Sampling from injured tissue as a blessing in disguise: tonic changes in cholinergic neurotransmission using microdialysis.

With the advent of enzyme-coated microelectrodes and the amperometric method for assessing synaptic acetylcholine (ACh) release [1], and following the demonstration of behavioral performance-associated cholinergic transients [2], microdialysis no longer appeared to be a useful or even justifiable method for measuring extracellular ACh concentrations. Numerous major limitations have been cited to plague this method, ranging from the implications of tissue injury resulting from the implantation of relatively large probes to the poor spatial definition of the area that contributes to recovered ACh. Moreover, microdialysis-based claims for (tonic) changes in ACh levels on the scale of minutes have been considered mere artifacts of the limits of detection of conventional analytical methods. The hypothesis that all cholinergic neurotransmission is phasic (scale of [milli-] seconds) contrasts with the hypothesis that low micromolar “ambient levels” of extracellular ACh are regulated by levels of acetylcholinesterase (AChE) [3]. Because our experiments using choline oxidase (CO)-coated microelectrodes as well as electrodes with co-coatings of AChE plus CO did not indicate the presence of non-hydrolized ACh even after (massive) neuronal depolarization [4], the presence - or at least the concentration - of such “ambient levels” of extracellular ACh has remain unsettled. Below we summarize evidence indicating that by using microdialysis we can observe systematic relationships between minutes-based extracellular ACh levels and behavioral/cognitive manipulations and performance, and that it is unlikely that these minute-based changes merely represent integrated cholinergic transients. We speculate that the pathological consequences of microdialysis probe insertion produces a sphere with reduced or absent AChE concentrations, allowing a tonic component of cholinergic neurotransmission to be detected by this method. Although the physiological significance of such measures remains unclear, their psychobiological relevance suggests that the aspects of the microdialysis method that were previously considered detrimental may in fact conspire to allow the measurement of meaningful, slow changes in cholinergic activity. Methods