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Department of Biomedical Sciences, University of Maryland Dental School and Department of Anatomy and Neurobiology and Program in Neuroscience, University of Maryland School of Medicine, Baltimore, Maryland 21201
Submitted 4 September 2003; accepted in final form 16 January 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Consistent with this suggestion, there is evidence that the properties of afferents innervating hollow organ structures are unique relative to those innervating somatic tissue. For example, there are both high and low threshold C- and A
-fiber afferents in the pelvic nerve that innervate the colon, yet both of these populations of afferents appear to be nociceptive in that they encode increasing stimulus intensity well into the noxious range and demonstrate sensitization in response to injury (Sengupta and Gebhart 1994
; Su et al. 1997). In contrast, C-fibers innervating somatic tissue tend to have a high threshold for activation. While afferent activation thresholds depend on the properties of the innervated tissue (Cooper 1993), there is also evidence that differences between visceral and somatic afferents reflect intrinsic properties of afferent subpopulations (Berkley et al. 1993). However, this issue has not been systematically investigated in dorsal root ganglia (DRG) neurons.
The excitability of nociceptive afferents may be dynamically regulated. This is particularly true in the presence of inflammation, where the excitability of nociceptive afferents increases (Davis et al. 1993; Sengupta et al. 1996). Inflammation-induced sensitization of afferents is also likely to reflect changes in properties intrinsic to the sensory neurons. Indeed, we (Gold et al. 1996) and others (Baccaglini and Hogan 1983; Nicol and Cui 1994) have previously demonstrated that isolated DRG neurons in vitro may be sensitized following the application of the inflammatory mediator prostaglandin E2 (PGE2). Interestingly, there was significant variability in both the magnitude and the pattern of PGE2-induced changes in neuronal excitability (Gold et al. 1996). The basis for this variability has yet to be investigated, but if it reflects differences between afferent subpopulations defined by target of innervation, another explanation for site-specific pain syndromes may be that there are differences in the response to injury between populations of afferents based on target of innervation.
We hypothesize that differences in the expression of pain syndromes in various parts of the body reflect, in part, unique properties of the afferents innervating different structures. Furthermore, we hypothesize that these unique properties influence excitability of afferents innervating naïve tissue as well as inflammation-induced changes in the excitability of these afferents. We have performed this study to begin to test these hypotheses. Specifically, we have recorded from isolated sensory neurons retrogradely labeled from either the glabrous skin of the hindpaw or the distal colon. Because the colon receives innervation from two spinal nerves, the pelvic, and the hypogastric/lumbar colonic, we recorded from two distinct populations of colonic afferents. We compared passive and active electrophysiological properties as well as changes in these properties in response to the prototypical inflammatory mediator, PGE2.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Identification of DRG neurons innervating the colon and glabrous skin of the hindpaw
Colonic DRG neurons were identified by the retrograde administration of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) into the descending colon. Labeling was performed as described previously (Gold et al. 2002). Briefly, rats were anesthetized with Na-pentobarbital (50 mg/kg ip). The descending colon was exposed by a midline laparotomy, and 20 µl DiI (2.5% in methanol) was injected at 10–15 sites into the colon wall with the aid of a dissecting microscope. DiI that visibly leaked from the injection site was wiped away with cotton swabs. The surgical wound was sutured in layers, and rats were allowed to recover from anesthesia. Neurons were studied 14–21 days after labeling. Cutaneous neurons innervating glabrous skin of the hindpaw were also labeled with DiI, except that the dye was dissolved in DMSO (170 mg/ml) and diluted 1:10 in 0.9% sterile saline. Five microliters of this solution was injected subcutaneously with a 30-g injection needle directed into the epithelium (area 
5 mm2). DiI-labeled neurons were easily identified under epifluorescence illumination with a Texas-red/rhodamine filter set.
Cell dissociation
The colon receives innervation from two spinal nerves: the pelvic and the hypogastric/lumbar colonic. These nerves arise from lumbosacral (LS: L6–S2) and thoracolumbar (TL: T12–L2) DRG, respectively. The site of injection in the hindpaw receives innervation via the sciatic nerve, which is comprised primarily of axons arising from L4 and L5 DRG. DRG neurons were prepared for recording as described previously (Gold et al. 2002). Briefly, rats were deeply anesthetized with a subcutaneous injection of rat cocktail (1 ml/kg of 55 mg/ml ketamine, 5.5 mg/ml xylazine, and 1.1 mg/ml acepromazine); TL and LS or L4 and L5 DRG were removed, and rats were subsequently killed by decapitation. DRG were desheathed in ice-cold MEM-BS composed of 90% minimal-essential-medium (MEM; Gibco BRL, Gaithersburg, MD), 10% heat-inactivated fetal bovine serum (BS), and 1,000 units/ml each of penicillin and streptomycin. DRGs were incubated 45 min at 37° C in 5 ml MEM, to which collagenase P (Roche Bioscience, Palo Alto, CA) had been added to a final concentration of 0.125% and bubbled with carbogen (95% O2-5% CO2). DRG were incubated 5 min at 37° C in Ca2+- and Mg2+-free Hanks balanced salt solution (GIBCO BRL) containing 0.25% trypsin (Worthington, Bristol, UK) and 0.025% EDTA (Sigma, St. Louis, MO). Trypsin activity was inhibited by the addition of MEM-BS containing 0.125% MgSO4, and DRG were dissociated by trituration with a fire-polished Pasteur pipette. DRG cells were plated onto glass coverslips, previously coated by a solution of 5 µg/ml mouse laminin (GIBCO BRL) and 0.1 mg/ml poly-L-ornithine (Sigma). The cells were incubated in MEM-BS at 37° C, 3% CO2, and 90% humidity for 2 h, at which point they were transferred to a HEPES-buffered L-15 media containing 10% BS and 5 mM glucose and stored at room temperature. TL and LS DRG were processed in parallel. Colonic and cutaneous neurons were isolated from different rats and therefore studied on different days. All neurons were studied between 2 and 7 h after removal from the animal.
Electrophysiology
Current-clamp recordings were performed using a HEKA EPC9 (HEKA Electonik, Lambrecht/Pfaz, Germany). Data were low-pass filtered at 5–10 kHz with a 4-pole Bessel filter and digitally sampled at 25–100 kHz. For voltage-clamp protocols, capacity transients were canceled, and series resistance was compensated (>80%). Electrodes (0.7–3 M
) were filled with (in mM) 140 K-Methansulphonate, 5 NaCl, 1 CaCl2, 2 MgCl2, 11 EGTA, 10 HEPES, 2 Mg-ATP, and 1 Li-GTP; pH was adjusted to 7.2 with Tris-base, and osmolality was adjusted to 310 mOsm with sucrose. Bath solution contained (in mM) 140 NaCl, 3 KCl, 2.5 CaCl2, 0.6 MgCl2, 10 HEPES, and 10 glucose; pH was adjusted to 7.4 with Tris-Base, and osmolality was adjusted to 325 mOsm with sucrose. All salts were obtained from Sigma. Sucrose was obtained from Gibco.
Experimental protocol
After formation of a tight seal (>5 G) and compensation of pipette capacitance with amplifier circuitry, whole cell access was established. Cell capacitance was determined with four hyperpolarizing pulses (10 ms) from –60 to –80 mV.
Passive electrophysiological properties (resting membrane potential and input resistance) were determined within 2 min of establishing whole cell access. Input resistance was determined by assessing current evoked in response to a 10-ms, 20-mV hyperpolarizing voltage step from –70 mV. The presence of spontaneous activity was assessed by observing the neuron under current clamp for 1 min, after which active electrophysiological properties were determined.
To assess properties of the action potential waveform (rate of rise and decay, duration, overshoot, and afterhyperpolarization magnitude and duration), a single action potential was evoked with depolarizing current injection (4-ms duration rectangular pulse at threshold intensity; Fig. 1). A 500-ms depolarizing current injection was used to assess neuronal excitability (action potential threshold, rheobase, and adaptation). Action potential threshold was defined as the maximum depolarization obtained in the absence of an action potential. Rheobase was defined the minimum depolarizing current injection necessary to evoke an action potential (Fig. 1). The total number of spikes evoked at threshold was used as a measure of adaptation. Finally, the response to suprathreshold stimuli was assessed by stimulating neurons with depolarizing current inject equal to 1.5, 2, 2.5, and 3 times rheobase (Fig. 1). To control for the effect of membrane potential on the availability of voltage-gated ion channels and therefore the secondary influence of membrane potential on active properties, current injection was used to hold neurons between at –55 and –60 mV.
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), a single protocol could be used to assess several aspects of excitability. After establishing the baseline action potential threshold, rheobase, and the response to suprathreshold stimuli with square-wave stimuli, the ramp stimulus was used to monitor stability over a period of 
3 min with stimuli applied with an interstimulus interval (ISI) of 1 min. The effects of PGE2 were only assessed in neurons demonstrating stable baseline excitability. Results obtained with both square-wave and ramp and hold stimuli were comparable. A neuron was considered sensitized by PGE2 if there was a decrease in threshold or rheobase or an increase in spike number at least 2 SD from baseline mean. The effects of PGE2 were analyzed relative to results obtained with the first application of the square-wave stimulus, either as a difference (i.e., After – Before) or as a percent of baseline [i.e., (After/Before) x 100]. PGE2 vehicle (0.1% ethanol in bath solution) had no effect on neuronal excitability in five neurons subsequently sensitized by PGE2 (data not shown). The presence of the hyperpolarization-activated current Ih was assessed in a subpopulation of neurons following completion of current-clamp experiments. The amplifier was switched back to voltage-clamp mode. Series resistance compensation (>80%) and whole capacitance compensation were employed via amplifier circuitry. Neurons were held at –60 mV. The presence of Ih was assessed with a 500-ms voltage step to –100 mV following a 500-ms prepulse to –50 mV. Inward current that developed over the 500 ms step to –100 mV was considered Ih. This current was completely and reversibly blocked by the application of 5 mM Cs+ to the bath solution (data not shown).
Data analysis
Action potential duration was determined at 0 mV. Maximum rates of rise and fall for the action potential were determined by taking the first derivative of the action potential waveform. Magnitude of the afterhyperpolarization was determined relative to the resting membrane potential at the largest potential obtained following the action potential. The afterhyperpolarization (AHP) was fitted with a single exponential equation to estimate the decay rate of the AHP. A paired t-test was used to assess the statistical significance of PGE2-induced changes in passive and active properties of colonic DRG neurons, while one- and two-way ANOVAs with a Holmak-Sidak post hoc test were used to assess the statistical significance of differences between cutaneous, TL, and LS neurons.
Drugs
PGE2 was dissolved in 100% ethanol, stored as a 10 mM stock solution at –20° C, and diluted in bath solution immediately prior to use. PGE2 and all other chemicals employed (unless otherwise stated) were obtained from Sigma.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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We studied 33 cutaneous neurons from six rats and 98 colonic neurons from an additional nine rats. Between 2 and 10 neurons were studied from each rat. Because there is a rough correlation between axon conduction velocity and cell body diameter for afferents innervating cutaneous tissue (Lawson et al. 1993; Lee et al. 1986), and we wished to compare putative nociceptive afferents innervating cutaneous tissue to those innervating the colon, we studied labeled cutaneous neurons with a small cell body size. Because membrane capacitance measured in the whole cell configuration is an accurate measure of cell body size, we used membrane capacitance to distinguish small from large neurons with a cut-off of 42 pF. This cut-off was chosen because it corresponds with a cell body diameter of 30 µm, based on a specific capacitance of 1.19 µF/cm2, which was determined with a regression analysis of data from over 500 DRG neurons in which cell body diameter and membrane capacitance had been determined. An additional correction factor of 6% was included because of the influence of DiI on capacitance measurements (Gold et al. 2002). The average cell capacitance of cutaneous DRG neurons was 33.2 ± 1.1 (SE) pF (n = 33; range, 20.34–42 pF). Because the colon is only innervated by afferents with slowly conducting axons (70% C-fibers and 30% A- fibers), we studied all colonic afferents. The average cell body capacitance of colonic DRG neurons was 67.8 ± 2.5 pF (n = 98; range, 19–131 pF); the difference between colonic and cutaneous DRG neurons with respect to cell body capacitance was statistically significant (P < 0.01). There was no statistically significant difference between TL and LS neurons with respect to cell capacitance; both were 6 pF (Table 1).
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Cutaneous DRG neurons were significantly different from colonic (both TL and/or LS) DRG neurons in a number of the electrophysiological properties assessed, including resting membrane potential (P < 0.01, cutaneous vs. LS only; Table 1), rheobase (P < 0.05, cutaneous vs. TL only; Fig. 2; this difference did not reach statistical significance if data were normalized with respect to cell body capacitance), action potential duration (P < 0.01; Table 1), action potential overshoot (P < 0.05; Table 1), maximum rate of action potential fall (P < 0.01; Table 1), and the magnitude of the AHP (P < 0.05, Table 1). Previous studies of somatic afferents indicate that there is a correlation between cell body size, axon conduction velocity, and action potential properties: larger neurons tend to give rise to more rapidly conducting axons, with narrower action potentials and smaller neurons tending to give rise to more slowly conducting axons with slower broader action potentials (Djouhri and Lawson 1999; Harper and Lawson 1985; Koerber et al. 1995; McCarthy and Lawson 1989; Ritter and Mendell 1992; Villiere and McLachlan 1996; Waddell and Lawson 1990). The differences between cutaneous and colonic neurons observed in this study are consistent with these previous observations of somatic afferents. However, the action potential rate of rise was similar between all three groups of neurons (Table 1) and is much closer to the rates observed in C-fibers (McCarthy and Lawson 1997) than in neurons giving rise to more rapidly conducting axons (Ritter and Mendell 1992). Importantly, the C-fiber–like action potential in colonic neurons was also consistent with our previous results, suggesting that the majority of DiI-labeled colonic DRG neurons give rise to unmyelinated axons (Gold et al. 2002). These observations suggest that we have studied primarily cutaneous and colonic C-fiber afferents that differ with respect to cell body size as well as other properties.
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In contrast to the marked differences between colonic and cutaneous neurons, TL and LS neurons were similar with respect to the majority of passive and active electrophysiological properties assessed (Table 1). There were, however, several notable exceptions. First, mean Rin of TL neurons was significantly larger than that of LS neurons (P = 0.01). Given that a hyperpolarizing current injection was used to assess Rin, this difference may reflect differences in the expression of the hyperpolarization-activated current Ih. Indeed, we assessed Ih with a 500-ms hyperpolarizing voltage-step from –50 to –100 mV in 20 TL and 18 LS neurons. Consistent with the difference between the two populations with respect to Rin, there is a higher density of Ih in LS neurons (2.7 ± 0.5 pA/pF at –100 mV) than in TL neurons (1.3 ± 0.2 pA/pF; P < 0.01). The difference in Ih may also have contributed to differences in the Rin between cutaneous neurons and LS neurons, because the density of Ih in cutaneous neurons was only 0.5 ± 0.1 pA/pF. Second, action potential threshold of LS neurons was significantly lower than that of TL neurons (Fig. 2; P < 0.05). Rheobase in LS neurons was also lower than that observed in TL neurons (Fig. 2, P < 0.05). Finally, suprathreshold stimuli resulted in a significantly greater number of evoked action potentials in TL neurons than in LS neurons (Fig. 2; P < 0.01), and the median slope of the stimulus response function was significantly greater in TL than in LS neurons (Fig. 2; P < 0.01).
PGE2-induced sensitization of cutaneous and colonic DRG neurons
Similar to results obtained in previous studies of unlabeled DRG neurons (Baccaglini and Hogan 1983; England et al. 1996; Gold et al. 1996; Nicol and Cui 1994), PGE2 increased the excitability of both cutaneous and colonic DRG neurons (Fig. 3). In general, the increase in excitability, or sensitization, was associated with a leftward shift in the stimulus response function, such that lower intensity stimuli were required to evoke action potentials and more action potentials were evoked in response to previously suprathreshold stimuli. Many neurons also demonstrated spontaneous activity.
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2 test). There were also differences in the nature of the sensitization. More than one-quarter of the colonic neurons (8 of 28) became spontaneously active following application of PGE2, while none of the cutaneous neurons demonstrated spontaneous activity. In addition, all but 1 of the colonic neurons sensitized by PGE2 demonstrated a decrease in rheobase, a decrease in action potential threshold, and an increase in the number of action potentials evoked in response to suprathreshold stimuli (Fig. 4), whereas only 4 of the 10 sensitized cutaneous neurons demonstrated changes in all of these properties. The remaining cutaneous neurons sensitized by PGE2 demonstrated either a decrease in rheobase (4 neurons) or an increase in action potentials evoked in response to suprathreshold stimuli (2 neurons). Interestingly, sensitization of cutaneous neurons was associated with an increase in the slope of the stimulus response function with little apparent decrease in threshold (Fig. 4). This change is analogous to that observed following sensitization of cutaneous nociceptive terminals in vivo (Andrew and Greenspan 1999).
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PGE2-induced changes in passive and active properties of cutaneous and colonic DRG neurons
PGE2-induced sensitization of cutaneous and colonic DRG neurons was associated with several significant changes in passive and active electrophysiological properties (Table 2). Interestingly, the pattern and/or magnitude of changes was different among subpopulations of sensory neurons. For example, the increase in input resistance observed in cutaneous and LS colonic neurons sensitized by PGE2 was not associated with a significant change in resting membrane potential in cutaneous neurons, while it was associated with a small but significant membrane depolarization in LS neurons. In contrast, PGE2 induced a significant decrease in resting membrane potential in the absence of a significant change in input resistance in TL neurons (Table 2). Similarly, there was no significant change in action potential overshoot, rate of rise, or rate of fall in cutaneous neurons, while there were significant changes in all three of these properties in TL and LS neurons (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Both passive and active electrophysiological properties reflect underlying ion channels. Consequently, differences between subpopulations of sensory neurons with respect to passive and active properties indicate that there are differences between subpopulations of neurons with respect to the functional properties of ion channels. For example, the difference between TL and LS colonic neurons with respect to input resistance is likely to reflect, at least in part, differences between these subpopulations of sensory neuron with respect to the density of Ih. However, these neurons must also differ with respect to the presence and/or density of at least one other ion channel active at rest. That is, because Ih is a nonselective cation current that drives membrane depolarization, a higher density of this current would, if it contributed to resting membrane potential, result in a more depolarized resting membrane potential. In contrast to this expectation, neurons with the highest density of Ih had the most hyperpolarized resting membrane potential.
Similarly, the larger AP overshoot, longer AP duration, slower AP fall, and smaller AHP magnitude observed in cutaneous neurons all suggest differences in the activity of at least one voltage- and/or Ca2+-dependent ion channel in this population of neurons compared with colonic neurons. The presence of a higher density of voltage-gated Na+ channels in cutaneous neurons would be consistent with the larger AP overshoot. However, a more likely explanation is that there is a lower density of a voltage- or Ca2+-dependent K+ channels in cutaneous neurons that contributes to AP repolarization. Subclasses of both of these channel subtypes have been shown to function in this capacity (Christian et al. 1994; Rudy 1988). Importantly, such an explanation accounts for all of the differences stated above as well as the lack of difference observed with respect to AP rise time.
Differences in the electrophysiological properties of TL and LS neurons indicated that there are also differences within subpopulations of colonic sensory neurons with respect to ion channels controlling excitability. A higher input resistance should, all else being equal, result in a lower rheobase. That this was not the case in TL neurons suggests that these neurons contain either a higher density of low-threshold voltage- or Ca2+-dependent K+ or Cl– channels or a lower density of voltage-gated Na+ channels. This latter possibility is unlikely given our previous (Gold et al. 2002) as well as unpublished observations indicating that there is no difference between TL and LS neurons with respect to the density or biophysical properties of either TTX-resistant or TTX-sensitive voltage-gated Na+ current.
That TL neurons may be less excitable in naïve animals than LS neurons was suggested by previous studies performed in vivo. In sensory innervation studies of both the cat colon (Janig and Koltzenburg 1991) and the rat uterus (Berkley et al. 1993), afferents in sympathetic nerves (lumbar colonic/hypogastric) had higher thresholds to mechanical stimuli than afferents in the pelvic nerve. Because we have assessed the excitability of colonic sensory neurons with depolarizing current injection, a stimulus that presumably by-passes "natural" transduction processes, our results suggest that at least some of the differences between TL and LS neurons observed in vivo reflect differences in the expression of ionic currents. Our observation that TL neurons are intrinsically less excitable than LS neurons may help explain the observation that, in the absence of inflammation, there appears to be little TL input into the CNS (Traub 2000).
The marked differences between cutaneous and colonic neurons with respect to the influence of the prototypical inflammatory mediator, PGE2, also suggest that there are differences between these populations of sensory neurons with respect to the underlying mechanisms mediating sensitization. Given the apparent differences between these populations of sensory neurons with respect to ion channels controlling passive and active electrophysiological properties in the absence of inflammation, it is reasonable to expect that PGE2-induced change in the biophysical properties of a single ion channel will impact the excitability of these different populations differently. However, the pattern of PGE2-induced changes in both passive and active electrophysiological properties suggests that different mechanisms underlie sensitization of cutaneous and colonic neurons. For example, PGE2 induced both an increase in input resistance and membrane depolarization in LS colonic sensory neurons, consistent with the closing of a K+ leak channel. In contrast, sensitization of cutaneous neurons was associated with a larger increase in input resistance than that observed in colonic neurons. However, this change in resistance was associated with no change in resting membrane potential, suggesting that either a different channel is closed in cutaneous neurons or there is an additional channel active at Vrest in cutaneous neurons that is modified in addition to the channel modified in colonic neurons. Similarly, sensitization of TL and LS colonic neurons was associated with a small but significant increase in AP overshoot and rate of rise and decrease in the AP rate of fall. Given that the presence of TTX-R INa enables AP broadening in sensory neurons (Blair and Bean 2002), these changes in active properties are consistent with the PGE2-induced increase in TTX-R INa that we have recently described in colonic sensory neurons (Gold et al. 2002). Furthermore, given that the same mechanism is likely to contribute to the sensitization of cutaneous sensory neurons (Khasar et al. 1998), the absence of similar changes in AP properties in cutaneous sensory neurons suggests that changes in other channels may compensate for the influence of an increase in TTX-R INa on the AP waveform. Thus, sensitization of cutaneous neurons was likely associated with the closing of a leak channel, modulation of a voltage-gated Na+ channel, and the inhibition of a voltage- or Ca2+-dependent channel, while sensitization of colonic neurons was associated with at least the closing of a different leak channel and the modulation of a voltage-gated Na+ channel.
There were also marked differences between TL and LS neurons with respect to the magnitude of PGE2-induced changes in excitability. However, the pattern of PGE2-induced changes in both passive and active properties was similar between these two populations of neurons. This observation suggests that the underlying mechanisms of sensitization were similar between the two populations and that differences between them with respect to the pattern of excitability changes reflect differences in density and/or properties of the ion channels active in these neurons in the absence of PGE2.
In summary, we have described striking differences between cutaneous and colonic sensory neurons with respect to the passive and active electrophysiological properties of these neurons as well as changes in these properties in response to the inflammatory mediator PGE2. These differences in electrophysiological properties suggest that the density and distribution of ion channels in sensory neurons varies systemically among subpopulations of sensory neurons defined by target of innervation. That PGE2-induced changes in excitability were associated with different changes in passive and active electrophysiological properties suggests that ion channels underlying sensitization of afferents also varies systematically among subpopulations of sensory neurons defined by target of innervation. This latter observation suggests that it may be possible, if not necessary, to treat pain arising from various parts of the body with therapeutic interventions specific for that region of the body.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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GRANTS
This work was supported by National Institute of Neurological Disorders and Stroke Grant NS-41384.
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FOOTNOTES |
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Address for reprint requests and other correspondence: M. S. Gold, Univ. of Maryland, Dental School, Dept. BMS, Rm. 5-A-12 HHH, 666 West Baltimore St., Baltimore, MD 21201 (E-mail: msg001{at}dental.umaryland.edu).
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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C. Ma, K. W. Greenquist, and R. H. LaMotte Inflammatory Mediators Enhance the Excitability of Chronically Compressed Dorsal Root Ganglion Neurons J Neurophysiol, April 1, 2006; 95(4): 2098 - 2107. [Abstract] [Full Text] [PDF]
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G. Wang, B. Tang, and R. J. Traub Differential Processing of Noxious Colonic Input by Thoracolumbar and Lumbosacral Dorsal Horn Neurons in the Rat J Neurophysiol, December 1, 2005; 94(6): 3788 - 3794. [Abstract] [Full Text] [PDF]
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T. Sugiura, K. Dang, K. Lamb, K. Bielefeldt, and G. F. Gebhart Acid-Sensing Properties in Rat Gastric Sensory Neurons from Normal and Ulcerated Stomach J. Neurosci., March 9, 2005; 25(10): 2617 - 2627. [Abstract] [Full Text] [PDF]
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M. J. Beyak, N. Ramji, K. M. Krol, M. D. Kawaja, and S. J. Vanner Two TTX-resistant Na+ currents in mouse colonic dorsal root ganglia neurons and their role in colitis-induced hyperexcitability Am J Physiol Gastrointest Liver Physiol, October 1, 2004; 287(4): G845 - G855. [Abstract] [Full Text] [PDF]
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in AP threshold) was significantly greater in TL neurons than in cutaneous neurons (P < 0.05). B: PGE2 induced a decrease in rheobase in cutaneous, LS, and TL neurons. The decrease in rheobase in TL neurons was significantly greater than that in cutaneous neurons. Data were analyzed as a percent of baseline: (rheobase after PGE2/rheobase before PGE2) x 100. Data were plotted as a box plot with the center line in the box as the median, the lower and upper edges of the box as the 25th and 75th percentiles, and the error bars as the 5th and 95th percentiles. The decrease in rheobase was significantly larger in TL neurons than in cutaneous neurons (P < 0.05). C: PGE2 induced an increase in the number of action potentials evoked (APs/500 ms) in cutaneous, LS, and TL neurons in response to suprathreshold stimulation. Increase in evoked action potentials was significantly greater in TL neurons than in LS or cutaneous neurons at 2 and 3 times rheobase (P < 0.05, 2-way ANOVA follow by Holm-Sidak test for post hoc comparisons). Filled and open symbols are stimulus response data prior to and after application of PGE2, respectively. Circles are cutaneous neurons, triangles are LS colonic neurons, and squares are TL colonic neurons.