A critical assessment of electrochemical procedures applied to the measurement of dopamine and its metabolites during drug-induced and species-typical behaviours

Abstract

Monoamine neurotransmitters and their metabolites are electro-oxidizable at the surface of electrodes implanted in brain, and this has raised the expectation that in-vivo electrochemical procedures may be used routinely to monitor dynamic changes in dopamine (DA), noradrenaline and serotonin during species-typical and drug-induced behaviours. This expectation is slowly becoming a reality as various laboratories refine electrochemical procedures and design recording electrodes that are selective for specific neurotransmitters. The most important step is to conduct rigorous assessments of these procedures in-vivo to confirm that they are indeed valid and reliable bioprobes for a given neurochemical species. The present review describes the different electrochemical procedures and recording electrode designs currently used to measure monoamines in freely moving animals. Emphasis is placed on recording electrodes used to monitor DA in the extracellular compartment of the brain. In-vitro and in-vivo validation studies, demonstrating the selectivity and stability of these electrodes, are discussed in detail with respect to five criteria: (a) voltammograms recorded in-vivo are identical to those recorded in-vitro; (b) independent confirmation of the species detected and agreement of concentration estimates with other neurochemical methods; (c) sufficient electrode sensitivity to measure impulse-flow dependent transmitter efflux; (d) the dependency of the electrochemical signal on the presence of an intact neuronal system; and (e) predictable and reproducible effects of drugs or other manipulations on the electrochemical signal. The final section discusses recent applications of in-vivo electrochemical procedures for the monitoring of DA neurotransmission in terminal regions of the mesotelencephalic DA system during: (a) operant behaviour reinforced by brain-stimulation reward or drugs of abuse; (b) species-typical motivated behaviours, as exemplified by feeding and sexual behaviour, and (c) in response to environmental stressors.