Enhanced behavioral sensitivity to the competitive GABA agonist, gaboxadol, in transgenic mice over‐expressing hippocampal extrasynaptic α6β GABA_A receptors

Abstract

The behavioral and functional significance of the extrasynaptic inhibitory GABAA receptors in the brain is still poorly known. We used a transgenic mouse line expressing the GABAA receptor α6 subunit gene in the forebrain under the Thy‐1.2 promoter (Thy1α6) mice ectopically expressing α6 subunits especially in the hippocampus to study how extrasynaptically enriched αβ(γ2)‐type receptors alter animal behavior and receptor responses. In these mice extrasynaptic α6β receptors make up about 10% of the hippocampal GABAA receptors resulting in imbalance between synaptic and extrasynaptic inhibition. The synthetic GABA‐site competitive agonist gaboxadol (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol; 3 mg/kg) induced remarkable anxiolytic‐like response in the light : dark exploration and elevated plus‐maze tests in Thy1α6 mice, while being almost inactive in wild‐type mice. The transgenic mice also lost quicker and for longer time their righting reflex after 25 mg/kg gaboxadol than wild‐type mice. In hippocampal sections of Thy1α6 mice, the α6β receptors could be visualized autoradiographically by interactions between gaboxadol and GABA via [35S]TBPS binding to the GABAA receptor ionophore. Gaboxadol inhibition of the binding could be partially prevented by GABA. Electrophysiology of recombinant GABAA receptors revealed that GABA was a partial agonist at α6β3 and α6β3δ receptors, but a full agonist at α6β3γ2 receptors when compared with gaboxadol. The results suggest strong behavioral effects via selective pharmacological activation of enriched extrasynaptic αβ GABAA receptors, and the mouse model represents an example of the functional consequences of altered balance between extrasynaptic and synaptic inhibition.

Molecular basis for the high THIP/gaboxadol sensitivity of extrasynaptic GABAA receptors

“…It has been strongly suggested, based on data from native eGABARs, that the constrained GABA analogue THIP is a selective, nanomolar affinity ligand; however, recombinant studies of ␦ subunit-containing GABARs have led to the conclusion that there are uniform populations of receptors with much higher THIP EC 50 ranging from 3 to 53 M (Adkins et al 2001;Brown et al 2002;Jia et al 2005;Mortensen et al 2010;Saarelainen et al 2008;Storustovu and Ebert 2006). Here, we reconcile this discrepant THIP sensitivity between native eGABARs and recombinant forms of eGABARs.…”

“…Prior studies have used single-component Hill equations to derive EC 50 for THIP, and these estimates range from 6 to 53 M for ␣4␤2/3␦ receptors (Adkins et al 2001;Brown et al 2002;Jia et al 2005;Storustovu and Ebert 2006) and 3.1 to 8.2 M for ␣6␤3␦ receptors (Saarelainen et al 2008;Storustovu and Ebert 2006). This work suggested that the eGABAR dependence of the behavioral and cellular actions of THIP could not be accounted for by subunit-specific differences in sensitivity.…”

“…To evaluate this issue in more detail, we turned to the conformationally constrained GABA analog 4,5,6,7-tetrahydroisoxazolo (5,4-c)pyridin-3(-ol) (THIP) (Krogsgaard-Larsen et al 1977). THIP has been suggested as a selective ligand for eGABARs (Belelli et al 2005;Cope et al 2005;Drasbek and Jensen 2006;Jia et al 2005;Winsky-Sommerer et al 2007), yet the reported THIP EC 50 of recombinant ␦ subunit-containing receptors range from 3 to 53 M (Adkins et al 2001;Brown et al 2002;Jia et al 2005;Mortensen et al 2010;Saarelainen et al 2008;Storustovu and Ebert 2006), which are far too high to account for this selectivity. We therefore decided to systematically compare the activity of THIP at recombinant and native GABARs.…”

Ketamine, But Not Phencyclidine, Selectively Modulates Cerebellar GABAAReceptors Containing α6 and δ Subunits

“…9B). Furthermore, 1 M THIP, a concentration that can selectively activate ␣6-and ␦-containing GABA A receptors (Storustovu and Ebert, 2006;Saarelainen et al, 2008), evoked 37.5 Ϯ 4.8 pA compared with 38.3 Ϯ 5.2 pA (n ϭ 11) of current for 1 M GABA within these dissociated granule neurons. These data suggest the presence of ␣6-and ␦-containing GABA A receptors within the dissociated granule neurons.…”

GABA Site Agonist Gaboxadol Induces Addiction-Predicting Persistent Changes in Ventral Tegmental Area Dopamine Neurons But Is Not Rewarding in Mice or Baboons

“…However, THIP demonstrated an opposite effect, as it induced a long-lasting conditioned place aversion. We used an unbiased conditioning procedure to eliminate possible interference of anxiety and anxiolysis during the test (File, 1986;Parker et al, 1998), although THIP has poor anxiolytic efficacy in wild-type mice (Saarelainen et al, 2008). Although changes in the locomotor activity could have interfered with the expression of place preference (Gremel and Cunningham, 2007), this is unlikely because the locomotor activity during the tests was similar for the low-dose groups not showing aversion or preference and for the highest dose (6 mg/kg) group showing aversion.…”