Labeling by [3H]1,3-di(2-tolyl)guanidine of two high affinity binding sites in guinea pig brain: evidence for allosteric regulation by calcium channel antagonists and pseudoallosteric modulation by sigma ligands.
Equilibrium binding studies with the sigma receptor ligand [3H]1,3-di(2-tolyl)guanidine ([3H]DTG) demonstrated two high affinity binding sites in membranes prepared from guinea pig brain. The apparent Kd values of DTG for sites 1 and 2 were 11.9 and 37.6 nM, respectively. The corresponding Bmax values were 1045 and 1423 fmol/mg of protein. Site 1 had high affinity for (+)-pentazocine, haloperidol, (R)-(+)-PPP, carbepentane, and other sigma ligands, suggesting a similarity with the dextromethorphan/sigma 1 binding site described by Musacchio et al. [Life Sci. 45:1721-1732 (1989)]. Site 2 had high affinity for DTG and haloperidol (Ki = 36.1 nM) and low affinity for most other sigma ligands. Kinetic experiments demonstrated that [3H]DTG dissociated in a biphasic manner from both site 1 and site 2. DTG and haloperidol increased the dissociation rate of [3H]DTG from site 1 and site 2, demonstrating the presence of pseudoallosteric interactions. Inorganic calcium channel blockers such as Cd2+ selectively increased the dissociation rate of [3H]DTG from site 2, suggesting an association of this binding site with calcium channels.
Rothman
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Unit on Receptor Studies, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland 20892.
Reid
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Mahboubi
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Kim
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De Costa
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Jacobson
A E
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Rice
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D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism / Q000648:ultrastructure*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D006146:Guanidines / Q000378:metabolism*; D006168:Guinea Pigs; D007700:Kinetics; D011957:Receptors, Opioid / Q000378:metabolism*; D017480:Receptors, sigma; D013329:Structure-Activity Relationship; D014316:Tritium
D002121:Calcium Channel Blockers; D006146:Guanidines; D011957:Receptors, Opioid; D017480:Receptors, sigma; D014316:Tritium; C050232:1,3-ditolylguanidine