Different neuroendocrine cell types in the pars intercerebralis of Periplaneta americana produce their own specific IGF-related peptides

Of the nine genes of the American cockroach, Periplaneta americana, coding for peptides related to insulin and insulin-like growth factor, seven show significant expression in the central nervous system as demonstrated by the polymerase chain reaction on reverse transcribed RNA. In situ hybridisation shows that five of those are expressed by cells in the pars intercerebralis. Antisera raised to the predicted peptides show that these cells are neuroendocrine in nature and project to the corpora cardiaca. Interestingly, and in contrast to Drosophila where a single cell type expresses a number of genes expressing several such peptides, there are at least three cell types that each express different genes. Whereas in Drosophila the neuroendocrine cells producing insulin-like peptides also express sulfakinins, the arthropod orthologs of gastrin and cholecystokinin, in Periplaneta the sulfakinins are produced by different cells. Other neuropeptides known to be produced by the pars intercerebralis in this and other insect species, such as the CRF-like diuretic hormone, neuroparsin, leucokinin or myosuppressin, neither colocalize with an insulin-related peptide. The differential expression of these peptides and the existence of multiple insulin receptors in this species implies a more complex regulation by insulin and IGF-related peptides in cockroaches than in the fruit fly.