Modulation of leg motoneuron activity in the stick insect: The importance of acetylcholine and octopamine and the role of subesophageal descending neurons

行走动物腿部肌肉组织的收缩是由运动神经元控制的。这些运动神经元的活动由强直性去极化驱动、来自腿部感觉器官和中枢神经系统神经元的相位结构输入以及神经调节影响决定。在竹节虫中，我们希望阐明紧张性兴奋的机制和食道下神经元在运动神经元活动调节中的作用。将毒蕈碱类乙酰胆碱激动剂匹罗卡品应用于竹节虫胸神经节，引起运动神经元的紧张性兴奋，激活振荡神经网络（CPGs）。CPGs对运动神经元活性的抑制作用。然而，当乙酰胆碱唤起张力兴奋时，受体的确切位置尚不清楚。我们想知道强直性兴奋是由位于运动神经元上还是位于运动前神经元上的肌动蛋白受体介导的。我们知道张力兴奋是由生物胺章鱼胺调节的。毒蕈碱受体的定位将有助于揭示章鱼胺调节的机制。章鱼胺被认为是无脊椎动物肾上腺素的对应物，在昆虫体内调节着大量不同的功能。例如，它调节行走和飞行行为以及肌肉和感觉器官的活动。在竹节虫体内，章鱼胺阻断了一种由尼古丁乙酰胆碱受体激活引起的内向电流。然而，目前尚不清楚是哪一种章鱼胺能神经元调节了昆虫胸椎神经节神经元的活动。因此，在本项目的第二部分，我们希望探索竹节虫食道下神经节下降神经元对腿运动神经元活动的调节。这些5-6个未配对神经元（DUM神经元，背侧未配对中位）已在一些直脚类昆虫中被鉴定为章鱼胺能细胞。这些细胞的轴突向下穿过两个胸连接细胞，其功能尚不清楚。直到最近，在一些初步实验中，我们才发现其中一些神经元可以调节感觉-运动的相互作用。具体来说，单个DUM神经元的激活部分阻断了胫骨伸肌运动神经元的活动，该活动是由模仿各自关节屈曲的感觉器官的刺激引起的。我们试图证实这一发现，并研究这种调制的机制。当感觉神经元利用乙酰胆碱作为传递器时，胆碱信号传递的阻滞是可能的。我们还试图测试这种调节是否取决于动物的行为状态，即静止动物与行走动物。此外，我们想知道其他感觉器官的作用和紧张兴奋是否也被调节。

The contractions of the leg musculature in a walking animal are controlled by motoneurons. The activity of these motoneurons is determined by tonic depolarizing drive, phasic structuring input from leg sense organs and neurons in the CNS and by neuro-modulatory influences. In the stick insect, we wish to elucidate the mechanisms of the tonic excitation and the role of subesophageal neurons in the modulation of motoneuron activity.Application of the muscarinic acetylcholine agonist pilocarpine to the thoracic ganglia of the stick insect evokes a tonic excitation in motoneurons and activates oscillatory neural networks (CPGs). Inhibition by CPGs structures motoneuron activity. However, the exact place of the receptors where acetylcholine acts when it evokes the tonic excitation is not known. We wish to find out whether the tonic excitation is mediated by muscarinc receptors located on motoneurons or on premotor neurons. We know that the tonic excitation is modulated by the biogenic amine octopamine. The localization of the muscarinic receptors will help in unrevealing the mechanisms of the octopaminergic modulation.Octopamine, which is seen as the invertebrate counterpart of adrenalin, modulates a plethora of different functions in insects. For example, it modulates walking and flying behavior and the activity of muscles and sense organs. In stick insects octopamine blocks an inward current induced by activation of a nicotinc acetycholine receptor. However, so far it is not known which octopaminergic neurons modulate the activity of neurons in the thoracic ganglia in insects. Therefore, in the second part of our project we wish to explore in stick insects the modulation leg motoneuron activity by descending neurons in the subesophageal ganglion. These 5-6 unpaired neurons (DUM neurons, dorsal unpaired median) have been identified as octopaminergic cells in some orthopteran insects. The axons of these cells descend through both thoracic connectives their function is not known. Only recently, in some pilot experiments we could show that some of these neurons modulate sensory-motor interaction. Specifically, the activation of a single DUM neuron partially blocks the activity in an extensor tibiae motoneuron that is evoked by stimulation of a sense organ that mimics flexion of the respective joint. We attempt to substantiate this finding and investigate the mechanisms of this modulation. A block of cholingeric signal transmission is possible as sensory neurons utilize acetylcholine as a transmitter. We also attempt to test whether the modulation depends on the behavioral state of the animal, i.e. resting animal vs. walking animal. In addition, we would like to find out whether effects by other sense organs and the tonic excitation are also modulated.