Thoracic dorsal funicular lesions affect the bouton patterns on, and diameters of, layer VB pyramidal cell somata in rat hindlimb cortex

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Abstract
The effect of spinal dorsal funicular lesions (T 12) upon the frequency of boutons on, and diameters of the somata of pyramidal cells in layer VB of hindlimb cortex was studied. Adult rats sustained bilateral damage to either the dorsal column (DC, n = 10) alone or DC combined with the corticospinal tract (CS) (DC+CS, n = 34) and were utilized 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperatively (DPO). Neurons randomly sampled from 44 lesioned and 13 unoperated cases were analyzed for the number of silver-impregnated boutons (Rasmussen method) on the circumference of the soma as well as diameters of the soma, nucleus, and nucleolus. Analyses of variance comparing across lesioned and normal groups were significant for bouton counts on the soma (P < 0.01), and diameters (long axis) of somata (P < 0.01) and their nuclei (P < 0.05). Both lesioned groups exhibited significant decreases from normal for these latter three parameters. With respect to survival time for the DC+CS-lesioned animals we noted the following: (1) Bouton counts on the soma significantly decreased below normal between 1 and 60 DPO; this decrease was most dramatic during the first three days postlesion. (2) Somal diameter (long axis) significantly decreased below normal between 2 and 120 DPO (except at 14 and 90 DPO). (3) Nuclear diameter (long axis) significantly decreased below normal only at 90 DPO. (4) Bouton counts on somata of neurons in layers VB and IV [Ganchrow and Bernstein, 1981] of hindlimb cortex correlated negatively and significantly across 120 postlesion days. The rapid shrinkage and reduced afferentation of layer VB somata during the first week following DC+CS lesions suggest initial, retrograde reactions to CS axotomy. Since bouton counts on layer VB somata were significantly less (P < 0.05) in DC- than DC+CS-lesioned rats, it is hypothesized that CS axotomy regulated a set-point for increased afferentation which was maintained on the shrunken somata between 7 and 120 DPO.