The effects of intracochlear cyanide and tetrodotoxin on the properties of single cochlear nerve fibres in the cat.

1. Tuning properties and spontaneous discharge rate of single cochlear fibres in the anaesthetized cat were determined under conditions where millimolar concentrations of KCN were instilled into the scala tympani. 2. Short-term effects on the tuning properties were obtained, in which the threshold of the low threshold sharply tuned tip segment of the frequency-threshold ('tuning') curve (f.t.c.) was elevated by up to 40 db, without changes in the threshold of the low frequency 'tail' segment of the f.t.c., or necessarily changes in the spontaneous and maximally evoked activity. These changes were accompanied by a shift of the characteristic frequency tip segment towards lower frequencies. All these effects could be reversed. 3. The long-term effects of repeated KCN instillations produced irreversible changes similar to the short-term effects. 4. These changes correlated well with depression of the amplitude of the gross cochlear action potential but not with the cochlear microphonic potential, both recorded at the round window. 5. Instillations of tetrodotoxin (TTX) rapidly reduced and blocked the cochlear fibre discharges without effects on their tuning, in contrast to the effects of KCN.