Abstracts

Rationale: A series of three randomized, placebo-controlled clinical trials have shown GW’s pharmaceutical formulation of cannabidiol (CBD) to be significantly superior to placebo in reducing seizure frequency in patients with Lennox-Gastaut and Dravet Syndromes. CBD exhibits a multimodal pharmacologic profile, wherein one mechanism of action may be to antagonize the G-protein coupled receptor GPR55, activated by the endogenous ligand lysophosphatidylinositol (LPI). LPI activation of GPR55 increases presynaptic Ca2+ and vesicular release at excitatory CA3-CA1 synapses, promoting the excitability of CA1 pyramidal neurons (Proc Natl Acad Sci USA 2013;110:5193–5198). Since LPI production is catalyzed by phospholipase A2, the activity of which is increased during epileptic seizures (Acta Neurol Scand 2002;106:258–262), GPR55 appears to be a promising target for the treatment of epileptic disorders. Methods: Here, we first studied CBD’s effect on GPR55 signalling ex vivo in acute brain slices taken from animals rendered chronically epileptic following lithium-pilocarpine induced status epilepticus and non-epileptic controls. Whole-cell patch-clamp recordings of hippocampal CA1 pyramidal neurons and interneurons were made and changes in LPI-mediated effects on frequency and waveform of miniature postsynaptic currents were examined in CBD- (10 µM) or CBD vehicle–treated slices. Secondly, in vivo mouse models of acute seizure were used to assess the role of GPR55 expression in seizure events and the effect of altered GPR55-mediated signaling upon the anticonvulsant potency of CBD. Results: The mean amplitude of miniature excitatory postsynaptic currents (mEPSCs) in epileptic slices, 6.6 ± 1.8 pA (n = 9), was decreased when compared to non-epileptic tissue, 14.9 ± 5.6 pA (n = 9, p < 0.001). The mEPSCs mean decay time constant was also longer in epileptic tissue, 11.2 ± 2.4 ms (n = 9) when compared to non-epileptic slices (14.5 ± 0.9 ms; n = 9, p < 0.05), suggesting alterations of postsynaptic amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPARs). However, changes in mEPSCs waveform in epileptic tissue were not affected by CBD (n = 9, p > 0.05).We confirmed previously published results (Proc Natl Acad Sci USA 2013;110:5193–5198) showing that GPR55 receptor activation by LPI evoked a 1.4-fold (± 0.1) relative increase of mEPSCs frequency in non-epileptic slices (n = 9, p < 0.05), an effect blocked by CBD. However, we showed for the first time that LPI effect was potentiated in epileptic CA1 pyramidal cells when compared to non-epileptic neurons, evoking a  2.8-fold (± 0.6) relative increase of mEPSC frequency (n = 9, p < 0.05), and that CBD remained able to fully inhibit this response. Conclusions: These results show that glutamatergic transmission at CA3-CA1 synapses is modified by epilepsy, potentially involving changes in AMPA receptor subunit composition. Our results also provide insights into a novel anticonvulsant mechanism of action of CBD by demonstrating that GPR55 function is enhanced by epilepsy in a manner that would worsen the disease state, but that CBD remains fully potent in epileptic brains. Funding: Funded by GW Research Ltd