NMDA AND NON-NMDA RECEPTOR MEDIATED INCREASE OF C-FOS MESSENGER-RNA IN DENTATE GYRUS NEURONS INVOLVES CALCIUM INFLUX VIA DIFFERENT ROUTES

We examined the effects of selective agonists of ionotropic excitatory amino acid (EAA) receptor subtypes on induction of the immediate early gene c-fos. We used in situ hybridization to measure c-fos mRNA and fura-2 imaging to measure intracellular calcium (Ca(i)2+) in individual dentate gyrus neurons maintained in vitro. Activation of either NMDA or non-NMDA receptor subtypes is sufficient to induce the rapid and dramatic increase of c-fos mRNA. Activation of either NMDA or non-NMDA receptors also induces a rapid and dramatic increase of Ca(i)2+, effects blocked by the removal or chelation of extracellular calcium (Ca(e)2+). c-fos mRNA induction by either receptor subtype is Ca2+ dependent, since chelation of Ca(e)2+ with EGTA prevents c-fos mRNA induction by both NMDA and non-NMDA receptor agonists. The increase in Ca(i)2+ induced by activating non-NMDA receptors is inhibited either by removal of extracellular sodium (Na(e)+) or by the voltage-sensitive calcium channel (VSCC) blocker nifedipine. By contrast, the increase of Ca(i)2+ induced by activating NMDA receptors is not inhibited by removal of Na(e)+ or nifedipine. Consistent with these effects on Ca(i)2+, nifedipine inhibits induction of c-fos mRNA by non-NMDA, but not by NMDA, receptor agonists. These findings indicate that Ca2+ serves as a second messenger coupling ionotropic EAA receptors with transcriptional activation of c-fos mRNA. The route of Ca2+ entry into dentate neurons, however, depends on the EAA receptor subtype stimulated. Non-NMDA receptor activation results in Ca2+ influx indirectly via VSCCs, whereas NMDA receptor activation results in Ca2+ influx directly through the NMDA channel itself. Since non-NMDA and NMDA receptors are colocalized at some synapses and can be activated simultaneously by synaptically released glutamate, the induction of c-fos mRNA by each receptor may reflect the activation of identical and therefore redundant programs of gene expression. Alternatively, the varying routes of Ca2+ entry following stimulation of EAA receptor subtypes may activate distinct signaling pathways that culminate in different programs of early gene expression and correspondingly different patterns of target gene expression.