Adenosine is a neuroprotective agent that inhibits neuronal activity and modulates neurotransmission. Previous research has shown adenosine gradually accumulates during pathologies such as stroke and regulates neurotransmission on the minute-to-hour time scale. Our lab developed a method using carbon-fiber microelectrodes to directly measure adenosine changes on a sub-second time scale with fast-scan cyclic voltammetry (FSCV). Recently, adenosine release lasting a couple of seconds has been found in murine spinal cord slices. In this study, we characterized spontaneous, transient adenosine release in vivo, in the caudate-putamen and prefrontal cortex of anesthetized rats. The average concentration of adenosine release was 0.17±0.01 µM in the caudate and 0.19±0.01 µM in the prefrontal cortex, although the range was large, from 0.04 to 3.2 µM. The average duration of spontaneous adenosine release was 2.9±0.1 seconds and 2.8±0.1 seconds in the caudate and prefrontal cortex, respectively. The concentration and number of transients detected do not change over a four hour period, suggesting spontaneous events are not caused by electrode implantation. The frequency of adenosine transients was higher in the prefrontal cortex than the caudate-putamen and was modulated by A1 receptors. The A1 antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine, 6 mg/kg i.p.) increased the frequency of spontaneous adenosine release, while the A1 agonist CPA (N6-cyclopentyladenosine, 1 mg/kg i.p.) decreased the frequency. These findings are a paradigm shift for understanding the time course of adenosine signaling, demonstrating that there is a rapid mode of adenosine signaling that could cause transient, local neuromodulation.

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Microelectrodes are typically used for neurotransmitter detection, but nanoelectrodes are not because there is a trade-off between spatial resolution and sensitivity, which is dependent on surface area. Cavity carbon nanopipette electrodes (CNPEs), with tip diameters of a few hundred nanometers, have been developed for nano-scale electrochemistry. Here, we characterize the electrochemical performance of CNPEs with fast-scan cyclic voltammetry (FSCV) for the first time. Dopamine detection is compared at cavity CNPEs, with a depth equivalent to a few radii, and open-tube CNPEs, an essentially infinite geometry. Open-tube CNPEs have very slow temporal response that changes over time as the liquid rises in the pipette. However, the cavity CNPEs have a fast temporal response to a bolus of dopamine that is not different than traditional carbon-fiber microelectrodes. Cavity CNPEs, with a tip diameter of 200-400 nm, have high currents because the small cavity traps and increases the local dopamine concentration. The trapping also leads to a FSCV frequency independent response and the appearance of cyclization peaks that are normally observed only with large concentrations of dopamine. CNPEs have high dopamine selectivity over ascorbic acid (AA) due to the repulsion of AA by the negative electric field at the holding potential and the irreversible redox reaction. In mouse brain slices, cavity CNPEs detected exogenously-applied dopamine, showing they do not clog in tissue. Thus, cavity CNPEs are promising neurochemical sensors that provide spatial resolution on the scale of hundreds of nanometers, useful for small model organisms or locating near specific cells.

Implantable neural microsensors have significantly advanced neuroscience research, but the geometry of most probes is limited by the fabrication methods. Therefore, new methods are needed for batch-manufacturing with high reproducibility. Herein, a novel method is developed using two-photon nanolithography followed by pyrolysis for fabrication of free-standing microelectrodes with a carbon electroactive surface. 3D-printed spherical and conical electrodes were characterized with slow scan cyclic voltammetry (CV). With fast-scan CV, the electrodes showed low dopamine LODs of 11±1 nm (sphere) and 10±2 nm (cone), high sensitivity to multiple neurochemicals, and high reproducibility. Spherical microelectrodes were used to detect dopamine in a brain slice and in vivo, demonstrating they are robust enough for tissue implantation. This work is the first demonstration of 3D-printing of free-standing carbon electrodes; and the method is promising for batch fabrication of customized, implantable neural sensors.

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