Therapeutic potentials of α6GABAAR PAMs based on patient studies and animal models of neuropsychiatric disorders

DisorderPatient Study/Animal ModelPathophysiological ChangesEffects of α6GABAAR PAMs
Angelman syndromePost-mortem (human)Cerebellar atrophy, PC, and granule cells loss and lower cerebellar GABA levels (Jay et al., 1991).# THIP (gaboxadol), a δGABAAR agonist, reduced PC abnormal firing and reduced cerebellar ataxia in Ube3a-deficient mice (Egawa et al., 2012).

* α6GABAR PAMs may achieve similar therapeutic effects in Ube3a-deficient mice and patients with Angelman syndrome.
Gene association/post-mortem (human)Functional deficit of UBE3A-encoded protein (Kishino et al., 1997; Lalande and Calciano, 2007).
Ube3a-deficient miceReduced tonic inhibition of cerebellar GCs causes abnormal PC firing and cerebellar ataxia (Egawa et al., 2012).
Down syndromePostmortem (human)Reduced cerebellar volume and fewer cerebellar GCs (Aylward et al., 1997; Baxter et al., 2000).
MRI (human)Smaller brain volume with disproportionately smaller cerebellum (Pinter et al., 2001).
Ts65Dn mouse modelSmaller tonic GABA currents, unchanged α6 subunits, and increased excitability in cerebellar GCs (Szemes et al., 2013).* THIP and α6GABAAR PAMs may restore tonic GABA transmission in cerebellar GCs and alleviate motor deficits in Ts65Dn mice and in patients with Down syndrome.
Essential tremorPostmortem (human)Cerebellar and PC abnormalities (Louis and Faust, 2020).# THIP suppressed harmaline-induced tremor in wild-type but not in Gabra6−/− or Gabrd−/− mice (Handforth et al., 2018), suggesting an involvement of α6βδ GABAARs.
fMRI (human)Excessive cerebellar activity (Sharifi et al., 2014).
Cerebellar PC VGAT deletion miceOnset of induced tremor in wild-type mice coincident with rhythmic PC firing. Tremorgenic agents had no effects in mice with impaired PC output (Brown et al., 2020).
# α6GABAAR PAMs (Compound 6 and LAU 463 and their deuterated derivatives DK-I-56-1 and DK-I-58-1, respectively) suppressed harmaline-induced action tremor in mice in a manner prevented by intracerebellar furosemide (Huang et al., 2021), suggesting an involvement of cerebellar α6GABAARs.
Tic disordersMRI (human)Reduced cerebellar gray matter volume (Tobe et al., 2010;
Sigurdsson et al., 2020).
# C. inerme leaf juice reduced intractable motor tics in a patient (Fan et al., 2009).
[11C]flumazenil PET (human)Widespread abnormalities of GABAARs in the brain, increase of GABAARs in bilateral cerebellum (Lerner et al., 2012).# This herb’s extract (Chen et al., 2012) and a bioactive constituent, hispidulin, (Huang et al., 2015), reduced tic-associated responses in mice. Hispidulin acted as an α6GABAAR PAM (Liao et al., 2016).
PPI test (human)PPI deficit in TS children (Swerdlow et al., 2001).
# α6GABAAR-selective PAM DK-I-56-1 reduced tic behaviors and PPI deficits in mouse models of tic disorders (Cadeddu et al., 2021).
ADHDMRI (human)Smaller superior cerebellar vermis (Mackie et al., 2007).* α6GABAAR PAMs may restore tonic GABA inhibition in cerebellar GCs and reduce ADHD phenotypes.
fMRI (human)Fronto-parieto-cerebellar deficits (Hart et al., 2012).
PPI test (human)PPI deficit in patients with ADHD (Ornitz et al., 1992).# α6GABAAR PAMs (Compound 6, hispidulin, Ro15-4513, loreclezole) significantly reversed PPI deficits in mice induced with methamphetamine (Chiou et al., 2018).
Gene associationGIT1 SNP is associated with ADHD phenotypes (Won et al., 2011).
Git1-knockout miceReduced tonic GABA currents in cerebellar GC and ADHD-like behaviors (Kim et al., 2017).
# DK-I-56-1 reversed PPI deficits in animal models of tic disorders (Cadeddu et al., 2021).
Obsessive-compulsive disorderMRI (human)Significant cerebellar enlargement (Pujol et al., 2004; Tobe et al., 2010; Brooks et al., 2016).# α6GABAAR PAMs (Compound 6, hispidulin, Ro15-4513, loreclezole) significantly reversed PPI deficits in mice induced with methamphetamine (Chiou et al., 2018).
fMRI (human)Decreased functional connectivity between the cerebral cortex and cerebellum in executive control and emotion-processing networks (Anticevic et al., 2014; Xu et al., 2019).
# DK-I-56-1 reversed PPI deficits in animal models of tic disorders (Cadeddu et al., 2021).
PPI test (human)PPI deficit (Swerdlow et al., 1993; Ahmari et al., 2012).
Huntington diseasePost-mortem
Cerebellar atrophy and loss of PCs correlate with motor deficits (Rub et al., 2013; Singh-Bains et al., 2019).# α6GABAAR-selective PAMs might eliminate PPI deficits (Chiou et al., 2018; Cadeddu et al., 2021) and possibly also reduce motor symptoms of Huntington disease.
MRI (human)Aberrant cerebellar diffusion and reduced cerebellar volume associated with impaired motor function and increased psychiatric symptoms (Rees et al., 2014).
PPI test (human)PPI deficit in patients with Huntington disease (Swerdlow et al., 1995).
SchizophreniaPost-mortem (human)Loss of cerebellar PCs (Maloku et al., 2010).# α6GABAAR-selective PAMs (Compound 6, hispidulin, Ro15-4513, loreclezole) rescued PPI deficits (Chiou et al., 2018).
Increased α6 subunit expression in the superior frontal cortex (Fatemi et al., 2017).
Decreased GAD67, GAD65, and GAT-1 and increased GABRA6 mRNA in the cerebellum. (Fatemi et al., 2005; Bullock et al., 2008). However, no change in GABRA6 mRNA and protein expression in the cerebellum (Fatemi et al., 2013).# Compound 6, DK-I-56-1, and hispidulin rescued social withdrawal and cognitive deficits (Chiou et al., 2019; Mouri et al., 2020) in mouse models mimicking schizophrenia.
MRI (human)Volume reductions in posterior superior vermis as well as in frontal and temporal lobe, thalamus, and left cerebellar hemisphere, supporting the model of cognitive dysmetria in schizophrenia (Volz et al., 2000; Okugawa et al., 2002).
Reduced cerebellar volume associated with negative and psychotic symptom duration and psychosocial impairment (Wassink et al., 1999).
fMRI (human)Impaired PFC-cerebellum functional connectivity associated with negative symptoms (Brady et al., 2019).
PPI test (human)PPI deficit (Braff et al., 1978; San-Martin et al., 2020).
PET (human)Dysfunctional prefrontal thalamic-cerebellar circuitry (Andreasen et al., 1996).
Gene association
GABRA6 rs3219151 C>T associated with schizophrenia susceptibility (Gong et al., 2013).
5q34 mutations are associated with schizophrenia and cause lower GABRA6 expression (Petryshen et al., 2005).
PCP-chronically treated ratsDecreased GAD65 and GAD67 and increased Gabra6 mRNA at cerebellar Golgi-granule synapses (Bullock et al., 2009).
Stress-associated disordersMRI (adolescents with early-life stress)Reduced cerebellar gray matter volume (Walsh et al., 2014; Lim et al., 2018).* α6GABA AR PAMs may attenuate functional deficits in the cerebellum and reduce symptoms of stress-associated disorders.
fMRI (adults with early-life stress)Reduced blood flow and neuronal activity in vermis (Anderson et al., 2002).
MRI (pediatric PTSD)Reduced cerebellar volume (De Bellis et al., 2015).
MRI (adult PTSD)Reduced cerebellar volume (Baldacara et al., 2012).
PPI test (human)PPI deficits in patients with premenstrual dysphoric disorder (Kask et al., 2008).
PPI deficits in women with PTSD (Pineles et al., 2016) and in trauma-affected refugees with PTSD (Meteran et al., 2019).
Gene associationGABRA6 rs3219151 T carriers had greater responses to stress (Uhart et al., 2004; Lynch et al., 2015).
GABRA6 rs3219151 T male carriers had higher cortisol levels and higher abdominal obesity (Rosmond et al., 2002).
GABRA6 rs3811995 C carriers had higher incidents of externalizing behaviors and substance use (Trucco et al., 2016).
Cerebellar GCs Gabrd-KO miceIncreased stress-related behaviors (Rudolph et al., 2020).
Anxiety disordersPPI test (human)PPI deficit in patients with panic disorder (Ludewig et al., 2002).# α6GABAAR PAMs (Compound 6, hispidulin, Ro15-4513, loreclezole) significantly reversed PPI deficits in mice induced with methamphetamine (Chiou et al., 2018).
Gene association (human)GABRA6 rs3219151 T carriers had higher anxiety, depression, and suicide risk after recent negative life events (Gonda et al., 2017).
GABRA6 rs3219151 C carriers associated with panic disorder, but T carriers had stronger reactions to a dynamic fearful face (Inoue et al., 2015).# DK-I-56-1 reversed PPI deficits in animal models of tic disorders (Cadeddu et al., 2021).
Panic disorder–associated miRNA reduced GABRA6 expression in a Luciferase-reporter assay (Muinos-Gimeno et al., 2011).* α6GABAAR PAMs may attenuate functional deficits in cerebellum and reduce symptoms of anxiety and panic disorders.
Cerebellar GCs Gabrd-KO miceIncreased anxiety-related behaviors (Rudolph et al., 2020).
Depression/bipolar disorderPost-mortem (human)Decreased cerebellar GAD65 and GAD67 (depression/bipolar disorder) (Fatemi et al., 2005).* α6GABAAR PAMs might compensate low GABA levels in cerebellum and ameliorate some symptoms of these disorders.
Loss of cerebellar PCs in bipolar disorder (Maloku et al., 2010).
* Increased α6 subunits in depression/bipolar patients might be an incomplete compensatory change due to impaired GABAergic transmission. α6GABAAR PAMs might compensate α6GABAAR deficits and ameliorate depressive symptoms.
Increased α6 subunits in the lateral cerebellum (major depression) (Fatemi et al., 2013).
Increased α6 subunits in the superior frontal cortex (bipolar disorder) (Fatemi et al., 2017).
MRI (human)Reduced cerebellar volume (bipolar disorder) (Baldacara et al., 2012).
Gene association (human)GABRA6 rs3219151 C-allele
associated with major depression (Inoue et al., 2015).
GABRA6 rs3811993 missense variant associated with bipolar disorder (Ament et al., 2015).
Adolescent rats with maternal separationDecreased Gabra6 mRNA and α6 subunits in the hippocampus and depressive-like behavior (Yang et al., 2016).
Hippocampal Gabra6-KD ratsInducing depressive-like behaviors (Yang et al., 2016).
Autism spectrum disorderPostmortem (human)Reduced cerebellar PC size and number (Fatemi et al., 2002a; Whitney et al., 2008).* α6GABA AR PAMs may compensate for reduced GABA and α6GABAAR levels and attenuate symptoms in autism spectrum disorders.
Decreased GAD65 and GAD67 in parietal and cerebellar cortices (Fatemi et al., 2002b; Yip et al., 2007).
Fewer α6, β2, δ, and γ2 subunits in the superior frontal cortex (Fatemi et al., 2014).
MRI (human)Reduced cerebellar gray matter in vermis associated with social and interaction deficits (Riva et al., 2013; D'Mello et al., 2015)
# α6GABAAR PAMs (Compound 6, hispidulin, Ro15-4513, loreclezole) significantly reversed PPI deficits in mice induced with methamphetamine (Chiou et al., 2018). # DK-I-56-1 reduced tic-like jerks in animal models of tic disorders (Cadeddu et al., 2021).
PPI (human)PPI deficit in children/adolescents but not adults (Cheng et al., 2018).
Alcohol use disorderPostmortem (chronic excessive alcoholics)Cerebellar atrophy (Torvik and Torp, 1986).* Cerebellar changes in humans and rats suggest that α6GABAAR PAMs may have some effects also in this disorder. Because of the multiple changes in other receptors and proteins, these effects are difficult to predict.
Reduced PC volume but increased granular layer in the cerebellum (Andersen, 2004).
Decreased cerebellar δ, but unchanged α6 protein and decreased PFC α6 and δ mRNA (Gatta et al., 2017; Gatta et al., 2021).
Gene association (human)α6Ser385 carriers in offspring of alcoholics had the lowest alcohol sensitivity during adolescence (Iwata et al., 1999) and were more vulnerable to become alcoholics later on (Schuckit et al., 1999; Hu et al., 2005).
GABRA6 rs3219151 T allele carriers were significantly associated with alcohol dependence (Loh et al., 1999; Sander et al., 1999; Radel et al., 2005; Sahni et al., 2019).
Alcohol-dependent patients carrying the GABRA6 rs3219151 TT or CC alleles differentially respond to naltrexone or acamprosate treatment, respectively (Ooteman et al., 2009).* Alcohol-dependent patients carrying the GABRA6 rs3219151 CC alleles might benefit from treatment with α6GABAAR PAMs.
Repeated ethanol-treated ratsIncreased cerebellar α6 but decreased α1 protein (Mhatre and Ticku, 1992; Morrow et al., 1992).
Alcohol-preferring ratsIncreased α6 and δ but decreased γ2 mRNAs in the periaqueductal gray and dorsal raphe nucleus (McClintick et al., 2015, 2016).
Trigeminal-related painTMJ inflammation (rats)CFA induced stronger TMJ pain responses in rats with reduced TG Gabra6 expression (Puri et al., 2012).* α6GABAARs PAMs may reduce TMJ or trigeminal-related orofacial pain.
Trigeminal neuropathic pain (rats)Infraorbital nerve ligation induced trigeminal neuropathy (Vasovic et al., 2019).
# α6GABAAR-selective PAM DK-I-56-1 suppressed infraorbital nerve ligation-induced nociceptive responses (Vasovic et al., 2019).
MigraineTGVS-activated ratsCapsaicin (i.c.) induces central and peripheral sensitization in the TGVS (Fan et al., 2018).# α6GABAAR-selective PAMs (Compound 6, Ro15-4513, loreclezole) reduced migraine-related peripheral and central sensitization of TGVS. These effects were mimicked by topiramate and inhibited by systemic furosemide (Fan et al., 2018).
Nitroglycerin-repeatedly treated miceDecreased GAD65, unchanged GAT1, and unchanged α6GABAARs in trigeminal ganglia (Tzeng et al., 2021).
# α6GABAAR-selective PAM (DK-I-56-1) abolished and prevented migraine-like grimaces in a manner mimicked by sumatriptan and topiramate, respectively, and blocked by systemic furosemide (Tzeng et al., 2021).
  • α6GABAAR, α6 subunit-containing GABAA receptor; ADHD, attention deficit hyperactivity disorder; CFA, complete Freund´s adjuvant; GAD65, glutamate decarboxylase 65 kDa; GAD67, glutamate decarboxylase 67 kDa; GAT-1, GABA transporter 1; GC, granule cell; GIT1/Git1, G-protein-coupled–receptor kinase-interacting protein-1; KO, knockout; i.c., intracisternally; ICR, Institute of Cancer Research; KO, knockout; MRI, magnetic resonance imaging; fMRI, functional magnetic resonance imaging; MRS, magnetic resonance spectroscopy; NTG, nitroglycerin; PAM, positive allosteric modulator; PC, Purkinje cells; PET, positron emission tomography; PFC, prefrontal cortex; PTSD, post-traumatic stress disorder; PPI, prepulse inhibition; SNP, single nucleotide polymorphism; TG, trigeminal ganglia; TGVS, trigemino-vascular system; THIP, 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridin-3-ol (gaboxadol); TMJ, temporomandibular joint; TS, Tourette syndrome; VGAT, vesicular GABA transporter; wt, wild type; *, hypothesis to be proven; #, hypothesis proven.