The rat preoptic area-anterior hypothalamic continuum (POA-AH) contains about 400-800 neurons that express the decapeptide GnRH and the 56-amino-acid GnRH- associated peptide. Originating from the olfactory placode, these neurons migrate and establish their final distribution and connections in the POA-AH several days before birth. The aim of the present study was to examine whether the biosynthesis of the mRNA encoding the precursor (proGnRH) common to GnRH and GnRH-associated peptide undergoes postnatal changes corresponding to the development of sexual maturation. The POA-AH content of proGnRH messenger RNA (mRNA) was followed from postnatal day 1 to day 90 in female and male Sprague-Dawley rats killed by decapitation between 1000-1200 h. Cytoplasmic RNA fractionated from individual POA-AH homogenates was purified using proteinase K digestion. Cytoplasmic proGnRH mRNA was quantitated simultaneously with cyclophilin mRNA (an internal standard control) using solution hybridization-RNase protection assay, with the protected fragments separated through polyacrylamide gel electrophoresis. In the POA-AH, the concentrations of proGnRH mRNA (femtograms mRNA per μg total RNA) increased significantly with age in both sexes (P < 0.001). In males, proGnRH mRNA levels increased by day 30 some 2-fold over the values of days 5 and 10, and the levels established on day 30 were maintained through adulthood. In females, the first rise in proGnRH mRNA levels occurred on day 30, followed by an additional increase on day 45 to levels seen in adulthood. Levels of proGnRH mRNA established in adulthood were significantly higher in females than in males (P < 0.03). The concentrations of cyclophilin mRNA (picograms mRNA per μg total RNA) remained essentially unchanged in both sexes during the same period of time when proGnRH mRNA levels were increasing. These results provide evidence for postnatal sex-related increases in the levels of proGnRH mRNA in the rat POA-AH, which are likely to reflect differential regulation by gonadal steroids.
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