The Androgen Status of Aging Male Rhesus Macaques*

Abstract

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.