Low-voltage activated Kv1.1 subunits are crucial for the processing of sound source location in the lateral superior olive in mice

Abstract

Voltage-gated potassium (Kv) channels containing Kv1.1 subunits are strongly expressed in neurons that fire temporally precise action potentials (APs). In the auditory system, AP timing is used to localize sound sources by integrating interaural differences in time (ITD) and intensity (IID) using sound arriving at both cochleae. In mammals, the first nucleus to encode IIDs is the lateral superior olive (LSO), which integrates excitation from the ipsilateral ventral cochlear nucleus and contralateral inhibition mediated via the medial nucleus of the trapezoid body. Previously we reported that neurons in this pathway show reduced firing rates, longer latencies and increased jitter in Kv1.1 knockout (Kcna1−/−) mice. Here, we investigate whether these differences have direct impact on IID processing by LSO neurons. Single-unit recordings were made from LSO neurons of wild-type (Kcna1+/+) and from Kcna1−/− mice. IID functions were measured to evaluate genotype-specific changes in integrating excitatory and inhibitory inputs. In Kcna1+/+ mice, IID sensitivity ranged from +27 dB (excitatory ear more intense) to −20 dB (inhibitory ear more intense), thus covering the physiologically relevant range of IIDs. However, the distribution of IID functions in Kcna1−/− mice was skewed towards positive IIDs, favouring ipsilateral sound positions. Our computational model revealed that the reduced performance of IID encoding in the LSO of Kcna1−/− mice is mainly caused by a decrease in temporal fidelity along the inhibitory pathway. These results imply a fundamental role for Kv1.1 in temporal integration of excitation and inhibition during sound source localization.

Figure 2. Kv1 expression in the mouse LSO

A and B, immunolabelling revealed both cellular and axonal Kv1.1 labelling in Kcna1^+/+ tissue (A and B), while no Kv1.1 expression was found in Kcna1^−/− tissue (C and D); nuclei were visualized with DAPI. There was intense staining of Kv1.2 in both Kcna1^+/+ (E and F) and Kcna1^−/− tissue (G and H). Co-labelling of juxtaparanodal Kv1 with nodal Na_V1.6 showed the typical triplet staining of the nodes of Ranvier in wild-type tissue (I and J) but not in Kcna1^−/− tissue (K and L). In the latter, only Na_V1.6 staining is present besides the DAPI. Low magnification images (20×) are shown in A, C, E, G, I and K; squares indicate areas magnified in B, D, F, H, J and L, respectively. Scale bars: low magnification: 100 μm, high magnification: 10 μm.

Figure 1. IID pathway of the mouse brainstem

A, sketch of the mouse brainstem IID pathway, binaural acoustic stimulation, and extracellular recordings from units in the LSO. B, response maps of one representative wild-type LSO unit in response to ipsilateral (light grey) or contralateral (dark grey) acoustic stimulation. The respective outermost lines show the frequency/intensity ranges that correspond to significant increase (excitation, light grey) or decrease (inhibition, dark grey) in firing rates relative to the units’ spontaneous rates. Additional contour lines show significant further increase or decrease of firing rates. The unit's CF and threshold were obtained from this analysis. This Kcna1^+/+ unit shows a strong overlap of excitatory and inhibitory response areas with largely identical CFs and thresholds. In some units, the inhibitory response area was much broader tuned than the excitatory one (not shown).

Figure 3. IID encoding in Kcna1^+/+ and Kcna1^−/− mice

A, dot raster display of the response of a representative unit to binaural stimulation (40 ms; CF at the respective ear); ipsilateral: 20 dB above threshold; contralateral: levels between 0 and 90 dB SPL in steps of 5 dB (presented in pseudorandom order; ordinate). Note that the firing rate decreased with increasing contralateral stimulus intensities. B, normalized firing rate (same unit as in A) plotted for different IIDs, i.e. ipsilateral minus contralateral intensity: positive IIDs representing higher ipsilateral and negative IIDs higher contralateral stimulus levels. Firing rate at ipsilateral stimulation exclusively was set to 100%. Data points were fitted by a 4-parametric sigmoid function (continuous line). From such fits, IID values corresponding to a 50% rate reduction (IID₅₀) and the dynamic range of the IID functions (grey shaded area) were determined for further analyses. C and D, IID functions obtained from the LSO of Kcna1^+/+ (C) and Kcna1^−/− mice (D). While in the Kcna1^+/+ LSO, IID functions are equally distributed between negative and positive IIDs, in Kcna1^−/− mice, IID functions do not seem to achieve 50% rate reduction for negative IIDs. This is also shown by the uneven distribution of IID₅₀ values between both genotypes (E). The inset in E features the range of absolute ipsilateral intensities used for the recording of IID functions in wild-type (grey) and knockout (white) LSO. F, within recordings from individual neurons, IID₅₀ values shifted towards more positive IIDs with increasing ipsilateral intensity. This increase was significantly larger in wild-types (grey). G, the Kcna1^−/− IID dynamic ranges (white) were larger and accompanied by shallower slopes.

Figure 4. Computational modelling of IID tuning in Kcna1^+/+ and Kcna1^−/− LSO neurons

A, excitatory (red, g_exc) and inhibitory (blue, g_inh) synaptic conductance and membrane potential (V, black trace below respective conductance traces) of an LSO neuron for different spike rates of the contralateral, inhibitory input (expressed as the ratio of contralateral and ipsilateral spike rates indicated on the left side). Parameters were: ipsilateral spike jitter s.d.0.15 ms, contralateral spike jitter s.d. 0.4 ms, relative mean latency 1.4 ms. B, APs (dots) of a simulated LSO neuron during 10 repetitions of a 40 ms stimulus under the conditions described in A. C, the resulting IID tuning curve of the simulated LSO neuron. The inset shows the firing rates of a prototypic VCN (black) and MNTB (grey) neuron for the respective input conditions. D and E, effects of changing the AP jitter (D) and first spike latency of spikes in the contralateral input (relative to the ipsilateral input) (E). All remaining parameters are the same as in A. Firing rate begins to decrease at lower contralateral rates in the presence of larger jitter and slopes of IID functions decrease. First-spike latency of contralateral spikes has a weaker influence on the response of the simulated LSO neuron. F, examples of simulated synaptic conductance and LSO spike trains (design same as in A) for an increased contralateral latency (top four traces) and jitter (bottom four traces). Dots indicate single APs. G, simulated IID tuning curves for the control case (as in A–C), for an increased contralateral jitter, for an increased latency, and for the combination of both. In all simulations, the ipsilateral input had a spike rate of 250 Hz, and the contralateral input was varied from 0 Hz (ratio = 0) to 500 Hz (ratio = 2).