Decreased expression of glial cell line-derived neurotrophic factor signaling in rat models of neuropathic pain

Abstract

1. In an attempt to clarify whether glial cell line-derived neurotrophic factor (GDNF), a survival factor for subpopulations of primary afferent neurons, is involved in the states of neuropathic pain, we observed changes in the expressions of GDNF and its signal-transducing receptor Ret after nerve injury in two rat models of neuropathic pain. 2. In the rats treated with sciatic nerve ligation (chronic constrictive injury (CCI) model) or spinal nerve ligation at L5 (SNL model), the thresholds of paw withdrawal in response to mechanical or heat stimuli began to decrease on the injured side within the first week after the operation and the decreases in the thresholds persisted for more than 2 weeks. 3. In CCI-treated rats, the GDNF contents in L4 and L5 dorsal root ganglia (DRGs) on the injured side were markedly decreased at day 7 after the operation and stayed at low levels at day 14. In SNL-treated rats, comparable reductions of GDNF levels in L4 and L5 DRGs on the injured side were observed at 14 postoperative days. 4. Significant decreases of the percentages of DRG neurons expressing Ret were also observed at L4 DRGs in CCI-treated rats at 7 and 14 postoperative days and in SNL-treated rats at 14 days. 5. In CCI- or SNL-treated rats, continuous intrathecal administration of GDNF (12 microg day-1) using an osmotic pump suppressed the increased sensitivities to nociceptive stimuli to control levels. 6. The present results suggested that the dysfunction of GDNF signaling in the nociceptive afferent system may contribute to the development and/or maintenance of neuropathic pain states.

Figure 1

Thresholds of foot withdrawal of the operated left side (closed circles) and intact right side (open circles) in response to mechanical or thermal stimuli applied to the corresponding hind paw pad in the rats treated with CCI (a) or SNL (b) procedure. ^*P<0.05, ^**P<0.01 and ^***P<0.001 compared with the values on control side at corresponding days by paired t-test; ⁺P<0.05, ⁺⁺P<0.01 and ⁺⁺⁺P<0.001 compared with the values at the preoperative day by Tukey's multiple comparisons. n=6 (CCI) and n=12 (SNL).

Figure 2

Changes in GDNF contents in the L4 (a) and L5 (b) DRGs of control (open circles), CCI-treated (closed squares) and SNL-treated rats (closed circles). ^*P<0.05 and ^**P<0.01 compared with the values of control rats by unpaired t-test. n=5–14.

Figure 3

Immunohistochemistry of Ret expression in the L4 DRGs at day 7 (a–c) and day 14 after the operation (d–f) of control, CCI-treated and SNL-treated rats. Ret-positive cells are stained in red with Alexor Fluor 594-labeled secondary antibody.

Figure 4

Changes in the percentages of Ret-expressing neurons in the L4 (a) and L5 (b) DRGs of control (open circles), CCI-treated (closed squares) and SNL-treated rats (closed circles). ^*P<0.05 and ^**P<0.01 compared with the values of control rats by unpaired t-test. n=4.

Figure 5

Effect of GDNF administration in the rats treated with CCI procedure. Thresholds of foot withdrawal were examined as in Figure 1. GDNF-containing saline (a; n=4) or saline alone (b; n=3) was applied intrathecally by an osmotic pump at day 5 as indicated by the horizontal bars. ^*P<0.05 and ^**P<0.01 compared with the values on the control side at the corresponding days by paired t-test; ⁺P<0.05, ⁺⁺P<0.01 and ⁺⁺⁺P<0.001 compared with the values at the preoperative day by Tukey's multiple comparisons.

Figure 6

Effect of GDNF administration in the rats treated with SNL procedure. Thresholds of foot withdrawal were examined as in Figure 1. GDNF-containing saline (a) or saline alone (b) was applied intrathecally by an osmotic pump at day 5 as indicated by the horizontal bars. ^*P<0.05, ^**P<0.01 and ^***P<0.001 compared with the values on the control side at the corresponding days by paired t-test; ⁺P<0.05 and ⁺⁺⁺P<0.001 compared with the values at the preoperative day by Tukey's multiple comparisons. n=6.