POSTER PRESENTATION Open Access Modeling the chemical dynamics of chloride ion indicators

Fluorescent indicators have shown promise as a relatively non-invasive probe to measure cytosolic ion concentra-tions in brain slices and neuron cultures using microscopy imaging. Most fluorescent ion indicators bind selectively with a certain ion in solution causing a decrease in fluores-cence in a process known as quenching. Under steady state conditions, a fluorescence measurement, made at a specific point and time, is directly related to the local ion concentration at the same point and time, typically via the Stern-Volmer relationship. However, this is usually no longer true under the dynamic conditions inside a cell when transmembrane currents are active. In our research, we are interested in measuring chloride (Cl-) channel currents because of their impli-cation in substance abuse mechanisms. Useful probes have been found in Cl- sensitive dyes, such as N-(6-methoxyquinolyl) acetoethyl ester (MQAE) [1], and 6-methoxy-N-ethylquinolinium (MEQ) [3], and var-iants of the Yellow Fluorescent Protein (YFP) useful as a Cl- biosensor in dendritic compartments [4]. We have modeled jointly the Cl- flow as part of a Hodgkin-Huxley type model and the interaction of Cl-with a generic binding indicator as a system of non-lin-ear differential equations derived from biophysical and chemical kinetic theory, using arbitrary parameters. Using the quenched indicator concentration as the given input signal, several approximate solutions can be derived for the corresponding output signal, the Cl- cur-rent of interest. To date, we have explored a simple quasi-linear approximation similar to that discussed in [2], as well as a best-fit model-based approximation in which the output is assumed to satisfy a sum of parame-terized exponential curves (an assumption consisten