Development of mustard oil-induced hyperalgesia in rats

Age-dependent changes in nociceptive responses were investigated using either the electromyogram (EMG) recorded from the hamstring muscle in response to electrical stimulation of the hind foot in spinal transected rats or measurement of the tail-flick (TF) reflex latency in intact rats. The development of hyperalgesia produced by topical application of mustard oil was subsequently studied. In experiments involving EMG recordings, rats were tested from day 2 to day 34 after birth (4-day interval) and as adults. In experiments involving measurement of the TF reflex, rats were tested from day 5 to day 30 after birth (5-day interval) and as adults. It was found that the latency and the duration of an early component of the EMG decreased with an increase in animal age, and was similar to adult animals at approximately 18 days after birth. The thermal tail withdraw threshold was lower in pups in comparison with older rats, and took more than 30 postnatal days to become similar to that of adult rats. Although nociceptive behaviors such as biting, body movement, and vocalization could be produced in intact rats by mustard oil in rats as young as 5 days old, the intensities of these responses were subjectively less than those of adult rats. Mustard oil application enhanced significantly the EMG response to electrical stimulation and the effect increased with increasing age. Similarly, mustard oil applied to a hind leg facilitated the TF reflex (decreased response latency). In both experiments, it took approximately 34-40 postnatal days for mustard oil-produced hyperalgesia to become similar to that of adult rats. These data confirm that nociceptive processing is not mature in the young animal and that a developmental period after birth is required for hyperalgesia-related mechanisms to mature.