Role of galanin in the gut.

The neuropeptide galanin is present in both the central and the peripheral nervous systems (l-10) (Table 1). In the central nervous system galanin is widely distributed throughout the spinal cord and the brain. In the spinal cord it is present at all levels and has been detected in large amounts in dorsal horn cells, nerve cell bodies intrinsic to the spinal cord, including neurons of lamina II, intermediate dorsal laminae, neurons around lamina X, and in subpopulations of lower motor neurons. Galanin-containing fibers are most prominent in the lumbosacral regions of the spinal cord (11). In the brain, galanin is widely distributed in the medulla (caudal trigeminal nucleus, nucleus tractus solitarius, and ventral surface), locus caeruleus, and midbrain. In the forebrain, galanin is present in thalamus, hypothalamus, and magnocellular neurosecretory neurons of supraoptic and paraventricular nuclei. The median eminence and hypothalamus have the highest concentrations of galanin. In the peripheral nervous system galanin is richly present in the terminal innervation of the gastrointestinal, respiratory, and urogenital tracts, the pancreas, and the adrenal medulla (6). Galanin is present in the neurons of the myenteric, submucous, and mucous plexuses of the gut. Galaninimmunoreactive nerves have been shown to be present in all layers of the gut wall and at all levels of the gastrointestinal tract (7,12) in the different species examined. Most of the galanin fibers are present in muscle layers; only a few occur in the mucosa. There are a few species differences regarding the distribution of galanin fibers. Galanin-immunoreactive cell bodies are present only in the submucous plexus of human and porcine gut. In the rat they are present in both the myenteric and submucous plexuses, but they are more numerous in the latter. Surgical and chemical denervation experiments suggest that galanin fibers are primarily intrinsic to the enteric nervous system (9,13). Galanin-containing axons are among the longest, projecting distances of 1.5-2 cm in the anal direction (13). Galanin may coexist with vasoactive intestinal polypeptide (VIP) in ganglion cells of the submucous plexuses of different species examined (7). In some cases, e.g., in porcine duodenum, all VIP-immunoreactive neuronal cell bodies of the submucous plexus may contain galanin. In other regions, two thirds of the VIP-immunoreactive cells may show colocalization with galanin, and only a few cells show galanin immunoreactivity alone. In the myenteric plexus of antrum and in the submucous plexus of the canine small intestine (14), it was reported that galaninimmunoreactive fibers always contained VIP immunoreactivity. In the pig, galanin-containing cell bodies may also contain substance P and calcitonin generelated peptide (15). Galanin has also been reported to coexist with acetylcholine (16), catecholamines, 5-hydroxytryptamine, y-aminobutyric acid and vasopressin, oxytocin, corticotropin-releasing factor, neurotensin, and VIP (17-22). Galanin immunoreactivity has also been shown to be present in the primary afferent fibers (3,21). Studies by Ch’ng et al. (11) showing the accumulation of galanin in the sacral regions of the spinal cord following thoracic transection suggest that galanin is present in the ascending pathway, originating from the sacral segments. These observations have been further confirmed by capsaicin pretreatment (23) and surgical deafferentation. Such findings suggest the