With the use of retrograde transport of horseradish peroxidase we confirmed the observation of Yamamoto and Shimoyama ([ 19771 Neurosci Lett. 5279-283) that Purkinje cells of the rabbit flocculus projecting to the medial vestibular nucleus are located in two discrete zones, FZIl and FZlv, that alternate with two other Purkinje cell zones, FZI and FZIII, projecting to the superior vestibular nucleus. The retrogradely labeled axons of these Purkinje cells collect in four bundles that occupy the corresponding floccular white matter compartments, FC1-4, that can be delineated with acetylcholinesterase histochemistry (Tan et al. [ 1995al J. Comp. Neurol., this issue). Anterograde tracing from small injections of wheat germ agglutin-hoseradish peroxidase in single Purkinje cell zones of the flocculus showed that Purkinje cell axons of FZrr travel in FC2 to terminate in the medial vestibular nucleus. Purkinje cell axons from FZI and FZIII occupy the FCI and FC3 compartments, respectively, and terminate in the superior vestibular nucleus. Purkinje cell axons from all three compartments pass through the floccular peduncle and dorsal group y. In addition, some fibers from FZI and FZIJ, but not from FZIII, arch through the cerebellar nuclei to join the floccular peduncle more medially. No anterograde tracing experiments were available to determine the projections of the FZ, and C2 zones. The functional implications of these results are discussed. Indexing terms: cerebellum, corticonuclear projection, Purkinje cell, group y, eye movements (c. l Y Y 5 wiley-Lis. Inc. The flocculus plays an important role in the control of eye movements by processing visual and vestibular information (Robinson, 1976; Ito et al., 1982a; Nagao, 1983; Ito, 1984). Via their projections to the vestibular nuclei, Purkinje cells of the flocculus inhibit certain groups of vestibulo-ocular relay cells (Ito et al., 1970; Baker et al., 1972; Fukuda et al., 1972; Highstein, 1973; Kawaguchi, 1985; Sat0 and Kawasaki, 1987, 1990a,b, 1991; Sato et al., 1988). The projection of the flocculus to the vestibular nuclei has been known since the studies of Dow (1936, 1938a,b) using the Marchi method in rat, cat, and monkey. Recent studies using modern tracing techniques showed that the flocculus sends major projections to the superior (SV) and medial vestibular nucleus ( M V ) , group y, the lateral cerebellar nucleus (LCN), and the basal interstitial nucleus and minor projections to the descending vestibular nucleus (DV) and the nucleus prepositus hypoglossi (Jansen and Brodal, 1940; Voogd, 1964; Angaut and Brodal, 1967; Alley, 1977; Haines, 1977; Yamamoto and Shimoyama, 1977; Balaban et al., 1981; Sat0 et al., 1982a,b; Carpenter and Cowie, 1985; Langer et al., 1985; Epema, 1990). Some of these studies have indicated that the Purkinje cell population of the flocculus consists of subsets arranged in zones that cross the folia of the flocculus and project to different vestibular nuclei. Yamamoto and Shimoyama (1977) found that the Purkinje cells of the flocculus of the rabbit that project to MV and SV are distributed in a striped pattern. They distinguished two Purkinje cell zones projecting to MV and two zones projecting t o SV. The two zones projecting to MV are interleaved with the two zones projecting to SV; the most rostra1 of these four zones projects to MV. Purkinje cells of a fifth zone, located in the caudal flocculus and also extending into folium p, were labeled from injections of horseradish peroxidase (HRP) in the lateral cerebellar nucleus (Yamamoto, 1978). In her later publications (YamaAccepted September 9,1994 Address reprint requests to J. Voogd, Department of Anatomy, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. 8 1995 WILEY-LISS, INC.
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