Synaptic mechanisms underlying reward seeking and compulsive drug use

D2自身受体而不是纹状体突触后D2受体在可卡因滥用易感性中起关键作用的证据 这项研究测试了一种假设，即纹状体和中脑神经元中低水平的D2受体可能协同作用，导致易受攻击的表型，增强了对可卡因的增强特性，并削弱了对消费的控制丧失。我们提出这一假说是基于我们已发表的工作的收集，这些工作表明，与纹状体神经元中的D2Rs相比，多巴胺神经元中的D2自身受体在回路中发挥不同的、有时是互补的作用。因此，我们推断，降低同一动物体内两种D2受体的水平将产生脆弱的回路，并导致可卡因增强特性，增强寻求可卡因的毅力，以及对不良后果不敏感的摄取。 此前，我们已经证明D2自身受体是多巴胺合成和释放以及多巴胺神经元活动的负调节因子。D2自身受体的丢失增强了可卡因的急性运动反应和可卡因自我给药行为的获得。 另一方面，我们还发现纹状体神经元中的D2受体调节纹状体神经元之间的侧向抑制的强度，当这些受体在低水平表达时，纹状体处于强有力的抑制之下。中脑多巴胺神经元D2自身受体的丢失或纹状体神经元D2受体的丢失会导致基底节突触生理的不同变化。我们发现，每一种受体都对可卡因驱动行为的选定变化起到了作用。该项目的目的是剖析纹状体和中脑的D2Rs对可卡因的行为反应和发展成瘾行为的易感性的贡献。 我们在纹状体的中棘神经元(MSN)(IMSN-D2HET)或中脑多巴胺神经元(AUTO-D2HET)或两者(Double-D2HET)中产生了定向单等位基因缺失的转基因小鼠。我们认为，患有药物使用障碍的患者表现出D2受体可用性的减少而不是完全丧失，因此部分敲除D2受体是一种更有信息量的操作，可以改善临床情况的表面有效性。我们培育了这些小鼠，使所有三种基因型和野生型的小鼠都能产下小鼠，并测试了一系列纹状体控制的多巴胺介导的行为和对可卡因的反应，包括静脉注射可卡因自我给药(IV-SA)。这些是研究结果的亮点：D2自身受体有助于纹状体中D2/3配体的结合，并主要负责D2/3激动剂(通过抑制DA释放)产生的运动抑制。纹状体神经元中的D2受体负责介导可卡因的急性效应及其与D1受体的协同作用。 纹状体神经元D2受体低的小鼠对D2/3激动剂的反应正常，纹状体D1样配体结合和行为反应上调，对可卡因的急性反应较低，但敏化反应增加。 D2自身受体低表达的小鼠纹状体D2/3结合减少，D2/3激动剂反应减少，纹状体D1样配体结合及其行为反应下调，对可卡因的急性反应和反复暴露的脱敏增加。 因此，我们的数据显示，D2自身受体水平低的小鼠和纹状体D2受体水平低的小鼠在与多巴胺和可卡因相关的行为方面往往表现出相反的表型。这项研究进一步表明，纹状体神经元(双D2HET小鼠)中D2自身受体和D2R的同时减少通常具有最温和的表型。因此，与我们最初的假设相反，即两个区域的低D2R将协同产生易受攻击的表型，我们发现D2R的平衡减少导致了最类似于野生型小鼠的行为表型。我们只在对可卡因的急性反应中发现了协同作用的证据。双D2HET在某些行为上类似于自动D2HET的改变，而在其他行为上模仿纹状体-D2HET的表型。这项研究使我们更好地了解纹状体和中脑D2R对多巴胺依赖行为的独特贡献以及对可卡因的反应，以更好地为治疗物质使用障碍的新疗法提供信息。

Evidence for a key role of D2 auto-receptors rather than striatal postsynaptic D2 receptors in the susceptibility for cocaine abuse This study tested the hypothesis that low levels of D2 receptors in both striatum and midbrain neurons might synergize to give rise to a vulnerable phenotype with enhanced reinforcing properties for cocaine and impair loss of control over consumption. We developed this hypothesis based on collection of our published work that showed that D2 auto-receptors in dopamine neurons play different and sometimes complementary roles in the circuitry in comparison to D2Rs in striatal neurons. Thus, we reasoned that lowering levels of both D2 receptors in the same animals would generate a vulnerable circuitry and lead to increased reinforcing properties of cocaine, increased perseverance in cocaine seeking and intake that is insensitive to adverse outcomes. Previously we had shown that D2 auto-receptors act as negative regulators of dopamine synthesis and release and of dopamine neuron activity. Loss of D2 auto-receptors enhanced cocaine acute locomotor response and the acquisition of cocaine self-administration behavior. On the other hand, we also showed that D2 receptors in striatal neurons regulate the strength of the lateral inhibition between striatal neurons and when these receptors are expressed at low levels, the striatum is under potent inhibition. Loss of D2 auto-receptors from midbrain dopamine neurons or loss of D2 receptors on striatal neurons produce different alterations of synaptic physiology in the basal ganglia. We showed that each of the receptors contribute to selected changes in cocaine-driven behaviors. The goal of this project was to dissect out the contribution of the D2Rs in the striatum and the midbrain to the behavioral response to cocaine and the vulnerability to develop addictive behaviors. We generated transgenic mice with targeted single-allele deletion of Drd2 gene in either the medium spiny neurons (MSN) of the striatum (iMSN-D2HET), or the midbrain dopamine neurons (auto-D2HET), or both (double-D2HET). We argue that patients suffering from substance use disorders display a reduction rather than complete loss of D2 receptor availability and as such partial knockdown of D2 receptors is a more informative manipulation with improved face-validity for the clinical condition. We bred these mice such that mice of all three genotypes and wildtype were produced as littermates and tested a battery of striatal-controlled dopamine-mediated behaviors and the response to cocaine, including intravenous cocaine self-administration (IV-SA). These are the highlights of the findings: D2 auto-receptors contribute to D2/3 ligand binding in the striatum and are largely responsible for locomotor suppression produced by d2/3 agonists (via inhibition of DA release). D2 receptors in striatal neurons are responsible for mediating the acute effects of cocaine and its actions synergized with D1 receptors. Mice with low D2 receptors in striatal neurons show normal response to D2/3 agonists, display upregulation of striatal D1-like ligand binding and behavioral response, and show lower acute response but increased sensitized response to cocaine. Mice with low expression of D2 auto-receptors have reduced D2/3 binding in the striatum, have reduced D2/3 agonist responses, display downregulation of striatal D1-like ligand binding and its behavioral response, and show increased acute response to cocaine and desensitization over repeated exposure. Thus, our data shows that mice with low D2 auto-receptors and mice with low levels of striatal D2 receptors often display opposite phenotypes with regard to dopamine-related and cocaine-related behaviors. The study further shows that simultaneous reduction of both D2 auto-receptors and D2R in striatal neurons (double-D2HET mice) most often have the mildest phenotypes. Thus, contrary to our initial hypothesis that low D2Rs in both regions will synergize to produce the vulnerable phenotype, we found that balanced reduction of D2Rs led to behavioral phenotypes that most resemble the wild-type mice. We only found evidence of synergism in the acute response to cocaine. Double-D2HET resembles auto-D2HET alterations on some behaviors while they mimic the phenotype of striatum-D2HET on other behaviors. This study gives us a better understanding of the unique contributions of striatal and midbrain D2R to dopamine-dependent behaviors and the response to cocaine to better inform new treatments for substance use disorders.