Effects of (-)-nicotine and (-)-cotinine on 6-hydroxydopamine-induced oxidative stress and neurotoxicity: relevance for Parkinson's disease.

In view of the apparent controversial properties of (-)-nicotine (NIC) in relation to both oxidative stress and neuroprotection, we studied the effects of NIC on hydroxyl radical (*OH) formation, oxidative stress production by 6-hydroxydopamine (6-OHDA) autoxidation in the presence and absence of ascorbate, and 6-OHDA neurotoxicity. Both NIC and (-)-cotinine (COT) exhibited increased *OH production during 6-OHDA autoxidation. Although the same effect was observed in *OH generation by the Fenton reaction (H2O2 + Fe2+), this reaction was completely prevented with the previous incubation of Fe2+ with NIC or COT. Furthermore, both NIC and COT demonstrated a capacity to be able to reduce the TBARS formation provoked in rat brain mitochondrial preparations by 6-OHDA autoxidation. This effect is assumed as a consequence of the action of NIC and COT on lipid peroxidation propagation. We treated with NIC (1mg/kg, i.p.) two 6-OHDA-induced rat models of Parkinson's disease. However, only in one of these models did we obtain clear evidence of a neuroprotective effect of NIC on nigrostriatal terminals, as revealed by immunohistochemistry against tyrosine hydroxylase. Thus, the antioxidant properties of both NIC and COT in relation to the lipid peroxidation induced by 6-OHDA autoxidation, together with their reported capacity to prevent the Fenton reaction, probably by sequestration of Fe2+, may contribute to an understanding of its neuroprotective properties. In addition, the reported capacity of both NIC and COT to increase the production of *OH by 6-OHDA autoxidation might help explain the controversial observation found under different experimental conditions.