Mice exhibited a marked suppression in motility (conditioned suppression) when placed in the same environment in which they had previously received an electric footshock. The present study was designed to investigate the effects of opiate agonists on the conditioned suppression in motility. Morphine partially reduced the conditioned suppression in motility while cyclazocine attenuated it in a dose-dependent manner. On the other hand, ethylketocyclazocine and methionine-enkephalin had no effect. These results suggest that sigma receptor may be more important than mu receptor for opiate-induced attenuation of the conditioned suppression in motility of mice.
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