Fig. 1.

Rats with long access to oxycodone excalate their drug intake during self-administration. a Experimental timeline of oxycodone self-administration (SA) experiments. Saline (n = 8), short-access (ShA) (n = 15) rats and long-access (LgA) (n = 15) rats were trained as described in the text. After the last day of SA, rats underwent cue-induced extinction test for 3 h on withdrawal days 5, 10, 19, and 31. b LgA rats escalate their oxycodone intake during the training sessions. c LgA rats show two distinct intake phenotypes, high (LgA-H) (n = 9) and low (LgA-L) (n = 6) drug takers during the escalation stage. d LgA-H rats show higher active lever presses in comparison to all drug-taking phenotypes. e–g ShA, LgA-L, and LgA-H rats take more drug during first hours of the three 3-h daily sessions. Individual statistics are described in Supplemental Figs. S1, S2, and S3. The values represent means ± SEM. Key to statistics: *, **, *** = p < 0.05, 0.01, 0.001, respectively, in comparison to saline rats; ^#, ^### = p < 0.05, 0.001, respectively, in comparison to ShA rats; ^$, ^$$, ^$$$ = p < 0.05, 0.01, 0.001, respectively, in comparison to LgA-L rats