Coupling of metabotropic glutamate receptors to phosphoinositide mobilisation and inhibition of cyclic AMP generation in the guinea-pig cerebellum

Abstract

1. The effects of metabotropic glutamate receptor (mGluR) agonists on cyclic nucleotide and phosphoinositide turnover were investigated in adult guinea-pig cerebellar slices by use of radioactive precursors. 2. L-Glutamate, 1-aminocyclopentane-1S,3R-dicarboxylate (1S,3R-ACPD) and RS-3,5-dihydroxyphenylglycine (DHPG) evoked concentration-dependent increases in the accumulation of [3H]-inositol phosphates with pEC50 values of 2.98 ± 0.02, 4.45 ± 0.06 and 4.47 ± 0.07, respectively. Maximal responses to these agents were 43 ± 8, 52 ± 12 and 84 ± 11% of the response to 1 mM histamine, respectively. 3. The phosphoinositide response to 1S,3R-ACPD was antagonized in the presence of (+)-α-methyl-4-carboxyphenylglycine, with a calculated pK(i) value of 3.55 ± 0.03. 4. Forskolin-stimulated accumulation of [3H]-cyclic AMP was not significantly altered in the presence of 10 μM DCG-IV or 300 μM 1S,3R-ACPD. Similarly, 300 μM 1S,3R-ACPD failed to alter isoprenaline (1 μM) or 2-chloroadenosine (2-CA, 30 μM)-stimulated accumulation of [3H]-cyclic AMP. 5. Forskolin-stimulated accumulation of [3H]-cyclic AMP was concentration-dependently inhibited in the presence of L-glutamate and L-serine-O-phosphate (L-SOP) with pIC50 values of 2.91 ± 0.17 and 2.86 ± 0.04 with maximal inhibitions of 47 ± 2 and 92 ± 3%, respectively. L-2-Amino-4-phosphonobutyrate (L-AP4) inhibited the forskolin response without saturating, evoking an inhibition of 71 ± 7% at 3 mM. 6. 2-CA-evoked accumulation of [3H]-cyclic AMP was also inhibited by L-glutamate and L-SOP with pIC50 values of 2.71 ± 0.03 and 2.72 ± 0.08 and maximal inhibitions of 51 ± 5 and 99 ± 0%, respectively. L-AP4 inhibited the 2-CA response without saturating, evoking an inhibition of 68 ± 1% at 3 mM. 7. Isoprenaline-evoked accumulation of [3H]-cyclic AMP was inhibited by L-glutamate and L-SOP with pIC50 values of 3.21 ± 0.01 and 2.96 ± 0.08 and maximal inhibitions of 88 ± 2 and 93 ± 3%, respectively. 8. These results suggest that the guinea-pig cerebellum expresses Group I and Group III mGluRs coupled to phosphoinositide turnover and inhibition of cyclic AMP generation, respectively.