Neuroplasticity in the rearrangement of olfactory tract fibers after neonatal transection in hamsters

Olfactory bulb efferent axons run caudally in the lateral olfactory tract (LOT) to end in a broad cortical field in the ventral forebrain. Principles governing the plastic rearrangement of this fiber population after early lesions were probed by cutting the tract in hamster pups and studying the distribution of surviving olfactory bulb projections in adulthood using silver and autoradiographic techniques. The spatial pattern of rearrangement proved to depend on the extent of the cut and also the age at which it was made. For example, after complete LOT section at seven days of age no bulb efferents appeared distal to the cut but the proximal projection increased in laminar thickness and spread laterally and medially beyond its normal cytoarchitectonic boundaries. This spread was less pronounced in animals with earlier or later lesions. After transection of only part of the LOT fibers at seven days of age the proximal input was similarly increased. Just distal to the transection, uncut fibers sprouted collaterals to fill the terminal sites vacated as a result of the lesion. In these cases, however, the farthest distal parts of the projection field lost their normal innervation. In a tentative interpretation of these data it is proposed that developing LOT fibers tend to conserve their total amount of axonal arbor. That is, when distal branches are pruned off surgically, the axon compensates by producing extra proximal branches. When an overabundance of proximal collaterals are produced in axons that have not been surgically pruned, the growth of more distal axonal branches is stunted in compensation.