(S)-4-carboxy-3-hydroxyphenylglycine activates phosphatidyl inositol linked metabotropic glutamate receptors in different brain regions of the neonatal rat

Abstract

In the present investigation, effects of several agonists and antagonists of metabotropic glutamate receptors (mGluRs) which are coupled to phosphatidyl inositol (PI) hydrolysis were evaluated in slices of neonatal rat hippocampus, striatum, cortex and cerebellum. The rank order of potency of agonists in the PI hydrolysis assay was identical in all brain regions: quisqualic acid (Quis) > (RS)-3,5-dihydroxyphenylglycine (3,5-DHPG) > 1S,3R-aminocyclopentane dicarboxylic acid (1S,3R-ACPD) > > l-glutamate (Glu). All agonists were equiefficacious in the four brain regions tested.

The responses to 3,5-DHPG, a highly selective Class I mGluR agonist, were attenuated by (S)-4-carboxyphenylglycine ((S)-4CPG), (+)-α-methyl-4-carboxyphenylglycine ((+)-MCPG) and 1-aminoindan-1,5-dicarboxylic acid (UPF-523) with a rank order of potency of (+)-MCPG ≥ (S)-4CPG ≥ UPF-523 in the different brain regions. These results suggest little selectivity among these putative mGluR antagonists in the different brain regions studied.

Interestingly, (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG), a compound reported to act as an antagonist at Class I mGluRs, produced concentration-dependent increases in PI hydrolysis in all four brain regions suggesting that (S)-4C3HPG acts as an agonist. In striatum, hippocampus and cortex, (S)-4C3HPG was equiefficacious to Quis, 3,5-DHPG, 1S,3R-ACPD and Glu. However, in the cerebellum, (S)-4C3HPG displayed weak agonist activity (37% of that of a maximally effective concentration of Quis). The effects of (S)-4C3HPG in the PI hydrolysis assay appeared to be mediated by the activation of an mGluR subtype since it was significantly blocked by (S)-4CPG, an mGluR antagonist. In addition, the agonistic effects of (S)-4C3HPG appear to be unrelated to inhibition of [³H]-Glu uptake into rat hippocampal or cerebellar synaptosomes. These results demonstrate a unique pharmacological profile of (S)-4C3HPG which can be interpreted as (S)-4C3HPG being a highly selective mGluR5 agonist or alternatively, that the effects of (S)-4C3HPG may be mediated through a novel Class I mGluR subtype(s), yet to be identified.