Physiology & Behavior, Vol. 16, pp. 543--546. Pergamon Press and Brain Research Pubi., 1976. Printed in the U.S.A. Sex Difference in the Effect of Dexamethasone on Open-Field Behavior in Rats: Gonadal Hormones ' JUSTIN M. JOFFE, JAMES A. MULICK, JOHN M. PETERSON Department of Psychology, University of Vermont, Burlington, VT 05401 AND JASENKA PAUNOVIC Department of Biology, Medical Faculty, University of Zagreb, Zagreb, Yugoslavia (Received 28 April 1975) JOFFE, J. M., J. A. MULICK, J. M. PETERSON AND J. PAUNOVIC. Sex difference in the effect ofdexamethasone on open-field behavior in rats: gonadal hormones. PHYSIOL. BEHAV. 16(5) 543-546, 1976. - Previous experiments indicated that dexamethasone phosphate (DEX) increases open-field defecation in females but not in males. To examine the question of whether this sex difference in response to DEX is due to sex differences in gonadal hormones, female rats were (a) ovariectomized; (b) ovariectomized and injected with testosterone propionate prior to and during open-field testing; or (c) sham operated. Half the females in each group received DEX in their drinking water during the 4 day period of testing. DEX produced the expected reductions in body weight, adrenal weight, and plasma corticosterone levels. Despite the differences between the three groups on a variety of physiological measures, DEX produced comparable increases in open-field defecation in all cases. This indicates that the sex difference in the response of open-field defecation to DEX is not dependent on sex differences in circulating gonadal hormones. Activity Body weight Corticosterone Defecation Dexamethasone phosphate Open-field behavior Ovarian hormones Ovariectomy Pituitary-adrenocortical system Sex differences Testosterone propionate EXPERIMENTAL manipulations of pituitary-adrenal status affect open-field behavior. Moyer found that adrenalec- tomy significantly increases open-field defecation in female rats [ 12] but that low doses of ACTH do not affect activity or defecation [13] and adrenal demedullation does not affect defecation or urination [14]. Joffe et al. [5] confirmed Moyer's [12] findings concerning adrenalec- tomy, and showed that the effect occurs in both male and female rats. In an attempt to determine which of the possible hormonal changes might be involved in the effects on open-field behavior, Mulick et al. [ 15 ] treated animals with dexamethasone phosphate (DEX), a synthetic gluco- corticoid known to produce strong inhibition of cortico- trophic stimulation of the adrenal cortex [ 1, 16, 19] and a decrease in adrenocorticosteroid production [2,17]. Since DEX is itself a potent glucocorticoid the pattern of pituitary-adrenocortical hormonal changes resulting from DEX administration is the reverse of that produced by adrenalectomy - the DEX treated animal has high circu- lating levels of steroids and low levels of ACTH. Never- theless DEX treatment had similar effects to adrenalectomy in female rats but not in males; that is, females showed a significant increase in open-field defecation [15]. That this f'mding was specific to the open field, and not simply a general effect on overall defecation, was indicated by the substantial and significant decrease in home-cage defecation observed in both male and female rats. Both males and females lost weight, and showed suppressed levels of plasma corticosterone; in addition males tested in an activity wheel showed an increase in activity similar to that reported by others [8]. The present experiment was designed to investigate whether or not the sex difference in the effect of DEX on open-field defecation was due to the difference in circu- lating gonadal hormones in normal male and female rats. This was done by comparing the effects of DEX treatment on the open-field behavior of ovariectomized females 1Supported by grant USPHS R01-05571 from the National Institute of Child Health and Human Development. The authors wish to thank Dr. Clement A. Stone of Merck, Sharp, and Dohme Research Laboratories for supplying the dexamethasone phosphate and Joel Najman for help with preparation of figures. A preliminary version of this paper was presented at the Fifteenth Annual Meeting of the Psychonomic Society, Boston, in November, 1974. Reprints may be obtained from Dr. J. M. Joffe, Department of Psychology, University of Vermont, Burlington, VT 05401. 543