Effect Of Drugs of Abuse On Synaptic Transmission In Nucleus Accumbens

伏隔核(NAC)是几类滥用药物的奖赏和成瘾特性的关键部位。因此，有必要了解大麻、可卡因、阿片类药物和特制药物等滥用药物对这一系统的生理作用，以及了解连接NAC的重要大脑回路。此外，众所周知，这个大脑核团还负责调节行为的动机方面。出于这个原因，NAC卷入了各种精神障碍，包括情绪和动机的改变，以及吸毒成瘾的过程。NAC中棘GABA能输出神经元(MSN)接受其他内源性MSN的神经支配，接受外源性的谷氨酸能神经支配。与MSN的GABA能和谷氨酸能突触都被滥用药物抑制，这表明这种作用可能有助于它们的奖赏特性。此外，已知滥用药物会增加NAC中多巴胺(DA)的释放。DA在调节NAC活动中的作用之一可能是有助于重复激活谷氨酸能传入后兴奋性传递的长期变化。然而，这种突触可塑性发展的确切机制，以及滥用药物如何改变这种突触可塑性，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(FSCV)记录技术在转基因和正常啮齿动物的脑片上进行记录。通过结合这些方法，我们希望能够同时监测DA水平的变化和突触可塑性的发展。我们最近的实验包括检测由腹侧被盖(VTA)DA神经元产生的兴奋性突触的突触特性，在转基因大鼠中，cre重组酶受酪氨酸羟基酶启动子的控制(TH-CRE大鼠)。因此，我们使用腺相关病毒(AAV-DIO-ChR2)选择性地表达了光激活蛋白-视紫红质-2(ChR-2)。由于许多酪氨酸羟基酶阳性(TH)的VTA神经元也表达囊泡性谷氨酸-2转运体(VGlut-2)，它们能够将DA和谷氨酸信号共同传递到NAC以及VTA靶向的其他脑区。在脑片实验中，我们发现FSCV可以在TH轴突的光激活(473 Nm)过程中检测到NAC中DA的释放，但不能在另一个接受VTA输入的大脑区域-外侧缰核(LHb)检测到DA的释放。然而，与这些实验相反，我们也发现光激活的谷氨酸介导的突触EPSCs在两个大脑区域都很容易检测到。这表明LHb和Nac的DA释放机制不同，而对谷氨酸释放的控制是不同的。随后对NAC和LHb中谷氨酸能EPSCs特性的研究表明，这些光激活突触电流的生物物理动力学明显不同，NAC中的这些电流表现出比NAC中的慢得多的衰减动力学。这些数据表明，这些TH神经元靶向的离子型谷氨酸受体在这些不同的大脑区域是不同的。计划中的实验将阐明这种差异的分子基础。

The nucleus accumbens (NAc) represents a critical site for the rewarding and addictive properties of several classes of abused drugs. Therefore, it is necessary to understand the actions of abused drugs such as marijuana, cocaine, opioids, and designer drugs on physiology of this system, as well as to understand the important brain circuits that connect with the NAc. In addition, this brain nucleus is known to mediate motivational aspects of behavior. For this reason the NAc has been implicated in a variety of psychiatric disorders that involve alterations in mood and motivation, as well as in the process of drug addiction. The NAc medium spiny GABAergic output neurons (MSNs) receive innervation from other intrinsic MSNs, and glutamatergic innervation from extrinsic sources. Both GABAergic and glutamatergic synapses onto MSNs are inhibited by abused drugs, suggesting that this action may contribute to their rewarding properties. In addition, abused drugs are known to increase the release of dopamine (DA)in the NAc. One role of DA in regulating NAc activity may be to contribute to the long-term changes in excitatory transmission observed following repetitive activation of glutamatergic afferents. However, the precise mechanisms through which such synaptic plasticity develops, and how drugs of abuse alter such synaptic plasticity, remain poorly understood. To investigate the actions of abused drugs in the NAc, we are utilizing both electrophysiological and fast scan cyclic voltammetry (FSCV) recording techniques in brain slices obrained from transgenic and normal rodents. By combining these approaches, we hope to be able to simultaneously monitor changes in DA levels and the development of synaptic plasticity. Our most recent experiments have involved examining the synaptic properties of excitatory synapses 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). Therefore, we selectively expressed the light-activated protein, channelrhodopsin-2 (ChR-2), using an adeno-associated virus containing the ChR-2 construct (AAV-DIO-ChR2). 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, as well as to other brain areas targeted by the VTA. In experiments with brain slices, we find that FSCV can detect DA release in the NAc during light-activation (473 nm) of TH+ axons, but not in the lateral habenula (LHb), another brain area receiving TH+ inputs from VTA. However, in contrast to these experiments, we also find that light-activated, glutamate-mediated synaptic EPSCs are readily detected in both brain regions. This suggestes that the mechanisms of DA release in the LHb differ from those in the NAc, whereas the control of glutamate release is distinct. Subsequent experiments examining the properties of glutamatergic EPSCs in both the NAc and LHb demonstrate that the biophysical kinetics of these light-activated synaptic currents differ significantly, with those in the NAc demonstrating much slower decay kinetic than those in the NAc. These data suggest that the ionotropic glutamate receptors targeted by these same TH+ neurons differ in these distinct brain regions. Planned experiments will elucidate the molecular basis for this difference.