Separation of the locomotor stimulant and discriminative stimulus effects of cocaine by its C-2 phenyl ester analog, RTI-15.

During a routine evaluation of several analogs of cocaine, we observed that the C-2 phenyl ester, RTI-15, appeared to suppress motor activity in rats. We subsequently examined RTI-15 for its cocaine-like stimulus effects as well as for its locomotor activity effects. RTI-15 dose-dependently generalized from the cocaine stimulus in rats trained to discriminate 10 mg/kg cocaine from saline with complete substitution (> or = 80% cocaine-lever responding) occurring at 24 mg/kg. During automated locomotor activity tests in mice, cocaine (3-60 mg/kg) dose-dependently increased activity counts and movement time across the entire 1 h test session. RTI-15, however, had little affect on activity counts and movement time from 10-30 mg/kg, and decreased these measures at 60 mg/kg, the highest dose tested. These results indicate that while changing the C-2 methyl ester of cocaine to a C-2 phenyl ester increases dopamine-transporter selectivity, it dissociates its locomotor activity effects from its discriminative stimulus effects suggesting that the underlying mechanisms mediating these effects are not identical.