Neuropeptide regulators of juvenile hormone synthesis: structures, functions, distribution, and unanswered questions

Juvenile hormones (JH), produced by the corpora allata, have an essential role in growth and development, morphogenesis, and reproductive processes of insects. The output of JH and circulating titer are required to be precisely regulated throughout the insect's life in response to developmental requirements and environmental factors. The synthesis of JH must be periodically turned off and on, or finely tuned, in a highly coordinated way. Except for a few key or intensely studied insect species, the control of synthesis of JH by regulatory peptides remains largely undefined and many of the details remain obscure. Several different classes of neuropeptide are believed to be involved in the regulation of corpus allatum function and hence JH output. In different insect species and at different stages of development, these regulatory peptides may include at least three types of inhibitory allatostatins, at least one type of stimulatory allatotropin, and perhaps several other, as yet largely undefined, additional neuropeptides. The details of how each of these peptides acts to affect JH production and their relationship to each other in the coordination of JH synthesis remain to be established. There are several insect orders for which almost nothing is known concerning the regulation of JH synthesis and the peptides that might be involved. Current proteomic and genomic studies are helping to redress this balance but at the same time posing new questions. Other neuropeptides are implicated in the regulation of JH production, and there is new evidence concerning the mode of action of allatotropins.