Lateral and Medial Olivocochlear Neurons Have Distinct Electrophysiological Properties in the Rat Brain Slice. KIYOHIRO FUJINO, KONOMI KOYANO and HARUNORI OHMORI. Department of Physiology and Department of Otolaryngology, Kyoto University Graduate School of Medicine, Kyoto 606-01, JAPAN.
APStracts 4:0044N, 1997.
ABSTRACT
Electrical properties of cochlear efferent (olivocochlear; OC) neurons were investigated using the whole-cell patch recording technique in slice preparations of the neonatal rat (postnatal days of 5 - 11). Lateral and medial OC (LOC and MOC) neurons were retrogradely labeled with a fluorescent tracer injected into the cochlea. Stained neurons were identified under a fluorescence microscope, and they were subjected to whole-cell recording. LOC and MOC neurons showed different electrophysiological properties. Both neurons showed spike trains of tonic pattern in response to injection of depolarizing current pulses at the resting membrane potential (-60 to -70 mV). However, when the membrane was slightly hyperpolarized (-72 to -76 mV), LOC neurons showed spike trains with a long first interspike interval (ISI), while MOC neurons showed spike trains with a long latency to the first spike. Extracellular application of 4-aminopyridine (4-AP; 0.5 - 2 mM) shortened these ISI and latency. In voltage clamp experiments, two transient outward currents with different (fast and slow) decay kinetics were observed in LOC neurons. The fast outward current (IA-LOC) was inactivated by the preceding depolarization, and decayed with a time constant (t) of 86 ms (at 0 mV). The preceding potential which reduced the current size to the half maximum (V1/2) was -72 mV. The slow current (IKD) decayed with t of 853 ms (at 0 mV). The IA- LOC was sensitive to 4-AP (2 mM), and was less sensitive to tetraethylammonium chloride (TEA; 20 mM). The IKD was partially blocked by TEA (20 mM), but was insensitive to 4-AP (2 mM). The recovery from inactivation of the IA-LOC was time dependent with a time constant (trec) of 32 ms at -90 mV. MOC neurons also showed a transient outward current which was consisted of a single transient component (IA-MOC) with a steady outward current. The IA-MOC was inactivated by the preceding depolarization. The decay time constant of the IA-MOC was 33 ms (at 0 mV), and the V1/2 was -75 mV. The IA-MOC was sensitive to 4-AP (0.5 - 1 mM). The time-dependent recovery from inactivation of the IA- MOC was faster than the IA-LOC, and trec was 15 ms at -90 mV. The different kinetics of transient outward currents between LOC and MOC neurons seems to be responsible for the difference in firing properties of these two neurons. 

Received 19 September 1996; accepted in final form 8 January 1997.
APS Manuscript Number J753-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997