Halothane enhances exocytosis of [3H]‐acetylcholine without increasing calcium influx in rat brain cortical slices

The effect of halothane on the release of [3H]-acetylcholine ([3H]-ACh) in rat brain cortical slices was investigated. Halothane (0.018 mM) did not significantly affect the basal and the electrical field stimulation induced release of [3H]-ACh. However, halothane (0.063 mM) significantly increased the basal release of [3H]-ACh and this effect was additive with the electrical field stimulation induced release of [3H]-ACh. The release of [3H]-ACh induced by 0.063 mM halothane was independent of the extracellular sodium and calcium ion concentration and was decreased by tetracaine, an inhibitor of Ca2+-release from intracellular stores or dantrolene, an inhibitor of Ca2+-release from ryanodine-sensitive stores. Using 2-(4-phenylpiperidino)-cyclohexanol (vesamicol), a drug that blocks the storage of ACh in synaptic vesicles, we investigated whether exocytosis of this neurotransmitter is involved in the effect of halothane. Vesamicol significantly decreased the release of [3H]-ACh evoked by halothane. It is suggested that halothane may cause a Ca2+ release from intracellular stores that increases [3H]-ACh exocytosis in rat brain cortical slices. Keywords: Acetylcholine, transmitter release, anaesthetics, volatile, halothane, rat brain cortical slices, ions, intracellular Ca2+ release, exocytosis Introduction There have been extensive efforts to characterize the mechanism of action of volatile anaesthetics, but their molecular and cellular actions are still a matter of debate. A possible site of action for general anaesthetics could be the presynaptic terminal as indicated by results showing that synaptic transmission is more sensitive to the effects of general anaesthetics than axonal conduction (Larrabee et al., 1952; Richards, 1983; Griffiths & Norman, 1993). Thus, the investigation of the effects of anaesthetics on the release of neurotransmitters may provide information on the mechanisms that contribute to the general effect of these substances during anaesthesia. Acetylcholine (ACh) is a transmitter implicated in memory, learning, cognitive behaviour and it is also of importance during sleep (Mitchell, 1963; Kanai & Szerb, 1965; Collier & Mitchell, 1967; Griffiths & Norman, 1993; Keifer et al., 1994; Winkler et al., 1995). Although there has been some reports of the action of volatile anaesthetics on ACh release, it is not clear how these drugs alter the release of this transmitter. Some authors reported significant inhibition of potassium-stimulated release of ACh in the presence of halothane (Johnson & Hartzell, 1985; Griffiths et al., 1995) while others observed no effect (Bazil & Minneman, 1989a,1989b). However, to our knowledge all studies examined the release of ACh from the central nervous system (CNS) using high potassium stimulation, and little effort has been made to follow the effects of anaesthetics on ACh release induced by electrical stimulation. In the course of an investigation in brain cortical slices, we observed that halothane increased the basal release of ACh ([3H]-ACh). When associated with electrical stimulation, the release of this transmitter was additive. Therefore, the aim of the present work was to further investigate the mechanisms involved on halothane-induced release of [3H]-ACh in rat brain cortical slices.