Effects of postnatal manipulation on nociception and morphine sensitivity in adult mice

The long-term effects of postnatal manipulation on nociception were studied in NMRI albino male mice. During the first two weeks of life, pups were re moved from their cage and deprived of maternal/nest odour for 15 min/day. T o evaluate pain sensitivity, adult mice exposed to this postnatal manipulat ion (CB group) were tail flick and formalin tested for acute and tonic pain , respectively. CB mice showed a reduced pain sensitivity both in tail-flic k and in formalin tests in comparison with control animals. Moreover, respo nsiveness to morphine (MO 1.0, 2.5, and 5.0 mg/kg, i.p.) in young (35 days old) and adult (90 days old) postnatally manipulated animals was evaluated with the tail-flick test: a decrease of the antinociceptive effects induced by morphine both in young and adult males was observed in postnatally mani pulated animals. Morphine induced significant analgesic effects in control mice at doses lower than those affecting nociceptive thresholds both in you ng and adult CB mice. In addition, young animals showed a higher sensitivit y to morphine than adults, independently of postnatal manipulation. The lon g-term effects of postnatal manipulation on nociception are discussed in te rms of involvement of the opioid system and of the characteristics of pup m anipulation. (C) 1999 Elsevier Science B.V. All rights reserved.