Sulfur-containing amino acids were found to inhibit norepinephrine-stimulated [3H]phosphoinositide hydrolysis in rat cortical slices. Of the amino acids tested, l-cysteine was the most potent, inhibiting the response by 42 and 85% at concentrations of 50 and 500 μM respectively. l-Cystine and l-serine-O-sulfate also inhibited the response to norepinephrine, but to a lesser degree than did l-cysteine. l-homocysteic acid slightly potentiated phosphoinositide hydrolysis at a concentration of 100μm, but caused inhibition at 500 μM. l-cysteine sulfinate produced effects intermediate to those of l-cysteine and l-homocysteic acid, having no effect on the response to norepinephrine at 50 μM, but causing 84% inhibition at 500 μM. The d-isomers of cysteine and homocysteic acid were much less potent than were the l-isomers. Examination of the time course of the inhibition of norepinephrine-stimulated [3H]phosphoinositide hydrolysis by l-cysteine showed that it was inhibited almost completely after 15, 30, 45 and 60min of incubation. l-Systeine and l-homocysteic acid caused similarly strong inhibitions of the production of [3H]inositol monophosphate, [3H]inositol bisphosphate and [3H]inositol trisphosphate. The hydrolysis of [3H]phosphoinositides stimulated by norepinephrine in slices from rat hippocampus and striatum were inhibited by l-cysteine to an extent similar to that occurring in cortical slices. These results demonstrate that several sulfur-containing amino acids, some of which have been proposed to be endogenous excitatory amino acid neurotransmitters, effectively modulate the response to norepinephrine of the phosphoinositide second messenger system in rat brain.
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