Colocalization of neuronal NO synthase with urotensins I and II in the caudal neurosecretory neurons and the urophysis of the teleost oreochromis niloticus: a gold immunoelectron microscopic study.

The intracellular distribution of neuronal nitric oxide synthase (nNOS) was studied in the caudal neurosecretory system of a teleost, Oreochromis niloticus (Cichlids), by means of post-embedding immunogold labeling with a polyclonal antibody directed against nNOS of human origin. Ultrastructural examination demonstrated that neuronal NOS-like molecules are distributed within the Dahlgren cell perikarya, the neurosecretory axons, and the urophysial axon terminals. In the neurosecretory somata, gold particles for nNOS were mainly cytosolic, whereas in the neurosecretory axons and axon terminals they were associated with the membrane and/or the dense core of neurosecretory granules. Double immunogold labelings for nNOS/urotensin I (UI) and nNOS/urotensin II (UII) demonstrated that nNOS-like molecules are colocalized with UI and/or UII in the neurosecretory granules contained within the urophysial terminals. The present findings suggest that both a soluble cytosolic and a particulate neuronal NOS are expressed in the caudal neurosecretory neurons. They confirm previous biochemical data on the same species.