Effect of injection duration on the acquisition of cocaine self‐administration in rats: the role of paired stimuli

Faster infusions of cocaine are more likely to support behavior related to abuse. However, this has not been a universal finding, possibly because other factors can mask the influence of infusion speed. Since a drug‐paired stimulus is known to facilitate acquisition of drug self‐administration, we investigated the role of a paired stimulus in rats trained with two different cocaine infusion durations. Rats were trained to self‐administer 1 mg/kg/injection cocaine on a fixed‐ratio 1 schedule. For two groups the injection duration was 1.8 sec (fast) and for two groups the duration was 100 sec (slow). For two of these four groups each injection was followed by a 110‐sec timeout that was signaled by a paired stimulus. Both these groups acquired self‐administration. The two other groups of rats were trained under identical conditions except that the timeout after the injection was not signaled. The rats trained with the fast infusion acquired self‐administration. However, most rats in the group trained with the slow un‐signaled infusion failed to acquire cocaine self‐administration. These results indicate infusing cocaine over a longer duration reduces its ability to support self‐administration. Pairing the infusion with a stimulus change can mask the effect of infusion duration. Supported by NIH/NIDA Intramural Research Funds.