Abstract/Summary

Over 50 million people worldwide have some form of epilepsy, characterised by seizures resulting from hyperexcitability of neuronal networks. Around a third of those with epilepsy are unable to control their seizures using current antiepileptic drugs or therapies, underscoring a large unmet clinical need for developing new pharmacotherapies. Cannabidiol (CBD), a nonpsychoactive component of the Cannabis sativa plant, has an anecdotal antiseizure history and has been gaining regulatory approval for epilepsy syndromes. However, its mechanism of action remains to be fully elucidated. The central adenosine signalling system has been proposed as a putative target for the CBD mechanism of action. Adenosine acts as a neuromodulator within the central nervous system through activation of the inhibitory adenosine 1 receptor (A1R), and release of adenosine is seen during seizures. This adenosine release provides an endogenous seizure termination mechanism. CBD has been shown to inhibit the bi-directional Equilibrative Nucleoside Transporter 1 (ENT1), which allows adenosine to transit cellular membranes by passive diffusion. This thesis uses in vitro techniques with a rat model of chronic epilepsy to test the hypothesis that prevention of adenosine reuptake through CBD blockade at ENT1, allowing for greater activation of the inhibitory A1R, underlies the antiepileptic efficacy of CBD. These techniques include functional electrophysiological recordings using multielectrode arrays to assess local field potentials in hippocampal slices, and enzymatic biosensors detecting adenosine concentration in the same slices. Additionally, qPCR, radioligand binding, and Western blotting were used to quantify any changes in expression of A1R, the adenosine 2A receptor (A2AR; which stimulates excitatory transmission), and ENT1 as a result of chronic epilepsy. Hippocampal slices taken from epileptic rats appeared to show a decrease in the ability of endogenously released adenosine to inhibit network activity, following seizure-like stimulation. Application of CBD (10 µM) returned adenosine potency to that seen in healthy hippocampus; however, peak adenosine release as measured by biosensors was significantly lower in the presence of CBD. Through radioligand binding, a decrease in A1R expression in epileptic hippocampus was found; a dysfunction in A1R activation was corroborated through use of an 8-CPT, an A1R antagonist, which potentiated field potentials significantly less in epileptic tissue than healthy. Additionally, CBD again appeared to positively modulate low concentrations of adenosine towards inhibition in epileptic hippocampal slices. While CBD did not appear to inhibit reuptake of seizure-associated adenosine in our assays, there appears to be a modulatory role it may play at adenosine receptors. Adenosine dysfunction in chronic epilepsy is indicated, but further experiments are required to characterise the contribution of the receptors, the transporter, and the interaction with CBD.