LUTEINIZING-HORMONE-RELEASING HORMONE GENE-EXPRESSION DIFFERS IN YOUNG AND MIDDLE-AGED FEMALES ON THE DAY OF A STEROID-INDUCED LH SURGE

LHRH mRNA levels were examined in young and middle-aged female rats at 4 times (10:00 h, 14:00 h, 18:00 h and 20:00 h) on the day of a stero id-induced LH surge by in situ hybridization with a digoxigenin-labele d riboprobe. Young, but not middle-aged females, exhibited dynamic tem poral changes in the number of LHRH mRNA positive neurons detected in the organum vasculosum of the lamina terminalis-preoptic area (OVLT-PO A) continuum. Specifically, fewer LHRH mRNA positive neurons were dete cted at 18:00 h compared with the number detected at 14:00 h and 20:00 h (P < 0.01) in the OVLT-POA of young females. All LHRH mRNA positive neurons present in 4 anatomically matched sections through the rostra l POA of young and middle aged animals were digitized for detailed com puter-assisted analysis of the hybridization reaction product. The mea n hybridization area (P < 0.00025) and integrated optical density per cell (P < 0.006) were reduced in middle-aged compared to young females consistent with a relative age-related decline in LHRH mRNA levels. M oreover, an age-related reduction in cellular and/or regional hybridiz ation area was noted at each of the time points examined (P < 0.05-P < 0.001). These data confirm earlier reports of dynamic changes ill LHR H mRNA levels on the day of an LH surge. Furthermore, they support a r ole for age-related alterations in LHRH gene expression in the disrupt ion of regular estrous cyclicity in middle-aged females. (C) 1997 Else vier Science B.V.