Evidence that protein kinase C activation is involved in the excitatory and facilitatory effects of bradykinin on canine visceral nociceptors in vitro

Depolarizing Effectors of Bradykinin Signaling in Nociceptor Excitation in Pain Perception

Inflammation is one of the main causes of pathologic pain. Knowledge of the molecular links between inflammatory signals and pain-mediating neuronal signals is essential for understanding the mechanisms behind pain exacerbation. Some inflammatory mediators directly modulate the excitability of pain-mediating neurons by contacting the receptor molecules expressed in those neurons. For decades, many discoveries have accumulated regarding intraneuronal signals from receptor activation through electrical depolarization for bradykinin, a major inflammatory mediator that is able to both excite and sensitize pain-mediating nociceptor neurons. Here, we focus on the final effectors of depolarization, the neuronal ion channels, whose functionalities are specifically affected by bradykinin stimulation. Particular G-protein coupled signaling cascades specialized for each specific depolarizer ion channels are summarized. Some of these ion channels not only serve as downstream effectors but also play critical roles in relaying specific pain modalities such as thermal or mechanical pain. Accordingly, specific pain phenotypes altered by bradykinin stimulation are also discussed. Some members of the effector ion channels are both activated and sensitized by bradykinin-induced neuronal signaling, while others only sensitized or inhibited, which are also introduced. The present overview of the effect of bradykinin on nociceptor neuronal excitability at the molecular level may contribute to better understanding of an important aspect of inflammatory pain and help future design of further research on the components involved and pain modulating strategies.

Transient receptor potentials (TRPs) and anaphylaxis

The transient receptor potential (TRP) superfamily consists of 28 members in mammals (27 in human) that act as polymodal sensors and ion channels. They regulate cellular calcium influx, generate depolarization thereby triggering voltage dependent cellular processes, and in turn they are critical in inducing the metabolic activities of cells. It is increasingly apparent that many of the inflammatory mediators released in allergic reactions involve at least two of these ion channels, the 'Vanilloid' TRPV1 and the 'Ankyrin" TRPA1. This review mainly focuses on TRPV1 and TRPA1 and the role they have in the allergic response and how these receptors may be influenced in exercise-induced anaphylaxis. The threshold to react to an allergen for mast cells and lymphocytes can be reduced by activating the melastatin channel TRPM4. This channel is briefly discussed in the context of allergy.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Role of the transient receptor potential vanilloid 1 in inflammation and sepsis

The transient receptor potential vanilloid 1 (TRPV1) is a thermoreceptor that responds to noxious temperatures, as well as to chemical agonists, such as vanilloids and protons. In addition, its channel activity is notably potentiated by proinflammatory mediators released upon tissue damage. The TRPV1 contribution to sensory neuron sensitization by proalgesic agents has signaled this receptor as a prime target for analgesic and anti-inflammatory drug intervention. However, TRPV1 antagonists have notably failed in clinical and preclinical studies because of their unwanted side effects. Recent reports have unveiled previously unrecognized anti-inflammatory and protective functions of TRPV1 in several diseases. For instance, this channel has been suggested to play an anti-inflammatory role in sepsis. Therefore, the use of potent TRPV1 antagonists as a general strategy to treat inflammation must be cautiously considered, given the deleterious effects that may arise from inhibiting the population of channels that have a protective function. The use of TRPV1 antagonists may be limited to treating those pathologies where enhanced receptor activity contributes to the inflamed state. Alternatively, therapeutic paradigms, such as reduction of inflammatory-mediated increase of receptor expression in the cell surface, may be a better strategy to prevent abrogation of the TRPV1 subpopulation involved in anti-inflammatory and protective processes.

Excitation and sensitization of nociceptors by bradykinin: what do we know?

Bradykinin is an endogenous nonapeptide known to induce pain and hyperalgesia to heat and mechanical stimulation. Correspondingly, it excites nociceptors in various tissues and sensitizes them to heat, whereas sensitizing effect on the mechanical response of nociceptors is not well established. Protein kinase C and TRPV1 contribute to the sensitizing mechanism of bradykinin to heat. In addition, TRPA1 and other ion channels appear to contribute to excitation caused by bradykinin. Finally, prostaglandins sensitize bradykinin-induced excitation in normal tissues by restoring desensitized responses due to the inhibition of protein kinase A.

Analgesic activity of a non-peptide imidazolidinedione somatostatin agonist: in vitro and in vivo studies in rat

Several lines of evidence support an important role for somatostatin receptors (SSTRs) in pain modulation. The therapeutic use of established SSTR peptide agonists for this indication is limited by their broad range of effects, need for intrathecal delivery, and short half-life. Therefore, the goal of the present study was to investigate the analgesic effect of SCR007, a new, highly selective SSTR2 non-peptide agonist. Behavioral studies demonstrated that paw withdrawal latencies to heat were significantly increased following intraplantar SCR007. Furthermore, both intraperitoneal and intraplantar injection of SCR007 significantly reduced formalin- and capsaicin-induced flinching and lifting/licking nociceptive behaviors. Recordings from nociceptors using an in vitro glabrous skin-nerve preparation showed that SCR007 reduced heat responses in a dose-dependent fashion, bradykinin-induced excitation, heat sensitization and capsaicin-induced excitation. In both the behavioral and single fiber studies, the SCR007 effects were reversed by the SSTR antagonist cyclo-somatostatin, demonstrating receptor specificity. In the single fiber studies, the opioid antagonist naloxone did not reverse SCR007-induced anti-nociception suggesting that SCR007 did not exert its effects through activation of opioid receptors. Analysis of cAMP/protein kinase A (PKA) involvement demonstrated that SCR007 prevented forskolin- and Sp-8-Br-cAMPS (a PKA activator)-induced heat sensitization, supporting the hypothesis that SCR007-induced inhibition could involve a down-regulation of the cAMP/PKA pathway. These data provide several lines of evidence that the non-peptide imidazolidinedione SSTR2 agonist SCR007 is a promising anti-nociceptive and analgesic agent for the treatment of pain of peripheral and/or central origin.

Mechanisms involved in the nociception produced by peripheral protein kinase c activation in mice

Protein kinase C (PKC) is able to phosphorylate several cellular components that serve as key regulatory components in signal transduction pathways of nociceptor excitation and sensitisation. Therefore, the present study attempted to assess some of the mechanisms involved in the overt nociception elicited by peripheral administration of the PKC activator, phorbol 12-myristate 13-acetate (PMA), in mice. The intraplantar (i.pl.) injection of PMA (16-1600 pmol/paw), but not its inactive analogue alpha-PMA, produced a long-lasting overt nociception (up to 45 min), as well as the activation of PKCalpha and PKCepsilon isoforms in treated paws. Indeed, the local administration of the PKC inhibitor GF109203X completely blocked PMA-induced nociception. The blockade of NK1, CGRP, NMDA, beta1-adrenergic, B2 or TRPV1 receptors with selective antagonists partially decreased PMA-induced nociception. Similarly, COX-1, COX-2, MEK or p38 MAP kinase inhibitors reduced the nociceptive effect produced by PMA. Notably, the nociceptive effect promoted by PMA was diminished in animals treated with an antagonist of IL-1beta receptor or with antibodies against TNFalpha, NGF or BDNF, but not against GDNF. Finally, mast cells as well as capsaicin-sensitive and sympathetic fibres, but not neutrophil influx, mediated the nociceptive effect produced by PMA. Collectively, the results of the present study have shown that PMA injection into the mouse paw results in PKC activation as well as a relatively delayed, but long-lasting, overt nociceptive behaviour in mice. Moreover, these results demonstrate that PKC activation exerts a critical role in modulating the excitability of sensory neurons.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

Characterization and function of TWIK-related acid sensing K+ channels in a rat nociceptive cell

We examined the properties of a proton sensitive current in acutely dissociated, capsaicin insensitive nociceptive neurons from rat dorsal root ganglion (DRG). The current had features consistent with K(+) leak currents of the KCNK family (TASK-1, TASK-3; TWIK-related acid sensing K(+)). Acidity and alkalinity induced inward and outward shifts in the holding current accompanied by increased and decreased whole cell resistance consistent with a K(+) current. We used alkaline solutions to open the channel and examine its properties. Alkaline evoked currents (AECs; pH 10.0-10.75), reversed near the K(+) equilibrium potential (-74 mV), and were suppressed 85% in 0 mM K(+). AECs were insensitive to Cs(+) (1 mM) and anandamide (1 microM), but blocked by Ba(++) (1 mM), quinidine (100 microM) or Ruthenium Red (10 microM). This pharmacology was identical to that of rat TASK-3 and inconsistent with that of TASK-1 or TASK-2. The TASK-like AEC was not modulated by PKA (forskolin, kappa opioid agonists U69593 and GR8696, somatostatin) but was inhibited by PKC activator phorbol-12-myristate-13 acetate (PMA). When acidic solutions were used, we were able to isolate a Ba(++) and Ruthenium Red insensitive current that was inhibited by Zn(++). This Zn(++) sensitive component of the proton sensitive current was consistent with TASK-1. In current clamp studies, acidic pH produced sensitive changes in resting membrane potential but did not influence excitability (pH 7.2-6.8). In contrast, Zn(++) produced substantial changes in excitability at physiological pH. Alkaline solutions produced hyperpolarization followed by proportional burst discharges (pH 10.75-11.5) and increased excitability (at pH 7.4). In conclusion, multiple TASK currents were present in a DRG nociceptor and differentially contributed to distinct discharge mechanisms.

Sensitization to mechanical stimulation by inflammatory mediators and by mild burn in canine visceral nociceptors in vitro

Hyperalgesia to mechanical stimulation and heat is commonly observed in inflamed conditions. Although sensitization to heat is well documented and its mechanism has also been well studied, it remains unclear whether and how nociceptors are sensitized to mechanical stimulation. Therefore we conducted in vitro investigation of which inflammatory mediators (bradykinin, histamine, prostaglandin E2, and protons) sensitize nociceptors to suprathreshold mechanical stimulation and at what concentrations. In addition, we studied the effects of possible second messengers for these mediators downstream of the receptors and also the effects of mild burn. Single polymodal receptor activities were recorded in canine testis-spermatic nerve preparations excised from deeply anesthetized dogs. Mechanical stimulation was applied to the identified receptive field for 10 s with a servo-controlled mechanical stimulator. Bradykinin at 0.001 microM induced neither excitation nor facilitation of the mechanical response; however, it facilitated the mechanical response at 0.01 microM and higher, levels at which significant excitation was also induced by bradykinin alone. Histamine excited the nociceptor and sensitized it to mechanical stimulation at 10 microM and higher. PG E(2) also sensitized the mechanical response, but starting at 1 microM, without inducing excitation by itself. The effects of two possible intracellular messengers for these mediators were studied using forskolin (10 microM), which increases intracellular cAMP, and a protein-kinase-C-stimulating phorbol ester, phorbol 12,13-dibutyrate (0.1 microM). Both substances reversibly facilitated the mechanical response of testicular polymodal receptors. In contrast, low-pH solution (pH: 6.6-4.5) seldom induced excitation and failed to facilitate the mechanical response. After 55 degrees C, 30-s heat stimulation, testicular polymodal receptors were sensitized to mechanical stimulation. These results demonstrated that inflammatory mediators and burn sensitized nociceptor responses to mechanical stimulation and provide support for the idea that peripheral nociceptor sensitization is a mechanism involved in hyperalgesia to mechanical stimulation in inflamed tissues.

In vivo evidence for a role of protein kinase C in peripheral nociceptive processing

1. The present study was designed to characterize the nociceptive response induced by protein kinase C (PKC) peripheral activation and to investigate if this biochemical event is important for the nociceptive response induced by formaldehyde, and bradykinin (BK). 2. Intraplantar injection of phorbol-12,13-didecanoate (PDD; 0.01, 0.1 or 1 microg), a PKC activator, but not of 4 alpha-PDD (inactive analogue), dose-dependently induced thermal hyperalgesia in rats. This response was not observed at the contralateral hindpaw. Intraplantar injection of PDD (0.01, 0.1 or 1 microg) also induced mechanical allodynia. In mice, injection of PDD (0.1 or 1 microg) into the dorsum of the hindpaw induced a spontaneous licking behaviour. 3. Intraplantar co-injection of chelerythrine (10 or 50 microg), a PKC inhibitor, attenuated the thermal hyperalgesia induced by PDD (0.1 microg) in rats. 4. The second phase of the nociceptive response induced by the injection of formaldehyde (0.92%, 20 microl) into the dorsum of mice hindpaws was inhibited by ipsi-, but not contralateral, pre-treatment with chelerythrine (1 microg). 5. Intraplantar injection of BK (10 microg) induced mechanical allodynia in rats. Ipsi- but not contralateral injection of bisindolylmaleimide I (10 microg), a PKC inhibitor, inhibited BK-induced mechanical allodynia. 6. In conclusion, this study demonstrates that PKC activation at peripheral tissues leads to the development of spontaneous nociceptive response, thermal hyperalgesia and mechanical allodynia. Most importantly, it also gives in vivo evidence that peripheral PKC activation is essential for the full establishment of the nociceptive response induced by two different inflammatory stimuli.

Sustained sensitization and recruitment of rat cutaneous nociceptors by bradykinin and a novel theory of its excitatory action

Excitation and sensitization to heat of nociceptors by bradykinin (BK) were examined using an isolated rat skin-saphenous nerve preparation. A total of 52 C-fibres was tested: 42 were mechano-heat sensitive (CMH) and 40% of them were excited and sensitized to heat by BK superfusion (10-5 M, 5 min) of their receptive fields; heat responses were augmented by more than five times and heat thresholds dropped to 36.4 degrees C, on average. Sixty per cent of the CMH did not respond to BK itself, but 3/4 of these units showed an increase in their heat responses by more than 100% following BK exposure. Ten high-threshold mechanosensitive C-fibres did not discharge upon BK application but following this five of them responded to heat in a well-graded manner. In all fibres, the sensitizing effect of BK was abolished within 9 min or less of wash-out, and it could be reproduced several times at equal magnitude, whereas the excitatory effect of BK regularly showed profound tachyphylaxis. Sustained superfusion (20 min) of BK induced a desensitizing excitatory response while superimposed heat responses showed constant degrees of sensitization. The large extent and high prevalence of BK-induced sensitization (almost 80% of all fibres tested) and de novo recruitment of heat sensitivity suggest a prominent role of BK not only in hyperalgesia but also in sustained inflammatory pain which may be driven by body or even lower local temperatures acting on sensitized nociceptors. Based on the latter assumption, a hypothesis is put forward that excludes a direct excitatory effect of BK on nociceptors, but assumes a temperature-controlled activation as a result of rapid and profound sensitization.

Induction of vanilloid receptor channel activity by protein kinase C

Capsaicin or vanilloid receptors (VRs) participate in the sensation of thermal and inflammatory pain. The cloned (VR1) and native VRs are non-selective cation channels directly activated by harmful heat, extracellular protons and vanilloid compounds. However, considerable attention has been focused on identifying other signalling pathways in VR activation; it is known that VR1 is also expressed in non-sensory tissue and may mediate inflammatory rather than acute thermal pain. Here we show that activation of protein kinase C (PKC) induces VR1 channel activity at room temperature in the absence of any other agonist. We also observed this effect in native VRs from sensory neurons, and phorbol esters induced a vanilloid-sensitive Ca2+ rise in these cells. Moreover, the pro-inflammatory peptide, bradykinin, and the putative endogenous ligand, anandamide, respectively induced and enhanced VR activity, in a PKC-dependent manner. These results suggest that PKC may link a range of stimuli to the activation of VRs.

Possible contribution of protein kinase C in the effects of histamine on the visceral nociceptor activities in vitro

To clarify the possible contribution of protein kinase C activation in histamine-induced excitation and sensitization of the heat response of testicular polymodal receptors, the effects of staurosporine, a protein kinase C inhibitor, and phorbol 12,13-dibutyrate, a protein kinase C activating phorbol ester, were studied in visceral polymodal receptors. Single polymodal receptor activities were recorded in vitro from testis-spermatic nerve preparations obtained from deeply anesthetized dogs (pentobarbital sodium, 30 mg/kg, i.v.). Histamine (10 microM)-induced excitation and facilitation of the heat response of polymodal receptors were both suppressed by staurosporine (1 microM), suggesting that activation of protein kinase C is involved in both these effects of histamine. Application of phorbol 12,13-dibutyrate (0.1 microM) mixed with histamine increased the histamine-induced excitation, whereas a 5 min application of phorbol 12,13-dibutyrate before histamine suppressed it. These results suggest that phorbol 12,13-dibutyrate-activated protein kinase C has inactivation as well as activation effects on the intracellular cascade connected to histamine receptors, and that the former has a slower time course.

Effect of diabetes on bradykinin-induced thermal hyperalgesia in mice

To investigate the role of protein kinase C in the attenuation of bradykinin-induced thermal hyperalgesia in diabetic mice, we examined the effects of a protein kinase C activator or inhibitor on the i.t. bradykinin-induced hyperalgesia in diabetic and non-diabetic mice. Intrathecal injection of bradykinin caused a transient antinociceptive effect, which diminished within 30 min, and then produced a thermal hyperalgesia, which lasted about 120 min, in non-diabetic mice. Although the duration of the antinociceptive phase was longer in diabetic mice than in non-diabetic mice, the hyperalgesic response was not observed in diabetic mice. The bradykinin-induced hyperalgesia was dose-dependently and significantly enhanced by pretreatment with calphostin C (0.3 to 3 pmol, i.t.), a specific protein kinase C inhibitor, in diabetic mice. However, calphostin C (3 pmol, i.t.) had no significant effect on bradykinin-induced hyperalgesia in non-diabetic mice. On the other hand, pretreatment with phorbol-12, 13-dibutyrate (12.5 to 50 pmol, i.t.), a protein kinase C activator, significantly and dose-dependently reduced bradykinin-induced hyperalgesia in non-diabetic mice. However, phorbol-12, 13-dibutyrate (50 pmol, i.t. ) had no significant effect on bradykinin-induced hyperalgesia in diabetic mice. These results suggest that the change in bradykinin-induced thermal hyperalgesia in diabetic mice may be due, at least in part, to the modification of nociceptive transmission in the spinal cord by the activation of protein kinase C.

The primary afferent nociceptor as pattern generator

One of the most important advances in our understanding of the pain experience was the introduction of the 'gate control' theory which stimulated analysis of activity pattern in nociceptive pathways and its modulation. Advances in cellular and molecular biology have recently begun to provide detailed information on the mechanisms of stimulus transduction within primary afferent nociceptors as well as mechanisms that modulate the transduction process. From these new insights into the sensory physiology of the nociceptive nerve ending emerges a concept of the primary afferent as the first site of pattern generation in the nociceptive pathway, in which dynamic tuning of gain in the mosaic of inputs to individual primary afferents occurs. The electrical properties of the nociceptor membrane that converts the generator potential to a pattern of action potentials is also actively adjusted. Our present understanding of the intracellular mechanisms that modulate the pattern of activity in nociceptive primary afferents is summarized, and implications for future efforts to unravel the meaning of patterning in nociceptor activity are discussed.

Specific involvement of PKC-epsilon in sensitization of the neuronal response to painful heat

Pain is unique among sensations in that the perceived intensity increases, or sensitizes, during exposure to a strong stimulus. One important mediator of sensitization is bradykinin (BK), a peptide released as a consequence of tissue damage. BK enhances the membrane ionic current activated by heat in nociceptive neurons, using a pathway that involves activation of protein kinase C (PKC). We find that five PKC isoforms are present in sensory neurons but that only PKC-epsilon is translocated to the cell membrane by BK. The heat response is sensitized when constitutively active PKC-epsilon is incorporated into nociceptive neurons. Conversely, BK-induced sensitization is suppressed by a specific peptide inhibitor of PKC-epsilon. We conclude that PKC-epsilon is principally responsible for sensitization of the heat response in nociceptors by bradykinin.

Primitivism and plasticity of pain--implication of polymodal receptors

Bio-warning and defense mechanisms play the most fundamental roles in living organisms. From an evolutionary point of view, nociceptive systems are very primitive and are richly provided with humoral signaling mechanisms of aboriginal humoral defense systems, as reflected in the primitive nature of the polymodal receptor, a poorly differentiated sensory receptor signaling nociceptive information. Recent advances in studies on pain have made it possible to explain neural mechanisms of pain systems under physiological conditions and reveal that there is a large gap between physiological and pathological pains. Protracted nociceptive inputs under pathological conditions induce plastic, either functional or structural, alterations in the nociceptive pathways. These plastic changes lead to crosstalk among the neural networks, including circuits related to motor, autonomic, or psychological functions. These plastic changes, once established, persist even after the original pain sources disappear in a memory-like fashion. Thus, it is revealed that chronic pain cannot be treated by blocking pain pathways, which is effective against acute pain, but require treatment from a multidisciplinary perspective.