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The Journal of Neurophysiology Vol. 84 No. 6 December 2000, pp. 2715-2724
Copyright ©2000 by the American Physiological Society
-Mediated Synaptic Transmission on Rat Hypoglossal
Motoneurons In Vitro
Biophysics Sector and Istituto Nazionale di Fisica della Materia Unit, International School for Advanced Studies (SISSA), 34014 Trieste, Italy
Donato, Roberta and
Andrea Nistri.
Relative Contribution by GABA or Glycine to
Cl-Mediated Synaptic Transmission on Rat Hypoglossal
Motoneurons In Vitro. J. Neurophysiol. 84: 2715-2724, 2000. The relative contribution by
GABA and glycine to synaptic transmission of motoneurons was
investigated using an hypoglossus nucleus slice preparation from
neonatal rats. Spontaneous, miniature, or electrically evoked
postsynaptic currents (sPSCs, mPSCs, ePSCs, respectively) mediated by
glycine or GABA were recorded under whole cell voltage clamp after
blocking excitatory glutamatergic transmission with kynurenic acid. The
overall majority of Cl
-mediated sPSCs was
glycinergic, while only one-third was GABAergic; 70 ± 10% of
mPSCs were glycinergic while 22 ± 8% were GABAergic. Tetrodotoxin (TTX) application dramatically reduced the frequency (and
slightly the amplitude) of GABAergic events without changing frequency
or amplitude of glycinergic sPSCs. These results indicate that, unlike
spontaneous GABAergic transmission, glycine-mediated neurotransmission
was essentially independent of network activity. There was a consistent
difference in the kinetics of GABAergic and glycinergic responses as
GABAergic events had significantly slower rise and decay times than
glycinergic ones. Such a difference was always present whenever sPSCs,
mPSCs, or ePSCs were measured. Finally, GABAergic and glycinergic mPSCs
were differentially modulated by activation of glutamate metabotropic
receptors (mGluRs), which are abundant in the hypoglossus nucleus. In
fact, the broad-spectrum mGluR agonist
(±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (50 µM), which in control solution increased the frequency of both
GABAergic and glycinergic sPSCs, enhanced the frequency of glycinergic
mPSCs only. These results indicate that on brain stem motoneurons,
Cl-mediated synaptic transmission is mainly due
to glycine rather than GABA and that GABAergic and glycinergic events
differ in terms of kinetics and pharmacological sensitivity to mGluR
activation or TTX.
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