Analysis of firing activity of VTA DA neurons in rats passively exposed to nicotine. A, B, Scatterplots showing the bursting activity as a function of firing rate of individual neurons for rats receiving continuous saline (pump-SAL) or nicotine (pump-NIC) infusion via osmotic pumps. Rats were exposed to continuous administration of nicotine via osmotic pumps (n = 7 rats, 93 neurons, osmotic pumps filled with nicotine at 9 mg/kg/d, for 6 d and recordings 24 h after withdrawal of the pumps). Total intake of nicotine was similar in pump-NIC and NIC groups (27.1 ± 2.2 and 20.5 ± 0.4 mg, respectively; Student's t test, NS). Control rats underwent similar surgical and recording procedures (n = 17 rats, 111 neurons, osmotic pumps filled with 0.9% NaCl). C, Histograms showing the distribution of VTA DA neuron firing rates in SAL and NIC rats. Scale on the y-axis of the histograms corresponds only to the histogram bars. Solid lines on each of the graphs represent best-fit normal distribution curves for the histogram data. Firing rate and bursting activity of VTA DA neurons for the pump-NIC groups did not differ from pump-SAL. Numbers refers to numbers of VTA DA neurons recorded.