SB-205384 Is a Positive Allosteric Modulator of Recombinant GABA_A Receptors Containing Rat α3, α5, or α6 Subunit Subtypes Coexpressed with β3 and γ2 Subunits

Abstract

Many drugs used to treat anxiety are positive modulators of GABA A receptors, which mediate fast inhibitory neurotransmission. The GABA A receptors can be assembled from a combination of at least 16 different subunits. The receptor's subunit composition determines its pharmacologic and functional properties, and subunit expression varies throughout the brain. A primary goal for new treatments targeting GABA A receptors is the production of subunit-selective modulators acting upon a discrete population of receptors. The anxiolytic 4-amino-7-hydroxy-2-methyl-5, 6,7,8,-tetrahydrobenzo[b]thieno [2,3-b]pyridine-3-carboxylic acid, but-2-ynyl ester (SB-205384) is widely considered to be selective for a3-containing GABA A receptors. However, it has been tested only on a1-, a2-, and a3-containing receptors. We examined the activity of SB-205384 at recombinant receptors containing the six different a subunits and found that receptors containing the a3, a5, and a6 subunits were potentiated by SB-205384, with the a6 subunit conferring the greatest responsiveness. Properties associated with chimeric a1/a6 subunits suggested that multiple structural domains influence sensitivity to SB-205384. Point mutations of residues within the extracellular N-terminal domain identified a leucine residue located in loop E of the agonist binding site as an important determinant of high sensitivity to modulation. In the a6 subunit the identity of this residue is species-dependent, with the leucine found in rat subunits but not in human. Our results indicate that SB-205384 is not an a3-selective modulator, and instead acts at several GABA A receptor isoforms. These findings have implications for the side-effect profile of this anxiolytic as well as for its use in neuronal and animal studies as a marker for contribution from a3-containing receptors.

A Pharmacogenetic ‘Restriction-of-Function’ Approach Reveals Evidence for Anxiolytic-Like Actions Mediated by α5-Containing GABAA Receptors in Mice

“…However, two recent studies failed to replicate the findings of the initial report by Dias et al (2005) regarding the subtype selectivity of TP003, and instead demonstrated comparable efficacies at all diazepam-sensitive α-subunits (Christian et al, 2015;de Lucas et al, 2015). Moreover, the presumably α3-selective compound SB-205384 (4-amino-7-hydroxy-2-methyl-5,6,7,8,-tetrahydrobenzo[b]thieno [2,3-b]pyridine-3-carboxylic acid, but-2-ynyl ester), which was shown to have anxiolytic-like actions (Navarro et al, 2006), was later found to be a positive modulator at α5-and α6-GABA A Rs in addition to α3-GABA A Rs (Heidelberg et al, 2013). Similarly, while the anxiogenic compound α3IA (6-(4-pyridyl)-5-(4-methoxyphenyl)-3-carbomethoxy-1-methyl-1H-pyridin-2-one) displays some selectivity for α3-GABA A Rs, it also possesses some efficacy at α2-GABA A Rs (Atack et al, 2005).…”

The efficacy of γ-aminobutyric acid type A receptor (GABA AR) subtype-selective positive allosteric modulators in blocking tetramethylenedisulfotetramine (TETS)-induced seizure-like behavior in larval zebrafish with minimal sedation

“…For example, flumazenil, an imidazobenzodiazepine that binds at the interface of α (nonselectively) and γ2 ( Kucken et al, 2003 ) is used clinically to treat benzodiazepine overdoses ( Seger, 2004 ). Heidelberg et al (2013) reports that SB-205384 most likely does not work via the same site as flumazenil by testing the modulation of α3ß3γ2 by flumazenil and SB-205384 in vitro . It is also reported that SB-205384 maintains some activity on α5 ( Heidelberg et al, 2013 ).…”

“… Heidelberg et al (2013) reports that SB-205384 most likely does not work via the same site as flumazenil by testing the modulation of α3ß3γ2 by flumazenil and SB-205384 in vitro . It is also reported that SB-205384 maintains some activity on α5 ( Heidelberg et al, 2013 ). Considering these two observations, the antagonism observed in this study could be occurring at the α5-γ2 interface; however, further testing is needed to confirm this.…”

The role of loops B and C in determining the potentiation of GABAA receptors by midazolam

“…For example, nonhypnotic drugs targeting the α2 and α3 subunits have been studied for their anxiolytic and analgesic effects . However, creating ligands that distinguish these two subunits remains difficult, as shown when an “α3‐specific” PAM (SB‐205384) was found to potentiate α6‐containing GABA A receptors even more strongly than α3 . Another way to distinguish different GABA A receptor subtypes is through the γ subunit.…”

“…41,45 However, creating ligands that distinguish these two subunits remains difficult, as shown when an "α3-specific" PAM (SB-205384) was found to potentiate α6-containing GABA A receptors even more strongly than α3. 46 Another way to distinguish different GABA A receptor subtypes is through the γ subunit. Although other γ subunits can form benzodiazepinesensitive receptors, the γ3 subunit is less prevalent (~14% of receptors), 47 and the γ1 subunit notably reduces the benzodiazepine affinity of the receptor.…”