Ultrastructural relation between cortical efferents and terminals containing enkephalin-like immunoreactivity in rat neostriatum

The interrelationships between cortical efferents and terminals containing enkephalin-like immunoreactivity (ELI) were examined by combining anterograde degeneration with electron microscopic immunocytochemistry in the adult rat neostriatum. Two days following unilateral removal of the cerebral cortex, the brains were fixed by aortic arch perfusion, then sectioned and processed for the immunocytochemical localization of an antiserum directed against methionine (Met⁵)-enkephalin. The observed relationships between the degenerating cortical efferents and immunocytochemically labeled terminals were of two types. In the first, the degenerating and ELI containing terminals converged on the same unlabeled dendrite or dendritic spine. In the second, terminal and preterminal axons of the ELI containing neurons had one surface directly apposed to the plasma membrane of a degenerating axon terminal. These findings support the concept that neurons containing opioid peptides and cortical efferents modulate the output of common recipient neurons and may also directly interact with each other through presynaptic axonal mechanisms in the rat neostriatum.