Extrasynaptic ?-GABAA receptors are high-affinity muscimol receptors.

ABSTRACT: Muscimol, the major psychoactive ingredient in the mushroom Amanita muscaria, has been regarded as a universal non-selective GABA-site agonist. Deletion of the GABAA receptor (GABAA R) ? subunit in mice (?KO) leads to a drastic reduction in high-affinity muscimol binding in brain sections and to a lower behavioral sensitivity to muscimol than their wild type counterparts. Here, we use forebrain and cerebellar brain homogenates from WT and ?KO mice to show that deletion of the ? subunit leads to a > 50% loss of high-affinity 5 nM [3 H]muscimol-binding sites despite the relatively low abundance of ?-containing GABAA Rs (?-GABAA R) in the brain. By subtracting residual high-affinity binding in ?KO mice and measuring the slow association and dissociation rates we show that native ?-GABAA Rs in WT mice exhibit high-affinity [3 H]muscimol-binding sites (KD ~1.6 nM on ?4?? receptors in the forebrain and ~1 nM on ?6?? receptors in the cerebellum at 22°C). Co-expression of the ? subunit with ?6 and ?2 or ?3 in recombinant (HEK 293) expression leads to the appearance of a slowly dissociating [3 H]muscimol component. In addition, we compared muscimol currents in recombinant ?4?3? and ?4?3 receptors and show that ? subunit co-expression leads to highly muscimol-sensitive currents with an estimated EC50 of around 1-2 nM and slow deactivation kinetics. These data indicate that ? subunit incorporation leads to a dramatic increase in GABAA R muscimol sensitivity. We conclude that biochemical and behavioral low-dose muscimol selectivity for ?-subunit-containing receptors is a result of low nanomolar-binding affinity on ?-GABAA Rs.

SUBMITTER: Benkherouf AY 

PROVIDER: S-EPMC6438731 | biostudies-literature | 2019 Apr

REPOSITORIES: biostudies-literature