Change in excitability and phenotype of substance P and its receptor in cat Abeta sensory neurons following peripheral inflammation

Mapping the neuroethological signatures of pain, analgesia, and recovery in mice

Ongoing pain is driven by the activation and modulation of pain-sensing neurons, affecting physiology, motor function, and motivation to engage in certain behaviors. The complexity of the pain state has evaded a comprehensive definition, especially in non-verbal animals. Here, in mice, we used site-specific electrophysiology to define key time points corresponding to peripheral sensitivity in acute paw inflammation and chronic knee pain models. Using supervised and unsupervised machine learning tools, we uncovered sensory-evoked coping postures unique to each model. Through 3D pose analytics, we identified movement sequences that robustly represent different pain states and found that commonly used analgesics do not return an animal's behavior to a pre-injury state. Instead, these analgesics induce a novel set of spontaneous behaviors that are maintained even after resolution of evoked pain behaviors. Together, these findings reveal previously unidentified neuroethological signatures of pain and analgesia at heightened pain states and during recovery.

The Physiological Basis of Cervical Facet-Mediated Persistent Pain: Basic Science and Clinical Challenges

Synopsis Chronic neck pain is a common condition and a primary clinical symptom of whiplash and other spinal injuries. Loading-induced neck injuries produce abnormal kinematics between the vertebrae, with the potential to injure facet joints and the afferent fibers that innervate the specific joint tissues, including the capsular ligament. Mechanoreceptive and nociceptive afferents that innervate the facet have their peripheral terminals in the capsule, cell bodies in the dorsal root ganglia, and terminal processes in the spinal cord. As such, biomechanical loading of these afferents can initiate nociceptive signaling in the peripheral and central nervous systems. Their activation depends on the local mechanical environment of the joint and encodes the neural processes that initiate pain and lead to its persistence. This commentary reviews the complex anatomical, biomechanical, and physiological consequences of facet-mediated whiplash injury and pain. The clinical presentation of facet-mediated pain is complex in its sensory and emotional components. Yet, human studies are limited in their ability to elucidate the physiological mechanisms by which abnormal facet loading leads to pain. Over the past decade, however, in vivo models of cervical facet injury that reproduce clinical pain symptoms have been developed and used to define the complicated and multifaceted electrophysiological, inflammatory, and nociceptive signaling cascades that are involved in the pathophysiology of whiplash facet pain. Integrating the whiplash-like mechanics in vivo and in vitro allows transmission of pathophysiological mechanisms across scales, with the hope of informing clinical management. Yet, despite these advances, many challenges remain. This commentary further describes and highlights such challenges. J Orthop Sports Phys Ther 2017;47(7):450-461. Epub 16 Jun 2017. doi:10.2519/jospt.2017.7255.

Role of satellite glial cells in gastrointestinal pain

Gastrointestinal (GI) pain is a common clinical problem, for which effective therapy is quite limited. Sensations from the GI tract, including pain, are mediated largely by neurons in the dorsal root ganglia (DRG), and to a smaller extent by vagal afferents emerging from neurons in the nodose/jugular ganglia. Neurons in rodent DRG become hyperexcitable in models of GI pain (e.g., gastric or colonic inflammation), and can serve as a source for chronic pain. Glial cells are another element in the pain signaling pathways, and there is evidence that spinal glial cells (microglia and astrocytes) undergo activation (gliosis) in various pain models and contribute to pain. Recently it was found that satellite glial cells (SGCs), the main type of glial cells in sensory ganglia, might also contribute to chronic pain in rodent models. Most of that work focused on somatic pain, but in several studies GI pain was also investigated, and these are discussed in the present review. We have shown that colonic inflammation induced by dinitrobenzene sulfonic acid (DNBS) in mice leads to the activation of SGCs in DRG and increases gap junction-mediated coupling among these cells. This coupling appears to contribute to the hyperexcitability of DRG neurons that innervate the colon. Blocking gap junctions (GJ) in vitro reduced neuronal hyperexcitability induced by inflammation, suggesting that glial GJ participate in SGC-neuron interactions. Moreover, blocking GJ by carbenoxolone and other agents reduces pain behavior. Similar changes in SGCs were also found in the mouse nodose ganglia (NG), which provide sensory innervation to most of the GI tract. Following systemic inflammation, SGCs in these ganglia were activated, and displayed augmented coupling and greater sensitivity to the pain mediator ATP. The contribution of these changes to visceral pain remains to be determined. These results indicate that although visceral pain is unique, it shares basic mechanisms with somatic pain, suggesting that therapeutic approaches to both pain types may be similar. Future research in this field should include additional types of GI injury and also other types of visceral pain.

Eosinophil-dependent skin innervation and itching following contact toxicant exposure in mice

BACKGROUND

Contact toxicant reactions are accompanied by localized skin inflammation and concomitant increases in site-specific itch responses. The role(s) of eosinophils in these reactions is poorly understood. However, previous studies have suggested that localized eosinophil-nerve interactions at sites of inflammation significantly alter tissue innervation.

OBJECTIVE

To define a potential mechanistic link between eosinophils and neurosensory responses in the skin leading to itching.

METHODS

BALB/cJ mice were exposed to different contact toxicants, identifying trimellitic anhydride (TMA) for further study on the basis of inducing a robust eosinophilia accompanied by degranulation. Subsequent studies using TMA were performed with wild type versus eosinophil-deficient PHIL mice, assessing edematous responses and remodeling events such as sensory nerve innervation of the skin and induced pathophysiological responses (ie, itching).

RESULTS

Exposure to TMA, but not dinitrofluorobenzene, resulted in a robust eosinophil skin infiltrate accompanied by significant levels of degranulation. Follow-up studies using TMA with wild type versus eosinophil-deficient PHIL mice showed that the induced edematous responses and histopathology were, in part, causatively linked with the presence of eosinophils. Significantly, these data also demonstrated that eosinophil-mediated events correlated with a significant increase in substance P content of the cutaneous nerves and an accompanying increase in itching, both of which were abolished in the absence of eosinophils.

CONCLUSIONS

Eosinophil-mediated events following TMA contact toxicant reactions increase skin sensory nerve substance P and, in turn, increase itching responses. Thus, eosinophil-nerve interactions provide a potential mechanistic link between eosinophil-mediated events and neurosensory responses following exposure to some contact toxicants.

Smad-interacting protein 1 affects acute and tonic, but not chronic pain

BACKGROUND

Smad-interacting protein 1 (also named Zeb2 and Zfhx1b) is a transcription factor that plays an important role in neuronal development and, when mutated, causes Mowat-Wilson syndrome (MWS). A corresponding mouse model carrying a heterozygous Zeb2 deletion was comprehensively analysed in the German Mouse Clinic. The most prominent phenotype was the reduced pain sensitivity. In this study, we investigated the role of Zeb2 in inflammatory and neuropathic pain.

METHODS

For this, we tested mutant Zeb2 animals in different models of inflammatory pain like abdominal constriction, formalin and carrageenan test. Furthermore, we studied the pain reactivity of the mice after peripheral nerve ligation. To examine the nociceptive transmission of primary sensory dorsal root ganglia (DRG) neurons, we determined the neuronal activity in the spinal dorsal horn after the formalin test using staining of c-Fos. Next, we characterized the neuronal cell population in the DRGs and in the sciatic nerve to study the effect of the Zeb2 mutation on peripheral nerve morphology.

RESULTS

The present data show that Zeb2 is involved in the development of primary sensory DRG neurons, especially of C- and Aδ fibres. These alterations contribute to a hypoalgesic phenotype in inflammatory but not in neuropathic pain in these Zeb2(+/-) mice.

CONCLUSION

Our data suggest that the under-reaction to pain observed in MWS patients results from a reduced responsivity to nociceptive stimulation rather than an inability to communicate discomfort.

Intercellular communication in sensory ganglia by purinergic receptors and gap junctions: implications for chronic pain

Peripheral injury can cause abnormal activity in sensory neurons, which is a major factor in chronic pain. Recent work has shown that injury induces major changes not only in sensory neurons but also in the main type of glial cells in sensory ganglia-satellite glial cells (SGCs), and that interactions between sensory neurons and SGCs contribute to neuronal activity in pain models. The main functional changes observed in SGCs after injury are an increased gap junction-mediated coupling among these cells, and augmented sensitivity to ATP. There is evidence that the augmented gap junctions contribute to neuronal hyperexcitability in pain models, but the mechanism underlying this effect is not known. The changes in SGCs described above have been found following a wide range of injuries (both axotomy and inflammation) in somatic, orofacial and visceral regions, and therefore appear to be a general feature in chronic pain. We have found that in cultures of sensory ganglia calcium signals can spread from an SGC to neighboring cells by calcium waves, which are mediated by gap junctions and ATP acting on purinergic P2 receptors. A model is proposed to explain how augmented gap junctions and greater sensitivity to ATP can combine to produce enhanced calcium waves, which can lead to neuronal excitation. Thus this simple scheme can account for several major changes in sensory ganglia that are common to a great variety of pain models.

Pressure and activity-related allodynia in delayed-onset muscle pain

OBJECTIVES

Muscle pain from different activities was tested with the muscle pain expected to vary in ways that may clarify mechanisms of activity-induced exacerbation of myofascial pain.

METHODS

Participants [N=20; 45% women; 23 y old (SD=2.09)] consented to participate in a 6 session protocol. Bilateral muscle pain ratings and pressure pain thresholds (PPTs) were collected before and for 4 days after lengthening (ie, eccentric) muscle contractions were completed with the nondominant elbow flexors to induce delayed-onset muscle pain. The muscle pain ratings were collected with the arms in several conditions (eg, resting, moving, and contracting in a static position) and PPTs were collected with the arms.

RESULTS

In the ipsilateral arm, muscle pain ratings at rest and during activity significantly increased whereas PPTs significantly decreased after the eccentrics (ηs=0.17 to 0.54). The greatest increases in pain occurred during arm extension without applied load, in which there was more stretching but less force than isometrics. In the contralateral arm, neither muscle pain nor PPTs changed from baseline.

DISCUSSION

These results resemble earlier electrophysiology studies showing differential sensitization across stimuli and support that increased depth of information about aggravating activities from clinical patients is needed.

Differing neurophysiologic mechanosensory input from glabrous and hairy skin in juvenile rats

Sensory afferents in skin encode and convey thermal and mechanical conditions, including those that threaten tissue damage. A small proportion of skin, the glabrous skin of the distal extremities, is specialized to explore the environment in fine detail. Aside from increased innervation density, little is known regarding properties of mechanosensory afferents to glabrous skin in younger animals that explain the exquisite precision and high contrast in rapidly sampling physical structures, including those that threaten injury. To assess this, we obtained intact neuronal intracellular recordings in vivo from 115 mechanosensitive afferent neurons from lumbar and thoracic dorsal root ganglia in juvenile rats. Two characteristics were unique to glabrous skin: a threefold higher proportion of fast-conducting to slow-conducting afferents that were high-threshold mechanosensitive nociceptors compared with hairy skin and a twofold faster conduction velocity of fast-conducting nociceptors compared with hairy skin. Additionally differences were found in mechanical thresholds between glabrous skin and hairy skin for each fiber type. These differences reflect and help explain the rapid response of skin specialized to explore the physical environment. Additionally, these results highlight potential limitations of using passive electrical properties and conduction velocity alone to characterize primary afferents without knowledge of the skin type they innervated.

Contribution of activated interleukin receptors in trigeminal ganglion neurons to hyperalgesia via satellite glial interleukin-1beta paracrine mechanism

The present study investigated whether under in vivo conditions, inflammation alters the excitability of nociceptive Adelta-trigeminal ganglion (TRG) neurons innervating the facial skin via a cytokine paracrine mechanism. We used extracellular electrophysiological recording with multibarrel-electrodes in this study, and complete Freund's adjuvant (CFA) was injected into the rat facial skin. The threshold for escape from mechanical stimulation applied to the whisker pad area in inflamed rats (2 days after CFA injection) was significantly lower than that in control rats. A total of 45 Adelta-nociceptive-TRG neurons responding to electrical stimulation of the whisker pad were recorded in pentobarbital-anesthetized rats. The number of Adelta-TRG neurons with spontaneous firings and their firing rate in inflamed rats were significantly larger than those in control rats. The firing rates of the Adelta-TRG neuronal spontaneous activity were current-dependently decreased by local iontophoretic application of an interleukin I receptor type I antagonist (IL-1ra) in inflamed rats, but not in controls, and current-dependently increased by iontophoretic application of interleukin 1beta (IL-1beta) in both control and inflamed rats. IL-1ra also inhibited Adelta-TRG neuron activity evoked by mechanical stimulation in the inflamed rats. The mechanical threshold of nociceptive-TRG neurons in inflamed rats was significantly lower than that in control rats, but was not significantly different between control and inflamed rats after application of an IL-1ra. These results suggested that inflammation modulates the excitability of nociceptive Adelta-TRG neurons innervating the facial skin via IL-1beta paracrine action within trigeminal ganglia. Such an IL-1beta release could be important in determining trigeminal inflammatory hyperalgesia.

Morphine inhibits herpetic allodynia through mu-opioid receptors induced in Abeta-fiber neurons

Peripheral action of mu-opioid receptor agonist inhibits mechanical allodynia in mice with herpetic pain. Mechanical allodynia is mainly mediated by Abeta fibers, whereas mu-opioid receptors are present in C and Adelta fibers. This study was conducted to address this discrepancy. Neonatal capsaicin treatment, which almost abolished aversive response to capsaicin, did not affect herpetic allodynia and antiallodynic effect of local injection of morphine. Although mu-opioid receptor was chiefly expressed in small-sized and medium-sized sensory neurons in naive mice, it was induced in large sensory neurons in mice with herpetic pain. Viral propagation in the sensory ganglion may induce mu-opioid receptor expression in Abeta fibers, which may be responsible for the inhibitory action of local opioids on mechanical allodynia in mice with herpetic pain.

Physiological Properties of Mouse Skin Sensory Neurons Recorded Intracellularly In Vivo: Temperature Effects on Somal Membrane Properties

Recent combined analyses of the structural, functional, and molecular attributes of individual skin sensory neurons using semi-intact in vitro preparations from mice have provided a wealth of novel insights into nociceptor biology. How these findings translate to more natural conditions nevertheless remains unresolved. Toward this end, intracellular recordings were obtained from 362 physiologically identified dorsal root ganglion (DRG) neurons in a new in vivo mouse preparation developed for combined structure/function analyses of individual skin sensory neurons. Recordings were conducted at thoracic levels in adult decorticate mice for comparison with in vitro findings from the same trunk region. In all, 270 neurons were recorded at DRG temperatures tightly regulated at normal core values to establish a baseline and 137 skin sensory neurons were included in detailed analyses of somal properties for comparisons with similar data obtained under reduced temperatures mirroring in vitro conditions. Recovery of Neurobiotin-labeled central projections was crucial for verifying perceived afferent identity of certain neurons. Further, profound temperature dependency was seen across diverse physiological properties. Indeed, the broad, inflected somal spikes normally viewed as diagnostic of myelinated nociceptors were found to be a product of reduced temperatures and were not present at normal core values. Moreover, greater complexity was observed peripherally in the mechanical and thermal sensitivity profile of nociceptive and nonnociceptive populations than that seen under in vitro conditions. This novel in vivo preparation therefore holds considerable promise for future analyses of nociceptor function and plasticity in normal and transgenic models of pain mechanisms.

Neurokinin-1 receptor in peripheral nerve terminals mediates thermal hyperalgesia

Neurokinin-1 receptor (NK-1) plays an important role in nociception. The present study was to explore whether activation of peripheral NK-1 receptor, especially expressed on primary sensory afferents, could induce hyperalgesia and sensitize C-type sensory afferents. (1) Intraplantar administration of NK-1 agonist [Sar9, Met(O2)11]SP (Sar-SP, 0.2, 1 nmol, 20 microl) produced significant thermal hyperalgesia and edema, which was blocked by co-injection of NK-1 antagonist WIN51,708 (10 nmol). But in the rats with compound 48/80 treatment for mast cell depletion, the Sar-SP-induced edema, but not hyperalgesia, was attenuated. (2) Close-arterial injection of Sar-SP (1 nmol, 0.1 ml) excited and sensitized sensory C afferents of the sural nerve to heat stimuli. The results suggest involvement of NK-1 receptors expressed on the peripheral afferent terminals in thermal hyperalgesia mediated by directly sensitizing C-type sensory afferents.

Activation of NK1 receptor of trigeminal root ganglion via substance P paracrine mechanism contributes to the mechanical allodynia in the temporomandibular joint inflammation in rats

The aim of this study was to investigate whether under in vivo conditions, temporomandibular joint (TMJ) inflammation alters the excitability of Abeta-trigeminal root ganglion (TRG) neuronal activity innervating the facial skin by using extracellular electrophysiological recording with multibarrel-electrodes. Complete Freund's adjuvant (CFA) was injected into the rat TMJ. Threshold for escape from mechanical stimulation applied to the whisker pad area in inflamed rats (2 days) was significantly lower than that in control rats. A total of 36 Abeta-TRG neurons responding to electrical stimulation of the whisker pad was recorded in pentobarbital-anesthetized rats. The number of Abeta-TRG neurons with spontaneous firings and their firing rate in TMJ inflamed rats were significantly larger than those in control rats. The firing rates of their spontaneous activity in the Abeta-TRG neurons were current-dependently decreased by local iontophoretic application of an NK1 receptor antagonist (L-703,606) in inflamed, but not non-inflamed rats. Their spontaneous activities were current-dependently increased by local iontophoretic application of substance P (SP) in control and inflamed rats. The mechanical response threshold of Abeta-TRG neurons in inflamed rats was significantly lower than that in control rats. The mechanical response threshold in inflamed rats after iontophoretic application of L-703,606 was not different from that in control rats. These results suggest that TMJ inflammation modulate the excitability of Abeta-TRG neurons innervating the facial skin via paracrine mechanism due to SP released from TRG neuronal cell body. Such a SP release may play an important role in determining the trigeminal inflammatory allodynia concerning the temporomandibular disorder.

Alterations of membrane properties and effects of d-serine on NMDA-induced current in rat anterior cingulate cortex neurons after monoarthritis

Accumulated evidence implicates the anterior cingulate cortex (ACC) in pain processing. The activation of the NMDA receptor requires the occupation of both the glutamate site and the glycine site. d-Serine released by astrocytes is presumed to be an endogenous ligand for the glycine site of the NMDA receptor. Using whole-cell patch clamp recording, membrane characteristics and effects of exogenous d-serine on NMDA-evoked currents were examined in neurons in ACC slices from normal and complete Freund's adjuvant-induced monoarthritic rats. Neurons from rats with monoarthritis exhibited more depolarized membrane potential, lower firing threshold, lower input resistance and higher slope conductance compared with normal rats. The NMDA-evoked currents were enhanced by d-serine (20 microM) in both normal (135.3+/-4.3% of control, p < 0.01) and arthritic (157.9 +/- 9.7% of control, p < 0.01) rats, respectively. The effect of d-serine was greater in arthritic rats than control rats (p < 0.05). These results suggest that inflammatory pain increased the excitability of ACC neurons, and that the NMDA receptor glycine sites in the ACC neurons were not saturated in either normal or inflammatory pain states.

Temporomandibular Joint Inflammation Potentiates the Excitability of Trigeminal Root Ganglion Neurons Innervating the Facial Skin in Rats

The aim of this study was to test the hypothesis that temporomandibular joint (TMJ) inflammation alters the excitability of trigeminal root ganglion (TRG) neurons innervating the facial skin, by using behavioral, electrophysiological, molecular, and immunohistochemical approaches. Complete Freund’s adjuvant (CFA) was injected into the rat TMJ to produce inflammation. The threshold for escape from mechanical stimulation applied to the orofacial area in TMJ-inflamed rats was significantly lower than that in naïve rats. The TRG neurons innervating the inflamed TMJ were labeled by 2% Fluorogold (FG) injection into the TMJ. The number of FG-labeled substance P (SP)-immunoreactive neurons in the inflamed rats was significantly increased compared with that in the naïve rats. On the other hand, medium- and large-diameter TRG neurons (>30 μm) innervating the facial skin were labeled by FG injection into the facial skin. In the FG-labeled cutaneous TRG neurons, the occurrence of SP (100 nM) induced membrane depolarization in inflamed rats (medium: 73.3%, large : 85.7%) was larger than that in the naïve rats (medium: 29.4%, large : 0%). In addition, SP application significantly increased the firing rate evoked by depolarizing pulses in the neurons of inflamed rats compared with those of naïve rats. Quantitative single-cell RT-PCR analysis showed the increased expression of mRNA for the NK1 receptor in FG-labeled TRG neurons in inflamed rats compared with that in naïve rats. The numbers of SP and NK1 receptors/neurofilament 200 positive immunoreactive TRG neurons innervating the facial skin (FG-labeled) in the inflamed rats were significantly increased compared with those seen in naïve rats. These results suggest that TMJ inflammation can alter the excitability of medium- and large-diameter TRG neurons innervating the facial skin and that an increase in SP/NK1 receptors in their soma may contribute to the mechanism underlying the trigeminal inflammatory allodynia in the TMJ disorder.

Morphine-induced decrease in mechanical allodynia is mediated by central, but not peripheral, opioid receptors in rats with inflammation

The aim of this study was to investigate the mechanism underlying the effect of morphine on allodynia to complete Freund's adjuvant-induced inflammation in rats. Morphine (5 mg/kg, i.v.) markedly inhibited the mechanical stimulation-induced nociceptive reflex of the gastrocnemius muscle in the inflamed hind-limb, and the inhibition was blocked by naloxone (1 mg/kg). Teased fiber recordings were made from the tibial nerve innervating the inflamed hindpaw. Morphine at the same dose did not affect the spontaneous firing rate of A-type fibers, whereas it markedly decreased the spontaneous firing of C-type fibers. The present data suggested that the central, but not peripheral, plasticity triggered by inflammation-induced facilitation of A(beta) fibers plays an important role in morphine-induced alleviation of allodynia, whereas activation of opioid receptor expression on the peripheral terminals of C fibers may contribute to morphine-induced alleviation of persistent pain of inflammation.

Involvement of spinal neurokinin-1 receptors in the maintenance but not induction of carrageenan-induced thermal hyperalgesia in the rat

The study was undertaken to assess the antihyperalgesic effect of L-732,138, (N-acetyl-L-tryptophan-3,5-bistrifluoromethyl benzyl ester), a non-peptide neurokinin-1 (NK1) receptor antagonist in rats when given intrathecally. The peripheral inflammation associated with behavioral hyperalgesia to a thermal stimulus was induced by intraplantar (i.pl.) injection of carrageenan. The thermal hyperalgesia was measured by paw withdrawal latency. Intrathecal (i.t.) injection of L-732,138 (100 nmol) at 3h after carrageenan markedly attenuated the paw withdrawal latency of the inflamed paw, but not that of the non-inflamed paw. L-732,138 (100 nmol, i.t.) given 10 min prior to carrageenan injection had no effect on the carrageenan-induced decrease in paw withdrawal latency to noxious thermal stimulus. The results demonstrate that NK1 receptor is involved in the maintenance but not the induction and development of thermal hyperalgesia evoked by carrageenan.

Prostaglandin E2 increases the expression of the neurokinin1 receptor in adult sensory neurones in culture: a novel role of prostaglandins

(1) Peripheral inflammation causes an increase in the proportion of primary afferent neurones that express neurokinin(1) (NK(1)) receptors for substance P (SP). This upregulation may contribute to the neuronal mechanisms of inflammatory pain. The aim of this study was to identify endogenous mediators that stimulate upregulation of NK(1) receptors in dorsal root ganglion (DRG) neurones. Cultured DRG neurones from the adult normal rat were exposed for 2 days to media that contained specific mediators, namely potassium in high concentration, prostaglandin E(2) (PGE(2)), somatostatin (SRIF), and compounds influencing second messenger cascades. After fixation neurones were labelled with an NK(1) receptor antibody. (2) Repetitive addition of the inflammatory mediator PGE(2) or dibutyryl-cyclic adenosine 3',5' monophophate (db-cAMP) to the culture medium enhanced the proportion of neurones with NK(1) receptor-like immunoreactivity from about 12% up to 40%. PGE(2)-induced upregulation was prevented by coadministration of PGE(2) and a protein kinase A inhibitor or SRIF to the medium. High potassium concentration, protein kinase C inhibitors and omission of nerve growth factor from the medium had no effect. (3) In calcium-imaging experiments, bath application of SP evoked increases of the intracellular calcium concentration in about 20% of the neurones. This proportion increased to about 40% after PGE(2)-pretreatment, but the increase was prevented when PGE(2) and SRIF were coadministered to the medium. (4) These data show that the expression of NK(1) receptor-like immunoreactivity in DRG neurones is regulated by the inflammatory mediator PGE(2). This upregulation depends on the intracellular adenylyl cyclase-protein kinase A pathway.

Cross-inhibition of mechanoreceptive inputs in dorsal root ganglia of peripheral inflammatory cats

Primary afferent neurons in mammalian dorsal root ganglia (DRGs) normally function as independent sensory communication elements. However, it has recently been shown that most DRG neurons are transiently activated when axons of neighboring neurons of the same ganglion are stimulated repetitively and the cross-depolarization contributes to this mutual cross-excitation. Here, we reported the cross-inhibition of mechanoreceptive information in DRG under peripheral inflammatory condition. Intracellular recordings were made in vivo from A-type afferent neurons in cat L(6-7) DRGs. Among spontaneously firing neurons both from control (Con) and carrageenan (Carg) injected cats, some A-type afferent neurons showed to have two distinct receptive fields on the hindpaw. Mechanical stimulation of one receptive field increased the ongoing activities, while stimulation of the other receptive field led to a decrease of spontaneous firings of the same neuron. These two distinct receptive fields are termed excitatory receptive field (ERF) and inhibitory receptive field (IRF), respectively. Peripheral inflammation significantly increased the prevalence of Abeta and Adelta neurons with two distinct receptive fields (Abeta: Con, 1.34%, n=149; Carg, 6.59%, n=182; P<0.05; Adelta: Con, 0%, n=138, Carg, 3.9%, n=102, P<0.05). Most interestedly, ERF stimulation-induced enhancement of cell firings can be suppressed by IRF stimulation. Similarly, IRF stimulation-induced decrease of cell discharges can be reversed by ERF stimulation. This interaction was not affected by cutting the dorsal roots at the place close to the recorded DRG. Preapplication of naloxone and yohimbine did not block the interaction. Taken together with previous reports, this intraganglionic cross-talking appears to be mediated by collision of retrograde spread of action potentials, or/and at least in part, by an activity-dependent diffusible excitatory substance released from neuronal somata and/or adjacent axons, and detected by neighboring cell somata.