General procedure described previously (8). Single-unit activity of VTA neurons was recorded extracellularly with glass micropipettes filled with 2% pontamine sky blue dissolved in 0.5 M sodium acetate (impedance 2-5 MΩ). Single units were isolated and identified according to previously published criteria (27,28). All neurons were antidromically identified as projecting to the nucleus accumbens shell by antidromic spikes elicited by stimulation of the shell of the NAc. An antidromic response was defined as the ability of evoked spikes to follow stimulation frequencies of >250 Hz, displaying constant latency and collision with spontaneously occurring spikes (29). Nicotine (0.2 mg/kg) was administered i.v. after 5-10 min of baseline recording. MethOEA (0, 5 or 10 mg/kg i.v.) was injected 4 min before nicotine. WY14643 (20 or 40 mg/kg i.p.) was injected ∼30 min before the start of recordings, MK886 (3 mg/kg i.p.) was injected 15 min before WY14643. Only one cell was recorded per rat.
