Elevated environmental temperature and methamphetamine neurotoxicity

Abstract

Amphetamines have been of considerable research interest for the last several decades. More recent work has renewed interest in the role of ambient temperature in both the toxicity and neurotoxicity of these drugs. We have determined that the striatal dopaminergic neurotoxicity observed in the mouse is linked in some fashion to both body and environmental temperature. Most studies of d-methamphetamine (d-METH) neurotoxicity are conducted at standard laboratory ambient temperatures (e.g., ∼21–22°C) and utilizing a repeated dosage regimen (e.g., three to four injections spaced 2 h apart). A lowering of the ambient temperature provides neuroprotection, while an elevation increases neurotoxicity. d-METH causes long-term depletions of striatal dopamine (DA) that are accompanied by other changes that are indicative of nerve terminal degeneration. These include argyrophilia, as detected by silver degeneration stains, and an elevation in glial fibrillary acidic protein (GFAP), a marker of reactive gliosis in response to injury, as well as a long-term decrease in tyrosine hydroxylase (TH) protein levels. Here we show that increasing the ambient temperature during and for some time following dosing increases the neurotoxicity of d-METH. Mice (female C57BL6/J) given a single dosage of d-METH (20 mg/kg s.c.) and maintained at the usual laboratory ambient temperature show minimal striatal damage (an ∼15% depletion of DA and an ∼ 86% increase in GFAP). Substantial striatal damage (e.g., an ∼70% depletion of DA and an ∼200% elevation in GFAP) was induced by this regimen if mice were maintained at 27°C for 24 or 72 h following dosing. An increase in neurotoxicity was also apparent in mice kept at an elevated temperature for only 5 or 9 h, but keeping animals at 27°C for 24 or 72 h was the most effective in increasing the neurotoxicity of d-METH. Our data show how a relatively minor change in ambient temperature can have a major impact on the degree of neurotoxicity induced by d-METH. Single-dose regimens may aid in uncovering the as yet unknown mechanism(s) of substituted amphetamine neurotoxicity because they reduce the inherent complexity present in repeated dosage regimens.