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A modeling study of the responses of the lateral superior olive to ipsilateral sinusoidally amplitude-modulated tones.

Wang L, Colburn HS - J. Assoc. Res. Otolaryngol. (2011)

Bottom Line: In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies.In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model.These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Center for Hearing Research, Boston University, Boston, MA 02215, USA.

ABSTRACT
The lateral superior olive (LSO) is a brainstem nucleus that is classically understood to encode binaural information in high-frequency sounds. Previous studies have shown that LSO cells are sensitive to envelope interaural time difference in sinusoidally amplitude-modulated (SAM) tones (Joris and Yin, J Neurophysiol 73:1043-1062, 1995; Joris, J Neurophysiol 76:2137-2156, 1996) and that a subpopulation of LSO neurons exhibit low-threshold potassium currents mediated by Kv1 channels (Barnes-Davies et al., Eur J Neurosci 19:325-333, 2004). It has also been shown that in many LSO cells the average response rate to ipsilateral SAM tones decreases with modulation frequency above a few hundred Hertz (Joris and Yin, J Neurophysiol 79:253-269, 1998). This low-pass feature is not directly inherited from the inputs to the LSO since the response rate of these input neurons changes little with increasing modulation frequency. In the current study, an LSO cell model is developed to investigate mechanisms consistent with the responses described above, notably the emergent rate decrease with increasing frequency. The mechanisms explored included the effects of after-hyperpolarization (AHP) channels, the dynamics of low-threshold potassium channels (KLT), and the effects of background inhibition. In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies. The model also suggests that the background inhibition alone, possibly from the medial nucleus of the trapezoid body, can account for the small rate decrease seen in some LSO neurons, but could not explain the large rate decrease seen in other LSO neurons at high modulation frequencies. In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model. These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

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A Rate–fm functions with varying KLT conductance for the HH-type LSO model with regular KLT channels. Different colors represent different values of KLT strength gKLT. Solid lines represent responses with inputs from the simplified AN model; dashed lines represent responses with inputs from the Earlab AN model (see “Results”). B tMTFs for the Earlab AN inputs and the HH-type LSO model with regular KLT channels. Blue, red, and green curves represent different KLT strength gKLT as indicated. Solid black curve represents the tMTF for the Earlab AN inputs, and the dashed black curve is the same as the solid black curve except a constant vertical shift to emphasize the lower 3-dB cutoff frequency in the LSO model relative to the Earlab AN input. For all cases shown (A and B), ratemean is 200 spikes/s, strE is 2.55 nS, and NE is 20.
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Fig7: A Rate–fm functions with varying KLT conductance for the HH-type LSO model with regular KLT channels. Different colors represent different values of KLT strength gKLT. Solid lines represent responses with inputs from the simplified AN model; dashed lines represent responses with inputs from the Earlab AN model (see “Results”). B tMTFs for the Earlab AN inputs and the HH-type LSO model with regular KLT channels. Blue, red, and green curves represent different KLT strength gKLT as indicated. Solid black curve represents the tMTF for the Earlab AN inputs, and the dashed black curve is the same as the solid black curve except a constant vertical shift to emphasize the lower 3-dB cutoff frequency in the LSO model relative to the Earlab AN input. For all cases shown (A and B), ratemean is 200 spikes/s, strE is 2.55 nS, and NE is 20.

Mentions: As in the case of the LSO model with inhibition, we also tested the LSO model with KLT channels using more realistic AN inputs. Figure 7 shows the simulation results for an average AN input rate ratemean equal to 200 spikes/s with two types of AN inputs. First, the simple AN model used to generate the results in Figure 6 was tested with a higher average rate (solid curves in Fig. 7A), and second, the Earlab AN model was used with the same higher average rate (dashed curves in Fig. 7A). Values for the KLT conductance parameter gKLT were chosen as before: a value of zero and two additional values to give rates of 100 spikes/s and 25 spikes/s for the 1,500-Hz modulation frequency. In Figure 7A, note first that the rate–fm functions with increasing gKLT are qualitatively similar to those with ratemean equal to 100 spikes/s (Fig. 6A), although the maximum RPH with higher input rate in Figure 7A is around 160 spike/s, much higher than the maximum RPH with lower input rate (Fig. 6A). Second, the rate–fm functions are similar with or without the fm-dependent synchronization in the inputs (comparing dashed and solid curves). The Earlab AN inputs only slightly increased the peak rate of the rate–fm functions (Fig. 7A, dashed lines). Therefore, the fm-dependent synchronization in the AN inputs does not appear to be essential for creating the rate decay in the rate–fm functions, but it may play a role in further increasing the RPH of the rate–fm functions.FIG. 7


A modeling study of the responses of the lateral superior olive to ipsilateral sinusoidally amplitude-modulated tones.

Wang L, Colburn HS - J. Assoc. Res. Otolaryngol. (2011)

A Rate–fm functions with varying KLT conductance for the HH-type LSO model with regular KLT channels. Different colors represent different values of KLT strength gKLT. Solid lines represent responses with inputs from the simplified AN model; dashed lines represent responses with inputs from the Earlab AN model (see “Results”). B tMTFs for the Earlab AN inputs and the HH-type LSO model with regular KLT channels. Blue, red, and green curves represent different KLT strength gKLT as indicated. Solid black curve represents the tMTF for the Earlab AN inputs, and the dashed black curve is the same as the solid black curve except a constant vertical shift to emphasize the lower 3-dB cutoff frequency in the LSO model relative to the Earlab AN input. For all cases shown (A and B), ratemean is 200 spikes/s, strE is 2.55 nS, and NE is 20.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3298618&req=5

Fig7: A Rate–fm functions with varying KLT conductance for the HH-type LSO model with regular KLT channels. Different colors represent different values of KLT strength gKLT. Solid lines represent responses with inputs from the simplified AN model; dashed lines represent responses with inputs from the Earlab AN model (see “Results”). B tMTFs for the Earlab AN inputs and the HH-type LSO model with regular KLT channels. Blue, red, and green curves represent different KLT strength gKLT as indicated. Solid black curve represents the tMTF for the Earlab AN inputs, and the dashed black curve is the same as the solid black curve except a constant vertical shift to emphasize the lower 3-dB cutoff frequency in the LSO model relative to the Earlab AN input. For all cases shown (A and B), ratemean is 200 spikes/s, strE is 2.55 nS, and NE is 20.
Mentions: As in the case of the LSO model with inhibition, we also tested the LSO model with KLT channels using more realistic AN inputs. Figure 7 shows the simulation results for an average AN input rate ratemean equal to 200 spikes/s with two types of AN inputs. First, the simple AN model used to generate the results in Figure 6 was tested with a higher average rate (solid curves in Fig. 7A), and second, the Earlab AN model was used with the same higher average rate (dashed curves in Fig. 7A). Values for the KLT conductance parameter gKLT were chosen as before: a value of zero and two additional values to give rates of 100 spikes/s and 25 spikes/s for the 1,500-Hz modulation frequency. In Figure 7A, note first that the rate–fm functions with increasing gKLT are qualitatively similar to those with ratemean equal to 100 spikes/s (Fig. 6A), although the maximum RPH with higher input rate in Figure 7A is around 160 spike/s, much higher than the maximum RPH with lower input rate (Fig. 6A). Second, the rate–fm functions are similar with or without the fm-dependent synchronization in the inputs (comparing dashed and solid curves). The Earlab AN inputs only slightly increased the peak rate of the rate–fm functions (Fig. 7A, dashed lines). Therefore, the fm-dependent synchronization in the AN inputs does not appear to be essential for creating the rate decay in the rate–fm functions, but it may play a role in further increasing the RPH of the rate–fm functions.FIG. 7

Bottom Line: In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies.In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model.These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Center for Hearing Research, Boston University, Boston, MA 02215, USA.

ABSTRACT
The lateral superior olive (LSO) is a brainstem nucleus that is classically understood to encode binaural information in high-frequency sounds. Previous studies have shown that LSO cells are sensitive to envelope interaural time difference in sinusoidally amplitude-modulated (SAM) tones (Joris and Yin, J Neurophysiol 73:1043-1062, 1995; Joris, J Neurophysiol 76:2137-2156, 1996) and that a subpopulation of LSO neurons exhibit low-threshold potassium currents mediated by Kv1 channels (Barnes-Davies et al., Eur J Neurosci 19:325-333, 2004). It has also been shown that in many LSO cells the average response rate to ipsilateral SAM tones decreases with modulation frequency above a few hundred Hertz (Joris and Yin, J Neurophysiol 79:253-269, 1998). This low-pass feature is not directly inherited from the inputs to the LSO since the response rate of these input neurons changes little with increasing modulation frequency. In the current study, an LSO cell model is developed to investigate mechanisms consistent with the responses described above, notably the emergent rate decrease with increasing frequency. The mechanisms explored included the effects of after-hyperpolarization (AHP) channels, the dynamics of low-threshold potassium channels (KLT), and the effects of background inhibition. In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies. The model also suggests that the background inhibition alone, possibly from the medial nucleus of the trapezoid body, can account for the small rate decrease seen in some LSO neurons, but could not explain the large rate decrease seen in other LSO neurons at high modulation frequencies. In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model. These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

Show MeSH