Two mechanisms of synaptic vesicle recycling in rat brain nerve terminals

KCl and 4-aminopyridine (4-AP) evoke glutamate release from rat brain cortical nerve terminals by voltage clamping or by Na(+) channel-generated repetitive action potentials, respectively. Stimulation by 4-AP but not KCl is largely mediated by protein kinase C (PKC). To determine whether KCl and 4-AP utilise the same mechanism to release glutamate, we correlated glutamate release with release of the hydrophobic synaptic vesicle (SV) marker FM2-10. A strong correlation was observed for increasing concentrations of KCl and after application of phorbol 12-myristate 13-acetate (PMA) or staurosporine. The parallel increase in exocytosis measured by two approaches suggested it occurred by a PKC-independent mechanism involving complete fusion of SVs with the plasma membrane. At low concentrations of 4-AP, alone or with staurosporine, glutamate and FM2-10 release also correlated. However, higher concentrations of 4-AP or of 4-AP plus PMA greatly increased glutamate release but did not further increase FM2-10 release. This divergence suggests that 4-AP recruits an additional mechanism of release during strong stimulation that is PKC dependent and is superimposed upon the first mechanism. This second mechanism is characteristic of kiss-and-run, which is not detectable by styryl dyes. Our data suggest that glutamate release in nerve terminals occurs via two mechanisms: (1) complete SV fusion, which is PKC independent; and (2) a kiss-and-run-like mechanism, which is PKC dependent. Recruitment of a second release mechanism may be a widespread means to facilitate neurotransmitter release in central neurons.