| Abstract
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The experiments herein examine the behavioral and physiological responses involved in the metabolic control of reproduction using Syrian hamsters ( Mesocricetus auratus) and sheep (Ovis aries) as animal models. The hamster reproductive cycle is sensitive to energy availability, and the present research investigates the relative importance of sensory signals from bulk intake, gastric fill, or caloric content on estrous cyclicity in Syrian hamsters. The caloric content of a laboratory chow diet was diluted with indigestible cellulose and fed to food-deprived, female hamsters throughout day 2 of the estrous cycle. The frequency of hamsters showing normal 4-day estrous cycles decreased with diet dilution, plasma insulin concentrations, and caloric intake. Bulk intake either increased or did not change significantly, and the rate of gastric emptying did not change significantly with diet dilution. The large body size in the sheep allows for measurement of the specific, rapid effects of diet manipulation on reproduction using pulsatile secretion of luteinizing hormone (LH) from the pituitary gland as a marker of reproductive function. Food restricted, reproductively suppressed, lean sheep were allowed free access to food and the recovery of pulsatile secretion of LH was observed. Recovery of pituitary LH secretion occurred in 2 phases: an initial, rapid stimulation of LH, and complete recovery by 9 days of the return to free access to food. Prior to the initial LH pulses, plasma ketone bodies first fell and then rose to levels significantly above those in food-restricted ewes. Significant changes in circulating insulin, glucose, and total ketone body concentrations, daily food intake and lean body mass preceded restoration of pulsatile LH secretion. Full recovery of pulsatile LH secretion was accompanied by increased expression if pro-opiomelanocortin (POMC) mRNA in the arcuate nucleus and decreased expression of neuropeptide Y (NPY) mRNA and ObRb, which encodes for the long-form of the leptin receptor. No changes in KiSS-1 in the arcuate nucleus accompanied the restoration of pulsatile LH secretion, but KiSS-1 expression in the medial preoptic area of re-fed animals resembled that of fed animals.
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