The androgen status of aging male rhesus macaques.

Whether the rhesus male experiences changes in the dynamics of testosterone (T) and LH secretion with age was investigated in six young (6-8 yr old; 7-12 kg) and six aged (22+ yr old; 9-11 kg) intact rhesus males. Each male was fitted with an indwelling jugular catheter, which was attached to a cranial platform and stainless steel cable tether. Catheters passed from a swivel device at the top of each cage through a wall to an adjoining room. On four occasions, 1.0-ml blood samples were obtained from each male every 20 min for 24 h for plasma T RIA and plasma LH bioassay. Plasma T and LH data were analyzed by the PULSAR program to detect hormone peaks. Mean 24-h plasma T levels were similar in young and aged males as were the pulse amplitudes of T; however, the pulsatile pattern of T concentrations was different between young and aged males. Young males displayed a marked nocturnal increase in both T concentration and number of T pulses. Aged males demonstrated a significant nocturnal elevation of plasma T concentration, but displayed a significant nocturnal diminution in both concentration and numbers of T pulses compared with young males at night. Although LH concentrations and the total number of LH pulses per 24 h were similar in young and aged males, marked age-related alterations were evident in the pulsatile pattern of LH levels. Unlike young males, aged males failed to display a daytime reduction in the number of LH pulses. Our data point to coexistent changes in hypothalamic sensitivity to the negative feedback effects of T and testicular responsiveness to LH as a function of age in the rhesus macaque.