Di. Evans et al., Differential actions of PKA and PKC in the regulation of glutamate releaseby group III mGluRs in the entorhinal cortex, J NEUROPHYS, 85(2), 2001, pp. 571-579
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.