Colocalization of VIP with Other Neuropeptides and Neurotransmitters in the Autonomic Nervous Systema

Since the first histochemical demonstration of VIP immunoreactivity in autonomic numerous reports have appeared showing VIP-immunoreactive fibers in almost every organ of those species studied.' The widespread distribution of nerve fibers containing VIP immunoreactivity together with its potent biological actions on autonomic effectors had led to the proposal that VIP acts as a neurotransmitter in some autonomic nerves?' The use of immunoneutralization of endogenously released VIP in pharmacological studies has lent support to the role of VIP and related peptides as neurotransmitter.610 The idea that VIP acts as a neurotransmitter fell on fertile ground because at the time of its discovery by Said and Mutt" it was known that substances other than acetylcholine and norepinephrine mediate autonomic neurotransmission in a variety of tissues." Over the last twelve years, in parallel with the discovery of VIP, numerous other peptides were found to be contained in autonomic neurons. Also, for some of them, evidence exists that they can act as neurotransmitters. The suggestion that neurons may utilize a cotransmitter in addition to the main neurotransmitteri3 led to research that compiled functional and morphological evidence confirming this possibility in the salivary glands. Lundberg" showed that both VIP and acetylcholine are contained in and released from the same postganglionic parasympathetic neurons. Thus the concept of cotransmission gave a new perspective to the increasing number of reports on the presence of peptides in autonomic n e ~ r 0 n s . I ~