Laminar difference in tetanus-induced increase of intracellular Ca2+ in visual cortex of young rats

Changes in intracellular Ca2+ evoked by electrical stimulation of the white matter were observed by means of microfluorometry with a Ca2+ indicator, rhod-2, in slice preparations of the visual cortex obtained from young rats. Tetanic stimulation at 5 Hz for 1 min induced a marked fluorescence increase, while single-shock stimulation did not induce a sizable increase in normal perfusate. The tetanus-induced increase took place in a column-like manner from layer VI near the stimulation site to layer II/III of the cortex, although it spread horizontally in layer II/III. The magnitude of fluorescence rise was largest in layer II/III of the cortex. Since N-methyl-D-aspartate (NMDA) receptors are known to exist only on neurons, the following results are taken to indicate that the fluorescent signal is derived mostly from postsynaptic neurons: Application of NMDA in the presence of tetrodotoxin induced a marked fluorescence increase with the same laminar bias as tetanic stimulation did, and the fluorescence increase by single-shock stimulation in Mg(2+)-free medium was almost completely blocked by an antagonist for NMDA receptors. These results support the hypothesis that input-associated entry of Ca2+ into postsynaptic neurons triggers processes for induction of long-term potentiation of synaptic efficacy.