Ontogeny of N-methyl-D-aspartate-induced current in cultured hippocampal neurons
Ujihara, H.; Albuquerque, E.X.
Journal of Pharmacology and Experimental Therapeutics 263(2): 859-867
1992
ISSN/ISBN: 0022-3565 PMID: 1432702 Document Number: 389982
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 mu-M) in the presence of glycine (1-100 mu-M) 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 EC-50 values for glycine were 1.8 and 0.3 mu-M 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 hippocamapal 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 Zn-2+ gave further evidence of a change in the NMDA receptor/channel properties related to maturation of the cultured neurons. Zn-2+ had open channel blocking and other negative allosteric effects that were also maturation-dependent. Thus, these findings led us to suggest that the characterization of ontogenetic alteration in the NMDA receptor channel may be critical for a better understanding of changes in physiological processes that occur with aging in the central nervous system.