Cellular localization of peptides in neural structures.

By means of the immunohistochemical technique of Coons and collaborators, numerous peptide-containing neurons have been observed in the brain, spinal cord and periphery. These neurons may contain peptides such as substance P, vasoactive intestinal polypeptide (VIP), enkephalin or somatostatin. Some systems are very extensive. For example, immunoreactive substance P has been observed in more than 30 cell groups in the central nervous system, in primary sensory neurons, in sensory neurons in the vagus nerve and in taste buds, and in intestinal neurons. Thus, one and the same peptide can be utilized at many different levels in the nervous system. Several examples are now known where a regulatory peptide occurs together with a classical transmitter, such as a catecholamine, in the same neuron, which suggests the possibility that a neuron can release more than one transmitter substance. Of particular interest is the occurrence of VIP in presumed cholinergic neurons innervating exocrine glands in the cat, and the coexistence of a cholecystokinin (CCK)-like peptide in dopamine neurons projecting mainly to limbic areas. In the former system VIP seems to be responsible mainly for vasodilation, whereas acetylcholine mainly causes secretion. Furthermore, combined infusion of both substances in very low doses results in a marked potentiation of the secretory and vasodilatory responses. Thus, we have an example where two putative transmitters, released from the same nerve endings, seem to cooperate to activate a physiological response (secretion). With regard to the central CCK/dopamine neurons the type of interaction between the two coexisting transmitter candidates is at present unclear. It is suggested that elucidation of different types of coexistence phenomena may advance our understanding of chemical transmission at synapses under normal and pathological conditions, and may lead to new approaches to the treatment of some nervous disorders.