Peter-Michael Bräunig

Morphological and molecular data argue for the labrum being non-apical, articulated, and the appendage of the intercalary segment in the locust

Our analysis of head segmentation in the locust embryo reveals that the labrum is not apical as often interpreted but constitutes the topologically fused appendicular pair of appendages of the third head metamere. Using molecular, immunocytochemical and retrograde axonal staining methods we show that this metamere, the intercalary segment, is innervated by the third brain neuromere-the tritocerebrum. Evidence for the appendicular nature of the labrum is firstly, the presence of an engrailed stripe within its posterior epithelium as is typical of all appendages in the early embryo. Secondly, the labrum is innervated by a segmental nerve originating from the third brain neuromere (the tritocerebrum). Immunocytochemical staining with Lazarillo and horseradish peroxidase antibodies reveal that sensory neurons on the labrum contribute to the segmental (tritocerebral) nerve via the labral nerve in the same way as for the appendages immediately anterior (antenna) and posterior (mandible) on the head. All but one of the adult and embryonic motoneurons innervating the muscles of the labrum have their cell bodies and dendrites located completely within the tritocerebral neuromere and putatively derive from engrailed expressing tritocerebral neuroblasts. Molecular evidence (repo) suggests the labrum is not only appendicular but also articulated, comprising two jointed elements homologous to the coxa and trochanter of the leg.

A simple fluorescent double staining method for distinguishing neuronal from non-neuronal cells in the insect central nervous system.

Being able to discriminate between neurons and non-neuronal cells such as glia and tracheal cells has been a major problem in insect neuroscience, because glia-specific antisera are available for only a small number of species such as Drosophila melanogaster and Manduca sexta. Especially developmental or comparative studies often require an estimate of neuron numbers. Since neuronal and glial cell bodies are in many cases indiscernible in situ, a method to distinguish neurons from non-neuronal cells that works in any given species is wanting. Another application is cell culturing. Cultured cells usually change their outward shape dramatically after being isolated so that it is frequently impossible to tell neurons and glia apart. Here, we present a simple method that uses a commercially available antiserum directed against horseradish peroxidase, which specifically stains neurons but no other cell type in every insect species investigated. Counterstaining with DAPI, a fluorescent chromophore that binds to double-stranded DNA in the nuclei of all cells, yields the total number of cells in a given sample. Thus, double labeled cells can be identified as neurons, cells that carry only DAPI staining are non-neuronal.

Actions of motor neurons and leg muscles in jumping by planthopper insects (hemiptera, issidae)

To understand the catapult mechanism that propels jumping in a planthopper insect, the innervation and action of key muscles were analyzed. The large trochanteral depressor muscle, M133b,c, is innervated by two motor neurons and by two dorsal unpaired median (DUM) neurons, all with axons in N3C. A smaller depressor muscle, M133a, is innervated by two neurons, one with a large‐diameter cell body, a large, blind‐ending dendrite, and a giant ovoid, axon measuring 50 μm by 30 μm in nerve N5A. The trochanteral levator muscles (M132) and (M131) are innervated by N4 and N3B, respectively. The actions of these muscles in a restrained jump were divisible into a three‐phase pattern. First, both hind legs were moved into a cocked position by high‐frequency bursts of spikes in the levator muscles lasting about 0.5 seconds. Second, and once both legs were cocked, M133b,c received a long continuous sequence of motor spikes, but the two levators spiked only sporadically. The spikes in the two motor neurons to M133b,c on one side were closely coupled to each other and to the spikes on the other side. If one hind leg was cocked then the spikes only occurred in motor neurons to that side. The final phase was the jump movement itself, which occurred when the depressor spikes ceased and which lasted 1 ms. Muscles 133b,c activated synchronously on both sides, are responsible for generating the power, and M133a and its giant neuron may play a role in triggering the release of a jump. J. Comp. Neurol. 518:1349–1369, 2010.

Structure of identified neurons innervating the lateral cardiac nerve cords in the migratory locust, Locusta migratoria migratorioides (Reiche and Fairmaire) (Orthoptera, Acrididae)

Abstract Our knowledge about the morphology of neurons innervating the lateral cardiac nerve cords (LCNCs) in migratory locusts, Locusta migratoria migratorioides (R. and F.) (Orthoptera, Acrididae) has increased considerably during recent years, mainly owing to immunocytochemical studies using antisera directed against members of insect neuropeptide families. In principle, there are three morphological types of neurons located within the CNS, which innervate the LCNCs in locusts: abdominal ganglia contain (i) bilaterally projecting, possibly unpaired neurons (BPNs) and (ii) paired, unilaterally projecting neurons. In addition, (iii) the LCNCs receive innervation from a pair of neurons, which is located within the suboesophageal ganglion. The axons of all three types of neurons project into the LCNCs via the segmental heart nerves, the most distal extensions of the dorsal segemental nerves of abdominal ganglia. When estimating the number of axons contained in one segmental heart nerve and formed by all central neurons so far identified, this number exceeds the number of axon profiles previously seen using the electron microscope. This indicates that most, or perhaps all central neurons projecting into the LCNCs, have been identified in these insects.

Neuronal connections between central and enteric nervous system in the locust, Locusta migratoria.

The number and location of neurons, in the central nervous system, that project into the frontal connective was studied in the locust by using retrograde neurobiotin staining. Staining one frontal connective revealed some 70 neurons in the brain. Most of these were located within both tritocerebral lobes. Additional groups of neurons were located within the deutocerebrum and protocerebrum. Some 60 neurons were labelled in the suboesophageal ganglion. These formed nine discernable populations. In addition, two neurons were located in the prothoracic ganglion and two neurons in the first abdominal neuromere of the metathoracic ganglion. Thus, some 250 neurons located within the head ganglia, and even neurons in thoracic ganglia, project into the ganglia of the enteric nervous system. This indicates that the coordination between the central and enteric ganglia is much more complex than previously thought. With the exception of some previously described dorsal unpaired median neurons and a few motor neurons in the head ganglia, the identity and function of most of these neurons is as yet unknown. Possible functions of the neurons in the thoracic ganglia are discussed.

Internal receptors in insect appendages project directly into a special brain neuropile

BackgroundThe great majority of afferent neurons of insect legs project into their segmental ganglion. Intersegmental projections are rare and are only formed by sense organs associated with the basal joints of the legs. Such intersegmental projections never ascend as far as the brain and they form extensive ramifications within thoracic ganglia. A few afferents of chordotonal organs of the subcoxal joints ascend as far as the suboesophageal ganglion.ResultsWe describe novel afferent neurons in distal segments of locust legs that project directly into the brain without forming ramifications in other ganglia. In the brain, the fibres terminate with characteristic terminals in a small neuropile previously named the superficial ventral inferior protocerebrum. The somata of these neurons are located in the tibiae and tarsi of all legs and they are located within branches of peripheral nerves, or closely associated with such branches. They are not associated with any accessory structures such as tendons or connective tissue strands as typical for insect internal mechanoreceptors such as chordotonal organs or stretch receptors. Morphologically they show great similarity to certain insect infrared receptors.We could not observe projections into the superficial ventral inferior protocerebrum after staining mandibular or labial nerves, but we confirm previous studies that showed projections into the same brain neuropile after staining maxillary and antennal nerves, indicating that most likely similar neurons are present in these appendages also.ConclusionBecause of their location deep within the lumen of appendages the function of these neurons as infrared receptors is unlikely. Their projection pattern and other morphological features indicate that the neurons convey information about an internal physiological parameter directly into a special brain neuropile. We discuss their possible function as thermoreceptors.

Neurons without dendrites? – A novel type of neurosecretory cell in locusts

Small-diameter nerves were found that are associated with the lateral peripheral nerves of the unfused abdominal ganglia of locusts. Such small nerves were observed in about 30% of all cases in Locusta migratoria, more than 60% in Schistocerca gregaria. Retrograde staining of these small nerves showed two somata in the posterior, lateral, and ventral region of an abdominal ganglion. These cells give rise to the small nerves that accompany the big lateral nerves and, on their surface, form putative neurohaemal release sites. Astonishingly the cells do not form any dendritic ramifications within the neuropile of the ganglia.