Nicotine and fluoxetine alter adolescent dopamine-mediated behaviors via 5-HT1A receptor activation

Abstract

Introduction: Abuse or misuse of tobacco, e-cigarettes, or antidepressants may have serious clinical consequences during adolescence, a sensitive period during brain development when the distinct neurobiology of adolescent serotonin (5-HT) and dopamine (DA) systems create unique behavioral vulnerabilities to drugs of abuse.

Methods: Using a pharmacological approach, we modeled the behavioral and neurochemical effects of subchronic (4-day) nicotine (60µg/kg, i.v.) or fluoxetine (1mg/kg, i.v.) exposure in adolescent and adult male rats.

Results: Nicotine and fluoxetine significantly enhance quinpirole-induced locomotor activity and initial cocaine self-administration in adolescents, but not adults. These effects were blocked by serotonin 5-HT_1A receptor antagonists, WAY-100,635 (100 µg/kg, i.v.) or S-15535 (300 µg/kg, i.v.). Neurochemical and anatomical autoradiographic analysis of 8-OH-DPAT-stimulated [³⁵S]GTPγS reveal that prior exposure to nicotine and fluoxetine results in both overlapping and distinct effects on regional 5-HT1A receptor activity. Both fluoxetine and nicotine enhance adolescent 5-HT1A receptor activity in the primary motor cortex (M1), whereas fluoxetine alone targets prefrontal cortical neurocircuitry and nicotine alone targets the amygdala.

Discussion: Given their different pharmacological profiles, comparison between WAY-100,635 and S-15535 indicates that postsynaptic 5-HT_1A receptors mediate the behavioral effects of prior nicotine and fluoxetine exposure. In addition, within the adolescent M1, maladaptive changes in 5-HT signaling and 5-HT_1A activity after nicotine or fluoxetine exposure may potentiate hyper-responsiveness to dopaminergic drugs and prime adolescent vulnerability for future substance abuse.

Keywords: adolescence; age-dependent; behavioral acquisition; cocaine; fluoxetine; nicotine; self-administration.

Figure 1

Age differences in the effect of fluoxetine and WAY-100,635 pretreatment on quinpirole-induced locomotion. (A) Quinpirole (0.4 mg/kg, i.p.) significantly induced locomotion in adolescents (***p < 0.001 vs. vehicle). Fluoxetine (1mg/kg, i.v.)-pretreated adolescents showed enhanced locomotor activity following administration of quinpirole (++p < 0.01 vs. saline), an effect that was blocked by pretreatment with WAY-100,635 (100 µg/kg, i.v.; +++p = 0.001 vs. FLX+WAY). WAY-100,635 pretreatment alone did not alter quinpirole-induced locomotor activity (n = 9 saline-vehicle, 9 saline-quinpriole, 10 FLX-vehicle, 10 FLX-quinpirole, 9 WAY-vehicle, 9 WAY-quinpirole, 11 FLX+WAY-vehicle, 9 FLX+WAY-quinpirole). (B) In adults, there were no significant effects of pretreatment or quinpirole on locomotor activity across groups (n = 9 saline-vehicle, 8 saline-quinpriole, 9 FLX-vehicle, 8 FLX-quinpirole, 8 WAY-vehicle, 8 WAY-quinpirole, 8 FLX+WAY-vehicle, 8 FLX+WAY-quinpirole). Data analyzed with analyzed by 3-way ANOVA, with Bonferroni post hoc tests. Bars represent mean ± SEM.

Figure 2

Fluoxetine pretreatment during adolescence enhanced cocaine self-administration, an effect blocked by co-administration of WAY-100,635. Fluoxetine (1mg/kg, i.v.)-pretreated adolescents had significantly higher reinforced responses for cocaine (0.5 mg/kg/inf, i.v.) compared to non-reinforced responses (***p < 0.001). Fluoxetine-pretreated adolescents also had greater total reinforced responses for cocaine compared to other pretreatment groups (+++p < 0.001 vs. saline, +p < 0.05 vs. WAY and FLX+WAY). WAY-100,635 (100 µg/kg, i.v.) alone did not alter cocaine self-administration. Data were analyzed by 2-way ANOVA, with repeated measures on Responses. Significant effects of reinforcement were analyzed by Bonferroni-corrected paired t-tests for each pretreatment group. Bars represent mean ± SEM. n = 13 saline, 14 FLX, 13 WAY, 11 FLX + WAY.

Figure 3

Acute 5-HT_1A receptor antagonism blocks enhancement of quinpirole-induced locomotion in nicotine- and fluoxetine-pretreated adolescent rats. Nicotine and fluoxetine pretreatment enhance quinpirole-induced locomotor activity (+++p < 0.001 vs. saline pretreatment). Acute administration of WAY-100,635 (100 µg/kg, i.v.) or S-15535 (300 µg/kg, i.v.) blocks the enhancement of quinpirole-induced locomotion in adolescents pretreated with nicotine (60 µg/kg, i.v.) or fluoxetine (1mg/kg, i.v.) (**p < 0.01 vs vehicle). Data analyzed with analyzed by 2-way ANOVA, with Bonferroni post hoc tests. Bars represent mean ± SEM. n = 25 saline-vehicle, 23 saline-WAY, 12 saline-S-15535, 28 nicotine-vehicle, 14 nicotine-WAY, 12 nicotine-S-15535, 12 fluoxetine-vehicle, 12 fluoxetine-WAY, 12 fluoxetine-S-15535.

Figure 4

Post-treatment 5-HT_1A and D₂ receptor activation mediate enhancement of cocaine self-administration after nicotine and fluoxetine pretreatment. (A) Adolescent rats pretreated with nicotine (60 µg/kg, i.v.) or fluoxetine (2 mg/kg, i.v.) had significantly greater reinforced responses compared to non-reinforced responses (***p < 0.001) and higher reinforced responses for cocaine (+++p < 0.001 vs. saline) (n = 21 saline-veh, 21 nicotine-veh, 23 fluoxetine-veh). Enhancements of cocaine self-administration are no longer present after (B) acute 5-HT_1A receptor antagonism with WAY-100,635 (100 µg/kg, i.v.) (n = 8 saline-WAY, 7 nicotine-WAY, 8 fluoxetine-WAY) or (C) acute D₂ receptor antagonism with L-741,626 (2 mg/kg, i.p.) (n = 12 saline-L-741626, 11 nicotine- L-741626, 11 fluoxetine- L-741626). Data were analyzed by 2-way ANOVA, with repeated measures on Responses. Significant effects of reinforcement were analyzed by Bonferroni-corrected paired t-tests for each pretreatment group. Bars represent mean ± SEM.