RT Journal Article
SR Electronic
T1 In Vivo Evidence for Dopamine-Mediated Internalization of  D<sub>2</sub>-Receptors after Amphetamine: Differential Findings with [<sup>3</sup>H]Raclopride versus [<sup>3</sup>H]Spiperone
JF Molecular Pharmacology
JO Mol Pharmacol
FD American Society for Pharmacology and Experimental Therapeutics
SP 456
OP 462
DO 10.1124/mol.63.2.456
VO 63
IS 2
A1 W. Sun
A1 N. Ginovart
A1 F. Ko
A1 P. Seeman
A1 S. Kapur
YR 2003
UL http://molpharm.aspetjournals.org/content/63/2/456.abstract
AB Competition with endogenous dopamine (DA) is usually invoked to explain changes in [11C]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 [3H]raclopride binding observed in the rat striatum after an amphetamine challenge reflects true competition with endogenous DA or agonist-mediated internalization of D2-receptors. We found that the amphetamine-induced decrease in [3H]raclopride binding is caused by a decrease in D2-receptor density (Bmax) with no change in affinity (Kd). In contrast, in the same tissue, neither the Bmax nor theKd were affected when measured with [3H]spiperone. Challenge with amphetamine not only decreased the number of D2-receptors but also eliminated the proportion (22%) of receptors usually in the high-affinity state. The addition of Gpp(NH)p had no effect onBmax, 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 [3H]raclopride and [3H]spiperone; however, in the case of [3H]spiperone, this was accompanied by a compensatory increase in binding in the intracellular compartment, whereas no increase was seen with [3H]raclopride. These data suggest that amphetamine releases dopamine, which binds to the high-affinity state of the D2-receptor, leading to its sequestration in some intracellular compartment; in this compartment, sequestered receptors are inaccessible to [3H]raclopride binding but can still be bound by [3H]spiperone. The American Society for Pharmacology and Experimental Therapeutics