Pharmacological Properties of GABA_A Receptors Containing γ1 Subunits

Abstract

GABA A receptors composed of ␣ 1 , ␤ 2 , ␥ 1 subunits are expressed in only a few areas of the brain and thus represent interesting drug targets. The pharmacological properties of this receptor subtype, however, are largely unknown. In the present study, we expressed ␣ 1 ␤ 2 ␥ 1 -GABA A receptors in Xenopus laevis oocytes and analyzed their modulation by 21 ligands from 12 structural classes making use of the two-microelectrode voltage-clamp method and a fast perfusion system. Modulation of GABA-induced chloride currents (I GABA ) was studied at GABA concentrations eliciting 5 to 10% of the maximal response. Triazolam, clotiazepam, midazolam, 2-(4-methoxyphenyl)-2, 3,5,6,7,8,9,10-octahydro-cyclohepta-(b) (CGS 9896), diazepam, zolpidem, and bretazenil at 1 M concentrations were able to significantly (Ͼ20%) enhance I GABA in ␣ 1 ␤ 2 ␥ 1 receptors. Methyl-6,7-dimethoxy-4-ethyl-␤-carboline-3-carboxylate, 3-methyl-6-[3-trifluoromethyl-phenyl]-1,2,4-triazolo [4,3-b]pyridazine (Cl 218,872), clobazam, flumazenil,, and 6-ethyl-7-methoxy-5-methylimidazo[1,2-a]pyrimidin-2-yl)phenylmethanone (Ru 32698) (1 M each) had no significant effect, and flunitrazepam and 2-phenyl-4-(4-ethyl-piperidinyl)-quinoline (PK 8165) inhibited I GABA . The most potent compounds triazolam, clotiazepam, midazolam, and CGS 20625 were investigated in more detail on ␣ 1 ␤ 2 ␥ 1 and ␣ 1 ␤ 2 ␥ 2S receptors. The potency and efficiency of these compounds for modulating I GABA was smaller for ␣ 1 ␤ 2 ␥ 1 than for ␣ 1 ␤ 2 ␥ 2S receptors, and their effects on ␣ 1 ␤ 2 ␥ 1 could not be blocked by flumazenil. CGS 20625 displayed the highest efficiency by enhancing at 100 M I GABA (␣ 1 ␤ 2 ␥ 2 ) by 775 Ϯ 17% versus 526 Ϯ 14% I GABA (␣ 1 ␤ 2 ␥ 1 ) and 157 Ϯ 17% I GABA (␣ 1 ␤ 2 ) (p Ͻ 0.05). These data provide new insight into the pharmacological properties of GABA A receptors containing ␥ 1 subunits and may aid in the design of specific ligands for this receptor subtype.GABA is the principal inhibitory neurotransmitter in the mammalian brain. It mediates fast synaptic inhibition by interaction with the GABA A receptor. GABA A receptors are ligandgated ion channels that are modulated by a large number of clinically relevant drugs such as benzodiazepines (BZs), barbiturates, neurosteroids, and anesthetics (Sieghart, 1995). They are assembled from individual subunits forming a pentameric structure. Nineteen isoforms of mammalian GABA A receptor subunits have been cloned: ␣ 1-6 , ␤ 1-3 , ␥ 1-3 , ␦, ⑀, p, 1-3 , and (Barnard et al., 1998;Simon et al., 2004). The major receptor subtype of the GABA A receptor in adults consists of ␣ 1 , ␤ 2 , and ␥ 2 subunits, and the most likely stoichiometry is two ␣ subunits, two ␤ subunits, and one ␥ subunit (Sieghart and Sperk, 2002). CGS 20625, 2-(4-methoxyphenyl)-2,3,5,6,7,8,9,10-octahydro-cyclohepta-(b) ABBREVIATIONS:

GABAA Receptors Containing Gamma1 Subunits Contribute to Inhibitory Transmission in the Central Amygdala

“…1) and receptors assembled with these subunits have a greatly reduced sensitivity to both diazepam and zolpidem and are not affected by DMCM (Fig. 3) (Khom et al 2006). These properties are similar to those of receptors present at synapses made by lateral inputs to CeAL neurons.…”

Markers in the 5′-Region of GABRG1 Associate to Alcohol Dependence and are in Linkage Disequilibrium with Markers in the Adjacent GABRA2 Gene

“…Other brain regions that show selective expression of g-1 (eg hypothalamic medial preoptic area) vs g-2 subunits (eg ventromedial nucleus of the hypothalamus) (Herbison and Fenelon, 1995;Nett et al, 1999) differ in GABA A pharmacologic properties and show opposite effects with respect to receptor modulation by anabolic steroids (Jorge-Rivera et al, 2000). Additional differences in the pharmacology of g-1-containing receptors include an insensitivity to the benzodiazepine antagonist flumazenil (Ymer et al, 1990;Khom et al, 2006) and an increased sensitivity of g-1-containing receptors to the neuroactive steroid allopregnanolone in transfected human embryonic kidney (HEK293) cells compared with g-2-containing receptors (Puia et al, 1993). Pharmacologic agents that are selective for g-1-containing GABA A receptors may be of clinical interest given the limited CNS distribution of this receptor subunit.…”

Expression of organic anion-transporting polypeptides 1B1 and 1B3 in ovarian cancer cells: Relevance for paclitaxel transport

“…Preparation of stage V-VI oocytes from X. laevis was performed as previously described [34]. [³H]paclitaxel were manipulated using glassware.…”