Cannabinoids: Do they have the potential to treat the symptoms of multiple sclerosis?

This article reviews the role of cannabinoids in inhibiting neurodegeneration in models of multiple sclerosis (MS). MS is a chronic, debilitating disease of the central nervous system (CNS), induced by autoimmunity-driven inflammation that leads to demyelination and thus disconnection of the normal transmission of nerve impulses. Despite the use of an array of immune modulating drugs that restore blood brain barrier function, disability continues in patients concomitant with the loss of axons in the spinal cord. MS patients therefore suffer neuropathic pain, spasticity and tremor. Anecdotal evidence suggests that MS patients using cannabis, though illegal, achieve symptomatic relief from neuropathic pain and spasticity associated with MS. The discovery of the endogenous cannabinoid (endocannabinoid) system that naturally exists in the body and which responds to cannabinoids to exert their effects has aided research into the therapeutic utility of cannabinoids. The endocannabinoid system consists of two G-protein coupled receptors cannabinoid receptor type-1 (CB₁) and CB₂. CB₁ is mainly expressed in the CNS and CB₂ is predominantly found in leukocytes, while an increasing number of potential ligands and endocannabinoid degradation molecules are being isolated. Several studies have highlighted the involvement of this system in regulating neurotransmission and its ability to prevent excessive neurotransmitter release, consistent with a capacity to provide symptomatic relief. In summary, antagonism of the CB₁ receptor pathway contributes to neuronal damage in chronic relapsing experimental allergic encephalomyelitis (EAE) and suppresses tremor and spasticity. The addition of exogenous CB₁ agonists derived from cannabis also afforded significant neuroprotection from the consequences of inflammatory CNS disease in EAE and experimental allergic uveitis models. Although clear neuroprotective benefits of cannabinoids have been demonstrated, the unwanted psychotropic effects need to be addressed. However, manipulating the endogenous cannabinoid system may be one way of eliciting beneficial effects without some or all of the unwanted side effects.