Serendipitous compound action potential oscillations reveal glycolytic astrocyte and oxidative axon interstitial K ⁺ buffering in central white matter

The principal processes that govern interstitial K⁺ ([K⁺]_o) buffering in mouse optic nerve (MON), a central white matter tract, either directly consume energy (Na⁺-K⁺-ATPase) or use transmembrane ion gradients created by energy-dependent pumps to enable the K⁺ fluxes that maintain a stable [K⁺]_o, and thus ready availability of utilisable energy substrate is vital in supporting MON function. We switched the artificial cerebrospinal fluid (aCSF) bathing isolated ex vivo MON from a glucose and physiological [K⁺] (3 mM) formulation to one in which glucose was replaced by lactate and [K⁺] was increased to supra-physiological concentrations ('stress aCSF'), to test the ability of an oxidative fuel to support astrocyte function when faced with the buffering-related increased energy demand that accompanies elevating [K⁺]_o. We recorded simultaneously the compound action potential (CAP) and [K⁺]_o with suction electrodes and ion-sensitive microelectrodes, respectively. Increases in aCSF [K⁺] were not matched by equivalent increases in [K⁺]_o, evidence of powerful buffering. The stress aCSF caused unexpected reciprocal CAP and [K⁺]_o oscillations and exhaustion of astrocyte energy reserves coupled with elevation of [K⁺]_o sufficient to activate axonal Na⁺ channels, the key factors required for their initiation. The oscillation profile was of a rise in [K⁺]_o towards aCSF [K⁺], followed by a restoration of [K⁺]_o towards baseline, driven by intermittent activation of the axonal Na⁺-K⁺-ATPase, a cyclical process that continued for several hours. These oscillations exposed the contrasting utility of lactate, supporting axonal CAPs and axonal dominance of buffering during the oscillations, but incapable of fuelling astrocyte function.