Suppression of Pulsatile Luteinizing Hormone and Testosterone Secretion during Short Term Food Restriction in the Adult Male Rhesus Monkey (Macaca mulatta)*

Abstract

To determine whether signals occurring during the early stages of undernutrition can have a suppressive effect on the central drive to the reproductive axis, the effects of 1 day of fasting on pulsatile LH and testosterone secretion were examined in adult male rhesus monkeys. Monkeys were maintained with chronic indwelling venous catheters on tether/swivel systems. One day of fasting caused a small weight loss of 0.1-0.2 kg, which represented a loss of 1-3% of the initial body weight. On a day of normal feeding monkeys showed a mean of 4.57 +/- 0.53 LH pulses/12 h (measured from 1900-0700 h). On a subsequent day of fasting LH pulse frequency was significantly reduced to 1.86 +/- 0.46 pulses/12 h (P less than or equal to 0.05). Likewise, there was a similar decrease in testosterone pulse frequency on a day of fasting. The substantial decrease in LH/testosterone pulse frequency was not caused by the intensive blood-sampling regimen, in that collection of blood samples for 12 h on 2 consecutive days of normal feeding did not result in a decrease in either LH or testosterone pulse frequency. Administration of exogenous GnRH in doses of 0.05-0.3 microgram/kg caused LH pulses of similar magnitudes on a day of normal feeding and a day of fasting, suggesting that the decrease in LH pulse frequency during the day of fasting reflects a decrease in GnRH stimulation of the pituitary rather than a loss of pituitary sensitivity to GnRH. Measurement of pulsatile LH across 3 consecutive days (e.g. a day of normal feeding, a day of fasting, and a day of refeeding) indicated that LH pulse frequency is slow before the time that the meal is missed on the second day and remains low throughout the day of fasting (normal feeding, 7 +/- 1.16 pulses/24 h; fasting, 3.33 +/- 0.33 pulses/24 h). Refeeding a normal meal at 1100 h on the third day resulted in an immediate and sustained increase in pulsatile LH secretion above normal frequency (11.07 +/- 0.33 pulses/24 h). These results indicate that very brief periods of undernutrition can significantly suppress the central drive to the reproductive axis in male rhesus monkeys, and this suppression can be rapidly reversed by refeeding. These findings argue against the hypothesis that undernutrition only suppresses central drive to the reproductive axis once a substantial amount of body weight has been lost.