Alcohol and Ganaxolone Suppress Tremor via Extra-Synaptic GABAA Receptors in the Harmaline Model of Essential Tremor

Abstract

Background: A long-standing question is why essential tremor often responds to non-intoxicating amounts of alcohol. Blood flow imaging and high-density electroencephalography have indicated that alcohol acts on tremor within the cerebellum. As extra-synaptic δ-subunit-containing GABA_A receptors are sensitive to low alcohol levels, we wondered whether these receptors mediate alcohol's anti-tremor effect and, moreover, whether the δ-associated GABA_A receptor α6 subunit, found abundantly in the cerebellum, is required.

Methods: We tested the hypotheses that low-dose alcohol will suppress harmaline-induced tremor in wild-type mice, but not in littermates lacking GABA_A receptor δ subunits, nor in littermates lacking α6 subunits. As the neurosteroid ganaxolone also activates extra-synaptic GABA_A receptors, we similarly assessed this compound. The harmaline mouse model of essential tremor was utilized to generate tremor, measured as a percentage of motion power in the tremor bandwidth (9-16 Hz) divided by background motion power at 0.25-32 Hz.

Results: Ethanol, 0.500 and 0.575 g/kg, and ganaxolone, 7 and 10 mg/kg, doses that do not impair performance in a sensitive psychomotor task, reduced harmaline tremor compared to vehicle-treated controls in wild-type mice but failed to suppress tremor in littermates lacking the δ or the α6 GABA_A receptor subunit.

Discussion: As cerebellar granule cells are the predominant brain site intensely expressing GABA_A receptors containing both α6 and δ subunits, these findings suggest that this is where alcohol acts to suppress tremor. It is anticipated that medications designed specifically to target α6βδ-containing GABA_A receptors may be effective and well-tolerated for treating essential tremor.

Highlights: How does alcohol temporarily ameliorate essential tremor? This study with a mouse model found that two specific kinds of GABA receptor subunits were needed for alcohol to work. As receptors with both these subunits are found mainly in cerebellum, this work suggests this is where alcohol acts to suppress tremor.

Keywords: alcohol; cerebellum; harmaline; receptor; tremor.

Figure 1

Alcohol effect on harmaline tremor. Motion power in groups of mice followed sequentially during 15-minute epochs at baseline (B), pre-treatment harmaline (H1, H2), and after vehicle or alcohol injection (arrow, E1–E4). (A) In δ^+/+ mice ethanol, 0.50 and 0.575 g/kg, suppressed tremor during E1 compared to vehicle controls but (B) not in δ ^–/– littermates. (C) Similarly, in α6^+/+ mice ethanol, 0.50 and 0.575 g/kg, suppressed tremor during E1 compared to vehicle controls but (D) not in α6 ^–/– littermates. * P > 0.05, ** P > 0.01, *** P > 0.001, ANOVA.

Figure 2

Ganaxolone effect on harmaline tremor. Motion power during baseline (B), pre-treatment harmaline (H1, H2), and after vehicle or ganaxolone injection (arrow, E1–E4). (A) In δ^+/+ mice ganaxolone, 7 and 10 mg/kg, suppressed tremor following injection ompared to vehicle controls but (B) not in δ ^–/– littermates. (C) Similarly, in α6^+/+ mice ganaxolone, 7 and 10 mg/kg, suppressed tremor compared to vehicle controls but (D) not in α6 ^–/– littermates. * P > 0.05, ** P > 0.01, *** P > 0.001, ANOVA.