Figure 1.
Unlimited daily access to heroin escalated heroin intake and decreased the excitability of brain reward systems. a, Heroin intake (±SEM; 20 μg per infusion) in rats during limited (1 h) or unlimited (23 h) self-administration sessions. ∗∗∗p < 0.001, main effect of access (1 or 23 h), two-way repeated measures ANOVA. b, Percentage change from baseline ICSS thresholds (±SEM) in control rats, which remained heroin naive for the duration of the experiment and had ICSS thresholds assessed at the same time points as the 23 h rats. c, Percentage change from baseline ICSS thresholds (±SEM) in 1 h rats. Daily post-thresholds (±SEM), assessed immediately after each heroin self-administration session, were lowered compared with prethresholds (±SEM), assessed immediately before each self-administration session in 1 h rats. ∗p < 0.05, main effect of heroin on reward thresholds, two-way repeated measures ANOVA. d, Percentage change from baseline ICSS thresholds (±SEM) in 23 h rats. Reward thresholds, assessed immediately after each daily 23 h self-administration session, became progressively more elevated as exposure to self-administered heroin increased across sessions. ∗p < 0.05, main effect of heroin on reward thresholds, two-way repeated measures ANOVA.
