Objective: To study the ultrastructural relationship between distal neurons contacting cere- brospinal fluid (CSF) in the dorsal raphe nucleus (DR) and neurons no-contacting CSF in the brain parenchyma. Methods: An antergrade and retrograde tracing of the cholera toxin subunit B with horseradish peroxidase (CB-HRP) and electron microscope. Results: There were mainly two synaptic types between neurons contacting CSF in DR and neurons no-contacting CSF in the brain parenchyma: A dendro (-)-dendritic (+) syna...