Both α2 and α3 GABA_A receptor subtypes mediate the anxiolytic properties of benzodiazepine site ligands in the conditioned emotional response paradigm

Abstract

Mice with point-mutated alpha2 GABAA receptor subunits (rendering them diazepam insensitive) are resistant to the anxiolytic-like effects of benzodiazepines (BZs) in unconditioned models of anxiety. We investigated the role of the alpha2 GABAA subtype in a model of conditioned anxiety. alpha2(H101R) and wildtype mice were trained in a conditioned emotional response (CER) task, in which lever-pressing for food on a variable interval (VI) schedule was suppressed during the presentation of a conditioned stimulus (CS+) that predicted footshock. The ability of diazepam, ethanol and pentobarbital to reduce suppression during the CS+ was interpreted as an anxiolytic response. Diazepam (0, 0.5, 1, 2, 4 and 8 mg/kg) induced a dose-dependent anxiolytic-like effect in wildtype mice. At high doses, diazepam (2, 4 and 8 mg/kg) was sedative in alpha2(H101R) mice. Analysis of the anxiolytic properties of nonsedative diazepam doses (0.5 and 1 mg/kg), showed that alpha2(H101R) mice were resistant to the anxiolytic effects of diazepam. Equivalent anxiolytic properties of pentobarbital (20 mg/kg) and ethanol (1 and 2 g/kg) were seen in both genotypes. These findings confirm the critical importance of the alpha2 GABAA subtype in mediating BZ anxiolysis. However, as a compound, L-838417, with agonist properties at alpha2, alpha3 and alpha5-containing receptors, gave rise to anxiolytic-like activity in alpha2(H101R) mice in the CER test, alpha3-containing GABA receptors are also likely to contribute to anxiolysis. Observations that alpha2(H101R) mice were more active, and displayed a greater suppression of lever pressing in response to fear-conditioned stimuli than wildtype mice, suggests that the alpha2(H101R) mutation may not be behaviourally silent.

Cocaine effects on mouse incentive-learning and human addiction are linked to α2 subunit-containing GABA A receptors

“…α2 −/− mice (31) and α2H101R "knock-in" mice (11,12) were gen- Locomotor Activity. Locomotor activity, and behavioral sensitization to cocaine (α2 −/− ) mice or Ro 15-4513 (α2H101R) mice were measured in circular runways, as previously described (32).…”

“…Importantly, the point mutation does not alter the sensitivity of the receptor to its natural ligand, GABA. As selective ligands for α2-containing receptors are not available, we used this knowledge to differentially facilitate transmission at α2-containing receptors, using knock-in mice (11,12) in which the normal H at position 101 within the benzodiazepine binding pocket of the α2 protein was replaced by R (α2H101R mice). In such mutant mice, Ro 15-4513 will continue to act as an inverse agonist at α1−, α3−, and α5-containing receptors, and as an agonist at α4-and α6-containing receptors, so that the only difference between WT and mutant mice will be the direction of Ro 15-4513s action at α2-containing receptors.…”

Seeking Structural Specificity: Direct Modulation of Pentameric Ligand-Gated Ion Channels by Alcohols and General Anesthetics

“…This approach has been applied to several pentameric ligand-gated ion channels and has been quite successful in linking specific receptor subunits with drug actions. One of the earliest and most elegant Ligand-Gated Ion Channel Alcohol Sites examples is the construction of knock-in mice with specific GABA A receptor subunits lacking actions of benzodiazepines or general anesthetics (Rudolph and Möhler, 2004;Morris et al, 2006;Tan et al, 2011). For example, these studies showed that benzodiazepine modulation of the a2 subunit of the GABA A receptor is critical for the antianxiety actions of these drugs and the general anesthetic action of propofol is due to action on GABA A receptors containing the b3 subunit (Jurd et al, 2003).…”

Bitter acids from hydroethanolic extracts of Humulus lupulus L., Cannabaceae, used as anxiolytic

“…Gamma-aminobutyric acid (GABAA) receptors had been widely studied since they are the site of action of a number of clinically important drugs, including benzodiazepines, barbiturates, and anesthetics (Morris et al, 2006). Benzodiazepines are the first-line drugs for the treatment of anxiety disorders, acting at the GABAA receptors which remain primary targets for novel anxiolytic compounds.…”