Amphetamine-sensitized animals show a marked increase in dopamine D2 high receptors occupied by endogenous dopamine, even in the absence of acute challenges

While a range of dopamine D-2-related behaviors are exaggerated in amphetamine-sensitized animals, studies of the dopamine D-2 receptor have reported either no change or a decrease in dopamine D-2 receptor density-especially when measured using radioraclopride. We hypothesized that a decrease in D-2 receptors may actually be "apparent" and that these receptors may still be present, but are noncompetitively "occupied" by endogenous dopamine. Animals sensitized to amphetamine (and their saline controls) were examined 4 weeks after their last injection. We first measured the [H-3]raclopride binding in vivo, and observed that sensitized animals showed a 29% lower level of raclopride binding in vivo, suggesting an apparently lower level of dopamine D-2 receptors. To assess the reason for this we examined the density of receptors (using Scatchard analysis in vitro) measured by [H-3]raclopride in the presence and absence of guanilylimidodiphosphate. This guanine nucleotide converts the dopamine-bound high-affinity state of D-2 receptors into low-affinity states, thereby making measurable the absolute density of the sites. As previously reported, the amphetaminesensitized animals showed a 31% lower number of D-2 receptors in conventional binding (B-max 15.6 vs. 22.7 pmol/g). However, with the addition of guanilylimidodiphosphate there was an equalization of both groups (B-max 25.9 vs. 25.6 pmol/g), revealing an additional 10.3 pmol/g in the sensitized animals, but only 2.9 pmol/g in saline controls. There were no changes in the dissociation constant of [H-3]raclopride for the receptors. The nearly four-fold increase of dopamine D-2 receptors in the high-affinity state occupied by dopamine may explain why amphetamine-sensitized animals show almost an order of magnitude greater response to dopamine-releasing challenges or dopamine agonists, even though the absolute receptor number is unchanged and the "apparent" receptor number is decreased. (C) 2002 Wiley-Liss, Inc.