Selective blockade of mGlu5 metabotropic glutamate receptors is protective against methamphetamine neurotoxicity

Abstract

Methamphetamine (MA), a widely used drug of abuse, produces oxidative damage of nigrostriatal dopaminergic terminals. We examined the effect of subtype-selective ligands of metabotropic glutamate (mGlu) receptors on MA neurotoxicity in mice. MA (5 mg/kg, i.p.; injected three times, every 2 hr) induced, 5 d later, a substantial degeneration of striatal dopaminergic terminals associated with reactive gliosis. MA toxicity was primarily attenuated by the coinjection of the noncompetitive mGlu5 receptor antagonists 2-methyl-6-(phenylethynyl)pyridine and (E)-2-methyl-6-styrylpyridine both at 10 mg/kg, i.p.). In contrast, the mGlu1 receptor antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (10 mg/kg, i.p.), and the mGlu2/3 receptor agonist (-)-2-oxa-4-aminocyclo[3.1.0]hexane-4,6-dicarboxylic acid (1 mg/kg, i.p.), failed to affect MA toxicity. mGlu5 receptor antagonists reduced the production of reactive oxygen species but did not reduce the acute stimulation of dopamine release induced by MA both in striatal synaptosomes and in the striatum of freely moving mice. We conclude that endogenous activation of mGlu5 receptors enables the development of MA neurotoxicity and that mGlu5 receptor antagonists are neuroprotective without interfering with the primary mechanism of action of MA.

Fig. 1.

Striatal DA (A), DOPAC (B), and HVA (C) levels in mice injected with saline or MA (5 mg/kg, i.p.; 3 times, 2 hr apart) alone or in combination with MPEP (10 mg/kg), SIB-1893 (10 mg/kg), LY379268 (1 mg/kg), and CPCCOEt (10 mg/kg). Values are mean ± SEM of 8–10 determinations. *p < 0.05 (one-way ANOVA + Dunnett's test for post hoc analysis), if compared with the respective “saline” values.

Fig. 2.

Immunohistochemical analysis of TH, DAT, and GFAP in the corpus striatum of mice injected with MA alone or in combination with MPEP or SIB-1893 (Fig. 1). Densitometric analysis of striatal TH or DAT immunostaining has been performed on comparable sections from five to six mice per group. *p < 0.05 (one-way ANOVA + Dunnett's test) versus MA alone.

Fig. 3.

Assessment of hydroxyl radical formation in the striatum of freely moving mice injected intraperitoneally with MA (A) and locally perfused with CPCCOEt (B), SIB-1757 (C), or MPEP (D) (all at 100 μm), since the beginning of the perfusion, or injected with MPEP (E) (10 mg/kg, i.p.) 20 min before MA. Thevertical arrows indicate the time of injection of the drugs. Values are means ± SEM of five determinations. *p < 0.05, if compared with the respective time points obtained in rats injected locally (B,C, F_(5.32) = 14.83;D, F_(5.32) = 27.82) or systemically (E, F_(5.32)= 28.88) with saline (dotted line) (two-way ANOVA + Dunnett's test). ^#p < 0.05, if compared with the respective values at 20 min (A) (one-way ANOVA + Dunnett's test).

Fig. 4.

Microdialysis study of extracellular striatal DA levels in mice injected intraperitoneally with MA (A) and locally perfused with CPCCOEt (B), SIB-1757 (C), or MPEP (D) (all at 100 μm), since the beginning of the perfusion, or injected with MPEP (E) (10 mg/kg, i.p.) 20 min before MA. Thevertical arrows indicate the time of injection of the drugs. Values are means ± SEM of five determinations. *p < 0.05 (one-way ANOVA + Dunnett's test) if compared with the respective values at 20 min.

Fig. 5.

Effect of MPEP, SIB-1757, or SIB-1893 (all at 5 μm) on [³H]DA release stimulated by 0.1 μmd-amphetamine (A) or 0.1 μm MA (B) in striatal synaptosomes. We have also used nomifensine (5 μm) to validate the model. Thearrows indicate the time of addition ofd-amphetamine or MA. mGlu5 receptor antagonists or nomifensine were added to the perfusate 20 min befored-amphetamine or MA. Values are means ± SEM of six to eight determinations. SIB-1757 and SIB-1893 had no effect on [³H]DA release on their own.