GABA Mediates Presynaptic Inhibition at Glycinergic Synapses in a Rat Auditory Brainstem Nucleus

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1. Many inhibitory nerve terminals in the mammalian anteroventral cochlear nucleus (AVCN) contain both glycine and GABA, but the reason for the co-localization of these two inhibitory neurotransmitters in the AVCN is unknown. We have investigated the roles of glycine and GABA at synapses on bushy cells in the rat AVCN, using receptor immunohistochemistry and electrophysiology.

2. Our immunohistochemical results show prominent punctate labelling of postsynaptic clusters of glycine receptors and of the receptor clustering protein gephyrin over the surface of bushy cells. In contrast, weak diffuse membrane immunolabelling of GABAA receptors was observed.

3. Whole-cell recordings from bushy cells in AVCN slices demonstrated that evoked inhibitory postsynaptic currents (IPSCs) were predominantly (81%) glycinergic, based on the decrease in amplitude of the IPSCs in bicuculline (10 μM). This observation was supported by the effect of strychnine (1 μM), which was to decrease the evoked IPSC (to 10% of control IPSC amplitude) and to produce a greater than 90% block of spontaneous miniature IPSCs.

4. These results suggest a minor role for postsynaptic GABAA receptors in bushy cells, despite a high proportion of GABA-containing terminals on these cells. Therefore, a role for metabotropic GABAB receptors was investigated. Activation of GABAB receptors with baclofen revealed a significant attenuation of evoked glycinergic IPSCs. The effect of baclofen was presynaptic, as indicated by a lack of change in the mean amplitude of spontaneous IPSCs.

5. Significantly, the decrease in the amplitude of evoked glycinergic IPSCs observed following repetitive nerve stimulation was reduced in the presence of the GABAB antagonist, CGP 35348. This indicates that synaptically released GABA can activate presynaptic GABAB receptors to reduce transmitter release at glycinergic synapses. Our results suggest specific pre- versuspostsynaptic physiological roles for GABA and glycine in the AVCN.