Differential modification of the rewarding effects of methamphetamine and cocaine by opioids and antihistamines.

We previously reported that the reinforcing effects of opioids are enhanced in combination with antihistamines. In the present study, effects of opioids and antihistamines on the reinforcing effects of psychostimulants such as methamphetamine and cocaine were investigated by utilizing the conditioned place preference procedure in rats. The place preference induced by methamphetamine was enhanced in combination with either morphine or chlorpheniramine, which produced additive and potentiative effects, respectively. In contrast, although the preference for cocaine was also enhanced by combination with these two drugs, morphine caused a potentiative effect and chlorpheniramine an additive one. In other words, the reinforcing effect of methamphetamine was differentially enhanced by opioids and antihistamines as compared to that of cocaine. These results suggest that the mechanism of reinforcing effect of methamphetamine is different from that of cocaine, resembling rather those of opioids.