Glycine antagonists structurally related to 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridin-3-ol inhibit binding of [3H]strychnine to rat brain membranes.
[3H]Strychnine binding to rat pons + medulla membranes was used as a measure of glycine receptors or glycine receptor-coupled chloride channels in vitro. A series of compounds structurally related to 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), which previously were shown to antagonize glycine responses in cat spinal cord, inhibited [3H]strychnine binding in micromolar concentrations. The most potent of these glycine antagonists, 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol (iso-THAZ), was also the most potent inhibitor of [3H]strychnine binding, with a Ki of 1,400 nM. The Ki value for strychnine was 7.0 nM, whereas the Ki value for the mixed gamma-aminobutyric acid (GABA)/glycine antagonist 3 alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (RU 5135) was only 4.6 nM. Sodium chloride (1,000 mM) enhanced the affinity of strychnine, brucine, isostrychnine, and the nonselective GABA antagonist pitrazepin for [3H]strychnine binding sites, whereas the affinities of glycine, beta-alanine, and taurine were reduced. These sodium chloride shifts, however, were not predictive of antagonist or agonist properties, since the sodium chloride shift for the glycine antagonist iso-THAZ and of the other THIP-related antagonists were similar to those of the glycine-like agonists. The various sodium chloride shifts show that different groups of ligands bind to glycine receptor sites in different ways.
D000818:Animals; D000838:Anions; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D002712:Chlorides / Q000494:pharmacology; D002794:Choline / Q000494:pharmacology; D018756:GABA Antagonists; D005998:Glycine / Q000037:antagonists & inhibitors*; D007555:Isoxazoles / Q000494:pharmacology*; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000378:metabolism*; D010080:Oxazoles / Q000494:pharmacology*; D011149:Pons / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D018009:Receptors, Glycine; D017981:Receptors, Neurotransmitter / Q000187:drug effects / Q000378:metabolism*; D012965:Sodium Chloride / Q000494:pharmacology; D013331:Strychnine / Q000037:antagonists & inhibitors / Q000378:metabolism*
D000838:Anions; D002712:Chlorides; D018756:GABA Antagonists; D007555:Isoxazoles; D010080:Oxazoles; D018009:Receptors, Glycine; D017981:Receptors, Neurotransmitter; D012965:Sodium Chloride; D013331:Strychnine; C015542:gaboxadol; D002794:Choline; D005998:Glycine