Metabotropic glutamate receptor activation produces extrapyramidal motor system activation that is mediated by striatal dopamine.

Abstract

Little is known about the in vivo function of the GTP-binding protein-coupled "metabotropic" excitatory amino acid (EAA) receptor. In vitro studies on agonist-induced brain phosphoinositide hydrolysis have shown that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid is a highly selective and efficacious metabotropic EAA agonist. We have recently reported that in vivo unilateral intrastriatal injection of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid induces transient extrapyramidal motor activation that manifests itself as contralateral turning. In this study, we fully characterized the onset of turning behavior following intrastriatal (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid injection and the possible involvement of striatal dopamine neurons in the mediation of this effect. Rats were anesthetized with the short-acting agent halothane to allow for rapid surgical recovery and thus early behavioral measurements. Intrastriatal (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 mumol/2 microliters) produced an incremental increase in contralateral turning starting at 1 h and plateauing 3-6 h after injection (peak effect, 39.1 +/- 6.7 rotations per 5 min). Dopamine depletion with alpha-methyl-DL-p-tyrosine (250 mg/kg i.p., 80% depletion) resulted in greater than 85% inhibition of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-induced contralateral turning. The dopamine antagonist haloperidol (0.3 mg/kg i.p.) produced 48% inhibition of the (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid response. In time course studies, turning behavior correlated with increases in levels of the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid. These results suggest a functional interaction between the metabotropic EAA receptor and the dopaminergic system in the striatum.