Figure 2.

Role of opioid receptors in vlPAG in extinction of conditioned freezing (experiment 1). A, Mean ± SEM percentage of freezing per 2 min period across the 5 d of extinction (open squares, saline; filled circles, naloxone). B, Mean ± SEM percentage of freezing during the first 4 min before CS onset on day 1 and average freezing across the 5 d of extinction training for the groups described in A (open squares, saline; filled circles, naloxone), as well as freezing during a drug-free test (Test 1) and a crossover test (Test II). SAL, Saline; NAL, naloxone.