Morphine attenuation of a conditioned emotional response in post-dependent rats.

Rats placed in a test chamber where they had received repetitive shocks the previous day significantly reduced their motor activity; this was taken as indicator of a conditioned emotional response. Morphine attenuated this conditioned suppression of motility, possibly due to a reduction of the anxiety associated with the expectation of the noxious stimuli. Previous morphine dependence (20 mg/kg daily for 26 days) did not modify the effect of the opioid on the conditioned suppression of motility. This fact suggests that the action of morphine on pain anticipatory anxiety is similar in non-dependent and in post-dependent rats. Opioids are considered to produce pain relief in part by decreasing the anticipatory anxiety. The present results thus indicate that this important component of the analgesic action of morphine is unchanged in post-addicts.