Model Introduction
Parkinson’s Disease (PD) is a common neurodegenerative disease in the middle-aged and elderly, characterized by the progressive degeneration and death of dopaminergic (DA) neurons in the substantia nigra pars compacta and the formation of Lewy bodies. Its pathogenesis is not fully understood but may involve environmental factors, genetics, mitochondrial dysfunction, oxidative stress, immune abnormalities, and apoptosis. Existing PD animal models are mainly divided into neurotoxic models, genetic models, and combinatorial models. Classic neurotoxic 6-OHDA models suffer from acute neuronal death and excessive injury, making them less suitable for studying the entire chronic progressive process. Mechanical injury to the Medial Forebrain Bundle (MFB) also leads to progressive degeneration of DA neurons, simulating the chronic process through MFB axotomy or midbrain hemitransection.
Research Applications
PD animal models support research into pathogenesis and evaluation of therapeutic effects. Neurotoxic models are widely used due to simple operation and low cost but often lack typical pathology like alpha-synuclein deposition. MPTP models have a more progressive course and better clinical alignment. Transgenic models allow study at the genetic and molecular levels. Mechanical MFB injury models are particularly suited for studying neuronal regeneration and PD prevention due to the gradual death of DA neurons.
Key Points of Experimental Design
Animal Selection: Rodents are most common for neurotoxic models due to cost and clinical similarity. C. elegans and Drosophila are used for genetic models due to short lifecycles. Primates are closest to humans but face high costs and ethical issues.
1) Mechanical MFB Injury Models (for chronic progressive pathology)
- (A) MFB Axotomy: Female Wistar rats (185–210g) are fixed in a stereotaxic frame. A hole is drilled (AP -3.8mm, ML 2.4mm). A retractable wire knife is inserted 8mm below the skull surface, extended 2.0–3.0mm, and manipulated to ensure full transection.
- Detection: At 4 weeks post-surgery, AMPH (5 mg/kg, i.p.) is administered to induce rotations; the number of rotations over 90 minutes is counted. Pathological and biochemical detections are consistent with the 6-OHDA lesion model.
- Remark: Survival rates of nigral DA neurons were approximately 44% and 50% on days 18 and 19 post-axotomy, respectively, simulating early-stage PD pathological changes. Retrograde tracing shows a success rate of 92–99%.
- (B) Midbrain Hemitransection: SD rats (200–250g) are fixed. A hole is drilled (AP -1mm, ML 0.5mm). A special 4mm wide blade is inserted 9mm at a 68° angle to the skull. At 8 weeks post-surgery, APO (0.25 mg/kg, s.c.) is used to induce contralateral rotations.
2) Neurotoxic Models
- (A) 6-OHDA Model: 6-hydroxydopamine is injected via stereotaxis into the substantia nigra, striatum, or MFB. It causes oxidative stress and inhibits respiratory complex I. The course is rapid and lacks alpha-synuclein deposition. Unilateral injection is usually preferred for behavioral observation and safety.
- (B) MPTP Model: MPTP is a lipophilic respiratory complex I inhibitor that crosses the blood-brain barrier. It is oxidized to MPP+ in astrocytes, which then enters DA neurons to inhibit ATP synthesis. This model is progressive and can show alpha-synuclein deposition. Mice are the most sensitive rodents to MPTP.
3) Genetic Models Common targets include autosomal dominant (SNCA, LRRK2) and autosomal recessive (PRKN, PINK1, DJ-1) genes.
- SNCA (alpha-synuclein): Most common; shows PD-like motor symptoms but rarely shows significant nigrostriatal nerve injury.
- PRKN (Parkin): Loss of function leads to mitochondrial protein disorder; activation can restore motor symptoms.
- PINK1: Related to mitochondrial stability; knockout mice can show olfactory and gait disorders (prodromal symptoms).
4) Combinatorial Models (Transgenic + Neurotoxin) Recent studies suggest transgenic animals are more susceptible to neurotoxins (e.g., MPTP on DJ-1 deficient mice). These models allow for the study of gene-environment interactions.
Key Detection Indicators
Behavioral Indicators:
- Rotational behavior:
- MFB axotomy: AMPH (5 mg/kg)-induced rotations (90-minute count) at 4 weeks.
- Hemitransection: APO (0.25 mg/kg)-induced contralateral rotations at 8 weeks.
- Motor symptoms in neurotoxic models evaluated by behavioral scales.
Pathological and Molecular/Biochemical Indicators:
- Degeneration and loss of nigrostriatal DA neurons.
- DA content depletion (detectable within 15 min of 6-OHDA injection, becoming more pronounced at 30–60 min; most neurons die within 3–5 days in acute models).
- Alpha-synuclein deposition (primarily in MPTP and some genetic/combinatorial models).
- Mitochondrial dysfunction markers (Complex I inhibition, decreased ATP synthesis).
- MFB mechanical injury: Progressive death of nigral DA neurons (e.g., survival rates of 44%–50% at days 18–19).
