LATERAL AND MEDIAL OLIVOCOCHLEAR NEURONS HAVE DISTINCT ELECTROPHYSIOLOGICAL PROPERTIES IN THE RAT-BRAIN SLICE

Electrical properties of cochlear efferent (olivocochlear) neurons wer e investigated with the use of the whole cell patch recording techniqu e in slice preparations of the neonatal rat (postnatal days 5-11). Lat eral and medial olivocochlear (LOC and MOC, respectively) neurons were retrogradely labeled with a fluorescent tracer injected into the coch lea. Stained neurons were identified under a fluorescence microscope, and they were subjected to whole cell recording. LOC and MOC neurons s howed different electrophysiological properties. Both showed spike tra ins of tonic pattern in response to injection of depolarizing current pulses at the resting membrane potential (-60 to -70 mV). However, whe n the membrane was slightly hyperpolarized (-72 to -76 mV), LOC neuron s showed spike trains with a long first interspike interval (ISI). whe reas MOC neurons showed spike trains with a long latency to the first spike. Extracellular application of 4-aminopyridine (4-AP; 0.5-2 mM) s hortened these ISIs and latencies. In voltage-clamp experiments. two t ransient outward currents with different (fast and slow) decay kinetic s were observed in LOC neurons. The fast outward current (IA-LOC) was inactivated by the preceding depolarization, and decayed with a time c onstant (tau) of 86 ms (at 0 mV). The preceding potential, which reduc ed the current size to the half-maximum (Vi(1/2)), was -72 mV. The slo w current (I-KD) decayed with a tau of 853 ms (at 0 mV). IA-LOC was se nsitive to 4-AP (2 mM), and was less sensitive to tetraethylammonium c hloride (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 IA-LO C was time dependent with a time constant (tau(rec)) of 32 ms at -90 m V. MOC neurons also showed a transient outward current that consisted of a single transient component (IA-MOC ) With a steady outward curren t. IA-MOC was inactivated by the preceding depolarization. Decay tau o f IA-MOC was 33 ms (at 0 mV), and V-1/2 was -75 mV. IA-MOC was sensiti ve to 4-AP (0.5-1 mM). The time-dependent recovery from inactivation o f IA-MOC was faster than that of IA-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 p roperties of these two neurons.