Figure 1. Experimental design and Addiction-like behavior.

Rats were split between a standard and enriched environment for the duration of the study (A). After two weeks, catheter surgery was performed followed by habituation to the test chambers and recovery for two days. Self-administration lasted for two weeks, at the end of which the progressive ratio test was performed. Following two weeks of abstinence, extinction reinstatement testing was performed for a day. Euthanasia was the next day, to ensure no drug was remaining at the time of tissue collection. (B) Rate of infusions (saline or heroin) per day over the last three days of acquisition demonstrated some variability in the standard saline group but a high rate of infusions in the heroin groups as compared to the enriched saline condition. (C) Sum of heroin (0.06 mg/0.2ml of heroin) infusions/3 h for standard (SE) heroin and enriched (EE) heroin across 12 trials. (C) Number of saline or heroin infusions earned during progressive ratio testing for standard saline, enriched saline, standard heroin, and enriched heroin. (E) Number of responses/1 h exhibited during the 6 h extinction session and (F) during the 1 h after heroin administration reinstatement session for standard saline, enriched saline, standard heroin, and enriched heroin. *p<0.05, ANOVA n=5–10/group, data are presented as mean (+/− SEM).