Measurement of intracellular free zinc in living cortical neurons: routes of entry.

We used the ratioable fluorescent dye mag-fura-5 to measure intracellular free Zn2+ ([Zn2+]i) in cultured neocortical neurons exposed to neurotoxic concentrations of Zn2+ in concert with depolarization or glutamate receptor activation and identified four routes of Zn2+ entry. Neurons exposed to extracellular Zn2+ plus high K+ responded with a peak cell body signal corresponding to a [Zn2+]i of 35-45 nM. This increase in [Zn2+]i was attenuated by concurrent addition of Gd3+, verapamil, omega-conotoxin GVIA, or nimodipine, consistent with Zn2+ entry through voltage-gated Ca2+channels. Furthermore, under conditions favoring reverse operation of the Na+-Ca2+ exchanger, Zn2+ application induced a slow increase in [Zn2+]i and outward whole-cell current sensitive to benzamil-amiloride. Thus, a second route of Zn2+ entry into neurons may be via transporter-mediated exchange with intracellular Na+. Both NMDA and kainate also induced rapid increases in neuronal [Zn2+]i. The NMDA-induced increase was only partly sensitive to Gd3+ or to removal of extracellular Na+, consistent with a third route of entry directly through NMDA receptor-gated channels. The kainate-induced increase was highly sensitive to Gd3+ or Na+ removal in most neurons but insensitive in a minority subpopulation ("cobalt-positive cells"), suggesting that a fourth route of neuronal Zn2+ entry is through the Ca2+-permeable channels gated by certain subtypes of AMPA or kainate receptors.