Elsevier

Neuroscience

Volume 163, Issue 1, 29 September 2009, Pages 456-465
Neuroscience

Systems Neuroscience
Research Paper
Altered CB1 receptor and endocannabinoid levels precede motor symptom onset in a transgenic mouse model of Huntington's disease

https://doi.org/10.1016/j.neuroscience.2009.06.014Get rights and content

Abstract

Huntington's disease (HD) is an inherited neurodegenerative disease characterised by cell dysfunction and death in the basal ganglia and cortex. Currently there are no effective pharmacological treatments available. Loss of cannabinoid CB1 receptor ligand binding in key brain regions is detected early in HD in human postmortem tissue [Glass M, Dragunow M, Faull RL (2000) The pattern of neurodegeneration in Huntington's disease: a comparative study of cannabinoid, dopamine, adenosine and GABA(A) receptor alterations in the human basal ganglia in Huntington's disease. Neuroscience 97:505–519]. In HD transgenic mice environmental enrichment upregulates the CB1 receptors and slows disease progression [Glass M, van Dellen A, Blakemore C, Hannan AJ, Faull RL (2004) Delayed onset of Huntington's disease in mice in an enriched environment correlates with delayed loss of cannabinoid CB1 receptors. Neuroscience 123:207–212]. These findings, combined with data from lesion studies have led to the suggestion that activation of cannabinoid receptors may be protective. However, studies suggest that CB1 mRNA may be decreased early in the disease progression in HD mice, making this a poor drug target. We have therefore performed a detailed analysis of CB1 receptor ligand binding, protein, gene expression and levels of endocannabinoids just prior to motor symptom onset (12 weeks of age) in R6/1 transgenic mice. We demonstrate that R6/1 mice exhibit a 27% decrease in CB1 mRNA in the striatum compared to wild type (WT). Total protein levels, determined by immunohistochemistry, are not significantly different to WT in the striatum or globus pallidus, but are significantly decreased by 19% in the substantia nigra. CB1 receptor ligand binding demonstrates significant but small decreases (<20%) in all basal ganglia regions evaluated. The levels of the endocannabinoid 2-arachidonoyl glycerol are significantly increased in the cortex (147%) while anandamide is significantly decreased in the hippocampus to 67% of WT. Decreases are also apparent in the ligand binding of neuronal D1 and D2 dopamine receptors co-located with CB1, while there is no change in GABAA receptor ligand binding. These results suggest that in this R6/1 mouse colony at 12 weeks there are only very small changes in CB1 protein and receptors and thus this would be an appropriate time point to evaluate therapeutic interventions.

Section snippets

Animals

R6/1 hemizygote males (Jackson Laboratory, Bar Harbor, ME, USA) were bred with CBB6 (CBA×C 57/B6) F1 females to establish the R6/1 colony at the Howard Florey Institute. The CAG repeats length of transgenic mice in the colony at the time of cohort generation was in the range 127–135 (Pathology Department, University of Melbourne, Melbourne, Australia). Offspring were polymerase chain reaction (PCR) genotyped with genomic DNA obtained from toe biopsies. Mice were group housed in mixed-genotypes

Acute cannabinoid drug treatment results in behavioural consequences in R6/1 mice at 12 weeks of age

A single acute dose of cannabinoid partial agonist THC or vehicle was administered to female WT (n=7) and R6/1 (n=4) littermates 2.5 h prior to a tail suspension test. Within treatment groups, there was no difference between genotypes in time spent immobile. THC-treatment caused a significant increase in immobility for both WT (P<0.001) and R6/1 (P<0.001) mice compared to vehicle-treated counterparts.

CB1 receptor mRNA is decreased in the striatum but unchanged in the hippocampus and motor cortex of R6/1 mice at 12 weeks of age

In situ hybridisation of DNA probes was carried out on coronal brain sections from R6/1 HD mice

Discussion

A full characterisation of the cannabinoid system and other similarly distributed neurotransmitter receptors in R6/1 mice at 12 weeks of age is valuable for understanding HD disease pathogenesis prior to onset of behavioural symptoms. In this study, behavioural, mRNA, protein and ligand binding results demonstrate that CB1 is present and active at 12 weeks of age although regional, selective changes in the cannabinoid system are occurring.

The R6/1 model is a well-characterised model of HD, with

Acknowledgments

We thank L. Schwarcz for critical reading of this manuscript and J. Bullock for assistance with tissue processing. This work was supported by grants from the Neurological Foundation of New Zealand (M.G. and R.L.M.F.), the Australian National Health and Medical Research Council (A.J.H.) and a University of Auckland Health Research Doctoral Scholarship (M.J.D.).

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