Localization and opiate receptor binding of enkephalin, CCK and ACTH/beta-endorphin in the rat central nervous system

We have examined the distributions of ACTH, CCK and enkephalin immunoreactive nerves in the rat central nervous system and compared the pattern obtained with that of opiate receptors. In addition, a radioreceptor assay has been employed for studying the possible functional interactions between these peptides and opiate receptors. Our results suggest that: (a) The distribution of enkephalin, CCK and ACTH/beta-endorphin immunoreactive nerve terminals is sufficiently similar to suggest functional interaction between these neuropeptides. (b) The CCK immunoreactive nerves display a distribution similar to that of enkephalin, but the main endogenous CCK forms do not bind to opiate receptors. However, opioid peptides and CCK may interact in many brain regions via binding to different, but functionally interacting receptors. (c) The ACTH immunoreactive nerves, which seem also to contain beta-endorphin, shows a partially overlapping distribution with the enkephalinergic systems. Further, ACTH and its fragments bind to opiate receptors. This suggests that ACTH could be an endogenous opioid ligand.