A DEVELOPMENTAL-STUDY ON STRESS-INDUCED ANTINOCICEPTION MEASURED BY THE TAIL ELECTRIC-STIMULATION TEST

The possible influence of weaning on the development of different neur al mechanisms involved in stress-induced antinociception (SIA) was stu died. Male Wisfar albino rats were used for studies on adult and pre- and postweanling rats of 20 and 25 days of age, respectively. Animals were stressed by warm-water (20C) swimming for 3-min periods. Antinoci ception wasd assessed by the tail electric stimulation test. The thres holds for the motor response (tail withdrawal) (TW), vocalization duri ng stimulus CV), and vocalization afterdischarge (VAD) were recorded. These responses are considered to be integrated at spinal, medulla obl ongata, and diencephalon-rhinencephalon levels, respectively. In 20-da y-old neonates, swimming stress only induced significant increases in the VAD thresholds that were not significantly reversed by naloxone (N AL) (1 mg/kg). Twenty-five-day-old rats showed increased thresholds fa r the three nociceptive responses after stress, the effects on TW and V being antagonized by NAL. Adult rats subjected to stress showed incr eased thresholds for the three responses, an effect that was antagoniz ed by NAL in all cases. These results suggest that the weaning period might be critical for the development of the mechanisms mediating SIA. Besides, a different involvement of opioid systems throughout develop ment, particularly in relation to the affective/emotional component of pain, is also suggested.