GENE-EXPRESSION IN A SUBPOPULATION OF LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH) NEURONS PRIOR TO THE PREOVULATORY GONADOTROPIN SURGE

Gene expression in luteinizing hormone-releasing hormone (LHRH) neuron s was analyzed during the periovulatory period to (1)characterize temp oral patterns of LHRH gene expression and their relationship(s) to gon adotropin surges, and (2) determine if any such changes are uniform or dissimilar at different rostrocaudal levels of the basal forebrain. T he number of neurons expressing mRNA for the decapeptide, and the rela tive degree of expression per cell were analyzed using in situ hybridi zation and quantitative image analysis. Rats were killed at 1800 hr on metestrus (Met), 0800 hr, 1200 hr, 1800 hr, and 2200 hr on proestrus (Pro), or 0200 hr, 0800 hr, and 1800 hr on estrus (E; n = 5-6 rats/gro up). All sections were processed for LHRH mRNA in a single in situ hyb ridization assay. Sections were atlas matched and divided into four ro strocaudal groups for analysis: vertical limb of the diagonal band of Broca (DBB), rostral preoptic area/organum vasculosum of the lamina te rminalis (rPOA/OVLT), medial preoptic area (mPOA), and suprachiasmatic /anterior hypothalamic area (SCN/AHA). Plasma LH and FSH levels from a il animals were analyzed by RIA. The labeling intensity per cell was s imilar among all time points at all four rostrocaudal levels. The numb er of cells expressing LHRH mRNA, however, varied as a function of tim e of death during the estrous cycle, and this temporal pattern varied among the four anatomical regions. At the level of the mPOA, the numbe r of cells was highest at 1200 hr on Pro, and then declined and remain ed low throughout the morning of E. At the level of the rPOA/OVLT, the greatest number of LHRH neurons was noted later in Pro, at 1800 hr, d ropping rapidly to lowest numbers at 2200 hr. No significant changes i n LHRH cell number occurred at the DBB or SCN/AHA levels. At all anato mical levels, the secondary surge of FSH was unaccompanied by any chan ge in the number of neurons expressing LHRH mRNA. These data demonstra te that (1) the number of detectable LHRH mRNA-expressing cells fluctu ates during the periovulatory period and (2) peak numbers of LHRH-expr essing cells are attained in the mPOA before the onset of the LH surge and before peak LHRH cell numbers are seen at more rostral levels. A model is proposed in which gene expression in this subpopulation of LH RH neurons may be activated by preovulatory estrogen secretion and acu tely reduced following the proestrous surge of progesterone.