Pathogenic variants in GABAA receptor subunit genes (GABR*) are important contributors to rare and common genetic epilepsies. Here, we present a comprehensive analysis of variants in GABRB1, which encodes the GABAA receptor β1 subunit, by revealing their functional implications, establishing genotype-phenotype correlations and evaluating treatment response. Clinical information on individuals carrying a GABRB1 variant was obtained through an international collaboration and literature review. Our cohort included 19 individuals (7 males, 12 females) from 15 families harbouring 13 different GABRB1 variants (11 missense, 1 indel, 1 stop). Functional analysis was performed using two-electrode voltage-clamp recordings in Xenopus laevis oocytes. For all 11 missense variants, α1β1γ2 GABAA receptors with a single mutant β1 subunit were used. Four missense variants were selected for further functional analysis using α5β1γ2 GABAA receptors with two mutant β1 subunits. Gain-of-function (GoF) effects, characterized by increased GABA-sensitivity, were observed for eight missense variants. Loss-of-function (LoF) effects were observed for one variant and no functional effects for two variants. Clinically, GoF variants were only observed in individuals with severe early-onset disease, including profound intellectual disability, hypotonia and early mortality. Additionally, cortical visual impairment, dysmorphisms and cortical atrophy were exclusive to this cohort. By integrating previously reported clinical data for variants in other GABR* genes, we validated that these features were associated with GoF variants more broadly. The only LoF variant was identified in a nuclear family with the relatively milder syndrome of genetic epilepsy with febrile seizures plus. Seizures were therapy-resistant in all individuals with GoF variants and a single individual with a LoF variant. The GABAergic anti-seizure medication (ASM) vigabatrin caused life-threatening side-effects in two individuals with GoF variants, while the sodium-channel blocker (SCB) lamotrigine exacerbated seizures in a single individual carrying a LoF variant. By integrating data from literature on all GABR* variants, we observed a potential dichotomy in treatment responses: GABAergic and broad-spectrum ASMs, such as valproate and levetiracetam, were more effective for individuals with LoF variants in GABR* genes, while SCBs showed greater benefit for GoF variants. Additionally, there is an increased risk of adverse effects of SCBs in LoF and vigabatrin in GoF variants. Our results highlight the importance of functional characterization of variants and clinical predictors in guiding treatment strategies for individuals with GABRB1 and other GABR* variants, although larger prospective studies are needed to confirm these observations.

BACKGROUND AND OBJECTIVES:Variants in the GABRG2 gene encoding the γ2 subunit of the γ-aminobutyric acid type A (GABAA) receptor are associated with a spectrum of epilepsy phenotypes. These range from simple febrile seizures to more severe conditions, including developmental and epileptic encephalopathies (DEEs). Despite previous analyses suggesting that pathogenic variants may lead to loss-of-function (LoF) receptors, a correlation between functional analysis and clinical phenotypic diversity remains elusive. We, therefore, aimed to determine why variants in the GABRG2 gene can lead to highly diverse phenotypes. METHODS:We assembled a cohort of unreported probands carrying presumed pathogenic GABRG2 variants. Electroclinical information was systematically collected, and electrophysiologic measurements were conducted for missense variants to explore potential alterations in receptor function. RESULTS:We examined 44 individuals with 35 GABRG2 variants (18 null and 17 missense). Functional assessments of the missense variants revealed that 9 caused LoF and 3 caused gain-of-function (GoF). The remaining 5 did not alter receptor function and are likely not pathogenic. Based on functional analysis and electroclinical data, 37 affected individuals were categorized into 3 groups: null LoF, missense LoF, and GoF variants. Among 19 individuals with null variants, epilepsy was diagnosed in 13, with a median onset of 14 months. The remaining 6 of 19 only had febrile seizures. Developmental delay/intellectual disability (DD/ID) was observed in 1 of 19 and psychiatric features in 4 of 18. By contrast, all 12 individuals with missense LoF variants suffered from epilepsy with a median onset of 15 months. Most common epilepsy diagnoses were febrile seizures plus in 4 of 12 and DEE in 4 of 12. DD/ID affected 9 of 12, and psychiatric features were diagnosed in 8 of 12. Statistical comparisons revealed that null variants were associated with a milder phenotype than missense LoF variants. Finally, 5 of 6 individuals with GoF variants had DEE characterized by early infancy onset at 2 months and severe/profound DD/ID. The sixth individual exhibited mild DD/ID and hypotonia without seizures. DISCUSSION:Our findings indicate that the severity of disease associated with pathogenic GABRG2 variants depends on the functional consequences of the variants. Null variants are associated with a mild phenotype and missense LoF variants with an intermediate phenotype while GoF variants can lead to severe phenotypes.

Recent discoveries have revealed that genetic variants in γ-aminobutyric acid type A (GABA A ) receptor subunits can lead to both gain-of-function (GOF) and loss-of-function (LOF) receptors. GABA A receptors, however, have a pseudosymmetrical pentameric assembly, and curiously diverse functional outcomes have been reported for certain homologous variants in paralogous genes (paralogous variants). To investigate this, we assembled a cohort of 11 individuals harboring paralogous M1 proline missense variants in GABRA1 , GABRB2 , GABRB3, and GABRG2. Seven mutations (α1 P260L , α1 P260S , β2 P252L , β3 P253L , β3 P253S , γ2 P282A , and γ2 P282S ) in α1β2/3γ2 receptors were analyzed using electrophysiological examinations and molecular dynamics simulations. All individuals in the cohort were diagnosed with developmental and epileptic encephalopathy, with a median seizure onset age of 3.5 mo, and all exhibited global developmental delay. The clinical data for this cohort aligned with established GABA A receptor GOF but not LOF cohorts. Electrophysiological assessments revealed that all variants caused GOF by increasing GABA sensitivity by 3- to 23-fold. In some cases, this was accompanied by LOF traits such as reduced maximal current amplitude and enhanced receptor desensitization. The specific subunit mutated and whether the mutation occurred in one or two subunits within the pentamer influenced the overall effects. Molecular dynamics simulations confirmed similar structural changes from all mutations, but with position-dependent asymmetry. These findings establish that paralogous variants affecting the 100% conserved proline residue in the M1 transmembrane helix of GABA A R subunits all lead to overall GOF traits. The unexpected asymmetric and mixed effects on receptor function have broader implications for interpreting functional analyses for multimeric ion-channel proteins.

Background Variants in GABRB2 , encoding the 0 2 subunit of the gamma-aminobutyric acid type A (GABA(A) ) receptor, can result in a diverse range of conditions, ranging from febrile seizures to severe developmental and epileptic encephalopathies. However, the mechanisms underlying the risk of developing milder vs more severe forms of disorder remain unclear. In this study, we conducted a comprehensive genotype - phenotype correlation analysis in a cohort of individuals with GABRB2 variants. Methods Genetic and electroclinical data of 42 individuals harbouring 26 different GABRB2 variants were collected and accompanied by electrophysiological analysis of the effects of the variants on receptor function. Findings Electrophysiological assessments of alpha 1 0 2 gamma 2 receptors revealed that 25/26 variants caused dysfunction to core receptor properties such as GABA sensitivity. Of these, 17 resulted in gain-of-function (GOF) while eight yielded loss-of-function traits (LOF). Genotype-phenotype correlation analysis revealed that individuals harbouring GOF variants suffered from severe developmental delay/intellectual disability (DD/ID, 74%), movement disorders such as dystonia or dyskinesia (59%), microcephaly (50%) and high risk of early mortality (26%). Conversely, LOF variants were associated with milder disease manifestations. Individuals with these variants typically exhibited fever-triggered seizures (92%), milder degrees of DD/ID (85%), and maintained ambulatory function (85%). Notably, severe movement disorders or microcephaly were not reported in individuals with loss-of-function variants. Interpretation The data reveals that genetic variants in GABRB2 can lead to both gain and loss-of-function, and this divergence is correlated with distinct disease manifestations. Utilising this information, we constructed a diagnostic fl owchart that aids in predicting the pathogenicity of recently identi fi ed variants by considering clinical phenotypes.

Abstract Genetic variants associated with developmental and epileptic encephalopathies have been identified in the GABRB3 gene that encodes the β3 subunit of GABAA receptors. Typically, variants alter receptor sensitivity to GABA resulting in either gain- or loss-of-function, which correlates with patient phenotypes. However, it is unclear how another important receptor property, desensitization, contributes to the greater clinical severity of gain-of-function variants. Desensitization properties of 20 gain-of-function GABRB3 variant receptors were evaluated using two-electrode voltage-clamp electrophysiology. The parameters measured included current decay rates and steady-state currents. Selected variants with increased or reduced desensitization were also evaluated using whole-cell electrophysiology in transfected mammalian cell lines. Of the 20 gain-of-function variants assessed, 13 were found to alter receptor desensitization properties. Seven variants reduced desensitization at equilibrium, which acts to worsen gain-of-function traits. Six variants accelerated current decay kinetics, which limits gain-of-function traits. All affected patients displayed severe clinical phenotypes with intellectual disability and difficult-to-treat epilepsy. Nevertheless, variants that reduced desensitization at equilibrium were associated with more severe clinical outcomes. This included younger age of first seizure onset (median 0.5 months), movement disorders (dystonia and dyskinesia), epilepsy of infancy with migrating focal seizures (EIMFS) and risk of early mortality. Variants that accelerated current decay kinetics were associated with slightly milder phenotypes with later seizure onset (median 4 months), unclassifiable developmental and epileptic encephalopathies or Lennox–Gastaut syndrome and no movement disorders. Our study reveals that gain-of-function GABRB3 variants can increase or decrease receptor desensitization properties and that there is a correlation with the degree of disease severity. Variants that reduced the desensitization at equilibrium were clustered in the transmembrane regions that constitute the channel pore and correlated with greater disease severity, while variants that accelerated current decay were clustered in the coupling loops responsible for receptor activation and correlated with lesser severity.

OBJECTIVE:Variants in GABRA1 have been associated with a broad epilepsy spectrum, ranging from genetic generalized epilepsies to developmental and epileptic encephalopathies. However, our understanding of what determines the phenotype severity and best treatment options remains inadequate. We therefore aimed to analyze the electroclinical features and the functional effects of GABRA1 variants to establish genotype-phenotype correlations. METHODS:Genetic and electroclinical data of 27 individuals (22 unrelated and 2 families) harboring 20 different GABRA1 variants were collected and accompanied by functional analysis of 19 variants. RESULTS:Individuals in this cohort could be assigned into different clinical subgroups based on the functional effect of their variant and its structural position within the GABRA1 subunit. A homogenous phenotype with mild cognitive impairment and infantile onset epilepsy (focal seizures, fever sensitivity, and electroencephalographic posterior epileptiform discharges) was described for variants in the extracellular domain and the small transmembrane loops. These variants displayed loss-of-function (LoF) effects, and the patients generally had a favorable outcome. A more severe phenotype was associated with variants in the pore-forming transmembrane helices. These variants displayed either gain-of-function (GoF) or LoF effects. GoF variants were associated with severe early onset neurodevelopmental disorders, including early infantile developmental and epileptic encephalopathy. INTERPRETATION:Our data expand the genetic and phenotypic spectrum of GABRA1 epilepsies and permit delineation of specific subphenotypes for LoF and GoF variants, through the heterogeneity of phenotypes and variants. Generally, variants in the transmembrane helices cause more severe phenotypes, in particular GoF variants. These findings establish the basis for a better understanding of the pathomechanism and a precision medicine approach in GABRA1-related disorders. Further studies in larger populations are needed to provide a conclusive genotype-phenotype correlation. ANN NEUROL 2023.

Normal brain function requires a tightly regulated balance between excitatory and inhibitory neurotransmissions. γ-Aminobutyric acid type A (GABAA ) receptors represent the major class of inhibitory ion channels in the mammalian brain. Dysregulation of these receptors and/or their associated pathways is strongly implicated in the pathophysiology of epilepsy. To date, hundreds of different GABAA receptor subunit variants have been associated with epilepsy, making them a prominent cause of genetically linked epilepsy. While identifying these genetic variants is crucial for accurate diagnosis and effective genetic counselling, it does not necessarily lead to improved personalised treatment options. This is because the identification of a variant does not reveal how the function of GABAA receptors is affected. Genetic variants in GABAA receptor subunits can cause complex changes to receptor properties resulting in various degrees of gain-of-function, loss-of-function or a combination of both. Understanding how variants affect the function of GABAA receptors therefore represents an important first step in the ongoing development of precision therapies. Furthermore, it is important to ensure that functional data are produced using methodologies that allow genetic variants to be classified using clinical guidelines such as those developed by the American College of Medical Genetics and Genomics. This article will review the current knowledge in the field and provide recommendations for future functional analysis of genetic GABAA receptor variants.

Vinpocetine is a synthetic derivative of the alkaloid vincamine and has been used as a dietary supplement for decades. Following a positive report of the use of vinpocetine in a patient with a loss-of-function GABRB3 variant, we here describe another patient with a loss-of-function GABRA1 variant (p.(Arg112Gln)) who benefited from vinpocetine treatment. This patient was diagnosed with autism spectrum disorder, psychiatric complications, and therapy-resistant focal epilepsy. Upon add-on treatment with 40 mg vinpocetine daily for 16 months, the patient experienced an overall improved quality of life as well as seizure freedom. Our findings corroborate that vinpocetine can attenuate epilepsy-associated behavioral issues in patients with loss-of-function GABAA receptor gene variants.

The orthosteric binding site of GABA-gated ion channels has been widely explored. Many residues in the binding site of GABA were studied. The interactions due to the binding of GABA into the binding site drive channel activation and determine the potency and efficacy of GABA response. The combined effect of a competitive ligand and GABA on GABA-ρ1 receptors has been poorly studied. Here, we used point mutations, molecular modeling, and electrophysiological studies to explore the role of two hydrophilic residues (Serine 168 and Serine 243) of the GABA-ρ1 receptors in response to the binding of GABA and other studied ligands. Our results suggested that Ser168 residue stabilizes either closed state or open conformation depending on the other determinant interactions of each state. On the other hand, Ser243 residue is predicted to form different inter-subunit interactions with residues in the adjacent subunit at different states of the channel. Our current findings enlighten us to reasonably explain the additive/inhibitive effects of applying a competitive ligand with GABA simultaneously. Understanding the mixed effect of potentiation and inhibition would facilitate the discovery of new drugs to work as a direct GABA's activity modulators with more selectivity at various subunits forming GABA-gated ion channels.

Many patients with developmental and epileptic encephalopathies present with variants in genes coding for GABA A receptors. These variants are presumed to cause loss-of-function receptors leading to reduced neuronal GABAergic activity. Yet, patients with GABA A receptor variants have diverse clinical phenotypes and many are refractory to treatment despite the availability of drugs that enhance GABAergic activity. Here we show that 44 pathogenic GABRB3 missense variants segregate into gain-of-function and loss-of-function groups and respective patients display distinct clinical phenotypes. The gain-of-function cohort ( n = 27 patients) presented with a younger age of seizure onset, higher risk of severe intellectual disability, focal seizures at onset, hypotonia, and lower likelihood of seizure freedom in response to treatment. Febrile seizures at onset are exclusive to the loss-of-function cohort ( n = 47 patients). Overall, patients with GABRB3 variants that increase GABAergic activity have more severe developmental and epileptic encephalopathies. This paradoxical finding challenges our current understanding of the GABAergic system in epilepsy and how patients should be treated.

GABAϱ receptors are distinctive GABAergic receptors from other ionotropic GABAA and metabotropic GABAB receptors in their pharmacological, biochemical, and electrophysiological properties. Although GABA-ϱ1 receptors are the most studied in this subfamily, GABA-ϱ2 receptors are widely distributed in the brain and are considered a potential target for treating neurological disorders such as stroke. The structure of GABA-ϱ2 receptors and their pharmacological features are poorly studied. We generated the first homology model of GABA-ϱ2 channel, which predicts similar major interactions of GABA with the binding-site residues in GABA-ϱ1 and GABA-ϱ2 channels. We also investigated the pharmacological properties of several GABA analogues on the activity of GABA-ϱ2 receptors. In comparison to their pharmacological effect on GABA-ϱ1 receptors, the activation effect of these ligands and their potentiation/inhibition impact on GABA response have interestingly shown inter-selectivity between the two GABA-ϱ receptors. Our results suggest that several GABA analogues can be used as research tools to study the distinctive physiology of GABA-ϱ1 and GABA-ϱ2 receptors. Furthermore, their partial agonist effect may hold promise for the future discovery of selective modulatory agents on GABAA receptors.

A potential link between GABRD encoding the delta subunit of extrasynaptic GABA(A) receptors and neurodevelopmental disorders has largely been disregarded due to conflicting conclusions from early studies. However, we identified seven heterozygous missense GABRD variants in 10 patients with neurodevelopmental disorders and generalized epilepsy. One variant occurred in two sibs of healthy parents with presumed somatic mosaicism, another segregated with the disease in three affected family members, and the remaining five occurred de novo in sporadic patients. Electrophysiological measurements were used to determine the functional consequence of the seven missense delta subunit variants in receptor combinations of alpha 1 beta 3 delta and alpha 4 beta 2 delta GABA(A) receptors. This was accompanied by analysis of electroclinical phenotypes of the affected individuals. We determined that five of the seven variants caused altered function of the resulting alpha 1 beta 3 delta and alpha 4 beta 2 delta GABA(A) receptors. Surprisingly, four of the five variants led to gain-of-function effects, whereas one led to a loss-of-function effect. The stark differences between the gain-of-function and loss-of function effects were mirrored by the clinical phenotypes. Six patients with gain-of-function variants shared common phenotypes: neurodevelopmental disorders with behavioural issues, various degrees of intellectual disability, generalized epilepsy with atypical absences and generalized myoclonic and/or bilateral tonic-clonic seizures. The EEG showed qualitative analogies among the different gain-of-function variant carriers consisting of focal slowing in the occipital regions often preceding irregular generalized epileptiform discharges, with frontal predominance. In contrast, the one patient carrying a loss-of-function variant had normal intelligence and no seizure history, but has a diagnosis of autism spectrum disorder and suffers from elevated internalizing psychiatric symptoms. We hypothesize that increase in tonic GABA-evoked current levels mediated by delta-containing extrasynaptic GABA(A) receptors lead to abnormal neurotransmission, which represent a novel mechanism for severe neurodevelopmental disorders. In support of this, the electroclinical findings for the gain-of-function GABRD variants resemble the phenotypic spectrum reported in patients with missense SLC6A1 (GABA uptake transporter) variants. This also indicates that the phenomenon of extrasynaptic receptor overactivity is observed in a broader range of patients with neurodevelopmental disorders, because SLC6A1 loss-of-function variants also lead to overactive extrasynaptic delta-containing GABA(A) receptors. These findings have implications when selecting potential treatment options, as a substantial portion of available antiseizure medication act by enhancing GABAergic function either directly or indirectly, which could exacerbate symptoms in patients with gain-of-function GABRD variants.

To the Editors: We were encouraged to read the recent publication by Vogel et al.1 in Epilepsia providing the first association between a de novo missense variant in GABRA4 and a neurodevelopmental disorder with earlyonset epilepsy.1 The GABRA4 gene encodes the α4 subunit of the γaminobutyric acid (GABA) type A receptor, and this subunit is relatively abundant in the cortex, hippocampus, and thalamus, all brain regions known to be involved in epilepsy.2,3 The α4 subunit primarily assembles with β and δ subunits to form, for example, α4β2δ receptors,4– 6 and these receptors are localized in extrasynaptic membranes where they respond to low ambient levels of GABA and spill over from synaptic release resulting in longlasting tonic inhibition of neuronal activity.7– 9 We recently discovered that pathogenic variants in GABRD, encoding the δ subunit, cause gainoffunction traits in α4β2δ receptors and interestingly one specific variant, GABRD p.Thr291Ile, is paralogous to the GABRA4 p.Thr300Ile variant.10 Intrigued by this observation, we extended the study of Vogel et al.1 with electrophysiological analysis of the GABRA4 p.Thr300Ile variant in combination with the δ subunit using previously described methodologies.10,11 The mean current amplitude obtained with a maximally efficacious concentration of GABA was increased by 6.2fold and the sensitivity to GABA was increased by ~10 fold for α4T300Iβ2δ vs wildtype receptors (Figure 1). Furthermore, the maximum estimated open probability was increased ~18fold, showing an increased ability of GABA to gate variant α4T300Iβ2δ receptors. Like Vogel et al.1 we observe that variant receptors display faster desensitization kinetics than wildtype receptors at high GABA concentrations (data not shown); however, α4β2δ receptors are extrasynaptic receptors that respond to low concentrations of GABA in the brain, and no obvious desensitization was observed with GABA concentrations below 1 μM. Thus despite the inherent complexity of receptor desensitization kinetics, we conclude that the increases in current amplitudes and sensitivity to GABA caused by the GABRA4 variant bestow extrasynaptic δcontaining receptors with gainoffunction properties. Of interest, the GABRA4 Thr300 amino acid position appears to be a hotspot for pathogenic variants in most if not all GABAAR subunit classes. Besides the paralog GABRD p.Thr291Ile variant mentioned above,10 we recently described the functional consequence of the paralogous variant in GABRB3 p.Thr287Ile.12,13 GABRB3 encodes the β3 subunit, and we observed that this epilepsyassociated variant also causes strong gainoffunction traits. Hence, a threonine to isoleucine substitution in this specific protein position appears to cause gainoffunction traits irrespective of the subunit type. When comparing the clinical manifestations, there are similarities as well as differences between the carriers of the GABRA4 variant1 and the paralog GABRD and GABRB3 variants.10,12 The GABRA4 variant, which was observed in mosaic state (17%) in a 5.5yearold girl, was associated with intractable nocturnal frontal lobe seizures (onset 3.5 years), dyspraxia, and attention deficit.1 In comparison, the GABRD variant was observed to cause earlyonset (1– 4 years) generalized epilepsy with intractable atypical absence seizures, various degrees of learning difficulties/intellectual disability, and attentiondeficit/ hyperactivity disorder (ADHD) in a mother and her twin sons.10 Finally, the GABRB3 variant was observed in a child with an unclassified developmental and epileptic encephalopathy (onset 3 months) with intractable tonic,

Background and PurposeGABAA receptors containing δ‐subunits are notorious for being difficult to study in vitro due to heterogeneity of expressed receptor populations and low GABA‐evoked current amplitudes. Thus, there are some published misconceptions and contradictory conclusions made regarding the pharmacology and stoichiometry of δ‐containing receptors. The aim of this study was to obtain robust homogenous expression of α1βδ receptors for in‐depth investigation.Experimental ApproachNovel δ‐containing pentameric concatenated constructs were designed. The resulting α1β2δ and α1β3δ GABAA receptor concatemers were investigated by two‐electrode voltage‐clamp electrophysiology using Xenopus laevis oocytes.Key ResultsFirst, while homogenous α1βδ GABAA receptor pools could not be obtained by manipulating the ratio of injected cRNAs of free α1, β2/3, and δ subunits, concatenated pentameric α1β2δ and α1β3δ constructs resulted in robust expression levels of concatemers. Second, by using optimised constructs that give unidirectional assembly of concatemers, we found that the δ subunit cannot directly participate in GABA binding and receptor activation. Hence, functional δ‐containing receptors are likely to all have a conventional 2α:2β:1δ stoichiometry arranged as βαβαδ when viewed counterclockwise from the extracellular side. Third, α1β2/3δ receptors were found to express efficiently in X. laevis oocytes but have a low estimated open probability of ~0.5% upon GABA activation. Because of this, these receptors are uniquely susceptible to positive allosteric modulation by, for example, neurosteroids.Conclusion and ImplicationsOur data answer important outstanding questions regarding the pharmacology and stoichiometry of α1δ‐containing GABAA receptors and pave the way for future analysis and drug discovery efforts.

Communication between nerve cells depends on the balance between excitatory and inhibitory circuits. GABA, the major inhibitory neurotransmitter, regulates this balance and insufficient GABAergic activity is associated with numerous neuropathological disorders including pain. Of the various GABAA receptor subtypes, the δ-containing receptors are particularly interesting drug targets in management of chronic pain. These receptors are pentameric ligand-gated ion channels composed of α, β and δ subunits and can be activated by ambient levels of GABA to generate tonic conductance. However, only a few ligands preferentially targeting δ-containing GABAA receptors have so far been identified, limiting both pharmacological understanding and drug-discovery efforts, and more importantly, understanding of how they affect pain pathways. Here, we systemically review and discuss the known drugs and ligands with analgesic potential targeting δ-containing GABAA receptors and further integrate the biochemical nature of the receptors with clinical perspectives in pain that might generate interest among researchers and clinical physicians to encourage analgesic discovery efforts leading to more efficient therapies.

BACKGROUND AND PURPOSE:Cannabis has been used to treat epilepsy for millennia, with such use validated by regulatory approval of cannabidiol (CBD) for Dravet syndrome. Unregulated artisanal cannabis-based products used to treat children with intractable epilepsies often contain relatively low doses of CBD but are enriched in other phytocannabinoids. This raises the possibility that other cannabis constituents might have anticonvulsant properties.EXPERIMENTAL APPROACH:We used the Scn1a+/- mouse model of Dravet syndrome to investigate the cannabis plant for phytocannabinoids with anticonvulsant effects against hyperthermia-induced seizures. The most promising, cannabigerolic acid (CBGA), was further examined against spontaneous seizures and survival in Scn1a+/- mice and in electroshock seizure models. Pharmacological effects of CBGA were surveyed across multiple drug targets.KEY RESULTS:The initial screen identified three phytocannabinoids with novel anticonvulsant properties: CBGA, cannabidivarinic acid (CBDVA) and cannabigerovarinic acid (CBGVA). CBGA was most potent and potentiated the anticonvulsant effects of clobazam against hyperthermia-induced and spontaneous seizures, and was anticonvulsant in the MES threshold test. However, CBGA was proconvulsant in the 6-Hz threshold test and a high dose increased spontaneous seizure frequency in Scn1a+/- mice. CBGA was found to interact with numerous epilepsy-relevant targets including GPR55, TRPV1 channels and GABAA receptors.CONCLUSION AND IMPLICATIONS:These results suggest that CBGA, CBDVA and CBGVA may contribute to the effects of cannabis-based products in childhood epilepsy. Although these phytocannabinoids have anticonvulsant potential and could be lead compounds for drug development programmes, several liabilities would need to be overcome before CBD is superseded by another in this class.

Analogues of methyllycaconitine (MLA) based on a (3-ethyl-9-methylidene-3-azabicyclo[3.3.1]nonan-1-yl) methanol template have been designed and synthesised that incorporate the modified ester sidechains distinct from that present in the natural product. Electrophysiology experiments using Xenopus oocytes expressing nicotinic acetylcholine receptors (nAChRs) revealed selected analogues served as non-competitive inhibitors that showed selectivity for the alpha 4 beta 2 over alpha 7 nAChR subtypes, and selectivity for the (alpha 4)(3)(beta 2)(2) over (alpha 4)(2)(beta 2)(3) stoichiometry. This study more clearly defines the biological effects of MLA analogues and identifies strategies for the development of MLA analogues as selective ligands for the alpha 4 beta 2 nAChR subtype.