Abstract
In Parkinson’s disease, dopamine-producing cells degenerate and therefore, dopamine is lost from the movement control areas in the brain (the striatum). Conventional oral replacement with dopamine-mimicking ‘agonists’, including levodopa, restores movement in the short term but essentially ‘floods’ the brain, disrupting natural dopamine timing. Consequently, after 3-5 years of use, these drugs induce undesirable side effects ranging from dyskinesias (uncontrolled excessive movements) to sudden complete cessation of movement (freezing). Our lab has developed a drug-delivery system using focused ultrasound and liposomes containing dopamine agonists, designed to reduce side effects by activating dopamine receptors via short pulses to target areas. The optimal doses of dopamine agonist pulses are unknown. We, therefore, sought to identify a motor threshold dose of a D1 and D2 receptor agonist, SKF 82958 and ropinirole, respectively, required to initiate motor behaviour in a unilateral sheep model of parkinsonism.
Four Romdale ewes received 6-OHDA neurotoxin lesions of the substantia nigra, verified by subcutaneous apomorphine. Dose-escalation studies for each agonist were performed intravenously to identify threshold and dose-response relationships. The optimal ropinirole turning response was estimated for each sheep and confirmed in randomised blinded single bolus trials. Turning behaviour and locomotion were recorded in the field using a harness- mounted GPS unit. Following completion of the behavioural studies, sheep were euthanised, and the brains were prepared for future histological analysis, to determine the extent of the dopamine lesion and correlate to behaviour.
In response to ropinirole, the cohort showed a variable turning response. One sheep achieved threshold turning in the contralateral direction, whereas two other sheep did not achieve the threshold criterion for turns in the contralateral direction. Threshold turning in the ipsilateral direction was observed in one sheep. A pilot study testing doses of SKF 82958 produced contralateral turning at a lower intensity than ropinirole in 3 sheep. The contrasting direction of turning after D1 and D2 agonist administration in sheep suggested a cooperative D1/D2 receptor interaction mediated locomotor activity in the dopamine-depleted striatum. One of the potential sources of variability in response considered was how successful the targeting of injections into the substantia nigra was at finding their targets. A pilot study of viral injections was made in three sheep to examine retrograde labelling from the substantia nigra to the striatum. These were, unfortunately, inconclusive due to the degree of autofluorescence.
The behavioural methodology for evaluating dopamine agonist-induced motor activity in this sheep model of Parkinson’s disease could be extended more conclusively to a larger cohort with an additional study of concurrent selective D1 and D2 agonist administration. In addition, more methodological development is required to improve the viral-mediated labelling and extend to anterograde tracing of dopamine cells. The results of future trials, together with the present study, will inform the application of the novel antiparkinsonian treatment in future translational experiments. The intention is to minimise the complications of conventional dopamine therapy, with the goal of improving the long-term quality of life of people living with Parkinson’s Disease and their support people.