Contribution of single-channel properties to the time course and amplitudevariance of quantal glycine currents recorded in rat motoneurons

The amplitude of spontaneous. glycinergic miniature inhibitory postsynaptic currents (mIPSCs) recorded in hypoglossal motoneurons (HMs) in an in vitro brain stem slice preparation increased over the first 3 postnatal weeks, f rom 42 +/- 6 pA in neonate (P0-3) to 77 +/- ii pA in juvenile (P11-18) HMs. Additionally, mIPSC amplitude distributions were highly variable: CV 0.68 +/- 0.05 (means +/- SE) for neonates and 0.83 +/- 0.06 for juveniles, We wi shed to ascertain the contribution of glycine receptor (GlyR)channel proper ties to this change in quantal amplitude and to the amplitude variability a nd time course of mIPSCs. To determine whether a postnatal increase in GlyR -channel conductance accounted for the postnatal change in quantal amplitud e, the conductance of synaptic GlyR channels was determined by nonstationar y, variance analysis of mIPSCs. It was 48 +/- 8 pS in neonate and 46 +/- 10 pS in juvenile HMs, suggesting that developmental changes in mIPSC amplitu de do not result from a postnatal alteration of GlyR-channel conductance. N ext we determined the open probability (P-open) of GlyR channels in outside -out patches excised from HMs to estimate the contribution of stochastic ch annel behavior to quantal amplitude variability. Brief (1 ms) pulses Of gly cine (I mM) elicited patch currents that closely resembled mIPSCs. The GlyR channels' P-open, calculated by nonstationary variance analysis of these c urrents, was similar to 0.70 (0.66 +/- 0.09 in neonates and 0.72 +/- 0.05 i n juveniles). The decay rate of patch currents elicited bq brief applicatio n of saturating concentrations of glycine (10 mM) increased postnatally, mi micking previously documented changes in mIPSC time course, Paired pulses o f glycine (10 mM) were used to determine if rapid GlyR-channel desensitizat ion contributed to either patch current time course or quantal amplitude va riability. Because we did not observe any fast desensitization of patch cur rents, we believe that fast desensitization of GlyRs underlies neither phen omenon. From our analysis of glycinergic patch currents and mIPSCs, ne draw three conclusions. First, channel deactivation is the primary determinant of glycinergic mIPSC time course, and postnatal changes in channel deactiva tion rate account for observed developmental changes in mIPSC decay rate. S econd, because GlyR-channel P-open is high, differences in receptor number between synapses rather than stochastic channel behavior are likely to unde rlie the majority of quantal variability seen at glycinergic synapses throu ghout postnatal development. Wr estimate the number of GlyRs available at a synapse to be on average 27 in neonate neurons and 39 in juvenile neurons. Third, this change in the calculated number of GlyRs at each synapse may a ccount for the postnatal increase in mIPSC amplitude.