Two-photon analysis of lead accumulation in rat cerebellar granule neurons

Lead (Pb2+) is a common pollutant and potent central neurotoxin. We have studied its pathways of permeation by two-photon fluorescence microscopy in rat cerebellar granule neurons loaded with the fluorescent dye indo-1. Pb2+ binds indo-1 with high affinity acting as a quencher. Its permeation through the neuronal membrane was indicated by a decrease of the fluorescence emission, which occurred even in resting condition. In the presence of 20 microM Pb2+, uptake reached a plateau level (approximately 45% of initial fluorescence) in 4 min and was partially antagonized by 25 microM lanthanum. Subsequent addition of a membrane permeant ionophore caused a further (>70%) quenching of the dye, suggesting that previous saturation was due to inactivation of the transport system. Intracellular Pb2+ concentrations were evaluated from the fluorescence intensity and this estimate indicated that the concentration of free Pb2+ sufficient to inactivate the transport system is close to 50 pM.