Trigeminocerebellar projection to the paramedian lobule with emphasis on the climbing fibre zones: a retrograde tracing study in the rabbit.

Projections of the trigeminal sensory nuclear complex (TSNC) onto the cerebellar paramedian lobule (PML) were investigated by the retrograde horseradish peroxidase (HRP) and fluorescent tracers (FB, DY) technique in the rabbit. Following injections of the tracers into the various regions of different sublobules of PML, the retrograde labelling pattern in TSNC was analyzed, especially in relation to the climbing fibre zones identified by retrograde labelling in the inferior olive (Zimny et al., 1989). The present results indicate that projections are bilateral with a clear ipsilateral preponderance. The major projections originate mainly from the dorsolateral and ventromedial regions of the principal trigeminal nucleus (Vp) except its caudal pole. Prominent input is also derived from the rostral (Vir) and caudal (Vic) pars interpolaris of the spinal trigeminal nucleus (Vsp). The cells of origin of the projections are present throughout the entire length of Vir with preference to dorsolateral and dorsal location. In Vic they tend to be distributed in the medial regions of its rostral half. Afferents form the pars oralis (Vo) of Vsp are moderate and arise mainly from the dorsomedial and dorsal regions in its caudal one-third. Projections from the pars caudalis (Vc) of Vsp and the mesencephalic trigeminal nucleus (Vmes) are absent. A topography exists in the trigeminocerebellar projections, however, no clear-cut evidence was obtained for a topical relationship between distribution of neurones in TSNC and projections sites in PML. On the other hand, certain correspondence could be found between subdivisions of TSNC and climbing fibre zones in PML. While zones C3 and C2-lateral appear to receive no trigeminal afferents, zone C1 is supplied by fibres from Vir and Vic. All subdivisions of TSNC project zones C2, D1 and D2. In sublobule f, medial (Dm) and lateral (Dl) composite zone D1 + D2 receive projections from Vic, or Vp and Vir, respectively. The present report provides new detailed data on the trigeminocerebellar system in the rabbit.