A Reevaluation of the Question: Is the Pubertal Resurgence in Pulsatile GnRH Release in the Male Rhesus Monkey (Macaca mulatta) Associated With a Gonad-Independent Augmentation of GH Secretion?

Abstract

A somatic signal has been posited to trigger the pubertal resurgence in pulsatile GnRH secretion that initiates puberty in highly evolved primates. That GH might provide such a signal emerged in 2000 as a result of a study reporting that circulating nocturnal GH concentrations in castrated juvenile male monkeys increased in a 3-week period immediately preceding the pubertal resurgence of LH secretion. The present study was conducted to reexamine this intriguing relationship, again in an agonadal model. Four castrated juvenile male monkeys were implanted with indwelling jugular catheters, housed in remote sampling cages, and subjected to 24 hours of sequential blood sampling (every 30 min) every 2 weeks from 19.5 to 22 months of age. Twenty-four-hour profiles of circulating GH concentrations were analyzed using the pulse detection algorithm, PULSAR, and developmental changes in pulsatile GH release with respect to the initiation of the pubertal rise of LH secretion (week 0; observed between 22.5 and 32 mo of age) were examined for significance by a repeated-measures ANOVA. Changes in the parameters of pulsatile GH secretion, including mean 24-hour GH concentration and GH pulse frequency and pulse amplitude for 3 (n = 4) and 6 (n = 3) months before week 0 were unremarkable and nonsignificant. These findings fail to confirm those of the earlier study and lead us to conclude that the timing of the pubertal resurgence of GnRH release in the male monkey is not dictated by GH. Reasons for the discrepancy between the two studies are unclear.

Figure 1.

Time courses of plasma LH concentrations in four agonadal male rhesus monkeys from approximately 20 months of age until the pubertal increase in the secretion of this gonadotropin was identified (indicated for each animal with the respective colored arrows). LH concentrations were measured weekly at 9:00 am and 9:00 pm and the mean weekly level is shown. In one animal (purple trace), LH was not measured at age 26 months and 1 week.

Figure 2.

The progression of pulsatile GH secretion in four agonadal male monkeys for approximately 3 months before the initiation of the pubertal increase in LH secretion (identified as wk 0 and shown in Figure 1). Circulating GH concentrations were measured every 30 minutes over a 24-hour period every other week. In three animals, the time of day at which sequential sampling was started alternated between the morning (8:30 am) and evening (8:30 pm) (top three animals). For the remaining animal, sequential sampling always started at 8:30 am. All GH profiles have been aligned to the morning start time to appreciate the lack of a diurnal variation in the secretion of this hormone.

Figure 3.

Parameters of pulsatile GH secretion for the 3 months preceding the initiation of the pubertal increase in LH secretion on week 0. Each data bar represents a 2-week period (for example, wk −2 includes data for wk −1 [see Figure 1] and wk 0 includes wk +1). None of the parameters of GH secretion showed significant changes for the 3 months preceding the initiation of the pubertal increase in LH secretion (P < .05, repeated measures ANOVA). Mean ± SE is shown.

Figure 4.

Parameters of pulsatile GH secretion for the 6 months preceding the initiation of the pubertal increase in LH secretion on week 0. Each data bar represents a 2-week period (for example, wk −2 includes data for wk −1 [see Figure 1] and wk 0 includes wk +1). None of the parameters of GH secretion showed significant changes for the 6 months preceding the initiation of the pubertal increase in LH secretion (P < .05, repeated measures ANOVA). Mean ± SE is shown.