Quantitative Chemical Measurements of Vesicular Transmitters with Electrochemical Cytometry

Electrochemical cytometry adds a new dimension to our ability to study the chemistry and chemical storage of transmitter molecules stored in nanometer vesicles. The approach involves the adsorption and subsequent rupture of vesicles on an electrode surface during which the electroactive contents are quantitatively oxidized (or reduced). The measured current allows us to count the number of molecules in the vesicles using Faraday's law and to correlate this to the amount of molecules released when single exocytosis events take place at communicating cells. The original format for this method involved a capillary electrophoresis separation step to singly address each vesicle, but we have more recently discovered that cellular vesicles tend to adsorb to carbon electrodes and spontaneously as well as stochastically rupture to give mostly single vesicle events. This approach, called impact electrochemical cytometry, even though the impact is perhaps not the important part of this process, has been studied and the vesicle rupture appears to be at the interface between the vesicle and the electrode and is probably driven by electroporation. The pore size and rate of content electrolysis are a function of the pore diameter and the presence of a protein core in the vesicles. In model liposomes with no protein, events appear extremely rapidly as the soft nanoparticles impact the electrode and the contents are oxidized. It appears that the proteins decorating the surface of the vesicle are important in maintaining a gap from the electrode and when this gap is closed electroporation takes place. Models of the event response times suggest the pores formed are small enough so we can carry out these measurements at nanotip electrodes and we have used this to quantify the vesicle content in living cells in a mode we call intracellular impact electrochemical cytometry. The development of electrochemical cytometry allows comparison between vesicle content and vesicular release and we have found that only part of the vesicle content is released in typical exocytotic cases measured by amperometry. This has led to the novel hypothesis that most exocytosis from dense core vesicles is via mechanism where vesicles fuse with the cell membrane, some content is released and then close again to be reloaded and reused. It leaves open the possibility that cells regulate release during individual events. This might be important in learning and memory and be a nonreceptor pharmaceutical target for brain-related disorders. Indeed, the concept of the chemo-brain observed in cisplatin-treated cancer patients appears to be at least in part the result of changing the fraction of transmitter released and we have been able to show this by using the combined amperometric measurement of release and electrochemical cytometry at model cells.