Caudal Neurosecretory System: A Physiologist's View

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Abstract
Several active principles (urotensins) have been isolated from teleost caudal neurosecretory systems. The original classification of these substances has been recently modified: There are at least two mammalian hypotensive factors (with no effects yet known in fish), possibly two smooth muscle contractants, arginine vasotocin, and factors affecting Na+ movement. A factor affecting lymph heart frequency has not yet been compared with other peptides. Evidence for the hormonal nature of some of these substances includes subcellular localization in granules and calcium-dependent release by depolarization. The physiology of the system is still not clear. The best evidence for an osmoregulatory role includes effects of extracts on renal diuresis and Na+ movement and changes in firing rates of caudal neurosecretory neurons by altered ionic composition of the blood. Most other evidence is subject to alternative explanation. Several observations suggest a role for this system in reproduction.

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