Reporting in today's Early Edition of the August
"Proceedings of the National Academy of Sciences",
researchers at the University of Texas at Austin and
Stanford University describe a novel approach that may help
scientists to better understand how alcohols and anesthetic
drugs interact with certain brain proteins. The report also
provides the strongest evidence to date that alcohols have
specific protein binding sites.
R. Adron Harris, Ph.D., Director, Waggoner Center for
Alcohol and Addiction Research, Institute for Cellular and
Molecular Biology, University of Texas at Austin, with
colleagues Maria Paola Mascia, Ph.D. (University of Texas)
and James R. Trudell, Ph.D. (Stanford University), developed
a way to attach an anesthetic analogue called propanethiol
to amino acid residues at a specific site in glycine and
GABA(A) receptors. GABA and glycine receptors are the
primary mediators of inhibitory neurotransmission in the
brain and spinal cord.
The new work adds weight to previous studies that suggested
that alcohols and anesthetic drugs exert some of their
effects by interacting with specific protein molecules in
the brain. Propanethiol (and also propyl-
methanethiosulfanate), the researchers found, irreversibly
enhances receptor function and obviates the ability of other
alcohols and anesthetics to potentiate receptor function.
"Today's report advances general medical understanding of
the basic pharmacology of alcohols and anesthetics," said
Enoch Gordis, M.D., Director of the National Institute on
Alcohol Abuse and Alcoholism (NIAAA). "Accumulating
knowledge of how beverage alcohol (ethanol) produces its
anesthetic and intoxicating effects at these receptors may
lead to new pharmacologic and behavioral interventions."
For years, prevailing wisdom held that, unlike drugs with a
single site of action, alcohols and anesthetics acted on
many nonspecific sites of the neuronal membrane. More recent
research has shown that these compounds act on specific
receptor proteins, including the glycine and GABA
neurotransmitter-activated ion channels. Defining the
precise mechanisms of these actions, however, has defied
traditional research methods, largely because of the low
affinities and rapid kinetics that characterize alcohol and
anesthetic compounds. To overcome these obstacles, Dr.
Harris and his colleagues used anesthetic alcohol analogues
capable of forming irreversible (covalent) bonds with
specific amino acids.
Previous studies suggested that alcohols and general
anesthetics interacted with the amino acids of the second
(TM2) and third (TM3) transmembrane portions of the glycine
and GABA receptors and that the presence of a serine amino
acid (S267) in TM2 and an alanine amino acid (A288) in TM3
was necessary for the alcohols and anesthetics to trigger
receptor response. To test whether either S267 and A288 in
the glycine receptor and equivalent residues (S270 and A291)
in the GABA(A) receptor indicated a critical binding site,
Dr. Harris and his colleagues changed S267 and A288 to
cysteine and tested whether propanethiol or proply-
methanethiosulfanate bound with cysteine at the critical
positions. If either S267 or A288 was a critical binding
site, the research team reasoned, the analogue should
irreversibly activate the altered receptors but reversibly
activate the unchanged receptors. The researchers found this
effect at a specific TM2 site in both glycine and GABA(A)
"While other possible explanations cannot be fully ruled
out, our results are extremely suggestive that the binding
of alcohols and anesthetics in a protein cavity formed in
part by a single amino acid is both necessary and sufficient
to enhance receptor function," said Dr. Harris. "This
indicates that anesthetics act by a mechanism closer to that
of traditional receptor-mediated pharmacology than was
previously thought. We believe this approach can help steer
future research to define anesthetic binding sites on other
The NIAAA, the National Institute of General Medical
Sciences, and the Texas Commission on Alcohol and Drug Abuse
supported the research. For alcohol research information,
please visit www.niaaa.nih.gov or telephone NIAAA
Reprints are available from the PNAS editorial office (tel.
202/334-2138). For additional information about the
research, please telephone S. John Mihic, Ph.D. (tel.
512/232-7174) through July 31 or Adron Harris, Ph.D. (tel.
512/232-2514) after July 31.