Fig. 1From: Metabolomics analysis of serum in a rat heroin self-administration model undergoing reinforcement based on 1H-nuclear magnetic resonance spectraHeroin self-administration. Active and inactive nose pokes during heroin self-administration over 9 consecutive training days and the reinforcement stage (n = 7, the data are expressed by mean ± SEM). Significant difference between the number of active poke and inactive poke in each session was indicated by ** (p < 0.01)Back to article page