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Neural Engineering

Neural Engineering - Poster Session B

Poster K5 - Cell-Type Specific Effects of Tetrahydrocannabinol on Epileptogenesis

Thursday, October 24, 2024
2:45 PM - 3:45 PM EST
Location: Exhibit Hall E, F & G

Introduction:  Epilepsy, a neurological disorder characterized by recurrent seizures, affects 50 million people worldwide. With 30% of patients resistant to conventional drugs, researchers are exploring alternative treatments, including cannabinoids. Tetrahydrocannabinol (THC), in particular, has garnered substantial interest because of its potential to reduce seizures. However, several studies have warned of potential pro-epileptic effects at higher doses.

We hypothesize that this dose-dependence stems from THC’s ability to suppress both glutamate and gamma-aminobutyric acid (GABA) release through cannabinoid type 1 (CB1) receptor activation on hippocampal neurons. The net effect of these competing pathways likely depends on THC concentration and differences in CB1 receptor expression and sensitivity between distinct cell types. This makes it difficult to elucidate the role of THC in epileptogenesis through systemic administration of the drug.

To overcome this challenge, we leveraged Drug Acutely Restricted by Tethering (DART.2), a technology that allows for cell-type specific pharmacology. We developed a cannabinoid-DART toolset, which we validated with Tango GPCR assays and whole-cell patch clamp electrophysiology. This toolset allowed us to deliver THC exclusively to inhibitory GABA neurons in the hippocampus of live mice, without impacting intermingled glutamate neurons. This enabled us to dissect the cell-type-specific impact of THC on seizure susceptibility in an acute model of epilepsy.

The insights from this study could inform the development of more targeted and effective epilepsy treatments that capitalize on the therapeutic potential of THC while mitigating its pro-convulsant effects.

Materials and

Methods:  Stereotaxic surgeries were performed to implant bipolar recording electrodes coated in Teflon in the hippocampal hilus of mice. For THC-DART.2 recipients, a viral injection was also administered at the electrode implantation site to express the HaloTag protein in inhibitory GABA interneurons. Additionally, a bilateral cannula was implanted in the lateral ventricles of these mice for ease of THC-DART.2 infusion later.

The baseline EEG signal of each mouse was recorded for 5 minutes in a faraday cage. 1s long biphasic electrical stimulations were delivered to the mice using an isolated high power stimulator. The stimulation intensity was incrementally increased by 20 μA until an electrographic seizure was observed. Once the first seizure threshold and duration were recorded, the mice either received a systemic injection of THC or an intracranial infusion of THC-DART.2 through the lateral ventricle.

Three hours after the first recording, electrical stimulations were again delivered to the mice using the same protocol. The second seizure threshold and duration were recorded.

Statistical analyses were performed using GraphPad Prism and included 2-way repeated measures ANOVA and 1-way ANOVA with Tukey multiple comparisons.

Results, Conclusions, and Discussions: We found that selective administration of THC-DART.2 to GABA interneurons, without impacting the excitatory mossy cells, significantly reduced the seizure threshold, indicating a pro-convulsant effect. By contrast, systemic administration of THC had no statistically significant effect on seizure threshold and duration compared to the control group.

The lack of effect observed with systemic THC administration aligns with the proposed competing mechanisms of CB1 receptor activation on inhibitory and excitatory pathways. Our finding that THC's effect on GABA interneurons is pro-convulsant also lends credence to this theory. Seizure susceptibility increased, as expected, because the disinhibition of the GABA interneurons was not counterbalanced by a suppression of mossy cell excitation. These results also incline us to hypothesize that selective administration of THC-DART.2 to mossy cells, on the other hand, will exert anti-convulsant effects, which remains to be investigated.

In conclusion, we developed a cannabinoid-DART toolset to explore the effects of THC on epileptogenesis in an acute model of seizure induction, providing insights into the complex interplay between the inhibitory and excitatory pathways of the CB1 system.

Future studies could investigate the effects of systemic THC in combination with rimonabant-DART.2 (a CB1 receptor antagonist) selectively administered to GABA interneurons. By blocking the binding of THC to CB1 receptors on GABA interneurons, this approach may allow us to isolate the effects of THC on excitatory mossy cells, furthering our understanding of this aspect of the THC-CB1 receptor mechanism.

Additionally, investigating the dose-dependence of the effects of systemic THC on seizure threshold and duration, as well as analyzing the dose-dependence of THC's effects on GABA interneurons vs those on mossy cells in a slice electrophysiology experiment could provide us with valuable insights that may guide future therapeutic strategies.

Acknowledgements (Optional):  I would like to express my heartfelt gratitude to my research mentor, Aryana, for her patient guidance and support throughout the course of this project!