Ontogeny of N-methyl-D-aspartate-induced current in cultured hippocampal neurons

The ontogeny of the N-methyl-D-aspartate (NMDA) subtype of glutamatergic receptor/ion channel was studied by examining whole cell currents evoked by NMDA in cultured hippocampal neurons 1 to 30 days after plating of cells from 18- to 20-day-gestation rat fetuses. We observed a maturation-dependent increase in conductance, compatible with an increased density of NMDA receptors, which is in agreement with previous binding data. The whole cell currents evoked by NMDA (10-100 microM) in the presence of glycine (1-100 microM) had two components that contributed to the peak amplitude. The first was a rapidly decaying current (fast component) and the second a slowly decaying current (slow component), their ratio depending upon glycine concentration. The EC50 values for glycine were 1.8 and 0.3 microM for the fast and slow components of the current, respectively. The quantitative analysis of these components indicated the existence of two distinct glycine sites, which differ in their affinity for glycine. The fast component originates from the action of glycine at the site with lower affinity. Moreover, the ratio of the fast to the slow component was also dependent on the time lapsed after plating of the fetal hippocampal neurons. The slow component became more predominant and the fast component less predominant along with cell maturation in culture, a phenomenon which reflects a change in the ratio of high- to low-affinity glycine binding sites. In addition, studies on Zn2+ gave further evidence of a change in the NMDA receptor/channel properties related to maturation of the cultured neurons.(ABSTRACT TRUNCATED AT 250 WORDS)