Synaptic vesicle dynamics in rat fast and slow motor nerve terminals

We have investigated whether rat motor nerve terminals with different in vi vo activity patterns also have different vesicle trafficking characteristic s. To do this, we monitored, using combined optical and electrical techniqu es, the rate of exocytosis (during different frequencies and patterns of ac tivity), the releasable pool size, and the recycle time of synaptic vesicle s in terminals on soleus (slow-twitch) and extensor digitorum longus [(EDL) ; fast-twitch] muscle fibers. EDL terminals had a higher initial quantal co ntent (QC) than soleus, but during tonic or phasic stimulation at 20-80 Hz, EDL QC ran down to a greater extent than soleus QC. By recording loss of f luorescence from exocytosing vesicles labeled with the dye FM1-43, EDL term inals were found to destain faster than those in soleus, Simultaneous intra cellular recording of end plate potentials, to count the number of vesicles released, permitted estimation of the total vesicle pool (VP) size and the recycle time by combining the optical and electrophysiological data. Soleu s vesicle pool was larger than EDL, but recycle time was not significantly different. These terminals, therefore, are adapted to their in vivo activit y patterns by alterations in QC and VP size but not recycle time.