Stimulation sites in periaqueductal gray, nucleus raphe magnus and adjacent regions effective in suppressing oral-facial reflexes

Formalin-induced c-fos expression in the brain of infant rats

In the fetal, infant, and adult rat, injury induces a well-defined behavioral response and induces c-fos expression in the spinal cord dorsal horn. There is more limited information about the processing of noxious stimulation in the infant brain. We describe here the appearance of the Fos protein in the brain of fetal and infant rats following formalin-induced injury. Regions were chosen for analysis with a special focus on brain loci that express c-fos in the adult. No Fos positive cells were found in the brains of fetuses; newborns did not show increased Fos expression after formalin injection in any structure examined. At 3 and 14 days of age, there was a significant increase in Fos staining induced by formalin in the ventral lateral medulla. In contrast, paraventricular and medial dorsal nuclei of the thalamus, the paraventricular nucleus of the hypothalamus, and periaqueductal gray of the midbrain showed increased levels of Fos protein only at 14 days of age. We hypothesize that this developmental pattern is related not only to the maturation of pain perception but also to development of autonomic and defensive reactions to pain in the infant.

PERSPECTIVE

Because the infant processes pain differently than the adult, knowledge of those differences informs pediatric clinical practice. Using Fos expression as a marker of neural activity in the rat, we show that the pattern of brain activation is immature at birth but is in place by 14 days of age.

Study of the neural basis of striatal modulation of the jaw-opening reflex

Previous experimental data from this laboratory demonstrated the participation of the striatum and dopaminergic pathways in central nociceptive processing. The objective of this study was to examine the possible pathways and neural structures associated with the analgesic action of the striatum. The experiments were carried out in rats anesthetized with urethane. The jaw-opening reflex (JOR) was evoked by electrical stimulation of the tooth pulp of lower incisors and recorded in the anterior belly of the digastric muscles. Intrastriatal microinjection of apomorphine, a nonspecific dopamine agonist, reduced or abolished the JOR amplitude. Electrolytic or kainic acid lesions, unilateral to the apomorphine-injected striatum, of the globus pallidus, substantia nigra pars reticulata, subthalamic nucleus and bilateral lesion the rostroventromedial medulla (RVM), blocked the inhibition of the JOR by striatal stimulation. These findings suggest that the main output nuclei of the striatum and the RVM may be critical elements in the neural pathways mediating the inhibition of the reflex response, evoked in jaw muscles by noxious stimulation of dental pulp.

Effect of low-level clenching and subsequent muscle pain on exteroceptive suppression and resting muscle activity in human jaw muscles

OBJECTIVE

To investigate the effects of muscle fatigue induced by low-level isometric jaw-clenching and subsequent glutamate-evoked muscle pain on the exteroceptive suppression (ES) response and resting electromyographic (EMG) activities in human jaw muscles.

METHODS

The resting EMG activity and the ESs were recorded before (baseline), after low-level jaw-clenching (Post1), after subsequent glutamate or isotonic saline injections into the left masseter (Post2), and 60 min after the clenching (Post3) in 23 healthy volunteers.

RESULTS

The late ES (ES2) showed more inhibition at Post1 compared with baseline (P<0.05). It was less inhibited after both types of injections (Post2), and increased at Post3 again (P<0.05) with no significant difference between the glutamate and isotonic saline sessions. The resting EMG activity increased at Post1 and Post2 (P<0.05). The glutamate injection further increased the resting EMG activity in the injected muscle (P<0.01).

CONCLUSIONS

Muscle fatigue influences inhibitory reflex pathways in jaw-closing muscles and subsequent acute muscle pain potentiates the local increase in the resting EMG activity of the painful muscle.

SIGNIFICANCE

Muscle fatigue which can be observed in patients with oral dysfunctions may interact with nociceptive regulation and influence the clinical presentation of jaw symptoms and function.

Descriptive and functional neuroanatomy of locus coeruleus-noradrenaline-containing neurons involvement in bradykinin-induced antinociception on principal sensory trigeminal nucleus

The present study was carried out in Wistar rats, using the jaw-opening reflex and dental pulp stimulation, to investigate noradrenaline- and serotonin-mediated antinociceptive circuits. The effects of microinjections of bradykinin into the principal sensory trigeminal nucleus (PSTN) before and after neurochemical lesions of the locus coeruleus noradrenergic neurons were studied. Neuroanatomical experiments showed evidence for reciprocal neuronal pathways connecting the locus coeruleus (LC) to trigeminal sensory nuclei and linking monoaminergic nuclei of the pain inhibitory system to spinal trigeminal nucleus (STN). Fast blue (FB) injections in the locus coeruleus/subcoeruleus region retrogradely labeled neurons in the contralateral PSTN and LC. Microinjections of FB into the STN showed neurons labeled in both ipsilateral and contralateral LC, as well as in the ipsilateral Barrington's nucleus and subcoeruleus area. Retrograde tract-tracing with FB also showed that the mesencephalic trigeminal nucleus sends neural pathways towards the ipsilateral PSTN, with outputs from cranial and caudal aspects of the brainstem. In addition, neurons from the lateral and dorsolateral columns of periaqueductal gray matter also send outputs to the ipsilateral PSTN. Microinjections of FB in the interpolar and caudal divisions of the STN labeled neurons in the caudal subdivision of STN. Microinjections in the STN interpolar and caudal divisions also retrogradely labeled serotonin- and noradrenaline-containing nucleus of the brainstem pain inhibitory system. Finally, the gigantocellularis complex (nucleus reticularis gigantocellularis/paragigantocellularis), nucleus raphe magnus and nucleus raphe pallidus also projected to the caudal divisions of the STN. Microinjections of bradykinin in the PSTN caused a statistically significant long-lasting antinociception, antagonized by the damage of locus coeruleus-noradrenergic neuronal fibres with (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) (DSP4), a neurotoxin that specifically depleted noradrenaline from locus coeruleus terminal fields. These data suggest that serotonin- and noradrenaline-containing nuclei of the endogenous pain inhibitory system exert a key-role in the antinociceptive mechanisms of bradykinin and the locus coeruleus is crucially involved in this effect.

Modulation of jaw reflexes induced by noxious stimulation to the muscle in anesthetized rats

Previous studies have shown that jaw reflexes and activity patterns of the jaw muscles were modulated in the presence of jaw muscle pain. However, there is no study comparing the modulatory effects on the jaw reflexes induced by noxious stimulation to the jaw muscle. To clarify this, effects of the application of mustard oil (MO), an inflammatory irritant, into the temporalis (jaw-closing) muscle on (1) jaw-opening reflex evoked by tooth pulp stimulation (TP-evoked JOR) as a nociceptive reflex, (2) jaw-opening reflex evoked by inferior alveolar nerve stimulation as a non-nociceptive reflex and (3) jaw-closing reflex evoked by trigeminal mesencephalic nucleus stimulation as a proprioceptive reflex were investigated in anesthetized rats. The MO application induced suppression of all reflexes, and the effect on the TP-evoked JOR was more prominent than on the other reflexes. To elucidate the involvement of endogenous opioid system for the suppressive effect, a systemic administration of naloxone following the MO application was conducted. The MO-induced suppressive effect on the TP-evoked JOR was reversed by the naloxone administration. The results suggest that noxious stimulation to the jaw muscle modulate jaw reflexes particularly for the nociceptive jaw-opening reflex, and the modulatory effect includes both facilitatory and inhibitory aspects. The results also suggest that pain modulatory systems such as the endogenous opioid system play a crucial role in the suppression of the nociceptive transmissions related to nociceptive reflexes, and in some pathological states, defense reflexes may not be evoked properly.

Striatal Inhibition of Nociceptive Responses Evoked in Trigeminal Sensory Neurons by Tooth Pulp Stimulation

The noxious evoked response in trigeminal sensory neurons was studied to address the role of striatum in the control of nociceptive inputs. In urethane-anesthetized rats, the jaw opening reflex (JOR) was produced by suprathreshold stimulation of the tooth pulp and measured as electromyographic response in the digastric muscle, with simultaneous recording of noxious responses in single unit neurons of the spinal trigeminal nucleus pars caudalis (Sp5c). The microinjection of glutamate (80 ηmol/0.5 μl) into striatal JOR inhibitory sites significantly decreased the Aδ and C fiber–mediated–evoked response (53 ± 4.2 and 43.6 ± 6.4% of control value, P < 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons ( n = 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information.

Modulation of sympathetic and somatomotor function by the ventromedial medulla

The ventromedial medulla is implicated in a variety of functions including nociceptive and cardiovascular modulation and the control of thermoregulation. To determine whether single microinjections into the ventromedial medulla elicit changes in one or multiple functional systems, the GABA(A) receptor antagonist bicuculline was microinjected (70 nl, 5-50 ng) into the ventromedial medulla of lightly anesthetized rats, and cardiovascular, respiratory, and nociceptive measures were recorded. Bicuculline microinjection into either the midline raphe or the laterally adjacent reticular nucleus simultaneously increased interscapular brown adipose tissue temperature, heart rate, blood pressure, expired [CO(2)], and respiration rate and elicited shivering. Bicuculline microinjection also decreased the noxious stimulus-evoked changes in heart rate and blood pressure, decreased the frequency of heat-evoked sighs, and suppressed the cortical desynchronization evoked by noxious stimulation. Although bicuculline suppressed the motor withdrawal evoked by noxious tail heat, it enhanced the motor withdrawal evoked by noxious paw heat, evidence for specifically patterned nociceptive modulation. Saline microinjections into midline or lateral sites had no effect on any measured variable. All bicuculline microinjections, midline or lateral, evoked the same set of physiological effects, consistent with the lack of a topographical organization within the ventromedial medulla. Furthermore, as predicted by the isodendritic morphology of cells in the ventromedial medulla, midline bicuculline microinjection increased the number of c-fos immunoreactive cells in both midline raphe and lateral reticular nuclei. In summary, 70-nl microinjections into ventromedial medulla activate cells in multiple nuclei and elicit increases in sympathetic and somatomotor tone and a novel pattern of nociceptive modulation.

Coeruleotrigeminal suppression of nociceptive sensorimotor function during inflammation in the craniofacial region of the rat

Descending action from the locus coeruleus (LC) on the trigeminal sensorimotor function was evaluated in a rat model of oral-facial inflammation. For the induction of oral-facial inflammation, mustard oil (20% solution in 20microl mineral oil) was injected into the region of the temporomandibular joint (TMJ). One week before testing, rats received bilateral lesions of the LC using a cathodal current. The electromyogram (EMG) threshold, which is the threshold intensity for the onset of EMG activity of the masseter muscle evoked by pressure on the TMJ region, was used in the present study as an indicator of the trigeminal sensorimotor function. Following mustard oil injection, in the LC-lesioned rats, EMG thresholds significantly decreased at 30min, which lasted up to 240min. In contrast, EMG thresholds in the LC-intact rats returned to the level before injection after 180min. Systemic naloxone (1.3mg/kg, i.v.) produced a further decrease of EMG thresholds in both the LC-intact and LC-lesioned rats. Under the existence of naloxone, EMG thresholds in the LC-lesioned rats were significantly lower than those of the LC-intact rats. These results suggest that oral-facial inflammation activates the coeruleotrigeminal modulating system and that an action of this system is independent of the opioid depressive mechanism.

Bloqueio anestésico do nervo occipital maior na profilaxia da migrânea

Na fisiopatologia da enxaqueca muitas estruturas estão envolvidas, sendo que o nervo trigêmeo pode ser considerado a estrutura principal. Com o objetivo de determinar a influência do nervo occipital maior (NOM) sobre o comportamento da enxaqueca, estudamos 37 pacientes que apresentavam crises de enxaqueca. Utilizando-se de um estudo duplo cego "cruzado" os pacientes foram submetidos a infiltração do NOM com bupivacaína 0,5% (BP) e soro fisiológicos 0,9% (SF), os efeitos clínicos após os bloqueios anestésicos foram avaliados: subjetivamente através da escala visual analítica para dor e objetivamente determinou-se os limiares de percepção dolorosa. A comparação entre os dois grupos (BP-SF) e (SF-BP) mostrou que: o número e a duração das crises em todos os momentos do estudo não mudaram; a intensidade das crises no grupo (BP-SF) foi menor somente depois da segunda infiltração (P=0,020), em todos os outros momentos não se observaram alterações significativas. Concluímos que o bloqueio anestésico com BP sobre o NOM não altera o número e a duração das crises de migrânea, porém promove uma redução média na intensidade das crises 60 dias após a sua infiltração. Os resultados mostrados sugerem que o NOM participa ativamente sobre a modulação nociceptiva durante as crises de enxaqueca sem aura.

Antinociceptive reflex alteration in acute posttraumatic headache following whiplash injury

Brainstem-mediated antinociceptive inhibitory reflexes of the temporalis muscle were investigated in 82 patients (47 F, 35 M, mean age 28.3 years, SD 9.4) with acute posttraumatic headache (PH) following whiplash injury but without neurological deficits, bone injury of the cervical spine or a combined direct head trauma on average 5 days after the acceleration trauma. Latencies and durations of the early and late exteroceptive suppression (ES1 and ES2) and the interposed EMG burst (IE) of the EMG of the voluntarily contracted right temporalis muscle evoked by ipsilateral stimulation of the second and third branches of the trigeminal nerve were analyzed and compared to a cohort of 82 normal subjects (43 F, 39 M, mean age 27.7 years, SD 7.1). Highly significant reflex alterations were found in patients with PH with a shortening of ES2 duration with delayed onset and premature ending as the primary parameter of this study, a moderate prolongation of ES1 and IE duration and a delayed onset of IE. The latency of ES1 was not significantly changed. These findings indicate that acute PH in whiplash injury is accompanied by abnormal antinociceptive brainstem reflexes. We conclude that the abnormality of the trigeminal inhibitory temporalis reflex is based on a transient dysfunction of the brainstem-mediated reflex circuit mainly of the late polysynaptic pathways. The reflex abnormalities are considered as a neurophysiological correlate of the posttraumatic (cervico)-cephalic pain syndrome. They point to an altered central pain control in acute PH due to whiplash injury.

Striatal modulation of the jaw opening reflex

The effect of striatal electrical and chemical conditioning stimulation (L-glutamate 80-160 nmoles/0.5 microl) on the jaw opening reflex (JOR) was studied in Sprague-Dawley male rats anesthetized with urethane. The JOR was evoked by stimulation of the tooth pulp of lower incisors. This response was suppressed by transection of the dental root, which indicates according with the bibliography, a specific activation of the pulp nerves. Three type of responses were obtained on the evoked JOR by conditioning stimulation of the striatum; being the main one the suppression of the reflex elicited by tooth pulp activation. A second type of response was an increase of the tooth-JOR amplitude. This effect was observed more frequently with glutamate stimulation rather than with electrical activation of the striatum. A third response was observed with chemical stimulation but not by electrical stimulation of the striatum. This was a triphasic response which consisted in an increase followed by an inhibition and a late increase of the tooth-JOR amplitude. A biphasic effect, an increase prior to a decrease of the JOR amplitude, was also recorded with a minor frequency. The distribution of effective sites for electrical and chemical stimulation within the striatum are mainly similar located in the rostral aspect of the nucleus, with the inhibitory sites in the middle of the nucleus and intermingled with the excitatory ones. The complex responses (tri/biphasic) were observed ventrally and caudally in the nucleus. On the basis of the results mentioned above, one could assume that the striatum is related to the modulation of the JOR evoked probably by nociceptive stimulation. However, activation of other type of fibers could not be ruled out.

Stimulation of the locus coeruleus suppresses trigeminal sensorimotor function in the rat

The nucleus locus coeruleus (LC) has been implicated in the modulation of the spinal sensorimotor function. The aim of the present study was to examine the effect of electrical stimulation of the LC on sensorimotor function in the trigeminal system. The following two cases of sensorimotor behaviors mediated by the trigeminal brainstem sensory nuclear complex were examined: (1) the activity of the masseter muscle evoked by pressure on the region of the temporomandibular joint (TMJ); and (2) the activity of the digastric muscle evoked by electrical stimulation of the tooth pulp, resulting in the jaw-opening reflex. In the first case, LC stimulation at 10, 30 and 50 microA resulted in a 70%, 68% and 55% reduction in the magnitude of electromyogram (EMG) activity of the masseter muscle compared with the control (without LC stimulation), respectively. The threshold intensity for the onset of masseter EMG activity increaced to 106%, 111% and 121% of the control with 10, 30 and 50 microA LC stimulation, respectively. In the second case, EMG magnitude in response to the digastric muscle decreased to 42% of the control when 30 microA of LC stimulation was delivered. These results suggest that descending influences from the LC can act in suppression of the trigeminal sensorimotor function.

Electrically-evoked inhibitory effects of the nucleus submedius on the jaw-opening reflex are mediated by ventrolateral orbital cortex and periaqueductal gray matter in the rat

In previous studies we have shown that electrical stimulation of the nucleus submedius inhibits the rat radiant heat-induced tail flick reflex, and that this antinociceptive effect is mediated by the ventrolateral orbital cortex and periaqueductal gray. The aim of the present study was to examine whether electrical stimulation of the nucleus submedius could inhibit the rat jaw-opening reflex, and to determine whether electrolytic lesions of the ventrolateral orbital cortex or the periaqueductal gray could attenuate the nucleus submedius-evoked inhibition. Experiments were performed on pentobarbital-anesthetized rats. The jaw-opening reflex elicited by electrical stimulation of the tooth pulp or the facial skin was monitored by recording the evoked digastric electromyogram. Conditioning stimulation was delivered unilaterally to the nucleus submedius 90 ms prior to each test stimulus to the tooth pulp. After that, electrolytic lesions were made in ventrolateral orbital cortex or periaqueductal gray, and the effect of nucleus submedius stimulation on the jaw-opening reflex was re-examined. Unilateral electrical stimulation of nucleus submedius was found to significantly depress the jaw-opening reflex (mean threshold of 28.0+/-1.4 microA, n = 48), and the magnitude of inhibition increased linearly when the stimulus intensity was increased from 20 to 70 microA, resulting in depression of the digastric electromyogram amplitude from 18.4+/-5.4% to 74.0+/-4.9% of the control (P < 0.01, n = 37). The onset of inhibition occured 60 ms after the beginning of nucleus submedius stimulation and lasted about 100 ms, as determined by varying the conditioning-test time interval. Furthermore, ipsilateral lesions of the ventrolateral orbital cortex or bilateral lesions of the lateral or ventrolateral parts of periaqueductal gray eliminated the nucleus submedius-evoked inhibition of the jaw-opening reflex. These data suggest that the nucleus submedius plays an important role in modulation of orofacial nociception, and provide further support for a hypothesis that the antinociceptive effect of nucleus submedius stimulation is mediated by ventrolateral orbital cortex and activation of a descending inhibitory system in the periaqueductal gray.

Inhibitory effects of electrical stimulation of ventrolateral orbital cortex on the rat jaw-opening reflex

In previous studies, we have shown that electrically or chemically evoked activation of the ventrolateral orbital cortex (VLO) depresses the rat tail-flick (TF) reflex, and this antinociceptive effect is mediated by the periaqueductal gray (PAG). The aim of the present study was to examine whether electrical stimulation of the VLO could inhibit the rat jaw-opening reflex (JOR), and to determine whether electrolytic lesions of the PAG could attenuate this VLO-evoked inhibition. Unilateral electrical stimulation of the VLO significantly depressed the JOR elicited by tooth pulp or facial skin stimuli, with a mean threshold of 30.5+/-2.3 microA (n=22). Increasing stimulation intensities from 30 to 80 microA resulted in greater reduction of the dEMG amplitude from 22.9+/-5.0% to 69.7+/-3.7% of the baseline value (P<0.01, n=22). The inhibitory effect appeared 50 ms after the beginning of VLO stimulation and lasted about 150 ms, as determined by varying the conditioning-test (C-T) time interval. Unilateral lateral or ventrolateral lesions of the PAG produced only a small attenuation of the VLO-evoked inhibition of the JOR, but bilateral lesions eliminated this inhibition. These findings suggest that the VLO plays an important role in modulation of orofacial nociceptive inputs, and provide further support for the hypothesis that the antinociceptive effect of VLO is mediated by PAG leading to activation of a brainstem descending inhibitory system and depression of nociceptive inputs at the trigeminal level. The role played by VLO in pain modulation is discussed in association with the proposed endogenous analgesic system consisting of medullary cord-Sm-VLO-PAG-medullary cord.

Functional changes of brainstem reflexes in Parkinson's disease. Conditioning of the blink reflex R2 component by paired and index finger stimulation

Recovery curves of the R2 component of the blink reflex have been studied in 10 control subjects and 13 parkinsonian patients both after ipsilateral paired stimulation of the supraorbital nerve and after index finger stimulation. In control subjects, both types of conditioning induced a comparable marked inhibition lasting more than 600 ms. In parkinsonian patients, inhibition was reduced after both conditionings. However, differences appeared in the magnitude of the changes: after paired stimulation, it was less significant (ANOVA and post-hoc Duncan's test: p = 0.04) than after index finger stimulation (p = 0.002). In that latter situation, the more marked reduction in inhibition is interpreted, in the light of current physiologic knowledge, by hypoactivity of the Nucleus Reticularis Giganto Cellularis (NRGC) which would make less efficient inhibitory interneurones in the trigemino-facial pathway. The results are thus compatible with the suggestion that NRGC is made indirectly less active in Parkinson's disease.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. I. Masticatory muscle motor systems

Oromotor behavior results from the complex interaction between jaw, facial, and lingual muscles. The experiments in this and subsequent papers identify the sources of multisynaptic input to the trigeminal, facial, and hypoglossal motor nuclei. In the current experiments, pseudorabies virus (PRV-Ba) was injected into the jaw-opening (anterior digastric and mylohyoid) and jaw-closing muscles (masseter, medial pterygoid, and temporalis) in bilaterally sympathectomized rats. Injection volumes ranged from 2 to 21 microl with average titers of 2.8 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were consistent between each of the individual muscles and muscle groups. A predictable myotopic labeling pattern was produced in the trigeminal motor nucleus (Mo 5). Transneuronally labeled neurons occurred in regions known to project directly to Mo 5 motoneurons including the principal trigeminal sensory and supratrigeminal areas, Kölliker-Fuse region, nucleus subcoeruleus, and the parvicellular reticular formation. Maximum survival times revealed polysynaptic connections from the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial pontine reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, the results define the structure of the multisynaptic brainstem neural circuits controlling mandibular movement in the rat.

Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nucleus stimulation

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests that activation of this pathway may alter the excitability of trigeminal motoneurons. Thus, we recorded intracellular responses from cat jaw-closing (JC) andjaw-opening (JO) alpha-motoneurons evoked by raphe stimulation and used a combination of intracellular staining of horseradish peroxidase (HRP) and immunohistochemistry at the light and electron microscopic levels to examine the distribution of contacts made by serotonin (5-HT)-immunoreactive boutons on the two motoneurons types. Electrical stimulation applied to the nucleus raphe pallidus-obscurus complex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) alpha-motoneurons and an EPSP with an action potential in JO (mylohyoid) alpha-motoneurons. The EPSP rise-times (time to peak) and half widths were significantly longer in the JC than in the JO motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO alpha-motoneurons were well stained with HRP. Contacts were seen between 5-HT-immunoreactive boutons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contacts were distributed widely in the proximal three-fourths of the dendritic tree of JC motoneurons but were distributed on more proximal dendrites in the JO motoneurons. At the electron microscopic level, synaptic contacts made by 5-HT-immunoreactive boutons on motoneurons were identified. The present study demonstrated that JC motoneurons receive stronger 5-HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO motoneurons.

An explanation for reflex blink hyperexcitability in Parkinson's disease. II. Nucleus raphe magnus

Hyperexcitable reflex blinks are a cardinal sign of Parkinson's disease. The first step in the circuit linking the basal ganglia and brainstem reflex blink circuits is the inhibitory nigrostriatal pathway (Basso et al., 1996). The current study reports the circuits linking the superior colliculus (SC) to trigeminal reflex blink circuits. Microstimulation of the deep layers of the SC suppresses subsequent reflex blinks at a latency of 5.4 msec. This microstimulation does not activate periaqueductal gray antinociceptive circuits. The brainstem structure linking SC to reflex blink circuits must suppress reflex blinks at a shorter latency than the SC and produce the same effect on reflex blink circuits as SC stimulation, and removal of the structure must block SC modulation of reflex blinks. Only the nucleus raphe magnus (NRM) meets these requirements. NRM microstimulation suppresses reflex blinks with a latency of 4.4 msec. Like SC stimulation, NRM microstimulation reduces the responsiveness of the spinal trigeminal nucleus. Finally, blocking the receptors for the NRM transmitter serotonin eliminates SC modulation of reflex blinks, and muscimol inactivation of the NRM transiently prevents SC modulation of reflex blinks. Thus, the circuit through which the basal ganglia modulates reflex blinking is (1) the substantia nigra pars reticulata inhibits SC neurons, (2) the SC excites tonically active NRM neurons, and (3) NRM neurons inhibit spinal trigeminal neurons involved in reflex blink circuits.

Effects of NK433, a new centrally acting muscle relaxant, on masticatory muscle reflexes in rats

The effects of (-)-(R)-2-methyl-3-(1-pyrrolidinyl)-4'- trifluoromethylpropiophenone monohydrochloride (NK433), a novel centrally acting muscle relaxant, on masticatory muscle reflexes were investigated in rats. NK433 inhibited the monosynaptic tonic vibration reflex of the masseter muscle and the polysynaptic tonic periodontal masseteric reflex. These reflexes are increased by gamma-motor activity. NK433 had a weak inhibitory effect on the polysynaptic jaw opening reflex evoked by electrical stimulation of the tooth pulp, which is little related with gamma-motor activity. Eperisone-HCl depressed the three types of masticatory muscle reflexes. When intravenously administered, eperisone-HCl was equipotent to NK433, but the effect of eperisone-HCl was shorter-lasting than that of NK433. The effect of intragastrically administered NK433 on the periodontal masseteric reflex was about three times stronger than that of eperisone-HCl. These results suggest that NK433 inhibits masticatory muscle reflexes controlled by the gamma-motor system and thus may ameliorate the temporomandibular joint syndrome in man.

Is the cuneiform nucleus a critical component of the mesencephalic locomotor region? An examination of the effects of excitotoxic lesions of the cuneiform nucleus on spontaneous and nucleus accumbens induced locomotion

The cuneiform nucleus and the pedunculopontine tegmental nucleus have both been suggested as possible sites for the mesencephalic locomotor region (MLR), an area from which controlled stepping on a treadmill can be elicited following electrical or chemical stimulation in a decerebrate animal. It has been shown that excitotoxic lesions of the pedunculopontine tegmental nucleus impair neither spontaneous locomotion nor locomotion induced by stimulation of the nucleus accumbens. Excitotoxic lesions of the cuneiform nucleus have not previously been investigated. Rats received either bilateral ibotenate or sham lesions of the cuneiform nucleus combined with bilateral implantation of guide cannulae aimed at the nucleus accumbens. On recovery from surgery spontaneous locomotion was tested, followed by accumbens-stimulated locomotion. For nucleus accumbens stimulation, each rat received bilateral microinjection of each of three doses of d-amphetamine (10.0, 20.0 and 30.0 micrograms) and a vehicle only injection. Locomotor activity was recorded following the injection. In comparison to the sham-lesioned group, the ibotenate-lesioned group showed no differences in either spontaneous or amphetamine-induced locomotor activity. These results suggest that, like the pedunculopontine tegmental nucleus, the cuneiform nucleus is not involved in the direct mediation of spontaneous or accumbens-induced locomotion, and thus is very unlikely to be the anatomical substrate of the MLR. The role of the cuneiform nucleus in other types of behavioural control is discussed.

Ventrolateral orbital cortex and periaqueductal gray stimulation-induced effects on on- and off-cells in the rostral ventromedial medulla in the rat

On- and off-cells of the rostral ventromedial medulla are thought to be involved in bulbospinal inhibition of ascending nociceptive information. Experiments were carried out in lightly anaesthetized rats to assess the effects of prefrontal cortex stimulation on the responses of neurons in the rostral ventromedial medulla. For comparison purposes, effects of periaqueductal gray stimulation were also investigated. Single unit activity was recorded in the rostral ventromedial medulla and on-, off- and neutral-cells were identified based on the tail nocifensor reflex to noxious heat. Short (0.1-1 s) and long (10-15 s) trains of bipolar electrical stimulation (100-300 Hz) were delivered to the ventrolateral orbital cortex of the rat forebrain and the periaqueductal gray. Short-train stimulation of the periaqueductal gray (including dorsolateral, ventrolateral and the dorsal raphé regions) excited 58% (25 of 43) of on-cells and 44% (seven of 16) of off-cells in the rostral ventromedial medulla. Long trains blocked the noxious stimulus-evoked pause of all seven off-cells tested and blocked the excitatory response of two, and enhanced one of three on-cells. Such stimulation also inhibited or abolished the tail-flick reflex at currents below 100 microA. Glutamate microinjections into the periaqueductal gray inhibited the noxious-evoked response of two off- and two on-cells and increased the tail-flick latency. Short-train stimulation of the ventrolateral orbital cortex (100-400 microA) excited eight of 25 on-cells and inhibited the ongoing activity of 10 of 14 off-cells. Long-train ventrolateral orbital cortex stimulation (5-15 s, 100-200 microA, 200-300 Hz) enhanced the noxious evoked responses of 10 of 11 on-cells, prolonged the noxious heat-evoked pause of all of four off-cells and decreased the tail-flick latency (pronociception). The results of this study support the proposed role of on- and off-cells in descending inhibition of nociception from the periaqueductal gray and implicate the ventrolateral orbital cortex in the control of this pathway.

Functional properties of cells in rat trigeminal subnucleus interpolaris following local serotonergic deafferentation

We have previously demonstrated increases in serotonin (5-HT) content and immunoreactivity within spinal trigeminal subnucleus interpolaris (SpVi) that are correlated with the functional changes observed in this subnucleus following adult infraorbital nerve (ION) transection. To assess the possible functional significance of this change, we have examined the influence of 5-HT afference upon the normal response properties of cells in SpVi. We employed local depletion of the transmitter, using 5,7-dihydroxtryptamine (5,7-DHT), in combination with extracellular single-cell recording. Chromatographic methods revealed a 97.6% depletion of 5-HT 24 hr after neurotoxin injection. Immunocytochemical procedures revealed depletion of 5-HT throughout SpVi. Physiological recordings were made from 403 SpVi cells in 5,7-DHT-injected rats and 387 cells in vehicle-injected rats. All recordings were made 19-27 hr after injection. Horseradish peroxidase (HRP) deposits from the recording electrode were used to mark recording tracks. 5-HT depletion did not influence receptive field (RF) location, size, or continuity, or the dynamic response characteristics of SpVi cells. It did, however, (1) alter the probability that certain types of somatosensory receptor surfaces would activate local-circuit neurons, and (2) influence the rate of firing of spontaneously active SpVi cells. There was a significant increase in the proportion of vibrissa-sensitive cells with infraorbital RF components, and a concurrent decrease in the proportion of guard-hair-sensitive cells. It therefore appears that 5-HT input to SpVi is necessary for some mechanoreceptive features of the normal functional organization of this area. These functional changes were interesting in that they were opposite to those found following adult ION transection, which increases 5-HT within SpVi. Thus, changes in 5-HT central afference to SpVi that follow ION damage may be responsible for at least one type of functional change observed following this peripheral lesion.

Reduction of temporalis exteroceptive suppression by peripheral electrical stimulation in migraine and tension-type headaches

Inhibition of the second exteroceptive suppression of temporalis muscle activity (ES2) produced by a preceding electrical stimulus applied at the index was studied in patients suffering from migraine without aura (MO), chronic (CTH) or episodic (ETH) tension-type headache. Each patient group comprised of 12 subjects was compared to a group of healthy controls. Mean duration of unconditioned ES2, measured on 10 averaged rectified responses after labial stimulation at a 0.1 Hz frequency, was reduced in CTH only. From stimulation intensities of 20 mA onward, peripheral-induced inhibition of temporalis ES2 was significantly more pronounced in both subtypes of tension-type headache compared to migraineurs or controls. After an index finger stimulus of 20 mA, temporalis ES2 was abolished in 83% of CTH, 67% of ETH, 25% of MO patients and 8% of controls, whereas unconditioned ES2 was present in all patients. Among 9 ETH patients with normal (> or = 32 msec) unconditioned ES2, 5 had total disappearance of ES2 after a 20 mA index stimulation. These results demonstrate that peripheral conditioning at 20 mA increases the diagnostic sensitivity of ES2 studies. They suggest that the changes observed in tension-type headache are due to hyperexcitability of the reticular nuclei which inhibit the medullary inhibitory interneurons mediating ES2.

Modulation of temporalis muscle exteroceptive suppression by limb stimuli in normal man

The effects of noxious and non-noxious limb stimulations on the second exteroceptive suppression of voluntary temporalis muscle activity (ES2) were studied in healthy human volunteers. Duration of temporalis ES2 was measured on averaged rectified responses obtained after stimulating the labial commissure at an intensity of 25 mA. Single peripheral electrical stimuli applied over nerve trunks or over the skin before the labial stimulus decreased ES2 duration. This effect was most pronounced after cutaneous stimuli, especially of the index finger, and it was not observed when the conditioning stimulus was a 10 second, high frequency train. For stimulation at the index finger, temporalis ES2 inhibition progressively increased with intensity from 10 mA to 40 mA; it was maximal for an interstimulus interval between 50 and 140 ms. After naloxone (0.4 mg or 4 mg, i.v.) there was a partial reversal of the index-induced ES2 depression, but this effect was not significant. Immersion of one hand in water heated at 47 degrees C produced a short-lasting ES2 reduction. These results are comparable, though not similar, to the inhibition of the digastric reflex (or jaw opening reflex) observed in animals after limb stimuli and to the depression of the spinal flexion reflex reported in man after heterotopic peripheral stimuli. Although peripheral stimuli were able by themselves to suppress temporalis EMG activity in some subjects, it is likely that they reduce labial-induced ES2 via activation of brainstem structures, such as periaqueductal gray matter or raphe magnus nucleus, which are thought to inhibit the medullary inhibitory interneurons mediating ES2.

Medullary on-cell activity during tail-flick inhibition produced by heterotopic noxious stimulation

Reflex responses and neuronal excitation elicited by noxious stimuli applied to a given body site can be inhibited by application of noxious stimulation to another, even distant body region. Such heterotopic noxious stimulation (HNS) has been proposed to act via 'diffuse noxious inhibitory controls' (DNIC) which involve supraspinal components. The so-called on-cells of the rostral ventromedial medulla (RVM) in rats are thought to facilitate nociceptive transmission. Experimental manipulations that inhibit on-cells also inhibit withdrawal reflexes and nociceptive thalamic responses. In the present study on-cell activity was recorded in relation to the tail flick (TF) elicited by noxious heat applied to the tail both before and during either immersion of a paw in water above 56 degrees C or application of strong pinch to various body regions. Such HNS elicited strong activation of on-cells, followed by depression even when HNS continued. When this depression was intense, tail-heating failed to elicit vigorous on-cell firing, and TF was retarded or abolished. These results are compatible with the hypothesis that antinociception elicited by HNS involves depression of on-cell firing and hence lack of facilitation of nociceptive transmission.

Modulation by central postsynaptic alpha 2-adrenoceptors of the jaw-opening reflex induced by orofacial stimulation in rats

1. The modulation by alpha 2-adrenoceptors of the jaw-opening reflex (digastric electromyographic responses) elicited by orofacial electrical stimulation (OF-JOR) in pentobarbitone anaesthetized rats was investigated. 2. Increasing doses of clonidine (0.1-1000 micrograms kg-1, i.v.) reduced, in a dose-dependent manner until abolition, the amplitude and duration of the OF-JOR and increased the latency to onset. The sum of amplitudes of the reflex was the most sensitive parameter to the inhibitory effects of clonidine (ED50 = 13.9 micrograms kg-1). 3. Pretreatment with the alpha 2-adrenoceptor antagonist, idazoxan (0.03-1 mg kg-1, i.v.), caused a dose-dependent shift (1.5 to 37 fold) to the right of the dose-response curve for clonidine without significant change of maximum inhibitory effect, in a manner compatible with competitive antagonism (ED50B = 29.0 micrograms kg-1). Pretreatment with yohimbine (0.3 mg kg-1, i.v.) also antagonized the inhibitory effect of clonidine on the OF-JOR. In contrast, the alpha 2-adrenoceptor antagonist ARC-239 (0.3 mg kg-1, i.v.) did not antagonize the effect of clonidine on the reflex. 4. In rats pretreated with reserpine (5 mg kg-1, s.c., 18 h) the OF-JOR was not modified, but the potency of clonidine in inhibiting the reflex was potentiated (ED50 value decreased to 6.8 micrograms kg-1) without a significant change of maximum inhibitory effect. 5. Increasing doses of amphetamine (0.1-3000 micrograms kg-1, i.v.) caused a dose-related, but partial, inhibition of the OF-JOR (ED50 = 135 micrograms kg-1; Emax = 67%). Pretreatment with idazoxan (0.1 mg kg-1, i.v.)induced a nine fold shift to the right of the dose-response curve for amphetamine, while treatment with the depleting drug alpha-methyl-p-tyrosine (150mg kg-1 daily, i.p., for 14 days) abolished the inhibitory effect of this indirect adrenoceptor agonist on the OF-JOR.6. Morphine (0.1-3000 microgkg-1, i.v.) also reduced the OF-JOR in a dose-dependent manner (ED50 value about 325 microg kg-1) but, in contrast to clonidine, it failed to inhibit the reflex fully (Emax = 48%).As expected, pretreatment with the opioid antagonist naloxone (1 mg kg-1, i.v.) abolished the inhibitory effect of morphine on the OF-JOR, while it did not alter that of clonidine.7. Chronic, but not acute, pretreatment with idazoxan (3 mg kg-1 daily, i.p. for 14 days) led to a marked potentiation of the inhibitory effect of clonidine on the OF-JOR (ED50 value decreased to 4.2 microg kg-1), without a significant change of maximum inhibitory effect.8. Together the results indicate that clonidine evokes a potent inhibition of the OF-JOR in rats through the activation of postsynaptic alpha2-adrenoceptors. It is suggested that this functional response represents a simple and useful in vivo model for studying various regulatory mechanisms of central alpha2-adrenoceptors.

The anterior pretectal nucleus: a proposed role in sensory processing

Four nuclei of the pretectal complex, the olivary pretectal nucleus, the medial pretectal nucleus, the nucleus of the optic tract and the posterior pretectal nucleus, all have a demonstrated role in visual function. In contrast, the anterior pretectal nucleus (APtN) has no inputs from retina and has few outputs to visual accessory nuclei. The APtN has connections with areas associated with sensory functions and it has been suggested that this nucleus may have a role to play in somatosensory processing. An increasing number of behavioural and electrophysiological studies support this view. Brief low-intensity electrical or chemical stimulation of the APtN causes antinociception in the tail flick test in both unanaesthetised and anaesthetised animals. This inhibition of the tail flick response is attenuated by naloxone, alpha-adrenoceptor antagonists and muscarinic cholinergic receptor antagonists. Electrical stimulation of the APtN is similarly effective in the paw pressure and formalin tests. APtN stimulation also causes a brief inhibition of the tooth pulp-evoked jaw opening reflex. studies with [C14]2-deoxyglucose indicate that peripheral noxious stimuli will cause an increase in metabolic activity within the APtN. Animals with electrodes placed in the APtN will self-administer electrical stimulation and this can reduce the aversive and autonomic effects of stimulating the ventromedial hypothalamus. Part of the antinociceptive effects of stimulating the APtN are due to a descending inhibition of spinal dorsal horn projection neurones. Multireceptive neurones deep in the dorsal horn are inhibited by APtN stimulation. In contrast, superficial projection neurones that respond to intense cutaneous stimuli are excited by APtN stimulation. The APtN receives an excitatory input from low-threshold afferents via the dorsal column pathway and a high-threshold excitatory drive from superficial cells projecting through the dorsolateral funiculus. The excitatory input from the dorsal columns may well participate in the long-term inhibition of spinal projection neurones evoked by dorsal column stimulation. These ascending excitatory pathways may also be important to the long-term activation of descending inhibition from the APtN.

Exteroceptive suppression of temporalis muscle activity: methodological and physiological aspects

In recent years studies of the suppression of EMG activity in temporalis muscle induced by stimulation in the trigeminal territory have opened new perspectives in headache research. The various methods that have been used in different laboratories are reviewed and some of the physiological modulations of temporalis exteroceptive suppression are described. Among different methods of recording, averaging 10 full-wave rectified EMG responses produces results with acceptable variability and discomfort. In order to obtain maximal responses the intensity of the stimulation should reach at least 20 mA. To avoid habituation of the second temporalis exteroceptive suppression period (ES2), the stimulation frequency has to be at 0.1 Hz or below. The level of voluntary contraction is not a critical variable as long as it reaches 50% of maximum. Some physiological variations of temporalis suppression are well documented. In females, ES2 is shorter during menstruation than at mid-cycle and correlated with the estradiol/progesterone ratio in plasma. Conditioning temporalis ES2 by a preceding peripheral stimulus markedly reduces its duration, which is partly reversible by naloxone. Various pharmacological agents are able to modify temporalis ES2: its duration is increased by 5-HT1 antagonists, but decreased by 5-HT uptake blockers; contradictory results have been obtained with acetylsalicylic acid. These results suggest that inhibitory brain-stem interneurons mediating temporalis ES2 are inhibited by serotonergic afferents, probably from the raphe magnus nucleus, and that the latter receives an excitatory input from the periaqueductal gray matter and other limbic structures, in part via opioid receptors.

The substantia innominata complex and the peripeduncular nucleus in orofacial dyskinesia: a pharmacological and anatomical study in cats

It has been shown that orofacial dyskinesia, i.e. a syndrome of abnormal involuntary movements of the facial muscles, can be elicited from the sub-commissural part of the globus pallidus and the adjoining dorsal parts of the extended amygdala in cats. Until now it is unknown whether the peripeduncular nucleus, which receives input from these structures according to anterograde tracing studies, plays a role in the funneling of orofacial dyskinesia to lower output stations. In the present study the connection of the subcommissural part of the globus pallidus and dorsal parts of the extended amygdala with the peripeduncular nucleus was investigated anatomically, using cholera toxin subunit B as a retrograde tracer, and functionally, using intracerebral injections of GABAergic compounds. The anatomical data show that the sub-commissural part of the globus pallidus and dorsal parts of the extended amygdala were marked by cholera toxin sub-unit B-immunoreactive cells following injections of this retrograde tracer into the peripeduncular nucleus. Thus, it could be confirmed that the peripeduncular nucleus receives input from the sub-commissural part of the globus pallidus and dorsal parts of the extended amygdala. Still, the orofacial dyskinesia elicited by local injections of the GABA antagonist picrotoxin (500 ng/0.5 microliters) into the sub-commissural part of the globus pallidus and dorsal extended amygdala was only in part attenuated by local injections of the GABA agonist muscimol (100 ng/l microliters) into the peripeduncular nucleus. Only the number of tongue protrusions was significantly attenuated, but not that of the ear and cheek movements. Furthermore, tongue protrusions, but no additional oral movements, were elicited by picrotoxin injections (375-500 ng) into the peripeduncular nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Anterior pretectal nucleus-induced modulatory effects on trigeminal brainstem somatosensory neurons

This study examined the effects of anterior pretectal nucleus (APT) conditioning stimulation on the activity of 92 functionally identified somatosensory brainstem neurons in the trigeminal (V) subnuclei oralis, interpolaris and caudalis of anesthetized rats. Conditioning stimulation inhibited most of the nociceptive neurons and some of the non-nociceptive neurons; facilitation was observed only in some non-nociceptive neurons. These data indicate that the ATP has modulatory effects on both nociceptive and non-nociceptive V somatosensory neurons.

Effects of lateral reticular nucleus stimulation on trigeminal sensory and motor neuron activity related to the jaw opening reflex

The effects of lateral reticular nucleus (LRN) stimulation on the responses to tooth pulp (TP) stimulation of neurons located in the trigeminal (V) sensory (39 units) and motor (33 units) nuclei were assessed in anesthetized rats. Only neurons which responded to TP stimulation with bursts of spikes that were in a constant temporal relationship with the digastric EMG signal were studied. The LRN-stimulating electrodes were positioned at optimal sites to suppress the TP-evoked jaw-opening reflex (JOR) recorded simultaneously with the neuronal activity related to it. It was found that: (1) the neurons in the V nucleus oralis responded to TP stimulation with 3-8 msec latency excitatory responses that were suppressed during LRN conditioning stimulation with a time course comparable to that of the JOR suppression, and (2) the neurons in the V nucleus motor responded to TP stimulation with 5-15 msec latency excitatory responses. This activity was suppressed during LRN-conditioning stimulation with a time course that parallels the inhibition of the activity of nucleus oralis neurons and of the JOR. However, assuming that the excitatory interneurons for the V motoneurons are located in the nucleus oralis, the suppression of this input by LRN may account for the lack of response in V motor neurons. Thus, we suggest that LRN inhibits the TP-evoked JOR by acting on the sensitive afferent limb of the reflex.

Periaqueductal gray matter and nucleus raphe magnus involvement in anterior pretectal nucleus-induced inhibition of jaw-opening reflex in rats

In previous studies we have shown that electrical stimulation of the cortex or anterior pretectal nucleus (APT) inhibits the jaw-opening reflex (JOR). In the present study we investigated whether these effects are mediated by a relay in the periaqueductal gray matter (PAG) or rostroventromedial medulla (RVM). Experiments were performed on chloralose-urethane anesthetized rats. The JOR which was elicited by electrical stimulation of the mandibular incisor tooth was monitored by recording the evoked digastric muscle activity. Conditioning stimulation (20 ms train of 0.2 ms pulses at 400 Hz) was delivered to the facial area of the sensorimotor cortex, APT, PAG or nucleus raphe magnus (NRM) 50 ms prior to the test stimulus to the tooth that evoked the JOR. In addition, the effects of microinjections of glutamate into APT, PAG and NRM on the tooth-evoked JOR were also evaluated. The inhibition of the JOR by electrical and glutamate conditioning stimulation was found to be most potent for activation of the NRM and least potent for the APT. Local anesthetic (2% lidocaine, 0.3-0.6 microliters) block of the PAG could partially, significantly (P less than 0.05) and reversibly reduce both the APT and cortical-induced depression of the JOR. Lidocaine block of the ventromedial pons reversibly reduced the PAG, APT and cortical-induced inhibition of the JOR (P less than 0.05). Lidocaine block of the lateral RVM had powerfully (P less than 0.01) and reversibly reduced the PAG-induced inhibition, but had only a small effect (P less than 0.05) on the APT-induced inhibition and no significant effect on the cortical-induced inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatodendritic morphology of on- and off-cells in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) contains two classes of physiologically defined neurons, on-cells and off-cells, that are implicated in nociceptive modulation. In a continuing effort to detail the neural circuitry that underlies the activity of these two distinct neuronal types, the somatodendritic morphology of on- and off-cells was studied in the cat, rat, and ferret. In lightly anesthetized animals, on-cells increased and off-cells decreased their discharge rate during a withdrawal reflex evoked by noxious stimuli. Following their physiological characterization by using intracellular recording, on- and off-cells were injected with either horseradish peroxidase or biocytin and their somatodendritic arborizations were examined. Labeled on- and off-cells included fusiform and stellate cells of all sizes as well as large multipolar neurons. Although the somatic shape of both on- and off-cells in RVM was heterogeneous, off-cells tended to be fusiform neurons whose long axis was oriented mediolaterally. The dendritic domains of both on- and off-cells extended bilaterally past the lateral edge of the trapezoid body or pyramid and ventrally to, and sometimes including, the trapezoid body or pyramid. In contrast to their extensive mediolateral spread, the dendritic domains of both cell types were limited to the ventral half of the reticular formation and were compressed along the rostrocaudal axis. The dendritic arbor of individual on- and off-cells extended well beyond the cytoarchitectonic boundaries of any single nuclear region, within the domain delineated as the RVM. The spatial domains of the dendritic arbors of on- and off-cells are further evidence that the on- and off-cells throughout the RVM constitute an integrated unit in the modulation of nociceptive transmission.

Characterization of descending inhibition and facilitation from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Descending influences on the spinal nociceptive tail-flick (TF) reflex produced by focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) were examined and characterized in rats lightly anesthetized with pentobarbital. Both inhibition and facilitation of the TF reflex were produced by electrical stimulation at identical sites in the NGC/NGC alpha; glutamate microinjection only inhibited the TF reflex. The chronaxie of stimulation for inhibition of the TF reflex was 169 +/- 28 microseconds. Inhibition of the TF reflex by stimulation was produced throughout the NGC and NGC alpha; intensities of stimulation for inhibition were least in the ventral NGC and in the NGC alpha. At threshold intensities of stimulation, inhibition of the TF reflex did not outlast the period of stimulation. Facilitation of the TF reflex was produced at many of the same sites at which stimulation inhibited the TF reflex, but always at lesser intensities of stimulation (mean, 10 microA vs. 43 microA for inhibition, n = 25). Stimulation in the NGC/NGC alpha at threshold intensities for facilitation or inhibition of the TF reflex did not significantly affect blood pressure. Strength-duration characterization of electrical stimulation and microinjection of glutamate into identical sites in the NGC and NGC alpha suggest that descending inhibition of the TF reflex results from activation of cell bodies in the NGC and NGC alpha.

Role of anterior pretectal nucleus in somatosensory cortical descending modulation of jaw-opening reflex in rats

Both the somatosensory cerebral cortex and anterior pretectal nucleus (APT) have been shown to produce descending modulation of trigeminal (V) and spinal somatosensory neurone and reflex activities. Since the APT receives a direct projection from the somatosensory cortex, experiments were performed to compare the effects of APT and somatosensory cortex stimulation on the jaw-opening reflex (JOR) and to examine the possible involvement of APT in corticofugal modulation. Conditioning stimulation of the ipsilateral or contralateral somatosensory cortex in chloralose-urethane anaesthetized rats induced inhibition of the JOR elicited by test stimulation of the maxillary skin or tooth pulp, at conditioning-test intervals between 30 and 200 ms. A similar time course of inhibition of the JOR was noted with APT conditioning stimulation. Local unilateral injection of 2% lidocaine (0.3-0.6 microliter) into APT could partially and reversibly reduce the ipsilateral cortically evoked inhibition of the JOR. Ibotenic acid (5 micrograms, 0.5 microliter)-induced unilateral lesions of APT and its adjoining structures also greatly reduced the ipsilateral cortically induced inhibition of the JOR. Histological reconstruction of APT lesion sites and data analysis indicated that this reduction in the corticofugal inhibition was proportionally and significantly related to the extent of damage to APT, but not to its adjoining structures. These findings collectively suggest that the cortically-induced inhibition of the JOR is at least partly mediated by a relay in the APT.

Non-reciprocal facilitation of trigeminal motoneurons innervating jaw-closing and jaw-opening muscles induced by iontophoretic application of serotonin in the guinea pig

Effects of iontophoretically applied serotonin (5-HT) and its antagonist, methysergide (MS), on masseter (a jaw-closer, MA.MNs) and anterior digastric motoneurons (a jaw-opener, AD.MNs) were studied in paralyzed guinea pigs, chloralose-anesthetized or decerebrate. Unitary activity was recorded with multibarrel capillary electrodes from MA.MNs and AD.MNs identified by antidromic spikes evoked by stimulation of the masseter and anterior digastric nerves, respectively. Under chloralose anesthesia, both MA.MNs and AD.MNs were almost quiescent, and application of 5-HT alone induced no changes in discharge of either of them. However, iontophoretically applied 5-HT increased the frequency of discharge induced by iontophoretic application of glutamate in 26 of 34 MA.MNs (76%) and 17 of 30 AD.MNs (59%) tested. MS depressed the glutamate-induced activity in 17 MA.MNs and 3 AD.MNs, respectively, in which 5-HT exerted a facilitatory effect on the glutamate-induced activity. In decerebrate preparations, the firing index of spikes of MA.MNs monosynaptically evoked by stimulation of the trigeminal mesencephalic nucleus was increased by 5-HT and decreased by MS. 5-HT also enhanced the discharge of MA.MNs induced by a ramp-and-hold stretch of the masseter muscle. We conclude that 5-HT alone does not excite either MA.MNs or AD.MNs, but potentiates the effect of excitatory inputs to them: 5-HT exerts a modulatory facilitatory action on trigeminal motoneurons.

Involvement of descending monoaminergic systems in the transmission of dental pain in the trigeminal nucleus caudalis of the rabbit

Perfusates were taken from the superficial layers of the subnucleus caudalis of the trigeminal sensory nuclear complex (SpVc), the first relay station of dental pain, with a push-pull cannula system and were assayed for endogenous serotonin (5-HT) and catecholamines by high-pressure liquid chromatography with an electrochemical detection. Spontaneous release of 5-HT and epinephrine was observed, while that of norepinephrine was not. Tooth pulp stimulation (ST) tended to increase the level of 5-HT in the perfusates. Pretreatment with morphine at a dose of 10 mg/kg (i.v.) significantly enhanced the release of 5-HT. However, there was no significant difference in morphine effect on the 5-HT level between stimulated and non-stimulated animals. Systemic administration of morphine (10 mg/kg i.v.) completely inhibited the release of immunoreactive substance P from the superficial layers of SpVc evoked by ST, and this inhibition was antagonized by local application of methysergide (10(-4) M). These results suggest that in the superficial layers of SpVc, morphine may primarily activate the descending 5-HT pathway which serves to modulate dental pain transmission.

Pertussis toxin inhibits morphine analgesia and prevents opiate dependence

Six days after intracerebroventricular pretreatment of rats with pertussis toxin (PTX 0.5 microgram/rat) there was a marked decrease in the antinociceptive effect of morphine, regardless of the route of opioid administration (into the periaqueductal gray matter, intrathecally or intraperitoneally) or the analgesic test used (tail flick and jaw opening reflex). PTX pretreatment also partially attenuated the naloxone-precipitated withdrawal syndrome in morphine-dependent rats, significantly reducing teeth chattering, rearing and grooming. These in vivo findings indicate that G-protein-dependent mechanisms are involved in morphine analgesia and dependence. The biochemical mechanism could be related to ADP ribosylation of Gi coupled to the adenylate cyclase system, but an interaction of PTX with other G-proteins linked to different second messengers or directly to ionic channels cannot be excluded.

Medullary on- and off-cell responses precede both segmental and thalamic responses to tail heating

Medullary on- and off-cell responses to tail heating were studied in lightly anesthetized rats. In 10 animals the electromyographic (EMG) activation of tail muscles was recorded simultaneously with on- or off-cells. The on-cell burst or the off-cell pause always preceded segmental EMG activation by about 0.5 sec. In turn, EMG activation preceded visible tail flick by about 0.09 sec. In 13 other animals ventrobasal thalamic unitary responses were recorded simultaneously with on- and off-cell responses. On-cell bursts or off-cell pauses always preceded thalamic responses by about 0.4 sec. These results support the notion that on- and off-cells play a regulatory role in both segmental and ascending nociceptive transmission.

Axonal trajectories and terminations of on- and off-cells in the cat lower brainstem

Two physiologically defined classes of pontomedullary raphe neurons were intracellularly labeled in order to determine the target nuclei of their axonal projections. In the lightly anesthetized cat, cells either increased (on-cells) or decreased (off-cells) their discharge rate during the paw withdrawal reflex evoked by noxious pinch or heat. On- and off-cells were injected with horseradish peroxidase and the initial course of labeled axons through the lower brainstem was reconstructed. On-cell projections to the pontomedullary raphe and medial reticular nuclei were sparse. On-cells projected densely to regions of the lateral reticular formation and the ventrolateral medulla at both rostral and caudal medullary levels. In general, on-cells had few collaterals and spare axonal swellings. In contrast to on-cells, most off-cells had axons that collateralized densely within the brainstem raphe and adjacent reticular formation. Such collaterals were either local, within the neuron's dendritic field, or distant, involving a projection of 1-8 mm. One off-cell had a dense terminal field within the sensory trigeminal complex, a projection that may subserve the inhibition of trigeminal sensory neurons produced by raphe magnus stimulation. Well-labeled off-cells had numerous collaterals and dense regions of axonal swellings. In summary, off-cells terminated densely in the raphe magnus and adjacent reticular formation whereas on-cells projected predominantly to the ventrolateral medulla, a region implicated in autonomic control. Local off-cell collaterals provide an anatomical substrate that would enable off-cells to coordinate the activity of on- and off-cells through synaptic contacts.

Inhibitory effect of stimulation of the anterior pretectal nucleus on the jaw-opening reflex

The anterior pretectal nucleus (APT) has been recently implicated in sensorimotor integration and has been shown to have suppressive influences on tail flick behaviour and on nociceptive responses of spinal dorsal horn neurones in rats. The present study tested the effect of stimulation of the APT on the rat's digastric jaw-opening reflex elicited by orofacial stimuli. Either ipsilateral or contralateral electrical stimulation at histologically confirmed sites within and immediately subjacent to the APT produced a suppression of the reflex that had an onset of 20-30 ms, peaked around 50 ms and lasted for 200-300 ms; in some cases, a brief period of reflex facilitation preceded the onset of inhibition and the inhibition was sometimes followed by a facilitatory period. No prolonged period of suppression induced by electrical stimulation was noted in these anaesthetized rats. The injection of monosodium glutamate at comparable sites within and subjacent to APT induced reflex suppression that lasted several minutes. These findings represent the first documentation of APT-induced modulation in the trigeminal sensorimotor system, but support recent evidence suggesting the involvement of APT in sensorimotor integration and modulation.

Changes in mechanoreceptive field properties of trigeminal somatosensory brainstem neurons induced by stimulation of nucleus raphe magnus in cats

Experiments were carried out on adult anesthetized cats in which the effects of nucleus raphe magnus (NRM) conditioning stimulation (20 ms) were tested on the responses evoked by orofacial stimuli in single brainstem neurons of trigeminal (V) subnucleus oralis. The NRM stimulation induced inhibition of the responses of 57 of 77 low-threshold mechanoreceptive (LTM) neurons and the one wide-dynamic range (WDR) neuron tested. The duration of the neuronal inhibition ranged from 300-600 ms and the mean threshold for inhibition ranged from 47.8 +/- 4.8 to 102.7 +/- 15 microA depending on the orofacial stimulation site (skin or tooth pulp) and form (mechanical or electrical) of cutaneous stimuli used to evoke neuronal responses. In 20 LTM neurons showing NRM-induced inhibition that were specifically examined for the effects of NRM stimulation on the mechanoreceptive field, one population (n = 11) showed shrinkage (mean 55 +/- 4.4% from control area) of the mechanoreceptive field while the remaining neurons (n = 9) showed no change in mechanoreceptive field size during NRM stimulation. The former group of neurons were also distinguished from the latter neurons by their significantly larger mechanoreceptive field and the activation of the majority of them by electrical stimuli applied outside their mechanoreceptive field. The responses of these neurons evoked by low-threshold inputs from the edge of the mechanoreceptive field were more sensitive to NRM conditioning stimulation than responses evoked from the mechanoreceptive field center, as judged by threshold, magnitude and duration of the NRM-induced inhibition. These findings underscore the sensitivity of LTM neurons to NRM influences. They also reveal a particular population of oralis neurons which have a differential sensitivity of low-threshold inputs evoked from the edge compared to the center of the mechanoreceptive field.

Putative nociceptive modulatory neurons in the dorsolateral pontomesencephalic reticular formation

The present study was undertaken to confirm the dorsolateral pontomesencephalic reticular formation (an area that includes the nucleus cuneiformis (NCF) and nucleus parabrachialis (NPB)) as a sensitive site for stimulation-produced antinociception and to investigate the possibility that there are cells in this region that show a change in activity that can be correlated with the occurrence of a nocifensive reflex (the tail-flick withdrawal response from noxious heat (TF)). Such cells have been previously demonstrated in the rostral ventromedial medulla (RVM). Extracellular single unit recording studies were made at sites from which it was possible to inhibit the TF with currents of 10 microA or less. Cellular response to 3 TF trials at 5 min intervals, and spontaneous activity over a 10 min period, were monitored for each unit. Of the cells encountered, 17% displayed an increase (on-cells) and 4% a decrease (off-cells) in activity that preceded the TF. The remaining cells were unaffected by the TF (neutral cells). On- and off-cells were found throughout the region from which TF suppression was observed but were most concentrated in the areas of the NCF and NPB. The presence of on- and off-cells in the NCF/NPB region as well as the RVM suggests that the input to RVM from NCF/NPB may be important in descending nociceptive modulation.

Synaptic substrates for enkephalinergic and serotoninergic interactions with dental primary afferent terminals in trigeminal subnucleus interpolaris: an immunocytochemical study using peroxidase and colloidal gold

Pain processing in the trigeminal complex has been thought to reside primarily in the spinal subnucleus caudalis (Vc). However, trigeminal tractotomies eliminating primary afferent input to Vc and severance of secondary trigemino-thalamic fibers from Vc do not disturb pain perception from the central face and oral cavity. Furthermore, large numbers of neurons that are highly responsive to noxious stimuli and suppressed by inputs from the periaqueductal gray and raphe complex have been identified in subnuclei interpolaris (Vi) and oralis (Vo). Therefore, the purpose of this study was to assess the distribution and spatial arrangements of nociceptive modulatory transmitters with nociceptive afferents and trigemino-thalamic relay cells in the rostral portion of the spinal trigeminal nuclear complex. The dental pulp contains predominantly nociceptors that project to all three subdivisions of the trigeminal spinal complex. These projections were visualized by anterograde transganglionic transport of horseradish peroxidase or by degeneration following administration of toxic ricin to the pulp chambers. The spatial arrangements of dental primary afferents with enkephalinergic (ENK) and serotoninergic (5HT) inputs was then assessed by employing avidin-biotin peroxidase and protein-A colloidal gold double-labeling immunocytochemistry. Trigemino-thalamic relay cells were also labeled by retrograde transport of HRP after stereotaxic injections into the ventrobasal thalamus. ENK and 5HT immunoreactivity was found in the ventrolateral quadrant and lateral margin of Vi, together with the adjacent interstitial nucleus (IN). This activity extended from the caudal pole of Vi and the periobex region, where it was most dense, rostrally to a position approximately 2.9 mm from the Obex. Neither ENK nor 5HT immunoreactivity was observed in Vo. Primary dental afferents projected into the ventromedial quadrant of rostral Vi and were found in the ventrolateral quadrant and dorsal aspect of the subnucleus farther caudally. They appeared as simple boutons with single contacts or as larger, sometimes scalloped terminals that formed multiple contacts. Postsynaptic elements were usually small dendritic profiles, although relay cell somata rarely received primary afferent inputs. Many primary afferents entered areas of synaptic clustering and contacted enkephalinergic dendrites, some of which were also postsynaptic to serotoninergic synapses. Alternatively, primary afferents contacted unlabeled processes that were also postsynaptic to the enkephalinergic element to form a triad arrangement. The least common occurrence was axo-axonic contacts in which enkephalinergic synapses were presynaptic to primary afferents. Both enkephalinergic and serotoninergic synaptic categories displayed round vesicles and generally formed asymmetric junctions.(ABSTRACT TRUNCATED AT 400 WORDS)

The nuclei of origin of monoaminergic, peptidergic, and cholinergic afferents to the cat nucleus reticularis magnocellularis: a double-labeling study with cholera toxin as a retrograde tracer

Using a sensitive double-immunostaining technique with nonconjugated cholera toxin B subunit (CT) as a retrograde tracer, we examined the cells of origin and the histochemical nature of afferents to the cat nucleus reticularis magnocellularis (Mc) of the medulla oblongata. After injections of CT confined to the Mc, we found that the major afferents to the Mc arise from: (1) the lateral part of the bed nucleus of the stria terminalis, the nucleus of the anterior commissure, the preoptic area, the central nucleus of the amygdala, the posterior hypothalamus, and the nucleus of the fields of Forel; (2) the Edinger-Westphal nucleus, the mesencephalic reticular formation, and the ventrolateral part of the periaqueductal grey; (3) the nuclei locus coeruleus alpha (LC alpha), peri-LC alpha, locus subcoeruleus, and reticularis pontis oralis and caudalis; (4) the caudal raphe nuclei; and (5) the nucleus reticularis ventralis of the medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of functionally identified nociceptive and nonnociceptive facial primary afferents and presynaptic excitability changes induced in their brain stem endings by raphe and orofacial stimuli in cats

The activity of 221 single primary afferent units was recorded extracellularly in the trigeminal (V) ganglion of chloralose-anaesthetized cats to examine the receptive field properties of nonnociceptive and nociceptive cutaneous afferents and the effect of conditioning stimulation of the raphe system and orofacial afferents on the antidromic excitability of their brain stem endings in V subnucleus caudalis. In addition to slowly adapting and rapidly adapting low-threshold mechanosensitive afferents, we functionally identified three classes of cutaneous nociceptive afferents: these included A-delta high-threshold mechanoreceptive afferents (A-delta HTMs), C-fiber high-threshold mechanoreceptive afferents (C-HTMs), and C-polymodal nociceptive afferents (CPNs). Most of the CPNs could be activated by light tactile stimuli as well as by heavy pressure and pinch and noxious radiant heat applied to their mechanoreceptive field which usually involved a localized spot (approximately 1 mm in diameter) of skin. In contrast, the C-HTMs and A-delta HTMs could not be activated by radiant heat stimuli although some did show sensitization which was also a feature of the CPNs; they did respond to noxious mechanical stimulation of a localized area of skin. We noted that orofacial conditioning stimulation could produce an increase in antidromic excitability which was considered a reflection of primary afferent depolarization (PAD) in both nociceptive and nonnociceptive afferents innervating the cat's facial skin; nonnoxious mechanical stimuli and electrical stimuli were particularly effective in the low-threshold mechanosensitive afferents and noxious mechanical and high-intensity electrical stimuli were especially effective in the cutaneous nociceptive afferents. Raphe conditioning stimulation also was very effective in inducing PAD in these nociceptive afferents; however, the raphe conditioning effects were not limited to these nociceptive afferents since PAD was also frequently demonstrated in the low-threshold mechanosensitive afferents.

Nucleus cuneiformis and pain modulation: anatomy and behavioral pharmacology

The anatomical substrate and behavioral pharmacology of stimulation-produced analgesia resulting from electrical stimulation of the pontomesencephalic nucleus cuneiformis (NCF) was determined in the present study. Maximum increase in nociceptive tail-flick latencies following NCF stimulation occurred during the first 5 min post stimulation and decreased afterwards. The increased reflex latency could be attenuated by prior treatment with the narcotic antagonist, naloxone or the cholinergic antagonist, scopolamine. The anatomical projections of NCF were identified in autoradiographic and histochemical studies. Ipsilateral fibers coursed caudal from the NCF injection site through the ventral pontine reticular formation to innervate nucleus raphe magnus and the ipsilateral nucleus magnocellularis. At rostral medullary levels fibers coursed dorsolateral to innervate the ipsilateral nucleus reticularis parvocellularis. Descending contralateral fibers crossed through the decussation of the superior cerebellar peduncle, then coursed ventrolaterally projecting to the contralateral nucleus magnocellularis. Two primary groups of ascending fibers were observed. The dorsally located group ascended through the central tegmental tract projecting to the dorsal raphe, ipsilateral periaqueductal gray, nucleus parafascicularis and centromedianus, the intermediolateral and lateral thalamic nuclei. The ventral group coursed ventrolateral from the injection site projecting to the substantia nigra, zona compacta, ventral tegmental area of Tsai, zona incerta, Fields of Forel, lateral hypothalamic nucleus and nucleus reuniens. These anatomic and behavioral data suggest that NCF plays an important role in sensory/motor integration relevant to pain transmission.