Exposure to mild stress enhances the reinforcing efficacy of intravenous heroin self-administration in rats.

The effect of a mild footshock on intravenous heroin self-administration was examined in male rats. Animals in the stress condition were exposed to 10 min of intermittent footshock (0.5 mA; 0.5 s on, with a mean off period of 40 s) before each of four daily self-administration sessions. Animals in the control group were not exposed to footshock. Following acquisition of heroin-reinforced behavior (100 micrograms/kg per infusion), during which no group differences emerged, animals were placed on a progressive ratio schedule of reinforcement and were subsequently tested under a decreasing series of doses. Animals exposed to footshock before each drug session had higher rates of lever pressing for heroin and achieved higher final ratios on the progressive ratio schedule than animals in the control group at the higher doses of heroin. Thus, under the conditions of this experiment, exposure to mild intermittent stress appeared to enhance the reinforcing efficacy of heroin. The parameters of footshock used in the present study, and its relation to drug availability may characterize conditions under which stress leads to increased opioid abuse.