INHIBITION OF DOPAMINE TRANSPORT IN RAT-BRAIN SYNAPTOSOMES BY VOLATILE ANESTHETICS

Background: Volatile anesthetics may depress transmission by altering synaptic concentrations of neurotransmitter. Microdialysis studies have found an increase in brain extracellular dopamine concentration on exposure to volatile anesthetics. We investigated the possibility that synaptosomal dopamine transport is reversibly inhibited by the halothane and isoflurane. Methods: Rat brain synaptosomes were incubated with 5 nM H-3-DA and increasing concentrations of anesthetic in Teflon-sealed microvials. Cocaine (100 muM) was used to quantify non-specific binding/uptake. Uptake was stopped by vacuum filtration and washing; label incorporation into synaptosomes was determined by liquid scintillation counting. H-3-DA release from preloaded synaptosomes also was studied in the presence of the anesthetic to allow distinction between uptake inhibition and release stimulation in the synaptosomes. Results: Both halothane and isoflurane inhibited the specific H-3-DA uptake in a concentration-dependent fashion with an IC50 of 0.72 +/- 0.14 mM for halothane and 2.24 +/- 0.85 mM for isoflurane. No stereoselectivity of isoflurane's action on Dopamine (DA) uptake was observed. The inhibition produced by halothane and isoflurane was kinetically characterized as noncompetitive, but full reversal was demonstrated after removal of the anesthetic from the incubation mixture. The anesthetics did not stimulate H-3-DA release from preloaded synaptosomes. Conclusions: These results demonstrate volatile anesthetic-induced inhibition of the dopamine transporter in this preparation of synaptosomes. The calculated IC50s suggest this inhibition occurs with clinically relevant concentrations of halothane but not with isoflurane. The results are consistent with and may explain the increase in extracellular dopamine concentrations demonstrated by microdialysis.