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Key points

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    Interaural time difference (ITD) is a major cue for sound source localization and is processed by detecting a coincidence of bilateral excitatory postsynaptic potentials (EPSPs) in the nucleus laminaris (NL) in birds.
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    The sharpness of coincidence detection (CD) depends on the speed and size of EPSPs. We found here a regulatory mechanism of EPSP size through the presynaptic mGluR activity.
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    The activation of mGluRs reduced the transmitter release and extent of excitatory postsynaptic current depression during tetanic stimulation, but improved the CD. Furthermore, the activity of mGluRs and their expression were graded along the tonotopic axis and were stronger toward the low frequency neurons of NL.
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    We proposed an idea that the presynaptic mGluRs may operate as a self-regulatory mechanism to optimize the size of EPSP and have roles in sharpening the CD. This regulatory mechanism may underlie the sound source localization particularly during a long-lasting sound in the NL.

Abstract  Interaural time difference (ITD) is a major cue for localizing a sound source and is processed in the nucleus laminaris (NL) in birds. Coincidence detection (CD) is a crucial step for processing ITD and critically depends on the size and time course of excitatory postsynaptic potentials (EPSPs). Here, we investigated a role of metabotropic glutamate receptors (mGluRs) in the regulation of EPSP amplitude and CD in the NL of chicks. A non-specific agonist of mGluRs ((±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid; t-ACPD) reduced the amplitude and extent of depression of excitatory postsynaptic currents (EPSCs) during a stimulus train, while the paired pulse ratio and coefficient of variation of EPSC amplitude were increased. In contrast, the amplitudes of spontaneous EPSCs were not affected, but the frequency was reduced. Thus, the effects of t-ACPD were presynaptic and reduced the release of neurotransmitter from terminals in the NL. Expression of group II mGluRs was graded along the tonotopic axis and was stronger towards the low frequency region in the NL. Both group II (DCG-IV) and group III (l-AP4) specific agonists reduced EPSC amplitude by presynaptic mechanisms, and the reduction was larger in the low frequency region; however, we could not find any effects of group I-specific agonists on EPSCs. The reduced EPSP amplitude in DCG-IV improved CD. A specific antagonist of group II mGluRs (LY341495) increased the amplitude of both EPSCs and EPSPs and enhanced the depression during a stimulus train, indicating constitutive activation of mGluRs in the NL. These observations indicate that mGluRs may work as autoreceptors and regulate EPSP size to improve CD in the NL.