Topographic organization of serotonergic dorsal raphe neurons projecting to the superior colliculus in the Mongolian gerbil (Meriones unguiculatus)

Recent evidence suggests that the dorsal raphe nucleus (DRN) of the brainst em is a collection of neuronal clusters having different neurochemical char acteristics and efferent projection patterns. To gain further insight into the neuroanatomic organization of the DRN, neuronal populations projecting to the superior colliculus (SC) were mapped in a highly visual rodent, the Mongolian gerbil (Meriones unguiculatus). Retrograde tracers Fluoro-Cold (F G) or cholera toxin subunit-B (CTB) were injected into the superficial laye rs of the SC, and serotonin (5-hydroxytryptamine, 5-HT) -positive cells wer e identified by using immunocytochemistry in the FG-injected animals. Based on its projections to the SC, the DRN was divided into five rostrocaudal l evels. In the rostral and middle levels of the DRN, virtually all FG-fined cells occurred in the lateral DRN, and 36-55% of 5-HT-immunoreactive (Ei-HT -ir) cells were also double-labeled with FG. Caudally, FG-filled cells occu rred in the lateral, ventromedial, and interfascicular DRN; and 44, 12, and 31% of 5-HT-ir cells, respectively, were also FG-filled. The dorsomedial D RN contained only a small proportion of FG-filled cells at its most caudal level and was completely devoid of FG-filled cells more rostrally. The CTB- injected animals showed a similar distribution of retrogradely labeled cell s in the DRN. Topographically, the dorsal tegmental nucleus and the laterod orsal tegmental nucleus appeared to be closely associated with B-HT-ir cell s in the caudal DRN. These results suggest that the lateral DRN and the ven tromedial/interfascicular DRN may be anatomically, morphologically, and neu rochemically unique subdivisions of the gerbil DRN. (C) 1999 Wiley-Liss, In c.