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Structural and Molecular Properties of Insect Type II Motor Axon Terminals.

ABSTRACT: A comparison between the axon terminals of octopaminergic efferent dorsal or ventral unpaired median neurons in either desert locusts (Schistocerca gregaria) or fruit flies (Drosophila melanogaster) across skeletal muscles reveals many similarities. In both species the octopaminergic axon forms beaded fibers where the boutons or varicosities form type II terminals in contrast to the neuromuscular junction (NMJ) or type I terminals. These type II terminals are immunopositive for both tyramine and octopamine and, in contrast to the type I terminals, which possess clear synaptic vesicles, only contain dense core vesicles. These dense core vesicles contain octopamine as shown by immunogold methods. With respect to the cytomatrix and active zone peptides the type II terminals exhibit active zone-like accumulations of the scaffold protein Bruchpilot (BRP) only sparsely in contrast to the many accumulations of BRP identifying active zones of NMJ type I terminals. In the fruit fly larva marked dynamic changes of octopaminergic fibers have been reported after short starvation which not only affects the formation of new branches ("synaptopods") but also affects the type I terminals or NMJs via octopamine-signaling (Koon et al., 2011). Our starvation experiments of Drosophila-larvae revealed a time-dependency of the formation of additional branches. Whereas after 2 h of starvation we find a decrease in "synaptopods", the increase is significant after 6 h of starvation. In addition, we provide evidence that the release of octopamine from dendritic and/or axonal type II terminals uses a similar synaptic machinery to glutamate release from type I terminals of excitatory motor neurons. Indeed, blocking this canonical synaptic release machinery via RNAi induced downregulation of BRP in neurons with type II terminals leads to flight performance deficits similar to those observed for octopamine mutants or flies lacking this class of neurons (Brembs et al., 2007).

SUBMITTER: Stocker B

PROVIDER: S-EPMC5867341 | biostudies-literature | 2018

REPOSITORIES: biostudies-literature

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Structural and Molecular Properties of Insect Type II Motor Axon Terminals.

Stocker Bettina B Bochow Christina C Damrau Christine C Mathejczyk Thomas T Wolfenberg Heike H Colomb Julien J Weber Claudia C Ramesh Niraja N Duch Carsten C Biserova Natalia M NM Sigrist Stephan S Pflüger Hans-Joachim HJ

Frontiers in systems neuroscience 20180319

A comparison between the axon terminals of octopaminergic efferent dorsal or ventral unpaired median neurons in either desert locusts (<i>Schistocerca gregaria</i>) or fruit flies (<i>Drosophila melanogaster</i>) across skeletal muscles reveals many similarities. In both species the octopaminergic axon forms beaded fibers where the boutons or varicosities form type II terminals in contrast to the neuromuscular junction (NMJ) or type I terminals. These type II terminals are immunopositive for bot ...[more]

PMID: 29615874

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Project description:Background Spinal motoneurons may become hyperexcitable after a stroke. Knowledge about motoneuron hyperexcitability remains clinically important as it may contribute to a number of phenomena including spasticity, flexion synergies, and abnormal limb postures. Hyperexcitability seems to occur more often in muscles that flex the wrist and fingers (forearm flexors) compared to other upper limb muscles. The cause of hyperexcitability remains uncertain but may involve plastic changes in motoneurons and their axons. Aim To characterize intrinsic membrane properties of flexor carpi radialis (FCR) motor axons after stroke using nerve excitability testing. Methods Nerve excitability testing using threshold tracking techniques was applied to characterize FCR motor axon properties in persons who suffered a first-time unilateral cortical/subcortical stroke 23 to 308 days earlier. The median nerve was stimulated at the elbow bilaterally in 16 male stroke subjects (51.4 ± 2.9 y) with compound muscle action potentials recorded from the FCR. Nineteen age-matched males (52.7 ± 2.4 y) were also tested to serve as controls. Results Axon parameters after stroke were consistent with bilateral hyperpolarization of the resting potential. Nonparetic and paretic side axons were modeled by a 2.6-fold increase in pump currents (IPumpNI) together with an increase (38%–33%) in internodal leak conductance (GLkI) and a decrease (23%–29%) in internodal H conductance (Ih) relative to control axons. A decrease (14%) in Na+ channel inactivation rate (Aah) was also needed to fit the paretic axon recovery cycle. “Fanning out” of threshold electrotonus and the resting I/V slope (stroke limbs combined) correlated with blood potassium [K+] (R = −0.61 to 0.62, p< 0.01) and disability (R = −0.58 to 0.55, p < 0.05), but not with spasticity, grip strength, or maximal FCR activity. Conclusion In contrast to our expectations, FCR axons were not hyperexcitable after stroke. Rather, FCR axons were found to be hyperpolarized bilaterally post stroke, and this was associated with disability and [K+]. Reduced FCR axon excitability may represent a kind of bilateral trans-synaptic homeostatic mechanism that acts to minimize motoneuron hyperexcitability.

Dataset Information

Structural and Molecular Properties of Insect Type II Motor Axon Terminals.

Publications

Structural and Molecular Properties of Insect Type II Motor Axon Terminals.

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