Voltammetric Recording and Surgical Preparation

Rats were deeply anesthetized with a mixture of ketamine hydrochloride (100 mg/kg) and xylazine hydrochloride (10 mg/kg), placed in a stereotaxic frame, and surgically prepared for voltammetric recording as described elsewhere (Day et al., 2010 (link); Sugam et al., 2012 (link); Saddoris et al., 2015b (link)). A guide cannula (Bioanalytical Systems) was positioned dorsally to the NAc core (AP +1.3 mm, ML –1.3 mm from bregma) or shell (AP +1.3 mm, ML –0.8 mm from bregma). Another guide cannula (for the Ag/AgCl reference electrode) was placed contralateral to the NAc cannula. A bipolar stimulating electrode was placed dorsally to the ventral tegmental area (VTA; AP –5.2 mm, ML –1.0 mm and DV –7.0 mm from bregma) and ipsilateral to the NAc cannula. Correct placement of the stimulating electrode in the VTA was determined by applying a range of stimulation parameters (12–24 biphasic pulses, 20–60 Hz) and observing tail movement. The stimulating electrode was lowered in increments of 0.1 mm until slight to no tail movement was observed at 60 Hz, 24 pulses. Stainless steel screws and dental cement were then used to secure all items. For 2 d postsurgery, rats were given an anti-inflammatory medication (meloxicam, 1 mg/kg) and were allowed access to food and water ad libitum.