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A novel electrochemical detection scheme for channel electrophoresis has been developed. A microfabricated array of individually addressed microelectrodes has been used as a site-specific amperometric detector for continuous separations in a buffer-filled rectangular channel. Continuous separations of picomole quantities of dopamine and catechol by channel electrophoresis with this new detection scheme have been demonstrated. Dopamine, an electroactive neurotransmitter, has been continuously monitored for 60 min with the electrochemical array detector. The new detection technique should allow dynamic chemical events in volume-limited microenvironments (i.e. single cells) to be examined with channel electrophoresis.

Characterization of a microfabricated electrochemical array detection scheme used for continuous electrophoretic separations in narrow channels is described. The amperometric detector consists of 100 platinum microelectrodes (95 µm wide, 1.2-2 mm long, 0.12 µm high, each spaced by 5 µm) and has been previously used in channel structures with internal heights as small as ∼21 µm. Here, the ability to differentiate both mass and concentration changes of dopamine is demonstrated in 8-µm-internal height channel structures with electrochemical detection. Characterization of the array detector to provide insight into the nonuniform sensitivity observed with the microfabricated electrophoresis-electrochemical array detection technique is detailed. Approaches to circumvent the nonuniform sensitivity of the microelectrode array are described. Finally, we push the limits of the fabrication technology with the construction and use of submicrometer internal height (∼0.6 µm) rectangular channel structures.

We have discovered a neuronal system that releases neurotransmitter via exocytosis from the cell body. In the large dopamine cell of the pond snail Planorbis corneus, depolarization induces rhythmic release of dopamine from the cell body. When a stimulant is applied extracellularly or intracellularly in situ to the cell body, transient dopamine concentration packets that appear in a bursting pattern are observed. Dopamine release is calcium dependent and release is on the time scale expected for exocytosis (2 to 4 msec rise times). Quantitation of individual events reveals an average of 818,000 molecules per exocytotic event. As many as 89,000 individual exocytotic events have been observed following a single stimulation of one cell. Neurotransmitter exocytosis from the neuronal cell body appears to represent an alternative form of neurocommunication to synaptic transmission.

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