Descending Octopaminergic Neurons in the Stick Insect: Their Inputs and their Output Effects on the Locomotor System

The neural networks controlling locomotion (walking) must exhibit a high degree of flexibility for task-specific adaptation of behavior to environmental influences and changes in internal state. Neuromodulatory influences are very important for this flexibility, as they can regulate the activity of all neurons in the walking system and the strengths of their synaptic connections. To fully understand the neural control of walking, it is crucial to identify the neurons that release neuromodulators and to determine their activity patterns during behavior and analyze the properties of their output effects. Octopamine, one such neuromodulator, is considered the invertebrate homolog to the vertebrate noradrenaline. It is a significant modulator in insect locomotor systems, both acting in the peripheral and central nervous systems. Octopamine modulates muscle metabolism, neuromuscular transmission, sensory sensitivity, excitability of motor neurons, and activity in the central pattern generating networks that control locomotion. The neural source of octopamine acting in the central nervous system of insect thoracic segments has not yet been identified. Thus, it is unknown to what extent effects of application of octopamine to thoracic ganglia in previous studies reflect the physiological role of octopamine. In the current thesis, I hypothesized that dorsal unpaired median neurons with bilaterally descending axons (desDUM neurons) are a source of octopaminergic modulation of activity in thoracic neural networks for the control of walking in the stick insect Carausius morosus. I revealed the morphology of desDUM neurons in the gnathal ganglion by intracellular staining. Employing the newly developed method of direct MALDI-TOF mass spectrometry, I could show that stick insect desDUM neurons are octopaminergic. Using semi-intact preparations and intracellular recordings of desDUM neurons, I found that they are phasically activated during six-legged walking and single-leg stepping. The phasic excitatory input to desDUM neurons during walking does not arise from coupling to activity of mesothoracic central pattern generating networks, but most likely from activation of mechanosensory organs of all six legs. Passive leg movement and stimulation of mesothoracic campaniform sensilla excited desDUM neurons. Furthermore, stimulation of the mesothoracic femoral chordotonal organ (fCO) had a weak excitatory influence on their activity. Further, I investigated the output effects of desDUM neurons on reflex-evoked, spontaneous, and centrally generated activity of mesothoracic motor neurons with activation of single desDUM neurons. I could show that distinct desDUM neurons mediate differential effects. Some neurons induce a decrease and others an increase, in the magnitude of reflex-induced motor neuron activity. The neurons which mediated an excitatory influence additionally increased the frequency of reversal of an fCO-induced postural reflex. Some desDUM neurons mediated an increase in the frequency of centrally generated rhythmic motor neuron activity. Collectively, the results of the current thesis provide a comprehensive characterization of desDUM neurons and their complex roles in the stick insect locomotor system. The identity of direct neural target structures for the modulatory action of desDUM neurons, as well as the net output effects of the entire population of desDUM neurons during walking remain open questions. In future experiments, genetic access to desDUM neurons could aid in the activation, silencing, or visualization of their activity, which would collectively contribute to comprehensive answers to the open questions.