Double-labeling study of axonal branching within the lateral reticulocerebellar projection in the rat.

Collateral axonal branching to the cerebellum from the lateral reticular nucleus (LRN) was studied in the rat by using the fluorescent double-labeling technique. Following injection of Fast Blue (FB) into the cerebellar cortex, followed 3 days later by injection of Nuclear Yellow (NY) into a different region of the cortex, single- and double-labeled cells were found within the LRN. Most LRN-cerebellar projections were bilateral with ipsilateral preponderance, except for the projection to the paramedian lobule, which was completely ipsilateral. The dorsolateral area of the magnocellular division of the LRN contained cells whose axons branch to terminate in the rostral anterior lobe and the caudal part of the ipsilateral paramedian lobule (hindlimb areas of the cerebellar cortex), while the medial area of the LRN contained cells that supply, via collateral axonal branching, the caudal area of the contralateral anterior lobe and the rostral part of the ipsilateral paramedian lobule (forelimb areas of the cerebellum). Branched LRN-cerebellar axons projected to both hemispheres and to both sides of the caudal anterior lobe. No axonal branching was evident in the LRN-cerebellar projection to the rostral anterior lobe. The projection to the anterior and posterior lobe vermis also contained collateral axonal branching.