Roles of peptides and other substances in cotransmission from vascular autonomic and sensory neurons.

Blood vessels may be innervated by up to three major classes of neurons: sympathetic vasoconstrictor neurons; sympathetic or parasympathetic vasodilator neurons; and peripheral fibres of small diameter sensory neurons, which can mediate vasodilation. Most vascular neurons utilise multiple transmitters, including neuropeptides and small nonpeptides such as ATP or nitric oxide, often in addition to noradrenaline or acetylcholine. Subpopulations of each major class of vascular neurons innervating different vascular segments may contain different combinations of neurotransmitters. Furthermore, the same population of neurons can release different cotransmitters in response to different patterns of stimulation. In general, peptides mediate slower and more long lasting changes in vascular resistance than do nonpeptides. Thus, autonomic and sensory neurons are well adapted to produce qualitatively different vascular effects in response to different types of afferent input. The major challenge for the future is to develop new antagonists for many of the substances colocalised in vascular neurons, particularly neuropeptides. These agents will allow us to precisely determine the relative roles of multiple cotransmitters, and are likely to provide therapeutic agents that can be targeted to specific regions of the vasculature.