Pharmacological properties of metabotropic glutamate receptors were studied in Xenopus oocytes injected with rat brain mRNA. trans-1-Amino-cyclopentyl-1,3-dicarboxylic acid (t-ACPD), a conformationally restricted analog of glutamate, induced oscillatory inward currents in mRNA-injected oocytes. These t-ACPD responses showed several characteristics identical to those of the other metabotropic responses including the metabotropic glutamate responses stimulated by quisqualate. d,l-2-Amino-3-phosphonopropionate (d,l-AP3) effectively suppressed the t-ACPD and ibotenate responses. However, quisqualate responses were not affected substantially by d,l-AP3. These findings suggest that the metabotropic glutamate receptors in the oocytes may be classified into at least two subtypes according to their pharmacological properties: one preferentially activated by quisqualate and insensitive to AP3, and the other activated by t-ACPD and ibotenate and antagonized by AP3.
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