Gonadotropin-releasing hormone mRNA in the rat: distribution and neuronal content over the estrous cycle and after castration of males.

The decapeptide gonadotropin-releasing hormone (GnRH) stimulates release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary. In the present study we used a 51-base oligonucleotide probe and in situ hybridization to study the neuronal content of GnRH mRNA at several time points in the estrous cycle and 7 days after castration of male rats. GnRH mRNA containing cells were found in the medial septum (SEPT), the vertical and horizontal limbs of the diagonal band of Broca (DBB), and throughout the preoptic area (POA) from the organum vasculosum of the lamina terminalis (OVLT) to its caudal merger with the anterior hypothalamus. The number of neurons producing detectable quantities of GnRH mRNA was not different either among females killed at 0700 h proestrus, 1000 h estrus, or 1900 h of diestrus 1 or between intact male rats and male rats killed 1 week after castration. We did, however, detect a significant difference in the number of GnRH mRNA producing neurons between males and females (P less than 0.05), where females had 20% more labeled cells. We detected no significant difference in the relative copy number of GnRH mRNA molecules (grains per labeled cell) either over the estrous cycle or between intact and castrate males. However, females overall had 24% more grains per labeled cell than males (P less than 0.05). These results suggest that gonadal steroid regulation of GnRH both over the estrous cycle and after short-term castration of males is mediated primarily by cellular processes subsequent to GnRH gene regulation. Furthermore, these results suggest that biosynthetic activity of GnRH is higher in females than in males.