Anatomical regeneration and behavioral recovery following crush injury of the trigeminal root in lamprey.

In normal larval lamprey, bilateral application of horseradish peroxidase (HRP) to the dorsal part of the anterior oral hood labeled subpopulations of trigeminal components on both sides of the brain; peripherally projecting motoneurons, medullary dorsal cells (sensory), and spinal dorsal cells (sensory), as well as centrally projecting afferents in the trigeminal descending tracts. Following unilateral crush injury of the right trigeminal root, HRP labeling of sensory and motor trigeminal components on the right side gradually increased with increasing recovery time, between 2 weeks and 12 weeks postcrush (PC). Axons of trigeminal motoneurons appeared to exhibit robust regeneration, whereas restoration of projections in the descending trigeminal tract ipsilateral to the injury was incomplete. Control experiments indicated that motor and sensory axons from the intact side of the oral hood did not sprout across the midline to the denervated side. Several results suggested that regenerated trigeminal sensory fibers made synapses with brain neurons that have direct or indirect inputs to reticulospinal (RS) neurons. Following a unilateral crush injury of the right trigeminal root, escape behavior in response to stimulation of the right side of the oral hood gradually returned to normal. Muscle recordings at various recovery times confirmed that anatomical regeneration of trigeminal sensory axons was functional. In addition, at 8 or 12 weeks PC, brief stimulation of the oral hood ipsilateral or contralateral to the crush injury elicited synaptic responses in RS neurons on either side of the brain, similar to that in normal animals. In the lamprey, compensatory mechanisms probably allow recovery of behavioral function despite incomplete regeneration of trigeminal sensory axons within the central nervous system.