GABAergic inhibition modulates intensity sensitivity of temporally patterned pulse trains in the inferior collicular neurons in big brown bats / 生理学报

The echolocating big brown bats (Eptesicus fuscus) emit trains of frequency-modulated (FM) biosonar signals with duration, amplitude, repetition rate, and sweep structure changing systematically during interception of their prey. In the present study, the sound stimuli of temporally patterned pulse trains at three different pulse repetition rates (PRRs) were used to mimic the sounds received during search, approach, and terminal stages of echolocation. Electrophysiological method was adopted in recordings from the inferior colliculus (IC) of midbrain. By means of iontophoretic application of bicuculline, the effect of GABAergic inhibition on the intensity sensitivity of IC neurons responding to three different PRRs of 10, 30 and 90 pulses per second (pps) was examined. The rate-intensity functions (RIFs) were acquired. The dynamic range (DR) of RIFs was considered as a criterion of intensity sensitivity. Comparing the average DR of RIFs at different PRRs, we found that the intensity sensitivity of some neurons improved, but that of other neurons decayed when repetition rate of stimulus trains increased from 10 to 30 and 90 pps. During application of bicuculline, the number of impulses responding to the different pulse trains increased under all stimulating conditions, while the DR differences of RIFs at different PRRs were abolished. The results indicate that GABAergic inhibition was involved in modulating the intensity sensitivity of IC neurons responding to pulse trains at different PRRs. Before and during bicuculline application, the percentage of changes in responses was maximal in lower stimulus intensity near to the minimum threshold (MT), and decreased gradually with the increment of stimulus intensity. This observation suggests that GABAergic inhibition contributes more effectively to the intensity sensitivity of the IC neurons responding to pulse trains at lower sound level.

Effects of backward masking on the responses of the inferior collicular neurons in the big brown bat, Eptesicus fuscus / 生理学报

Temporal features of sound convey information vital for behaviors as diverse as speech recognition by human and echolocation by bats. However, auditory stimuli presented in temporal proximity might interfere with each other. Although much progress has been made in the description of this phenomenon from psychophysical studies, the neural mechanism responsible for its formation at central auditory structures especially at the inferior colliculus (IC), a midbrain auditory nucleus which practically receives massive bilateral projections from all the major auditory structures in the brainstem, remains unclear. This study was designed to investigate it in vivo by using electrophysiological recording from the inferior collicular neurons of the big brown bat, Eptesicus fuscus. In our results, the responses of 12 (38%, n= 31) neurons to the test sound (leading sound) were obviously inhibited by the masker (lagging sound). The inhibitory effects in these neurons were correlated with the inter-stimulus level difference (SLD) and the inter-stimulus onset asynchrony (SOA) interval. The strength of backward masking increased with the masker intensity increasing, the test sound intensity decreasing and the SOA interval shortening. There were no obvious effects of backward masking on the responses of many other neurons (52%, 16/31), and yet in a part of these neurons, the neural inhibition of responses to the test sound was observed at the special SLD and the special SOA intervals. Moreover, few of the 31 sampled IC neurons (10%, 3/31) displayed facilitating responses to the test sound at the special SLD and the special SOA intervals. These data demonstrate that a lot of IC neurons are involved in the generation of the backward masking of acoustical perception. It is conjectured that the temporal dynamic integration between the leading inhibitory inputs evoked by the masker sound and the excitatory inputs evoked by the test sound might play a key role in shaping the acoustical response characteristics of the IC neurons.