LOCALIZED CONTRIBUTION OF N-METHYL-D-ASPARTATE RECEPTORS TO SYNAPTIC INPUT-INDUCED RISE OF CALCIUM IN APICAL DENDRITES OF LAYER II III NEURONS IN RAT VISUAL-CORTEX/

To examine the role of N-methyl-D-aspartate receptors in the synaptic input-induced increase in Ca2+ in layer II/III neurons of visual corte x, Ca2+ imaging with a relatively low-affinity indicator was carried o ut simultaneously with whole-cell recordings of synaptic activity in c ortical slices of young rats. Electrical stimulation of layer IV elici ted excitatory postsynaptic potentials that generated action potential s concomitantly with a marked rise of Ca2+ signal in apical dendrites of 24 pyramidal cell-like neurons. Dendritic regions about 50 mu m fro m the soma (peak regions) consistently showed the strongest increase i n signal. The application of an N-methyl-D-aspartate receptor antagoni st, DL-2-amino-5-phosphonovalerate, reduced the Ca2+ signal in peak re gions but did not in other regions in nine of the 15 neurons tested. I n all the dendritic regions in which Ca2+ signal was measured, the mag nitude of the increase in signal was related linearly to the number of action potentials, suggesting that voltage-dependent Ca2+ channels ac tivated by action potentials may also be involved in the increase. In 25 of 33 neurons which were voltage-clamped at -80 mV and perfused wit h the Mg2+-free solution, layer TV stimulation could induce a local ri se in Ca2+ signal in apical dendrites. This rise was blocked by the N- methyl-D-aspartate receptor antagonist almost completely. Such an anta gonist-sensitive Ca2+ rise was still seen even when the membrane poten tial was held at +10 mV when voltage-dependent Ca2+ channels were expe cted not to be activated by excitatory synaptic inputs. These results suggest that N-methyl-D-aspartate receptors are involved in the synapt ic input-induced rise in Ca2+ at postsynaptic sites in apical dendrite s of layer II/III neurons, and the increase may be boosted through vol tage-dependent Ca2+ channels activated by action potentials. (C) 1998 IBRO. Published by Elsevier Science Ltd.