Increased expression of GABAA receptor subunits associated with tonic inhibition in patients with temporal lobe epilepsy

Abstract

Epilepsy animal models indicate pronounced changes in the expression and rearrangement of GABAA receptor subunits in the hippocampus and in para-hippocampal areas, including widespread downregulation of the subunits α5 and δ, and up-regulation of α4, subunits that mediate tonic inhibition of GABA. In this case-control study we investigated changes in the expression of subunits α4, α5 and δ in hippocampal specimens of drug resistant temporal lobe epilepsy (TLE) patients who underwent epilepsy surgery. Using in situ-hybridization, immunohistochemistry and α5-specific receptor autoradiography, we characterized expression of the receptor subunits in specimens from patients with and without Ammon’s horn sclerosis compared to post mortem controls. Expression of the α5-subunit was abundant throughout all subfields of the hippocampus including the dentate gyrus, sectors CA1 and CA3, the subiculum and pre- and parasubiculum. Significant but weaker expression was detected for subunits α4 and δ notably in the granule cell/molecular layer of control specimens, but was faint in the other parts of the hippocampus. Expression of all three subunits was similarly altered in sclerotic and non-sclerotic specimens. Respective mRNA levels were increased by about 50 to 80% in the granule cell layer compared with post mortem controls. Subunit α5 mRNA levels and immunoreactivities were also increased in the sector CA3 and in the subiculum. Autoradiography for α5-containing receptors using [3H]L-655,708 as ligand showed significantly increased binding in the molecular layer of the dentate gyrus in non-sclerotic specimens. Increased expression of the α5 and δ subunits is in contrast to the previously observed down-regulation of these subunits in different epilepsy models, whereas increased expression of α4 in temporal lobe epilepsy patients is consistent with that in the rodent models. Our findings indicate increased tonic inhibition likely representing an endogenous anticonvulsive mechanism in temporal lobe epilepsy. GABAA receptors containing α4, α5 and/or δ subunits receptors are located extrasynaptically and mediate volume transmission with tonic inhibition. Sperk et al. show markedly increased expression of these subunits in the hippocampus of temporal lobe epilepsy patients indicating sustained tonic inhibition as a possible endogenous anticonvulsive mechanism.

Cys-loop receptors on cannabinoids: All high?

“…Furthermore, RT-PCR revealed a significant difference in the expression of GABAAR α1, α2, and α4 subunits, whereas the expression of α1 subunit mRNA was significantly lower and the expression of α2 and α4 subunits were significantly higher in Dravet syndrome patients compared to controls ( Ruffolo et al, 2018 ). Preclinical research further substantiates both causative and compensatory roles of changes in expression levels (citations, e.g., in multiple animal models of epilepsy ( Brooks-Kayal et al, 1998 ; Sperk, 2007 ; Sperk et al, 2021 ).…”

“…Not only for nAChRs, but also for specific GABR gene products, post-mortem changes have been observed. GABAAR-α4 subunit mRNA and protein concentration was increased in hippocampal specimens of drug resistant temporal lobe epilepsy (TLE) patients who underwent epilepsy surgery ( Sperk et al, 2021 ). The GABAAR α4 subunit concentration in epileptic tissue was three times higher than compared to control tissue across multiple pathologies such as focal cortical dysplasia (FCD), tuberous sclerosis complex (TSC), ganglioglioma, and gliosis, suggesting enhanced α4 subunit expression may occur due to recurrent seizures rather than epilepsy etiology ( Joshi et al, 2020 ).…”

Regulation of Inhibitory Signaling at the Receptor and Cellular Level; Advances in Our Understanding of GABAergic Neurotransmission and the Mechanisms by Which It Is Disrupted in Epilepsy

“…Replication of these studies, along with more recent immunohistochemical datasets from surgically resected tissue of patients with intractable epilepsy, have produced inconsistent findings that often do not validate earlier reports ( Schwarzer et al, 1997 ; Tsunashima et al, 1997 ; Shumate et al, 1998 ; Houser and Esclapez, 2003 ; Pirker et al, 2003 ; Peng et al, 2004 ; Nishimura et al, 2005 ; Zhang et al, 2007 ; Drexel et al, 2013 ; González et al, 2013 , 2015 ; Stefanits et al, 2019 ; Sperk et al, 2021 ). Additionally, recent discoveries of mutations in epilepsy syndromes in ion channels that would be expected to promote increased inhibition, such as SCN1A and potassium channel gain of function mutations, along with upregulation of inhibition in models of absence epilepsy, suggest that E/I imbalance as a general predisposing factor for epilepsy may not be a sufficient descriptor of the condition itself ( Miri et al, 2018 ; Shao et al, 2019 ; Owen et al, 2021 ).…”

“…More recent studies have found mutations in α5 specifically in cases of epileptic encephalopathy ( Hernandez et al, 2019 ). Additionally, several decades of studies in animal models, and more recently in tissue resected from human cases of TLE, report alterations in the levels of varying receptor subunits, which may represent compensatory responses or alternatively, pathological changes that serve to contribute to epileptogenesis ( González et al, 2013 , 2015 ; Sperk et al, 2021 ). Mixed results, however, are reported in the literature that may reflect differences in the particular epilepsy model that was used and/or the time of data collection since the onset of SE.…”

“…Alterations in β1-β3 transcript and protein levels, as well as additional α subunits, have been described across numerous studies, but display less consistent patterns of change ( Schwarzer et al, 1997 ; Tsunashima et al, 1997 ; Nishimura et al, 2005 ; Drexel et al, 2013 ; González et al, 2013 , 2015 ). Results from studies conducted in tissue resected from human TLE patients undergoing surgical resection as a treatment for epilepsy, have also been varied and sometimes inconsistent with findings from animal models ( Shumate et al, 1998 ; Pirker et al, 2003 ; Stefanits et al, 2019 ; Sperk et al, 2021 ). However, it may be important to keep in mind that in studies conducted using animal models, even with tissue harvested from animals with “chronic” epilepsy, the duration and severity of epilepsy, and the resultant remodeling of neural circuits, is likely less extensive and variable than the alterations present in resected human tissue.…”