Reversal of fasting-induced suppression of luteinizing hormone (LH) secretion in male rhesus monkeys by intragastric nutrient infusion: evidence for rapid stimulation of LH by nutritional signals.

Short periods of fasting (1-2 days) significantly suppress the frequency of pulsatile LH secretion in adult male rhesus monkeys. Resumption of food intake (i.e. refeeding) after fasting rapidly restores pulsatile LH secretion. We hypothesized that the resumption of LH secretion after refeeding results from the metabolic consequences of food intake rather than from relief of the psychological stress associated with fasting. To differentiate between these two possible types of signals, we attempted to provide fasting monkeys with the nutritional signals of a refeed meal without relieving the psychological stress associated with fasting. To do this, we provided monkeys with a refeed meal via indwelling gastric cannulae while continuing to deny them access to monkey chow. Nine adult male rhesus monkeys with indwelling gastric and venous catheters were fasted for 1 day, and on the following day they received a liquid nutrient infusion (equal in caloric content and macronutrient composition to a normal meal) at the normal feeding time (1100 h) via the gastric cannulae. Blood samples were collected from 0600-2400 h at 20-min intervals. Nutrient infusions caused a restoration of pulsatile LH secretion that was not different from the restoration caused by refeeding a normal meal of Purina monkey chow. In contrast, LH secretion remained suppressed on a second day of fasting. Additionally, monkeys receiving nutrient infusions displayed behavioral agitation similar to fasting monkeys, while monkeys refed monkey chow showed very little agitation. These findings support the hypothesis that nutritional/metabolic signals, rather than the relief of psychological stress, lead to the resumption of pulsatile LH secretion after refeeding.