Dataset Information

Pathogenesis and pathophysiology of autosomal dominant sleep-related hypermotor epilepsy with S284L-mutant α4 subunit of nicotinic ACh receptor.

ABSTRACT:

Background and purpose

The mechanisms causing spontaneous epileptic seizures, including carbamazepine-resistant/zonisamide -sensitive seizures and comorbidity in autosomal dominant sleep-related hypermotor epilepsy (ADSHE) are unclear. This study investigated functional abnormalities in thalamocortical transmission in transgenic rats bearing rat S286L-mutant Chrna4 (S286L-TG) of α4 subunit of the nicotinic ACh receptor (nAChR) that corresponds to the human S284L-mutant CHRNA4.

Experimental approach

Effects of carbamazepine and zonisamide on epileptic discharges of S286L-TG rat were measured using telemetry electrocorticogram. Transmission abnormalities of L-glutamate and GABA in thalamocortical pathway of S286L-TG rats were investigated using multiprobe microdialysis and ultra-high-performance liquid-chromatography.

Key results

Epileptic discharges in S286L-TG rats were reduced by zonisamide but not by carbamazepine, similar to that of S284L-ADSHE patients. Carbamazepine unaffected functional abnormality in transmission of S286L-TG rats. However, zonisamide was able to compensate for the attenuated S286L-mutant nAChR induced GABA release in frontal-cortex, without affecting attenuated thalamocortical glutamatergic transmission. Excitatory effects of S286L-mutant nAChR on thalamocortical transmission were attenuated compared with those of wild-type nAChR. Loss-of-function of S286L-nAChR enhanced transmission in thalamocortical motor pathway by predominantly attenuating GABAergic transmission. However, it attenuated transmission in thalamocortical cognitive pathway by reducing inhibitory GABAergic and excitatory glutamatergic transmission.

Conclusion and implications

Our results suggest that functional abnormalities of S286L-nAChR are associated with intra-frontal and thalamocortical transmission, possibly contributing to the pathogenesis of ADSHE-seizure and comorbidity of S284L-ADSHE. Selective compensation of impaired GABAergic transmission by zonisamide (but not by carbamazepine) in frontal cortex may be involved, at least partially, in carbamazepine-resistant ADSHE-seizure of S284L-ADSHE patients.

SUBMITTER: Fukuyama K

PROVIDER: S-EPMC7161548 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Sleep-related hypermotor epilepsy (SHE) is a group of seizure disorders prominently associated with mutations in nicotinic acetylcholine receptors (nAChR). The most prevalent central nervous system nAChR subtype contains ?4 and ?2 subunits, in two ratios. (?4?2)2?2-nAChR have high agonist sensitivity (HS-isoform), whereas (?4?2)2?4-nAChR agonist responses exhibit a small high-sensitivity, and a predominant low-sensitivity, phase of function (LS-isoform). Multiple non-synonymous mutations in the second and third transmembrane domains of ?4 and ?2 subunits are associated with SHE. We recently demonstrated that two additional, SHE-associated, missense mutations in the major cytoplasmic loops of these subunits [?4(R336H) and ?2(V337G)] cause increased macroscopic function-per receptor. Here, we use single-channel patch-clamp electrophysiology to show that these mutations influence single-channel amplitudes and open- and closed-state kinetics. Pure populations of HS- or LS-isoform ?4?2-nAChR were expressed by injecting either 1:10 or 30:1 ?4:?2 cRNA ratios, respectively, into Xenopus laevis oocytes. Functional properties of the resulting mutant ?4?2-nAChR isoforms were compared to their wildtype counterparts. ?4(R336H) subunit incorporation minimally affected single-channel amplitudes, whereas ?2(V337G) subunit incorporation reduced them significantly in both isoforms. However, for both mutant subunits, increased function-per-receptor was predominantly caused by altered single channel kinetics. The ?4(R336H) mutation primarily destabilizes desensitized states between openings. By contrast, the ?2(V337G) mutation principally stabilizes receptor open states. The use of naturally-occurring and physiologically-impactful mutations has allowed us to define valuable new insights regarding the functional roles of nAChR intracellular domains. Further mechanistic context is provided by intracellular-domain structures recently published for other members of the Cys-loop receptor superfamily (?3?4-nAChR and 5-HT3AR).