Mallinckrodt Pharmacology Research Unit
Because general anesthetics dissociate rapidly from their binding sites on ion channels, new approaches were needed for locating their sites. We have developed agents that are normal general anesthetics until exposure to light of a specific wavelength renders them reactive and they combine with the amino acid residues at their binding site. We have synthesized a photoactivable derivative of one of the most potent general anesthetics employed in clinical practice, etomidate. In collaborations with workers at the HMS and UCLA, we have located azietomidate's binding site on both nicotinic acetylcholine and GABAA receptors. Other Unit researchers have used high time resolution electrophysiology to show that etomidate acts as an allosteric co-agonist of GABAA receptors, binding to both the closed and open conformation of the receptor. General anesthetics act selectively on open ion channels, implying that their binding sites must have the highest affinity for channels in these transient states. We have therefore developed a novel time-resolved photolabeling method and located such a transient site for the first time on the nicotinic acetylcholine receptor.
For the last decade, researchers in our field focused almost exclusively on the Cys-loop ligand-gated ion channels, particularly the GABAA receptor, believing that inhaled anesthetics act with little molecular specificity. However, Unit researchers have now shown that nonpolar anesthetics such as cyclopropane, while inactive at GABAA receptors, potently inhibit excitatory NMDA and nicotinic ion channels. This result challenges current thinking and opens up the prospect of developing novel anesthetic compounds that act selectively at receptor targets within the central nervous system. Another novel anesthetic target that Unit investigators are studying is the tandem pore family of potassium channels, specifically their role in anesthetic-induced respiratory depression as well as the mechanism of anesthetic action on them. The role of intracellular signal transduction in modulating channels is little explored. One hypothesis is that anesthetic-induced prevention of ischemia-reperfusion injury is mediated by protein kinase C delta (PKC). Unit researchers have shown there is an anesthetic site on the phorbol ester-binding domain of PKCdelta and have obtained a high-resolution crystal structure of it. They have also shown that the molecular basis for the action of ethanol in producing fetal alcohol spectrum disorder is associated with alcohol sites on the neural cell adhesion molecule, L1.