Characterization of metabotropic glutamate receptors negatively linked to adenylyl cyclase in brain slices

We have characterized the pharmacological profile of activation of metabotropic glutamate receptors negatively linked to adenylyl cyclase (mGluR _↓cAMP) in brain slices. Among the putative mGluR agonists, (2_S,1′_R,2′_R,3′_R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and (1_S,3_R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), were the most potent inhibitors of forskolin-stimulated cAMP formation in hippocampal slices, followed by ibotenate, l-2-amino-3-phosphonopropionate (AP3), quisqualate, l-glutamate and β-N-methylamino-l-alanine (BMAA). Inhibition of forskolin-stimulated cAMP formation by DL-2-amino-4-phosphonobutanoate (AP4) was biphasic, suggesting that the drug interacts with more than one mGluR _↓cAMP subtype. Both l-AP4 and l-serine-O-phosphate (a restricted analogue of AP4) were much more effective in inhibiting forskolin-stimulated cAMP formation than their d-isomers, indicating that interaction of these drugs with the mGluR _↓cAMP is stereoselective. Despite the fact that DCG-IV and ibotenate behave as NMDA receptor agonists, their effect was insensitive to MK-801. The regional pattern of expression of mGluR _↓cAMPs, as estimated by using 1_S,3_R-ACPD as an agonist, did not correlated with the steady-state levels of mGluR2 mRNA. Thus, 1_S,3_R-ACPD inhibited forskolin-stimulated cAMP in slices from hippocampus, cerebral cortex, corpus striatum, olfactory tubercle or hypothalamus, but not in slices from olfactory bulb or cerebellum; in contrast, mGluR2 mRNA levels were high in the olfactory bulb and very low in the corpus striatum. 1_S,3_R-ACPD also inhibited forskolin-stimulated cAMP formation in cortical membranes, excluding the involvement of trans-synaptic mechanisms in the activity of mGluR _↓cAMPs. Finally, 1_S,3_R-ACPD not only failed to reduce, but rather enhanced, norepinephrine or N-ethylcarboxamidoadenosine (NECA)-stimulated cAMP formation in hippocampal slices, indicating the existence of multiple levels of interaction between mGluRs and adenylyl cyclase activity.