Conditioned taste aversions induced by the combination of heroin and cocaine (speedball)

Speedball (heroin and cocaine) is a prevalent drug combination. It is possible that either drug impacts the perceived rewarding effects of the other, increasing their combined use. Interestingly, the aversive effects of a drug also contribute to its overall acceptability, basically as a factor that might limit drug intake. Accordingly, the present study examined the effects of heroin on cocaine-induced aversions (Experiment 1) and cocaine on heroin-induced aversions (Experiment 2) to determine if a weakening of aversions might occur which could impact the use of the drug combination. In Experiment 1, male Sprague-Dawley rats were given access to a novel saccharin solution and then injected with various doses of cocaine (10, 18 or 32 mg/kg, SC) immediately followed by injections of vehicle or heroin (3.2 mg/kg, IP). This conditioning day was followed by three days of water recovery which constituted one conditioning trial, for a total of four trials. On the day following the final water recovery day, all subjects were given a test of the aversion to saccharin. On the aversion test, aversions induced by the combination of heroin and the low dose of cocaine (10 mg/kg) were significantly greater than that induced by cocaine alone. At the two higher doses of cocaine, the aversion induced by the combination was comparable to that induced by cocaine alone. In Experiment 2, male Sprague-Dawley rats were given access to a novel saccharin solution and then injected with vehicle or cocaine (10 mg/kg) prior to injections of varying doses of heroin (1.8, 3.2, 5.6 or 10 mg/kg). The conditioning procedure was identical to Experiment 1. On the aversion test, aversions induced by the combination were never significantly different from aversions induced by heroin alone at any dose. Given that aversions induced by the combination were not consistently weaker than the aversions induced by the individual drug(s) alone suggests that the use of the combination is not likely due to a weakening of the aversive effects of either heroin or cocaine. Instead, these results suggest that the combination increases the rewarding effects of the individual drug(s), an effect that impacts its overall acceptability.