Here’s an overview of a research study which received a 2019 research grant from the IETF:
Principal Investigator: Adrian Handforth, M.D.
Veterans Affairs Greater Los Angeles Healthcare System
Behind the cerebral hemispheres lies the cerebellum, a structure involved in precise motor control. Brain imaging studies performed on ET patients over 20 years ago showed that the cerebellum has a higher metabolic rate than normal, suggesting that brain cells here are abnormally active in ET. It has been found that low oral doses of alcohol that suppress tremor reduce this high metabolic rate in cerebellum down towards normal, suggesting that alcohol suppresses tremor by inhibiting cerebellar brain cells. The most abundant type of brain cell in the cerebellum is the granule cell. The ability of these cells to excite cerebellar outflow pathways is limited by the neurotransmitter GABA that is released by local brain cells.
GABA is the brain’s main inhibitory neurotransmitter, and it works by activating GABA receptors. Brain GABA receptors are not uniform but differ according to their subunit composition. They all contain 2 alpha subunits, of which there are 6 types, 2 beta (3 types), and either a delta (only 1 type) or a gamma (3 types) subunit. Cerebellar granule cells contain most of the brain’s alpha6-beta-delta receptors. This is interesting, because if a drug can be found that is specific in activating these receptors, it might be an effective and well-tolerated treatment for ET. Dr. Adrian Handforth and colleagues at VA Greater Los Angeles Healthcare System previously found that a GABA delta-receptor-specific drug, gaboxadol, reduced tremor in the harmaline mouse model of ET (mice who were injected with the drug harmaline to induce tremor). But, it did not work if the mice lacked the GABA receptor alpha6 or delta subunit. While this finding suggests this approach is on the right track, a problem is that gaboxadol at higher doses will also activate alpha4-beta-delta receptors in the rest of the brain, causing off-target side effects such as sleepiness. To date, there are no drugs that activate only alpha6-beta-delta receptors, and thereby act on cerebellar granule cells selectively. However, these cells are also special in possessing most of the brain’s alpha6-beta-gamma2 GABA receptors. As it happens, drugs have been created that are specific for these receptors. This creates an opportunity to find out whether a drug that inhibits cerebellar granule cells relatively selectively will suppress tremor effectively in doses that are well tolerated.
The drug known as Compound 6 selectively activates only alpha6-beta-gamma2 receptors and has already been shown to affect cerebellar granule cell function when injected systemically but has not been tested on tremor. In initial experiments, Dr. Handforth’s team will find out what dose levels are associated with psychomotor impairment in mice. Only doses at which all mice pass will be used in subsequent tremor experiments. Tremor will be induced in normal (wild-type) mice with the drug harmaline. Tremor in the harmaline model has similar tremor circuitry to ET and responds to medications used to treat ET. Compound 6 dose levels that suppress tremor will be determined and compared with dosages needed to cause psychomotor impairment. Mice lacking the GABA receptor alpha6 subunit will also be tested.
It is anticipated that Compound 6 will be found to suppress harmaline-induced tremor in wild-type mice, but not in mice lacking alpha6 GABA receptor subunits, indicating that Compound 6 is likely suppressing tremor by inhibiting cerebellar granule cells, where most brain alpha6-containing GABAA receptors are located. More importantly, because Compound 6 was devised to activate alpha6-beta-gamma2 GABA receptors with exquisite selectivity, receptors located almost only on cerebellar granule cells, it is anticipated that Compound 6 will suppress tremor in doses much less than those that cause impairment. Full safety and toxicology testing of Compound 6 has not been done, so that it is not yet known whether it could be given to humans. Nonetheless, Dr. Handforth believes that this work will provide proof-of-concept evidence that an alpha6-beta-gamma2 GABA receptor-selective medication might be highly effective and well-tolerated for ET.
Progress Report February 2020
Subtitle: Evaluation of an α6βγ2 GABA Receptor-Specific Drug as Potential Therapy
Adrian Handforth, MD
Progress Report Feb 27, 2020
Evidence suggests that in essential tremor the oscillatory circuit includes cerebellar granule cells, and that these cells are too active in this condition. Their activity is controlled by mossy fiber excitatory inputs and by GABA locally released by Golgi neurons. The inhibitory action of GABA is mediated by synaptic α6βγ2 and extra-synaptic α6βδ GABAA receptors. The cerebellar granule cell is the main site within brain that expresses α6 receptors of either class. We previously found that THIP, low-dose ethanol, and the neurosteroid ganaxolone suppress tremor in the mouse harmaline model in an α6- and a δ-dependent way, suggesting they act on the cerebellar granule cell. Unfortunately, they also act on extra-synaptic α4βδ GABAA receptors that are dominant in the brain outside the cerebellum, causing side effects. We are not aware of any α6βδ-selective drug. However, an alternative strategy for inhibiting cerebellar granule cells relatively specifically is to employ drugs selective for synaptic α6βγ2 GABAA receptors, as these are also mainly expressed on cerebellar granule cells. We therefore examined Compound 6 (PZ-II-029), which has been shown in vitro to act as a positive allosteric modulator of α6β3γ2 receptors, and less potently of synaptic α6β2γ2 receptors. Little in vivo work has been performed with Compound 6. Doses of 3 and 10 mg/kg have been reported to affect pre-pulse inhibition in mice, while the deuterated form has been found to exert analgesia in a rat model of trigeminal neuralgia. Based on its receptor action, we anticipated that Compound 6 would suppress tremor in doses that do not cause psychomotor impairment.
Aim 1: Effect of genotype on straight wire test response to Compound 6. The goal is to find the highest dose of Compound at which 6/6 mice pass the straight wire test, a sensitive test of psychomotor impairment, in α6 wild-type (WT) and in α6 knockout (KO) mice.
Aim 2: Effect of Compound 6 on harmaline tremor in mice with or without GABAA receptor α6 subunit. The goal is to assess whether Compound 6, in doses that were found in Aim 1 not to cause failures on the straight wire test, suppresses tremor in the harmaline mouse model.
Mice in our α6 colony have been back-crossed with C57BL6 mice for 10 generations. For experiments, littermate WT and KO offspring of heterozygote parents are utilized. Compound 6, synthetized in Belgrade by Dr. Miroslav Savic, and provided by Dr Margot Ernst of Austria, was dissolved in 85% saline, 14% propylene glycol, and 1% Tween 80, and injected intraperitoneally.
Straight wire test. Compound 6 in various doses was injected and mice tested every 10 minutes for up to 1 hour. The mouse is suspended by its front paws from a 2-mm diameter wire and the time taken to bring a hind paw up to the wire noted. Mice normally do so within 10 seconds, whereas sedated or ataxic mice fail to do this or fall off. The doses at which 6/6 pass all tests are determined. Only doses at which 6/6 pass the test or lower doses are to be used in harmaline experiments. This is a sensitive test of psychomotor impairment.
Harmaline tremor. The mouse is placed on an elevated platform on a cylinder that rests on a chamber floor fitted with a seismic detector and allowed to habituate before collecting motion power data. The mouse is free to move within the confines of the platform. After subcutaneous injection of harmaline, 20 mg/kg, tremor is observed to develop and stabilize, then motion power again accessed for 15 minutes. Compound 6 or vehicle is then administered. Motion power then is collected for five more 15-minute epochs separated by 5-minute rests in the home cage. Harmaline tremor is associated with an increase in motion power at 10-16 Hz. The tremor measure is motion power at 10-16 Hz as a percentage of overall motion power at 0.5-32 Hz to control for activity level. Percentages of 20% to 35% are seen with normal activity whereas higher values indicate tremor at 10-16 Hz.
Aim 1: We determined that doses of 10 mg/kg and 20 mg/kg were associated with all 6/6 mice passing the straight wire test. We did not try higher doses in order to preserve the Compound 6 supply, which was provided as a 100 mg initial shipment.
Aim 2: Compound 6 was found to suppress harmaline tremor in doses of 10 mg/kg and 20 mg/kg. The pattern of tremor suppression was unusual in that it was brief, lasting one or two 15-minute epochs, but the onset of this suppression varied, so that it could occur in the first post-drug 15-minute epoch, or in the second or third post-injection 15-minute epoch. Therefore, we used as a measure the minimum motion power percentage (MPP) among the first three post-drug epochs. As shown in the table below, α6 WT mice among the vehicle-, 10 mg/kg-, and 20 mg/kg-treated groups displayed comparable MPP values during pre-harmaline baseline and harmaline pre-drug epochs, but the groups receiving 10 mg/kg or 20 mg/kg showed a reduction of MPP compared to the vehicle-treated group, reflecting tremor suppression. No such reduction occurred in α6 KO mice.
Table 1: Effect of Compound 6, a α6βγ2-selective positive modulator, on harmaline tremor
Genotype Sample Baseline Harmaline Post-drug least p value
& Treatment size pre-harmaline pre-drug motion power
Vehicle 8 24.4 ± 2.4 81.4 ± 4.1 78.5 ± 2.9
10 mg/kg 9 26.3 ± 3.1 77.9 ± 4.1 53.6 ± 7.5 0.001
20 mg/kg 10 33.6 ± 6.5 79.9 ± 3.1 52.9 ± 4.8 0.0005
Vehicle 9 27.0 ± 1.7 82.7 ± 3.9 80.0 ± 1.6
10 mg/kg 9 27.2 ± 2.4 82.9 ± 4.0 86.7 ± 2.9 NS
20 mg/kg 10 32.0 ± 1.9 90.4 ± 2.9 84.1 ± 2.9 NS
Data represent motion power percentage of the tremor bandwidth power divided by overall motion power
(10-16 Hz)/(0.5-32 Hz) x 100. Means and SEM are shown. Post-drug least motion power refers to least motion power during any of three post-drug 15-minute epochs. Comparisons with vehicle, Student’s t test.
The initial results shown in Table 1 indicate that Compound 6, a selective positive allosteric modulator of α6βγ2 GABAA receptors, suppresses tremor in the harmaline model of essential tremor. Doses of 10 and 20 mg/kg exerted similar efficacy, suggesting that doses higher than 10 m/kg do not add to efficacy. The variable timing of tremor suppression and brief action likely reflect variable absorption, as the compound comes out of solution easily, and rapid degradation or re-distribution to fat stores. Nonetheless, the findings importantly indicate proof of principle that drugs designed to activate this receptor selectively can suppress tremor, so that this strategy may be fruitful for developing new therapies for essential tremor.
We wish to examine the effect of a lower dose on tremor in order to understand better the relationship between dose and efficacy. The collaborators are preparing another 100 mg shipment of Compound 6, which will enable us to complete both the harmaline and straight wire testing. Higher doses in the straight wire test will allow us to compare the highest dose tolerated in the straight wire test with those that suppress tremor. The PI is grateful to the IETF for supporting this work.
Conclusion Summary 2021
Background: Considerable work has identified the cerebellum as a state of physiological abnormality in essential tremor (ET). In previous work funded by IETF we explored the potential role of certain inhibitory receptors that respond to GABA. GABAA receptors are not uniform, but differ according to their subunit composition. They all contain 2 α (alpha) subunits, of which there are 6 types, 2 β (beta, 3 types), and either a δ (delta, only 1 type) or a γ (gamma, 3 types) subunit. We have been interested in GABAA receptors containing both δ and α6 subunits, as such receptors are found mainly on cerebellar granule cells, the most abundant type of cell in the cerebellum. In previous work we found that low-dose alcohol, ganaxolone, and gaboxadol each suppresses tremor in a mouse model of ET in which tremor is induced with the drug harmaline, but not if the mice lack the δ or the α6 GABAA receptor subunit. Unfortunately, there is no drug that specifically activates only δ GABAA receptors containing α6 subunits. Therefore, given that cerebellar granule cells are also the main brain site expressing α6βγ2 GABAA receptors, we turned out attention to the idea that a drug that activates such receptors will suppress tremor. The candidate compound was a drug known as Compound 6, reported to selectively activate α6βγ2 receptors. This drug had been shown to affect cerebellar granule cell function when injected systemically. We predicted that Compound 6 would suppress tremor at doses much lower than those that cause psychomotor impairment, and that this action would depend on the presence of α6 subunits.
Methods: The straight wire test was used to assess what doses of Compound 6 do or do not interfere with the ability of mice to hang on to a wire for 10 seconds at 10-minute intervals over an hour. This is a sensitive test of psychomotor performance. In the harmaline tremor experiment, mice received harmaline to induce tremor, then received either vehicle or Compound 6 in various doses intraperitoneally. The tremor measure was motion power in the 9-16 Hz tremor bandwidth divided by background 0.25-32 Hz motion power, as accessed with a seismic detector and digitized by software. Wild-type (WT) and littermate α6 knockout (KO) mice were assessed.
Results: Due to low solubility, the highest dose that was tested in the straight wire test was 20 mg/kg, at which 6/6 mice passed. In harmaline experiments with WT mice, 10 mg/kg strongly suppressed tremor, with 2 mg/kg exerting mild tremor suppression, and 5 mg/kg moderate tremor suppression. Similarly, Compound 6 caused dose-dependent tremor suppression in α6 KO mice.
Conclusions: Compound 6 exerts strong tremor suppression in well-tolerated doses. Our results indicate that, contrary to our prediction, this action does not depend on the presence of the GABAA receptor α6 subunit. The mechanism of anti-tremor action is thus uncertain. Nonetheless, the results suggest that this compound, and potentially others in its class, may hold promise as therapy for ET.
- Rationale and Relevance to Essential Tremor
During tremor an oscillatory circuit involves prefrontal and frontal cortex, pons, cerebellum, and thalamus (1) As the sole output from cerebellar cortex; Purkinje cells receive a massive excitatory input via parallel fibers from granule cells, which in turn receive massive excitatory inputs from ascending mossy fibers. Evidence that this entire cerebellar cortical system is excessively active in ET is provided by imaging scans that demonstrated increased blood flow (2). Because cerebellar metabolism is dominated by the vast cerebellar granule cell population, this finding suggests these cells are more active in ET. It was also found that when subjects were given a non-intoxicating dose of alcohol sufficient to suppress tremor, with a blood level of 0.04 g/dl, the cerebellar hyper-metabolism was reduced towards normal. (2). The findings were interpreted as indicating that alcohol reduces excitatory drive by granule cells on Purkinje cells, leading to reduction of tremor. In support of this interpretation, high-density EEG has indicated that alcohol reduces tremor by acting in the cerebellum (3). Infarcts of the pons, which destroy mossy fibers and remove the main excitatory input to granule cells, abolishes tremor (4). The Golgi cell, as the main source of inhibition to cerebellar granule cells, is positioned to play a critical role in controlling cerebellar activity, and hence tremor, by releasing GABA. This brain site is unique in its intense expresses α6βδ and α6βγ2 GABAA receptors. GABAA receptors form two main classes. Synaptic receptors are pentameric structures containing 2 α, 2 β, and a γ subunit, and mediate fast signaling. There are 6 types of α and 3 types of β subunits. Extrasynaptic receptors are also pentameric, but utilize a δ rather than a γ subunit, usually contain α4 or α6 subunits, are sensitive to very low GABA levels, and induce tonic inhibitory currents. In contrast to δ GABAA receptors elsewhere in brain that usually contain α4, those of cerebellar granule cells utilize α6 subunits (5). GABAA receptor subunit mapping reveals very intense α6 expression that is almost restricted to cerebellum, with only light staining in certain other CNS areas, such as sensory brainstem nuclei (6). The α6 KO state results in 50% loss of cerebellar GABAA receptors, with a severe loss of δ, and 40% depletion of γ2 (7). As expected, tonic inhibitory currents are lost in cerebellar granule cells (8). Despite these considerable changes, α6 KO mice are agile, show no motor deficits, and exhibit normal harmaline-induced tremor. This is explained by the finding that in α6 KO mice there is a compensatory increase in voltage-independent potassium conductance, so that granule cells are normally excitable (8).
Ethanol enhances GABA currents at concentrations as low as 3 mM in α6β3δ receptors in recombinant systems (9). Slices of cerebellar granule cells respond with enhanced tonic currents to ethanol at 10 mM, but not if taken from δ KO mice (10) or from α6 KO mice (8). This level is below the legal driving limit of 0.08 g/dL (17.3 mM). Reasoning that this may be the mechanism by which alcohol suppresses tremor in ET, we found that low-dose alcohol suppresses tremor in the harmaline model in WT mice but not in mice lacking the α6 or the δ GABAA receptor subunit (unpublished). Similarly, we found that gaboxadol, which activates α6βδ receptors at sub-micromolar levels (11), and induces tonic currents in slices of cerebellar granule cells in δ- and α6-subunit dependent fashion, also suppresses harmaline tremor in mice and requires δ and α6 GABAA (12). Neurosteroids are activate tonic currents (13). The neurosteroid ganaxolone exerts anxiolytic effects in mice, but not if they lack the δ subunit (14). We found that ganaxolone reduces harmaline tremor in WT mice but not in littermate mice lacking the α6 or the δ GABAA receptor subunit (unpublished).
Collectively, these results indicate that the inhibition of cerebellar granule cells, as by activating α6βδ receptors, is a valid strategy to treat tremor. However, a drawback is that neither alcohol, ganaxolone, nor gaboxadol are α6βδ-receptor selective, as they each also activate α4βδ GABAA receptors. That is a major problem, because the concurrent activation of the later receptors is prone to cause adverse effects such as somnolence, imbalance, addiction, disinhibition, and other effects. Selective activation of α6βδ GABAA receptors to inhibit overactive cerebellar granule cells represents a promising strategy for treating tremor but unfortunately is so far not feasible, as no compounds are available that activate α6βδ GABAA receptors with high selectively.
An alternative strategy for selective inhibition of cerebellar granule cells is to stimulate GABAA α6βγ2 receptors, as these are intensely expressed by these cells. The pyrazoloquinolinones are a class of compounds suggested to hold promise as anxiolytics as they exert exceedingly low sedation (15). Studies with derivatives on recombinant GABAA receptors expressed on oocytes showed that Compound 6 (PZ-II-029) displays high affinity for the α6βγ2 receptor, and acts as a positive allosteric modulator, so that it does not exert intrinsic activating activity, but enhanced GABA-induced currents (16, 17). The α6βγ2 GABAA synaptic receptor is expressed mainly, but not solely, in cerebellar granule cells, activated by GABA released by Golgi neurons. Compound 6 has over 1000 times higher affinity for α6βγ2 GABAA receptors than for any other GABAA receptor, but does have weak affinity for α3βγ2 receptors (17).
Methamphetamine causes disruption of prepulse inhibition of the acoustic startle response, an effect that is blocked by intraperitoneal (i.p.) injection of Compound 6 in doses of 3 or 10 mg/kg. This effect of Compound 6 is itself blocked by intracerebellar injection of furosemide, which blocks α6 GABAA receptors (18). Compound 6 thus appears to affect α6βγ2 receptors in the cerebellum when given systemically.
There are limitations to the evidence supporting the notion that Compound 6 may exert in vivo selectivity for α6βγ2 GABAA receptors as is seen in recombinant systems. Validation with cerebellar granule cell slices has not been performed. In the prepulse inhibition studies, α6 knockout (KO) mice were not tested. The effect of furosemide on blocking Compound 6’s effect on prepulse inhibition may have been downstream from the site of Compound 6’s action. The extent to which Compound 6 affects non-GABA receptors is not known.
Nonetheless, given the extraordinary selectivity for recombinant α6βγ2 GABAA receptors, we chose to study Compound 6 for anti-tremor efficacy, as a vehicle for evaluating this attractive therapeutic target. In addition, we assessed whether the α6 GABAA receptor subunit is required for its anti-tremor action, if any.
To induce tremor in mice, we used the harmaline model, which is not a model of ET the disease, but a symptom-based model of action tremor. Harmaline tremor has considerable physiological, anatomic, and pharmacological overlap with tremor of ET. Harmaline tremor in rodents responds to drugs that suppress tremor in ET (propranolol, primidone, alcohol, benzodiazepines, gabapentin, zonisamide, 1-octanol, γ-hydroxybutyrate), while being exacerbated by drugs that worsen ET (tricyclics, caffeine). Like ET, harmaline causes cerebellar hyper-metabolism, including Purkinje cells, the cortex, and deep cerebellar nuclei, all implicated in ET. Cerebellar granule cells are also activated during harmaline tremor, as indicated by c-fos labelling (19, 20). Thalamic deep brain stimulation suppresses harmaline tremor (21), as in ET. False positives have occurred, in which drugs that suppress harmaline tremor do not reduce tremor in ET patients (22). Many of these disparities were likely from the use of doses that caused non-specific psychomotor impairment in animals.
Our first aim was to assess the effect of Compound 6 on psychomotor impairment, utilizing the straight wire test, a highly sensitive test for psychomotor compromise in mice (23). The purpose of this experiment was to determine the highest dose of Compound 6 at which 6/6 mice passed all straight wire tests. Only doses at which all mice pass were to be used in subsequent harmaline experiments.
The second aim was to assess whether Compound 6 suppresses harmaline-induced tremor in doses not associated with impairment on the straight wire test, and whether any anti-tremor effect depended on the presence of the α6 GABAA receptor subunit.
- Research Methods and Procedures
Breeding: Our α6 colony, on a C57BL6 background, has been backcrossed for 10 generations with C57BL6 mice in our laboratory. To generate mice for experiments, littermate α6 WT and α6 KO mice were bred from α6 heterozygote parents. For polymerase chain reaction (PCR) genotyping, ear snips were collected before 3 weeks of age and sent to Transnetyx, Inc. (Memphis, TN). Mice were housed with ad libitum access to food and water on a 12:12 h light:dark cycle; temperature 22-24 degrees Celsius. Both males and females were used.
Drugs: Harmaline was obtained from Sigma-Aldrich, and Compound 6 was donated by Dr. Margot Ernst (Center for Brain Research, Medical University Vienna, Vienna, Austria). Harmaline was dissolved in saline and administered subcutaneously as 20 mg/kg in 4 ml/kg. Compound 6 was dissolved by sonicating and warming in a mixture of 85% distilled water, 14% propylene glycol, and 1% Tween 80 (Sigma-Aldrich), and was given i.p. in various doses in a volume of 10 ml/kg, except for the dose 20 mg/kg, which was given as 20 ml/kg, as the maximum solubility of the drug in this solution is approximately 1 mg/ml.
Straight wire test: In this test for impairment, Compound 6 in various doses was injected and mice tested every 10 minutes for up to 1 hour. Each mouse was suspended by its front paws from a 2-mm diameter wire and the time taken to bring a hind paw up to the wire noted. Mice normally do so within 10 seconds, whereas sedated or ataxic mice fail to do this or fall off. The dosage at which 6/6 pass all tests was be determined. Only doses at which 6/6 pass the straight wire test or lower doses were used in harmaline experiments. This is a sensitive test of psychomotor impairment. For example, we found that 3 mg/kg gaboxadol is the maximum dose at which 6/6 pass (12); doses of 4-6 mg/kg are known to affect the EEG in mice (24).
Harmaline tremor: Motion activity was measured with a Convuls-1 Sensing Platform (Columbus Instruments, Columbus, OH), that has a load sensor, connected to a Grass amplifier (Grass Instruments, West Warwick RI) with 1 and 70 Hz filter settings. Motion power was digitally analyzed using Spike2 software (Cambridge Electronic Design UK) to transform data into frequency spectra. The motion power percentage (MPP) is the tremor bandwidth divided by overall motion: (9-16 Hz power)/(0.25-32 Hz power) x 100.
Each mouse was placed on a 9.5-inch high 3.2-inch wide cylinder situated on the tremor platform, allowed 20 minutes to habituate, then 15 minutes of pre-harmaline baseline motion power data collected. The exposed location has an alerting effect, encouraging mice to move about, eliciting action tremor. Harmaline was then injected, and motion power measurement initiated 10 minutes later when full tremor had stabilized. Harmaline tremor was then recorded for 15 minutes to verify an adequate tremor response, defined as an MPP increase of at least 30% over pre-harmaline baseline. MPP is usually 25-35% during baseline, then rises to 60-90% after harmaline injection, and remains high for at least an hour. Compound 6 or vehicle was then injected after the end of the harmaline epoch, then motion power collection initiated 10 minutes later, and measured in 5 successive epochs of 15 minutes each, separated by 5-minute rest periods in the home cage (E1 to E5). Data from mice that failed to show an adequate harmaline tremor response were not used (approximately 10-20% of mice). Animals were exposed to harmaline only once.
Straight wire testing. The highest dose tested was 20 mg/kg, at which 6/6 WT mice passed. Higher doses could not be assessed as Compound 6’s limited solubility, a maximum of 1 mg/ml, meant that higher doses would have exceeded our institution’s permitted maximum volume limit of 25 ml/kg i.p. in mice. Thus the maximal tolerated dose was not determined but is at least 20 mg/kg, given that 6/6 WT mice passed at this dose.
Harmaline tremor. In the table below, motion power data, expressed motion power in the tremor bandwidth (9-16 Hz) as a percentage of overall motion power (0.25-32 Hz) are provided during the pre-harmaline baseline epoch, during the pre-treatment harmaline tremor epoch, and at 10-45 minutes after vehicle or Compound 6 administration in various doses (combining data from post-injections epochs E1 and E2), and at 50-85 minutes post-injection (combining epochs E3 and E4).
Table 1: Harmaline tremor in GABAA receptor α6 subunit wild-type and α6 knockout mice
after vehicle or Compound 6.
Treatment n Pre-Harmaline Pre-Treatment Post-injection
Baseline Harmaline 10-45 min 50-85 min
GABAA receptor α6 subunit wild-type (α6+/+)
Vehicle 16 31.8 ± 2.3 73.7 ± 2.4 71.3 ± 3.6 77.0 ± 2.1
10 mg/kg 11 32.9 ± 2.2 76.3 ± 3.3 34.2 ± 5.5*** 52.6 ± 5.9***
5 mg/kg 11 27.0 ± 3.1 75.3 ± 3.1 49.4 ± 4.5*** 58.3 ± 4.2***
2 mg/kg 15 33.0 ± 3.0 75.2 ± 2.5 63.0 ± 4.7 63.6 ± 4.3**
GABAA receptor α6 subunit knock-out (α6-/-)
Vehicle 11 36.4 ± 1.3 68.6 ± 2.0 69.1 ± 2.1 71.7 ± 2.7
10 mg/kg 11 31.8 ± 3.1 74.6 ± 2.9 40.4 ± 6.3*** 56.9 ± 6.1*
5 mg/kg 11 36.6 ± 1.7 79.7 ± 2.3 46.7 ± 5.2*** 58.1 ± 7.1
2 mg/kg 15 33.3 ± 2.3 75.7 ± 1.6 58.0 ± 4.9 61.6 ± 4.9
Means ± S.E.M are shown. * P<0.05 ** P<0.01 *** P <0.001, Student’s t-test.
Comparisons are with vehicle-treated group.
These results indicate that at doses well below 20 mg/kg that was found to be tolerated on the straight wire test, a sensitive test of psychomotor impairment, Compound 6 reduces harmaline-induced tremor. The effect is seen at both 10-45 and at 50-85 minutes after injection, but the drug is more effective during the earlier post-injection period. Tremor suppression was dose-related, with 10 mg/kg exerting marked tremor suppression, 2 mg/kg minimal suppression, and 5 mg/kg acting with moderate efficacy. Contrary to prediction, tremor suppression occurred in α6 subunit KO mice, and to a degree comparable to that seen in WT mice.
We found that Compound 6 effectively suppresses harmaline tremor at doses in the well-tolerated range, below the 20 mg/kg dose at which 6/6 mice passed the straight wire test, suggesting that Compound 6 may exert clinical efficacy for essential tremor at well-tolerated doses. This is in keeping with the reputation of pyrazoloquinolinones, of which Compound 6 is a member, for lacking sedative effects (15). The results also imply that the Compound enters the brain and exerts efficacy within minutes, suppresses tremor for up to an hour in mice.
Surprisingly, given Compound 6’s reported very high and selective affinity for the α6βγ2 GABAA receptor compared to other GABAA receptors (16, 17), our α6 KO mice responded comparably to WT littermates with tremor suppression on administration of Compound 6. This result might be due to allosteric activation of α3βγ2 receptors, as Compound 6 does bind to these receptors and enhances GABA currents at high concentrations (17). A recent publication has reported that compounds that selectively activate α3 GABAA receptors suppress harmaline tremor (25). Compound 6 might be another example of a compound that suppresses tremor by activating α3 GABAA receptors. As these receptors exert anxiolytic effects, this mechanism could be attractive for ET patients. On the other hand, the binding profile of Compound 6 has been explored only to a limited degree, and it is possible it suppresses tremor through an unidentified non-GABA receptor mechanism.
We believe the present results indicate that Compound 6 (and structurally related compounds) hold promise in the treatment of essential tremor. These findings represent an early step. More needs to be known about the safety and binding profile of this interesting compound and others in its class.
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