The Highs and Lows of the Endocannabinoid System—Another Piece to the Epilepsy Puzzle?

Commentary

Positron Emission Tomography Imaging of Cerebral Glucose Metabolism and Type 1 Cannabinoid Receptor Availability During Temporal Lobe Epileptogenesis in the Amygdala Kindling Model in Rhesus Monkeys.

Cleeren E, Casteels C, Goffin K, Koole M, Van Laere K, Janssen P, Van Paesschen W. Epilepsia 2018;59:959–970.

OBJECTIVE: We investigated changes in the endocannabinoid system and glucose metabolism during temporal lobe epileptogenesis. METHODS: Because it is rarely possible to study epileptogenesis in humans, we applied the electrical amygdala kindling model in nonhuman primates to image longitudinal changes in type 1 cannabinoid receptor (CB1R) binding and cerebral glucose metabolism. Two rhesus monkeys received [18 F]-MK-9470 and fluorodeoxy-glucose-positron emission tomography ([18 F]-FDG -PET) scans in each of the 4 kindling stages to quantify relative changes over time of CB1R binding and cerebral glucose metabolism in vivo. We constructed z-score images relative to a control group (n = 8), and considered only those changes measured in both kindled animals by calculating the binary conjunction image per kindling stage. RESULTS: The seizure-onset zone exhibited an increased CB1R binding and a decreased glucose metabolism, which both aggravated gradually in extent and intensity throughout kindling. The ipsilateral thalamus and insula showed hypometabolism that coincided with an increase and a decrease in CB1R binding, respectively. These changes also became gradually more severe throughout kindling and overlapped with ictal perfusion changes during the final stage of amygdala kindling, with hyperperfusion in the ipsilateral thalamus and hypoperfusion in the ipsilateral insula. SIGNIFICANCE: The observed changes in CB1R binding may reflect a combination of a protective mechanism of neurons against seizure activity that becomes stronger over time to combat more severe seizures, and on the other hand, a process of epileptogenesis that facilitates seizure activity and generalization, depending on the cell type involved in those specific regions. This study provides unique evidence that the CB1R is dynamically and progressively involved from the start of mesial temporal lobe epileptogenesis.

A word of caution, highs and lows are possible when the endocannabinoid system (ECS) is in play. The components of ECS are omnipresent in our bodies and have very divergent roles. Modulating ECS may have therapeutic potential in many human maladies, including psychiatric disorders (e.g., depression, posttraumatic stress disorder, anxiety, or schizophrenia), neurologic conditions, including epilepsy and neurodegenerative processes, diabetes and its complications, obesity, pain management, cancer treatment, graft versus host disease, treatment of chemotherapy side effects, and so on. The list is long, and it is constantly growing.

Over the years, rodent and other animal basic science work has made great strides toward understanding the divergent roles of ECS and the dynamics of its receptors (CB₁R and CB₂R) and neurotransmitters (anandamide [AEA], and 2-arachidon-oylglycerol [2-AG]). Better understanding of the role of ECS is particularly important in epilepsy where the commercial availability of a new antiseizure treatment, cannabidiol (CBD; Epidiolex, Greenwich Biosciences, Carlsbad, CA), is imminent. Thus, the study by Cleeren et al. is important, as it provides additional and needed information on the role of ECS in epilepsy.

Cannabis extracts have been used for the treatment of epilepsy for centuries (1). Yet, until recently, this empirical use was not linked to a known mechanism of action. Of the two main and most frequently investigated compounds derived from the cannabis plant, the mechanism of action of tetrahydrocannabinol (THC) is relatively clear and well documented (via CB₁R distributed mainly centrally and CB₂R distributed mainly peripherally). However, the mechanism of action of CBD is still under investigation with effects mediated via multiple receptors and channels including TRPV1, GPR55, and modulation of adenosine (2–6). Yet, both THC and CBD appear to modulate ECS. Further, basic science studies have documented the involvement of endocannabinoid receptors and endocannabinoids in epileptogenesis as well as seizure generation and maintenance (7). While animal data are clear in that CBD has anticonvulsant properties in the majority of studies, with only few studies showing no effect, THC has been demonstrated to also have proconvulsant effects in addition to anticonvulsant properties (8). Further, there is evidence of increased CB₁ receptor protein expression in epileptic rodent hippocampus and increased 2-AG in response to pilocarpine-induced seizures, indicating that endogenous cannabinoid tone modulates seizure termination via CB₁R and that plasticity of ECS occurs in this animal model of epilepsy (9).

While animal data are important, we also need to know how they translate into human epilepsy. Of course, experimental data are not available; hence, we need to resort to retrospective human data. One study has shown a significant reduction of AEA in the cerebrospinal fluid of patients with new onset temporal lobe epilepsy (TLE) who were naïve to treatment when compared with healthy controls (10). Those findings, in general, support the notion that endocannabinoids may be preventing seizures from occurring and that dysfunction of the endocannabinoid system may be involved in the pathogenesis of TLE. Another study has examined whether seizures/epilepsy resulted in long-term reorganization of the human ECS (11). These authors found, in hippocampi of epileptic patients when compared with hippocampi of patients without neurologic disorders, significant downregulation of CB₁R mRNA; there was also reduction of an enzyme responsible for the production of 2-AG. In addition, there was reduction of the CB₁R in the epileptic hippocampus. Put together, these findings indicate impairment of ECS in the epileptic hippocampus and facilitation of the network excitability by ECS dysfunction. The third study performed PET examination of the CB₁R in patients with TLE associated with hippocampal sclerosis (12). Using [¹⁸F]MK-9470 (same as in the study by Cleeren et al.), those authors were able to show increased CB₁R availability in the ipsilateral temporal lobe, which also negatively correlated with the time since the last seizure. Other areas that showed decreased CB₁R availability were insular cortex bilaterally, more ipsilateral than contralateral. The authors' interpretation was that the increased CB₁R availability in the ictal onset zone may be a protective mechanism against hyperexcitability and seizure activity, and/or it may contribute to the process of epileptogenesis while decreases in CB₁R availability may play role in ictal spread inhibition.

So, how do the animal and human data mesh with the data from rhesus monkeys in the study by Cleeren et al.? As the authors state, the rhesus monkey kindling model of TLE is not only much closer phylogenetically to human epilepsy than the rodent model, it is also relatively easy to study because of the slow progression of kindling. Here, the authors electrically kindled two monkeys and observed prospectively and longitudinally the changes in [¹⁸F]MK-9470 and [¹⁸F]FDG PET studies as the epileptic process progressed through all stages of epilepsy development. Not surprisingly, they observed with [¹⁸F]FDG PET global and progressive decrease in brain glucose metabolism with maximum decreases in the ipsilateral insula and temporal lobe with less consistent increases in other areas. Concurrently, they observed gradually increasing spatially and in intensity CB₁R binding in the seizure onset zone and in the ipsilateral thalamus and decreases in the ipsilateral insula. Thus, the observed changes in the final stages of the epileptogenesis in this rhesus monkey model were very similar to the changes in the chronic stages of TLE observed in human epilepsy (12). Hence, this study, in conjunction with the rodent and human data, provides evidence of the dynamic involvement of ECS in epileptogenesis.

Of course, despite its importance to the field, this study also has shortcomings. The authors only collected data on two monkeys, which resulted in them using conjunction analyses to show commonalities in the tracer uptake between animals and groups. This is not optimal, but the best they could do with the limited sample—future studies, if performed, will need larger sample sizes to better evaluate the progressive and evolving changes. Further, they observed high variability in the signal obtained with both tracers that necessitated scaling of the metabolic scanning results; baseline variability of the [¹⁸F]MK-9470 PET in rhesus monkey is not available. Both of these issues could have affected the results of the study. Further, scanning was conducted within a very short time after seizures that, by some, could be considered early postictal rather than interictal scanning—scanning subjects so close to the ictal events may have exacerbated or altered the observed changes especially between early stages where seizures were not spontaneous and later stages of epileptogenesis. Finally, while the authors observed changes in the process of epileptogenesis, they did not introduce intervention (e.g., CBD) to evaluate whether such intervention could modulate the process. Nevertheless, this is a valuable study that provides evidence that CB₁R and ECS are intimately involved in the process of epileptogenesis. Further testing of the effects of interventions on this process is needed to determine whether ECS modulation can change epileptogenesis in a meaningful way.