Modulation of synaptic input by GABAB receptors improves coincidence detection for computation of sound location

Abstract

Interaural time disparities (ITDs) are the primary cues for localisation of low-frequency sound stimuli. ITDs are computed by coincidence-detecting neurones in the medial superior olive (MSO) in mammals. Several previous studies suggest that control of synaptic gain is essential for maintaining ITD selectivity as stimulus intensity increases. Using acute brain slices from postnatal day 7 to 24 (P7–P24) Mongolian gerbils, we confirm that activation of GABAB receptors reduces the amplitude of excitatory and inhibitory synaptic currents to the MSO and, moreover, show that the decay kinetics of IPSCs are slowed in mature animals. During repetitive stimuli, activation of GABAB receptors reduced the amount of depression observed, while PSC suppression and the slowed kinetics were maintained. Additionally, we used physiological and modelling approaches to test the potential impact of GABAB activation on ITD encoding in MSO neurones. Current clamp recordings from MSO neurones were made while pharmacologically isolated excitatory inputs were bilaterally stimulated using pulse trains that simulate ITDs in vitro. MSO neurones showed strong selectivity for bilateral delays. Application of both GABAB agonists and antagonists demonstrate that GABAB modulation of synaptic input can sharpen ITD selectivity. We confirmed and extended these results in a computational model that allowed for independent manipulation of each GABAB-dependent effect. Modelling suggests that modulation of both amplitude and kinetics of synaptic inputs by GABAB receptors can improve precision of ITD computation. Our studies suggest that in vivo modulation of synaptic input by GABAB receptors may act to preserve ITD selectivity across various stimulus conditions.

Figure 5. ITD sensitivity is modulated by GABA_BR activation

A, schematic of bilateral stimulation protocol used to simulate ITDs in vitro (see Methods). B, representative current clamp traces from the peak and +0.2 ms sITD time points from a MSO neurone (P13). Total spike counts are the sum of 10 trials at each simulated ITD. C, sITD tuning curves constructed using data from the neurone in B. D, population data for sITD experiments show a significant decrease in sITD tuning curve half-width with application of 0.1 μm baclofen that was recovered after washout (P < 0.05, n= 25).

Figure 6. Blocking GABA_BR activity broadens sITD tuning curves

A, CGP55845 (2 μm) application (diamonds) broadens sITD tuning curves relative to control (squares) in a P18 neurone. Recovery from inactivation is shown with open squares. B, population data show significant broadening of sITD half-width during CGP55845 application (*P < 0.01, n= 12). C, sequential application of baclofen and CGP55845 demonstrates time course of effect on ITD half-width.

Figure 3. Modulation of GABA_BRs affects rates of depression of EPSCs during repetitive stimuli

A, average of 20 trials of 100 Hz stimulus trains. B, normalized traces of control and baclofen condition show the reduction in depression during activation of GABA_BRs. C, population data for depression at each pulse during the stimulus shows the reduction in depression during application of baclofen and CGP55845. The percent depression at the 10th pulse was significantly different from control with baclofen (P < 0.01, n= 8) and CGP55845 (P < 0.01, n= 15) application. D, population data for percent depression values at three stimulus frequencies. EPSC trains in baclofen showed significantly less depression at all frequencies tested (50 Hz, n= 7; 200 Hz, n= 3). CGP55845 application also mildly but significantly reduced EPSC depression at 50 (n= 7) and 100 Hz (at 200 Hz, P= 0.06; n= 3). **P < 0.01, *P < 0.05.

Figure 4. Depression rates of IPSCs are affected by modulation of GABA_BRs during repetitive stimuli

A, average of 20 trials of 50 Hz stimulus trains. B, normalized traces of control and baclofen condition show the reduction in depression during activation of GABA_BRs. Additionally the slowing of IPSC kinetic seen in unitary responses are maintained in responses throughout the stimulus train. C, population data (100 Hz trains) for depression at each pulse during the stimulus shows the reduction in depression during application of baclofen and CGP55845. Percent depression at the 10th pulse was significantly different from control in baclofen (P < 0.01, n= 6) and CGP55845 (P < 0.05, n= 4). D, population data for percent depression at the two frequencies tested for IPSC train stimuli. Similar data patterns are observed at each frequency where both baclofen and CGP55845 reduced the amount of depression observed during the train (50 Hz, n= 5). **P < 0.01, *P < 0.05.

Figure 1. GABA_BR activation suppresses evoked EPSCs in MSO principal neurones

A, average of 10 EPSC traces from a P18 MSO neurone evoked via stimulation of contralateral AVCN fibres show amplitude suppression during baclofen (10 μm) application and no change with application of CGP55845 (2 μm). B, normalised traces from A show no difference in kinetics between treatments. C, baclofen (Bac) significantly reduces amplitude of EPSCs (P < 0.001, n= 16) in P17–P24 MSO neurones, while CGP55845 has no effect (P > 0.05, n= 11). D, EPSC τ_decay values are unchanged by application of baclofen or CGP55845 (P > 0.05). E, effect of baclofen on ESPC τ_decay does not correlate with age (r= 0.272, P= 0.237, n= 26), dashed line indicates ratio value of 1, indicative of no change in τ_decay with baclofen treatment. r value is Pearson correlation.

Figure 2. Activation of GABA_BRs suppresses IPSC amplitude and slows kinetics

A, average traces of IPSCs in each condition. B, normalised average traces from A depict baclofen effect on IPSC kinetics. C, population data from P17–P24 MSO neurones reveals that IPSC amplitudes were significantly reduced by application of baclofen (P < 0.001, n= 18), but remained unchanged in CGP55845 (P > 0.05). D, baclofen significantly increased τ_decay kinetics (P < 0.001). E, effect of baclofen on τ_decay correlates with age (r= 0.687, P < 0.001, n= 30). Ea, normalised IPSC traces in control (black) and baclofen (grey) show a decrease in τ_decay at younger ages (top, triangle), and an increase in τ_decay at older ages (bottom, square) with GABA_BR activation. r value is Pearson correlation.

Figure 7. Model parameters for excitatory and inhibitory inputs

A, modelled excitatory input rates are derived based on published AVCN input/output functions adapted from Joris et al. (1994), continuous line. Data are expressed as both firing rate and entrainment values. Computational ITD simulations used three sample intensities on the entrainment curve to simulate MSO input near threshold (30 dB, triangle), at moderate intensity (45dB, square) and high intensity near saturation (60 dB, circle). B, input/output function for gerbil MNTB neurones adapted from Kopp-Scheinpflug et al. (2008). The same three intensity conditions are sampled as in the excitation only model shown in A.

Figure 8. IPSC kinetics and PSC amplitude reduction independently shape ITD tuning in the E/EI model

A–C, model output for low, moderate and high intensities, respectively, under each treatment condition (symbols: control, black filled squares; kinetic only, red triangles; amplitude only, blue circles; combined modulation, green diamonds). Aa–Ca, mean firing rate ± SEM for the model output under each condition. Ab–Cb, normalised firing rate ± SEM for the model output under each condition. Ac–Cc, histograms of normalised half-width results at each intensity. Kinetic modulation of IPSCs alone improved ITD selectivity similarly to amplitude alone conditions at moderate and high intensities. Ad–Cd, firing rate (FR) modulation for each condition for the biologically relevant range of ITDs (±130 μs) indicated by grey bars in columns a and b. Firing rate modulation increased in all GABA_B-activated conditions relative to control at 45 and 60 dB. D, comparison of the contribution of improvement in ITD sensitivity for each GABA_B-dependent effect modelled at three intensities. The kinetic modulation of IPSCs has a greater impact on selectivity as intensity increases, while the amplitude modulation is most potent at low intensities. Interestingly, the combined influence of these modulations is fairly stable across intensity.

Figure 9. GABA_BR-dependent improvements in ITD selectivity are maintained over a range of input parameters in the E/EI model

A, half-width values for each condition at the 5 different inhibitory lead times. ITD selectivity improved with GABA_BR-dependent modulations regardless of inhibitory lead time. B, half-width values at the three different ratios of excitatory to inhibitory input magnitude. GABA_BR-dependent changes in PSCs led to a decrease in half-width regardless of the relative input magnitude but ITD selectivity was greatest at the ratio where inhibition dominated.