Preloading in vivo: a rapid and reliable method for measuring gamma-aminobutyric acid and glutamate fluxes by microdialysis.

In vivo microdialysis was used in conjunction with a novel dual-label preloading method, to monitor changes in extracellular levels of gamma-aminobutyric acid (GABA) and glutamate in the striatum of conscious, unrestrained rats. [3H]GABA and [14C]glutamate were applied in the dialysis stream for a preloading period of 30 min, after which dialysis perfusion was continued for up to 6 h, and dialysate samples were collected for scintillation counting. Veratridine (Vtd: 100 microM in the dialysate) caused significant rises in both 3H and 14C content measured in the dialysates, the majority of which remained associated with the preload GABA and glutamate, respectively. The Vtd-stimulated release of GABA and glutamate measured in this way was blocked by tetrodotoxin and was Ca2+ dependent. Thus, by reproducing results obtained using other techniques, we have shown that the preloading method provides a quick and reliable method for measuring the effects of drugs on the release of neurotransmitter GABA and glutamate in vivo by dyalisis. It should enable sample times as low as 1 min to be used, thus allowing resolution of transient stimulated responses taking place over a time course of minutes.