# Tonic and Phase Amoremetric Monitoring of Dopamine Using Microelectrode Arrays in Rat Striatum
## 4 Discussion  
In vivo monitoring of neurotransmitters in the CNS during the last decade with ceramic-based MEAs has primarily focused on the measurement and quantification of non-electroactive chemical species (e.g., glutamate) through enzyme-mediated conversion to an electroactive reporter molecule (i.e., H\({}_{2}\)O\({}_{2}\)) [30; 34; 43; 44; 45]. Undoubtedly, the success of these approaches for the determination of phasic and tonic neurotransmitter levels, can be directly attributed to the multi-site configuration of the MEAs [29; 32]. More recently, Nafion(r)-coated MEAs with m-PD electroplated on select platinum recording sites were used for the measurement of brain nitric oxide [53]. Here, S2 MEAs were successfully utilized to measure phasic and tonic DA levels in the anesthetized rat dSTR--extending the abilities of MEA technology for in vivo monitoring. The confirmation of a hypodopaminergic state in the 6-OHDA lesion model--attenuated DA uptake, impaired evoked release of DA and depleted resting DA levels--observed with the S2 MEAs provided a sound platform and proof-of-concept for further exploring the utility of this approach. Figure 5: Upper plots (**A**–**C**) show the self-referenced dopamine peak amplitude at the 3 depths for subjects 1–3; bar is the average amplitude of the spikes in each region. Lower traces (**D**–**F**) are self-referenced DA concentration versus time plots for the three test subjects following administration of d-amphetamine. Green arrows indicate the time that the d-amphetamine was administered. Note the high signal-to- noise of these signals that are not filtered or averaged. Typical detection limits of these signals, based on a signal-to-noise of 3, typically ranged from 0.3 to 1 nanomolar, rivaling the recent report of [PERSON] et al., 2019 [51]. In addition to repelling negatively charged molecules (e.g., AA), thus reducing high-level background currents in vivo, Nafion(r) concentrates positively charged molecules (e.g., DA) near the electrode surface [28; 31; 32]. During the last decade, the design and fabrication of MEAs has continued to evolve alongside steps towards understanding the unique performance abilities of the device [54]. When compared with earlier MEA designs, the enhanced performance of the MEA types used here to measure DA can likely be attributed to improved precision and reproducibility of the Pt recording sites due to the MEA fabrication process [29]. Subtle differences between individual recording site performances within a single MEA device, as well as between MEA devices, are not unexpected since individual pads retain unique physical properties post-fabrication. In this regard, we have recently shown that the nanostructured surface topography of individual pads contributes to an electroactive area that is greater than the geometric area [41]. However, dissimilarities regarding the thickness of Nafion(r) following dip-coating, which may produce differential diffusion layers, may also contribute to small differences in the responsiveness of individual recording sites. In addition to confirming the utility of Nafion(r) for repelling major CNS interferents, our findings here further support the utility of m-PD for removing larger organic molecules (e.g., AA), including the analyte of interest (i.e., DA), for the incorporation of self-referencing approaches [33]. Both Nafion and m-PD were employed on the sentinel recording sites to achieve the self-referencing configuration to selectively measure resting DA levels This dual-selective layer makes the Pt recording sites of the MEAs essentially unresponsive to electroactive species that are anionic and larger than H\({}_{2}\)O\({}_{2}\). When the sentinel signal is subtracted from the Nafion-only site a 'pure' catecholamine signal is achieved. In addition, m-PD can be precisely coated onto recording sites even if they are in close proximity (within microns) to other sites. The resulting electrode pairs have excellent selectivity over anionic species such as DOPAC and ascorbate.