Long-term depression induced by postsynaptic group II metabotropic glutamate receptors linked to phospholipase C and intracellular calcium rises in rat prefrontal cortex.

We have previously shown (Otani et al., 1999b) that bath application of (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV), the agonist of group II metabotropic glutamate receptors (mGluRs), induces postsynaptic Ca2+-dependent long-term depression (LTD) of layer I-II to layer V pyramidal neuron glutamatergic synapses of rat medial prefrontal cortex. In the present study, we examined detailed mechanisms of this DCG IV-induced LTD. First, the group II mGluR antagonist (RS)-alpha-methylserine-O-phosphate monophenyl ester blocked DCG IV-induced LTD, and another group II agonist (2S,3S,4S)-CCG/(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine-induced LTD, suggesting that LTD is indeed mediated by the activation of group II mGluRs. Second, DCG IV-induced LTD was blocked by the NMDA receptor antagonist AP-5, whereas DCG IV did not potentiate NMDA receptor-mediated synaptic responses. Interruption of single test stimuli during DCG IV application blocked DCG IV-induced LTD. These results suggest that small NMDA receptor-mediated responses evoked by single synaptic stimuli contribute to DCG IV-induced LTD. Third, DCG IV-induced LTD was blocked or reduced by the following drugs: phospholipase C inhibitor U-73122 (bath-applied or postsynaptically injected), postsynaptically injected IP3 receptor blocker heparin, phospholipase D-linked mGluR blocker PCCG-13, PKC inhibitor RO318220, postsynaptically injected PKC inhibitor PKC(19-36), and PKA inhibitor KT-5720. Fourth, fluorescent Ca2+ analysis techniques revealed that DCG IV increases Ca2+ concentration in prefrontal layer V pyramidal neurons. These Ca2+ rises and the LTD were both blocked by postsynaptic heparin in the same cells. Taken together, these results suggest that postsynaptic group II mGluRs, linked to phospholipase C and probably also phospholipase D, induce LTD through postsynaptic PKC activation and IP3 receptor-mediated postsynaptic increases of Ca2+ concentration.