Could cannabidiol be used as an alternative to antipsychotics?
Introduction
Schizophrenia is a complex mental health disorder that poses serious complications to the individual and to the society in terms of health care costs. This disease typically leads to low quality of life, long term disability, and reduced ability to work (Makinen et al., 2008). The median lifetime prevalence and incidence of schizophrenia are estimated at 0.72% and 15.2 per 100 000 individuals a year respectively (McGrath et al., 2008). However, the relative risk of this disease varies greatly across different locations and ethnicity, and has been shown to be particularly elevated among migrant group (Bhugra, 2004, Bhugra and Becker, 2005). Symptoms of schizophrenia usually emerge during adolescence and early adulthood, and are clustered into tree main categories; negative, positive, and cognitive. Negative symptoms, which are some of the most difficult to diagnose, refer to reduced emotional and behavioral response (Patel et al., 2014). Among them, blunted affect and anhedonia are the most strongly manifested in schizophrenic patients, but other symptoms such as reduced motivation and social interaction are also commonly observed (Makinen et al., 2008). In contrast to negative symptoms, positive symptoms are generally more easily identified. They are characterized by hallucinations, delusion, disorganized thinking and movement disturbances (Patel et al., 2014). Finally, cognitive deficits in schizophrenia include reduced episodic memory, poor executive functioning, low processing speed and disorganized speech, ultimately impairing the ability of the individual to properly think and communicate (Patel et al., 2014, Sheffield et al., 2014).
One of the most studied and recognized model of schizophrenia is the dopamine hypothesis (Howes and Kapur, 2009). This model attributes the symptoms of the disease to a hyperfunction of dopaminergic signaling in the mesolimbic system. It draws evidence from the observation that a large number of antipsychotics have the ability to block dopamine D2 receptors (Seeman and Lee, 1975, Howes and Kapur, 2009). Antipsychotics are divided into two classes: the typical (or first generation) and the atypical (or second generation). Although being efficient in managing some symptoms of schizophrenia, treatment with typical antipsychotics usually results in extrapyramidal side effects such as spasticity and tardive dyskinesia (Tandon and Jibson, 2002), whereas treatment with atypical antipsychotics leads to severe complications including weight gain, sedation and hyperlipidemia (Ucok and Gaebel, 2008).
Besides the dopamine hypothesis, evidence also support the notion that a dysregulation of the endocannabinoid system (ECS) may be linked to the pathophysiology of schizophrenia. The ECS is a signaling system that is mainly composed of the cannabinoid receptors types 1 (CB1) and type 2 (CB2), the endogenous ligands N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), as well as the hydrolytic enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) (Parsons and Hurd, 2015). Evidence supporting the role of the ECS in schizophrenia comes in part from post-mortem, imaging, and neurochemical studies showing abnormal brain levels of cannabinoid receptors and endocannabinoid ligands during the course of schizophrenia (Vigano et al., 2009, Dalton et al., 2011, Muguruza et al., 2013). Namely, some studies have shown that patients with schizophrenia-like symptoms have increased endocannabinoid signaling as a result of higher cannabinoid receptor density in certain brain areas (Dalton et al., 2011, Jenko et al., 2012). In addition, chronic administration of delta-9-tetrahydrocannabinol (THC), which acts as an agonist to cannabinoid receptors, was shown to precipitate the occurrence of schizophrenia-like symptoms in healthy subjects (D’Souza et al., 2004), and exacerbate the core psychotic and cognitive deficits in antipsychotic-treated schizophrenic patients (D’Souza et al., 2005). These findings led to the formulation of the “cannabinoid hypothesis of schizophrenia”, which speculates that a hyperactivity of the ECS may contribute to the development of schizophrenia-like symptoms. As such, the use of compounds that modulate its activity could have a tremendous potential for therapeutic interventions. One such compound is cannabidiol (CBD), a non-psychotropic component of cannabis. CBD was recently suggested as a promising antipsychotic agent because of its ability to prevent the actions of THC, the main constituent of cannabis (Niesink and van Laar, 2013). However, despite recent advances in research, the antipsychotic effects of CBD are not fully understood, and there are still debates as to whether this compound should be used as a treatment for schizophrenic patients.
The aim of this review is to provide a better understanding of the current treatment approaches used for schizophrenia, and to discuss whether CBD could be used as an effective line of treatment for this debilitating disease. This review first starts by describing evidence supporting a role for dopamine and endocannabinoids in schizophrenia, and provides an overview of the most commonly encountered side effects associated with currently used antipsychotics. This paper also draws on findings of animal studies and clinical trials exploring the therapeutic profile of CBD in schizophrenia, and discusses whether this compound could be used as an alternative to antipsychotics.
Section snippets
Dopamine in schizophrenia
The two dopaminergic pathways that are implicated in the onset of schizophrenia-like symptoms are the mesolimbic and mesocortical pathways. The mesolimbic pathway is comprised of dopaminergic projections that originate from the ventral tegmental area (VTA), and that innervate regions of the brain like the nucleus accumbens (NAc), which plays a key role in regulating the positive symptoms of schizophrenia. On the other hand, the mesocortical dopaminergic pathway is involved in emotional and
Typical and atypical antipsychotics: the end of an era?
Typical and atypical antipsychotics are currently the two main types of medication used for the treatment of schizophrenia. They both have a successful track record in treating hallucinations, paranoia, and other symptoms of schizophrenia. Atypical antipsychotics are currently considered a better option than conventional drugs because they are less prone to induce secondary motor effects, and are associated with overall improved subjective experience and compliance to treatment regimen (Lambert
The ECS in schizophrenia
Because of major issues related to the use of antipsychotics, several studies have explored neurotransmitter systems outside of the standard dopamine hypothesis that could have potential implications in schizophrenia. One such system is the ECS. The belief that the ECS might be implicated in schizophrenia symptomatology comes from longitudinal studies suggesting that chronic exposure to cannabis, especially during adolescence, increases the risk of psychotic and schizophrenia-like symptoms
CBD: a promising antipsychotic medication?
Since the discovery of cannabinoid receptors and endogenous ligands, there has been growing interest in exploring the ECS as a new target for pharmacotherapy. Knowing that brain alterations in cannabinoid receptors and endocannabinoids are strongly associated with the development of psychotic symptoms, the use of selective compounds that regulate their levels may represent a promising avenue for schizophrenia research (Kucerova et al., 2014, Fakhoury, 2016). As it turned out, modulating the
Conclusion
Despite significant progress in research, schizophrenia remains a debilitating disorder that presents a serious challenge to clinical therapeutics. As antipsychotics failed to meet expectations, the search for more effective therapies in still under way. The studies presented in this paper have shown that CBD could effectively reverse the positive, negative and cognitive symptoms of schizophrenia in animal models of the disease. In addition, findings from clinical trials suggest that CBD is
Conflict of interest
The author states that there is no conflict of interest.
Contributor
Marc Fakhoury.
Role of the funding source
Not available for this type of study.
Acknowledgments
The author is recipient of an award from the Natural Sciences and Engineering Research Council (NSERC) of Canada.
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