TY - JOUR 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 SP - 456 LP - 462 DO - 10.1124/mol.63.2.456 VL - 63 IS - 2 AU - W. Sun AU - N. Ginovart AU - F. Ko AU - P. Seeman AU - S. Kapur Y1 - 2003/02/01 UR - http://molpharm.aspetjournals.org/content/63/2/456.abstract N2 - 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 ER -