An Investigation of Behavioral Processes Influencing Reinforcer Motivation Following Intermittent-Access Training

Following intermittent-access (IntA) self-administration, rats show increased cocaine motivation compared to continuous-access (ContA) self-administration training. This effect has been attributed to rapid cocaine spikes that occur once every 30 min on IntA schedules. However, few studies have investigated the role of learning that develops differentially following IntA or ContA training. Exp. 1 addressed this by using a within-subjects design where the same rats self-administered cocaine on the IntA and short-access (ShA) procedures on alternating sessions in separate contexts. Rats showed evidence of context-specific behavior by escalating cocaine intake in the IntA context but not the ShA context. Additionally, rats earned more progressive ratio infusions in the IntA context compared to the ShA context following 11 sessions. Exps. 2 and 3 studied the influence of the stimulus-response (S-R) association (Exp. 2) or the stimulus-outcome (S-O) association (Exp. 3) on progressive ratio responding in IntA and ContA groups when saccharin was the reinforcer. Exp. 2 tested the role of the S-R association by comparing progressive ratio performance between IntA and ContA rats under two test conditions. One test was given with the same lever press response used during training. Another test was given using a briefly trained nosepoke response. The IntA group earned more reinforcers than the ContA group on the test with the nosepoke response, but not the lever press response, indicating that the S-R association was not critical for the IntA motivation effect. Exp. 3 used a method similar to that of Exp. 2 except the briefly trained nosepoke response was used for both progressive ratio tests. Under one test condition, the light + noise cues used to signal IntA availability periods (and present during the entirety of ContA sessions) were present; during a second test condition these cues were removed. Therefore, this design tested the role of the S-O association on progressive ratio responding between IntA and ContA groups. IntA rats earned more reinforcers than ContA rats regardless of test condition, indicating that the S-O association was not critical for the effect. It is suggested that IntA training generally increases the value of the reinforcer compared to ContA training.