Hydroxyl Radical‐Mediated Oxidation of Serotonin: Potential Insights into the Neurotoxicity of Methamphetamine

Abstract

When incubated with a hydroxyl radical (HO•)‐generating system (ascorbic acid/Fe2+‐EDTA/O2/H2O2), 5‐hydroxytryptamine (5‐HT; serotonin) is rapidly oxidized initially to a mixture of 2,5‐, 4,5‐, and 5,6‐dihydroxytryptamine (DHT). The major reaction product is 2,5‐DHT, which at physiological pH exists as its keto tautomer, 5‐hydroxy‐3‐ethylamino‐2‐oxindole (5‐HEO). Rapid autoxidation of 4,5‐DHT gives tryptamine‐4,5‐dione (T‐4,5‐D), which reacts with the C(3)‐centered carbanion of 5‐HEO to give 3,3′‐bis(2‐aminoethyl)‐5‐hydroxy‐[3,7′‐bi‐1H‐indole]‐2,4′,5′‐3H‐trione (7). The latter slowly cyclizes to 3′‐(2‐aminoethyl)‐1′,6′,7′,8′‐tetrahydro‐5‐hydroxyspiro[3H‐indole‐3,9′‐[9H]pyrrolo[2,3‐f]quinoline]‐2,4′,5′(1H)‐ trione (9). A minor amount of T‐4,5‐D dimerizes to give 7,7′‐bi‐(5‐hydroxytryptamine‐4‐one) (7,7′‐D). In the presence of GSH, the reaction of T‐4,5‐D with 5‐HEO is diverted and, in the presence of sufficient concentrations of this tripeptide, completely blocked. This is because GSH preferentially reacts with T‐4,5‐D to give 7‐S‐glutathionyltryptamine‐4,5‐dione (11). The results of this investigation suggest that 5,6‐DHT, 5‐HEO, 7, and 9 are products unique to the HO•‐mediated oxidation of 5‐HT. Thus, the observation of other investigators that 5,6‐DHT is formed in the brains of rats following a large dose of methamphetamine (MA) suggests that this drug might evoke HO• formation. However, the present in vitro study indicates that 5,6‐DHT is a rather minor, unstable product of the HO•‐mediated oxidation of 5‐HT and suggests that detection of 5‐HEO, 7/9, and 11 in rat brain following MA administration could provide additional support for HO• formation. Furthermore, one or more of the intermediates and major products of oxidation of 5‐HT by HO• might, in addition to 5,6‐DHT, contribute to the MA‐induced degeneration of serotonergic neurons.

Protective Actions of the Vesicular Monoamine Transporter 2 (VMAT2) in Monoaminergic Neurons

“…While the indole structure of serotonin is less susceptible to quinone formation, the addition of a hydroxyl group creates the possibility for further oxidation to an ortho-or metaquinone, causing adduction to macromolecules and terminal destruction [185]. Injection of 5,6-dihydroxytryptamine or 5,7-dihydroxytryptamine destroys serotonin neurons, and it has been suggested that serotonin can be oxidized to these compounds in vivo [186][187][188]. Experimental evidence suggests that in Alzheimer's disease and disorders of copper storage, serotonin oxidation produces 4,5-dihydroxytryptamine which further oxidizes to compounds that inhibit acetylcholine esterase and cause oxidative stress [189][190][191][192].…”

“…Experimental evidence suggests that in Alzheimer's disease and disorders of copper storage, serotonin oxidation produces 4,5-dihydroxytryptamine which further oxidizes to compounds that inhibit acetylcholine esterase and cause oxidative stress [189][190][191][192]. Furthermore, the production of neurotoxic oxidation products of cytosolic serotonin has been proposed as a possible mechanism of methamphetamine neurotoxicity in these neurons [187,191,193].…”

Fenfluramine‐induced serotonergic neurotoxicity in mice: lack of neuroprotection by inhibition/ablation of nNOS

“…The mechanism underlying the development of serotonergic neurotoxicity is unclear. Endogenous formation of 5,6-dihydroxytryptamine due to 5-HT excess release following the administration of substituted amphetamines (Commins et al 1987), and the generation of other products of 5-HT oxidation (Wrona et al 1995) may be involved in degeneration of 5-HT nerve terminals.…”

“…The mechanism underlying the development of serotonergic neurotoxicity is unclear. Endogenous formation of 5,6-dihydroxytryptamine due to 5-HT excess release following the administration of substituted amphetamines (Commins et al 1987), and the generation of other products of 5-HT oxidation (Wrona et al 1995) may be involved in degeneration of 5-HT nerve terminals.Other amphetamine analogs, such as methamphetamine (METH) and 3,4-methylenedioxy-methamphetamine (MDMA) cause varying degrees of dopaminergic and serotonergic neurotoxicity, depending on the species examined. Evidence suggests the involvement of free radicals and, specifically, reactive oxygen species in amphetamines-induced dopaminergic neurotoxicity (Cadet and Brannock 1998;Hanson et al 1998).…”

Serotonin neurotoxins — past and present

“…This may not be surprising when one considers that . OH-mediated oxidation by nonenzymatic procedures in vitro (e.g., by ascorbic acid/Fe 2+ -EDTA/O 2 /H 2 O 2 ,) results in the formation of a number of products (2,5-DHT, 4,5-DHT, 5-OH-3-ethylamino-2-oxoindole, 5-HEO, tryptamine-4,5-dione) among which 5,6-DHT was only a minor, unstable component (Wrona et al, 1995a). Similar intermediates are generated by electrochemically driven and tyrosinase-O 2 -mediated oxidation (the enzyme that catalyzes 5,6-o-indolequinone formation from SERT in melanocytes).…”