Document Type


Publication Date

April 2006


This paper has been peer reviewed. It is the authors' final version prior to publication in the European Journal of Neuroscience 23(8): 2067-2077, April 2006. The published version is available at Copyright is retained by Blackwell Publishing, Inc.


We previously demonstrated that the opioid peptide, enkephalin, and corticotropin-releasing factor (CRF) are occasionally co-localized in individual axon terminals but more frequently converge on common dendrites in the locus coeruleus (LC). To further examine potential opioid co-transmitters in CRF afferents, we investigated the distribution of proopiomelanocortin (POMC), the precursor that yields the potent bioactive peptide, ß-endorphin, with respect to CRF immunoreactivity using immunofluorescence and immunoelectron microscopic analyses of the LC. Coronal sections were collected through the dorsal pontine tegmentum of rat brain and processed for immunocytochemical detection of POMC and CRF or tyrosine hydroxylase (TH). POMC-immunoreactive processes exhibited a distinct distribution within the LC as compared to the enkephalin family of opioid peptides. Specifically, POMC fibers were enriched in the ventromedial aspect of the LC with fewer fibers present dorsolaterally. Immunofluorescence microscopy showed frequent co-existence of POMC and CRF in varicose processes that overlapped TH-containing somatodendritic processes in the LC. Ultrastructural analysis showed POMC immunoreactivity in unmyelinated axons and axon terminals. Axon terminals containing POMC were filled with numerous large dense core vesicles. In sections processed for POMC and TH, approximately 29% of POMC-containing axon terminals (n = 405) targeted dendrites that exhibited immunogold-silver labeling for TH. Whereas, sections processed for POMC and CRF showed that 27% of POMC-labeled axon terminals (n = 657) also exhibited CRF immunoreactivity. Taken together, these data indicate that a subset of CRF afferents targeting the LC contain POMC and may be positioned to dually impact LC activity.