Acute study on the neuronal excitability of the cochlear nuclei of the guinea pig following electrical stimulation.

To help deaf patients who cannot benefit from the cochlear implant due to interruption of the auditory nerve, a central auditory prosthesis has been developed to directly stimulate the cochlear nucleus in the brainstem. The electrode array lies on the surface of the cochlear nucleus and is designed to stimulate at 250 pulses/sec. To examine the safety of this prosthesis, guinea pig cochlear nuclei were stimulated acutely with bipolar surface electrodes using charge-balanced biphasic current pulses at rates of 250, 500 or 1,000 pulses/s and charge intensities of 1.8, 2.8, 3.5 or 7.1 microC/phase cm(-2). The electrically evoked auditory brainstem response (EABR) was used to monitor neuronal excitability of the cochlear nuclei following this acute electrical stimulation. Respiration rate and body temperature were also monitored during the experiment. The amplitudes and latencies of the EABR waves were measured and compared among the before, during and after stimulation periods. The results showed that respiration rate and body temperature remained within normal limits for the duration of the acute stimulation. During and after electrical stimulation, no change was found in the EABR waveform, dynamic ranges and threshold with up to 6 h direct continuous stimulation of the cochlear nucleus. There was no significant change in the amplitudes and latencies of the EABR waves after stimulation. However, a slight temporary reduction in the amplitude of the EABR waves was observed at 30-60 min during the course of acute stimulation using the highest charge density (7.1 microC/phase cm(-2)). This reduction showed a stronger correlation with the stimulus current, charge/phase and charge density than threshold. The present findings suggest that acute bipolar electrical stimulation with surface electrodes at rates up to 1,000 pulses/s and charge density up to 7.1 microC/phase cm(-2) is safe for neuronal excitability of the cochlear nucleus in guinea pig.