Differential actions of PKA and PKC in the regulation of glutamate releaseby group III mGluRs in the entorhinal cortex

In a previous study we showed that activation of a presynaptically located metabotropic glutamate receptor (mGluR) with pharmacological properties of mGluR4a causes a facilitation of glutamate release in layer V of the rat en torhinal cortex (EC) in vitro. In the present study we have begun to invest igate the intracellular coupling linking the receptor to transmitter releas e. We recorded spontaneous alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropi onic acid receptor-mediated excitatory postsynaptic currents (EPSCs) in the whole cell configuration of the patch-clamp technique, from visually ident ified neurons in layer V. Bath application of the protein kinase A (PKA) ac tivator, forskolin, resulted in a marked facilitation of EPSC frequency, si milar to that seen with the mGluR4a specific agonist, ACPT-1. Preincubation of slices with the PKA inhibitor H-89 abolished the effect of ACPT-1, as d id preincubation with the adenylate cyclase inhibitor, SQ22536. Activation of protein kinase C (PKC) using phorbol 12 myristate 13-acetate (PMA) did n ot affect sEPSC frequency; however, it did abolish the facilitatory effect of ACPT-1 on glutamate release. A robust enhancement of EPSC frequency was seen in response to bath application of the specific PKC inhibitor, GF 1092 03X. Both H-89 and the group III mGluR antagonist (RS)-alpha -cyclopropyl-4 -phosphonophenylglycine (CPPG) abolished the effects of GF 109203X. These d ata suggest that in layer V of the EC, presynaptic group III mGluRs facilit ate release via a positive coupling to adenylate cyclase and subsequent act ivation of PKA. We have also demonstrated that the PKC system tonically dep resses transmitter release onto layer V cells of the EC and that an interac tion between mGluR4a, PKA, and PKC may exist at these synapses.