Volatile Anesthetic Effects on Glutamate versus GABA Release from Isolated Rat Cortical Nerve Terminals: Basal Release

Abstract

The effects of three volatile anesthetics (isoflurane, enflurane, and halothane) on basal release of glutamate and GABA from isolated rat cerebrocortical nerve terminals (synaptosomes) were compared using a dual isotope superfusion method. Concentration-dependent effects on basal release differed between anesthetics and transmitters. Over a range of clinical concentrations (0.5-2ϫ minimum alveolar concentration), basal glutamate release was inhibited by all three anesthetics, whereas basal GABA release was enhanced (isoflurane) or unaffected (enflurane and halothane). These effects may represent a balance of stimulatory and inhibitory mechanisms between transmitters and anesthetics. There were no significant differences between anesthetic effects on basal release in the absence or presence of external Ca 2ϩ , whereas intracellular Ca 2ϩ buffering limited volatile anesthetic inhibition of basal glutamate release. Although these results demonstrate fundamental differences in anesthetic effects on basal release between glutamatergic and GABAergic nerve terminals, all three volatile anesthetics at clinical concentrations consistently reduced the ratio of basal glutamate to GABA release. These actions may contribute to the net depression of glutamatergic excitation and potentiation of GABAergic inhibition characteristic of general anesthesia.

Modulation of a voltage-gated Na+channel by sevoflurane involves multiple sites and distinct mechanisms

“…In rat neurohypophysial nerve terminal preparations, isoflurane inhibits Na + currents and dampens action potentials (44). This inhibition reduces neurotransmitter release (45)(46)(47)(48), which may play a critical role in mediating key physiological features of general anesthesia in vivo. In addition, Na V channel modulation may explain anesthetic agentinduced immobilization (19,20).…”

Nicotinic receptor-evoked hippocampal norepinephrine release is highly sensitive to inhibition by isoflurane

“…Synaptosomes were co-labelled with either [ 14 C]glutamate (1 mM; American Radiolabeled Chemicals) or [ 14 C]GABA (2 mM; PerkinElmer, Boston, MA, USA), collected by centrifugation for 10 min at 20 000 g at 48C, loaded into release chambers, and superfused at 378C with KHB using a modified Brandel SF-12 apparatus. 10 Release was evoked by 2 min pulses of 100 mM (-) nicotine or 5 mM 4-aminopyridine (4AP) in the presence of varying concentrations of isoflurane, nAChR antagonists, or the Na þ channel antagonist tetrodotoxin (TTX), all applied 12 min before the release stimulus. Radioactivity in 1 min fractions was determined by liquid scintillation spectrometry using BioSafe II scintillation cocktail (RPI, Mt Prospect, IL, USA) with quench correction (Beckman-Coulter, Fullerton, CA, USA).…”

“…10 Release was quantified as summed fractional release above baseline (sum DFR), normalized to the mean of all assay controls, and analysed using ANOVA or the Student t-test. Concentration-effect data were fitted to sigmoidal curves using Prism version 5.0a (GraphPad, San Diego, CA, USA).…”

“…Isolated nerve terminals (synaptosomes) were prepared from adult male Sprague-Dawley rats (200-280 g, n¼42) as described previously 10 under National Institutes of Health guidelines as approved by the Weill Cornell Medical College IACUC. Briefly, brains were rapidly removed from rats anaesthetized with 80% CO 2 /20% O 2 , the hippocampus was removed and homogenized in ice-cold 0.32 M sucrose with a motor-driven (500 rpm) Teflon-glass homogenizer, the homogenate was centrifuged for 2 min at 4000 g, and the resulting supernatant was centrifuged through a sucrose gradient to demyelinate and pellet the isolated hippocampal nerve terminals.…”

Involvement of 5-hydroxytryptamine type 3 receptors in sevoflurane-induced hypnotic and analgesic effects in mice

“…Recently, ligand-gated ion channels have emerged as the most promising molecular target for inhalation anesthetics [8,15,23,30]. Various in vitro studies have reported that enhancement of inhibitory neurotransmitter function or inhibition of excitatory neurotransmitter function is a plausible method to induce anesthesia [1,30,32,33].…”