Rapid calcium-dependent exocytosis underlies neurotransmitter release from nerve terminals. Despite the fundamental importance of this process, neither the relationship between presynaptic intracellular calcium ion concentration ([Ca2+]i) and rate of exocytosis, nor the maximal rate of secretion is known quantitatively. To provide this information, we have used flash photolysis of caged Ca2+ to elevate [Ca2+]i rapidly and uniformly in synaptic terminals, while measuring membrane capacitance as an index of exocytosis and monitoring [Ca2+]i with a Ca(2+)-indicator dye. When [Ca2+]i was abruptly increased to > 10 microM, capacitance rose at a rate that increased steeply with [Ca2+]i. The steepness suggested that at least four calcium ions must bind to activate synaptic vesicle fusion. Half-saturation was at 194 microM, and the maximal rate constant was 2,000-3,000 s-1. A given synaptic vesicle can exocytose with high probability within a few hundred microseconds, if [Ca2+]i rises above 100 microM. These properties provide for the extremely rapid signalling required for neuronal communication.

SUMMARY1. Physiological noise in the visual transduction mechanism was studied by recording membrane current from single rod outer segments in pieces of isolated toad retina.2. The inward current in darkness showed spontaneous fluctuations which disappeared during the response to bright light.3. The dark noise consisted of two components, a continuous fluctuation of rms amplitude about 0X2 pA and occasional discrete events about 1 pA in size.4. Intervals between discrete events followed the exponential distribution expected of a Poisson process with a mean rate of about one event per 50 see (20 'C).5. The amplitude and power spectrum of the discrete events resembled those of single photon effects in the same rod, suggesting that discrete events may arise from spontaneous activation of single rhodopsin molecules.6. The temperature dependence of the mean frequency of occurrence of discrete events gave an activation energy of 22 kcal mole-', probably characteristic of thermal isomerization of rhodopsin.7. The variance of the continuous component of the dark noise rose linearly with the length of the outer segment drawn into the suction electrode, indicating that this component is generated in the outer segment.8. The power spectrum of a rod's continuous noise was usually fitted by the square of a Lorentzian with the same time constant as that of the four first-order delays in the cell's single photon response. The shot effects composing the continuous component thus appear to be shaped by two of four sequential processes in transduction.9. The variance and spectrum of the continuous noise are interpreted to reflect shot effects about 1/400 the size of a single photon effect occurring at a frequency of 6 x 103 sec-'. 10. A rod's flash sensitivity was halved by a steady light I. giving about 8 photoisomerizations sec. The much lower mean rate of discrete events indicates that I. in increment sensitivity experiments on individual receptors is not set by thermal activation of rhodopsin.11. Values of sensitivity and time-to-peak flash response collected from many cells

Voltage-dependent sodium channels are uniformly distributed along unmyelinated axons, but are highly concentrated at nodes of Ranvier in myelinated axons. Here, we show that this pattern is associated with differential localization of distinct sodium channel alpha subunits to the unmyelinated and myelinated zones of the same retinal ganglion cell axons. In adult axons, Na(v)1.2 is localized to the unmyelinated zone, whereas Na(v)1.6 is specifically targeted to nodes. During development, Na(v)1.2 is expressed first and becomes clustered at immature nodes of Ranvier, but as myelination proceeds, Na(v)1.6 replaces Na(v)1.2 at nodes. In Shiverer mice, which lack compact myelin, Na(v)1.2 is found throughout adult axons, whereas little Na(v)1.6 is detected. Together, these data show that sodium channel isoforms are differentially targeted to distinct domains of the same axon in a process associated with formation of compact myelin.

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