Determination of the relative potency of two excitant amino acids

Recent reports have suggested that some cells in the CNS differ in their relative sensitivity to iontophoretically applied aspartate and glutamate. Attempts to investigate this phenomenon present difficulties since transport numbers for the same substance show considerable variation among different micropipette barrels. A method is described which allows corrections to be made for differences in efflux rates from different barrels. Efflux of tritiated glutamate and aspartate into saline for each pipette is measured by liquid scintillation counting before and after its use in vivo. Calibration curves of efflux (pmol min ^?1) against current (nA min^?1) are then constructed for each barrel containing amino acid. Using these calibrations, log dose-response curves for aspartate and glutamate can be plotted for each neurone by derivation from the apparent dose-response curves in which firing rate is plotted against log current (nA).When the method was tested on brainstem neurones it was found that the apparent relative sensitivity of most cells was changed following correction, and in some cases reversed.