Making, Testing, and Using Potassium Ion Selective Microelectrodes in Tissue Slices of Adult Brain

Potassium ions contribute to the resting membrane potential of cells and extracellular K⁺ concentration is a crucial regulator of cellular excitability. We describe how to make, calibrate and use monopolar K⁺-selective microelectrodes. Using such electrodes enables the measurement of electrically evoked K⁺ concentration dynamics in adult hippocampal slices.

Potassium ions significantly contribute to the resting membrane potential of cells and, therefore, extracellular K⁺ concentration is a crucial regulator of cell excitability. Altered concentrations of extracellular K⁺ affect the resting membrane potential and cellular excitability by shifting the equilibria between closed, open and inactivated states for voltage-dependent ion channels that underlie action potential initiation and conduction. Hence, it is valuable to directly measure extracellular K⁺ dynamics in health and diseased states. Here, we describe how to make, calibrate and use monopolar K⁺-selective microelectrodes. We deployed them in adult hippocampal brain slices to measure electrically evoked K⁺ concentration dynamics. The judicious use of such electrodes is an important part of the tool-kit needed to evaluate cellular and biophysical mechanisms that control extracellular K⁺ concentrations in the nervous system.

Potassium ion concentrations are tightly regulated in the brain, and their fluctuations exert a powerful influence on the resting membrane potential of all cells. In light of these critical contributions, an important goal of biology is to determine the cellular and biophysical mechanisms that are used to tightly regulate the concentration of K⁺ in the extracellular space in different organs of the body^1,2. An important requirement in these studies is the ability to measure K⁺ concentrations accurately. Although many components which contribute to potassium homeostasis in the brain in hea....

All animal experiments were conducted in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals and were approved by the Chancellor's Animal Research Committee at the University of California, Los Angeles. All mice were housed with food and water available ad libitum in a 12 h light-dark environment. All animals were healthy with no obvious behavioral changes, were not involved in previous studies, and were sacrificed during the light cycle. Data for experiments wer.......

For selective measurement of extracellular K⁺, we prepared ion-selective microelectrodes coated with a hydrophobic layer through silanization of clean borosilicate glass pipettes (Figure 1A). This coating enables the K⁺ ionophore containing valinomycin to rest at the tip of the electrode and permit only K⁺ flux through a narrow opening at the electrode tip (Figure 1B). After priming the electrodes.......

The method that we describe here has allowed us to assess K⁺ dynamics in response to electrical stimulation of Schaffer collaterals in acute hippocampal slices from adult mice. Our method of preparing K⁺ ion selective microelectrodes is similar to earlier described procedures^12,13,14,15. However, this method has advantages over alternative electrode configurations in that .......

The Khakh lab was supported by NIH MH104069. The Mody lab was supported by NIH NS030549. J.C.O. thanks the NIH T32 Neural Microcircuits Training Grant(NS058280).