Sciatic nerve injuries alter electrophysiological and behavioral responses in rats

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Sciatic nerve injuries alter electrophysiological
and behavioral responses in rats

• Gh. A. Hamidi, H. Manaheji, M. Janahmadi, S. M.
  Noorbakhsh, M. Salami

Dept. of Physiology and Pharmacology, School of
Medicine, Kashan University of Medical Sciences,
Kashan I. R. Iran. Dept. of Physiology,
Neurosciences Research Center, School of
Medicine, Shaheed Beheshti University of Medical
Sciences, Tehran I. R. Iran
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Introduction

• Neuropathic pain arising from peripheral never
  injury can result in increased sensitivity to
  both noxious and non-noxious stimuli
• neuropathic pain which is characterized by
  spontaneous burning pain with accompanying
  allodynia (a decreased pain threshold, so that
  innocuous stimuli become painful), and
  hyperalgesia (an increased response to noxious
  stimuli)

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Numbers of animal nerve injury models of
neuropathic pain have been developed
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• The etiology and underlying mechanisms of such
  pain are poorly understood and existing
  treatments are often ineffective.
• Hence, it is important to increase our
  understanding of the mechanisms that underlie
  neuropathic pain states in order to identify
  strategies for development of effective
  therapies.

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• Shortly after injury a peripheral nerve many
  damaged sensory neurons produce spontaneous
  dischrches that enter the spinal cord and
  sensitized dorsal horn neurons.
• Therefore the sensitization of dorsal horn
  neurons by ectopic discharge input is the
  critical step in the development and maintenance
  of neuropathic pain.
• On the other hand, the analysis of compound
  action potential (CAP) of the peripheral nerve
  trunk after electrical stimulation has became a
  standard tool for investigation of the function
  of the nerve trunks

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• The main aims of study the present were to
  demonstrate the behavioral signs that represent
  allodynia, hyperalgesia and analysis of
  electrophysiological responses in the CCI
  (chronic constriction injury) and SNI (Spared
  nerve injury) models

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Methods

• Animals
• Groups
• Surgery
• Behavioral tests

Experiments were performed on male Sprague-Dawley
rats 230-280g
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Groups

• Chronic constriction injury (CCI) model
• Sham group for CCI model
• Spared nerve injury (SNI) model
• Sham group for SNI model

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Surgery

• The rats were anaesthetized with sodium
  pentobarbital (50 mg/kg, i.p). Behavioral and
  electrophysiological studies were performed in
  separate groups of rats.

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• In the chronic constriction injury (CCI) model
  the common sciatic nerve was exposed and became
  freed of adhering tissue for about 7mm. Four
  ligatures (4-0 chromic gut) were tied loosely
  around the nerve with a 1-1.5 mm interval between
  ligatures so that circulation through the
  superficial epineurial vasculature was not
  totally interrupted.

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• Spared nerve injury (SNI) model developed by
  Decosterd and woolf (2000), involves a lesion of
  two of the three terminal branches of the sciatic
  nerve (tibial and common peroneal nerve) Leaving
  the remaining sural nerve intact.
• Same procedure was performed in separated sham
  groups except that no ligatures were tied. The
  rats were housed individually in cages after
  surgery.

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Behavioral tests

• In the behavioral studies the animals were tested
  for mechano-allodynia and heat-hyperalgesia. The
  mechanical and heat stimulations were applied
  through von Frey filament and radiant heat,
  respectively. Behavioral tests were conducted on
  the animals prior to surgery (the day 0( , and,
  3, 7, 14, 21 and 28 days post-operation.

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Mechanical allodynia test
Animals were placed on an elevated wire grid and
the plantar surface of the paw was stimulated
with a series of ascending force via von Frey
filaments. The stimulation was applied three
times consecutively, pushing down on the hind paw
until the rat withdrew its paw or the fiber
bowed. The withdrawal threshold was for the
smallest filament size which evoked at least two
withdrawal responses during three consecutive
applications with the same filament. Each
filament applied for approximately 1 s and the
interstimulus intervals was about 5 s
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Radiant Heat test

• The rats stood upon an elevated glass floor and a
  moveable radiant heat source beneath the floor
  was aimed at the hind paw. The stimulus onset
  activated a timer controlled by a photocell and
  the hind paw withdrawal reflex interrupted
  photocell's light, automatically stopped the
  timer and terminated the stimulus. The heat
  stimulation was repeated 3 times at an interval
  of 5-10 min for the injured (the left) and intact
  (the right) paws. A maximal cut off of 22 s was
  used to prevent tissue damage. The mean latency
  of the withdrawal responses for each foot was
  calculated. Then, the values for the left side
  were subtracted from those for the right side and
  the difference was considered as hyperalgesia
  sign in the injured paw, if it was negative. .

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Electrophysiological recording
Electrophysiological responses were recorded at
the day 14 post-operation. In the CCI model
electrical stimulations were applied on either
sides of a constriction in common sciatic nerve
and compound action potential (CAP) were recorded
by an electrode placed proximally to the
constriction. In the SNI model CAPs were recorded
only proximally to axotomized sciatic nerve.
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Results
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Mechanical allodynia test
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Asterisks indicate a significant difference
(plt0.01, plt0.001) between the CCI and Sham
groups. Crosses indicate a significant difference
(plt0.001) between the SNI and Sham groups.
Closed squares indicate a significant difference
(plt0.05) between the CCI and SNI groups.
The subjects developed a noticeable
hypersensitivity to innocuous mechanical
stimulation (von Frey filament) of the lateral
surface of the hind paw (sural nerve territory)
in SNI model and medial surface of the hind paw
in CCI model 72 h after surgery. Figure a bow
shows that there is an enhanced response to the
stimulus in CCI to comparison by the sham group
at the day 7 (Plt0.01) and 14, 21, 28 (Plt0.001).
The SNI group showed an increased mechanical
allodynia compared to the sham group at the day
3, 7, 14, 21 and 28 (Plt0.001). The magnitude of
responses to mechanical allodynia was
significantly increased in SNI compared with CCI
group at the day 3 (Plt0.05)
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Thermal hyperalgesia test
Asterisks indicate a significant difference
(plt0.05, plt0.01, plt0.001) between the CCI
and Sham groups. Crosses indicate a significant
difference (plt0.05, plt0.01 and plt0.001)
between the SNI and Sham groups.
When noxious radiant heat was applied on the hind
paw, rats lifted their feet followed by aversive
behavior such as licking the stimulated paw. The
SNI group vs. sham group showed increased thermal
hyperalgesia at the day 3 (Plt0.05), 7 (Plt 0.01),
14 (Plt0.05), 21 (Plt 0.05) and 28 (Plt0.01). The
CCI vs. sham group showed a significant increase
at the day 7 (Plt0.05), 14 (Plt0.001), 21 (Plt0.01)
and 28 (Plt0.01). Thermal hyperalgesia show no
significant difference in CCI group when compared
with SNI group at the day 3, 7, 14, 21 and 28 .
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Fig. a bow Histograms of CV (A), amplitude (B)
and rising time (C) of the CAPs recorded from the
sciatic nerve proximal (CP) and distal (CD) to
the level of the constriction in the CCI model.
Asterisks indicate significant differences
between CP or CD with sham. Crosses indicate
significant difference between CD and
CP. plt0.05, plt0.01, plt0.001 plt0.05 and
plt0.001
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Fig. a bow Histograms of CV (A), amplitude (B)
and rising time (C) of the CAPs recorded from the
sciatic nerve proximal (SP) and spared (SS) to
the level of the axotomized in the SNI model.
Asterisks indicate significant differences
between SP or SS with sham. plt0.05, plt0.01,
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• The present study investigated some
  electrophysiological and behavioral properties of
  neuropathic pain in CCI and SNI models. There was
  a considerable difference in the magnitude of
  behavioral signs of each models. We observed
  sensitized responses after 3 days and reaches to
  maximum by 14th day. Bennett and Xie 1988
  suggested abnormal pain responses can be detected
  2 days after the injury reach peak severing in
  10-18 days, and last for about two months.

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Conclusion

• In briefly, the present data demonstrate that
  the CCI and SNI procedures result in severe
  changes in behavioral responses in whether
  hyperalgesia or allodynia. It seems that the
  sciatic nerve injury yields changes in the
  behavioral responses which are in accordance with
  the electrophysiological events occurred only in
  the distal part of the ligation site in the CCI
  model.

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Thank you
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