Characterising the anti-convulsant effects of CBD and CBDV on layer II of the medial entorhinal cortex of rat and human brain tissue in vitro

Abstract

Epilepsy is a common and serious neurological disorder, which manifests in seizures, and has an incidence of approximately 1% of the world population. In developed nations, most instances are relatively well controlled through the use of anti-epileptic drugs (AEDs). For around a third of cases, AEDs are ineffective, resulting in poorly maintained seizures otherwise known as refractory, or drug-resistant epilepsy (DRE). Currently, treatment of DRE often requires neurosurgery to be performed to resect seizure generating foci. Historically, such treatment was used as a last resort due to the invasive and higher risk nature of neurosurgery. More recently, however, surgical intervention has been performed much earlier in order to achieve better long-term patient outcomes. Notwithstanding this, DRE presents a major and as of yet, unmet clinical need for new and effective antiepileptic drugs to be found. In vitro and in vivo electrophysiological methods have been used investigate epilepsy for many years. Neuronal network oscillations and single cell patching recordings between physiological and pathophysiological samples provide a basis to compare alterations between normal and epileptic brain tissue. In terms of electrophysiological approaches, the hippocampus and entorhinal cortex (EC) are two of the most commonly studied areas of the brain, especially in relation to temporal lobe epilepsy (TLE). Plant derived cannabinoids – phytocannabinoids – have been proposed as effective AEDs for DRE cases. In particular the non-psychoactive phytocannabinoids cannabidiol (CBD) and cannabidivarin (CBDV), with recent clinical trials supporting this claim. The present study is an investigation into whether CBD and CBDV are suitable and effective AEDs, and to identify their mechanism(s) of action. Electrophysiological recordings of medial entorhinal cortex (mEC) layer II principal cells have been studied due to their relative importance and participation in TLE. Alterations in oscillatory rhythms and single cell responses were compared between RISE afflicted epileptic rats (SE rats) and wild type, age matched controls (AMC). Experiments on human tissue resected from children with TLE were also performed, concurrent with the rodent experiments. Key findings from this project show CBD(V) suppressant effects on induced gamma oscillations, in an age- and disease-dependent manner in rat tissue, suggesting damping of neuronal network excitability. Further to this, CBD induces increased GABA inhibition onto rat medial entorhinal principal cells as evidenced by increases in mean median decay times and inhibitory charge transfer across the postsynaptic membrane, while CBDV did not show this effect. The effects of CBD were effectively blocked by both GABAAR and NMDAR antagonists, suggesting interaction with both of these receptors to exert the response. CBD also showed additive effect to low-dose benzodiazepine and barbiturate agonists and a ceiling effect at higher doses, suggestive of an allosteric action on the GABAAR. Similar effects were also noted in the human tissue cells, suggestive of an analogous mechanism of action in humans. Hence, we postulate that CBD is acting at both postsynaptic GABAARs, as a positive allosteric modulator (PAM) and, at pre-/postsynaptic NMDARs, either directly or indirectly, to positively influence GABA signalling mechanisms causing an increase in inhibitory activity at postsynaptic principal cells resulting in decreased neuronal excitability.