Abstract

It has been hypothesized that the emotionality of the female rat is reduced at estrus. In confirmation of previous research, it was found that administration of estradiol benzoate (EB;20 mug/kg) to female rats of the Maudsley Reactive (MR) strain increased open-field activity and decreased open-field defecation. In addition, ovariectomy increased open-field defecation in MR females. Supporting the generality of these findings, hormone administration reduced open-field defecation and increased open-field activity in intact females of a genetically heterogeneous background. Additional studies suggested that the decrease in open-field defecation at estrus is dependent on estrogenic suppression of food intake. Hormone replacement decreased food intake in the period preceding the open-field test, and colonic contents were also lower in rats treated with EB+P (progesterone) immediately after the completion of the open-field test. These changes were seen in both MR and genetically heterogeneous females. It was concluded that the decrease in open-field defecation at estrus may be mediated by a reduction in food intake and a consequent decrease in colonic contents and that the validity of the defecation response as a measure of emotionality may be seriously questioned under these circumstances.

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