Swim‐stress‐induced antinociception in young rats

Abstract

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. 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). 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. 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. 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. 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. 1989 British Pharmacological Society