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Retraction of Spinule‐type Neurites from Carp Retinal Horizontal Cell Dendrites during Dark Adaptation Involves the Activation of Ca2+/Calmodulin‐dependent Protein Kinase II

The formation of spinules at the terminal dendrites of retinal horizontal cells with the onset of light and their subsequent retraction during darkness is a remarkable example of synaptic plasticity where sensory experience modifies reversibly, and on a time scale of minutes the ultrastructure of synaptic connectivity. The signals and the subsequent intracellular cascades underlying the prominent morphological alterations are only partially understood. We show here that lowering the external calcium concentration did prevent dark‐ and AMPA‐induced retraction of spinules in an eyecup preparation. Furthermore, spinule retraction was prevented in vivo by the injection of calmidazolium, an inhibitor of calmodulin, into the eyeball, and also by the injection of KN‐62, an inhibitor of Ca2+/calmodulin‐dependent protein kinase II (CaMkll). We conclude that local Ca2+ influx through AMPA‐gated channels followed by activation of CaMkll is an important step for spinule retraction during dark adaptation. The phosphorylation patterns of phosphoproteins derived from purified horizontal cells was affected by the inhibitors of camodulin and CaMkll respectively. Some of the affected phosphoproteins appeared to be cytoskeleton‐associated proteins, including GAP‐43. Based on these observations, a putative scenario for the retraction of spinules is proposed.