Developmental changes of calcium currents in the visual cortex of the cat.

During a critical period of postnatal development the visual cortex of kittens is susceptible to experience-dependent modifications of neuronal response properties. Evidence is accumulating that these modifications are triggered by a transient neuronal calcium (Ca) influx. To further investigate this issue we measured extracellular Ca concentrations with ion-sensitive microelectrodes and compared the magnitude and the distribution of stimulus-evoked Ca fluxes in slices of the visual cortex of 4- to 5-week-old kittens and of 6-month-old adult cats. Stimulation of the white matter at 15 Hz for 8 s caused transient decreases of the extracellular Ca concentration (delta Cao) in slices of both age groups and in all cortical layers. However, there were developmental changes in the laminar distribution of the delta Cao: in kittens, they were maximal in layer IV whereas in adult cats they were most pronounced in the supragranular layers. The ratios between the amplitudes of delta Cao in layer IV and the supragranular layers were 1.65 +/- 0.26 in kittens and 0.43 +/- 0.2 in adult cats. These changes in laminar distribution resemble the laminar specific decay of neuronal malleability and parallel the developmental redistribution of 1,4-Dihydropyridine-sensitive Ca channels. Because of these correlations we interpret our findings as support for the hypothesis that experience-dependent modifications are triggered by Ca influx.