Estradiol and testosterone differently affect visceral pain-related behavioural responses in male and female rats.

In the study of pain, the presence of sex differences is well known, with female subjects being more affected in a number of chronic painful conditions; however, the underlying mechanisms and the involvement of gonadal hormones, are still controversial. This study evaluated visceral pain in a validated rat model of artificial calculosis and the effects of estradiol and testosterone administration. Adult male and female rats were divided into groups and treated with one of the substances or Oil (vehicle) for 5 days, starting 2 days before surgery, with half receiving an artificial calculosis (Stone) and half only a sham (Sham) procedure. The animals' behaviour (ureteral crises, frequency and duration) were recorded for 72 h; estradiol and testosterone plasma levels were determined in all groups at the end of the observation period. After surgery, only Stone rats showed ureteral pain crises, with a significant sex difference in the Oil-treated groups in which the number and duration of crises were higher in females than in males. This difference was not present in the estradiol-treated groups in which ureteral crises were decreased only in females while testosterone treatment had no effect in either sex. Estradiol and testosterone plasma levels were affected by treatments in both sexes. These results confirm that, also in this model of visceral pain, females experience more pain than males; moreover, they show that supraphysiological levels of estradiol, but not of testosterone, are analgesic only in females. A dose and sex-dependent efficacy of gonadal hormones is suggested and discussed. Copyright (c) 2009 European Federation of International Association for the Study of Pain Chapters. Published by Elsevier Ltd. All rights reserved.