Effect Of Drugs of Abuse On Synaptic Transmission In Nucleus Accumbens

伏隔核（NAc）是一个重要的大脑区域，涉及动机和介导几种滥用药物的奖励和成瘾特性。此外，由于其在动机过程中的作用，NAc也与涉及情绪、动机改变的精神疾病有关。因此，有必要了解这一大脑区域的基本功能，以及大麻、可卡因、阿片类药物和设计药物等滥用药物对该系统的作用。NAc主要由中棘、gaba能输出神经元（msn）组成，这些神经元接受来自其他msn的神经支配，谷氨酸神经支配来自许多其他脑区，多巴胺神经支配来自中脑腹侧。严重的是，连接到msn上的GABA和谷氨酸突触都被几种滥用药物抑制，这表明这种行为可能有助于它们的奖励特性，长期暴露于药物会改变内在回路和外在输入的功能。此外，已知滥用药物会增加NAc中多巴胺（DA）的释放，这可能有助于在谷氨酸传入事件重复激活后观察到的兴奋传递的长期变化。尽管人们对NAc对行为的影响了解甚多，但这种突触可塑性影响行为的确切机制，以及滥用药物如何改变突触过程，仍然知之甚少。为了研究滥用药物在NAc中的作用，我们利用电生理和快速扫描循环伏安法（FSCV）记录结合光生技术对转基因和正常啮齿动物的脑切片进行了研究。通过结合这些方法，我们可以同时监测DA水平的变化和突触可塑性的发展。在酪氨酸羟化酶启动子控制cre重组酶的转基因大鼠（TH-Cre大鼠）中，目前的实验涉及检测由腹侧被盖（VTA）DA神经元产生的NAc msn上的兴奋性突触的突触特性。当使用含有ChR-2构建体的腺相关病毒（AAV-DIO-ChR2）时，这允许选择性地表达光激活蛋白，如通道视紫红质-2 （ChR-2）。由于许多酪氨酸羟化酶阳性（TH+） VTA神经元也表达泡状谷氨酸-2转运体（VGlut-2），它们能够将DA和谷氨酸信号共同传递到NAc。证实了这一点，我们发现ChR2的光激活在病毒注射到VTA后唤起NAc壳中谷氨酸介导的突触EPSCs。然后将这些EPSP特性与ChR2注入其他靶向NAc通路（如腹侧海马、基底外侧杏仁核和内侧前额叶皮质）所引起的特性进行比较。我们发现由这些输入引起的NAc的谷氨酸神经支配的大小差异很大，并且这些输入的生物物理特性是可区分的。我们还发现，长期暴露于大麻的精神活性成分- δ -9-四氢大麻酚（THC）会不同程度地改变这些NAc输入的强度，以及它们产生突触可塑性的能力。具体而言，暴露于THC超过2周的时间会导致前额叶对NAc壳的输入显着减弱，并大大增强来自海马腹侧和杏仁核基底外侧的输入。有趣的是，尽管由VTA引起的谷氨酸输入NAc的总体强度没有改变，但这一途径的生物物理特性发生了改变，表明发生了稳态补偿。在NAc输入中观察到的变化模式与在人类大麻使用者中进行的脑成像研究报告一致，并且表明慢性四氢大麻酚使用导致皮质对NAc的控制丧失。这对认知和情绪处理具有重要意义，也与人类的行为变化一致。因此，我们假设我们发现的变化与被诊断为大麻使用障碍的人的行为和精神变化有关。

The nucleus accumbens (NAc) is a critical brain area involved in motivation and mediates the rewarding and addictive properties of several classes of abused drugs. Additionally, because of its role in motivational processes, the NAc is also implicated in psychiatric disorders that involve alterations in mood, motivation. Therefore, it is necessary to understand the basic function of this brain region, as well as the actions of abused drugs such as marijuana, cocaine, opioids, and designer drugs on this system. The NAc is largely comprised of medium spiny, GABAergic output neurons (MSNs) that receive innervation from other MSNs, glutamate innervation from many additional brain areas, and dopamine innervation from the ventral midbrain. Acutely, both GABA and glutamate synapses onto MSNs are inhibited by several classes of abused drugs, suggesting that this action may contribute to their rewarding properties, and long-term exposure to drugs alters the function of both intrinsic circuits and extrinsic inputs. In addition, abused drugs are known to increase the release of dopamine (DA)in the NAc, and this likely contributes to long-term changes in excitatory transmission observed following repetitive activation of glutamatergic afferents. Although much is known about the contributions of the NAc to behavior, the precise mechanisms in which such synaptic plasticity contributes to behavior, and how abused drugs alter synaptic processes remains poorly understood. To investigate the actions of abused drugs in the NAc, we are utilizing electrophysiological and fast scan cyclic voltammetry (FSCV) recording combined with optogentic techniques in brain slices obrained from transgenic and normal rodents. By combining these approaches, we can simultaneously monitor changes in DA levels and the development of synaptic plasticity. Current experiments involve examining the synaptic properties of excitatory synapses onto NAc MSNs arising from ventral tegmental (VTA)DA neurons in transgenic rats in which cre recombinase is under control of the tyrosine hydroxylase promoter (TH-Cre rats). This permits selective expression of the light-activated proteins, such as channelrhodopsin-2 (ChR-2), when an adeno-associated virus containing the ChR-2 construct (AAV-DIO-ChR2) is used. As many tyrosine hydroxylase positive (TH+) VTA neurons also express the vesicular glutamate-2 transporter (VGlut-2) they are capable of co-transmitting DA and glutamate signals to the NAc. Confirming this, we find that light-activation of ChR2 evokes glutamate-mediated synaptic EPSCs in the NAc shell following virus injections into the VTA. These EPSP properties are then compared to those evoked by ChR2 injected into other pathways targeting the NAc, such as the ventral hippocampus, basolateral amygdala and medial prefrontal cortex. We find that the magnitude of glutamate innervation of the NAc arising from these inputs varies widely, and that the biophysical properties of these inputs are distinguishable. We also find that long-term exposure to the psychoactive component of marijuana, delta-9-tetrahydrocannabinol (THC) differentially alters the strength of these NAc inputs, as well as their capacity to generate synaptic plasticity. Specifically, exposure to THC over a 2 week period causes a significant weakening of prefrontal inputs to the NAc shell, and greatly strengthens inputs arising from the ventral hippocampus and basolateral amygdala. Interestingly, although the overall strength of glutamate input to the NAc arising from VTA is not changed, the biophysical properties of this pathway are altered in a way that suggests that homeostatic compensation occurs. The pattern of changes observed in the inputs to the NAc are consistent with those reported in brain imaging studies conducted in human marijuana users, and suggest a loss of cortical control over the NAc with chronic THC use. This has important implications for cognition and emotional processing that is also consistent with behavioral changes seen in humans. Therefore, we hypothesize that the changes we have identified are related to behavioral and psychiatric changes seen in humans that are diagnosed with cannabis use disorder.