Visualization of peptide-immunoreactive processes on serotonin-immunoreactive cells using two-color immunoperoxidase staining.

The sequential application of the peroxidase-antiperoxidase (PAP) technique with nickel-intensified DAB and with DAB alone was used to visualize black peptide-immunoreactive endings on amber serotonin-immunoreactive cells in 1-2-micron paraffin sections of the hamster medulla. Met-enkephalin- and substance P-positive terminals were present on serotonin cells in the raphe nuclei, the ventral reticular formation, and in the nucleus interfascicularis hypoglossi. The presence of enkephalin-immunoreactive endings on medullary serotonin-immunoreactive cells correlates with the analgesia and autonomic changes that result from the application of morphine or met-enkephalin to the medulla.