Functional characterization of a mouse model for central post-stroke pain

Abstract

Background: Stroke patients often suffer from a central neuropathic pain syndrome called central post-stroke pain. This syndrome is characterized by evoked pain hypersensitivity as well as spontaneous, on-going pain in the body area affected by the stroke. Clinical evidence strongly suggests a dysfunction in central pain pathways as an important pathophysiological factor in the development of central post-stroke pain, but the exact underlying mechanisms remain poorly understood. To elucidate the underlying pathophysiology of central post-stroke pain, we generated a mouse model that is based on a unilateral stereotactic lesion of the thalamic ventral posterolateral nucleus, which typically causes central post-stroke pain in humans.

Results: Behavioral analysis showed that the sensory changes in our model are comparable to the sensory abnormalities observed in patients suffering from central post-stroke pain. Surprisingly, pharmacological inhibition of spinal and peripheral key components of the pain system had no effect on the induction or maintenance of the evoked hypersensitivity observed in our model. In contrast, microinjection of lidocaine into the thalamic lesion completely reversed injury-induced hypersensitivity.

Conclusions: These results suggest that the evoked hypersensitivity observed in central post-stroke pain is causally linked to on-going neuronal activity in the lateral thalamus.

Keywords: central post-stroke pain; mouse model; pain; stroke.

Figure 1.

Sensory changes following unilateral microinjection of kainate into the VPL nucleus of the thalamus. (a) Mechanical threshold to von Frey filaments applied to the plantar surface of the contralateral hind paw after thalamic kainate or saline injection. Area under the curve (AUC) of the mechanical stimulus-response curve is shown on the right. (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; n = 6–10 mice per group.) (b) Withdrawal latencies on a noxious cold plate (−2℃) of kainate-injected and saline-injected mice. AUC of the stimulus-response curves is shown on the right. (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; *P < 0.05, unpaired, two tailed, t test; n = 6–9 mice per group.) (c) Withdrawal latencies to infrared heat applied to the plantar paw surface of kainate-injected and saline-injected mice. AUC of the stimulus-response curves is shown on right. (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; *P < 0.05, unpaired, two tailed, t test; n = 6–9 mice per group.) (d) Immunostaining reveals significant loss of NeuN-immunoreactivity in the right thalamic VPL nucleus seven days after kainate injection. HC, hippocampus; HT, hypothalamus; VPL, ventral posterolateral nucleus of the thalamus; VPM, ventral posteromedial nucleus of the thalamus; IC, internal capsule. Scale bar: 1 mm.

Figure 2.

Sensory changes following unilateral microinjection of collagenase into the VPL nucleus of the thalamus. (a) Mechanical threshold to von Frey filaments applied to the plantar surface of the contralateral hind paw after thalamic collagenase or saline injection. Area under the curve (AUC) of the mechanical stimulus-response curve is shown on the right. (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; n = 6–8). (b) Withdrawal latencies to a noxious cold plate (−2℃) of collagenase-injected and saline-injected mice. AUC of the stimulus-response curves is shown on the right. (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; *P < 0.05, unpaired, two tailed, t test; n = 5–10). (c) Percentage of time of a 10 min observation period spent on a plate set at a non-noxious cold temperature (18℃) vs. a plate at 30℃ of collagenase-injected animals vs. controls (*P < 0.05, unpaired two tailed t test; n = 4–11). (d,e) Withdrawal latencies to infrared heat applied to the plantar paw surface of collagenase-injected and control mice. AUC of the stimulus-response curves is shown in (e). (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; *P < 0.05, unpaired, two tailed, t test; n = 6–10.) (f) Hemorrhage in the right thalamus, including the ventral posterolateral nucleus (VPL), three days after collagenase injection. HC, hippocampus; HT, hypothalamus; VPL, ventral posterolateral nucleus of the thalamus; VPM, ventral posteromedial nucleus of the thalamus; IC, internal capsule. Scale bar: 1 mm.

Figure 3.

Blocking TRPV1 positive C-fibers does not reverse mechanical hypersensitivity induced by thalamic collagenase injection. (a) Timeline of thalamic collagenase injection and intraplantar QX-314/capsaicin injection. (b) Injection of QX-314/capsaicin significantly increases mechanical threshold in saline-injection mice but fails to do so in collagenase-injected animals (^tP < 0.05 as compared to basal; *P < 0.05 as compared to corresponding control mice; repeated measures ANOVA, post-hoc Fisher’s test; n = 6–8).

Figure 4.

Ablation of lamina I/III NK1 receptor positive neurons in the spinal cord does not affect induction or maintenance of hypersensitivity associated with thalamic collagenase injection. (a) Timeline of spinal NK1 receptor positive neuron ablation by intrathecal SSP-SAP injection and induction of hypersensitivity by thalamic collagenase injection. (b, c) SSP-SAP treatment 28 days prior to thalamic collagenase injection does not prevent development of mechanical (b) or cold (c) hypersensitivity as compared to the corresponding SAP treated control mice. (d) Timeline showing the time points of hypersensitivity induction by thalamic collagenase injection and pharmacological intervention by intrathecal SSP-SAP injection. (e, f) Mechanical (e) and cold (f) hypersensitivity is maintained to a similar degree in SSP-SAP injected animals and controls. (g) Immunohistochemistry with anti-NK1-receptor antibody demonstrating SSP-SAP induced ablation of lamina I NK1 receptor positive neurons in the spinal cord. Scale bar: 300 µm. Data were analyzed via repeated measures ANOVA, post-hoc Fisher’s test in b–f; n = 4–5 mice per group; *P < 0.05 compared to controls.

Figure 5.

Lidocaine microinjection into the injured thalamic VPL nucleus reverses mechanical and cold hypersensitivity induced by thalamic collagenase injection. (a) Timeline of thalamic collagenase injection and intrathalamic lidocaine injection using a guide cannula. (b,c) Lidocaine but not saline microinjection into the injured VPL nucleus reverses mechanical (b) and noxious cold (c) hypersensitivity induced by intrathalamic collagenase injection. (*P < 0.05 compared to controls; repeated measures ANOVA, post-hoc Fisher’s test; n = 4).