Enkephalin-like immunoreactivity in rat area postrema: ultrastructural localization and coexistence with serotonin

The ultrastructure of enkephalin-containing neurons and their capacity to take-up [³H]serotonin were examined in the area postrema. Untreated adult rats and rats with intraventricular infusions of 10^-4 M tritiated serotonin, 5-hydroxytryptamine ([³H]5-HT) were perfused with 4% paraformaldehyde and 0.2-0.5% glutaraldehyde. Coronal Vibratome sections through the area postrema from both groups of animals were immunocytochemically labeled with an antiserum to leucine Leu⁵-enkephalin. The sections from the animals infused with the isotope subsequently were processed for autoradiography. Enkephalin-like immunoreactivity (ELI) was detected in perikarya, dendrites, axons and axon terminals most frequently located along the ventricular and ventrolateral portions of the area postrema. The labeled perikarya were few in number and were characterized by a thin rim of cytoplasm containing peroxidase immunoreactivity. Dendrites and terminals containing ELI formed synapses primarily with unlabeled axon terminals and dendrites, respectively. However, terminals containing ELI also formed synaptic junctions with other unlabeled axon terminals. Appositions between enkephalin-containing processes and modified glia were occasionally seen near the ventricular surface. In sections processed for both immunocytochemistry and autoradiography, approximately 5% of the terminals containing ELI showed uptake of [³H]5-HT. We conclude that neurons containing ELI are primarily, but not exclusively, associated with other intrinsic neurons or afferents in the rat area postrema and that some of the enkephalin-labeled terminals have the capacity to take-up serotonin. Specificity of uptake of [³H]5-HT in neurons containing endogenous serotonin and factors which may contribute to the low probability of detecting both peroxidase and autoradiographic markers in single sections are discussed.