Fig. 1: Pavlovian experimental procedure.

a Schematic of operant chamber for training/recording in Experiment 1⁷⁶. b Example waveform showing stability over a two-hour recording session. Color corresponds to action potential amplitude. c Pavlovian conditioning of three 5-s tones predicting sucrose (blue), remifentanil (green), or nothing (neutral, red). d Example responses of putative dopamine responses to cued sucrose reward (top), reward omission (middle), and uncued reward (bottom). e Rasters and post-stimulus time histogram (PSTH) showing example dopamine neuron firing after remifentanil infusion (2 infusions of 4 μg/kg remifentanil) with onset of remifentanil effect at ~10 s post-infusion. f PSTH for neuronal data of onset and duration of effect of remifentanil at doses of 0.5 to 16 μg/kg/infusion prior to training (activation terminates by ~2.5 min for 4 μg/kg). g Total time (left) and probability (right) of rat in sucrose reward well during sucrose and neutral cue presentations in opioid-naive rats (N = 4 rats, n = 22 units, two-tailed Wilcoxon test, z = -2.105, p < 0.0001). Lines and shades represent means ± SEM. h Same as h but in opioid-exposed animals after sucrose, remifentanil, and neutral cues (N = 7 rats, n = 50 units, Friedman test, F = 75.36, p < 0.0001 with Dunn’s multiple comparison test: sucrose vs. remifentanil p > 0.9999, sucrose vs. neutral p < 0.0001, remifentanil vs. neutral, p < 0.0001). Source data are provided as a Source Data file.