Mice exhibited a marked suppression of motility (conditioned suppression) when placed in the same environment in which they had previously received an electric footshock. Furthermore, chronic morphine (mu agonist)-pretreated mice, as well as chronic vehicle-pretreated mice, exhibited the conditioned suppression but chronic ethylketocyclazocine (kappa agonist)- and pentazocine (kappa agonist)-pretreated mice did not. On the other hand, in the synaptic membranes of the chronic vehicle- and morphine-pretreated mice showing conditioned suppression, the specific binding of [3H]phencyclidine (sigma agonist) and [3H]naloxone (mu antagonist) significantly increased, while the specific binding of [3H]ethylketocyclazocine did not change compared to those of the corresponding control groups. However, in the chronic pentazocine- and ethylketocyclazocine-pretreated groups showing non-conditioned suppression, the specific binding of [3H]phencyclidine and [3H]naloxone were not altered. Moreover, a decrease of [3H]ethylketocyclazocine binding was observed in the chronic pentazocine-pretreated group. These results suggest that the binding function of different opioid receptor subtypes may be altered differently by stress, and that the kappa receptor may be important for the conditioned suppression of motility in mice.
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