Figure 1.

Typical pretraining approaches confound drug self-administration experiments in mice. Previous sucrose-paired cues support conditioned reinforcement. Panel A: Illustration of operant training procedures. Phase 1, operant training: Active nose pokes delivered a sucrose pellet concurrent with a 5-s cue light on a fixed ratio (FR) 1 and FR2 schedule of reinforcement. Phase 2, conditioned reinforcement: Mice were switched from sucrose responding to an FR1 schedule—like would be seen in cocaine self-administration training— however, active pokes resulted in only a 5-s cue light presentation, with no reinforcer. In all phases, inactive nose pokes were recorded but had no programmed consequence. Panel B: Active and inactive nose pokes during FR1 and FR2 sucrose training. Mice showed higher responding on the active nose poke compared to inactive and increased active responding during FR2. Panel C: Sucrose pellets earned across training. Animals earned reinforcers at a stable rate across FR1 and FR2 schedules of reinforcement. Panel D: Cumulative record of active and inactive responses across training sessions. Panel E: Discrimination index across sucrose training plotted as active/total responses. Panel F: Active and inactive nose pokes during FR1 during conditioned reinforcement. Mice maintained active responding for the presentation of the cue alone for 5 consecutive days. Panel G: Cue presentations earned across sessions. Animals earned stable cue presentations over 5 consecutive days. Panel H: Discrimination index across cue testing (Days 1–6) showing that mice still met discrimination criteria with no reinforcer present and maintained discrimination indices comparable to final day for sucrose training (Day 0). Panel I: Number of active nose pokes for sucrose or for cues alone (with no reinforcer present) are indistinguishable under an FR1 schedule. Panel J: Mice showed similar bias (measured by Logb) for active nose poke when reinforced by sucrose or cue light presentations alone. Data reported as mean ± standard error of the mean. ** p < .01. *** p < .001. **** p < .0001. Multiple comparisons were corrected for using Bonferroni correction of alpha threshold.