Halothane and isoflurane increase spontaneous but reduce the N-methyl-D-aspartate-evoked dopamine release in rat striatal slices: evidence for direct presynaptic effects.

Abstract

Experimental data suggest that volatile anesthetics induce significant changes in extracellular dopamine concentrations in the striatum, a restricted but functionally important brain area. In the present study, the authors used a superfused slice preparation to examine the effects of halothane and isoflurane on both spontaneous and N-methyl-D-aspartate (NMDA)-evoked dopamine release in the striatum, and whether these effects involved actions of these anesthetics mediated by gamma-aminobutyric acid receptors in this structure. Radioactivity collected from 5-min fractions was compared in the absence (basal release) or presence (evoked release) of NMDA alone and combined with various pharmacologic or anesthetic agents in slices of the dorsolateral striatum and synaptosomes of the whole striatum preloaded with 3H-dopamine and superfused with artificial cerebrospinal fluid. In tetrodotoxin-treated striatal slices, halothane and isoflurane significantly increased dopamine basal release (EC50 = 0.33 mM and 0.41 mM for halothane and isoflurane, respectively). Both agents decreased the NMDA-evoked dopamine release in both the absence (IC50 = 0.15 mM and 0.14 mM for halothane and isoflurane, respectively) and presence (IC50 = 0.15 mM for both halothane and isoflurane) of tetrodotoxin in slices, and in synaptosomes (IC50 = 0.19 mM for both halothane and isoflurane). NMDA-induced dopamine release was significantly enhanced by bicuculline, a gamma-aminobutyric acid receptor antagonist. Halothane and isoflurane inhibitory effects on NMDA-evoked dopamine release were significantly reduced in the presence of bicuculline. These results indicate that halothane and isoflurane decrease the NMDA-evoked dopamine release by acting directly at dopamine terminals in striatal slices. They support the involvement of both depression of presynaptic NMDA receptor-mediated responses and enhancement of gamma-aminobutyric acid receptor-mediated responses in these effects.