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The Journal of Neurophysiology Vol. 88 No. 3 September 2002, pp. 1393-1399
Copyright ©2002 by the American Physiological Society
in Rats With Chronic Compression of the Lumbar Ganglia
Department of Anesthesiology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Liu, Baogang,
Huiqing Li,
Sorin J. Brull, and
Jun-Ming Zhang.
Increased Sensitivity of Sensory Neurons to Tumor Necrosis Factor
in Rats With Chronic Compression of the Lumbar Ganglia.
J. Neurophysiol. 88: 1393-1399, 2002.
Proinflammatory cytokines may sensitize primary sensory neurons and
facilitate development of neuropathic pain processes after peripheral
nerve injury. The goal of this study was to determine whether responses
of dorsal root ganglion (DRG) neurons to exogenous tumor necrosis
factor 
(TNF-) are altered in a chronically compressed DRG (CCD)
injury model. Extracellular recordings from teased dorsal root
microfilaments demonstrated that acute topical application of TNF-
to the DRG for 15 min evoked C- and A
-fiber responses in both normal
and CCD rats. However, the response latency was significantly shorter,
and the peak discharge rate was higher, in CCD fibers than in normal
fibers. Intracellular recordings from small- and large-sized neurons
showed that TNF- induced greater depolarization and greater decrease
in rheobase in CCD neurons than in normal neurons. The proportion of
both small- and large-sized neurons that were responsive to TNF-
increased significantly after CCD injury. Furthermore, TNF- altered
the discharge patterns of large, spontaneously active neurons in
addition to enhancing their discharge rates. However, the
depolarization caused by TNF- in such neurons was minor (<2 mV).
Inflammatory cytokines such as TNF- increased the sensitivity of
sensory neurons in normal and CCD rats. The CCD injury itself, on the
other hand, increased neuronal responses to inflammatory cytokines.
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Lumbar disk herniation in humans is often associated with severe low back pain and sciatica resulting from chemical irritation with or without mechanical compression of the dorsal root ganglia (DRG) and/or nerve roots. Endogenous inflammatory agents are believed to play a critical role in the generation of pain and hyperalgesia, because antiinflammatory drugs have been shown to be very effective in relieving pain in patients with radiculopathy.
Among several inflammatory agents that have been detected in the
herniated disk, tumor necrosis factor (TNF-), a potent proinflammatory cytokine, is believed to be important in the initiation of local inflammation (Igarashi et al. 2000
). Recent
behavioral studies in our laboratory demonstrated that chronic local
application of TNF- to the normal, intact ganglion significantly
increased cutaneous sensitivity to mechanical stimulation (Homma
et al. 2002). In vitro electrophysiological recordings from
uninjured primary sensory neurons indicate that acute topical
application of TNF- to the ganglion induces protein kinase A
(PKA)-mediated activities in DRG neurons with myelinated or
unmyelinated axons (see companion paper). Similar effects were reported
previously when TNF- was applied directly to nerve trunks
(Sorkin et al. 1997) or injected into peripheral
receptive fields in vivo (Junger and Sorkin 2000).
There is evidence that TNF- and other inflammatory cytokines such as
interleukin-1 may directly modulate the activity in various classes
of neurons. TNF- was found to reduce K+
conductance in Aplysia (Sawada et al. 1990),
and retinal ganglion neurons (Diem et al. 2001). On a
slower time scale (24 h instead of 20-60 min), TNF- affects calcium
currents in cultured sympathetic (Soliven and Albert
1992) and hippocampal neurons (Furukawa and Mattson
1998).
TNF- can be synthesized and released by a variety of cell types
during inflammatory as well as neuropathic processes (Creange et
al. 1997; Tchelingerian et al. 1993;
Wagner et al. 1998) and contributes to the development
of pain and hyperalgesia in animal models of local inflammation or
peripheral neuropathy (Cunha et al. 1992; DeLeo
and Colburn 1995; Sommer et al. 1998a;
Wagner et al. 1998; Woolf et al. 1997).
It has been demonstrated that antiserums, or soluble receptors that
reduce endogenous TNF- in neuropathic animal models, reduce thermal
as well as mechanical hyperalgesia (Lindenlaub et al.
2000; Sweitzer et al. 2001).
Recently it was reported that rats develop cutaneous hyperalgesia to
radiant heat and tactile stimuli on the plantar surface of the foot
after chronic compression injury of the ipsilateral DRG produced by
implantation of a metal rod in each of the L4 and
L5 intervertebral foramina (Song et al.
1999). Ectopic discharges originating in the compressed ganglia
were electrophysiologically recorded in vitro from 9% of the
myelinated dorsal root fibers (Song et al. 1999). The
patterns of ectopic discharge were similar to those recorded from
primary sensory neurons with transected peripheral axons
(Burchiel 1984; DeSantis and Duckworth
1982; Devor 1994; Wall and Gutnick
1974; Zhang et al. 1997). Enhanced excitability
was demonstrated in all three types of sensory neurons with or without
spontaneous activity (Zhang et al. 1999). This DRG
chronic compression preparation provides a human model of DRG
compression from an acutely herniated lumbar disk, spinal stenosis,
tumors, or other injuries or diseases of the spinal cord.
In the present study, using extracellular and intracellular techniques,
we compared the sensitivity of DRG neurons to exogenously applied
TNF- between normal rats and rats that had been subjected to chronic
compression injury of the DRG.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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CCD injury animal model
Young female Sprague-Dawley rats weighing 100 g at the time
of surgery were anesthetized with intraperitoneal injection of pentobarbital sodium (40 mg/kg ip). After a midline incision was made
from L3 to L6, the right
paraspinal muscles were separated from the transverse processes and the
L4 and L5 intervertebral foramina were exposed. In each of 47 rats, an L-shaped stainless steel
rod (4 × 2 mm in length and 0.6 mm diam) was inserted
unilaterally into each foramen at an angle of 30° to the midline
without exposing the ganglia. The incision was then closed in layers
and prophylactic Augmentin (7.52 g to 500 ml drinking water,
amoxicillin/clavulanate potassium, SmithKline Beecham Pharmaceuticals,
Philadelphia, PA) was given to all rats daily for 
3 postoperative
days. An additional 39 normal, unoperated rats were used as control.
Extracellular electrophysiological recording
Extracellular recordings were obtained from 17 normal and 27 CCD
rats on postoperative days 7 to 14. As described previously (Zhang et al. 1997), the rats were first anesthetized
with pentobarbital sodium (40 mg/kg ip). The right
L4 and L5 ganglia with
attached dorsal roots (length: ~2 cm) and sciatic nerve (length: ~3
cm) were dissected surgically and placed in a recording chamber after the capsule was carefully peeled off. The DRG was perfused at a rate of
5 ml/min with oxygenated artificial cerebrospinal fluid (ACSF)
containing (in mM): 130 NaCl, 3.5 KCl, 1.25 NaH2PO4, 24 NaHCO3, 10 dextrose, 1.2 MgCl2, and 1.2 CaCl2
(pH = 7.3). The perfusion solution was heated to maintain the bath
temperature at 37°C. The dorsal root was placed to extend out of this
chamber into an adjacent mineral oil-filled chamber where microfilament recordings were performed. The spinal/sciatic nerve (length: ~40 mm)
was placed in an adjacent chamber containing mineral oil and was
positioned in contact with a bipolar stimulating electrode. Each
chamber contents were separated by petroleum jelly (Vaseline) to
prevent the solutions from different chambers from intermixing.
Dorsal root microfilaments were teased apart under a dissecting microscope. The proximal end of a dissected microfilament was placed on a fine silver electrode for single-fiber recording. The discharges of single fibers were displayed on a digital oscilloscope and collected via an interfaced Spike 2 data-acquisition system (Cambridge Electronic Design, Cambridge, U.K.) on an interfaced Pentium III PC. The conduction velocity (CV) of each fiber was measured via electrical stimulation delivered to the sciatic nerve.
Recombinant human TNF- (R & D Systems, Minneapolis, MN) was
dissolved in 0.1% bovine serum albumin in buffered saline to a
concentration of 100 ng/ml and stored at 
80°C in aliquots (10 µl)
for later use. TNF- (1 ng/ml) was applied topically to the DRG for
15 min after a 15-min baseline recording of dorsal root fibers
with/without spontaneous activity, followed by washout with ACSF for 30 min.
Microelectrode intracellular recording
After surgical dissection, the right L4 or
L5 DRG from each of 22 normal and 20 CCD rats was
placed in the recording chamber and mounted on the stage of an
upright microscope (BX50-WI, Olympus, Japan). A U-shaped stainless
steel wire on which three to four fine nylons fibers spanned the two
sides was used to gently hold the ganglion immersed at the bottom of
the chamber. The DRG was continuously perfused with oxygenated ACSF at
a rate of 2 ml/min, and the temperature was maintained at 37 ± 1°C as described previously (Zhang et al. 1999).
DRG cells were visualized under differential interference contrast
(DIC) through a CCD camera (Hamamatsu, Japan). Intracellular electrophysiological recordings were made from each cell with a
microelectrode filled with 2.5 M potassium acetate (pH = 7.2). Satisfactory recordings were obtained with electrodes of 50-80 M
.
Before electrode penetration, the DRG soma was visually classified according to its diameter as small (
30 µm) or large ( 50 µm). The electrophysiological data were collected with the use of
single-electrode continuous current clamp (AxoClamp-2B, Axon
Instruments, Foster City, CA) and analyzed with Clampex 8 software
(Axon Instruments).
After a stabilization-recording period of 3 min, TNF- at 0.001 ng/ml
was applied to the DRG for 5 min, followed by washout with ACSF for 30 min. A TNF- concentration of 0.001 ng/ml was chosen as it was the
lowest dose tested in the extracellular studies (see the companion
paper). To compare the sensitivity of normal and CCD rat neurons to
TNF-, we measured the changes in the threshold current, action
potential (AP) threshold, resting membrane potential (Vm), input resistance
(Rin) and afterhyperpolarization (AHP) of each DRG cell (Czeh et al. 1977;
Pellegrino et al. 1984) after topical application of
TNF- for 5 min (Fig. 1).
Vm was first measured 3 min after a
stable recording was obtained and was measured again after 5 min of
TNF- application. Depolarizing currents of 0.05-4.0 nA (100-ms
pulse duration) were delivered in increments of 0.05 nA until an action
potential (AP) was evoked. The threshold current (rheobase) was defined
as the minimum current required to evoke an AP. The AP voltage
threshold was defined as the first point on the rising phase of the
spike at which the change in voltage exceeded 50 mV/ms. The duration of
the AP was measured at the AP threshold level. The AP amplitude was
measured between the peak and the AP threshold. The
Rin for each cell was obtained from
the slope of a steady-state I-V plot in response to a series of hyperpolarizing currents of 100-ms duration, delivered in decreasing steps of 0.05 nA from 0.2 to 2 nA. The AHP amplitude was measured from the valley peak to the baseline; and the AHP duration was measured
at amplitude half way between.
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Data acquisition and statistical analyses
A mean basal ("control") discharge rate was computed as the
mean number of spikes/s (± SE) for 15 min before delivery of TNF-. For each fiber, the maximal or peak effect of TNF- on the fiber's activity was defined as the discharge rate in the 5-min interval (spikes · s
1 · 5 min1) following drug administration that
exhibited the greatest increase from the peak basal rate. Student's
t-test was used to compare the different peak discharges and
the response latencies between normal and CCD fibers. Paired
t-test or Wilcoxon signed-rank test was used to compare the
changes in AP parameters before and after topical TNF- application.
2 test or Mann-Whitney rank-sum test was used
to compare the incidence of neuronal response to TNF- between normal
and CCD rats. A P value of <0.05 was considered
statistically significant.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Extracellular microfilament recording
C FIBERS FROM NORMAL AND CCD RATS. A total of 40 C fibers with CV <2 m/s were selected for this series of experiments, 18 from normal rats and 22 from CCD rats. Eight of the 18 normal and 9 of the 22 CCD fibers were spontaneously active prior to drug application, while the remaining fibers were initially quiescent.
As shown in Fig. 2A, TNF-
evoked responses in 11 of 13 quiescent fibers from CCD rats and in 4 of
10 quiescent fibers from normal rats (P < 0.05, 2 test). The peak discharge rates of the 11 CCD fibers that responded to TNF- averaged 0.68 ± 0.32 spikes/s, which is significantly higher than the average discharge rate
in normal rats (0.09 ± 0.03 spikes/s, n = 4; Fig.
2B). Furthermore, the response latency for TNF- to evoke
discharges was shorter in CCD rats (13.4 ± 4.2 min,
n = 11) than in normal rats (22.9 ± 7.9 min,
n = 4; P < 0.05, Student's
t-test; Fig. 2C). Seven of 11 CCD fibers and 3 of
4 normal fibers recovered completely after washout with ACSF.
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also enhanced the discharge rate in eight of nine spontaneously
active CCD fibers and in all eight spontaneously active normal fibers
(Fig. 2D). The mean basal discharge rates prior to TNF-
application were 0.03 ± 0.01 and 0.04 ± 0.02 spikes/s for
CCD and normal neurons, respectively. After TNF- application, the
mean peak discharge rate of the nine CCD fibers increased to 0.20 ± 0.21 spikes/s, which was higher than that of normal fibers
(0.13 ± 0.16 spikes/s; P = NS, Student's
t-test). Figure 2D shows that a similar response
was induced by TNF- applications in spontaneously active C fibers
from normal and CCD rats. Similar to quiescent fibers, the response
latency of spontaneously active fibers was shorter in CCD neurons than
normal neurons (Fig. 2C).
A FIBERS FROM NORMAL AND CCD RATS.
A total of 36 A fibers (CV > 15 m/s) were tested in this
series of experiments, 18 CCD fibers (8 quiescent and 10 spontaneously active) and 18 normal fibers (15 quiescent and 3 spontaneously active).
The mean basal discharge rate for the 10 spontaneously active CCD
fibers prior to TNF- application was 6.0 ± 3.6 spikes/s. Following TNF- application, six of eight (75%) quiescent CCD fibers
responded after a mean latency of 16.6 ± 4.4 min. Only two of
these six fibers returned to electrical silence after 30-40 min
washout with ACSF. In contrast, only 4 of 15 (27%) of the normal
quiescent fibers responded to TNF- application and did so after a
much longer latency (23.3 ± 7.4 min) than CCD fibers (P < 0.05, Student's t-test). Seven of 10 (70%) spontaneously active CCD fibers responded to TNF- application
by increasing discharge rate from an average of 5.4 ± 3.1 to
12.0 ± 5.6 spikes/s (P < 0.01, Wilcoxon
signed-rank test, n = 10). Discharge of one fiber was
inhibited; two fibers did not shown significant changes in discharge
rate. One of three (33%) normal spontaneously active fibers responded
with a slight increase in discharge rate following TNF- application.
Microelectrode intracellular recording
A total of 68 small neurons (39 normal and 29 CCD) and 68 large neurons (22 normal, 35 quiescent CCD, and 11 spontaneously active CCD) were studied.
SMALL SIZE DRG NEURONS FROM NORMAL AND CCD RATS.
Normal neurons.
Acute topical application of TNF- significantly increased
(depolarized) membrane potential of normal neurons from 61.2 ± 1.7 mV prior to TNF- application to 59.1 ± 1.9 mV 5 min
after TNF- application (P = 0.006, Wilcoxon
signed-rank test, n = 39; Fig.
3A). Of 39 normal neurons, 23 were depolarized, 9 were hyperpolarized, and the remaining 7 neurons
did not respond to TNF-. For the depolarized neurons, the mean
increase was 4.6 ± 0.7 mV (n = 23; 1-13 mV
range). Acute topical application of TNF- decreased the rheobase in
19 of 39 neurons, increased it in 9 neurons, and induced no changes in
the remaining 11 neurons. The mean rheobase for all the neurons tested
decreased significantly, from 0.69 ± 0.06 to 0.59 ± 0.06 nA
after TNF- application (P = 0.004, Wilcoxon signed-rank tests; Fig. 3B). In addition, the mean maximal
depolarizing rate was decreased significantly from 161 ± 11.3 to
146 ± 8.1 mV/s (P = 0.017, Wilcoxon signed-rank
test), and the AP amplitude decreased from 50.2 ± 1.9 to
47.6 ± 1.8 ms (P = 0.034, paired t-test). There was no significant change in the action
potential threshold, the maximal repolarizing rate, or the
afterhyperpolarization after 5 min of TNF- application.
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application by increasing the membrane potential and decreasing rheobase. The mean membrane potential increased from 62.6 ± 1.6 to 58.4 ± 1.6 mV (P < 0.001, paired
t-test, n = 29; Fig. 3A). Of the
29 small CCD neurons tested, 27 neurons were depolarized by TNF-,
and 2 neurons were minimally hyperpolarized (<2 mV). The
depolarization for the 27 neurons averaged 5 ± 0.8 mV, ranging between 1 and 15 mV. TNF- decreased the rheobase in 20 of 29 neurons, increased it in 4 neurons, and produced no change in 5 neurons. For all 29 neurons tested, the rheobase decreased from an
average of 0.64 ± 0.06 to 0.53 ± 0.06 nA (P < 0.001, paired t-test; Fig. 3B). There were no
significant changes in any other action potential parameters.
CCD vs. normal neurons.
Compared to normal, uninjured neurons, small CCD neurons were more
sensitive to TNF-. TNF--induced changes in membrane potential (5.0 vs. 4.6 mV, P > 0.05, Mann-Whitney rank-sum test)
and rheobase (0.11 vs. 0.08 nA, P > 0.05, Mann-Whitney
rank-sum test) were both slightly greater in CCD than normal neurons;
the proportion of neurons that were depolarized by TNF- was
significantly higher in CCD (93%) than in normal rats (59%;
P < 0.05, 2 test). Similarly,
the rheobase was decreased by TNF- in 49% of normal neurons and
69% of CCD neurons (P < 0.05, 2 test; Fig. 3C).
LARGE-SIZED DRG NEURONS FROM NORMAL AND CCD RATS.
Normal neurons.
Acute topical application of TNF- produced minor depolarization in
11 of 22 (50%) normal large neurons. The depolarization in the 11 responsive neurons averaged 1.55 ± 0.45 mV (1- to 6-mV range).
The mean membrane potential immediately before and 5 min after TNF-
application was 64.1 ± 1.6 and 63.7 ± 1.4 mV,
respectively (P > 0.05, paired t-test).
Only 3 of 22 (14%) tested neurons showed a decreased rheobase (<0.2
nA) after a 5-min of TNF- application; the rest of the neurons were
either slightly hyperpolarized or no change. No statistical difference
was observed in any other action potential parameters (Fig.
4, A and C-E).
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induced a greater effect on the membrane
properties than in normal neurons (Fig. 4, A and
B). Thirty of 35 (86%) CCD neurons were depolarized after
topical application of TNF-. As a result, the mean membrane
potential for the 35 neurons tested increased (depolarized) from
64.6 ± 1.3 to 60.8 ± 1.4 mV (P < 0.001, paired t-test, n = 35; Fig.
4C). The depolarization averaged 5.1 ± 0.6 mV, ranging
between 1 and 17 mV (depolarized cells only). Of the rest of the
neurons that were not depolarized by TNF- application, two neurons
were slightly hyperpolarized, and three neurons did not respond to
TNF-. In 21 of 35 (60%) neurons, the rheobase decreased; for the
group of 35 neurons, the mean rheobase decreased from a control value
of 0.92 ± 0.07 to 0.81 ± 0.06 nA (P = 0.007, paired t-test) following TNF- application (Fig. 4,
B and D). In addition, the action potential
duration was increased by TNF- application from an average of 0.8 0 ± 0.05 to 0.85 ± 0.06 ms (P = 0.016, paired t-test, n = 35; Fig. 4E).
There were no significant changes in other action potential parameters
of CCD neurons.
Spontaneously active CCD neurons.
Eleven large spontaneously active neurons with different discharge
patterns were studied. Of the 11 neurons, 3 exhibited low-frequency irregular discharges; 3 neurons had regular bursting discharges; and
the remaining 5 neurons had irregular high-frequency discharges. Topical application of TNF- to the DRG for 4-10 min significantly increased the discharge rate of all three neurons that exhibited low-frequency discharges. After washout with ACSF, the responses in two
of these three neurons returned to basal level within 13 and 20 min,
respectively (Fig. 5A). One
neuron failed to recover after a 30-min ACSF washout. TNF- also
enhanced the discharge rate of the three neurons with regular bursting
discharges, with an average of 2.5-mV depolarization. In addition, the
discharge pattern of two neurons changed from a regular bursting
discharge to an irregular high-frequency discharge following TNF-
application and recovered after washout with ACSF for 10 and 30 min,
respectively (Fig. 5B). Among the remaining five neurons,
three depolarized (average of 1.2 mV), one neuron did not change, and
another one hyperpolarized. However, there was no significant change in
the discharge rate or the discharge patterns after TNF- application. For all the spontaneously active neurons tested (n = 11), a slight but significant depolarization was induced by TNF-
application. The mean membrane potential before and after TNF-
application was 66.2 ± 2.8 and 64.4 ± 2.7 mV,
respectively (P = 0.02, paired t-test).
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as demonstrated by a higher percentage of neurons that were
depolarized by TNF- in CCD (86%) than in normal neurons (50%;
P < 0.05, 2 test). Similarly,
60% of CCD neurons responded to TNF- application with decreased
rheobase, while in normal neurons, the responsive proportion was 14%.
In addition, the action potential duration was increased in 60% of CCD
neurons, which is greater than in normal neurons (30%;
P < 0.05, 2 test; Fig.
4F).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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This study demonstrates that acute topical application of TNF-
evokes action potentials and enhances neuronal excitability of DRG
neurons, whether or not previously exposed to compression injury.
However, CCD neurons are more sensitive to TNF- application than
normal neurons.
Extracellular microfilament recording
Both A and C fibers from control and CCD rats responded to
topical application of TNF-. The response of silent A and C fibers to TNF- application was similar in CCD rats. The latency of
TNF-2-evoked responses was shorter in CCD than in normal neurons, and
the mean peak discharge rate was higher in CCD than in normal neurons.
However, the response magnitude was lower in spontaneously active
fibers than in quiescent ones. The reasons are unknown; however, as
discussed in the companion paper, it is possible that in spontaneously
active neurons with or without compression injury, certain ion channels
(e.g., K+ channels) responsible for
TNF--evoked responses might have been partially inactivated prior to
chemical application, leading to a reduced response to TNF- application.
Dose-response comparisons would provide further information about the
relative differences in TNF sensitivity between normal and CCD fibers
and help determine if CCD neurons have a lower response threshold.
However, the long excitatory effect of TNF- after a single
application, and the difficulties in maintaining a reliable recording
of C-fibers for over 2-3 h limited our ability to conduct such an
experiment. The concentration of TNF- (1 ng/ml) may be higher than
the physiological level in the rat sciatic nerve (George
1999). However, under certain pathologic conditions, DRG
neurons can be exposed to relatively high concentrations of TNF-
released from herniated nucleus pulposus (Igarashi et al. 2000). In addition, in our extracellular recording experiments, the concentration of TNF- that actually reached the somata in the
DRG could be considerably lower than that in the perfusion solution.
Responses of DRG neurons to TNF- at lower concentrations (as shown
in the companion paper) suggest a physiological effect of TNF-.
Intracellular recording
Low concentration TNF- was used during the intracellular
recording experiments to detect possible changes in the membrane properties of DRG neurons, which are not measurable with extracellular recording techniques. Results are in agreement with current
extracellular studies: chronic compression injury enhanced TNF-
sensitivity of DRG neurons, demonstrating that CCD neurons are
hyperexcitable not only to electrical (Zhang et al.
1999) but also to chemical stimulation.
TNF- enhanced excitability of normal DRG neurons with either
myelinated or unmyelinated axons. The decreased rheobase is probably
caused by depolarization, as no changes in action potential threshold
or Rin were observed in response to
TNF- application. The decreased maximal depolarization rate of small
DRG neurons can be explained by lowered action potential amplitude
without alteration in action potential duration. In small CCD neurons, no significant changes in action potential duration, action potential amplitude, or maximal depolarization rate were observed on TNF- application. This lack of response could be due to changes in action
potential configurations by compression injury prior to TNF-
application (Zhang et al. 1999).
TNF- increased the discharge rate of spontaneously active large DRG
neurons with only minor depolarization (1-2 mV). This suggests that
low degrees of depolarization induced by lower concentrations of
TNF- may not be enough to generate spontaneous activity in normal
DRG neurons but may be sufficient to evoke action potentials or enhance
spontaneous activity in cells that are hyperexcitable following
peripheral nerve or lumbar ganglia injury.
The latency for TNF- to evoke action potentials as observed in
extracellular fiber recordings is longer than the latency to induce
depolarization as obtained in our intracellular recording experiments.
The reasons are twofold: first, in the intracellular study, most cells
that were tested were on the surface of the ganglion, whereas in
extracellular fiber recording study, it is very likely that a large
number of cells were located beneath the superficial layer. Second, it
is possible that during the extracellular recording study, a certain
level of depolarization may have occurred before the generation of spikes.
The ionic mechanisms underlying TNF--induced depolarization are not
clear. However, evidence suggests strongly that
K+ conductance is involved in changes in neuronal
excitability following peripheral nerve injury (Everill and
Kocsis 1999). Increased neuronal excitability caused by a low
K+ conductance is normally accompanied by an
increase in the input resistance and a decrease in the rheobase
(Bal and McCormick 1993). In the current study, TNF-
did not induce significant changes in the input resistance but
significantly decreased the rheobase, which likely resulted from
depolarization of the membrane potential. This hypothesis is supported
by previous studies in which TNF- decreased K+
conductance in Aplysia abdominal neurons (Sawada et
al. 1990) and retinal ganglion neurons (Diem et al.
2001). In the companion paper, we report that TNF--induced
sensory responses are blocked by specific antagonists of PKA pathway.
Thus it is possible that when deposited at the ganglia, TNF-
activates the PKA pathway and induces phosphorylation of certain types
of K+ channel and results in their closure. The
prolonged action potential duration found in large DRG neurons supports
this hypothesis.
The mechanisms underlying the enhanced sensitivity of CCD neurons to
TNF- are not clear. There is evidence that the levels of TNF- and
TNF- receptors are increased in peripheral nerves that underwent
previous loose ligation injury (Shubayev and Myers 2000). Other studies showed that injection of antibody to
TNF- receptor1 or TAPI (which is believed to downregulate TNF-
receptor 1 protein level) can reduce thermal and mechanical allodynia
(Sommer et al. 1997, 1998b). In a recent study from our
own laboratory, we reported that local administration of mixed, soluble
TNF- receptors partially reversed the CCD-induced mechanical
hyperalgesia. An alternative explanation for the enhanced neuronal
TNF- sensitivity after CCD injury could be the lower pH of the
intracellular environment induced by the inflammatory reaction. This
local inflammation could have facilitated the self-embedding of TNF-
into the cell membrane to form mini pores, a hypothesis suggested by
Kagan et al. (1992). Finally, changes in potassium
activity due to compression injury, if any, could directly alter the
sensitivity of DRG neurons to TNF-.
In summary, results from our present study indicate that TNF- may
further enhance excitability of CCD neurons and that CCD sensory
neurons become more sensitive to exogenous TNF-. Our results suggest
that clinically, inflammatory cytokines (such as those released from
the herniated disks) may cause pain and hyperalgesia by activating
sensory neurons of the lumbar ganglia.
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ACKNOWLEDGMENTS |
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This work was supported by National Institute of Neurological Disorders and Stroke Grant R01NS-39568A.
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FOOTNOTES |
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Address for reprint requests: J.-M. Zhang, Dept. of Anesthesiology, Slot 515, University of Arkansas for Medical Sciences, 4301 W. Markham St., Little Rock, AR 72205 (E-mail: ZhangJunming{at}uams.edu).
Received 10 January 2002; accepted in final form 31 May 2002.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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 X. Jin and R. W. Gereau IV Acute p38-Mediated Modulation of Tetrodotoxin-Resistant Sodium Channels in Mouse Sensory Neurons by Tumor Necrosis Factor-{alpha} J. Neurosci., January 4, 2006; 26(1): 246 - 255. [Abstract] [Full Text] [PDF]
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B. Liu and J. C. Eisenach Hyperexcitability of Axotomized and Neighboring Unaxotomized Sensory Neurons Is Reduced Days After Perineural Clonidine at the Site of Injury J Neurophysiol, November 1, 2005; 94(5): 3159 - 3167. [Abstract] [Full Text] [PDF]
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Y. Li, A. Ji, E. Weihe, and M. K.-H. Schafer Cell-Specific Expression and Lipopolysaccharide-Induced Regulation of Tumor Necrosis Factor {alpha} (TNF{alpha}) and TNF Receptors in Rat Dorsal Root Ganglion J. Neurosci., October 27, 2004; 24(43): 9623 - 9631. [Abstract] [Full Text] [PDF]
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 T. L. Jones and L. S. Sorkin Calcium-Permeable {alpha}-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid/Kainate Receptors Mediate Development, but Not Maintenance, of Secondary Allodynia Evoked by First-Degree Burn in the Rat J. Pharmacol. Exp. Ther., July 1, 2004; 310(1): 223 - 229. [Abstract] [Full Text] [PDF]
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M. Schafers, D. H. Lee, D. Brors, T. L. Yaksh, and L. S. Sorkin Increased Sensitivity of Injured and Adjacent Uninjured Rat Primary Sensory Neurons to Exogenous Tumor Necrosis Factor-alpha after Spinal Nerve Ligation J. Neurosci., April 1, 2003; 23(7): 3028 - 3038. [Abstract] [Full Text] [PDF]
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