Chronic agonist exposure induces down-regulation and allosteric uncoupling of the gamma-aminobutyric acid/benzodiazepine receptor complex.
Using [3H]flunitrazepam as a probe for the benzodiazepine-sensitive modulator site located on the gamma-aminobutyric acid (GABA)A receptor complex, we have investigated the cellular regulation of the GABAA receptor in neuronal cultures derived from embryonic chick brain. Treatment of cultures with 1 mM GABA for 48 hr causes a reversible 35% decrease in the number of [3H]flunitrazepam binding sites with no change in affinity. The EC50 for chronic GABA-induced down-regulation is 94 microM and the half-time is 25 hr. The effect of GABA is blocked by SR-95531, a GABAA receptor antagonist, and mimicked by muscimol but not baclofen. Consistent with the decrease in [3H]flunitrazepam binding, chronic GABA exposure causes a 43% decrease in the binding of [35S]t-butylbicyclophosphorothionate, a ligand for the receptor-associated chloride ionophore. In addition to chronic GABA-induced down-regulation, allosteric interactions between GABA and benzodiazepine recognition sites are uncoupled by 34%. The half-time and pharmacology for chronic GABA-induced uncoupling is indistinguishable from that for GABA-induced down-regulation, consistent with the hypothesis that the action of GABA at a common site induces both down-regulation and uncoupling.
Roca
D J
DJ
Department of Anatomy and Cell Biology, State University of New York, Brooklyn 11203.
Rozenberg
I
I
Farrant
M
M
Farb
D H
DH
D000494:Allosteric Regulation; D000818:Animals; D001921:Brain / Q000166:cytology; D001643:Bridged Bicyclo Compounds / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002478:Cells, Cultured; D002642:Chick Embryo; D015536:Down-Regulation; D005445:Flunitrazepam / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D009118:Muscimol / Q000494:pharmacology; D011724:Pyridazines / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000187:drug effects / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000008:administration & dosage*
D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D011724:Pyridazines; D011963:Receptors, GABA-A; D009118:Muscimol; D005680:gamma-Aminobutyric Acid; D005445:Flunitrazepam; C037476:tert-butylbicyclophosphorothionate; C049853:gabazine