# Tonic and Phase Amoremetric Monitoring of Dopamine Using Microelectrode Arrays in Rat Striatum
## 3 Results
### Proof-of-Concept for MEA-Based In Vivo Measures of Phasic and Tonic DA Levels  
The high level of spatial-temporal resolution imparted by in vivo electrochemical technologies with microelectrodes has been instrumental to shaping current views of phasic DA neurotransmission in the areas of release and uptake [11]. The ability of S2 MEAs to measure phasic changes in extracellular DA was determined in the dSTR of anesthetized rats with and without a 6-OHDA lesion. The in vivo MEA response was tracked following the local application of DA (100 nL; 200 uM, pH 7.4) in the dSTR (data not shown). In addition to detecting exogenous DA, MEAs successfully captured active DA uptake--indicated by the descending portion of the curve. In keeping with previous observations from our laboratory using carbon fiber microelectrodes, amplitude-matched DA signals were cleared more slowly in the lesioned vs. non-lesioned dSTR [41]. Figure 2 shows the in vivo MEA response to repeated applications of KCl (100 nL; 120 mM, pH 7.4) in the dSTR, which stimulates neurotransmitter release from nerve terminals. Ejection of KCl in the non-lesioned dSTR stimulated endogenous DA release (peak amplitude in uM: first stimulation, 4.3; second stimulation, 3.1). Importantly, KCl-evoked DA release in the lesioned dSTR was significantly reduced for both the first and second applications of KCl (peak amplitude in uM based on the calibration slope calculated in pA/uM: first stimulation, 0.1; second stimulation, 0.05). In addition, while the rat striatum has norepinephrine and serotonin nerve terminals that can be detected by the MEAs, the KCl-evoked signals are dramatically reduced in the lesioned striatum showing that the signal recorded is likely predominantly DA. The MEAs are known to respond to serotonin and norepinephrine in vitro and in vivo, but the levels of these neurotransmitters in vivo are 40-1000 folds lower than the DA based on the tissue levels of the monoamines [42]. Collectively, these data support the use of MEAs for traditional measures of in vivo DA signals. The inability of in vivo electrochemical techniques to measure tonic DA levels continues to be a major shortcoming and necessitates that a complementary approach (e.g., microdialysis) be used to study resting DA levels [7]. While the combined approach escalates the complexity, cost and commitment to a study, inclusion of one approach and not the other may only be appropriate for certain experimental designs. MEAs have been routinely utilized to quantify both evoked and resting glutamate levels in the CNS [30; 32; 33; 34; 43; 44; 45; 46]. Therefore, we employed 4-channel S2 MEAs in initial studies aimed at providing proof-of-concept for measurement of tonic DA levels in the dSTR of anesthetized rats. Preliminary studies determined that self-referencing alone did not always remove the intrinsic background current of S2 MEA recording sites in vivo (data not shown). Therefore, we utilized the small size of the MEA device to record electrochemical signals in brain regions that differ with regard to dopaminergic innervation--DA innervation is low in the primary motor cortex and high in the dSTR--in conjunction with the self-referencing approach to establish a baseline electrochemical signal in the brain microenvironment (Figure 3A). MEAs were first positioned in the cortex, and after establishing a stable baseline signal (typically \(\geq\)90 min), S2 MEAs were lowered ventrally to a final recording position in the dSTR (Figure 3A). Initially, the self-referenced baseline signal measured by DA sites appeared to be comparable between the cortex and dSTR (Figure 3B). However, Figure 3B shows that the dSTR baseline signal is actually greater (i.e., flipped) vs. the cortical baseline signal (in vivo baseline current, in pA: cortex, 71.7; dSTR, 73.0). Thus, by first recording in the primary motor cortex,