Grinevich Valery, Brecht Michael, Osten Pavel
Department of Molecular Neurobiology, Max Planck Institute for Medical Research, 69120 Heidelberg, Germany.
J Neurosci. 2005 Sep 7;25(36):8250-8. doi: 10.1523/JNEUROSCI.2235-05.2005.
The mammalian motor cortex typically innervates motor neurons indirectly via oligosynaptic pathways. However, evolution of skilled digit movements in humans, apes, and some monkey species is associated with the emergence of abundant monosynaptic cortical projections onto spinal motor neurons innervating distal limb muscles. Rats perform skilled movements with their whiskers, and we examined the possibility that the rat vibrissa motor cortex (VMC) projects monosynaptically onto facial motor neurons controlling the whisker movements. First, single injections of lentiviruses to VMC sites identified by intracortical microstimulations were used to label a distinct subpopulation of VMC axons or presynaptic terminals by expression of enhanced green fluorescent protein (GFP) or GFP-tagged synaptophysin, respectively. Four weeks after the injections, GFP and synaptophysin-GFP labeling of axons and putative presynaptic terminals was detected in the lateral portion of the facial nucleus (FN), in close proximity to motor neurons identified morphologically and by axonal back-labeling from the whisker follicles. The VMC projections were detected bilaterally, with threefold larger density of labeling in the contralateral FN. Next, multiple VMC injections were used to label a large portion of VMC axons, resulting in overall denser but still laterally restricted FN labeling. Ultrastructural analysis of the GFP-labeled VMC axons confirmed the existence of synaptic contacts onto dendrites and somata of FN motor neurons. These findings provide anatomical demonstration of monosynaptic VMC-to-FN pathway in the rat and show that lentivirus-based expression of GFP and GFP-tagged presynaptic proteins can be used as a high-resolution neuroanatomical tracing method.
哺乳动物的运动皮层通常通过多突触通路间接支配运动神经元。然而，人类、猿类和一些猴类中熟练的手指运动的进化与大量单突触皮质投射到支配远端肢体肌肉的脊髓运动神经元的出现有关。大鼠通过胡须进行熟练的运动，我们研究了大鼠触须运动皮层（VMC）是否单突触投射到控制胡须运动的面部运动神经元上的可能性。首先，通过皮质内微刺激确定的VMC位点单次注射慢病毒，分别通过增强型绿色荧光蛋白（GFP）或GFP标记的突触素的表达来标记VMC轴突或突触前终末的一个独特亚群。注射四周后，在面神经核（FN）外侧部分检测到轴突和假定突触前终末的GFP和突触素-GFP标记，靠近通过形态学鉴定以及从胡须毛囊进行轴突逆向标记确定的运动神经元。VMC投射在双侧均被检测到，对侧FN中的标记密度大三倍。接下来，多次VMC注射用于标记大部分VMC轴突，导致FN标记总体上更密集但仍局限于外侧。对GFP标记的VMC轴突的超微结构分析证实了在FN运动神经元的树突和胞体上存在突触接触。这些发现提供了大鼠中VMC到FN单突触通路的解剖学证据，并表明基于慢病毒的GFP和GFP标记的突触前蛋白表达可作为一种高分辨率神经解剖学追踪方法。
