Fluorescence microscopy of stimulated Zn(II) release from organotypic cultures of mammalian hippocampus using a carbonic anhydrase-based biosensor system

We demonstrate here that electrical stimulation of organotypic cultures of rat hippocampus results in the prompt release of significant amounts of Zn( II) by a fluorescence microscopic method. The fluorescence imaging of free Zn(II) is achieved using a highly selective biosensing indicator system con sisting of human apo-carbonic anhydrase II (apoCAII) and a fluorescent aryl sulfonamide inhibitor of the enzyme, ABD-N. The apoenzyme and ABD-N in the absence of Zn(II) exhibit weak, reddish fluorescence typical of the ABD-N alone; when Zn(II) is added it binds to the apoenzyme (k(D) = 4 pM), which strongly promotes binding of ABD-N to the holoenzyme (K-D = 0.9 mu M). Bind ing of ABD-N to the holoenzyme results in a 9-fold increase in apparent qua ntum yield, significant blue shifts in excitation and emission, an increase in average fluorescence lifetime, a 4-fold increase in the ratio of intens ities at 560 and 680 nm, and a large increase in anisotropy. Prior to stimu lation, cultures immersed in phosphate-buffered saline with glucose and apo CAII with ABD-N emitted negligible fluorescence, but within 20 s after elec trical stimulation a diffuse cloud of greenish fluorescence emerged and sub sequently covered most of the culture, indicating release of zinc into the extracellular medium. (C) 2000 Elsevier Science B.V. All rights reserved.