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

• Applicant: Privatdozent Dr. Joachim Schmidt
• Subject Area: Cognitive, Systems and Behavioural Neurobiology
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 263884427

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.

• DFG Programme: Research Grants
• Participating Person: Professor Dr. Ansgar Büschges