Real-time extracellular measurement of neurotransmitters in conscious sheep

Measurements of neurotransmitters in conscious animals have been restrictive in real-time. The use of specific enzymes within an amperometric probe, based upon a microdialysis membrane, may overcome some of these problems. I report the use of such a probe, with different perfusions of enzymes, to allow real-time measurement of glutamate, catecholamines and indoleamines, in conscious animals. At an adjacent site microdialysis sample collections were made concurrently and neurotransmitters measured in the dialysate. Both probes were positioned within the somatosensory cortex. Values obtained by amperometric probes were similar to those in dialysate samples for glutamate, catecholamines and indoleamines during basal and stimuli related collections. Amperometric measurements showed higher peak concentrations and better time resolution than dialysate sampling. This reflects sampling differences. Application of external stimuli increased extracellular concentrations of glutamate, catecholamines and indoleamines, in both forms of sampling. Dopamine measurements did not correlate well between the two forms of sampling. This may reflect the non-specificity of the enzyme (dopamine-beta-hydroxylase) used in the amperometric probe. This combination, of microdialysis and amperometry, offers a useful tool for real-time neurotransmitter studies in vivo.