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INNOVATIVE METHODOLOGY

Fabrication and Use of High-Speed, Concentric H⁺- and Ca²⁺-Selective Microelectrodes Suitable for In Vitro Extracellular Recording

Department of Neurosurgery, and Department of Physiology and Neuroscience, New York University School of Medicine, New York, New York

Submitted 9 March 2006; accepted in final form 26 April 2006

Ion-selective microelectrodes (ISMs) have been used extensively in neurophysiological studies. ISMs selective for H⁺ and Ca²⁺ 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 [Ca²⁺] transients in cat cortex, but has not been subsequently used. Here, we describe a method for the rapid fabrication of concentric pH- and Ca²⁺-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 Ca²⁺-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 [Ca²⁺] transients are shown to arise two- to threefold faster, and attain amplitudes two- to fourfold greater, when recorded by concentric versus conventional ISMs. The advantage of concentric ISMs for studies of ion transport and ion diffusion is discussed.

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