Pharmacology of metabotropic glutamate receptor inhibition of cyclic AMP formation in the adult rat hippocampus.

Phosphoinositide-linked metabotropic glutamate receptors have been well characterized in a variety of CNS tissues. In this study the pharmacology of metabotropic glutamate receptors negatively coupled to cAMP formation was investigated in cross-chopped slices of the adult rat hippocampus. Excitatory amino acid agonists and antagonists were examined for effects on forskolin (30 microM)-simulated cAMP formation. The selective metabotropic glutamate agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and various nonselective metabotropic/ionotropic agonists were found to inhibit forskolin-stimulated cAMP formation. Inhibition of cAMP formation was observed using 1S,3R-ACPD (57% at 100 microM), quisqualate (92% at 500 microM), ibotenate (44% at 500 microM), L-glutamate (41% at 1000 microM), and L-aspartate (59% at 1000 microM). Inhibition of forskolin-stimulated cAMP formation induced by these agonists was observed even in the presence of the ionotropic antagonists MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione. Up to 500 microM of the ionotropic agonists N-methyl-D-aspartate, AMPA, and kainate did not inhibit forskolin-stimulated cAMP formation. L-2-amino-3-phosphonopropionate (500 microM) greatly inhibited the stimulatory effect of 1S,3R-ACPD on phosphoinositide hydrolysis, even in the presence of forskolin. However when measuring cAMP formation, L-2-amino-3-phosphonoproprionate (500 microM) mimicked the effect of 1S,3R-ACPD, producing 64% inhibition of forskolin-stimulated cAMP. These studies show that in the adult rat hippocampus metabotropic glutamate receptors that are negatively linked to cAMP formation have a pharmacology that is distinct from ionotropic glutamate receptors and phosphoinositide-linked metabotropic glutamate receptors.