Cell type and conduction velocity of rat primary sensory neurons with substance P-like immunoreactivity

Morphological and biochemical characterisation of sensory neurons infected in vitro with rabies virus

This work was aimed at the morphological and biochemical characterisation of the most susceptible neuronal subpopulation to rabies virus (RABV) infection. Adult mouse DRG cultures were infected with RABV and double-processed for viral antigen detection and neuropeptides: calcitonin gene-related peptide (CGRP), galanin (GAL), substance P (SP), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). It was found that 56% of the neurons in culture were small (diameter < 20 microm) but, in spite of this, 69% of the infected neurons had intermediate and large diameters (> or = 20 microm). More than 50% of infected neurons expressed NPY, VIP or SP, whereas no association was found between infected neurons and the presence of CGRP or GAL. Despite SP having been shown to be a small neuron marker, it was found that RABV infects medium and large-sized SP positive cells. RABV preference for larger neurons could explain part of the neuropathogenesis since it can be suggested that, following a rabid accident, the virus uses large neurons (mainly innervating muscle and joints) in vivo to be transported later on to the central nervous system.

Modification of classical neurochemical markers in identified primary afferent neurons with Aβ-, Aδ-, and C-fibers after chronic constriction injury in mice

It is functionally important to differentiate between primary afferent neurons with A-fibers, which are nociceptive or nonnociceptive, and C-fibers, which are mainly nociceptive. Neurochemical markers such as neurofilament 200 (NF200), substance P (SP), and isolectin B4 (IB4) have been useful to distinguish between A- and C-fiber neurons. However, the expression patterns of these markers change after peripheral nerve injury, so that it is not clear whether they still distinguish between fiber types in models of neuropathic pain. We identified neurons with Abeta-, Adelta-, and C-fibers by their conduction velocity (corrected for utilization time) in dorsal root ganglia taken from mice after a chronic constriction injury (CCI) of the sciatic nerve and control mice, and later stained them for IB4, SP, calcitonin gene-related peptide (CGRP), NF200, and neuropeptide Y (NPY). NF200 remained a good marker for A-fiber neurons, and IB4 and SP remained good markers for C-fiber neurons after CCI. NPY was absent in controls but was expressed in A-fiber neurons after CCI. After CCI, a group of C-fiber neurons emerged that expressed none of the tested markers. The size distribution of the markers was investigated in larger samples of unidentified dorsal root ganglion neurons and, together with the results from the identified neurons, provided only limited evidence for the expression of SP in Abeta-fiber neurons after CCI. The extent of up-regulation of NPY showed a strong inverse correlation with the degree of heat hyperalgesia.

Substance P initiates NFAT-dependent gene expression in spinal neurons

Persistent hyperalgesia is associated with increased expression of proteins that contribute to enhanced excitability of spinal neurons, however, little is known about how expression of these proteins is regulated. We tested the hypothesis that Substance P stimulation of neurokinin receptors on spinal neurons activates the transcription factor nuclear factor of activated T cells isoform 4 (NFATc4). The occurrence of NFATc4 in spinal cord was demonstrated with RT-PCR and immunocytochemistry. Substance P activated NFAT-dependent gene transcription in primary cultures of neonatal rat spinal cord transiently transfected with a luciferase DNA reporter construct. The effect of Substance P was mediated by neuronal neurokinin-1 receptors that coupled to activation of protein kinase C, l-type voltage-dependent calcium channels, and calcineurin. Interestingly, Substance P had no effect on cyclic AMP response element (CRE)-dependent gene expression. Conversely, calcitonin gene-related peptide, which activated CRE-dependent gene expression, did not activate NFAT signaling. These data provide evidence that peptides released from primary afferent neurons regulate discrete patterns of gene expression in spinal neurons. Because the release of Substance P and calcitonin gene-related peptide from primary afferent neurons is increased following peripheral injury, these peptides may differentially regulate the expression of proteins that underlie persistent hyperalgesia.

Inhibition by spinal mu- and delta-opioid agonists of afferent-evoked substance P release

Opioid mu- and delta-receptors are present on the central terminals of primary afferents, where they are thought to inhibit neurotransmitter release. This mechanism may mediate analgesia produced by spinal opiates; however, when they used neurokinin 1 receptor (NK1R) internalization as an indicator of substance P release, Trafton et al. (1999) noted that this evoked internalization was altered only modestly by morphine delivered intrathecally at spinal cord segment S1-S2. We reexamined this issue by studying the effect of opiates on NK1R internalization in spinal cord slices and in vivo. In slices, NK1R internalization evoked by dorsal root stimulation at C-fiber intensity was abolished by the mu agonist [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) (1 microM) and decreased by the delta agonist [D-Phe2,5]-enkephalin (DPDPE) (1 microM). In vivo, hindpaw compression induced NK1R internalization in ipsilateral laminas I-II. This evoked internalization was significantly reduced by morphine (60 nmol), DAMGO (1 nmol), and DPDPE (100 nmol), but not by the kappa agonist trans-(1S,2S)-3,4-dichloro-N-mathyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride (200 nmol), delivered at spinal cord segment L2 using intrathecal catheters. These doses of the mu and delta agonists were equi-analgesic as measured by a thermal escape test. Lower doses neither produced analgesia nor inhibited NK1R internalization. In contrast, morphine delivered by percutaneous injections at S1-S2 had only a modest effect on thermal escape, even at higher doses. Morphine decreased NK1R internalization after systemic delivery, but at a dose greater than that necessary to produce equivalent analgesia. All effects were reversed by naloxone. These results indicate that lumbar opiates inhibit noxious stimuli-induced neurotransmitter release from primary afferents at doses that are confirmed behaviorally as analgesic.

Dorsal root ganglion neurones in culture: a model system for identifying novel analgesic targets?

Ion channels represent attractive targets in the development of novel analgesics for the treatment of pain. Dorsal root ganglion (DRG) neurones in culture can share characteristics with nociceptors in vivo and are frequently used to investigate the ion channels that underlie the transduction of noxious stimuli into electrical activity during sensory processing. In this article, I describe the methods used to prepare cultures of DRG neurones including the procedures for the dissection, enzymatic dissociation and plating. The criteria used to identify putative nociceptors in vitro are reviewed and using the M-current as an example I highlight how potential analgesic targets can be identified by combining the use of the voltage clamp technique with the use of selective pharmacological agents.

GABAA receptor facilitation of neurokinin release from primary afferent terminals in the rat spinal cord

Our goal was to test the following hypotheses: 1) GABA(A) receptors facilitate neurokinin release from primary afferent terminals; 2) they do this by suppressing an inhibitory effect of GABA(B) receptors; 3) the activation of these two receptors is controlled by the firing frequency of primary afferents. We evoked neurokinin release by stimulating the dorsal root attached to spinal cord slices, and measured it using neurokinin 1 receptor (NK1R) internalization. Internalization evoked by root stimulation at 1 Hz (but not at 100 Hz) was increased by the GABA(A) receptor agonists muscimol (effective concentration of drug for 50% of the increase [EC50] 3 microM) and isoguvacine (EC50 4.5 microM). Internalization evoked by root stimulation at 100 Hz was inhibited by the GABA(A) receptor antagonists bicuculline (effective concentration of drug for 50% of the inhibition [IC50] 2 microM) and picrotoxin (IC50 243 nM). Internalization evoked by incubating the root with capsaicin (to selectively recruit nociceptive fibers) was increased by isoguvacine and abolished by picrotoxin. Therefore, GABA(A) receptors facilitate neurokinin release. Isoguvacine-facilitated neurokinin release was inhibited by picrotoxin, low Cl-, low Ca2+, Ca2+ channel blockers and N-methyl-D-aspartate receptor antagonists. Bumetanide, an inhibitor of the Na(+)-K(+)-2Cl- cotransporter, inhibited isoguvacine-facilitated neurokinin release, but this could be attributed to a direct inhibition of GABA(A) receptors. The GABA(B) agonist baclofen inhibited NK1R internalization evoked by 100 Hz root stimulation (IC50 1.5 microM), whereas the GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl) phosphinic acid (CGP-55845) increased NK1R internalization evoked by 1 Hz root stimulation (EC50 21 nM). Importantly, baclofen inhibited isoguvacine-facilitated neurokinin release, and CGP-55845 reversed the inhibition of neurokinin release by bicuculline. In conclusion, 1) GABA(B) receptors located presynaptically in primary afferent terminals inhibit neurokinin release; 2) GABA(A) receptors located in GABAergic interneurons facilitate neurokinin release by suppressing GABA release onto these GABA(B) receptors; 3) high frequency firing of C-fibers stimulates neurokinin release by activating GABA(A) receptors and inhibiting GABA(B) receptors, whereas low frequency firing inhibits neurokinin release by the converse mechanisms.

Modulation of voltage-dependent calcium channels by neurotransmitters and neuropeptides in parasympathetic submandibular ganglion neurons

The control of saliva secretion is mainly under parasympathetic control, although there also could be a sympathetic component. Sympathetic nerves are held to have a limited action in secretion in submandibular glands because, on electrical stimulation, only a very small increase to the normal background, basal secretion occurs. Parasympathetic stimulation, on the other hand, caused a good flow of saliva with moderate secretion of acinar mucin, plus an extensive secretion of granules from the granular tubules. The submandibular ganglion (SMG) is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. Neurotransmitters and neuropeptides acting via G-protein coupled receptors (GPCRs) change the electrical excitability of neurons. In these neurons, many neurotransmitters and neuropeptides modulate voltage-dependent calcium channels (VDCCs). The modulation is mediated by a family of GPCRs acting either directly through the membrane delimited G-proteins or through second messengers. However, the mechanism of modulation and the signal transduction pathway linked to an individual GPCRs depend on the animal species. This review reports how neurotransmitters and neuropeptides modulate VDCCs and how these modulatory actions are integrated in SMG systems. The action of neurotransmitters and neuropeptides on VDCCs may provide a mechanism for regulating SMG excitability and also provide a cellular mechanism of a variety of neuronal Ca(2+)-dependent processes.

Cutaneous and Colonic Rat DRG Neurons Differ With Respect to Both Baseline and PGE2-Induced Changes in Passive and Active Electrophysiological Properties

This study was designed to test the hypotheses that pain syndromes associated with specific body regions reflect unique properties of sensory neurons innervating these regions and/or unique responses of these afferents to tissue damage. Acutely dissociated adult rat dorsal root ganglia (DRG) neurons retrogradely labeled from either the colon or the glabrous skin of the hindpaw were studied by whole cell patch-clamp recording in current-clamp mode. Two populations of colonic afferent neurons were studied: pelvic afferents (arising from L6, S1, and S2 DRG = LS DRG) and hypogastric/lumbar colonic afferents (arising from T13, L1, and L2 DRG = TL DRG). Passive and active electrophysiological properties were studied before and after application prostaglandin E2 (PGE2). We observed marked differences between cutaneous and colonic sensory neurons with respect to baseline passive and active electrophysiological properties as well as both the magnitude and pattern of PGE2-induced changes in excitability, passive, and active properties. There were also significant differences between TL and LS neurons with respect to baseline and PGE2-induced changes in several passive and active electrophysiological properties. Our results suggest that differences between cutaneous and colonic neurons reflect differences in pattern and/or density of ionic currents present in the plasma membrane. More interestingly, the ionic currents underlying the PGE2-induced sensitization of cutaneous neurons appeared to differ from those underlying the sensitization of colonic neurons. The implication of this observation is that it may be possible, in fact necessary, to treat pain arising from specific body regions with unique therapeutic interventions.

Demonstration of substance P, calcitonin gene-related peptide, and protein gene product 9.5 containing nerve fibers in human cervical facet joint capsules

STUDY DESIGN

Human cervical facet joint capsules were evaluated by immunohistochemistry.

OBJECTIVES

To study the neuropeptide innervation of the cadaveric cervical facet joint capsules.

SUMMARY OF BACKGROUND DATA

Various clinical and biomechanical studies indicate a role for cervical facet joint capsules in the etiology of neck pain. However, studies on innervation of these capsules are very limited. There is also a dearth of studies on the neuropeptide nature of this innervation.

METHODS

Facet joint capsules harvested from unembalmed cadavers were studied by the avidin biotin peroxidase method for the presence of nerve fibers. Neuropeptide innervation was investigated by using antisera to substance P and calcitonin gene-related peptide. Antisera to protein gene product 9.5 (PGP 9.5), a general neuronal marker, were also used.

RESULTS

In a study of 12 human cervical facet joint capsules, short segments of substance P were observed in 6 capsules, while fibers reactive to calcitonin gene-related peptide were demonstrated in 7 capsules. Nerve fibers immunoreactive to protein gene product 9.5 were also observed in 9 of the 14 capsules studied. Protein gene product 9.5 reactive fibers were the most extensively distributed fibers, observed as bundles and also as single fibers.

CONCLUSIONS

An abundance of protein gene product 9.5 reactive nerve fibers indicates an extensive innervation of the cervical facet joint capsules. The presence of substance P and calcitonin gene-related peptide reactive nerve fibers in a population of these lends credence to cervical facet joint capsules as a key source of neck pain.

c-Src kinase activation regulates preprotachykinin gene expression and substance P secretion in rat sensory ganglia

Increased synthesis of substance P (SP) in the dorsal root ganglia (DRG) and enhanced axonal transport to and secretion from the primary afferent sensory neurons might enhance pain signalling in the spinal dorsal horn by modifying pronociceptive pathways. IL-1beta increases SP synthesis by enhancing the expression of preprotachykinin (PPT) mRNA encoding for SP and other tachykinins in the DRG. Stimulation of IL-1 receptor by IL-1beta may induce the phosphorylation of tyrosine residues in many effector proteins through the activation of p60c-src kinase. The hypothesis that the synthesis of SP in and secretion from the primary sensory ganglia are regulated by the activation of p60c-src kinase induced by IL-1beta was tested. Pretreatment of DRG neurons in culture with herbimycin A, genistein or PP2, three structurally different nonreceptor tyrosine kinase inhibitors that act by different mechanisms, decreased the kinase activity of p60c-src induced by the activation of IL-1 receptor. PP3, a negative control for the Src family of tyrosine kinase inhibitor PP2 had no effect. Herbimycin A and genistein also decreased IL-1beta-induced expression of PPT mRNA-encoding transcripts and the levels of SP-li synthesized in the cells and secreted into the culture medium in a concentration-dependent manner. SB 203580 [a p38 mitogen-activated protein kinase (p38 MAPK) inhibitor] and PD 98059 (a p44/42 MAPK kinase inhibitor) were ineffective in modulating IL-1beta-induced SP synthesis and secretion, and p60c-src kinase activity in DRG neurons. Whereas, IL-1 receptor antagonist and cycloheximide inhibited IL-1beta-evoked secretion of SP-like immunoreactivity (SP-li), actinomycin D decreased it significantly but did not entirely abolish it. These findings show that phosphorylation of specific protein tyrosine residue(s) following IL-1 receptor activation might play a key role in IL-1beta signalling to modulate PPT gene expression and SP secretion in sensory neurons. In view of the role of SP as an immunomodulator, these studies provide a new insight into neural-immune intercommunication in pain regulation in the sensory ganglia through the IL-1beta-induced p60c-src activation.

Neurokinin release produced by capsaicin acting on the central terminals and axons of primary afferents: relationship with n-methyl-d-aspartate and gabab receptors

Capsaicin stimulates neurokinin release in the spinal cord when applied both centrally and peripherally. To determine whether these two actions have different mechanisms, we measured neurokinin 1 receptor (NK1R) internalization in rat spinal cord slices elicited by incubating the whole slice or just the dorsal root with capsaicin. NK1R internalization produced by incubating the slices with capsaicin was abolished by the NK1R antagonist RP-67580, by the vanilloid receptor 1 (VR1) antagonist capsazepine, and by eliminating Ca(2+) from the medium, but was not affected by the Na(+) channel blocker lidocaine. Therefore, the internalization was due to neurokinin release mediated by Ca(2+) entry through VR1 receptors, but did not require the firing of action potentials. Incubating the root with capsaicin produced NK1R internalization in the ipsilateral dorsal horn that was abolished when capsazepine or lidocaine was included in, or when Ca(2+) was omitted from, the medium surrounding the root. Therefore, the internalization was mediated by Ca(2+) entry in the axons through VR1, and required firing of action potentials. The efficacy of capsaicin when applied to the root (36+/-3%) was lower than when applied to the slice (91+/-3%), but its potency was the same (0.49 microM and 0.37 microM, respectively). We also investigated whether presynaptic N-methyl-D-aspartate (NMDA) and GABA(B) receptors modulate these two actions of capsaicin. Neither the NMDA receptor blocker MK-801 nor the GABA(B) agonist baclofen decreased NK1R internalization produced by 1 microM capsaicin applied to the slices, but they inhibited the internalization produced by 0.3 microM capsaicin applied to the slices or 1 microM capsaicin applied to the root. Therefore, capsaicin can produce neurokinin release from primary afferents 1) by a direct action on their central terminals and 2) by increasing the firing of action potentials on their axons. The first effect largely bypasses other modulatory mechanism, but the second does not.

Relationship between capsaicin-evoked substance P release and neurokinin 1 receptor internalization in the rat spinal cord

The relationship between substance P release and the activation of its receptor in the spinal cord remains unclear. Substance P release is usually measured by radioimmunoassay, whereas the internalization of the neurokinin 1 (NK1) receptor has been used to assess its activation by noxious stimuli. Our objective was to compare substance P release and NK1 receptor internalization produced by capsaicin in rat spinal cord slices. Superfusion of the slices with capsaicin for 3 min produced a gradual increase in substance P release that peaked 3-7 min afterward, and then decreased to baseline levels. The concentration-response curve for capsaicin was biphasic, with concentrations above 10 microM producing significantly less release. The effective concentration for 50% of response (EC(50)) for capsaicin, calculated from its stimulatory phase, was 2.3 microM. However, the potency of capsaicin to elicit NK1 receptor internalization in the same slices was one order of magnitude higher (EC(50)=0.37 microM) in lamina I, probably because NK1 receptors become saturated at relatively low concentrations of substance P. The potency of capsaicin to produce internalization was progressively lower in lamina III (EC(50)=1.9 microM) and lamina IV (EC(50)=14.5 microM), suggesting that neurokinins released in laminae I-II become diluted as they diffuse to the inner dorsal horn. To study the correlation between these two measures, we plotted substance P release against NK1 receptor internalization and fitted a saturation binding function to the points. The correlation was good for laminae I (R(2)=0.82) and III (R(2)=0.78), but it was poor (R(2)=0.35) for lamina IV because NK1 receptor internalization kept on increasing at high concentrations of capsaicin, whereas substance P release decreased. In conclusion, amounts of substance P able to activate NK1 receptors may fall under the threshold of detection of radioimmunoassay. Conversely, radioimmunoassay often detects levels of substance P release well over those required to saturate NK1 receptors in the superficial dorsal horn, but that may be able to activate these receptors in nearby regions of the spinal cord.

Galanin-immunoreactive cells and their relation to calcitonin gene-related peptide-, substance P- and somatostatin-immunoreactive cells in rat lumbar dorsal root ganglia

We report upon the distribution of galanin-immunoreactive (GAL-IR) cells in the lumbar dorsal root ganglia (DRG) of the rat, and upon the distribution of GAL-IR cells, which also contain calcitonin gene-related peptide (CGRP)-, substance P (SP)- and somatostatin (SOM)-immunoreactivity. Neuropeptide-immunoreactive lumbar DRG cells were 55.8% for CGRP, 12.7% for SP, and 6.5% for GAL in lumbar DRG cells. There was no significant difference between the right and left DRGs (L1-L6) for any neuropeptide-immunoreactive cell (P < 0.01). In terms of size distribution, CGRP-immunoreactive cells were identified below 1500 microm2, and SP-, and GAL-IR cells below 600 microm2. Neuropeptide immunoreactive cells showed various immunoreactivities in the cytoplasm according to each neuropeptide. CGRP and SP immunoreactive cells were colocalized with GAL immunoreactive cells in the serial sections about 83.3 and 60% respectively, but SOM colocalizing with GAL-IR cells were not in evidence. The current results confirm and extend previous results, and show that neuropeptides can coexist in single sensory neurones of the rat DRG. In addition, our results demonstrate that the normal distribution of some neurotransmitters modulating sensory action in Wistar Kyoto rat, make this model more prone to develop neuropathic pain than Sprague-Dawley rat.

Chemical responsiveness and histochemical phenotype of electrophysiologically classified cells of the adult rat dorsal root ganglion

Whole cell patch recordings were obtained from medium diameter (35-45 microm) dorsal root ganglion neurons. Using electrophysiological parameters, we were able to subclassify acutely dissociated dorsal root ganglion cells into three uniform (types 5, 6 and 9) and one mixed class (type 8) of neurons. All subtypes (types 5, 6, 8 and 9) had broad action potentials (7.0+/-0.2, 5.2+/-0.4, 7.3+/-0.5 and 6.0+/-0.4 ms) and exceptionally long afterhyperpolarizations (112+/-9, 178+/-19, 124+/-31 and 204+/-33 ms). Long afterhyperpolarizations have been linked to mechanically insensitive (silent) nociceptors by other laboratories [Djouhri et al., J. Physiol. 513 (1998) 857-872]. Chemosensitivity varied among cell classes. Cell types 5, 8 and 9 were capsaicin sensitive (45+/-13, 87+/-30 and 28+/-13 pA/pF; 5 microM) groups, while the type 6 cell was capsaicin insensitive. All cell types expressed ASIC-like (acid sensing ion channel) amiloride sensitive, proton-activated currents with a threshold of pH 6.8 and a peak near pH 5.0. All medium sized cells were sensitive to ATP (50 microM) and exhibited the 'mixed' form of ATP-gated current [Burgard et al., J. Neurophysiol. 82 (1999) 1590-1598; Grubb and Evans, Eur. J. Neurosci. 11 (1999) 149-154]. Immunohistochemistry performed on individual cells indicated the expression of both P2X(1) and P2X(3) subunits. Electrophysiologically defined classes were histochemically uniform. All types were examined for the presence of substance P (SP), calcitonin gene related peptide (CGRP) and binding of isolectin B4 (IB4). All subtypes expressed CGRP immunoreactivity. Types 5 and 8 co-expressed SP and CGRP immunoreactivity and also bound IB4. Subtypes 6 and 9 were positive for neurofilament m. It is likely that these cells represent major classes of myelinated and unmyelinated peptide expressing nociceptors.

Voltage-dependent Ca2+ currents in rat cardiac dorsal root ganglion neurons

This study presents the kinetic and pharmacological properties of voltage-gated Ca(2+) currents in anatomically defined cardiac dorsal root ganglion (DRG) neurons in rats. The neurons were labelled by prior injection of fluorescent tracer Fast Blue into the pericardial sack. There were three distinct groups of neurons with respect to cell size: small (27% of total; cell capacitance <30 pF), medium (65% of total; capacitance 30-80 pF) and large neurons (8% of total; capacitance >80 pF). The properties of Ca(2+) currents were tested in small and medium-sized neurons. In large neurons currents could not be adequately controlled and were not analysed. Ca(2+) currents did not completely inactivate during 100 ms depolarising voltage steps. The activation thresholds in small (-36.9+/-1.3 mV) and medium (-39.0+/-1.3 mV) size neurons were similar. Current densities were 105.8 pA/pF in small and 97.4 pA/pF in large neurons and also did not differ. The kinetic properties of activation and inactivation did not differ between small and medium-sized cardiac DRG neurons. At membrane potentials between -50 and -60 mV (the expected resting membrane potential in these neurons) 55 to 70% of Ca(2+) currents in small and medium-sized neurons were available for activation. Both, small and medium-sized neurons expressed similar proportions of L (7.5%), N (25%) and P/Q (36%) type Ca(2+) currents. We conclude that small and medium-sized cardiac DRG neurons are homogeneous with respect to the expression and properties of voltage-gated Ca(2+) currents. Voltage-gated Ca(2+) currents probably play an important role in action potential generation in cardiac DRG neurons due to their availability for activation at resting membrane potential, their high density and voltage threshold close to the threshold for voltage-gated Na(+) currents.

Spinal neurons that possess the substance P receptor are required for the development of central sensitization

In previous studies, we have shown that loss of spinal neurons that possess the substance P receptor (SPR) attenuated pain and hyperalgesia produced by capsaicin, inflammation, and nerve injury. To determine the role of SPR-expressing neurons in modulating pain and hyperalgesia, responses of superficial and deep lumbar spinal dorsal horn neurons evoked by mechanical and heat stimuli and by capsaicin were made after ablation of SPR-expressing neurons using the selective cytotoxin conjugate substance P-saporin (SP-SAP). Morphological analysis and electrophysiological recordings were made after intrathecal infusion of vehicle, saporin alone, or SP-SAP. SP-SAP, but not vehicle or SAP alone, produced an approximately 62% decrease in SPR-expressing neurons in the dorsal horn. Loss of SPR-expressing neurons diminished the responses of remaining neurons to intraplantar injection of capsaicin. Peak responses to 10 microg of capsaicin were approximately 65% lower in animals pretreated with SP-SAP compared with controls. Additionally, sensitization to mechanical and heat stimuli that normally follows capsaicin was rarely observed. Importantly, responses to mechanical and heat stimuli in the absence of capsaicin were not altered after SP-SAP treatment. In addition, nociceptive neurons did not exhibit windup in the SP-SAP-treated group. These results demonstrate that SPR-expressing neurons located in the dorsal horn are a pivotal component of the spinal circuits involved in triggering central sensitization and hyperalgesia. It appears that this relatively small population of neurons can regulate the physiological properties of other nociceptive neurons and drive central sensitization.

Expression and kinetic properties of Na(+) currents in rat cardiac dorsal root ganglion neurons

The expression and properties of voltage-gated Na(+) currents in cardiac dorsal root ganglion (DRG) neurons were assessed in this study. Cardiac DRG neurons were labelled by injecting the Fast Blue fluorescent tracer into the pericardium. Recordings were performed from 138 cells. Voltage-dependent Na(+) currents were found in 115 neurons. There were 109 neurons in which both tetrodotoxin-sensitive (TTX-S, blocked by 1 microM of TTX) and tetrodotoxin-resistant (TTX-R, insensitive to 1 microM of TTX) Na(+) currents were present. Five cells expressed TTX-R current only and one cell only the TTX-S current. The kinetic properties of Na(+) currents and action potential waveform parameters were measured in neurons with cell membrane capacitance ranging from 15 to 75 pF. The densities of TTX-R (110.0 pA/pF) and TTX-S (126.1 pA/pF) currents were not significantly different. Current threshold was significantly higher for TTX-R (-34 mV) than for TTX-S (-40.4 mV) currents. V(1/2) of activation for TTX-S current (-19.6 mV) was significantly more negative than for TTX-R current (-9.2 mV), but k factors did not differ significantly. V(1/2) and the k constant for inactivation for TTX-S currents were -35.6 and -5.7 mV, respectively. These values were significantly lower than those recorded for TTX-R current for which V(1/2) and k were -62.3 and -7.7 mV, respectively. The action potential threshold was lower, the 10-90% rise time and potential width were shorter before than after the application of TTX. Based on this we drew the conclusion that action potential recorded before adding tetrodotoxin was mainly TTX-S current dependent, while the action potential recorded after the application of toxin was TTX-R current dependent. We also found 23 cells with mean membrane capacitance ranging from 12 to 35 pF (the smallest labelled DRG cells found in this study) that did not express the Na(+) current. The function of these cells is unclear. We conclude that the overwhelming majority of cardiac dorsal root ganglion neurons in which voltage-dependent Na(+) currents were present, exhibited both TTX-S and TTX-R Na(+) currents with remarkably similar expression and kinetic properties.

Sensory neuron addition in juvenile rat: time course and specificity

The time course and specificity of neuron addition to lumbar dorsal root ganglia (DRGs) L(4)-L(6) of rats was investigated. By using methods validated by three-dimensional reconstructions, profile counts in paraffin sections of nucleoli within a nucleus were 36% greater in 100-day-old (P100) rats than in 1-day-old (P1) rats. Adult values were reached by P50. Added neurons fell disproportionately into the population of neurons whose size was below that of the mean size within the ganglion. The biochemical characteristics of small neurons were used to determine whether added neurons fall into particular subpopulations. In DRGs, L(3) and L(4), the number of neurons immunoreactive to substance P (SP) or calcitonin-gene-related peptide (CGRP) or that bound the lectin isolectin B4 (IB4) was determined. Between P5 and P100, the number of SP-stained neurons increased by 2,280 (40% increase), CGRP-stained neurons increased by 6,080 (70% increase), and IB4-stained neurons increased by 6,900 (90% increase). The increase in the number of neurons stained for CGRP or IB4 was more than twice the number of neurons found to be added to these ganglia, indicating that coexpression of these markers as well as neuron number may be developmentally regulated during postnatal life.

Activity-Dependent Changes in Rat Ventral Horn Neurons in vitro; Summation of Prolonged Afferent Evoked Postsynaptic Depolarizations Produce a d-2-Amino-5-Phosphonovaleric Acid Sensitive Windup

The synaptic responses of lumbar ventral horn neurons including identified flexor motoneurons, to graded stimulation of peripheral nerves have been recorded in vitro in the young rat spinal cord-hindlimb preparation. Single shock stimulation of low threshold myelinated afferents evoked short latency (< 20 ms) short duration (< 1.0 s, 391 +/- 42 ms n=43 SEM) compositive mono- and polysynaptic potentials. Recruitment of both thinly myelinated (A delta) and unmyelinated (C) afferent fibres elicited a prolonged postsynaptic depolarization (> 1 s) in all cells. In the majority of cells (67.4%), this depolarization exceeded 4.0 s in duration (8.01 +/- 0.4 s, n=26, maximum 14 s). In the remainder, shorter responses were evoked (< 3.0 s, mean=1.74 +/- 0.4 s, n=18). In those cells where the postsynaptic response to a single A delta or C fibre strength stimulus exceeded 4 s, low frequency (0.5 - 1.0 Hz) repetitive stimulation resulted in a temporal summation of the postsynaptic depolarizations, which generated a cumulatively increasing depolarization. This incrementing depolarization was sufficient in 33% of the cells to produce a progressive increase in spike discharge (windup). On cessation of the train of stimuli the depolarization decayed slowly (65 +/- 27 s). The N-methyl d-aspartic acid (NMDA) receptor antagonist d-2-amino-5-phosphonovaleric acid (d-APV) reduced the duration and amplitude of the prolonged postsynaptic depolarizations elicited by a single shock stimulation of small diameter afferents by 57% and 50% respectively. A smaller effect was produced on the low threshold afferent evoked early excitatory postsynaptic potentials (EPSP) (3% decrease in amplitude and 24% decrease in duration). In the presence of d-APV the cumulatively incrementing depolarization produced by repetitive stimulation was substantially reduced and windup failed to occur. Activity-dependent amplifications of primary afferent evoked responses in spinal neurons therefore involves a temporal summation of d-APV sensitive prolonged postsynaptic depolarizations.

Neonatal Anti-NGF Treatment Reduces the Adelta- and C-Fibre Evoked Vasodilator Responses in Rat Skin: Evidence That Nociceptor Afferents Mediate Antidromic Vasodilatation

Electrical stimulation of cutaneous nerve Adelta-fibres can lead to increases in skin blood flow (Jänig and Lisney, J. Physiol. (Lond.), 415, 477 - 486, 1989). Here we have examined this phenomenon in adult rats treated neonatally with antisera to nerve growth factor between postnatal days 2 and 14. This treatment forces many Adelta nociceptor afferents to take on the phenotype of low-threshold D-hair afferents (Lewin et al., J. Neurosci., 12, 1896 - 1905, 1992). In animals treated this way we found a parallel decrease in the ability of Adelta-fibres to increase skin blood flow. The increase in blood flow evoked by C-fibre stimulation was also reduced, but no change was seen in the ability of C-fibres to elicit neurogenic extravasation in skin. These data may be taken as evidence that in rats, amongst the cutaneous Adelta-fibres, nociceptor but not D-hair afferents are capable of producing blood flow changes in the skin.

Regulation of nociceptive neurons by nerve growth factor and glial cell line derived neurotrophic factor

Nociceptive dorsal root ganglion (DRG) cells can be divided into three main populations, namely (1) small diameter non-peptide-expressing cells, (2) small-diameter peptide-expressing (calcitonin gene related peptide (CGRP), substance P) cells, and (3) medium-diameter peptide-expressing (CGRP) cells. The properties of these cell populations will be reviewed, with a special emphasis on the expression of the vanilloid (capsaicin) receptor VR1 and its regulation by growth factors. Cells in populations 1 and 2 express VR1, a nonselective channel that transduces certain nociceptive stimuli and that is crucial to the functioning of polymodal nociceptors. Cells in population 1 can be regulated by glial cell line derived neurotrophic factor (GDNF) and those in populations 2 and 3 by nerve growth factor (NGF). In vivo, DRG cells express a range of levels of VR1 expression and VR1 is downregulated after axotomy. However, treatment with NGF or GDNF can prevent this downregulation. In vitro, DRG cells also show a range of VR1 expression levels that is NGF and (or) GDNF dependent. Functional studies indicate that freshly dissociated cells also show differences in sensitivity to capsaicin. The significance of this is not known but may indicate a difference in the physiological role of cells in populations 1 and 2.

Substance P and calcitonin gene-related peptide immunoreactive sensory DRG neurons innervating the lumbar intervertebral discs in rats

The rat L5/6 disc is innervated from T13 to L6 dorsal root ganglia (DRGs) multisegmentally. Sensory fibers from T13, L1 and L2 DRGs have been reported to innervate through the paravertebral sympathetic trunks, whereas those from L3 to L6 DRGs innervate directly through sinuvertebral nerves on the posterior longitudinal ligament (PLL). The presence of substance P (SP)- and calcitonin gene-related peptide (CGRP)-immunoreactive (ir) nerve fibers has been demonstrated in the lumbar intervertebral discs, but their percentages in DRG neurons have not been studied. Fluoro-gold (F-G) labeled neurons innervating the L5/6 disc were distributed throughout DRGs from T13 to L6 levels. Of F-G labeled neurons innervating the L5/6 disc, the percentage of SP-ir T13 to L6 DRG neurons was 30%, and that of CGRP-ir neurons was 47%. The mean cross-sectional area of the cell of SP-ir neurons was 696+/-66 microm2 (mean +/- S. E.), and that of CGRP-ir neurons was 695+/-72 microm2 (mean +/- S. E.). SP- and CGRP-ir were mainly observed in small neurons. The percentages of SP- or CGRP-ir neurons in L1 and L2 DRGs innervating the L5/6 disc were not different from those in L3, L4 or L5 DRGs. In the physiological condition in rats, DRG neurons at all levels may have the same significant role in pain sensation of the disc.

Postnatal changes in membrane properties of mice trigeminal ganglion neurons

Intracellular recordings from neurons in the mouse trigeminal ganglion (TG) in vitro were used to characterize changes in membrane properties that take place from early postnatal stages (P0-P7) to adulthood (>P21). All neonatal TG neurons had uniformly slow conduction velocities, whereas adult neurons could be separated according to their conduction velocity into Adelta and C neurons. Based on the presence or absence of a marked inflection or hump in the repolarization phase of the action potential (AP), neonatal neurons were divided into S- (slow) and F-type (fast) neurons. Their passive and subthreshold properties (resting membrane potential, input resistance, membrane capacitance, and inward rectification) were nearly identical, but they showed marked differences in AP amplitude, AP overshoot, AP duration, rate of AP depolarization, rate of AP repolarization, and afterhyperpolarization (AHP) duration. Adult TG neurons also segregated into S- and F-type groups. Differences in their mean AP amplitude, AP overshoot, AP duration, rate of AP depolarization, rate of AP repolarization, and AHP duration were also prominent. In addition, axons of 90% of F-type neurons and 60% of S-type neurons became faster conducting in their central and peripheral branch, suggestive of axonal myelination. The proportion of S- and F-type neurons did not vary during postnatal development, suggesting that these phenotypes were established early in development. Membrane properties of both types of TG neurons evolved differently during postnatal development. The nature of many of these changes was linked to the process of myelination. Thus myelination was accompanied by a decrease in AP duration, input resistance (R(in)), and increase in membrane capacitance (C). These properties remained constant in unmyelinated neurons (both F- and S-type). In adult TG, all F-type neurons with inward rectification were also fast-conducting Adelta, suggesting that those F-type neurons showing inward rectification at birth will evolve to F-type Adelta neurons with age. The percentage of F-type neurons showing inward rectification also increased with age. Both F- and S-type neurons displayed changes in the sensitivity of the AP to reductions in extracellular Ca(2+) or substitution with Co(2+) during the process of maturation.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

The DRASIC cation channel contributes to the detection of cutaneous touch and acid stimuli in mice

Cation channels in the DEG/ENaC family are proposed to detect cutaneous stimuli in mammals. We localized one such channel, DRASIC, in several different specialized sensory nerve endings of skin, suggesting it might participate in mechanosensation and/or acid-evoked nociception. Disrupting the mouse DRASIC gene altered sensory transduction in specific and distinct ways. Loss of DRASIC increased the sensitivity of mechanoreceptors detecting light touch, but it reduced the sensitivity of a mechanoreceptor responding to noxious pinch and decreased the response of acid- and noxious heat-sensitive nociceptors. The data suggest that DRASIC subunits participate in heteromultimeric channel complexes in sensory neurons. Moreover, in different cellular contexts, DRASIC may respond to mechanical stimuli or to low pH to mediate normal touch and pain sensation.

Vanilloid receptor homologue, VRL1, is expressed by both A- and C-fiber sensory neurons

The capsaicin receptor (VR1) homologue, VRL1, is thought to be responsible for transducing high-threshold heat responses in Adelta-fiber neurons. In the present study, the expression of VRL1 by A- or C-fiber sensory neurons in rats was investigated by using a VRL1 and 200 kDa neurofilament (NF200, an A-fiber marker) double immunohistochemical staining method. Approximately 46% of VRL-positive neurons were NF200 positive. Though double-labeled neurons tended to be medium to large, many VRL1 single-labeled neurons were large. Dense VRL1 immunoreactivity was also found in laminae I and II of the spinal dorsal horn, where nociceptive Adelta- and C-fibers normally terminate. These results suggest that both C-fiber and Adelta-fiber primary sensory neurons express VRL1, and VRL1 may play an important role in nociception.

Somatic noxious mechanical stimulation induces Fos expression in the postsynaptic dorsal column neurons in laminae III and IV of the rat spinal dorsal horn

This study was conducted to ascertain the possible expression of Fos-like immunoreactivity (Fos-LI) in the postsynaptic dorsal column (PSDC) neurons in response to noxious mechanical stimulation of the forepaw glabrous area of normal rats. For this purpose, Fos immunohistochemistry along with Fluoro-Gold (FG) retrograde tracing was utilized. After repeated noxious pinching of the forepaw glabrous area, there was a marked increase in number of Fos-LI neurons in the dorsal horn, including Rexed's laminae III and IV, at C5-T1 spinal cord segments ipsilateral to the stimulation. Between segments C5 and T1, about 40% of the Fos-LI neurons in laminae III and IV were distributed at segment C7. In the rats subjected to the noxious pinch coupled with FG injection into the right cuneate nucleus, PSDC neurons double labeled with Fos and FG were localized in the ipsilateral laminae III and IV extending from segment C5 to T1, with about 70% of them distributed at segments C6 and C7. At segment C6 or C7, double-labeled neurons made up about 10% of the PSDC neurons that projected their axons to the cuneate nucleus. Most of the double-labeled neurons appeared fusiform with their primary dendrites projected dorso-ventrally. The present results suggest that the morphologically distinct, subclasses of PSDC neurons in spinal laminae III and IV may contribute to the central transmission of mechanical nociceptive information through the dorsal column into the cuneate nucleus.

Inflammatory mediators potentiate ATP-gated channels through the P2X(3) subunit

The P2X(3) receptor is an ATP-gated ion channel predominantly expressed in nociceptive neurons from the dorsal root ganglion. P2X(3) receptor channels are highly expressed in sensory neurons and probably contribute to the sensation of pain. Kinetics of P2X(3) currents are characterized by rapid desensitization (<100 ms) and slow recovery (>20 s). Thus, any mechanism modulating rate of desensitization and/or recovery may have profound effect on susceptibility of nociceptive neurons expressing P2X(3) to ATP. Here we show that currents mediated by P2X(3) receptor channels and the heteromeric channel P2X(2/3) composed of P2X(2) and P2X(3) subunits are potentiated by the neuropeptides substance P and bradykinin, which are known to modulate pain perception. The effect is mediated by the respective neuropeptide receptors, can be mimicked by phorbol ester and blocked by inhibitors of protein kinases. Together with data from site-directed mutagenesis our results suggest that inflammatory mediators sensitize nociceptors through phosphorylation of P2X(3) and P2X(2/3) ion channels or associated proteins.

Equal proportions of small and large DRG neurons express opioid receptor mRNAs

Previous studies have reported that the mRNAs encoding the cloned mu-opioid receptor (MOR1) and the cloned delta-opioid receptor (DOR1) are expressed in the dorsal root ganglia (DRG) of rats. In the present study, we determined the sizes of DRG neurons expressing DOR1 and MOR1 mRNAs and examined whether or not DRG neurons were likely to be the source of the DOR1 and MOR1 immunoreactivity previously observed in the spinal dorsal horn. DRG neurons were labeled in five male Sprague-Dawley rats by applying Fluoro-Gold (FG) topically to the dorsal root entry zone. Five-micrometer cryostat sections were cut, and in situ hybridization was performed using full-length cRNA probes labeled with 35S-UTP. The distribution of sizes of DRG neuronal profiles (1372 neuronal profiles were evaluated) ranged from 98 to 2081 microm(2) and was similar to those found in previous reports. Of 583 retrogradely labeled neuronal profiles in DRGs, 246 (40 +/- 14%, mean +/- SD, n = 5) expressed MOR1 mRNA. Of 789 DRG cell profiles from sections that were hybridized for DOR1 mRNA, 687 (85 +/- 18%, mean +/- SD, n = 5) were labeled for DOR1. The proportion of DRG cell profiles expressing DOR1 mRNA was significantly higher than that expressing MOR1 mRNA (P < 0.0001, chi-square test). No significant differences were observed between small (less than or = 700 microm(2)) and large (> 700 microm(2)) FG-labeled neurons in the proportions labeled for either MOR1 mRNA (202/497 vs. 44/86, P > 0.2, chi-square test) or DOR1 mRNA (555/651 vs. 132/138, P > 0.3, chi-square test). Most FG-labeled neurons that expressed either MOR1 mRNA or DOR1 mRNA (82.1 and 80.8%, respectively) were smaller than 700 microm(2). In addition to cells expressing a single opioid receptor, individual DRG neurons were observed that expressed both MOR1 and DOR1. In a sample of 25 DRG neurons expressing MOR1-mRNA, 23 also expressed DOR1 mRNA. Within the spinal cord itself, DOR1 and MOR1 mRNAs had different patterns of expression. Both were expressed in the dorsal horn, but of the two, only MOR1 message was expressed in the superficial dorsal horn. We conclude that both small and large DRG neurons express DOR1 and MOR1 mRNAs, but most cells expressing these mRNAs are small. In addition, some DRG neurons express both MOR1 and DOR1 mRNAs. Finally, both DOR1 and MOR1 in the spinal dorsal horn originate, at least in part, from DRG neurons.

Distribution of P2X1, P2X2, and P2X3 receptor subunits in rat primary afferents: relation to population markers and specific cell types

We determined the co-expression of immunoreactivity (IR) for ATP-receptor subunits (P2X1, P2X2, and P2X3), neuropeptides, neurofilament (NF), and binding of the isolectin B(4) from Griffonia simplicifolia type one (GS-I-B(4)) in adult dorsal root ganglion neurons. P2X1-IR was expressed primarily in small DRG neurons. Most P2X1-IR neurons expressed neuropeptides and/or GS-I-B(4)-binding, but lacked NF-IR. P2X1-IR overlapped with P2X3-IR, though each was also found alone. P2X2-IR was expressed in many P2X3-IR small neurons, as well as a group of medium to large neurons that lacked either P2X3-IR or GS-I-B(4)-binding. A novel visible four-channel fluorescence technique revealed a unique population of P2X2/3-IR neurons that lacked GS-I-B(4)-binding but expressed NF-IR. Co-expression of P2X1, and P2X3 in individual neurons was also demonstrated. We examined P2X subunit-IR on individual recorded neurons that had been classified by current signature in vitro. Types 1, 2, 4 5, and 7 expressed distinct patterns of P2X-IR that corresponded to patterns identified in DRG sections, and had distinct responses to ATP. Types with rapid ATP currents (types 2, 5, and 7) displayed P2X3-IR and/or P2X1-IR. Types with slow ATP currents (types 1 and 4) displayed P2X2/3-IR. Type 1 neurons also displayed P2X1-IR. This study demonstrates that the correlation between physiological responses to ATP and the expression of particular P2X receptor subunits derived from expression systems is also present in native neurons, and also suggests that novel functional subunit combinations likely exist.

Nerve growth factor-induced phenotypic switch in guinea pig airway sensory neurons

Immunohistochemistry was combined with retrograde tracing techniques to characterize the effect of nerve growth factor (NGF) on substance P (SP) producing vagal neurons innervating the guinea pig trachea. Fast blue dye instilled into the trachea retrogradely labeled nerve cell bodies located in the nodose and jugular ganglia. In untreated guinea pigs > 99% of the SP-containing neurons labeled with fast blue were located in the jugular ganglia. The SP-positive neurons were small in diameter (23 +/- 1 microm) and were negative for neurofilament immunoreactivity. The fast-blue-positive neurons in the nodose ganglia, by contrast, were large in diameter (40 +/- 3 microm) and were negative for SP immunoreactivity and positive for neurofilament immunoreactivity. After NGF-beta injections into the tracheal wall, approximately 10% of the large-diameter nodose neurofilament-positive neurons projecting fibers to the trachea became SP-positive (p < 0.05). We previously demonstrated that nodose nerve endings supplying the trachea are exquisitely mechanically sensitive, but capsaicin- and bradykinin-insensitive. These results suggest that NGF not only increases SP expression in airway neurons, but changes the neuronal phenotype such that large, capsaicin-insensitive nodose neurons with fast-conducting "Adelta" fibers provide a component of the tachykinergic innervation.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Nerve injury-induced mechanical but not thermal hyperalgesia is attenuated in neurokinin-1 receptor knockout mice

Mice lacking the gene encoding for substance P and neurokinin A, or the NK-1 receptor, exhibit alterations in behavior to various acute nociceptive stimuli. However, behavioral responses of NK-1 mutant animals have not been well characterized in models of chronic pain. We studied the behavioral responses of NK-1 knockout and wild-type control mice to thermal and mechanical stimuli before and after inducing chronic neuropathic pain by unilateral ligation of the L5 spinal nerve. Mechanical hyperalgesia was evaluated by determining the frequency of withdrawal to von Frey monofilaments applied to the hind paws. Nerve injury-induced hyperalgesia to thermal stimuli was examined by determining responses to radiant heat and cooling stimuli. The contribution of the sympathetic nervous system to mechanical hyperalgesia was evaluated by administering 3 mg/kg phentolamine, an alpha-adrenergic antagonist, subcutaneously. Following spinal nerve injury, withdrawal frequencies to mechanical stimulation increased in wild-type mice within 1 day and persisted during the 9-week observation period, whereas in the knockout mice, withdrawal frequencies did not increase significantly. In contrast, withdrawal latencies to radiant heat decreased up to 2 weeks after nerve injury in both the NK-1 and the wild-type mice. Similarly, the increase in withdrawal frequency to the cooling stimuli following the nerve injury was not different in the NK-1 knockout and wild-type mice. Mechanical hyperalgesia in the wild-type mice was not reversed by systemic administration of phentolamine, suggesting that the pain is not sympathetically maintained. The results indicate that NK-1 receptors contribute to the development of mechanical, but not thermal, hyperalgesia in neuropathic pain.

Capsaicin-sensitive afferents are involved in signalling transneuronal effects between cutaneous sensory nerves

The aim of the present study was to investigate changes in contralateral nerves associated with peripheral nerve injuries. Transection and subsequent regeneration of the saphenous nerve on one side caused a suppression of the ability of the contralateral saphenous nerve to produce a neurogenic plasma extravasation response. This effect was transient, and was first evident two weeks after injury, reaching its maximum at four weeks, but was no longer detectable at eight weeks. This change was paralleled by a decrease in the content of substance P, a neuropeptide involved in neurogenic plasma extravasation, in the contralateral nerve. The neurotoxin capsaicin was used to deplete the nerve of a subclass of C-fibres, namely the polymodal nociceptor afferents. Pretreatment of the nerve to be lesioned with capsaicin was sufficient to significantly attenuate the changes in the plasma extravasation response and substance P content observed on the contralateral side. The effectiveness of the capsaicin treatment was confirmed by histological examination. These results strongly suggest that changes observed at a site distant from the location of the nerve injury are dependent on the integrity of capsaicin-sensitive C-fibre afferents within the injured nerve. Furthermore, given that the contralateral nerve has commonly been used as the control for an injury conducted on the homologous nerve or muscle on the opposite side of the body, the underlying assumption being that the contralateral nerve remained unchanged, the present findings emphasize the need for separate groups of control animals which have undergone no surgical procedures.

Abnormal substance P release from the spinal cord following injury to primary sensory neurons

The neuropeptide substance P (SP) modulates nociceptive transmission within the spinal cord. Normally, SP is uniquely contained in a subpopulation of small-calibre axons (Adelta- and C-fibres) within primary afferent nerve. However, it has been shown that after nerve transection, besides being downregulated in small axons, SP is expressed de novo in large myelinated Abeta-fibres. In this study we investigated whether, following peripheral nerve injury, SP was released de novo from the spinal cord after selective activation of Abeta-fibres. Spinal cords with dorsal roots attached were isolated in vitro from rats 2 weeks following distal sciatic axotomy or proximal spinal nerve lesion (SNL). The ipsilateral dorsal roots were electrically stimulated for two consecutive periods at low- or high-threshold fibre strength, spinal cord superfusates were collected and SP content was determined by radioimmunoassay. SNL, but not axotomized or control rat cords, released significant amounts of SP after selective activation of Abeta-fibres. Not only do these data support the idea that Abeta myelinated fibres contribute to neuropathic pain by releasing SP, they also illustrate the importance of the proximity of the lesion to the cell body.

Chronic pain is associated with increased TrkA immunoreactivity in spinoreticular neurons

Repetitive noxious stimulation leads to permanent adaptive changes of central pathways involved in the genesis and integration of nociception. Several classes of neurotrophic factors that affect brain plasticity are also involved in the regulation of sensory functions in adulthood. To investigate a putative role of nerve growth factor (NGF) in central plasticity linked to chronic pain, modifications in immunoreactivity (IR) for the high-affinity NGF receptor, TrkA, were studied at spinal levels in a rat model of inflammatory chronic pain, adjuvant-induced arthritis (AIA). We report a specific increase in the number of TrkA-IR profiles in laminae V-VI at lumbar levels L3 and L4 in arthritic rats. Tract tracing using FluoroGold injections in the ventrobasal complex of the thalamus and in the brainstem showed that these increased TrkA-IR profiles are spinoreticular neurons. Dual labeling with calcitonin gene-related peptide or substance P showed that TrkA-IR neurons were mainly located in projection fields of small- to medium-sized primary afferent fibers, which convey nociceptive inputs. These results suggest that TrkA-containing neurons of the spinal dorsal horn participate in the first central relay of transmission of nociceptive information to supraspinal centers. Enhanced numbers of TrkA-IR neurons during AIA strongly support the hypothesis of a participation of NGF in adaptive mechanisms of central nociceptive pathways observed in chronic pain states.

Primary afferent fibers that contribute to increased substance P receptor internalization in the spinal cord after injury

Upon noxious stimulation, substance P (SP) is released from primary afferent fibers into the spinal cord where it interacts with the SP receptor (SPR). The SPR is located throughout the dorsal horn and undergoes endocytosis after agonist binding, which provides a spatial image of SPR-containing neurons that undergo agonist interaction. Under normal conditions, SPR internalization occurs only in SPR+ cell bodies and dendrites in the superficial dorsal horn after noxious stimulation. After nerve transection and inflammation, SPR immunoreactivity increases, and both noxious as well as nonnoxious stimulation produces SPR internalization in the superficial and deep dorsal horn. We investigated the primary afferent fibers that contribute to enhanced SPR internalization in the spinal cord after nerve transection and inflammation. Internalization evoked by electrical stimulation of the sciatic nerve was examined in untreated animals, at 14 days after sciatic nerve transection or sham surgery and at 3 days after hindpaw inflammation. Electrical stimulation was delivered at intensities to excite Abeta fibers only, Abeta and Adelta fibers or A and C fibers as determined by the compound action potential recorded from the tibial nerve. Electrical stimuli were delivered at a constant rate of 10 Hz for a duration of 5 min. Transection of the sciatic nerve and inflammation produced a 33.7 and 32.5% increase in SPR and immunoreactivity in lamina I, respectively. Under normal conditions, stimulation of Adelta or C fibers evoked internalization that was confined to the superficial dorsal horn. After transection or inflammation, there was a 20-24% increase in the proportion of SPR+ lamina I neurons that exhibited internalization evoked by stimulation of Adelta fibers. The proportion of lamina I SPR+ neurons that exhibited internalization after stimulation of C-fibers was not altered by transection or inflammation because this was nearly maximal under normal conditions. Moreover, electrical stimulation sufficient to excite C fibers evoked SPR internalization in 22% of SPR+ lamina III neurons after nerve transection and in 32-36% of SPR+ neurons in lamina III and IV after inflammation. Stimulation of Abeta fibers alone never evoked internalization in the superficial or deep dorsal horn. These results indicate that activation of small-caliber afferent fibers contributes to the enhanced SPR internalization in the spinal cord after nerve transection and inflammation and suggest that recruitment of neurons that possess the SPR contributes to hyperalgesia.

Localization of central cannabinoid CB1 receptor messenger RNA in neuronal subpopulations of rat dorsal root ganglia: a double-label in situ hybridization study

In situ hybridization histochemistry was used to show the distribution of messenger RNA for central cannabinoid CB 1 receptors in dorsal root ganglia of the rat. CB1 messenger RNA was highly expressed in neuronal subpopulations of rat dorsal root ganglia. The phenotypes of neurons that express messenger RNA for CB1 were subsequently examined by combining a 35S-labeled ribonucleotide probe for CB1 messenger RNA with digoxigenin-labeled riboprobes for preprotachykinin A (substance P precursor), alpha-calcitonin gene-related peptide and preprosomatostatin (somatostatin precursor) messenger RNAs. Qualitative examination revealed expression of CBI messenger RNA predominantly in medium-and large-sized cells distributed throughout the dorsal root ganglia. The majority of neurons expressing substance P messenger RNA were CB1 messenger RNA negative and smaller in size than the CB1 messenger RNA-positive cells. Only 13% of substance P messenger RNA-positive cells expressed CB1 messenger RNA. A similar degree of co-localization was observed with alpha-calcitonin gene-related peptide: 10% of cells expressing messenger RNA for this neuropeptide were CB1 messenger RNA positive. Co-localization of CB1 and somatostatin messenger RNAs was observed in less than 0.5% of somatostatin messenger RNA-positive cells. The data suggest that subpopulations of neurons in rat dorsal root ganglia are capable of synthesizing cannabinoid receptors and inserting them on terminals in the superficial dorsal horn. These findings provide anatomical evidence for cannabinoid modulation of primary afferent transmission. Although an anatomical basis for cannabinoid-mediated suppression of release of neurogenic peptides from nociceptive primary afferents is provided, our results demonstrate that the majority of CB messenger RNA-positive neurons in the dorsal root ganglia contain transmitters and/or neuromodulators other than the neuropeptides examined herein.

Substance P receptor (neurokinin-1)-expressing neurons in lamina I of the spinal cord encode for the intensity of noxious stimulation: a c-Fos study in rat

The substance P receptor neurokinin-1 is expressed by a subset of neurons in the rat spinal cord. We have combined immunostaining for Fos, a marker of noxious peripheral stimulation, and neurokinin-1 to examine whether nociceptive signals from particular peripheral tissues (skin, muscle or knee joint) or activity generated by nerve injury or formalin-induced inflammation are preferentially modulated by substance P. Our results indicate that superficial and deep spinal neurokinin-1-positive neurons process nociceptive information in markedly different ways. In lamina I, the number of double-labelled neurons was positively correlated with the intensity of the stimulus (defined by the total Fos count) and was not directly related to any particular peripheral target. However, in the deeper layers of the spinal cord (V-X), there was no such correlation, and stimulation of joint nociceptors and formalin-induced inflammation produced the greatest proportion of Fos/neurokinin-1 co-localization, suggesting a particular role for substance P in the mediation of joint pain and inflammatory hyperalgesia. Thus, lamina I neurokinin-1 receptor-bearing neurons appear to be involved in intensity discriminative aspects of pain, whereas the deep neurokinin-1 cells are involved in spatial localization or the detection of particular nociceptive submodalities.

Origin of sensory and autonomic innervation of the rat temporomandibular joint: a retrograde axonal tracing study with the fluorescent dye fast blue

Previous studies that have used retrograde axonal tracers (horseradish peroxidase alone or conjugated with wheat germ agglutinin) have shown that the temporomandibular joint (TMJ) is supplied with nerve fibers originating mainly from the trigeminal ganglion, in addition to other sensory and sympathetic ganglia. The existence of nerve fibers in the TMJ originating from the trigeminal mesencephalic nucleus is unclear, and the possible innervation by parasympathetic nerve fibers has not been determined. In the present work, the retrograde axonal tracer, fast blue, was used to elucidate these questions and re-evaluated the literature data. The tracer was deposited in the supradiscal articular space of the rat TMJ, and an extensive morphometric analysis was performed of the labeled perikaryal profiles located in sensory and autonomic ganglia. This methodology permitted us to observe labeled small perikaryal profiles in the trigeminal ganglion, clustered mainly in the posterior-lateral region of the dorsal, medial, and ventral thirds of horizontal sections, with some located in the anterior-lateral region of the ventral third. Sensory perikarya were also labeled in the dorsal root ganglia from C2 to C5. No labeled perikaryal profiles were found in the trigeminal mesencephalic nucleus. On the other hand, autonomic labeled perikaryal profiles were distributed in the sympathetic superior cervical and stellate ganglia, and parasympathetic otic ganglion. Our results confirmed those of previous studies and also demonstrated that: (i) there is a distribution pattern of labeled perikaryal profiles in the trigeminal ganglion; (ii) some perikaryal profiles located in the otic ganglion were labeled; and (iii) the trigeminal mesencephalic nucleus did not show any retrogradely labeled perikaryal profiles.

Labeling of calcitonin gene-related peptide and substance P increases in subnucleus caudalis of rabbit during maxillary sinusitis

Calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) and substance P-like immunoreactivity (SP-LI) were evaluated in subnucleus caudalis following induction of sinusitis. Three days after induction, intensity of labeling for CGRP-LI and SP-LI increased in ipsilateral subnucleus caudalis. Labeling for CGRP-LI and SP-LI appeared normal at later time points (20 and 28 days). Early changes in these neuropeptides may contribute to the inflammatory process and painful symptoms accompanying sinusitis.

Altered nociception, analgesia and aggression in mice lacking the receptor for substance P

The peptide neurotransmitter substance P modulates sensitivity to pain by activating the neurokinin-1 (NK-1) receptor, which is expressed by discrete populations of neurons throughout the central nervous system. Substance P is synthesized by small-diameter sensory 'pain' fibres, and release of the peptide into the dorsal horn of the spinal cord following intense peripheral stimulation promotes central hyperexcitability and increased sensitivity to pain. However, despite the availability of specific NK-1 antagonists, the function of substance P in the perception of pain remains unclear. Here we investigate the effect of disrupting the gene encoding the NK-1 receptor in mice. We found that the mutant mice were healthy and fertile, but the characteristic amplification ('wind up') and intensity coding of nociceptive reflexes was absent. Although substance P did not mediate the signalling of acute pain or hyperalgesia, it was essential for the full development of stress-induced analgesia and for an aggressive response to territorial challenge, demonstrating that the peptide plays an unexpected role in the adaptive response to stress.

Hypoalgesia in mice with a targeted deletion of the tachykinin 1 gene

The tachykinin neuropeptides, substance P and substance K, are produced in nociceptive primary sensory neurons and in many brain regions involved in pain signaling. However, the precise role and importance of these neuropeptides in pain responses has been debated. We now show that mice that cannot produce these peptides display no significant pain responses following formalin injection and have an increased pain threshold in the hotplate test. On the other hand, the mutant mice react normally in the tail flick assay and acetic acid-induced writhing tests. These results demonstrate that substance P and/or substance K have essential functions in specific responses to pain.

Substance P regulates Ih via a NK-1 receptor in vagal sensory neurons of the ferret

Substance P (SP) hyperpolarizes approximately 80% of ferret vagal sensory neurons (nodose ganglion neurons) via NK-1 receptor-mediated activation of a potassium current (IK). A depolarizing current activated by membrane hyperpolarization could minimize the SP-induced hyperpolarization. Such a current exists in 65% of the nodose neurons (n = 264). In this study, we examine this current and how it can interact with SP-induced membrane hyperpolarizations. This slowly developing, noninactivating inward current, designated Ih, was activated maximally at about -120 mV and had a reversal potential value of -23 +/- 4.4 mV (n = 4). The time course of activation followed voltage-dependent, monoexponential kinetics. Steady-state activation curves derived from tail current analysis were well fit by a Boltzmann equation yielding a half-activation potential (V1/2) of-77 +/- 1.5 mV and a ks value of 18 +/- 0.5 (n = 8). In the presence of 1 mM cesium, the current was completely abolished. These parameters are consistent with those derived for Ih in other neurons. Substance P (200 nM) reduced the magnitude of Ih elicited by membrane hyperpolarizations to about -110 mV but did not affect the magnitude of Ih elicited by hyperpolarizations to more negative potentials. Tail current analysis revealed that this effect was the result of a SP-induced shift of the Ih activation curve to more negative membrane potentials. The V1/2 value for Ih was shifted by -20 +/- 1.4 mV in the presence of SP with no change in ks (18 +/- 0.7; n = 5). The SP effect on Ih, like its effect on IK, was blocked reversibly by 10 nM CP99,994, a NK-1 antagonist, and was mimicked by the NK-1 agonist Ac-[Arg6, Sar9, Met(O2)11]SP(6-11) (ASMSP; 200 nM). Ih was not affected by NK-2 or NK-3 selective agonists (n = 4 for each) nor was the effect of SP on Ih reduced by an NK-2 antagonist (n = 4). These results show that SP activates a NK-1 receptor coupled to the Ih channel. Thus NK-1 receptor activation in ferret vagal afferents not only leads to membrane hyperpolarization but it also can enhance synergistically this inhibitory effect by decreasing Ih.

Cultured rat sensory neurones express functional tachykinin receptor subtypes 1, 2 and 3

The neuropeptide substance P (SP) is known to play a key role in peripheral nociceptive processes. We investigated the in vitro pharmacological characteristics of functional tachykinin receptors expressed in dorsal root ganglia (DRG) sensory neurones by analysing intracellular free calcium concentration changes induced after stimulation by SP or specific tachykinin agonists. We observed that about 37% of the tested neurones were responsive to either SP or an NK1-, NK2- or NK3-specific agonist. Tachykinin-responsive neurones had a small soma diameter (<20 microm) and were sensitive to capsaicin. These results suggest the presence of NK1, NK2 and NK3 receptors in noxious sensory neurones.

Neurokinin 1 receptor internalization in spinal cord slices induced by dorsal root stimulation is mediated by NMDA receptors

The excitability of spinal neurons that transmit pain is modulated by glutamate and substance P (SP). Glutamate is an excitatory neurotransmitter in the dorsal horn, and its effects are enhanced by SP acting on neurokinin 1 receptors (NK1Rs). We assessed activation of NK1Rs by studying their internalization in spinal cord slices. NK1Rs were localized in sections from the slices by using immunohistochemistry combined with fluorescence and confocal microscopy. Incubating the slices with SP induced internalization in most NK1R-positive neurons in laminae I, IIo, and X and in half of NK1R-positive neurons in laminae III-V. SP-induced internalization was abolished by the specific NK1R antagonist L-703,606 (1 microM). Stimulating the dorsal root with long-duration (0.4 msec) pulses evoked EPSPs in dorsal horn neurons with latencies consistent with the conduction speed of A partial differential- and C-fibers. High-frequency (100 Hz) stimulation of the dorsal root with these pulses induced NK1R internalization in neurons in laminae I-IIo of the stimulated side of the slice but not in the contralateral side or in other laminae. Stimulation at lower frequencies (1 and 10 Hz) failed to elicit significant internalization, suggesting that the release of SP is frequency-dependent. Internalization produced by the 100 Hz tetanus was mimicked by NMDA and blocked by an NMDA antagonist, 2-amino-5-phosphonopentanoic acid, but not by the AMPA and kainate antagonist CNQX. The NK1R antagonist L-703,606 abolished the internalization produced by 100 Hz stimulation or NMDA. Therefore, the release of SP in the dorsal horn appears to be controlled by NMDA receptors.

Relationship of substance P to afferent characteristics of dorsal root ganglion neurones in guinea-pig

1. The relationship between the afferent properties and substance P-like immunoreactivity (SP-LI) of L6 and S1 dorsal root ganglion (DRG) neuronal somata was examined in anaesthetized guinea-pigs. Glass pipette microelectrodes filled with fluorescent dyes were used to make intracellular recordings and to label DRG somata. The dorsal root conduction velocity (CV) and the afferent receptive properties of each unit were categorized according to criteria established in other species. Categories included a variety of low threshold mechanoreceptive classes, innocuous thermoreceptive and several nociceptive classes. Nociceptive units were further subdivided on the basis of CV and the locus of the receptive field (superficial cutaneous, deep cutaneous or subcutaneous). 2. SP-LI was determined using the avidin-biotin complex method and the relative staining intensity determined by image analysis. The possible significance of labelling intensity is discussed. Clear SP-LI appeared in twenty-nine of 117 dye-labelled neurones. All SP-LI positive units with identified receptive properties were nociceptive but not all categories of nociceptors were positive. The intensity of SP-LI labelling varied, often systematically, in relation to afferent properties. There was a tendency for nociceptive neurones with slower CVs and/or smaller cell bodies to show SP-LI. 3. Nineteen of fifty-one C fibre neurones showed SP-LI. Fewer than half the C polymodal nociceptors (CPMs) were positive. The most intensely labelled units were the deep cutaneous nociceptors and some of the CPMs in glabrous skin. C low threshold mechanoreceptors and cooling-sensitive units did not show SP-LI. 4. Ten of sixty-six A fibre neurones exhibited SP-LI, including eight of sixteen A delta nociceptors and two of fifteen A alpha/beta nociceptors. A fibre neurones exhibiting SP-LI included seven of eight deep cutaneous mechanical nociceptors and some superficial cutaneous mechano-heat nociceptors of hairy skin. In contrast, none of twenty superficial cutaneous A high threshold mechanoreceptor units or the thirty-five A fibre low threshold units (D-hair and other units) showed detectable SP-LI. 5. We conclude that SP-LI labelling in guinea-pig DRG neurones is related to (a) afferent receptive properties, (b) the tissue in which the peripheral receptive terminals are located, (c) the CV and (d) the soma size.