Research reportAlterations in dopamine and benzodiazepine receptor binding precede overt neuronal pathology in mice modelling early Huntington disease pathogenesis
Introduction
The progressive neurodegenerative disorder, Huntington disease (HD), is an autosomal dominant disease caused by the expansion of a polymorphic CAG triplet repeat sequence within exon 1 of the HD gene [17]. The expansion mutation results in an extended polyglutamine stretch within the N-terminus of the ubiquitously expressed protein called huntingtin [17]. The initial pathological basis of HD is selective degeneration of the medium spiny neurones within the striatum [16], although progression of the disease is associated with degeneration of additional brain regions, most prominently, the cerebral cortex [43]. Currently, it is not understood how the presence of mutant huntingtin leads to the cell-selective aspects of HD pathology.
The profound neuronal loss observed in end-stage disease must clearly be a major causal factor in the clinical picture. However, the disease course is so prolonged that vulnerable neurones may be disposed to long periods of dysfunction prior to cell death. The demonstration that a small number of symptomatic HD patients show no overt neuropathology (Grade 0) [42], [43] argues that massive striatal cell loss is not an absolute pre-requisite for onset of disease. A number of functional imaging studies using positron emission tomography (PET) have demonstrated reduced striatal glucose metabolism in the early stages of human HD [1], lending support to the idea that synaptic activity and, therefore, neuronal function is compromised early in HD. The precise nature of the cellular or biochemical changes that mediate neuronal dysfunction is not well understood. However, since neurotransmitters and their receptors have a central role in maintaining the normal operation of neostriatal circuitry, some effort has been invested in determining whether neurotransmitter systems are altered in HD brain tissue.
Studies of human postmortem brain tissue from HD cases have indicated changes in a variety of neurotransmitter receptors, including those associated with dopamine, acetylcholine, GABA and glutamate [9], [10], [15], [31]. Although some early stage cases have been investigated [15], it is possible that the deficits detected may reflect loss of specific sub-populations of neurones rather than a primary role in the disease process. The advent of functional imaging studies has allowed investigation of HD patients throughout their disease course. These studies have indicated that altered brain neurochemistry is present early; PET studies have revealed reduced levels of dopamine D2 receptors in the caudate and putamen of asymptomatic mutation carriers, coincident with reduced glucose metabolism [2], [3]. Studies using PET ligands have also showed altered opioid and benzodiazepine receptor binding early in HD [19], [46]. Whilst supporting the hypothesis that primary deficits in neurotransmitter systems may contribute to neuronal dysfunction in early HD, subtle cell loss could still account for the deficits.
In the past, one of the obstacles hampering studies in this field of was the paucity of appropriate tissue for analysis. However, the development of genetic models of HD provides in vivo systems that can be used to examine pathological cascades that culminate in clinical symptoms. Accordingly, we have used a knock-in HD mouse model [38] to investigate molecular and cellular changes involved in early HD pathogenesis. The insertion of a perfect CAG repeat tract into exon 1 of the mouse Hdh gene has provided the opportunity to explore the consequences of the HD mutation in its appropriate genomic and protein context. Previous studies of these mice have demonstrated several phenotypic changes including behavioural and motor abnormalities [21], [38], as well as abnormalities of long-term potentiation in the hippocampus [41]. The phenotypic and cellular changes appear to occur before frank neurodegeneration as histological and immunohistochemical analyses have failed to reveal any evidence of overt neuronal loss in the striatum of 18-month-old HD mice [38].
In the present study, we have determined the density of dopamine, opioid and benzodiazepine receptor binding sites in the brains of knock-in HD mice using quantitative ligand binding autoradiography. The ligands selected for this study were based on those used previously in human brain imaging studies of pre-end stage disease patients and we focused on the striatum as the region of primary pathology. In order to investigate the presence of potential genetic modifiers of the early disease process, a post hoc subgroup analysis of animals with different genetic backgrounds was performed.
Section snippets
Animals
The generation of the knock-in HD mice used in the current study has been described previously [38]. All experiments were performed on female progeny of the founder mouse line Hdh4/Q80 inbred onto either a C57BL/6 (N6–7 generation) or FVB/N (N4 generation) genetic background. A total of 15 HD and 15 wild-type mice was used. The genotype of the mice was determined by PCR analysis of tail DNA biopsies using standard procedures [38]. All experiments were carried out using 17- to 18-month-old
Results
Autoradiograms of total binding for the different ligands are presented in Fig. 1 for illustrative purposes. On visual inspection, differences in the levels of binding between HD and wild-type mice were not marked. However, the quantitative analysis data revealed there to be statistically significant differences between the two groups.
Discussion
The present study demonstrates that 18-month-old knock-in HD mice expressing full-length mutant huntingtin appropriately have reduced levels of D2 receptor binding sites in the striatum and increased levels of benzodiazepine receptor binding sites in the striatum and cerebral cortex. There is also a trend towards increased opioid binding densities in the striatum and, to a lesser extent, in the cortex of the HD mice, although these changes do not reach statistical significance. All these
Acknowledgments
The authors are grateful to Colin Hughes, Dennis Duggan, Margaret Ennis and staff at the Wellcome Surgical Institute for technical assistance and The University of Glasgow Dynamic Mutation Group and Professor J. McCulloch for helpful discussion. This work was funded by the Hereditary Disease Foundation. L.K. was supported by a studentship from The Huntington's Disease Association of Great Britain.
References (46)
- et al.
Advances in the understanding of early Huntington's disease using the functional imaging techniques of PET and SPET
Mol. Med. Today
(1998) - et al.
The distribution of GABAA-benzodiazepine receptors in the basal ganglia in Huntington's disease and in the quinolinic acid-lesioned rat
Prog. Brain Res.
(1993) - et al.
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
(2000) - et al.
Huntingtin–protein interactions and the pathogenesis of Huntington's disease
Trends Genet.
(2004) - et al.
Receptor subtype-specific modulation by dopamine of glutamatergic responses in striatal medium spiny neurons
Brain Res.
(2003) - et al.
Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice
Cell
(1996) - et al.
Alterations in the benzodiazepine receptor of Huntington's diseased human brain
Brain Res.
(1979) - et al.
Heterogeneous dopamine receptor changes in early and late Huntington's disease
Neurosci. Lett.
(1991) - et al.
Evidence for a preferential loss of enkephalin immunoreactivity in the external globus pallidus in low grade Huntington's disease using high resolution image analysis
Neuroscience
(1995) - et al.
Huntington's disease progression, PET and clinical observations
Brain
(1999)
Striatal glucose metabolism and dopamine D2 receptor binding in asymptomatic gene carriers and patients with Huntington's disease
Brain
Dopamine D1 and D2 receptor gene expression in the striatum in Huntington's disease
Ann. Neurol.
Neuromodulatory actions of dopamine in the neostriatum are dependent upon the excitatory amino acid receptor subtypes activated
Proc. Natl. Acad. Sci. U. S. A.
Facilitated glutamatergic transmission in the striatum of D2 dopamine receptor-deficient mice
J. Neurophysiol.
Altered brain neurotransmitter receptors in transgenic mice expressing a portion of an abnormal human huntington disease gene
Proc. Natl. Acad. Sci. U. S. A.
Altered neurotransmitter receptor expression in transgenic mouse models of Huntington's disease
Philos. Trans. R. Soc. Lond., B
Excitatory amino acid binding sites in the caudate nucleus and frontal cortex of Huntington's disease
Ann. Neurol.
Dopamine selects glutamatergic inputs to neostriatal neurons
Synapse
The mouse brain in stereotaxic coordinates
D1 and D2 dopamine receptor-regulated gene expression of striatonigral and striatopallidal neurons
Science
PET study of the pre-and post-synaptic dopaminergic markers for the neurodegenerative process in Huntington's disease
Brain
Evidence for degenerative and regenerative changes in neostriatal spiny neurons in Huntington's disease
Science
A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes
Cell
Cited by (17)
Altered reactivity of central amygdala to GABA<inf>A</inf>R antagonist in the BACHD rat model of Huntington disease
2017, NeuropharmacologyCitation Excerpt :Indeed, in early-stage HD patients, GABAAR binding is decreased in the caudate nucleus (Faull et al., 1993; Penney and Young, 1982; Reisine et al., 1979; Walker et al., 1984), whereas GABAAR density is increased in the putamen (Holthoff et al., 1993; Kunig et al., 2000; Pinborg et al., 2001). Similarly, increased levels of GABAAR binding has been observed in the striatum and cerebral cortex in 5 months old and 18 months old mice models of HD (Glass et al., 2004; Kennedy et al., 2005). An increased frequency of spontaneous GABAergic synaptic currents in striatum medium spiny neurons from 5 weeks old HD mice has also been reported (Cepeda et al., 2004), possibly reflecting compensatory up-regulation to reduce the excessive firing in striatum medium spiny neurons or in cortical neurons (Kennedy et al., 2005).
Rodent genetic models of Huntington disease
2008, Neurobiology of DiseaseHuntington's disease: Modeling the gait disorder and proposing novel treatments
2008, Journal of Theoretical BiologyNormal electrical properties of hippocampal neurons modelling early Huntington disease pathogenesis
2007, Brain ResearchCitation Excerpt :The electrophysiological signature of HD pathogenesis, exemplified by the pre-symptomatic decrease in the early phase of evoked cortical sensory potentials, also correlates with mutation length in patients (Beniczky et al., 2002). The HD mice used in the current study inherited an expanded 72–80 CAG repeat length mutation that results in mild symptoms including modest motor dysfunction which likely mirror the early stages of human HD pathogenesis (Shelbourne et al., 1999, Kennedy et al., 2003, Kennedy et al., 2005). Whilst the majority of clinical symptoms of HD are classically attributed to striatal degeneration, recent evidence points to a much more widespread degeneration occurring in early to mid-stage HD across many brain structures including the hippocampal formation (Rosas et al., 2003).
Alteration of GABAergic neurotransmission in Huntington's disease
2018, CNS Neuroscience and Therapeutics