Department / Division Affiliations
Molecular & Medical Pharmacology
ACCESS Program: Dept. of Molecular & Medical Pharmacology,
Brain Research Institute
A. Structure and function of GABA-A receptor proteins.
1) Basis of receptor pharmacological heterogeneity: different subtypes of receptors differing in various pharmacological properties are produced from structural isoforms of the heteropentameric protein containing different subunit composition. We study this with protein chemistry, immunochemistry, cell culture, genetically engineered mice, and electrophysiology of cloned recombinant receptors expressed in heterologous cells such as frog oocytes and mammalian cell lines. Such studies can tell us about the mechanism of action of drugs active on the brain via the GABA system, including sedatives, anesthetics, antiepileptics, and anti-anxiety agents, and help to develop better new medications. One of the GABA receptor subunits beta3 was shown by us to be a gene for human absence epilepsy and a contributor to the genetic neurodevelopmental disease Angelman syndrome.
2) Functional domains in the GABA-A receptor/chloride channel(GABAR): we identified amino acid residues in the GABAR protein that are involved in ligand binding using affinity labeling with radioligands and microsequencing, and developed a new model of the agonist and modulatory drug sites of interaction in the protein that could convert binding energy into conformational changes regulating ion channel gating. We study this with site-directed mutagenesis and expression of mutant receptors as above. We have contributed new information regarding not only the site of action of agonists (GABA and analogs), but also important CNS drugs that act via sites on the GABAR for benzodiazepines (like valium); we were the first to identify binding sites for general anesthetics (like etomidate, propofol, barbiturates, neurosteroids, and volatile agents), and ethanol. We discovered novel sites for low dose ethanol action on certain GABA-A receptor subtypes containing the delta subunit and located extrasynaptically (see also faculty Martin Wallner biosketch). We are studying the mechanism of action of ethanol on the GABARs using homology models of the protein structure based on recent crystal structures published for related proteins.
B. An animal model of alcoholism with general relevance to drug dependence and epileptogenesis: we gave rats chronic intermittent administration of ethanol (CIE) to produce multiple episodes of intoxication and withdrawal, resulting in persistent severe withdrawal signs, assessed as an increased seizure susceptibility, using the GABA blocking convulsant, pentylenetetrazol, and increased anxiety, tolerance to alcohol action, and difficulties with sleep, signs of human alcoholism (along with increased drinking). The animals resemble those made pre-epileptic, or `kindled', by chronic stimulation with subconvulsant levels of electrical activation or convulsant drugs. This seems to be a useful model of human addictive behavior as well as an interesting example of brain plasticity in response to environmental stressors. The behavioral alterations can be explained by hypofunction of GABA-mediated inhibition, involving subunit switches in GABARs and therefore physiological and pharmacological properties, demonstrated in hippocampus and elsewhere. This aberrant plasticity, or maladaptation, appears to contribute to the molecular explanation of drug dependence. Finally, we are exploring traditional Asian herbal medicines for substance which might be developed as medications for alcohol use disorders. We (see also faculty Jing Liang biosketch) found one plant Hovenia with such properties and identified an ingredient dihydromyricetin (DHM) which is very promising. It blocks alcohol action, withdrawal, development of dependence, and increased drinking in rats, which we hope can be translated into action in humans