Molecular actions of propofol on human 5-HT3A receptors:: enhancement as well as inhibition by closely related phenol derivatives

BACKGROUND: 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. METHODS: 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. RESULTS: When applied in equilibrium (60 s before and during the 5-HT pulse), propofol inhibited human 5-HT3, receptors (IC50 = 18 +/- 1.0 mu M). In equilibrium, the less hydrophobic 2-isopropylphenol was surprisingly a similarly potent inhibitor of human 5-HT,, receptors (IC50 = 17 +/- 3.2 mu M), whereas phenol was considerably less potent (IC50 = 1.6 +/- 0.2 rnM). 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 = 35 ms and TIN-2 = 4.8 s), 2-isopropylphenol (TIN-1 = 64 ms and TIN-2 = 6.6 s); and phenol (TIN-1 < 10 ms, TIN-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 mu M, whereas the slow effects were not sensitive to 5-HT. CONCLUSIONS: 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 less potent than expected from its hydrophobic properties. Underlying mechanisms appear to involve the phenolic hydroxyl group, hydrophobic interactions, and steric restrictions.