Developmental changes of the distribution of binding sites for organic Ca2+-channel blockers in cat visual cortex

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Abstract
There are indications that during a critical period of visual cortex development Ca2+-fluxes from extra- to intracellular compartments serve as a trigger signal for experience-dependent changes of neuronal response properties. In this study we investigate the possibility of a relation between the time course of the critical period and age-dependent changes in the density and topographical distribution of Ca2+-channels. As a marker for the latter we used Ca2+-channel blockers of the 1,4-Dihydropyridine (1,4-DHP) class since these are supposed to bind to voltage-dependent Ca2+-channels. We used the tritiated 1,4-DHP derivative 3H-PN 200 110 for autoradiographic determination of 1,4-DHP binding sites in the visual cortex of adult cats and kittens ranging in age from two to ten weeks. The binding of 3H-PN 200 110 to slide-mounted tissue sections was saturable and of high affinity. The overall density of specific 3H-PN 200 110 binding sites decreased during development and their laminar distribution underwent marked changes: in young kittens specific binding was accentuated in lower layer IV, whereas in adult cats the supragranular layers were most intensely labeled. Dark rearing did not affect these developmental changes of 3H-PN 200 110 binding sites. The time course of the reduction of 1,4-DHP binding sites correlates well with that of the age-dependent decrease of the susceptibility to experience-dependent modifications. We consider this result as compatible with the hypothesis that use-dependent modifications of the response properties of cortical neurons involve changes in the Ca2+-fluxes from extra- to intracellular compartments.

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