A phospho-deficient & alpha;3 glycine receptor mutation alters synaptic glycine and GABA release in mouse spinal dorsal horn neurons

Abstract

Glycine receptors (GlyRs), together with GABAA receptors, mediate postsynaptic inhibition in most spinal cord and hindbrain neurons. In several CNS regions, GlyRs are also expressed in presynaptic terminals. Here, we analysed the effects of a phospho-deficient mutation (S346A) in GlyR α3 subunits on inhibitory synaptic transmission in superficial spinal dorsal horn neurons, where this subunit is abundantly expressed. Unexpectedly, we found that not only were the amplitudes of evoked glycinergic inhibitory postsynaptic currents (IPSCs) significantly larger in GlyRα3(S346A) mice than in mice expressing wild-type α3GlyRs (GlyRα3(WT) mice), but so were those of GABAergic IPSCs. Decreased frequencies of spontaneously occurring glycinergic and GABAergic miniature IPSCs (mIPSCs) with no accompanying change in mIPSC amplitudes suggested a change in presynaptic transmitter release. Paired-pulse experiments on glycinergic IPSCs revealed an increased paired-pulse ratio and a smaller coefficient of variation in GlyRα3(S346A) mice, which together indicate a reduction in transmitter release probability and an increase in the number of releasable vesicles. Paired-pulse ratios of GABAergic IPSCs recorded in the presence of strychnine were not different between genotypes, while the coefficient of variation was smaller in GlyRα3(S346A) mice, demonstrating that the decrease in release probability was readily reversible by GlyR blockade, while the difference in the size of the pool of releasable vesicles remained. Taken together, our results suggest that presynaptic α3 GlyRs regulate synaptic glycine and GABA release in superficial dorsal horn neurons, and that this effect is potentially regulated by their phosphorylation status.