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.