Preferentially impaired neurotransmitter release sites not their discreteness compromise the validity of microdialysis zero-net-flux method

Intracerebral microdialysis is a popular technique for studying neurochemistry and neural circuits in various brain regions. Recent studies called into question the validity of the microdialysis zero-net-flux (ZNF) method by suggesting that this method significantly underestimates the basal level of extracellular dopamine as a result of the discreteness of dopamine release sites as well as the preferential damage to dopamine release over uptake. To identify which factor is most important in undermining the microdialysis ZNF measurements and the extent of underestimation, two mathematical models were developed to explore the influences of the discrete nature and the probe-induced impairment in the neurotransmitter release. The two models differ in their characterizations of the transmitter release as spatially discrete and homogeneous, respectively. Simulations using physiologically reasonable parameters for striatal dopamine systems indicate that the preferential release site damage surrounding the implanted probe is the most important determinant to the underestimation of the microdialysis ZNF concentration. Under normal physiological conditions, the discreteness of neurotransmitter release sites is of minor importance, except when neuronal degeneration occurs. It is concluded that homogeneous models can adequately describe microdialysis operating processes as long as the corresponding tissue damage parameters in such models are appropriately incorporated.