['The nicotinic acetylcholine receptors (nAChRs) in the central nervous system may be a potential target for the anesthetic effects of thiopental. We evaluated the mechanism of action of thiopental on the human alpha7 nAChR by using 2-electrode voltage clamp methodology. Concentration response curves for agonist were prepared in the presence of 25-250 microM of thiopental. Inhibition by the S- and R-thiopental enantiomers was compared with inhibition by racemic thiopental. We found that thiopental acts as a competitive inhibitor at the human alpha7 nAChR. Inhibition is independent of membrane potential and the K(i(apparent)) is 13 microM of thiopental. The clinical 50% effective concentration for thiopental in humans is 25 microM. Thus, with a K(i(apparent)) of 13 microM, inhibition of the human alpha7 nAChR is within a clinically relevant range. The S- and R-enantiomers of thiopental cause inhibition indistinguishable from the inhibition caused by racemic thiopental. This discordance makes it unlikely that the alpha7 nAChR plays a role in loss of righting reflex induced by thiopental in mice, although nicotinic inhibition by thiopental may mediate other anesthetic effects and side effects.'] Is thiopental a competitive inhibitor at the human alpha7 nicotinic acetylcholine receptor?