Silva et al. Journal of Neuroinflammation (2015) 12:10
DOI 10.1186/s12974-014-0216-1
RESEARCH
JOURNAL OF
NEUROINFLAMMATION
Open Access
The spinal anti-inflammatory mechanism of
motor cortex stimulation: cause of success
and refractoriness in neuropathic pain?
Guilherme D Silva1†, Patrícia SS Lopes1†, Erich T Fonoff1,2 and Rosana L Pagano1*
Abstract
Background: Motor cortex stimulation (MCS) is an effective treatment in neuropathic pain refractory to
pharmacological management. However, analgesia is not satisfactorily obtained in one third of patients. Given the
importance of understanding the mechanisms to overcome therapeutic limitations, we addressed the question:
what mechanisms can explain both MCS effectiveness and refractoriness? Considering the crucial role of spinal
neuroimmune activation in neuropathic pain pathophysiology, we hypothesized that modulation of spinal astrocyte
and microglia activity is one of the mechanisms of action of MCS.
Methods: Rats with peripheral neuropathy (chronic nerve injury model) underwent MCS and were evaluated with a
nociceptive test. Following the test, these animals were divided into two groups: MCS-responsive and MCSrefractory. We also evaluated a group of neuropathic rats not stimulated and a group of sham-operated rats. Some
assays included rats with peripheral neuropathy that were treated with AM251 (a cannabinoid antagonist/inverse
agonist) or saline before MCS. Finally, we performed immunohistochemical analyses of glial cells (microglia and
astrocytes), cytokines (TNF-α and IL-1β), cannabinoid type 2 (CB2), μ-opioid (MOR), and purinergic P2X4 receptors in
the dorsal horn of the spinal cord (DHSC).
Findings: MCS reversed mechanical hyperalgesia, inhibited astrocyte and microglial activity, decreased
proinflammatory cytokine staining, enhanced CB2 staining, and downregulated P2X4 receptors in the DHSC
ipsilateral to sciatic injury. Spinal MOR staining was also inhibited upon MCS. Pre-treatment with AM251 blocked the
effects of MCS, including the inhibitory mechanism on cells. Finally, MCS-refractory animals showed similar CB2, but
higher P2X4 and MOR staining intensity in the DHSC in comparison to MCS-responsive rats.
Conclusions: These results indicate that MCS induces analgesia through a spinal anti-neuroinflammatory effect and
the activation of the cannabinoid and opioid systems via descending inhibitory pathways. As a possible explanation
for MCS refractoriness, we propose that CB2 activation is compromised, leading to cannabinoid resistance and
consequently to the perpetuation of neuroinflammation and opioid inefficacy.
Keywords: Motor cortex, Epidural stimulation, Neuropathic pain, Glia, Cannabinoids, Neuroinflammation,
Spinal cord, Rats
* Correspondence: rosana.lpagano@hsl.org.br
†
Equal contributors
1
Laboratory of Neuromodulation and Experimental Pain, Hospital Sírio
Libanês, Rua Coronel Nicolau dos Santos, 69, 01308-060 São Paulo, SP, Brazil
Full list of author information is available at the end of the article
© 2015 Silva et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain
Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article,
unless otherwise stated.
Silva et al. Journal of Neuroinflammation (2015) 12:10
Background
Neuropathic pain is defined as pain caused by a primary lesion of the somatosensory system [1]. Classic symptoms
include allodynia (pain in response to innocuous thermal
or mechanical stimuli), hyperalgesia (exaggerated pain in
response to noxious stimuli), and spontaneous pain (pain
in the absence of noxious stimulation) [2-4]. These symptoms can be explained by immunogenic and neurogenic
mechanisms such as glial activation, proinflammatory cytokine release, and differential activation of receptors [3,5,6].
Proinflammatory cytokines, such as IL-1β and TNF-α, are
secreted by activated astrocytes and microglia, enhancing
glutamatergic transmission and disinhibiting GABAergic
interneurons in the dorsal horn of the spinal cord (DHSC).
Through this process, spinal synaptic transmission is increased, leading to neuropathic pain sensitization [7-9].
The cannabinoid and purinergic systems modulate the
inflammatory response of glial cells. Cannabinoid type 2
receptor (CB2) activation in glial cells decreases IL-1β and
TNF-α release [7,10,11], while the binding of adenosine
triphosphate (ATP) to purinergic P2X4 receptors in microglia leads to the secretion of inflammatory mediators [12].
Epidural motor cortex stimulation (MCS) has been proposed as a treatment for neuropathic pain that is refractory
to clinical management (i.e. tricyclic antidepressants, antiepileptic drugs, or opioids) [13-18]. Moreover, MCS is a
nondestructive, adjustable, and reversible therapy for central and peripheral pain [19]. Although MCS is considered
a promising strategy for drug-resistant neuropathic pain,
one third of patients do not respond satisfactorily [20], and
its mechanism of action remains controversial. In rats and
humans, both the opioid system and descending analgesic
pathways have been implicated in MCS-induced analgesia
[21-27]. Even though neuroimmunomodulation processes
have been implicated in the pathophysiology of neuropathic pain, their underlying mechanisms in MCS-induced
analgesia remain poorly understood. In this study, using
an experimental model of peripheral neuropathy, we investigated whether modulation of spinal glial inflammatory
response is involved in the mechanisms of MCS-induced
analgesia. We also studied whether the differential involvement of cannabinoid, opioid, and purinergic systems plays
a role in the effectiveness and refractoriness to MCS.
Methods
Page 2 of 11
submitted to 15 minutes of MCS and, at the end of this
period, still under stimulation, they were re-evaluated on
the nociceptive test. Sham-operated and neuropathic rats
that were not stimulated were also evaluated. To evaluate
the cannabinoid involvement, animals with peripheral
neuropathy were treated with cannabinoid receptor antagonist or saline, and after 1 hour, they were stimulated and
re-evaluated with the nociceptive test. Following these
experiments, animals were divided into eight groups:
sham-operated, CCI (non-stimulated), CCI + MCS (MCSresponsive), and CCI + MCS (MCS-refractory); or CCI +
Saline, CCI + Saline + MCS, CCI + AM251 + MCS, and
Naive + AM251. Immediately after the last nociceptive
evaluation, all animals were anesthetized, perfused, and
their spinal cords were processed for immunohistochemical analysis.
Animals
Male Wistar rats weighing between 180 and 220 g were
housed in acrylic boxes (3 rats per box) for at least 2 days
before initiating experimental procedures. Animals were
maintained in a controlled environment under 12/12 hour
light/dark cycle at room temperature (22° ± 2°C) with
wood shavings and free access to water and rat chow pellets. The experimental procedures were in accordance
with the guidelines for the ethical use of animals in research involving pain and nociception [28], and were approved by the Ethics Committee on the Use of Animals at
Hospital Sírio Libanês (São Paulo, Brazil), under protocol
number CEUA 2012/24.
Neuropathic pain induction
Rats received an inhalational general anesthetic (halothane, 2.5%) and were subjected to sciatic nerve
chronic constriction injury (CCI), according to previously described methods [29]. Using sharp and blunt
dissection, the sciatic nerve of the right paw was exposed at the midline of the thigh. Proximal to the sciatic nerve’s trifurcation (about 7 mm), 4 loose ligatures
were placed around the nerve using 4.0 catgut chrome
wire. The skin was closed with 4.0 nylon. In shamoperated rats, the sciatic nerve was exposed but not
compressed.
Experimental design
Electrode implantation and electrical stimulation
parameters
Under anesthesia, peripheral neuropathy (or sham surgery)
was performed on the right hind limb of adult rats (day 0).
After 1 week, transdural electrodes were implanted over
the left motor cortex involving the functional area of the
right hind limb (day 7). After another week, the nociceptive
test (described in the Measuring mechanical hyperalgesia
section below) was performed in awake animals (day 14).
After that, a group of rats with peripheral neuropathy was
One week after the induction of peripheral neuropathy or
sham surgery, rats received local (2% lidocaine 100 μL, subcutaneously) and general (ketamine/xylazine 0.5/2.3 mg/kg,
intramuscularly) anesthesia. Whenever necessary, supplementary doses of ketamine were administered to the animals. Guided by a map developed by our group [30], two
transdural stainless steel electrodes were placed under
stereotaxic conditions on the primary motor cortex over
Silva et al. Journal of Neuroinflammation (2015) 12:10
the functional area of the hind limb. Two fixation screws
and acrylic polymer were used to stabilize the implant
and to ensure electrical isolation. One week after surgical implantation, electrical stimulation was delivered in
a single 15-minute session with the following parameters: 60 Hz, 1.0 V, and 210 μs (Medtronic electrical stimulator, Minneapolis, MN, USA), which has been previously
shown to reverse the neuropathic pain in rats [31].
Pharmacological treatment
Fourteen days after CCI, animals were treated with
AM251, a cannabinoid receptor antagonist/inverse agonist
(1 mg/kg, intraperitoneally; Sigma-Aldrich, St Louis, MO,
USA) [32] or with saline solution (400 μL, 0.9% NaCl in
water) 1 hour before the nociceptive test or MCS. Naive
animals injected with AM251 were also evaluated. The experimenter was blind to the treatments.
Measuring mechanical hyperalgesia
The mechanical nociceptive threshold was determined
using a pressure apparatus on the right hind paw (Insight
Ltda., Ribeirão Preto, São Paulo, Brazil), as previously described [33]. Briefly, the mass (in grams) required to induce a withdrawal response represented the nociceptive
threshold. The nociceptive test was carried out on the
14th day following CCI or sham surgery, before (initial
measurement) and during MCS (final measurement).
Upon antagonist treatment, the nociceptive threshold was
evaluated before CCI (initial measurement), 14 days
after CCI (final measurement), and 1 hour after antagonist or saline administration during MCS (final
measurement + MCS). The results were analyzed by
comparing the initial and final measurements to each
other. Investigators were blind to group identification.
To reduce stress, rats were habituated to the testing
procedure the day preceding the experiment. According
to the presence or absence of analgesia (as measured by
significant changes in nociceptive threshold), the MCS
group was further divided into ‘MCS-responsive’ and
‘MCS-refractory’ groups.
Page 3 of 11
mouse anti-GFAP (glial fibrillary acidic protein,1:1,000,
G3893, Sigma-Aldrich, St Louis, MO, USA), rabbit antiIba-1 (1:1,000, 019-19741, Wako Chemicals, Richmond,
VA, USA), goat anti-IL-1β (1:500, AF-501-NA, R & D Systems, Minneapolis, MN, USA), rabbit anti-TNF-α (1:500,
AB1837P, Calbiochem, San Diego, CA, USA), rabbit antiCB2 (1:500, 101550, Cayman, Ann Arbor, USA), goat antiMOR-1 (μ-opioid receptor 1, 1:500, sc-7488, Santa Cruz
Biotechnology, Santa Cruz, CA, USA), and rabbit antiP2X4 (1:500, APR-002, Alomone Labs, Jerusalem, Israel)
diluted in 0.3% of Triton X-100 containing 5% normal
donkey serum. Then, sections were incubated for 2 hours
at room temperature with biotinylated secondary antibodies (1:200, Jackson ImmunoResearch, Bar Harbor, ME,
USA) or fluorescent secondary antibodies: fluorescein
isothiocyanate (FITC) (green) or tetramethylrhodamine
(TRITC) (red) (1:100, Jackson ImmunoResearch, Bar
Harbor, ME, USA). Sections with biotinylated antibodies
were incubated for 2 hours at room temperature, with
avidin-biotin complex (1:100, ABC Elite kit, Vector Labs,
Burlingame, CA, USA), and visualized with 0.05% diaminobenzidine tetrahydrochloride (DAB, Sigma-Aldrich, St
Louis, MO, USA) and 0.03% (final concentration) hydrogen peroxide in PB. Then, sections were mounted on glass
slides, air-dried, dehydrated, and coverslipped with Permount (Fisher Scientific, Pittsburgh, PA, USA). Sections
with fluorescent antibodies were mounted on slides and
coverslipped with mounting medium. For both assays,
samples were washed between each step (3 × 5 minutes).
Finally, images were captured utilizing a fluorescence/light
microscope (E1000, Nikon, Melville, NY, USA) and Zen
software (Carl Zeiss, Oberkochen, Germany).An area
encompassing the DHSC (L4-L5 dorsal horn, ipsilateral to
CCI) was outlined and immunoreactivity intensities above
background were semiquantified using ImageJ software
(National Institutes of Health; http://rsbweb.nih.gov/ij/).
For each assay, total number of positive profile (immunostained particles), including neuropil and cell bodies, was
used to provide a mean fluorescence value (compared with
pre-defined threshold) in five tissue sections per animal
and four to six animals per group.
Immunohistochemistry
Immediately after the last nociceptive test, rats were
deeply anesthetized with ketamine and xylazine and then
subjected to transcardiac perfusion with saline solution,
followed by 4% paraformaldehyde (PFA) dissolved in
0.1 M phosphate buffer (PB). The lumbar spinal cords
(L2-L5 segments) were collected and post-fixed in PFA for
4 hours, followed by incubation with 30% sucrose solution
for 48 hours at 4°C. Coronal sections (30 μm) were cut on
a freezing microtome. Heat-induced epitope retrieval
(HIER) in citrate buffer (10 mM) was performed at 75°C
for 30 minutes. Next, tissue sections were incubated for
48 hours at 4°C, with the following primary antibodies:
Statistical analysis
Results are expressed as means ± standard error of
the mean (SEM). Data were analyzed with GraphPad
Prism (La Jolla, CA, USA) statistical software using
ANOVA (analysis of variance) followed by Tukey post
hoc tests. Three or four groups were compared: shamoperated, CCI (non-stimulated), CCI + MCS (MCSresponsive), and CCI + MCS (MCS-refractory); or CCI +
Saline, CCI + Saline + MCS, CCI + AM251 + MCS, and
Naive + AM251. In all cases, P < 0.05 was considered
statistically significant.
Silva et al. Journal of Neuroinflammation (2015) 12:10
Results and discussion
Does MCS modulate proinflammatory cytokine release
and spinal glial cell activation?
Similar to results previously reported [31], CCI caused
mechanical hyperalgesia and MCS reversed this response
in 88% of cases (Figure 1A). Three animals (12%) that
did not respond to MCS were analyzed separately as the
CCI + MCS (MCS-refractory) group. We found no difference between the CCI + MCS (MCS-responsive) and
CCI + MCS (MCS-refractory) groups on the initial measure of hyperalgesia (prior to MCS).
In an attempt to elucidate the modulation of glial cell
activity in MCS-induced analgesia, we evaluated Iba-1
(microglial marker) and GFAP (astrocytic marker) immunolabeling in the DHSC ipsilateral to the CCI. We observed a marked increase in the density of Iba-1 and
Page 4 of 11
GFAP-positive cells in the DHSC of CCI rats in comparison to sham-operated rats. MCS reversed both Iba1 and
GFAP increased reactivity in neuropathic rats (Additional
file 1), suggesting an association between MCS and
changes in activation pattern of the spinal cord astrocytes
and microglia. Furthermore, we observed both astrocyte
hyperplasia (increased number of cells) and hypertrophy
(increased cell volume), in the DHSC of the CCI group as
compared with the sham-operated group, consistent with
astrocyte activation [34,35]. In morphological analysis,
MCS reversed the astrocyte hypertrophy but not hyperplasia. Microglial hyperplasia was clear in CCI compared to
sham-operated, characterizing microglial activation [36].
However, the effect of MCS on hyperplasic and hypertrophic responses of spinal microglia, as compared with
the CCI group, was not easily detected in our results. To
Figure 1 Motor cortex stimulation (MCS), analgesia, and their relationship with spinal cytokine release. (A) Nociceptive threshold
evaluated in the right hind paw of sham-operated rats (Sham), animals with sciatic nerve chronic constriction injury (CCI), and animals with CCI
submitted to MCS (CCI + MCS). CCI was performed in the right paw and cortical electrodes were implanted in the left hemisphere. The nociceptive
test was performed 14 days after sham surgery or CCI (IM, initial measurement), and again after 15 minutes, regardless of whether MCS was performed
or not (FM, final measurement). Quantification of TNF-α (B) and IL-1β (C) immunofluorescence-staining in the dorsal horn of the spinal cord (DHSC),
ipsilateral to sham or CCI (n = 5 images per animal). Values represent means ± SEM (n = 5 animals per group). *P < 0.05, compared to the sham group.
Photomicrographs illustrating glial fibrillary acidic proten (GFAP) (red), IL-1β (green), GFAP and IL-1β colocalization (yellow) in the DHSC of sham
(D), CCI (E), and CCI + MCS (responsive) (F) animals. Scale bars: 130 μm.
Silva et al. Journal of Neuroinflammation (2015) 12:10
solve this problem, we used P2X4 staining as a marker of
microglial activation and the results are discussed below.
Considering that glial proinflammatory cytokines mediate
the initiation and maintenance of neuropathic pain in animal models [37,38], we evaluated the presence of TNF-α
and IL-1β in the DHSC. Fluorescence microscopy analysis
revealed that TNF-α (Figure 1B and Additional file 2B)
and IL-1β (Figure 1C and Additional file 2E) immunoreactivity was more intense in the CCI group as compared with the sham-operated group (Figure 1B and C,
and Additional file 2). Increased cytokine immunoreactivity was completely reversed by MCS (Figure 1B and C,
and Additional file 2). These findings are consistent with
the observed analgesic effects, suggesting a correlation between MCS and downregulation of central sensitization
mechanisms, as cytokines levels and glia (astrocytes and
microglia) activation patterns. We hypothesize that, similar
to the mechanism associated with drugs (xanthine derivative KMUP-1, fucoidan, montelukast) [39-41], hyperalgesia
in neuropathic pain is attenuated upon suppression of
spinal neuroinflammation. To the best of our knowledge,
this is the first study to show the spinal anti-inflammatory
effect of MCS.
Accumulating evidence has shown that activated astrocytes contribute to maintaining chronic pain sensitization
through IL-1β release in the spinal cord under peripheral
Page 5 of 11
nerve injury [42-44]. In agreement with these findings,
we observed colocalization of IL-1β and astrocytes
(GFAP) in the CCI group (Figure 1E), but not in the
CCI + MCS (MCS-responsive) (Figure 1F) or shamoperated animals (Figure 1D). Thus, we propose that
the anti-neuroinflammatory effects of MCS depend on
downregulation of glial cell activity.
Can modulatory systems (cannabinoid, opioid, and
purinergic) explain the positive effect or refractoriness of
MCS?
Endocannabinoids inhibit the neuroinflammatory response [45], and cannabinoid receptor agonists have been
shown to have beneficial effects in several animal models
of neuropathic pain [46,47]. In an attempt to evaluate the
participation of the endocannabinoid system in MCSinduced analgesia, animals were treated with the cannabinoid receptor antagonist/inverse agonist AM251 prior
to cortical stimulation. We observed that the analgesic effect of MCS was completely inhibited in animals pretreated with AM251 (Figure 2A). To verify the role of glia
in this response, we evaluated GFAP-positive cells in CCI,
CCI + saline + MCS, and CCI + AM251 + MCS groups. As
demonstrated by the increased GFAP staining, astrocytes remained activated after pre-treatment with AM251
(Figure 2B-D), suggesting that the endocannabinoid system
Figure 2 Participation of the cannabinoid system in motor cortex stimulation (MCS)-induced analgesia. (A) Nociceptive threshold
evaluated in the right hind paw of rats with peripheral neuropathy pre-treated with saline (CCI + saline), chronic constriction injury (CCI) rats
pre-treated with saline and submitted to MCS (CCI + Saline + MCS), and CCI rats pre-treated with cannabinoid receptor antagonist AM251 and
submitted to MCS (CCI + AM251 + MCS). CCI was performed in the right paw and cortical electrodes were implanted in the left hemisphere. The
nociceptive test was performed before CCI (IM, initial measurement), and 14 days after CCI (FM, final measurement). One hour after injection of
saline (400 μL) or AM251 (1 mg/kg; intraperitoneally), animals were submitted or not to MCS and re-evaluated on the nociceptive test
(FM + MCS). Naive rats pre-treated only with AM251 were also evaluated. Values represent the mean ± SEM (n = 5 per group). *P < 0.05 compared
with IM. Photomicrographs illustrating glial fibrillary acidic protein (GFAP) immunofluorescence in the dorsal horn of the spinal cord (DHSC) of
CCI (B), CCI + Saline + MCS (C), and CCI + AM251 + MCS (D) animals.
Silva et al. Journal of Neuroinflammation (2015) 12:10
is involved in MCS-induced analgesia by inhibiting astrocyte activity. Given that (1) MCS inhibits astrocyte and
microglial activation; (2) the CB2 receptor, rather than
CB1, is directly involved in astrocyte and microglial inhibition in neuropathic pain conditions [47-49]; and (3)
AM251 can act as a CB2 inverse agonist [50,51], we next
evaluated the CB2 labeling pattern in the spinal cord of
MCS-responsive and MCS-refractory animals.
While the CB2 receptor is the main receptor for cannabinoid signaling in astrocytes and microglia, the MOR is
found exclusively in DHSC neurons [11,52-54]. While the
former decreases cytokine release (IL-1β and TNF-α) from
astrocytes and microglia [55,56], the latter diminishes
neurotransmitter secretion (glutamate, substance P) in the
DHSC upon activation of the descending analgesic pathway [57,58]. Because both mechanisms play a crucial role
Page 6 of 11
in neuropathic pain pathophysiology, the cannabinoid antagonist/inverse agonist AM251 as well as the opioid antagonist naloxone [24] can compromise MCS-induced
analgesia.
Cannabinoid and opioid receptor regulation work in
opposite directions: while CB2 operates in a positive
feedback loop in which receptor activation leads to increased receptor expression, MOR is downregulated
with increased opioid transmission, and its internalization correlates with the MOR-mediated postsynaptic inhibitory effect [11,59]. Given that microglial cells are
activated [48,49,52] and spinal opioid neurotransmitters
are depleted during chronic pain [60,61], we observed,
as expected, a more intense staining for both spinal CB2
(Figure 3A, C) and MOR (Figure 3F, H) receptors in rats
with peripheral neuropathy in comparison to sham-
Figure 3 Spinal cannabinoid receptor type 2 (CB2) and μ-opioid receptor (MOR) staining and its correlation with the effectiveness of
motor cortical stimulation (MCS). Quantification of CB2 (A) and MOR (F) immufluorescence-staining in the dorsal horn of the spinal cord
(DHSC), ipsilateral to sham surgery or chronic constriction injury (CCI), in sham-operated rats (Sham), CCI rats with neuropathic pain (CCI), CCI rats
in which MCS reversed the neuropathic pain (CCI + MCS, responsive), and CCI rats unresponsive to MCS (CCI + MCS, refractory). Values represent
the mean ± SEM (n = 5 images per animal, n = 3 to 5 animals per group). *P < 0.05 compared to the sham group. Photomicrographs illustrating
CB2 (B-E) and MOR (G-J) immunofluorescence in the DHSC, ipsilateral to sham surgery or CCI, in sham (B, G), CCI (C, H), CCI + MCS (responsive)
(D, I), and CCI + MCS (refractory) (E, J) animals. Scale bars: 130 μm.
Silva et al. Journal of Neuroinflammation (2015) 12:10
operated rats (Figure 3A, B, F, G). Also, MCS further increased CB2 (Figure 3A, D) and decreased MOR reactivity
(Figure 3F, I) in MCS-responsive animals, results that are
coherent with the activation of opioid and cannabinoid
systems and their roles in the reversal of neuropathic pain
[62-65]. Unexpectedly, no difference in spinal CB2 immunoreactivity was observed between MCS-responsive and
MCS-refractory groups (Figure 3A, E). On the other hand,
MOR staining in unresponsive rats was partially inhibited
Page 7 of 11
when compared with responsive and sham-stimulated
rats (Figure 3F, J). Considering that (1) the inflammatory state modulates opioid release in neuropathic pain
conditions [66]; and (2) astrocytes and microglia remained
activated in MCS-refractory animals, a plausible explanation for the partial reversion of MOR upregulation in this
group is that inflammatory mediators secreted by activated
astrocytes and microglia inhibited opioid release of opioid
peptides.
Figure 4 Spinal purinergic system involvement in analgesia induced by motor cortex stimulation (MCS). (A) Quantification of P2X4
immufluorescence-staining in the dorsal horn of the spinal cord (DHSC), ipsilateral to sham surgery or chronic constriction injury (CCI), in
sham-operated rats (Sham), CCI rats with neuropathic pain (CCI), CCI rats in which MCS reversed the neuropathic pain (CCI + MCS, responsive),
and CCI rats unresponsive to MCS (CCI + MCS, refractory). Values represent means ± SEM (n = 5 images per animal, n = 3 to 5 animals per group).
*P < 0.05 compared to the sham group. #P < 0.05 compared to the sham and CCI groups. Photomicrographs illustrating P2X4 immunofluorescence in
the DHSC, ipsilateral to sham surgery or CCI, in sham (B), CCI (C), CCI + MCS (responsive) (D), and CCI + MCS (refractory) (E) animals. Scale
bars: 130 μm.
Silva et al. Journal of Neuroinflammation (2015) 12:10
To better understand these results, we evaluated the purinergic receptor P2X4, which is expressed in activated
microglia but not in neurons or astrocytes [67,68]. Animals
with peripheral neuropathy showed a significant increase in
spinal P2X4 reactivity (Figure 4A, C), which was partially
reversed in MCS-responsive rats (Figure 4A, D) when compared with sham-operated animals (Figure 4A, B). Surprisingly, P2X4 staining in the CCI + MCS (MCS-refractory)
animals fell somewhere between staining in CCI rats and
CCI + MCS (MCS-responsive) rats (Figure 4A, E). Thus,
even though cannabinoid transmission was similar regardless of the success or failure of MCS treatment, MCS inhibitory effects on astrocytes and microglia were reduced in
the MCS-refractory group. We hypothesize that CB2
Page 8 of 11
activation may be compromised in MCS-refractory animals, probably due to altered intracellular signaling that
prevents the inhibition of spinal microglia activity, leading
to cannabinoid resistance.
Cannabinoid resistance is implicated in opioid inefficacy
[69], since neuroinflammation persists if cannabinoid
modulation fails to suppress astrocyte and microglia activity. Considering this intimate relationship between cannabinoid and opioid systems in DHSC and the fact that MCS
activates descending analgesic pathways [21,25,26,70] and
inhibits spinal nociceptive neurons [31,71], we propose
that the following spinal circuitry is involved in MCSinduced analgesia: activation of spinal cannabinoid neurons causes them to release endocannabinoids, which
Figure 5 Motor cortex stimulation (MCS) effectiveness and refractoriness: cannabinoids, opioids, and neuroinflammation. (A) In the
chronic constriction injury (CCI) group, activated astrocytes and microglia release cytokines (IL-1β and TNF-α) that enhance nociceptive neuron
transmission in the dorsal horn of the spinal cord (DHSC), resulting in neuropathic pain. (B) In CCI + MCS (responsive) group, MCS activates the
spinal cannabinoid and opioid interneurons. Endocannabinoids bind to CB2 receptors and inhibit cytokine secretion by glial cells, while endogenous
opioids interact with μ-opioid receptors (MOR) receptors and suppress neuronal transmission, thus reverting neuropathic pain. (C) In CCI + MCS
(refractory) group, endocannabinoids fail to suppress astrocyte and microglia activation (cannabinoid resistance), maintaining proinflammatory cytokine
release (perpetuation of neuroinflammation), and thus compromising opioid effectiveness (opioid inefficacy). Hence, spinal pain transmission persists,
preventing analgesia.
Silva et al. Journal of Neuroinflammation (2015) 12:10
inhibit astrocyte and microglia activity, thus decreasing
neuroexcitatory cytokine secretion and suppressing nociceptive neuron excitability through opioid activation
(Figure 5).
A possible explanation for MCS effectiveness in some
individuals who are refractory to conventional pharmacological treatments (opioid analgesic, tricyclic antidepressants, and antiepileptic drugs) is that such treatments do
not simultaneously modulate cannabinoid and opioid systems in such a manner as to block neuroexcitatory astrocyte and microglia signals and, at the same time, suppress
the nociceptive neuron excitability. Future studies should
further investigate the mechanisms involved in ‘cannabinoid resistance’: are different receptors involved or are there
intracellular signaling abnormalities? Are gene polymorphisms or transcription factor mutations involved?
Conclusion
Our results indicate that the spinal anti-neuroinflammatory
effect of MCS is responsible, at least in part, for the induced analgesia. In addition, the results reinforce that MCS
reverses neuropathic pain through the activation of descending analgesic pathways. Our data suggest that through
the cannabinoid system, MCS inhibits spinal astrocyte and
microglia activity, decreasing proinflammatory cytokine secretion and, thus, neuroinflammation; through the opioid
system, MCS suppresses spinal nociceptive neuron excitability and, hence, transmission of persistent pain. The fact
that inflammation decreases the efficacy of opioids suggests
that both of these mechanisms play a role in MCS-induced
analgesia. A possible explanation for MCS refractoriness in
some individuals with neuropathic pain is that spinal CB2
activation is compromised, leading to cannabinoid resistance and consequently to the perpetuation of neuroinflammation and opioid inefficacy.
Additional files
Additional file 1: Motor cortex stimulation and spinal glial cells.
Photomicrographs illustrating Iba-1 (A-C) and GFAP (D-F) staining in the
DHSC of sham-operated rat (A, D), rat with sciatic nerve chronic constriction
injury (CCI) (B, E), and rat with CCI submitted to cortical stimulation that was
MCS-responsive (C, F). CCI was performed in the right paw and cortical
electrodes were implanted in the left hemisphere. Sections illustrate the
DHSC ipsilateral to sham surgery or CCI. Scale bars: 100 μm (A-C) and
130 μm (D-F).
Additional file 2: Motor cortex stimulation and spinal cytokine
release. Photomicrographs illustrating TNF-α (A-C) and IL1-β (D-F)
staining in the DHSC of sham-operated rat (A, D), rat with sciatic nerve
chronic constriction injury (CCI) (B, E), and rat with CCI submitted to
cortical stimulation, MCS-responsive (C, F). CCI was performed in the right
paw and cortical electrodes were implanted in the left hemisphere.
Sections illustrate the DHSC ipsilateral to sham surgery or CCI. Scale
bars:130 μm.
Abbreviations
ANOVA: analysis of variance; ATP: adenosine triphosphate; CB2: cannabinoid
receptor type 2; CCI: chronic constriction injury; DAB: diaminobenzidine
Page 9 of 11
tetrahydrochloride; DHSC: dorsal horn of the spinal cord; FITC: fluorescein
isothiocyanate; FM: final measurement; GFAP: glial fibrillary acidic protein;
HIER: heat-induced epitope retrieval; IL-1β: interleukin-1β; IM: initial
measurement; MCS: motor cortex stimulation; MOR: μ-opioid receptor;
PB: phosphate buffer; PFA: paraformaldehyde; TNF-α: tumor necrosis factor-α;
TRITC: tetramethylrhodamine.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
GDS performed the peripheral neuropathy induction surgery and
immunohistochemistry assays, and drafted the manuscript. PSSL performed
electrode implantation and the nociceptive tests. ETF critically reviewed the
work. RLP conceived the study, participated in its design and coordination,
and drafted the manuscript. All authors read and approved the final
manuscript.
Acknowledgments
The authors would like to acknowledge Dr. Raquel Chacon Ruiz Martinez for
her excellent assistance with figure preparation and undergraduate student
Cristiane Cagnoni Ramos, who contributed to the pilot study. This research
was supported by the Fundação de Amparo a Pesquisa do Estado de São
Paulo (FAPESP: 2009/50772-4 and 2012/11925-2) and Hospital Sírio Libanês.
Author details
1
Laboratory of Neuromodulation and Experimental Pain, Hospital Sírio
Libanês, Rua Coronel Nicolau dos Santos, 69, 01308-060 São Paulo, SP, Brazil.
2
Division of Functional Neurosurgery, Department of Neurology, University of
São Paulo School of Medicine, Rua Dr Ovídio Pires de Campos, 785,
01060-970 São Paulo, SP, Brazil.
Received: 9 September 2014 Accepted: 5 December 2014
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