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Vol. 300, Issue 1, 162-171, January 2002
Department of Physiology and Neuroscience, Medical University of
South Carolina, Charleston, South Carolina
The regulation of extracellular glutamate in the nucleus
accumbens by group II metabotropic glutamate receptors (mGluR2/3) was
examined in vivo. Stimulation of mGluR2/3 with
2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or N-acetylaspartylglutamate reduced
extracellular glutamate levels. Conversely, blockade of mGluR2/3 by
LY143495 or (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) increased extracellular glutamate, an effect antagonized by the coadministration of APDC. These effects likely involve both vesicular and nonvesicular glutamate, because the increase in
glutamate by APICA or the decrease by APDC was prevented by blocking
N-type calcium channels and the release of glutamate after potassium-induced membrane depolarization was antagonized by
APDC. In addition, blockade of the cystine-glutamate exchange, a major
nonvesicular source of extracellular glutamate, by
(S)-4-carboxyphenylglycine blocked the effects induced
by either APDC or APICA. However, blockade of Na+ channels
by tetrodotoxin or Na+-dependent glutamate transporters by
DL-threo-
-benzyloxyaspartate failed to
affect the alterations in extracellular glutamate by APICA or APDC,
respectively. Group II mGluRs are Gi-coupled and coperfusion with the cAMP-dependent protein kinase (PKA) activator Sp-cAMPS blocked the reduction in glutamate by APDC and the PKA inhibitor Rp-cAMPS prevented the elevation in glutamate by APICA. Taken
together, these data support three conclusions: 1) group II mGluRs
regulate both vesicular and nonvesicular release of glutamate in the
nucleus accumbens, 2) there is tonic in vivo stimulation of mGluR2/3 by
endogenous glutamate, and 3) modulation of group II mGluRs of
extracellular glutamate is Ca2+- and PKA-dependent.
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