Pharmacological Characterization of Agonists at δ-Containing GABA_AReceptors: Functional Selectivity for Extrasynaptic Receptors Is Dependent on the Absence of γ₂

Abstract

Several groups have characterized the pharmacology of ␣ 4 -or ␣ 6 ␤ 3 ␦-containing GABA A receptors expressed in different cell systems. We have previously demonstrated that the pharmacological profiles of a series of GABA A receptor agonists are highly dependent on the ␣ subunit and little on the ␤ and ␥ subunits, so to further understand the contribution of the different subunits in the GABA A receptor complex, we characterized a series of full agonists, partial agonists, and antagonists at ␣ 4 ␤ 3 , ␣ 4 ␤ 3 ␦, and ␣ 6 ␤ 3 ␦ receptors expressed in Xenopus oocytes. Little or no difference was seen when the compounds were compared at ␣␤-and ␣␤␦-containing receptors, whereas a significant reduction in both potency and relative efficacy was observed compared with ␣␤␥-containing receptors described in the literature. These data clearly confirm that the presence of the ␦ subunit in heterotrimeric receptors is a strong determinant of the increased pharmacological activity of compounds with agonist activity. The very similar agonist pharmacology of ␣␤-and ␣␤␦-containing receptors, which is significantly different from that of ␣␤␥-containing receptors, shows that whereas the presence of a ␥ subunit impairs the response to an agonist stimulation of the ␣␤ receptor complex, the ␦ subunit does not affect this in any way. Taken together, these data are well in line with the idea that ␣ 4 ␤ 3 ␦ may contribute to the pharmacological action of exogenously applied agonists and may explain why systemically active compounds such as gaboxadol and muscimol in vivo appear to act as selective extrasynaptic GABA A agonists.

Studies of ethanol actions on recombinant δ-containing γ-aminobutyric acid type A receptors yield contradictory results

“…Experiments expressing human cRNAs in oocytes yielded similar results with respect to ethanol modulation of α 4 β 3 δ GABA A -Rs; the human subunits presented a pharmacological profile consistent with the literature, explored in more detail in a companion paper (Stórustovu and Ebert, 2006). The L(tk -) cell line stably expressing human α 4 β 3 δ and α 4 β 3 γ 2S showed lower overall sensitivity to ethanol potentiation, but again no differences between α 4 β 3 δ and α 4 β 3 γ 2S GABA A -Rs.…”

“…Several neuroactive steroids (5α-pregnane-3α-ol-20-one, alphaxalone, and 5α-pregnane-3α,21-diol-20-one or THDOC) were more potent modulators of GABA in α 4 β 3 δ than in α 4 β 3 γ 2 GABA A -Rs. More recent studies of human and rat α 4 β 3 δ expressed in oocytes replicated many of these findings (Borghese et al, 2006;Stórustovu and Ebert, 2006).…”

Neurosteroid Agonist at GABAA Receptor Induces Persistent Neuroplasticity in VTA Dopamine Neurons

“…We first searched for slightly sedative doses of GAN after acute injections to locate the closest match to our previous study with THIP (Vashchinkina et al, 2012) that has a full agonistic profile at extrasynaptic GABA A receptors (Storustovu and Ebert, 2006). We found that GAN 30 mg/ kg produced a transient reduction in locomotor activity of young mice, whereas a 10 mg/kg dose did not affect activity (Figure 1a and b).…”

“…Thereafter, body temperature was recorded at different intervals (ranging from 15 to 30 min) after an i.p. injection of vehicle, GAN (30 mg/kg), or 4,5,6,7-tetrahydroisoxazolol[4,5-c]pyridine-3-ol (THIP, gaboxadol; 6 mg/ kg), a selective GABA-site agonist with a full agonist profile at d-subunit-containing GABA A receptors (Storustovu and Ebert, 2006). THIP has shown to induce neuroplasticity in VTA DA neurons but no reward-like behavior in CPP test in mice (Vashchinkina et al, 2012).…”

Extrasynaptic GABAAReceptors Couple Presynaptic Activity to Postsynaptic Inhibition in the Somatosensory Thalamus

“…Indeed, immunogold studies suggest a limited expression the ␣4 subunit within synaptic regions of dentate granule cells (Sun et al, 2007;Zhang et al, 2007). However, in the thalamus, both functional studies demonstrating inhibitory actions for Ro154513 (a partial benzodiazepine "agonist" with positive allosteric activity at ␣4␤x␥2 receptors and negative modulatory activity at equivalent receptors incorporating ␣1-3 or ␣5 subunits) (Wafford et al, 1996) and immunohistochemical evidence highlighting an apparent lack of colocalization between ␣4-containing receptors and synaptic markers Kralic et al, 2006;Peden et al, 2008) argue against such an interpretation.…”