A SWITCHING ROLE OF POSTSYNAPTIC CALCIUM IN THE INDUCTION OF LONG-TERM POTENTIATION OR LONG-TERM DEPRESSION IN VISUAL-CORTEX

Long-term potentiation (LTP) and depression. (LTD) are considered to b e an initial step in processes governing experience-dependent changes in neuronal function in cerebral neocortex. As a mechanism for the ind uction of LTP and LTD, it is hypothesized that an input-associated ris e of Ca2+ beyond a certain threshold at postsynaptic sites leads to LT P while a lower rise below the threshold leads to LTD. To test this Ca 2+-switching hypothesis, the method of microscopic fluorometry with Ca 2+ indicators such as fura-2 has been employed. In this review, proble ms with this fura-2 method are described, and results obtained with ot her indicators having weaker Ca2+-chelating action are mentioned brief ly. Experimental results indicating the involvement of Ca2+/calmodulin -dependent protein kinase II (CaMKII) and protein phosphatase (calcine urin) are also reviews, and a model that includes the spatiotemporal d ynamics of Ca2+ and the intracellular location of both enzymes as vari ables ils proposed as a modification of the Ca2+-switching hypothesis.