(+)-MCPG induces PKCε translocation in cortical synaptosomes through a PLD-coupled mGluR

We have tested whether different agonists of metabotropic glutamate receptors could induce translocation of selective protein kinase C isozymes in nerve terminals. In rat cortical synaptosomes 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 100 μM) induced an increase in translocation to 124.6 ± 5.7% of basal unstimulated conditions of the Ca⁺⁺-independent protein kinase Cε, but not of the Ca⁺⁺-dependent isozyme β. This effect was counteracted by 1-aminoindan-1,5-dicarboxylic acid (100 μM), an antagonist of metabotropic glutamate receptor 1. On the other hand, (+)-α-methyl-4-carboxyphenylglycine [(+)-MCPG], an antagonist of metabotropic glutamate receptors group I and II, did not antagonize the effect of 1S,3R-ACPD, and per se induced a translocation of protein kinase Cε of 164 ± 17.7% of basal unstimulated conditions. Because the (+)-MCPG induction of protein kinase Cε translocation was not antagonized by 1-aminoindan-1,5-dicarboxylic acid, it is suggested that 1S,3R-ACPD and (+)-MCPG activate this signal transduction pathway through distinct membrane receptors. Indeed (2-[2″-carboxy-3'-phenylcyclopropyl]glycine)-13 (300 nM), a new compound known to antagonize metabotropic glutamate receptors coupled to phospholipase D, was able to antagonize protein kinase Cε translocation induced by (+)-MCPG. Moreover (+)-MCPG directly induced phospholipase D activity, measured as [³H]phosphoethanol production in cortical synaptosomes. These data suggest that in cortical nerve terminals (i) distinct metabotropic glutamate receptors, coupled to different signal transduction pathways, are present, (ii) (+)-MCPG is able to induce protein kinase Cε translocation, and that (iii) a metabotropic glutamate receptor associated to phospholipase D might influence translocation of protein kinase C in a calcium-independent manner.