The effects of lesions of medullary midline and lateral reticular areas on inhibition in the dorsal horn produced by periaqueductal grey stimulation in the cat

Divergent Modulation of Nociception by Glutamatergic and GABAergic Neuronal Subpopulations in the Periaqueductal Gray

The ventrolateral periaqueductal gray (vlPAG) constitutes a major descending pain modulatory system and is a crucial site for opioid-induced analgesia. A number of previous studies have demonstrated that glutamate and GABA play critical opposing roles in nociceptive processing in the vlPAG. It has been suggested that glutamatergic neurotransmission exerts antinociceptive effects, whereas GABAergic neurotransmission exert pronociceptive effects on pain transmission, through descending pathways. The inability to exclusively manipulate subpopulations of neurons in the PAG has prevented direct testing of this hypothesis. Here, we demonstrate the different contributions of genetically defined glutamatergic and GABAergic vlPAG neurons in nociceptive processing by employing cell type-specific chemogenetic approaches in mice. Global chemogenetic manipulation of vlPAG neuronal activity suggests that vlPAG neural circuits exert tonic suppression of nociception, consistent with previous pharmacological and electrophysiological studies. However, selective modulation of GABAergic or glutamatergic neurons demonstrates an inverse regulation of nociceptive behaviors by these cell populations. Selective chemogenetic activation of glutamatergic neurons, or inhibition of GABAergic neurons, in vlPAG suppresses nociception. In contrast, inhibition of glutamatergic neurons, or activation of GABAergic neurons, in vlPAG facilitates nociception. Our findings provide direct experimental support for a model in which excitatory and inhibitory neurons in the PAG bidirectionally modulate nociception.

Patterns of fos expression in the rostral medulla and caudal pons evoked by noxious craniovascular stimulation and periaqueductal gray stimulation in the cat

Functional imaging studies and clinical evidence suggest that structures in the brainstem contribute to migraine pathophysiology with a strong association between the brainstem areas, such as periaqueductal gray (PAG), and the headache phase of migraine. Stimulation of the superior sagittal sinus (SSS) in humans evokes head pain. Second-order neurons in the trigeminal nucleus that are activated by SSS stimulation can be inhibited by PAG stimulation. The present study was undertaken to identify pontine and medullary structures that respond to noxious stimulation of the superior sagittal sinus or to ventrolateral PAG stimulation. The distribution of neurons expressing the protein product (fos) of the c-fos immediate early gene were examined in the rostral medulla and caudal pons of the cat after (i) sham, (ii) stimulation of the superior sagittal sinus, (iii) stimulation of the superior sagittal sinus with PAG stimulation, or (iv) stimulation of the PAG alone. The structures examined for fos were the trigeminal nucleus, infratrigeminal nucleus, reticular nuclei, nucleus raphe magnus, pontine blink premotor area, and superior salivatory nucleus. Compared with all other interventions, fos expression was significantly greater in the trigeminal nucleus and superior salivatory nucleus after SSS stimulation. After PAG with SSS stimulation, on the side ipsilateral to the site of PAG stimulation, fos was significantly greater in the nucleus raphe magnus. These structures are likely to be involved in the neurobiology of migraine.

Effect of acupuncture on pain management in patients before and after lumbar disc protrusion surgery--a randomized control study

Management of acute and chronic low back and leg pain often includes the use of acupuncture. The effectiveness of this form of therapy is dependent upon compliance, which in turn is dependent on availability, response, treatment of proper acupoints, and the placebo effect. We hypothesized that classical acupuncture would be more effective than placebo acupuncture. One hundred and thirty-two patients with acute and chronic low back and leg pain were examined before and after surgery for lumbar disc protrusion. Diagnosis was based on CT and MRT findings. Patients received acupuncture drug-free throughout the study period. The visual analogue scale was used to assess pain intensity before and after (i.e. 30 min. 60 min. 2 h and 6 h) acupuncture. Classical acupuncture resulted in a significant reduction in pain that become increasingly stronger during the 6h study period. Placebo acupuncture lead to same early pain relief that did not reach statistic significant and then declined thereafter.

Involvement of the caudal medulla in negative feedback mechanisms triggered by spatial summation of nociceptive inputs

In the rat, applying noxious heat stimuli to the excitatory receptive fields and simultaneously to adjacent, much larger, areas of the body results in a surface-related reduction in the responses of lumbar dorsal horn convergent neurons. These inhibitory effects induced by spatial summation of nociceptive inputs have been shown to involve a supraspinally mediated negative feedback loop. The aim of the present study was to determine the anatomic level of integration of these controls and hence to ascertain what relationships they might share with other descending controls modulating the transmission of nociceptive signals. The responses of lumbar convergent neurons to noxious stimulation (15-s immersion in a 48 degrees C water bath) applied to increasing areas of the ipsilateral hindlimb were examined in several anesthetized preparations: sham-operated rats, rats with acute transections performed at various levels of the brain stem, and spinal rats. The effects of heterotopic noxious heat stimulation (tail immersion in a 52 degrees C water bath) on the C-fiber responses of these neurons also were analyzed. The electrophysiological properties of dorsal horn convergent neurons, including their responses to increasing stimulus surface areas, were not different in sham-operated animals and in animals the brain stems of which had been transected completely rostral to a plane -2. 8 mm remote from interaural line (200 micron caudal to the caudal end of the rostral ventromedial medulla). In these animals, increasing the stimulated area size from 4.8 to 18 cm2 resulted in a 35-45% reduction in the responses. In contrast, relative to responses elicited by 4.8 cm2 stimuli, responses to 18 cm2 were unchanged or even increased in animals with transections at more caudal level and in spinal animals. Inhibitions of the C-fiber responses elicited by heterotopic noxious heat stimulation were in the 70-80% range during conditioning in sham-operated animals and in animals with rostral brain stem transections. Such effects were reduced significantly (residual inhibitions in the 10-20% range) in animals with transections >500 micron caudal to the caudal end of the rostral ventromedial medulla and in spinal animals. It is concluded that the caudal medulla constitutes a key region for the expression of negative feed-back mechanisms triggered by both spatial summation of noxious inputs and heterotopic noxious inputs.

The pontine A5 noradrenergic cells which project to the spinal cord dorsal horn are reciprocally connected with the caudal ventrolateral medulla in the rat

A disynaptic pathway linking the caudal ventrolateral medulla (VLM) to the spinal cord via the A5 noradrenergic cell group of the pons has recently been described in the rat. In the present work, the projections of the A5 to the VLM and to the spinal dorsal horn were studied with double-tracing techniques combined with immunostaining of the noradrenaline-synthesizing enzyme dopamine-beta-hydroxylase. Cholera toxin subunit B (CTb) injected into the VLM and fluoro-gold injected into the spinal dorsal horn produced double retrograde labelling of A5 neurons immunoreactive for dopamine-beta-hydroxylase, which received appositions of fibre varicosities labelled anterogradely with CTb injected into the VLM. After injecting CTb into the A5, retrogradely labelled neurons occurred in the VLM. These neurons were contacted by anterogradely labelled fibres from the A5 group. These observations indicate that the VLM cells acting upon the A5 spinally projecting neurons, which are likely to exert an alpha2-adrenoreceptor-mediated inhibition on the spinal cord, are targeted by collaterals of the A5 spinal cord-bound axons. The A5-VLM pathway may be the anatomical substrate of a negative feedback circuit whereby the modulatory action of the VLM on the spinal cord is self-inhibited through activation of the A5.

Excitatory amino acid antagonists in the rostral ventromedial medulla inhibit mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM). Morphine analgesia elicited from the PAG is respectively decreased by selective serotonergic and opioid receptor antagonists administered into the RVM, and increased by RVM neurotensin antagonists. Since glutamate and excitatory amino acid (EAA) receptors are also active in the RVM, the present study evaluated whether either competitive (AP7) or non-competitive (MK-801) N-methyl-D-aspartate (NMDA) antagonists or a kainate/AMPA (CNQX) antagonist microinjected into the RVM altered morphine (2.5 micrograms) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was markedly reduced on both tests after RVM pretreatment with either AP7 (0.01-1 microgram, 0.08-7.8 nmol) or MK-801 (0.03-3 micrograms, 0.04-4.4 nmol). In contrast, small but significant reductions in mesencephalic morphine analgesia occurred on the jump test following CNQX (0.5 microgram, 2.2 nmol) in the RVM. NMDA antagonists did not markedly alter either basal nociceptive thresholds following RVM administration, or mesencephalic morphine analgesia following administration into medullary placements lateral or dorsal to the RVM. These data implicate EAA and particularly NMDA receptors in the RVM in modulating the transmission of opioid pain-inhibitory signals from the PAG.

The spinal transmission of nociceptive information: modulation by the caudal medulla

Multiple descending systems for pain control originate from the rostral medulla and midbrain. These systems are involved in the antinociceptive action produced by opioids. One category of descending inhibitory controls is activated specifically by noxious stimuli and has been termed diffuse noxious inhibitory controls. These controls have been described in both animal and man, but their supraspinal circuitry has not been fully localized. To determine the supraspinal level of integration of nociceptor activated controls and hence their potential relationships with previously described descending controls, we studied in halothane-anesthetized rats the effects of transections performed at various levels in the brainstem. The physiological properties of dorsal horn convergent neurons, including supraspinally-mediated inhibitory processes elicited by heterotopic noxious stimuli, i.e. diffuse noxious inhibitory controls, were not altered in rats in which the brainstem had been completely transected up to 200 microns caudal to the caudal end of the rostral ventromedial medulla. In contrast, the spontaneous activity of these neurons was significantly enhanced and the inhibitory phenomena significantly reduced in animals with transections more than 500 microns caudal to the caudal end of the rostral ventromedial medulla. These effects were not related to cardiovascular changes induced by the transections. These data indicate that some tonic descending inhibitory controls and diffuse noxious inhibitory controls depend upon connections in the caudal medulla. It is proposed that this area constitutes another level from which the transmission of nociceptive information can be modulated and that it acts co-operatively with previously described modulatory systems in the spinal cord and at more rostral levels of the brainstem.

The role of alpha 2-adrenoceptors of the medullary lateral reticular nucleus in spinal antinociception in rats

We attempted to find out the role of alpha 2-adrenoceptors of the medullary lateral reticular nucleus (LRN) in antinociception in rats. Spinal antinociception was evaluated using the tail-flick test, and supraspinal antinociception using the hotplate test. Antinociceptive effects were determined following local electric stimulation of the LRN, and following microinjections of medetomidine (an alpha 2-adrenoceptor agonist; 1-10 micrograms), atipamezole (an alpha 2-adrenoceptor antagonist; 20 micrograms) or lidocaine (4%) into the LRN. The experiments were performed using intact and spinalized Hannover-Wistar rats with a unilateral chronic guide cannula. Electric stimulation of the LRN as well as of the periaqueductal gray produced a significant spinal antinociceptive effect in intact rats. Medetomidine (1-10 micrograms), when microinjected into the LRN, produced no significant antinociceptive effect in the tail-flick test in intact rats. However, following spinalization, medetomidine in the LRN (10 micrograms) produced a significant atipamezole-reversible antinociceptive effect in the tail-flick test. In the hot-plate test, medetomidine (10 micrograms) in the LRN produced a significant atipamezole-reversible increase of the paw-lick latency in intact rats. Microinjection of atipamezole (20 micrograms) or lidocaine alone into the LRN produced no significant effects in the tail-flick test. The results are in line with the previous evidence indicating that the LRN and the adjacent ventrolateral medulla is involved in descending inhibition of spinal nocifensive responses. However, alpha 2-adrenoceptors in the LRN do not mediate spinal antinociception but, on the contrary, their activation counteracts antinociception at the spinal cord level.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycine and GABAA antagonists reduce the inhibition of primate spinothalamic tract neurons produced by stimulation in periaqueductal gray

Amino acids are demonstrated to be important neurotransmitters mediating the inhibitory transmission from nucleus raphe magnus to spinal nociceptive dorsal horn neurons. In this study, the role of glycine and GABA in the inhibitory processes evoked by stimulation in periaqueductal gray (PAG) of responses of primate spinothalamic tract (STT) neurons to cutaneous mechanical and thermal stimuli was investigated by examining the effects of strychnine and bicuculline, antagonists of glycine and GABAA receptors, respectively, introduced into the dorsal horn through a microdialysis fiber. The inhibitory effects of iontophoretic application of glycine and GABAA agonists on STT cell activity evoked by noxious mechanical stimulation of the skin were selectively blocked by their specific antagonist, strychnine or bicuculline, infused into the dorsal horn. Similarly, intra-spinal application of strychnine or bicuculline resulted in a significant reduction in the PAG stimulation-induced inhibition of responses of STT cells to cutaneous stimuli. This reduction was mainly on the PAG-induced inhibition of the responses to noxious mechanical stimuli. Our results suggest that glycinergic and GABAergic inhibitory interneurons in the spinal cord dorsal horn synapsing on STT cells are activated during stimulation in PAG and contribute to descending antinociceptive actions.

Nuclei within the rostral ventromedial medulla mediating morphine antinociception from the periaqueductal gray

The relative contributions of nuclei within the rostral ventromedial medulla (RVM) involved in mediating morphine induced antinociception from the periaqueductal gray (PAG) were examined. Lidocaine injections (4%) at the time of morphine's maximal response were used to provide a localized neural block and were administered in the nucleus raphe magnus/reticularis gigantocellularis pars alpha (RMg/GiA; commonly referred to as RMg), reticularis gigantocellularis (Gi) and reticularis paragigantocellularis lateralis (LPGi). Microinjection of morphine (6 nmol; 0.5 microliter) into the PAG of awake rats produced an inhibition of the tail-flick reflex that was maximal after 30 min. This response was unaffected by a single medial lidocaine injection (0.5 microliter) into the RMg/GiA or Gi, bilateral injections into the Gi or LPGi or triple injections that included both the RMg/GiA and LPGi. A partial, non-significant block of morphine's response was observed either by bilateral injections (0.5 microliter) into both the Gi and LPGi (% inhibition = 16.4 +/- 24.8) or by bilateral injections in the LPGi and a single medial injection into the Gi (% inhibition = 41.5 +/- 29.8). However, injection of a greater volume of lidocaine (1 microliter) into the RMg/GiA or bilaterally into the LPGi affected adjacent medial and lateral tissue, and completely inhibited morphine's response. Furthermore, triple injections of lidocaine (0.5 microliter) into the Gi or bilateral injections (0.5 microliter) into the Gi and a single medial injection into the RMg/GiA completely blocked morphine's antinociceptive response. These results indicate that morphine antinociception from the PAG is mediated by a large volume of tissue in the RVM containing nuclei located both medially and laterally. Additionally, the principal nuclei involved in this response appear to be the Gi and RMg/GiA.

Localization of the antinociceptive and antianalgesic effects of neurotensin within the rostral ventromedial medulla

Triple microinjections of neurotensin (10 nmol each), which occupied a large volume of tissue within the nucleus reticularis gigantocellularis (Gi), produced an inhibition of the tail-flick reflex in awake rats. This effect was less than that previously observed by a single injection (10 nmol) into the nucleus raphe magnus (RMg) (see ref. [25]). Bilateral injections of neurotensin (10 nmol each) into the nucleus reticularis paragigantocellularis lateralis (LPGi) had no effect. The neurotensin antagonist [D-Trp11]-neurotensin (3 pmol) was previously found to enhance morphine, but not beta-endorphin antinociception from the periaqueductal gray (PAG) when injected into the RMg. A similar enhancement of morphine, but not beta-endorphin antinociception from the PAG was observed in the current study by [D-Trp11]-neurotensin injections into the bilateral LPGi, bilateral Gi, or medial Gi. These data suggest that neurotensinergic projections from the PAG function in an antianalgesic manner throughout the RVM during morphine, but not beta-endorphin antinociception. The antinociceptive effect of neurotensin, on the other hand, is more localized.

Descending projections from the medullary dorsal reticular nucleus make synaptic contacts with spinal cord lamina I cells projecting to that nucleus: an electron microscopic tracer study in the rat

An ultrastructural study is made of the synaptic contacts occurring between structures labelled anterogradely and retrogradely in the superficial dorsal horn following injections of cholera toxin subunit B or horseradish peroxidase in the dorsal reticular nucleus of the medulla oblongata of the rat. Both tracers revealed labelled axonal boutons in lamina I with round synaptic vesicles and a few large granular vesicles making asymmetrical synaptic contacts upon labelled somata and dendrites. After injections of Phaseolus vulgaris leucoagglutinin in the dorsal reticular nucleus, labelled boutons identical to those revealed by the two other tracers were presynaptic to unlabelled somata and dendrites. In addition, dorsoreticular neurons were labelled retrogradely following injections of cholera toxin subunit B into the superficial dorsal horn of the cervical enlargement. These observations show the occurrence of a reciprocal connection between dorsal reticular and lamina I neurons. Considering the putative excitatory nature of the axodendritic contacts in lamina I, a positive feedback circuit is suggested, whereby the nociceptive signals transmitted to the dorsal medullary reticular formation by marginal neurons are intensified.

Reversible tetracaine block of rat periaqueductal gray (PAG) decreases baseline tail-flick latency and prevents analgesic effects of met-enkephalin injections in nucleus paragigantocellularis (PGC)

One micrograms of tetracaine in the rat periaqueductal gray (PAG) produced a decline in baseline tail-flick latencies (hyperalgesia) from about 5 to 3.5 s over the course of 9 min, after which the latencies increased to about 4.5 s. One micrograms of Met-enkephalin in PGC caused an expected increase in latencies (analgesia) from about 4.25 to 6.2 s in 9 min, with recovery to 4.7 s after 15 min post-injection. Giving the preceding 2 nanoinjections simultaneously led to an essentially total block of the PGC analgesia. A control injection in PAG simultaneous with a Met-enkephalin injection in PGC did not block the latter's analgesic effect. Single control (artificial cerebrospinal fluid) injections in PAG or PGC were without effect. The hyperalgesic effect of PAG tetracaine supports the involvement of PAG in normally occurring, tonic descending pain inhibition. The block of PGC met-enkephalin analgesia by distant injection of tetracaine in PAG supports the necessity of PAG integrity for PGC analgesic function.

Morphological characterization of marginal (lamina I) neurons immunoreactive for substance P, enkephalin, dynorphin and gamma-aminobutyric acid in the rat spinal cord

Neurons of the rat spinal cord were immunostained for substance P, enkephalin and dynorphin in colchicine-treated animals, and for gamma-aminobutyric acid (GABA). Lamina I stained cells were classified in the four neuronal groups of our previous morphological classification of marginal cells (See Lima and Coimbra, 1986), according to their configuration in the three main anatomical planes. Most lamina I cells exhibiting substance P-immunoreactivity belonged in the group of flattened neurons. Most enkephalinergic cells were pyramidal neurons, while GABA-immunoreactive cells included all multipolar stained neurons and some fusiform neurons. Dynorphin-immunoreactive cells could be fusiform, pyramidal or flattened. The different neurochemical nature and supraspinal projection patterns are suggestive of functional specificity for each group. It is likely that each immunocytochemical subset in each cell group includes tract cells acting at their projection target and intrinsic neurons with local functional roles.

Descending adrenergic input to the primate spinal cord and its possible role in modulation of spinothalamic cells

The present study focuses on 3 different aspects of the descending adrenergic system in the primate: (1) the distribution of adrenergic fibers and terminals in the spinal cord, (2) the source of this input and (3) the possible physiological effects of this system on spinal nociceptive processing. Antibodies to the enzyme phenylethanolamine-N-methyltransferase (PNMT) were employed to map the distribution of epinephrine-containing axonal profiles in the primate spinal cord. Smooth longitudinally oriented fibers were localized to the outer edge of the lateral funiculus. PNMT-containing axonal enlargements were distributed to the superficial dorsal horn, intermediate gray matter and the region surrounding the central canal at all spinal cord levels. PNMT-immunostained profiles were also observed in the intermediolateral cell column. A double labeling study employing retrograde transport of HRP from the spinal cord and PNMT immunohistochemistry identified a small population of HRP-PNMT-labeled neurons in the 'C1' region at the levels of the medulla and ponto-medullary junction. Thus, these cells are a probable source of adrenergic input to the spinal cord. Electrophysiological studies demonstrated that iontophoresis of epinephrine onto identified primate spinothalamic tract neurons in the lumbar dorsal horn resulted in inhibition of the glutamate-induced firing of these cells. The data from these studies support the hypothesis that adrenergic (PNMT-containing) cells in the caudal brainstem project to all levels of the cord and may contribute to descending modulation of nociceptive processing at these levels.

Efferent projections of the periaqueductal gray in the rabbit

The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

The spino-latero-reticular system of the rat: projections from the superficial dorsal horn and structural characterization of marginal neurons involved

The projections of the superficial dorsal horn to the lateral reticular nucleus of the medulla oblongata of the rat, and the morphological types of spinal cord lamina I neurons involved were studied after injecting the retrograde tracer cholera toxin subunit B in the caudal portion of the lateral reticular nucleus. Only injection sites located in the lateral part of the lateral reticular nucleus caused retrograde cell labelling in the superficial dorsal horn (laminae I-III). However, injection sites covering the lateral half of the lateral reticular nucleus and the region intermediate between its lateral border and the ventrocaudal tip of the trigeminal spinal nucleus also labelled cells in the neck of the dorsal horn. In contrast, injection sites confined to the intermediate region gave rise to an almost exclusive cell labelling in laminae I-III. Because the lateral part of the lateral reticular nucleus and the adjoining lateral region are rich in noradrenergic cells, it is suggested that these may be the specific targets of laminae I-III neurons. On the basis of the solid dendritic filling achieved, labelled lamina I cells were classified structurally. Most were fusiform cells (80%) and a minority pyramidal or flattened cells (10% each). Since fusiform cells also project selectively to the parabrachial nuclei, which together with the lateral reticular nucleus have been implicated in respiratory and cardiovascular reflexes, it is suggested that this cell type may convey nociceptive input originating autonomic responses. The pyramidal cells project also in large numbers to the mesencephalic periaqueductal gray which, like the lateral reticular nucleus, exerts descending inhibition on the dorsal horn nociceptive neurons. This suggests that this cell type may activate the spinal-midbrain pain modulatory loops centred on both nuclei.

The role of glutamate in opiate descending inhibition of nociceptive spinal reflexes

The present experiment examined descending inhibition of the nociceptive tail-flick reflex produced by microinjection of morphine and glutamate into the periaqueductal gray (PAG) matter and the neurotransmitters mediating the inhibition at the level of the nucleus raphe magnus (NRM). The longlasting opiate analgesia was significantly reduced by microinjection of excitatory amino acid antagonists 1-(p-chlorobenzoyl)-piperazine-2,3-dicarboxylate (PCB, 3.25 mumol) or DL-2-amino-5-phosphono-valerate (APV, 25.38 mumol) into the NRM, whereas the short-lived glutamate analgesia was not. This indicates that although both opiate and non-opiate analgesia may originate in the PAG, the former is relayed through the NRM, whereas the latter is relayed by additional or different nuclei in the medulla. Two observations shed light on the question which receptors mediate the above effect in the NRM. First, PCB blocked morphine analgesia at doses that were 8 times lower than doses of APV that were effective. Second, analgesia produced by injection of glutamate into the NRM was antagonized by PCB (3.25 mumol), whereas APV (25.38 mumol) failed to do so. Together these results indicate that kainate/quisqualate, but not N-methyl-D-aspartate (NMDA), receptors are implicated in the NRM as a relay station in opiate descending inhibition.

The synergistic effect of concurrent spinal and supraspinal opiate agonisms is reduced by both nociceptive and morphine pretreatment

The antinociceptive effect of morphine administered into the periaqueductal gray (PAG), the intrathecal space (ITH) and concurrently, into both sites (in a 1:1 dose ratio), was assessed in 1) nontolerant rats, 2) rats made tolerant to the effect of morphine on the tail-flick (TF) test and 3) rats that were tested on the TF during chronic saline administration. In nontolerant rats, concurrent morphine injections produced a multiplicative antinociceptive effect (ED50 = 0.392 microgram, total dose) relative to that obtained after separate PAG (ED50 = 2.8 micrograms) or ITH (ED50 = 6.7 micrograms) injections. The multiplicative effect of concurrent morphine administration was significantly reduced in rats made tolerant to morphine (one 3 mg/kg SC injection and TF test per day for six days). Opiate synergy was also reduced but to a smaller extent in rats that were repeatedly tested on the TF during chronic saline administration (one SC injection and TF test per day for six days). Neither chronic morphine nor saline pretreatment altered the dose-response function to intrathecal morphine. However, both morphine and saline pretreatment significantly reduced the antinociceptive effect of morphine administered into the PAG. The data indicate that concurrent morphine administration into the PAG and ITH space results in a synergistic antinociceptive action which is reduced by performance of the nociceptive response, even in the absence of opiate administration. We suggest that the decrease in opiate synergism produced by nociceptive assessment (behavioral tolerance) is mediated supraspinally, while the additional decline resulting from morphine administered in conjunction with the nociceptive tests (opiate tolerance) is mediated by a combined action at spinal and supraspinal sites.

Distribution of phenylethanolamine N-methyltransferase cell bodies, axons, and terminals in monkey brainstem: an immunohistochemical mapping study

Adrenaline (epinephrine) is an important candidate transmitter in descending spinal control systems. To date intrinsic spinal adrenergic neurons have not been reported; thus adrenergic input is presumably derived from brainstem sites. In this regard, the localization of adrenergic neurons in the brainstem is an important consideration. Maps of adrenergic cell bodies and to a lesser extent axons and terminal fields have been made in various species, but not in monkeys. Thus, the present study concerns the organization of adrenergic systems in the brainstem of a monkey (Macaca fascicularis) immunohistochemically mapped by means of an antibody to the enzyme phenylethanolamine N-methyltransferase (PNMT). PNMT-immunostained cell bodies are distributed throughout the medulla in two principal locations. One concentration of labeled cells is in the dorsomedial medulla and includes the nucleus of the solitary tract (NTS), the dorsal motor nucleus of the vagus (X), and an area ventral to X in a region of the reticular formation (RF) known as the central nucleus dorsalis (CnD) of the medulla. A few scattered cells are observed in the periventricular gray just ventral to the IVth ventricle and on midline in the raphe. The second major concentration of PNMT-immunostained cells is located in the ventrolateral RF, lateral and dorsolateral to the inferior olive (IO), including some cells in the rostral part of the lateral reticular nucleus (LRN). Terminal fields are located in the NTS, X, area postrema (AP), and the floor of the IVth ventricle in the medulla and pons. A light terminal field is also observed in the raphe, particularly raphe pallidus (RP). A heavy terminal field is present in locus coeruleus (LC). Fibers labeled for PNMT form two major fiber tracts. One is in the dorsomedial RF extending as a well-organized bundle through the medulla, pons, and midbrain. A second tract is located on the ventrolateral edge of the medulla and caudal pons. Fibers in this tract appear to descend to the spinal cord. A comparison with maps of other catecholamine neurons in primates is discussed, confirming that the distribution of the adrenergic system in monkeys is similar to that described in the human.

Electrophysiological identification of spinally projecting neurons in the lateral reticular nucleus of the rat

Eighty-four neurons in the caudal ventrolateral medullary reticular formation were antidromically activated by the stimulation of the dorsolateral funiculus in 49 urethane-anesthetized rats. Of 76 neurons, 37 had no spontaneous discharge. Of the neurons that had spontaneous discharges, 80% had firing rates between 0.1 and 15 Hz. The average conduction velocity, determined among 70 neurons, was 15.20 +/- 1.23 m/s, and 87% had conduction velocities within the range of 2-30 m/s. This study further confirms the existence of spinally-projecting neurons in the lateral reticular nucleus (LRN) of the caudal medulla, and some of them are probably responsible for the descending controls of nociception from the LRN.

Inhibition of the responses of cat dorsal horn neurons to noxious skin heating by stimulation in medial or lateral medullary reticular formation

Responses of single lumbar dorsal horn units to noxious radiant heating (50 degrees C, 10 s) of glabrous footpad skin were recorded in cats anesthetized with sodium pentobarbital and 70% nitrous oxide. The heat-evoked responses of 37/40 units were reduced during electrical stimulation (100 ms trains, 100 Hz, 3/s, 25-600 microA) in the medullary nucleus raphe magnus (NRM) and/or in laterally adjacent regions of the medullary reticular formation (MRF). Inhibition was elicited by stimulation in widespread areas of the medulla, but with greatest efficacy at ventrolateral sites. The magnitude of inhibition increased with graded increases in medullary stimulation intensity. Mean current intensities at threshold for inhibition or to produce 50% inhibition were higher for NRM than for MRF sites. Units' responses to graded noxious heat stimuli increased linearly from threshold (42-43 degrees C) to 52 degrees C. During NRM (5 units) or ipsilateral MRF stimulation (7 units), responses were inhibited such that the mean temperature-response functions were shifted toward higher temperatures with increased thresholds (1.5 degrees and 1 degree C, respectively) and reduced slopes (to 60% of control). Contralateral MRF stimulation had a similar effect in 4 units. Inhibitory effects of NRM and MRF stimulation were reduced (by greater than 25%) or abolished in 4/6 and 5/12 units, respectively, following systemic administration of the serotonin antagonist methysergide. Inhibitory effects from NRM, ipsi- and contralateral MRF were reduced or abolished in 2/9, 4/8 and 6/9 cases, respectively, following systemic administration of the noradrenergic antagonist phentolamine. These results confirm and extend previous studies of medullospinal inhibition and the role of monoamines, and are discussed in terms of analgesic mechanisms.

Absence of tonic supraspinal control of substance P release in the substantia gelatinosa of the anaesthetized cat

Antibody microprobes were used to measure immunoreactive substance P (irSP) release in the substantia gelatinosa of the lower lumbar spinal cord of barbiturate-anaesthetized cats. Release of irSP was produced by noxious peripheral stimuli. Such release was not altered by blocking spinal conduction at the first lumbar segment by cooling or transecting the spinal cord. The results suggest that the release of irSP from the central terminals of nociceptors is not subject to tonic supraspinal inhibition.

The effects of periaqueductal gray and nucleus raphe magnus stimulation on the spontaneous and noxious-evoked activity of lateral reticular nucleus neurons in rabbits

In urethane-anesthetized rabbits the effects of periaqueductal gray (PAG) and nucleus raphe magnus (NRM) stimulation on the spontaneous and noxious-evoked activity of the lateral reticular nucleus (LRN) neurons were studied. The PAG and the NRM stimulating electrodes were located in the optimal sites for suppressing the jaw-opening reflex (JOR) evoked by the tooth pulp stimulation. It was found that the 12% of neurons tested were affected by one or both stimuli. A total of 80 responsive neurons (52% antidromically activated by the cerebellum) were analyzed. Out of these neurons, 31 showed a convergence to both stimuli, 43 responded only to PAG and 6 only to NRM. Noxious heat stimulation of the contralateral foot was effective in altering the activity of 60% of these neurons. The PAG and NRM stimuli modified the noxious-evoked responses in most of these units. While the excitation was the predominant effect on the spontaneous activity (52 cells), the inhibition was predominant on the noxious-evoked activity (29 cells). These results indicate the presence of connections from PAG and NRM to LRN, probably devoted to the processing of the nociceptive information.

Brain-stem areas tonically inhibiting dorsal horn neurones: studies with microinjection of the GABA analogue piperidine-4-sulphonic acid

In barbiturate anaesthetized cats, tonic inhibition of the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferents was measured by reversibly cooling the spinal cord at the thoraco-lumbar junction. Tonic inhibition was reduced by microinjection of the GABA analogue, piperidine-4-sulphonic acid (2.5 nM in 0.5 microliter) mainly at AP -7, L 2-5 and V -8 to -10. This area in the ventrolateral medulla is just ventral to the facial nucleus and has been shown to be important in cardiovascular control, particularly in relation to fear-defence reactions. It is proposed that tonic inhibition of the nociceptive responses of spinal neurones is part of such a reaction in response to the trauma of surgery. Since previous experiments had shown that the ventrolateral medulla was important in spinal inhibition produced by PAG stimulation, these experiments support the proposal that analgesia does not occur in isolation but is part of a complex behavioural response of an animal in a potentially injurious environment.

Opioid, cholinergic and alpha-adrenergic influences on the modulation of nociception from the lateral reticular nucleus of the rat

The lateral reticular nucleus (LRN) has been identified as an area in the caudal medulla involved in the centrifugal modulation of spinal nociceptive transmission and withdrawal reflexes. The data presented in this report further support a role for the LRN in the modulation of nociceptive responses. It was confirmed in the present study that focal electrical stimulation in the LRN inhibits the nociceptive tail-flick (TF) reflex at low intensities of stimulation in lightly pentobarbital-anesthetized rats. Aversive effects, however, were typically produced at similar and higher intensities of stimulation in the LRN in the same rats in the awake state. It was also determined that an inhibitory modulation of nociceptive responses organized both spinally and supraspinally could be activated independently by muscarinic cholinergic or opioid mechanisms in the LRN. Microinjection of morphine into the LRN in conscious rats produced an antinociception in both TF and hot plate (HP) tests which could be attenuated significantly by naloxone, but not atropine, previously microinjected into the same site in the LRN. Carbachol microinjected into the LRN also produced an antinociception which was attenuated significantly by atropine but not naloxone previously microinjected into the same site in the LRN. In contrast, the microinjection of clonidine or norepinephrine into the LRN either did not affect or shortened significantly response latencies in the TF and HP tests. These results further establish that the LRN contributes to the modulation of nociception. Opioid and cholinergic influences in the LRN appear to independently activate inhibition of responding to nociceptive stimuli organized either spinally or supraspinally, although descending inhibition was most clearly activated. An action at alpha 2 adrenoceptors in the LRN, conversely, produces an hyperalgesia as reflected by shortened latencies to respond in TF and HP tests.

Inhibition of the responses of neurons in the rat spinal cord to noxious skin heating by stimulation in midbrain periaqueductal gray or lateral reticular formation

Single units responsive to noxious heating of glabrous hindfoot skin were recorded in the lumbar spinal cord of rats anesthetized with sodium pentobarbital. Unit responses to heat stimuli (e.g. 50 degrees C, 10 s) delivered at 2-min intervals were stable and were markedly suppressed during stimulation (100-ms pulse trains at 100 Hz, 3/s, 25-400 microA) in the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF). Inhibition did not appear to outlast the midbrain stimulation period. By systematically varying the position of an array of 3 or 5 stimulating electrodes, we observed that stimulation in PAG and subjacent tegmental areas, as well as in widespread regions of the LRF bilaterally, suppressed unit responses to noxious skin heating. The degree of suppression of unit responses increased with graded increases in PAG or LRF stimulation intensity. LRF appeared to be more efficacious than PAG stimulation, based on lower currents at threshold, as well as on significantly greater slopes in current-inhibition plots for LRF compared to PAG stimulation. Unit responses increased linearly with graded increases in stimulus temperature from 42 to 54 degrees C. Slopes of temperature-response lines were reduced during PAG stimulation with no change in response threshold, while temperature-response lines were shifted in a parallel manner toward higher temperatures during LRF stimulation with an increase in response threshold. The results suggest that differential inhibitory systems are activated by PAG or LRF stimulation and are discussed in relation to previous findings in the cat and as a possible mechanism of stimulation-produced analgesia.

Inhibition of spinal nociceptive transmission accompanies cardiovascular changes from stimulation in diencephalic 'defence' regions of cats

In anaesthetized cats, diencephalic regions were electrically stimulated while recording evoked responses from lumbar dorsal horn neurones and cardiovascular parameters. Neuronal responses to impulses evoked electrically in unmyelinated primary afferents were inhibited by stimulation in many diencephalic regions. Responses to non-noxious cutaneous stimulation (hair deflection) were inhibited at relatively few sites. Indirect circulatory measurements showed that this selective spinal inhibition was accompanied by increases in cardiac output and muscle blood flow but reduced cutaneous perfusion. This association between selective inhibition of nociceptive spinal neuronal responses and a cardiovascular response pattern associated with the defence reaction supports the proposal that analgesia could be an important component of defensive behaviour.

Hypothalamic inhibition of rat dorsal horn neuronal responses to noxious skin heating

The responses of single lumbar dorsal horn neurons to noxious radiant heat stimuli (42-54 degrees C, 10 sec, 1/2 min) applied to glabrous hind paw skin were recorded in rats anesthetized with sodium pentobarbital. Unit responses to 50 or 52 degrees C stimuli were constant over time and were consistently and powerfully inhibited during bipolar stimulation (three 100 msec trains/sec at 100 Hz, 200 microA) in the medial hypothalamus. Inhibition was also evoked by stimulation in medial and ventrobasal thalamic nuclei, lateral hypothalamus and adjacent cerebral peduncle, and amygdala. Inhibition increased with graded increases in intensity of hypothalamic stimulation, with a mean inhibitory threshold of 71 +/- 43 (S.D.) microA for 13 units. The responses of dorsal horn units to graded increases in the temperature of noxious heat stimuli were inhibited during hypothalamic stimulation, such that slopes of the linear temperature-response functions were reduced with no change in response threshold (mean: approximately 44 degrees C). Inhibition was blocked or reduced in 4/7 units following systemic administration of the 5-hydroxytryptamine (5-HT) antagonist methysergide. The results confirm and extend previous work in the cat and are discussed in relation to analgesic mechanisms.

The effect of local and systemic application of dopaminergic agents on tail flick latency in the rat

Dopamine (DA) is thought to have a neurotransmitter role in the spinal cord of the rat. Intrathecal administration of the DA receptor agonist apomorphine has previously been shown to reduce nocifensive responses. The present experiments investigated the site of action of apomorphine, and the mechanisms by which DA agonists apparently produce antinociception. Small doses of apomorphine (40-80 micrograms/kg) increased the tail flick latency (TFL) in lightly anaesthetised rats when given intrathecally and intravenously but not intracerebroventricularly. This effect is probably mediated via D2 receptors since the D2 agonist LY171555 had a similar effect whereas the D1 agonist SK&F 38393 was inactive. Furthermore the D2 antagonist sulpiride blocked the effects of apomorphine and LY171555. The spinal monosynaptic reflex was not modified by 150 micrograms/kg apomorphine suggesting that sensory rather than motor processes are being influenced. Pretreatment with the serotonin receptor antagonist methysergide prevented the apomorphine induced increase in TFL. It is concluded that exogenously applied DA agonist can result in antinociception in the spinal cord and that this effect may be dependent upon activity in the spinal 5-hydroxytryptaminergic and noradrenergic systems.

Methysergide and spinal inhibition from electrical stimulation in the periaqueductal grey

In barbiturate anaesthetized cats, electrical stimulation in the periaqueductal grey (PAG) selectively inhibited the excitation of multireceptive dorsal horn neurons by noxious heating of the skin or by electrical stimulation of unmyelinated primary afferents. Intravenous methysergide (0.2-1.0 mg/kg) had opposing effects on uninhibited responses, increasing excitation by noxious heat but reducing responses to C fibre stimulation. Evidence was obtained that the increases to noxious heat resulted from increased firing of peripheral nociceptors secondary to decreased cutaneous blood flow. Intravenous methysergide reduced inhibition from PAG stimulation. When administered electrophoretically from micropipettes however, methysergide did not reduce such inhibition. The mechanism whereby systemic methysergide reduces PAG spinal inhibition is unknown and cannot necessarily be related to a blockade of spinally released 5-hydroxytryptamine.

Response of serotonin-containing neurons in nucleus raphe magnus to morphine, noxious stimuli, and periaqueductal gray stimulation in freely moving cats

Extracellular single-unit recordings were made in nucleus raphe magnus in unanesthetized, unrestrained cats. Discharge of serotonergic neurons in this region was increased when animals were aroused by noxious stimuli such as pinch and radiant heating of the tail, but these cells were not specifically nociceptive. Peristimulus time histograms indicated that stimulation in the periaqueductal gray was excitatory but alveolar nerve stimulation at a noxious current intensity was no more effective than nonnoxious nerve stimulation in activating serotonergic unit discharge: Similarly, stressful treatments such as physical restraint increased the discharge of some serotonergic neurons, but these cells were activated during any period of behavioral arousal whether or not arousal was the result of aversive treatment. Injection of Formalin into the paw produced pain lasting about 30 min without increasing serotonergic unit discharge above rates observed during undisturbed active waking behavior. The activity of serotonergic neurons was not increased by an analgesic dose of morphine (2 mg/kg, i.p.). These results then are not consistent with the hypothesis that morphine analgesia depends on activation of serotonergic neurons in nucleus raphe magnus or that these cells are specifically involved in modulation of nociception. These neurons may, however, be involved in nociceptive control within the context of a general modulation of sensorimotor processes by serotonin in the central nervous system. We did observe neurochemically unidentified neurons in the medulla whose discharge was more specifically activated by aversive stimuli and also by morphine. It is possible that these neurons are more directly involved in the mediation of opiate and/or stress-induced analgesia.

Ventrolateral medullary lesions block the antinociceptive and cardiovascular responses elicited by stimulating the dorsal periaqueductal grey matter in rats

In rats anaesthetised with Saffan (Glaxovet), inhibition of the tail flick reflex evoked by electrical stimulation in the dorsal periaqueductal grey matter (PAG) was accompanied by an increase in blood pressure, tachycardia, vasodilatation in hind limb muscle, an increase in respiration, pupillodilatation and widening of the palpebral fissure. Stimulation deeper in the PAG and in the tegmentum ventral to it produced analgesia but without this pattern of autonomic changes. The antinociceptive, cardiovascular and respiratory effects of PAG stimulation were abolished by bilateral lesions in the ventrolateral medulla in the area which lies ventromedial to the facial nucleus (i.e., in nucleus paragigantocellularis lateralis, RPGL). Lesions in nucleus raphe magnus (NRM), or bilateral lesions of nucleus reticularis gigantocellularis (RG) and paragigantocellularis (RPG) or a combination of a lesion in RPGL with one in NRM or the contralateral RG or RPG did not block the effects of stimulating in the dorsal PAG. These findings are discussed in relation to the role of the PAG in mediating behavioural responses to stress.

Relative contributions of the nucleus raphe magnus and adjacent medullary reticular formation to the inhibition by stimulation in the periaqueductal gray of a spinal nociceptive reflex in the pentobarbital-anesthetized rat

The organization in the brainstem of descending pathways of spinal inhibition was examined in the lightly pentobarbital-anesthetized rat. Thresholds for focal electrical stimulation-produced inhibition of the spinal nociceptive tail flick (TF) reflex were determined at one stimulation site in the midbrain periaqueductal gray and three sites in the rostral medulla: nucleus raphe magnus, and the adjacent medullary reticular formation contralateral and ipsilateral to the stimulating electrode in the periaqueductal gray. Lidocaine (0.5 microliter, 4%) was subsequently microinjected in the same and other medullary loci in the same coronal plane to produce a time-limited, reversible functional neural block. The functional block produced by 0.5 microliter of lidocaine microinjected in the medulla was determined to have a radius of 0.5 mm and was maximally efficacious during the first 30 min after its intramedullary microinjection. The stimulation threshold in the periaqueductal gray for inhibition of the TF reflex was not increased significantly when either the nucleus raphe magnus was fully blocked by lidocaine microinjected in three dorsoventral positions 1.0 mm apart or when the medullary reticular formation ipsilateral and contralateral were simultaneously fully blocked. Not until the nucleus raphe magnus and medullary reticular formation ipsilateral were simultaneously blocked by lidocaine was the stimulation threshold in the periaqueductal gray for inhibition of the TF reflex significantly increased. An increase in the periaqueductal gray stimulation threshold twice as great resulted when the nucleus raphe magnus and both the ipsilateral and contralateral medullary reticular formations were all simultaneously blocked by lidocaine. These results indicate that: (1) the nucleus raphe magnus is not a necessary bulbar relay in a descending antinociceptive pathway activated by stimulation in the midbrain periaqueductal gray; and (2) descending inhibitory pathways activated in the periaqueductal gray course medially as well as laterally in the rat ventral medulla.