Firing Properties of Chopper and Delay Neurons in the Lateral Superior Olive of the Rat

Neurons in the lateral superior olivary nucleus (LSO) respond to acoustic stimuli with the "chopper response", a regular repetitive firing pattern with a short and precise latency. In the past, this pattern has been attributed to dendritic integration of synaptic inputs. We investigated a possible contribution of intrinsic membrane properties using intracellular recording techniques in a tissue slice preparation. We found two electrophysiological classes of neurons in the LSO. Chopper neurons responded to depolarizing current pulses with a single onset spike at short, precise latency close to threshold and with repetitive, regular, but accommodating discharges at greater current intensities. An emphasis of response onset and subsequent rate accommodation resulted from the activation of a voltage- and time-dependent sustained outward rectification in a range depolarized from rest. Responses to hyperpolarizing pulses were characterized by an inward rectification, which caused a depolarizing voltage sag in a range negative to -65 mV. Peristimulus time histograms were multimodal, and discharge regularity was evident in narrow unimodal interspike interval time histograms and low coefficients of variation. The accommodation time course was usually fit best by two exponentials with time constants of tau1=3-8 ms and tau2=32-97 ms. Delay neurons responded with a regular repetitive firing to depolarization by current pulses. However, repetitive spike discharge occurred with a prolonged, variable delay of 25-180 ms. High current intensities evoked an additional onset spike with short, precise latency. Activation of a transient outward conductance in the depolarized voltage range caused an early repolarization, which terminated as a depolarizing ramp, reaching spike threshold after the delay. Flat peristimulus time histograms characterized the repetitive discharge in spite of narrow unimodal interspike interval time histograms and low coefficients of variation. Intracellular neurobiotin injections revealed morphological differences between these classes. Chopper neurons were large and fusiform, with a bipolar dendritic distribution oriented perpendicular to the curvature of the LSO. Delay neurons were small and spherical, with highly branched tortuous dendritic arbours of bipolar origin and variable orientation. Chopper and delay neurons are probably LSO principal cells and lateral olivocochlear efferent neurons, respectively. Our findings suggest that the pattern of firing activity of LSO neurons to sound, in vivo, is determined to a large extent by intrinsic membrane properties. Somato-dendritic integration of synaptic inputs are fundamental to the encoding of interaural sound differences, but membrane non-linearities play an important role in determining postsynaptic response patterns.