Parafascicular nucleus-raphe projections and termination patterns in the rat

Thalamic projections from the parafascicularis nucleus to the raphe system were studied by means of anterograde techniques, utilizing both biocytin and dextran amine in rats. Both tracers injections in the parafascicularis nucleus resulted in the labeling of descending bundles of fibers running along the brainstem. Labeled terminal fields were found in all the raphe nuclei except the nucleus raphe pallidus. Three different types of labeled terminals (numerous small boutons, less numerous large claw-like terminals, varicosities in close apposition to blood vessel walls) originating from the parafascicular nucleus were present in the raphe system. Ultrastructural data suggest an inhibitory nature for the parafascicular-raphe projections. Our results confirm and extend previous retrograde data by indicating the trajectories, the terminal fields, the fine structure of terminal axonal arborizations and boutons. Based on previous retrograde data and our observations, we conclude that the relationship between the parafascicular nucleus and raphe system is reciprocal and concerns most of the raphe nuclei, suggesting that parafascicularis cell population may be involved in many of the functions ascribed to the raphe system