ATP is required at an early step in compensatory endocytosis in synaptic terminals

Whole-terminal capacitance measurements were used to examine membrane retrieval that follows Ca2+-triggered exocytosis in single synaptic terminals. Exocytosis was followed by endocytosis only when the internal solution contained a hydrolyzable analog of ATP. ATP-gamma -S, a poorly hydrolyzable ATP analog, did not support endocytosis but instead produced a rapid and profound inhibition of membrane retrieval. Under similar conditions, the GTP analogs GTP-gamma -S and GDP-beta -S failed to block endocytosis, suggesting that ATP is the preferred substrate. Furthermore, the requirement for ATP was independent of the role of ATP in regulating intraterminal Ca2+, and the role of Ca2+ in endocytosis was different from that of ATP. The results suggest a direct, acute requirement for ATP hydrolysis in compensatory fast endocytosis in synaptic terminals. Given that the capacitance technique detects changes in membrane surface area, ATP must be required for the membrane fission step or at a step that is a prerequisite for membrane fission.