Antagonism of excitatory amino acid-induced responses and of synaptic excitation in the isolated spinal cord of the frog

1. A range of compounds has been tested for excitatory amino acid agonist or antagonist activity and for effects on synaptic activity on isolated hemisected spinal cords of frogs. 2. L-Monoamino dicarboxylic acids of chain length up to 8 carbon atoms (L-alpha-aminosuberate) were all agonists. 3. Within a series of D-monoamino dicarboxylic acids, and with diamino dicarboxylic acids (mainly unresolved mixtures of diasteroisomers), there was a progression from agonist activity, for compounds of chain length equal to or shorter than glutamate, to antagonist activity, for compounds of longer chain length equal to or shorter than glutamate, to antagonist activity, for compounds of longer chain length, D-alpha-Aminosuberate (D alpha SD) was the most potent antagonist. 4. The antagonist actions of these substances showed a Mg2+--like selectivity with respect to depolarizations produced by different excitants. N-methyl-D-aspartate (NMDA) was the most susceptible agonist and quisqualate and kainate the least susceptible. Responses to other excitatory amino acids, including L-glutamate and L-aspartate, showed intermediate sensitivity to the antagonists. 5. A parallelism was observed between the relative potencies of mono- and diamino dicarboxylic acids as NMDA antagonists and their relative potencies as depressants of synaptic responses. 6. The results support the concept of different types of excitatory amino acid receptors, with NMDA and its antagonists acting predominantly on one type. These NMDA receptors are probably transmitter receptors activated by an excitatory amino acid transmitter.

Quisqualamine, a novel gamma-aminobutyric acid (GABA) related depressant amino acid

A new substnace, quisqualamine, the decarboxylated analogue of quisqualic acid, predictably depressed electrical activity of neurons of the frog and rat spinal cord in vitro and of the mouse spinal cord in vivo. In the in vitro preparations, the action of quisqualamine was associated with a prolonged depolarization of primary afferent terminals which was sensitive to blockade by picrotoxin and bicuculline and which was also depressed by strychnine. This suggests an interaction of quisqualamine with presynaptic receptors for both GABA and beta-alanine. Post-synaptic actions of quisqualamine, which were less marked than those at presynaptic sites, also appeared to be predominantly GABA-mimetic in vitro, though a sensitivity to the GABA-antagonist bicuculline could not be demonstrated in vitro.

THE STRUCTURE OF LINSEED MUCILAGE: PART II

Linseed mucilage has been separated into an acidic and a neutral fraction. The acidic fraction was further separated, by the use of cupric acetate solution, into two fractions, CuI and CuII. Fraction CuI contained L-rhamnose, L-galactose, and D-galacturonic acid. The methylated reduced polysaccharide gave on hydrolysis 2,3,4-tri-O-methyl-L-rhamnose, 3,4-di-O-methyl-L-rhamnose, 4-O-methyl-L-rhamnose, 2,3,4,6-tetra-O-methyl-D-galactose, 2,3,6-tri-O-methyl-D-galactose, and 2,3-di-O-methyl-D-galactose (?); L-galactose was lost during the methylation process. Periodate oxidation studies on the material indicated that the polymer was composed of a main chain of L-rhamnose units with most of the L-galactose units attached as non-reducing end groups. Fraction CuII contained L-rhamnose, L-fucose, L-galactose, and D-galacturonic acid. The methylated reduced polysaccharide gave on hydrolysis 2,3,4-tri-O-methyl-L-fucose, 2,3,4,6-tetra-O-methyl-L-galactose, 2,3,6-tri-O-methyl-D-galactose, 4-O-methyl-L-rhamnose, L-rhamnose, and possibly 2,3-di-O-methyl-D-galactose and 3-O-methyl-D-galactose. Periodate oxidation studies and a degradation by the Smith procedure indicated the presence of a L-rhamnose backbone with L-fucose and L-galactose units attached as non-reducing end groups.The neutral fraction yielded a periodate-oxidizable material after one Smith-type degradation. Periodate oxidation studies indicated that the degraded material was branched. Methylation of the degraded polysaccharide followed by hydrolysis yielded 2,3,4-tri-O-methyl-D-xylose, 2,3-di-O-methyl-D-xylose, 2,4-di-O-methyl-D-xylose, 4-O-methyl-D-xylose, D-xylose, and traces of 2,3,4-trt-O-methyl- or 2,5-di-O-methyl-L-arabinose, 2,4-di-O-methyl-D-xylose, and 3-O-methyl-D-xylose. The main backbone of the degraded polysaccharide appeared to consist of (1 → 4)-linked D-xylose units. Linkages of the (1 → 3) type were also present. The smaller fragments from the Smith-type degradation, L-arabinose, 2-O-α-L-arabinosyl glycerol, and glycerol were characterized. A partial acid hydrolysis of the neutral fraction yielded a number of oligosaccharides.