Effects of decreased serum estradiol or progesterone level on thermal pain threshold in female rats

Various studies indicate that gonadal hormones exert modulatory effects on pain threshold. The aim of this study was to investigate the role of estradiol or progesterone in the response by female rats to thermal nociceptive stimulation. In this laboratory experimental study, thirty 7 week old albino (Wistar) rats were randomly divided into control, sham and ovariectomised groups of 10 each. After 10, 20 or 40 days thermal pain threshold was measured through tail immersion test (55°C water bath). The pain threshold was measured as the time required eliciting a flick of the tail which was called analgesia time. Serum estradiol, progesterone or prolactin levels were also simultaneously measured by radioimmunoassay method. Data were statistically analyzed and compared between groups using ANOVA. There was no significant change in serum estradiol or progesterone levels as well as analgesia time 10 days after ovariectomy; however, a significant decrease was observed 20 or 40 days after operation compared with control female rats (p<0.001, p<0.05 or p<0.001, respectively). Also there was no significant change in serum prolactin level 10 or 20 days after ovariectomy compared with control rats; however, a significant decrease was observed 40 days compared with 20 days after ovariectomy (p<0.05). Our findings clearly indicate that depletion of female gonadal hormones 20 or 40 days after ovariectomy modulates the pain-induced behavioral responses related to thermal nociception in female animals.