ENLARGING EFFECTS OF ESTRADIOL ON THE NUCLEAR VOLUME OF NEURONS IN THE HYPOTHALAMUS DURING AGING

Neuronal nuclear volumes (NNVs) were measured in the medial preoptic n ucleus (MPN), anterior hypothalamic area (AHA) and arcuate nucleus (AR N) of young adult, middle-aged, and old rats of both sexes. The NNVs i n the darkly stained sexual-dimorphic nucleus of the preoptic area (SD N-POA) and the lighter staining surrounding area (non-SDN-POA) within the MPN were measured separately. Intact young and middle-aged female rats had larger NNVs than those of the males in SDN-POA, non-SDN-POA a nd AHA but not in ARN. During aging, only intact old female rats manif ested significant NNV shrinkage in all the measured areas. Long-term t reatment with estradiol benzoate (EB) caused a significant enlargement of the NNVs in non-SDN-POA and ARN of middle-aged and old male rats a s well as the NNVs in SDN-POA, non-SDN-POA and ARN of old female rats. The enlarging effect of EB on NNVs in both SDN-POA and non-SDN-POA of female rats could be prevented by ovariectomy. Furthermore, NNVs in S DN-POA and non-SDN-POA of ovariectomized female rats were even smaller than those of the age-matched intact female rats. These results indic ate that: (1) the NNVs of MPN and ARN in male and female rats were enl arged after long-term exposure of physiological dose of estradiol; (2) the enlarging effects of EB on NNV in MPN can explain why the NNV of intact female rats is larger than that of males, and (3) during aging, the sex-specific shrinkage of NNVs in MPN, AHA and ARN of female fats may be due to an intrinsic aging process rather than long-term effect s of EB.