Patterns of reafferentation in rat nucleus gracilis after thoracic dorsal column lesions

The effect of a bilateral, dorsal column lesion (T12) on the frequency of boutons on nucleus gracilis (NG) neurons was studied. Rats were killed 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperative (DPO), and bouton counts were taken on the soma, or for 5- and 10-μm distances on the proximal dendrite branchings from the soma, in the rostral (level of area postrema), middle, and caudal NG. Significant changes in boutons on the soma occurred mainly during the first week postlesion, in the caudal and middle NG. Only at 7 DPO were the counts significantly less than normal; they increased dramatically at 90 DPO. Bouton frequency correlated significantly during 120 days postlesion between the caudal and middle NG. These data are consistent with previous reports on the distribution in the NG of hind limb afferent fibers coursing in the thoracic dorsal columns; the increase in boutons at 90 DPO is supported by data in other systems. All NG levels showed changes in bouton counts on the proximal dendrite: (i) during the first week postlesion, bouton frequency decreased at 1 and 3 DPO, and increased at 2 and 7 DPO, significantly; (ii) at 7 and 2 DPO the counts were significantly higher than at most longer postlesion times; (iii) only bouton counts at 5 μm on the proximal dendrite (rostral NG) correlated significantly, and negatively, with counts on the soma during 120 days postlesion. These data emphasize the dynamic changes in afferentation of NG neurons during the first postlesion week, which parallel the progressive stages of bouton degeneration and phagocytosis as reported in several sensory systems.