Long term exposure to norharman exacerbates 6-hydroxydopamine-induced parkinsonism: Possible involvement of L-type Ca2+ channels

Abstract β-Carbolines (BCs) are considered as endogenous neurotoxins that may contribute to the pathogenesis of Parkinson's disease (PD). However, several lines of evidences show that these compounds have neuroprotective effect. This study was designed to assess effect of long term exposure to norharman, a BC compound which in mammalian brain occurs at high levels in the substantia nigra, on the progress of parkinsonism induced by 6-hydroxydopamine (6-OHDA). Animals were daily treated by norharman at doses 100, 200 and 1000 μg/kg (i.p.) just before to four weeks after the intrastriatal injection of 6-OHDA. Statistical analysis of apomorphine-induced rotation tests demonstrates that treatment by norharman at doses 200 and 1000 μg/kg for four weeks exacerbates significantly behavioral symptoms of the parkinsonism. To explore mechanisms by which norharman affects nigral dopaminergic cells, we studied the role of L-type Ca 2+ channels. For this purpose, animals were daily treated with either L-type Ca 2+ channel blocker of nifedipine at doses 2 and 5 mg/kg (i.p.) or nifedipine together with norharman before to four weeks after the 6-OHDA injection. While treatment with nifedipine improved behavioral symptoms of the parkinsonism, treatment with both nifedipine and norharman had no affect on these symptoms. This data indicates that long term exposure to BCs promote nigral dopaminergic cell death possibly through L-type Ca 2+ channels.