Morphological Characterization of Neurons Projecting to the Ring Gland in the Larval Blow Fly, Protophormia terraenovae

The cephalic neuroendocrine system of the larval blow fly, Protophormia terraenovae was studied by backfills using either horseradish peroxidase or NiCl2, and peptide immunocytochemistry. Backfills through a proximal part of the ring gland mainly revealed three groups of neurons: (1) neurons with somata in the pars intercerebralis (PI) of the protocerebrum, (2) neurons with somata in the pars lateralis (PL) of the protocerebrum, and (3) neurons with somata in the subesophageal ganglion (SEG). Dense arborization was found mainly in the superior protocerebral, tritocerebral and SEG neuropils. Backfills through a distal part of the ring gland exclusively revealed two types of neurons with somata in the PL, viz., those with ipsilateral projections and those with contralateral projections to the ring gland. Antisera against cholecystokinin-8, FMRFamide, and Gryllus bimaculatus pigment-dispersing factor labeled cells in the PI and PL as well as fibers in the ring gland and aorta. Anti-cholecystokinin-8 and anti-FMRFamide antisera also labeled cells in the SEG. These results suggest that cephalic neurons projecting to the ring gland and aorta receive information at restricted regions in the superior protocerebral, tritocerebral and SEG neuropils, and that they release cholecystokinin-8, FMRFamide, and pigment-dispersing factor-like peptides from the ring gland and aorta into the hemolymph as neurohormones, or locally in the ring gland and aorta to regulate the production and/or release of hormones by the gland cells.