Vasopressin- and proctolin-like immunoreactive efferent neurons in blowfly abdominal ganglia: development and ultrastructure

In the neural sheath of the fused thoracicoabdominal ganglia of the blowfly Calliphora erythrocephala, extensive neurohaemal areas can be seen in the electron microscope. A separate set of neurohaemal areas located in the sheath of the lateral abdominal nerve roots contain neural terminals of at least three morphological types. To determine which bioactive substances are stored and possibly released from the neurons supplying these neurohaemal areas, we applied a large number of antisera raised against different neuropeptides of invertebrate and mammalian type. Antisera to two types of neuropeptides react with neurons innervating the sheath of the abdominal nerve roots: antisera to lysine-vasopressin and proctolin. There are only 14-24 vasopressin-like immunoreactive (VPLI) neurons in the entire nervous system of Calliphora. These are all restricted to a bilateral cluster in the fused abdominal ganglia. From this cluster, the neurohaemal areas in abdominal nerve roots are supplied. Proctolin-like immunoreactivity (PLI) can be seen in a large number of neurons in the nervous system of blowflies. The supply of PLI terminals to the abdominal nerve roots is from 12 to 14 neurons in a bilateral cluster of abdominal PLI neurons. It is clear from light- and electron-microscopic immunocytochemistry that the two antisera label two separate populations of neurons that form overlapping terminals in the neural sheath. The immunoreactive terminals are located just below the permeable acellular basal lamina of the neural sheath. Hence, it is likely that at least two different bioactive peptides can be released neurohormonally into the circulation. An additional set of four efferent PLI neurons send axons into the medial abdominal nerve. These do not form neurohaemal terminals in the nerve root, but may innervate the hindgut. Also in the larval nervous system, VPLI and PLI neurons can be recognized. In the larva, the peptide-containing neurons are segmentally arranged. The 14 larval VPLI neurons supply segmental abdominal nerves with axons that run inside the nerves to their targets. During metamorphosis, the segmental nerves fuse and the VPLI axons invade the neural sheath where they arborize and form varicose terminals. About the same number of PLI neurons could be detected in the abdominal ganglia of larval and adult flies. Only for a set of four caudal PLI neurons could efferent axons be traced in the larva. These axons run inside the medial abdominal nerves. The same four PLI neurons, with the same axonal projections, can be recognized in the adults.