K⁺ currents produce P₁ in the RW CAP: Evidence from DC current bias, K⁺ channel blockade and recordings from cochlea and brainstem

Tone-burst-evoked compound action potentials (CAP) from the guinea pig round window (RW) are altered by DC current injection through the RW. The CAP waveform consists of a series of interleaved negative and positive peaks (N ₁, P₁, N₂, P₂ etc.) of decreasing amplitude. During positive DC current injection (around +50 μA) the positive peaks are depressed substantially and there is an overall negative baseline shift of the waveform following the N₁. Negative current injection (around -50 μA) increased the positive peaks, in particular P₁, and produced an overall positive baseline shift following the N₁ peak. Results support our hypothesis that the first and dominant N₁ peak in the RW CAP is due to depolarising Na⁺ currents into the primary afferent dendrites and axons within the cochlea, and that the P ₁ potential is largely due to the exit of the hyperpolarising K ⁺ currents in the same cells. We have reached this conclusion on the basis of the sign and latency of the N₁ and P₁ components at the RW, beneath the myelin layers around the spiral ganglion cells, at the internal auditory meatus (IAM) within the brain case, and on the basis of the differential susceptibility of the various peaks to perfusion of lidocaine in the cochlear nucleus, sectioning of the cochlear nerve at the IAM, application of the K⁺ channel blockers 4-amino-pyridine and tetraethylammonium within the cochlea, and DC current biasing at the RW.