Pathways of Cadmium Influx in Mammalian Neurons

The Influx of the toxic cation Cd2+ was studied in fura 2-loaded rat cerebellar granule neurons. In cells depolarized with Ca2+-free, high-KCI solutions, the fluorescence emission ratio (R) increased in the presence of 100 ?M Cd2+. This increase was fully reversed by the Cd2+ chelator tetrakis(2-pyridylmethyl)ethylenediamine, indicating a cadmium influx into the cell. The rate of increase, dR/dt, was greatly reduced (67 ± 5%) by 1 ?M nimodipine and enhanced by 1 ?M Bay K 8644. Concurrent application of nimodipine and ?-agatoxin IVA (200 nM) blocked Cd2+ permeation almost completely (88 ± 5%), whereas ?-conotoxin MVIIC (2 ?M) reduced dR/dt by 24 ± 8%. These results indicate a primary role of voltage-dependent calcium channels in Cd2+ permeation. Stimulation with glutamate or NMDA and glycine also caused a rise of R in external Cd2+. Simultaneous application of nimodipine and ?-agatoxin IVA moderately reduced dR/dt (25 ± 3%). NMDA-driven Cd2+ entry was almost completely prevented by 1 mM Mg2+, 50 ?M memantine, and 10 ?M 5,7-dichlorokynurenic acid, suggesting a major contribution of NMDA-gated channels in glutamate-stimulated Cd2+ influx. Moreover, perfusion with ?-amino-3-hydroxy-5-methylisoxazole-4-propionate caused a slow increase of R. These results suggest that Cd2+ permeates the cell membrane mainly through the same pathways of Ca2+ influx.