Suppression of Harmaline Tremor by Activation of an Extrasynaptic GABA_A Receptor: Implications for Essential Tremor

Abstract

Background: Metabolic imaging has revealed excessive cerebellar activity in essential tremor patients. Golgi cells control cerebellar activity by releasing gammaaminobutyric acid (GABA) onto synaptic and extrasynaptic receptors on cerebellar granule cells. We postulated that the extrasynaptic GABA A receptor-specific agonist THIP (gaboxadol; 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) would suppress tremor in the harmaline model of essential tremor and, since cerebellar extrasynaptic receptors contain a6 and d subunits, would fail to do so in mice lacking either subunit. Methods: Digitally measured motion power, expressed as 10-16 Hz power (the tremor bandwidth) divided by background 8-32 Hz motion power, was accessed during pre-harmaline baseline, pre-THIP harmaline exposure, and after THIP administration (0, 2, or 3 mg/kg). These low doses were chosen as they did not impair performance on the straight wire test, a sensitive test for psychomotor impairment. Littermate d wild-type and knockout (Gabrd +/+ , Gabrd-/-) and littermate a6 wild-type and knockout (Gabra6 +/+ , Gabra6-/-) mice were tested. Results: Gabrd +/+ mice displayed tremor reduction at 3 mg/kg THIP but not 2 mg/kg, and Gabra6 +/+ mice showed tremor reduction at 2 and 3 mg/kg. Their respective subunit knockout littermates displayed no tremor reduction compared with vehicle controls at either dose. Discussion: The loss of anti-tremor efficacy with deletion of either d or a6 GABA A receptor subunits indicates that extrasynaptic receptors containing both subunits, most likely located on cerebellar granule cells where they are highly expressed, mediate tremor suppression by THIP. A medication designed to activate only these receptors may display a favorable profile for treating essential tremor.

Extrasynaptic δ‐GABAA receptors are high‐affinity muscimol receptors

“…For example, recent work suggested that α6βδ‐selective agonists might be useful in the clinic as antitremor medications (Handforth et al . ).…”

Positive modulation of cerebellar α6GABAAreceptors for treating essential tremor: a proof-of-concept study in harmaline-treated mice

“…3, B & C) suggests that the α6GABAARs in the cerebellum are the site of action of Compound 6. A previous study indicated that gaboxadol, a selective agonist of the δ-subunit containing GABAARs (δGABAARs), including the α6βδ subtype of GABAARs, significantly suppressed harmaline-induced tremor in wild type, but not in Gabra6 -/or Gabrd -/mice (Handforth et al, 2018), suggesting that the α6βδGABAAR subtype is involved in harmaline-induced tremor. Our previous electrophysiological study on oocyte expressing various recombinant GABAAR subtypes demonstrated that Compound 6 exerted its PAM activity at both α6βγ2 and α6βδ containing α6GABAARs although having a lower efficacy for the α6βδ subtype (Chiou et al, 2018).…”

“…This tremor-burrowing measurement alternation was repeated for several cycles as indicated in each experiment. As harmaline-induced tremor is an action tremor, alternations were designed to refresh the alertness of the mouse once back in the tremor chamber, enhancing tremor consistency by replacing the rest period described in previous studies (Handforth et al, 2018;Martin et al, 2005) with the burrowing activity measurement.…”

“…Action tremor was induced in mice by harmaline in accordance with the protocol described previously with minor modifications (Handforth et al, 2018;Martin et al, 2005). The frequency and intensity of tremor were measured and analyzed using a Tremor Monitor (San Diego Instruments, SDI, CA, USA) as reported previously (White et al, 2016), which can accurately differentiate tremor activity from ambulatory/stereotyped movements and global motion activity (Paterson et al, 2009).…”

Medical management of myoclonus-dystonia and implications for underlying pathophysiology

“…There is debate over whether phasic inhibition via synaptic GABA-A receptors (activated by benzodiazepines) or tonic inhibition via extrasynaptic GABA-A receptors (predominantly activated by alcohol) is the critical pathogenic mechanism in ET [100]. Recent animal studies have shown that gaboxadol, a highly selective agonist of extrasynaptic GABA-A receptors, is effective at suppressing tremor in a harmaline-induced murine model of ET [101]. A phase 2 trial of allopregnanolone, a progesterone metabolite, which activates both synaptic and extrasynaptic GABA-A receptors is currently underway (Clinicaltrials.gov: NCT02277106).…”