GABA is a modulator, rather than a classical transmitter, in the medial nucleus of the trapezoid body-lateral superior olive sound localization circuit.

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 GABAA receptors (GABAA Rs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked inhibitory postsynaptic currents and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABAA Rs, 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 GABAB Rs. GABAB R activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABAA Rs as demonstrated by application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a δ-subunit-specific GABAA R 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 GABAA Rs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.