Molecular Actions of Propofol on Human 5-HT3A Receptors: Enhancement as Well as Inhibition by Closely Related Phenol Derivatives

5-Hydroxytryptamine type 3 (5-HT3) receptors are excitatory ligand-gated ion channels which are involved in postoperative nausea and vomiting. They are depressed by the anesthetic propofol, which, in contrast, enhances the activity of inhibitory ligand-gated ion channels such as gamma-aminobutyric acid type A receptors and glycine receptors. To investigate the molecular mechanisms responsible for these contrasting actions, we examined the kinetics of the action of propofol and its lesser hydrophobic derivatives 2-isopropylphenol and phenol on human 5-HT3A receptors. Human embryonic kidney 293 cells containing stably transfected cDNA of the human 5-HT3A receptor subunit were patch clamped (excised outside-out patches). Drugs were applied with a fast solution exchange system (within 2 ms) and their concentrations were determined by high performance liquid chromatography. When applied in equilibrium (60 s before and during the 5-HT pulse), propofol inhibited human 5-HT3A receptors (IC50 = 18 +/- 1.0 microM). In equilibrium, the less hydrophobic 2-isopropylphenol was surprisingly a similarly potent inhibitor of human 5-HT3A receptors (IC50 = 17 +/- 3.2 microM), whereas phenol was considerably less potent (IC50 = 1.6 +/- 0.2 mM). Varying the duration of drug application before currents were elicited, and then applying 5-HT still in the presence of the drug revealed that fast and slow processes contributed to the (equilibrium) effects of propofol (tau(IN-1) = 35 ms and tau(IN-2) = 4.8 s), 2-isopropylphenol (tau(IN-1) = 64 ms and tau(IN-2) = 6.6 s), and phenol (tau(IN-1) < 10 ms, tau(IN-2) = 20.4 s). When applied transiently together with 5-HT (open channel application), propofol depressed currents and accelerated the 5-HT-induced desensitization significantly, whereas, in contrast, 2-isopropylphenol and phenol increased currents and slowed desensitization. Slowed desensitization was also observed for 5-hydroxyindole (1 mM), a 5-HT derivative, but not for benzene. The fast effects of phenol, 2-isopropylphenol, and propofol were more pronounced when the 5-HT concentration was decreased from 30 to 3 microM, whereas the slow effects were not sensitive to 5-HT. At least two separate inhibitory actions on 5-HT3A receptors could be identified for propofol, whereas the enhancing action seen for the two related smaller phenol derivatives could no longer be detected. 5-HT-dependent and 5-HT-independent interactions could be distinguished for all three drugs. Propofol was