Mechanoreceptive Field Research Articles

A Fovea for Pain at the Fingertips

SummaryThe spatial resolution of sensory systems is not homogeneous across their receptive surfaces. For example, tactile acuity is greatest on the fingertips, reflecting the high innervation density and small mechanoreceptive fields in this area [1, 2]. In contrast, pain is considered to lack any equivalent to the tactile fovea on the fingertips, where the density of nociceptive fibers is remarkably low [3]. Here, by combining psychophysics with histology, we show that this established notion is incorrect. By delivering small-diameter nociceptive-specific laser pulses to human volunteers, we discovered that (1) the spatial acuity for pain is higher on the fingertips than on proximal skin regions such as the hand dorsum, and (2) this distal-proximal gradient for pain is comparable to that for touch. In contrast, skin biopsies in the same participants showed that the intraepidermal nerve fiber density is lower in the fingertips than in the hand dorsum. The increased spatial acuity for pain on the fingertips therefore cannot be explained simply by peripheral innervation density. This finding is, however, consistent with the existence of fine-grained maps of nociceptive input to individual digits in the human primary somatosensory cortex [4].Video

Pregabalin Suppresses Nociceptive Behavior and Central Sensitization in a Rat Trigeminal Neuropathic Pain Model

The aim of this study was to determine whether pregabalin affects nociceptive behavior and central sensitization in a trigeminal neuropathic pain model. A partial infraorbital nerve transection (p-IONX) or sham operation was performed in adult male rats. Nociceptive withdrawal thresholds were tested with von Frey filaments applied to the bilateral vibrissal pads pre- and postoperatively. On postoperative day 7, the behavioral assessment was conducted before and at 30, 60, 120, and 180 minutes after and 24 hours after pregabalin (.1, 1, 10, 100 mg/kg intraperitoneally) or saline injection. The effects of pregabalin or saline were also examined on the mechanoreceptive field and response properties of nociceptive neurons recorded in the medullary dorsal horn at postoperative days 7 to 10. Reduced withdrawal thresholds reflecting bilateral mechanical allodynia were observed in p-IONX rats until postoperative day 28, but not in sham-operated rats. At postoperative day 7, pregabalin significantly and dose-dependently reversed the reduced mechanical withdrawal thresholds in p-IONX rats. Pregabalin also attenuated central sensitization of the neurons, as reflected in reversal of their reduced activation threshold, increased responses to pinch/pressure, and enhanced stimulus-response function. This study provides the first documentation that pregabalin attenuates the mechanical allodynia and central sensitization that characterize this trigeminal neuropathic pain model, and supports its clinical use for treating craniofacial neuropathic pain. PerspectiveTrigeminal nerve injury in rats produced facial mechanical hypersensitivity and trigeminal central sensitization of medullary dorsal horn neurons that were markedly attenuated by systemically administered pregabalin, suggesting its potential clinical utility for orofacial neuropathic pain.

Central α-adrenoceptors contribute to mustard oil-induced central sensitization in the rat medullary dorsal horn

Our previous studies have demonstrated that application of the inflammatory irritant mustard oil (MO) to the tooth pulp produces trigeminal central sensitization that includes increases in mechanoreceptive field size and responses to noxious stimuli and decrease in activation threshold in brainstem nociceptive neurons of trigeminal subnucleus caudalis (the medullary dorsal horn, MDH). The aim of the present study was to test if central noradrenergic processes are involved in the central sensitization of MDH neurons and if α1-adrenoceptors or α2-adrenoceptors or both are involved. In urethane/α-chloralose-anesthetized rats, the activity of extracellularly recorded and functionally identified single nociceptive neurons in the MDH was studied. Continuous intrathecal (i.t.) superfusion of the adrenergic modulator guanethidine and α-adrenoceptor blocker phentolamine or selective α1-adrenoceptor antagonist prazosin over the medulla strongly attenuated all three MO-induced parameters of central sensitization in the MDH nociceptive neurons, compared to phosphate-buffered saline (as vehicle control). In contrast, i.t. superfusion of the selective α2-adrenoceptor antagonist yohimbine had little effect on the mechanoreceptive field expansion and the decreased mechanical activation threshold, and indeed facilitated responses to noxious stimuli of sensitized nociceptive neurons. Superfusion of each of the four chemicals alone did not affect baseline nociceptive neuronal properties. These findings provide the first documentation of the involvement of central noradrenergic processes in MDH in the development of the central sensitization, and that α1- and α2-adrenoceptors may be differentially involved.

Activation of peripheral P2X receptors is sufficient to induce central sensitization in rat medullary dorsal horn nociceptive neurons

Central sensitization and purinergic receptor mechanisms have been implicated as important processes in acute and chronic pain conditions following injury or inflammation of peripheral tissues. This study has documented that application of the P2X1,2/3,3 receptor agonist αβ-meATP (100mM) to the rat tooth pulp induces central sensitization in medullary dorsal horn nociceptive neurons that is reflected in significant increases in mechanoreceptive field size and responses to noxious stimuli and decreased mechanical activation threshold. Furthermore, these responses can be blocked by pulp application of the P2X1,2/3,3 antagonist TNP-ATP and also attenuated by medullary application of TNP-ATP. These results suggest that activation of P2X1,2/3,3 receptors in orofacial tissues plays a critical role in producing central sensitization in medullary dorsal horn nociceptive neurons.

Site‐dependent and state‐dependent inhibition of pruritogen‐responsive spinal neurons by scratching

The relief of itch by scratching is thought to involve inhibition of pruritogen-responsive neurons in the spinal cord. We recorded the responses of superficial dorsal horn neurons in mice to intradermal injection of the pruritogens chloroquine and histamine. Scratching within an area 5-17 mm distant from the injection site, outside of the units' mechanoreceptive fields (off-site), significantly inhibited chloroquine-evoked and histamine-evoked responses without affecting capsaicin-evoked firing. This is consistent with observations that scratching at a distance from a site of itch is antipruritic. In contrast, scratching directly at the injection site (within the receptive field; on-site) had no effect on chloroquine-evoked neuronal firing, but enhanced the same neurons' responses to intradermal injection of the algogen capsaicin. Moreover, neuronal responses to histamine were enhanced during on-site scratching, and this was followed by suppression of firing below baseline levels after termination of scratching. Scratching thus inhibits pruritogen-responsive neurons in a manner that depends on the input modality (i.e. pain vs. histamine-dependent or histamine-independent itch) and skin location.

Central sensitization of nociceptive neurons in rat medullary dorsal horn involves purinergic P2X7 receptors.

Central sensitization is a crucial process underlying the increased neuronal excitability of nociceptive pathways following peripheral tissue injury and inflammation. Our previous findings have suggested that extracellular adenosine 5'-triphosphate (ATP) molecules acting at purinergic receptors located on presynaptic terminals (e.g., P2X2/3, P2X3 subunits) and glial cells are involved in the glutamatergic-dependent central sensitization induced in medullary dorsal horn (MDH) nociceptive neurons by application to the tooth pulp of the inflammatory irritant mustard oil (MO). Since growing evidence indicates that activation of P2X7 receptors located on glia is involved in chronic inflammatory and neuropathic pain, the aim of the present study was to test in vivo for P2X7 receptor involvement in this acute inflammatory pain model. Experiments were carried out in anesthetized Sprague-Dawley male rats. Single unit recordings were made in MDH functionally identified nociceptive neurons for which mechanoreceptive field, mechanical activation threshold and responses to noxious stimuli were tested. We found that continuous intrathecal (i.t.) superfusion over MDH of the potent P2X7 receptor antagonists brilliant blue G and periodated oxidized ATP could each significantly attenuate the MO-induced MDH central sensitization. MDH central sensitization could also be produced by i.t. superfusion of ATP and even more effectively by the P2X7 receptor agonist benzoylbenzoyl ATP. Superfusion of the microglial blocker minocycline abolished the MO-induced MDH central sensitization, consistent with reports that dorsal horn P2X7 receptors are mostly expressed on microglia. In control experiments, superfusion over MDH of vehicle did not produce any significant changes. These novel findings suggest that activation of P2X7 receptors in vivo may be involved in the development of central sensitization in an acute inflammatory pain model.

Intrinsic membrane properties of spinal dorsal horn neurones modulate nociceptive information processingin vivo

The dorsal horn of the spinal cord is the first central relay where nociceptive inputs are processed. Based on the expression and modulation of intrinsic electrophysiological properties in in vitro slice preparations, dorsal horn neurones (DHNs) display different discharge patterns (tonic, plateau or rhythmic), which shape the neurone's response to sensory inputs. However, it is unclear whether intrinsic properties play any role in sensory processing in vivo. Using in vivo patch clamp recordings in the adult rat, we here examine whether these intrinsic properties are present, and to what extent they determine the DHN response to natural stimulation. We focused primarily on wide dynamic range neurones in deep laminae. These cells displayed a multicomponent peripheral receptive field, comprising an excitatory firing zone, a low-probability firing fringe, and adjacent inhibitory zones. Deep DHNs presented similar intrinsic properties to those observed in vitro, including plateau potentials. These plateaus, underlying high frequency accelerating discharges and after-discharges, were triggered by mechanical stimulation of the excitatory receptive field. Persistent activities induced by activation of plateau potentials were interrupted by stimulation of peripheral inhibitory zones. Moreover, we show that plateau activation is necessary for the expression of windup in response to repetitive, nociceptive stimulation. Finally, using the spinal nerve ligation model of neuropathy, we demonstrate a significant increase in the proportion of plateau neurones in deep dorsal laminae. Our data, therefore, establish that intrinsic amplification properties are expressed within intact spinal circuits and suggest their involvement in neuropathy-induced hyperexcitability of deep DHNs.

Neonatal capsaicin treatment modulates experience‐dependent plasticity in the rat barrel cortex

Previous studies have reported that capsaicin-induced C-fiber depletion results in expansion of low threshold somatosensory mechanoreceptive fields. Here we used this paradigm to investigate its effect on experience-dependent plasticity in the barrel cortex of rats. All but the D2 vibrissa were first plucked on postnatal day 0 (P0), P5, or P8, and kept plucked for a period of 30 days before being allowed to regrow for 7-9 days prior to the recording session. To assess receptive field characteristics the spared D2 principal whisker (PW) and the deprived D1 adjacent whisker (AW) were moved either singly or in concert, neuronal responses being recorded in layers IV and V of the D2 barrel. In vehicle-treated rats, PW-evoked ON responses (layer IV) were increased only in those animals that first had their vibrissae plucked on P0, whereas AW-evoked ON responses (layers IV and V) were decreased in the P0, P5, and P8 groups. In the capsaicin-treated animals, PW-evoked ON responses (layer IV) were increased in all three groups, but no decrease was recorded in the AW-evoked ON (layers IV and V) responses. In the vehicle- and capsaicin-treated animals, the greatest decrease in inhibitory interactions was observed in the P5 and P0 groups, respectively. These findings indicate that, following the induction of experience-dependent plasticity, the resultant changes in excitatory and integrative circuits can be further influenced by C-fiber depletion.

Central Sensitization Induced in Trigeminal and Upper Cervical Dorsal Horn Neurons by Noxious Stimulation of Deep Cervical Paraspinal Tissues in Rats With Minimal Surgical Trauma

Objective This study investigated if central sensitization is induced in the trigeminal subnucleus caudalis (also termed the medullary dorsal horn) and C1 and C2 dorsal horns by noxious stimulation of deep upper cervical paraspinal tissues in a preparation relatively free of surgical trauma. Methods Adult male Sprague-Dawley rats (275-450 g) were anesthetized intraperitoneally. Animals were then placed in a stereotaxic frame; a small cutaneous incision was made 3 to 4 mm near the bregma in the midline, and an opening into the skull was prepared by a 1/32-inch drill, 1 mm to the left from the midline. An epoxylite-coated tungsten microelectrode was introduced at an 18° angle to enter this small opening on the skull and was then carefully advanced about 16 mm through cortex, cerebellum, and brainstem to reach subsequently histologically confirmed sites in the Vc and upper cervical (C1 and C2) dorsal horn region. Thirty-three, 27, and 15 neurons recorded in medullary, C1, and C2 dorsal horns, respectively, of chloralose/urethane-anesthetized rats were activated by noxious stimulation of mechanoreceptive fields involving V1, V2, and/or V3 trigeminal nerve territories. The inflammatory irritant mustard oil was injected into the deep paraspinal tissues at the level of the left C1-C2 joint. Pre and postinjection receptive field (RF) sizes were mapped by nonnoxious mechanical stimuli and noxious mechanical and heat stimuli. Results A 30- to 50-minute increase (mean, 165% ± 38.1%) in RF size postinjection for 62% of neurons tested was demonstrated, suggesting central sensitization; for most (>70%) neurons, the RF expanded caudally into cervically innervated tissues. Conclusions These findings provide the first documentation that deep cervical nociceptive inputs can induce central sensitization in medullary and C1/C2 dorsal horns and suggest that these effects may reflect mechanisms contributing to deep cervical pain and its referral.

Glutamate and capsaicin effects on trigeminal nociception II: Activation and central sensitization in brainstem neurons with deep craniofacial afferent input

We examined the effect of the peripheral application of glutamate and capsaicin to the temporomandibular joint (TMJ) in influencing the activation and central sensitization of TMJ-responsive nociceptive neurons in the trigeminal subnucleus caudalis/upper cervical cord (Vc/UCC). The activity of single neurons activated by noxious mechanical stimulation of the TMJ was recorded in the Vc/UCC of 49 halothane-anesthetized male rats. Cutaneous mechanoreceptive field (RF), cutaneous mechanical activation threshold (MAT), and TMJ MAT of each neuron were assessed before and after injection of 0.5 M glutamate (or vehicle) and 1% capsaicin (or vehicle) into the TMJ. A total of 49 nociceptive neurons (37 nociceptive-specific, 12 wide-dynamic-range) that could be activated by blunt noxious mechanical stimulation of the TMJ were studied. When injected alone, glutamate and capsaicin activated and induced central sensitization (reflected in cutaneous RF expansion and cutaneous and/or TMJ MAT reduction) in most Vc/UCC neurons. Following glutamate injection, capsaicin evoked greater activity and less cutaneous/TMJ MAT reduction compared with capsaicin alone, whereas capsaicin abolished all subsequent glutamate-evoked activity and depressed cutaneous RF expansion in most neurons. Glutamate effects on deep afferents and Vc/UCC neurons were analogous since glutamate sensitized afferent and neuronal responses to capsaicin but the desensitizing effects of capsaicin on glutamate-evoked excitability of Vc/UCC neurons contrast with the lack of capsaicin-induced modulation of glutamate-evoked afferent excitability [ Lam, D.K., Sessle, B.J., Hu, J.W., 2008a. Glutamate and capsaicin effects on trigeminal nociception I: activation and peripheral sensitization of deep craniofacial nociceptive afferents. Brain Res. doi:10.1016/j.brainres.2008.11.029], suggesting that peripheral and central sensitization may be differentially involved in the nociceptive effects of glutamate and capsaicin applied to deep craniofacial tissues.

Surgical incision can alter capsaicin-induced central sensitization in rat brainstem nociceptive neurons

Surgical trauma can affect spinal neuronal excitability, but there have been no studies of the effects of surgical cutaneous injury on central nociceptive processing of deep afferent inputs evoked by noxious stimuli such as capsaicin. Thus our aim was to test the effect of surgical cutaneous incision in influencing central sensitization induced by capsaicin injection into the temporomandibular joint (TMJ). The activity of single nociceptive neurons activated by noxious mechanical stimulation of the TMJ was recorded in the trigeminal subnucleus caudalis/upper cervical cord of halothane-anesthetized rats. The cutaneous mechanoreceptive field (RF), cutaneous mechanical activation threshold (MAT) and TMJ MAT of neurons before and after both surgical cutaneous incision alone and capsaicin injection were compared with results of incision and lidocaine pretreatment of the facial skin overlying the TMJ and capsaicin injection into the TMJ. Incision itself induced a barrage of neuronal spikes and excitability increases reflecting central sensitization (cutaneous RF expansion, cutaneous MAT reduction) in most neurons tested whereas lidocaine pretreatment significantly attenuated the barrage and central sensitization. Capsaicin injection into the TMJ induced cutaneous RF expansion, cutaneous MAT reduction and TMJ MAT reduction following lidocaine pretreatment of the cutaneous incision site whereas capsaicin injection following incision alone not only failed to induce further central sensitization but also decreased the existing incision-induced central sensitization (no cutaneous RF expansion, increased cutaneous MAT and TMJ MAT) in most neurons tested. These findings suggest that central sensitization induced by capsaicin alone or by cutaneous incision alone can readily occur in TMJ-responsive nociceptive neurons and that following incision-induced excitability increases, capsaicin may result in a temporary suppression of nociceptive neuronal changes reflecting central sensitization.

Modulation of Trigeminal Spinal Subnucleus Caudalis Neuronal Activity Following Regeneration of Transected Inferior Alveolar Nerve in Rats

Modulation of trigeminal spinal subnucleus caudalis neuronal activity following regeneration of transected inferior alveolar nerve in rats. To clarify the neuronal mechanisms of abnormal pain in the face innervated by the regenerated inferior alveolar nerve (IAN), nocifensive behavior, trigeminal ganglion neuronal labeling following Fluorogold (FG) injection into the mental skin, and trigeminal spinal subnucleus caudalis (Vc) neuronal properties were examined in rats with IAN transection. The mechanical escape threshold was significantly higher at 3 days and lower at 14 days after IAN transection, whereas head withdrawal latency to heat was significantly longer at 3, 14, and 60 days after IAN transection. The number of FG-labeled ganglion neurons was significantly reduced at 3 days after IAN transection but increased at 14 and 60 days. The number of wide dynamic range (WDR) neurons with background (BG) activity was significantly higher at 14 and 60 days after IAN transection compared with naïve rats, and the number of WDR and low-threshold mechanoreceptive (LTM) neurons with irregularly bursting BG activity was increased at these two time points. Mechanically evoked responses were significantly larger in WDR and LTM neurons 14 days after IAN transection compared with naïve rats. Heat- and cold-evoked responses in WDR neurons were significantly lower at 14 days after transection compared with naïve rats. Mechanoreceptive fields were also significantly larger in WDR and LTM neurons at 14 and 60 days after IAN transection. These findings suggest that these alterations may be involved in the development of mechanical allodynia in the cutaneous region innervated by the regenerated IAN.

Selective Role for TRPV4 Ion Channels in Visceral Sensory Pathways

Although there are many candidates as molecular mechanotransducers, so far there has been no evidence for molecular specialization of visceral afferents. Here, we show that colonic afferents express a specific molecular transducer that underlies their specialized mechanosensory function: the transient receptor potential channel, vanilloid 4 (TRPV4). We found TRPV4 mRNA is highly enriched in colonic sensory neurons compared with other visceral and somatic sensory neurons. TRPV4 protein was found in colonic nerve fibers from patients with inflammatory bowel disease, and it colocalized in a subset of fibers with the sensory neuropeptide CGRP in mice. We characterized the responses of 8 subtypes of vagal, splanchnic, and pelvic mechanoreceptors. Mechanosensory responses of colonic serosal and mesenteric afferents were enhanced by a TRPV4 agonist and dramatically reduced by targeted deletion of TRPV4 or by a TRP antagonist. Other subtypes of vagal and pelvic afferents, by contrast, were unaffected by these interventions. The behavioral responses to noxious colonic distention were also substantially reduced in mice lacking TRPV4. These data indicate that TRPV4 contributes to mechanically evoked visceral pain, with relevance to human disease. In view of its distribution in favor of specific populations of visceral afferents, we propose that TRPV4 may present a selective novel target for the reduction of visceral pain, which is an important opportunity in the absence of current treatments.

Involvement of glia in central sensitization in trigeminal subnucleus caudalis (medullary dorsal horn)

Central sensitization is a crucial mechanism underlying the increased excitability of nociceptive pathways following peripheral tissue injury and inflammation. We have previously demonstrated that the small-fiber excitant and inflammatory irritant mustard oil (MO) applied to the tooth pulp produces glutamatergic- and purinergic-dependent central sensitization in brainstem nociceptive neurons of trigeminal subnucleus caudalis (Vc). Recent studies have implicated both astrocytes and microglia in spinal nociceptive mechanisms, showing, for example, that inhibition of spinal astroglial metabolism or spinal microglial p38MAPK activation can attenuate hyperalgesia in inflammatory pain models but have not tested effects of glial inhibitors on central sensitization in functionally identified spinal nociceptive neurons. The aim of the present study was to determine whether glial cells are involved in the MO-induced central sensitization in Vc nociceptive neurons, by examining the effects of intrathecally applied SB203580 (SB), an inhibitor of p38MAPK, and fluoroacetate (FA), an inhibitor of the astroglial metabolic enzyme aconitase. During continuous superfusion of phosphate-buffered saline over Vc, MO application to the pulp-induced central sensitization in Vc nociceptive neurons reflected in significant increases in cutaneous mechanoreceptive field (RF) size and responses to noxious mechanical stimuli and a decrease in mechanical activation threshold. The i.t. application of SB or FA markedly attenuated the MO-induced increases in pinch RF size and responses to noxious stimuli and the decrease in activation threshold. Neither SB nor FA application significantly affected the baseline (i.e., pre-MO application) RF and response properties. These results suggest that glial metabolic processes are important in the development of Vc central sensitization.

Central sensitization in thalamic nociceptive neurons induced by mustard oil application to rat molar tooth pulp

We have recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces central sensitization that is reflected in changes in spontaneous activity, mechanoreceptive field (RF) size, mechanical activation threshold, and responses to graded mechanical stimuli applied to the neuronal RF in trigeminal brainstem subnucleus caudalis and subnucleus oralis. The aim of this study was to test whether central sensitization can be induced in nociceptive neurons of the posterior thalamus by MO application to the pulp. Single unit neuronal activity was recorded in the ventroposterior medial nucleus (VPM) or posterior nuclear group (PO) of the thalamus in anesthetized rats, and nociceptive neurons were classified as wide dynamic range (WDR) or nociceptive-specific (NS). MO application to the pulp was studied in 47 thalamic nociceptive neurons and found to excite over 50% of the 35 VPM neurons tested and to produce significant long-lasting (over 40 min) increases in spontaneous activity, cutaneous pinch RF size and responses to graded mechanical stimuli, and a decrease in threshold in the 29 NS neurons tested; a smaller but statistically significant increase in mean spontaneous firing rate and decrease in activation threshold occurred following MO in the six WDR neurons tested. Vehicle application to the pulp did not produce any significant changes in six VPM NS neurons tested. MO application to the pulp produced pronounced increases in spontaneous activity, pinch RF size, and responses to mechanical stimuli, and a decrease in threshold in three of the six PO neurons. In conclusion, application of the inflammatory irritant MO to the tooth pulp results in central sensitization of thalamic nociceptive neurons and this neuronal hyperexcitability likely contributes to the behavioral consequences of peripheral inflammation manifesting as pain referral, hyperalgesia and allodynia.

Central sensitization induced in thalamic nociceptive neurons by tooth pulp stimulation is dependent on the functional integrity of trigeminal brainstem subnucleus caudalis but not subnucleus oralis

We have previously demonstrated that application of the inflammatory irritant mustard oil (MO) to the rat molar tooth pulp induces central sensitization in nociceptive neurons within the contralateral ventroposterior medial (VPM) nucleus and posterior nuclear group (PO) of the thalamus as well as brainstem subnucleus caudalis (Vc) and subnucleus oralis (Vo). Since Vc and Vo are important relays of pulp afferent input to thalamus, the aim of this study was to test if local application of the synaptic blocker CoCl 2 to Vc or Vo influences the pulp-induced thalamic central sensitization. The activity of 32 nociceptive-specific (NS) neurons within the rat VPM and immediately adjacent PO was recorded. Spontaneous activity, mechanoreceptive field (RF), mechanical activation threshold and evoked responses to graded mechanical stimuli were assessed before and after MO application to the pulp. MO application evoked immediate but short-lasting neuronal discharges in 21 of the 32 NS neurons tested, as well as central sensitization reflected in significant and long-lasting (> 60 min) RF expansion, decrease in activation threshold, and increase in graded pinch-evoked responses in all 32 NS neurons. CoCl 2 applied to the ipsilateral Vc significantly attenuated these pulp-induced changes for 20 min or more. In contrast, CoCl 2 applied to the ipsilateral Vo did not reverse this MO-induced central sensitization. Isotonic saline applied to Vc or Vo was also ineffective. These findings indicate that central sensitization induced in nociceptive neurons within VPM and PO by noxious stimulation of the tooth pulp is dependent upon the functional integrity of Vc but not Vo.

Activation of splanchnic and pelvic colonic afferents by bradykinin in mice

Lumbar splanchnic (LSN) and sacral pelvic (PN) nerves convey different mechanosensory information from the colon to the spinal cord. Here, we determined whether these pathways differ also in their chemosensitivity to bradykinin. Using a novel in vitro mouse colon preparation, serosal afferents were recorded from the LSN and PN and distinguished based on their mechanosensitivity to von Frey filaments (70-4000 mg) and insensitivity to colonic stretch (1-5 g) or fine mucosal stroking (10 mg). Bradykinin was applied into a ring around mechanoreceptive fields. The LSN and PN afferents had different dynamic responses to mechanical stimuli: PN afferents required lower intensity stimuli, evoked larger responses, and displayed more maintained responses than LSN afferents. Bradykinin (1 micromol L-1) excited 66% (27 of 41) of LSN afferents. Responses to probing were potentiated after bradykinin. The concentration-dependent (EC50: 0.16 micromol L-1) response was reversed by the B2-receptor antagonist HOE-140 (10 nmol L-)). Twelve bradykinin responsive afferents were mechanically insensitive. More LSN serosal afferents responded to bradykinin than PN afferents (11%, P<0.001) , with larger responses (P<0.05). No mechanically insensitive PN afferents were recruited by bradykinin. Bradykinin potently stimulates most splanchnic serosal afferents via B2-receptors, but few pelvic afferents. Mechanically insensitive afferents recruited by bradykinin are exclusive to the LSN.

Mechanoreceptive field and response properties of nociceptive neurons in ventral posteromedial thalamic nucleus of the rat

The somatotopic organization, mechanoreceptive field (RF) and response properties of nociceptive neurons in the ventral posteromedial thalamic nucleus (VPM) were studied in urethane/α-chloralose anesthetized rats. Microelectrode recordings were made of the activity of single neurons within VPM, and neurons were identified as either nociceptive [wide-dynamic-range (WDR), nociceptive-specific (NS)] or non-nociceptive [low-threshold mechanoreceptive (LTM)]. Of the 350 neurons tested, 317 responded to orofacial mechanical stimulation; all the RFs were contralateral. The incidence of NS neurons was significantly higher than that of WDR neurons (21.2% versus 4.2%) but LTM neurons were most common (62.9%). WDR and NS neurons were scattered among the LTM neurons in the VPM. NS neurons were significantly denser in rostral and caudal portions of VPM; most of the WDR neurons (87%) were located in the core of VPM. Neurons with an ophthalmic division RF were located most dorsally and those with a mandibular division RF most ventrally. The RF of most neurons (77%) was located in the maxillary region, with the RF of nociceptive neurons mainly in the intraoral region and that of most LTM neurons in the perioral region. The majority of LTM, WDR and NS neurons tested had a low spontaneous activity and there was a significant difference in spontaneous firing between WDR and NS neurons, and between WDR and LTM neurons. The mean mechanical activation threshold of NS neurons (195 ± 11.2 g) was significantly higher than that of WDR neurons (4.7 ± 1.3 g). Response magnitude and peak firing frequency to graded mechanical stimuli were significantly higher in the WDR neurons. These findings indicate significant differences in the spontaneous activity, RF and response properties between WDR and NS neurons in VPM. However, they also indicate that both types of nociceptive neurons can encode the spatial properties and intensity of a noxious orofacial stimulus and, thus, might contribute to the sensory-discriminative component of pain.

Effects of GABA A receptor inhibition on response properties of barrel cortical neurons in C-Fiber-depleted rats

C-fiber depletion results in expansion of low threshold somatosensory mechanoreceptive fields. In this study, we investigated the role of intact C-fibers in GABAA-mediated inhibition in barrel cortical neurons. We used electronically controlled mechanical stimulation of whiskers to quantitatively examine the responses of barrel cells to whisker displacements. After systemic injection of picrotoxin neuronal responses were recorded at 5 min intervals for 20 min and then at 10 min intervals for 100 min. Picrotoxin injection caused a 3-fold increase in response magnitude of adjacent whisker stimulation and 1.4-fold increase in response magnitude of principal whisker stimulation with a maximum enhancement 50 min after the injection. There was no significant change in spontaneous activity following picrotoxin injection. The response enhancement and receptive field expansion observed in normal rats were completely absent in the C-fiber-depleted rats. These results suggest that the GABAA-mediated inhibition that modulates the receptive field functional organization of the barrel cortex depends on intact C-fibers.

Craniofacial inputs to upper cervical dorsal horn: Implications for somatosensory information processing

The aim of this study was to characterize the properties of somatosensory neurons in the first 2 cervical spinal dorsal horns (C1 and C2 DHs) and compare them with those previously described for the rostral subnucleus caudalis (rVc). A total of 74 nociceptive neurons classified as wide-dynamic-range (WDR) or nociceptive-specific (NS), as well as 72 low-threshold mechanoreceptive (LTM) neurons, was studied in urethane/chloralose-anesthetized rats. The majority of LTM neurons were located in laminae III/IV and had a small mechanoreceptive field (RF) that included the posterior face and cervical tissues. In contrast, the nociceptive neurons were located in laminae I/II or V/VI, and the RF of each C1 and C2 DH nociceptive neuron included a part of the face and in 47% of them the RF included a region supplied by upper cervical afferents. There was a gradual caudal shift in the neuronal RF from nasal/intraoral tissues towards the neck as recording sites progressed from rVc to C1 and C2 DHs. In contrast to LTM neurons, many C1 and C2 DH nociceptive neurons received mechanosensitive convergent afferent inputs from cervical and craniofacial deep tissues (e.g., tongue muscles or temporomandibular joint), and over 50% could be activated by hypoglossal (XII) nerve electrical stimulation. We propose that C1 and C2 DHs represent part of the caudal extension of the Vc, and that Vc and C1 and C2 DHs may act together as one functional unit to process nociceptive information from craniofacial and cervical tissues, including that from deep craniofacial tissues.