The Journal of Neurophysiology Vol. 84 No. 5 November 2000, pp. 2317-2329
Copyright ©2000 by the American Physiological Society

Role of Synaptic Inputs in Determining Input Resistance of Developing Brain Stem Motoneurons

 ¹Department of Neuroscience and  ²Department of Psychiatry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260; and  ³Department of Physiology and Pharmacology, Oregon Health Sciences University, Portland, Oregon 97201

Núñez-Abades, Pedro A., John M. Pattillo, Tracy M. Hodgson, and William E. Cameron. Role of Synaptic Inputs in Determining Input Resistance of Developing Brain Stem Motoneurons. J. Neurophysiol. 84: 2317-2329, 2000. 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 (₀) 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 ₀. Addition of 10 µM bicuculline to a strychnine perfusate failed to further increase either R_n or ₀. 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 ₀ in the oldest age group. The proportion of change in ₀ 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.