Common and specific inhibitory motor neurons innervate the intersegmental muscles in the locust thorax

Gamma-aminobutyric acid in the honey bee mushroom bodies - is inhibition the wellspring of plasticity?

Structural plasticity is the hallmark of the protocerebral mushroom bodies of adult insects. This plasticity is especially well studied in social hymenopterans. In adult worker honey bees, phenomena such as increased neuropil volume, increased dendritic branching, and changes in the details of synaptic microcircuitry are associated with both the onset of foraging and the accumulation of foraging experience. Prior models of the drivers of these changes have focused on differences between the sensory environment of the hive and the world outside the hive, leading to enhanced excitatory (cholinergic) inputs to the intrinsic neurons of the mushroom bodies (Kenyon cells). This article proposes experimental and bioinformatics-based approaches for the exploration of a role for changes in the inhibitory (GABAergic) innervation of the mushroom bodies as a driver of sensitive periods for structural plasticity in the honey bee brain.

Regulation of coordinated muscular relaxation in Drosophila larvae by a pattern-regulating intersegmental circuit

Typical patterned movements in animals are achieved through combinations of contraction and delayed relaxation of groups of muscles. However, how intersegmentally coordinated patterns of muscular relaxation are regulated by the neural circuits remains poorly understood. Here, we identify Canon, a class of higher-order premotor interneurons, that regulates muscular relaxation during backward locomotion of Drosophila larvae. Canon neurons are cholinergic interneurons present in each abdominal neuromere and show wave-like activity during fictive backward locomotion. Optogenetic activation of Canon neurons induces relaxation of body wall muscles, whereas inhibition of these neurons disrupts timely muscle relaxation. Canon neurons provide excitatory outputs to inhibitory premotor interneurons. Canon neurons also connect with each other to form an intersegmental circuit and regulate their own wave-like activities. Thus, our results demonstrate how coordinated muscle relaxation can be realized by an intersegmental circuit that regulates its own patterned activity and sequentially terminates motor activities along the anterior-posterior axis.

Neuromuscular transmitter candidates of a centipede (Lithobius forficatus, Chilopoda)

Background

The 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.

Results

Glutamate 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.

Conclusions

Our 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.

Motor innervation pattern of labral muscles of Locusta migratoria

The current study investigates the motor innervation pattern of labral muscles in the adult locust and tries to interpret the results in the light of the hypothesis that the labrum phylogenetically developed by the fusion of paired appendages associated with the intercalary segment. Using Neurobiotin™ as a retrograde neuronal tracer, specific motor nerves or individual labral muscles were stained. Results show that the labral muscles receive innervation from tritocerebrum and suboesophageal ganglion. The axons of many motor neurons use three different pathways to cross the midline in the periphery to innervate ipsi- and contralateral muscles. Intracellular recordings from fibers of individual muscles and simultaneous recordings from motor neurons imply that the labral muscles lack inhibitory innervation. The location of motor neurons in both tritocerebrum and suboesophageal ganglion supports the notion that the labrum is innervated by the so-called intercalary segment. That many of the efferent axons cross the midline in the periphery might be explained by the hypothesis that the labrum derives from a fusion of appendages.

Inhibitory motoneurons in arthropod motor control: organisation, function, evolution

Miniaturisation of somatic cells in animals is limited, for reasons ranging from the accommodation of organelles to surface-to-volume ratio. Consequently, muscle and nerve cells vary in diameters by about two orders of magnitude, in animals covering 12 orders of magnitude in body mass. Small animals thus have to control their behaviour with few muscle fibres and neurons. Hexapod leg muscles, for instance, may consist of a single to a few 100 fibres, and they are controlled by one to, rarely, 19 motoneurons. A typical mammal has thousands of fibres per muscle supplied by hundreds of motoneurons for comparable behavioural performances. Arthopods—crustaceans, hexapods, spiders, and their kin—are on average much smaller than vertebrates, and they possess inhibitory motoneurons for a motor control strategy that allows a broad performance spectrum despite necessarily small cell numbers. This arthropod motor control strategy is reviewed from functional and evolutionary perspectives and its components are described with a focus on inhibitory motoneurons. Inhibitory motoneurons are particularly interesting for a number of reasons: evolutionary and phylogenetic comparison of functional specialisations, evolutionary and developmental origin and diversification, and muscle fibre recruitment strategies.