A Lie Detector Test for Presynaptic Capacitance Measurements
چکیده
tance measurements by Mennerick and Matthews (1996), which was interpreted to represent vesicles docked at the plasma membrane. After a delay that depended on the magnitude of the calcium current, the fast component was followed by a second, slower wave of gluta-Capacitance Measurements vesicles tethered to synaptic ribbons (von Gersdorff et al., 1996). The reserve was depleted within a few hun-Measurements of membrane capacitance are a main-dred milliseconds during weak depolarizations that did stay of studies of exocytosis in single cells. However, not strongly activate calcium channels. With strong de-capacitance reflects both addition and retrieval of mem-polarization, the two components merged into a single brane, and an increase in capacitance can be taken as transient burst of glutamate release that decayed to an index of the amount of release only if exocytosis and baseline within 100 ms during sustained depolarization, endocytosis do not overlap in time. Because of this even though presynaptic calcium influx continued un-potential problem, workers in the field have long ap-abated. preciated the need for an independent index of the re-One interesting remaining question is how the results lease of secreted substances, in parallel with capaci-from capacitance measurements—and now from gluta-tance measurements. For cells that release oxidizable mate release—can be reconciled with the very different substances such as norepinephrine, electrochemical conclusions reached from studies of uptake of the fluo-detection of the released molecules is commonly used rescent membrane dye FM1-43 in retinal bipolar cells. for this purpose (e.g., Albillos et al., 1997). For nonoxidiz-On the one hand, capacitance and glutamate release able neurotransmitters such as glutamate, however, measurements demonstrate rapid fusion of a limited some other detection scheme is needed. In a paper pool of vesicles, driven by microdomain levels of calcium published in this issue of Neuron, von Gersdorff and (Heidelberger et al., 1994). On the other hand, uptake colleagues (1998) have met this need, by using gluta-of FM1-43 was interpreted to indicate continuous exo-mate receptors of a " postsynaptic " detector cell to de-cytosis for minutes of a very large population of synaptic termine whether presynaptic capacitance changes tell vesicles, driven by submicromolar calcium (Lagnado et the truth about glutamate release from the terminal. The ex-In a series of elegant and technically demanding experiments of von Gersdorff and colleagues leave little periments, von Gersdorff et al. patch clamped single doubt that capacitance measurements indeed give an glutaminergic synaptic terminals of retinal bipolar neu-accurate indication of …