Differential expression of the p75 nerve growth factor receptor in glia and neurons of the rat dorsal root ganglia after peripheral nerve transection

Abstract

Sympathetic nerve terminals on blood vessels within the dorsal root ganglia sprout after sciatic nerve lesions in the rat. The mechanism underlying this phenomenon is not clear, but might be predicted to involve nerve growth factor or its homologs because these factors are known to trigger collateral sprouting of undamaged sympathetic noradrenergic terminals. We have found that sciatic nerve lesions lead to a decreased expression of neuronal p75, the low-affinity receptor for the neurotrophins, but an increased expression of glial p75 in ipsilateral dorsal root ganglia. Intriguingly, the increased expression of p75 was found primarily in association with glia surrounding large-diameter neurons, which are those associated with the noradrenergic sprouts. A smaller but significant glial response was also found in contralateral ganglia. The glial response in ipsilateral ganglia could be mimicked by ventral, but not dorsal, root transection. The dorsal root lesion-induced glial responses in contralateral ganglia were greater than those induced by ventral root or sciatic nerve lesions. Combined lesions of dorsal root and either ventral root or sciatic nerve did not prevent the glial responses of ipsilateral ganglia, suggesting that a peripheral signal is involved. Colocalization studies indicate that tyrosine hydroxylase-immunoreactive nerve sprouts were associated with p75-immunoreactive glial cells. Thus, increased glial synthesis of p75 might provide an explanation for the abnormal growth of sympathetic fibers in dorsal root ganglia after peripheral nerve injury.

Fig. 1.

Schematic diagrams showing different nerve lesion paradigms (see Materials and Methods).

Fig. 2.

Effects of sciatic nerve transection on the expression of p75-ir within L5 DRG. 1, A section from sham-operated rats showing subpopulations of large and small p75-ir neurons. 2, A section from contralateral DRG 8 weeks after sciatic nerve transection showing increased expression of p75-ir in fibers and glia surrounding some large neurons. 3–6, One, two, four, and eight weeks, respectively, after ipsilateral sciatic nerve transection showing decreased expression of p75-ir in neurons and increased expression in nerve bundles and glia. Asterisksindicate a neuron with surrounding ring of immunoreactivity; openarrows indicate clusters of p75-ir glial cells; filledarrows indicate p75-ir nerve bundles. Scale bar, 50 μm.

Fig. 3.

Effect of sciatic nerve transection on the percentage of p75-ir neurons in L5 DRG. The numbers of p75-ir neurons were counted as described in Materials and Methods. The data are plotted as mean ± SEM (n = 4). The percentage of p75-ir neurons was significantly reduced after sciatic nerve transection (asterisks indicate p < 0.05 compared with control).

Fig. 4.

Effect of sciatic nerve transection on the number of p75-ir ring structures surrounding neurons. The neurons surrounded by p75-ir glia or nerve fibers were counted as described in Materials and Methods and are plotted as mean ± SEM (n = 4). The percentage of neurons with ring structures in ipsi- and contralateral DRG after sciatic nerve lesion was significantly increased with postlesion time (asterisks indicate p < 0.05 compared with control).

Fig. 5.

Colocalization of p75-ir with GFAP-ir in the ipsilateral (panels 1, 2, 5,6) and contralateral (panels 3, 4) L5 DRG after sciatic nerve transection. Panels 1,3, and 5 are sections stained with mouse antibodies to p75 followed by AMCA-conjugated secondary antibodies, andpanels 2, 4, and 6 are the same sections stained with rabbit antibodies to GFAP followed by FITC-conjugated secondary antibodies. Asterisks indicate neurons immunoreactive for p75 but negative for GFAP-ir;arrowheads in 1–4 indicate large neurons with p75-ir ring structures that were also immunoreactive for GFAP;arrowheads in 5 and 6 indicate clusters of glial cells that are both p75-ir and GFAP-ir; openarrows in 1 and 2 indicate a small neuron without p75-ir ring structure but with GFAP-ir. Scale bars:1–4 (shown in 2), 100 μm; 5,6 (shown in 6), 50 μm.

Fig. 6.

Effects of ventral root, dorsal root, and combined dorsal and ventral root transections on p75-ir in L5 DRGs.1, A micrograph showing normal expression of p75 in the sham-operated DRGs. 2, A micrograph showing ring structures around large neurons from an animal with an L5 ventral root transection one week earlier. An open arrow indicates intact dorsal root, and an arrowhead indicates lesioned ventral root.3, 4, Micrographs of the respective ipsi- and contralateral DRG from an animal with a transection of dorsal root of the L5 spinal ganglion one week earlier. Arrowheads in3 indicate intensely labeled DRG neurons. 5,6, Micrographs of respective ipsi- and contralateral DRG from an animal with combined lesion of dorsal and ventral root of the L5 spinal ganglion 1 week earlier. Scale bar, 200 μm.

Fig. 7.

Effects of nerve lesions on the numbers of p75-ir ring structures in the DRGs ipsi- and contralateral to the lesions. The surgical procedures were as described in Materials and Methods.Shaded bars, Ipsilateral DRGs; filledbars, contralateral DRGs. Sci. n, Sciatic nerve division; VR, ventral root section;DR, dorsal root section. ANOVA: F = 31.7, p < 0.01; *p < 0.01 compared with control; ⁺p < 0.05 compared with DRG contralateral to sciatic nerve division alone (filledbar).

Fig. 8.

Colocalization of TH-ir axons with p75-ir glia and nerve fibers in ipsilateral DRG 1 week after sciatic nerve lesion. Pairs of micrographs (1, 2; 3,4; 5, 6) are each of the same section using different filters. Large arrowheads in1, 3, and 5 indicate TH-ir nerve fibers; small arrowheads in 2 indicate p75-ir satellite cells associated with a large neuron surrounded by TH-ir nerve terminals (1); arrowheads in4 indicate nerve bundles immunoreactive for p75 in the same site as TH-ir fibers (3); arrowheads in6 indicate p75-ir colocalized with a TH-ir nerve fiber. Scale bar (shown in 1), 100 μm. In 1,3, and 5, which show TH-ir fibers, significant autofluorescence of sensory neurons was observed with the filter for FITC.