Swim‐stress‐induced antinociception in young rats

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Abstract
1 Opioid and non-opioid mechanisms have been implicated in the phenomenon of stress-induced antinociception in adult rodents. We have studied stress-induced antinociception in developing rats and characterized differences in the neurochemical basis of this effect in pre- and post-weanling animals. 2 Twenty and 25 day old rats were stressed using warm water (20°C) swimming for 3 or 10 min periods and antinociception was assessed by the tail immersion test (50°C). 3 A 3 min swim in 20 and 25 day old rats produced marked antinociception which was blocked by naloxone, Mr 1452, 16-methyl cyprenorphine and levallorphan but not Mr 1453 or N-methyl leval-lorphan. The δ-opioid receptor antagonist ICI 174,864 attenuated stress-induced antinociception in 25 day old rats but was without effect in 20 day old animals. 4 A 10 min swim in 25 day old rats produced antinociception which was non-opioid in nature. In contrast, antinociception was not observed in 20 day old rats after a 10 min swim-stress. 5 Pretreatment of animals with dexamethasone blocked 3 min swim-stress antinociception in 20 and 25 day old animals but had no effect on antinociception induced by a 10 min swim. 6 Swim-stress-induced antinociception can be observed in young rats and dissociated into opioid and non-opioid types dependent on the duration of swimming stress. The non-opioid type appears to develop more slowly and cannot be observed in preweanling rats. The opioid type is a predominantly μ-receptor phenomenon in preweanling animals but δ-receptor components are observable in postweanling rats.