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

综合神经科学实验室突触药理学部分的研究继续关注神经调节和可塑性的机制以及酒精和其他滥用药物对这些神经元功能的影响。我们的主要兴趣是背侧纹状体（DS）的功能，这是一个涉及动作控制和选择以及动作学习的大脑区域。 纹状体 GABA 突触的长期抑制 我们继续研究纹状体突触的长期突触抑制（LTD）。 实验室之前的工作表明，LTD 发生在纹状体固有的 GABA 能突触处。 这种LTD可以通过中低频率的传入激活来诱导，并且需要内源性大麻素信号传导。 纹状体投射神经元（中棘神经元或 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 的轴突侧支，以及主要在 MSN 胞体附近突触的快速尖峰中间神经元。 我们使用光遗传学技术，通过在两种神经元亚型中选择性表达通道视紫红质 2 并用光激活该通道，独立激活两个 GABA 能输入。 这种方法揭示了这些 GABA 能突触的两种类型的 LTD。 当突触后 MSN 的膜电位接近称为“下调状态”的正常静息电位时，LTD 发生在两组输入处，并且不需要激活电压门控钙通道。 然而，当 MSN 膜电位去极化至接近体内观察到的“上状态”时，LTD 仅发生在 MSN-MSN 突触处，并且似乎涉及 L 型钙通道激活。 在所有条件下，LTD 都是由作用于 CB1 受体的内源性大麻素介导的。 目前的实验正在探索上态和下态 GABAergic LTD 所涉及的不同机制。 这些研究表明，内源性大麻素可以根据传入输入和突触后神经元的状态产生不同的突触可塑性模式。 这些机制有可能通过不同方式影响 MSN 树突上兴奋性突触附近的抑制，或体细胞附近 GABA 能突触控制的神经元激活，以多种方式塑造纹状体输出。 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) 的 GABA 突触传递的影响。我们观察到 EtOH 通过突触前机制抑制 DLS MSN 中的 GABA 能突触传递，同时增强 DMS 中的传递，这表明 EtOH 在两个纹状体亚区域中的作用机制不同。我们目前正在使用药理学方法以及上述光遗传学技术来确定这些不同的 EtOH 效应是否发生在 MSN 的不同传入输入处。 这些相反的作用是令人惊讶的，并且可能表明 EtOH 抑制了对于目标导向行动很重要的 DMS 的输出，同时增强了参与习惯形成的 DLS 的输出。 我们对小鼠和猕猴的慢性乙醇影响的持续研究揭示了突触变化，可能导致长期、习惯性饮酒和滥用。在这两种模型生物中，长期接触 EtOH 会导致 DLS 和 DMS（分别大致相当于猴子的壳核和尾状核）中 GABA 能突触传递的持续减少。 在猴子身上的发现特别有趣，因为这种模型生物的饮酒量很大且时间较长，与人类饮酒有许多共同特征。 这些猴子饮酒的一个显着特征是，动物在饮酒两年后会形成高度规律的酒精摄入量，并且饮酒持续时间增加，并且它们的血液酒精水平在三年内逐渐升高。 我们在这些猴子的壳核 MSN 中观察到 GABA 能传输的减少与每只猴子的平均酒精摄入量密切相关。 我们还观察到，长期饮用乙醇的猴子壳核中谷氨酸能突触传递增加，树突棘（谷氨酸突触部位）数量增加。 长期饮酒后，壳核 MSN 的内在神经元兴奋性也会增加。 最近的研究结果扩展了我们的知识，表明饮酒较早的猴子的 GABA 能突触变化更大，饮酒是人类酒精使用障碍的已知危险因素。我们在这些猴子中观察到的突触变化似乎使壳核过度兴奋，这是由于 GABA 能抑制减少和内在和谷氨酸能突触兴奋性增加的综合作用。 考虑到壳核和相关电路在重复性习惯行为的发展和产生中的作用，我们假设过度兴奋的壳核有助于在猴子中观察到的高酒精负荷下维持不变的饮酒模式。 我们还在继续研究胎儿/产后早期接触乙醇对纹状体功能的影响。该项目的灵感来自于有关胎儿酒精谱系障碍 (FASD) 患者皮质纹状体功能改变的报告。我们在妊娠期和产后早期采用乙醇蒸气暴露法。 电生理学实验表明，在胎儿/出生后早期接触 EtOH 后，成年小鼠的 DLS MSN 中 GABA 能突触传递减少。 在这些小鼠中，由于急性暴露于乙醇而产生的 GABA 能传输的正常减少消失了，这表明对乙醇作用产生了某种耐受性。 有趣的是，通常由 CB1 受体激活产生的突触抑制在胎儿/出生后早期暴露于 EtOH 的小鼠 DLS MSN 中也消失了。 初步研究表明，与其他突触前 G 蛋白偶联受体相比，这种效应对 CB1 具有选择性，并且 eCB 对 CB1 受体的异常强直激活可能是生命早期接触 EtOH 的结果。 我们的平行行为研究表明，生命早期的乙醇暴露也会损害小鼠的习惯学习，并破坏纹状体神经元活动的相关变化。 这些实验揭示了纹状体突触传递的变化，可能导致 FASD 中皮质纹状体回路的异常。

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. Long-term depression at striatal GABAergic synapses We have continued our studies of long-term synaptic depression (LTD) at synapses in striatum. Previous work in the laboratory indicated that LTD occurs at GABAergic synapses intrinsic to the striatum. This LTD can be induced by afferent activation at low-moderate frequencies, and requires endocannabinoid signaling. The two predominant GABAergic inputs to striatal projection neurons (medium spiny neurons or 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 have used optogenetic techniques to activate the two GABAergic inputs independently, by expressing channel rhodopsin 2 selectively in the two neuronal subtypes and activating this channel with light. This approach has revealed two types of LTD at these GABAergic synapses. When the membrane potential of the postsynaptic MSN is near the normal resting potential called the "down-state", LTD occurs at both sets of inputs, and does not require activation of voltage-gated calcium channels. However, when the MSN membrane potential is depolarized to values near the "up-states" seen in vivo, LTD occurs exclusively at MSN-MSN synapses, and appears to involve L-type calcium channel activation. Under all conditions, LTD is mediated by endocannabinoids acting at CB1 receptors. Current experiments are exploring the different mechanisms involved in up-state versus down-state GABAergic LTD. These studies have revealed that endocannabinoids can produce differential patterns of synaptic plasticity depending on the afferent input and the state of the postsynaptic neuron. These mechanisms have the potential to shape striatal output in multiple ways by differentially affecting inhibition near excitatory synapses on the MSN dendrites, or neuronal activation controlled by GABAergic synapses near the soma. Ethanol actions at striatal GABAergic synapses We are also continuing our studies of ethanol (EtOH) effects on GABAergic synaptic transmission in dorosolateral (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 are currently using pharmacological approaches, as well as the optogenetic techniques described above, to determine if these differential EtOH effects occur at different afferent inputs to MSNs. These opposing effects 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. Our continuing studies of chronic EtOH effects in mice and macaque monkeys have revealed synaptic changes that could contribute to prolonged, habitual alcohol use and abuse. In both model organisms, chronic EtOH exposure leads to a maintained decrease in GABAergic synaptic transmission in DLS and DMS (roughly equivalent to putamen and caudate nucleus, respectively, in monkeys). The findings in monkey are particularly interesting, as the alcohol drinking in this model organism is heavy and prolonged, sharing many features with human drinking. One notable feature of drinking in these monkeys is that animals develop highly regular alcohol intake with increased duration of intake bouts after 2 years of drinking, and the blood alcohol levels they achieve gradually escalate over a three year period. The decreased GABAergic transmission we have observed in putamen MSNs from these monkeys is strongly correlated with the average alcohol intake for each individual monkey. We have also observed increases in glutamatergic synaptic transmission and increased numbers of dendritic spines (the sites of glutamatergic synapses) in the putamen of the chronic EtOH-drinking monkeys. Intrinsic neuronal excitability is also increased after prolong drinking in the putamen MSNs. Recent findings have extended our knowledge by showing that GABAergic synaptic changes are greater in monkeys with earlier onset of drinking, a known risk factor for human alcohol use disorders. The synaptic changes that we have observed in these monkeys appear to render the putamen nucleus hyperexcitable, due to a combination of decreased GABAergic inhibition and increased intrinsic and glutamatergic synaptic excitability. Given the role of the putamen and associated circuitry in development and production of repetitive habitual actions, we hypothesize that the hyperexcitable putamen contributes to maintained, invariant patterns of drinking with high alcohol loading observed in the monkeys. We are also continuing to study effects of fetal/early-postnatal EtOH exposure on striatal function. This project was stimulated by reports implicating altered corticostriatal function in humans with fetal alcohol spectrum disorder (FASD). We are using vapor exposure to EtOH during gestation and the early postnatal period. Electrophysiological experiments have revealed decreased GABAergic synaptic transmission in DLS MSNs examined in adult mice following fetal/early postnatal EtOH exposure. The normal decrease in GABAergic transmission produced by acute exposure to EtOH is lost in these mice, suggesting development of some sort of tolerance to EtOH actions. Interestingly, the synaptic depression normally produced by CB1 receptor activation is also lost in the fetal/early postnatal EtOH-exposed mouse DLS MSNs. Preliminary studies indicate that this effect is selective for CB1 in comparison to to other presynaptic G protein-coupled receptors, and that an abnormal tonic activation of CB1 receptors by eCBs may occur as a consequence of this early life EtOH exposure. Our parallel behavioral studies indicate that this early-life EtOH exposure also impairs habit learning in the mice, and disrupts the associated changes in striatal neuronal activity. These experiments have revealed changes in striatal synaptic transmission that could contribute to abnormalities in the corticostriatal circuitry seen in FASD.