Fig. 8.

Proposed model of inside-out and outside-out types of resealed vesicles and working hypothesis of presynaptic InsP₃ receptor channel in plasma membranes of nerve terminals. A, Inside-out type of resealed SPM vesicles. B, Outside-out type of resealed SPM vesicles. C, In this model, there is a major calcium channel, a voltage-operated calcium channel (VOC), and a relatively minor calcium channel, InsP₃ receptor, in nerve terminals involved in the Ca²⁺ influx. When agonist (kyotorphin) binds to the presynaptic receptor, G_i1 and PLC are activated. Produced InsP₃ activates the InsP₃receptor located in plasma membranes of nerve terminals, followed by gating of the calcium channel. Organelles in nerve terminals may not play important roles in the InsP₃-evoked Ca²⁺ mobilization.