Abstract
BackgroundThe neuromuscular junction is the chemical synapse where motor neurons communicate with skeletal muscle fibers. Whereas vertebrates and many invertebrates use acetylcholine as transmitter at the neuromuscular junction, in those arthropods examined up to now, glutamate and GABA are used instead. With respect to taxon sampling in a phylogenetic context, there is, however, only a limited amount of data available, focusing mainly on crustaceans and hexapods, and neglecting other, arthropod groups. Here we investigate the neurotransmitter equipment of neuromuscular synapses of a myriapod, Lithobius forficatus, using immunofluorescence and histochemical staining methods.ResultsGlutamate and GABA could be found colocalised with synapsin in synaptic boutons of body wall and leg muscles of Lithobius forficatus. Acetylcholinesterase activity as a marker for cholinergic synapses was found abundantly in the central nervous system and also in some peripheral nerves, but not at neuromuscular junctions. Furthermore, a large number of leg sensory neurons displayed GABA-immunofluorescence and was also labeled with an antiserum against the GABA-synthesizing enzyme, glutamate decarboxylase.ConclusionsOur data indicate that glutamate and GABA are neurotransmitters at Lithobius forficatus neuromuscular junctions, whereas acetylcholine is very unlikely to play a role here. This is in line with the concept of glutamate as excitatory and GABA as the main inhibitory neuromuscular transmitters in euarthropods. Furthermore, we have, to our knowledge for the first time, localized GABA in euarthropod leg sensory neurons, indicating the possibility that neurotransmitter panel in arthropod sensory systems may be far more extensive than hitherto assumed.
Highlights
The neuromuscular junction is the chemical synapse where motor neurons communicate with skeletal muscle fibers
The cell body rind was not stained for acetylcholinesterase staining (AChE) or synapsin-IR
Cell bodies labeled for gamma-aminobutyric acid (GABA) (Fig. 2d) or the GABA synthetizing enzyme, glutamate decarboxylase (GAD, Fig. 2e) appeared to be more uniformly distributed within the cell body rind (Fig. 2d)
Summary
The neuromuscular junction is the chemical synapse where motor neurons communicate with skeletal muscle fibers. [1,2,3,4,5,6]) have demonstrated that glutamate is the most likely fast neurotransmitter at excitatory synapses onto skeletal muscles This is in contrast to the vertebrates and many other bilaterian invertebrates. Some muscle fibers in arthropods are innervated by excitatory synapses, in which glutamate is the Langeloh et al Frontiers in Zoology (2018) 15:28 neurotransmitter, and by inhibitory motor neurons, which use gamma-aminobutyric acid (GABA) [14,15,16,17]. On the input side of the arthropod nervous system and again, in contrast to the vertebrates, acetylcholine (ACh) seems to be the predominant classical transmitter in mechanosensory and chemosensory neurons of crustaceans [20,21,22,23] and hexapods [24,25,26,27,28,29]
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