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 s timuli with the "chopper response", a regular repetitive firing pattern wit h a short and precise latency. In the past, this pattern has been attribute d to dendritic integration of synaptic inputs. We investigated a possible c ontribution 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 c urrent pulses with a single onset spike at short, precise latency close to threshold and with repetitive, regular but accommodating discharges at grea ter 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. Response s to hyperpolarizing pulses were characterized by an inward rectification, which caused a depolarizing voltage sag in a range negative to -65 mV. Peri stimulus time histograms were multimodal, and discharge regularity was evid ent in narrow unimodal interspike interval time histograms and low coeffici ents of variation. The accommodation time course was usually fit best by tw o exponentials with time constants of tau(1)=3-8 ms and tau(2)=32-97 ms. De lay neurons responded with a regular repetitive firing to depolarization by current pulses. However, repetitive spike discharge occurred with a prolon ged, variable delay of 25-180 ms. High current intensities evoked an additi onal onset spike with short, precise latency. Activation of a transient out ward conductance in the depolarized voltage range caused an early repolariz ation, which terminated as a depolarizing ramp, reaching spike threshold af ter the delay. Flat peristimulus time histograms characterized the repetiti ve discharge in spite of narrow unimodal interspike interval time histogram s and low coefficients of variation. Intracellular neurobiotin injections r evealed morphological differences between these classes. Chopper neurons we re large and fusiform, with a bipolar dendritic distribution oriented perpe ndicular to the curvature of the LSO. Delay neurons were small and spherica l, with highly branched tortuous dendritic arbours of bipolar origin and va riable orientation. Chopper and delay neurons are probably LSO principal ce lls and lateral olivocochlear efferent neurons, respectively. Our findings suggest that the pattern of firing activity of LSO neurons to sound, in viv o, is determined to a large extent by intrinsic membrane properties. Somato -dendritic integration of synaptic inputs are fundamental to the encoding o f interaural sound differences, but membrane non-linearities play an import ant role in determining postsynaptic response patterns.