Chemical activators of sensory neurons

Molecular basis of dental sensitivity: The odontoblasts are multisensory cells and express multifunctional ion channels

Odontoblasts are the dental pulp cells responsible for the formation of dentin. In addition, accumulating data strongly suggest that they can also function as sensory cells that mediate the early steps of mechanical, thermic, and chemical dental sensitivity. This assumption is based on the expression of different families of ion channels involved in various modalities of sensitivity and the release of putative neurotransmitters in response to odontoblast stimulation which are able to act on pulp sensory nerve fibers. This review updates the current knowledge on the expression of transient-potential receptor ion channels and acid-sensing ion channels in odontoblasts, nerve fibers innervating them and trigeminal sensory neurons, as well as in pulp cells. Moreover, the innervation of the odontoblasts and the interrelationship been odontoblasts and nerve fibers mediated by neurotransmitters was also revisited. These data might provide the basis for novel therapeutic approaches for the treatment of dentin sensibility and/or dental pain.

Unmasking of spiral ganglion neuron firing dynamics by membrane potential and neurotrophin-3

Type I spiral ganglion neurons have a unique role relative to other sensory afferents because, as a single population, they must convey the richness, complexity, and precision of auditory information as they shape signals transmitted to the brain. To understand better the sophistication of spiral ganglion response properties, we compared somatic whole-cell current-clamp recordings from basal and apical neurons obtained during the first 2 postnatal weeks from CBA/CaJ mice. We found that during this developmental time period neuron response properties changed from uniformly excitable to differentially plastic. Low-frequency, apical and high-frequency basal neurons at postnatal day 1 (P1)-P3 were predominantly slowly accommodating (SA), firing at low thresholds with little alteration in accommodation response mode induced by changes in resting membrane potential (RMP) or added neurotrophin-3 (NT-3). In contrast, P10-P14 apical and basal neurons were predominately rapidly accommodating (RA), had higher firing thresholds, and responded to elevation of RMP and added NT-3 by transitioning to the SA category without affecting the instantaneous firing rate. Therefore, older neurons appeared to be uniformly less excitable under baseline conditions yet displayed a previously unrecognized capacity to change response modes dynamically within a remarkably stable accommodation framework. Because the soma is interposed in the signal conduction pathway, these specializations can potentially lead to shaping and filtering of the transmitted signal. These results suggest that spiral ganglion neurons possess electrophysiological mechanisms that enable them to adapt their response properties to the characteristics of incoming stimuli and thus have the capacity to encode a wide spectrum of auditory information.

Distinct subclassification of DRG neurons innervating the distal colon and glans penis/distal urethra based on the electrophysiological current signature

Spinal sensory neurons innervating visceral and mucocutaneous tissues have unique microanatomic distribution, peripheral modality, and physiological, pharmacological, and biophysical characteristics compared with those neurons that innervate muscle and cutaneous tissues. In previous patch-clamp electrophysiological studies, we have demonstrated that small- and medium-diameter dorsal root ganglion (DRG) neurons can be subclassified on the basis of their patterns of voltage-activated currents (VAC). These VAC-based subclasses were highly consistent in their action potential characteristics, responses to algesic compounds, immunocytochemical expression patterns, and responses to thermal stimuli. For this study, we examined the VAC of neurons retrogradely traced from the distal colon and the glans penis/distal urethra in the adult male rat. The afferent population from the distal colon contained at least two previously characterized cell types observed in somatic tissues (types 5 and 8), as well as four novel cell types (types 15, 16, 17, and 18). In the glans penis/distal urethra, two previously described cell types (types 6 and 8) and three novel cell types (types 7, 14, and 15) were identified. Other characteristics, including action potential profiles, responses to algesic compounds (acetylcholine, capsaicin, ATP, and pH 5.0 solution), and neurochemistry (expression of substance P, CGRP, neurofilament, TRPV1, TRPV2, and isolectin B4 binding) were consistent for each VAC-defined subgroup. With identification of distinct DRG cell types that innervate the distal colon and glans penis/distal urethra, future in vitro studies related to the gastrointestinal and urogenital sensory function in normal as well as abnormal/pathological conditions may be benefitted.

Mast cells and the cyclooxygenase pathway mediate colonic afferent nerve sensitization in a murine colitis model

INTRODUCTION

Intestinal inflammation alters colonic afferent nerve sensitivity which may contribute to patients' perception of abdominal discomfort. We aimed to explore whether mast cells and the cyclooxygenase pathway are involved in altered afferent nerve sensitivity during colitis.

METHODS

C57Bl6 mice received 3% dextran-sulfate sodium (DSS) in drinking water for 7 days to induce colitis. Control animals received regular water. On day 8 inflammation was assessed in the proximal colon by morphology and histology. Extracellular afferent nerve discharge was recorded from the mesenteric nerve of a 2 cm colonic segment. Subgroups were treated in vitro with the mast cell stabilizer doxantrazole (10⁻⁴M) or the cyclooxygenase inhibitor naproxen (10⁻⁵M).

RESULTS

DSS colitis resulted in morphological and histological signs of inflammation. At baseline, peak firing was 11±2 imp s⁻¹ in colitis segments and 5±1 imp s⁻¹ in uninflamed control segments (p<0.05; mean ± SEM; each n=6). In colitis segments, afferent nerve discharge to bradykinin (0.5 μM) was increased to 47±7 compared to 23±6 imp s⁻¹ in recordings from non-inflamed control tissue (p<0.05). Mechanosensitivity during luminal ramp distension (0-80 cm H₂O) was increased reaching 24±5 imp s⁻¹ at 80 cm H₂O during colitis compared to 14±2 in non-inflamed controls (p<0.05). Doxantrazole or naproxen reduced afferent discharge to bradykinin and luminal ramp distension in colitis segments to control levels.

CONCLUSION

Intestinal inflammation sensitizes mesenteric afferent nerve fibers to bradykinin and mechanical stimuli. The underlying mechanism responsible for this sensitization seems to involve mast cells and prostaglandins.

Chemosensory properties of the trigeminal system

The capacity of cutaneous, including trigeminal endings, to detect chemicals is known as chemesthesis or cutaneous chemosensation. This sensory function involves the activation of nociceptor and thermoreceptor endings and has a protective or defensive function, as many of these substances are irritants or poisonous. However, humans have also developed a liking for the distinct sharpness or pungency of many foods, beverages, and spices following activation of the same sensory afferents. Our understanding of the cellular and molecular mechanisms of chemosensation in the trigeminal system has experienced enormous progress in the past decade, following the cloning and functional characterization of several ion channels activated by physical and chemical stimuli. This brief review attempts to summarize our current knowledge in this field, including a functional description of various sensory channels, especially TRP channels, involved in trigeminal chemosensitivy. Finally, some of these new findings are discussed in the context of the pathophysiology of trigeminal chemosensation, including pain, pruritus, migraine, cough, airway inflammation, and ophthalmic diseases.

Activation of CaMKII and ERK1/2 contributes to the time-dependent potentiation of Ca2+ response elicited by repeated application of capsaicin in rat DRG neurons

When capsaicin is applied repeatedly to dorsal root ganglion (DRG) neurons for brief periods (10-15 s) at short intervals (5-10 min), the evoked responses rapidly decline, a phenomenon termed tachyphylaxis. In addition to this phenomenon, the present study using Ca(2+) imaging revealed that repeated application of capsaicin to rat dissociated DRG neurons at longer intervals (20-40 min) or during multiple applications at short intervals elicited an enhancement of the responses, termed potentiation. The potentiation occurred in 50-60% of the capsaicin-responsive cells, on average representing a 20- to 30% increase in the peak amplitude of the Ca(2+) signal, and was maximal at a 40-min application interval. An analysis of the mechanisms underlying potentiation revealed that it was suppressed by block of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) with 5 μM KN-93 or block of the activation of extracellular signal-regulated kinase (ERK) 1/2 with 2 μM U-0126. Lowering the extracellular Ca(2+) concentration from 2 to 1 mM or pretreatment with deltamethrin (1 μM), which blocks calcineurin and tachyphylaxis, enhanced potentiation. Potentiation was not affected by: 1) inhibition of protein kinase C or protein kinase A, 2) block of the three subtypes of neurokinin receptors, or 3) block of the trafficking of transient receptor potential V1 channel to the membrane. These results indicate that the potentiation is a slowly developing Ca(2+)-modulated process that is mediated by a complex intracellular signaling pathway involving activation of CaMKII and ERK1/2. Potentiation may be an important peripheral autosensitization mechanism that occurs independently of the pronociceptive effects of inflammatory mediators and neurotrophic factors.

Cartilage degeneration is associated with augmented chemically-induced joint pain in rats: a pilot study

BACKGROUND

Osteoarthritis arising from cartilage degeneration is the most common cause of joint pain. However, the relationship between joint pain and cartilage degeneration is not well understood.

QUESTIONS/PURPOSES

We asked whether the inflammatory mediators participate in the joint pain in the presence of cartilage degeneration.

METHODS

We observed electromyographic responses of hindlimb flexors to four inflammatory mediators (bradykinin, ATP, acetylcholine, and serotonin) injected in normal rat knees and in those with monosodium iodoacetate (MIA)-induced arthritis.

RESULTS

Joint cartilage of all the rats with MIA-induced arthritis histologically showed severe degeneration. We observed greater magnitude and longer duration responses in the MIA-induced arthritis than normal joints with all four mediators.

CONCLUSIONS

The data suggested nociceptors in osteoarthritic joints are more sensitive to inflammatory mediators than in normal joints. Such nociceptive sensitization to inflammatory mediators may participate in the joint pain in osteoarthritis.

A novel herbal preparation desensitizes mesenteric afferents to bradykinin in the rat small intestine

Herbal preparations are evolving as promising agents for the treatment of functional gastrointestinal disorders which are considered to be secondary to visceral hypersensitivity. We aimed to determine whether a new combination of six herbal extracts reduces the sensitivity of intestinal afferents in rat. Male Wistar rats (250-350 g, n = 6 per group) were gavaged with either vehicle or 2.5, 5 or 10 mL kg(-1) of STW 5-II, a herbal preparation which contains six extracts. Two hours later, animals were anaesthetized and extracellular multi-unit mesenteric afferent nerve recordings were obtained in the proximal jejunum in vivo. Afferent discharge to 5-hydroxy-tryptamine (5-HT) (5, 10, 20 and 40 microg kg(-1), i.v.), luminal distension (0-60 mmHg) and bradykinin (BK) (15, 30 and 60 microg kg(-1), i.v.) was recorded. At baseline, spontaneous afferent discharge was not different following pretreatment with the various doses of STW 5-II compared with vehicle. The pressure-dependent increase in afferent discharge to intraluminal ramp distension and the dose-dependent increase in afferent firing following 5-HT were also uninfluenced by STW 5-II pretreatment. In contrast, the afferent nerve responses to 15, 30 and 60 microg kg(-1) of BK were reduced following 10 mL kg(-1) STW 5-II with peaks at 106 +/- 19, 153 +/- 22 and 156 +/- 25 imp s(-1) compared with 160 +/- 15, 228 +/- 14 and 220 +/- 16 imp s(-1) following vehicle pretreatment (mean +/- SEM, P < 0.05). Intestinal afferent sensitivity to BK which plays a prime role in nociception was reduced following STW 5-II. Thus, STW 5-II may be of therapeutic use for conditions that involve neuronal hypersensitivity and the release of BK in the intestine.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Metabolic changes in the thalamus after spinal cord injury followed by proton MR spectroscopy

Our study followed the changes in thalamic nuclei metabolism, hindlimb sensitivity to thermal stimulation, and locomotor function after spinal cord injury (SCI). MR spectroscopy (MRS) was used to examine the thalamic nuclei of rats 1 day before and 1, 3, 6, and 15 days after SCI or sham surgery. All animals were tested before MRS measurements for motor performance and thermal sensitivity. SCI induced by balloon compression caused complete paraplegia from the first to third day, followed by partial functional recovery during the second week. MRS revealed an increase in N-acetylaspartate (NAA) concentration in the thalamic nuclei on the first day after SCI, which decreased by the third day. The data also showed an increase in inositol (Ins), glutamate, and creatine (Cr) concentrations on the third day postinjury; the Ins concentration remained elevated on the sixth day. In sham-operated animals an increase in NAA concentration was observed on the sixth and fifteenth days after surgery and an increase in Cr concentration on the third day. A positive correlation between Ins concentration and hindlimb sensitivity in both SCI and sham-operated animals suggests changes in glial activity, while changes in NAA levels may indicate the response of thalamic neuronal cells to injury.

Concentration-detection functions for eye irritation evoked by homologous n-alcohols and acetates approaching a cut-off point

We measured the concentration-detection (i.e., psychometric) functions for the eye irritation evoked by three homologous n-alcohols (1-nonanol, 1-decanol and 1-undecanol) and two homologous acetates (nonyl and decyl acetate). A vapor delivery device based on a dynamic dilution of stimuli in nitrogen served to present various concentrations of each compound, including the undiluted vapor, to the subjects (n >or= 26). Delivered concentrations were quantified by gas chromatography. Detection probability (P) was assessed via a three-alternative, forced-choice procedure and quantified on a scale ranging from P = 0.0 (chance detection) to P = 1.0 (perfect detection). Flow rate to the eye equaled 2.5 l/min and time of exposure was 6 s. The functions for 1-undecanol and decyl acetate plateaued at P approximately 0.5 and P approximately 0.25, respectively, such that further increases in concentration failed to increase detection notably. Thus, both series reached a break point, or cut-off, in the detection of ocular irritation. The present outcome provides additional evidence that the cut-off does not rest on the low vapor concentration of the homolog but, more likely, on the homolog exceeding a critical molecular dimension(s), which prevents it from interacting effectively with the appropriate receptors.

Electrophysiological and pharmacological validation of vagal afferent fiber type of neurons enzymatically isolated from rat nodose ganglia

An unavoidable consequence of enzymatic dispersion of sensory neurons from intact ganglia is loss of the axon and thus the ability to classify afferent fiber type based upon conduction velocity (CV). An intact rat nodose ganglion preparation was used to randomly sample neurons (n=76) using the patch clamp technique. Reliable electrophysiological and chemophysiological correlates of afferent fiber type were established for use with isolated neuron preparations. Myelinated afferents (approximately 25%) formed two groups exhibiting strikingly different functional profiles. One group (n=10) exhibited CVs in excess of 10 m/s and narrow (<1 ms) action potentials (APs) while the other (n=9) had CVs as low as 4m/s and broad (>2 ms) APs that closely approximated those identified as unmyelinated afferents (n=57) with CVs less than 1m/s. A cluster analysis of select measures from the AP waveforms strongly correlated with CV, producing three statistically unique populations (p<0.05). These groupings aligned with our earlier hypothesis (Jin et al., 2004) that a differential sensitivity to the selective purinergic and vanilloid receptor agonists can be used as reliable pharmacological indicators of vagal afferent fiber type. These metrics were further validated using an even larger population of isolated (n=240) nodose neurons. Collectively, these indicators of afferent fiber type can be used to provide valuable insight concerning the relavence of isolated cellular observations to integrated afferent function of visceral organ systems.

Cutoff in detection of eye irritation from vapors of homologous carboxylic acids and aliphatic aldehydes

Using neat vapors of selected homologous aldehydes (decanal, undecanal, dodecanal) and carboxylic acids (pentanoic, hexanoic, heptanoic, octanoic, nonanoic), we explored the point where a certain homolog (and all larger ones) becomes undetectable by eye irritation (i.e. by ocular chemesthesis). This phenomenon has been observed in other homologous series that also reach a break-point, or cutoff, in chemesthetic detection. Participants (11<or=n<or=32) were tested using a three-alternative, forced-choice procedure. Flow rate to the eye equaled 4 or 8 l/min and time of exposure was 6 s. The outcome showed that dodecanal and heptanoic acid were the shortest undetectable homologs. When the vapor concentration of the stimuli was increased by heating the liquid source to 37 degrees C, homologs located before the cutoff point (e.g. hexanoic acid) became readily detected by all subjects, whereas homologs located at the cutoff remained largely undetected. In addition, a comparison of calculated values of eye irritation thresholds for aldehydes and acids (from a successful model of ocular chemesthetic potency) with values of saturated vapor concentration at 23 and 37 degrees C indicated that the vapor concentration of dodecanal and heptanoic acid should have been enough to produce detection. The outcome suggests that the cutoff observed does not result from a low vapor concentration but from limitations in the structure or dimension(s) of the molecules that render them unsuitable to interact effectively with human chemesthetic receptors.

Noxious compounds activate TRPA1 ion channels through covalent modification of cysteines

The nervous system senses peripheral damage through nociceptive neurons that transmit a pain signal. TRPA1 is a member of the Transient Receptor Potential (TRP) family of ion channels and is expressed in nociceptive neurons. TRPA1 is activated by a variety of noxious stimuli, including cold temperatures, pungent natural compounds, and environmental irritants. How such diverse stimuli activate TRPA1 is not known. We observed that most compounds known to activate TRPA1 are able to covalently bind cysteine residues. Here we use click chemistry to show that derivatives of two such compounds, mustard oil and cinnamaldehyde, covalently bind mouse TRPA1. Structurally unrelated cysteine-modifying agents such as iodoacetamide (IA) and (2-aminoethyl)methanethiosulphonate (MTSEA) also bind and activate TRPA1. We identified by mass spectrometry fourteen cytosolic TRPA1 cysteines labelled by IA, three of which are required for normal channel function. In excised patches, reactive compounds activated TRPA1 currents that were maintained at least 10 min after washout of the compound in calcium-free solutions. Finally, activation of TRPA1 by disulphide-bond-forming MTSEA is blocked by the reducing agent dithiothreitol (DTT). Collectively, our data indicate that covalent modification of reactive cysteines within TRPA1 can cause channel activation, rapidly signalling potential tissue damage through the pain pathway.

Design and Characterization of a Noncompetitive Antagonist of the Transient Receptor Potential Vanilloid Subunit 1 Channel With In Vivo Analgesic and Anti-inflammatory Activity

Vanilloid receptor subunit 1 (TRPV1) is an integrator of physical and chemical stimuli in the peripheral nervous system. This receptor plays a key role in the pathophysiology of inflammatory pain. Thus, the identification of receptor antagonists with analgesic and anti-inflammatory activity in vivo is an important goal of current neuropharmacology. Here, we report that [L-arginyl]-[N-[2,4-dichlorophenethyl]glycyl]-N-(2,4-dichlorophenethyl) glycinamide (H-Arg-15-15C) is a channel blocker that abrogates capsaicin and pH-evoked TRPV1 channel activity with submicromolar activity. Compound H-Arg-15-15C preferentially inhibits TRPV1, showing marginal block of other neuronal receptors. Compound H-Arg-15-15C acts as a noncompetitive capsaicin antagonist with modest voltage-dependent blockade activity. The compound inhibited capsaicin-evoked nerve activity in afferent fibers without affecting mechanically activated activity. Notably, administration of compound H-Arg-15-15C prevented the irritant activity of a local administration of capsaicin and formalin and reversed the thermal hyperalgesia evoked by injection of complete Freund's adjuvant. Furthermore, it attenuated carrageenan-induced paw inflammation. Compound H-Arg-15-15C specifically decreased inflammatory conditions without affecting normal nociception. Taken together, these findings demonstrate that compound H-Arg-15-15C is a channel blocker of TRPV1 with analgesic and anti-inflammatory activity in vivo at clinically useful doses and substantiate the tenet that TRPV1 plays an important role in the etiology of chronic inflammatory pain.

PERSPECTIVE

This study reports the design of a potent TRPV1 noncompetitive antagonist that exhibits anti-inflammatory and analgesic activity in preclinical models of acute and chronic pain. This compound is a lead for analgesic drug development.

Biological and biochemical properties of the Brazilian Potamotrygon stingrays: Potamotrygon cf. scobina and Potamotrygon gr. orbignyi

Stingrays of the family Potamotrygonidae are widespread throughout river systems of South America that drain into the Atlantic Ocean. Some species are endemic to the most extreme freshwater environment of the Brazil and cause frequent accidents to humans. The envenomation causes immediate, local, and intense pain, soft tissue edema, and a variable extent of bleeding. The present study was carried out in order to describe the principal biological and some biochemical properties of the Brazilian Potamotrygon fish venoms (Potamotrygon cf. scobina and P. gr. orbignyi). Both stingray venoms induced significant edematogenic and nociceptive responses in mice. Edematogenic and nociceptive responses were reduced when the venom was incubated at 37 or 56 degrees C. The results showed striking augments of leukocytes rolling and adherent cells to the endothelium of cremaster mice induced by both venoms. The data also presented that injection of both venoms induced necrosis, low level of proteolytic activity, without inducing haemorrhage. But when the venoms of both stingray species were injected together with their mucus secretion, the necrotizing activity was more vigorous. The present study provided in vivo evidence of toxic effects for P. cf. scobina and P. gr. orbignyi venoms.

Chemical Boundaries for Detection of Eye Irritation in Humans from Homologous Vapors

In a series of experiments, we looked at a "cutoff" effect for the detection of eye irritation from neat vapors of homologous n-alkylbenzenes and 2-ketones. Stimuli comprised pentyl, hexyl, and heptyl benzene, 2-dodecanone, and 2-tridecanone, presented to each eye at 4 and 8 l/min for 6 sec, using a three-alternative forced-choice procedure against blanks. Detection probability corrected for chance (i.e., detectability) decreased with carbon chain length such that heptyl benzene and 2-tridecanone were virtually undetectable, irrespective of flow rate to the eye. Heating both stimuli sources to 37 degrees C (body temperature) from 23 degrees C (room temperature) increased vapor concentration by 5.0 and 6.9 times, respectively, for heptyl benzene and 2-tridecanone. Still, both chemicals failed to show increased detection for 13 of the 21 participants. In addition, plots of experimentally measured and calculated eye irritation thresholds as a function of carbon chain length for each series indicated that, based on the trend, the concentration of the two cutoff homologs at 37 degrees C should have been high enough to allow detection. Taken together, the results suggest that these cutoffs rest on limitations related to the dimension of the molecules rather than on limitations related to their vapor concentration. For example, the stimulus molecule could exceed the size that allows it to fit into the receptor pocket of a receptive protein. Plots of calculated molecular dimensions across homologous alkylbenzenes, from ethyl to dodecylbenzene, and across 2-ketones, from 2-octanone to 2-octadecanone, provided additional support to the above conclusion.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

STW 5 (Iberogast) reduces afferent sensitivity in the rat small intestine

INTRODUCTION

A limited number of drugs are available for the treatment of functional dyspepsia and irritable bowel syndrome. The efficacy of STW 5 (Iberogast) was previously shown in clinical trials. Since visceral hypersensitivity seems to be the prime pathomechanism of functional gastro-intestinal disorders, the aim of this study was to explore whether STW 5 reduces intestinal afferent sensitivity in the upper gastrointestinal tract.

METHODS

Two groups of male Wistar rats were pretreated with either the herbal preparation STW 5 or its vehicle (30.8% ethanol). Then, after 2h, general anesthesia was induced by pentobarbitone (60 mg kg(-1)i.p.) and extracellular multi-unit afferent recordings were obtained from mesenteric afferents innervating the proximal jejunum. The intestinal afferent nerve response to increasing doses of 5-HT and bradykinin were quantified as well as afferent discharge following a ramp distension of the adjacent intestinal loop from 0 to 60 cm H(2)O.

RESULTS

Afferent discharge to 5-HT and bradykinin increased dose-dependently. Following the different doses of 5-HT, the peak in afferent nerve discharge was always reduced after pretreatment with STW 5 compared to controls with a response of 110+/-5 imp s(-1) after STW 5 and 128+/-3 in vehicle controls at the maximum dose (40 microg kg(-1); p<0.05; mean+/-SEM). For bradykinin, afferent responses were reduced following STW 5 at the 20 and 40 microg kg(-1) dose but not at 10 microg kg(-1) (40 microg kg(-1)176+/-7 imp s(-1) following STW 5 versus 200+/-6 imp s(-1) in controls; p<0.05). The ramp distension of the intestinal loop stimulated a rise in intestinal afferent nerve discharge that was always lower in the STW 5 pretreated group compared to vehicle controls with the exception of the discharge rate at the pressure level of 0 and 20 cm H(2)O (all other pressures up to 60 cm H(2)O p<0.05).

CONCLUSIONS

Sensitivity of intestinal afferents to mechanical and chemical stimuli is reduced following treatment with the herbal preparation STW 5. This mechanism may help to explain why STW 5 relieves dyspeptic and bowel symptoms in patients.

Functional aspects and mechanisms of TRPV1 involvement in neurogenic inflammation that leads to thermal hyperalgesia

Neurogenic inflammation is produced by overstimulation of peripheral nociceptor terminals by injury or inflammation of tissues. Excessive activity of sensory neurons produces vasodilation, plasma extravasation and hypersensitivity. Mechanistically, neurogenic inflammation is due to the release of substances from primary sensory nerve terminals that act directly or indirectly at the peripheral terminals, either activating or sensitizing nociceptors, endothelial cells and immunocytes. Notably, small-diameter sensory neurons that are sensitive to capsaicin play a key role in the generation of neurogenic inflammation. The cloning of the vanilloid receptor 1 (TRPV1) has been a breakthrough that has propelled our understanding of the molecular mechanisms involved in neurogenic inflammation. TRPV1 pivotally contributes to the integration of various stimuli and modulates nociceptor excitability, thus making it a true gateway for pain transduction. In addition, TRPV1 is the endpoint target of intracellular signalling pathways triggered by inflammatory mediators. Phosphorylation-induced potentiation of TRPV1 channel activity, along with an incremented TRPV1 surface expression are major events underlying the nociceptor activation and sensitization that leads to thermal hyperalgesia. The important contribution of TRPV1 receptor to the onset and maintenance of neurogenic inflammation has validated it as a therapeutic target for inflammatory pain management. As a result, the development of specific TRPV1 antagonists is a central focus of current drug discovery programs.

Sensory nerves, neurogenic inflammation and pain: missing components of alternative irritation strategies? A review and a potential strategy

The eyes and skin are highly innervated by sensory nerves; stimulation of these nerves by irritants may give rise to neurogenic inflammation, leading to sensory irritation and pain. Few in vitro models of neurogenic inflammation have been described in conjunction with alternative skin and eye irritation methods, despite the fact that the sensory innervation of these organs is well-documented. To date, alternative approaches to the Draize skin and eye irritation tests have proved largely successful at classifying severe irritants, but are generally poor at discriminating between agents with mild to moderate irritant potential. We propose that the development of in vitro models for the prediction of sensory stimulation will assist in the re-classification of the irritant potential of agents that are under-predicted by current in vitro strategies. This review describes the range of xenobiotics known to cause inflammation and pain through the stimulation of sensory nerves, as well as the endogenous mediators and receptor types that are involved. In particular, it focuses on the vanilloid receptor, its activators and its regulation, as these receptors function as integrators of responses to numerous noxious stimuli. Cell culture models and ex vivo preparations that have the potential to serve as predictors of sensory irritation are also described. In addition, as readily available sensory neuron cell line models are few in number, stem cell lines (with the capacity to differentiate into sensory neurons) are explored. Finally, a preliminary strategy to enable assessment of whether incorporation of a sensory component will enhance the predictive power of current in vitro eye and skin testing strategies is proposed.

Chemical mediators enhance the excitability of unmyelinated sensory axons in normal and injured peripheral nerve of the rat

Ectopic excitation of nociceptive axons by chemical mediators may contribute to symptoms in neuropathic pain. In this study, we have measured the excitability of unmyelinated rat C-fiber axons in isolated segments of sural nerves under different experimental conditions. (1) We demonstrate in normal rats that several mediators including ATP, serotonin (5-HT), 1-(3-chlorophenyl)biguanide (5-HT3 receptor agonist), norepinephrine, acetylcholine and capsaicin alter electrophysiological parameters of C-fibers which indicate an increase of axonal excitability. Other mediators such as histamine, glutamate, prostaglandin E(2) and the cytokines tumor necrosis factor alpha, interleukin-1beta and interleukin-6 did not produce such effects. (2) The effects of several mediators were tested after peripheral nerve injury (partial ligation or spared nerve injury). Sural nerves from such animals did not show significant changes when compared with controls. (3) We tested whether the effects of chemical mediators on axonal excitability are due to actions on the sensory C-fiber afferents or the postganglionic sympathetic efferents. In order to distinguish these effects, we performed surgical sympathectomy of the lumbar sympathetic chain, including the L3, L4 and L5 ganglia. Sympathectomy did not markedly influence the effects of mediators on axonal excitability (except that the norepinephrine effect was significantly diminished). In conclusion, our data suggest a constitutive rather than inducible expression of axonal receptors for some chemical mediators on the axonal membrane of unmyelinated fibers. Most of the changes in axonal excitability take place in sensory C-fiber afferents rather than in postganglionic sympathetic efferents. Thus, it is possible that certain immune and glial cell mediators released in or around the nerve following injury or inflammation influence the excitability of intact nociceptive fibers. This mechanism could contribute to ectopic excitation of axons in neuropathic pain.

Expression of PDE5 splice variants during ontogenesis of chick dorsal root ganglia

Cyclic GMP (cGMP)-binding cGMP-specific phosphodiesterase (PDE5) activity was found in chick dorsal root ganglia (DRG). PDE5 expression was studied at different stages of development: in embryonic day 10 (E10) and E18 embryos and in 5-day post-hatching chick (P5). The presence of PDE5 was suggested by the ion exchange chromatography elution profile in E18 DRG extracts, where cGMP-specific hydrolytic calmodulin-independent activity was found; in other stages, this activity coeluted with the PDE1 calmodulin-stimulated isoform characterized previously. Inhibition studies supported the hypothesis that the newly identified PDE activity belongs to the PDE5 isoform. Western blot analysis using a PDE5-specific antibody was also carried out and revealed the presence of three specific immunoreactive bands with apparent molecular weights of 98, 93, and 86 kDa, corresponding to the three described splice variants (PDE5A1, PDE5A2, and PDE5A3). The expression in DRG of the three PDE5 isoforms was also confirmed by RT-PCR. Developmental regulation of PDE5 was revealed by the immunoblot analysis at different stages; expression was very low at E10 but an overall substantial increase occurred between E10-18 (about 12-fold, considering the three PDE5 isoforms together). Differences were revealed, however, when a single PDE5 isoform was considered. PDE5A1 and PDE5A3 showed an increase at all stages although more pronounced between E10-18, whereas PDE5A2 underwent a marked increase (about 38-fold) in the first period and remained nearly constant between E18 and P5. This is the first evidence of PDE5 in sensory neurons, and the distinct temporal expression patterns of enzyme isoforms may indicate different physiologic roles in developing and mature chick DRG.

The herbal preparation STW 5 (Iberogast) desensitizes intestinal afferents in the rat small intestine

INTRODUCTION

Visceral hypersensitivity in the upper gastrointestinal tract is a potential pathomechanism of functional dyspepsia. The herbal preparation STW 5 (Iberogast) provides symptomatic relief for this condition. We aimed to investigate whether STW 5 modulates intestinal afferent sensitivity.

METHODS

The herbal preparation STW 5 or vehicle (30.8% ethanol) were administered orally in male Wister rats. After 2 h animals were anaesthetized and extracellular multi-unit intestinal afferent nerve recordings were secured from the neurovascular bundle of the mesentery in the proximal jejunum. Afferent discharge to ramp distension of the intestinal loop (0-60 cm H2O) and dose-response curves for i.v. bradykinin (10, 20 and 40 microg kg(-1)) and 5-HT (5, 10, 20 and 40 microg kg(-1)) were recorded.

RESULTS

Baseline discharge was not different between the vehicle and treatment group. Ramp distension was followed by a pressure dependent increase in afferent nerve discharge that was decreased following STW 5 pretreatment for all distending pressures reaching 147 +/- 8 impulses s(-1) (imp s(-1)) following STW 5 vs 171 +/- 5 imp s(-1) following vehicle at 60 cm H2O (mean +/- SEM; P < 0.05). A dose-dependent increase in afferent discharge was observed for 5-HT and bradykinin. Following STW 5 pretreatment, afferent discharge was reduced at all doses of 5-HT to 110 +/- 5 at the maximum dose after STW 5 and 128 +/- 3 imp s(-1) in controls (all P < 0.05). Afferent discharge to bradykinin was similarly reduced at 20 and 40 microg kg(-1) but not at 10 microg kg(-1) of bradykinin with a discharge rate of 176 +/- 7 imp s(-1) following STW 5 and 200 +/- 6 imp s(-1) in controls at 40 microg kg(-1) (P < 0.05).

CONCLUSIONS

The preparation STW 5 reduces intestinal afferent nerve discharge following chemical and mechanical stimuli, while baseline discharge is not affected. This effect of STW 5 on afferent sensitivity may contribute to its therapeutic relief of dyspeptic symptoms.

Oxidizing reagent copper-o-phenanthroline is an open channel blocker of the vanilloid receptor TRPV1

The TRPV1 channel plays an important role in generating nociceptive signals in mammalian primary sensory neurons. It consists of 838 amino acids with six transmembrane segments (TM1-TM6), a pore-forming loop between TM5 and TM6 and N- and C- terminals located intracellularly. It is a homotetramer and forms a nonselective cationic channel that can be opened by capsaicin, weak acids and noxious heat. There are 18 cysteines (Cys), three of which are located on the extracellular side of the receptor in and around the region of the pore-forming loop. We report that the TRPV1 channel in transfected HEK293T cells and in cultured rat DRG neurons is blocked in the open state by an oxidizing agent Cu-o-phenanthroline complex (Cu:Phe). The effects of Cu:Phe are concentration dependent ( IC50 = 5.2 : 20.8 microm ) and fully reversible. Cu:Phe applied immediately before exposure to an acidic solution, capsaicin or noxious heat is without effect. Substitutions of the extracellular Cys residues (616, 621, 634) by glycine individually or together do not alter the blocking effects of Cu:Phe suggesting that disulfide cross-linking does not represent the underlying mechanism. It is suggested that the complex Cu:Phe, a bulky, positively charged molecule, represents a very effective and reversible open channel blocker of TRPV1.

Glial activation and pathological pain

Pain is a sensation we have all experienced. For most of us, the pain has been temporary. However, for patients with pathological pain, the pain experience is unending, with little hope for therapeutic relief. Pathological pain is characterized by an amplified response to normally innocuous stimuli, and an amplified response to acute pain. Pathological pain has long been described as the result of dysfunctional neuronal activity. While neuronal functioning is indeed altered, there is significant evidence showing that exaggerated pain is regulated by the activation of astrocytes and microglia. In exaggerated pain, astrocytes, and microglia are activated by neuronal signals including substance P, glutamate, and fractalkine. Activation of glia by these substances leads to the release of mediators that then act on other glia and neurons. These include a family of proteins called "proinflammatory cytokines" released from microglia and astrocytes. These cytokines have been shown to be critical mediators of exaggerated pain. Some patients with pathological pain also report "extra-territorial" and/or "mirror" image pain. That is, exaggerated pain is experienced not only in the area of trauma. In extra-territorial pain, pain is also perceived as arising from neighboring healthy tissues outside of the site of trauma. In the rare cases of mirror-image pain, such pain is perceived as arising from the healthy, corresponding body part on the opposite side of the body. New data suggest that activation of astrocyte communication via gap junctions may mediate such spread of pain. While traditional therapies for pathological pain have focused on neuronal targets, the following review describes glia as newly recognized mediators of exaggerated pain, and as new therapeutic targets. Moreover, the glial-neuronal interactions discussed here are likely not exclusive to pain, but rather are likely to play significant roles in other behavioral phenomena.

Bradykinin decreases K(+) and increases Cl(-) conductances in vagal afferent neurones of the guinea pig

Bradykinin (BK) is an inflammatory mediator that can excite and sensitize primary afferent neurones. The nature of the ionic channels underlying the excitatory actions of BK is still incompletely understood. Using whole-cell patch-clamp recording from acutely dissociated nodose ganglion neurones (NGNs) we have examined the ionic mechanism responsible for BK's excitatory effect. Bath-applied BK (0.1 microM) depolarized the membrane potential (29 +/- 3.1 mV, n= 7), evoked action potentials, and induced an inward ionic current (I(BK)) with two distinctive membrane conductances (g(m)). Initially, g(m) decreased; the ionic current associated with this g(m) had a reversal potential (E(rev)) value of -87 +/- 1.1 mV (n= 26), a value close to E(K) (-89 mV). Subsequently, g(m) increased; the ionic current associated with this g(m) had an estimated E(rev) of 49 +/- 4.3 mV (n= 23). When the second component was isolated from the first component, by replacing [K(+)](o) with Cs(+), E(rev) was 20 +/- 4.7 mV (n= 10). Replacing external NaCl with NMDG-Cl or choline-Cl, or reducing [Ca(2+)](o) did not significantly diminish I(BK). After replacing external NaCl with sodium isethionate, E(rev) for the second component shifted to 56 +/- 8.8 mV (n= 4), a value close to the E(Cl) (66 mV). The second component was inhibited by intracellular BAPTA or by bath application of niflumic acid (100 microM), a Ca(2+)-activated Cl(-) channel blocker. These results suggest that the first and second components of I(BK) are produced by a decrease in K(+) conductance and an increase in Ca(2+)-activated Cl(-) conductance, respectively. The BK-evoked Cl(-) conductance in NGNs may be the first demonstration of an inflammatory mediator exciting primary afferents via an anion channel.

Inflammation and hyperalgesia induced by nerve injury in the rat: a key role of mast cells

Inflammatory cells and their mediators are known to contribute to neuropathic pain following nerve injury. Mast cells play a key role in non-neural models of inflammation and we propose that mast cells and their mediators (in particular histamine) are important in the development of neuropathic pain. In rats, where the sciatic nerve was partially ligated, we showed that stabilisation of mast cells with sodium cromoglycate reduced the recruitment of neutrophils and monocytes to the injured nerve and suppressed the development of hyperalgesia. Treatment with histamine receptor antagonists suppressed the development of hyperalgesia following nerve injury and alleviated hyperalgesia once it was established. These results suggest that mast cell mediators such as histamine released within hours of nerve injury contribute to the recruitment of leukocytes and the development of hyperalgesia.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.

Characterization of neuronal nicotinic acetylcholine receptors in the membrane of unmyelinated human C-fiber axons by in vitro studies

Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small-fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C fibers by <or=30%. This increase in axonal excitability was accompanied by a rise in the free intracellular Ca2+ concentration. The following rank order of potency for agonists was found: epibatidine >> 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 subunits in the axonal membrane of unmyelinated human C fibers. In addition, the observations suggest that the axonal membrane of C fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.

ATP and sensory transduction in the enteric nervous system

ATP is a neurotransmitter in the central and peripheral nervous systems and is also involved in peripheral inflammation and transmission of the sensation of pain. Recently, the regulated release of ATP from non-neuronal sources has been shown to play a role in the activation of sensory nerve terminals. Within the enteric nervous system, which is present in the wall of the gastrointestinal tract, ATP plays three major roles. ATP acts as an inhibitory transmitter from the enteric motor neurons to the smooth muscle via P2Y receptors. ATP is released as an excitatory neurotransmitter between enteric interneurons and from the interneurons to the motor neurons via P2Y and P2X receptors. Finally, ATP may act as a sensory mediator, from epithelial sources to the intrinsic sensory nerve terminals. Thus, ATP participates in the transduction of sensory stimuli from the gut lumen and in the subsequent initiation and propagation of enteric reflexes.

Immediate sensory nerve-mediated respiratory responses to irritants in healthy and allergic airway-diseased mice

The immediate responses of the upper respiratory tract (URT) to the irritants acrolein and acetic acid were examined in healthy and allergic airway-diseased C57Bl/6J mice. Acrolein (1.1 ppm) and acetic acid (330 ppm) vapors induced an immediate increase in flow resistance, as measured in the surgically isolated URT of urethane-anesthetized healthy animals. Acrolein, but not acetic acid, induced a small URT vasodilatory response. In awake spontaneously breathing mice, both vapors induced a prolonged pause at the start of expiration (a response mediated via stimulation of nasal trigeminal nerves) and an increase in total respiratory specific airway flow resistance, the magnitude of which was similar to that observed in the isolated URT. Both responses were significantly reduced in animals pretreated with large doses of capsaicin to defunctionalize sensory nerves, strongly suggesting a role for sensory nerves in development of these responses. The breathing pattern and/or obstructive responses were enhanced in mice with ovalbumin-induced allergic airway disease. These results suggest that the primary responses to acrolein and acetic acid vapors are altered breathing patterns and airway obstruction, that sensory nerves play an important role in these responses, and that these responses are enhanced in animals with allergic airway disease.

Topical and peripherally acting analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, alpha-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and alpha-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

A role for TRPV1 in bradykinin-induced excitation of vagal airway afferent nerve terminals

Using single-unit extracellular recording techniques, we have examined the role of the vanilloid receptor-1 (VR1 aka TRPV1) in bradykinin-induced activation of vagal afferent C-fiber receptive fields in guinea pig isolated airways. Of 17 airway C-fibers tested, 14 responded to bradykinin and capsaicin, 2 fibers responded to neither capsaicin nor bradykinin, and 1 fiber responded to capsaicin but not bradykinin. Thus, every bradykinin-responsive C-fiber was also responsive to capsaicin. Bradykinin (200 microl of 0.3 microM solution) evoked a burst of approximately 130 action potentials in C-fibers. In the presence of the TRPV1 antagonist capsazepine (10 microM), bradykinin evoked 83 +/- 9% (n = 6; P < 0.01) fewer action potentials. Similarly, the TRPV1 blocker, ruthenium red (10 microM), inhibited the number of bradykinin-evoked action potentials by 75 +/- 10% (n = 4; P < 0.05). In the presence of 5,8,11,14-eicosatetraynoic acid (10 microM), an inhibitor of lipoxygenase and cyclooxygenase enzymes, the number of bradykinin-induced action potentials was reduced by 76 +/- 10% (n = 6; P < 0.05). Similarly, a combination of the 12-lipoxygenase inhibitor, baicalein (10 microM) and the 5-lipoxygenase inhibitor ZD2138 [6-[3-fluoro-5-[4-methoxy-3,4,5,6-tetrahydro-2H-pyran-4-yl])phenoxy-methyl]-1-methyl-2-quinolone] (10 microM) caused significant inhibition of bradykinin-induced responses. Our data suggest a role for lipoxygenase products in bradykinin B(2) receptor-induced activation of TRPV1 in the peripheral terminals of afferent C-fibers within guinea pig trachea.

Expression of vesicular glutamate transporters in rat lumbar spinal cord, with a note on dorsal root ganglia

Three vesicular glutamate transporters (VGLUTs) have been recently identified and their distribution has been mapped in various brain areas. In the present study, we used morphological approaches to investigate their expression in the rat lumbar spinal cord and dorsal root ganglia. Our results show a complementary distribution of VGLUT-expressing fibers in the spinal cord, with no overlapping in nerve endings. In the dorsal horn, VGLUT1 is most abundant in mechanosensory/proprioceptive deep afferent fibers. VGLUT2 and VGLUT3 are expressed only at moderate levels in primary sensory afferent fibers and are not used by central projections of nociceptive neurons. VGLUT1 and VGLUT2 mRNAs are mainly segregated in superficial laminae but colocalized in deeper laminae. Weak expression of VGLUT3 mRNA is only detected in deep laminae. The colocalization of VGLUT1 and VGLUT2 transcripts in most sensory neurons of the dorsal root ganglia is not in agreement with the clear segregation between the proteins in their spinal projections. Such a discrepancy suggests targeting mechanisms specific for each transporter and/or a distinct regulation of their translation. In the ventral horn, the expression of VGLUT1 and VGLUT2 mRNAs in motoneuron perikarya suggests the possible unexpected role of glutamate in the vertebrate neuromuscular junction. These results demonstrate the existence of different subpopulations of glutamate nerve terminals in the rat lumbar spinal cord and suggest that functionally distinct subsets of excitatory glutamatergic neuronal networks are involved in sensory processing and motor control.

Protein kinase C isoform antagonism controls BNaC2 (ASIC1) function

We explored the involvement of protein kinase C (PKC) and its isoforms in the regulation of BNaC2. Reverse transcriptase PCR evaluation of PKC isoform expression at the level of mRNA revealed the presence of alpha and epsilon/epsilon' in all glioma cell lines analyzed; most, but not all cell lines expressed delta and zeta. No messages were found for the betaI and betaII isotypes of PKC in the tumor cells. Normal astrocytes expressed beta but not gamma. The essential features of these results were confirmed at the protein level by Western analysis. This disproportionate pattern of PKC isoform expression in glioma cell lines was further echoed in the functional effects of these PKC isoforms on BNaC2 activity in bilayers. PKC holoenzyme or the combination of PKCbetaI and PKCbetaII isoforms inhibited BNaC2. Neither PKCepsilon nor PKCzeta or their combination had any effect on BNaC2 activity in bilayers. The inhibitory effect of the PKCbetaI and PKCbetaII mixture on BNaC2 activity was abolished by a 5-fold excess of a PKCepsilon and PKCzeta combination. PKC holoenzymes, PKCbetaI, PKCbetaII, PKCdelta, PKCepsilon, and PKCzeta phosphorylated BNaC2 in vitro. In patch clamp experiments, the combination of PKCbetaI and PKCbetaII inhibited the basally activated inward Na(+) conductance. The variable expression of the PKC isotypes and their functional antagonism in regulating BNaC2 activity support the idea that the participation of multiple PKC isotypes contributes to the overall activity of BNaC2.

Effect of capsaicin on voltage-gated currents of trigeminal neurones in cell culture and slice preparations

Effects of capsaicin on voltage-gated currents were examined in vitro by whole-cell patch-clamp recordings from small neurones of rat trigeminal ganglia either in slice preparations or in different cell cultures. Cells were classified as sensitive to capsaicin if they responded with inward current and/or conductance change to the agent in nanomolar concentration. Capsaicin (150 to 330 nM) in sensitive cells reduced the mixed inward current evoked by depolarizing step or ramp commands in all preparations. In cultured cells, the inward current was depressed to 32.78 +/- 26.42% (n = 27) of the control. Both the tetrodotoxin-sensitive and -resistant inward currents were affected. The data support the concept that capsaicin besides acting on VR-1 receptors inhibits also some voltage gated channels. In 34 cultured cells, capsaicin increased the slope conductance to 170.5 +/- 68%. Percentage of capsaicin sensitive cells observed in nerve growth factor-treated cultured cell populations was higher (77.8%) than in the two other preparations (14.3 or 38.8%). It is concluded that 1) depression of the voltage-gated currents may play an important role in the functional desensitization of the sensory receptors and in the analgesic effect induced by the agent and 2) cell body of sensory neurones under native condition seems less sensitive to capsaicin then that of cells cultured in the presence of nerve growth factor.

ATP as a putative sensory mediator: activation of intrinsic sensory neurons of the myenteric plexus via P2X receptors

The mucosal terminals of sensory neurons intrinsic to the wall of the intestine are sensitive to the chemical environment within the lumen. Lumenal stimuli probably release sensory mediators from the mucosal epithelium, which then activate the nerve terminals indirectly. Here, we tested the idea that ATP activates intrinsic sensory nerve terminals in a way consistent with its being a sensory mediator. We made intracellular recordings from intrinsic sensory neurons located in the myenteric plexus [identified as AH neurons, which are neurons with a long-lasting afterhyperpolarization following the action potential (AP)], located within 1 mm of intact mucosa. Focal electrical stimulation of the mucosa was used to locate and map regions innervated by each neuron. Application of ATP (1-2 mm in the pressure pipette) to these regions elicited trains of APs that originated at the sensory terminals. ATP-gamma-S produced a similar response, but alpha,beta-methylene ATP and 2-methylthio-ATP were only weakly active. The P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',5'-disulphonic acid (PPADS) (60 microm in the bath) abolished the APs evoked by ATP and ATP-gamma-S but spared similar responses evoked by 5-hydroxytryptamine (5-HT). Another P2 receptor antagonist suramin (100 microm in the bath) did not significantly change the number of APs evoked by ATP. Either ATP or alpha,beta-methylene ATP desensitized the ATP-evoked APs; 50% recovery occurred after approximately 5 sec. The number of APs evoked by ATP was reduced, but not abolished, by the selective 5-HT3 receptor antagonist granisetron (1 microm in the bath). ATP was applied to the cell bodies of sensory neurons to investigate whether the cell bodies express the same P2X receptor as the terminals. ATP evoked a fast depolarization associated with a reduction in input resistance and a reversal potential of -11 mV. This depolarization was potentiated by suramin and blocked by PPADS. We conclude that activation of an atypical excitatory P2X receptor by ATP triggers AP generation in the mucosal processes of the sensory neurons; endogenous 5-HT release may also contribute to activation of the nerve terminals. A similar P2X receptor exists on the cell body of the sensory neuron. Together, these data are consistent with a role for ATP as a sensory mediator in gastrointestinal chemosensory transduction.

A sensory neuron subpopulation with unique sequential survival dependence on nerve growth factor and basic fibroblast growth factor during development

We characterized a subpopulation of dorsal root ganglion (DRG) sensory neurons that were previously identified as preferential targets of enkephalins. This group, termed P-neurons after their "pear" shape, sequentially required nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) for survival in vitro during different developmental stages. Embryonic P-neurons required NGF, but not bFGF. NGF continued to promote their survival, although less potently, up to postnatal day 2 (P2). Conversely, at P5, they needed bFGF but not NGF, with either factor having similar effects at P2. This trophic switch was unique to that DRG neuronal group. In addition, neither neurotrophin-3 (NT-3) nor brain-derived neurotrophic factor influenced their survival during embryonic and postnatal stages, respectively. The expression of NGF (Trk-A) and bFGF (flg) receptors paralleled the switch in trophic requirement. No single P-neuron appeared to coexpress both Trk-A and flg. In contrast, all of them coexpressed flg and substance P, providing a specific marker of these cells. Immunosuppression of bFGF in newborn animals greatly reduced their number, suggesting that the factor was required in vivo. bFGF was present in the DRG and spinal cord, as well as in skeletal muscle, the peripheral projection site of P-neurons, as revealed by tracer DiIC(18)3. The lack of requirement of NT-3 for survival and immunoreactivity for the neurofilament of 200 kDa distinguished them from muscle proprioceptors, suggesting that they are likely to be unmyelinated muscle fibers. Collectively, their properties indicate that P-neurons constitute a distinct subpopulation of sensory neurons for which the function may be modulated by enkephalins.

Neurogenic inflammation and particulate matter (PM) air pollutants

Exposure to a class of airborne pollutants known as particulate matter (PM) is an environmental health risk of global proportions. PM is thought to initiate and/or exacerbate respiratory disorders, such as asthma and airway hyper-responsiveness and is epidemiologically associated with causing death in the elderly and those with pre-existing respiratory, or cardiopulmonary disease. Plausible mechanisms of action to explain PM inflammation and its susceptible sub-population component are lacking. This review describes a series of published studies which indicate that PM initiates airway inflammation through sensory neural pathways, specifically by activation of capsaicin-sensitive vanilloid (e.g. VRI) irritant receptors. These acid-sensitive receptors are located on the sensory C nerve fibers that innervate the airways as well as on various immune and non-immune airway target cells. The activation of these receptors results in the release of neuropeptides from the sensory terminals that innervate the airways. Their interactions with airway target cells, result in signs of inflammation (e.g. bronchoconstriction, vasodilation, histamine release, mucous secretion etc.). Our data have linked the activation of the VR1 receptors to the surface charge carried on the colloidal particulates which constitute PM pollution. Related studies have examined how genetic and non-genetic factors modify the sensitivity of these irritant receptors and enhance the inflammatory responsiveness to PM. In summary, this review proposes a mechanism by which neurogenic elements initiate and sustain PM-mediated airway inflammation. Although neurogenic influences have been appreciated in normal airway homeostasis, they have not, until now, been associated with PM toxicity. The sensitivity of the sensory nervous system to irritants and its interactions with pulmonary target tissues, should encourage neuroscientists to explore the relevance of neurogenic influences to toxic disorders involving other peripheral target systems.

Capsaicin-induced relaxation in rabbit coronary artery

In the present study mechanism of inhibitory effects of capsaicin on the contractility of rabbit coronary artery were studied by measurement of isometric tension and intracellular Ca2+ concentration. Capsaicin (1 microM to 30 microM) relaxed the coronary artery pre-contracted with prostaglandin (PG) F2alpha (1 microM) in a concentration-dependent manner. The PGF2alpha-induced increase in intracellular Ca2+ concentration was also inhibited. The effects of capsaicin were readily reversed by washing capsaicin from the bath. Capsaicin-induced relaxation was not attenuated by pretreatment with capsazepine (1 microM), a blocker of vanilloid receptor or ruthenium red (1 microM), a blocker of non-selective cation channel. Previous exposure to a high concentration of capsaicin (100 microM) or repeated application of capsaicin did not eliminate the relaxation response to subsequent application of capsaicin. Increasing the external K+ concentration to 80 mM significantly attenuated the capsaicin-induced relaxation with simultaneous change in intracellular Ca2+ concentration. Pretreatment with iberiotoxin (100 nM), a blocker of Ca2+-activated K+ channel, only partially inhibited the capsaicin-induced relaxation. However, application of 4-aminopyridine (4-AP, 1 mM), a blocker of delayed rectifier K+ current significantly inhibited the capsaicin-induced relaxation with concomitant attenuation of the effect on intracellular Ca2+ concentration. These results indicate that capsaicin may have a direct relaxing effect on the smooth muscle contractility, and relaxation may be due to activation of the 4-AP-sensitive, delayed rectifier K+ channels in the rabbit coronary artery.

Hyperalgesia due to nerve injury: role of neutrophils

The hypothesis that the early inflammatory cell, the neutrophil, contributes to the hyperalgesia resulting from peripheral nerve injury was tested in rats in which the sciatic nerve was partially transected on one side. The extent and time-course of neutrophilic infiltration of the sciatic nerve and innervated paw skin after partial nerve damage was characterized using immunocytochemistry. The number of endoneurial neutrophils was significantly elevated in sections of operated nerve compared to sections of sham-operated nerve for the entire period studied, i.e. up to seven days post-surgery. This considerable elevation in endoneurial neutrophil numbers was only observed at the site of nerve injury. Depletion of circulating neutrophils at the time of nerve injury significantly attenuated the induction of hyperalgesia. However, depletion of circulating neutrophils at day 8 post-injury did not alleviate hyperalgesia after its normal induction. It is concluded that endoneurial accumulation of neutrophils at the site of peripheral nerve injury is important in the early genesis of the resultant hyperalgesia. The findings support the notion that a neuroimmune interaction occurs as a result of peripheral nerve injury and is important in the subsequent development of neuropathic pain.