Development of miniature implantable electrode and telemeter for long-term recording of sympathetic efferent nerve activity

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It is important for better understanding of sympathetic control of the cardiovascular system to measure sympathetic outflows to various vascular beds and analyze the long-term changes in sympathetic outflow following a given intervention. The aim of this study was to develop a new implantable microelectrode and a miniature telemetry device for stable and long-term recording of sympathetic neural discharges in freely moving small animals. We have made two types of the novel bipolar cuff electrodes with electro-conductive rubber or water-absorbent polymer, which can be applied to the renal nerve in the rat. When the microelectrodes were embedded in saline, electrical impedance of the electrodes was 2.8-9.4 kΩ in a frequency range of 0.05-3 kHz, which corresponded to the band of sympathetic discharge. Either electrode showed stable electrical impedance over several weeks, suggesting that the impedance was low enough to record sympathetic nerve activity with low background noise as long as … More the electrode had good contact with a nerve bundle. Because the present electrodes were of small size (volume, 2mm^3) and handled easily, it took a shorter time to implant each electrode on the nerve without any nerve injury due to drying or mechanical damage and the electrodes worked well as long as being used in the acute type of experiments. Therefore the novel electrodes may be useful for recording of sympathetic nerve discharge in conscious animals. Furthermore, we have attempted to develop a miniature telemetry transmitter, which can be implanted in small animals. Special care was taken to reduce the background noise as much as possible and transmit sympathetic nerve signal without any distortion. The noise level of the novel telemetry system was several microvolts and its signal frequency band was 0.05-3 kHz, indicating that the present novel telemetry system is able to record sympathetic nerve discharge in conscious animals. However, the transmitter was too large to be implanted in smaller animals than the cat or rabbit, because its size was 4 x 5 x 0.5cm. It will be necessary to reduce the size of the transmitter by using one-tip integrated circuit. Less