CHANGES IN PAIN THRESHOLD DURING THE REPRODUCTIVE-CYCLE OF THE FEMALERAT

Responsiveness to pain was determined in female rats across the whole reproductive cycle using the tail-flick test. When tested immediately after mating, pain thresholds were unaltered, whereas 10 min later ani mals typically demonstrated hyperalgesia (Experiment 1). They also dem onstrated hyperalgesia during most of pregnancy, and had significantly lower pain thresholds than the unmated controls except for the 24 h b efore parturition, when a sudden increase in tail-flick latencies was recorded (Experiment 2). Pain thresholds were also significantly lower throughout the nursing period but increased significantly when dams w ere separated from their litters for 6 h, and returned to premating ba seline values within 24 h of weaning (Experiment 3). These findings co nfirm and extend earlier reports that female reproductive state may si gnificantly modify responsiveness to noxious stimuli, and it is sugges ted that differences between the results of this and previous studies may be at least partly explained by the relatively stress-free test pr ocedure adopted here.