Multiple zonal projections of the nucleus reticularis tegmenti pontis to the cerebellar cortex of the rat

Compartmentalization (alternating labelled and unlabelled stripes) of mossy fibre terminals was found in the cerebellar cortex after iontophoretic injections of biotinylated dextran amine into discrete regions of the nucleus reticularis tegmenti pontis (NRTP). The zonal pattern was only observed when volumes of nuclear tissue ranging from 4.5 x 106 to 17.66 x 106 microm3 were impregnated. Up to nine compartments (i.e. up to five stripes separated by four interstripes) were found in crus I and in vermal lobule VI. Up to seven compartments (four stripes and three interstripes) were found in crus II; up to five compartments (three stripes and two interstripes) were identified in the lobulus simplex, the paraflocculus and vermal lobules IV, V and VII; up to three compartments (two stripes and one interstripe) were identified in the paramedian lobule and, finally, up to two compartments (one stripe and one interstripe) were identified in the copula pyramidis, in the flocculus and in vermal lobules II, III, VIII and IX. The projections of the NRTP are arranged according to a divergent/convergent projection pattern. From single injections in the NRTP, projections were traced to a set of cortical stripes widely distributed over the cerebellar cortex. The set of stripes labelled from different regions of the NRTP partially overlapped but complete overlap was never found. This finding revealed that the topographic combination of the projections of the NRTP to the cerebellar cortex is specific for each region of the NRTP. Finally, the projections to single cortical areas were arranged according to a pattern of compartmentalization that is specific for each cortical area, independent of the site of injection in the NRTP and of the number of stripes evident in the cortex.