Complete conversion of brain D2 dopamine receptors from the high- to the low-affinity state for dopamine agonists, using sodium ions and guanine nucleotide.
Since previous work had shown that brain D2 3,4-dihydroxyphenylethylamine (dopamine) receptors were only partly converted from their high-affinity state to their low-affinity state, we here tested whether it was possible to obtain a complete 100% conversion of these receptors into their low-affinity state. It was first essential to resolve the components of [3H]spiperone binding to dopaminergic sites and nondopaminergic sites in rat striatal homogenates. In the presence of 50 microM S-sulpiride (to occlude the dopaminergic sites), therefore, we first determined that the residual binding of [3H]spiperone (approximately 20%) was inhibited by serotonergic agonists much more effectively than dopamine or noradrenaline, thus identifying the serotonergic component of [3H]spiperone binding. Thus, dopamine (or ADTN) inhibited the binding of [3H]spiperone at a high-affinity site (with dissociation constant of 10 nM dopamine), at a low-affinity site (with dissociation constant of 2,000 nM dopamine), and at the serotonergic site (with dissociation constant of 50,000 nM dopamine). In the absence of sodium ions, the high-affinity site was about 50% occupied by [3H]spiperone, and guanine nucleotide had no effect on this proportion. In the presence of 120 mM NaCl, however, the high-affinity site was reduced to 15% and guanine nucleotide completely eliminated this high-affinity site, 100% of the sites having been completely converted to their low-affinity state. Using [3H]N-propyl-norapomorphine to label the high-affinity state of the dopamine receptor, 50% conversion into the low-affinity state occurred at 45 mM LiCl, 69 mM NaCl, and 202 mM KCl. We conclude that it is possible to convert brain D2 dopamine receptors completely into their low-affinity state, in the presence of NaCl and a guanine nucleotide, providing that appropriate allowance is made for the serotonergic component of [3H]spiperone binding.
Grigoriadis
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Seeman
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D000818:Animals; D001058:Apomorphine / Q000031:analogs & derivatives / Q000378:metabolism; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology*; D066298:In Vitro Techniques; D007650:Ketanserin; D008297:Male; D010880:Piperidines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011954:Receptors, Dopamine / Q000187:drug effects* / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology*; D013134:Spiperone / Q000378:metabolism; D013764:Tetrahydronaphthalenes / Q000378:metabolism; D014316:Tritium
D006150:Guanine Nucleotides; D010880:Piperidines; D011954:Receptors, Dopamine; D013764:Tetrahydronaphthalenes; D014316:Tritium; D013134:Spiperone; C011639:ADTN; C009074:N-n-propylnorapomorphine; D007650:Ketanserin; D012964:Sodium; D001058:Apomorphine