Nitric oxide and neuromodulation in the caudal neurosecretory system of teleosts.

Although evidence exists that nitric oxide (NO) mediates neuroendocrine secretion in mammals, the involvement of NO in the neuroendocrine regulation of non-mammalian vertebrates has yet to be investigated in detail. The present review conveys several recent data, suggesting that NO plays a modulatory role in the caudal neurosecretory system (CNSS) of teleosts. The presence and distribution of neuronal NO synthase (nNOS) was demonstrated in the CNSS of the Nile tilapia Oreochromis niloticus by means of NADPHd histochemistry, NOS immunohistochemistry, NOS immunogold electron microscopy, the citrulline assay for NOS activity and Western blot analysis. NO production by the caudal spinal cord homogenates was also evaluated by the oxyhemoglobin assay. On the whole, these findings indicate that caudal neurosecretory cells express NOS enzymes and presumably produce NO as a cotransmitter. Moreover, the comparison of the nNOS distribution with that of urotensins I and II (UI and UII) suggests that neurosecretory Dahlgren cells belong to two different functional subpopulations: a population of UI/UII secreting nitrergic neurons and a population of non-nitrergic neurons, which principally secrete UII. These results implicate NO as a putative modulator of the release of urotensins from the neurosecretory axon terminals. Therefore, like in mammals, NO appears to influence neuroendocrine secretion in teleosts.

Development of the caudal neurosecretory system of the nile tilapia Oreochromis niloticus: an immunohistochemical and electron microscopic study.

The development of the caudal neurosecretory system (CNSS) of the Nile tilapia, Oreochromis niloticus, has been investigated by means of UI/oCRF (urotensin I/ovine corticotropin-releasing factor) immunohistochemistry and transmission electron microscopy. UI-like immunoreactive perikarya and fibers are first detected in the caudal spinal cord of larval fish about 4 days after hatching (stage 21). In the region of the future urophysis two bundles of strongly immunoreactive neurosecretory fibers are observed. At this stage, neurosecretory axons terminate on the meninx sheath of the spinal cord with immature neurosecretory terminals. The histogenesis of the urophysis begins at stage 24. The future neurohemal organ consists of a small ventral swelling of the spinal cord, which is associated with dilated vessels. At this stage, neurosecretory axons terminate on the basal lamina of the ingrowing blood vessels. Further development occurs by means of progressive branching of vessels and the concomitant increase in the number of neurosecretory terminals. In the caudal spinal cord, immunoreactive neurons also increase in number and progressively differentiate morphologically. Typical features of the mature CNSS are recognizable in 4-month-old juveniles. Data suggest that in tilapia both the synthesis and the release of urophysial hormones begin before morphogenesis of the neurohemal organ takes place.