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

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Abstract
While a range of dopamine D2‐related behaviors are exaggerated in amphetamine‐sensitized animals, studies of the dopamine D2 receptor have reported either no change or a decrease in dopamine D2 receptor density—especially when measured using radioraclopride. We hypothesized that a decrease in D2 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 [3H]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 D2 receptors. To assess the reason for this we examined the density of receptors (using Scatchard analysis in vitro) measured by [3H]raclopride in the presence and absence of guanilylimidodiphosphate. This guanine nucleotide converts the dopamine‐bound high‐affinity state of D2 receptors into low‐affinity states, thereby making measurable the absolute density of the sites. As previously reported, the amphetamine‐sensitized animals showed a 31% lower number of D2 receptors in conventional binding (Bmax 15.6 vs. 22.7 pmol/g). However, with the addition of guanilylimidodiphosphate there was an equalization of both groups (Bmax 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 [3H]raclopride for the receptors. The nearly four‐fold increase of dopamine D2 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. Synapse 46:235–239, 2002.