Cannabinoid receptor messenger RNA levels decrease in a subset of neurons of the lateral striatum, cortex and hippocampus of transgenic Huntington's disease mice

Cannabinoid receptor signalling in neurodegenerative diseases: a potential role for membrane fluidity disturbance

Type-1 cannabinoid receptor (CB(1)) is the most abundant G-protein-coupled receptor (GPCR) in the brain. CB(1) and its endogenous agonists, the so-called 'endocannabinoids (eCBs)', belong to an ancient neurosignalling system that plays important functions in neurodegenerative and neuroinflammatory disorders like Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. For this reason, research on the therapeutic potential of drugs modulating the endogenous tone of eCBs is very intense. Several GPCRs reside within subdomains of the plasma membranes that contain high concentrations of cholesterol: the lipid rafts. Here, the hypothesis that changes in membrane fluidity alter function of the endocannabinoid system, as well as progression of particular neurodegenerative diseases, is described. To this end, the impact of membrane cholesterol on membrane properties and hence on neurodegenerative diseases, as well as on CB(1) signalling in vitro and on CB(1) -dependent neurotransmission within the striatum, is discussed. Overall, present evidence points to the membrane environment as a critical regulator of signal transduction triggered by CB(1) , and calls for further studies aimed at better clarifying the contribution of membrane lipids to eCBs signalling. The results of these investigations might be exploited also for the development of novel therapeutics able to combat disorders associated with abnormal activity of CB(1).

Metabolic and type 1 cannabinoid receptor imaging of a transgenic rat model in the early phase of Huntington disease

Several lines of evidence imply early alterations in metabolic and endocannabinoid neurotransmission in Huntington disease (HD). Using [(18)F]MK-9470 and small animal PET, we investigated for the first time cerebral changes in type 1 cannabinoid (CB1) receptor binding in vivo in pre-symptomatic and early symptomatic rats of HD (tgHD), in relation to glucose metabolism, morphology and behavioral testing for motor and cognitive function. Twenty-three Sprague-Dawley rats (14 tgHD and 9 wild-types) were investigated between the age of 2 and 11 months. Relative glucose metabolism and parametric CB1 receptor images were anatomically standardized to Paxinos space and analyzed voxel-wise. Volumetric microMRI imaging was performed to assess HD neuropathology. Within the first 10 months, bilateral volumes of caudate-putamen and lateral ventricles did not significantly differ between genotypes. Longitudinal- and genotype evolution showed that relative [(18)F]MK-9470 binding progressively decreased in the caudate-putamen and lateral globus pallidus of tgHD rats (-8.3%, p≤1.1×10(-5) at 5 months vs. -10.9%, p<1.5×10(-5) at 10 months). In addition, relative glucose metabolism increased in the bilateral sensorimotor cortex of 2-month-old tgHD rats (+8.1%, p≤1.5×10(-5)), where it was positively correlated to motor function at that time point. TgHD rats developed cognitive deficits at 6 and 11 months of age. Our findings point to early regional dysfunctions in endocannabinoid signalling, involving the lateral globus pallidus and caudate-putamen. In vivo CB1 receptor measurements using [(18)F]MK-9470 may thus be a useful early biomarker for HD. Our results also provide evidence of subtle motor and cognitive deficits at earlier stages than previously described.

Worsening of Huntington disease phenotype in CB1 receptor knockout mice

Huntington's disease (HD) is a progressive neurodegenerative genetic disorder which leads to motor, cognitive and psychiatric disturbances. The primary neuropathological hallmark is atrophy of the striatum. Cannabinoid CB1 receptors (CB1Rs) are particularly enriched in the striatum and previous works indicate their early loss of expression in HD, even before symptom occurrence. However, pathophysiological significance of this loss of expression remains unclear. In addition, whether specific modulation of CB1R is able to mitigate striatal neuron fate in HD remains currently controversial. In order to gain further insights on the potential role of CB1R in HD physiopathology, we evaluated the pathophysiological consequences of a genetic deletion of CB1R in the N171-82Q transgenic model and following 3-nitropropionic (3NP) intoxication. Taken together our data demonstrate that CB1R knockout (1) worsens motor performances in N171-82Q mice and (2) increases mouse susceptibility to 3NP. These results suggest that functional changes in CB1R may contribute to the physiopathological development of HD.

Brain networks in Huntington disease

Recent studies have focused on understanding the neural mechanisms underlying the emergence of clinical signs and symptoms in early stage Huntington disease (HD). Although cell-based assays have focused on cell autonomous effects of mutant huntingtin, animal HD models have revealed alterations in the function of neuronal networks, particularly those linking the cerebral cortex and striatum. These findings are complemented by metabolic imaging studies of disease progression in premanifest subjects. Quantifying metabolic progression at the systems level may identify network biomarkers to aid in the objective assessment of new disease-modifying therapies and identify new regions that merit mechanistic study in HD models.

Longitudinal behavioral, cross-sectional transcriptional and histopathological characterization of a knock-in mouse model of Huntington's disease with 140 CAG repeats

The discovery of the gene mutation responsible for Huntington's disease (HD), huntingtin, in 1993 allowed for a better understanding of the pathology of and enabled the development of animal models. HD is caused by the expansion of a polyglutamine repeat region in the N-terminal of the huntingtin protein. Here we examine the behavioral, transcriptional, histopathological and anatomical characteristics of a knock-in HD mouse model with a 140 polyglutamine expansion in the huntingtin protein. This CAG 140 model contains a portion of the human exon 1 with 140 CAG repeats knocked into the mouse huntingtin gene. We have longitudinally examined the rearing behavior, accelerating rotarod, constant speed rotarod and gait for age-matched heterozygote, homozygote and non-transgenic mice and have found a significant difference in the afflicted mice. However, while there were significant differences between the non-transgenic and the knock-in mice, these behaviors were not progressive. As in HD, we show that the CAG 140 mice also have a significant decrease in striatally enriched mRNA transcripts. In addition, striatal neuronal intranuclear inclusion density increases with age. Lastly these CAG 140 mice show slight cortical thinning compared to non-transgenic mice, similarly to the cortical thinning recently reported in HD.

Huntington's disease and Group I metabotropic glutamate receptors

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary body movement, cognitive impairment and psychiatric disturbance. A polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein is the genetic cause of HD. Htt protein interacts with a wide variety of proteins, and htt mutation causes cell signaling alterations in various neurotransmitter systems, including dopaminergic, glutamatergic, and cannabinoid systems, as well as trophic factor systems. This review will overview recent findings concerning htt-promoted alterations in cell signaling that involve different neurotransmitters and trophic factor systems, especially involving mGluR1/5, as glutamate plays a crucial role in neuronal cell death. The neuronal cell death that takes place in the striatum and cortex of HD patients is the most important factor underlying HD progression. Metabotropic glutamate receptors (mGluR1 and mGluR5) have a very controversial role in neuronal cell death and it is not clear whether mGluR1/5 activation either protects or exacerbates neuronal death. Thus, understanding how mutant htt protein affects glutamatergic receptor signaling will be essential to further establish a role for glutamate receptors in HD and develop therapeutic strategies to treat HD.

The endocannabinoid system as a target for the treatment of neurodegenerative disease

The Cannabis sativa plant has been exploited for medicinal, agricultural and spiritual purposes in diverse cultures over thousands of years. Cannabis has been used recreationally for its psychotropic properties, while effects such as stimulation of appetite, analgesia and anti-emesis have lead to the medicinal application of cannabis. Indeed, reports of medicinal efficacy of cannabis can been traced back as far as 2700 BC, and even at that time reports also suggested a neuroprotective effect of the cultivar. The discovery of the psychoactive component of cannabis resin, Delta(9)-tetrahydrocannabinol (Delta(9)-THC) occurred long before the serendipitous identification of a G-protein coupled receptor at which Delta(9)-THC is active in the brain. The subsequent finding of endogenous cannabinoid compounds, the synthesis of which is directed by neuronal excitability and which in turn served to regulate that excitability, further widened the range of potential drug targets through which the endocannabinoid system can be manipulated. As a result of this, alterations in the endocannabinoid system have been extensively investigated in a range of neurodegenerative disorders. In this review we examine the evidence implicating the endocannabinoid system in the cause, symptomatology or treatment of neurodegenerative disease. We examine data from human patients and compare and contrast this with evidence from animal models of these diseases. On the basis of this evidence we discuss the likely efficacy of endocannabinoid-based therapies in each disease context.

Loss of striatal type 1 cannabinoid receptors is a key pathogenic factor in Huntington's disease

Endocannabinoids act as neuromodulatory and neuroprotective cues by engaging type 1 cannabinoid receptors. These receptors are highly abundant in the basal ganglia and play a pivotal role in the control of motor behaviour. An early downregulation of type 1 cannabinoid receptors has been documented in the basal ganglia of patients with Huntington's disease and animal models. However, the pathophysiological impact of this loss of receptors in Huntington's disease is as yet unknown. Here, we generated a double-mutant mouse model that expresses human mutant huntingtin exon 1 in a type 1 cannabinoid receptor-null background, and found that receptor deletion aggravates the symptoms, neuropathology and molecular pathology of the disease. Moreover, pharmacological administration of the cannabinoid Δ(9)-tetrahydrocannabinol to mice expressing human mutant huntingtin exon 1 exerted a therapeutic effect and ameliorated those parameters. Experiments conducted in striatal cells show that the mutant huntingtin-dependent downregulation of the receptors involves the control of the type 1 cannabinoid receptor gene promoter by repressor element 1 silencing transcription factor and sensitizes cells to excitotoxic damage. We also provide in vitro and in vivo evidence that supports type 1 cannabinoid receptor control of striatal brain-derived neurotrophic factor expression and the decrease in brain-derived neurotrophic factor levels concomitant with type 1 cannabinoid receptor loss, which may contribute significantly to striatal damage in Huntington's disease. Altogether, these results support the notion that downregulation of type 1 cannabinoid receptors is a key pathogenic event in Huntington's disease, and suggest that activation of these receptors in patients with Huntington's disease may attenuate disease progression.

Cannabinoids and Dementia: A Review of Clinical and Preclinical Data

The endocannabinoid system has been shown to be associated with neurodegenerative diseases and dementia. We review the preclinical and clinical data on cannabinoids and four neurodegenerative diseases: Alzheimer’s disease (AD), Huntington’s disease (HD), Parkinson’s disease (PD) and vascular dementia (VD). Numerous studies have demonstrated an involvement of the cannabinoid system in neurotransmission, neuropathology and neurobiology of dementias. In addition, several candidate compounds have demonstrated efficacy in vitro. However, some of the substances produced inconclusive results in vivo. Therefore, only few trials have aimed to replicate the effects seen in animal studies in patients. Indeed, the literature on cannabinoid administration in patients is scarce. While preclinical findings suggest causal treatment strategies involving cannabinoids, clinical trials have only assessed the suitability of cannabinoid receptor agonists, antagonists and cannabidiol for the symptomatic treatment of dementia. Further research is needed, including in vivo models of dementia and human studies.

Type 1 cannabinoid receptor mapping with [18F]MK-9470 PET in the rat brain after quinolinic acid lesion: a comparison to dopamine receptors and glucose metabolism

PURPOSE

Several lines of evidence imply early alterations in metabolic, dopaminergic and endocannabinoid neurotransmission in Huntington's disease (HD). Using [18F]MK-9470 and small animal PET, we investigated cerebral changes in type 1 cannabinoid (CB1) receptor binding in the quinolinic acid (QA) rat model of HD in relation to glucose metabolism, dopamine D2 receptor availability and amphetamine-induced turning behaviour.

METHODS

Twenty-one Wistar rats (11 QA and 10 shams) were investigated. Small animal PET acquisitions were conducted on a Focus 220 with approximately 18 MBq of [18F]MK-9470, [18F]FDG and [11C]raclopride. Relative glucose metabolism and parametric CB1 receptor and D2 binding images were anatomically standardized to Paxinos space and analysed voxel-wise using Statistical Parametric Mapping (SPM2).

RESULTS

In the QA model, [18F]MK-9470 uptake, glucose metabolism and D2 receptor binding were reduced in the ipsilateral caudate-putamen by 7, 35 and 77%, respectively (all p<2.10(-5)), while an increase for these markers was observed on the contralateral side (>5%, all p<7.10(-4)). [18F]MK-9470 binding was also increased in the cerebellum (p=2.10(-5)), where it was inversely correlated to the number of ipsiversive turnings (p=7.10(-6)), suggesting that CB1 receptor upregulation in the cerebellum is related to a better functional outcome. Additionally, glucose metabolism was relatively increased in the contralateral hippocampus, thalamus and sensorimotor cortex (p=1.10(-6)).

CONCLUSION

These data point to in vivo changes in endocannabinoid transmission, specifically for CB1 receptors in the QA model, with involvement of the caudate-putamen, but also distant regions of the motor circuitry, including the cerebellum. These data also indicate the occurrence of functional plasticity on metabolism, D2 and CB1 neurotransmission in the contralateral hemisphere.

Endocannabinoid system: emerging role from neurodevelopment to neurodegeneration

The endocannabinoid system, including endogenous ligands ('endocannabinoids' ECs), their receptors, synthesizing and degrading enzymes, as well as transporter molecules, has been detected from the earliest stages of embryonic development and throughout pre- and postnatal development. ECs are bioactive lipids, which comprise amides, esters and ethers of long chain polyunsaturated fatty acids. Anandamide (N-arachidonoylethanolamine; AEA) and 2-arachidonoylglycerol (2-AG) are the best studied ECs, and act as agonists of cannabinoid receptors. Thus, AEA and 2-AG mimic several pharmacological effects of the exogenous cannabinoid delta9-tetrahydrocannabinol (Delta(9)-THC), the psychoactive principle of cannabis sativa preparations like hashish and marijuana. Recently, however, several lines of evidence have suggested that the EC system may play an important role in early neuronal development as well as a widespread role in neurodegeneration disorders. Many of the effects of cannabinoids and ECs are mediated by two G protein-coupled receptors (GPCRs), CB1 and CB2, although additional receptors may be implicated. Both CB1 and CB2 couple primarily to inhibitory G proteins and are subject to the same pharmacological influences as other GPCRs. This new system is briefly presented in this review, in order to put in a better perspective the role of the EC pathway from neurodevelopment to neurodegenerative disorders, like Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. In addition, the potential exploitation of antagonists of CB1 receptors, or of inhibitors of EC metabolism, as next-generation therapeutics is discussed.

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

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 GABA(A) 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.

The endocannabinoid system as a target for the treatment of motor dysfunction

There is evidence that cannabinoid-based medicines that are selective for different targets in the cannabinoid signalling system (e.g. receptors, inactivation mechanism, enzymes) might be beneficial in basal ganglia disorders, namely Parkinson's disease (PD) and Huntington's disease (HD). These benefits not only include the alleviation of specific motor symptoms [e.g. choreic movements with cannabinoid receptor type 1 (CB(1))/transient receptor potential vanilloid type 1 agonists in HD; bradykinesia with CB(1) antagonists and tremor with CB(1) agonists in PD], but also the delay of disease progression due to the neuroprotective properties demonstrated for cannabinoids (e.g. CB(1) agonists reduce excitotoxicity; CB(2) agonists limit the toxicity of reactive microglia; and antioxidant cannabinoids attenuate oxidative damage). In addition, extensive biochemical, anatomical, physiological and pharmacological studies have demonstrated that: (i) the different elements of the cannabinoid system are abundant in basal ganglia structures and they are affected by these disorders; (ii) the cannabinoid system plays a prominent role in basal ganglia function by modulating the neurotransmitters that operate in the basal ganglia circuits, both in healthy and pathological conditions; and (iii) the activation and/or inhibition of the cannabinoid system is associated with important motor responses that are maintained and even enhanced in conditions of malfunctioning and/or degeneration. In this article we will review the available data regarding the relationship between the cannabinoid system and basal ganglia activity, both in healthy and pathological conditions and will also try to identify future lines of research expected to increase current knowledge about the potential therapeutic benefits of targeting this system in PD, HD and other basal ganglia disorders.

Neocortical expression of mutant huntingtin is not required for alterations in striatal gene expression or motor dysfunction in a transgenic mouse

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disease caused by an expanded polyglutamine tract in the ubiquitously expressed huntingtin protein. Clinically, HD is characterized by motor, cognitive and psychiatric deficits. Striking degeneration of the striatum is observed in HD with the medium spiny neurons (MSNs) being the most severely affected neuronal subtype. Dysfunction of MSNs is marked by characteristic changes in gene expression which precede neuronal death. The ubiquitous expression of the huntingtin protein raises the question as to whether the selective vulnerability of the MSN is cell-autonomous, non-cell-autonomous, or a combination thereof. In particular, growing evidence suggests that abnormalities of the cortex and corticostriatal projections may be primary causes of striatal vulnerability. To examine this issue, we developed transgenic mice that, within the forebrain, selectively express a pathogenic huntingtin species in the MSNs, specifically excluding the neocortex. These mice develop a number of abnormalities characteristic of pan-cellular HD mouse models, including intranuclear inclusion bodies, motor impairment, and changes in striatal gene expression. As this phenotype develops in the presence of normal levels of brain-derived neurotrophic factor and its major striatal receptor, tropomyosin-related kinase B, these data represent the first demonstration of in vivo cell-autonomous transcriptional dysregulation in an HD mouse model. Furthermore, our findings suggest that therapies targeted directly to the striatum may be efficacious at reversing some of the molecular abnormalities present in HD.

Cortical expression of brain derived neurotrophic factor and type-1 cannabinoid receptor after striatal excitotoxic lesions

An involvement of one particular neurotrophin, namely, the brain-derived neurotrophic factor (BDNF), has been demonstrated in the pathophysiology Huntington's disease. Type-1 cannabinoid (CB1) receptor has been postulated to upregulate BDNF gene transcription. To better understand the relationship between CB1 and BDNF levels in a situation where the striatum is degenerating, we studied, by dual label immunofluorescence, the distribution of CB1 and BDNF in cortical neurons projecting to the striatum in our rat quinolinic acid model of striatal excitotoxicity. We completed our study with quantitative analyses of BDNF protein levels and CB1 binding activity in the cortex. We show that, 2 weeks post lesion, cortical neurons contain more BDNF compared with controls and to earlier time points. Such BDNF up-regulation coincides with a higher binding activity and an increased protein expression of CB1. We suggest that after excitotoxic lesions, CB1 might, at least transiently, upregulate BDNF in the attempt to rescue striatal neurons from degeneration.

The effect of neurodegenerative diseases on the subventricular zone

During brain development, one of the most important structures is the subventricular zone (SVZ), from which most neurons are generated. In adulthood the SVZ maintains a pool of progenitor cells that continuously replace neurons in the olfactory bulb. Neurodegenerative diseases induce a substantial upregulation or downregulation of SVZ progenitor cell proliferation, depending on the type of disorder. Far from being a dormant layer, the SVZ responds to neurodegenerative disease in a way that makes it a potential target for therapeutic intervention.

Short- and long-term plasticity of the endocannabinoid system in neuropsychiatric and neurological disorders

The activity of the endocannabinoid system, in terms of the levels of the endocannabinoids and of cannabinoid receptors, or of the functional coupling of the latter to a biological response, undergoes to remodelling during pathological conditions. In the CNS, these changes, depending also on the nature of the disorder, can be transient or long-lasting, occur only in those tissues involved in the pathological condition and usually aim at restoring the physiological homeostasis by reducing excitotoxicity, inflammation and neuronal death. However, during chronic disorders, prolonged activation of the endocannabinoid system might also contribute to the symptoms of the pathology. Whilst acute changes of the tissue levels of the endocannabinoids reflect the "on demand" nature of their biosynthesis and release, and hence are effected mostly through regulation of the biosynthetic enzymes, chronic changes seem to be mostly due to longer-lasting alterations in the expression of anabolic and catabolic enzymes. The possibility of obtaining therapeutic advantage from endocannabinoid plasticity in neuropsychiatric and neurological disorders is discussed in this review article.

Expression of Cannabinoid CB1 Receptors in Models of Diabetic Neuropathy

A clearer understanding of the mechanisms underlying the development and progression of diabetic neuropathy is likely to indicate new directions for the treatment of this complication of diabetes. In the present study we investigated the expression of cannabinoid CB(1) receptors in models of diabetic neuropathy. PC12 cells were differentiated into a neuronal phenotype with nerve growth factor (NGF) (50 ng/ml) in varying concentrations of glucose (5.5-50 mM). CB(1) receptor expression was studied at the mRNA level by reverse transcriptase-polymerase chain reaction (RT-PCR) and at the protein level via immunohistochemical and Western blot analysis. CB(1) expression was also compared in dorsal root ganglia (DRG) removed from streptozotocin-induced diabetic rats versus control animals. Total neurite length induced by NGF was reduced in cells cultured in 20 to 50 mM glucose at day 6 (P < 0.01 versus 5.5 mM; n = 6). Cell viability assays conducted in parallel on day 6 confirmed that the total cell numbers were not significantly different among the various glucose concentrations (P = 0.86; n = 12). RT-PCR, immunohistochemical, and Western blot analysis all revealed down-regulation of the CB(1) receptor in cells treated with high glucose (P < 0.05; n = 4-5 for each), and in DRG removed from diabetic rats compared with controls (P < 0.01; n = 5 for immunohistochemistry, and n = 3 for Western blot). These results suggest that high glucose concentrations are associated with decreased expression of CB(1) receptors in nerve cells. Given the neuroprotective effect of cannabinoids, a decline in CB(1) receptor expression may contribute to the neurodegenerative process observed in diabetes.

Symptom-related changes of endocannabinoid and palmitoylethanolamide levels in brain areas of R6/2 mice, a transgenic model of Huntington's disease

Previous studies have shown an impairment of the endocannabinoid system in experimental models of Huntington's disease. In transgenic R6/2 mice, created by inserting exon 1 of the human IT15 mutant gene into the mouse, and exhibiting 150 CAG repeats as well as signs of HD, a progressive decline of CB(1) receptor expression and an abnormal sensitivity to CB(1) receptor stimulation have been reported. Here, by using isotope-dilution liquid chromatography-mass spectrometry, we investigated whether the levels of three endogenous neuroprotective substances, the endocannabinoids anandamide (AEA) and 2-arachidonoylglycerol (2-AG), and palmitoylethanolamide (PEA), are altered in different brain areas of transgenic R6/2 versus wild-type (WT) mice at two different disease phases, i.e. in pre-symptomatic (4.5 weeks) or overtly symptomatic (10 weeks) R6/2 mice versus age-matched WT mice (n=4/group). Except for a approximately 25% decrease in 2-AG levels in the cortex, no significant changes in endocannabinoid and PEA levels were observed in pre-symptomatic R6/2 versus WT mice. By contrast, in symptomatic R6/2 mice the levels of all three compounds were significantly (approximately 30-60%) decreased in the striatum, whereas little changes were observed in the hippocampus, and a approximately 28% decrease of 2-AG levels, accompanied by a approximately 50% increase of AEA levels, was found in the cortex. These findings show that endocannabinoid levels change in a disease phase- and region-specific way in the brain of R6/2 mice and indicate that an impaired endocannabinoid system is a hallmark of symptomatic HD, thus suggesting that drugs inhibiting endocannabinoid degradation might be used to treat this disease.

Cannabinoids and neuroprotection in basal ganglia disorders

Cannabinoids have been proposed as clinically promising neuroprotective molecules, as they are capable to reduce excitotoxicity, calcium influx, and oxidative injury. They are also able to decrease inflammation by acting on glial processes that regulate neuronal survival and to restore blood supply to injured area by reducing the vasoconstriction produced by several endothelium-derived factors. Through one or more of these processes, cannabinoids may provide neuroprotection in different neurodegenerative disorders including Parkinson's disease and Huntington's chorea, two chronic diseases that are originated as a consequence of the degeneration of specific nuclei of basal ganglia, resulting in a deterioration of the control of movement. Both diseases have been still scarcely explored at the clinical level for a possible application of cannabinoids to delay the progressive degeneration of the basal ganglia. However, the preclinical evidence seems to be solid and promising. There are two key mechanisms involved in the neuroprotection by cannabinoids in experimental models of these two disorders: first, a cannabinoid receptor-independent mechanism aimed at producing a decrease in the oxidative injury and second, an induction/upregulation of cannabinoid CB2 receptors, mainly in reactive microglia, that is capable to regulate the influence of these glial cells on neuronal homeostasis. Considering the relevance of these preclinical data and the lack of efficient neuroprotective strategies in both disorders, we urge the development of further studies that allow that the promising expectatives generated for these molecules progress from the present preclinical evidence till a real clinical application.

Transcriptional signatures in Huntington's disease

While selective neuronal death has been an influential theme in Huntington's disease (HD), there is now a preponderance of evidence that significant neuronal dysfunction precedes frank neuronal death. The best evidence for neuronal dysfunction is the observation that gene expression is altered in HD brain, suggesting that transcriptional dysregulation is a central mechanism. Studies of altered gene expression began with careful observations of postmortem human HD brain and subsequently were accelerated by the development of transgenic mouse models. The application of DNA microarray technology has spurred tremendous progress with respect to the altered transcriptional processes that occur in HD, through gene expression studies of both transgenic mouse models as well as cellular models of HD. Gene expression profiles are remarkably comparable across these models, bolstering the idea that transcriptional signatures reflect an essential feature of disease pathogenesis. Finally, gene expression studies have been applied to human HD, thus not only validating the approach of using model systems, but also solidifying the idea that altered transcription is a key mechanism in HD pathogenesis. In the future, gene expression profiling will be used as a readout in clinical trials aimed at correcting transcriptional dysregulation in Huntington's disease.

Molecular mechanisms mediating pathological plasticity in Huntington's disease and Alzheimer's disease

Neurodegenerative diseases such as Huntington's disease and Alzheimer's disease, although very different in etiology, share common degenerative processes. These include neuronal dysfunction, decreased neural connectivity, and disruption of cellular plasticity. Understanding the molecular mechanisms underlying the neural plasticity deficits in these devastating conditions may lead the way toward new therapeutic targets, both disease-specific and more generalized, which can ameliorate degenerative cognitive deficits. Furthermore, investigations of 'pathological plasticity' in these diseases lend insight into normal brain function. This review will present evidence for altered plasticity in Huntington's and Alzheimer's diseases, relate these findings to symptomatology, and review possible causes and commonalities.

Neuropharmacology of the endocannabinoid signaling system-molecular mechanisms, biological actions and synaptic plasticity

The endocannabinoid signaling system is composed of the cannabinoid receptors; their endogenous ligands, the endocannabinoids; the enzymes that produce and inactivate the endocannabinoids; and the endocannabinoid transporters. The endocannabinoids are a new family of lipidic signal mediators, which includes amides, esters, and ethers of long-chain polyunsaturated fatty acids. Endocannabinoids signal through the same cell surface receptors that are targeted by Delta(9)-tetrahydrocannabinol (Delta(9)THC), the active principles of cannabis sativa preparations like hashish and marijuana. The biosynthetic pathways for the synthesis and release of endocannabinoids are still rather uncertain. Unlike neurotransmitter molecules that are typically held in vesicles before synaptic release, endocannabinoids are synthesized on demand within the plasma membrane. Once released, they travel in a retrograde direction and transiently suppress presynaptic neurotransmitter release through activation of cannabinoid receptors. The endocannabinoid signaling system is being found to be involved in an increasing number of pathological conditions. In the brain, endocannabinoid signaling is mostly inhibitory and suggests a role for cannabinoids as therapeutic agents in central nervous system (CNS) disease. Their ability to modulate synaptic efficacy has a wide range of functional consequences and provides unique therapeutic possibilities. The present review is focused on new information regarding the endocannabinoid signaling system in the brain. First, the structure, anatomical distribution, and signal transduction mechanisms of cannabinoid receptors are described. Second, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. Finally, the role of the endocannabinoid signaling system in the CNS and its potential as a therapeutic target in various CNS disease conditions, including alcoholism, are discussed.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

Decreased association of the transcription factor Sp1 with genes downregulated in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease caused by expansion of a polyglutamine tract within the huntingtin protein. Transcriptional dysregulation has been implicated in HD pathogenesis; recent evidence suggests a defect in Sp1-mediated transcription. We used chromatin immunoprecipitation (ChIP) assays followed by real-time PCR to quantify the association of Sp1 with individual genes. We find that, despite normal protein levels and normal to increased overall nuclear binding activity, Sp1 has decreased binding to specific promoters of susceptible genes in transgenic HD mouse brain, in striatal HD cells, and in human HD brain. Genes whose mRNA levels are decreased in HD have abnormal Sp1-DNA binding, whereas genes with unchanged mRNA levels have normal levels of Sp1 association. Moreover, the altered binding seen with Sp1 is not found with another transcription factor, NF-Y. These findings suggest that mutant huntingtin dissociates Sp1 from target promoters, inhibiting transcription of specific genes.

DNA microarray analysis of striatal gene expression in symptomatic transgenic Huntington's mice (R6/2) reveals neuroinflammation and insulin associations

Huntington's disease (HD) is an inherited, progressive neurodegenerative disorder caused by CAG repeat expansion in the gene that codes for the protein huntingtin. The underlying neuropathological events leading to the selectivity of striatal neuronal loss are unknown. However, the huntingtin mutation interferes at several levels of normal cell function. The complexity of this disease makes microarray analysis an appealing technique to begin the identification of common pathways that may contribute to the pathology. In this study, striatal tissue was extracted for gene expression profiling from wild-type and symptomatic transgenic Huntington mice (R6/2) expressing part of the human Huntington's disease gene. We interrogated a 15 K high-density mouse EST array not previously used for HD and identified 170 significantly differentially expressed ESTs in symptomatic R6/2 mice. Of the 80 genes with known function, 9 genes had previously been identified as altered in HD. 71 known genes were associated with HD for the first time. The data obtained from this study confirm and extend previous observations using DNA microarray techniques on genetic models for HD, revealing novel changes in expression in a number of genes not previously associated with HD. Further bioinformatic analysis, using software to construct biological association maps, focused attention on proteins such as insulin and TH1-mediated cytokines, suggesting that they may be important regulators of affected genes. These results may provide insight into the regulation and interaction of genes that contribute to adaptive and pathological processes involved in HD.

Plant, Synthetic, and Endogenous Cannabinoids in Medicine

Although used for more than 4000 years for recreational and medicinal purposes, Cannabis and its best-known pharmacologically active constituents, the cannabinoids, became a protagonist in medical research only recently. This revival of interest is explained by the finding in the 1990s of the mechanism of action of the main psychotropic cannabinoid, Delta9-tetrahydrocannabinol (THC), which acts through specific membrane receptors, the cannabinoid receptors. The molecular characterization of these receptors allowed the development of synthetic molecules with cannabinoid and noncannabinoid structure and with higher selectivity, metabolic stability, and efficacy than THC, as well as the development of antagonists that have already found pharmaceutical application. The finding of endogenous agonists at these receptors, the endocannabinoids, opened new therapeutic possibilities through the modulation of the activity of cannabinoid receptors by targeting the biochemical mechanisms controlling endocannabinoid tissue levels.

Brain-specific factors in combination with mutant huntingtin induce gene-specific transcriptional dysregulation

Mutant huntingtin lowered steady-state levels of DARPP-32 mRNA in the brain but not kidney of R6 transgenic HD mice by repressing transcription from one of two promoters. The activity of DARPP-32 promoter deletion constructs were lower in the presence of mutant huntingtin in immortalized striatal cell lines but no difference in transcription factor binding to the promoter was detected. The activity of CMV, TK and HPRT promoters was also affected by mutant huntingtin in these cell lines. Transient transfection experiments demonstrated that short-term expression of mutant huntingtin exerted a cell- and promoter-specific transcriptional repression. In in vitro experiments, transcription of the CMV promoter was reduced in the presence of striatal proteins and mutant huntingtin. It is likely that select combinations of trans-acting factors, co-activators and components of the Pol II holoenzyme acting in concert provide the basis for both the gene- and tissue-specific effects of mutant huntingtin.

GENE–ENVIRONMENT INTERACTIONS, NEURONAL DYSFUNCTION AND PATHOLOGICAL PLASTICITY IN HUNTINGTON'S DISEASE

Huntington's disease (HD) is a fatal autosomal dominant disorder in which there is progressive neurodegeneration producing motor, cognitive and psychiatric symptoms. The dynamic mutation that causes the disease is common to numerous other brain disorders, which may share similar pathogenic mechanisms. Much progress has been made in the past decade in understanding how a trinucleotide (CAG) repeat expansion, encoding an expanded polyglutamine tract in the huntingtin protein, induces dysfunction at molecular and cellular levels. The present review integrates various lines of experimental evidence in an attempt to move towards a unifying mechanistic framework, which may explain the pathogenesis of HD, from molecular through to neuronal network and behavioural levels. Recent evidence, using transgenic mouse models, also suggests that environmental factors can modify the onset and progression of HD. The effects of specific environmental manipulations are discussed in the context of gene-environment interactions and experience-dependent plasticity in the healthy and diseased brain, particularly the cerebral cortex.

Intrastriatal rAAV-mediated delivery of anti-huntingtin shRNAs induces partial reversal of disease progression in R6/1 Huntington's disease transgenic mice

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by the presence of an abnormally expanded polyglutamine domain in the N-terminus of huntingtin. We developed a recombinant adeno-associated viral serotype 5 (rAAV5) gene transfer strategy to posttranscriptionally suppress the levels of striatal mutant huntingtin (mHtt) in the R6/1 HD transgenic mouse via RNA interference. Transient cotransfection of HEK293 cells with plasmids expressing a portion of human mHtt derived from R6/1 transgenic HD mice and a short-hairpin RNA directed against the 5' UTR of the mHtt mRNA (siHUNT-1) resulted in reduction in the levels of mHtt mRNA (-75%) and protein (-60%). Long-term in vivo rAAV5-mediated expression of siHUNT-1 in the striatum of R6/1 mice reduced the levels of mHtt mRNA (-78%) and protein (-28%) as determined by quantitative RT-PCR and Western blot analysis, respectively. The reduction in mHtt was concomitant with a reduction in the size and number of neuronal intranuclear inclusions and a small but significant normalization of the steady-state levels of preproenkephalin and dopamine- and cAMP-responsive phosphoprotein 32 kDa mRNA. Finally, bilateral expression of rAAV5-siHUNT-1 resulted in delayed onset of the rear paw clasping phenotype exhibited by the R6/1 mice. These results suggest that a reduction in the levels of striatal mHtt can ameliorate the HD phenotype of R6/1 mice.

The paradigm of Huntington's disease: therapeutic opportunities in neurodegeneration

Despite a relatively small number of affected patients, Huntington's disease (HD) has been a historically important disease, embodying many of the major themes in modern neuroscience, including molecular genetics, selective neuronal vulnerability, excitotoxicity, mitochondrial dysfunction, apoptosis, and transcriptional dysregulation. The discovery of the HD gene in 1993 opened the door to the mechanisms of HD pathogenesis. Multiple pathologic mechanisms have been discovered, each one serving as a potential therapeutic target. HD thus continues to serve as a paradigmatic disorder, with basic bench research generating clinically relevant insights and stimulating the development of therapeutic human trials.

Novel therapeutic targets for Huntington’s disease

Huntington's disease (HD) is a fatal autosomal-dominant disorder involving progressive motor, cognitive and psychiatric symptoms. HD is one of a large family of neurodegenerative diseases caused by a trinucleotide (CAG) repeat mutation, encoding an expanded tract of glutamines in the disease protein. HD was one of the first neurological disorders for which accurate transgenic models were created, allowing mechanisms of pathogenesis to be explored at molecular, cellular and behavioural levels. In the last decade, the understanding of molecular and cellular changes which occur in HD prior to onset of symptoms, and at early and late stages of disease progression, has been greatly expanded. A wide range of potential molecular targets for therapeutic intervention have been identified, associated with a variety of cellular processes including gene transcription, protein trafficking, protein degradation, protein-protein interactions, glutamatergic synaptic transmission, presynaptic signalling, postsynaptic signalling, synaptic plasticity, dopaminergic and neurotrophic modulation of synaptic function, experience-dependent neurogenesis, mitochondrial function and oxidative metabolism. Presymptomatic testing for the HD gene mutation necessitates future development of novel therapeutics aimed at delaying onset of symptoms, as well as slowing or reversing disease progression.

Structure, expression and regulation of the cannabinoid receptor gene (CB1) in Huntington's disease transgenic mice

Loss of cannabinoid receptors (CB1) occurs prior to neurodegeneration in Huntington's disease (HD). The levels and distribution of CB1 RNA were equivalent in 3-week-old mice regardless of genotype demonstrating that the specific factors and appropriate chromatin structure that lead to the transcription of CB1 were present in the striatum of young R6/2 and R6/1 transgenic HD mice. The expression of the mutant HD transgene led progressively to decreased steady-state levels of CB1 mRNA in neurons of the lateral striatum, which was dependent on the size of the CAG repeat and relative expression of the gene encoding mutant huntingtin (HD). Although it is known that the coding region of CB1 is contained within a single exon in mice, rats and humans, the 5'-untranslated region of the mouse gene remained to be defined. CB1 mRNA is encoded by two exons separated by an 18.4-kb intron. Transcription of CB1 occurred at multiple sites within a GC-rich promoter region upstream of exon 1 encoding the 5'-UTR of CB1. There was no difference in the selection of specific transcription initiation sites associated with higher levels of CB1 expression in the striatum compared to the cortex or between the striata of wild-type and HD transgenic mice. The progressive decline in CB1 mRNA levels in R6 compared to wild-type mice was due to decreased transcription, which is consistent with the hypothesis that mutant huntingtin exerts its effects by altering transcription factor activity. The cell-specific conditions that allow for increased transcription of CB1 in the lateral striatum compared to other forebrain regions from all transcription start sites were affected by the expression of mutant huntingtin in a time-dependent manner.

Arvanil, a hybrid endocannabinoid and vanilloid compound, behaves as an antihyperkinetic agent in a rat model of Huntington's disease

The present study was designed to examine whether arvanil (N-arachidonoyl-vanillyl-amide), an endocannabinoid/vanilloid structural "hybrid", might provide symptom relief in the rat model of Huntington's disease (HD) generated by bilateral intrastriatal application of 3-nitropropionic acid (3-NP), where previous evidence suggests that hybrid cannabinoid/vanilloid compounds might be effective. As expected, arvanil did reduce ambulation, stereotypic activity, and number of hole entries, and increased the inactivity, in control rats. It was also active in 3-NP-lesioned rats, where, despite its lowering effects on stereotypic activity and number of hole entries, arvanil reduced the hyperkinesia (increased ambulation) typical of these rats, and also increased the inactivity, these two effects being more moderate than those found in control rats. Arvanil caused its antihyperkinetic effects in 3-NP-lesioned rats presumably by enhancing excitatory transmission at the globus pallidus, since it increased glutamate content in this region. This contrasts with its effects in control rats where arvanil enhanced GABA transmission at the globus pallidus. In summary, arvanil does alleviate hyperkinesia typical of HD, although it also affects locomotion in normal rats. Nevertheless, considering the lack of efficacious pharmacological treatments in this basal ganglia disorder, our findings might provide the basis for the development of more specific drugs against HD.

Increased calbindin-D28k immunoreactivity in striatal projection neurons of R6/2 Huntington's disease transgenic mice

Striatal degeneration in Huntington's disease (HD) is associated with increases in perikaryal calbindin immunolabeling in yet-surviving striatal projection neurons. Since similar increases have also been observed in surviving striatal projection neurons after intrastriatal injection of the excitotoxin quinolinic acid, the increased calbindin in HD striatum has been interpreted to suggest an excitotoxic process in HD. We used immunolabeling to assess if calbindin is elevated in striatal projection neurons of R6/2 HD transgenic mice. These mice bear exon 1 of the human huntingtin gene with 144 CAG repeats and show some of the neuropathological signs (e.g., neuronal intranuclear inclusions) and clinical traits (e.g., wasting prior to early death) of HD. We found an increased frequency of calbindin-immunoreactive neuronal perikarya in the striatum of 6- and 12-week-old R6/2 mice compared to wild-type controls. This increase was most notable in the normally calbindin-poor dorsolateral striatum. We found no significant changes in the total area of striatum occupied by the calbindin-negative striosomes and no consistent changes in striatal calbindin mRNA. The increase in calbindin in R6/2 striatal neurons was thus limited to the matrix compartment, and it may be triggered by increased Ca2+ entry due to the demonstrated heightened NMDA sensitivity of these neurons. The data further support the similarity of R6/2 mice to HD, and are consistent with the occurrence of an excitotoxic process in striatum in both.

Abnormal sensitivity to cannabinoid receptor stimulation might contribute to altered gamma-aminobutyric acid transmission in the striatum of R6/2 Huntington's disease mice

BACKGROUND

One of the earliest neurochemical alterations observed in both Huntington's disease (HD) patients and HD animal models is the dysregulation of the endocannabinoid system, an alteration that precedes the development of identifiable striatal neuropathology. How this alteration impacts striatal synaptic transmission is unknown.

METHODS

We measured the effects of cannabinoid receptor stimulation on gamma-aminobutyric acid (GABA)-ergic synaptic currents recorded from striatal neurons of R6/2 HD mice in the early phase of their disease.

RESULTS

The sensitivity of striatal GABA synapses to cannabinoid receptor stimulation is severely impaired in R6/2 HD mice. In particular, whereas in control animals activation of cannabinoid CB1 receptors results in a significant inhibition of both evoked and spontaneous GABA-mediated synaptic events by a presynaptic mechanism, in R6/2 mice this treatment fails to reduce GABA currents but causes, in contrast, a slight increase of spontaneous inhibitory postsynaptic currents (sIPSCs).

CONCLUSIONS

Experimental HD was also associated with enhanced frequency of sIPSCs, a result consistent with the conclusion that loss of cannabinoid-mediated control of GABA transmission might contribute to hyperactivity of GABA synapses in the striatum of HD mice. Accordingly, spontaneous excitatory postsynaptic currents, which were not upregulated in R6/2 mice, were still sensitive to cannabinoid receptor stimulation.

Cannabinoid CB1 receptors in the basal ganglia and motor response to activation or blockade of these receptors in parkin-null mice

The endocannabinoid transmission becomes overactive in the basal ganglia in Parkinson's disease (PD), as reported in patients and animal models of this disease. In the present study, we examined the status of cannabinoid CB(1) receptors in the basal ganglia of female and male Park-2 knockout mice, a genetic model of PD that progresses with no neuronal death and that may be considered representative of early and presymptomatic parkinsonian deficits. We found an increase in the density of CB(1) receptors in the substantia nigra compared to wild-type animals with no changes in other basal ganglia, although this occurred only in females. Despite this increase, the motor inhibition caused by the acute administration of the cannabinoid agonist Delta(9)-tetrahydrocannabinol to Park-2 knockout female mice was markedly of lesser magnitude compared with the response found in wild-type animals. By contrast, the administration of the CB(1) receptor antagonist SR141716 resulted in a hyperkinetic response in parkin-null mice, response that was almost absent in wild-type animals and that was accompanied by a decrease in tyrosine hydroxylase activity in the caudate-putamen. However, parkin-null male mice exhibited normal levels of CB(1) receptors in the substantia nigra and the remaining basal ganglia, with the only exception of a small decrease in the lateral part of the caudate-putamen. This was associated with an increase in mRNA levels for superoxide dismutase in this structure. In addition, the administration of Delta(9)-tetrahydrocannabinol to parkin-null male mice caused a motor inhibition that was significantly greater than in the case of their wild-type counterparts, and that was accompanied by an increase in tyrosine hydroxylase activity in the caudate-putamen. In summary, extending the data obtained in humans and animal models of basal ganglia neurodegeneration, changes in CB(1) receptors were also observed in parkin-null mice, a model of PD that may be considered representative of early stages of this disease. These changes are associated with differences in behavioral responses to cannabinoid agonists or antagonists between Park-2 knockout and wild-type mice, although parkin-null mice exhibited evident gender-dependent differences for both levels of CB(1) receptors and motor responses to agonists or antagonists.

Cannabinoid control of motor function at the basal ganglia

Classic and novel data strengthen the idea of a prominent role for the endocannabinoid signaling system in the control of movement. This finding is supported by three-fold evidence: (1) the abundance of the cannabinoid CB1 receptor subtype, but also of CB2 and vanilloid VR1 receptors, as well as of endocannabinoids in the basal ganglia and the cerebellum, the areas that control movement; (2) the demonstration of a powerful action, mostly of an inhibitory nature, of plant-derived, synthetic, and endogenous cannabinoids on motor activity, exerted by modulating the activity of various classic neurotransmitters; and (3) the occurrence of marked changes in endocannabinoid transmission in the basal ganglia of humans affected by several motor disorders, an event corroborated in animal models of these neurological diseases. This three-fold evidence has provided support to the idea that cannabinoid-based compounds, which act at key steps of the endocannabinoid transmission [receptors, transporter, fatty acid amide hydrolase (FAAH)], might be of interest because of their potential ability to alleviate motor symptoms and/or provide neuroprotection in a variety of neurological pathologies directly affecting basal ganglia structures, such as Parkinson's disease and Huntington's chorea, or indirectly, such as multiple sclerosis and Alzheimer's disease. The present chapter will review the knowledge on this issue, trying to establish future lines for research into the therapeutic potential of the endocannabinoid system in motor disorders.

The biosynthesis, fate and pharmacological properties of endocannabinoids

The finding of endogenous ligands for cannabinoid receptors, the endocannabinoids, opened a new era in cannabinoid research. It meant that the biological role of cannabinoid signalling could be finally studied by investigating not only the pharmacological actions subsequent to stimulation of cannabinoid receptors by their agonists, but also how the activity of these receptors was regulated under physiological and pathological conditions by varying levels of the endocannabinoids. This in turn meant that the enzymes catalysing endocannabinoid biosynthesis and inactivation had to be identified and characterized, and that selective inhibitors of these enzymes had to be developed to be used as (1) probes to confirm endocannabinoid involvement in health and disease, and (2) templates for the design of new therapeutic drugs. This chapter summarizes the progress achieved in this direction during the 12 years following the discovery of the first endocannabinoid.

Mutant huntingtin affects the rate of transcription of striatum-specific isoforms of phosphodiesterase 10A

Huntington's disease (HD) is caused by the inheritance of a copy of the gene encoding mutant huntingtin with an expanded CAG repeat. Phosphodiesterase 10A (PDE10A) mRNA decreases in transgenic HD mice expressing exon 1 of the human huntingtin gene (HD). The mouse PDE10A mRNA is expressed through alternative splicing and polyadenylation in a tissue-specific manner and that transcription of striatal PDE10A mRNA is driven by two promoters. PDE10A2 is the predominant isoform of the gene is expressed in the striatum. Using in situ hybridization and quantitative RT-PCR, we determined that decreased steady-state levels of PDE10A2 mRNA were caused by an altered transcription initiation rate rather than by post-transcriptional mRNA instability in HD mice. Transcription from three initiation sites located within a 50-bp region in the PDE10A2-specific promoter was differentially affected by the presence of the mutant huntingtin transgene. The mouse and human PDE10A2 promoters are highly conserved with respect to the relative position of cis-regulatory elements. Several transcription factors that have been shown to interact with mutant huntingtin, including Sp1, neuron restrictive silencing factor, TATA-binding protein and cAMP-response element binding protein, are unlikely to be involved in mutant huntingtin-induced PDE10A2 transcriptional dysregulation.

Immunolocalization of CB1 receptor in rat striatal neurons: a confocal microscopy study

Several lines of evidence indicate that cannabinoids, among other functions, are involved in motor control. Although cannabinoid receptors (CB(1)) mRNA has been observed in medium-sized spiny neurons of the striatum, a description of the precise localization of CB(1) at a protein level among striatal cells is still lacking. Therefore, we performed immunohistochemical studies with light and confocal microscopy to identify neuronal subpopulations that express CB(1) and to assess the distribution of the receptor within these neurons. In our single label light microscopy study, CB(1) was observed in most medium-sized neurons of the caudate-putamen. However, CB(1) was also present in large-sized neurons scattered throughout the striatum. Our dual-label study showed that 89.3% of projection neurons in matrix contain CB(1), and that 56.4% of projection neurons in patch are labeled for CB(1). To investigate the presence of CB(1) among the different subclasses of striatal interneurons we performed a double-labeling study matching CB(1) and each of the striatal interneuron markers, namely, choline acetyl-transferase, parvalbumin, calretinin, and nitric oxide synthase. Our double-label study showed that most parvalbumin immunoreactive interneurons (86.5%), more than one-third (39.2%) of cholinergic interneurons, and about one-third (30.4%) of the NOS-positive neurons are labeled for CB(1). Calretinin-immunolabeled neurons were devoid of CB(1).

The endocannabinoid system and its therapeutic exploitation

The term 'endocannabinoid' - originally coined in the mid-1990s after the discovery of membrane receptors for the psychoactive principle in Cannabis, Delta9-tetrahydrocannabinol and their endogenous ligands - now indicates a whole signalling system that comprises cannabinoid receptors, endogenous ligands and enzymes for ligand biosynthesis and inactivation. This system seems to be involved in an ever-increasing number of pathological conditions. With novel products already being aimed at the pharmaceutical market little more than a decade since the discovery of cannabinoid receptors, the endocannabinoid system seems to hold even more promise for the future development of therapeutic drugs. We explore the conditions under which the potential of targeting the endocannabinoid system might be realized in the years to come.

Cannabis et récepteurs cannabinoïdes : de la physiopathologie aux possibilités thérapeutiques

BACKGROUND

Although cannabis has been used as a medicine for several centuries, the therapeutic properties of cannabis preparations (essentially haschich and marijuana) make them far most popular as a recreational drugs.

STATE OF THE ART

Scientific studies on the effects of cannabis were advanced considerably by the identification in 1964 of cannabinoid D9-tetrahydrocannadinol (THC), recognized as the major active constituent of cannabis. Cloning of the centrally located CB1 receptor in 1990 and the identification of the first endogenous ligand of the CB1 receptor, anandamide, in 1992 further advanced our knowledge.

PERSPECTIVE AND CONCLUSIONS

Progress has incited further research on the biochemistry and pharmacology of the cannabinoids in numerous diseases of the central nervous system. In the laboratory animal, cannabinoids have demonstrated potential in motion disorders, demyelinizing disease, epilepsy, and as anti-tumor and neuroprotector agents. Several clinical studies are currently in progress, but therapeutic use of cannabinoids in humans couls be hindered by undesirable effects, particularly psychotropic effects. CB1 receptor antagonists also have interesting therapeutic potential.

Delayed onset of huntington′s disease in mice in an enriched environment correlates with delayed loss of cannabinoid CB1 receptors

Huntington's disease (HD) is a late onset progressive genetic disorder characterised by motor dysfunction, personality changes, dementia and premature death. The disease is caused by an unstable expanded trinucleotide (CAG) repeat encoding a polyglutamine stretch in the IT15 gene for huntingtin, a protein of unknown function. Transgenic mice expressing exon one of the human HD gene with an expanded polyglutamine region develop many features of human HD. Exposure of these mice to an "enriched" environment delays the onset of motor disorders and slows disease progression [Nature 404 (2000) 721]. We have compared the levels of receptor binding of a range of basal ganglia neurotransmitter receptors believed to be important in HD, in normal mice and R6/1 transgenic HD mice housed in either enriched or standard laboratory environments. HD mice housed in a normal environment show a loss of cannabinoid CB1 and dopamine D1 and D2 receptors in the striatum and the corresponding output nuclei of the basal ganglia. HD mice exposed to an enriched environment show equivalent loss of D1 and D2 receptors as their "non-enriched" counterparts; in contrast, the "enriched" mice show significantly less depletion of CB1 receptors. In the brains of humans diagnosed with HD cannabinoid CB1 receptors are selectively lost from the basal ganglia output nuclei prior to the development of other identifiable neuropathology [Neuroscience 97 (2000) 505]. Our results therefore show that an enhanced environment slows the rate of loss of one of the first identifiable neurochemical deficits of HD. This suggests that delaying the loss of CB1 receptors, either by environmental stimulation or pharmacologically, may be beneficial in delaying disease progression in HD patients.

Gene-environment interplay in neurogenesis and neurodegeneration

Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene-environment interplay. There are many identified genetic determinants for so-called genetic disorders, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in either knock-in or knock-out mice. However, there are similarly, many identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. Constituent cellular defense mechanisms responsive to the challenge of increased reactive oxygen species represent only one crossroad whereby environment can influence genetic predisposition. In this paper we highlight some of the major neurodegenerative disorders and discuss possible links of gene-environment interplay. The process of adult neurogenesis in brain is also presented as an additional element that influences gene-environment interplay. And the so-called priming processes (i.e., production of receptor supersensitization by repeated drug dosing), is introduced as yet another process that influences how genes and environment ultimately and co-dependently govern behavioral ontogeny and outcome. In studies attributing the influence of genetic alteration on behavioral phenotypy, it is essential to carefully control environmental influences.

Striatal phosphodiesterase mRNA and protein levels are reduced in Huntington′s disease transgenic mice prior to the onset of motor symptoms

Inheritance of a single copy of the gene encoding huntingtin (HD) with an expanded polyglutamine-encoding CAG repeat leads to neuronal dysfunction, neurodegeneration and the development of the symptoms of Huntington's disease (HD). We have found that the steady-state mRNA levels of two members of the phosphodiesterase (PDE) multi-gene family decrease over time in the striatum of R6 transgenic HD mice relative to age-matched wild-type littermates. Phosphodiesterase 10A (PDE10A) mRNA and protein levels decline in the striatum of R6/1 and R6/2 HD mice prior to motor symptom development. The rate of reduction in PDE10A protein correlates with the rate of decline of the message and the decrease in PDE10A mRNA and protein is more rapid in R6/2 compared with R6/1 mice. Both PDE10A protein and mRNA, therefore, decline to minimum levels prior to the onset of overt physical symptoms in both strains of transgenic mice. Moreover, protein levels of PDE10A are decreased in the caudate-putamen of grade 3 HD patients compared with age-matched neuropathologically normal controls. Striatal PDE1B mRNA levels also decline in R6/1 and R6/2 HD mice; however, the decrease in striatal PDE10A levels (>60%) was greater than that observed for PDE1B and immediately preceded the onset of motor symptoms. In contrast, PDE4A mRNA levels are relatively low in the striatum and do not differ between age-matched wild-type and transgenic HD mice. This suggests that the regulation of PDE10A and PDE1B, but not PDE4A, mRNA levels is dependent on the relative expression of or number of CAG repeats within the human HD transgene. The loss of phosphodiesterase activity may lead to dysregulation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) levels in the striatum, a region of the brain that contributes to the control of movement and cognition.

Complex alteration of NMDA receptors in transgenic Huntington's disease mouse brain: analysis of mRNA and protein expression, plasma membrane association, interacting proteins, and phosphorylation

We analyzed NMDA receptor subunit mRNAs, proteins, and anchoring proteins in mice transgenic for exon 1 of the HD gene. R6/2 mice had decreased levels of mRNAs encoding epsilon1 and epsilon2 NMDA receptor subunits (mouse orthologs of rat NR2A and NR2B subunits), but not the zeta1 subunit (mouse ortholog of NR1), as assessed by gene expression profiling and Northern blotting. In situ hybridization resolved mRNA decreases spatially to the CA1 field of hippocampus. Western blotting revealed decreases in plasma membrane-associated epsilon1 and epsilon2 subunits in hippocampus, and decreases in plasma membrane-associated zeta1 subunit in cortex and hippocampus. In addition, PSD-95 and alpha-actinin-2, proteins essential for anchoring NMDA receptors, were decreased. Finally, we found a decreased level of tyrosine-phosphorylated epsilon1 subunit, another determinant of NMDA receptor trafficking, in R6/2 hippocampus. Taken together, these data demonstrate multiple levels of NMDA receptor dysregulation, including abnormalities in mRNA expression levels, receptor stoichiometry, protein phosphorylation, and receptor trafficking.