Developmental changes and ocular dominance plasticity in the visual cortex.

There is a shift in ocular dominance of cells recorded in the visual cortex which occurs after closure of one eye during a critical period lasting from eye opening to puberty. Three criteria distinguish factors that are crucially related to ocular dominance plasticity: 1) the factor should be more concentrated or active at the peak of the critical period; 2) dark rearing, which makes the cortex less plastic early in the critical period and more plastic late in the critical period, should have a similar effect on the factor, and 3) antagonists or inhibitors of the factor should block ocular dominance plasticity. The second criterion can be used to distinguish activity-related factors that may simply increase or decrease with development from factors that are more specifically related to plasticity. Two factors currently fulfill these criteria, namely N-methyl-D-asparate (NMDA) receptors and protein kinase A (PKA). PKA and NMDA receptors are linked through calcium, since calcium influx through the NMDA receptor increases the production of cyclic AMP by calcium-sensitive adenylate cyclase, which in turn activates PKA. PKA is specifically involved, since protein kinase G and protein kinase C antagonists do not inhibit ocular dominance plasticity. However, NMDA agonists and PKA activators by themselves are not known to bring back plasticity. Thus there may be two or more pathways for ocular dominance plasticity acting in parallel with each other: for example, metabotropic glutamate receptors may act in parallel with NMDA receptors to change calcium levels within the cell.