Cellular localization of the insect prothoracicotropic hormone: In vitro assay of a single neurosecretory cell

The distribution of prothoracicotropic hormone in the pupal brain of Manduca sexta has been determined by an in vitro assay for prothoracic gland activation. Prothoracicotropic activity was observed in both the brain and retrocerebral complex, but predominantly in the dorosolateral regions of the protocerebrum. Of the two groups of neurosecretory cells present in this area of the brain, only the two lateral type III neurosecretory cells exhibited significant prothoracicotropic hormone activity. Further analysis revealed that the neurohormone was localized in only one of the two type III cells, suggesting that a single neurosecretory cell in each hemisphere is the source of the hormone at the stage examined (day 0). Prothoracicotropic hormone activity was detected in both the corpora allata and the corpora cardiaca, but the corpora allata contained 2 to 9 times the activity of the corpora cardiaca, depending on developmental stage. The significantly higher level of activity in the corpora allata suggests that they may be the neurohemal organs through which the prothoracicotropic hormone of Manduca is released.