Effects of heroin/cocaine combinations in rats trained to discriminate heroin or cocaine from saline

The effects of heroin and cocaine administered alone or in combination were examined in rats trained to discriminate either heroin (0.56 mg/kg IP; n = 6) or cocaine (5.6 mg/kg IP; n = 6) from saline. Heroin (0.032-1.8 mg/kg) substituted completely for the heroin training stimulus in all six heroin- trained rats, but failed to substitute for cocaine in any of the cocaine- trained rats. Cocaine (0.1-32 mg/kg) substituted completely for the cocaine training stimulus in all six cocaine-trained rats, and substituted for heroin in two of six heroin-trained rats. The opioid antagonist naltrexone (0.01- 1.0 mg/kg) antagonized the discriminative stimulus effects of heroin, but naltrexone at doses up to 10 mg/kg had no effect on the discriminative stimulus effects of cocaine. The dopamine receptor antagonist flupenthixol (0.032-0.56 mg/kg) attenuated the discriminative stimulus effects of heroin and completely blocked the discriminative stimulus effects of cocaine. When heroin-cocaine combinations were administered to the heroin-trained rats, cocaine (1-5.6 mg/kg) did not significantly alter the mean heroin dose- effect curve. Similarly, in the cocaine-trained rats, heroin (0.1-0.56 mg/kg) did not significantly alter the mean cocaine dose-effect curve. These results suggest that combinations of heroin and cocaine usually produce discriminative stimulus effects similar to either heroin or cocaine alone.