Complete Conversion of Brain D2 Dopamine Receptors from the High‐ to the Low‐Affinity State for Dopamine Agonists, Using Sodium Ions and Guanine Nucleotide

Abstract

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 μMS-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 NaCl, 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.