Effects of Uptake 2 inhibition on Levodopa-induced movements in the hemiparkinsonian rat model

Abstract

Fifty years since its introduction, L-DOPA (Levodopa) remains the ‗gold-standard‘ for the treatment of Parkinson‘s disease (PD). This drug replenishes the neurotransmitter dopamine that is largely lost in the course of the disease. However, after prolonged use of the drug, patients develop motor complications such as the ‗wearing-off‘ effect and dyskinesia. To prevent or minimise these complications, strategies that aim at prolonging the half-life of L-DOPA and smoothing out the fluctuating dopamine concentrations are needed. One way to achieve this goal is to inhibit the activity of monoamine transporters that remove L-DOPA-derived dopamine from the extracellular space. Uptake 2 is a low affinity, high capacity transporter that supports the high affinity, low capacity Uptake 1 monoamine transporters (including dopamine transporters, DAT). The aim of this study was to investigate the effects of inhibiting Uptake 2 by decynium-22 (D-22) on L-DOPA-induced rotations in hemiparkinsonian rats, using the automated video-tracking system, EthoVision XT. Microinjections of the neurotoxin that damages the nigrostriatal pathway were performed with the NeuroStar stereotaxic robotic system. This system was first optimised so that the injected toxin could reach to desired brain targets. The hemiparkinsonian model was created by a unilateral injection of 6-OHDA (10 μg in 3 μL) into the medial forebrain bundle, a brainstem structure consisting of axons from the Substantia Nigra pars compacta projecting to the striatum. Using this model, apomorphine-induced rotation tests were first conducted to indirectly assess the extent of the lesion. In some animals, the extent of the lesion was also verified by analysing the loss of tyrosine hydroxylase immunoreactivity in the lesioned striatum. Rats were given 20 mg/kg (i.p.) L-DOPA alone or together with 0.1 or 1 mg/kg D-22 (i.p.). Neither of the D-22 doses potentiated L-DOPA-induced rotations during the 2 hour testing period. However, animals that were given the higher D-22 dose had a tendency to rotate more during the first 30 min after drug administration. Further studies are needed to investigate the potential of D-22 and other Uptake 2 inhibitors in potentiating L-DOPA-induced rotation and other movements in animal models of PD. If successful, these studies could lead to improved L-DOPA therapy in PD patients.