5-HYDROXY-3-ETHYLAMINO-2-OXINDOLE IS NOT FORMED IN RAT-BRAIN FOLLOWING A NEUROTOXIC DOSE OF METHAMPHETAMINE - EVIDENCE THAT METHAMPHETAMINEDOES NOT INDUCE THE HYDROXYL RADICAL-MEDIATED OXIDATION OF SEROTONIN

Oxygen radicals have been implicated in the neurodegenerative and othe r neurobiological effects evoked by methamphetamine (MA) in the brain. It has been reported that shortly after a single large subcutaneous d ose of MA to the rat, the serotonergic neurotoxin 5,6-dihydroxytryptam ine (5,6-DHT) is formed in the cortex and hippocampus. This somewhat c ontroversial finding suggests that MA potentiates formation of the hyd roxyl radical (HO.) that oxidizes 5-hydroxytryptamine (5-HT) to 5,6-DH T, which, in turn, mediates the degeneration of serotonergic terminals . A major and more stable product of the in vitro HO.-mediated oxidati on of 5-HT is 5-hydroxy-3-ethylamino-2-oxindole (5-HEO). In this inves tigation, a method based on HPLC with electrochemical detection (HPLC- EC) has been developed that permits measurement of very low levels of 5-HEO in rat brain tissue in the presence of biogenic amine neurotrans mitters/metabolites. After intracerebroventricular administration into rat brain, 5-HEO is transformed into a single major, but unknown, met abolite that can be detected by HPLC-EC. One hour after administration of MA (100 mg/kg s.c.) to the rat, massive decrements of 5-HT were ob served in all regions of the brain examined (cortex, hippocampus, medu lla and pens, midbrain, and striatum). However, 5-HEO, its unidentifie d metabolite, or 5,6-DHT were not detected as in vivo metabolites of 5 -HT. MA administration, in particular to rats pretreated with pargylin e, resulted in the formation of low levels of N-acetyl-5-hydroxytrypta mine (NAc-5-HT) in all brain regions examined. These results suggest t hat MA does not potentiate the HO.-mediated oxidation of 5-HT. Further more, the rapid MA-induced decrease of 5-HT might not only be related to oxidative deactivation of tryptophan hydroxylase, as demonstrated b y other investigators, but also to the inhibition of tetrahydrobiopter in biosynthesis by NAc-5-HT. The massive decrements of 5-HT evoked by MA are accompanied by small or no corresponding increases in 5-hydroxy indole-3-acetic acid (5-HIAA) levels. This is due, in part, to the rel atively rapid clearance of 5-HIAA from the brain and monoamine oxidase (MAO) inhibition by MA. However, the loss of 5-HT without correspondi ng increases in its metabolites point to other mechanisms that might d eplete the neurotransmitter, such as oxidation by superoxide radical a nion (O-2(.-)), a reaction that in vitro does not generate 5-HEO or 5, 6-DHT but rather another putative neurotoxin, tryptamine-4,5-dione. On e hour after administration, MA evokes large depletions of norepinephr ine (NE) throughout the brain but somewhat smaller decrements of dopam ine (DA) that are restricted to the nigrostriatal pathway. Furthermore , MA evokes a major shift in the metabolism of both NE and DA from the pathway mediated by MAO to that mediated by catechol-O-methyltransfer ase. The profound and widespread effects of MA on the noradrenergic sy stem, but more anatomically localized influence on the dopaminergic sy stem, suggests that NE in addition to DA, or unusual metabolites of th ese neurotransmitters, might play roles in the neurodegenerative effec ts evoked by this drug.