Keith Miller, Ph.D.
Anesthesia, Critical Care and Pain Medicine, Mass General Research Institute
Mallinckrodt Professor of Pharmacology in the Department of Anaesthesia
Harvard Medical School
• Molecular mechanisms of action of general anesthetics
• Location and allosteric action of anesthetic sites on ligand-gated ion channels such as GABAA receptors
• Action of xenon on depression
Description of Research
My laboratory focuses on elucidating the molecular mechanisms by which general anesthetics act to produce anesthesia and its side effects. Our overall hypothesis is that anesthesia is caused by action on neuronal pentameric ligand-gated ion channels. Anesthesia may result from general anesthetics enhancing the action of inhibitory receptors (GABAA or Glycine receptors), inhibiting the action of excitatory receptors (nicotinic, 5HT3A or glutamate receptors) or a mixture of both actions.
Our current focus is the synaptic GABA(A) receptors. They consist of five roughly homologous subunits arranged centro–symmetrically around a central ion–conducting channel in the order β–α–β–α–γ. Each subunit consists of an extra cellular domain that binds the agonist, a transmembrane domain of four helices, and an intracellular domain. Working with collaborators in an NIGMS–funded Program Project Grant, over the last decade using analogs of general anesthetic that are photolabels, we found that intravenous general anesthetics bind in the transmembrane domain of GABA(A) receptors between two or more of the five subunit interfaces. Binding is often limited to particular subunit interfaces. Etomidate and propofol bind between β and α subunits (the two β+/α– interfaces). A high affinity mephobarbital derivative binds at the single γ+/β– subunit interface and the single α+/β– interface; at higher concentrations propofol can bind there too. It has proved hard to find an anesthetic that binds the fifth interface (the α+/γ– “orphan” interface), and this is an active area of research. Steroid anesthetics bind in the two β+/α– interfaces, but much more intracellular in the transmembrane domain than etomidate; the two sites do not overlap.
The location of these sites on synaptic receptors has important implications when using mixtures of general anesthetics. We have recently reported that the potency of mixtures of general anesthetics that bind at the same sites is additive, whereas that of mixtures of two or more anesthetics that all bind at non-overlapping sites is synergistic.
The GABA(A)R has three main conformations: (1) the resting closed state; (2) the GABA-bound open state, and (3) the desensitized state. Anesthetics bind to the latter two states, prolonging GABA-currents and enhancing desensitization. A photoactivable convulsant barbiturate we developed only binds to one anesthetic–site (the γ+/β– interface) and consequently it binds exclusively to the closed resting state. Clearly, the subtle differences in the structure of these inter-subunit binding sites in different conformations is important. Currently my collaborators and I are focused on determining the molecular details of anesthetics and convulsants bound in their sites in order to design and synthesize more potent intravenous anesthetics and agents with properties between those of anesthetic and convulsant that may perhaps prove to be antagonists of anesthetic action. As a first step, we recently reported the structure of a synaptic GABA(A)R in a lipid bilayer.
Xenon is an unusual gaseous anesthetic that acts, like ketamine, on a different class of ligand-gated ion channel, the NMDA receptors. We in the Department of Anesthesia, Critical Care & Pain Medicine are collaborating with MGH psychiatrists, together we aim to test the hypothesis that xenon is superior to ketamine in the treatment of major depressive disorder and bipolar depression.