A RELATIVE DEPLETION OF LUTEINIZING-HORMONE-RELEASING HORMONE WAS OBSERVED IN THE MEDIAN-EMINENCE OF YOUNG BUT NOT MIDDLE-AGED RATS ON THE EVENING OF PROESTRUS

Computer-assisted analysis was used to examine LHRH reaction product i n the median eminence of young and middle-aged rats prior to and after the expected peak of the LH surge on proestrus. The area of LHRH reac tion product was analyzed in 5 rostral-caudal levels (A-E) of the medi an eminence. The relative depletion of LHRH in the median eminence of young females on the evening compared to the afternoon of proestrus su ggested LHRH neurosecretion in conjunction with the preovulatory LH su rge. The pattern of depletion observed further suggested that LHRH rel ease may occur preferentially from restricted regions of the median em inence or in a coordinated wave-like pattern. Four of the five levels of the median eminence exhibited a relative decrease in LHRH on the ev ening of proestrus in young females, and this time-related difference in LHRH reaction product was statistically significant in median emine nce levels B and C. In contrast, little evidence of a relative depleti on in LHRH reaction product from early to late proestrus was observed in the median eminence of aging animals. Moreover, the concentration o f the densest LHRH reaction product appeared diminished in the median eminence of middle-aged compared to young females at the time points e xamined in the present study. The age-related differences observed in LHRH reaction product in the median eminence may contribute to the att enuated LH surge documented in middle-aged female rats. Whether these changes in LHRH immunoreactivity can be attributed to age-related alte rations in afferents received by LHRH neuronal cell bodies or terminal s or to intrinsic deficits in signalling mechanisms within LHRH neuron s remains to be determined. Computer-assisted analysis of the immunocy tochemical data enabled the assessment of relative changes in reaction product within specific elements of LHRH neurons in precise regions o f the median eminence.