Dopamine peak voltammograms

Fig. 14. Peak voltammograms of dopamine at graphite electrodes. (A) Linear potential sweep (200 mV/sec), 500 pM dopamine in pH 7.4 phosphate buffer (B) cyclic voltammogram of same solution as A. Dotted line—see text.

FIGURE 25-33 Dopamine release during cocaine selfadministration, Rats were trained to press a lever owhead) to receive a small intravenous Injection of cocaine. The lower trace shows changes in dopamine. Peak 1 indicates an increase in dopamine before the rat pressed the lever. The two peaks (2) indicate transients that occurred after the lever press. Underneath the trace, the dark blue bar marks the time the audiovisual cues associated with the lever press were on. The light blue bar indicales when the pump was activated to deliver cocaine. The cyclic voltammogram (black) of behaviorally evoked dopamine matches the electrically evoked voltammogram (blue). (Adapted with permission from P. E. M. Phillips, Nature, 2003, A22. 614.)... 

Several publications on electrochemical mechanistic studies of the oxidative transformations of catecholamines followed the contribution by R. N. Adam s group (256) and involved a-methyldopamine, a-methylnor-adrenaline, dopamine (257), a-methyldopa, 5,6-dihydroxy-2-methylin-dole (255), and dopa (259). These studies (257) (Scheme 5), which confirmed the validity of the melanization scheme by Mason and Raper (Ref. 7, p. 50), explored the pH effect on the sequence of events that characterize the electrooxidation of catecholamines. Thus, the cyclic voltammogram in I M HCIO4 (pH 0.6) shows only peaks corresponding to the catechol-quinone redox couple as the protonation of the amino group prevents the cyclization step. 

The two cyclic voltammograms in the upper left of the figure show that dopamine was released both by behavioral stimulation (black irace) and cocaine administration (blue trace). The blue trace in the center shows the conccniration of dopamine as a function of time calculated from the peak currents of the vollam-mograms. The trace was extracted from 200 discrete cyclic voltammograms acquired over the 20-sccond pe-ri(xl shown. The peak at position I shows that there... 

The voltammogram obtained from the difference between that recorded before and during stimulation demonstrates that the detected substance Is dopamine. The current from the peak oxidation potential for dopamine recorded from sequential voltammograms demonstrates that the concentration of dopamine increases during stimulation, and then rapidly decreases after the stimulation is over. The disappearance of dopamine Is due to uptake back Into dopamine cells (21). The appearance of dopamine during the stimulation Is a combination of the synaptic release of this compound as well as the concurrent uptake. The combined effect of both of these factors, plus diffusion, can be modelled as Indicated by the solid line in Figure 8. 

Fig. 8. Oxidation current from a microvoltammetric electrode placed in the caudate nucleus of an anesthetized rat during an electrical stimulation of the medial forebrain bundle. Top chronoampero-metric current ya. time at 0.5 V ys. SCE. Bottom difference of voltammogram obtained at the peak (circles), compared to voltammograms obtained for dopamine (DA) and ascorbic acid (AA) in vitro. Current scales for voltammograms i = 16 pA, 25 yM dopamine i = 28 pA, 200 jjM ascorbic acid i 32 pA, stimulation. Insert chronoamperometric current ys. time for an Identical experiment with 2-s intervals between measured points. Reprinted with permission from reference 34 (copyright 1983 by the AAAS).

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