All drugs were diluted to their final concentration in aCSF and applied by superfusion. For experiments requiring the use of selective opioid agonists/antagonists, these drugs were applied cumulatively and sequentially, with the opioid receptor agonist/antagonist order combination varying between experiments to avoid bias. Concentrations of: naloxone (nalox; 10 μM), deltorphin II (delt; 300 nM), ICI174864 (ICI; 1 μM), CTAP (1 μM), naltrindole (nal, 10 nM) (all purchased from Tocris), DAMGO (1 μM), norBNI (10 nM), U69593 (U69; 3 μM) thiorphan (10 μM) (all Abcam Biochemicals), bestatin (10 μM, Cayman Chemicals) and captopril (1 μM, Sigma) remained the same for all experiments, while met-enkephalin (ME, Bachem; 100 nM, 300 nM, 500 nM, 10 μM, 30 μM) concentration was varied where indicated. BMS-986187 (1 μM) was from Bristol-Myer Squibb Co.