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 in 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 reaction product was analyzed in 5 rostral-caudal levels (A-E) of the median eminence. The relative depletion of LHRH in the median eminence of young females on the evening compared to the afternoon of proestrus suggested LHRH neurosecretion in conjunction with the preovulatory LH surge. The pattern of depletion observed further suggested that LHRH release may occur preferentially from restricted regions of the median eminence or in a coordinated wave-like pattern. Four of the five levels of the median eminence exhibited a relative decrease in LHRH on the evening of proestrus in young females, and this time-related difference in LHRH reaction product was statistically significant in median eminence levels B and C. In contrast, little evidence of a relative depletion in LHRH reaction product from early to late proestrus was observed in the median eminence of aging animals. Moreover, the concentration of the densest LHRH reaction product appeared diminished in the median eminence of middle-aged compared to young females at the time points examined in the present study. The age-related differences observed in LHRH reaction product in the median eminence may contribute to the attenuated LH surge documented in middle-aged female rats. Whether these changes in LHRH immunoreactivity can be attributed to age-related alterations in afferents received by LHRH neuronal cell bodies or terminals or to intrinsic deficits in signalling mechanisms within LHRH neurons remains to be determined. Computer-assisted analysis of the immunocytochemical data enabled the assessment of relative changes in reaction product within specific elements of LHRH neurons in precise regions of the median eminence.