Immunocytochemical colocalizations of serotonin, aromatic L-amino acid decarboxylase and polypeptide hormones in A- and PP-cells of the chicken endocrine pancreas.

The colocalization of serotonin and aromatic L-amino acid decarboxylase (AADC) in the avian pancreatic polypeptide-containing PP-cells and glucagon-storing A-cells of the chicken endocrine pancreas was investigated using combined pre-embedding immuno-peroxidase and post-embedding immunogold electron microscopic immunocytochemistry. The avian pancreatic polypeptide-immunoreactive cells manifested by the labeling of immunogold particles on secretory granules were also immunoreactive with antisera directed against serotonin and AADC, an enzyme involved in the synthesis of serotonin. In PP-cells immunoreactivity against the anti-serotonin serum was stronger in secretory granules than in the cytoplasmic matrix, whereas immunoreaction with the anti-AADC serum was observed to be more intense in the cytoplasmic matrix. Immunoreactions with the serotonin and AADC antisera were also found in secretory granules of glucagon-storing A-cells. These results indicate that serotonin is co-stored within secretory granules of both A- and PP-cells, and that AADC is localized within secretory granules of A-cells, and may be present in the cytoplasmic matrix of PP-cells. It is probable that serotonin is synthesized and released simultaneously with secretory granules from both A- and PP-cells of the chicken endocrine pancreas.