VIP terminals, axons, and neurons: distribution throughout the length of monkey and cat spinal cord

The distribution of vasoactive intestinal polypeptide (VIP) was mapped by peroxidase immunocytochemistry in the spinal cords of seven Macaca fascicularis monkeys and two cats. The animals were perfusion fixed with different chemicals. Those that were perfused with either a Zamboni fixative or 5% acrolein had significantly greater immunoreactivity outside the sacral cords; those fixed with 4% paraformaldehyde had little in nonsacral regions. VIP-like immunoreactive (VIP) axons and terminals were found in the superficial dorsal horn, reticular nucleus of lamina V, intermediomedial nucleus, and lamina X at all levels from C2 to S4; a few axons and terminals were also seen in the ventral horn. Axons were found in Lissauer's tract at all levels, and axons appeared in the dorsolateral and ventrolateral white matter at midthoracic levels; in the lumbosacral cord the number and extent of axons in the lateral and ventral white matter increased progressively in a caudal direction. VIP neurons were identified in thoracic intermediate gray lateral to the central canal and in the intercalatus (IC) and intermediolateral (IML) nuclei. Electron microscopy of the VIP terminals in laminae I and II of the cervical cord revealed they contain small round vesicles and many large granular vesicles; some are glomerular terminals and most form asymmetrical synaptic contacts onto dendrites. These results indicate VIP is much more widely distributed in the spinal cord than previously thought; VIP may be associated with both visceral thoracic and lumbosacral afferents, and with other afferents in the cervical cord; VIP neurons are present in the thoracic intermediate gray; and VIP axons in the ventral and lateral white matter indicate that the spinal cord is supplied in part by VIP sources other than primary afferents.