Seeman Philip, Tallerico Teresa, Ko Françoise, Tenn Catherine, Kapur Shitij
Department of Pharmacology, University of Toronto, Toronto, Ontario, Canada M5S 1A8.
Synapse. 2002 Dec 15;46(4):235-9. doi: 10.1002/syn.10139.
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 [(3)H]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 [(3)H]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 amphetamine-sensitized 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 [(3)H]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.
虽然一系列与多巴胺D(2)相关的行为在苯丙胺致敏动物中被夸大，但对多巴胺D(2)受体的研究报告称，多巴胺D(2)受体密度要么没有变化，要么有所下降——尤其是使用放射性雷氯必利测量时。我们推测，D(2)受体的减少可能实际上是“表观”的，这些受体可能仍然存在，但被内源性多巴胺非竞争性地“占据”。对苯丙胺致敏的动物（及其生理盐水对照组）在最后一次注射后4周进行检查。我们首先在体内测量了[(3)H]雷氯必利结合，观察到致敏动物体内雷氯必利结合水平低29%，表明多巴胺D(2)受体水平明显较低。为了评估其原因，我们在有无鸟苷酰亚胺二磷酸的情况下，通过[(3)H]雷氯必利测量了受体密度（使用体外Scatchard分析）。这种鸟嘌呤核苷酸将多巴胺结合的D(2)受体的高亲和力状态转化为低亲和力状态，从而使这些位点的绝对密度可测量。如先前报道，在传统结合中，苯丙胺致敏动物的D(2)受体数量低31%（B(max)为15.6对22.7 pmol/g）。然而，加入鸟苷酰亚胺二磷酸后，两组趋于相等（B(max)为25.9对25.6 pmol/g），显示致敏动物额外增加了10.3 pmol/g，而生理盐水对照组仅增加了2.9 pmol/g。[(3)H]雷氯必利与受体的解离常数没有变化。被多巴胺占据的高亲和力状态下多巴胺D(2)受体增加近四倍，这可能解释了为什么苯丙胺致敏动物对多巴胺释放挑战或多巴胺激动剂的反应几乎大一个数量级，尽管绝对受体数量不变且“表观”受体数量减少。
