In vivo simultaneous intra- and extracellular potassium recordings using a micro-optrode

This technique proposes a new approach to correlate intra- and extracellular variations of the ionic concentrations in vivo by means of tapered optical waveguides coupled to standard electrophysiological electrodes to monitor in vivo simultaneously the intracellular and extracellular K+ concentration as well as the neighboring field potential. The optical fibers were tapered to a final diameter of approximately 10 mu m and were used to guide the excitation light deep into the tissue and to collect the fluorescence emanating from the intracellular milieu. This fiber was coupled to a double barrel ion-sensitive electrode forming a micro-optrode with a final diameter around 15 mu m. The method was successfully used to record the intracellular K+ evolution with the fluorescent indicator PBFI during three states: normal sleep-like patterns, paroxysmal seizures, and coma. While we could not disclose any phasic fluctuations of the intracellular K+ during normal sleep patterns, they were clearly present during seizures and coma. In the majority of cases (58%), paroxysmal discharges were associated with positive variations of the intracellular fluorescence of 62 +/- 5% corresponding to extracellular K+ increases of 2.04 +/- 0.4 mM. In the remaining cases (42%) intracellular K+ dropped by 44.4 +/- 12% for an extracellular K+ increase of 2.62 +/- 0.47 mM. We suggest that this differential behavior might reflect different cellular populations (glia vs. neurons, respectively). Comatose states were accompanied by an extracellular drop of K+ of 1.31 +/- 0.13 mM, which was reflected, in all cases, by an intracellular K+ increase of 39 +/- 4%. (C) 2010 Elsevier B.V. All rights reserved.