DEVELOPMENT OF GLYCINE- AND GABA-GATED CURRENTS IN RAT SPINAL MOTONEURONS. Gao, Bao-Xi, and Lea Ziskind-Conhaim. Department of Physiology and Center for Neuroscience, University of Wisconsin Medical School, Madison, WI 53706.
APStracts 2:0083N, 1995.
SUMMARY AND CONCLUSIONS
1) Developmental changes in glycine- and GABA-activated currents were studied in spinal motoneurons of embryonic and neonatal rats using whole-cell recording techniques. 2) Pressure ejection of glycine or GABA onto motoneuron somata produced Cl - -mediated inward currents and membrane depolarizations. During embryonic development, the average amplitude of GABA-gated currents was three-fold larger than that of glycine-gated currents, but due to a large eight-fold postnatal increase in glycine-activated currents, similar currents were produced by both amino acids after birth. 3) At all ages, the decay of glycine- and GABA-gated currents best fit one-exponential curve, and their time constants were similar. The average decay time constant decreased by two- fold after birth. 4) The ionic specificity of glycine- and GABA-gated channels was studied to determine whether the large amplitude of GABA-activated currents in embryonic motoneurons resulted from the contribution of an outward HCO 3 - movement. Manipulations of Cl - and HCO 3 - concentrations produced changes in the reversal potentials of glycine and GABA that were similar to the calculated changes in the equilibrium potentials of Cl - . This suggested that glycine- and GABA-gated currents were Cl - specific, and HCO 3 - movement did not contribute more to the current generated by GABA than that produced by glycine. Development of Glycine- and GABA-Gated Currents J433-4 5) Glycine- and GABA-gated currents were associated with several-fold increases in membrane conductance. The conductance increase generated by GABA in embryonic motoneurons was seven-fold larger than that generated by glycine, but similar conductance changes were produced by both amino acids after birth. 6) Despite the induced depolarizations, activation of glycine and GABA receptors was associated with inhibition of spontaneous potentials and decreased motoneuron excitability. Our data suggested that glycine and GABA acted as inhibitory neurotransmitters by increasing Cl - conductance and effectively shunting incoming excitatory currents. 

Received 14 July 1994; accepted in final form 8 March 1995.
APS Manuscript Number J433-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 19 April 1995.