Heteromeric Nicotinic Inhibition by Isoflurane Does Not Mediate MAC or Loss of Righting Reflex

Background Neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in the mechanism of action of isoflurane as they are inhibited at subanesthetic concentrations. Despite clear evidence for nicotinic inhibition at relevant isoflurane concentrations, it is unclear what behavioral result ensues, if any. Methods The authors have modeled two behaviors common to all general anesthetics, immobility and hypnosis, as minimum alveolar concentration that prevents movement in response to a supramaximal stimulus (MAC) and loss of righting reflex (LORR). They have tested the ability of nicotinic pharmacologic modulators and congenital absence of most heteromeric nAChRs to affect concentration of isoflurane required for these behaviors. Results Neither mecamylamine, 5 mg/kg, nor chlorisondamine, 10 mg/kg, affected isoflurane MAC. Nicotine caused a small decrease in MAC. None of the above agents had any effect on the concentration of isoflurane required for LORR. Mice genetically engineered to lack the beta 2 nicotinic gene product were not different in MAC or LORR from controls. Conclusions Nicotinic antagonists do not cause MAC or LORR. Inhibition of nicotinic acetylcholine receptors by isoflurane is not likely related to its ability to provide immobility and hypnosis in a surgical setting. This is perhaps not surprising as the inhibition of nAChRs in vitro is complete at an isoflurane concentration equal to one half of MAC. Nicotinic inhibition may, however, be involved in anesthetic behaviors such as amnesia and analgesia, which occur at lower anesthetic concentrations.