Amino acid transmitters in the mammalian central nervous system

144 D.R. CURTIS and G.A.R. JOHNSTON :
1973), glycine usually being more effective than GABA when the amino acids
are administered near cell bodies, with a reverse order of relative potency when
the amino acids were administered more dorsally near dendrites (Rabbit: ALa'-
MANN, BRUGGENCATE, and SONNHOF, 1972). Both of these amino acids, and flalanine
and b-aminovaleric acid, increase the membrane conductance of hypoglossal
motoneurones and hyperpolarize these cells (BRUGGENCATE and SONNHOF,
1972). Furthermore the inhibitory effect of glycine is selectively blocked by strychnine
(Cat: BRUGGENCATE and NONNHOF, 1972. Rat: DUGGAN et a1.,1973) and
that of GABA by picrotoxin (BRUGGENCATE and SONNHOF, 1972) and bicuculline
(DUGGAN et al., 1973). Since IPSP's recorded from hypoglossal motoneurones
in response to impulses in the lingual nerve are reduced by strychnine (Cat:
Intravenous 0.08 mg/kg, MORIMOTO, TAKATA, and KAWAMURA, 1968), and inhibition
evoked by impulses in the glossopharyngeal nerve, and associated with
chloride sensitive IPSP's, can be reduced by intravenous or electrophoretic strychnine
but not by electrophoretic bicuculline (Rat: DUGGAN et al., 1973), glycine
can be assumed to be an inhibitory transmitter influencing hypoglossal motoneurones
most probably at axosomatic synapses. The relatively higher sensitivity
of the dendritic regions of these cells to GABA may be associated with an
inhibitory transmitter function of this amino acid at axodendritic synapses (see
ALTMANN et al., 1972). It is thus significant that impulses in high threshold hypoglossal
afferents have been reported to produce long latency and duration chloridesensitive
IPSP's in hypoglossal motoneurones, an inhibitory effect blocked by
picrotoxin (2mg/kg) but not by strychnine (0.2ms/ks, Cat: MORIMOTO and
KAWAMURA, 1972).
4.10.2. Reticular Formation
In the brain stem and medullary reticular formation most investigations have
been concerned with anatomically located "reticular" neurones, although the
sensitivity of antidromically identified reticulospinal neurones to depressant
amino acids has been correlated with the operation of certain inhibitory pathways.
Medullary reticulospinal neurones are approximately equally sensitive to L-
glutamate and L-aspartate (Cat: TEB~:ClS, 1973b)as are reticular neurones (Cat:
H/SSLI and TEB~CIS, 1970). In both of these studies DL-homocysteate was a
more potent excitant than either naturally occurring acidic amino acid, although
another report suggests that unlike the situation elsewhere in the nervous
system, medullary reticular neurones excited by DL-homocysteate were not
always excited by L-glutamate, and vice versa (Cat: BOAKES et al., 1970).
Lysergic acid diethylamide (LSD) has been reported to block the excitation
of medullary reticular neurones by 5-hydroxytryptamine and L-glutamate, the
excitatory action of acetylcholine, DL-homocysteate, noradrenaline and of L-
glutamate on cells depressed by 5-hydroxytryptamine, being unaffected (Cat:
BOAKES et al., 1970). In contrast, in the rat, LSD reduced the effect of L-glutamate
on neurones within the dorsal and median raph6 nuclei which were depressed
by 5-hydroxytryptamine (Rat: AGHAJANIAN, HAIGLER, and BLOOM, 1972).