Presynaptic glycine receptors (GlyRs) have been implicated in the regulation of glutamatergic synaptic transmission. Here, we characterized presynaptic GlyR-mediated currents by patch-clamp recording from mossy fiber boutons (MFBs) in rat hippocampal slices. In MFBs, focal puff-application of glycine-evoked chloride currents that were blocked by the GlyR antagonist strychnine. Their amplitudes declined substantially during postnatal development, from a mean conductance per MFB of approximately 600 pS in young to approximately 130 pS in adult animals. Single-channel analysis revealed multiple conductance states between approximately 20 and approximately 120 pS, consistent with expression of both homo- and hetero-oligomeric GlyRs. Accordingly, estimated GlyRs densities varied between 8-17 per young, and 1-3 per adult, MFB. Our results demonstrate that functional presynaptic GlyRs are present on hippocampal mossy fiber terminals and suggest a role of these receptors in the regulation of glutamate release during the development of the mossy fiber--CA3 synapse.
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