Parkinson's Disease | Causes & Pathophysiology | Part 2

Key Concepts

Parkinson's Disease, Dopaminergic Neurons, Substantia Nigra (Pars Compacta), GABAergic Neurons, Direct Pathway, Indirect Pathway, D1 Receptors, D2 Receptors, Akinesia, Bradykinesia, Tremors, Rigidity, Cholinergic Neurons, Acetylcholine, LRRK2 Gene, Endosome-Lysosome Pathway, Alpha-Synuclein, Lewy Bodies, Park2 Gene, E3 Ubiquitin Ligase, Proteasome, DJ-1 Gene, Oxidative Stress, Mitochondrial Dysfunction, Reactive Oxygen Species (ROS), PINK1 Gene, Tau Protein, Neurofibrillary Tangles, Insecticides (DDT, Rotenone), MPTP.

Parkinson's Disease Pathophysiology

Impact of Dopaminergic Neuron Damage

• Damage Location: The primary issue in Parkinson's disease is the damage to dopaminergic neurons within the pars compacta of the substantia nigra.
• Dopamine Reduction: This damage leads to a decrease in dopamine release onto GABAergic neurons in the striatum.
• Direct Pathway Effects (D1 Receptors):
  • Less dopamine stimulation of D1 receptors on GABAergic neurons.
  • Reduced action potentials in these GABAergic neurons projecting to the Globus Pallidus Internus (GPi) and Substantia Nigra pars reticulata (SNr).
  • Less GABA release from these neurons, leading to less inhibition of the GPi/SNr.
  • Increased activity in the GPi/SNr, resulting in more GABA release onto the thalamus.
  • Inhibition of thalamic neurons, decreasing action potentials sent to the cerebral cortex.
  • Reduced stimulation of the primary motor cortex via the corticospinal-thalamic tract, leading to decreased impulses to muscles.
• Indirect Pathway Effects (D2 Receptors):
  • Less dopamine inhibition of D2 receptors on GABAergic neurons.
  • Increased action potentials in these GABAergic neurons projecting to the Globus Pallidus Externus (GPe).
  • More GABA release onto the GPe, leading to increased inhibition of the Subthalamic Nucleus (STN).
  • Reduced action potentials in the STN, resulting in less glutamate release onto the GPi/SNr.
  • Decreased inhibition of the GPi/SNr, leading to increased GABA release onto the thalamus.
  • Inhibition of thalamic neurons, decreasing action potentials sent to the cerebral cortex.
• Overall Motor Cortex Impact: Both pathways result in decreased stimulatory input to the motor cortex, leading to reduced impulses to muscles.

Akinesia and Bradykinesia

• Cause: The combined effect of the direct and indirect pathway dysfunction due to dopamine loss is the primary cause of akinesia (difficulty initiating movement) and bradykinesia (slowness of movement).
• Manifestations:
  • Shuffling gait.
  • Difficulty stopping movement once initiated.
  • Postural instability (retropulsion - difficulty recovering balance when pushed).
  • Masked face (reduced facial expression) due to affected facial muscles.

Tremors and Rigidity

• Cholinergic-Dopaminergic Imbalance:
  • In a healthy state, dopamine and acetylcholine are balanced, preventing tremors and rigidity.
  • In Parkinson's, dopamine levels decrease due to neuron degeneration, disrupting this balance.
  • The relative increase in cholinergic activity contributes to tremors and rigidity.
• Reverberating Circuits: Tremors are also attributed to altered reverberating circuits within the basal ganglia, which are normally modulated by dopamine.

Hypothetical/Theoretical Causes of Parkinson's Disease

Genetic Mutations

• LRRK2 Gene:
  • Mutation disrupts the endosome-lysosome pathway, impairing the degradation of alpha-synuclein.
  • Alters signaling mechanisms involving Ras and MAP kinase, affecting vesicular transport and dopamine release.
  • Increases phosphorylation of tau protein, leading to neurofibrillary tangles.
• Park2 Gene:
  • Encodes for an E3 ubiquitin ligase, which normally tags alpha-synuclein for degradation by the proteasome.
  • Mutation leads to accumulation of alpha-synuclein and Lewy body formation.
• DJ-1 Gene:
  • Affects proteins that control oxidative stress (antioxidant proteins) and mitochondrial function.
  • Mutation leads to increased reactive oxygen species (ROS) and mitochondrial dysfunction.
• PINK1 Gene:
  • Linked to both DJ-1 and tau protein accumulation, suggesting overlapping mechanisms.

Environmental Factors

• Insecticides: Exposure to insecticides containing DDT or rotenone is linked to increased risk of Parkinson's disease.
• MPTP:
  • MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) is a neurotoxin.
  • A case study involved a person synthesizing a synthetic opioid (MPPP) that was contaminated with MPTP.
  • Injection of the contaminated drug led to acute onset of Parkinson's disease due to destruction of dopaminergic neurons.

Synthesis/Conclusion

Parkinson's disease is characterized by the degeneration of dopaminergic neurons in the substantia nigra, leading to a cascade of effects on the direct and indirect pathways of the basal ganglia. This results in motor symptoms such as akinesia, bradykinesia, rigidity, and tremors. The underlying causes are complex and multifactorial, involving genetic mutations (LRRK2, Park2, DJ-1, PINK1) that disrupt cellular processes like protein degradation, mitochondrial function, and oxidative stress management, as well as environmental factors like exposure to insecticides and neurotoxins such as MPTP. The imbalance between dopamine and acetylcholine further contributes to the tremors and rigidity seen in Parkinson's patients. Diagnosis is primarily based on clinical evaluation and response to L-DOPA treatment.