Innovative Methodology Fabrication and Use of High-Speed, Concentric H - and Ca -Selective Microelectrodes Suitable for In Vitro Extracellular Recording

Fedirko, Nataliya, Nataliya Svichar, and Mitchell Chesler. Fabrication and use of high-speed, concentric H and Ca -selective microelectrodes suitable for in vitro extracellular recording. J Neurophysiol 96: 919–924, 2006; doi:10.1152/jn.00258.2006. Ion-selective microelectrodes (ISMs) have been used extensively in neurophysiological studies. ISMs selective for H and Ca are notable for their sensitivity and selectivity, but suffer from a slow response time, and susceptibility to noise because of the high electrical resistance of the respective ion exchange cocktails. These drawbacks can be overcome by using a “coaxial” or “concentric” inner micropipette to shunt the bulk of the ion exchanger resistance. This approach was used decades ago to record extracellular [Ca ] transients in cat cortex, but has not been subsequently used. Here, we describe a method for the rapid fabrication of concentric pHand Ca -selective microelectrodes useful for extracellular studies in brain slices or other work in vitro. Construction was simplified compared with previous implementations, by using commercially available, thin-walled borosilicate glass, drawing an outer barrel with a rapid taper (similar to a patch pipette), and by use of a quick and reliable silanization procedure. Using a piezoelectric stepper to effect a rapid solution change, the response time constants of the concentric pH and Ca -electrodes were 14.9 1.3 and 5.3 0.90 ms, respectively. Use of these concentric ISMs is demonstrated in rat hippocampal slices. Activity-dependent, extracellular pH, and [Ca ] transients are shown to arise twoto threefold faster, and attain amplitudes twoto fourfold greater, when recorded by concentric versus conventional ISMs. The advantage of concentric ISMs for studies of ion transport and ion diffusion is discussed.