Endocannabinoid System as a New Therapeutic Avenue for the Treatment of Huntington’s Disease

Abstract

Huntington’s disease (HD) is a progressive, neurodegenerative disorder manifested by chorea as well as a variety of psychiatric abnormalities. Up to this date, only symptomatic treatment exists. Therefore, there is an urgent need for further therapies. Several neuroanatomical circuits are involved in the pathophysiology of HD, mainly the dopaminergic system. Animal studies and limited studies in humans have shown that abnormalities in the endocannabinoid system could also play an important role in the pathophysiology of HD. These findings have important clinical implications since cannabis-based medicines could potentially be used in the treatment of HD. The aim of this chapter is to summarize the current state of the research regarding the involvement of the endocannabinoid system in HD.

1. Introduction

Huntington’s disease (HD) is a neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline as well as psychiatric disturbance [1, 2]. The prevalence of HD is estimated to be between 0.4 and 5.70 per 100,000. Since HD is a genetic disorder, the prevalence depends strongly on the study population and it is higher in Europe, North America, and Australia than in Asia [3]. HD is caused by a dominantly inherited CAG repeat expansion in the huntingtin gene (HTT). The disease develops in individuals bearing a number of repetitions greater than 40, whereby greater CAG repeats found in the huntingtin gene are associated with early-onset forms of the disorder, fast rate of disease progression, and the most severe neurological deficits [4].

The mean age of HD onset is around 40 years, meanwhile the Juvenile Onset Huntington’s Disease (JOHD), occurs in individuals bearing more than 60 CAG repeats, which usually starts at the age of 21. HD eventually leads to death 15–20 years after the symptomatic onset [5]. It is believed that mutant huntingtin (mHTT) affects many cellular functions and leads to cell death, preferentially subpopulations of GABAergic medium spiny projection neurons and neurons in the cerebral cortex [1, 6]. This leads to imbalances in diverse neurotransmission, including the dopaminergic (DA) and glutaminergic systems. In the early stages of HD, DA neurotransmission is increased, whereas expression of DA receptors is reduced. However, in the course of the disease DA neurotransmission decreases. In turn, time-dependent abnormal DA neurotransmission affects glutamate receptor modulation, which may cause excitotoxicity [7, 8]. As DA plays a crucial role in the control of coordinated movements, motivation, and reward as well as cognitive function, alterations in DA balance in the striatum and provoke neurological and psychiatric symptoms of HD. The early stages of the disease are often characterized by chorea, followed by akinesia, while dystonia is more typical for the late stages [9]. Major non-motor symptoms include apathy and depression, anxiety, irritability, or aggressive behavior [9]. Impairment in cognitive functioning eventually ending in dementia, which has been mentioned by George Huntington in his first report, is another integral part of the disease [10]. Until today, there is no cure for HD, and treatment is only symptomatic, targeting mainly dopaminergic and glutaminergic systems [11].

2. Possible role of the endocannabinoid system in Huntington’s disease

Over the last 30 years, the endocannabinoid system (ECS) has emerged as an important neuromodulatory system, which could be efficiently targeted in a number of neurological diseases, including HD [8, 12, 13]. The primary cannabinoid receptor subtypes are cannabinoid receptors type 1 (CB1) and type 2 (CB2). The CB1 receptor is a protein-coupled receptor, highly expressed in the central nervous system (CNS), particularly in the neocortex, hippocampus, basal ganglia, cerebellum, and brainstem. In addition to its CNS location, CB1 has also been identified in numerous peripheral tissues and cell types [14]. On the other hand, the CB2 receptor is expressed mainly outside CNS, predominantly in the immune system. However, it has also been identified in the CNS, especially in the glial cells and brainstem neurons [15, 16]. The abovementioned high distribution of the CB1 receptor in basal ganglia indicates an indispensable role of the ECS in the control of movements by inhibitory modulation of other neurotransmitter systems [16]. Moreover, the CB1 receptors regulate glutamatergic neurotransmission under both physiological and pathological conditions and thus are able to downregulate excitotoxic glutamate release [17].

3. Studies in animal models

Studies in animal models suggest that the pathogenesis of HD may be related to an early and widespread reduction in the ECS, particularly to the loss of CB1 receptors [16, 18, 19] and decreased endocannabinoid levels in the striatum, which in turn may lead to hyperkinesia [19]. The administration of substances, which increase endocannabinoid activity led to a significant improvement of motor disturbances in a rat model of HD [16, 20]. In particular, Lastres-Becker et al. [17] hypothesized that substances that increase the endocannabinoid activity could be applied for the treatment of hyperkinetic symptoms. To test this hypothesis the authors created a rat model of HD through bilateral striatal injections of 3-nitropionic acid that leads to impaired striatal GABAergic neurotransmission. As a result, these rats started suffering from abnormal movements followed by motor depression. In addition, they demonstrated that the severity of motor hyperkinesias was correlated with decreased concentration of several neurotransmitters, such as GABA, dopamine, and their metabolites. Moreover, mRNA levels for the CB1 receptor were depleted in the caudate-putamen of 3-nitropropionic acid (3-NP) injected rats. In addition, the authors demonstrated a reduction in CB1 receptor binding in the caudate-putamen, the globus pallidus, and also substantia nigra. Finally, the administration of AM404, an inhibitor of endocannabinoid uptake, led to the alleviation of motor disturbances. The same group from Madrid [21] explored the status of CB1 receptors in the HD94 transgenic mouse model of HD. To investigate this problem, the authors analyzed mRNA levels of the CB1 receptor and the number of specific binding sites, and the activation of GTP-binding proteins by the CB1 receptor agonist. As a result, they have demonstrated that mRNA transcripts of the CB1 receptor were significantly decreased in selected regions of the brain, such as caudate in the HD transgenic mice compared to controls. This depletion was correlated with a marked reduction of reception density in the caudate, globus pallidus, and substantia nigra pars reticulata. In addition, the efficacy of CB1 receptor activation was depleted in the globus pallidus and there was a trend toward a decrease in substantia nigra.

Another significant contribution was done by the group from the Autonomous University in Madrid led by Isabel Lastres-Becker [22]. The scientists used a previously mentioned rat model of HD for this purpose created via bilateral intrastriatal injections of 3-NP. As a result, CB1 receptor binding and activation of GTP-binding proteins were also reduced in the basal ganglia. In parallel, the authors demonstrated a significant decrease of two endocannabinoids, anandamide and 2-arachidonoylglycerol in the striatum of affected rats, while there was an increase in anandamide concentration in the substantia nigra. Importantly, both CB1 receptors concentration, as well as endocannabinoid levels, were not changed in the cerebral cortex. Another study by the same group [23] has shown that compounds acting at the endocannabinoid systems reduce hyperkinesia in a rat model of HD. In particular, they applied AM404, an inhibitor of the endocannabinoid reuptake, which was able to reduce hyperkinesia and provoke recovery from neurochemical deficits.

As for exocannabinoids used in the treatment of neurological and psychiatric disorders, in one study [24], delta9-tetrahydrocannabinol (THC), a nonselective cannabinoid receptor agonist, and SR141716, a selective antagonist for the CB1 receptor, were tested in an animal model of HD. Surprisingly enough, the administration of THC increased malonate-induced striatal lesions, but SR141716 enhanced the same effect to an even greater extent. Another study examined the long-term effects of exocannabinoid exposure in animal models of HD. In this case, they used transgenic mice R6/1 of HD and administered THC for 8 weeks. This chronic treatment preserved CB1 receptors in the R6/1 striatum, suggesting that the manipulation of endocannabinoid levels warrants further exploration.

Similarly, Sagredo et al. [25] examined the neuroprotective effect of cannabinoids in rats with 3NP striatal lesions. To tackle this question, the authors used the CB1 agonist arachidonyl-2-chloroethylamide (ACEA), the CB2 agonist HU-308, and cannabidiol (CBD). Interestingly enough, the application of CBD, but not ACEA or HU-308 reversed the effects of 3NP. In particular, CBD reversed 3NP-induced reductions in GABA contents and mRNA levels of substance P (SP), neuronal-specific enolase (NSE), and superoxide dismutase-2 (SOD-2). The authors concluded that CBD has neuroprotective values, but mainly on striatal neurons projecting to substantia nigra. This neuroprotective effect was not reversed by the CB1 receptor antagonist SR141716. Pintor et al. [26] demonstrated that the cannabinoid receptor agonist, WIN 55,212–2, attenuates the effects induced by quinolinic acid (QA) in the rat striatum. In this study, QA was introduced in the rat striatum and this, in turn, led to the reproduction of clinical features typical for HD. The administration of WIN 55,212–2 blocked the increase in extracellular glutamate induced by QA. During in vivo experiment, WIN 55,212–2 significantly improved the striatal damage induced by QA, but no effect was observed on a behavioral ground. Valdeolivas et al. [27] also explored the neuroprotective potential of cannabinoids in an experimental model of HD. In particular, they investigated Sativex®, a combination of tetrahydrocannabinol (THC) and CBD at a ratio of 1:1, to monitor the potential neuroprotective effects of cannabinoids. The authors applied both histological and biochemical markers. As a result, the application of malonate in the striatum led to an increase in edema, while Sativex® reduced it. Moreover, Sativex® led to a reduction in neurodegeneration and glial activation. Furthermore, the authors found that both CB1 and CB2 receptors are involved in the positive effects of cannabinoids on HD symptoms. Similar findings were reported by Sagredo et al. [28], who used an animal model of HD to examine the potential neuroprotective effects of compounds influencing the endocannabinoid system. Interestingly enough, only compounds activating CB2 receptors had neuroprotective effects. The authors confirmed this statement by using the selective CB2 receptor antagonist, SR144528, which, in turn, led to increased vulnerability to malonate. What is more, the activation of CB2 receptors reduced the levels of tumor necrosis factor-alpha (TNF-alpha) that had been increased in the malonate-induced model of HD.

Another study by de Lago et al. [29] examined whether arvanil, an endocannabinoid „hybrid,” could lead to symptom reduction in the rat model of HD. It was demonstrated that arvanil reduced ambulation and stereotypic movements. The same group [30] demonstrated that UCM707, an inhibitor of the anandamide uptake, could be used as a symptom control agent in an animal model of HD and multiple sclerosis (MS), but failed to delay the disease progression.

Furthermore, a number of other studies have suggested that therapies with CB-activating compounds might lead to neuroprotective effects against excitotoxic striatal toxicity through both CB receptor-mediated and independent effects [21, 31, 32, 33, 34, 35]. However, in several studies, no benefit or even exacerbation of neurotoxicity could be observed [22, 25, 29].

An overview of studies investigating the relevance of the endocannabinoid system in HD pathogenesis in animal models is shown in Table 1.

Reference	Model	Substance	Outcome
Lastres-Becker et al. [17]	3 NP rats	AM404 (eCBRI)	AM404 reduced motor hyperactivity and improved toxin-induced GABA and dopamine deficits.
Lastres-Becker et al. [23]	3 NP rats	AM404 (eCBRI) VDM11 (eCBRI) AM374 (inhibitor of endocannabinoid hydrolysis) Capsaicin (VR1 agonist) CP55,940 (CB1R and CB2R agonist)	AM404 reduced hyperkinesia in lesioned animals; VDM11 and AM374 did not improve hyperkinesia. Capsaicin and CP55,940 reduced hyperkinesia. Capsaicin improved GABA and dopamine deficits in basal ganglia.
Lastres-Becker et al. [24]	Malonate rats	THC SR141716A (CB1R antagonist)	Exacerbation of neurotoxicity.
Lastres-Becker et al. [36]	3NP rats	THC	Neuroprotection.
De Lago et al. [21]	3 NP rats	Arvanil (eCBRI, CB1R and TRPV1 agonist)	Arvanil showed anti-hyperkinetic effects and increased the content of glutamate in the globus pallidus.
Pintor et al. [26]	Quinolinic acid rats	WIN55,212-2 (CB1R and CB2R agonist) AM-251 (CB1R antagonist)	WIN55,212-2 showed neuroprotective effects and AM-251 reversed them.
De Lago et al. [30]	Malonate rats	UCM707 (eCBRI)	Reduction of hyperkinetic activity and increase both glutamate and GABA levels in the globus pallidus. No neuroprotection.
Sagredo et al. [25]	3 NP rats	CBD	Neuroprotection.
Sagredo et al. [28]	1. Malonate mice; 2. CB2R knockout mice	HU-308 (CB2R agonist) SR144528 (CB2R antagonist) CBD ACEA (CB1R agonist)	HU-308 was neuroprotective and reduced proinflammatory markers (TNF-alpha). These effects were reversed by SR144528. CBD and ACEA were not neuroprotective.
Palazuelos et al. [35]	Mice expressing mHTT or quinolinic acid exposure	HU-308	HU-308 reduced quinolinic acid neurotoxicity.
Sotter et al. [34]	Pheochromocytoma cells expressing mHHT	HU210 (CB1R and CB2R agonist)	HU210 caused small, but significant increase of cell survival. It excerted potentially toxic effects including increased huntingtin aggregation.
Dowie et al. [37]	R6/1 transgenic mice	HU210 THC	HU210 and THC did not improve motor symptoms. HU210 treatment was associated with seizures.
Valdeolivas S et al. [27]	Malonate rats	THC/CBD SR141716A AM630 (CB2R antagonist)	THC/CBD was neuroprotective. SR141716 and AM630 reduced its neuroprotective effects

Table 1.

Studies investigating the relevance of endocannabinoid system in HD pathogenesis in animal models. Studies are presented in chronological order.

HD: Huntington disease; CB1R: cannabinoid receptor type 1; CB2R: cannabinoid receptor type 2; 3 NP mice: 3-nitropropionic acid; eCBRI: endocannabinoid re-uptake inhibitor; TRPV1: the transient receptor potential cation channel subfamily V member 1 (TrpV1); GABA: γ-aminobutyric acid; THC: tetrahydrocannabinol; CBD: cannabidiol; and ACEA: arachidonyl-2-chloroethylamide.

4. Clinical research

The post-mortem examination of brain tissue in individuals with HD as well as PET imaging studies in vivo showed that CB1 receptors are severely reduced in all regions of the basal ganglia in comparison to other receptor changes in HD, which strengthens the hypothesis of a possible role of cannabinoids in the progression of neurodegeneration in HD [38, 39].

First reports of using cannabinoids in patients with HD were contradictory [24, 28, 30]. In 1991, Consroe et al. conducted the first double-blind randomized cross-over study to evaluate the efficacy and safety of oral CBD (10 mg/kg/day for 6 weeks) in 15 neuroleptic-free patients with HD [28]. The therapeutic response was evaluated with the use of the Marsden and Quinn chorea severity scale [40]. In this study, no statistically significant improvement has been shown. There was also no significant difference between the CBD and placebo groups in terms of side effects. In 1999 Müller-Vahl et al. published a case of a 58-year-old male with HD who was treated with a single dose of 1.5 mg of a CB1 agonist, nabilone. In this individual, a severe deterioration of chorea was observed [24]. In 2006, Curtis et al. described a case of a 43-year-old female, whose chorea and irritability improved after medication with 1 mg of nabilone [30]. A double-blind placebo-controlled randomized cross-over trial using nabilone was conducted in 2009 by the same author. This time 37 patients were treated with 1 mg or 2 mg of nabilone daily for 5 weeks. For primary measures, the patients were assessed with Unified Huntington’s Disease Rating Scale (UHDRS) total motor score and UHDRS subsections for chorea, cognition and behavior, and neuropsychiatric inventory (NPI) for secondary measures. There were no statistically significant differences in total UHDRS between the groups. However, statistically, significant improvements were noted for the UHDRS chorea scale and the neuropsychiatric inventory. There were no statistical differences reported between the 1 and 2 mg. Adverse effects were reported for placebo and nabilone similarly. There was one Serious Adverse Event (SAE) related to nabilone—one of the patients withdrew due to severe sedation. Importantly, no psychoses were reported [23]. In 2016, the results of a study conducted by Moreno et al. using nabiximols in the treatment of HD were published [36]. Nabiximols (tradename Sativex®) is an oromucosal spray, containing 2.7 mg THC and 2.5 mg cannabidiol (CBD) per puff licensed in most European countries for symptomatic treatment of multiple sclerosis [35]. Both Sativex and placebo were handed to 25 patients in a form of an oral spray, to be administered up to 12 sprays/day for 12 weeks. The main aim of the study was to investigate the safety of nabiximols in HD patients, assessed by the absence of SAE and lack of impairment of motor, cognitive, behavioral, and functional scales during the active treatment. The secondary objective was a clinical improvement of Unified Huntington Disease Rating Scale scores. As a result, safety and tolerability were confirmed. No statistically significant improvement in UHDRS in the nabiximols group was noted with respect to the placebo group. Moreover, no significant changes in the biomarkers could be observed [35].

An overview of all available studies investigating the efficacy and safety of CBM in HD is provided in Table 2.

Reference	Number of patients (sex)	Age (mean)	Substance	Study design	Outcome
Consroe et al. [41]	15 (8 male, 7 female)	No data	CBD	Double-blind, randomized cross-over study	No significant improvement No relevant side effects
Müller-Vahl et al. [42]	1 male	58	Nabilone	Case report	Deterioration of chorea
Curtis et al. [43]	1 female	43	Nabilone	Case report	Improvement of chorea and irritability
Curtis et al. [44]	44 (22 male, 22 female)	52	Nabilone	Double-blind, placebo-controlled, cross-over study	Improvement of the UHDRS-chorea; 1 SAE – sedation
Moreno et al. [45]	25 (14 male, 11 female)	47.6	Nabiximols	Double-blind, randomized, cross-over, placebo-controlled, pilot trial	No SAE or clinical worsening; no significant improvement; no significant changes of biomarkers

Table 2.

An overview of studies investigating efficacy and safety of CBM in HD.

CBM: cannabis based medicine; HD: Huntington disease; SAE: severe adverse events; CBD: cannabidiol; UHDRS: United Huntington Disease Rating Scale; and SAE: serious adverse events.

5. Safety profile of cannabis-based medicines in patients with HD

Even today, very little is known about the safety of CBM in patients with HD due to the limited number of studies exploring this issue. However, the available preliminary results suggest that the safety profile of CBM in HD is similar to that in other groups of patients. A recently conducted meta-analysis, including diverse populations of patients treated with CBM, showed that administration of cannabinoids can be associated with a greater risk of adverse events (AE), including serious adverse events (SAE) [46]. The most common short-term AEs included dizziness, dry mouth, nausea or vomiting, fatigue, somnolence, euphoria, vomiting, disorientation, drowsiness, confusion, loss of balance, and hallucinations. So far, there has been no study evaluating the long-term AEs of cannabinoids [46]. Up to this point, only two CBM-related SAEs in HD have been reported and both occurred after the treatment with nabilone. A 58-year-old male described by Müller-Vahl experienced an exacerbation of chorea. Moreover, the patient noticed the deterioration of short-term memory [42]. During the study performed by Curtis et al. [44], one of the patients experienced severe sedation and had to withdraw from the trial. Importantly, none of the patients enrolled in this study suffered from exacerbation of chorea or psychosis. The most frequent AE was drowsiness and forgetfulness. In the recent study conducted by Moreno et al. [45], dizziness or disturbance in attention were the two most common AEs. No serious alterations in psychiatric or neurological conditions of the participants were noted [45].

6. Conclusions

There is increasing evidence that the endocannabinoid system is a new promising therapeutical target in patients with HD. However, larger well-designed controlled studies are urgently needed to confirm the efficacy and safety of this treatment.