Inferior Protocerebrum Research Articles

Neuroanatomy and functional morphology of peripheral receptor neurones with direct projections into the protocerebrum of the brains of the locust and a jewel beetle

AbstractProjections from peripheral receptors directly into the protocerebrum of insects have only been little studied. Retrograde staining of nerves from the antennae, maxillary palps and legs has revealed some fibres that project into the central areas of the protocerebrum. In the case of the antennae and palps, it was not known which receptors were responsible for these projections. In the legs of locusts, multipolar neurons (MN) with characteristic terminal dendritic masses (TDM) have been described to project into a neuropil called “superior ventral inferior protocerebrum” (SVIP). However, such neurons have only been found in the abdominal infrared organs of the Australian fire beetle Merimna atrata, where they function as thermoreceptors. In several orthopterans, fibres from the antennae and palps also project into the SVIP. The present work suggests that the multipolar neuron from the infrared organ of Merimna also projects into the protocerebrum, possibly into a ventral region functionally analogous to the SVIP. No MNs but single scolopidia were found in the tips of the antennae and palps of locusts, apparently responsible for projections into the SVIP, where they probably function as receptors for haemolymph pressure.

Odd‐skipped labels a group of distinct neurons associated with the mushroom body and optic lobe in the adult Drosophila brain

Olfactory processing has been intensively studied in Drosophila melanogaster. However, we still know little about the descending neural pathways from the higher order processing centers and how these connect with other neural circuits. Here we describe, in detail, the adult projections patterns that arise from a cluster of 78 neurons, defined by the expression of the Odd-skipped transcription factor. We term these neurons Odd neurons. By using expression of genetically encoded axonal and dendritic markers, we show that a subset of the Odd neurons projects dendrites into the calyx of the mushroom body (MB) and axons into the inferior protocerebrum. We exclude the possibility that the Odd neurons are part of the well-known Kenyon cells whose projections form the MB and conclude that the Odd neurons belong to a previously not described class of extrinsic MB neurons. In addition, three of the Odd neurons project into the lobula plate of the optic lobe, and two of these cells extend axons ipsi- and contralaterally in the brain. Anatomically, these cells do not resemble any previously described lobula plate tangential cells (LPTCs) in Drosophila. We show that the Odd neurons are predominantly cholinergic but also include a small number of γ-aminobutyric acid (GABA)ergic neurons. Finally, we provide evidence that the Odd neurons are a hemilineage, suggesting they are born from a defined set of neuroblasts. Our anatomical analysis hints at the possibility that subgroups of Odd neurons could be involved in olfactory and visual processing.

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.

Comparative anatomy of pigment-dispersing hormone-immunoreactive neurons in the brain of orthopteroid insects

In a comparative study, the anatomy of neurons immunoreactive with an antiserum against the crustacean β-pigment-dispersing hormone was investigated in the brain of several orthopteroid insects including locusts, crickets, a cockroach, and a phasmid. In all species studied, three groups of neurons with somata in the optic lobes show pigment-dispersing hormone-like immunoreactivity. Additionally, in most species, the tritocerebrum exhibits weak immunoreactive staining originating from ascending fibers, tritocerebral cells, or neurons in the inferior protocerebrum. Two of the three cell groups in the optic lobe have somata at the dorsal and ventral posterior edge of the lamina. These neurons have dense ramifications in the lamina with processes extending into the first optic chiasma and into distal layers of the medulla. Pigment-dispersing hormone-immunoreactive neurons of the third group have somata near the anterior proximal margin of the medulla. These neurons were reconstructed in Schistocerca gregaria, Locusta migratoria, Teleogryllus commodus, Periplaneta americana, and Extatosoma tiaratum. The neurons have wide and divergent arborizations in the medulla, in the lamina, and in several regions of the midbrain, including the superior and inferior lateral protocerebrum and areas between the pedunculi and α-lobes of the mushroom bodies. Species-specific differences were found in this third cell group with regard to the number of immunoreactive cells, midbrain arborizations, and contralateral projections, which are especially prominent in the cockroach and virtually absent in crickets. The unusual branching patterns and the special neurochemical phenotype suggest a particular physiological role of these neurons. Their possible function as circadian pacemakers is discussed.

Neuroarchitecture of the central complex in the brain of the locust Schistocerca gregaria and S. americana as revealed by serotonin immunocytochemistry

The central complex is a prominent structure in the insect brain, yet its anatomical organization and functional role is still poorly understood. To facilitate investigations on the physiology of the central complex, this study describes its anatomical organization in the brain of locusts (Schistocerca gregaria and Schistocerca americana) based on an investigation of serotonin immunocytochemistry. Most subdivisions of the central complex including the protocerebral bridge, several layers in the upper division of the central body, and the noduli of the central body are innervated by serotonin-immunoreactive neurons, while the lower division of the central body does not exhibit serotonin-like immunoreactivity. Several types of serotonin-immunoreactive neurons can be distinguished. A system of about 60 columnar neurons innervates the protocerebral bridge, layer III of the upper division of the central body, and the noduli. A group of 15-20 bilateral pairs of serotonin-immunoreactive neurons connects the posterior optic tubercles with the protocerebral bridge. About ten pairs of neurons with somata in the inferior protocerebrum innervate layer Ia of the upper division of the central body. In addition, large-field neurons arborize in layers Ia and Ib of the upper division of the central body and in the lateral accessory lobes. The detailed mapping of serotonin immunoreactivity provides further insight into the anatomical organization of the central complex and suggests that serotonin is a major neuroactive substance within this brain structure.