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Structure and pharmacology of vertebrate GABA(A) receptor subtypes.
INT.REV.NEUROBIOL. 38 95-138 (1995).
2. RUDOLPH, U., CRESTANI, F. AND MOHLER, H.
GABA(A) receptor subtypes: dissecting their pharmacological functions.
TRENDS PHARMACOL.SCI. 22 188-194 (2001).
3. BARNARD, E.A., SKOLNICK, P., OLSEN, R.W., MOHLER, H., SIEGHART, W.,
BIGGIO, G., BRAESTRUP, C., BATESON, A.N. AND LANGER, S.Z.
International Union of Pharmacology: XV. Subtypes of gamma-aminobutyric acid
function.
PHARMACOL.REV. 50 291-313 (1998).
4. BONNERT, T.P., MCKERNAN, R.M., FARRAR, S., LE BOURDELLES, B.,
HEAVENS, R.P., SMITH, D.W., HEWSON, L., RIGBY, M.R., SIRINATHSINGHJI, D.J.,
BROWN, N., WAFFORD, K.A. AND WHITING, P.J.
Theta, a novel gamma-aminobutyric acid type A receptor subunit.
PROC.NATL.ACAD.SCI.U.S.A. 96 9891-9896 (1999).
5. ASHCROFT, F.M.
GABA(A) receptors.
IN ION CHANNELS AND DISEASE, ACADEMIC PRESS, 2000, PP.325-336.
6. PRITCHETT, D.B., SONTHEIMER, H., SHIVERS, B.D., YMER, S., KETTENMANN, H.,
SCHOFIELD, P.R. AND SEEBERG, P.H.
Importance of a novel GABA(A) receptor subunit for benzodiazepine
pharmacology.
NATURE 338 582-585 (1989).
7. ZHANG, D., PAN, Z.H., AWOBULUYI, M. AND LIPTON, S.A.
Structure and function of GABA(C) receptors: a comparison of native versus
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| Documentation | Gamma-aminobutyric acid type (GABA) is the major inhibitory neurotransmitter
in the vertebrate central nervous system (CNS). It modulates inhibitory tone
by interacting with two classes of receptor: ionotropic type A (GABAA) and
metabotropic type B (GABAB) receptors [1]. GABAA receptors are pentameric
membrane proteins that operate GABA-gated chloride channels [2] and belong
to the ligand-gated ion channel superfamily [1]. Eight types of receptor
subunit have been cloned, with multiple subtypes within some classes: alpha
1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [3,4].
Subunits are typically 50-60kDa in size and comprise a long N-terminal
extracellular domain; 4 putative transmembrane (TM) domains; and a large
cytoplasmic loop connecting the third and fourth TM domains [5]. Amongst
family members, the large cytoplasmic loop displays the most divergence in
terms of primary structure, the TM domains showing the highest level of
sequence conservation [6].
The effects of GABA on GABAA receptors are modulated by a range of
therapeutically important drugs, including barbiturates, anaesthetics and
benzodiazepines (BZs), which include the widely prescribed drugs librium and
valium [1,5]. Furthermore, they are characterised by their sensitivity
towards a selective antagonist, bicuculline [7]. A second type of ionotropic
GABA receptor has been identified that is insensitive to bicuculline and
classical GABAA modulators but has an enhanced affinity for GABA. This
receptor was found to be composed principally of rho subunits and was termed
'GABAC' in recognition of its altered pharmacology [8]. Despite this
distinction, rho subunits are generally considered to be part of the GABAA
family of receptor proteins due to similarities in sequence and topology.
Whilst early studies supported the view that rho subunits assembled to form
a homopentamer, it has been shown that a mutant rho 1 protein is able to
coassemble with GABAA gamma 2 subunits as well as the glycine receptor alpha
subunit [7]. Rho subunit mRNA occurs prominently in both human and rat
retina [3], each subunit showing a characteristic pattern of spatial
expression. In rat retina, rho 1 mRNA has been detected only in bipolar
cells, whereas rho 2 transcripts have been detected in both bipolar and
ganglion cells. In retinal tissues, expression of rho 3 mRNA is exclusive to
ganglion cells [7]. Reverse transcriptase PCR (RT-PCR) and in situ
hybridisation have shown rho transcripts also to be present in other regions
of the brain, specifically those involved in visual signal processing, such
as the superior colliculus and visual cortex [7].
GABAARRHO is a 3-element fingerprint that provides a signature for GABAA
receptor rho subunits. The fingerprint was derived from an initial alignment
of 9 sequences: the motifs were drawn from conserved regions spanning the
full alignment length, focusing on those sections that characterise rho
subunits but distinguish them from the rest of the GABAA receptor subunit
family - motif 1 lies at the N-terminus; and motifs 2 and 3 reside within
the long cytoplasmic loop connecting TM domains 3 and 4 [7]. A single
iteration on SPTR40_18f was required to reach convergence, no further
sequences being identified beyond the starting set.
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