Diurnal rhythmicity of gonadotropin-releasing hormone gene expression in the rat

The release of the decapeptide gonadotropin-releasing hormone (GnRH) from neuroterminals in the median eminence varies depending upon the stage of development, time of day, reproductive season, and day of the ovarian cycle. With respect to the biosynthetic events responsible for such alterations in GnRH release, we have reported that increases in GnRH mRNA occur concomitantly with or precede the proestrous GnRH/LH surge in female rats, and the pubertal increase in GnRH. Because a hallmark of puberty is the development of nocturnal increases in gonadotropin/GnRH release, in the present study, we examined whether a rhythm in GnRH gene expression occurs that may be responsible for this rhythm of neurosecretion. Additionally, we extended the previous finding of diurnal rhythms of GnRH gene expression in cycling females to adult male rats. The mechanism for these changes, i.e. GnRH gene transcription, RNA processing or a posttranscriptional mechanisms was also elucidated. In experiment I, GnRH gene expression was measured in female rats at different stages of puberty [P25 (juvenile); approximately P37 (day of vaginal opening); and P45 (postpubertal)] at 10.00, 14.00, 18.00, 22.00, 2.00 and 6.00 h. GnRH mRNA levels increased significantly during pubertal development, independent of any changes in GnRH gene transcription or RNA processing, indicating a posttranscriptional mechanism. GnRH mRNA levels did not vary with time of day in these pubertal rats, indicating that the diurnal rhythm of GnRH release that develops during puberty is probably a posttranslational event. In experiment II, GnRH gene expression was measured in adult male rats at the same 4-hour intervals. GnRH mRNA levels varied significantly with time of day: they increased from 10.00 to 14.00 h, remained elevated through 22.00 h, then decreased significantly to the nadir at 10.00 h. The mechanism for these changes appears to involve processing or stability of the GnRH mRNA in the nucleus. These results indicate that GnRH mRNA levels are regulated differentially with time of day depending upon age and gender.