wilmington mass are lightly anesthetized with halothane decapitated and the brains quickly removed to ice cold oxygenated medium 95 o 2 5 co 2 124 mm nacl 2 mm kcl 1 25 mm nah 2 po 4 26 mm nahco 3 10 mm d glucose 2 mm mgso 4 7h 2 o and 2 mm cacl 2 ph 7 4 the ventral tegmental area is blocked and glued using cyanoacrylate to the stage of a lancer vibratome series 1000 filled with ice cold medium coronal slices 350 u03bc are cut and placed in oxygenated medium 22 u00b0 c for 1 hour prior to recording for recording slices are placed on a nylon net in a recording chamber where they are completely submerged in continually flowing medium at 35 u00b0 c u2212 1 ml min all drugs are applied by switching the perfusion medium to a solution containing the drug spontaneous extracellular action potentials are recorded using 0 9 saline filled glass pipettes 6 8 mw firing rates are plotted on line in bins of 10 or 20 seconds and alterations in firing rate are calculated using average rates over 2 min epochs before and after drug application concentration response curves are constructed and analyzed by linear regression i shaped concentric microdialysis probes are constructed out of dialysis fiber molecular weight cut off of 18 000 300 um o d hospal the netherlands occluded at one end with epoxy resin and attached to fused silica microtubing the probes 9 mm long with 2 mm length of exposed dialysis membrane are implanted into the nucleus accumbens ap 1 7 ml 1 2 dv 8 0 of male sprague dawley rats 300 350 g anesthetized with ketamine 75 mg kg and xylazine 10 mg kg following surgery rats are placed in.