The contribution of synaptic input to input resistance was examined in 208 developing genioglossal motoneurons in 3 postnatal age groups (5-7 day, 13-16 day, and 18-24 day) using sharp electrode recording in a slice preparation of the rat brain stem. High magnesium (Mg2+; 6 mM) media generated significant increases (21-38%) in both the input resistance (R(n)) and the first time constant (τ0) that were reversible. A large percent of the conductance blocked by high Mg2+ was also sensitive to tetrodotoxin (TTX). Little increase in resistance was attained by adding blockers of specific amino acid (glutamate, glycine, and GABA) transmission over that obtained with the high Mg2+. Comparing across age groups, there was a significantly larger percent change in R(n) with the addition of high Mg2+ at postnatal days 13 to 15 (P13-15; 36%) than that found at P5-6 (21%). Spontaneous postsynaptic potentials were sensitive to the combined application of glycine receptor antagonist, strychnine, and the GABA(A) receptor antagonist, bicuculline. Application of either 10 μM strychnine or bicuculline separately produced a reversible increase in both R(n) and τ0. Addition of 10 μM bicuculline to a strychnine perfusate failed to further increase either R(n) or τ0. The strychnine/bicuculline-sensitive component of the total synaptic conductance increased with age so that this form of neurotransmission constituted the majority (>60%) of the observed percent decrease in R(n) and τ0 in the oldest age group. The proportion of change in τ0 relative to R(n) following strychnine or high magnesium perfusate varied widely from cell to cell and from age to age without pattern. Based on a model from the literature, this pattern indicates a nonselective distribution of the blocked synaptic conductances over the cell body and dendrites. Taken together, the fast inhibitory synapses (glycine, GABA(A)) play a greater role in determining cell excitability in developing brain stem motoneurons as postnatal development progresses. These findings suggest that synaptically mediated conductances effect the membrane behavior of developing motoneurons.
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