Sex differences in tail-flick latency of non-stressed and stressed rats

The present study was conducted to assess whether there are sex differences between male and female subjects in their response to noxious stimuli under non-stressed and stressed conditions. Tail-flick latency assay was used as an experimental tool on 12 adult male and 12 adult female Sprague-Dawley rats before immobilization (i.e., non-stressed condition) and after 30, 120, 240, and 360 min of continuous immobilization (i.e., stressed conditions). It was found that the non-stressed female rats exhibited significantly longer response latency to noxious thermal stimuli than the non-stressed male rats. Stressed conditions caused by immobilization of the animal inside a ventilated restrainer significantly prolonged the tail-flick latency thresholds of both sexes. Female and male rats did not develop any adaptation after 120, 240, and 360 min of immobilization compared to their 30 min of immobilization, as demonstrated by the tail-flick assay. Moreover, the difference in the tail-flick latency between male and female rats was reduced as the duration of immobilization in the restrainer lengthened. At 30 min of immobilization, the significant difference between the sexes reduced to p < .05 compared to p < .01 before immobilization, and to statistically non-significant differences after 120, 240, and 360 min of immobilization. Results from this study suggested that female rats had longer tail-flick latency than male rats in non-stressed environment, and the difference in tail-flick latency after immobilization between the sexes became less significant with time. In addition, 360 min of immobilization was not enough to adapt to the restrainer, as shown by the tail-flick assay.