Relation of Aging to Hypothalamic LHRH Content and Serum Gonadal Steroids in Female Rats

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Abstract
The relationships of age and reproductive state to hypothalamic LHRH [luteinizing hormone releasing hormone] content and gonadal steroids were studied in young and old female rats. LHRH content in the medial basal hypothalamus (MBH) of young female rats was maximal on the morning of proestrus (PE), but fell to a nadir on PE afternoon. MBH-LHRH rose again on the morning of estrus (E) and then dropped by 1000 h on diestrous day 2 (DE-2). Old constant estrous (CE) rats had MBH-LHRH levels that were significantly lower than in young E rats, while old pseudopregnant-like (PP) rats had intermediate MBH-LHRH levels, not significantly different from those in young E or DE-2 rats. LHRH content in the MBH of old anestrous (AN) rats was lower than in any other group. Anterior hypothalamic (AH)-LHRH in old CE, PP and AN rats were similar and did not differ from those in young E ror DE-2 rats. Serum estradiol and progesterone peaked at PE in young cycling rats, but showed no cyclic variations in old rats, but showed no cyclic variations in old rats. Old CE and PP rats had moderate estradiol levels comparable to those in young DE-2 rats, and AN rats had low estradiol levels comparable to those in young E rats. Old PP rats had higher progesterone levels than old CE or young E rats. Old AN rats had progesterone levels lower than in any other group. Sufficient LHRH is apparently present in old CE or PP rats to maintain basal LH [luteinizing hormone] secretion, but the stimulus for cyclic LHRH release is lacking. Low to undectable LH levels seen in old AN rats may be the result of the low LHRH levels seen in these rats.