Background: Presynaptic effects of general anaesthetics are not well characterized. We tested the hypothesis that isoflurane exhibits transmitter-specific effects on neurotransmitter release from neurochemically and functionally distinct isolated mammalian nerve terminals.
Methods: Nerve terminals from adult male rat brain were prelabelled with [(3)H]glutamate and [(14)C]GABA (cerebral cortex), [(3)H]norepinephrine (hippocampus), [(14)C]dopamine (striatum), or [(3)H]choline (precursor of [(3)H]acetylcholine; striatum). Release evoked by depolarizing pulses of 4-aminopyridine (4AP) or elevated KCl was quantified using a closed superfusion system.
Results: Isoflurane at clinical concentrations (<0.7 mM; ~2 times median anaesthetic concentration) inhibited Na(+) channel-dependent 4AP-evoked release of the five neurotransmitters tested in a concentration-dependent manner. Isoflurane was a more potent inhibitor [expressed as IC(50) (SEM)] of glutamate release [0.37 (0.03) mM; P<0.05] compared with the release of GABA [0.52 (0.03) mM], norepinephrine [0.48 (0.03) mM], dopamine [0.48 (0.03) mM], or acetylcholine [0.49 (0.02) mM]. Inhibition of Na(+) channel-independent release evoked by elevated K(+) was not significant at clinical concentrations of isoflurane, with the exception of dopamine release [IC(50)=0.59 (0.03) mM].
Conclusions: Isoflurane inhibited the release of the major central nervous system neurotransmitters with selectivity for glutamate release, consistent with both widespread inhibition and nerve terminal-specific presynaptic effects. Glutamate release was most sensitive to inhibition compared with GABA, acetylcholine, dopamine, and norepinephrine release due to presynaptic specializations in ion channel expression, regulation, and/or coupling to exocytosis. Reductions in neurotransmitter release by volatile anaesthetics could contribute to altered synaptic transmission, leading to therapeutic and toxic effects involving all major neurotransmitter systems.