Long-term changes induced by developmental handling on pain threshold: effects of morphine and naloxone.

Mice pups were exposed daily to a stress-producing procedure (handling and saline injection) during the first 3 weeks of life. At 25 and 45 days of age, they were tested for differences in the tail-flick and hot-plate tests. The results indicate that chronic handling procedures during developmental stages can produce a long-lasting increase of the threshold for painful stimuli. This phenomenon is completely prevented by naloxone pretreatment and has enhancing effects on morphine analgesia, thus suggesting that postnatal handling can exert long-lasting interference on the sensitivity of some opioid receptor populations.