Central vs. peripheral metabolic control of estrous cycles in Syrian hamsters. I. Lipoprivation

Metabolic energy availability has profound effects on reproduction in a wide variety of species. We have been studying the effects of fasting on estrous cycles in Syrian hamsters as a model system for metabolic control of reproduction. In previous experiments, a 48-h period of fasting inhibited estrous cycles in lean, but not fat, hamsters. In fat hamsters the effects of fasting may have been offset by the presence of high circulating levels of free fatty acids mobilized from lipids in adipose tissue. Consistent with this idea fat hamsters treated with the inhibitor of fatty acid oxidation methyl palmoxirate (MP) showed fasting-induced anestrus. Experiment 1 was designed to examine whether vagally transmitted signals are critical for the inhibitory effects of fasting and MP treatment. Lean or fat hamsters that had received bilateral subdiaphragmatic vagotomy or sham surgery were fasted and treated with MP or vehicle. In vagotomized and sham-operated hamsters, estrous cycles were inhibited in lean fasted hamsters and in fat fasted hamsters treated with MP, but not in fat fasted hamsters treated with vehicle. Thus the results of experiment 1 indicated that vagally transmitted signals about peripheral fatty acid availability are not critical for the effects of these particular metabolic challenges on estrous cycles in Syrian hamsters. In experiment 2, hamsters without food were allowed to ingest pure glucose or fructose solutions or vegetable shortening. One-half of each group was treated with an inhibitor of glucose utilization, 2-deoxy-D-glucose (2-DG), or vehicle. If ingestion of fructose or shortening, but not glucose, had protected hamsters from 2-DG-induced anestrus, this might have indicated that peripheral fuel availability is critical for anestrus. On the contrary, 2-DG treatment induced anestrus regardless of the type of fuel ingested. Neither experiment yielded results that implicated changes in peripheral fuel availability as a critical signal in metabolic control of estrous cycles.