Convulsant action of intracerebroventricularly administered l-kynurenine sulphate, quinolinic acid and other derivatives of succinic acid, and effects of amino acids: structure-activity relationships.

Succinic acid, and the derivatives which have been tested, i.e. phthalic, l-glutamic, l-aspartic, citric and laevulinic acids, in doses of 5,2,5,20,20 and 50 micrograms, respectively, as well as l-kynurenine sulphate and quinolinic acid, injected into the brain ventricles in mice, induced clonic seizures. When administered intraperitoneally or orally they antagonize strychnine-induced seizures. The convulsant effect of l-kynurenine sulphate was observed only when the pH of the solution was 4.0 or lower, while the actions of the other drugs tested were not dependent upon pH. Control intraventricular injections of 5 microliters of 0.06 N sulphuric acid (equivalent to 16.7 micrograms of SO4(2-), the same dose as was injected when a convulsant dose of 50 micrograms of l-kynurenine was administered) induced seizures in 25% of the mice. It is suggested that the convulsant effect of quinolinic acid and l-kynurenine sulphate is related to a moiety of succinic acid (O = C-C-C-C = O). The effect of l-kynurenine sulphate comprises the action of the cation of kynurenine and that of the anion of sulphuric acid. Reduction of the convulsant effect of l-kynurenine sulphate by alpha-amino acids and taurine is presumably related to a "shielding" of both amino- and carbonyl groups of 1-kynurenine. There is no evidence that l-kynurenine sulphate, in small doses, shares the central effects of the inhibitory amino acids.