General procedure described previously (30). Rats were surgically implanted with a concentric dialysis probe aimed at the shell of the nucleus accumbens [anterior +2.0 and lateral 1.1 from bregma, vertical −8.0 from dura] (31). Experiments were performed on freely-moving rats 20-24 h after the surgical implant. Ringer's solution (147.0 mM NaCl, 2.2 mM CaCl2, 4.0 mM KCl) was delivered at a constant flow rate of 0.5 μl/min. Collection of dialysate samples (10 μl) started after 90 min, with samples collected every 20 min and immediately analyzed by an HPLC system coupled to electrochemical detection in order to quantify dopamine. Rats were treated only after stable dopamine values (<10% variability) were obtained for at least three consecutive samples. Probe location in the nucleus accumbens shell was determined histologically after each experiment and only data from rats with correct probe placement were analyzed. WY14643 (0, 20 or 40 mg/kg i.p.) was injected 20 min before nicotine (0.4 mg/kg s.c.) or cocaine (3 mg/kg i.p.), and methOEA (0, 5 or 10 mg/kg i.p.) was injected 40 min before nicotine (0.4 mg/kg s.c.). MK886 (3 mg/kg i.p.) was injected i.p. 20 min before WY14643 or methOEA.
