title: Capacitance measurements in the mouse rod bipolar cell identify a pool of releasable synaptic vesicles.
abstract: 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 nonmammalian 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 Ca(2+) 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 (200-1,600 Hz) on the measured capacitance increase. The magnitude of the capacitance response increased with Ca(2+) entry until a plateau was reached at a spatially averaged intraterminal calcium of about 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.
authors.affiliation: Department of Neurobiology and Anatomy, MSB 7.046, University of Texas Medical School at Houston, 6431 Fannin Street, Houston, TX 77025, USA.
authors.forename: Zhen-Yu
authors.initials: ZY
authors.lastname: Zhou
authors.forename: Qun-Fang
authors.initials: QF
authors.lastname: Wan
authors.forename: Pratima
authors.initials: P
authors.lastname: Thakur
authors.forename: Ruth
authors.initials: R
authors.lastname: Heidelberger
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology; D020013:Calcium Signaling / Q000502:physiology; D025802:Electric Capacitance; D004594:Electrophysiology; D005089:Exocytosis / Q000502:physiology; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D007700:Kinetics; D051379:Mice; D008810:Mice, Inbred C57BL; D051245:Retinal Bipolar Cells / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000166:cytology / Q000502:physiology*; D013572:Synaptic Vesicles / Q000502:physiology*
chemical_list: D002118:Calcium