The distribution of neuropeptides as revealed by immunohistochemical techniques suggests two varieties of neural circuits in which the peptides play a role. The first of these generalized systems is thought to participate in neuroendocrine regulation, since the peptides are found in high concentrations in terminals adjacent to vascular elements that drain either to the anterior pituitary or to systemic circulation. Peptides released from such neurohemal sites may act as hormones upon target cells. Alternatively, they may act via axo-axonic interactions with other peptidergic terminals which liberate “true” hormones. Neuropeptides are also found in circuits of neurons engaged in interneuronal comrnunication. In the case of the enkephalins, it is especially noteworthy that their distribution in nerve terminals parallels, to a great extent, the distribution of opiate receptors. Thus, it is likely that neuropeptides also play a significant role in interneuronal communication - perhaps as neurotransmitters.
Bibliographical noteFunding Information:
Work reviewed from the author’s laboratory was supported by grants from the Graduate School, University of Minnesota, USPHS DA-02148 and a Scholar in Neuroscience Award to R.E. from the McKnight Foundation. The technical expertise of L. Grouse and T. Mullet and the secretarial assistance of J. L. Mattson are gratefully acknowledged.