NO solution to high salt intake

Nitric oxide, normally toxic at high concentrations, is now known to be involved in a number of functions within the nervous system of many animals. New research being presented today at the Society for Experimental Biology conference reveals for the first time that nitric oxide is also present within the neurosecretory system of fish and may help them cope with changes in environmental salinity.

Within the mammalian nervous system it was thought that nerve cells communicated exclusively using `traditional` neurotransmitters – small peptide molecules which travel between nerve cells binding to their surface and causing them to become electrically excited. It is now believed that a new class of transmitter exists – nitric oxide (NO). As a gas, NO is able to penetrate the cell and act directly within it, modulating its activity and allowing a rapid reaction to environmental change. This transmitter has been implicated in a variety of nervous functions from olfaction -the sense of smell – to hormone release.

The presence and activity of nitric oxide has, in the last 10 years, been demonstrated in almost every species of animal, says Dr Carla Cioni of `La Sapienza` University, Rome. At the conference in Swansea, Dr Cioni will show that NO may play a role within the neurosecretory system of fish. Fish possess two neurosecretory systems – essentially nerve cells which are able to release hormones – in the brain and, strangely, the tail. The system in the tail is known as the urophysis and produces urotensins. These proteins are released into the blood and cause circulatory changes which may help the fish to cope with changes in salinity.

Dr Cioni, and colleague Dr Bordieri, have been able to identify the presence of a specific enzyme, neuronal NO synthase, within these cells. This enzyme plays a crucial role in producing nitric oxide. Dr Cioni suggests that the production of this gas may modulate the release of urotensins into the bloodstream thus altering their concentration within the blood and their effect on blood pressure. Support for this theory has come from collaborative work with British scientists. It seems that the electrical (nervous) activity of the fish`s neurosecretory cells can be altered artificially by adding, or removing NO. In the presence of excess NO activity increases, and in its absence it decreases, lending considerable support to Dr Cioni`s theory.

“The next stage of our research to determine whether NO is directly involved in salinity regulation, where fish adjust to varying salinity as they move through different waters. But it seems clear that the NO system is a virtually universal phenomenon within the nervous systems of animals.”