Elsevier

Hearing Research

Volume 104, Issues 1–2, February 1997, Pages 112-126
Hearing Research

Research paper
Two sources of inhibition affecting binaural evoked responses in the rat's inferior colliculus : the dorsal nucleus of the lateral lemniscus and the superior olivary complex

https://doi.org/10.1016/S0378-5955(96)00182-7Get rights and content

Abstract

The present study was undertaken to determine the influence of two extrinsic sources of inhibition on auditory binaural evoked responses recorded from the rat's inferior colliculus. The first source, the dorsal nucleus of the lateral lemniscus (DNLL), is predominantly GABAergic and has both ipsi- and contralateral projections to the central nucleus of the inferior colliculus (ICC). The second, the superior olivary complex (SOC), has a large glycinergic projection from the lateral superior olive (LSO) to the ipsilateral ICC. Thus, both structures are candidates for imposing an inhibitory effect on responses in the ICC. Neural activity was experimentally blocked by local injection of the excitatory amino acid antagonist, kynurenic acid (KYNA), into either DNLL or SOC. Binaural evoked responses were recorded from the ICC as the intensity of the sound in the ipsilateral ear was increased. Interaural intensity difference functions based on the amplitude of the evoked responses were generated before and after the KYNA injection. An injection into the contralateral DNLL greatly reduced the response suppression produced by stimulation of the ipsilateral ear. Injection into the ipsilateral DNLL, however, had no effect. Injection into the ipsilateral SOC reduced the amount of binaural suppression but the effect was apparent only in cases with surgical transection of the contralateral lateral lemniscus at a level below the DNLL. These data support the conclusion that binaural responses in the rat's ICC are shaped by inhibitory projections from both contralateral DNLL and ipsilateral SOC.

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      However, when the masker is presented and the ipsilateral chatter leads the contralateral one, binaural unmasking of IC FFRs is significantly reduced by blocking excitatory glutamate transmissions in the contralateral DNLL (Fig. 5C), suggesting that GABAergic projections from the contralateral DNLL play a role in binaurally unmasking IC FFRs. It has been well known that GABAergic inhibitory inputs to the IC shape binaural responses of individual IC neurons (Burger and Pollak, 2001; Kelly and Li, 1997; Kidd and Kelly, 1996; Li and Kelly, 1992; Van Adel et al., 1999). Also, Lin and Feng (2003) have reported that iontophoretic application of bicuculline, a GABAA receptor antagonist, into the frog IC markedly degraded binaural processing involved in spatial unmasking of the IC.

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    1

    Present address: Laboratory of Visual and Auditory Neurosciences, Psychology Department, Queen's University, Kingston, Ontario K7L 3N6, Canada.

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