Striatal dopamine release in vivo following neurotoxic doses of methamphetamine and effect of the neuroprotective drugs, chlormethiazole and dizocilpine.

Abstract

1. Administration to rats of methamphetamine (15 mg kg-1, i.p.) every 2 h to a total of 4 doses resulted in a neurotoxic loss of striatal dopamine of 36% and of 5-hydroxytryptamine (5-HT) in the cortex (43%) and hippocampus (47%) 3 days later. 2. Administration of chlormethiazole (50 mg kg-1, i.p.) 15 min before each dose of methamphetamine provided complete protection against the neurotoxic loss of monoamines while administration of dizocilpine (1 mg kg-1, i.p.) using the same dose schedule provided substantial protection. 3. Measurement of dopamine release in the striatum by in vivo microdialysis revealed that methamphetamine produced an approximate 7000% increase in dopamine release after the first injection. The enhanced release response was somewhat diminished after the third injection but still around 4000% above baseline. Dizocilpine (1 mg kg-1, i.p.) did not alter this response but chlormethiazole (50 mg kg-1, i.p.) attenuated the methamphetamine-induced release by approximately 40%. 4. Dizocilpine pretreatment did not influence the decrease in the dialysate concentration of the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) produced by administration of methamphetamine while chlormethiazole pretreatment decreased the dialysate concentration of these metabolites still further. 5. The concentration of dopamine in the dialysate during basal conditions increased modestly during the course of the experiment. This increase did not occur in chlormethiazole-treated rats. HVA concentrations were unaltered by chlormethiazole administration. 6. Chlormethiazole (100-1000 microM) did not alter methamphetamine (100 microM) or K+ (35 mM)-evoked release of endogenous dopamine from striatal prisms in vitro. 7. Several NMDA antagonists prevent methamphetamine-induced neurotoxicity; however chlormethiazole is not an NMDA antagonist. Inhibition of striatal dopamine function prevents methamphetamine-induced toxicity of both dopamine and 5-HT pathways. Therefore the attenuation of the enhanced dopamine release which occurs in animals given chlormethiazole may be associated with the protective action of this drug against methamphetamine-induced neurotoxicity.