Operant behavior before electrophysiological recordings. A, Timeline of experiment. Animals were recorded 24 h after the following: (1) a single nicotine self-administration session (NIC-1 d, n = 5), (2) eight sessions of nicotine self-administration (NIC-8 d, n = 8), (3) extended training (NIC-60 d, n = 8; SAL-60 d, n = 7; yoked, n = 5), (4) extinction (Ext, n = 6), or (5) abstinence (Abst, n = 7). B, Number of visits in the active hole. Each bar represents the average number over the 3 last days before electrophysiological recording for each experimental group (except the “1 day” group). C, NIC-60 d rats showed a strong preference for the active hole. ***p < 0.001. Data are mean + SEM.