The Journal of Neurophysiology Vol. 77 No. 5 May 1997, pp. 2788-2804
Copyright ©1997 The American Physiological Society

Lateral and Medial Olivocochlear Neurons Have Distinct Electrophysiological Properties in the Rat Brain Slice

Kiyohiro Fujino^{1, 2}, Konomi Koyano¹, and Harunori Ohmori¹

¹ Department of Physiology and ² Department of Otolaryngology, Faculty of Medicine, Kyoto University, Kyoto 606-01, Japan

Fujino, Kiyohiro, Konomi Koyano, and Harunori Ohmori. Lateral and medial olivocochlear neurons have distinct electrophysiological properties in the rat brain slice. J. Neurophysiol. 77:2788-2804, 1997. Electrical properties of cochlear efferent (olivocochlear) neurons were investigated with the use of the whole cell patch recording technique in slice preparations of the neonatal rat (postnatal days 5-11). Lateral and medial olivocochlear (LOC and MOC, respectively) 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 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), whereas 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 ISIs and latencies. In voltage-clamp experiments, two transient outward currents with different (fast and slow) decay kinetics were observed in LOC neurons. The fast outward current (I_A-LOC) was inactivated by the preceding depolarization, and decayed with a time constant () of 86 ms (at 0 mV). The preceding potential, which reduced the current size to the half-maximum (V_1/2), was 72 mV. The slow current (I_KD) decayed with a  of 853 ms (at 0 mV).I_A-LOC was sensitive to 4-AP (2 mM), and was less sensitive to tetraethylammonium chloride (TEA; 20 mM). I_KD was partially blocked by TEA (20 mM), but was insensitive to 4-AP (2 mM). The recovery from inactivation of I_A-LOC was time dependent with a time constant (_rec) of 32 ms at 90 mV. MOC neurons also showed a transient outward current that consisted of a single transient component (I_A-MOC) with a steady outward current. I_A-MOC was inactivated by the preceding depolarization. Decay  of I_A-MOC was 33 ms (at 0 mV), and V_1/2 was 75 mV. I_A-MOC was sensitive to 4-AP (0.5-1 mM). The time-dependent recovery from inactivation of I_A-MOC was faster than that of I_A-LOC, and _rec 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.

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