We have recently characterized a novel oxidation product of serotonin (5-hydroxytryptamine, 5-HT), tryptamine-4,5-dione, which increases 5-HT efflux from striatum and hippocampus and causes selective neuronal death. Exposure of striatal synaptosomes or the major brain guanine nucleotide-binding regulatory proteins Gi and Go to [3H]tryptamine-4,5-dione resulted in the radiolabeling of a major band with an apparent molecular mass equivalent to that of the alpha subunits of Gi and Go (approximately 40,000). The binding of [35S]guanosine-5'-O-(3-thiotriphosphate) ([35S]GTP-gamma-S) to Gi and Go and pertussis toxin-catalyzed [32P]ADP-ribosylation of the G protein alpha subunits were both inhibited in a dose-dependent manner by tryptamine-4,5-dione. Thus, neurotoxins such as tryptamine-4,5-dione may exert their effects through specific interactions with G proteins.