Brief exposure to a mild stressor enhances morphine-conditioned place preference in male rats. 
Exposure to moderate tail shock [3, 0.75 s, 1 mA, 20 s interstimulus interval (ISI)] can enhance pain reactivity (hyperalgesia) in rats.  This hyperalgesia reflects an unconditioned response that transfers across contexts and is associated with enhanced Pavlovian fear conditioning to aversive unconditioned stimuli (US).  It is possible that moderate shock also enhances learning about appetitive stimuli such as a reinforcing drug.  The present study examined the effect of moderate shock exposure on unconditioned psychomotor activation and appetitive conditioning using a morphine place-preference task.  During training, rats were given moderate shock or restraint and then received subcutaneous morphine at one of four doses (0.0, 0.2, 1.0, or 5.0 mg/kg) and were transferred to a conditioning apparatus.  Five hours later, animals were given discrimination training in a different context.  Animals received 2 days of training, each separated by a day of testing for preference.  To test the impact of shock on psychomotor activation, subjects were given shock or restraint and one of two doses of morphine (0.0 mg/kg or 5.0 mg/kg) and placed in a box to monitor activity.  Vehicle-treated shocked rats showed a conditioned place aversion.  Subjects that received morphine showed a dose-dependent place preference that was facilitated by moderate shock exposure.  Shock also enhanced the motor activation produced by morphine.  These results indicate that the affective state produced by moderate shock has a negative valence that is sufficient to support a conditioned place aversion.  This state is associated with a general sensitization that enhances processing of appetitive US.