Nicotine hydrogen tartrate (Sigma, St Louis, MO) was dissolved in 0.9% physiological saline solution. F344 and HIV-1Tg rats were injected subcutaneously (s.c.) with either saline or nicotine once per day (0.4 mg/kg/day) for 27 d before tissue collection, a procedure commonly used to study pharmacological effects of nicotine in our and other groups (Gutala et al., 2006 (link), Matta et al., 2007 (link), Nesil et al., 2015 (link)). The concentration of nicotine was calculated as nicotine free base, pH 7.0, according to Matta et al. (Matta et al., 2007 (link)) and based on our previous reported (Song et al., 2015 , Nesil et al., 2015 (link)).
On Day 28, the animals were sacrificed and brain regions were collected. Using a rat brain matrix (Kent Scientific, Torrington, CT), 1 mm slices were taken from each brain. The slices that contained the prefrontal cortex (PFC), ventral tegmental area (VTA) and nucleus accumbens (NAc) were identified according to the rat brain atlas (Paxinos and Watson, 2005 ). Tissue from the regions of interest was collected from each brain using a 1.5 mm brain punch (Stoelting, Wood Dale, IL), and stored at −80°C until use.
On Day 28, the animals were sacrificed and brain regions were collected. Using a rat brain matrix (Kent Scientific, Torrington, CT), 1 mm slices were taken from each brain. The slices that contained the prefrontal cortex (PFC), ventral tegmental area (VTA) and nucleus accumbens (NAc) were identified according to the rat brain atlas (Paxinos and Watson, 2005 ). Tissue from the regions of interest was collected from each brain using a 1.5 mm brain punch (Stoelting, Wood Dale, IL), and stored at −80°C until use.