A comparative study on pain sensation in morphine-dependent male and female rats in the presence and absence of gonads using formalin test

Various physiological parameters including level of sex steroids undergo alterations following chronic administration of morphine. In this study, the effect of chronic administration of morphine on phasic and tonic pain was studied in morphine-dependent male and female rats in the presence and absence of gonads using formalin test. In addition, for evaluation of differences in dependency, withdrawal signs were observed using naloxone hydrochloride. For dependency induction, morphine sulphate was administered in drinking water for a period of 30 days. The results showed that although chronic pain is significantly greater in female rats than male ones but this pain increases in male dependent rats and decreases in female dependent animals. Thus, no gender differences were found between male and female dependent rats. Furthermore, gonadectomy led to a significant decrease in chronic pain only in male dependent rats. Meanwhile, withdrawal signs were significantly greater in female dependent rats than male ones and gonadectomy did not influence these signs. It can be concluded that following morphine addiction, pain increases in male and decreases in female rats and morphine dependency is not affected by sex hormones.