Abstract
THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol) is a GABAA receptor agonist with varying potencies and efficacies at γ‐subunit‐containing receptors. More importantly, THIP acts as a selective superagonist at δ‐subunit‐containing receptors (δ‐GABAARs) at clinically relevant concentrations. Evaluation of THIP as a potential anticonvulsant has given contradictory results in different animal models and for this reason, we reevaluated the anticonvulsive properties of THIP in the murine pentylenetetrazole (PTZ) kindling model. As loss of δ‐GABAAR in the dentate gyrus has been associated with several animal models of epilepsy, we first investigated the presence of functional δ‐GABAA receptors. Both immunohistochemistry and Western blot data demonstrated that δ‐GABAAR expression is not only present in the dentate gyrus, but also the expression level was enhanced in the early phase after PTZ kindling. Whole‐cell patch‐clamp studies in acute hippocampal brain slices revealed that THIP was indeed able to induce a tonic inhibition in dentate gyrus granule cells. However, THIP induced a tonic current of similar magnitude in the PTZ‐kindled mice compared to saline‐treated animals despite the observed upregulation of δ‐GABAARs. Even in the demonstrated presence of functional δ‐GABAARs, THIP (0.5–4 mg/kg) showed no anticonvulsive effect in the PTZ kindling model using a comprehensive in vivo evaluation of the anticonvulsive properties.
Highlights
A low-ambient c-Aminobutyric acid (GABA) (c-aminobutyric acid) concentration in the extracellular space continuously activates extrasynaptic GABAA receptors, resulting in a persistent inhibitory current causing tonic inhibition (Semyanov et al 2004; Farrant and Nusser 2005; Glykys and Mody 2007a,b)
Injections of 2.5–10 mg/kg THIP (Cremers and Ebert 2007). This THIP concentration has been shown in other neuronal cell types to activate extrasynaptic GABA receptor subtype A (GABAAR) but not synaptic GABAARs (Drasbek and Jensen 2006; Herd et al 2009)
In this study we have shown that GABAAR d-subunit expression is not compromised in the dentate gyrus in the early phase after PTZ kindling, we found the subunit to be upregulated
Summary
A low-ambient GABA (c-aminobutyric acid) concentration in the extracellular space continuously activates extrasynaptic GABAA receptors, resulting in a persistent inhibitory current causing tonic inhibition (Semyanov et al 2004; Farrant and Nusser 2005; Glykys and Mody 2007a,b). THIP induces tonic inhibition by activation of d-containing extrasynaptic GABAA receptors (Herd et al, 2009) and has functional d-GABAAR selectivity at recombinant and native GABAA receptors in clinically relevant concentrations (Drasbek and Jensen 2006; Storustovu and Ebert 2006; Herd et al 2009). Downregulation of functional d-GABAARs in kindling models could potentially account for the lack of anticonvulsive effect of THIP seen in kindling models (Lo€scher and Schwark 1985; Hansen et al 2004). In this study we investigated the d-subunit expression and THIP-induced tonic inhibition in the dentate gyrus after PTZ kindling and reevaluated the anticonvulsive effect of THIP in the murine PTZ kindling model using a more comprehensive assessment of seizure activity. Some of the results have been published in abstract form (Simonsen et al 2010)
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