GABA co-release guides the functional maturation of glycinergic synapses in an auditory sound localization circuit.

In the mammalian brainstem and spinal cord, glycine is the primary inhibitory neurotransmitter. However, during development, many glycinergic neurons also co-release the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Although the acute effects of GABA co-release on immature synaptic transmission have been increasingly characterized, its role in synapse maturation and circuit formation remains poorly understood. Here, we investigated the developmental roles of GABA co-release at glycinergic synapses from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO), an auditory pathway essential for binaural integration and sound localization. During the first two postnatal weeks, MNTB-LSO synapses co-release GABA and undergo pronounced synaptic and circuit refinement. Using conditional knockout mice with severely diminished GABA co-release from MNTB neurons, we found that key aspects of circuit refinement, including synaptic silencing and strengthening, occurred normally. However, a disruption of GABA co-release resulted in significantly larger quantal amplitudes and a reduced readily releasable vesicle pool, impairing the high fidelity and temporal precision of synaptic transmission, which are essential for accurate binaural processing. These results reveal a critical developmental role for GABA co-release in shaping the functional synaptic architecture of glycinergic synapses involved in sound localization. Significance Statement: Glycinergic neurons in the brainstem and spinal cord often co-release GABA during development, but the role of this co-release in circuit refinement or synapse maturation remains poorly understood. This study found that disruption of developmental GABA co-release at the MNTB-LSO synapse, a key part of the sound localization circuit, did not affect topographic refinement by synapse elimination or strengthening. However, impaired GABA co-release prevented synapses from developing the characteristic features that enable the high-fidelity, temporally accurate transmission required for sound localization. This highlights a critical role for developmental GABA co-release in shaping the specialized functional architecture of glycinergic synapses critical for binaural processing and sound localization.