Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), a metabolite of
dopamine, may act as an endogenous
neurotoxin and contribute to the etiology of
Parkinson's disease (PD). The inverse relationship between smoking and PD prompted our previous investigation and the report of protective effects of
nicotine against
salsolinol-induced toxicity in cultured SH-SY5Y cells (Copeland et al., Neurotox. Res. 8:289, 2005). These cells, derived from human
neuroblastoma cells, express dopaminergic activity and are used as a model of nigral dopaminergic cells, the major site of pathology in PD. The purpose of the current study was to investigate whether apoptotic or antiapoptotic mechanisms were responsible for the observed effects of
salsolinol and
nicotine, respectively. Moreover, it was of interest to determine whether the actions of
nicotine are mediated through
nicotinic receptors. SH-SY5Y cells were exposed to 0.4 or 0.7 mM
salsolinol with and without pretreatment in combination of 0.1 mM
nicotine and 0.1 mM
mecamylamine and were exposed for 24 and 48 h. Various parameters including cell cycle perturbations (reflected in
propidium iodide DNA staining); cell cycle regulator
retinoblastoma protein (reflected in the Western blot), apoptosis (reflected in
annexin V/
propidium iodide staining followed by flow cytometry) were analyzed.
Salsolinol caused an arrest of the cells in G1-phase of cell cycle and an increase in apoptotic indices, whereas pretreatment with
nicotine attenuated or completely blocked the effects of
salsolinol.
Nicotine effects in turn, were totally blocked by
mecamylamine (0.1 mM). The results suggest that apoptosis is a major mechanism for
salsolinol-induced toxicity and that antiapoptotic effects of
nicotine, mediated by
nicotinic receptors, may play a primary role in its
neuroprotective effects. Hence,
nicotinic agonists in combination with other antiapoptotic agents may be of substantial benefit in at least a subpopulation of Parkinson patients.