Single low doses of MPTP decrease tyrosine hydroxylase expression in the absence of overt neuron loss

Abstract

Parkinson’s disease (PD) is the second most common age-related neurodegenerative disease. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a prototypical neurotoxicant used in mice to mimic primary features of PD pathology including striatal dopamine depletion and dopamine neuron loss in the substantia nigra pars compacta (SNc). In the literature, there are several experimental paradigms involving multiple doses of MPTP that are used to elicit dopamine neuron loss. However, a recent study reported that a single low dose caused significant loss of dopamine neurons. Here, we determined the effect of a single intraperitoneal injection of one of three doses of MPTP (0.1, 2 and 20 mg/kg) on dopamine neurons, labeled by tyrosine hydroxylase (TH+), and total neuron number (Nissl+) in the SNc using unbiased stereological counting. Data reveal a significant loss of neurons in the SNc (TH+ and Nissl+−) only in the group treated with 20 mg/kg MPTP. Groups treated with lower dose of MPTP (0.1 and 2 mg/kg) only showed significant loss of TH+ neurons rather than TH+ and Nissl+. Striatal dopamine levels were decreased in the groups treated with 2 and 20 mg/kg MPTP and striatal terminal markers including, TH and the dopamine transporter (DAT), were only decreased in the groups treated with 20 mg/kg MPTP. These data demonstrate that lower doses of MPTP likely result in loss of TH expression rather than actual dopamine neuron loss in the SN. This finding reinforces the need to measure both total neuron number along with TH+ cells in determining dopamine neuron loss.

Striatal dopamine in Parkinson disease: A meta‐analysis of imaging studies

“…23 If the same applies to humans, the present estimates of binding losses are an accurate representation of the actual measures in vitro and postmortem, at least for DAT and VMAT2. 25,26 Although there are differences in the detected magnitude of dopaminergic function between neuropathological and imaging studies, the trajectories of the decline appear similar. 24 The striking contrast between neuropathology (nearly absent fibers) and neuroimaging (approximately 50% of function preserved) could be due to heavy general compensatory changes in the terminal area of the nigrostriatal tract in PD, such as a broad upregulation of dopaminergic function in vivo or a phenotypic downregulation of dopaminergic fiber markers as measured postmortem.…”

“…24 The discrepancy could also be due to methodological differences, as postmortem studies may underestimate dopaminergic function because of reduced tyrosine hydroxylase expression or rapid metabolism of dopamine in the postmortem brain. 25,26 Although there are differences in the detected magnitude of dopaminergic function between neuropathological and imaging studies, the trajectories of the decline appear similar. In any case, the present combined functional neuroimaging data show that there is relevant presynaptic dopamine metabolism in the striatum of PD patients several years after motor symptom onset, and there seems to be detectable function even after 20 years of disease progression.…”

Alternative microglial activation is associated with cessation of progressive dopamine neuron loss in mice systemically administered lipopolysaccharide

“…The substantia nigra pars compacta (SN) was defined based on TH + cells, as outlined previously (Alam et al, 2017; Baquet et al, 2009; Bradner et al, 2013). A counting frame of 50 μm × 50 μm with framing space of 200 μm and a height of 10 μm was chosen.…”

Pharmacological inhibition of CSF1R by GW2580 reduces microglial proliferation and is protective against neuroinflammation and dopaminergic neurodegeneration

“…28 Briefly, endogenous peroxidases were quenched in 40 μm sections by 75% methanol and 2.5% H 2 O 2 and sections were blocked (4% normal serum, 0.6% Triton X-100, and 5% BSA in 1×-PBS) and incubated with either rabbit anti-TH polyclonal antibody at 1:1000, rabbit anti-Iba1 antibody at 1:800, or chicken anti-GFAP antibody at 1:1000 for 24 hours at 4°C. 28 Briefly, endogenous peroxidases were quenched in 40 μm sections by 75% methanol and 2.5% H 2 O 2 and sections were blocked (4% normal serum, 0.6% Triton X-100, and 5% BSA in 1×-PBS) and incubated with either rabbit anti-TH polyclonal antibody at 1:1000, rabbit anti-Iba1 antibody at 1:800, or chicken anti-GFAP antibody at 1:1000 for 24 hours at 4°C.…”

“…Further, the attenuated Nos2 mRNA levels from Figure 4B would most likely arise from the reduced microglial proliferation following MPTP, since astrocyte iNOS levels were not significantly different in the MPTP + GW2580 treatment group. 28 Further, mice treated with MPTP exhibited significant motor deficits compared to saline-treated mice. 58 We found significant loss of the TH + and Nissl + neurons in the SN of mice treated with acute MPTP ( Figure 6C,D), demonstrating neuronal cell death and not loss of TH expression.…”