Alterations in Proestrous LH, FSH, and Prolactin Surges in Middle-Aged Rats (41356)

Plasma LH, FSH, and prolactin were measured in middle-aged (7- to 9-month-old) breeder and young (3-4 month old) female rats on proestrus. Sixty-eight percent of the middle-aged rats exhibited regular 4-day estrous cycles; the remainder showed irregular cycles 4-7 days in length. All young rats exhibited at least two consecutive 4-day cycles prior to use. The right atria of all rats were cannulated under ether anesthesia between 0700 and 0800 hr (14:10 light-dark cycle, lights on at 0400 hr). Blood was drawn at 0900 hr and at hourly intervals from 1200 to 2200 hr; each rat was bled a maximum of nine 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 hr were significantly higher in middle-aged female rats. These data indicate that alterations in the proestrous gonadotropin and prolactin surges occur early during the aging process in the female rat. Such changes may be important in the age-related transition to estrous acyclicity.