NEONATAL TREATMENT WITH NALOXONE CAUSES PERMANENT HYPERALGESIA IN RATS

The effect of treatment with naloxone early in life on pain responsive ness was studied in Wistar rats. Litters of six rats were divided equa lly into groups of 3 pups receiving daily naloxone (50 mg/kg, sc) and 3 pups receiving saline from the 3rd to 18th day of life. On days 30, 50, 70 and 90, one group of animals previously injected during sucklin g with naloxone (N = 21) and another with saline (N = 21) were submitt ed to the hot-plate test to measure the latency to paw licking. Other groups of rats also treated during suckling with naloxone (N = 13) and saline (N = 14) were assessed for the antinociceptive effect of morph ine (10 mg/kg, sc). The naloxone group displayed a lower latency than the saline group in all test sessions and a diminished analgesic respo nse to morphine. The results indicate that the use of naloxone (an ant agonist opioid) during suckling, the brain growth spurt period, facili tates a long-lasting increased pain responsiveness and alters antialge sic mechanisms. In this respect, the opioid and non-opioid effects of naloxone on the ontogeny of neural systems should be taken into accoun t.