Voluntary nicotine consumption triggers in vivo potentiation of cortical excitatory drives to midbrain dopaminergic neurons

Abstract

Active response to either natural or pharmacological reward causes synaptic modifications to excitatory synapses on dopamine (DA) neurons of the ventral tegmental area (VTA). Here, we examine these modifications using nicotine, the main addictive component of tobacco, which is a potent regulator of VTA DA neurons. Using an in vivo electrophysiological technique, we investigated the role of key components of the limbic circuit, the infralimbic cortex (ILCx) and the bed nucleus of the stria terminalis (BNST), in operant behaviors related to nicotine reward. Our results indicated that nicotine self-administration in rats, but not passive delivery, triggers hyperactivity of VTA DA neurons. The data suggest that potentiation of the ILCx-BNST excitatory pathway is involved in these modifications in VTA DA neurons. Thus, recruitment of these specific excitatory inputs to VTA DA neurons may be a neural correlate for the learned association between active responding and the reward experience.

Figure 1.

Voluntary nicotine consumption elevates VTA DA neuronal activity. A, Nicotine self-administration in the NIC group during a 2 month training under fixed-ratio schedules of reinforcement (FR1, FR2, and FR5). When responding was stable, the nicotine unit dose was increased from 30 to 60 μg/kg/infusion (arrow). Data are shown as mean ± SEM responses on the active (red circles) and inactive (white circles) nose-pokes. B, Oscilloscope traces of impulses from three VTA DA neurons showing typical recordings of two regular and one fast-firing VTA DA neuron in SAL, YOKED, and NIC rats, respectively. C, Analysis of firing activity of VTA DA neurons in SAL, YOKED, and NIC rats. Bar charts show firing rates as a function of the treatment that each group received. D, Histograms showing the distribution of VTA DA 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. E, Analysis of the bursting activity of VTA DA neurons in SAL, YOKED and NIC rats. Bar charts show percentage of spikes in bursts as a function of the treatment that each group received. F, The frequency (left axis) or burst firing (right axis) was independent of the amount of nicotine self-administered. The dot chart represents the average firing rate (closed circles) and the average bursting activity (open circles) for all the VTA DA neurons recorded in each rat as a function of the total amount of self-administered nicotine. There is no correlation between bursting or firing activity and the total amount of nicotine self-administered by the NIC rats. Pearson correlation calculation; firing rates: Pearson r = 0.20, R² = 0.04; bursting activity: Pearson r = 0.16, R² = 0.03. A one-sample Kolmogorov–Smirnov test was used to test normal distribution. ***p < 0.001 versus VTA DA activity in SAL rats. Numbers refer to numbers of VTA DA neurons recorded.

Figure 2.

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.

Figure 3.

Voluntary nicotine consumption potentiates cortical excitatory drives to the VTA: analysis of input/output tests on BNST neurons projecting to the VTA after stimulation of ILCx in SAL, YOKED, and NIC rats. A, Diagram of the stimulation protocol used in this experiment. Here, the ILCx was stimulated with a 100-pulse train and BNST neurons projecting to the VTA were identified by antidromic activation from the VTA. B, Ten consecutive sweeps showing antidromic response of a BNST neuron from VTA stimulation. Stimulation of the VTA 30 ms after spontaneous spikes elicit driven spikes (black traces). Driven spikes are occluded for similar stimuli delivered 5 ms after spontaneous impulses indicating collision between spontaneous and driven spikes (red traces). C, Increasing intensity of stimulation current in ILCx evoked higher response magnitude of BNST neurons projecting to the VTA. Cortical excitatory strength onto BNST neurons projecting to the VTA was enhanced only in NIC rats. The slope of the relationship between injected current and evoked firing rate is similar in SAL and YOKED animals. Numbers of neurons recorded in each group are mentioned in brackets. ***p < 0.001 versus excitatory responses in SAL rats. D, E, Graphs illustrating the effects of treatment (SAL, YOKED, and NIC rats) on inhibitory response duration (D) and on basal activity frequency (E). Treatment had no effect on inhibition or on basal activity in SAL, YOKED, or NIC rats. F, Typical PSTHs illustrate responses of BNST neurons projecting to the VTA on ILCx stimulation (1000 μA). In this PSTHs, the stimulus is delivered at time 0 (gray vertical line). Each PSTH consists of 100 trials individually illustrated in the associated raster. Bin width, 5 ms. Numbers in brackets refers to numbers of BNST neurons recorded.