Role of trace amine-associated receptor 1 in nicotine's behavioral and neurochemical effects

Abstract

Nicotine addiction and abuse remains a global health issue. To date, the fundamental neurobiological mechanism of nicotine addiction remains incompletely understood. Trace amine-associated receptor 1 (TAAR1) is thought to directly modulate dopaminergic system and are thought to be a neural substrate underlying addictive-like behaviors. We aimed to investigate the role of TAAR1 in nicotine addictive-like behaviors. TAAR1 expression after nicotine treatment was evaluated by western blotting. c-Fos immunofluorescence and in vivo fast-scan cyclic voltammetry were used to examine the activation of brain regions and dopamine release, respectively. We then thoroughly and systematically examined the role of TAAR1 in mediating nicotine-induced sensitization, nicotine discrimination, nicotine self-administration, nicotine demand curve, and the reinstatement of nicotine-seeking. Local pharmacological manipulation was conducted to determine the role of TAAR1 in the nucleus accumbens (NAcs) in the reinstatement of nicotine-seeking. We found that the expression of TAAR1 protein was selectively downregulated in the NAc, with no change in either dorsal striatum or prefrontal cortex. TAAR1 activation was sufficient to block nicotine-induced c-Fos expression in the NAc, while also reducing nicotine-induced dopamine release in the NAc. Systemic administration of TAAR1 agonists attenuated the expression and development of nicotine-induced sensitization, nicotine self-administration, the reinstatement of nicotine-seeking, and increased the elasticity of nicotine demand curve, while intra-NAc infusions of a TAAR1 agonist was sufficient to attenuate nicotine reinstatement. Moreover, TAAR1-knockout rats showed augmented cue-induced and drug-induced reinstatement of nicotine-seeking. These results indicated that modulation of TAAR1 activity regulates nicotine addictive-like behaviors and TAAR1 represents a novel target towards the treatment of nicotine addiction.

Fig. 1

Effects of nicotine treatment on TAAR1 expression, and effects of TAAR1 activation on nicotine-induced c-Fos expression. a–c Nicotine exposure significantly reduced the level of TAAR1 protein in the NAc (n = 7–8 per group), but not in the CPU (n = 7–8 per group) or the PFC (n = 6–7 per group). d TAAR1 agonist RO5166017 attenuated nicotine-induced behavioral sensitization. e TAAR1 agonist RO5166017 reduced nicotine-induced increase in the number of c-Fos-positive cells in the NAc. No difference among all groups was found in the PFC. f Representative images of c-Fos staining. AC anterior commissure. Data are expressed as mean ± SEM; *p < 0.05, ***p < 0.001, compared with saline or saline-vehicle group; ^#p < 0.05, ^##p < 0.01, compared with nicotine-vehicle group. Saline-vehicle, n = 5; other groups, n = 6 per group

Fig. 2

TAAR1 activation reduced nicotine-induced dopamine release in the NAc. a Representative color plots of electrically evoked dopamine. Data are presented as 5 s before and 10 s after electrical stimulation. Dopamine concentration changes are apparent in the color plots for their oxidation (~0.65 V, indicated by dotted white line) and reduction (~−0.29 V, indicated by solid white line) potentials. b Representative concentration vs. time traces of dopamine release evoked by electrical stimulation of the VTA (20 Hz, 60 pulses, 300 µA). c TAAR1 agonist RO5166017 reduced dopamine release and prevented nicotine-induced dopamine release in the NAc. [DA]_max maximal dopamine level. Data are expressed as mean ± SEM; ***p < 0.001, compared with saline-vehicle group; ^###p < 0.01, compared with nicotine group. Vehicle and nicotine groups, n = 4; RO and RO + nicotine groups, n = 5

Fig. 3

Effects of TAAR1 activation on nicotine-induced behavioral sensitization, nicotine discrimination, nicotine self-administration, and nicotine demand curve. a TAAR1 agonist RO5263397 reduced the expression of nicotine-induced sensitization (vehicle, n = 9; 3.2 and 10 RO, n = 10 per group). b TAAR1 agonist RO5263397 attenuated acute nicotine-induced hyperactivity and the development of nicotine-induced sensitization (vehicle, n = 8; 3.2 RO, n = 9; 10 RO, n = 10). c TAAR1 agonist RO5263397 and nicotinic receptor antagonist mecamylamine dose-dependently attenuated the average percentage of responding on the nicotine-appropriate lever (n = 8 per group). d TAAR1 agonist RO5263397 only at the dose of 17.8 mg/kg decreased rates of responding. Mecamylamine had no effect on rates of responding. e TAAR1 agonist RO5166017 and RO5263397 dose-dependently reduced nicotine self-administration (saline rats, n = 6; nicotine rats, n = 9). f TAAR1 agonist RO5263397 shifted the nicotine demand curve downward, reduced estimate of consumption at zero price (Ǫ₀), and increased the essential value (α) of nicotine (n = 9). Symbols above “V” represent vehicle. Data are expressed as mean ± SEM; *p < 0.05, **p < 0.01, ***p < 0.001, compared with vehicle

Fig. 4

Role of TAAR1 in cue-induced and drug-induced reinstatement of nicotine-seeking. a Timeline for this experiment. b Rats that self-administered nicotine (n = 9 per group) consistently responded more in the active lever compared to rats in the saline group (n = 7). Successful extinction was achieved after six sessions of extinction. c TAAR1 agonist RO5166017 attenuated cue-induced reinstatement of nicotine-seeking. d TAAR1 agonist RO5166017 decreased priming-induced reinstatement of nicotine-seeking. e No difference was found during nicotine self-administration or extinction between WT and TAAR1-KO rats (n = 6–7 per group). f TAAR1-KO rats showed higher responses during cue-induced reinstatement of nicotine-seeking. g TAAR1-KO rats showed higher responses during nicotine priming-induced reinstatement of nicotine-seeking. Data are expressed as mean ± SEM; *p < 0.05, ***p < 0.001, compared with saline-vehicle group or WT group; ^###p < 0.001, compared with nicotine-saline group. WT wild type, KO knockout.

Fig. 5

Local pharmacological activation of TAAR1 in the NAc decreased the reinstatement of nicotine-seeking. a Lever presses on the last 5 days of training. b Successful extinction was achieved after six sessions of extinction. c Microinjection of RO5166017 into the NAc significantly reduced the reinstatement of nicotine-seeking (n = 11 per group). d Microinjection of RO5166017 into the NAc did not affect locomotor activity. e Atlas of injection site. Gray dots are for vehicle group and dark dots are for RO5166017 group. Data are expressed as mean ± SEM; *p < 0.05, compared with vehicle group