Glial cell line-derived neurotrophic factor-mediated enhancement of noradrenergic descending inhibition in the locus coeruleus exerts prolonged analgesia in neuropathic pain

Abstract

Background and purpose: The locus coeruleus (LC) is the principal nucleus containing the noradrenergic neurons and is a major endogenous source of pain modulation in the brain. Glial cell line-derived neurotrophic factor (GDNF), a well-established neurotrophic factor for noradrenergic neurons, is a major pain modulator in the spinal cord and primary sensory neurons. However, it is unknown whether GDNF is involved in pain modulation in the LC.

Experimental approach: Rats with chronic constriction injury (CCI) of the left sciatic nerve were used as a model of neuropathic pain. GDNF was injected into the left LC of rats with CCI for 3 consecutive days and changes in mechanical allodynia and thermal hyperalgesia were assessed. The α2 -adrenoceptor antagonist yohimbine was injected intrathecally to assess the involvement of descending inhibition in GDNF-mediated analgesia. The MEK inhibitor U0126 was used to investigate whether the ERK signalling pathway plays a role in the analgesic effects of GDNF.

Key results: Both mechanical allodynia and thermal hyperalgesia were attenuated 24 h after the first GDNF injection. GDNF increased the noradrenaline content in the dorsal spinal cord. The analgesic effects continued for at least 3 days after the last injection. Yohimbine abolished these effects of GDNF. The analgesic effects of GDNF were partly, but significantly, inhibited by prior injection of U0126 into the LC.

Conclusions and implications: GDNF injection into the LC exerts prolonged analgesic effects on neuropathic pain in rats by enhancing descending noradrenergic inhibition.

Figure 1

Effects of GDNF injection into the LC on neuropathic pain. (A) Mechanical allodynia and thermal hyperalgesia were assessed on the ipsilateral and contralateral hind paws of CCI rats. GDNF (1.5 μg) or saline was repetitively administered into the ipsilateral LC as indicated by arrows. **P < 0.01, ***P < 0.001: compared with saline treatment. Paw withdrawal threshold was assessed using the Mann–Whitney U-test with Bonferroni correction, while latency was evaluated using the unpaired t-test with Bonferroni correction; n = 4–6. (B) Schematic depiction of the microinjection sites. The numbers on the left of the coronal sections represent the distances from the bregma. The different symbols represent the tip sites in CCI rats injected with GDNF or saline and in CCI rats injected with GDNF outside the LC. (C) A representative image of the cannula track in the LC. Scale bar, 100 μm.

Figure 2

Acute effects of GDNF injection into the LC on neuropathic pain. (A) Mechanical allodynia and thermal hyperalgesia were assessed on the ipsilateral hind paw of CCI rats. A single dose of GDNF (1.5 μg) or saline was injected into the ipsilateral LC on day 7 after CCI. **P < 0.01; compared with saline treatment; unpaired t-test with Bonferroni correction; n = 4–6. (B) Noradrenaline content was assayed in the fifth lumbar dorsal spinal cord of CCI rats 24 h after GDNF or saline injection. *P < 0.05; compared with saline treatment; unpaired t-test; n = 3–6.

Figure 3

Dose-dependent analgesic effects of GDNF. GDNF or saline was administered once a day from day 7 to 9 after CCI. Mechanical allodynia and thermal hyperalgesia were assessed on the ipsilateral hind paw of CCI rats on day 10. *P < 0.05, **P < 0.01, ***P < 0.001; compared with saline. Differences in paw withdrawal threshold and latency were evaluated using the Steel test and Dunnett's test, respectively; n = 4–11.

Figure 4

Effects of intrathecal administration of the α₂-adrenoceptor antagonist yohimbine on the analgesic effects of GDNF. Yohimbine or saline was intrathecally administered on day 10 in CCI rats after three GDNF injections. Mechanical allodynia and thermal hyperalgesia were assessed on the ipsilateral hind paw 20 min after the yohimbine injection. **P < 0.01, ***P < 0.001; compared with saline treatment. Paw withdrawal threshold was evaluated using the Mann–Whitney U-test with Bonferroni correction, while latency was assessed using the unpaired t-test with Bonferroni correction; n = 7–8.

Figure 5

Effects of the MEK inhibitor U0126 on the analgesic effects of GDNF. U0126 or vehicle was injected into the LC 30 min prior to GDNF injection. Both U0126 and GDNF were injected once a day on days 7 to 9 as indicated by the arrows. *P < 0.05, **P < 0.01, ***P < 0.001; compared with vehicle. Paw withdrawal threshold was evaluated using the Mann–Whitney U-test with Bonferroni correction, while latency was assessed using the unpaired t-test with Bonferroni correction; n = 7.