Pubertal Changes in Gonadotropin-Releasing Hormone and Proopiomelanocortin Gene Expression in the Brain of the Male Rat*

Abstract

Pubertal development in mammals is in part attributable to a brain-dependent process, whereby increased pulsatile GnRH secretion leads to the awakening of the entire reproductive system. However, the brain mechanisms controlling this event are unknown. The apparent increase in GnRH secretion at puberty could reflect an autonomous change in the activity of GnRH neurons themselves or in the afferent networks leading to GnRH neurons. If there were a significant increase in the secretion of GnRH with puberty onset, we hypothesized that there would be a commensurate increase in the biosynthetic capacity of GnRH neurons to meet the increasing demand. We tested this hypothesis by comparing the level of cellular prepro-GnRH mRNA (GnRH mRNA) observed between prepubertal (25-day-old; n = 5) and adult (75-day-old; n = 4) male rats by in situ hybridization. We detected no significant change with puberty in GnRH mRNA signal levels in any of the anatomical areas examined, which included the vertical limb of the diagonal band of Broca, medial septum, lateral preoptic area, and medial preoptic area. Given the variance of our analytical technique, we determined that there was a greater than 90% probability that we would have detected a 20% increase in GnRH mRNA had there been one. Endogenous opioid peptides have been implicated in timing the onset of puberty in the rat, with the argument being that a loss in opioid tone could effect a disinhibition of GnRH secretion. One opioid peptide, beta-endorphin, is among several peptides cleaved from the precursor POMC. We hypothesized that with puberty, POMC neurons in the arcuate nucleus would have an attenuated capacity to produce beta-endorphin. We tested this hypothesis by comparing cellular pre-POMC mRNA (POMC mRNA) levels in the arcuate nuclei of prepubertal (n = 6) and adult (n = 7) male rats with in situ hybridization. We observed an increase in POMC mRNA levels with puberty; prepubertal rats had relative POMC mRNA signal levels of 119 +/- 10 grains/cell, while adult rats contained 167 +/- 12 grains/cell (P less than 0.02). This increase in cellular POMC mRNA was confined to the rostral portion of the arcuate nucleus. We conclude that the GnRH gene is fully expressed well before the time of normal puberty onset and that the increase in POMC mRNA that occurs with the onset of puberty may be important for the development of pulsatile GnRH secretion.

Rat RFRP-3 Alters Hypothalamic GHRH Expression and Growth Hormone Secretion but Does Not Affect KiSS-1 Gene Expression or the Onset of Puberty in Male Rats

“…Puberty is defined as the initial increase in pulsatile GnRH release; however, there does not appear to be a change in GnRH gene transcription between prepubertal and adult ages [28]. The mechanism of the pubertal increase in pulsatile GnRH release is not clear; however, one theory is that GnRH release is intrinsically inhibited in prepubertal animals.…”

“…The mechanism of the pubertal increase in pulsatile GnRH release is not clear; however, one theory is that GnRH release is intrinsically inhibited in prepubertal animals. The pro-opiomelanocortin gene was a potential candidate as the prepubertal GnRH inhibitory agent; however, further evidence did not support this theory [28]; thus, the identity of this GnRH inhibitor has remained elusive. Recently, a potential candidate GnRH inhibitor was discovered in the avian brain and designated as the so-called GnIH [9, 29].…”

Sex differences in the effect of prepubertal GALP infusion on growth, metabolism and LH secretion

“…The ontogeny of neuropeptide gene expression in the Arcuate nucleus (Arc) is critical for the neuroendocrine control of feeding, metabolism, and reproduction [19][20][21][22][23][24][25][26][27][28][29]. In adult rats, GALP is expressed in the Arc of the hypothalamus and GALP gene expression is regulated by leptin, insulin, and metabolic fuels [6,30,31].…”

The neuroendocrine timing of puberty

“…These findings in the mouse are complemented by many studies in the rat. In an in situ hybridisation study in the male rat, no increases in GnRH mRNA levels were detected at the time of the pubertal increase in plasma testosterone, that is, after 25 days of age (Wiemann et al 1989). However, significant increases were observed at earlier ages, as determined by solution hybridisation RNAse protection assays (Gore et al 1996) or by PCR-based assays of hypothalamic extracts (Dutlow et al 1992).…”