Dorsal raphe stimulation produces inhibitory effect on trigeminal nucleus neurons

Current advances in orthodontic pain

Orthodontic pain is an inflammatory pain that is initiated by orthodontic force-induced vascular occlusion followed by a cascade of inflammatory responses, including vascular changes, the recruitment of inflammatory and immune cells, and the release of neurogenic and pro-inflammatory mediators. Ultimately, endogenous analgesic mechanisms check the inflammatory response and the sensation of pain subsides. The orthodontic pain signal, once received by periodontal sensory endings, reaches the sensory cortex for pain perception through three-order neurons: the trigeminal neuron at the trigeminal ganglia, the trigeminal nucleus caudalis at the medulla oblongata and the ventroposterior nucleus at the thalamus. Many brain areas participate in the emotion, cognition and memory of orthodontic pain, including the insular cortex, amygdala, hippocampus, locus coeruleus and hypothalamus. A built-in analgesic neural pathway—periaqueductal grey and dorsal raphe—has an important role in alleviating orthodontic pain. Currently, several treatment modalities have been applied for the relief of orthodontic pain, including pharmacological, mechanical and behavioural approaches and low-level laser therapy. The effectiveness of nonsteroidal anti-inflammatory drugs for pain relief has been validated, but its effects on tooth movement are controversial. However, more studies are needed to verify the effectiveness of other modalities. Furthermore, gene therapy is a novel, viable and promising modality for alleviating orthodontic pain in the future.

The striatum and pain modulation

The aim of this review was to give a general aspect of the sensorial function of the striatum related to pain modulation, which was intensively studied in our laboratory. We analyse the effect of electrical and chemical stimulation of the striatum on the orofacial pain, especially that produced by tooth pulp stimulation of the lower incisors. We demonstrated specific sites within the nucleus which electrical or chemical stimulation produced inhibition of the nociceptive jaw opening reflex. This analgesic action of the striatum was mediated by activation of its dopamine D(2) receptors and transmitted through the indirect pathways of the basal ganglia and the medullary dorsal reticular nucleus (RVM) to the sensorial nuclei of the trigeminal nerve. Its mechanism of action was by inhibition of the nociceptive response of the second order neurons of the nucleus caudalis of the V par.

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.

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.

Dorsal raphe nucleus and locus coeruleus neural networks and the elaboration of the sweet-substance-induced antinociception

In order to investigate the effects of monoaminergic neurons of the dorsal raphe nucleus (DRN) and locus coeruleus (LC) on the elaboration and control of sweet-substance-induced antinociception, male albino Wistar rats weighing 180-200 g received sucrose solution (250 g/L) for 7-14 days as their only source of liquid. After the chronic consumption of sucrose solution, each animal was pretreated with unilateral microinjection of ibotenic acid (1.0 microg/0.2 microL) in the DRN or in the LC. The tail withdrawal latencies of the rats in the tail-flick test were measured immediately before and 7 days after this treatment. The neurochemical lesion of locus coeruleus, but not of DRN neural networks with ibotenic acid, after the chronic intake of sweetened solution, decreased the sweet-substance-induced antinociception. These results indicate the involvement of noradrenaline-containing neurons of the LC in the sucrose-induced antinociception. We also consider the possibility of DRN serotonergic neurons exerting some inhibitory effect on the LC neural networks involved with the elaboration of the sweet-substance-induced antinociception.

Direct projections from the midbrain periaqueductal gray and the dorsal raphe nucleus to the trigeminal sensory complex in the rat

It is well known that analgesia can be produced by stimulation of the midbrain periaqueductal gray and dorsal raphe nucleus. This stimulation-produced analgesia may operate, at least partly, through direct projections to nociceptors in the primary sensory nuclei. In the present study, direct projections from the midbrain periaqueductal gray and dorsal raphe nucleus to the trigeminal sensory complex were studied in the rat. After injection of Phaseolus vulgaris leucoagglutinin into the periaqueductal gray or dorsal raphe nucleus, terminal labeling was found in the principal sensory trigeminal nucleus and the oral, interpolar and caudal subnuclei of the spinal trigeminal nucleus, bilaterally with an ipsilateral predominance. The terminal labeling was prominent in the ventrolateral parts of the trigeminal sensory complex; it was particularly marked in the principal sensory trigeminal nucleus and laminae I and III of the caudal subnucleus of the spinal trigeminal nucleus. After injection of cholera toxin B subunit into the principal sensory trigeminal nucleus or one of the subnuclei of the spinal trigeminal nucleus, retrogradely labeled cells were seen in the periaqueductal gray and dorsal raphe nucleus, bilaterally with an ipsilateral dominance. In the periaqueductal gray they were most often seen in the ventrolateral and dorsolateral subdivisions, but no particular topographic organization was detected in the distribution of retrogradely labeled neurons in the periaqueductal gray and dorsal raphe nucleus after tracer injection into each subdivision of the trigeminal sensory complex. After injections of Fast Blue and Diamidino Yellow, respectively, into the principal sensory trigeminal nucleus and the caudal subnucleus of the spinal trigeminal nucleus on one side of the brain, a number of doubly labeled neurons were found in the periaqueductal gray and dorsal raphe nucleus, bilaterally with an ipsilateral dominance. The results indicate that a considerable number of neurons in the periaqueductal gray and dorsal raphe nucleus send projection fibers to the trigeminal sensory complex, and that some of them send their axons to both the principal sensory trigeminal nucleus and the caudal subdivision of the spinal trigeminal nucleus by way of axon collaterals. These projections may subserve suppression of the activity of nociceptive neurons in the trigeminal system.

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.

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.

Opioid-mediated inhibition from the subnucleus caudalis of spinal trigeminal nucleus to the neurons in the subnucleus oralis

The influence of enkephalin-containing neurons in the subnucleus caudalis of the spinal trigeminal nucleus (STN caudalis) on the subnucleus oralis of the STN (STN oralis) was examined using chloral hydrate-anesthetized rats. In the STN oralis, conditioning stimuli applied to the STN caudalis 10-50 ms preceding the test stimulus inhibited spikes produced by tooth pulp stimulation in type B interneuron, which was activated by orthodromic stimulation but not by thalamic stimulation, without affecting those of the relay neuron. When the type B interneurons were further classified into type B1 and type B2 neurons, which were characterized by the occurrence of the STN caudalis-induced inhibition with long and short latencies, respectively, microiontophoretically applied naloxone reduced the STN caudalis-induced inhibition of th orthodromic spikes of type B1 interneurons with little effects on type B2 interneuron. Furthermore, naloxone-reversible inhibition of tooth pulp-induced spikes of the type B1 interneurons were also observed during iontophoretic application of enkephalin. These results suggest that the type B1 interneurons in the STN oralis are inhibited by opioid peptides-containing neurons in the STN caudalis.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.

Beta-receptor involvement in locus coeruleus-induced inhibition of spinal trigeminal nucleus neurons: microiontophoretic and HRP studies

Microiontophoretic and HRP studies were performed on cats anesthetized with alpha-chloralose to determine whether or not the locus coeruleus (LC)- and noradrenaline (NA)-induced inhibition of relay neurons in the subnucleus oralis of the spinal trigeminal nucleus (STN) is mediated by beta-adrenergic receptors. The inhibition of orthodromic spike generation upon intracranial trigeminal nerve stimulation by LC conditioning stimulation and microiontophoretically applied NA (100-200 nA) was antagonized during microiontophoretic application of sotalol, a beta-adrenergic antagonist, but not affected by phentolamine, an alpha-adrenergic antagonist. When HRP at doses of 300-500 nA was applied for 5-15 min to the immediate vicinity of the STN relay or interneuron, which was electrophysiologically identified by stimulating the ipsilateral trigeminal nerve and contralateral medial lemniscus, the injection site was localized to an area 0.3 mm in diameter and HRP-reactive cells were found in the ipsilateral LC, dorsal raphe nucleus and periaqueductal gray ventral to the aqueduct. These results strongly suggest that NA released from the nerve terminals of LC cells inhibits transmission in the STN relay neuron via beta-adrenergic receptors.

Effects of serotonin and morphine on spontaneous and evoked firing of nociceptive neurons in the trigeminal spinal nucleus of rats

Spontaneously firing neurons that were responsive to noxious face pinch or noxious heat were studied in the trigeminal spinal nucleus of the rat brain. These neurons responded with either an increase or decrease in firing rate. In these neurons serotonin (5-hydroxytryptamine; 5-HT) apparently acts through two mechanisms to attenuate the response to a noxious stimulus. One mechanism is mimicked by morphine; these two drugs block the response to the noxious stimuli without having a consistent effect on spontaneous firing. The effects of the two drugs were somewhat selective depending on the noxious stimulus used and the effect of the noxious stimulus; morphine and 5-HT were more effective in blocking the increase in firing rate evoked by the face pinch but 5-HT and morphine were more effective in blocking the decrease in firing rate evoked by the noxious heat stimulus. Interestingly, the direction of the response to a particular noxious stimulus frequently predicted whether or not both morphine and 5-HT would act on the same or different neurons. A second mechanism by which 5-HT, but not morphine, acted was to change the spontaneous firing in a direction opposite that evoked by the noxious stimulus. This type of effect apparently modulated the response to a noxious stimulus by changing the spontaneous firing rate such that a noxious stimulus had to be more intense before it could significantly alter the neuronal firing in the opposite direction. Morphine occasionally produced a change in firing pattern in neurons; this effect remains to be documented more extensively.

Differences in the responses of raphe nuclei to repetitive somatosensory stimulation

The responses of single units in raphe (R.) nuclei dorsalis, magnus, pallidus, and obscurus to repetitive stimulation of peripheral nerves were studied in the chloralose-anesthetized cat. Low-intensity electrical stimuli (1.5T) which activated the large-diameter fibers were applied to the common peroneal and lateral gastrocnemius nerves at 0.25, 0.5, 1 and 2 Hz. Responses evoked at each frequently were compared with the control response evoked at 0.1 Hz. All units isolated demonstrated response decrements during periods of stimulation greater than or equal to 0.5 Hz. The long-term effects of repetitive stimulation, however, varied among the four nuclei. After 150 stimulus presentations at 0.5, 1, or 2 Hz, sensory responsiveness decreased in R. dorsalis but was enhanced in caudal R. obscurus units. Changes in the responsiveness of the other two nuclei were not significantly different from control. Responses to twin pulses indicated that R. neurons are not well suited to relay rapid, repetitive stimuli. The functional significance of these observations has implications for the role of the raphe in habituation and in the modulation of sensory traffic to higher centers. Raphe responses are also contrasted to the known responses of reticular formation neurons to repetitive stimulation.

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

Electrode penetrations were made in the mesencephalon and caudal brainstem at the levels of the periaqueductal gray matter (PAG) and nucleus raphe magnus (NRM) in chloralose-anaesthetized or decerebrate cats. In a systematic fashion, mesencephalic and brainstem loci at every 1 mm of vertical depth were electrically stimulated in a series of mediolateral or anteroposterior electrode penetrations to determine the lowest stimulation threshold at each locus for suppressing the digastric jaw-opening reflex evoked by tooth pulp or infraorbital nerve stimulation; at some loci, the threshold current required for suppressing infraorbital nerve-evoked neck reflexes was also determined. Stimulation at sites within large regions of the mesencephalon and caudal brainstem was effective in suppressing these reflexes at less than 4 X the lowest threshold for reflex suppression in each animal. However, in these regions the areas of lowest threshold in the mesencephalon generally corresponded to the ventrolateral PAG and adjacent nucleus cuneiformis and part of the lateral reticular formation (LRF) and in the caudal brainstem they corresponded to NRM and the adjacent nuclei reticularis magnocellularis (RMC) and gigantocellularis (RGC). These findings suggest that there may be mesencephalic and caudal brainstem areas in addition to PAG and NRM that are equally effective in modulating reflex activity.

Catecholamines and serotonin in the caudal medulla of the rat: combined neurochemical-histofluorescence study

The distribution of monoamine transmitters among the nuclei of the caudal medulla was determined through the combined use of the Falck-Hillarp histofluorescence method for cellular localization of catecholamines and serotonin and quantitative micropunch neurochemical assays of norepinephrine (NE), dopamine (DA) and 5-hydroxytryptamine (5-HT) content using high performance liquid chromatography with electrochemical detection. These combined methods permit a direct correlation between neurotransmitter levels and fluorescent fiber and perikaryal profiles. Eight medullary nuclei were sampled: (1) dorsal motor nucleus of X and ventral nucleus solitarius, (2) the dorsomedial reticular formation, (3) the hypoglossal nucleus, (4) the ventromedial reticular formation, (5) the nucleus ambiguus, (6) nuclei raphe obscurus and pallidus, (7) the inferior olivary nucleus and (8) the descending (spinal) nucleus of V. Micropunched regions containing neurons which contribute projections to or receive sensory input from the vagus nerve were found to contain relatively high levels of NE, DA and 5-HT, consistent with the high density of catecholamine and 5-HT containing terminals observed in these nuclei. The dorsal motor nucleus of X and ventral nucleus solitarius contained the highest levels of NE, DA and 5-HT (218 +/- 10, 31 +/- 2 and 75 +/- 8 pmole/mg protein, respectively), whereas the lowest of these amines were found in the descending (spinal) nucleus of V (28 +/- 1, 3.6 +/- 0.8 and 32 +/- 2 pmole/mg protein, respectively). In general the distribution of catecholamines and 5-HT determined by micropunch/microchemical assay agrees well with the distribution of monoamine terminals detected by fluorescence histochemical techniques.

A comparative study of selective stimulation of raphe nuclei in the cat in inhibiting dorsal horn neuron responses to noxious stimulation

Consistent inhibition of cord nociceptive neurons was obtained at low levels of stimulation (5 V or 50-100 micro A) within raphe magnus. Less consistently and with higher stimulus intensities, inhibition was observed on stimulating raphe pallidus. Still less frequently, and generally only with stimulation in the 20 V or 500 micro A range, inhibition was observed in raphe dorsalis, raphe obscurus and centralis superior. No inhibition could be obtained by stimulation in linearis intermedias or linearis rostralis. Nearly all midline sites where inhibition of cord nociceptive neurons was observed were those within or in immediate proximity to raphe nuclei.

Inhibitory mechanisms of the hyper-irritability caused by picrotoxin in the rat

Topically applied norepinephrine, dopamine, serotonin, GABA and glycine, and systemically administered clonidine, L-DOPA (plus carbidopa) and 5-hydroxytryptophan completely suppressed the cutaneous hyper-irritability produced in the trigeminal sensory distribution by picrotoxin overlying the caudal medulla. Cholinergic agents and apomorphine were ineffective. Of the positive compounds, norepinephrine, serotonin and GABA showed the shortest latencies and norepinephrine and serotonin required the lowest concentrations in order to inhibit the hyper-irritability. If L-DOPA (plus carbidopa) was injected after pre-treatment with FLA-63, the effects of L-DOPA did not appear. Similar depression of the hyper-irritability was caused by electrical stimulation of the central gray. The inhibitory effects of stimulation of the central gray was suppressed after administration of tetrabenazine, but again it produced markedly by injection of L-DOPA. From these observations it was concluded that the hyper-irritability could be suppressed by serotonergic as well as noradrenergic fibers terminating at the spinal trigeminal nucleus caudalis. The potential clinical use of L-DOPA in patients with hyperesthesia is discussed.

Effects of stimulation of the raphe nuclei on muricide behavior in rats

Mouse-killing behavior was induced in male Wistar rats by chronic isolation. Selective stimulation of the dorsal raphe nucleus markedly reduced this aggressive behavior. On the other hand, stimulation of the median raphe nucleus produced no changes in muricide behavior. Our results indicate that dorsal and median raphe nuclei function differentially in the regulation of muricide behavior with the dorsal raphe playing a critical role in this behavioral pattern.

The raphe nuclei of the rabbit brain stem

The raphe nuclei of the rabbit brain stem were found in the midline and adjacent reticular formation of the medulla, pons, and mesencephalon. Nuclei raphe obscurus, pallidus, and magnus were located in the medulla. Nucleus raphe pontis and the caudal portion of nuclei raphe dorsalis and centralis superior were present in the pons. The rostral portion of nuclei raphe dorsalis and centralis superior, and nuclei linearis caudalis and intermedius were present in the msencephalon. Wings of neurons extended from the midline clusters of raphe neurons into the adjacent reticular formation. These wings of neurons contained serotonergic perikarya which were cytoarchitecturally indistinguishable from the midline neurons. A detailed localization of these nuclei is presented in atlas form. These raphe nuclei contained heterogeneous populations of neurons which varied in the size, shape and density of the cell bodies. In addition, the dendritic branching, specific orientation of dendrites, and appearance of spines were distinct for each of the raphe nuclei. Individual raphe nuclei often contained several subpopulations of neurons characterized by unique spatial configuration and orientation. The main morphological similarities of the raphe nuclei are location in or adjacent to the midline, the presence of serotonergic cell bodies in all raphe nuclei except the linear nuclei, and heterogeneous cell populations.

A raphe dendrite bundle in the rabbit medulla

A Golgi-Cox, histofluorescence, and electron microscopic examination of the serotonergic raphe nuclei of the rabbit medulla has revealed a large, vertically-oriented midline dendrite bundle extending from the floor of the fourth ventricle to the ventral boundary of nucleus raphe pallidus. The bundle was confined to the medulla, and averaged 150-200 micrometer in width in the adult. This dendrite bundle received contributions from four major sources: (1) Dendrites of midline and paramedian neurons of nucleus raphe obscurus; (2) Dendrites of midline and paramedian neurons of nucleus raphe pallidus; (3) Shafts from tanycytes located on the midline floor of the fourth ventricle; and (4) Dendrites from neurons of the medullary reticular formation. Perikarya and dendrites of serotonergic raphe neurons frequently abutted tanycyte shafts, midline bhood vessels, and perikarya and dendrites of other raphe neurons. The tanycyte shafts extended from the floor of the fourth ventricle into the bundle, and often ran the entire length of the bundle, where they intertwined themselves among neurons and dendrites of the medullary raphe nuclei. This study suggests that neurons of the medullary raphe may be influenced by communication channels including dendro-dendritic contacts within the midline bundle, fourth ventricular cerebrospinal fluid-borne influences through tanycyte shafts, blood-borne influences through the direct neuronal-vascular relationship in the raphe, and traditionally described axonal contacts impinging upon raphe neurons. We suggest that the raphe neurons might act as both neurons and endocrine-neural transducer cells.

Attenuation of morphine analgesia in rats with lesions of the locus coeruleus and dorsal raphe nucleus

The nociceptive reflex activity and analgesic effect of morphine were studied in rats using the hind paw stimulation test. The stimulation threshold was significantly increased in animals with bilateral destruction of the locus coeruleus (LC), and was reduced after lesion of the dorsal raphe nucleus (DR). LC lesions produced a selective lowering of noradrenaline (NA) content in the forebrain, while DR lesions resulted in a reduction in serotonin levels. Lesioning both LC and DR significantly reduced both NA and serotonin contents even when the stimulation threshold was not altered. Morphine produced a significant and dose-dependent elevation of the stimulation threshold in sham-operated animals, while morphine analgesia was almost completely inhibited by destruction of LC, DR and both the nuclei. These results imply that a depression of LC-mediated noradrenergic tone results in a decreased sensitivity to painful stimuli, whereas a reduction of raphe-derived serotonergic tone produces the opposite effect against LC. It is suggested, however, that both of these monoamines from the LC and DR are necessary for the analgesic effect of morphine.