High-dose methamphetamine treatment alters presynaptic GABA and glutamate immunoreactivity

Abstract

The goal of this study was to determine if high-dose methamphetamine treatment altered presynaptic immunoreactivity for the amino acid neurotransmitters GABA and glutamate within the basal ganglia. Methamphetamine (15 mg/kg every 6 h, four doses) treatment in rats resulted in severe hyperthermia and a long-lasting (four weeks) depletion of striatal dopamine content (>80%). Severe dopamine loss correlated with a decrease in the density of presynaptic immunolabeling for GABA one week post-drug, and an increase after four weeks. Although no changes were seen in presynaptic striatal glutamate immunoreactivity, there was a significant increase in the percentage of glutamate-immuno-positive terminals associated with perforated postsynaptic densities. Rats given the same dose of methamphetamine but prevented from becoming hyperthermic showed less severe dopamine depletions and a lack of ultrastructural or immunocytochemical changes. In addition, induction of hyperthermia in the absence of drug decreased immunolabeling within mitochondria, but had no effect on dopamine content, morphology or nerve terminal immunoreactivity. Altered presynaptic GABA immunolabeling and terminal size were found in both the striatum and globus pallidus, suggesting that dynamic changes occur in the striatopallidal pathway following methamphetamine-induced dopamine loss. In addition, ultrastructural changes in glutamate-positive synapses which have been correlated with increased synaptic activity were found. These results are similar to changes in GABA and glutamate synapses that follow nigrostriatal dopamine loss in 6-hydroxydopamine-lesioned animals and in Parkinson's disease, and provide the first direct evidence that methamphetamine-induced dopamine loss alters the GABAergic striatopallidal pathway. Exposure to either methamphetamine or prolonged hyperpyrexia decreased mitochondrial Immunoreactivity, indicating that hyperthermia may contribute to methamphetamine toxicity by affecting energy stores.