Vol. 63, Issue 2, 456-462, February 2003

In Vivo Evidence for Dopamine-Mediated Internalization of D₂-Receptors after Amphetamine: Differential Findings with [³H]Raclopride versus [³H]Spiperone

W. Sun, N. Ginovart, F. Ko, P. Seeman, and S. Kapur

PET Centre, Centre for Addiction and Mental Health, Toronto, Ontario, Canada (W.S., N.G., S.K.); and Departments of Psychiatry (N.G., S.K.) and Pharmacology (W.S., F.K., P.S.), University of Toronto, Ontario, Canada

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 (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 [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 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 [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.