Oxidation Potentials in CNS Measurements and Specificity of Response

There are literally hundreds of chemical compounds in the brain, but, fortunately, only a few are oxidizable in the potential range available. This potential window is set by the background limits, about —0.3 to + 1.0 V. 

If one were dealing with the solution electrochemistry under carefully controlled conditions, a table of p values of oxidizable compounds with an uncertainty of about 20—30 mV could be prepared. Much experi- 

A realistic compilation of the ease of oxidation of the compounds of interest is given in Fig. 17 where ranges, rather than values, are shown. (The abbreviation for each substance is approximately centered at the average value of p, but any given substance oxidizes in approximately the range of potentials enclosed by the vertical dotted lines.) 

Section A of Fig. 17 contains endogenous brain constituents. Those compounds in boldface type are routinely detected in vivo and form the basis of most of the work to date. In each case their endogenous potentials have been verified by matching with microinjections of the specific substance next to the detecting electrode. The potentials given for the 

Section B (Fig. 17) shows some commonly used drugs which are electroactive. The a-methyltryptamine was detected at + 0.8 V by Marsden (1980) in 5-HT studies. Several monoamine oxidase inhibitors are electroactive, but there is so far no evidence that they can be detected in vivo and, even if they are, it is at potentials sufficiently positive not to interfere with most studies. Chlorpromazine and haloperidol have solution Ep values about + 0.6 V and + 0.7 V, respectively. Clozapine oxidizes at about + 0.4 V. Since neuroleptics (and other drugs) are extensively metabolized by the time they appear in significant concentrations in the CNS, the potentials of the parent compounds may not be too relevant—but no data are available for metabolites. In any event, there is no evidence that any of these drugs are detected in CNS voltammetry. 

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