GABA is a Modulator, Rather than a Classical Transmitter, in the MNTB‐LSO Sound Localization Circuit

Key points

The LSO, a brainstem hub involved in sound localization, integrates excitatory and inhibitory inputs from the ipsilateral and the contralateral ear, respectively. In gerbils and rats, inhibition to the LSO reportedly shifts from GABAergic to glycinergic within the first three postnatal weeks. Surprisingly, we found no evidence for synaptic GABA signalling during this time window in mouse LSO principal neurons. However, we found that presynaptic GABA_BRs modulate Ca²⁺ influx into MNTB axon terminals, resulting in reduced synaptic strength. Moreover, GABA elicited strong responses in LSO neurons that were mediated by extrasynaptic GABA_ARs. RNA sequencing revealed highly abundant δ subunits, which are characteristic for extrasynaptic receptors. Whereas GABA increased the excitability of neonatal LSO neurons, it reduced the excitability around hearing onset. Collectively, GABA appears to control the excitability of mouse LSO neurons via extrasynaptic and presynaptic signalling. Thus, GABA acts as a modulator, rather than as a classical transmitter.

Abstract

GABA and glycine mediate fast inhibitory neurotransmission and are coreleased at several synapse types. Here we assessed the contribution of GABA and glycine in synaptic transmission between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO), two nuclei involved in sound localization. Whole‐cell patch‐clamp experiments in acute mouse brainstem slices at postnatal days (P) 4 and 11 during pharmacological blockade of GABA_A receptors (GABA_ARs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked IPSCs and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABA_ARs, corroborating the conclusion that GABA does not act synaptically in the mouse LSO. Imaging experiments revealed reduced Ca²⁺ influx into MNTB axon terminals following activation of presynaptic GABA_BRs. GABA_BR activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABA_ARs as demonstrated by application of THIP, a δ‐subunit‐specific GABA_AR agonist. RNA sequencing showed high mRNA levels for the δ subunit in the LSO. Moreover, GABA transporters GAT‐1 and GAT‐3 appear to control extracellular GABA. Finally, we show an age‐dependent effect of GABA on the excitability of LSO neurons. Whereas tonic GABA increased the excitability at P4, leading to spike facilitation, it decreased the excitability at P11 via shunting inhibition through extrasynaptic GABA_ARs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.

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