Immunohistochemical Localization Of 5-HT And Its Synthesizing Enzyme TPH In The Brain Of The Beetle Harmonia Axyridis

Applied rabbit anti-5-HT and mouse monoclonal anti-TPH antisera, the localization of5-HT and its synthesizing enzyme TPH in the visual system, midbrain, and suboesophagealganglion of the beetle Harmonia axyridis (Coleoptera: Coccinellidae) was studied byStreptavidin-Peroxidase immunohistochemical method on Colophony-Paraffin embeddedserial sections. An overall observation reveals that the brain structure of the beetle is relativelydistinct. The optic lobe is developed, and there exists lobula plate;the optic tubercle iscomplex. The glomeruli structure of antennal lobe is not classical. Accordingly, the mushroombody calyces have meager, and the lobes are relatively simple either. The central complex isprominent. The beetle showed notable 5-HT-like immunoreactivity throughout the wholebrain, which was also strongly labelled by TPH antibody, with the pattern matching that of5-HT. 5-HT-like positive fibers were resolved in all areas of the brain, among which thelamina and suboesophageal ganglion showed the most remarkable dark brownimmunostaining. While, all major neuropil regions, in general, showed low level backgroundafter TPH immunostaining for TPH mainly existing in cytoplasm. The number of 5-HT-likeimmunoreactive neurons was relatively small but possessed extensive varicose arborizations.Most of 5-HT-like immunoreactive neurons were tangential neurons and bilaterallysymmetrical distributed. The neurocyte type, number, distribution and projection of TPH-IRneurons were similar to that of 5-HT. In the visual system, 5-HT/TPH-like immunoreactiveneurons (ca.35 per hemisphere) were mainly situated between the optic lobe and lateralprotocerebrum, the neurites of which either descended into the optic neuropils or ascendedinto the protocerebrum, thus possibly integrating the optic lobes and other brain regions. Inthe protocerebrum, 5-HT/TPH-like immunoreactive neurons (ca. 25) mainly distributed dorsalto the protocerebrum beside the Kenyon cells. Their axons directly projected into theprotocerebrum as well as the optic lobe. Only 1 single 5-HT/TPH-like immunoreactiveinterneuron was localized near the dorsal antennal lobe, contributing to mostimmunoreactivity in the antennal lobe. In the suboesophageal ganglion, 5-HT/TPH-likeimmunoreactive neurons (ca.15) mainly distributed in the lateral and ventro-lateral area, withprocesses projecting into the suboesophageal ganglion neuropil. The collective resultsindicated that 5-HT plays an important role in the visual information processing andmodulates diurnal rhythm mainly via the distant and extensive pattern. In some specific areas,5-HT is colocalized and may serve as cotransmitters with GABA. In addition, 5-HT content inthe visual system is probably closely related to the physiological accommodation of insectlight-dark adaptation. Besides, in the calyxless condition, the mushroom body lobes of thebeetle probably exert the function of receiving both of the optic and olfactory information.Prominently, specific TPH may be present in the brain of the beetle, responsible for thesynthesis of central 5-HT. The positive TPH signals in the brain of the beetle Harmoniaaxyridis is also likely to reveal the phylogenetic relationships of aromatic amino acidhydroxylase superfamily that TPH-PAH duplication occurred at least before the emergence ofarthropods.