The animal protocols were approved by Zhejiang Chinese Medical University Animal Ethics Committee.
1. Animals
2. CPIP model establishment
3. Nocifensive behavioral tests
After placing the O-ring on the ankle, the ipsilateral hind paw skin showed cyanosis, an indication of tissue hypoxia (Figure 1A). After cutting the O-ring, the ipsilateral hind paw began to fill with blood and showed robust swelling, which demonstrated an intense sign of hyperemia (Figure 1A). The paw swelling gradually diminished and returned to normal 48 h after the ischemic/reperfusion procedure (two-way ANOVA with Sidak post-hoc test, Figure 1B). All of these signs are consistent with previous studies6,12.
Then, mechanical allodynia was measured using a von Frey hair test. The ipsilateral hind paw of the CPIP group exhibited obvious mechanical allodynia 1 day after model establishment compared to the sham group. The mechanical allodynia of the ipsilateral hind paw persisted until 13 days of the observation timeframe (two-way ANOVA with Sidak post-hoc test, Figure 1C). The contralateral hind paw of the CPIP group also displayed mechanical hyperalgesia similar to the ipsilateral hind paw, lasting for 13 days (two-way ANOVA with Sidak post-hoc test, Figure 1D).
Thermal hyperalgesia was then measured using a Hargreaves' test. Bilateral hind paws of CPIP rats exhibited significantly reduced withdrawal latency in response to noxious thermal stimuli, a sign of thermal hyperalgesia, compared to the sham group rats (two-way ANOVA with Sidak post hoc test, Figure 1E,F). This observation is consistent with previous studies11,12. The thermal hyperalgesia of the ipsilateral hind paw persisted until the end of the observation timeframe, whereas the thermal hyperalgesia of the contralateral hind paw lasted for 7 days (two-way ANOVA with Sidak post-hoc test, Figure 1E,F). The above results suggest that CPIP rats develop robust and persistent mechanical and thermal hypersensitivities, consistent with previous observations11,12.
CRPS patients exhibited an obvious increased response to capsaicin-induced pain in affected areas20. It was then examined whether the CPIP rat model can recapitulate this phenomenon. Nocifensive behavior in CPIP rats was observed in response to intraplantar capsaicin (a TRPV1 agonist) injection into the ipsilateral hind paw. First, the nocifensive responses of the rats when a vehicle was injected were tested. The sham group showed a slight nocifensive response to vehicle injection, whereas the CPIP group showed a significantly higher response compared to the sham group (one-way ANOVA with Sidak post-hoc test, Figure 2). Furthermore, capsaicin injection resulted in robust nocifensive response in the sham group (Figure 2). More importantly, CPIP rats showed significantly higher responses to capsaicin injection than the sham group (Figure 2). These data suggest that CPIP rats exhibited enhanced nocifensive responses to capsaicin, a phenomenon mimicking human patients with CRPS-I.
Figure 1: The CPIP rat model showed thermal and mechanical pain hypersensitivities in bilateral hind limbs. (A) Typical images taken during different time points (during ischemia, 10 min after reperfusion, and 7 days later). (B) Ipsilateral hind paw thickness measurements of both CPIP and sham groups. (C,D) 50% paw withdraw threshold (50% PWT, index of mechanical hyperalgesia) of ipsilateral (C) and contralateral (D) hind paws of rats. (E,F) Paw withdrawal latency (PWL, index of thermal hyperalgesia) of ipsilateral (E) and contralateral (F) hind paws of rats (n = 8 rats per group, **p < 0.01). Results are expressed as mean ± SEM (two-way ANOVA followed by Sidak post-hoc test). Please click here to view a larger version of this figure.
Figure 2: CPIP rats exhibited more nocifensive behaviors compared to sham rats in response to intraplantar capsaicin injection into ipsilateral hind paws. The cumulated time the rats spent licking, biting, or flinching the hind paws was calculated over 10 min after vehicle (0.1% DMSO in PBS, intraplantar) or capsaicin (10 nmol in 50 μL volume, intraplantar) injection (n = 8 rats per group, **p < 0.01). Results are expressed as mean ± SEM (one-way ANOVA followed by Sidak post-hoc test). Please click here to view a larger version of this figure.
1.5 ml Eppendorf tube | Eppendorf | 22431021 | |
DMSO | Sigma-Aldrich | D1435 | |
Capsaicin | APEXBIO | A3278 | |
Digital caliper | Meinaite | NA | |
O-ring | O-Rings West | Nitrile 70 Durometer | 7/32 in. internal diameter |
Plantar Test Apparatus | UGO Basile, Italy | 37370 | |
von Frey filaments | UGO Basile, Italy | NC12775 |
Complex regional pain syndrome type-I (CRPS-I) is a neurological disease that causes severe pain among patients and remains an unresolved medical condition. However, the underlying mechanisms of CRPS-I have yet to be revealed. It is known that ischemia/reperfusion is one of the leading factors that causes CRPS-I. By means of prolonged ischemia and reperfusion of the hind limb, the rat chronic post-ischemia pain (CPIP) model has been established to mimic CRPS-I. The CPIP model has become a well-recognized animal model for studying the mechanisms of CRPS-I. This protocol describes the detailed procedures involved in the establishment of the rat model of CPIP, including anesthesia, followed by ischemia/reperfusion of the hind limb. Characteristics of the rat CPIP model are further evaluated by measuring the mechanical and thermal hypersensitivities of the hind limb as well as the nocifensive responses to acute capsaicin injection. The rat CPIP model exhibits several CRPS-I-like manifestations, including hind limb edema and hyperemia in the early stage after establishment, persistent thermal and mechanical hypersensitivities, and increased nocifensive responses to acute capsaicin injection. These characteristics render it a suitable animal model for further investigation of the mechanisms involved in CRPS-I.
Complex regional pain syndrome type-I (CRPS-I) is a neurological disease that causes severe pain among patients and remains an unresolved medical condition. However, the underlying mechanisms of CRPS-I have yet to be revealed. It is known that ischemia/reperfusion is one of the leading factors that causes CRPS-I. By means of prolonged ischemia and reperfusion of the hind limb, the rat chronic post-ischemia pain (CPIP) model has been established to mimic CRPS-I. The CPIP model has become a well-recognized animal model for studying the mechanisms of CRPS-I. This protocol describes the detailed procedures involved in the establishment of the rat model of CPIP, including anesthesia, followed by ischemia/reperfusion of the hind limb. Characteristics of the rat CPIP model are further evaluated by measuring the mechanical and thermal hypersensitivities of the hind limb as well as the nocifensive responses to acute capsaicin injection. The rat CPIP model exhibits several CRPS-I-like manifestations, including hind limb edema and hyperemia in the early stage after establishment, persistent thermal and mechanical hypersensitivities, and increased nocifensive responses to acute capsaicin injection. These characteristics render it a suitable animal model for further investigation of the mechanisms involved in CRPS-I.
Complex regional pain syndrome type-I (CRPS-I) is a neurological disease that causes severe pain among patients and remains an unresolved medical condition. However, the underlying mechanisms of CRPS-I have yet to be revealed. It is known that ischemia/reperfusion is one of the leading factors that causes CRPS-I. By means of prolonged ischemia and reperfusion of the hind limb, the rat chronic post-ischemia pain (CPIP) model has been established to mimic CRPS-I. The CPIP model has become a well-recognized animal model for studying the mechanisms of CRPS-I. This protocol describes the detailed procedures involved in the establishment of the rat model of CPIP, including anesthesia, followed by ischemia/reperfusion of the hind limb. Characteristics of the rat CPIP model are further evaluated by measuring the mechanical and thermal hypersensitivities of the hind limb as well as the nocifensive responses to acute capsaicin injection. The rat CPIP model exhibits several CRPS-I-like manifestations, including hind limb edema and hyperemia in the early stage after establishment, persistent thermal and mechanical hypersensitivities, and increased nocifensive responses to acute capsaicin injection. These characteristics render it a suitable animal model for further investigation of the mechanisms involved in CRPS-I.