In vivo evidence for dopamine-mediated internalization of D2-receptors after amphetamine: differential findings with [3H]raclopride versus [3H]spiperone.
Competition with endogenous dopamine (DA) is usually invoked to explain changes in [(11)C]raclopride binding observed after amphetamine administration in animals and humans. This account has recently been questioned because a number of inconsistencies have been reported that contradict it. In the present study, we investigated whether the decrease in [(3)H]raclopride binding observed in the rat striatum after an amphetamine challenge reflects true competition with endogenous DA or agonist-mediated internalization of D(2)-receptors. We found that the amphetamine-induced decrease in [(3)H]raclopride binding is caused by a decrease in D(2)-receptor density (B(max)) with no change in affinity (K(d)). In contrast, in the same tissue, neither the B(max) nor the K(d) were affected when measured with [(3)H]spiperone. Challenge with amphetamine not only decreased the number of D(2)-receptors but also eliminated the proportion (22%) of receptors usually in the high-affinity state. The addition of Gpp(NH)p had no effect on B(max), suggesting that these receptors were not just noncompetitively bound with dopamine at the cell-surface. Subcellular fractionation studies showed that amphetamine treatment led to a decrease in radioligand binding in the cell-surface fraction for both [(3)H]raclopride and [(3)H]spiperone; however, in the case of [(3)H]spiperone, this was accompanied by a compensatory increase in binding in the intracellular compartment, whereas no increase was seen with [(3)H]raclopride. These data suggest that amphetamine releases dopamine, which binds to the high-affinity state of the D(2)-receptor, leading to its sequestration in some intracellular compartment; in this compartment, sequestered receptors are inaccessible to [(3)H]raclopride binding but can still be bound by [(3)H]spiperone.
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PET Centre, Centre for Addiction and Mental Health, Toronto, Ontario, Canada.
Ginovart
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Ko
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Seeman
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Kapur
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D000661:Amphetamine / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D004298:Dopamine / Q000502:physiology*; D018492:Dopamine Antagonists / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004705:Endocytosis / Q000187:drug effects*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008297:Male; D020891:Raclopride / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017448:Receptors, Dopamine D2 / Q000187:drug effects / Q000502:physiology*; D013134:Spiperone / Q000494:pharmacology; D013347:Subcellular Fractions; D013997:Time Factors; D014316:Tritium
D018492:Dopamine Antagonists; D017448:Receptors, Dopamine D2; D014316:Tritium; D006165:Guanylyl Imidodiphosphate; D020891:Raclopride; D013134:Spiperone; D000661:Amphetamine; D004298:Dopamine