Specific uptake through dopamine transporter (DAT) followed by the inhibition of the mitochondrial complex-I have been accepted as the cause of the specific dopaminergic toxicity of MPP+. However, MPP+ is taken up into many cell types through other transporters suggesting that in addition to the efficient uptake, intrinsic vulnerability of dopaminergic cells may also contribute to their high sensitivity to MPP+ and similar toxins. To test this possibility, two simple cyanines were employed in a comparative study based on their unique characteristics and structural similarity to MPP+. Here we show that they freely accumulate in dopaminergic (MN9D and SH-SY5Y) as well as in liver (HepG2) cells, but are specifically and highly toxic to dopaminergic cells with IC50s in the range of 50–100 nM demonstrating that they are about 1000 fold more toxic than MPP+ under similar experimental conditions. They cause mitochondrial depolarization non-specifically, but increase the ROS specifically in dopaminergic cells leading to the apoptotic cell death parallel to MPP+. These and other findings suggest that the specific dopaminergic toxicity of these cyanines is due to the inherent vulnerability of dopaminergic cells towards mitochondrial toxins that lead to the excessive production of ROS. Therefore the specific dopaminergic toxicity of MPP+ must also at least be partly due to the specific vulnerability of dopaminergic neurons. Thus, these cyanines could be stronger in vivo dopaminergic toxins than MPP+ and their in vivo toxicities must be evaluated.

We have recently reported that simple lipophilic cationic cyanines are specific and potent dopaminergic toxins with a mechanism of toxicity similar to the Parkinsonian toxin MPP+. In the present study a group of fluorescent lipophilic cyanines have been used to further exploit the structure-activity relationship of the specific dopaminergic toxicity of cyanines. Here we report that all cyanines tested were highly toxic to dopaminergic MN9D cells with IC50s in the range of 60–100 nM and not toxic to non-neuronal HepG2 cells parallel to the previously reported for 2,2’- and 4,4’-cyanines. All cyanines non-specifically accumulate in the mitochondria of both MN9D and HepG2 cells at high concentrations, inhibit the mitochondrial complex-I with the inhibition potencies similar to the potent complex I inhibitor, rotenone. They increase the reactive oxygen species (ROS) production specifically in dopaminergic cells causing apoptotic cell death. These and other findings suggest that the complex-I inhibition, the expression of low levels of antioxidant enzymes, and presence of high levels of oxidatively labile radical propagator, dopamine, could be responsible for the specific increase in ROS production in dopaminergic cells. Thus, the predisposition of dopaminergic cells to produce high levels of ROS in response to mitochondrial toxins together with their inherent greater demand for energy may contribute to their specific vulnerability towards these toxins. The novel finding that cyanines are an unusual class of potent mitochondrial toxins with specific dopaminergic toxicity suggest that their presence in the environment could contribute to the etiology of PD similar to MPP+ and rotenone.