| Abstrakt: |
Studies were conducted between December 1976 and February 1978 examining the influence of season and photoperiod on the sensitivity of female rats to the inhibitory effects of adrenocorticotropin (ACTH) on sexual maturation. Female rats were reared from birth in either a 14 h light:10 h dark schedule (14 hL:10 hD) or an 8 h light:16 h dark schedule (8 hL:16 hD). ACTH was administered daily, starting at 21 days of age and continuing until vaginal opening (VO) or day 40. Rats were examined for age of VO, estrous stage at VO, body weight at VO, age at first estrus and the number of ova ovulated at first estrus. ACTH significantly delayed VO and the age of first estrus in rats raised in 8 hL:16 hD schedules. Body weight at VO was also reduced in these animals but ova counts were not altered by ACTH treatment. In rats raised in 8 hL:16 hD, ACTH had a significant seasonal effect on the age of VO, age of first estrus and the estrous stage at VO. Rats were most responsive to the retarding effects of ACTH on VO and first estrus during the winter months and ACTH had little effect on these parameters during the summer. During the winter months, a larger number of ACTH-treated rats were in diestrus at VO compared with summer when the predominant smear was either proestrus or estrus. In 14 hL:l0 hD schedules, ACTH significantly delayed the age of first estrus, but had only marginal effects on the other parameters of sexual maturation. No seasonal differences in response to ACTH were observed in this photoperiod. These data show that there is a seasonal responsiveness to the effects of ACTH on sexual maturation which is operational only in a reduced photoperiod. |
