Biochemical and pathological correlates of cognitive and behavioural change in DLB/PDD

Dementia with Lewy bodies (DLB) and Parkinson's disease dementia (PDD) are second only to Alzheimer's disease (AD) in frequency. In particular it is evident that up to 80% of people with PD will develop dementia towards the end of their life. While the neurobiology of movement disorder has been well studied in PD, much less attention has been given to mechanisms underlying the cognitive and behavioural symptoms associated with DLB and PDD. To date, the best correlate of cognitive impairment appears to be cortical Lewy bodies; however, new emphasis has been placed on small aggregates of synuclein. Furthermore, very few studies have attempted to investigate the neurochemical correlates of behavioural disorders in DLB/PDD and whether these are similar or distinct from AD. Aggregated alpha-synuclein forms the core component of Lewy bodies, a major pathological feature of Parkinson's-related conditions. The 26S proteasome is an ATP-dependent protease that catalyses the breakdown of alpha-synuclein. Previous studies have implicated alterations in the proteasome in PD. Furthermore, proteasome inhibitors have been reported to induce alpha-synuclein aggregation and Lewy body-like inclusions, resulting in neuronal loss both in vitro and in vivo. Our preliminary results indicate that selective alterations in the expression of proteosome sub-units are a feature of both DLB and PDD, while changes in activity are restricted to PDD. Depression is a common symptom in DLB/PDD, yet the evidence base for standard treatment with SSRIs is limited. In contrast to previous studies of AD, our results indicate that there is no association between depression and the 5-HT transporter, while there was a significant increase in the number of 5-HT1A receptors in those DLB/PDD patients with depression. These data may provide an insight into the lack of success of current treatments and suggest alternative approaches.