LAMINAR DIFFERENCE IN TETANUS-INDUCED INCREASE OF INTRACELLULAR CA2-CORTEX OF YOUNG-RATS( IN VISUAL)

Changes in intracellular Ca2+ evoked by electrical stimulation of the white matter were observed by means of microfluorometry with a Ca2+ in dicator, rhod-2, in slice preparations of the visual cortex obtained f rom 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 t ook place in a columnlike manner from layer VI near the stimulation si te to layer II/III of the cortex, although it spread horizontally in l ayer II/III. The magnitude of fluorescence rise was largest in layer I I/III of the cortex. Since N-methyl-D-aspartate (NMDA) receptors are k nown to exist only on neurons, the following results are taken to indi cate that the fluorescent signal is derived mostly from postsynaptic n eurons: Application of NMDA in the presence of tetrodotoxin induced a marked fluorescence increase with the same laminar bias as tetanic sti mulation did, and the fluorescence increase by single-shock stimulatio n in Mg2+-free medium was almost completely blocked by an antagonist f or NMDA receptors. These results support the hypothesis that input-ass ociated entry of Ca2+ into postsynaptic neurons triggers processes for induction of long-term potentiation of synaptic efficacy.