Alterations in Proestrous LH, FSH, and Prolactin Surges in Middle-Aged Rats

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Abstract
Plasma LH [luteinizing hormone], FSH and prolactin were measured in middle-aged (7-9-mo. old) breeder and young (3-4 mo. old) female rats on proestrus. Of the middle-aged rats, 68% exhibited regular 4-day estrous cycles; the remainder showed irregular cycles 4-7 days in length. All young rats exhibited at least 2 consecutive 4 day cycles prior to use. The right atria of all rats were cannulated under ether anesthesia between 0700-0800 h (14:10 light-dark cycle, lights on at 0400 h). Blood was drawn at 0900 h and at hourly intervals from 1200-2200 h; each rat was bled a maximum of 9 times. Plasma LH and FSH rose significantly later and peak concentrations were significantly lower in middle-aged rats. Baseline prolactin concentrations at 0900 and 2000-2200 h were significantly higher in middle-aged female rats. Alterations in the proestrous gonadotropin and prolactin surges evidently occur early during the aging process in the female rat. Such changes may be important in the age-related transition to estrous acyclicity.