Sigma-1 receptor deficiency reduces MPTP-induced parkinsonism and death of dopaminergic neurons

Sigma-1 receptor (σ₁R) has been reported to be decreased in nigrostriatal motor system of Parkinson''s disease patients. Using heterozygous and homozygous σ₁R knockout (σ₁R^+/− and σ₁R^−/−) mice, we investigated the influence of σ₁R deficiency on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-impaired nigrostriatal motor system. The injection of MPTP for 5 weeks in wild-type mice (MPTP-WT mice), but not in σ₁R^+/− or σ₁R^−/− mice (MPTP-σ₁R^+/− or MPTP-σ₁R^−/− mice), caused motor deficits and ~40% death of dopaminergic neurons in substantia nigra pars compacta with an elevation of N-methyl-d-aspartate receptor (NMDAr) NR2B phosphorylation. The σ₁R antagonist NE100 or the NR2B inhibitor Ro25-6981 could alleviate the motor deficits and the death of dopaminergic neurons in MPTP-WT mice. By contrast, MPTP-σ₁R^+/− mice treated with the σ₁R agonist PRE084 or MPTP-σ₁R^−/− mice treated with the NMDAr agonist NMDA appeared to have similar motor deficits and loss of dopaminergic neurons as MPTP-WT mice. The pharmacological or genetic inactivation of σ₁R suppressed the expression of dopamine transporter (DAT) in substantia nigra, which was corrected by NMDA. The activation of σ₁R by PRE084 enhanced the DAT expression in WT mice or σ₁R^+/− mice. By contrast, the level of vesicular monoamine transporter 2 (VMAT2) in σ₁R^+/− mice or σ₁R^−/− mice had no difference from WT mice. Interestingly, MPTP-WT mice showed the reduction in the levels of DAT and VMAT2, but MPTP-σ₁R^−/− mice did not. The inactivation of σ₁R by NE100 could prevent the reduction of VMAT2 in MPTP-WT mice. In addition, the activation of microglia cells in substantia nigra was equally enhanced in MPTP-WT mice and MPTP-σ₁R^−/− mice. The number of activated astrocytes in MPTP-σ₁R^−/− mice was less than that in MPTP-WT mice. The findings indicate that the σ₁R deficiency through suppressing NMDAr function and DAT expression can reduce MPTP-induced death of dopaminergic neurons and parkinsonism.Parkinson''s disease (PD) is a neurodegenerative disorder characterized by motor symptoms, including bradykinesia and tremor, and a progressive loss of dopaminergic neurons in substantia nigra pars compacta (SNpc).^{1, 2} Sigma-1 receptor (σ₁R), previously named the opioid receptor sigma-1, is found primarily in motoneurons localized in the brainstem and spinal cord.³ The σ₁R is expressed in dopaminergic neurons and astrocytes.⁴ The σ₁R agonist PRE084 has been reported to exert neurorestorative effects on 6-hydroxydopamine (6-OHDA)-induced parkinsonism.⁴ Using positron emission tomography, the σ₁R-binding sites are found to be reduced in the brains of early-phase PD patients.⁵ However, the influence of σ₁R deficiency on the pathogenesis of PD has not yet been reported.Dopamine toxicity is involved in the etiology of PD.⁶ The σ₁R-binding sites on dopaminergic nerve terminals are involved in increasing dopamine release by enhancing N-methyl-d-aspartate receptors (NMDAr).⁷ The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) metabolized to 1-methyl-4-phenylpyridinium in glial cells selectively impairs dopaminergic neurons in SNpc through disrupting respiratory enzymes and causing oxidative damage.⁸ The dopamine transporter (DAT), a high-affinity transmembrane protein, is responsible for dopamine reuptake from the synaptic cleft and the transportation of 1-methyl-4-phenylpyridinium into dopaminergic nerve terminals.⁹ The σ₁R is co-expressed with DAT in dopaminergic neurons.⁴ Furthermore, the low density of DAT has been confirmed in the brains of PD patients.⁵The activation of σ₁R enhances the Ca²⁺ influx across NMDAr through increasing the phosphorylation of NR2B or the trafficking NMDAr to the plasma membrane.^{10, 11} The NMDAr NR2B inhibitor can attenuate MPTP- or 6-OHDA-induced parkinsonian symptoms and neurodegeneration.¹² The σ₁R deficiency has been demonstrated to reduce Aβ-induced neuronal cell death through suppressing NR2B phosphorylation.¹³ The inflammation is a predominant aspect of PD, manifested by glial activation with the expression of pro-inflammatory mediators.¹⁴ Sustained neuro-inflammation can exacerbate the degeneration of dopaminergic neurons.¹⁵ The blockade of σ₁R has been reported to inhibit methamphetamine-induced astrogliosis.¹⁶ Moreover, the 6-OHDA-induced spontaneous rotations or decline of dopaminergic fibers in σ₁R knockout mice seem to be less than those in wild-type (WT) mice.⁴ Paquette et al. reported that the blockade of σ₁R could attenuate abnormal involuntary movements induced by 6-OHDA.¹⁷In this study, we employed heterozygous and homozygous σ₁R knockout (σ₁R^+/− and σ₁R^−/−) mice to investigate the influence of σ₁R deficiency on MPTP-induced parkinsonism and death of dopaminergic neurons, and the underlying molecular mechanisms. Using the experimental PD models of MPTP-treated σ₁R^+/− mice and σ₁R^−/− mice, the present study provides in vivo evidence that the σ₁R deficiency through suppressing NMDAr function and DAT expression can attenuate MPTP-induced dopaminergic neurodegeneration and parkinsonism.