title: Group II metabotropic glutamate receptors modulate extracellular glutamate in the nucleus accumbens.
abstract: 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-beta-benzyloxyaspartate failed to affect the alterations in extracellular glutamate by APICA or APDC, respectively. Group II mGluRs are G(i)-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 Ca(2+)- and PKA-dependent.
authors.affiliation: Department of Physiology and Neuroscience, Medical University of South Carolina, Charleston, South Carolina 29425, USA. xizheng@musc.edu
authors.forename: Zheng-Xiong
authors.initials: ZX
authors.lastname: Xi
authors.forename: David A
authors.initials: DA
authors.lastname: Baker
authors.forename: Hui
authors.initials: H
authors.lastname: Shen
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Carson
authors.forename: Peter W
authors.initials: PW
authors.lastname: Kalivas
mesh_terms: D027322:Amino Acid Transport System X-AG / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism; D008297:Male; D017551:Microdialysis; D009714:Nucleus Accumbens / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018094:Receptors, Metabotropic Glutamate / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism*
chemical_list: D027322:Amino Acid Transport System X-AG; D002352:Carrier Proteins; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D007473:Ion Channels; D018094:Receptors, Metabotropic Glutamate; C108822:metabotropic glutamate receptor 2; C102718:metabotropic glutamate receptor 3; D018698:Glutamic Acid; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium