20.2.2.1 Introduction to Parkinson’s Disease

Normal ageing is associated with a loss of dopaminergic neurones. However, there is a greater loss of dopaminergic neurones in Parkinson’s disease, which was first described by Parkinson. Thus, a 70-80% loss of dopaminergic neurones is associated with Parkinson’s disease. Pathologically there is loss of dopaminergic neurones in substantia nigra (which is part of the basal ganglia). Also, pathological studies show the appearance of Lewy bodies, which are bits of protein dumped in the cell.

The cause of Parkinson’s disease is usually sporadic. Sporadic just means irregularly or random, and is a term used when it is not know why the disease has occurred. Parkinson’s disease can be genetic. The antipsychotic agents (which are potent D₂ receptor antagonists) can cause symptoms similar to Parkinson’s disease called parkinsonism.

The clinical syndrome that distinguishes Parkinson’s disease consists of 4 parts. Firstly, there is bradykinesia (which is a slowness/poverty of movement). Secondly, there is muscular rigidity, which makes starting a movement difficult. There may also be resting tremors but these usually abate during voluntary movement. Thus, if a resting tremor occurs during sitting, get up and moving around usually gets rid of the tremor. Lastly, there is impairment of balance, with disturbances of gait (movement of limbs) and the subjects with Parkinson’s disease lean forward. Impairment of balance leads to an increased risk of falling.

The main treatment of Parkinson’s disease is to enhance the dopaminergic system. This is an obvious approach as there is underactivity of the dopaminergic system in Parkinson’s disease.

Normally, the control of movement in the substantia nigra is a balance between the cholinergic system and the dopaminergic system (Figure 20.4). When the dopaminergic system degenerates, the cholinergic system is left unopposed, and predominates. Thus, an overactivity of the cholinergic system is observed. Consequently, the other approach to treating Parkinson’s disease is inhibiting the cholinergic muscarinic neurotransmission.

Figure 20.4 Control of movement (Copyright QUT, Sheila Doggrell)