EXCITATORY ACTIONS OF THE METABOTROPIC EXCITATORY AMINO-ACID RECEPTOR AGONIST, TRANS-(+/-)-1-AMINO-CYCLOPENTANE-1,3-DICARBOXYLATE (T-ACPD), ON RAT THALAMIC NEURONS INVIVO

The metabotropic excitatory amino acid receptor agonist trans-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate (t-ACPD) was applied to rat ventrobasal thalamic neurons by iontophoresis. This agonist typically evoked an excitatory response which was slower in onset and of longer duration than responses to the other excitatory amino acid agonists, N-methyl-aspartate, kainate or (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. Responses to t-ACPD were resistant to the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid and kynurenate. These results suggest that t-ACPD may exert its effects via the so-called 'metabotropic' excitatory amino acid receptor. The putative antagonists at this receptor, D-2-amino-4-phosphono-butyrate (D-AP4), L-2-amino-4-phosphono-butyrate (L-AP4) and L-2-amino-3-phosphono-propionate (L-AP3), were able to reduce responses to t-ACPD under certain circumstances. However, such antagonism was always accompanied by similar reductions in excitatory responses to other agonists. These non-selective effects would appear to limit the usefulness of AP4 and AP3 as antagonists of t-ACPD.