Enhanced excitability and suppression of A-type K+ currents in joint sensory neurons in a murine model of antigen-induced arthritis

Abstract

Pain is a dominant symptom of rheumatoid arthritis (RA) and its adequate treatment represents a major unmet need. However, the cellular mechanisms that drive arthritis pain are largely unexplored. Here, we examined the changes in the activity of joint sensory neurons and the associated ionic mechanisms using an animal model of antigen-induced arthritis (AIA). Methylated-bovine serum albumin (mBSA), but not vehicle challenge, in the ankle of previously immunized mice produced time-dependent symptoms of arthritis, including joint inflammation, primary mechanical hyperalgesia in the ipsilateral ankle, and secondary mechanical and heat hyperalgesia in the ipsilateral hindpaw. In vivo electrophysiological recordings revealed that Dil-labeled joint sensory neurons in AIA mice exhibited a greater incidence of spontaneous activity, mechanically evoked after-discharges, and/or increased responses to mechanical stimulation of their receptive fields, compared to control animals. Whole-cell recordings in vitro showed that AIA enhanced the excitability of joint sensory neurons. These signs of neuronal hyperexcitability were associated with a significant reduction in the density of A-type K⁺ currents. Thus, our data suggest that neuronal hyperexcitability, brought about in part by reduced A-type K⁺ currents, may contribute to pain-related behaviors that accompany antigen-induced arthritis and/or other antigen-mediated diseases.

Enhanced excitability and suppression of A-type K+ currents in joint sensory neurons in a murine model of antigen-induced arthritis

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Lintao Qu, & Michael J. Caterina,,

Pain is a dominant symptom of rheumatoid arthritis (RA) and its adequate treatment represents a major unmet need. However, the cellular mechanisms that drive arthritis pain are largely unexplored. Here, we examined the changes in the activity of joint sensory neurons and the associated ionic mechanisms using an animal model of antigen-induced arthritis (AIA). Methylated-bovine serum albumin (mBSA), but not vehicle challenge, in the ankle of previously immunized mice produced time- the ipsilateral ankle, and secondary mechanical and heat hyperalgesia in the ipsilateral hindpaw. In vivo electrophysiological recordings revealed that Dil-labeled joint sensory neurons in AIA mice exhibiteda greater incidence of spontaneous activity, mechanically evoked after-discharges, and/or increased recordings in vitro showed that AIA enhanced the excitability of joint sensory neurons. These signs + currents. Thus, our data suggest that neuronal hyperexcitability, brought about in part by reduced A-type K+ currents, may contribute to pain-related behaviors that accompany antigen-induced arthritis and/or other antigen-mediated diseases.

Rheumatoid arthritis (RA) is a common chronic autoimmune disease characterized by bone destruction and joint inammation1. Joint pain is a predominant clinical feature of RA and represents a signicant health burden2,3. However, the underlying mechanisms that drive arthritis pain are largely unexplored.

Antigeninduced arthritis (AIA) is one of the most extensively used animal models for studies of mechanisms underlying RA-induced pain4,5. It is an easily reproducible and translational immunization model in which arthritis is induced by exogenous antigens, such as ovalbumin6 or methylated bovine serum albumin (mBSA)7,8. Compared with arthritis models induced by Complete Freunds Adjuvant (CFA) or carrageenan, the AIA model is driven by both the innate and adaptive immune systems and in this way better mimics some of the major histological and immunological features of human RA, including joint swelling, bone destruction and hypernociception4.

In addition, unlike collagen-induced arthritis, collagen antibody-induced arthritis or the K/BxN serum transfer model, the AIA model generates RA-like pathology only in one joint, which facilitates the evaluation of pain-like behaviors4. Thus, this model oers certain advantages for the studies of RA-associated pain.

The joint structure is richly innervated by nociceptive bers (A and C) that are likely to be the sources of painful inputs to the spinal cord during RA9. Enhanced excitability of joint nociceptors is thought to contribute to the ongoing pain and hyperalgesia that accompany this disorder and other immune-related diseases in humans. Several studies have documented the eects of proinammatory cytokines (e.g., interleukin (IL)-1, IL-6, and IL-17) on arthritis-associated pain behaviors, on the sensitivity of teased peripheral joint aerents to joint rotation in the AIA model, or on the excitability of cultured dissociated joint sensory neurons1014. In some cases, these studies have also described associated changes in the expression of cytokine receptors and transduction

Department of Neurosurgery, Neurosurgery Pain Research Institute, Johns Hopkins University School of Medicine, Department of Biological Chemistry, Johns Hopkins University School of Medicine,

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channels11,14,15. Moreover, spontaneous activity (SA) and increased mechanical sensitivity were observed in joint sensory aerents in a rat model of osteoarthritis, and were implicated in the maintenance of osteoarthritis pain16,17. However, to our knowledge, there have been no studies addressing the possible existence of such spontaneous activity or of pathological aer-discharges in intact individual joint sensory neurons in models of AIA. Thus, much remains to be learned about the neurophysiological changes associated with joint pain in AIA.

Voltage-gated K+ (Kv) channels are widely expressed in primary sensory neurons and play critical roles in the regulation of neuronal excitability. Kv currents recorded from dorsal root ganglion (DRG) neurons consists of two major subtypes, transient A-type K+ currents (IA) and sustained delayed rectier K+ currents (IK), both of which have been implicated in the generation of pain sensation18. Knockdown of A-type K+ channel expression in primary sensory neurons induced mechanical hyperalgesia19. In addition, there is increasing evidence that the expression and activity of Kv channels in primary sensory neurons are downregulated under inammatory and neuropathic pain conditions and that these channels are involved in the maintenance of a chronic pain state2022.

Consistent with this notion, CFA- induced joint inammation produced downregulation of A-type K+ channels in joint-innervating sensory neurons, possibly contributing to mechanical allodynia in the inamed joint23,24.

However, whether similar changes also occur in joint sensory neurons in the context of AIA remains unknown.

In the present study, we employed both in vitro and in vivo approaches to examine changes in the excitability of joint sensory neurons in a murine AIA model induced by mBSA. Moreover, we specically investigated the possibility of alterations in K+ channels in joint sensory neurons in this model.

Results

mBSA challenge induced arthritis accompanied by pain-like behaviors. Intraarticular (i.a.) injection of mBSA to the ankle joint of mice that had previously been systemically sensitized to this antigen induced progressive arthritis, characterized by strong joint swelling, consistent with previous ndings with mBSA and other antigens8,13. Ankle diameter was signicantly increased on day 1 aer mBSA challenge as compared with saline-treated controls (Fig.1A, p< 0.01, two-way ANOVA with Bonferroni post hoc test). In addition, mice with AIA exhibited pronounced mechanical hyperalgesia to pressure applied to the inamed ankle, as compared to control animals (Fig.1B; p< 0.01; two-way ANOVA with Bonferroni post hoc test). The compression-withdrawal thresholds decreased to a nadir 1 day aer AIA induction and gradually returned to control levels by day 14. However, no signicant changes in mechanical threshold were observed in the contralateral ankle aer AIA (p > 0.05, two-way ANOVA). Compared with control animals, AIA in the ankle also lowered the threshold for responsiveness to punctate mechanical stimulation in the glabrous skin of the ipsilateral hindpaw, indicative of secondary mechanical hyperalgesia remote from the inamed site (Fig.1C; p< 0.01, two-way ANOVA with Bonferroni post hoc test). No such signicant changes occurred in the contralateral hindpaw aer AIA (p>0.05, two-way ANOVA). In addition, withdrawal latency to noxious heat in the glabrous hindpaw skin was reduced ipsilaterally one day aer mBSA challenge and was fully resolved by day 10 (Fig.1D; p< 0.05, two-way ANOVA with Bonferroni post hoc test). The paw thermal hyperalgesia was not observed in either hindpaw of control mice and did not extend to the contralateral hindpaw of AIA mice (p> 0.05, two-way ANOVA).

mBSA challenge produced spontaneous activity in a portion of joint-innervating DRG neurons in vivo. Joint pain may result from the activation and sensitization of nociceptive nerve bers that supply the joint. We therefore asked whether joint-innervating DRG neurons became hyperexcitable aer AIA. To address this issue in a physiologically relevant setting, we employed an in-vivo recording preparation in which the activity in the somata of individual sensory neurons in DRG can be interrogated with preservation of both peripheral and spinal connections25. Following retrograde labeling of joint innervating sensory neurons with DiI, extracellular electrophysiological recordings were obtained from Dil-labeled mechanosensitive sensory neurons with a RF within the vehicle- or mBSA-treated ankle (Fig.2AC). A total of 22 and 24 joint sensory neurons were recorded on day 1 aer vehicle and mBSA challenge, respectively. All the neurons tested had CVs within the ranges of C-(1.3m/s) or A-bers (1.315m/s). The mean CVs of C- or A- bers were similar between control and AIA mice (Fig.2A; p > 0.05, unpaired t-test). In control mice, all joint sensory neurons tested (including 7 C- and 15 A- bers) were silent in the absence of exogenous stimuli, with no detectable SA (Fig.2D). In contrast, 4 of 24 (16.7%) joint sensory neurons in AIA mice (including 11C- and 13 A- bers) exhibited SA (Fig.2D). All these 4 neurons with SA were C- bers. The proportion of spontaneously ring neurons was signicantly greater in AIA mice compared to control animals (Fig.2E; p< 0.05, Fishers exact test).

Since mechanical sensitization of joint sensory neurons has been proposed be a critical neuronal mechanism of mechanical hyperalgesia, we next examined whether the responses of joint sensory neurons to mechanical stimuli were enhanced aer AIA. To avoid the confounding of spontaneous ring, we exclusively focused on joint sensory aerents that did not exhibit SA. Three of 20 (15.0%) joint sensory neurons in AIA mice displayed abnormal aer-discharges in response to punctate mechanical stimulation of their RF. Of these 3 neurons, 2 were C- bers while the remaining one was A- ber. In contrast, no mechanically evoked aer-discharges occurred in 22 joint sensory neurons recorded in control mice (Fig.2F). The proportion of joint sensory neurons with abnormal mechanically evoked aer-discharges was signicantly larger in AIA mice as compared to control animals (Fig.2G; p< 0.05, Fishers exact test). Overall, 7 of 24 joint aerents in AIA mice exhibited abnormal activity (SA and mechanically evoked aer-discharges) whereas it was not observed in all 22 joint aerents tested in control animals (p< 0.05; Fishers exact test). In addition, we compared the mechanical sensitivities of joint sensory neurons that did not exhibit either SA or aer-discharges for two groups (Fig.2H). The mean number of APs evoked by each mechanical force (5 mN to 40 mN) was signicantly greater in joint sensory neurons of AIA mice than those of controls (Fig.2I; p< 0.01, two-way ANOVA with Bonferroni post hoc test). These results indicate that the excitability and mechanical sensitivity of the peripheral terminals of a portion of joint sensory neurons were enhanced by AIA.

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Figure 1. mBSA challenge induces joint swelling and pain-like behaviors in previously immunized mice . Antigen-induced arthritis (AIA) was produced in the right ankle. (A) Ankle diameter was signicantly increased one day aer mBSA (30g in 10l saline) was injected into the ankle of immunized mice (Ipsi-AIA, n= 1213 mice), whereas injection of saline alone (vehicle) had no signicant eects (Ipsi-Ctrl, n=13 mice). There were no signicant changes in joint diameter on the contralateral (Contra) side. (B) Primary mechanical withdrawal thresholds were evaluated when the ankle joint was compressed using calibrated electronic forceps. Withdrawal threshold for mechanical stimulation of the ankle joint was signicantly lower in mBSA-challenged mice (n= 1213 mice) compared to control animals (n= 13 mice). (C,D) Mice with AIA exhibited lower paw withdrawal thresholds for mechanical stimulation of glabrous paw skin with von-Frey laments (C) n=8 mice for each group), and shorter paw withdrawal latencies for noxious heat stimulation of the ipsilateral glabrous paw skin compared to controls (D) control: n= 13 mice; AIA: n= 1213 mice). There were no signicant dierences between groups in joint mechanical threshold or in paw skin mechanical or heat thresholds on the contralateral side. *p< 0.01, AIA versus control; #p< 0.01 versus baseline at day 0, Two-way ANOVA with repeated measures followed by Bonferroni adjustments.

mBSA challenge increased the excitability of dissociated joint-innervating DRG neurons. To further investigate the eects of AIA on the excitability of joint nociceptors, whole-cell recordings were made from acutely dissociated, retrogradely DiI-labeled DRG neurons that supply the ankle joint 1 day aer i.a injection of mBSA (AIA) or saline (control) (Fig.3). In comparison with controls, joint sensory neurons from AIA mice exhibited signicantly more depolarized resting membrane potentials (Fig.3; p < 0.001, unpaired t-test). The average rheobase of joint sensory neurons from AIA mice was markedly lower than that in control animals (Fig.3, p < 0.05, unpaired t-test). In addition, the numbers of APs evoked at twice rheobase were signicantly greater in joint sensory neurons from AIA mice (Fig.3; p< 0.001; unpaired t-test). In contrast, no signicant dierences were observed in input resistance (Fig.3) or cell capacitance (control: 16.9 0.8 pF, n = 11; AIA: 17.61.3pF, n= 20) of joint sensory neurons from the two groups (p> 0.05, unpaired t-test).

mBSA challenge reduced voltage-gated K+ currents. Voltage-gated K+ (Kv) channels are crucial for controlling neuronal excitability by regulating AP threshold and ring rate26. We therefore next examined whether the increased neuronal excitability in AIA mice was due to alterations in Kv currents. Command potentials delivered aer a holding potential of 100mV were used to evoke a total voltage-activated K current (Itotal),

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Figure 2. AIA produces spontaneous activity and enhanced mechanical sensitivity in joint sensory neurons. (A), Distribution of the recorded C and A bers innervating the ankle of control and AIA mice.

C and A bers had conduction velocities (CVs)< 1.3m/s (below blue line) and 1.315m/s (between blue and dark line), respectively. There were no signicant dierences in mean CVs of joint aerents between control and AIA mice. p> 0.05 versus control, unpaired t-test. (B) Cell body of a DiI uorescent neuron (arrow) in control mouse suctioned against the tip of a recording pipette. (C) Top, Receptive eld (RF) of joint sensory neuron shown in (B). Bottom, action potential (AP; open arrow) evoked by electrical stimulation (arrow) to the RF revealed a CV of 0.41m/s. (D) Representative extracellular current (Ie) traces and AP markers indicate the presence of abnormal spontaneous activity (SA) in joint sensory neurons of AIA mice but not those of control animals. (E) Prevalence of SA in AIA versus controls. *p< 0.05, Fishers exact test. (F) Responses of joint sensory neurons in control and AIA mice to a 2s, 10mN mechanical stimulus delivered via a 100 m probe. The neurons innervating mBSA-challenged ankle were initially silent but showed prolonged aer-discharges following mechanical stimulation. No mechanically evoked aer-discharges occurred in joint sensory neurons of control animals. (G) Prevalence of mechanically evoked aer-discharges in joint sensory neurons of AIA mice versus controls. * p< 0.05, Fishers exact test. (H) Representative responses of joint sensory neurons of control and AIA mice to mechanical stimulation of their RF with von Frey laments (100 m tip diameter) at the indicated bending forces. (I) The mean number of APs evoked by mechanical stimuli 2s in duration was greater in joint sensory neurons of AIA mice compared to those of controls. *p<0.01, **p<0.001 versus control, #p< 0.01 versus 5mN, two-way ANOVA with repeated measures followed by Bonferroni adjustments. In all panels, numbers of neurons tested are in parentheses.

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Figure 3. mBSA challenge increases the excitability of joint sensory neurons. (A) Representative tracesof APs elicited at rheobase and twice rheobase in Dil-labeled joint sensory neurons from control (Ctrl) and AIA mice. mBSA challenge caused a reduction in rheobase while increasing the number of APs evoked by a2x rheobase current injection. (B) Joint sensory neurons from AIA mice exhibited a more depolarized resting membrane potential (RMP), lower mean rheobase, and greater number of action potentials at 2X rheobase, as compared to the control group. No signicant dierence in input resistance (Rin) was observed between groups.

Cell numbers tested are indicated in parentheses. *p<0.05, **p< 0.001 versus control animals, unpaired t-test.

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Figure 4. AIA reduces the IA density in joint sensory neurons. Join innervating DRG neurons were retrogradely labeled with Dil. (A) Representative traces of voltage-gated K+ currents in two joint sensory neurons, one from a control (Ctrl) mouse (top) and the other from a AIA mouse (bottom). Total K+ currents (Itotal; le) were elicited by s series of 500-ms test pulses from 60 to +50mV in 10-mV steps, preceded by a 500-ms prepulse of 100mV (inset). Delayed rectier K+ (IK) currents (middle) were generated using the same series of test pulses but preceded by a 500-ms prepulse of 40mV. A-type K+ (IA) currents (right) were obtained by digitally subtracting IK from Itotal. (B) The peak current densities of Itotal (le) and IA (right) were signicantly smaller in AIA mice (closed circles, n= 16 neurons), as compared to controls (open circles, n=18 neurons). There were no signicant dierences in IK density (middle) between two groups. *p< 0.001 versus control, oneway repeated measures analysis of variance with Tukey post hoc comparisons.

which included both IK and IA (Fig.4A). A holding potential at 40 mV was used to inactivate IA and thereby isolate IK. Subsequent subtraction of the current traces evoked by the two holding potentials then allowed us to calculate IA (Fig.4A). The mean peak current densities of Itotal and IA were signicantly lower in joint sensory neurons from AIA mice (n= 13) compared to those in neurons from control animals (n= 16) (Fig.4B; p<0.01, repeated one-way ANOVA with Bonferroni post hoc test). In contrast, there were no signicant dierences in IK density between the two treatment groups (Fig.4B; p> 0.05, repeated one-way ANOVA).

We further tested whether the reduction in IA density observed in AIA group was due to signicant alterations in the voltage-dependence of activation or inactivation of K+ currents. The voltage-dependence of IA activation was not signicantly dierent between control and AIA groups (Fig.5A; p > 0.05, unpaired t-test). The mean activation midpoint (V1/2act) of IA did not signicantly dier between the two groups (Fig.5A; p>0.05, unpaired t-test). There also was no signicant dierence in the mean slope factor for IA activation (Fig.5A; p > 0.05, unpaired t-test). In addition, mBSA challenge did not signicantly aect the voltage-dependence of steady-state inactivation of IA (Fig.5B; p > 0.05; unpaired t-test). No signicant dierences were observed between control and AIA groups in the mean values of V1/2 inact or the slope factor for IA current inactivation (Fig.5B; p > 0.05, unpaired t-test).

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Figure 5. Eects of AIA on voltage-dependent activation and steady-state inactivation of IA in joint sensory neurons. (A) For activation curves, normalized conductance (G/Gmax) was plotted against test pulse voltage and tted to a Boltzmann function. AIA (closed circles) did not alter activation curve of IA compared with the control (Ctrl; open circles) in joint sensory neurons. Bar graph shows that there were no signicant changesin V1/2 or k for IA activation between two groups. (B) For inactivation curves, a long (500ms) conditionalstep of various voltages from 120mV to 0mV in 10mV increment was followed by a testing pulse (300ms)to +50mV (inset). Normalized current (I/Imax) was plotted against conditional step potentials and tted toa negative Boltzmann function. No changes in the inactivation curve of IA in joint sensory neurons occurred aer AIA (closed circles) compared to control (Ctrl; open circles) mice. Bar graph showed that there were no signicant dierences in V1/2 or k for IA inactivation between control and AIA mice. Numbers of neurons tested are in parentheses (A,B). p> 0.05 versus control, unpaired t-test.

Discussion

Although the inammatory component of AIA has received considerable attention, little is known of the eects of AIA on the functional properties of primary sensory neurons that innervate the joint. Our study specically focused on pain-related behaviors and changes in the excitability of joint sensory neurons following AIA. We provide several observations that support the presence of enhanced excitability of these neurons during AIA. First, in vivo electrophysiological recordings revealed that a proportion of DRG neurons innervating the mBSA-challenged ankle joint, but not those in control animals, became spontaneously active. SA in nociceptors has also been reported in models of joint and cutaneous inammation, and peripheral nerve injury16,25,2729, but

this is to our knowledge the rst demonstration of SA in the AIA model. Second, a subset of sensory neurons with their receptive elds in the inamed ankle exhibited abnormal mechanically evoked aer-discharges. Third, even among neurons without SA or aer-discharges, a greater mean number of mechanically evoked APs were observed in joint sensory neurons aer mBSA challenge, indicating mechanical hypersensitivity these neurons in the context of AIA. Fourth, the dissociated cell bodies of joint sensory neurons became hyperexcitable under arthritic conditions, as indicated by a depolarized resting membrane potential, a signicant decrease in rheobase and an increase in the number of action potential discharges evoked at twice rheobase.

Collectively, our ndings both reinforce and extend those of previous studies on the mechanisms underlying joint pain in AIA. Furthermore, whereas in prior studies, neuronal activity was measured from teased peripheral joint nerve bers10,11,16,30, our present study included in vivo recordings from the somata of anatomically intact, retrogradely labeled DRG neurons innervating the ankle. These ndings thus lay the foundation for future studies in which this assay can be combined with genetic labeling25 and optogenetic methods to study nociceptive processing in specic subsets of joint sensory neurons.

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In this study, we characterized the mechanical sensitivity of joint sensory aerents using von-Frey laments rather than joint rotation, since there are no commercially available devices to produce consistent and quantiable angles and forces of ankle joint rotation in the mouse. The kinetics of force delivery in our study might have been subject to small variations upon repetition because von-Frey stimuli were applied by hand. However, such punctate mechanical stimuli have been commonly used to determine mechanical sensitivity of joint sensory neurons under various pathologic conditions16,31,32. In addition, we conned our recordings to DRG neurons that had been retrogradely labeled by DiI injected into the ankle joint. For our in vivo recordings, this enrichment strategy was coupled with restriction to neurons with mechanical RFs on the ankle joint. Although the precise anatomical structures innervated by these sensory neurons were not identied in the present study, it is likely that they had at least one branch projecting to the joint and that they innervated the tissues immediately beneath the site of von Frey lament stimulation.

Kv channels play critical roles in controlling neuronal excitability by determining RMP and aecting rheobase and spike frequency33,34. Our ndings demonstrated that AIA caused a signicant decrease in IA magnitude. This change in IA may account in part for the hyperexcitability of joint sensory neurons in the context of AIA. Possible mechanisms for reduced IA density include changes in channels properties and down-regulation of IA channel expression in joint sensory neurons. Since we observed no signicant alterations in the kinetics of activation or inactivation of IA in joint sensory neurons from AIA mice, it appears more likely that the suppression of IA was due to the down-regulation of IA channel expression. In DRG neurons, ve subtypes of IA channels have been identied: Kv1.4, Kv3.4, Kv4.1, Kv4.2 and Kv.4.319,35,36. Among these, the Kv1.4 channel is the dominant IA subunit expressed in nociceptors36. Further investigations are required to determine the molecular basis of changes in IA produced by AIA. Reductions in IA have been proposed to underlie increased neuronal excitability in other models of peripheral inammation. For example, joint inammation induced by CFA caused a decrease in both IA magnitude and the expression of Kv 1.4 in small-diameter trigeminal ganglion neurons23,24. The mechanisms whereby peripheral inammation reduces the activity and/or expression of IA channels require further investigation. The inammatory milieu surrounding primary joint aerent ber terminals in the setting of AIA contains numerous inammatory mediators37, such as IL-1 and glial cell line-derived neurotrophic factor (GDNF). The latter may be retrogradely transported to cell bodies of DRG neurons and contribute to the reduced IA observed in present study. Indeed, both GDNF and IL-1 have been demonstrated to reduce IA in nociceptors38,39. In addition, autoantibodies against neuronal Kv channels have been detected in certain autoimmune diseases, and might decrease the activity of Kv channels to produce neuronal hyperexcitability40. Although the decrease of IA we observed likely contributes to enhanced excitability in AIA, we cannot rule out the involvement of other ion channels and/or inammatory mediators. Since no signicant changes in input resistance of joint sensory neurons occurred during AIA, it is unlikely that leak channels are involved in the observed neuronal hyperexcitability.

AIA is a reproducible model that replicates many features of joint pathology and associated pain symptoms observed in human RA4. Surprisingly, few studies have assessed primary mechanical hyperalgesia in the joints of mice with AIA, although both primary and secondary hypernociception were observed in the rats with AIA11,13,15.

In this study, we showed that challenging previously immunized mice with mBSA induced primary mechanical hyperalgesia in the ankle, that lasted for approximately 1 week, a time course similar to that of joint swelling. Consistent with previous studies8,15, AIA mice exhibited secondary mechanical and heat hyperalgesia in the ipsi-lateral hindpaw. Secondary hyperalgesia is thought to arise from central sensitization, a state in which a given input to the spinal cord results in a larger relative pain response. In some but not all previous studies, secondary mechanical hypersensitivity also developed in the contralateral hindpaw5,41. However, contralateral eects were not observed in our study. There are several possible relationships among the neurophysiological and behavioral AIA-associated changes described in this study. The increased excitability recorded in the cell bodies of joint sensory neurons from AIA mice in vitro may be linked to the hyperexcitability of their peripheral terminals in vivo. This hyperexcitability, in turn, could account for the abnormal mechanically evoked aer-discharges and/or mechanical hypersensitivity observed when their RFs were mechanically stimulated in vivo. Increased mechanical sensitivity of joint aerents themselves might reasonably be expected to contribute to the behavioral signs of primary mechanical hyperalgesia and allodynia. In addition, however, SA and mechanically evoked aer-discharges might not only provide nociceptive inputs that trigger pain perception, but might also contribute to the establishment and maintenance of central sensitization. Previous studies have demonstrated the existence of spinal hyperexcitability following knee joint inammation, and provided evidence that proinammatory cytokines such as tumor necrosis factor (TNF) contribute to such sensitization42. SA in nociceptors has been shown to induce and/or maintain central sensitization43,44. Moreover, inputs from joint and muscle nociceptors may produce a longer-lasting central sensitization than input from cutaneous nociceptors45,46. Coupled with the additional action potentials of mechanically-associated aer-discharges, SA in the AIA model might therefore contribute to pain by multiple mechanisms.

In conclusion, our results demonstrate that AIA causes neuronal hyperexcitability and suppression of IA in joint sensory neurons that might contribute to joint pain associated with AIA. These ndings may suggest new strategies for the treatment of pain accompanying AIA as well as other antigen-mediated disorders. Since no single experimental model of arthritis recapitulates all aspects of human RA4, however, further experiments will be needed to understand which of these neurophysiological changes are common across models.

Methods

Animals. C57BL/6 male mice used in the study were 2 to 3 months of age and weighed 2030 g and were originally obtained from Jackson Laboratories (Bar Harbor, ME). All experimental procedures were approved by the Institutional Animal Care and Use Committee of Johns Hopkins University School of Medicine and were in accordance with the guidelines provided by the National Institute of Health and the International Association for the Study of Pain.

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Model of antigen-induced arthritis (AIA). The antigen, mBSA (Sigma, St. Louis, MO), was used to elicit arthritis in the mouse as a model of human RA, as described previously8,13. Briey, Mice were sensitized with 500 g of mBSA in 200 l of an emulsion containing 100 l saline and 100 l CFA (1 mg/ml) and delivered by subcutaneous (s.c.) injection to the caudal back skin with a sterile syringe and 25G needle. The mice were boosted with the same preparations on day 7. Immunized mice were challenged on day 21 by i.a. injection of mBSA (30g; 10 l in saline) or saline alone (vehicle) to the right ankle of the hindlimb. Ankle diameter was measured with digital calipers before and aer the induction of AIA as an indicator of joint inammation.

Behavioral testing. All the behavioral tests of the mBSA immunized mice were performed on day 0 (immediately before mBSA challenge) and up to day 14 following i.a injection of mBSA or saline vehicle. Since ankle joint inammation was obvious during the course of AIA, the behavioral tester could not be blinded to the treatments. Primary mechanical hyperalgesia in both ankle joints was measured by applying ascending forces to the ankle with electronic blunt forceps (IITC Inc., Woodland Hills, CA). The cuto force was set at 350 g to avoid joint damage. The mechanical threshold was dened as the force at which the mouse withdrew its hindlimb forcefully or vocalized11,47. The mechanical threshold in the joint was averaged over three measurements obtained at intervals of at least 5 min. Secondary mechanical hyperalgesia in the glabrous skin of both paws was evaluated using the up-down von Frey lament assay48. Secondary thermal hyperalgesia in the glabrous hind paw skin was assessed by measuring withdrawal latency to noxious heat stimuli delivered using a radiant heat source49. Heat response latencies were averaged over three measurements obtained at intervals of at least 3min.

For in vivo and vitro studies, DRG cell bodies with their aerent bers innervating vehicle (saline)- or mBSA-treated ankle joints were identied by the presence of a retrogradely transported red uorescent dye, DiI (Sigma, St. Louis, MO). Dil injected into the right ankle (2.5mg /ml, 10l in 25% ethanol) at least 1 week prior to i.a. injection of mBSA or saline.

In- vivo electrophysiological recordings. The properties of DRG neurons innervating the hind limb ankles of mice were recorded in vivo on day 1 aer the induction of AIA using extracellular recording as described25. Briey, under isourane anesthesia delivered via intratracheal ventilation (SomnoSuite, Kent Scientic Corp., Torrington, CT), the lumbar spinal column was exposed and a laminectomy performed at the L2-L6 levels. The L4 or L5 DRG was exposed and superfused with warm (~37 C) oxygenated articial cerebrospinal uid (ACSF) at ow rate of 3 ml/min within a pool formed by a ring to which the skin was sewn. The ACSF contained (in mM): 130 NaCl, 3.5 KCl, 24 NaHCO3, 1.25 NaH2PO4, 1.2 MgCl2, 1.2 CaCl2, and 10 dextrose.

The solution was bubbled with 95% O2 and 5% CO2 and had a pH of 7.4 and an osmolarity of 290~310 mOsm. Aer removal of the epineurium, the neurons on the surface of the DRG were viewed by reection microscopy on a Nikon FN1 upright microscope equipped with a mercury light source (Nikon, Mellvile, NY) and an infrared camera (DAGE-MTI, Michigan City, IN). Epiuorescence imaging was also used to identify DiI-labeled joint-innervating neurons.

Extracellular recordings were made on individual DRG cell bodies using a polished suction micropipette electrode with a tip of 2030 m. Pipettes were pulled from borosilicate glass capillaries (Sutter Instruments; Novato, CA) using a P97 micropipette puller(Sutter Instruments). The occurrence of action potentials (APs) was recorded extracellularly using a Multiclamp 700B amplier and pCLAMP10 soware (Molecular Device, Sunnyvale, CA). The peripheral receptive eld (RF) of an individual joint-innervating DRG neuron was identied by probing the skin over and around the exposed ankle with a hand-held blunt glass probe. Only DRG neurons that had a mechanical RF in the ankle were included. The mechanical sensitivity of joint sensory aerents was assessed by poking their RF with a set of calibrated von Frey monolaments with a xed tip diameter (100m). Each monolament was applied for 2s with an inter-stimulus interval of 3min. Mechanical responses were quantied as the mean number of evoked APs during 2 s mechanical stimulation16. Conduction velocity (CV) was obtained by electrically stimulating the RF with two wire electrodes and calculated by dividing the conduction distance between the stimulation electrode and the soma of the recorded neurons by the latency to a spike peak. The bers with CV of 1.3 m/s or less were classied as C-bers whereas those with CV between 1.3 and 15m/s were classied as A-bers30. In this study, C ber and A bers were preferentially studied. A neuron was classied as spontaneously active only if spontaneous ongoing discharges occurred during a 3min period without any external stimulation50.

Culture of dissociated DRG neurons. On day 1 aer the induction of AIA, L4-L5 lumbar DRGs, ipsi-lateral to either the saline- or mBSA-treated ankle, were harvested and placed in oxygenated complete saline solution (CSS) for cleaning and then mincing25,51. CSS consisted of (in mM): 137 NaCl, 5.3 KCl, 1 MgCl2, 3 CaCl2,25 Sorbitol, and 10 HEPES, adjusted to pH 7.2 with NaOH. For 20min the DRGs were digested with 0.35 U/ml of Liberase TM (Roche Diagnostics Corp., Indianapolis, IN) and then for 15 min with 25 U/ml of Liberase TL (0.25 U/ml; Roche Diagnostics Corp.) and papain (30 U/ml, Worthington Biochemical, Lakewood, NJ) in CSS containing 0.5mM EDTA at 37C. The tissue was triturated with a re-polished Pasteur pipette. The DRG neurons were suspended in DMEM medium containing 1mg/ml trypsin inhibitor and 1mg/ml bovine serum albumin (Sigma) and then plated onto poly-D-lysine/laminin coated glass. The DMEM medium contained equivalent amounts of DMEM and F12 (Life Technologies Corp., Grand Island, NY) with 10% FCS (Sigma) and 1% penicillin and streptomycin (Invitrogen). The cells were maintained in 5% CO2 at 37 C in a humidied incubator and used between 1824h aer plating.

Whole-cell recordings were made from small-diameter (25m) joint-innervating DRG neurons identied by the uorescence of Dil using a Nikon TE200 inverted epiuorescence microscope. Electrophysiological recordings were performed at

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room temperature (2022 C) using a Axopatch 200B amplier with pClamp 10 soware (Molecular Device, Sunnyvale, CA), as described52,53. Signals were sampled at 10kHz or 20kHz and ltered at 2kHz. Patch pipettes had a resistance of 34M. The series resistance was routinely compensated at 6080%.

Resting membrane potential (RMP) was recorded for each neuron in current clamp mode aer stabilization (within 3min). A neuron was included only if the RMP was more negative than 40mV and the spike overshoot was >15 mV. APs were evoked by a series of depolarizing current steps, each 500 ms duration, in increments of 50 pA up to 1 nA delivered through the recording electrode. The number of APs evoked by a suprathreshold stimulus was estimated by injecting a 500-ms depolarizing current of a magnitude at twice rheobase. Input resistance was obtained from the slope of a steady-state current-voltage plot in response to a series of hyperpolarizing currents steps from 200 to 50pA in increments of 50pA. For current clamp recordings, the internal solution contained (in mM): 120 K+-gluconate, 20 KCl, 1 CaCl2, 2 MgCl2, 11 EGTA, 10 HEPES-K+, 2 MgATP, with pH adjusted to 7.2 using Tris-base and osmolarity adjusted to 290300 mOsm with sucrose. The external solution contained the following (in mM): 145 NaCl, 3 KCl, 2 MgCl2, 2 CaCl2, 10 glucose, and 10 HEPES, pH adjusted at 7.4 with NaOH. The liquid junction potential of 11mV was corrected.

Kv currents were recorded in voltage clamp mode using the same internal solution as above, but the bath

solution contained (in mM) 140 Choline Cl, 3 KCl, 1 CaCl2, 1 MgCl2, 0.1 CdCl2, 10 HEPES, and 10 glucose. The pH was adjusted to 7.4 with Tris base, and the osmolarity was adjusted to 300310mOsm54. IA and IK were separated biophysically by manipulating the holding potentials. Total K+ currents (Itotal) were evoked by a series of test pulses, each 500-ms in duration, from 60 mV to +50 mV in 10-mV steps, preceded by a 500-ms prepulse to 100 mV54. IK was isolated by using a holding membrane potential of 40 mV. IA was obtained by digitally subtracting the IK component from Itotal. Linear leakage currents were digitally subtracted on-line using a P/4 procedure. To avoid the contamination of IK, steady-state inactivation of IA was examined in presence of 25mM TEA and elicited using a series of 500ms prepulses from 120 to 10mV with 10mV increments, followed by a 300ms test pulse of +60mV55,56.

Data analysis. Electrophysiological data were analyzed using pClamp 10 soware and Origin 6.0 (OriginLab, Northampton, MA). For the analysis of voltage versus current relationships, the current density (pA/pF) was calculated by dividing the peak current by the cell capacitance. For the analysis of steady-state activation, K+ conductance (G) at each test pulse voltage (V) was calculated from the corresponding current (I) using the equation G=I/(VVrev), where Vrev is the reversal potential of K+ current (100mV). Activation curves were obtained by plotting normalized conductance (G/Gmax) against test pulse voltage (V) and then tting with Boltzmann functions in the forms of G/Gmax=1/{1+exp[(V1/2act V)/k]}, where Gmax is the maximal K+ conductance, V1/2act is the voltage for half-maximum activation and k is the slope factor.

For the analysis of steady-state inactivation of IA, peak inward currents (I) obtained from steady-state inactivation protocol were normalized to the maximal peak current (Imax) and tted with a negative Boltzmann function of the form: I/Imax=1/{1+exp[(VV1/2 inact)/k]}, where V represents the inactivation prepulse potential, V1/2 inact

represents the voltage at which activation is half-maximal and k is the slope factor.

Data were presented as mean s.e.m. Statistical analyses were performed using a students t-test, one-way or two-way ANOVA with repeated measures followed by Bonferroni adjustments for pairwise comparisons or Tukeys post hoc test as appropriate. Comparisons of proportions were made using Fishers exact test. Signicance was set at p< 0.05. The type of statistical test used for each comparison is indicated in the gure legend.

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Acknowledgements

This work was supported by a Johns Hopkins Blaustein Pain Research Grant (LQ) and by the Neurosurgery Pain Research Institute at Johns Hopkins.

Author Contributions

L.Q. conceived of the project, designed the experiments, conducted behavioral tests and in vivo and in vitro electrophysiological experiments, analyzed the data, and wrote the manuscript; and M.J.C. facilitated experimental design and analysis and revised the manuscript.

Additional Information

Competing nancial interests: The authors declare no competing nancial interests.

How to cite this article: Qu, L. and Caterina, M. J. Enhanced excitability and suppression of A-type K+ currents in joint sensory neurons in a murine model of antigen-induced arthritis. Sci. Rep. 6, 28899; doi: 10.1038/ srep28899 (2016).

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