The long-term impact of footshock stress on addiction-related behaviors in rats

We investigated the impact of electric shocks - frequently used to model post-traumatic stress disorder in rodents - on behaviors relevant to drug abuse in rats. Rats exposed to 10 shocks of 3 mA over 5 min showed a robust conditioned fear 28 days later, which confirms the traumatic nature of shock exposure. A different set of rats was studied in the conditioned place preference paradigm beginning with the 27th post-shock day. 10 mg/kg morphine induced a marked place preference in both shocked and non-shocked rats. Although the magnitude of place preference was not affected, extinction was markedly delayed in shocked rats. We also investigated tolerance to the hyperthermic effects of morphine. A low dose (5 mg/kg) that was administered 4 weeks after shock exposure robustly increased body temperature in both shocked and non-shocked rats. Repeated injections resulted in a mild tolerance in non-shocked controls; yet, morphine readily increased body temperature in these rats on the 5th day of injections. In contrast, the temperature-heightening effect of morphine was abolished in shocked rats after 2 days. Thus, shock exposure considerably delayed the extinction of place preference induced by, and dramatically accelerated the tolerance to the effects of, morphine. Our study shows that electric shocks durably affect behavior in tests relevant to drug abuse in conjunction with the development of post-traumatic stress disorder-like behavioral dysfunctions.