Immunohistochemical Localization Of 5-HT And Its Synthesizing Enzyme TPH In The Brain And Suboesophageal Ganglion Of The Three Beetles

The structure and 5-HT immunoreactivity in the visual system, midbrain, and suboesophageal ganglion of three beetles, Ambrostoma quadriimpressum, Henosepilachna vigintioctomaculata and Oxycetonia jucunda, were first studied by means of colophony-paraffin embedding serial section technique and streptevidin-peroxidase immunohistochemical method. The results showed that the brains of these three taxonomically closely related beetles were remarkably different in composition and size. Mushroom bodies and antennal lobes in Oxycetonia jucunda were conspicuous. Calyces and lobes of the mushroom bodies were much developed. In contrast, calyces of Ambrostoma quadriimpressum and Henosepilachna vigintioctomaculata were extremely undeveloped. However, the postretinal fibres and circumpharyngeal nerves of Ambrostoma quadriimpressum were highly developed. In the three beetles, 5-HT immunoreactivity was present in all neuropils of the brain and the suboesophageal ganglion. The number of 5-HT immunoreactive neurons was relatively small but possessed extensive varicose arborizations. Most of 5-HT immunoreactive neurons were tangential neurons and bilaterally symmetrical distributed. The pattern of 5-HT immunoreactivity and the localization of immunoreactive somata which often clustered into groups were similar among these beetles, while the immunoreactivity intensity was distinct, especially in the lamina and suboesophageal. These groups were mainly situated between the medulla and the procerebrum over the anterior surface of the lobulla (group G1), ventral to the lobulla (group G2), in the dorsal area of the lobulla (group G3), in the antero-ventral edge of the medulla(group G4), in the posterior cortex of the calyces(group G5), the dorso-lateral cortex of the antennal lobe(group G6) and anterior-ventral to suboesophageal ganglion(group G7). The results suggested that the three beetles had given rise to adaptive radiation under the evolutionary pressure because of the long-term different life styles and living environments in which the taxonomic status of Ambrostoma quadriimpressum was relatively low. The similarity of the pattern of 5-HT immunoreactivity and localization of some positive somata among the three beetles raised the possibility that 5-HT seemed to serve similar physiological function in different insects. Furthermore, 5-HT might be involved in modulating the ingestion by regulating muscular activity and visual sensitivity.