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The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2556-2564
Copyright ©1999 by the American Physiological Society
Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201
Morishita, Wade and
Bradley E. Alger.
Evidence for Endogenous Excitatory Amino Acids as Mediators in
DSI of GABAAergic Transmission in Hippocampal CA1. J. Neurophysiol. 82: 2556-2564, 1999. Depolarization-induced suppression of inhibition (DSI) is a process
whereby brief ~1-s depolarization to the postsynaptic membrane of
hippocampal CA1 pyramidal cells results in a transient suppression of
GABAAergic synaptic transmission. DSI is triggered by a
postsynaptic rise in [Ca2+]in and yet is
expressed presynaptically, which implies that a retrograde signal is
involved. Recent evidence based on synthetic metabotropic glutamate
receptor (mGluR) agonists and antagonists suggested that group
I mGluRs take part in the expression of DSI and raised the
possibility that glutamate or a glutamate-like substance is the
retrograde messenger in hippocampal CA1. This hypothesis was tested,
and it was found that the endogenous amino acids
L-glutamate (L-Glu) and L-cysteine
sulfinic acid (L-CSA) suppressed
GABAA-receptor-mediated inhibitory postsynaptic currents (IPSCs) and occluded DSI, whereas L-homocysteic acid
(L-HCA) and L-homocysteine sulfinic acid
(L-HCSA) did not. Activation of metabotropic kainate
receptors with kainic acid (KA) reduced IPSCs; however, DSI was not
occluded. When iontophoretically applied, both L-Glu and
L-CSA produced a transient IPSC suppression similar in
magnitude and time course to that observed during DSI. Both DSI and the actions of the amino acids were antagonized by
(S)-
-methyl-4-carboxyphenylglycine ([S]-MCPG), indicating that the
effects of the endogenous agonists were produced through activation of
mGluRs. Blocking excitatory amino acid transport significantly
increased DSI and the suppression produced by L-Glu or
L-CSA without affecting the time constant of recovery from
the suppression. Similar to DSI, IPSC suppression by L-Glu
or L-CSA was blocked by N-ethylmaleimide
(NEM). Moreover, paired-pulse depression (PPD), which is unaltered
during DSI, is also not significantly affected by the amino acids.
Taken together, these results support the glutamate hypothesis of DSI
and argue that L-Glu or L-CSA are potential
retrograde messengers in CA1.
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