Tyrosine hydroxylase-like immunoreactivity in the brain of fifth instar Rhodnius prolixus Stål (Hemiptera: Reduviidae)

Cytoarchitecture of histamine-, dopamine-, serotonin- and octopamine-containing neurons in the cricket ventral nerve cord

The present article provides a comparative neuroanatomical description of the cellular localization of the biogenic amines histamine, dopamine, serotonin and octopamine in the ventral nerve cord of an insect, namely the cricket, Gryllus bimaculatus. Generally, different immunocytochemical staining techniques reveal a small number of segmentally distributed immunoreactive (-IR) amine-containing neurons allowing single cell reconstruction of prominent elements. Aminergic neurons share common morphological features in that they innervate large portions of neurophil and often connect different neuromeres by intersegmental 'wide-field' projections of varicose appearance. In many cases aminergic terminals are also found on the surface of peripheral nerves suggesting additional neurohemal release sites. Despite such morphological similarities histological analysis demonstrates for any given amine functionally distinct neuron types with specific innervation patterns establishing discrete pathways. Histamine-IR interneurons are characterized by both ascending and descending projections forming central and peripheral terminals. The descending branches from dopamine-IR cells mainly converge within the terminal ganglion, whereas serotonin-IR interneurons with ascending projections often terminate within the brain. Serotonin is also present in sensory and motor neurons. In contrast to other aminergic neurons, most octopamine-IR cells represent unpaired neurons projecting through motor nerves of the soma-containing neuromere. Octopamine-IR cells with intersegmental branches are only rarely found. Based on these findings, a colocalization of different amines within the same neuron seems to be unlikely to occur in the cricket ventral nerve cord. With respect to the neuroanatomical description of amine-containing neurons known physiological effects of biogenic amines and their possible neuromodulatory functions in insects are discussed.

Embryonic and postembryonic development of serial homologous neurons in the subesophageal ganglion of Tenebrio molitor (Insecta: Coleoptera)

Neuroblast pattern, engrailed expression and proliferation in the subesophageal neuromers of the beetle Tenebrio molitor are characterized throughout embryogenesis. The proliferation of neuroblasts has been studied throughout postembyronic development. Serotonin, crustacean cardioactive peptide and tyrosine hydroxylase-like-immunoreactive neurons are characterized and their neuronal development has been studied. There is an initial posterior-anterior gradient in neuroblast segregation leading to a reduced number of neuroblasts in the frontal subesophageal neuromer. The study of the engrailed expression shows that only the anterior subfraction of the neuromeral neuroblast configuration is reduced, whereas the posterior two rows of engrailed-positive neuroblasts are not affected during the first 40% of embryogenesis. The overall number of proliferations in the first subesophageal neuromer reaches only 30-50% of the value found in each of the other two neuromers. The analysis of serotonin and crustacean cardioactive peptide immunoreactivity allows the identification of serial homologous neurons which persist from the early embryo to the adult stage. In the different gnathal neuromers, these neurons form structurally highly similar projection patterns, but show different extensions of their arborizations, corresponding to the relative size of each neuromer. Structural homologies between subesophageal and thoracic neuromers are discussed.