Peptides for calling? An immunohistochemical study of the avian n. intercollicularis

We used two phylogenetically distant avian species (dark-eyed junco, hyemalis, Passeriformes; domestic pigeon, Columba livia, Columbiformes) to determine the immunocytochemical distribution of opioid (leucine-enkephalin, dynorphin B) and non-opioid (adrenocorticotropic hormone) peptides in the n. intercollicularis (ICo), a midbrain region which plays a central role in the control of vocalizations. We found that, in both species, the peptides under study are present as fibers and terminal-like structures, and are similarly distributed. The n. dorsomedialis intercollicularis division contains much less immunoreactivity than the rest of the ICo. Based on this and previous studies, we propose that opioid peptides, possibly transported from the preoptic and hypothalamic regions of the diencephalon to the ICo, regulate vocal behavior by altering respiratory function rather than syringeal mechanisms.