Dopamine D3 receptor and beta-2 containing nicotinic acetylcholine receptor are critical in mediating nicotine effects on dopaminergic neurons morphology

Tobacco smoking is associated with increased risk for dependence and reduced risk for Parkinson’s disease, effects probably mediated by nicotine exposure. In both conditions the dopaminergic system is critically involved, but details regarding the mechanisms of such involvement are not fully understood. Activation of nicotinic acetylcholine receptors (nAChR) expressed by dopaminergic neurons is known to result in neurochemical and behavioural sensitization in rodents. Sensitization could be associated with changes of morphology as long lasting adaptation. Using primary cultures of mouse mesencephalic dopaminergic neurons we recently showed a role for D3 receptor (D3R) in mediating morphogenetic changes produced by dopaminergic agonists. In the present work we extended our observations to nicotine. Mesencephalic neurons isolated from E13 mouse embryos were cultured 5 days before pharmacological testing. Exposure to nicotine for 72 hrs increased the dendritic arborization and the soma size. These effects were blocked by mecamylamine and dihydro-β-erythroidine, but not by methyllycaconitine. The involvement of α4β2 nAChR was supported by the lack of nicotine-induced structural plasticity in dopaminergic neurons from α4KO mice. Moreover, nicotine triggered the activation of MEK/ERK and Akt-mTOR pathways. These effects were dependent upon functional D3R since nicotine was inactive both in cultures from D3KO mice or following pharmacologic blockade with D3R antagonist SB277011A. These observations were corroborated in vivo by morphometrical assessment of mesencephalic dopaminergic neurons of P1 newborns exposed to nicotine during pregnancy. Nicotine increased the soma area of wild-type but not of D3KO mice, supporting the translational value of our studies in primary culture