexigenic peptides (exendin-4 and amylin) to either attenuate hyperphagia and weight regain or promote greater weight loss in weight-reduced DIO rats. In different experiments, optimal dosing regimens for continuous SC and intermittent IP administration of leptin (determined in Specific Objective 1) will be administered for 3 weeks, either alone or with intermittent IP infusion of exendin-4 at a dosing regimen shown previously to be most effective in producing a sustained reduction in daily food intake in DIO rats. Information gained from these experiments will be crucial for determining whether leptin replacement significantly increases the ability of exendin-4 to reduce food intake and body weight in the DIO rats, whether this effect is dependent on method of leptin delivery, or correlated with change in plasma leptin or state of leptin sensitivity. In a subsequent experiment, the other potent anorexigenic peptide amylin will be administered by intermittent IP infusion alone and with either continuous SC or intermittent IP administration of leptin, depending on which method was most effective in previous experiments. Accomplishment of these specific objectives will provide crucial evidence toward identifying whether leptin replacement by continuous SC or intermittent IP infusion, either alone or with exendin-4 or amylin, can produce a sustained decrease in daily food intake and body weight in a rodent model of DIO.