Synaptic integration and intrinsic firing properties of basal ganglia neurons

我们的实验室研究位于中脑的多巴胺释放神经元的整合和兴奋性的细胞和亚细胞原理。该实验室的一个主要兴趣是识别中脑多巴胺神经元功能独特的亚群，并了解这些神经元如何对基底节回路做出贡献。为此，实验室的一个项目专注于了解抑制性输入如何控制黑质致密部(SNC)内的多巴胺能神经元亚群。在厌恶事件期间，SNC多巴胺能神经元暂停其活动以作出反应。活体实验表明，这些神经元中的一部分发射动作电位的反弹爆发，或在令人厌恶的活动暂停后显示反弹的钙活动。然而，这种行为背后的局部神经回路目前尚不清楚。我们一直使用双光子成像和局部光基因激活来从功能上将基底节核的抑制性输入映射到多巴胺神经元。我们比较了纹状体(纹状体和基质)、苍白球(Pvalb和Lhx6)和黑质网状部(SNR)中五(5)个独立的基因定义的抑制亚群的强度和位置。我们发现纹状体的输入选择性地抑制了多巴胺神经元的腹侧投射SNR树突。尽管这种连接孤立于SNR树突，但它对整个细胞施加了强大的控制，暂停了动作电位，促进了反弹。此外，我们发现纹状体的输入通过激活GABA-B受体来促进反弹放电，GABA-B受体使SNR树突强烈超极化。因此，纹状体对黑质多巴胺神经元的抑制是产生反弹放电的最佳位置。 在第二个项目中，我们开始直接测试轴突受体及其对轴突兴奋性和最终多巴胺释放的影响。具体地说，GABA-A受体调节某些神经元的递质释放，包括潜在的多巴胺神经元，但这种调节的机制尚有争议。为了解决这一知识缺口，我们从多巴胺能神经元轴突的断端进行了直接记录，包括从背侧纹状体内的分支轴突进行记录。我们的结果为GABA-A受体介导的电导的存在提供了确凿的证据。与它们在胞体的功能相比，我们发现轴突GABA-A受体正在去极化，氯翻转电位为-56 mV，而静息膜电位为-68 mV。此外，我们还发现，GABA-A受体的激活通过分流抑制降低了传播动作电位的幅度。最后，我们发现，安定，一种广谱的苯二氮卓类药物，降低了纹状体多巴胺神经元轴突的输入阻力，表明这些药物的作用机制被低估。总之，对多巴胺神经元轴突的直接记录表明，GABA-A受体是轴突动作电位传播和多巴胺释放的重要调节器。此外，这些受体是苯二氮卓类药物的靶标，也可能是其他靶向GABA-A受体的药物，如乙醇和巴比妥酸盐。 除了这两个主要项目外，我们还发表了一项研究，发现钠泄漏通道NALCN是SNC多巴胺能神经元自发放电的主要驱动因素(Philippart和Khaliq，eLife 2018)。重要的是，我们发现多巴胺D2受体和GABA-B受体都通过抑制NALCN负性调节多巴胺能神经元的活动。因此，本研究确定NALCN是多巴胺能神经元中一种新的效应Gi/o蛋白偶联受体。 最后，实验室的工作人员在自己的职业生涯中取得了成功。例如，今年5月，丽贝卡·埃文斯获得了大脑倡议K99奖。她还被要求在西班牙巴塞罗那的计算神经科学组织和GRS儿茶酚胺会议上发表演讲，在那里，她是被选为在GRC主要会议上发表演讲的两名GRS发言者之一。Paul Kramer还在GRC儿茶酚胺会议上展示了他的工作，取得了巨大的成功。最后，艾米丽·特韦德尔最近在今年夏天完成了她的学士学位后奖学金，并将于今年秋天作为学生进入加州大学旧金山分校参加他们的神经科学研究生项目。为了填补这一空缺职位，一位新的毕业后研究员亚历山大·苏哈雷夫加入了实验室。

Our laboratory studies the cellular and subcellular principles of integration and excitability in dopamine-releasing neurons located in the midbrain. A major interest of the lab is identifying functionally unique subpopulations of midbrain dopamine neurons and understanding how these neurons contribute to the basal ganglia circuit. To this end, one project in the lab focuses on understanding how inhibitory inputs may control subpopulations of dopaminergic neurons within the substantia nigra pars compacta (SNc). SNc dopaminergic neurons pause their activity in response to during aversive events. In vivo experiments show that a subset of these neurons fire rebound bursts of action potentials or show rebound calcium activity following the aversive pause in activity. However, the local neural circuits that underly this behavior are currently unknown. We have been using two-photon imaging and local optogenetic activation to functionally map the inhibitory inputs from basal ganglia nuclei onto dopamine neurons. We compare the strength and location of five (5) separate genetically-defined inhibitory subpopulations in the striatum (striosome and matrix), globus pallidus (Pvalb and Lhx6), and substantia nigra pars reticulata (SNr). We find that the striosomal inputs selectively inhibit the ventrally-projecting SNr dendrite of the dopamine neurons. Although isolated to the SNr dendrite, this connection exerts strong control over the entire cell, pausing action potentials and facilitating rebound firing. Furthermore, we find that striosomal input facilitates rebound firing through activation of GABA-B receptors, which strongly hyperpolarize the SNr dendrite. Therefore, inhibition from striosomes onto SNc dopamine neurons is optimally placed to produce rebound firing. In a second project, we set out to directly test for axonal receptors and their influence over axonal excitability and ultimately dopamine release. Specifically, GABA-A receptors modulate transmitter release in some neurons, including potentially dopamine neurons, but the mechanisms of this modulation are debated. To address this knowledge gap, we performed direct recordings from the cut ends of dopaminergic neuron axons, including from branching axons within the dorsal striatum. Our results provide definitive evidence for the existence of GABA-A receptor-mediated conductances. In contrast to their function at the soma, we found axonal GABA-A receptors were depolarizing with a chloride reversal potential of -56 mV relative to resting membrane potential of -68 mV. In addition, we found that activation of GABA-A receptors decreased the amplitude of a propagating action potential through shunting inhibition. Finally, we found that diazepam, a broad-spectrum benzodiazepine, decreased the input resistance of striatal dopamine neuron axons, suggesting an underappreciated mechanism of action for these drugs. In conclusion, direct recordings from dopamine neuron axons demonstrate that GABA-A receptors are important modulators of axonal action potential propagation and dopamine release. In addition, these receptors are targets of benzodiazepines, as well as potentially other drugs that target GABA-A receptors like ethanol and barbiturates. Aside from these two major projects, we have had one study published that identified a sodium leak channel, NALCN, as the main driver of spontaneous firing in SNc dopaminergic neurons (Philippart and Khaliq, eLife 2018). Importantly, we found that both dopamine D2 receptors as well as GABA-B receptor negatively modulate the activity of dopaminergic neurons through inhibition of NALCN. Therefore, this study identifies NALCN as a novel effector Gi/o protein coupled receptors in dopaminergic neurons. Lastly, the personnel in the lab are finding success in their own professional careers. This May, for example, Rebekah Evans was awarded the Brain Initiative K99 grant. She was also asked to deliver talks at the Organization for Computational Neuroscience in Barcelona Spain and at the GRS Catecholamines where she was one of two GRS speakers chosen to deliver a talk at the main GRC meeting. Paul Kramer also presented his work at the GRC Catecholamines meeting with great success. Lastly, Emily Twedell has recently completed her postbaccalaureate fellowship this summer and will enrolling this Fall as a student at UCSF in their Neuroscience Graduate Program. To fill this open position, a new postbaccalaureate fellow, Alexander Sukharev, has joined the lab.