THE NEURONAL SYSTEM OF THE SACCUS VASCULOSUS OF TROUT (SALMO-TRUTTA FARIO AND ONCORHYNCHUS-MYKISS) - AN IMMUNOCYTOCHEMICAL AND NERVE TRACING STUDY

The neuronal system of the saccus vasculosus of two species of trout w as studied with immunocytochemical methods and carboindocyanine-dye (D iI) tract-tracing. The cerebrospinal-fluid-contacting neurons of the s accus were immunoreactive for gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD) and neuropeptide Y (NPY). Immunostaining of alternate sections of the saccus vasculosus of fry with anti-GAD and a nti-NPY indicated that these substances were colocalized. The tractus sacci vasculosi and the neuropil of the nucleus sacci vasculosi were a lso immunoreactive to these substances. The GABA, GAD, and neuropeptid e Y immunoreactivity of the saccus vasculosus system appeared early in trout ontogeny. After applying Dil to various levels of the tractus s acci vasculosi of adult trout, we observed massive bilateral saccular projections to the nucleus sacci vasculosi and could follow the course of the sacco-thalamic tract. This tract extended in the subependymal region of the thalamus rostral to the nucleus sacci vasculosi and spli t into two small tracts that reached the subhabenular-preoptic region. Sacco-thalamic fibers formed extensive periependymal plexuses along t heir trajectory. Interestingly, no clear evidence of the existence of a saccopetal system was obtained. On the basis of these results, we po stulate that the saccus vasculosus system modulates the function of ce nters of the posterior tubercle and periventricular thalamus.