Abstract

The projection of the magnocellular red nucleus (RNm) to the region of the accessory abducens nucleus (AABD) was traced in rabbit using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). In one set of animals, recordings of antidromic responses from RNm neurons elicited by electrical stimulation of the rubrospinal tract were used to localize injections of WGA-HRP for orthograde labeling of RNm terminals. In a different 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 terminate 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 AABD, they could overlap with dendrites that extend in the lateral direction. RNm efferents terminate more extensively within the posterior AABD, overlapping within both dendritic and cell body regions of the nucleus. Even in this posterior region, however, RNm efferents were distributed primarily over the lateral half of the nucleus. These data show that RNm can monosynaptically influence the AABD, through primarily its lateral and posterior aspects. Our findings also show that a major target of RNm efferents is the reticular cell population located lateral to the AABD, suggesting that the RNm also may affect AABD motoneuronal output indirectly through its projection to reticular cells.

Author Keywords: accessory abducens nucleus; retractor bulbi muscle; nictitating membrane; classical conditioning; WGA-HRP.

*1 This research was supported by the National Institutes of Mental Health, MH51722 and MH00343.

*2 We gratefully thank Drs. Peter Land and Julia Bassett for their helpful advice with regard to certain phases of the analysis.

^f1 Address reprint requests to Theodore W. Berger, 500 Olin Hall, Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089-1451.