Posthearing developmental refinement of temporal processing in principal neurons of the medial superior olive

In mammals, principal neurons of the medial superior olive (MSO) exhibit biophysical specializations that enable them to detect sound localization cues with microsecond precision. In the present study, we used whole-cell patch recordings to examine the development of the intrinsic electrical properties of these neurons in brainstem slices from postnatal day 14 (P14) to P38 gerbils. In the week after hearing onset (P14-P21), we observed dramatic reductions in somatic EPSP duration, input resistance, and membrane time constant. Surprisingly, somatically recorded action potentials also dramatically declined in amplitude over a similar period (38 +/- 3 to 17 +/- 2 mV; tau = 5.2 d). Simultaneous somatic and dendritic patch recordings revealed that these action potentials were initiated in the axon, which primarily emerged from the soma. In older gerbils, the rapid speed of membrane voltage changes and the attenuation of action potential amplitudes were mediated extensively by low voltage-activated potassium channels containing the Kv1.1 subunit. In addition, whole-cell voltage-clamp recordings revealed that these potassium channels increase nearly fourfold from P14 to P23 and are thus a major component of developmental changes in excitability. Finally, the electrophysiological features of principal neurons of the medial nucleus of the trapezoid body did not change after P14, indicating that posthearing regulation of intrinsic membrane properties is not a general feature of all time-coding auditory neurons. We suggest that the striking electrical segregation of the axon from the soma and dendrites of MSO principal neurons minimizes spike-induced distortion of synaptic potentials and thus preserves the accuracy of binaural comparisons.