[ARTICLE] Endocannabinoids: A Promising Impact for Traumatic Brain Injury – Full Text

Abstract

The endogenous cannabinoid (endocannabinoid) system regulates a diverse array of physiological processes and unsurprisingly possesses considerable potential targets for the potential treatment of numerous disease states, including two receptors (i.e., CB1 and CB2 receptors) and enzymes regulating their endogenous ligands N-arachidonoylethanolamine (anandamide) and 2-arachidonyl glycerol (2-AG). Increases in brain levels of endocannabinoids to pathogenic events suggest this system plays a role in compensatory repair mechanisms. Traumatic brain injury (TBI) pathology remains mostly refractory to currently available drugs, perhaps due to its heterogeneous nature in etiology, clinical presentation, and severity. Here, we review pre-clinical studies assessing the therapeutic potential of cannabinoids and manipulations of the endocannabinoid system to ameliorate TBI pathology. Specifically, manipulations of endocannabinoid degradative enzymes (e.g., fatty acid amide hydrolase, monoacylglycerol lipase, and α/β-hydrolase domain-6), CB1 and CB2 receptors, and their endogenous ligands have shown promise in modulating cellular and molecular hallmarks of TBI pathology such as; cell death, excitotoxicity, neuroinflammation, cerebrovascular breakdown, and cell structure and remodeling. TBI-induced behavioral deficits, such as learning and memory, neurological motor impairments, post-traumatic convulsions or seizures, and anxiety also respond to manipulations of the endocannabinoid system. As such, the endocannabinoid system possesses potential drugable receptor and enzyme targets for the treatment of diverse TBI pathology. Yet, full characterization of TBI-induced changes in endocannabinoid ligands, enzymes, and receptor populations will be important to understand that role this system plays in TBI pathology. Promising classes of compounds, such as the plant-derived phytocannabinoids, synthetic cannabinoids, and endocannabinoids, as well as their non-cannabinoid receptor targets, such as TRPV1 receptors, represent important areas of basic research and potential therapeutic interest to treat TBI.

 

Introduction

Traumatic brain injury accounts for approximately 10 million deaths and/or hospitalizations annually in the world, and approximately 1.5 million annual emergency room visits and hospitalizations in the US (). Young men are consistently over-represented as being at greatest risk for TBI (). While half of all traumatic deaths in the USA are due to brain injury (), the majority of head injuries are considered mild and often never receive medical treatment (). Survivors of TBI are at risk for lowered life expectancy, dying at a 3⋅2 times more rapid rate than the general population (). Survivors also face long term physical, cognitive, and psychological disorders that greatly diminish quality of life. Even so-called mild TBI without notable cell death may lead to enduring cognitive deficits (). A 2007 study estimated that TBI results in $330,827 of average lifetime costs associated with disability and lost productivity, and greatly outweighs the $65,504 estimated costs for initial medical care and rehabilitation (), demonstrating both the long term financial and human toll of TBI.

The development of management protocols in major trauma centers () has improved mortality and functional outcomes (). Monitoring of intracranial pressure is now standard practice (), and advanced MRI technologies help define the extent of brain injury in some cases (). Current treatment of major TBI is primarily managed through surgical intervention by decompressive craniotomy () which involves the removal of skull segments to reduce intracranial pressure. Delayed decompressive craniotomy is also increasingly used for intractable intracranial hypertension (). The craniotomy procedure is associated with considerable complications, such as hematoma, subdural hygroma, and hydrocephalus (). At present, the pathology associated with TBI remains refractive to currently available pharmacotherapies () and as such represents an area of great research interest and in need of new potential targets. Effective TBI drug therapies have yet to be proven, despite promising preclinical data () plagued by translational problems once reaching clinical trials ().

The many biochemical events that occur in the hours and months following TBI have yielded preclinical studies directed toward a single injury mechanism. However, an underlying premise of the present review is an important need to address the multiple targets associated with secondary injury cascades following TBI. A growing body of published scientific research indicates that the endogenous cannabinoid (endocannabinoid; eCB) system possesses several targets uniquely positioned to modulate several key secondary events associated with TBI. Here, we review the preclinical work examining the roles that the different components of the eCB system play in ameliorating pathologies associated with TBI.

The Endocannabinoid (eCB) System

Originally, “Cannabinoid” was the collective name assigned to the set of naturally occurring aromatic hydrocarbon compounds in the Cannabis sativa plant (). Cannabinoid now more generally refers to a much more broad set of chemicals of diverse structure whose pharmacological actions or structure closely mimic that of plant-derived cannabinoids. Three predominant categories are currently in use; plant-derived phytocannabinoids (reviewed in ), synthetically produced cannabinoids used as research () or recreational drugs (), and the endogenous cannabinoids, N-arachidonoylethanolamine (anandamide) () and 2-AG ().

These three broad categories of cannabinoids generally act through cannabinoid receptors, two types of which have so far been identified, CB1 () and CB2 (). Both CB1 and CB2 receptors are coupled to signaling cascades predominantly through Gi/o-coupled proteins. CB1 receptors mediate most of the psychomimetic effects of cannabis, its chief psychoactive constituent THC, and many other CNS active cannabinoids. These receptors are predominantly expressed on pre-synaptic axon terminals (), are activated by endogenous cannabinoids that function as retrograde messengers, which are released from post-synaptic cells, and their activation ultimately dampens pre-synaptic neurotransmitter release (). Acting as a neuromodulatory network, the outcome of cannabinoid receptor signaling depends on cell type and location. CB1 receptors are highly expressed on neurons in the central nervous system (CNS) in areas such as cerebral cortex, hippocampus, caudate-putamen (). In contrast, CB2 receptors are predominantly expressed on immune cells, microglia in the CNS, and macrophages, monocytes, CD4+ and CD8+ T cells, and B cells in the periphery (). Additionally, CB2 receptors are expressed on neurons, but to a much less extent than CB1 receptors (). The abundant, yet heterogeneous, distribution of CB1 and CB2 receptors throughout the brain and periphery likely accounts for their ability to impact a wide variety of physiological and psychological processes (e.g., memory, anxiety, and pain perception, reviewed in ) many of which are impacted following TBI.

Another unique property of the eCB system is the functional selectivity produced by its endogenous ligands. Traditional neurotransmitter systems elicit differential activation of signaling pathways through activation of receptor subtypes by one neurotransmitter (). However, it is the endogenous ligands of eCB receptors which produce such signaling specificity. Although several endogenous cannabinoids have been described () the two most studied are anandamide () and 2-AG (). 2-AG levels are three orders of magnitude higher than those of anandamide in brain (). Additionally, their receptor affinity () and efficacy differ, with 2-AG acting as a high efficacy agonist at CB1 and CB2 receptors, while anandamide behaves as a partial agonist (). In addition, anandamide binds and activates TRPV1 receptors (), whereas 2-AG also binds GABAA receptors (). As such, cannabinoid ligands differentially modulate similar physiological and pathological processes.

Distinct sets of enzymes, which regulate the biosynthesis and degradation of the eCBs and possess distinct anatomical distributions (see Figure Figure11), exert control over CB1 and CB2 receptor signaling. Inactivation of anandamide occurs predominantly through FAAH (), localized to intracellular membranes of postsynaptic somata and dendrites (), in areas such as the neocortex, cerebellar cortex, and hippocampus (). Inactivation of 2-AG proceeds primarily via MAGL (), expressed on presynaptic axon terminals (), and demonstrates highest expression in areas such as the thalamus, hippocampus, cortex, and cerebellum (). The availability of pharmacological inhibitors for eCB catabolic enzymes has allowed the selective amplification of anandamide and 2-AG levels following brain injury as a key strategy to enhance eCB signaling and to investigate their potential neuroprotective effects.

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FIGURE 1
Endocannabinoid system cell localization by CNS cell type. Endocannabinoid functional specialization among CNS cell types is determined by the cellular compartmentalization of biosynthetic and catabolic enzymes (biosynthesis by NAPE and DAGL-α, -β, catabolism by FAAH and MAGL). Cellular level changes in eCB biosynthetic and catabolic enzymes as a result of brain injury have yet to be investigated, though morphological and molecular reactivity by cell type is well documented.

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