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 cortex, Ca2+ imaging with a relatively low-affinity indicator was carried out simultaneously with whole-cell recordings of synaptic activity in cortical slices of young rats. Electrical stimulation of layer IV elicited excitatory postsynaptic potentials that generated action potentials concomitantly with a marked rise of Ca2+ signal in apical dendrites of 24 pyramidal cell-like neurons. Dendritic regions about 50 microm from the soma (peak regions) consistently showed the strongest increase in signal. The application of an N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovalerate, reduced the Ca2+ signal in peak regions but did not in other regions in nine of the 15 neurons tested. In all the dendritic regions in which Ca2+ signal was measured, the magnitude of the increase in signal was related linearly to the number of action potentials, suggesting that voltage-dependent Ca2+ channels activated by action potentials may also be involved in the increase. In 25 of 33 neurons which were voltage-clamped at 80 mV and perfused with the Mg2+-free solution, layer IV stimulation could induce a local rise in Ca2+ signal in apical dendrites. This rise was blocked by the N-methyl-D-aspartate receptor antagonist almost completely. Such an antagonist-sensitive Ca2+ rise was still seen even when the membrane potential was held at +10 mV when voltage-dependent Ca2+ channels were expected not to be activated by excitatory synaptic inputs. These results suggest that N-methyl-D-aspartate receptors are involved in the synaptic input-induced rise in Ca2+ at postsynaptic sites in apical dendrites of layer II/III neurons, and the increase may be boosted through voltage-dependent Ca2+ channels activated by action potentials.