Patterns of afferentation in rat ventroposterolateral nucleus after thoracic dorsal column lesions

The secondary transneuronal effect of bilateral, dorsal column lesion (T12) on the frequency of boutons on cells in the lateral ventroposterolateral nucleus (VPL) was studied. Rats were killed 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperative (DPO), and bouton counts (Rasmussen stain) were made on the soma, or along 5- and 10-μm segments of the proximal dendrite branching from soma, in the rostral two-thirds of the VPL. The soma diameter also was measured of those neurons chosen for bouton counts on the circumference of the soma. Significant, increased afferentiation along a 5-μm segment of the proximal dendrite occurred during the first 3 days postlesion compared with longer survival times. Along the 10-μm segment, as at 5 μm on the proximal dendrite, bouton counts at 2 DPO were significantly higher than at 30 and 120 DPO. This rapid change in afferentation during the first week postlesion mirrors similar effects reported in the nucleus gracilis of these same cases. Bouton counts along the 5-μm segment of the proximal dendrite decreased significantly to less than normal at 30 and 120 DPO, indicating the nonstable and protracted character of afferentation after the distal cord lesion. A significant, positive correlation was obtained between boutons on the soma and proximal dendrite (5 μm segment) in cases with lesions, suggesting parallel innervation patterns for these adjacent neuronal components during 120 postoperative days. There was no significant change in the bouton counts or diameter of the soma during postoperative days. These results suggest that lesion of the thoracic dorsal column may result in subsequent rapid bouton patterning on the lateral VPL neurons, presumably mediated by their medial lemniscal afferent fibers. The postlesion source(s) of afferentation remains an intriguing, and as yet unanswered, question.