Synaptic integration and intrinsic firing properties of basal ganglia neurons

黑质多巴胺神经元以低速率紧张性地放电，导致动作电位反向传播。因此，我们已经开始研究背景紧张性放电的速率如何影响树突定位的电导和突触驱动的放电反应。在我们的第一项研究中，我们惊讶地发现，将强直频率从1 Hz增加到6 Hz产生的Ca信号比通过单个棘波诱发的Ca瞬变的线性总和（或我们称之为Ca超线性）预测的大三倍。此外，双光子谷氨酸释放和突触刺激表明，快速紧张性放电增强NMDA受体依赖的树突钙信号，并加强突发放电输出。这些结果表明，背景放电率的调制精确地调谐树突钙信号，并提供了一个简单而强大的机制来动态调节突触输入的增益。该研究的手稿已提交审查（Hage和Khaliq，提交）。 在奖赏行为中，多巴胺神经元在兴奋性突触输入的驱动下发出动作电位的爆发。使用基因敲除小鼠的行为实验以及在体内进行的单细胞记录已经确定NMDA受体是爆发放电的关键介质。强烈支配丘脑底核的神经元以10-30 Hz的高基础频率放电，并在短时间内以高达100 Hz的频率放电，这就提出了在重复突触输入的条件下如何释放谷氨酸和随后激活突触受体的问题。因此，我们正在研究在高频刺激模式下从丘脑底核到SNc多巴胺神经元的突触传递。我们正在发现谷氨酸溢出和突触外NMDA受体激活的可能作用。然而，令人惊讶的是，而不是简单地促进兴奋的SNc神经元可能是预期的，我们的数据表明，溢出也触发了负反馈机制，其功能是通过突触前II组代谢型受体（mGluRII）依赖性抑制突触输入，以减少尖峰输出。我们预计将在2014年秋季提交这些结果的手稿（Girasole，Hage和Khaliq，准备中）。 去年，我们被批准在小鼠身上进行颅内注射，并花了大量的时间来确定注射部位的坐标。目前，我们正在使用这个程序来逆行标记特定的多巴胺亚群的基础上轴突投射放电模式（如。前额叶皮层、丘脑核等）。到目前为止，我们发现多巴胺神经元（中皮层与中脑）的兴奋性存在显着差异，我们目前正在研究这些差异背后的离子机制。我们预计这项研究的实验将于2014年秋季完成，并期待在2015年初提交这份手稿（Tarfa和Khaliq）。 我们在实验室人员配备方面继续取得良好进展。目前实验室有五（5）名成员- Travis Hage，博士后研究员; Rebekah Evans，博士后研究员; Rahilla Tarfa，研究生研究员; Yujie Richie Sun，博士后研究员;和Renshu Sherry Zhang，研究技术员。埃文斯博士今年1月开始在实验室工作。我们还参与了与Ellen Sidransky博士和Andres Buonanno博士（NIH）的合作项目。

Substantia nigra dopamine neurons fire tonically at low rates resulting in action potential backpropagation. Therefore, we have begun to look at how the rate of background tonic firing influences dendritically-located conductances and synaptically-driven firing responses. In our first study, we were surprised to find that increasing the tonic rate from 1 to 6 Hz generated Ca signals up to three-fold greater than predicted by linear summation of single spike-evoked Ca-transients (or what we call a Ca supralinearity). In addition, two-photon glutamate-uncaging and synaptic stimulation revealed that fast tonic firing enhances NMDA receptor-dependent dendritic Ca signaling and intensifies burst firing output. These results show that modulation of background firing rate precisely tunes dendritic Ca signaling and provides a simple yet powerful mechanism to dynamically regulate the gain of synaptic inputs. A manuscript of this study has been submitted for review (Hage and Khaliq, submitted). During reward behaviors, dopamine neurons fire bursts of action potentials driven by excitatory synaptic input. Behavioral experiments using knockout mice as well as single-cell recordings performed in vivo have established that NMDA receptors are key mediators of burst firing. The subthalamic nucleus, which strongly innervates the compacta, contains neurons that fire at high basal rates of 10-30 Hz and fire bursts as high as 100 Hz for brief periods, raising the question of how release of glutamate and subsequent activation of synaptic receptors occurs under conditions of repetitive synaptic input. Therefore, we are examining synaptic transmission from the subthalamic nucleus to SNc dopamine neurons during high-frequency stimulation patterns. We are finding a likely role of glutamate spillover and activation of extrasynaptic NMDA receptors. Surprisingly, however, rather than simply promoting excitation of SNc neurons as might be expected, our data suggest that spillover also triggers a negative feedback mechanism that functions to reduce spiking output through presynaptic group II metabotropic receptor (mGluRII) dependent inhibition of synaptic input. We expect to submit a manuscript of these results this Fall 2014 (Girasole, Hage, and Khaliq, in preparation). We were approved to perform intracranial injections in mice last year and spent a significant amount of time working out the coordinates for injection sites. Currently, we are using this procedure to retrogradely label particular dopamine subpopulations based on the axonal projection firing pattern (eg. prefrontal cortex, nucleus accumbens, etc). So far, we find significant differences in the excitability of dopamine neurons (mesocortical vs mesoaccumbal), and we are currently studying the ionic mechanisms underlying those differences. We expect to have the experiments for this study complete this Fall 2014 and look to submit this manuscript early in 2015 (Tarfa and Khaliq). We continue to make good progress in staffing the laboratory. There are currently five (5) members of the lab - Travis Hage, postdoctoral fellow; Rebekah Evans, postdoctoral fellow; Rahilla Tarfa, graduate fellow; Yujie Richie Sun, postbaccalaureate fellow; and Renshu Sherry Zhang, research technician. Dr. Evans started working the lab earlier this year in January. We are also participating in collaborative projects with Dr. Ellen Sidransky and Dr. Andres Buonanno (NIH).