Zn2+ currents are mediated by calcium-permeable AMPA/kainate channels in cultured murine hippocampal neurones

Permeation of the endogenous cation Zn2+ through calcium-permeable AMPA/kainate receptor-gated (Ca-A/K) channels might subserve pathological and/or physiological signalling roles. Voltage-clamp recording was used to directly assess Zn2+ flux through these channels on cultured murine hippocampal neurones. Ca-A/K channels were present in large numbers only on a minority of neurones (Ca-A/K+ neurones), many of which were GABAergic. The presence of these channels was assessed in whole-cell or outside-out patch recording as the degree of inward rectification of kainate-activated currents, quantified via a rectification index (RI = G+40/G-60), which ranged from <0.4 (strongly inwardly rectifying) to >2 (outwardly rectifying). The specificity of a low RI as an indication of robust Ca-A/K channel expression was verified by two other techniques, kainate-stimulated cobalt-uptake labelling, and fluorescence imaging of kainate-induced increases in intracellular Ca2+. In addition, the degree of inward rectification of kainate-activated currents correlated strongly with the positive shift of the reversal potential (V(rev)) upon switching to a sodium-free, 10 mM Ca2+ buffer. With Zn2+ (3 mM) as the only permeant extracellular cation, kainate-induced inward currents were only observed in neurones that had previously been identified as Ca-A/K+. A comparison between the V(rev) observed with 3 mM Zn2+ and that observed with Ca2+ as the permeant cation revealed a P(Ca)/P(Zn) of approximately 1.8. Inward currents recorded in 3 mM Ca2+ were unaffected by the addition of 0.3 mM Zn2+, while microfluorimetrically detected increases in the intracellular concentration of Zn2+ in Ca-A/K+ neurones upon kainate exposure in the presence of 0.3 mM Zn2+ were only mildly attenuated by the addition of 1.8 mM Ca2+. These results provide direct evidence that Zn2+ can carry currents through Ca-A/K channels, and that there is little interference between Ca2+ and Zn2+ in permeating these channels.