Spinal NF-κB and Chemokine Ligand 5 Expression during Spinal Glial Cell Activation in a Neuropathic Pain Model
Spinal NF-B and Chemokine Ligand 5 Expression during Spinal Glial Cell Activation in a Neuropathic Pain Model
Qin Yin 0 1
Qin Fan 0 1
Yu Zhao 0 1
Ming-Yue Cheng 0 1
He Liu 0 1
Jing Li 0 1
Fei- Fei Lu 0 1
Jin-Tai Jia 0 1
Wei Cheng 0 1
Chang-Dong Yan 0 1
0 1 Xuzhou Medical College , Xuzhou , China , 2 Jiangsu Province Key Laboratory of Anesthesiology and Center for Pain Research and Treatment; Jiangsu Province Key Laboratory of Anesthesiology, Xuzhou, China 3 Affiliated Hospital of Xuzhou Medical College , Xuzhou , China , 4 Affiliated Heping Hospital of Changzhi Medical College , Changzhi , China
1 Academic Editor: Michael Costigan, Boston Chil- dren's Hospital and Harvard Medical School , UNITED STATES
Competing Interests: The authors have declared
that no competing interests exist.
The NF-B pathway and chemokine (C-C motif) ligand 5 (CCL5) are involved in pain
modulation; however, the precise mechanisms of their interactions in chronic neuropathic pain
have yet to be established.
The present study examined the roles of spinal NF-B and CCL5 in a neuropathic pain
model after chronic constriction injury (CCI) surgery. CCI-induced pain facilitation was
evaluated using the Plantar and von Frey tests. The changes in NF-B and CCL5 expression
were analyzed by immunohistochemistry and Western blot analyses.
Spinal NF-B and CCL5 expression increased after CCI surgery. Repeated intrathecal
infusions of pyrrolidine dithiocarbamate (PDTC, a NF-B inhibitor) decreased CCL5
expression, inhibited the activation of microglia and astrocytes, and attenuated CCI-induced
allodynia and hyperalgesia. Intrathecal injection of a CCL5-neutralizing antibody attenuated
CCI-induced pain facilitation and also suppressed spinal glial cell activation after CCI
surgery. However, the CCL5-neutralizing antibody did not affect NF-B expression.
Furthermore, selective glial inhibitors, minocycline and fluorocitrate, attenuated the hyperalgesia
induced by intrathecal CCL5.
The inhibition of spinal CCL5 expression may provide a new method to prevent and treat
nerve injury-induced neuropathic pain.
Neuropathic pain is a therapeutic challenge and is often associated with peripheral nerve injury
with characteristic pain facilitation. Previous studies have suggested that chemokines play an
essential role in glial cell activation, inflammatory pain and neuropathic pain . Glial
selective inhibitors partially antagonize pain hypersensitivities and the up-regulation of chemokines
in different pain models . Nevertheless, the neuroimmune mechanisms that mediate glial
cell activation in neuropathic pain are still unknown.
Chemokine (C-C motif) ligand 5 (also CCL5) is secreted by macrophages, platelets, and
glial cells in the central nervous system (CNS) . Furthermore, intracistemal injection of
CCL5 remarkably increased the duration and amount of scratching in the itching model .
When the midbrain periaqueductal grey (PAG) receives a CCL5 injection, apparent
hyperalgesia is observed . These results highlight the significance of chemokines in the CNS .
Studies have previously demonstrated that CCL5 may play a role in different pain models in
the spinal cord . Activating the NF-B pathway often promotes the activation of a
series of genes and neurotransmitters, which leads to chemokine secretion and pain
hypersensitivities [22, 23]. Intrathecal infusion of the NF-B inhibitor (pyrrolidine dithiocarbamate,
PDTC) delays and reverses pain facilitation in neuropathic pain .
However, the precise mechanisms of the NF-B pathway and the interactions between
NFB and CCL5 in chronic neuropathic pain have yet to be established. NF-B inhibition may
attenuate pain facilitation via CCL5 inhibition at the spinal level. We investigated the underlying
mechanisms of the expression and inhibition of glial cell activation as well as NF-B and CCL5
and their interactions in the spine in a neuropathic pain model following CCI surgery.
Male SD rats (250280 grams, 68 weeks) were housed in groups of 2 in clear plastic cages
with solid floors covered with 36 cm of soft bedding (sawdust) and were maintained in
controlled environments (21 2C; 6070% relative humidity; 12 h dark/light cycles with ad
libitum access to food and water). The rats were acclimatized for three days before any empirical
procedures. All testing procedures were approved by the Animal Ethics Committee of Xuzhou
Medical College. All experiments were conducted in compliance with the institutional
A CCI-induced neuropathic pain model was established according to a previously described
method . Four chromic gut ligatures were loosely created around the left sciatic nerve after
anesthesia (pentobarbital 50 mg/kg, i.p.). Sham-operated animals underwent the same surgical
procedure, but no ligatures were placed around the nerve. The animals were allowed to recover
for 72 hours to ensure the well-being of the rats after the CCI surgery. Only rats that exhibited
a normal gait were included in the experiments.
Lumbosacral intrathecal catheters were constructed and implanted as detailed in a previous
study . This method avoids pressure on the spine and the reactive ensheathment during
surgery. The catheter was utilized to thread caudally from the cisterna magna after anesthesia
(pentobarbital, 50 mg/kg, i.p.). The catheter locations were verified by visual inspection after
the behavioral analysis. Only the data obtained from rats in which the distal ends of the
catheter were located at the lumbo-sacral spinal level were analyzed.
Pyrrolidine dithiocarbamate (PDTC), minocycline and fluorocitrate were obtained from Sigma
(St. Louis, MO, USA). The normal goat IgG, anti-CCL5 neutralizing antibody and recombinant
rat CCL5 were purchased from R&D Systems (Minneapolis, MN, USA). Anti-rat CCL5, rabbit
anti-rat NF-B p65 and mouse anti-rat -actin were obtained from Santa Cruz (Santa Cruz,
CA, USA). Fluorescein isothiocyanate (FITC)-conjugated IgG and tetraethyl rhodamine
isothiocyanate (Jackson Immunolab, West Grove, PA, USA), glial fibrillary acidic protein (GFAP,
Millipore, Bedford, MA, USA), ionized calciumbinding adapter molecule 1 (Iba-1, Abcam),
and neuronal specific nuclear protein (NeuN, neuronal marker, NOVUS) were purchased. The
dosages of intrathecal drugs and peptides were chosen according to former studies [17, 29] and
our preliminary tests.
The rats were placed on a 5 5 mm wire mesh grid floor, and testing was conducted blindly
with respect to the group. Consistent with the Chaplan study , mechanical allodynia was
observed by withdrawal responses using von Frey incitation after a 30-min accommodation.
The von Frey filaments were inserted through the mesh floor bottom and were applied to the
middle of the plantar surface of the hind paw with a weight of 4.0, 6.0, 8.0, 10.0, and 15.0 g
(Stoelting, Wood Dale, IL, USA). The 50% paw withdrawal threshold was determined using
Chaplans up-down method as previously described.
The rats were placed on top of a glass sheet and covered with a clear cage. After adapting for 30
minutes, thermal hyperalgesia was evaluated by withdrawal latency using the Plantar Test
Analgesia Meter (BEM-410A, Chinese Academy of Medical Sciences Medical Research Institute
of Biology). The radiant heat source was positioned under the glass sheet and applied to the
plantar surface of the hind paw. The withdrawal latencies of the hind paws were measured five
times at 5 min intervals. Data are presented as the mean latency of the last three stimulations
. A cut-off latency of 25 s was set for each measurement to avoid tissue damage.
The left (ipsilateral to the CCI side) L45 spinal cord segments were collected for Western blot
analyses. The total protein was extracted from the spinal segments, and 20 g of extracts were
separated by 1015% SDS-PAGE and transferred to a PVDF membrane. The membrane was
blocked with 5% nonfat dry milk and incubated with mouse anti-rat CCL5 (1:100) and rabbit
anti-rat NF-B (1:1000) or mouse anti-rat -actin (1:1000) primary antibodies. The membrane
was washed and incubated with alkaline phosphatase (ALP)-conjugated goat anti-rabbit or
goat anti-mouse secondary antibody and treated with the NBT/BCIP Western blotting
substrate (Promega Corporation, Madison, WI, USA). All Western blots were performed at least
three times, and the data were consistent among the experiments. A previous method was
utilized to calculate the density of the band area . An equivalent-sized square was drawn to
quantify the density around each band, and the background surrounding the band was
subtracted. -actin expression was utilized as an internal control, and the protein level was
standardized to the -actin level.
The L45 spinal segments were post-fixed in fixative for 24 h at 4C and immersed in 30%
sucrose in PBS for 2448 h at 4C for cryoprotection. A frozen longitudinal slice (2030 m) was
prepared. The section was blocked with 10% donkey serum in PBS and was incubated with the
mouse anti-rat CCL5 (1:50) and rabbit anti-rat NF-B (1:100) antibodies overnight at 4C.
The antibodies against the proteins of spinal cord cells include NeuN (1:600), GFAP (1:300),
and Iba-1 (1:400). The sections were incubated in specific secondary antibodies that were
conjugated with FITC-conjugated IgG (1:200) or tetraethyl rhodamine isothiocyanate (1:200) for
120 min at 4C and then washed in PBS. The primary antibody was omitted in the negative
control. All sections were cover-slipped with a mixture of 50% glycerin in 0.01 M PBS and then
viewed under a Leica fluorescence microscope. The images were captured with a CCD spot
camera. The cell counts may not sufficiently reflect activation due to the complex morphology
of the neurons and gliocytes and the immunoreactive staining associated with cell bodies and
their processes. Therefore, the optical density of the immunoreactive staining was measured
with the Leica Qwin 500 image analysis system (Germany). The relative density of the images
was determined by subtracting the background density in each image. Six spinal L45 sections
were randomly selected from each animal for densitometric analysis to obtain the mean density
for each animal.
The data in the results section are presented as the mean standard error (S.E.M.). All
experiments were performed blindly. The Tukeys post-hoc test in one-way ANOVA was utilized to
perform multiple comparisons between all groups tested. The StudentNewmanKeuls
posthoc test and repeated measures ANOVA in two-way ANOVA were utilized to analyze the
post-drug time course measures in the behavioral tests. Statistical significance was established
at P < 0.05.
Repeated intrathecal use of PDTC (1000 pmol/d) did not change the paw withdrawal threshold
(WT) or paw withdrawal latency (WL) in the sham + PDTC 1000 pmol/d group (ANOVA,
P > 0.05). Compared with the rats in the CCI + saline group, the WT and WL of the CCI +
PDTC group (100 pmol/d and 1000 pmol/d, on days 02 or day 47) were dose-dependently
increased (two-way ANOVA, P < 0.01) (Fig. 1).
PDTC suppressed the CCI-induced glial cell activation and NF-B and
CCL5 expression in the spinal segments
The ipsilateral L45 spinal cord segments were collected on day 7 after CCI surgery and
were examined. Western blot analysis indicated that the NF-B and CCL5 expression
remarkably increased in the ipsilateral spinal segments compared with the sham group (ANOVA,
P < 0.01) (Fig. 2A).
According to the western blot (Fig. 2B) and immunohistochemistry (Fig. 2C) analysis,
PDTC attenuated the CCI-induced changes in NF-B and CCL5 expression, as demonstrated
by the decreased intensity of NF-B and CCL5 (ANOVA, P < 0.01).
PDTC attenuated the up-regulation of spinal GFAP and Iba-1 following CCI surgery as
shown by the reduced intensity of GFAP and Iba-1 staining (ANOVA, P < 0.01, comparing
the CCI + saline and CCI + PDTC 1000 pmol/d groups) (Fig. 3).
CCL5-neutralizing antibodies delayed and attenuated CCI-induced
allodynia and hyperalgesia
Repeated intrathecal infusions of the CCL5-neutralizing antibody did not change the WT or
WL in the sham + CCL5-neutralizing antibody group (ANOVA, P > 0.05). Compared with
the animals in the CCI + control IgG group, the WT and WL of the CCI + CCL5-neutralizing
antibody group (1 g/d and 4 g/d; on days 02 or day 47) were significantly increased
(twoway ANOVA, P < 0.01) (Fig. 4).
The CCL5-neutralizing antibody attenuated the CCI-induced glial cell
activation but not NF-B expression
In contrast to the CCI + control IgG group, the CCL5-neutralizing antibody (4 g/d)
attenuated the increase in CCL5 expression (ANOVA, P < 0.01) but did not affect NF-B expression
(ANOVA, P > 0.05, Fig. 5).
The mean optical densities of spinal Iba-1 and GFAP immunoreactivity were greatly
increased in the CCI rats compared with the sham group. Pre-administration of control IgG did
not alter the CCI-induced glial cell activation. The CCL5-neutralizing antibody (4 g/d)
Figure 2. Spinal NF-B and CCL5 expression after CCI surgery and intrathecal use of PDTC in the ipsilateral L45 spinal cord (mean S.E.M.,
n = 3). Time-course of spinal NF-B and CCL5 expression after CCI surgery. *P < 0.05, **P < 0.01 vs. the sham group (AI, II). Intrathecal administration of
PDTC 47 days following CCI surgery inhibited the increase in NF-B and CCL5 expression (western-blot (B I, II) and immunohistochemistry (C I, II)). The
ipsilateral L45 spinal cord segments were collected on day 7 after surgery. **P < 0.01 vs. the sham group; #P < 0.01 vs. the CCI + saline group. (a) Sham
group; (b) Sham + saline group; (c) Sham + PDTC, 1000 pmol group; (d) CCI group; and (e) CCI + saline group; (f) CCI + PDTC, 1000 pmol group. Scale
bar = 100 m.
suppressed the activation of spinal microglia and astrocytes caused by the CCI surgery, as
demonstrated by the decreased mean optical density of the GFAP and Iba-1 (ANOVA, P < 0.01)
Double immunofluorescence of NF-B and CCL5 with microglia,
astrocytes, and neurons
Dual labeling indicates that the CCL5-IR and NF-B-IR cells represented neurons, microglia
and astrocytic cells, as these cell types also co-expressed NeuN, Iba-1 and GFAP in the
ipsilateral L45 spinal cord on day 7 after CCI surgery. Dual staining also indicates that NF-B was
co-localized with CCL5 in the medial ipsilateral dorsal horn (Fig. 7).
Minocycline or fluorocitrate attenuated the CCL5-induced hyperalgesia
according to the Hargreaves test
Treatment with minocycline (Fig. 8A, B), fluorocitrate (Fig. 8C, D) or the vehicle did not affect
the WL compared with the baseline values (ANOVA, P > 0.05). Intrathecal infusions of CCL5
produced obvious hyperalgesia (CCL5 main effect), and treatment with minocycline or
fluorocitrate blocked the CCL5-induced hyperalgesia (two-way ANOVA, P < 0.01).
No effects were observed with the intrathecal administration of normal saline or CCL5 (0.2 g)
(ANOVA, P > 0.05). The WT and WL in the CCI + PDTC group were significantly increased
compared with the CCI + saline group (ANOVA, P < 0.01). The effects of PDTC were
attenuated by CCL5 (0.2 g, i.t., 15 min before PDTC) (two-way ANOVA, P < 0.01) (Fig. 8E, F).
In the present study, we found that the increase in spinal CCL5 after CCI surgery occurred in
parallel with the glial cell activation of the spinal cords and the development of neuropathic
pain. Intrathecal administration of CCL5-neutralizing antibody delayed and attenuated the
initiation of pain hypersensitivities following CCI surgery, and the CCL5-neutralizing antibody
inhibited CCI-induced glia activation in the spinal cords. Inhibition of microglia activation or
astrocyte activation relieved the intrathecal CCL5-induced pain facilitation. Therefore,
CCL5induced pain facilitation was regulated by microglia or astrocyte activation in the spine.
Several studies have demonstrated that glial cells (microglia and astrocytes) and neurons
secrete CCL5. The involvement of CCL5 [14, 15, 17] and its receptors (including CCR1 ,
CCR3 and CCR5 [1820, 36, 37, 38]) has also been observed in different pain models .
CCL5 plays a specific role in the complex chemical interaction between glial cells and neurons
and helps maintain CNS homeostasis, as may other chemokines. After CCL5-induced
activation, microglia secretes glial-excitatory transmitters, leading to astrocytic activation. Varieties
of neuro- and glial-excitatory transmitters are secreted by activated microglia and astrocytes
, which may lead to the initiation and maintenance of neuropathic pain. Therefore, the
prevention of CCL5 and glial cell activation blocks the occurrence and development of
CCIinduced pain hypersensitivities.
Furthermore, we also showed that the intrathecal administration of PDTC attenuated the
CCI-induced glial cell activation and increases in NF-B and CCL5 expression. The intrathecal
injection of CCL5 partially attenuated the analgesic effects of PDTC in CCI rats, suggesting
that the decrease in CCL5 expression and glial cell activation may be involved in the
antinociceptive mechanisms of PDTCs analgesic effects. Our data have extended the results of
previously published studies [22, 25, 26, 29, 47, 48] by showing that PDTC produces analgesic
effects in chronic models via the inhibition of spinal NF-B and CCL5 expression and the
activation of spinal glia and by indicating that the NF-B-CCL5 pathway mediates neuropathic
pain through the regulation of CCL5 expression.
In various types of pain, NF-B mediates immune and inflammatory responses via the
regulation of genes that can encode proinflammatory cytokines, adhesion molecules, and
chemokines in the spinal cords [23, 26]. Microglia activation may first lead to a series of spinal
immune responses. In addition, NF-B inhibition by PDTC reduces the expression of spinal
CX3CR1 in a CCI model  and the expression of spinal COX-2 in the SNL model. The
changes in TNF-, IL-6 and interleukin (IL)-1A in the CSF were remarkably related to the
changes in NF-B in the gp120-injected rats . The negative mediation of NF-B on these
pro-inflammatory factors and chemokines may explain the analgesic effects of PDTC. The data
presented above may explain why PDTC suppressed the up-regulation of CCL5 and glia
activation after CCI surgery and why intrathecal administration of CCL5 partially attenuated the
anti-nociceptive effects of PDTC in CCI rats. Our data also indicate that NF-B may not be the
sole neurotransmitter involved in spinal glial cell activation (microglia or astrocyte activation)
in the CCI rats.
In conclusion, our data provide new evidence supporting the hypothesis that spinal NF-B
and CCL5 play a role in the induction and development of neuropathic pain through glial cell
activation. Inhibition of spinal CCL5 may offer a novel method to prevent and treat nerve
injury-induced neuropathic pain.
Conceived and designed the experiments: CDY QY WC. Performed the experiments: QF YZ
QY WC JL. Analyzed the data: WC QF QY MYC. Contributed reagents/materials/analysis
tools: QF FFL HL JTJ YZ. Wrote the paper: WC MYC.
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