European Journal of Anaesthesiology 1998, 15, 224–229 PHARMACOLOGICAL STUDY Isoflurane reduces synaptic glutamate release without changing cytosolic free calcium in isolated nerve terminals M. Larsen, E. T. Valø, J. Berg-Johnsen* and I. A. Langmoen Institute for Surgical Research and Department of Neurosurgery, National Hospital and *Department of Neurosurgery, Ulleva ˚ l Hospital, University of Oslo, Oslo, Norway Summary reduced by 56, 43 and 36% in response to isoflurane 0.5, 1.5 and 3.0%, respectively (for all: P<0.05). Mem- The molecular mechanism of volatile anaesthetic ac- brane depolarization evoked a rise in cytosolic free tion on presynaptic glutamate release is not clear. An calcium of ≈34%. Addition of isoflurane (0.5, 1.5 and inhibitory effect on voltage-gated calcium channels 3.0%) produced no significant change in cytosolic free has been proposed. The present study examines the calcium. These results indicate that the isoflurane- effect of isoflurane on cytosolic free calcium and syn- induced reduction in presynaptic glutamate release is aptic glutamate release from isolated nerve terminals. caused by other mechanisms than blocking voltage- Synaptosomes from rat cerebral cortex were used. gated calcium channels. As the release is inversely Glutamate was measured with a continuous fluo- dose-dependent, two or more mechanisms could be rometric measurement in a spectrophotometer as the involved. fluorescence of NADPH and calcium as the fluor- escence of fura-2. Isoflurane reduced the calcium- Keywords:  , isoflurane; , dependent glutamate release evoked by membrane cerebral cortex, synaptosomes; , pre- depolarization with 4-aminopyridine in an inversely synaptic; . dose-dependent manner. The glutamate release was Introduction been shown that isoflurane depresses the activity of thin unmyelinated fibres and excitatory synaptic trans- Sherrington [1] reported at the beginning of the cen- mission [10,11] and hyperpolarizes the postsynaptic tury that chloroform blocks reflex activity in the spinal neurone [12,13]. Quantitatively, the effect on synaptic cord at lower concentrations than those required to transmission seems to be the most important [10,14]. block impulse propagation along nerve trunks. Later Glutamate is the most important excitatory trans- studies have shown that volatile anaesthetics depress mitter [15] in mammalian brain. Isoflurane reduces excitatory synaptic transmission in the superior cer- both synaptic (calcium-dependent) and non-synaptic vical ganglion [2], the spinal cord [3] and in various (calcium-independent) evoked release of glutamate parts of the central nervous system [4–8]. from hippocampal slices [16]. Isoflurane also increases Isoflurane, a halogenated methyl-ethyl-ether and an the uptake of glutamate into presynaptic terminals isomer of enflurane, is widely used because of its [17]. The mechanism behind the anaesthetic-induced advantages compared with older volatile anaesthetics. reduction of the glutamate release during synaptic Utilizing the rat hippocampal slice technique [9] it has transmission has not been clarified. Because of the importance of the calcium influx through voltage- Accepted October 1997 Correspondence: M. Larsen. gated calcium channels in neurotransmitter release 224  1998 European Academy of Anaesthesiology