Stizolobic acid, a competitive antagonist of the quisqualate-type receptor at the crayfish neuromuscular junction

Stizolobic acid on frog spinal cord; possible species dependent activation of excitatory amino acid receptors

1. We examined the effects of stizolobic acid, an amino acid isolated from a plant, Stizolobium hassjoo, on the binding of [3H]glutamic acid and [3H]kainic acid to synaptosomes from frog spinal cords and on the depolarization at the ventral roots of frog spinal cords. 2. Stizolobic acid inhibited the binding of [3H]kainic acid more potently than that of [3H]glutamic acid. 3. Among stizolobic acid derivatives, 3-Br-stizolobic acid was the most potent inhibitor of the binding of [3H]kainic acid, but the inhibitory potency was 100 times weaker than that of kainic acid. 4. Stizolobic acid and its derivatives could cause depolarization of the ventral root of frog spinal cord in a dose dependent manner, and 3-Br-stizolobic acid was a more potent inducer of depolarization than kainic acid, but the dose dependency of 3-Br-stizolobic acid was a little different from that of kainic acid. 5. The results suggest that stizolobic acid and its analogues act as a kainic acid agonist in frog spinal cord. 6. The present results and others indicate that stizolobic acid may interact with the different types of excitatory amino acid receptors dependent on the species of animals.

A metabotropic l-Glutamate receptor agonist: Pharmacological difference between rat central neurones and crayfish neuromuscular junctions

1. 2S,3S,4S-2-(carboxycyclopropyl)glycine (L-CCG-I), a conformationally restricted glutamate analogue, is a potent metabotropic L-glutamate receptor agonist in the mammalian central nervous system. 2. Depolarizing actions of L-CCG-I and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) in the newborn rat spinal motoneurone are temperature-sensitive, and are not depressed by 3-[(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP) and/or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. L-CCG-I and trans-ACPD induced oscillatory responses in Xenopus oocytes injected with rat brain mRNA. Oocytes with oscillatory responses to L-CCG-I and trans-ACPD showed reversal potential of about -20 mV, which was very close to the equilibrium potential of chloride ions. 4. In rat hippocampal synaptoneurosomes, L-CCG-I stimulated phosphoinositide hydrolysis in a concentration dependent manner. L-CCG-I was less potent than quisqualate but more potent than trans-ACPD. 5. At low concentrations, L-CCG-I did not cause any depolarization of newborn rat spinal motoneurones, but reduced substantially amplitudes of monosynaptic reflexes. 6. At the crayfish neuromuscular junction L-CCG-I, acting presynaptically, reduced the amplitude of excitatory junctional potentials. This action was prevented by application of picrotoxin but not pertussis toxin. The actions of trans-ACPD differ from those of either L-CCG-I or ibotenate at the crayfish neuromuscular junction. 7. L-CCG-I has a potential to provide further useful information on metabotropic L-glutamate receptor function.

Excitatory action of a plant extract, stizolobic acid, in the isolated spinal cord of the rat

Actions of stizolobic acid, stizolobinic acid and their derivatives were examined on the isolated spinal cord of the newborn rat. The responses were recorded from the ventral root. Stizolobic acid and its bromo-derivatives caused a depolarizing response in a dose-dependent manner. Stizolobinic acid was considerably less potent than stizolobic acid. Depolarizing responses to stizolobic acid and its bromo-derivatives were not affected by the existence of Mg2+ and specific N-methyl-D-aspartate (NMDA) antagonists. Kynurenate depressed responses to stizolobic acid. These results suggest that stizolobic acid is a new excitatory amino acid in the mammalian central neurons which binds preferably to other receptors than the NMDA-type receptor.