Pontocerebellar projection to the rabbit paramedian lobule by means of axonal collaterals: evidence for intralobular connections.

In this study, we utilized a double retrograde axonal tracing technique to investigate the possible existence of collateralized axonal projections from the pontine nuclei (PN) to the rostral (rPML) and caudal (cPML) parts of cerebellar paramedian lobule in the rabbit, known to be the forelimb and hindlimb regions, respectively. Following injections of fluorescent tracers Fast Blue (FB) and Diamidino Yellow (DY) within rPML and cPML, respectively, substantial numbers of FB and DY single labeled neurons were found in the dorsolateral, paramedian, lateral and peduncular pontine nuclei bilaterally with a very clearcut contralateral preponderance. No labeling was observed in the ventral pontine nucleus. Extensive areas of overlap of FB or DY labeled neurons indicated that no somatotopical relationship existed in projection from PN to the two functionally different PML target regions. In addition, a small number of double FB + DY labeled neurons was detected in the common areas of FB and DY single labeling in PN. These neurons give rise to pontocerebellar projections to rPML and cPML simultaneously by way of axonal collaterals and thus they may play a role in the coordination of unilateral forelimb and hindlimb movements.