PARAFASCICULAR NUCLEUS-RAPHE PROJECTIONS AND TERMINATION PATTERNS IN THE RAT

Thalamic projections from the parafascicularis nucleus to the raphe sy stem were studied by means of anterograde techniques, utilizing both b iocytin and dextran amine in rats. Both tracers injections in the para fascicularis nucleus resulted in the labeling of descending bundles of fibers running along the brainstem. Labeled terminal fields were foun d in all the raphe nuclei except the nucleus raphe pallidus. Three dif ferent types of labeled terminals (numerous small boutons, less numero us large claw-like terminals, varicosities in close apposition to bloo d vessel walls) originating from the parafascicular nucleus were prese nt in the raphe system. Ultrastructural data suggest an inhibitory nat ure for the parafascicular-raphe projections. Our results confirm and extend previous retrograde data by indicating the trajectories, the te rminal fields, the fine structure of terminal axonal arborizations and boutons. Based on previous retrograde data and our observations, we c onclude that the relationship between the parafascicular nucleus and r aphe system is reciprocal and concerns most of the raphe nuclei, sugge sting that parafascicularis cell population may be involved in many of the functions ascribed to the raphe system.