Differences of alteration in opioid systems induced by conditioned suppression and electric footshock in mice.

The motility, pain-threshold and opioid receptor activities of the synaptic membrane in mice showing conditioned suppression of motility were compared with those in mice given only electric footshock. Electric footshock caused analgesia and a decrease in motility, both of which were partially reversed by administration of high doses of naloxone. In contrast, mice exhibited a marked suppression of motility (conditioned suppression) but not analgesia when placed in the same environment 24 hr after the electric footshock in which the animals received the electric footshock. In the electric footshock group, the [3H]-naloxone binding capacity at low affinity site was increased. These results suggest that the increase in [3H]-naloxone binding capacity may play an important role in the behavioral changes of electric footshock group, but not conditioned suppression group.