Lateral and medial olivocochlear neurons have distinct electrophysiological properties in the rat brain slice

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 (tau) 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 (I(KD)) decayed with a tau 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 (tau(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 tau of I(A-MOC) was 33 ms (at 0 mV), and V1/2 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 tau(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.