Research reportLoss of cannabinoid CB1 receptor expression in the 6-hydroxydopamine-induced nigrostriatal terminal lesion model of Parkinson's disease in the rat
Introduction
The discovery in 1960 of a severe depletion of the neurotransmitter dopamine from the caudate and putamen of post-mortem Parkinsonian brains [17] revealed the first pharmacological target for this hitherto untreatable motor disorder. In 1961, it was reported that intravenous administration of the dopamine precursor, levodopa, to Parkinsonian patients was capable of reversing the akinesia associated with the disease [3]. Cotzias et al. [7] subsequently confirmed the anti-Parkinsonian effects of orally administered levodopa, but they also reported that long-term administration of levodopa caused the development of abnormal involuntary movements termed dyskinesias. Since then, the pharmacological treatment of Parkinson's disease has remained centred on the dopaminergic system despite the limitations and side effects associated with this approach.
The endocannabinoid system has recently emerged as a potential alternative target for the treatment of Parkinson's disease [5]. This proposition is based on a number of converging lines of evidence: firstly, brain regions involved in the control of movement such as the basal ganglia (especially the substantia nigra) possess the highest density of the CB1 subtype of cannabinoid receptor [10], [36] and the highest concentration of the endocannabinoids, anandamide and 2-arachidonylglycerol in the brain [4], [20]; secondly, plant-derived, synthetic and endogenous cannabinoids exert inhibitory effects on motor activity in both humans [51] and experimental animals [8], [45]. It has also been reported that endocannabinoid levels [41] and CB1 receptor expression are altered in Parkinson's disease patients [27], [31].
For Parkinson's disease, the most commonly used rat models are the unilateral 6-hydroxydopamine lesion models, where the catecholaminergic neurotoxin is administered directly into the nigrostriatal systemāmost commonly to the axons as they ascend along the medial forebrain bundle, or to the nigrostriatal terminals where they innervate the striatum. These models are suitable for testing the hypothesis that targeting the endocannabinoid system could be a valid therapeutic target for the treatment of the Parkinsonian motor disorder because they are associated with a well-characterised behavioural syndrome reminiscent of the human condition [48]. However, since there is some evidence that experimental Parkinsonian lesions induce alterations in the endocannabinoid system [23], [44], this could have an impact on the interpretation of studies that seek to determine the anti-Parkinsonian potential of cannabinoid drugs in these models, particularly if the alterations do not model the changes which are known to occur in the human condition [27], [31]. Therefore, it is vital to determine the effect of 6-hydroxydopamine-induced hemi-Parkinsonism on cannabinoid receptor expression if these models are to be used for validating the endocannabinoid system as a therapeutic target for Parkinson's disease.
Therefore, the aim of this study was to determine if there are any changes in CB1 receptor expression in the substantia nigra in the two most commonly used rat models of Parkinson's diseaseāthose in which the nigrostriatal pathway is lesioned by unilateral axonal or terminal injection of 6-hydroxydopamine.
Section snippets
Animals
Male Lister hooded rats (nĀ =Ā 57, Charles River, UK) were used in this study, weighing 225ā250Ā g at the start of the experiment. They were housed under a 12Ā h light:dark cycle in a room maintained at 21Ā Ā±Ā 2Ā Ā°C and had access to food and water ad libitum. All procedures were carried out under license from the Irish Department of Health and Children, were approved by the Animal Ethics Committee of the National University of Ireland, Galway, and were in compliance with the European Communities Council
CB1 receptor expression
In accordance with previous literature [16], [40], [49], the CB1 subtype of cannabinoid receptor was strongly expressed in the pars reticulata region of the substantia nigra and there was minimal overlap with the dopaminergic (tyrosine hydroxylase immunoreactive) neurons in the parscompacta (Fig. 1A).
Effect of axonal nigrostriatal lesion on CB1 receptor expression
As expected, injection of 6-hydroxydopamine into the nigrostriatal neurons at the axonal level in the medial forebrain bundle caused near-complete loss of tyrosine hydroxylase immunopositive cells
Discussion
This study sought to determine if there were any changes in CB1 receptor expression in the substantia nigra in the two most commonly used rat models of Parkinson's disease. Hemi-Parkinsonism was induced in male rats by either axonal or terminal injection of the catecholamine neurotoxin 6-hydroxydopamine and the density of CB1 receptor immunoreactivity in the substantia nigra was analysed over 28 days of post-lesion. Although CB1 receptor expression was transiently increased in the axonal lesion
Conflict of interest
The authors declare that they have no competing financial interests.
Acknowledgements
S. Walsh and K. Mnich are the recipients of EMBARK PhD studentships from the Irish Research Council for Science, Engineering and Technology. The assistance of Ms. Teresa Moloney and Mr. Padraig Mulcahy is gratefully acknowledged.
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