Altered Negative Feedback Response to Ovariectomy and Estrogen in Prepubertal Restricted-diet Rats 1

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Abstract
Studies were conducted to explore the hypothesis that the delayed sexual maturation of female rats induced by reduced food intake (R) may result partially from an altered negative feedback response to estrogen. Animals were placed on 60% of normal food intake at 20 days of age. Controls (C) were fed ad lib. Rats were used for 3 different experiments at 31-32 days of age. In experiment I, rats were ovariectomized (OVX) and injected s.c. for 4 days with varying doses of estradiol benzoate (EB). They were killed the day after the last injection. In experiment II, rats were ovariectomized and killed in groups at 4, 12, 24, 48, 72 and 120 h after OVX. In experiment III, they were castrated and 1 wk later received a single injection of 0.5 .mu.g EB. Groups were killed at 1, 2, 4, 8 and 24 h after injection. Sera from all experiments were assayed for FSH, luteinizing hormone (LH) and prolactin. Results of experiment I indicate that the efficacy of EB for suppressing LH, but not FSH, secretion is increased significantly in R rats. In Experiment II, OVX resulted in a delayed increase in serum LH, but not FSH, concentrations of R rats when compared to C animals. Results of experiment III indicate a delayed, but more prolonged, suppression of LH secretion by EB in R rats when compared to C rats. Prolactin secretion, on the other hand, increased earlier in R rats. The negative feedback response of the LH releasing system to EB evidently is modified by chronic underfeeding in the prepubertal female rat and may be responsible for the slower sexual maturation.