Modulation of leg motoneuron activity in the stick insect: The importance of acetylcholine and octopamine and the role of subesophageal descending neurons
Final Report Abstract
Calcium-imaging project. In earlier work we have shown that a tonic depolarisation of stick insect motor neurons during walking is Ca2+ dependent. Using Ca2+ imaging of OGB-1 backfilled mesothoracic retractor motor neurons we could show Ca2+ oscillations in the motor neurons in the presence of the muscarinic ACh agonist pilocarpine. Pilocarpine activates CPGs that control leg motor neurons activity. The oscillations were uniform in all regions of interest, which covered primary and secondary neurites. The oscillations persisted when the Na+-channel blocker QX 314 was backfilled into the motor neurons. Injection of QX 314 into motor neurons by intracellular electrodes confirmed that spike activity is blocked in motor neurons whereas membrane oscillations persisted. These results indicate that low threshold Ca2+ currents might contribute to the Ca2+ oscillations in pilocarpine. In addition to motor neurons, 4-5 local efferent octopaminergic DUM neurons were backfilled with the Calcium-indicator OGB-1. The DUM neurons exhibited Ca2+ oscillations with much longer periods than observed in motor neurons. A DUM neuron’s oscillations were not coupled to retractor motor neuron activity and usually not to oscillations in the other DUM neurons. Because the activities in efferent stick insect DUM neurons that innervate muscles in the same leg known to be coupled to a stepping leg’s stance we hypothesize that CPGs that drive different DUM neurons are synchronized during walking by input from leg sense organs. Descending dorsal unpaired median neurons (desDUM neurons). In earlier work we have shown that stepping evoked activity of motor neurons is increased in the presence of octopamine and that blocking octopamine receptors in thoracic ganglia decreases leg motor neuron activity during stepping. We now identified octopaminergic neurons that modulate leg motor neuron activity. Using MALDI-TOF mass spectrometry we could show that 5-6 desDUM neurons in the gnathal ganglion contain octopamine (OA). Axons of desDUM neurons project into the thoracic ganglia and thus provide a potential source of OA. DesDUM neurons receive excitatory input from tactile stimuli that can elicit walking and they receive excitatory input during walking that is coupled to a leg’s stance phase. Our data indicate that leg campaniform sensilla (strain receptors) are a likely source for providing the respective information. In comparison, the input of the fCO (femoral chordotonal organ; movement sensor) is minute. CPGs in the mesothoracic ganglion do not affect desDUM neuron activity. DesDUM neuron activity increases spike activity evoked by stimulation of campaniform sensilla in excitatory retractor motor neurons and in the inhibitory motor neuron CI1. We could also show that desDUM neurons either increased od weakend spike activity evoked by stimulation of the fCO in extensor tibiae motor neurons. Increased motor neuron responses were accompanied by an increased likelihood of reversed reflexes responses in extensor tibiae motor neurons as can be observed in active animals.
Publications
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(2019) Descending octopaminergic neurons modulate sensory-evoked activity of thoracic motor neurons in stick insects. Journal of neurophysiology 122 (6) 2388–2413
Stolz, Thomas; Diesner, Max; Neupert, Susanne; Hess, Martin E.; Delgado-Betancourt, Estefania; Pflüger, Hans-Joachim; Schmidt, Joachim
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(2015) Subesophageal modulation of thoracic motor activity in the stick insect. 11th Göttingen Meeting of the German Neuroscience Society
Stolz T., Schmidt J.
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(2016) Descending modulation of motor activity in the stick insect. Janelia Farm Conference: Motor Control Circuits
Stolz T, Diesner M, Neupert S, Schmidt J
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(2016) Descending modulation of thoracic motor activity in the stick insect. Janelia Farm Conference: Motor Control Circuits. Annual meeting of the Society for Neuroscience
Stolz T, Diesner M, Neupert S, Schmidt J
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(2017) Descending modulation of thoracic motor activity in the stick insect. 12th Göttingen Meeting of the German Neuroscience Society
Stolz T, Diesner M, Neupert S, Schmidt J
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(2018). Calcium imaging of CPG-evoked activity in efferent neurons of the stick insect. PLoS ONE 13(8):e0202822
Goldammer J, Mantziaris C, Büschges A, and Schmidt J
