Localization of ATPase activity in dendritic spines of the cerebral cortex

Abstract

An ATPase activity has been demonstrated in dendritic spines of the adult rat cerebral cortex using cerium to capture inorganic phosphate that is liberated during the enzymatic hydrolysis of ATP. Small pieces of cerebral cortex were fixed and incubated in a standard incubation medium containing both Ca2+ and Mg2+ at pH 7.2; other modifications of the incubation medium are described below. Electron microscopic examination of the cerium phosphate reaction product showed an electron dense precipitate localized in the cytoplasm of the spine behind the postsynaptic density. Whereas the postsynaptic density, itself, is not reactive, dense reaction product is seen immediately underneath the postsynaptic density and extending into the subsynaptic web. Reaction product is also associated with membranous cisternae within the dendritic spine. The reaction occurred in the presence of Ca2+ and Mg2+ and either of these two ions alone. However, virtually no reaction product is seen when the tissue was incubated in a medium devoid of Ca2+ and Mg2+, or in a medium containing Mg2+ and EGTA, suggesting that trace Ca2+ is necessary, but not sufficient for the reaction. Addition of p-chloromercurobenzoate, which selectively blocks SH groups, inhibited the reaction in the presence of Ca2+ and Mg2+, or both of these ions. The effect of pH on the reaction was determined using a lead precipitation method. The reaction occurred at pH 9.2 in the presence of Ca2+ alone. In the presence of Mg2+ alone, the reaction product appeared somewhat reduced at this pH. The presence of an ATPase activity, which is dependent upon Ca2+ in dendritic spines where actin and actin-binding proteins have also been localized, suggests that this activity may be involved in the dynamics of cytoskeletal function leading to shape changes in dendritic spines and synapses, as seen with various physiological and behavioral paradigms.