Suppression of luteinizing hormone secretion during short-term fasting in male rhesus monkeys: the role of metabolic versus stress signals.

One day of fasting leads to a significant suppression of pulsatile LH secretion in adult male rhesus monkeys that is evident within the first 4-6 h after a missed meal. Restricting food intake may suppress the central drive to the reproductive axis via metabolic signals occurring during the transition from a fed, energy storing state to a fasted, energy mobilizing state or via signals associated with the "psychological stress" imposed by withholding food. To distinguish between these two general types of signals, we chose the experimental strategy of providing monkeys with a great excess of calories on the day before fasting (i.e. overfeeding on the day before fasting) in order to maintain them in a metabolically fed state on the following day when they were fasted, without preventing the psychological stress associated with fasting. We reasoned that if metabolic signals are responsible for causing the fasting-induced suppression of LH secretion, overfeeding on the day before fasting would prevent the fasting-induced suppression of LH secretion. Eight adult male rhesus monkeys with indwelling venous catheters were studied on three occasions: on a day of normal feeding (receiving approximately 30 pellets of monkey chow at 1100 h on the day of the study), on a day of fasting after a day of normal feeding, and on a day of fasting after a day of overfeeding (receiving one meal of 100 pellets on the day before the study). Pulsatile LH secretion on a day of fasting after a day of normal feeding (0.4 +/- 0.2 pulses/6 h, measured from 1200-1800 h) was significantly suppressed, P < 0.05, compared to a day of normal feeding (1.8 +/- 0.3 pulses/6 h). In contrast, on a day of fasting after a day of overfeeding there was no significant difference in LH secretion measured from 1200-1800 h (1.5 +/- 0.3 pulses/6 h) compared to a day of normal feeding. Overfeeding the day before fasting prevented the normal fasting-induced suppression of plasma T3 concentrations and delayed the normal fasting-induced decline in plasma insulin concentrations. Overfeeding did not prevent the display of agitated behavior that we have previously documented in monkeys when they are fasted. These results support the hypothesis that the signal(s) which suppresses normal LH secretion after brief periods of fasting is related to the metabolic status of the body during the transition from a fed to a fasted state, rather than a function of the psychological state imposed by withholding food.