Sensory processing in a thermal afferent pathway

Extracellular recordings were made from cold-receptive afferent fibers in the trigeminal ganglion of rats anesthetized with halothane. By applying a standardized series of steady or changing temperatures to the receptive fields, we recorded the static and dynamic responses of the afferents. Comparable recordings were made from neurons in the marginal layer of the caudal trigeminal nucleus onto which the cold fibers synapse. The static and dynamic responses of the afferent fibers were reproduced faithfully by the second-order neurons, but at a much higher level of activity. Ganglionectomy silenced the second-order cells. Their continuous high level of activity appears to depend on the tonic input from the afferent fibers and not on any intrinsic circuits in the medulla.

Observations on facial nociception in a monkey after destruction of the rostral part of the trigeminal sensory nuclear complex

Facial sensibility was assessed in a cynomolgus monkey in whom the trigeminal main sensory nucleus and the rostral part of the trigeminal nucleus oralis on one side had been destroyed. The animal responded equally to noxious mechanical stimuli applied to the two sides of the face; no deficiency in nociception could be detected. This finding suggests that the synaptic connections made by nociceptor afferents from the face in rostral parts of the trigeminal sensory nuclear complex are not essential for facial nociception. These observations are also consistent with the opinion that, from a functional point of view, facial nociceptor afferents have their principal synaptic connections in the nucleus caudalis of the trigeminal spinal complex.

Two types of rostroventral medulla neurons projecting to the trigeminal spinal nucleus as differentiated by the response to antidromic activation and the histological location

A population of neurons in the rostroventral medulla, which send their axons to the subnucleus oralis of the trigeminal spinal nucleus of rats, could be differentiated into two types on the basis of their location and the variability of antidromic latency during repetitive stimulation at 10 Hz. Type A neurons were mostly located in the raphe magnus and were activated antidromically with a relatively long latency, which gradually increased during repetitive stimulation. By contrast, type B neurons were located in the medial portion of the gigantocellular reticular nucleus, and responded with a relatively short, stable antidromic latency.

Primary projections of the trigeminal nerve in two species of sturgeon: Acipenser oxyrhynchus and Scaphirhynchus platorynchus

Horseradish peroxidase histochemical studies of afferent and efferent projections of the trigeminal nerve in two species of chondrostean fishes revealed medial, descending and ascending projections. Entering fibers of the trigeminal sensory root project medially to terminate in the medial trigeminal nucleus, located along the medial wall of the rostral medulla. Other entering sensory fibers turn caudally within the medulla, forming the trigeminal spinal tract, and terminate within the descending trigeminal nucleus. The descending trigeminal nucleus consists of dorsal (DTNd) and ventral (DTNv) components. Fibers of the trigeminal spinal tract descend through the lateral alar medulla and into the dorsolateral cervical spinal cord. Fibers exit the spinal tract throughout its length, projecting to the ventral descending trigeminal nucleus (DTNv) in the medulla and to the funicular nucleus at the obex. Retrograde transport of HRP through sensory root fibers also revealed an ascending bundle of fibers that constitutes the neurites of the mesencephalic trigeminal nucleus, cell bodies of which are located in the rostral optic tectum. Retrograde transport of HRP through motor root fibers labeled ipsilateral cells of the trigeminal motor nucleus, located in the rostral branchiomeric motor column.

Activity of trigeminothalamic neurons in medullary dorsal horn of awake monkeys trained in a thermal discrimination task

We analyzed the activity of 51 trigeminothalamic neurons in the medullary dorsal horn (trigeminal nucleus caudalis) of monkeys during the performance of behavioral tasks requiring the monkeys to discriminate innocuous and noxious thermal stimuli applied to the face and to detect the onset of visual stimuli. Static properties of trigeminothalamic neurons in behaving monkeys were similar to those in anesthetized monkeys. Responses to passively presented mechanical and thermal stimuli, receptive-field properties, and conduction velocities did not differ in the awake and anesthetized states. For most wide dynamic range and nociceptive-specific trigeminothalamic neurons, there was a negative correlation between the magnitude of thermally evoked activity and behavioral latencies to discriminate 47 and 49 degrees C stimuli. Thus, both groups of neurons provide information that could be used by the monkey to discriminate noxious thermal stimuli. The magnitude of thermal responses of trigeminothalamic neurons was modulated by the behavioral significance of the stimulus. Behaviorally relevant thermal stimuli presented during the thermal discrimination task produced a greater neuronal response than equivalent irrelevant thermal stimuli presented between behavioral trials or presented while the monkey performed the visual detection task. Neurons whose activity is modulated by behavioral state are likely to be involved in discrimination of thermal stimuli, since the activity of these neurons correlates with the behavioral response to the stimuli and information from the modulated neurons is sent to the thalamus. Some trigeminothalamic neurons that exhibited somatosensory responses also responded to behaviorally relevant stimuli and events associated with trial initiation and receipt of reward in the behavioral tasks. Similar events outside a behavioral task evoked no neuronal responses. These task-related responses were similar to those described previously for medullary dorsal horn neurons not identified as to projection sites (14).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of injectable anaesthetics on responses to L-glutamate and on spontaneous synaptic activity in lamprey reticulo-spinal neurones

Intracellular recordings were made from reticulo-spinal cells in the medulla of lamprey ammocoetes; potential changes in response to iontophoretically applied L-glutamate were measured before, during and after the preparation was superfused with anaesthetic solutions. Of the anaesthetics pentobarbitone, ketamine, alphaxalone/alphadolone (Saffan) and metomidate, only pentobarbitone (greater than 10 microM) had a consistent dose-related depressant effect on glutamate responses. Spontaneous excitatory postsynaptic potentials (e.p.s.ps) and inhibitory postsynaptic potentials (i.p.s.ps) were diminished in frequency by high concentrations (1 mM) of all anaesthetics. Anaesthetic concentrations of all drugs also reduced i.p.s.ps; for e.p.s.ps this was true of pentobarbitone (100 microM) immediately, and of ketamine (370 microM) and alphaxalone (10-30 microM) after a transitory increase in activity. Consideration of the results in the light of previous observations on inhibitory responses suggests a basis for some of the excitatory side effects of these compounds, assuming that the equivalent mammalian cells are similarly affected.

Evidence for peripheral, but not central modulation of trigeminal cold receptive cells in the rat

The effects of locus coeruleus (LC), periaqueductal grey (PAG) and segmental stimulation (all of which are known to inhibit convergent nociceptive cells), were tested on the activity of cold receptive cells in the trigeminal system of the rat. LC and PAG stimulation from sites which inhibited convergent nociceptive cells had no effect on cells with cold receptive input in the trigeminal nucleus caudalis. Electrical or mechanical segmental stimulation caused suppression of activity in cold receptive trigeminal nucleus neurons. Recording from the trigeminal ganglion showed this suppression to be a property of the primary afferent cold receptors themselves and therefore it is not analogous to the proposed mechanism for the segmental inhibition of convergent nociceptive neurons.

Functional organization of trigeminal subnucleus interpolaris: nociceptive and innocuous afferent inputs, projections to thalamus, cerebellum, and spinal cord, and descending modulation from periaqueductal gray

In view of continuing uncertainties concerning the organization, afferent inputs, and projection sites of neurons in the subnucleus interpolaris of the trigeminal (V) spinal tract nucleus, the characteristics of 222 single neurons in and adjacent to the subnucleus were examined electrophysiologically in adult cats anesthetized with chloralose. Neurons were tested for orthodromic responsiveness to a variety of stimuli that included nonnoxious tactile stimuli, noxious mechanical and radiant-heat stimuli, and graded electrical stimulation of the skin, mucosa, tooth pulp, and masseter nerve. Antidromic activation techniques were also used to determine if the functionally identified neurons projected directly to the contralateral posterior thalamus, ipsilateral cerebellum, or cervical spinal cord. In addition, the periaqueductal gray matter (PAG) was stimulated to test for conditioning influences from the PAG on orthodromic responses to noxious and nonnoxious oral-facial stimuli. Interpolaris neurons were somatotopically arranged in subnucleus interpolaris in a pattern conforming in general to the medially facing, inverted-head representation characteristic of other parts of the V brain stem sensory nuclear complex. On the basis of their responsiveness to cutaneous stimuli, the neurons could be functionally classified as either cutaneous nociceptive or low-threshold mechanoreceptive (LTM) neurons. The LTM neurons constituted the major neuron type, accounting for over 75% of our neuron sample. Most of them had a localized mechanoreceptive field of less than 100 mm2 in area that was restricted to one V division, and they had skin-evoked response latencies indicative of afferent input predominantly from A-beta cutaneous afferents. A population of nociceptive neurons was also encountered in the lateral, marginal region of interpolaris and at its medial or ventral border with the reticular formation. These neurons were of two types: nociceptive-specific (NS) neurons, which did not respond to nonnoxious stimuli but which required noxious stimuli for their activation; and wide dynamic range ( WDR ) neurons, which responded to both noxious and nonnoxious stimuli applied to the facial skin. Most had an ipsilateral receptive field that was greater than 100 mm2 in area and that often involved two or three V divisions. Their properties generally conformed to those previously described for nociceptive neurons in the medullary dorsal horn (V subnucleus caudalis) and spinal cord dorsal horn. Interpolaris neurons of all classes (LTM, WDR , and NS) were found to have direct axonal projections to the thalamus, cerebellum, and spinal cord.(ABSTRACT TRUNCATED AT 400 WORDS)

Topography of spinal, dorsal column nuclear, and spinal trigeminal projections to the pontine gray in rats

Several studies using a variety of animals have reported conflicting evidence concerning the distribution, laterally, and indeed the presence of ascending projections to the pontine nuclei. In an attempt to clarify this issue, projections to the pontine nuclei from the spinal cord, dorsal column nuclei, and spinal trigeminal nucleus were investigated with anterograde methods, i.e., the Fink-Heimer technique and/or autoradiography, in Long-Evans black-hooded rats. Results revealed that dorsal column nuclear projections to the contralateral pontine gray terminate predominantly in two regions--one in the caudal aspect of the medial pontine subdivision and another overlapping the ventral and lateral subdivisions. Within the medial and ventral lateral nuclear regions, fibers from nucleus cuneatus primarily terminated more rostrally to afferents from the nucleus gracilis. Spinal trigeminal projections terminated most heavily within the contralateral pontine gray at midpontine levels. Similar to the dorsal column nuclear projections, trigeminal afferents were observed in the medial and ventrolateral subdivisions, although these terminations were rostral and dorsal to areas receiving cuneatus input. Additional projections from the spinal trigeminal nuclei to the contralateral ventral peduncular nucleus were also observed. In comparison to the above-mentioned pontine afferents, both high cervical and midthoracic spinal cord lesions produced a similar pattern of axonal degeneration in the ipsilateral pontine gray which overlapped substantially with gracilis inputs. The observed topographic distribution pattern of ascending afferents to pontine gray confirm and extend previous findings which in general have only briefly described these pathways.

The periaqueductal gray projections to the rat spinal trigeminal, raphe magnus, gigantocellular pars alpha and paragigantocellular nuclei arise from separate neurons

Possible collateral branches of periaqueductal gray axons which distribute to the nucleus raphe magnus, nucleus reticularis paragigantocellularis, nucleus reticularis gigantocellularis pars alpha and the spinal trigeminal nucleus were analyzed with the double fluorescent retrograde tracer technique. With the exception of a small number of double-labeled neurons observed in the periaqueductal gray following injections of fluorescent dyes into the nuclei reticularis paragigantocellularis and gigantocellularis pars alpha, no double-labeled cells were found in this midbrain region following injections of tracers into various combinations of the above 4 nuclear groups. The results of this investigation indicate that these 4 brainstem nuclei are innervated predominantly by separate neuronal populations within the periaqueductal gray.

Direct projections from vestibular nuclei to facial nucleus in cats

Postsynaptic potentials were recorded from motoneurons in the facial nucleus in response to stimulation of the vestibular and trigeminal nerves. The motoneurons were identified by antidromic activation from their peripheral axons. Disynaptic excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) and mixed EPSP/IPSPs were recorded in response to vestibular nerve stimulation, ranging in latency from 0.9 to 2.1 ms, with most at 1.5 ms. Activity in secondary vestibular axons recorded within the facial nucleus occurred at a latency of 0.7-1.1 ms. The amplitudes of the vestibular postsynaptic potentials were small, generally less than a millivolt, but double shocks produced marked summation. The average time to peak of ipsilateral vestibular EPSPs, 1.1 ms, was faster than that of either ipsilateral IPSPs, 1.6 ms, or contralateral EPSPs, 1.4 ms. The double-spiked vestibular activity was detectable in double-peaked PSPs. Disynaptic EPSPs, ranging in latency from 2.0 to 3.0 ms, were recorded in response to trigeminal nerve stimulation. The average time to peak was 1.3 ms. The multiple-spiked activity of the trigeminal neurons was detectable in multipeaked EPSPs. Inhibitory ipsilateral effects (Vi IPSPs) were recorded twice as often as excitatory ipsilateral effects (Vi EPSPs), being found in 29% versus 15% of the motoneurons. Contralateral effects were found in 13% of the motoneurons studied, and almost all were excitatory. Analysis of synaptic potential shapes suggested that the excitatory and inhibitory vestibular synapses probably contact distal dendrites preferentially, with the excitatory connections being somewhat closer to the soma. The trigeminal inputs probably contact the facial motoneurons more extensively near the soma. Horseradish peroxidase was injected into the facial nucleus, and retrograde uptake by vestibular neurons was studied. The majority of filled vestibular neurons was ipsilateral to the injection site, especially in the medial vestibular nucleus, ventral y group, and supravestibular nucleus. On the contralateral side, filled vestibular cells were found almost exclusively in the medial nucleus. Filled cells were also noted in the trigeminal nucleus, predominantly ipsilaterally at all rostrocaudal levels. We have demonstrated monosynaptic projections to facial motoneurons from both vestibular and trigeminal nuclei. The trigeminal input is likely to be involved in facial reflexes, especially blinking and grimacing. The afferent vestibular population overlaps that going to the oculomotor and cervical motoneurons; these projections may be collaterals of single vestibular neurons.4+.

Mechanisms regulating the activity of facial nucleus motoneurones--2. Synaptic activation from the caudal trigeminal nucleus

Field and postsynaptic potentials of facial motoneurones evoked by stimulation of the caudal trigeminal nucleus were studied in cats by means of extra- and intracellular recording. Mono- and polysynaptic input onto facial motoneurones from the caudal trigeminal nucleus were shown. Four types of responses were distinguished: excitatory postsynaptic potentials generating a single action potential; a gradual shift of depolarization inducing multiple discharges; a rhythmic discharge of action potentials appearing at a low level of depolarization; excitatory postsynaptic potentials or a sequence of excitatory and inhibitory postsynaptic potentials. Multiple discharge was shown to appear as a result of effective summation of high frequency excitatory influences from efferent neurones of the caudal trigeminal nucleus projecting into the facial nucleus. Factors facilitating the development of gradual depolarization are: dendritic localization of synaptic terminals, dendritic origin of after-depolarizing processes and the high input resistance of the facial motoneurone membrane. It is thought that specific features of facial motoneurones and properties of afferent inputs are supposed to provide high sensitivity of neuronal organization of the facial nucleus to afferent signals as well as wide diversity in controlling its activity.

Periaqueductal gray inhibition of trigeminal subnucleus caudalis unitary responses evoked by dentine and nonnoxious facial stimulation

The possible pain inhibitory effects of periaqueductal gray (PAG) stimulation were investigated in cats anesthetized with Nembutal and immobilized with Flaxedil. Unitary responses evoked by electrical stimulation of the upper canine dentine and by cutaneous facial noxious and nonnoxious stimuli were recorded extracellularly from the trigeminal subnucleus caudalis. A bipolar electrode was introduced into the PAG to test the effects of PAG excitation on the trigeminal response to dentine (TRED) and cutaneous nonnoxious stimulation. In some experiments, a similar electrode was lowered into the contralateral posterior thalamus to study the antidromic activation of subnucleus caudalis cells and the effects of thalamic stimulation on the TRED. Dentine stimulation evoked brief (6- to 15-ms) bursts of 1 to 10 spikes with 3- to 25-ms latencies. Most units (88%) were also activated by cutaneous facial stimulation. Stimulation of the posterior thalamus had no effect on the TRED or on responses to cutaneous stimulation, but activated antidromically 10% of the units. In 71% of the units PAG stimulation inhibited the TRED. In some of those cases (12%), the inhibitory effect persisted 30- to 60 s. The PAG stimulation could produce paradoxical effects, potentiating the TRED evoked by threshold intensity and inhibiting the TRED elicited by suprathreshold stimulation. About one-half the PAG points evoked detectable effects. Their location had no clear topographical distribution, although ventral sites were more potent than dorsal sites. Responses evoked by nonnoxious facial stimulation were also inhibited by the PAG.

Descending inhibitory influences from periaqueductal gray, nucleus raphe magnus, and adjacent reticular formation. II. Effects on medullary dorsal horn nociceptive and nonnociceptive neurons

1. This study examined the inhibitory effects elicited by brain stem stimulation on the somatosensory responses of trigeminal medullary dorsal horn (subnucleus caudalis of the spinal trigeminal nucleus) neurons. Single-unit extracellular recordings were obtained in chloralose-anesthetized cats. Neurons were classified as wide dynamic range (WDR), nociceptive specific (NS), or low-threshold mechanoreceptive (LTM). Conditioning stimuli were delivered to the periaqueductal gray (PAG), nucleus cuneiformis (CU), nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC), and nucleus reticularis magnocellularis (NMC). 2. Over 97% of the neurons tested could be inhibited by stimulation in all regions except PAG. Stimulation in the PAG inhibited 91% of the neurons tested. There was no statistically significant difference in the incidence of inhibition of WDR and NS nociceptive (noci) neurons and the LTM nonnociceptive (nonnoci) neurons. 3. Mean stimulation intensities necessary to produce inhibition were determined for each neuron from each stimulation site. The current thresholds necessary to inhibit the responses of noci neurons were found to be significantly lower, on the average, than those of nonnoci neurons at stimulation sites in the PAG, CU, and NGC. 4. Inhibition of the responses of WDR neurons required a lower mean current than for NS neurons but was statistically significant only for PAG and NGC. Thresholds for inhibiting the responses of NS neurons were similar to those for inhibiting the responses of LTM neurons for all regions except CU, where LTM thresholds were markedly but not significantly higher. 5. Stimulation thresholds were found to be lowest in NMC, while in NGC, NRM, and CU they were all similar and slightly higher. Stimulation in the PAG required the highest currents to produce inhibition. 6. These results indicate that stimulation in NRM and PAG not only inhibits the responses of noci neurons but also those of nonnoci neurons. Furthermore, stimulation in reticular regions adjacent to NRM and PAG is frequently even more effective in inhibiting the responses of both noci and nonnoci neurons. In addition, WDR neurons are more effectively inhibited than NS or LTM neurons. These results are compared with those obtained using similar methods in cat lumbar dorsal horn.

Analgesia for orofacial nociception produced by morphine microinjection into the spinal trigeminal complex

Morphine sulfate (0.75 micrograms) was microinjected into (rat) rostral and caudal trigeminal areas singly and simultaneously, using cannula-electrode combinations. Also, 0.5 micrograms or 1.0 micrograms of morphine was injected into nucleus reticularis paragigantocellularis (PGC). Both single trigeminal nuclear microinjections significantly elevated the latency to a defensive face-rub reaction to noxious facial heat, bilaterally. There was no summation effect with the conjoint injection of 1.5 micrograms total to rostral and caudal nuclear areas. The 0.5 micrograms injection in PGC had a significantly larger effect than did the 0.75 microgram trigeminal injections. The caudal but not the rostral trigeminal injections did elevate the threshold for aversive reaction to caudal trigeminal nuclear stimulation of the injected tissue. This finding contrasts with the previously reported fact that as much as 1.0 microgram of morphine in PGC has no effect on this aversive reaction threshold to brain stimulation.

The properties of cells in the cat trigeminal main sensory and spinal subnuclei activated by mechanical stimulation of the periodontium

Neurophysiological exploration of the trigeminal sensory complex was done on 42 cats under ketamine anaesthesia, paying special attention to units receiving a periodontal input. Among 492 cells recorded in the trigeminal sensory complex, 73 responded to mechanical stimulation of the periodontium and were precisely localized histologically. Thalamic stimulation was also delivered to the ipsi and contralateral ventro-posterior nucleus to test for antidromic responses. Results of this systematic study were plotted on reference drawings of the full extent of the trigeminal sensory complex.

Brain stem projections to the facial nucleus of the rat

Horseradish peroxidase was injected into the medial and lateral columns of the facial nucleus of the rat. Following medial injections, cells were labelled by retrograde transport in the ipsilateral spinal trigeminal nucleus (caudalis) both medial vestibular nuclei, contralateral midbrain reticular formation and nucleus of the lateral lemniscus. The periaqueductal grey, interstitial nucleus and nucleus of Darkschewitch were also labelled ipsilaterally. Injections into the lateral column of the facial nucleus labelled the spinal trigeminal nucleus (oralis) and parabrachial nuclei ipsilaterally and the Darkschewitch and red nuclei contralaterally.

Effects of 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG626) on the spinal trigeminal nucleus, ventral posteromedial nucleus, and sensory cortex

Effects of 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1 (2H)-phthalazinone (EG626) on the spinal trigeminal nucleus (STN), ventral posteromedial nucleus (VPM), and sensory cortex were examined in cats anesthetized with alpha-chloralose in comparison with the effects of morphine. EG626 produced a dose-dependent inhibition of the polysynaptic components of the cortical field potentials upon VPM stimulation and either facilitatory or inhibitory effects on the polysynaptic components of the VPM field potential upon stimulation of the medial lemniscus, while the drug failed to affect the STN field potential with trigeminal nerve stimulation. Morphine inhibited the postsynaptic components of the STN field potentials and to a lesser extent, the polysynaptic components of the cortical field potential; and the effects of morphine on the VPM field potential were similar to those seen with EG626. Pretreatment of the animal with naloxone antagonized the facilitatory effect on the VPM field potentials produced by morphine, but not those by EG626. Morphine and EG626 induced either a prolonged increase in the blood flow or transient increase followed by a decrease in the blood flow in the VPM. These results suggest that EG626 may impair the polysynaptic transmission and/or neuron excitability in the sensory cortex and the VPM at least partly due to the change in blood flow there as does morphine. Unlike morphine, however, EG626 did not produce any obvious effect on the STN.

Trigeminal evoked potentials in the cat

Electrical stimulation of the infraorbital nerve in the cat resulted in a series of far-field evoked potentials at the vertex. The wave form of these potentials is similar to the auditory brain stem evoked response and the somatosensory far-field response; it is multicomponent, the amplitudes of individual components are of the order of 1 microV, and the latencies are all less than 4 msec. The anatomical origins of these evoked potentials were investigated. The results indicated that contributions due to the afferent trigeminal nerve, the principal sensory nucleus, and the spinal trigeminal nucleus are involved.

Evaluation of (D-Pro2, D-Trp7,9)-substance P as an antagonist of substance P responses in the rat central nervous system

The (D-Pro2, D-Trp7,9) analogue of substance P has been tested for substance P antagonist activity in the caudal trigeminal nucleus in vivo, and in the isolated spinal cord in vitro. In neither case was the analogue found to be a specific antagonist of substance P, although the analogue did have weak antagonist actions in the isolated guinea-pig ileum preparation. It is concluded that the analogue is not a suitable tool for the identification of putative substance P systems in the spinal cord or the caudal trigeminal nucleus of the rat.

The corticotrigeminal projection in the cat. A study of the organization of cortical projections to the spinal trigeminal nucleus

The projection from the cerebral cortex to the spinal trigeminal nucleus has been studied light microscopically in adult cats. Both orthograde degeneration and orthograde intra-axonal labeling techniques have been applied. Our results indicate that the projection from the coronal gyrus (face area of primary somatosensory cortex) to the spinal trigeminal complex is somatotopically organized. In subnucleus caudalis this somatotopy is organized dorsoventrally and appears to match the somatotopic distribution of the divisional trigeminal afferents. Hence cortical fibers originating from the posterior coronal gyrus (upper representation) project ventrolaterally into caudalis where division I trigeminal afferents terminate. Likewise cortical fibers from the anterior coronal gyrus (jaw and tongue representation) terminate dorsomedially in caudalis to overlap with division III trigeminal afferents. In contrast, the distribution of corticofugal afferents to the rostral spinal trigeminal subnuclei (pars interpolaris and oralis) is organized mediolaterally. Therefore in these subnuclei the cortical projection does not appear to overlap the dorsoventral lamination of the divisional trigeminal afferents. In addition, our results suggest that the cortical projection to subnucleus caudalis includes fibers which terminate in the marginal zone (lamina I) and its extensions into the spinal trigeminal tract (the interstitial cells of Cajal). We have been unable to document a projection from the proreate gyrus to the spinal trigeminal complex.

An ionophoretic study of the responses of rat caudal trigeminal nucleus neurones to non-noxious mechanical sensory stimuli

1. Extracellular recordings of the responses of single caudal trigeminal nucleus neurones to non-noxious and noxious facial stimuli and to ionophoretically applied L-glutamate, L-aspartate and acetylcholine were made in urethane anaesthetized rats. 2. Neurones excited by non-noxious mechanical stimuli were located primarily in the magnocellular part of nucleus caudalis, whereas neurones excited by both noxious and non-noxious stimuli were located either ventromedially to the magnocellular part of nucleus caudalis or superficially to the substantia gelatinosa. 3. Both L-aspartate and L-glutamate were found to excite all neurones tested in nucleus caudalis. In contrast, however, acetylcholine was found to excite only 31% of the neurones tested. 4. Responses of nucleus caudalis neurones to non-noxious sensory stimulation were not antagonized by the excitatory amino acid antagonist D-alpha-aminoadipate, but were antagonized by cis-2, 3-piperidine dicarboxylate and gamma-D-glutamylglycine, two excitatory amino acid antagonists with a broader spectrum of action. 5. It is concluded that the chemical synaptic transmitter of non-nociceptive mechanoreceptive primary afferent fibres to nucleus caudalis may be a ligand for an excitatory amino acid receptor other than a D-alpha-aminoadipate-sensitive receptor. The synaptic receptor may thus be of the kainate or quisqualate type, and the transmitter possibly L-glutamate, L-aspartate or an as yet unidentified substance.

Facial thermal input in the caudal trigeminal nucleus of rats reared at 30 degrees C

1. Rats reared from birth in air at 30 degrees C showed a decreased ability to maintain colonic temperature when exposed to 10 degrees C as compared with rats reared at 20 degrees C. This difference was not due to physical factors affecting heat loss, such as surface area or fur thickness. 2. In anaesthetized rats extracellular recordings were made in trigeminal nucleus caudalis from higher order neurones with input from facial cold and warm receptors. A systematic search on a grid pattern showed there was no difference between heat-reared and control rats in facial receptive fields or in the abundance, extent and somatotopic distribution of thermal neurones in the nucleus. In both groups almost all the neurones were excited only by facial cooling. 3. When single cold neurones were tested quantitatively by the application of controlled temperature changes to their receptive fields on the face there was no difference in the static temperature/discharge rate relationship between the two groups of rats. 4. The results suggest that the observed difference in ability to regulate body temperature is not attributable to differences in skin-temperature reception at the level of the trigeminal nucleus.

The encoding of thermal stimuli by diffuse noxious inhibitory controls (DNIC)

The relationship between stimulus intensity and its efficacy in inducing diffuse noxious inhibitory controls (DNIC) was investigated in anaesthetized rats by using thermal stimulation of the tail for conditioning dorsal horn convergent neuronal responses to C fibres emanating from the hindpaw extremity. The threshold for obtaining inhibition of the neuronal responses ranged between 40 and 44 degrees C and there was a highly significant correlation between noxious temperatures (44-52 degrees C) and the degree of inhibition. These data provide support for the notion of an involvement of inhibitory processes (DNIC) in the signalling of pain by convergent neurones.

Identification of ultrasonic vocalization substrates determined by electrical stimulation applied to the medulla oblongata in the rat

The aim of the present investigation was to identify the neural structures, within the rat medulla, that are responsible for rodent ultrasound production. Sound producing substrates were found to be located in the reticular formation and some cranial nerve nuclei as well as several other nuclei situated in the lateral and dorsomedial portions of the medulla. To estimate the degree of involvement in the generation of ultrasound signals, the sound response latencies were measured for each structure. The lateral reticular nucleus and the facial nucleus showed latencies that were significantly shorter than those for other nuclei, and they were assumed to have a primary part in rodent ultrasound production. Audible sounds of considerably longer latencies were produced exclusively by stimulation of the trigeminal spinal tract nucleus. No ultrasounds could be obtained in this region. These results were discussed in terms of innervations of the facial and laryngeal musculature by the specific neural structures. Present results were also discussed with reference to the roles of the bulbar monoaminergic neurons projecting to the spinal cord and the role of ascending nociceptive pathways.

Inhibitory actions produced by local electrical stimulation in the caudal spinal trigeminal nucleus in rat

Inhibitory actions induced by local electrical stimulation (LES) on the tooth pulpal afferent activities were investigated in the caudal part of the rat spinal trigeminal nucleus. For the LES, ipsilateral Yin-Hsiang (intrasegmental point) or Ho-Ku (extrasegmental point) was used as a cathodal point which was stimulated electrically by a single pulse of 0.1 msec in duration or by 0.1 msec-pulse train at 45 Hz for 15 min. We found at least three types of inhibitions in the caudal trigeminal nucleus: Type I--this inhibition is the most forceful, caused by naloxone-reversible endogenous opiate system with a slow onset and prolonged aftereffect. This inhibition is presumably postsynaptic action. Type II--this is evoked by postsynaptical acting inhibition and begins within milliseconds after the stimulus is applied. Type III--this inhibition is elicited by presynaptic action and also begins within milliseconds after the onset of the stimulation. Type I and II inhibitions are evoked by stimulating either intra- or extrasegmental LES points, however, Type III is produced by stimulation of intrasegmental and rarely provoked by extrasegmental point stimulation. Naloxone failed to reverse Type II and III inhibitions. During LES, Type I to III inhibitions co-work for producing the suppressive effect and after the cessation of its stimulation, only Type I inhibition produces the so-called aftereffect of the LES.

Changes of the jaw opening reflex activity by electroacupuncture stimulation in rat

After electroacupuncture stimulation applied to bilateral Ho-Ku points in the rat forepaw, jaw opening reflex to tooth pulp stimulation was fairly suppressed, but that to gingiva, upper lip or lower lip stimulation was scarcely affected. By direct stimulation of afferent sensory pathway in the spinal trigeminal nucleus, jaw opening reflex was also evoked. Suppressive effects of electroacupuncture were more markedly observed when the jaw opening reflex was evoked by stimulating the caudal part of the spinal trigeminal nucleus than when the reflex was evoked by stimulation of the rostral area of the nucleus. One of the main action sites of the electroacupuncture on the jaw opening reflex was the caudal spinal trigeminal nucleus, which is the secondary neuron level in the trigeminal noxious sensory pathway, suggesting that electroacupuncture can provoke the pain suppressive effects strongly in the lower part of the brain with the higher brain function being intact. Therefore, this observation is thought to be well coincident with clinical data that acupuncture produces analgesia without affecting patients' consciousness during surgical operations.

Quantitative relations between noxious stimulus intensity and magnitude of central afferent evoked responses in rat

Relations between intensities of electrical noxious stimulus applied to the tooth pulp and magnitudes of evoked responses at various relay stations of tooth pulp afferents were investigated with lightly anesthetized rats. The stimulus-response function in each relay station and the cerebral cortex follows the power function only in the range of lower, but noxious, stimulus intensities; however, it deviates from it in the range of higher ones, the exponent of the function becoming smaller. The higher the relay station locates, the smaller the exponent value is.