D2 Receptor Antagonists Do Not Modify Guanine Nucleotide‐Sensitive Interactions Between Dopamine and D1 Dopamine Receptors Under In Vitro Conditions

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Abstract
This study investigated possible D1/D2 interactions in rat and bovine striatal tissue by examining the effects of D2 antagonists on the action of dopamine at D1 dopamine receptors. In addition, the extent to which D2 antagonists may induce an agonist low-affinity state of the D1 receptor was evaluated in comparison with the effects of the guanine nucleotide analogue 5'-guanylyl-imidodiphosphate [Gpp(NH)p]. In saturation experiments dopamine caused a dose-dependent decrease in rat striatal and bovine caudate D1 receptor density. This effect of dopamine, which has been shown to be sensitive to Gpp (NH)p, was not altered by pretreatment with either of the selective D2 antagonists eticlopride (200 nM) or domperidone (200 nM). Results from displacement experiments show that the affinity of dopamine for D1 receptors, and the proportion of receptors in an agonist high-affinity state, are reduced by Gpp(NH)p (100 microM) but not by eticlopride. A molar excess of dopamine (100 microM) promotes the dissociation of (+/-)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-o l ([3H]SCH 23390) from rat striatal D1 receptors at a rate that is significantly slower than when dissociation is initiated using 1 microM piflutixol. After pretreament with Gpp(NH)p, [3H]SCH 23390 dissociation induced by dopamine occurred at an even slower rate. Pretreatment with eticlopride had no effect on the dopamine-induced rate of [3H]SCH 23390 dissociation. These results indicate that all experimental approaches detected dopamine effects at D1 receptors that are Gpp(NH)p sensitive and D2 antagonist insensitive and provide no evidence to support a D1/D2 link operating at the receptor level.

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