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Amino Acid
Neurotransmitters
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Article Contents
. Use of Certain Amino Acids as Neurotransmitters
Jeremy M Henley, University of Bristol, Bristol, UK
. Glutamate: The Major Excitatory Neurotransmitter in
the Mammalian Brain
Amino acid neurotransmitters are single amino acid residues that are released from
presynaptic nerve terminals in response to an action potential, cross the synaptic cleft, and
bind to specific receptor proteins on the postsynaptic membrane to elicit a postsynaptic
response.
Use of Certain Amino Acids as
Neurotransmitters
Rapid communication between neurons at chemical
synapses is mediated by specific receptors which translate
the chemical signal of the neurotransmitter into a change in
membrane potential at the postsynaptic cell. l-Glutamate,
g-aminobutyric acid (GABA) and glycine are all well
established neurotransmitters, and the ion channels
activated by these amino acids account for the overwhelming majority of the fast synaptic neurotransmission
in the central nervous system (CNS). Furthermore, the
receptors mediating the actions of these transmitters play a
central role in a wide range of physiological and
pathophysiological brain processes and thus represent
useful targets for therapeutic drug development. lGlutamate is an excitatory neurotransmitter which acts
at several subtypes of specialized receptor to elicit
depolarization of the postsynaptic cell. GABA and glycine
are both inhibitory neurotransmitters which act at specific
receptors and generally cause the hyperpolarization of the
postsynaptic neuron. Both l-glutamate and GABA also
activate separate classes of guanine nucleotide-binding
(G)-protein-coupled receptors – the metabotropic (mGlu)
and GABAB receptors respectively. However, the properties of these G protein-coupled receptors are not discussed
in this article. (see Neurotransmitters.) (see Metabotropic glutamate receptors.)
. Glutamate-activated Ion Channel Receptors
. AMPA Receptors
. Kainate Receptors
. NMDA Receptor
. GABA: The Major Inhibitory Neurotransmitter in the
Mammalian Brain
. Synthesis of GABA by Decarboxylation of Glutamate
. GABA-activated Ion Channel Receptors (GABAA
Receptors)
. Glycine: The Major Inhibitory Neurotransmitter in the
Mammalian Spinal Cord
. Glycine-activated Ion Channel Receptors
. Summary
is involved in nearly every aspect of CNS function.
(see Glutamate as a neurotransmitter.)
Glutamate-activated Ion Channel
Receptors
In the mammalian CNS three distinct classes of ionotropic
glutamate receptors exist. These were initially named
according to their functional sensitivity to the selective
agonists N-methyl-d-aspartate (NMDA), kainate (KA)
and quisqualate (quisqualate receptors have subsequently
been renamed a-amino-3-hydroxy-5-methyl-4-isoxazole
propionate (AMPA) receptors owing to the greater
selectivity of the latter ligand). Collectively, AMPA and
kainate receptors are often called non-NMDA receptors.
For detailed reviews on ionotropic glutamate receptors
and extensive reference lists see Hollmann and Heinemann
(1994) and Bettler and Mulle (1995). (see Ion channels: ligand
gated.)
Glutamate: The Major Excitatory
Neurotransmitter in the Mammalian
Brain
AMPA Receptors
The excitatory effects of l-glutamate were first observed
more than 40 years ago, and since that time it has been
demonstrated conclusively that l-glutamate is the predominant excitatory neurotransmitter in the CNS. Glutamate is the transmitter at more than 95% of excitatory of
synapses and, therefore, glutamatergic neurotransmission
The pharmacological characterization of AMPA receptors
was fully established by the selective defining agonist
AMPA. More recently, however, (S)-5-fluorowillardiine
(FW) has been shown in functional and binding studies to
have a higher selective potency towards AMPA receptors
than AMPA itself. The most widely used competitive
Pharmacology
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Amino Acid Neurotransmitters
antagonists for AMPA receptors are 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo(F)quinoxaline (NBQX). The rank order
of ligands binding to the agonist recognition site on AMPA
receptors is quisqualate 4 FW 4 AMPA 4 CNQX 5
glutamate @ kainate. However, no compounds that bind
to this site provide complete selectivity between kainate
and AMPA receptors. (see AMPA receptors.)
N-terminal
Lobe 1
Lobe 2
C-terminal
Extracellular
Intracellular
Biochemistry and molecular biology
AMPA receptors are pentameric assemblies of the four
subunits GluR1–GluR4. Each subunit comprises approximately 900 amino acid residues and has a molecular weight
of about 100 kDa. Sequence identity between the four
subunits is around 70%. Alternative splicing of a
messenger ribonucleic acid (mRNA) exon sequence
encoding 38 amino acids just preceding the last membrane
domain IV (MD-IV) produces ‘flip’ and ‘flop’ isoforms. In
addition, a splice cassette has also been detected in the Cterminal region of GluR4 and GluR2. RNA editing of
adenosine to guanosine nucleotides also occurs in the
GluR1–4 subunits. Of particular importance is editing at
glutamine 586 in the putative channel-lining domain. This
changes the codon CAG (glutamine/Q) to CGG (arginine/
R) and, within the AMPA receptor subunits, occurs only in
GluR2. Expression of edited GluR2(R) subunits in the
AMPA receptor complex changes the receptor channel
from being Ca2 1 permeable to being Ca2 1 impermeable.
Editing levels of the Q/R site (i.e. the relative numbers of
edited and nonedited GluR2 subunits) approach 100% in
the adult, and changes in this level may have important
implications for neurodegenerative conditions such as
Alzheimer disease. (see Alternative splicing: cell-type-specific and
developmental control.) (see RNA editing.) (see Alzheimer disease.)
The topology of ionotropic glutamate receptor subunits
was originally believed to be similar to that of nicotinic
acetylcholine, GABAA and glycine receptor subunits (see
Figure 1): namely four transmembrane domains (TMD), an
intracellular loop region between the last two TMDs and
extracellular N- and C-terminals. Recent evidence, however, supports a three TMD model where MD-2 does not
cross the membrane but rather forms a re-entrant loop
from inside the membrane. This segment has significant
homology to the corresponding P region of voltage-gated
K 1 channels. The agonist binding domain has been
proposed to be formed from regions of both the Nterminal region (lobe 1) and the extracellular loop (lobe 2).
Sequence analysis in these regions suggest that the ligand
binding domains of GluRs evolved from bacterial lysine–
arginine–ornithine (LAOBP) and glutamine (Q-BP) binding proteins. Three transmembrane spanning domains
locate the C-terminus on the inside of the cell, a topology
consistent with immunocytochemical studies using antiserum directed against the C-terminus of GluR1, in vitro
2
N-terminal
C-terminal
(a)
(b)
Figure 1 Topology of (a) glutamate and (b) g-aminobutyric acid type A
(GABAA) and glycine receptor subunits. Based on sequence analysis and
topology, glutamate receptor subunits do not share a common ancestral
origin with GABAA and glycine receptors. The latter two receptors appear to
be more closely related to the extensively characterized nicotinic
acetylcholine receptor superfamily than to the glutamate receptor
superfamily.
translation–translocation studies and the distribution of
phosphoserine residues on the subunits. (see Voltage-gated
potassium channels.)
Electrophysiology and function
AMPA receptors channel conductances are due to Na 1 ,
K 1 and, in certain cases, Ca2 1 ion flux across the
membrane. The AMPA receptor displays characteristic
discrete unitary channel currents of about 20 pS. lGlutamate, AMPA and kainate elicit channel conductances in native receptors, however, the profiles of the
responses differ. l-Glutamate and AMPA produce strong
desensitizing responses, whereas kainate-induced responses are nondesensitizing, resulting in larger steadystate currents than AMPA. Recombinant AMPA receptors display the same differential desensitization to AMPA
and kainate. Flip/flop alternative splicing and R/G editing
of subunits within recombinant receptors alters the
desensitization rates and amplitude of agonist-induced
responses. Receptors with differing subunit compositions
display alterations in their I/V relationships and ion
selectivity. All homomeric or heteromeric combinations
of AMPA receptor subunits that lack the edited GluR2
subunit display inward rectification and significant Ca2 1
permeability. The presence of the edited GluR2(R) subunit
produces a receptor that displays a linear or outwardly
rectifying I/V relationship and is impermeable to Ca2 1 .
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Amino Acid Neurotransmitters
Kainate Receptors
Pharmacology
Kainate and domoate remain the most widely used
agonists for kainate receptors. Although domoate shows
a greater potency at native and most recombinant kainate
receptors (not GluR7; see below for subunit descriptions),
like kainate itself it also activates AMPA receptors.
Concanavalin A, which blocks receptor desensitization,
is the most selective kainate receptor compound. Some
halogenated derivatives of willardiine show selectivity for
kainate receptors. (S)-5-trifluoromethylwillardiine is the
most potent willardiine at kainate receptors, whereas (S)5-iodowillardiine is the most selective of the series for
kainate receptors. (RS)-2-amino-3(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA), a compound initially developed as an AMPA agonist, displaces [3H]kainate
binding from recombinant GluR5-containing receptors
with a low nanomolar Ki and is more potent than kainate
itself at expressed homomeric GluR5 receptors. The first
reported kainate-selective antagonist was 5-nitro-6,7,8,9tetrahydrobenzo[g]indole-2,3-dione-3-oxime (NS-102),
although this is less effective than the AMPA/kainate
antagonists CNQX and NBQX. Recently decahydroisoquinoline compounds have been shown to be subunit
specific antagonists. The AMPA receptor noncompetitive
antagonist, LY293558, is also an antagonist of homomeric
GluR5, but not GluR6, responses. Another decahydroisoquinoline, LY294486, selectively inhibits [3H]kainate
binding to homomeric GluR5 receptors, with little effect on
recombinant GluR6, GluR7 and KA2 receptors. Unlike
LY293558, LY294486 has a selectivity towards GluR5
receptors compared with AMPA receptors. (see AMPA
receptors.)
Biochemistry and molecular biology
The subunit sizes, topology and receptor stoichiometry for
assembly for kainate receptors are similar to those
described for AMPA receptor subunits (Figure 1). The
complementary deoxyribonucleic acid (cDNA) encoding
GluR5 was the first kainate receptor subunit to be cloned,
and it shows 40% sequence homology to the AMPA
receptor subunits GluR1–4. Four other kainate receptor
subunits (GluR6, GluR7, KA1 and KA2) have been
identified, and these can be divided into two categories
based on structural homology, affinity for [3H]kainate and
the ability to form functional homomeric channels.
GluR5–7 display 75% homology, whereas KA1 and
KA2 are 68% homologous; the homology between the
GluR5–7 group and KA1/KA2 is about 45%.
There are two alternative splice variants of GluR5
(GluR5-1 and GluR5-2), the former containing an additional 15 amino acids in the extracellular N-terminal
region. Three further C-terminal splice variants of
GluR5-2 have been identified. Two C-terminal alternative
splice variants of GluR7 (a and b) have also been reported.
No RNA editing or alternative splicing has been reported
for KA1 or KA2 subunits. Like GluR2, GluR5 and GluR6
are RNA edited at a glutamine/arginine (Q/R) site. Unlike
the GluR2 subunit, however, significant proportions of
unedited (Q) kainate subunits are present in both the
embryonic and adult CNS. GluR6 can also undergo
further editing at two more sites situated in the first
hydrophobic domain region: isoleucine to valine (I/V) and
tyrosine to cysteine (Y/C). Thus, there are eight different
splice combinations of GluR6 which are all present to
varying extents in the CNS, although the edited GluR6(R/
V/C), the least Ca2 1 permeable combination, is the most
abundantly expressed in the adult CNS. (see mRNA splicing:
regulated and dierential.)
Electrophysiology and function
The characterization of responses mediated by recombinant kainate receptors preceded that of native receptors in
the CNS. Both Q/R splice variants of GluR6 form
functional homomeric receptors but only the unedited
form of GluR5 (GluR5-Q) is functional. Homomeric
GluR7 receptors (both a and b) can form functional
channels but these display a very low affinity for glutamate.
GluR7a receptors are insensitive to AMPA. Also, despite a
very high affinity for domoate, GluR7a receptors are
functionally insensitive to this agonist. Kainate elicits a fast
onset and rapid desensitization of response for all the
functional recombinant receptors, but the sensitivity to
AMPA and deactivation to domoate differs markedly.
The physiological roles for kainate receptors have been
difficult to investigate. However, it has been shown recently
that native postsynaptic kainate receptors can be activated
by high-frequency electrical stimulation (e.g. 20 shocks at
100 Hz) of the mossy fibre pathway but not of the
associational or commissural pathway in hippocampus.
As yet, no physiological function has been determined for
the kainate receptor-induced depolarization of CA3
neurons other than it may act to increase the flexibility of
synaptic integration. There is growing evidence for
presynaptic kainate receptors, which appear to inhibit
neurotransmitter release. Kainate elicits a dose-dependent
decrease in l-glutamate release from rat hippocampal
synaptosomes and depresses glutamatergic synaptic transmission. Brief exposure to kainate inhibits Ca2 1 -dependent [3H]l-glutamate release by up to 80%. Inhibition is
reversed by CNQX and NS-102, but not by the AMPAselective antagonist GYKI52466. Activation of kainate
receptors also downregulates GABAergic transmission in
hippocampal CA1 neurons, suggesting that presynaptic
kainate receptors act as negative-feedback regulators.
(see Neurotransmitter receptors in the postsynaptic neuron.)
(see Glutamatergic synapses: molecular organization.)
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3
Amino Acid Neurotransmitters
NMDA Receptor
Pharmacology
In contrast to the non-NMDA receptors, highly selective
antagonists for the NMDA receptors such as D( 2 )-2amino-5-phosphonopentanoic acid have been available for
a number of years. Analogous to the GABAA receptor, a
number of discrete allosteric binding domains other than
the neurotransmitter site are present within the NMDA
receptor complex. These include a strychnine-insensitive
glycine modulatory site, a polyamine site at which
spermine and spermidine act as partial agonists and, like
glycine, leading to a facilitation of NMDA-mediated
responses. A number of sites within the channel pore have
been identified which include a Zn2 1 and a dissociative
anaesthetic binding domain. Functional activity of the
receptor can be blocked by antagonism of these sites. Most
importantly, an Mg2 1 site is present within the channel
pore, and this confers the crucial functional characteristic
of voltage-sensitive activation (see below). (see NMDA
receptors.) (see GABAA receptors.)
Biochemistry and molecular biology
The size and topology of the NMDAR1 subunit is similar
to those of the AMPA and kainate subunits despite a
relatively low sequence homology (approximately 25%).
NMDAR2 subunits are larger, possessing a particularly
large intracellular C-terminal domain which appears to be
important for protein–protein interactions controlling
targeting, localization, anchoring, etc. So far, four other
NMDA receptor subunits, NMDAR2A to NMDAR2D,
have been isolated; they have an approximate sequence
homology of 55–70% with one another, compared with
only a 20% homology with NMDAR1. In common with
AMPA and kainate subunits, the NMDAR1 subunit is
subject to splicing, which can generate at least eight
separate isoforms (NMDAR1a–h). Splice variants have
also been reported for some, but not all, of NMDAR2
subunits. (see Protein quaternary structure: subunit-subunit interactions.)
Electrophysiology and function
Recombinant homomeric NMDAR1 receptors expressed
in Xenopus oocytes form channels that are pharmacologically and functionally similar to native receptors. However, current amplitudes are significantly smaller than
those obtained in oocytes injected with brain poly(A) 1
RNA. None of the NMDAR2 subunits produce functional
homomeric channels, but coexpression with NMDAR1
produces ligand-gated channels with conductances much
larger than homomeric NMDAR1 channels. NMDA
receptor channels are permeable to Na 1 , K 1 and Ca2 1 .
The channel kinetics are much slower than those of AMPA
4
receptors, but channel conductances are higher (approximately 40 pS) because of the Ca2 1 permeability. Application of glycine alone has no effect, but NMDA-evoked
current amplitude and frequency of channel opening are
substantially increased when glycine is present, leading to
the proposal that glycine acts as a cotransmitter at NMDA
receptors. NMDA receptors play a critical role in the
induction of most forms of long-term potentiation (LTP),
an extensively studied form of synaptic plasticity. At
resting membrane potentials, NMDA receptor channels
are blocked by Mg2 1 , but the block is relieved at the
depolarized postsynaptic membrane potentials that occur
following activation of nearby AMPA receptors. The
consequent influx of Ca2 1 through the NMDA receptors
initiates cellular processes that underlie the increase in
synaptic transmission. (see Translation of mRNAs in Xenopus
oocytes.) (see Glycine as a neurotransmitter.) (see Long-term
potentiation.) (see Calcium signalling and regulation of cell function.)
GABA: The Major Inhibitory
Neurotransmitter in the Mammalian
Brain
GABA mediates its actions via two major classes of
integral membrane receptors which are classified according
to their signal transduction mechanisms. GABAA receptors are ligand-gated chloride channels, whereas GABAB
receptors are G protein-coupled receptors which share
some homology to metabotropic glutamate receptors. For
specialized reviews on GABAA receptors and extensive
references, see Stephenson (1995) and McKernan and
Whiting (1996). (see GABA as a neurotransmitter.)
Synthesis of GABA by Decarboxylation
of Glutamate
Of the neurotransmitter amino acids glutamate, glycine
and GABA, only GABA is uniquely synthesized in
neurons that use it as a neurotransmitter. Glutamate and
glycine are used for general protein synthesis, and these
amino acids can be made by enzymes present in all cells.
GABA is synthesized from glutamate by the enzyme
glutamic acid decarboxylase (GAD), which is present only
in neurons that release GABA. Therefore, antibodies
directed against GAD are used as specific markers for
GABAergic neurons. Following release into the synaptic
cleft, GABA is taken up by specialized transporter proteins
into the nerve terminal or glial cells, and is metabolized by
the enzyme GABA transaminase. (see Amino acid biosynthesis.) (see Neurotransmitter transporters.)
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Amino Acid Neurotransmitters
GABA-activated Ion Channel Receptors
(GABAA Receptors)
form a distinct subclass, termed GABAC receptors.
(see GABAB receptors.)
Pharmacology
Electrophysiology and function
GABAA receptors mediate most fast inhibitory neurotransmission in the mammalian brain. They are notable –
and clinically important – because of the number of
allosteric modulatory sites present on the receptor complex. Modulators of GABAA receptors include, among
other classes of compounds, benzodiazepines, barbiturates
and neurosteroids. Clinically important benzodiazepines
such as diazepam (Valium) are anxiolytic because they
facilitate inhibitory neurotransmission via GABAA receptors, although they do not themselves activate the
channels. Rather, the mechanism of benzodiazepine action
is to increase the frequency of channel opening by GABA.
Barbiturates such as ( 2 )-pentobarbital facilitate GABA
neurotransmission by increasing the mean open time of the
GABAA receptor chloride channel. Neurosteroids potentiate GABAA receptor responses by prolonging the open
time of individual channels. The effects of each of the
modulators are additive, indicating distinct allosteric sites
for each class of enhancer mentioned. (see GABAA receptors.)
(see Benzodiazepines.) (see Drugs and the synapse.)
GABA exerts an influence on a wide range of CNS
functions. In general, activation of GABA receptors
inhibits neuronal activity by the opening of an integral
chloride channel, which causes hyperpolarization. In
young animals, however, GABAA receptors can be
stimulatory, for example in developing neurons where
there is a high intracellular Cl 2 concentration and opening
of the channels allows an efflux of the anion and
consequent depolarization. Because of the useful anxiolytic actions of benzodiazepines, GABAA receptors are
known to be involved in the anxiety and stress responses.
GABAA receptor dysfunction has been implicated in
epilepsy, and stimulation of the fast GABAergic transmission may be of therapeutic benefit. The loss of acetylcholine terminals in Alzheimer disease may also be indirectly
influenced by blocking GABAA receptors, as acetylcholine
activity of most central cholinergic synapses is modulated
by GABA. (see Chloride channels.) (see Action potential: ionic
mechanisms.) (see Mood disorders.) (see Acetylcholine and GABA
receptors.)
Biochemistry and molecular biology
Glycine: The Major Inhibitory
Neurotransmitter in the Mammalian
Spinal Cord
Proteins present in the GABAA receptor complex were
originally biochemically purified from mammalian brain
by benzodiazepine affinity chromatography. From these
studies two subunit polypeptides were identified, designated the a subunit, with an apparent molecular mass (Mr)
of 53 000 kDa and a b subunit with an Mr of 58 000 kDa.
Partial amino acid sequence obtained from the purified
protein was used to isolate and clone cDNAs encoding
GABAA receptor subunits. Once the initial cDNA
sequences were available, specific probes were generated
and used to isolate multiple related cDNAs by lowstringency hybridization library screening.
Mammalian GABAA receptors comprise pentameric
assemblies. There are 13 known subunits (six a subunits,
three b subunits, three g subunits and one d subunit), so
there is a large number of possible subunit permutations
for pentameric receptors. When expressed alone the
subunits do not form receptors efficiently, suggesting that
homomeric receptors do not occur in vivo. Co-immunoprecipitation studies, however, have indicated that a
restricted number of subunit combinations appears to
exist in rat brain, with a1b2g2 being the most abundant
receptor combination (approximately 43% of receptors).
In contrast, a related rho subunit (r), which is located
mainly in the retina, efficiently forms homomeric channels
with properties similar to GABA receptors present in
invertebrates. It has been proposed that these channels
Glycine receptors are widely distributed throughout the
CNS; however, whereas GABA is the predominant
inhibitory transmitter in the higher regions of the CNS,
glycine is the main inhibitory neurotransmitter in the
spinal cord and brainstem. Glycine receptors display a
nanomolar affinity for strychnine. For specialized reviews
and references, see Betz (1992) and Rajendra et al. (1997).
(see Glycine as a neurotransmitter.)
Glycine-activated Ion Channel
Receptors
Pharmacology
Glycine receptors mediate most fast inhibitory neurotransmission in the mammalian spinal cord and brainstem.
The potency of different a and b amino acids at activating
glycine receptors is glycine @ b-alanine 4 taurine @ lalanine, l-serine 4 proline. The plant alkaloid strychnine
is the best characterized high-affinity competitive antagonist which binds at low nanomolar concentrations.
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5
Amino Acid Neurotransmitters
Strychnine poisoning very effectively abolishes glycinergic
neurotransmission, leading to hyperexcitation of the
motor system and muscular convulsions. [3H]Strychnine
is the standard radioligand marker for glycine receptors,
and its regional distribution in the CNS shows a marked
gradient within the neuroaxis. The most dense [3H]strychnine binding is in the brainstem and spinal cord. (see Glycine
receptors.)
Biochemistry and molecular biology
Like GABAA receptors, glycine receptors were first
isolated biochemically, in this case by amino-strychnine
affinity chromatography. Purified receptor was found to
comprise a and b subunits with Mr values of 48 000 and
58 000 Da. It was proposed that the receptors comprise
pentameric assemblies of a and b subunits (probable
stoichiometry a3b2). Photoaffinity labelling with
[3H]strychnine demonstrated that the ligand binding site
resides on the a subunit. An additional protein of Mr
93 000 Da co-purifies with the glycine receptor, and this has
been shown to be a receptor-associated protein named
gephyrin. Gephyrin is a cytoplasmic peripheral membrane
protein which is crucial for the activity-dependent synaptic
localization and clustering of glycine receptors.
(see Synapses.)
The first cDNA encoding a glycine receptor subunit was
reported at the same time as the first GABAA receptor
subunit cDNA. As with GABAA, multiple glycine receptor
subunit clones have been isolated but, unlike GABAA,
these have been confined to the two classes (a and b)
originally identified by protein biochemistry. Four a
subunit variants (a1–4) have now been cloned, and these
have been shown to be under tight developmental control.
In rat neonates a2 subunits predominate, whereas in adults
a3 subunits are prevalent. Alternative splice isoforms of a1
and a2 are also present. As yet, only a single b subunit
cDNA has been found. The glycine receptor subunit
polypeptides share the same structural features as the
GABAA subunits and have about the same percentage
residue conservation to GABAA subunits as seen between
the different subclasses of GABAA subunits themselves.
Immunocytochemistry with subunit specific antibodies
shows a localization consistent with, but not entirely
identical to, that of [3H]strychnine binding. In particular a
subunit immunoreactive neurons have been identified in
many higher brain areas. Curiously, the b subunit mRNA
is expressed at relatively high levels throughout the CNS,
including areas that show neither [3H]strychnine binding
nor a subunit gene expression. These observations have led
to the proposal that either additional glycine receptor
subunits exist which assemble with the b subunit to form
strychnine-insensitive receptors, or that the b subunit can
co-assemble with subunits of other receptors such as
GABAA or ionotropic glutamate receptors.
6
Electrophysiology and function
Like glutamate and GABAA receptors, the channel
properties of the glycine receptor have been investigated
in detail using patch-clamp electrophysiological techniques. In membrane patches from rat spinal cord neurons,
glycine causes bursts of single-channel activity with a main
subconductance state of 45 pS. When expressed in Xenopus
oocytes, most a-subunit homo-oligomers form robust
chloride channels which are blocked by strychnine.
However the a2* splice isoform of a2 is insensitive to
strychnine. The a2* splice isoform is highly expressed in
neonatal rat spinal cord but it is replaced by the a2 form at
about 2 weeks after birth, and this change corresponds to
the appearance of [3H]strychnine binding. Strychnine
sensitivity has been shown to be dependent on residue
glycine 167: when this residue was substituted by sitedirected mutagenesis to glutamine, strychnine sensitivity
was absent. (see Patch clamp and the revolution in cellular
neurobiology.) (see Alternative splicing: cell-type-specific and developmental control.)
An inherited form of myoclonus found in Poll Hereford
cattle has been reported which is characterized by the loss
of [3H]strychnine binding sites in the spinal cord. Similarly,
a mutant mouse strain spastic also has reduced [3H]strychnine binding sites and has been shown to contain a
mutation on chromosome 3 which interferes with the
production of the a1 glycine receptor subunit. Motor
function disorders in these animals resemble human
diseases such as hyperkinesia and spastic paraplegia,
raising the possibility that glycine receptor defects could
be implicated in these conditions. (see Motor system organization.) (see Movement disorders.)
Summary
In terms of the proportion of synapses at which they
operate, amino acids are by far the most important
neurotransmitters in the mammalian CNS. There has been
an explosion in the rate of development in the synthesis of
subtype- and, latterly, subunit-specific agonists and
antagonists and the molecular biological and electrophysiological characterization of native and recombinant
amino acid receptors. The consequent advances in knowledge have revealed a staggering heterogeneity of receptor
subunits, and thus of possible combinations within the
assembled receptor complex. Additional mechanisms such
as alternative splicing and RNA editing increase the
possible receptor diversity yet further. Importantly, many
receptor subunit combinations for the GABAA and
glutamate receptors have been shown to possess different
channel properties. It has been proposed that this huge
potential reservoir of receptor subunit combinations is
available to allow particular neuronal pathways to possess
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Amino Acid Neurotransmitters
customized receptors and even to allow individualization
of synapses. Which combinations of receptors actually
exist in mammalian brain, and how their physiological and
pathophysiological functions differ, remains one of the
most pressing questions in neuroscience. (see Receptor transduction mechanisms.) (see Ion channels.) (see Synaptic plasticity:
short term.) (see Cellular neuromodulation.)
References
Bettler B and Mulle C (1995) AMPA and kainate receptors. Neuropharmacology 34: 123–140.
Betz H (1992) Structure and function of inhibitory glycine receptors.
Quarterly Reviews of Biophysics 25: 381–394.
Hollmann M and Heinemann S (1994) Cloned glutamate receptors.
Annual Review of Neuroscience 17: 31–108.
McKernan RM and Whiting PJ (1996) Which GABAA receptor
subtypes really exist in the brain. Trends in Neuroscience 19: 139–143.
Rajendra S, Lynch JW and Schofield PR (1997) The glycine receptor.
Pharmacology and Therapeutics 73: 121–146.
Stephenson FA (1995) The GABAA receptors. Biochemical Journal 310:
1–9.
Further Reading
Ben-Ari Y, Khazipov R, Leinekugel X, Caillard O and Gaiarsa JL (1997)
GABAA, NMDA and AMPA receptors: a developmentally regulated
‘menage a trois’. Trends in Neuroscience 20: 523–529.
Betz H (1991) Glycine receptors: heterogeneous and widespread in the
mammalian brain. Trends in Neuroscience 14: 458–461.
Collingridge GL and Watkins JC (eds) (1994) The NMDA Receptor, 2nd
edn. Oxford: Oxford University Press.
Enna SJ and Bowery NG (eds) (1997) The GABA Receptors, 2nd edn.
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