Interstitial K⁺ ([K⁺]_o) regulation is required to counteract activity-induced increases in [K⁺]_o that result from release of K⁺ from neural compartments. Such buffering has been assumed as the sole responsibility of astrocytes in central tissue. Astrocytes display a range of transporters, pumps, and ion channels that facilitate this buffering. Evolution has taken advantage of such transient [K⁺]_o elevations which act as a molecule signalling astrocytes a neuronal need for energy substrate, provided by lactate release from astrocytes.

The ex vivo optic nerves from 74 adult female CD-1 mice were sustained in a superfusion chamber bathed with artificial cerebrospinal fluid (aCSF) containing 3 mM [K⁺] and 95% O₂ to support conduction over a period of many hours. All experiments were approved by the University of Nottingham Animal Care and Ethics Committee and were carried out in accordance with the Animals (Scientific Procedures) Act 1986 under the appropriate authority of establishment (NON ASPA 2321). The stimulus evoked compound action potential (CAP) and [K⁺]_o were simultaneously recorded with suction electrodes, and ion sensitive micro-electrodes, respectively.

When the ex vivo mouse optic nerve was exposed to a stress aCSF that contained elevated [K⁺] (9 mM) with glucose substituted by lactate, unique spontaneous recurrent reciprocal [K⁺]_o and CAP oscillations occurred. The oscillations started within 9.1 ± 5.6 min (n = 11) of exposure to stress aCSF, at a frequency of 0.34 ± 0.06 min⁻¹, and lasted for several hours. Powerful buffering ensured that [K⁺]_o did not match aCSF [K⁺], e.g. in 9 mM aCSF [K⁺] the [K⁺]_o was 4.44 ± 1.3 mM (n = 4). The mechanism of the oscillations was deduced via application of metabolic inhibitors, and transporter and ion channel blockers. The onset of the oscillations coincided with cessation of astrocytic metabolism, where the CAP fell coincident with an elevation in interstitial [K⁺]_o at a rate of 2.72 ± 1.31mM min⁻¹ (n = 5) towards aCSF [K⁺]. This elevated [K⁺]_o activated voltage gated Na⁺ channels, leading to axonal Na⁺ influx and activation of the axonal Na-K ATPase. The resulting Na⁺ efflux from the axon to the interstitial space was matched with an influx of K⁺ from the interstitial space into the axon, reflected in a fall in [K⁺]_o at a rate of 1.40 ± 0.71mM min⁻¹ (n = 5). This was a transient effect, which stopped when [K⁺]_o fell below the threshold required for Na⁺ channel activation.

This cyclical oscillatory behaviour continued indefinitely until either glucose (n = 6) or 3 mM K⁺ (n = 5) were restored to the aCSF. These oscillations exposed the inability of astrocytes to utilise interstitial lactate as a energy substrate, and also highlighted the previously unrecognised extremely powerful axonal [K⁺]_o buffering.