Whole-cell patch-clamp recording reveals subthreshold sound-evoked postsynaptic currents in the inferior colliculus of awake bats

Abstract

The inferior colliculus receives excitatory and inhibitory input from parallel auditory pathways that differ in discharge patterns, latencies, and binaural properties. Processing in the inferior colliculus may depend on the temporal sequence in which excitatory and inhibitory synaptic inputs are activated and on the resulting balance between excitation and inhibition. To explore this issue at the cellular level, we used the novel approach of whole-cell patch-clamp recording in the midbrain of awake bats (Eptesicus fuscus) to record EPSCs or IPSCs. Sound-evoked EPSCs were recorded in most neurons. These EPSCs were frequently preceded by an IPSC, followed by an IPSC, or both. These findings help explain the large latency range and transient responses that characterize inferior colliculus neurons. The EPSC was sometimes followed by long-lasting oscillatory currents, suggesting that a single brief sound sets up a pattern of altered excitability that persists far beyond the duration of the initial sound. In three binaural neurons, ipsilateral sound evoked a large IPSC that partially or totally canceled the EPSC evoked by contralateral sound. In one binaural neuron with ipsilaterally evoked IPSCs, contralaterally evoked IPSCs occurred in response to frequencies above and below the neuron's best frequency. Thus, both monaural and binaural interactions can occur at single inferior colliculus neurons. These results show that whole-cell patch-clamp recording offers a powerful means of understanding how subthreshold processes determine the responses of auditory neurons.

Fig. 1.

Response of an IC neuron to a 10 msec pure-tone burst, showing a short-latency IPSC followed by an EPSC. Recording was in voltage-clamp mode, with the cell held at its resting membrane potential of −65 mV. On this and subsequent figures, thevertical scale bar indicates current in picoamperes (pA) and the thick bar on the horizontal axis indicates the duration of the stimulus. In this and subsequent figures, upward deflections represent outward current or IPSCs. Downward deflections represent inward current or EPSCs.

Fig. 2.

Growth of inhibition and increase in spike latency in response to 5 msec pure-tone stimuli as SPL was increased. The SPL was increased from 45 dB (10 dB above threshold, top trace) to 85 dB. First-spike latency is indicated by the arrow on each trace. The resting potential of this neuron was −70 mV. Spikes have been truncated.

Fig. 3.

The same neuron illustrated in Figure 1 responded to a 20 msec pure tone with a small, short-latency IPSC (latency = 8.2 msec) followed by an EPSC correlated with sound offset (latency to spike = 29.4 msec). The EPSC was followed by a second IPSC (arrow) that lasted for ∼20 msec. To better illustrate the late IPSC, a thin line is drawn through the baseline. Spikes have been truncated.

Fig. 4.

Inhibition of spiking in a neuron with a high rate of spontaneous activity. The top trace shows spontaneous activity in the absence of a stimulus; the middle andbottom traces show the neuron’s response to 50 msec pure tones. The middle trace was recorded in voltage-clamp mode, the bottom trace in current-clamp mode. The resting potential of this neuron was −65 mV.

Fig. 5.

Response of an IC neuron that exhibited oscillations after the excitatory response evoked by a 20 msec pure tone. The top trace was recorded in the absence of a stimulus. The bottom trace shows the neuron’s response to a tone. The oscillations (arrows) occurred at a rate of ∼30 Hz and persisted for at least 100 msec. The resting potential of this neuron was −64 mV.

Fig. 6.

Responses of an IC neuron to a 5 msec pure tone at different frequencies. Note that frequencies are arranged in nonsequential order. The response to a tone at the neuron’s best frequency (26 kHz, bottom trace) was almost exclusively excitatory. A tone above best frequency (29 kHz, middle trace) elicited a long-latency IPSC. A tone below best frequency (25 kHz, top trace) elicited a short-latency IPSC. This neuron’s resting potential was −70 mV. Spikes have been truncated.

Fig. 7.

Responses of an IC neuron to a 5 msec pure tone at different sound levels. The traces are arranged with low sound levels at the bottom and high sound levels at the top. There are no spikes in the top or bottom trace. The arrow points to the IPSC that persisted at high sound levels above the upper excitatory threshold of the neuron. This neuron had a resting potential of −59 mV. Spikes in the trace at 34 dB SPL have been truncated.

Fig. 8.

Responses of the same IC neuron illustrated in Figure 1 to a 40 msec pure tone at different sound levels. The traces are arranged with low levels at the bottom and high levels at the top. The arrow points to the IPSC that was present at low sound levels.

Fig. 9.

Responses of the same IC neuron shown in Figure 1to pure tones of different durations. The traces are arranged with short durations at the top and long durations at thebottom. The bar above each trace indicates the sound duration.

Fig. 10.

Responses of an IC neuron to 100 msec SAM signals modulated at three different rates. The carrier frequency was 26 kHz, and the sound level was 35 dB SPL. The modulation rates are indicated at the right of each trace. The resting potential of the neuron was −67 mV.

Fig. 11.

Responses of the same neuron illustrated in Figure 10 to changes in the rise time of a 100 msec pure tone at 26 kHz and 35 dB SPL. Shaded symbols indicate the rise time over which sound amplitude was increased.

Fig. 12.

Binaural interaction in an IC neuron that responded to contralateral sounds with an EPSC and ipsilateral sounds with an IPSC. In the traces in the left column, the contralateral sound is a 5 msec pure tone at 27 kHz and 45 dB SPL, either alone (top trace) or in combination with a simultaneous ipsilateral tone of the same frequency at the sound levels indicated. The traces on the right show the responses to ipsilateral tones alone. The resting potential of this neuron was −70 mV. Spikes in all traces have been truncated.