We asked whether GABA(A) and NMDA receptors may act in synergy in neonatal hippocampal slices, at a time when GABA exerts a depolarizing action. The GABA(A) receptor agonist isoguvacine reduced the voltage-dependent Mg2+ block of single NMDA channels recorded in cell-attached configuration from P(2-5) CA3 pyramidal neurons and potentiated the Ca2+ influx through NMDA channels. The synaptic response evoked by electrical stimulation of stratum radiatum was mediated by a synergistic interaction between GABA(A) and NMDA receptors. Network-driven Giant Depolarizing Potentials, which are a typical feature of the neonatal hippocampal network, provided coactivation of GABA(A) and NMDA receptors and were associated with spontaneous and synchronous Ca2+ increases in CA3 pyramidal neurons. Thus, at the early stages of development, GABA is a major excitatory transmitter that acts in synergy with NMDA receptors. This provides in neonatal neurons a hebbian stimulation that may be involved in neuronal plasticity and network formation in the developing hippocampus.

The NMDA subtype of glutamate receptors (NMDAR) at excitatory neuronal synapses plays a key role in synaptic plasticity. The extracellular signal-regulated kinase (ERK1,2 or ERK) pathway is an essential component of NMDAR signal transduction controlling the neuroplasticity underlying memory processes, neuronal development, and refinement of synaptic connections. Here we show that NR2B, but not NR2A or NR1 subunits of the NMDAR, interacts in vivo and in vitro with RasGRF1, a Ca(2+)/calmodulin-dependent Ras-guanine-nucleotide-releasing factor. Specific disruption of this interaction in living neurons abrogates NMDAR-dependent ERK activation. Thus, RasGRF1 serves as NMDAR-dependent regulator of the ERK kinase pathway. The specific association of RasGRF1 with the NR2B subunit and study of ERK activation in neurons with varied content of NR2B suggests that NR2B-containing channels are the dominant activators of the NMDA-dependent ERK pathway.

The synapse contains densely localized and interacting proteins that enable it to adapt to changing inputs. We describe a Ca2+-sensitive protein complex involved in the regulation of AMPA receptor synaptic plasticity. The complex is comprised of MUPPI, a multi-PDZ domain-containing protein; SynGAP, a synaptic GTPase-activating protein; and the Ca2+/calmodulin-dependent kinase CaMKII. In synapses of hippocampal neurons, SynGAP and CaMKII are brought together by direct physical interaction with the PDZ domains of MUPP1, and in this complex, SynGAP is phosphorylated. Ca2+CaM binding to CaMKII dissociates it from the MUPP1 complex, and Ca2+ entering via the NMDAR drives the dephosphorylation of SynGAP. Specific peptide-induced SynGAP dissociation from the MUPP1-CaMKII complex results in SynGAP dephosphorylation accompanied by P38 MAPK inactivation, potentiation of synaptic AMPA responses, and an increase in the number of AMPAR-containing clusters in hippocampal neuron synapses. siRNA-mediated SynGAP knockdown confirmed these results. These data implicate SynGAP in NMDAR- and CaMKII-dependent regulation of AMPAR trafficking.