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1 Cell Biology and Anatomy, New York Medical College, Valhalla, New York, United States; Neurobiology and Anatomy, University of Texas Medical School at Houston, Houston, Texas, United States
2 Neurobiology and Anatomy, University of Texas Medical School at Houston, Houston, Texas, United States
* To whom correspondence should be addressed. E-mail: ruth.heidelberger{at}uth.tmc.edu.
The mouse is an important model system for understanding the molecular basis of neuronal signaling and diseases of synaptic communication. However, the best-characterized retinal ribbon-style synapses are those of non-mammalian vertebrates. To remedy this situation, we asked whether it would be feasible to track synaptic vesicle dynamics in the isolated mouse rod bipolar cell using time-resolved capacitance measurements. The results demonstrate that membrane depolarization triggered an increase in membrane capacitance that was Ca2+-dependent and restricted to the synaptic compartment, consistent with exocytosis. The amplitude of the capacitance response recorded from the easily-accessible soma of an intact mouse rod bipolar cell was identical to that recorded directly from the small synaptic terminal, suggesting that in the carefully-selected cohort of cells presented here, axonal resistance was not a significant barrier to current flow. This supposition was supported by the analysis of passive membrane properties and a comparison of membrane capacitance measurements in cells with and without synaptic terminals and reinforced by the lack of an effect of sine-wave frequency (200-1600 Hz) on the measured capacitance increase. The magnitude of the capacitance response increased with Ca2+ entry until a plateau was reached at a spatially-averaged intraterminal calcium of approximately 600 nM. We interpret this plateau, nominally 30 fF, as corresponding to a releasable pool of synaptic vesicles. The robustness of this measure suggests that capacitance measurements may be used in the mouse rod bipolar cell to compare pool size across treatment conditions.
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