Electrochemical carbon fiber-based technique for simultaneous recordings of brain tissue PO₂, pH, and extracellular...

A method for simultaneous electrochemical detection of brain tissue PO₂ (P_tO₂) and pH changes together with neuronal activity using a modified form of fast cyclic voltammetry with carbon fiber electrodes is described. This technique has been developed for in vivo applications and recordings from discrete brain nuclei in experimental animals. The small size of the carbon fiber electrode (⌀7 microm, length <100 microm) ensures minimal disruption of the brain tissue and allows recordings from small brain areas. Sample rate (up to 4 Hz) is sufficient to resolve rapid changes in P_tO₂ and pH that follow changes in neuronal activity and metabolism. Rapid switching between current and voltage recordings allows combined electrochemical detection and monitoring of extracellular action potentials. For simultaneous electrochemical detection of P_tO₂ and pH, two consecutive trapezoidal voltage ramps are applied with double differential-subtraction of the background current. This enables changes in current caused by protons and oxygen to be detected separately with minimal interference between the two. The profile of P_tO₂ changes evoked by increases in local neuronal activity recorded using the described technique was very similar to that of blood-oxygen-level-dependent responses recorded using fMRI. This voltammetric technique can be combined with fMRI and brain vessel imaging to study the metabolic mechanisms underlying neurovascular coupling response with much greater spatial and temporal resolution than is currently possible.