Dopamine and Norepinephrine Tissue Levels in the Developing Limbic Brain Are Impacted by the Human CHRNA 6 3'-UTR Single-Nucleotide Polymorphism (rs2304297) in Rats

Abstract

We previously demonstrated that a genetic single-nucleotide polymorphism (SNP, rs2304297) in the 3' untranslated region (UTR) of the human CHRNA6 gene has sex- and genotype-dependent effects on nicotine-induced locomotion, anxiety, and nicotine + cue-induced reinstatement in adolescent rats. This study aims to investigate how the CHRNA6 3'-UTR SNP influences dopaminergic and noradrenergic tissue levels in brain reward regions during baseline and after the reinstatement of drug-seeking behavior. Naïve adolescent and adult rats, along with those undergoing nicotine + cue reinstatement and carrying the CHRNA6 3'-UTR SNP, were assessed for dopamine (DA), norepinephrine (NE), and metabolites in reward pathway regions. The results reveal age-, sex-, and genotype-dependent baseline DA, NE, and DA turnover levels. Post-reinstatement, male α6^GG rats show suppressed DA levels in the Nucleus Accumbens (NAc) Shell compared to the baseline, while nicotine+ cue-induced reinstatement behavior correlates with neurotransmitter levels in specific brain regions. This study emphasizes the role of CHRNA6 3'-UTR SNP in the developmental maturation of the dopaminergic and noradrenergic system in the adolescent rat brain, with tissue levels acting as predictors of nicotine + cue-induced reinstatement.

Keywords: DA turnover; addiction; adolescence; nicotine-induced reinstatement; nucleus accumbens.

Figure 1

Sex and Genotype Differences in Naïve Adolescent (PN 32) and Adult (PN 60) DA, NE, and Turnover Profile in the Humanized CHRNA6 3′-UTR SNP rats. A complex interplay between sex, genotype, and sex in DA and NE regulation in males (A–G) and females (H–N). Adult α6^CC and adolescent (Adol) α6^GG show higher DA levels when compared to adolescent α6^CC males in the PFC (A) and NAc Core (C); while adult males α6^CC display increased DA levels when compared to adolescent α6^CC and adult α6^GG males in the dCPu (B). Within the NAc Core, adolescent α6^GG also exhibit greater DA when compared to adult α6^GG. NE is also elevated in adult males α6^CC and adolescent α6^GG in the mPFC compared to their respective counterparts (D). Adult α6^CC exhibit greater NE when compared to adolescent α6^CC and adult α6^GG in the BLA (E). Adolescent α6^GG males display greater NE in the LC when compared to adolescent α6^CC and adult α6^GG males (F). Adult males α6^CC display greater DOPAC/DA turnover ratios when compared to adolescent α6^CC and adult α6^GG (G). On the contrary, adult α6^GG females show elevated NE and DA in the mPFC (H), dCPu (I), and BLA (J,K) when compared to adolescent α6^GG females. Adult α6^CC females show greater NE when compared to adolescent α6^CC females. Potential differences in DA metabolism are indicated by greater DOPAC/DA turnover ratio and were observed for adult females α6^GG when compared to adolescent α6^GG females in the mPFC and LC (L–M). Furthermore, in the LC, HVA/DA turnover was greater for adult α6^GG females when compared to adult α6^CC and adolescent α6^GG (N). mPFC = medial Prefrontal cortex, dCPu = dorsal Caudate putamen, NAc = Nucleus accumbens (Shell and Core), VTA Ventral tegmental area, IPN = Interpeduncular nucleus, LC = Locus coeruleus, circles = adolescents (Adol), squares = adults * p < 0.05, ** p < 0.01, *** p < 0.001 α6^GG vs. α6^CC. All data presented as mean ± SEM. N = 8–10/group.

Figure 2

Correlations Between NE, DA, and HVA/DA Turnover Ratio and Reinstatement Behavior in the CHRNA6 3′-UTR males. (A) α6 3′-UTR SNP genotype dependently influences nicotine + cue-primed reinstatement, with α6^GG males more impacted than α6^CC males (~PN47). Data represent the mean (±SEM) of nicotine + cue-seeking responding in male, α6^GG vs. α6^CC. * p < 0.05, ** p < 0.01 vs. Extinction; ++ p < 0.01 α6^GG vs. α6^CC Pearson correlation of NE, DA, and HVA/DA turnover males α6^GG. Positive correlation between Nicotine + cue-primed reinstatement and increased NE levels in the LC (B) and DA levels in the BLA (C) in α6^GG males. On the contrary, a negative correlation between nicotine + cue-primed reinstatement and DA levels in the VTA (D). A lower HVA/DA turnover ratio in the mPFC and LC in α6^GG males suggests a greater nicotine + cue behavioral response (E,F). N = 5–9/group. * p < 0.05, ** p < 0.01 vs. Extinction; ++ p < 0.01 α6^GG vs. α6^CC.

Figure 3

Age and genotype differences in the NAc Shell in the CHRNA6 3′-UTR SNP knock-in rats. DA (pg/μg) levels in the NAc shell, a brain region associated with reward, pleasure, and addition, are presented as the mean (±SEM) of naïve adolescents and adults, as well as nicotine + cue-seeking CHRNA6 3′-UTR SNP knock-in rats. Nicotine-seeking GG male rats exhibit substantially decreased DA levels when compared to naïve adolescent and adult CHRNA6 3′-UTR SNP knock-in rats. Open circles = adolescents; open squares = adults, and closed squares = reinstatement animals. N = 7–10. ** p < 0.01 vs. adolescents; + p < 0.05 vs. adults.