Social crowding enhances aversiveness naloxone in rats.

If endorphins mediate various behavioural states including reactions to stress and social affect, then experimental manipulation of such states should alter the stimulus properties of opioid antagonists. The results of experiments in which the aversiveness of naloxone was increased by chronic environmental stress produced by rearing and maintaining male rats under conditions of severe social crowding, support this theoretical proposition. All rats acquired aversions to flavoured solutions that had been presented for 15 min immediately prior to a systemic injection of naloxone. The degree of the conditioned aversion was greater in rats maintained in crowded conditions. Transferring rats from crowded conditions to individual accommodation 3 weeks before the start of aversive conditioning did not attenuate the increased aversiveness of the antagonist. It is suggested that social interaction under conditions of chronic crowding induces a prolonged increase in the level of tonic activity in endorphinergic systems.