Endocrine Correlates of Sexual Development in the Male Monkey, Macaca fascicularis*

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Abstract
To increase our understanding of the physiology of sexual maturation in the primate, we have studied various endocrine correlates of reproductive development in the male monkey, Macaca fascicularis. In a cross-sectional study, we examined 80 monkeys, ranging in age from 2 weeks to more than 8 yr. We weighed, measured testis size, and obtained a single morning blood sample from all animals. Serum LH and testosterone (T) levels were measured by bioassay and RIA, respectively. Based on the criteria of age and testis size, we classified the monkeys into five study groups: neonates, older infants, juveniles, peripubertals, and adults. To characterize hormone secretory patterns and to search for evidence of episodic LH and T secretion during development, we selected animals at random (n ≥ 6) from each study group and bled them at 20- to 30-min intervals during evening hours (between 1900–2300 h). To assess pituitary responsiveness to LHRH during development, we administered LHRH (5 μg/kg, iv bolus) to groups of neonatal, older infant, juvenile, and adult animals (n ≥ 4). LH and T concentrations were integrated over the 90-min postinfusion period. Analysis of serum LH and T levels measured in single morning blood samples showed significantly higher levels of both LH and T in neonatal and older infants, peripubertals, and adults compared to juveniles (P < 0.02). The LH responses to LHRH infusions were significantly greater in neonates and adults compared to those in older infants and juveniles (P< 0.0006). Neither the neonatal vs. the adult nor the older infant vs. the juvenile LH responses differed significantly from one another (P> 0.05). All peripubertal and adult animals showed evidence of episodic LH and T secretion during the evening sampling interval; each T secretory episode was either coincidental with, or preceded by, a LH increment with a lag time of 30 min or less. Six of six infants and three of eight juveniles demonstrated evidence of episodic variation in serum LH concentrations over the sampling interval; five of six infants and seven of eight juveniles showed episodic variation in serum T levels. These data suggest that episodic LH secretion occurs throughout the life cycle, and that amplitude modulation of episodic LH secretion, caused by a changing amplitude of episodic LHRH secretion, is an important characteristic of the shifts between developmental stages.