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Vol. 59, Issue 1, 144-152, January 2001
-Aminobutyric Acid Type B Receptors with Specific Heterodimer
Composition and Postsynaptic Actions in Hippocampal Neurons Are Targets
of Anticonvulsant Gabapentin Action
Merck Frosst Center for Therapeutic Research, Kirkland, Canada
(G.Y.K.N., R.S., J.W., J.Y., M.B., L.T., K.B., K.M., G.P.O.); Centre de
recherche en sciences neurologiques et Département de
physiologie, Université de Montréal, Montréal, Canada
(S.B., J.-C.L.); Institut de Cardiologie de Montréal et le Groupe
de Recherche sur le Système Nerveux Autonome, Université de
Montréal, Montréal, Canada (N.E., T.E.H.); and Merck Sharp
& Dohme Research Laboratories, Terlings Park, Harlow, Essex CM20 2QR,
UK (L.A., A.S., R.M.)
-Aminobutyric acid (GABA) activates two qualitatively different
inhibitory mechanisms through ionotropic GABAA multisubunit chloride channel receptors and metabotropic GABAB G
protein-coupled receptors. Evidence suggests that pharmacologically
distinct GABAB receptor subtypes mediate presynaptic
inhibition of neurotransmitter release by reducing Ca2+
conductance, and postsynaptic inhibition of neuronal excitability by
activating inwardly rectifying K+ (Kir) conductance.
However, the cloning of GABAB gb1 and gb2 receptor genes
and identification of the functional GABAB gb1-gb2 receptor
heterodimer have so far failed to substantiate the existence of
pharmacologically distinct receptor subtypes. The anticonvulsant, antihyperalgesic, and anxiolytic agent gabapentin (Neurontin) is a
3-alkylated GABA analog with an unknown mechanism of action. Here we
report that gabapentin is an agonist at the GABAB gb1a-gb2 heterodimer coupled to Kir 3.1/3.2 inwardly rectifying K+
channels in Xenopus laevis oocytes. Gabapentin
was practically inactive at the human gb1b-gb2 heterodimer, a novel
human gb1c-gb2 heterodimer and did not block GABA agonism at these
heterodimer subtypes. Gabapentin was not an agonist at recombinant
GABAA receptors as well. In CA1 pyramidal neurons of rat
hippocampal slices, gabapentin activated postsynaptic K+
currents, probably via the gb1a-gb2 heterodimer coupled to inward rectifiers, but did not presynaptically depress monosynaptic
GABAA inhibitory postsynaptic currents. Gabapentin is the
first GABAB receptor subtype-selective agonist identified
providing proof of pharmacologically and physiologically distinct
receptor subtypes. This selective agonism of postsynaptic
GABAB receptor subtypes by gabapentin in hippocampal
neurons may be its key therapeutic advantage as an anticonvulsant.
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