Amino acid transmitters in the mammalian central nervous system

Amino Acid Transmitters in the Mammalian Central Nervous System 111
into protein, GABA (and glycine) stimulate protein synthesis in cell free extracts
of immature rat brain (BAXTER, TEWARI, and RAEBURN, 1972). An extensive
review of GABA literature has been published (SYTINSKY, 1972).
GABA Metabolism. GABA is synthesised by the decarboxylation of L-glutamate.
L-glutamate decarboxylase (GAD, EC. 4.1.1.15) exists in at least two
forms: GAD I and GAD II. GAD I, the form more widely studied, requires
pyridoxal phosphate as a cofactor and is strongly inhibited by anions such as
chloride, and by carbonyl trapping reagents such as thiosemicarbazide (ROBERTS
and KURIYAMA, 1968). GAD I is relatively insensitive to product inhibition by
GABA, but GABA may repress the synthesis of GAD I (Mouse: SZE, 1970;
SZE and LOVELL, 1970). GAD I appears to be localised predominantly within
the axoplasm of particular nerve endings and is not found in appreciable amounts
in non-neural tissues. GAD II is relatively insensitive to inhibition by anions,
is stimulated by carbonyl trapping reagents and is found in various non-neural
tissues (Human: HABER, KURIYAMA, and ROBERTS, 1970. Rabbit: KURIYAMA,
HABER, and ROBERTS, 1970).
GABA is degraded by the action of a specific GABA transaminase (4-aminobutyrate
:2-oxoglutarate aminotransferase, GABA-T, EC. 2.6.1.19) to succinic
semialdehyde, which is in turn oxidised to succinate by a specific dehydrogenase.
A number of multiple forms of GABA-T have been reported (Mouse, rat: WAKS-
MAN and BLOCH, 1968), and GABA-T activity has been found in non-neural
tissues (Rat: CACIAPPO, PANDOLFO, and DI CHIARA, 1959). GABA-T transaminates
some other substances related to GABA, including/~-alanine and 8-
amino-valeric acid, but not 7-amino-fl-hydroxybutyric acid or taurine (Mouse:
WAKSMAN and ROBERTS, 1965. Rat: SYTINSKY and VASILIJEV, 1970; BEART and
JOHNSTON, 1973 a). Many inhibitors of GABA-T activity, including amino-oxyacetic
acid, hydroxylamine and 3-hydrazinopropionic acid, increase brain GABA
levels (BAXTER, 1970; IVERSEN, 1972) but, as with inhibitors of GAD I, cause
and effect are difficult to correlate.
The metabolic pathway from 2-oxoglutarate to succinate via L-glutamate,
GABA and succinic semialdehyde constitutes the "GABA-shunt", i.e. an alternative
pathway to that through succinyl-coenzyme A in the normal tricarboxylic
acid cycle. It has been estimated that in rat brain slices the GABA-shunt constitutes
about 8% of the total carbon flux of the tricarboxylic acid cycle (B~LAZS,
MACHIYAMA, HAMMOND, JULIAN, and RICHTER, 1970). In vivo experiments with
rats indicate that the rate of GABA synthesis in the brain is about 5 pmole/g/h
(VAN GELDER, 1966).
Histochemical methods have been described for the localization of GABA
(WoLMAN, 1971), GABA-T (VAN GELDER, 1965a) and succinic semialdehyde
dehydrogenase (SIMS, WEITSEN, and BLOOM, 1971). Use has also been made of
the distribution of radiolabelled thiosemicarbazide as an indicator of GABA
metabolism (CSILLIK, GEREBTZOFF, KISS, and KNYIH~,R, 1971).
GABA Transport. GABA is taken up into brain slices by a saturable "high
affinity" process (Km approx. 10-5 M), which is structurally specific, dependent
on temperature and requires sodium ions (ELLIOTT and VAN GELDER, 1958;
ROBERTS and KURIYAMA, 1968; IVERSEN and NEAL, 1968; COHEN and LAJTHA,
1972). "Low affinity" uptake (Km approx. 10-4M) of GABA has also been