Projection of the magnocellular red nucleus to the region of the accessoryabducens nucleus in the rabbit

The projection of the magnocellular red nucleus (RNm): to the region of the , accessory abducens nucleus (AABD) was traced in rabbit using the bidirect ional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). In one set of animals, recordings of antidromic responses from RNm neurons elicit ed by electrical stimulation of the rubrospinal tract were used to localize injections of WGA-HRP for orthograde labeling of RNm terminals. In a diffe rent set of animals, horseradish peroxidase was injected into the retractor bulbi muscle to retrogradely label motoneurons of the AABD. The positions of RNm fibers and terminals were examined and compared to the locations and distribution of AABD cell bodies and labeled dendrites. Analyses revealed that along the entire rostrocaudal extent of the AABD, RNm efferents termin ate primarily lateral to, or in the lateral aspects of, labeled motoneurons . For the rostral AABD, RNm efferents terminate only lateral to the nucleus . Although the terminals are not positioned to contact cell bodies of the A ABD, they could overlap with dendrites that extend in the lateral direction . RNm efferents terminate more extensively within the posterior AABD, overl apping within both dendritic and cell body regions of the nucleus. Even in this posterior region, however, RNm efferents were distributed primarily ov er the lateral half of the nucleus. These data show that RNm can mono synap tically influence the AABD, through primarily its lateral and posterior asp ects. Our findings also show that a major target of RNm efferents is the re ticular cell population located lateral to the AABD, suggesting that the RN m also may affect AABD motoneuronal output indirectly through its projectio n to reticular cells. (C) 2001 Academic Press.