New Therapies for Essential Tremor from Cannabidiols Mechanisms

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Project Summary/Abstract Essential tremor affects 0.5-0.9% of the general population and about 4.5% above age 65. Medications for tremor are re-purposed drugs found empirically, often lack potency, and are often not well tolerated. In our program we study how drugs that suppress tremor work and through what molecular targets. Many patients report that cannabidiol suppresses tremor. Cannabidiol is non-psychotropic and has shown clinical efficacy for epilepsy, but may cause adverse effects, and induces hepatic enzymes. More effective and well-tolerated medications for tremor, based on cannabidiol’s mechanism, are desirable. To assess tremor in mice, we use the harmaline tremor model, which has many similarities to essential tremor. Our pilot data indicate that cannabidiol robustly suppresses harmaline-induced tremor, consistent with clinical observations. Cannabidiol has many actions, but the main receptors activated by it directly or indirectly are cannabinoid receptors type 1 and 2, the type 1 vanilloid receptor, and the serotonin type 1a receptor. By co-administering specific receptor antagonists along with cannabidiol, it is possible to determine the mechanism of action. In pilot experiments we found that activation of vanilloid 1 and serotonin 1a receptors both mediate cannabidiol's anti-tremor effect. As a vanilloid 1 agonist suppresses tremor, an effect blocked by a serotonin 1a receptor antagonist, it appears that vanilloid 1 receptor activation by cannabidiol is upstream to serotonin 1a receptor-mediated tremor suppression. In Aim 1 we will seek to replicate these findings and, also find out whether mice lacking the serotonin 1a receptor (knockouts) fail to show tremor suppression by cannabidiol or the vanilloid 1 receptor agonist. Much of cannabidiol's mechanism is due to inhibition of fatty acid amide hydrolase, leading to elevation of the endogenous cannabinoid anandamide, which then activates several receptors. We found in pilot experiments that a drug that inhibits this enzyme also suppresses tremor, an effect that requires both vanilloid 1 and serotonin 1a receptor activation. This suggests that inhibitors of fatty acid amide hydrolase could be used clinically to treat tremor. In Aim 2 we will seek to replicate these findings, using drugs and knockout mice to explore the role of vanilloid 1 and serotonin1a receptors in tremor suppression through inhibition of this enzyme and also assess whether anandamide administration suppresses tremor through the same mechanisms. An important potential tremor therapy stemming from cannabidiol's mechanisms is the administration of drugs that activate serotonin 1a receptors, which generally inhibit neuronal firing. When located on serotonin cell bodies, serotonin 1a autoreceptor activation reduces cell firing and serotonin release, thereby reducing activation of excitatory post-synaptic serotonin 2a receptors. Post-synaptic serotonin 1a receptors play a role in antidepressant and other actions. In pilot studies we found that both autoreceptor- and postsynaptic- preferring serotonin 1a agonists suppress tremor, while a drug that stimulates both receptors was even more effective. In Aim 3 we shall replicate these experiments and, also determine whether repeated dosing causes any of these serotonin agonists to lose their ability to suppress tremor. We will test them in serotonin 1a knockout mice to confirm that they are in fact suppressing tremor by activating the serotonin 1a receptor. This program is expected to lead to the identification of new therapies for essential tremor: inhibitors of fatty acid amide hydrolase, anandamide-like drugs, and serotonin 1a receptor agonists. These therapies are anticipated to be potent and well tolerated, should be available soon for clinical trials, and may also be beneficial for anxiety and depression, conditions that are common in Veterans with essential tremor.