Protein kinase C activity and substrate (F1/GAP-43) phosphorylation in developing cat visual cortex.

Protein kinase C (PKC) and substrate proteins such as F1/GAP-43 have been previously implicated in the synaptic plasticity of long-term potentiation (LTP). As a first step in determining whether they participate in the plasticity observed during the critical period of visual cortex development, we have studied cytosol and membrane PKC activity as well as the endogenous phosphorylation of visual cortical proteins in cat cortical areas 17, 18 at postnatal days 1 and 3, weeks 1, 3, 5, 7, 9, 13, 28 and 51, and adult year 5. There was an 8.4 to 10.9 fold increase in cytosolic PKC activity relative to day 1 level during the critical period of synaptic plasticity (weeks 3-13) which then dramatically decreased back to 2.5-fold of day 1 level by week 51. This was near the adult level of cytosolic PKC. Since there was an increase of 1.8- to 2.1-fold in membrane PKC activity during the critical period, this argues against a PKC translocation event and for an increase in enzyme synthesis. Endogenous phosphorylation in the same visual cortex tissue revealed an increase in protein F1 phosphorylation during the critical period. This level of PKC substrate activity was maintained in the adult providing a mechanism for plasticity in adult cat visual cortex.