Abstract
Microelectrode recordings (extracellular) from caudal neurosecretory units have been utilized to determine the reactivity of the caudal (urophysial) system to various osmotic and ionic changes. Detection of the neurosecretory units for recording of electrical activity was based on their long-duration action potentials. Results of intravenous injection of hypotonic (Na +-free) and hypertonic (NaCl, NaNO 3, Na 2SO 4, sucrose, choline chloride, tetramethylammonium chloride) solutions indicate the presence of two kinds of differentially responsive neurosecretory units: those activated by high Na + concentration and those activated by Na +-free solutions. A small group of neurosecretory fibers and all non-neurosecretory fibers tested failed to respond to any osmotic manipulation. Our results indicate that it is not change in osmotic pressure per se which alters the electrical activity of caudal neurosecretory units, but rather changes in Na + concentration regardless of the accompanying anion. Transection of the spinal cord demonstrates that the caudal neurosecretory neurons are synaptically controlled by a center located probably in the brain. A feedback loop is proposed in an attempt to delineate the possible role of the caudal neurosecretory system in ionoregulation.
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