Connections between the dorsal raphe nucleus and a hindbrain region consisting of the cochlear nucleus and neighboring structures

Previous studies have shown that neurons in the raphe nuclei respond to aco ustic stimuli. The present study investigated connections between the dorsa l raphe nucleus (DRN) and a hindbrain region consisting of the cochlear nuc leus (CN) and neighboring structures. A mixture of one or more tracers (cho lera toxin B, biotinylated dextran amine (BDA), and H-3-leucine) was inject ed into the cat DRN. Retrograde-labeling results are presented whereby a ne w structure, to be called the juxta-acoustico-floccular fascicle (JAFF), is identified. The JAFF is surrounded by the CN, flocculus, lateral cerebella r nucleus, lateral vestibular nucleus, and restiform body. The JAFF is clos ely associated with the infracerebellar nucleus (ICN). Labeled neurons proj ecting to the DRN were concentrated in the JAFF, embedded among axons. Less numerous labeled neurons were in the ICN and CN. Anterograde-labeling resu lts are presented showing fibers labeled with BDA or with BDA and H-3-leuci ne in the CN, cochlear nerve and vestibular nerve. indicating that the DRN projects to these structures. The ascending and descending connections betw een the DRN and the above hindbrain region may mediate a reflex that may al ter the sensitivity of the auditory system in response to biologically sali ent (e.g. threatening or attractive) stimuli.