High resolution recording of local field currents simultaneously with sound-evoked calcium signals by a photometric patch electrode in the auditory cortex field L of the chick

Abstract

Background: Functioning of the brain is based on both electrical and metabolic activity of neural ensembles. Accordingly, it would be useful to measure intracellular metabolic signaling simultaneously with electrical activity in the brain in vivo.

New method: We innovated a PhotoMetric-patch-Electrode (PME) recording system that has a high temporal resolution incorporating a photomultiplier tube as a light detector. The PME is fabricated from a quartz glass capillary to transmit light as a light guide, and it can detect electrical signals as a patch electrode simultaneously with a fluorescence signal.

Results: We measured the sound-evoked Local Field Current (LFC) and fluorescence Ca²⁺ signal from neurons labeled with Ca²⁺-sensitive dye Oregon Green BAPTA1 in field L, the avian auditory cortex. Sound stimulation evoked multi-unit spike bursts and Ca²⁺ signals, and enhanced the fluctuation of LFC. After a brief sound stimulation, the cross-correlation between LFC and Ca²⁺ signal was prolonged. D-AP5 (antagonist for NMDA receptors) suppressed the sound-evoked Ca²⁺ signal when applied locally by pressure from the tip of PME.

Comparison with existing methods: In contrast to existing multiphoton imaging or optical fiber recording methods, the PME is a patch electrode pulled simply from a quartz glass capillary and can measure fluorescence signals at the tip simultaneously with electrical signal at any depth of the brain structure.

Conclusion: The PME is devised to record electrical and optical signals simultaneously with high temporal resolution. Moreover, it can inject chemical agents dissolved in the tip-filling medium locally by pressure, allowing manipulation of neural activity pharmacologically.

Keywords: Auditory cortex; Calcium signaling; Field L; Local field current; NMDA receptors; Photometric patch electrode.