Immunohistochemical Localization Of 5-HT And Its Synthesizing Enzyme TPH In The Brain Of The Last Stadium Larvae (V/6) Of Antheraea Pernyi

Antheraea pernyi, a species that undergoes complete metamorphosis, is a classic model for studying the changes of insect brain system during individual development. Antheraea pernyi has five larvae stages. Larvae on day 6 of the fifth (V/6; last) stadium tend to stay still, physiologically ready to enter the pupae stage. By means of colophony-paraffin embedding serial section technique and streptevidin-peroxidase immunohistochemical method, the brain structure of V/6 larvae and the immunoreactivity pattern of 5-HT and its rate-limiting enzyme, tryptophan hydroxylase, had been studied. The results showed that the brain of last stadium larvae was consisted of main neuropils such as optic anlage, mushroom body, central complex, lateral accessory lobe, larval antennal center and tritocerebrum, which was similar to that of the imago as revealed by earlier research, while there were several basic differences besides the size of brain: the brain of larvae rotated posteriorly about 90°, causing the calyces of the mushroom body changing from opening toward the dorsal surface of the brain to opening toward the posterior surface, and theα-lobe from extending frontally to dorsally. There was no differentiation of the upper central body and lower central body in the larval brain, and most areas of the optic center were not developed and the major neuropils such as lamina, medulla and lobulla could not be observed. There were relatively few 5-HT immunoreactive neurons in the larval brain, which were clustered into 5 groups. These groups were located in the larval optic center (group PDL), in the media protocerebrum (group PM), anterior to protocerebrum (group PA), anterior to tritocerebrum (group TA) and lateral to tritocerebrum (group TL). It is interesting that these 5-HT immunoreactive neurons were also present TPH immunoreactivity, while there was little different between 5-HT and TPH immunoreactive pattern in the whole brain neuropils. Compared with the results of adult brain structure, 5-HT and TPH immunoreactivity, these results raised the possibility that pupae stage was the key stadium for restructuring larval brain and 5-HT neurons; 5-HT neurons seemed to be conserved between the stages of the larval brain development; as a classic neurotransmitter, 5-HT might be involved in modulating the visual sensitivity and courtship behavior.