New experimental and clinical links between the hippocampus and the dopaminergic system in Parkinson's disease

Parkinson's disease is a common progressive neurodegenerative disease, of which the main neuropathological hallmark is dopaminergic neuronal loss. Increased attention has been directed towards non-motor symptoms in Parkinson's disease, such as cognitive impairment and behavioural disorders. Clinical and experimental findings support the view that the hippocampus, a temporal lobe structure involved in physiological learning and memory, is also implicated in the cognitive dysfunction seen in some patients with Parkinson's disease. Moreover, emerging data suggest interactions between the dopaminergic systems and the hippocampus in synaptic plasticity, adaptive memory, and motivated behaviour. This structure is also implicated in the pathophysiology of other non-motor symptoms, such as impulse control disorders, anosmia, and fatigue. Evidence from clinical observations and experimental studies suggest a complex hippocampal cross-talk among the dopaminergic and other transmitter systems. Furthermore, neurotrophic factors might interact with the hippocampal dopaminergic system having possible implications on the non-motor symptoms seen in patients with Parkinson's disease.