The Role of Somatosensory Information in a Constrained Locomotor Task

Abstract

Abstract The purpose of this project was to study the role of somatosensory information in the performance of a constrained locomotor task by rats and to further examine the influence of structural recovery in the somatosensory thalamus, specifically the ventral posterolateral nucleus (VPL). Groups of rats were trained to traverse an elevated, one inch bar for a reward. The time taken to run across the bar (run time) was used as a measure of the success of the goal-directed behavior. The movement pattern of the hindlimb during the swing phase of the locomotor task was quantified from videotape on Preoperative (PRE) Day 15 and during the 46-day postoperative period. The movement pattern was characterized using six different parameters: the area, the X and Y values of the centroid under the normalized curve of the hindlimb trajectory, the vertical displacement of the hindlimb in the flexion and extension phases of the swing cycle, the maximum instantaneous hindlimb velocity, and the proportion of time spent in the acceleration versus deceleration phases of the swing cycle. In order to disrupt the central pathways for somatosensory information, lesions were made in (i) the right gracile nucleus (GN) (n = 18), (ii) bilateral GN (n = 7), (iii) the right GN and the left VPL (n = 6), and (iv) bilateral VPL (n = 8), and (v) sham-operated animals (n = 5). The run time and the pattern of the hindlimb swing cycle were used as measures of loss and recovery of function. Only the bilateral VPL group showed an impairment in run time and they recovered by Postoperative (POST) Week 4. All groups demonstrated an impairment in initial flexion of the hindlimb during the swing cycle that recovered in the right GN group only. On POST Day 49, the right GN, bilateral GN, and the sham groups received injections of 5% WGA-HRP into both CN to determine the location of these projections in VPL. The CN projections were not redistributed into the gracile area of VPL after GN lesions. Since our previous study (24) had shown the number of synpases in VPL returned to normal after dorsal column nuclei (DCN) lesions by POST Day 50, the recovery of the number of synapses alone was not sufficient to restore the normal gait pattern, while the recovery of the run time preceded the complete recovery of the number of synapses. The recovery of the hindlimb movement pattern during this constrained locomotor task may require the central processing of specific lemniscal information bilaterally by the VPL while behavioral recovery may be subserved by other neural mechanisms, such as the disinhibition of VPL neurons as a result of DCN lesions, allowing the VPL neurons to be more responsive to alternative inputs or physiological adaptations in other ascending systems.