Vasoactive intestinal polypeptide in sacral primary sensory pathways in the cat

Unmyelinated sensory axons in the sacral spinal cord may play a role in bla dder reflexes under certain pathological conditions. Previous data suggeste d vasoactive intestinal polypeptide (VIP) might be contained exclusively in sensory C-fibers, some of which innervate the bladder. This study was unde rtaken to describe the morphology of these VIP fibers in the sacral cord of the cat. VIP immunoreactivity was confined to unmyelinated axons observed at several levels of the sensory pathway including the dorsal root ganglia, dorsal roots, Lissauer's tract, and the lateral collateral pathway. A comb ination of light and electron microscopic observations showed VIP-immunorea ctive fibers with labeled varicosities and synaptic terminals in laminae I, IIo, V, VII, and X. VIP-immunolabeled varicosities had a mean diameter of 1.6 mu m (range = 0.11-7.4 mu m, S.D. = 1.01, n = 311) with a small percent age (8%) being relatively large (3-7.4 mu m). VIP varicosities contained a mixture of small clear vesicles (CLV) and large dense core vesicles (LDV). Although most varicosities contained a moderate number of LDVs (14.86 LDVs/ mu m(2)), some varicosities contained a large number of LDVs, whereas other s contained very few. Varicosities that possessed synaptic specializations were classed as terminals and were divided into three morphological classes . Two of these resembled Gray's Type I terminal, whereas a third was simila r to the Gray's Type II terminal. There was no consistent relationship betw een vesicle content of the terminal and the type of synaptic contact it pos sessed. This study shows that in the sacral spinal cord of the cat, VIP ter minals originate only from C-fibers, terminate primarily in laminae I and V , and exhibit a variety of morphologies consistent with heterogeneous origi ns and functions of the lower urinary tract. J. Comp. Neurol. 407:381-394, 1999. (C) 1999 Wiley-Liss, Inc.