Phasic Dopamine Release Magnitude Tracks Individual Differences in Sensitization of Locomotor Response following a History of Nicotine Exposure

Abstract

Smoking remains the primary cause of preventable death in the United States and smoking related illness costs more than $300 billion annually. Nicotine (the primary reinforcer in cigarettes) causes changes in behavior and neurochemistry that lead to increased probability of relapse. Given the role of mesolimbic dopamine projections in motivation, substance use disorder, and drug relapse, we examined the effect of repeated nicotine on rapid dopamine signals in the nucleus accumbens (NAc) of rats. Adult, male Sprague-Dawley rats were exposed to nicotine (0.2 or 0.4 mg/kg, subcutaneous) once daily for 7 days. On day 8, dopamine release and uptake dynamics, and their modulation by nicotinic receptor agonists and antagonists, were assessed using fast scan cyclic voltammetry in the NAc core. Nicotine exposure decreased electrically-stimulated dopamine release across a range of stimulation frequencies and decreased α6β2-containing nicotinic receptor control over dopamine release. Additionally, nicotine locomotor sensitization correlated with accumbal dopamine modulation by nicotine and mecamylamine. Taken together, our study suggests that repeated exposure to nicotine blunts dopamine release in the NAc core through changes in α6β2 modulation of dopamine release and individual differences in the sensitivity to this outcome may predict variation in behavioral models of vulnerability to substance use disorder.

Figure 1

Chronic nicotine administration lowers dopamine signaling. (A) Experimental timeline of locomotor assessments, nicotine injections, and voltammetry. Rats were given subcutaneous injections of saline or 0.2 mg/kg or 0.4 mg/kg nicotine for seven days, with locomotor assessment on Days 1 and 7. On the eighth day, brains were extracted and ex vivo voltammetry was used to examine dopamine release in the nucleus accumbens core. (B) Chronic exposure to nicotine lowers electrically-stimulated single pulse dopamine release compared to saline. (C) Nicotine decreases both dopamine release, but does not differ between doses. (D) Maximal rate of dopamine uptake (V_max) is unaffected by nicotine exposure. Bars and symbols represent means ± SEMs, *p < 0.05.

Figure 2

Chronic nicotine alters α6β2 nAChR modulation of dopamine following single pulse stimulation. (A) Chronic nicotine exposure did not alter the effect of a desensitizing concentration of nicotine (500 nM) or (B) MEC [a non-selective nAChR antagonist (2 μM)] on single pulse dopamine release in the NAc core. (C) DHβE [a selective β2 nAChR antagonist (500 nM)] decreased dopamine release significantly more in saline than nicotine treated rats. (D) Chronic nicotine exposure also blunted the decrease in single pulse dopamine release following application of α-Ctx MII [a selective α6 nAChR antagonist (100 nM)] followed by DHβE. This order was used to differentiate the effect of α6 and non-α6 nAChRs. Bars and symbols represent means ± SEMs, *p < 0.05.

Figure 3

α6 nAChR modulation of dopamine release is altered following chronic nicotine. (A) Schematic of local circuitry and nAChRs located in the NAc core. (B) Nicotine (500 nM) decreased dopamine release to single pulse and low frequency stimulation, but not the highest stimulation frequency. Chronic nicotine exposure did not change nicotine-induced modulation of dopamine release. (C) MEC (2 μM) decreased dopamine release to single pulse and low frequency stimulation in both saline and nicotine treated animals. (D) DHβE (500 nM) and (E) α-Ctx MII (100 nM) also modulate dopamine release in a frequency-dependent manner, but dopamine release is higher in rats with chronic nicotine exposure and shows facilitation at higher frequencies. (F) The application of DHβE following α-Ctx MII does not significantly change dopamine release. Bars and symbols represent means ± SEMs, *p < 0.05. Note: Not all significant interactions are visually represented.

Figure 4

Chronic nicotine does not alter enhancement of dopamine phasic:tonic ratios caused by nAChR modulation on a slice. Nicotine (A), MEC (B), DHβE (C), and α-ctx (D) all enhance dopamine phasic:tonic ratios (5 P @ 20 Hz/1 P) equally in both saline and nicotine treated animals.

Figure 5

The magnitude of nicotine-induced sensitization predicts the effects of nicotine on dopamine release to phasic firing in the NAc. (A) Nicotine increases post-injection locomotion significantly more following repeated nicotine exposure. Repeated saline and acute nicotine do not alter locomotion. Inset: Repeated injections of nicotine significantly changed nicotine-induced locomotion, while repeated injections of saline did not change locomotion following a saline injection. (B,C) The magnitude of nicotine-induced locomotor sensitization is not predicted by baseline dopamine release following 5 Hz (B) or 20 Hz (C) stimulations. (D) Magnitude of nicotine-induced locomotor sensitization is not predicted by changes in tonic (5 Hz) stimulations following bath application of nicotine (500 nM), (E) but is predicted by nicotine-induced changes to dopamine release following phasic (20 Hz) stimulation. (F) Similarly, magnitude of nicotine locomotor sensitization was not correlated with MEC-induced (2 μM) changes in dopamine release to tonic stimulation, (G) but did positively correlate with changes to phasic dopamine release following MEC application. Bars and symbols represent means  ± SEMs, *p < 0.05.