Analysis of induction mechanisms of intrinsic optical signals by simultaneous measurement of extracellular K~ concentration and space volume.

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It is known that neuronal excitation causes cell swelling (or a decrease in extracellular space volume)in the central nervous system.It is thought that cell swelling underlies changes in light scattering, called inirinsic optical signals(lOSs), recorded from slices not stained with dyes.Others and we have reported that furosemide-sensitive cation-chloride cotransporters play an important role in the induction of lOSs.There are two types of cotransporters, Na-K-2C1 and K-Cl cotransporters, and the latter has 4 isoforms (KCC1〜4).We have set up a new reaJ-time optical imaging system using a cooled CCD camera to measure lOSs.We have recorded lOSs induced by changes in externai K~ concentration. We have obtained the following results.1.lOSs could be induced by small (<2mM) changes in K^+ concentration, and the amplitude of the responses was dependent on the amount of K^+ concentration changes.Lowering of K^+ concentration caused reversed responses.In addition, K^+-induced lOSs were not inhibited by addition of TTX.2.K^+-induced lOSs were suppressed under the conditions (application of furosemide or in low CL solution) to inhibit the cotransporters.3.K^+-induced lOSs were not much affected in low Na^+ solution.4.K^+-induced lOSs were little inhibited by blockers of the inwardly-rectifying K^+ channels.These results indicate that in the dorsal horn of the spinal cord the K-Cl cotransporters are mainly involved in the induction of lOSs and that KCC2, which expresses specifically in neurons, may be the likely candidate for the responses.