THIP, a Hypnotic and Antinociceptive Drug, Enhances an Extrasynaptic GABAA Receptor-mediated Conductance in Mouse Neocortex

Abstract

THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) is a selective GABA(A) receptor agonist with a preference for delta-subunit containing GABA(A) receptors. THIP is currently being tested in human trials for its hypnotic effects, displaying advantageous tolerance and addiction properties. Since its cellular actions in the neocortex are uncertain, we studied the effects of THIP on neurons in slices of frontoparietal neocortex of 13- to 19-day-old (P13-19) mice. Using whole-cell patch-clamp recordings, we found that the clinically relevant THIP concentration of 1 muM induced a robust tonic GABA(A)-mediated current in layer 2/3 neurons. In comparison, only a minute tonic current was induced by mimicking in vivo endogenous GABA levels. Miniature IPSCs were not affected by 1 muM THIP suggesting an extrasynaptic site of action. The EC(50) for THIP was 44 muM. In accordance with the stronger expression of delta-containing receptors in superficial neocortical layers, THIP induced a 44% larger tonic current in layer 2/3 than in layer 5 neurons. Finally, monitoring spontaneously active neocortical neurons, THIP caused an overall depression of inhibitory activity, while enhancing excitatory activity prominently. Our studies suggest that THIP activates an extrasynaptic GABA(A) receptor-mediated conductance in the neocortex, which may alter the cortical network activity.

Excitability of cortical neurons depends upon a powerful tonic conductance in inhibitory networks

“…Recently, others have shown that low concentrations of the GABA superagonist THIP selectively enhances tonic inhibition through extrasynaptic GABA A receptors (Drasbek and Jensen, 2006;Ortinski et al, 2006). There is also evidence that the GABA A receptor-mediated tonic current is cell-type selective (Semyanov et al, 2003).…”

“…THIP is a GABA analog but has an increased affinity for δ-containing subunits of the GABA A receptor (Sundstrom-Poromaa et al, 2002). At low concentrations (<30 µM) THIP activates these extrasynaptic receptors (tonic inhibition) without affecting the synaptic GABA A receptors that are associated with phasic inhibition (Drasbek and Jensen, 2006). In the present study, we show that LTS cells are modulated by tonic inhibition; we show an 8 nS change in conductance caused by activation of tonic currents in LTS cells by THIP application.…”

Thalamic δ-Subunit Containing GABAAReceptors Promote Electrocortical Signatures of Deep Non-REM Sleep But Do Not Mediate the Effects of Etomidate at the ThalamusIn Vivo

“…Many commonly used anesthetic and sedating agents enhance neuronal inhibition via interactions with binding sites on GABA A receptors, including a subset containing the ␦-subunit (␦GABA A R) that are localized extrasynaptically (Belelli et al, 2005;Cope et al, 2005;Farrant and Nusser, 2005;Drasbek and Jensen, 2006).…”

“…The concentrations of THIP used in this study were selected based on clinically relevant doses in humans, with serum and plasma concentrations in the same range as our effective concentrations enhancing sleep quality and sedation (Madsen et al, 1983;Faulhaber et al, 1997). Additionally, the effects of THIP at synaptic GABA A Rs are minimal at these concentrations, whereas there is modulation of extrasynaptic GABA A Rs (Brown et al, 2002;Belelli et al, 2005;Jia et al, 2005;Drasbek and Jensen, 2006). The concentrations for etomidate were selected after consideration of the plasma concentrations that yield hypnosis in humans and loss of righting reflex in mice, with concentrations in this same range (1-5 M) eliciting both of these effects (De Paepe et al, 1999;Benkwitz et al, 2007).…”

Extrasynaptic GABAA receptors in the crosshairs of hormones and ethanol

“…High densities of α4/δ subunitcontaining GABA A Rs are found in the thalamus, striatum, hippocampal dentate gyrus, olfactory bulb, and layer 2-3 of the neocortex. Several studies have confirmed using pharmacological approaches, null mutant mice or both that in the neurons where δ subunits are present, these GABA A Rs are responsible for the mediation of tonic inhibition (Wei et al 2003;Stell et al 2003;Wei et al 2004;Jia et al 2005;Drasbek and Jensen 2006;Glykys and Mody 2006). …”