The
prothoracicotropic hormone is an insect
neuropeptide released into the hemolymph to signal molting and metamorphosis through its stimulation of steroidogenesis. The only known source of the
prothoracicotropic hormone in the tobacco hornworm, Manduca sexta, has been a group of lateral cerebral neurosecretory cells, the L-NSC III. In this study, the developmental and spatial distribution of the
prothoracicotropic hormone was examined throughout the life cycle of Manduca. In common with many vertebrates and invertebrates in which
neuropeptides are located in several regions within the central nervous system (CNS), the
prothoracicotropic hormone phenotype in Manduca is expressed by CNS neurons in addition to the L-NSC III. These neurons are located in the brain, frontal
ganglion, and subesophageal
ganglion. One cerebral neurosecretory cell group, the ventromedial neurons, expresses the
prothoracicotropic hormone phenotype and the behavioral
neurohormone,
eclosion hormone. Whereas the L-NSC III and the ventromedial neurons express the
peptide phenotype throughout the life cycle, the other neurons express the
peptide only during the embryonic and larval stages. This precise spatial and temporal expression of the
prothoracicotropic hormone by different groups of neurosecretory cells raises the possibility that in Manduca the
peptide may, in addition to its known neuroendocrine function, play other physiological roles in different ways at different stages of the life cycle.