Synaptic Transmission: Modulation, Plasticity And Effects Of Drugs Of Abuse

在整合神经科学实验室中，突触药理学部分的研究继续集中在神经调节和可塑性的潜在机制以及酒精和其他滥用药物对这些神经元功能的影响。我们主要感兴趣的是背侧纹状体(DS)的功能，这是一个参与动作控制和选择以及动作学习的大脑区域。 阿片剂/阿片受体介导的纹状体突触的长期抑郁。 我们继续研究纹状体突触的长期突触抑制(LTD)。实验室以前的工作表明，不同的突触前G蛋白偶联受体(GPCRs)可以启动对纹状体中棘神经元(MSN)突触处谷氨酸释放的长期抑制。这些受体的主要特性是激活Gi/o型G蛋白。阿片肽，如脑啡肽和强啡肽，是阿片受体的内源性激动剂。三种经典的阿片受体是GI/O偶联的MU、Delta和Kappa亚型(分别为MORS、DORS和KORS)。这些受体存在于纹状体，包括这个脑区的突触前元件。阿片肽脑啡肽和强啡肽在纹状体中也很丰富，它们的表达被分离在两种不同的MSN亚型中，分别称为直接通路和间接通路。阿片肽及其同源受体参与了奖赏的神经机制，因此我们有兴趣确定它们是如何影响纹状体的突触传递的，因为这个大脑区域在奖赏相关行为中发挥了作用。当从小鼠或大鼠的脑片中记录MSN时，我们观察到三种ORs中的任何一种激活都会抑制谷氨酸能突触传递。不同受体的拮抗剂抵消了亚型选择性激动剂的影响，因此不同的ORs似乎独立地作用于抑制传递。我们的发现表明，突触抑制主要是由于神经递质释放减少所致。令人惊讶的是，OR激动剂诱导的抑郁是长期的，即使激动剂的应用已经终止，也会持续长达30分钟。使用已知可降解脑啡肽和强啡肽的特定多肽酶的阻滞剂，可以发现由OR介导的长期突触抑制，表明内源性神经肽可以激活这些反应。阿片类激动剂被用于治疗疼痛，因此，了解在体内接触此类药物是否会改变阿片介导的纹状体有限公司是很重要的。事实上，在体内注射一种广泛使用的阿片类止痛剂羟考酮24小时内，从动物制成的纹状体切片中不能诱导阿片剂有限公司。到目前为止，我们的发现表明，内源性阿片类药物在谷氨酸能纹状体突触产生一种形式的LTD，其特性类似于激活Gi/o偶联GPCRs的其他神经递质产生的特性。此外，这种突触可塑性可能在OR激动剂的治疗过程中被阻止或阻断，也可能在习惯性药物使用和阿片成瘾的形成中发挥作用。 乙醇对纹状体GABA能突触的作用 我们还在继续研究乙醇(Etoh)对背外侧(DLS)和背内侧纹状体(DMS)GABA能突触传递的影响。我们的观察结果表明，乙醇通过突触前机制抑制DLS MSN的GABA能突触传递，而增强DMS的突触传递，提示EtoH在这两个纹状体亚区的作用机制不同。我们已经开始使用光遗传学技术来确定这些不同的乙醇效应是否发生在MSN的不同传入输入上。纹状体MSN的两个主要GABA能输入来自主要在MSN树突上突触的其他MSN的轴突侧枝，以及主要在MSN胞体附近突触的快尖峰中间神经元。我们可以通过选择性地在两个神经元亚型中表达视紫红质2通道，并用光激活这两个通道，从而独立地激活这两个GABA能输入。使用这种方法，我们发现乙醇抑制了DLS中两个神经元亚型对MSN的GABA能传入。我们现在开始在DMS中使用同样的方法。DLS和DMS的相反效应令人惊讶，这可能表明乙醇抑制了DMS的输出，而DMS的输出是目标导向行为的重要组成部分，而DLS的输出与习惯形成有关。

Research within the Laboratory for Integrative Neuroscience, Section on Synaptic Pharmacology, continues to focus on mechanisms underlying neuromodulation and plasticity and the effects of alcohol and other drugs of abuse on these neuronal functions. Our main interest is the function of the dorsal striatum (DS), a brain region involved in action control and selection, as well as action learning. Opiates/Opiate Receptor-Mediated Long-Term Depression at Striatal Synapses. We have continued our studies of long-term synaptic depression (LTD) at synapses in striatum. Previous work in the laboratory indicated that different presynaptic G-protein-coupled receptors (GPCRs) can initiate long-lasting depression of glutamate release at synapses onto striatal medium spiny neurons (MSNs). The main property shared by these receptors is activation of Gi/o-type G proteins. Opioid peptides, such as enkephalin and dynorphin, are endogenous agonists for the opiate receptors. The three classic opiate receptors are the Gi/o-coupled mu, delta and kappa subtypes (MORs, DORS and KORs respectively). These receptors are present in striatum, including on presynaptic elements in this brain region. The opiate peptides enkephalin and dynorphin are also abundant in striatum, where their expression is segregated in two different MSN-subtypes known as the direct and indirect pathways MSNs, respectively. The opiate peptides and their cognate receptors are implicated in neural mechanisms of reward, and we were thus interested in determining how they affect synaptic transmission in the striatum, as this brain region has roles in reward-related behaviors. When recording from MSNs in brain slices from mouse or rat, we have observed that activation of any of the three ORs produces inhibition of glutamatergic synaptic transmission. Antagonists of the different receptors counteract the effects of subtype-selective agonists, and thus the different ORs appear to act independently to depress transmission. Our findings indicate that synaptic depression is mainly due to decreased neurotransmitter release. Surprisingly, the OR agonist-induced depression is long-lasting, persisting for up to 30 min even when agonist application has been terminated. Using blockers of specific peptidases that are known to degrade enkephalin and dynorphin reveals long-lasting synaptic depression that is OR-mediated, indicating that the endogenous neuropeptides can activate these responses. Opiate agonists are used therapeutically for pain treatment, and thus it is important to know if in vivo exposure to such drugs alters opiate-mediated striatal LTD. Indeed, opiate LTD is not inducible in striatal slices made from animals within 24 hr of a single in vivo injection of oxycodone, a widely-used opiate analgesic. Our findings to date indicate that endogenous opiates produce a form of LTD at glutamatergic striatal synapses that has properties similar to that produced by other neurotransmitters that activate Gi/o-coupled GPCRs. Furthermore, this synaptic plasticity may be prevented or occluded during therapeutic treatment with OR agonists, and could also play roles in habitual drug use and the development of opiate addiction. Ethanol actions at striatal GABAergic synapses We are also continuing our studies of ethanol (EtOH) effects on GABAergic synaptic transmission in dorsolateral (DLS) and dorsomedial striatum (DMS). Our observation that EtOH inhibits GABAergic synaptic transmission in DLS MSNs via a presynaptic mechanism, while potentiating transmission in DMS, suggests differential mechanisms of EtOH action in the two striatal subregions. We have begun using optogenetic techniques to determine if these differential EtOH effects occur at different afferent inputs to MSNs. The two predominant GABAergic inputs to striatal MSNs come from axon collaterals of other MSNs that synapse mainly on the MSN dendrites, and from the fast-spiking interneurons that synapse mainly near the MSN soma. We can activate the two GABAergic inputs independently by expressing channel rhodopsin 2 selectively in the two neuronal subtypes and activating this channel with light. Using this approach, we find that EtOH inhibits GABAergic input to MSNs from both neuronal subtypes in the DLS. We are now beginning to use the same approach in the DMS. The opposing effects in DLS and DMS are surprising, and may indicate that EtOH suppresses the output of the DMS that is important for goal-directed actions, while enhancing the output of DLS which is involved in habit formation.