Acupuncture and diffuse noxious inhibitory controls: naloxone-reversible depression of activities of trigeminal convergent neurons

Recordings were made from convergent neurons in trigeminal nucleus caudalis of the rat. These neurons could be activated by both innocuous and noxious mechanical stimuli applied to their excitatory receptive fields on the ipsilateral part of the muzzle. Percutaneous application of suprathreshold, 2 ms square-wave electrical stimuli to the centre of the excitatory field resulted in responses to A- and C-fibres being observed. The effects on these responses of manual acupuncture performed by a traditional Chinese acupuncturist at the "Zusanli" point on the right hindlimb were compared with the effects induced by acupuncture applied at a non-acupoint, next to "Zusanli". In addition, the effects of acupuncture were compared with the inhibitory effects evoked by noxious thermal stimulation of the left hindlimb on the responses of the same neurons. This last type of inhibition has been described previously by our group and termed diffuse noxious inhibitory controls. Acupuncture, either applied at "Zusanli" or at a non-acupoint and noxious thermal stimulation induced similar strong inhibitory effects on the C-fibre-evoked responses of trigeminal convergent neurons (77.9 +/- 4.4%; 72.5 +/- 4.6% and 78.5 +/- 3.6% inhibition, respectively) and these inhibitions were followed by long-lasting aftereffects. In addition, both the acupuncture- and noxious thermal stimulation-evoked inhibitions were significantly reduced by systemic naloxone (0.4 mg/kg, i.v.). Since the antinociceptive effects elicited by acupuncture (i) had a similar magnitude and time-course to those evoked by noxious thermal stimulation, (ii) exhibited a lack of topographical specificity and (iii) involved an opioidergic link, we would suggest that, at least in our experimental conditions, acupuncture manoeuvres trigger the neuronal mechanisms involved in diffuse noxious inhibitory controls.     

Diffuse noxious inhibitory controls (DNIC) in animals and in man.

Some neurones in the dorsal horn of the spinal cord are strongly inhibited when a nociceptive stimulus is applied to any part of the body, distinct from their excitatory receptive fields. This phenomenon was termed "Diffuse Noxious Inhibitory Controls" (DNIC). DNIC influence only convergent neurones: the other cell types which are found in the dorsal horn, including specific nociceptive neurones, are not affected by this type of control. In normal conditions, these inhibitions can be triggered only by conditioning stimuli which are nociceptive. The inhibitions are then extremely potent, affect all the activities of the convergent neurones and persist, sometimes for several minutes, after the removal of the conditioning stimulus. In fact, only activity of A delta- or A delta- and C-peripheral fibres can trigger DNIC. DNIC are sustained by a complex loop which involves supraspinal structures since, unlike segmental inhibitions, they can not be observed in animals in which the cord has previously been transsected at the cervical level. The ascending and descending limbs of this loop travel respectively through the ventro-lateral and dorso-lateral funiculi respectively. We proposed that DNIC result from the physiological activation of some brain structures putatively involved in descending inhibition. However, lesions of the following structures did not modify DNIC: Periaqueductal grey (PAG), Cuneiform nucleus, Parabrachial area, locus coeruleus/subcoeruleus, rostral ventromedial medulla (RVM) including Raphe Magnus, Gigantocellularis and Paragigantocellularis nuclei. By contrast, lesions of Subnucleus Reticularis Dorsalis (SPD) in the caudal medulla strongly reduced DNIC. Both electrophysiological and anatomical data support the involvement of SRD neurones in spin-bulbo-spinal loop(s). Indeed, they are unresponsive to visual, auditory or proprioceptive stimulation but are preferentially or exclusively activated by nociceptive stimuli with a "whole-body receptive field"; they encode precisely the intensity of cutaneous and visceral stimulation within the noxious range and are exclusively activated by cutaneous A delta- or A delta- and C-fibre peripheral volleys; they send descending projections through the dorsolateral funiculus that terminate in the dorsal horn at all levels of the spinal cord. In man, exactly analogous results have been obtained by means of combined psychophysical measurements and recordings of nociceptive reflexes. Electrical stimulation of the sural nerve at the ankle simultaneously induces a nociceptive reflex in a flexor muscle of the knee (the RIII reflex) and a painful sensation from the territory of the nerve. Painful heterotopic conditioning stimuli, no matter whether thermal, mechanical or chemical in nature, depress both the reflex and the associated painful sensation, with stronger effects being observed with more intense conditioning stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)     

Changes in c-Fos expression induced by noxious stimulation in the trigeminal spinal nucleus caudalis and C1 spinal neurons of rats after hyperbaric exposure.

The present study aims to test the hypothesis that hyperbaric exposure inhibits nociceptive processing in the trigeminal spinal nucleus caudalis and C1 spinal neurons. We investigated the c-Fos-like immunoreactivity of the brainstem and upper cervical spinal cord (C1 region) following an injection of mustard oil (15 microliters of 20%) into the nasal mucosa of pentobarbital anesthetized rats after exposure to hyperbaric (2-atmospheres, 1 h) and normobaric pressures. After the hyperbaric exposure, the mean number of Fos-immunoreactive neurons in the ipsilateral laminae I-II and III-IV of the trigeminal spinal nucleus caudalis were significantly lower than those in the normobaric condition. Similarly, the mean number of c-Fos positive neurons in the superficial layer (I-II) of the ipsilateral C1 segment were significantly reduced as compared with that in the normobaric condition. When treated with the vehicle alone, no significant difference was detected in the numbers of c-Fos positive neurons in the trigeminal spinal nucleus caudalis and C1 regions between hyperbaric and normobaric conditions. These results suggest that hyperbaric exposure may attenuate nociceptive signals from the area innervated by the trigeminal nerves at the level of both the trigeminal spinal nucleus caudalis and C1 dorsal horn.     

Effects of GABA receptor antagonist on trigeminal caudalis nociceptive neurons in normal and neonatally capsaicin-treated rats

We have recently demonstrated that significant increases in cutaneous mechanoreceptive field (RF) size and spontaneous activity occur in nociceptive neurons of trigeminal subnucleus caudalis (Vc, the medullary dorsal horn) of adult rats depleted of C-fiber afferents by neonatal treatment with capsaicin. These neuronal changes in capsaicin-treated (CAP) rats are suggestive of central neuroplasticity and involve N-methyl-D-aspartic acid (NMDA) receptor mechanisms. The present study examined whether the GABA(A) receptor antagonist bicuculline (BIC) or the GABA(B) receptor antagonist 2-hydroxysaclofen (SAC) can influence the RF properties and activity of Vc nociceptive neurons classified as either nociceptive-specific or wide-dynamic range in CAP adult rats or in neonatally vehicle-treated (CON) rats. C-fiber depletion was confirmed in the CAP rats by a significant decrease in plasma extravasation of Evans blue dye in a skin area receiving topical application of mustard oil, a small-fiber excitant and inflammatory irritant. As previously reported, marked increases in cutaneous RF size and spontaneous activity occurred in Vc nociceptive neurons of adult CAP rats, compared with CON rats. GABA(A) receptor blockade by BIC (i.t.) in CON rats produced a significant increase in spontaneous activity and in pinch RF size and tactile RF size (or appearance of a tactile area in the RF of nociceptive-specific neurons), as well as a significant lowering of the mechanical threshold and a significant enhancement of responses to pinch stimuli applied to the RF. In CAP rats, GABA(A) receptor blockade also produced significant changes similar to those documented in CON rats, except for a paradoxical and significant decrease in pinch RF size and no noticeable changes in responses to pinch stimuli. GABA(B) receptor blockade by SAC (i.t. ) did not produce any significant changes in Vc nociceptive neurons in either CON or CAP rats. These results suggest that GABA(A) receptor-mediated inhibition may be involved in maintaining the functional expression of Vc nociceptive neuronal properties in normal conditions, and that in animals depleted of their C-fiber afferents, some features of this GABA(A) receptor-mediated modulation may be disrupted such that a GABA(A) receptor-mediated excitation is manifested.     

Effects of hypnosis on diffuse noxious inhibitory controls

The neurophysiological mechanisms of hypnotic analgesia are still under debate. It is known that pain occurring in one part of the body (counterstimulation) decreases pain in the rest of the body by activating the diffuse noxious inhibitory controls (DNICs). The aim of this study was to explore the effects of hypnosis on both pain perception and heterotopic nociceptive stimulation. The A forms of both the Harward Group Scale of Hypnotic Susceptibility and the Stanford Hypnotic Susceptibility Scale were administered to 50 healthy students. Twenty subjects were selected and assigned to two groups: group A, consisting of 10 subjects with high hypnotic susceptibility; and group B, consisting of 10 subjects with low hypnotic susceptibility. The subjects were then randomly assigned first to either a control session or a session of hypnotic analgesia. The nociceptive flexion reflex (RIII) was recorded from the biceps femoris muscle in response to stimulation of the sural nerve. The subjective pain threshold, the RIII reflex threshold, and the mean area with suprathreshold stimulation were determined. Heterotopic nociceptive stimulation was investigated by the cold-pressor test (CPT). During and immediately after the CPT, the subjective pain threshold, pain tolerance, and mean RIII area were determined again. The same examinations were repeated during hypnosis. Hypnosis significantly reduced the subjective pain perception and the nociceptive flexion reflex. It also increased pain tolerance and reduced pain perception and the nociceptive reflex during the CPT. These effects were found only in highly susceptible subjects. However, the DNIC's activity was less evident during hypnosis than during the CPT effects without hypnosis. Both hypnosis and DNICs were able to modify the perception of pain. It seems likely that DNICs and hypnosis use the same descending inhibitory pathways for the control of pain. The susceptibility of the subject is a critical factor in hypnotically induced analgesia.     

Effects of previous noxious stimulus applied to remote areas on noxious stimulus-evoked c-Fos expression in the rat trigeminal nucleus caudalis

Noxious stimulus-evoked c-Fos expression in the spinal dorsal horn is modulated by noxious stimuli applied previously to remote areas of the body. To confirm the existence of such modulation in c-Fos expression in the trigeminal system, changes in c-Fos expression in the trigeminal nucleus caudalis induced by formalin injection into the rat whisker pad were examined by previously injecting formalin into different areas (contralateral whisker pad, ipsilateral or contralateral forepaw) of the body. Formalin injection-evoked c-Fos expression in this nucleus was significantly reduced by previous formalin injection into the contralateral whisker pad or ipsilateral forepaw but not into the contralateral forepaw. The interval between the two injections of formalin that produced a maximal reduction of formalin injection-evoked c-Fos expression was 1 h, and the reduction of c-Fos expression was less when the interval of the two noxious stimuli was longer or shorter than 1 h. These results suggested that noxious stimulus-evoked c-Fos expression in the trigeminal nucleus caudalis is reduced by noxious stimulus applied previously to remote areas, and the reduction is dependent on the area of previous noxious stimulation and interval between the two noxious stimuli.     

Endotoxin-induced uveitis causes long-term changes in trigeminal subnucleus caudalis neurons

Endotoxin-induced uveitis (EIU) is commonly used in animals to mimic ocular inflammation in humans. Although the peripheral aspects of EIU have been well studied, little is known of the central neural effects of anterior eye inflammation. EIU was induced in male rats by endotoxin or lipopolysaccharide (LPS, 1 mg/kg ip) given 2 or 7 days earlier. Neurons responsive to mechanical stimulation of the ocular surface were recorded under barbiturate anesthesia at the trigeminal subnucleus interpolaris/caudalis (Vi/Vc) transition and subnucleus caudalis/cervical cord (Vc/C1) junction, the main terminal regions for corneal nociceptors. Two days after LPS, Vc/C1 units had reduced responses to histamine, nicotine, and CO₂ gas applied to the ocular surface, whereas unit responses were increased 7 days after LPS. Those units with convergent cutaneous receptive fields at Vc/C1 were enlarged 7 days after LPS. Units at the Vi/Vc transition also had reduced responses to histamine and CO₂ 2 days after LPS but no enhancement was seen at 7 days. Tear volume evoked by CO₂ was reduced 2 days after LPS and returned toward control values by 7 days, whereas CO₂-evoked eye blinks were normal at 2 days and increased 7 days after LPS. These results indicate that a single exposure to endotoxin causes long-term changes in the excitability of second-order neurons responsive to noxious ocular stimulation. The differential effects of EIU on tear volume and eye blink lend further support for the hypothesis that ocular-sensitive neurons at the Vi/Vc transition and Vc/C1 junction regions mediate different aspects of pain during intraocular inflammation.     

Encoding of nociceptive thermal stimuli by diffuse noxious inhibitory controls in humans

1. It has previously been shown that, in normal humans, heterotopic painful thermal conditioning stimuli induce parallel increase in the thresholds of a spinal nociceptive flexion reflex (RIII reflex) and the concurrent sensation of pain elicited by electrical stimulation of the sural nerve. On the basis of analogous animal studies, we proposed that such phenomena could be related to diffuse noxious inhibitory controls (DNIC), which have been described in the rat. The present study, which was carried out on normal volunteer subjects, was particularly concerned with the extent and temporal characteristics of the depressive effects of DNIC triggered by painful thermal stimuli on RIII reflex activity. In addition, because it was possible that these depressive effects could have resulted from a direct postsynaptic inhibition of motoneurons, a second part of the study was aimed at determining whether or not the heterotopic noxious thermal stimuli also affected the excitability of alpha-motoneurons, as monitored by the monosynaptic Hoffmann reflex (H reflex) technique. 2. In the 11 subjects under study, application of moderate, nonnoxious temperatures (40-44 degree C) to the contralateral hand (via a thermoregulated and agitated waterbath) did not modify the RIII reflex nor the associated sensation of pain. By contrast noxious temperatures clearly depressed the RIII reflex and the concurrent sensation of pain, both during and after the conditioning procedure (CP), in a direct linear relationship to the temperature of the waterbath in the 45-47 degree C range; the maximal depressive effect was observed with the highest conditioning temperature. A significant relationship was also found between the extent of the RIII depression during the CP and that during a 10-min period of post-CP observation. 3. The depressive effects observed on both the RIII reflex and pain were not associated with clear change in autonomic functions. Respiration remained stable during the sessions, with no significant relationship between the temperatures of the waterbath and respiratory rate. Heart rate was slightly but significantly increased during the immersion of the hand in the 46 or 47 degree C waterbaths; this increase, however, ceased with the end of CP. 4. Application of thermal conditioning stimuli produced a slight but nonsignificant increase of the monosynaptic H reflex during the first minute of CP, no matter what was the temperature of the waterbath. However, there were no subsequent variations during the 6-min period of post-CP observation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Change in mechanical receptive field properties induced by GABAA receptor activation in the trigeminal spinal nucleus caudalis neurons in rats

The purpose of the present study was to characterize the effect of a local GABAergic inhibitory mechanism on the mechanical receptive field properties of trigeminal spinal nucleus caudalis (SpVc) neurons by iontophoretic application of a gamma-aminobutyric acidA (GABA(A))-antagonist and -agonist. A total of 24 SpVc neurons that responded to orofacial mechanical stimulation were extracellularly recorded by means of multibarrel microelectrodes in urethane-anesthetized rats. The GABA(A) antagonist bicuculline (30 nA, 5 min) enhanced the activities of SpVc neurons (20/24) induced by both touch/pressure and pinch stimuli and also lowered the mechanical stimulation threshold (touch/pressure). Spontaneous discharges in these neurons (20/24) were significantly increased after bicuculline application. Eighteen out of 24 SpVc neurons showed signs of expansion of the receptive field size after iontophoretic application of bicuculline. These changes showed a current-dependent manner and were reversed in approximately 15-20 min. Iontophoretic application of the GABA(A) agonist muscimol induced a current-related inhibition of neuronal activity elicited by touch/pressure and pinch stimuli as well as a decrease in the size of receptive fields. The facilitation of evoked responses and receptive field expansion of SpVc neuron induced by bicuculline application were blocked by coapplication of muscimol (50 nA, 5 min). These results suggest that a local mechanism acting via GABA(A) receptors normally exerts a tonic inhibition of mechanoreceptive transmission in the trigeminal spinal nucleus neurons and this effect may limit responsiveness and size of receptive fields.     

Effect of trigeminal subnucleus caudalis cold block on the cerebrovascular-evoked responses of rostral trigeminal complex neurons

The technique of reversible cold block was used to explore the possibility that the trigeminal subnucleus caudalis (Vc) influences the responses of rostral trigeminal brainstem nuclear complex (TBNC) neurons to stimulation of the cerebrovasculature. Reversible cold block of Vc was found to abolish totally the responses of many rostral TBNC neurons to stimulation of the cerebrovasculature. The remaining neurons were not affected by the cold block. These data suggest that some rostral TBNC neurons may receive an indirect input from the cerebrovasculature via Vc while other rostral TBNC neurons receive a direct input from the cerebrovasculature.     

Functional type I vanilloid receptor expression by substantia gelatinosa neurons of trigeminal subnucleus caudalis in mice

The aim of this study was to investigate the existence of functional TRPV1 receptor by substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc), which is implicated in the processing of nociceptive information from orofacial regions. The direct membrane effects of a TRPV1 receptor agonist, capsaicin, were examined by gramicidin-perforated patch clamp recording using a trigeminal brainstem slice preparation containing Vc from immature mice. Capsaicin (1–2 μM) induced a membrane depolarization in 58 out of 71 SG neurons tested (82%). Capsaicin-induced depolarization was maintained in 20 out of 32 (63%) SG neurons in the presence of amino acid and voltage-dependent sodium channel blockers (10 μM CNQX, 20 μM AP-5, 0.5 μM TTX, 50 μM picrotoxin and 2 μM strychnine). In addition, capsaicin-induced depolarization was maintained in the presence of L-732,138 (1 μM), an NK1 receptor antagonist, in 14 out of 17 neurons (82%) tested. The capsaicin-induced depolarizing effects were blocked by a TRPV1 receptor antagonist, capsazepine (10 μM). These results indicate that a sub-population of SG neurons in the Vc express functional TRPV1 receptors, and that capsaicin can directly activate the TRPV1 receptor on the postsynaptic membrane of SG neurons.     

Cross-desensitization of responses of rat trigeminal subnucleus caudalis neurons to cinnamaldehyde and menthol

Most cold-sensitive subnucleus caudalis (Vc) neurons are also excited by the TRPM8 agonist menthol and the TRPA1 agonist cinnamaldehyde (CA). We investigated how interactions among menthol, CA and noxious cooling and heating of the tongue affected responses of superficial Vc units recorded in thiopental-anesthetized rats. Units responded to 1% CA which enhanced cold- and heat-evoked responses 5 min later. They responded more strongly to 10% CA which initially depressed cold responses, followed by enhancement at 5 min without affecting responses to heat. Following 10% CA, the mean response to 1% menthol was significantly lower than when menthol was tested first. After menthol, the subsequent response to CA was significantly weaker compared to the mean CA-evoked response when it was tested first. These results demonstrate mutual cross-desensitization between CA and menthol. The response to CA was enhanced following prior application of 10% ethanol (menthol vehicle). Prior application of menthol did not prevent the biphasic effect of 10% CA on cold-evoked responses, nor did prior application of CA prevent menthol enhancement of cold-evoked responses. Responses to noxious heat were unaffected by 10% CA and menthol regardless of the order of chemical presentation. These data indicate that superficial Vc neurons receive convergent input from primary afferents expressing TRPM8 and TRPA1. The mutual cross-desensitization between CA and menthol, and differential modulation of cold- vs. heat-evoked responses, suggests a direct inhibition of TRPM8 and TRPA1 expressed in peripheral nerve endings by CA and menthol, respectively, rather than a central site of interaction.     

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.     

Chronic inflammation and estradiol interact through MAPK activation to affect TMJ nociceptive processing by trigeminal caudalis neurons

The mitogen-activated protein kinase/extracellular regulated kinase (MAPK/ERK) pathway plays a key role in mediating estrogen actions in the brain and neuronal sensitization during inflammation. Estrogen status is a risk factor in chronic temporomandibular muscle/joint (TMJ) disorders; however, the basis for this relationship is not known. The present study tested the hypothesis that estrogen status acts through the MAPK/ERK signaling pathway to alter TMJ nociceptive processing. Single TMJ-responsive neurons were recorded in laminae I–II at the spinomedullary (Vc/C_1-2) junction in naïve ovariectomized (OvX) female rats treated for 2 days with high-dose (20 μg/day; HE2) or low-dose estradiol (2 μg/day; LE2) and after chronic inflammation of the TMJ region by complete Freund's adjuvant for 12–14 days. Intra-TMJ injection of ATP (1 mM) was used to activate Vc/C_1-2 neurons. The MAPK/ERK inhibitor (PD98059, 0.01–1 mM) was applied topically to the dorsal Vc/C_1-2 surface at the site of recording 10 min prior to each ATP stimulus. In naïve HE2 rats, low-dose PD98059 caused a maximal inhibition of ATP-evoked activity, whereas even high doses had only minor effects on units in LE2 rats. By contrast, after chronic TMJ inflammation, PD98059 produced a marked and similar dose-related inhibition of ATP-evoked activity in HE2 and LE2 rats. These results suggested that E2 status and chronic inflammation acted, at least in part, through a common MAPK/ERK-dependent signaling pathway to enhance TMJ nociceptive processing by laminae I–II neurons at the spinomedullary junction region.     

Dyspnea as a noxious sensation: inspiratory threshold loading may trigger diffuse noxious inhibitory controls in humans

Dyspnea, a leading respiratory symptom, shares many clinical, physiological, and psychological features with pain. Both activate similar brain areas. The neural mechanisms of dyspnea are less well described than those of pain. The present research tested the hypothesis of common pathways between the two sensations. Six healthy men (age 30-40 yr) were studied. The spinal nociceptive flexion reflex (RIII) was first established in response to electrical sural stimulation. Dyspnea was then induced through inspiratory threshold loading, forcing the subjects to develop 70% of their maximal inspiratory pressure to inhale. This led to progressive inhibition of the RIII reflex that reached 50 +/- 12% during the fifth minute of loading (P < 0.001), was correlated to the intensity of the self-evaluated respiratory discomfort, and had recovered 5 min after removal of the load. The myotatic H-reflex was not inhibited by inspiratory loading, arguing against postsynaptic alpha motoneuron inhibition. Dyspnea, like pain, thus induced counterirritation, possibly indicating a C-fiber stimulation and activation of diffuse noxious inhibitory descending controls known to project onto spinal dorsal horn wide dynamic range neurons. This confirms the noxious nature of certain types of breathlessness, thus opening new physiological and perhaps therapeutic perspectives.     

GABAA receptor-mediated tonic currents in substantia gelatinosa neurons of rat spinal trigeminal nucleus pars caudalis

In the present study, we describe GABAA receptor-mediated tonic inhibitory currents in the substantia gelatinosa (SG) region of rat spinal trigeminal nucleus pars caudalis (Vc). The GABA(A) receptor-mediated tonic currents were identified by bath-application of the GABAA receptor antagonists, picrotoxin (1mM), SR95531 (100microM) and bicuculline (100microM). All three antagonists completely blocked outward spontaneous (phasic) inhibitory postsynaptic currents, but only picrotoxin and bicuculline induced a significant (>5pA) inward shift of holding currents at a holding potential (Vh) of 0mV in 60-70% of SG neurons, revealing the existence of tonic outward currents. The tonic currents were resistant to further the blockades of glycine receptors or those in addition to glutamate receptors and voltage-dependent sodium channels. An acute bath-application of THDOC (0.1microM), the stress-related neurosteroid, did enhance tonic currents, but only in a small population of SG neurons. In addition, slices incubated with THDOC for 30min increased the probability of neurons with significant tonic currents. The GABAergic tonic inhibition demonstrated in this study may play a significant role in the sensory processing system of the Vc.     

Age-related changes in the effects of 5-hydroxytryptamine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis

The trigeminal subnucleus caudalis (Vc) is the critical brainstem relay site of orofacial nociceptive processing to higher brain centers. The descending serotonergic pathway from the brainstem exerts inhibitory or facilitatory effects on nociceptive transmission in the spinal dorsal horn and the Vc, and SG neurons of the Vc exhibit hyperpolarization, no response or depolarization in response to 5-hydroxytryptamine (5-HT) application. In this study, we examined age-related changes in the effects of 5-HT on SG neurons of the Vc using immature, peripubertal and adult male mice and gramicidin-perforated patch recordings under the current-clamp mode. In the three age groups, hyperpolarization was the major response in SG neurons exhibiting membrane potential changes in response to 5-HT application. The proportion of the SG neurons responding to 5-HT by hyperpolarization was significantly higher in the immature (20/27) than in the adult mice (10/26; P<0.05). The proportion of SG neurons showing no response to 5-HT was significantly higher in the peripubertal (11/21) and the adult mice (13/26) compared with the immature mice (5/27). The amplitude of 5-HT-induced hyperpolarization significantly decreased with increasing postnatal age (correlation coefficient=-0.43, P<0.05). The mean amplitude of 5-HT-induced hyperpolarization was significantly higher in the immature mice (-9.7±1.1 mV, n=20) than in the peripubertal (-5.3±1.0 mV, n=10) and the adult mice (-5.4±0.9 mV, n=10; both P<0.05). These results suggest that the descending serotonergic modulatory influence over the orofacial nociceptive processing in the Vc may change during postnatal development and postnatal age of three weeks is a critical period for changes in 5-HT-induced hyperpolarizing effects in mice.     

Suppressive effects of Neiting acupuncture on toothache: an experimental analysis on Fos expression evoked by tooth pulp stimulation in the trigeminal subnucleus pars caudalis and the periaqueductal gray of rats

To clarify the antinociceptive mechanism of acupuncture on acute pain, c-fos protein (Fos) expression induced by tooth pulp stimulation was immunohistochemically examined in the spinal trigeminal subnucleus pars caudalis (spVc) and the periaqueductal gray (PAG) of rats with or without Neiting acupuncture. The central projection of trigeminal ganglion neurons innervating in the tooth pulp was examined by tract-tracing method with horseradish peroxidase-conjugated wheat germ agglutinin (WGA-HRP). Central terminals from the first maxillary molar tooth were labeled transganglionically in the dorsomedial part of spVc with WGA-HRP. Numerous numbers of Fos-immunoreactive (Fos-ir) cells were found in the spVc and PAG by stimulation of the tooth pulp with acetic acid or saline. Neiting acupuncture significantly reduced the Fos expression in the spVc induced by tooth pulp stimulation. On the other hand, Neiting acupuncture evoked many Fos-ir cells in the PAG. The present results suggest that Neiting acupuncture activated PAG neurons that sent descending inhibitory fibers to medullo-spinal nociceptive neurons, and reduced the number of Fos-expressed neurons in the trigeminal subnucleus pars caudalis mediating noxious information from teeth to the higher central nervous system.     

Changes in duodenal motility produced by noxious mechanical stimulation of the skin in rats

Three major pressure components were recorded in the duodenum of anesthetized rats by the balloon method: (1) small, fast, rhythmic waves which corresponded to the pendular movement of the duodenum; (2) large, slow, rhythmic waves which corresponded to the peristaltic movement; (3) the baseline component of the pressure.Noxious stimulation was applied to the skin of the neck, chest and abdomen, but only abdominal stimulation produced changes in duodenal pressure. The following three reflexes were observed whose efferent pathways were proven to be through sympathetic nerves and whose major pathway was apparently propriospinal: (1) a decrease in the amplitude of the small, fast, rhythmic waves; (2) a decrease in frequency of the large, slow, rhythmic waves; (3) a decrease in baseline of duodenal pressure.