Postnatal development of glycinergic synaptic transmission and biophysical properties of glycine receptor-channels

Maturation of glycinergic synaptic transmission to hypoglossal motoneurons (HMs) was examined in an in vitro rat brainstem slice preparation. Motoneurons were placed in three age groups: neonate (P0-3), intermediate (P5-8), and juvenile (P10-18). Glycine receptor (GlyR)-channel subunit composition changes postnatally; I employed in situ hybridization techniques to document expression patterns of GlyR subunit mRNAs in the hypoglossal nucleus (n. XII) throughout this period.; GlyR-channel kinetics changed postnatally: juvenile channels had shorter mean burst durations than neonatal channels. Consequently, the decay time course of glycinergic inhibitory postsynaptic currents (IPSCs) became faster. The mean amplitude of spontaneous miniature IPSCs (mIPSCs) increased, and mIPSC amplitude distribution variance was large throughout the first three postnatal weeks. Gramicidin perforated-patch recordings of glycine-evoked currents revealed a postnatal change in reversal potential.; Glycinergic inhibitory postsynaptic potentials (IPSPs) also became shorter with development. I investigated the interaction between synaptic currents and motoneuron electrotonic properties which produces IPSPs, and found that synaptic current time course determines that of the IPSP. Glycinergic PSPs are depolarizing and prolonged in neonate HMs and become faster and hyperpolarizing during the first three postnatal weeks.; These results gave rise to two questions which were answered by examination of macroscopic patch currents: first, what properties of GlyR-channels govern the time course of mIPSCs, and, second, can stochastic channel behavior account for some of the observed variance of mIPSC amplitudes?; Brief (1 ms) pulses of glycine elicited patch currents that closely resembled mIPSCs. Patch current decay rate increased postnatally, mimicking mIPSCs. As I did not observe any significant rapid desensitization of patch current responses, I conclude that channel deactivation is the primary determinant of glycinergic mIPSC time course, and changes in deactivation rate underlie developmental changes in mIPSC decay. Although the ability of glycine to activate GlyRs increased postnatally, the channels' maximal open probability did not. Because GlyR-channel open probability is high, differences in receptor number between synapses, rather than stochastic channel behavior, underlie the majority of quantal variance throughout postnatal development. I estimate the number of GlyRs at a synapse to be 27 in neonate neurons and 50 in juvenile neurons.