Contribution of active zone subpopulation of vesicles to evoked and spontaneous release

Our previous work on Drosophila synapses has suggested that two vesicle pop ulations possessing different recycling pathways, a fast pathway emanating from the active zone and a slower pathway emanating from sites away from th e active zone, exist in the terminal. The difference in recycling time betw een these two pathways has allowed us to create a synapse that possesses th e small, active zone subpopulation without the larger, nonactive zone popul ation. Synapses were depleted using the temperature-sensitive endocytosis m utant, shibire, which reversibly blocks vesicle recycling at the restrictiv e temperature. In the depleted state, both the excitatory junction potentia l (EJP) and spontaneous release are abolished. After shibire-induced deplet ion. the active zone population begins to reform within 30 s at the permiss ive temperature. whereas the nonactive zone population does not begin to re form until similar to 10-15 min later. Evoked release recovered at approxim ately the same time as the active zone population. During the time when the active zone population existed in the terminal without the nonactive zone population, enough transmitter release was available to sustain a normal ev oked response for many minutes at frequencies above those produced during n ormal activity (flight) by this motor neuron. When only the active zone pop ulation existed in the terminal, the frequency of spontaneous release was g reatly attenuated and possessed abnormal release characteristics. Spontaneo us release recovered its predepletion frequency and release characteristics only after the nonactive zone population was reformed.