BIOSYNTHESIS OF GONADOTROPIN-RELEASING-HORMONE DURING THE RAT ESTROUS-CYCLE - A CELLULAR ANALYSIS

In an attempt to understand the regulatory events that control the syn thesis of gonadotropin-releasing hormone (GnRH) during the estrous cyc le of the rat, we undertook a cellular analysis of time of translation of GnRH mRNA into protein. A specific antiserum, Rb 1076, which recog nizes both the extended proGnRH as well as the processed form of the d ecapeptide was used for immunocytochemical staining. A cell was consid ered to be actively translating the pro-GnRH mRNA if elements of the r ough endoplasmic reticulum (RER), including the outer nuclear envelope , were filled with reaction product. A synthetically quiescent cell co ntained only immunopositive neurosecretory granules. Cycling rats were killed at various times and all GnRH cells scored as being RER positi ve (+) or negative (-). On the morning of estrus almost all GnRH neuro ns in 5 out of 6 of the animals studied were synthesizing their unique peptide. The immunostaining in many of the cells at this time was ver y pale, suggesting a prior depletion. This was the only time point exa mined where near uniformity among individuals in a group was observed. At all other times considerable heterogeneity was observed among anim als within a group. For example, at 17.30 h on the afternoon of proest rus 50% or more of the GnRH neurons were RER+ in half of the animals; the other half had values of 16-45% RER+. Synthetically active and ina ctive cells were found in close proximity in all animals. No regional differences were observed; all GnRH cell subpopulations from the level of the diagonal band of Broca through the hypothalamus reflected the population as a whole. These results suggest that only after the preov ulatory surge of LH are all GnRH neurons synchronized to initiate synt hesis of this neuropeptide.