NITRIC-OXIDE SYNTHASE AND VASOPRESSIN IN RAT CIRCUMVENTRICULAR ORGANS- AN IMMUNOHISTOCHEMICAL STUDY

The distribution of immunoreactivity to neuronal nitric oxide synthase (nNOS) and vasopressin (AVP) was studied in the circumventricular org ans of the female rat, The occurrence of NOS immunoreactivity showed c orrespondence to nicotinamide dinucleotide phosphate diaphorase reacti vity, a previously used but less specific marker for neuronal NOS. nNO S immunolabeling was detected in the two mast rostrally located circum ventricular organs - the organum vasculosum of the lamina terminalis a nd the subfornical organ. In the latter, AVP immunoreactivity was obse rved in some cell bodies, which also were nNOS-immunoreactive. In the median eminence and the neurohypophysis there were large amounts of nN OS- and AVP-immunoreactive nerve fibers, which often displayed similar ities in distribution and morphology. Within the pineal gland, only ve ry few nNOS-immunoreactive varicose terminals were observed, which ran along blood vessels. nNOS immunoreactivity was also seen in the epith elium of the choroid plexus, whereas no nNOS immunoreactivity could be found in the subcommissural organ or in the area postrema, The presen t demonstration of nNOS and AVP immunoreactivity in the subfornical or gan, median eminence, and neurohypophysis, and the occurrence of nNOS immunoreactivity also in the choroid plexus and organum vasculosum of the lamina terminalis, provides a morphological background for a funct ional role for nitric oxide in water homeostatic mechanisms, both as e xecuted through the hypothalamohypophyseal system and via the producti on of cerebrospinal fluid.