Morphine and memory in DBA/2 mice: effects of stress and of prior experience.

In a first set of experiments, immediately post-training morphine (1.0 or 2.5 but not 0.5 mg/kg) treatment, or immobilization stress (30 or 60 but not 15 min) impaired memory processes of non-pretrained DBA/2 mice tested in a passive avoidance box. The effects were naloxone-reversible and time-dependent (they were absent in mice injected with morphine, or immobilized, starting 120 min after training). No effect was evident in no-footshock groups injected with morphine (2.5 mg/kg) or immobilized (60 min), thus showing lack of proactive influence of the treatments on performance. In a second set of experiments, in which pretrained animals were used, both morphine and immobilization stress were less effective in disrupting memory processes of mice. In both sets of experiments a per se ineffective stress enhanced the effects of morphine. A number of possible hypotheses concerning the results obtained are examined. In particular the possible role of emotional factors in the effects of morphine on memory is discussed.