Multiple actions of propofol on alphabetagamma and alphabetadelta GABAA receptors

GABAA receptors are predominantly composed of alphabetagamma and alphabetadelta isoforms in the brain. It has been proposed that alphabetagamma receptors mediate phasic inhibition, whereas alphabetadelta receptors mediate tonic inhibition. Propofol (2,6-di-isopropylphenol), a widely used anesthetic drug, exerts its effect primarily by modulating GABAA receptors; however, the effects of propofol on the kinetic properties of alphabetagamma and alphabetadelta receptors are uncertain. We transfected human embryonic kidney (HEK293T) cells with cDNAs encoding rat alpha1, alpha6, beta3, gamma2L, or delta subunits and performed whole-cell patch-clamp recordings to explore this issue. Propofol (3 microM) increased GABA concentration-response curve maximal currents similarly for both alpha1beta3gamma2L and alpha6beta3gamma2L receptors, but propofol increased those for alpha1beta3delta and alpha6beta3delta receptors differently, the increase being greater for alpha1beta3delta than for alpha6beta3delta receptors. Propofol (10 microM) produced similar alterations in alpha1beta3gamma2L and alpha6beta3gamma2L receptor currents when using a preapplication protocol; peak currents were not altered, desensitization was reduced, and deactivation was prolonged. Propofol enhanced peak currents for both alpha1beta3delta and alpha6beta3delta receptors, but the enhancement was greater for alpha1beta3delta receptors. Desensitization of these two isoforms was not modified by propofol. Propofol did not alter the deactivation rate of alpha1beta3delta receptor currents but did slow deactivation of alpha6beta3delta receptor currents. The findings that propofol reduced desensitization and prolonged deactivation of gamma2L subunit-containing receptors and enhanced peak currents or prolonged deactivation of delta subunit-containing receptors suggest that propofol enhancement of both phasic and tonic inhibition may contribute to its anesthetic effect in the brain.