Amino acid transmitters in the mammalian central nervous system

Amino Acid Transmitters in the Mammalian Central Nervous System 131
in the presence of amino-oxyacetic acid (Rat: HOKFELT and LJUNGDAHL,
1971 b).
Electrophoretically administered GABA depresses the firing of neurones in
the many regions of the cat rhinencephaton which have been examined--CA1,
CA2, CA3 (STEEANIS, 1964; BISCOE and STRAUGHAN, 1966; CURTIS et al., 1971b):
dentate gyrus (STEFANIS, 1964): septum (HERz and GOGOLAK, 1965): amygdala
(STRAUGHAN and LEGGE, 1965): pyriform cortex (LEGGE, RANDI~, and STRAUGnAN,
1966); and similar results have been observed in the rat (STEINER and
RUE, 1967). Although GABA was a more effective depressant of the firing of
pyramidal cells than either glycine or fl-alanine (CURTIS et al., 197 lb), hyperpolarization
by GABA has yet to be demonstrated. However, bicuculline blocks the
inhibition of pyramidal cells by GABA (and by fl-alanine, CURTIS et al., 1971b)
and reduces the prolonged synaptic inhibition (ANDERSEN, ECCLES, and LOYNING,
1964) of these neurones which follows excitation of basket cells (CURTIS, FELIX,
and MCLENNAN, 1970C). This synaptic inhibition is not affected by strychnine
(ANDERSEN et al., 1963), an alkaloid which appeared to have little selectivity
as an amino acid antagonist in this region (CURTISet al., 1971b). The effects
of systemic or topical convulsants on hippocampal responses are complex (BAKER,
KRATKY and BENEDICT, 1965; GESSI, RABINI, and VOLTA, 1967).
4.4. Retina
Although a number of amino acids are present in the vertebrate retina, together
with relevant enzymes and transport processes (LOLLEr, 1969), the precise nature
of the neurones synthesising and releasing particular amino acids has yet to
be determined. The organization and structure of the vertebrate retina has been
reviewed recently by DOWLING (1970).
The high levels of both GABA and GAD in the ganglion cell layer of the
rabbit were considered to reflect technical difficulties in the physical separation
of retinal layers rather than an association of the amino acid with excitatory
neurones (KuRIYAMA, SISKEN, HABEN and ROBERTS, 1968). The distribution of
the enzyme and amino acid in the frog retina led to the suggestion that amacrine,
and probably also horizontal, cells may synthesise GABA (GRAHAM, 1972). Glutamate
is evenly distributed between the receptor, bipolar and ganglion cell layers
of the frog retina, and the levels are not significantly different in retinae adapted
to light or to darkness. In contrast GABA levels in both bipolar and ganglion
cell layers are decreased in dark adapted frog retina (GRAHAM, BAXTER, and
LOLLEY, 1970). Similarly, in the goldfish, light stimulation enhances the synthesis
of GABA from glutamate (LAM, 1972).
Retinal tissue takes up amino acids, both in vivo and in vitro, and use has
been made of radiolabelled compounds to determine both the kinetics of the
processes (GABA-Rat: GOODCHILD and NEAL, 1973: high affinity, STARR and
VOADEN, 1972a; NEAL and STARR, 1973. Glycine-Rabbit: high affinity, BRUUN
and EHINGER, 1972. Taurine-Rat: low affinity, STARR and VOADEN, 1972 b ; Chicken:
PASANTES-MORALES, KLETHI, URBAN, and MANDEL, 1972. Glutamate-Rat:
high affinity, NEAL and WHITE, 1971) and sites of amino acid accumulation.