Lateral reticular regions and the descending control of dorsal horn neurones of the cat: selective inhibition by electrical stimulation

Adjunctive effect of the serotonin 5-HT2C receptor agonist lorcaserin on opioid-induced antinociception in mice

Opioid-sparing adjuncts are treatments that aim to reduce the overall dose of opioids needed to achieve analgesia, hence decreasing the burden of side effects through alternative mechanisms of action. Lorcaserin is a serotonin 5-HT_2C receptor (5-HT_2CR) agonist that has recently been reported to reduce abuse-related effects of the opioid analgesic oxycodone. The goal of our studies was to evaluate the effects of adjunctive lorcaserin on opioid-induced analgesic-like behavior using the tail-flick reflex (TFR) test as a mouse model of acute thermal nociception. We show that whereas subcutaneous (s.c.) administration of lorcaserin alone was inactive on the TFR test, adjunctive lorcaserin (s.c.) significantly increased the potency of oxycodone as an antinociceptive drug. This effect was prevented by the 5-HT_2CR antagonist SB242084. A similar lorcaserin (s.c.)-induced adjunctive phenotype was observed upon administration of the opioid analgesics morphine and fentanyl. Remarkably, we also show that, opposite to the effects observed via s.c. administration, intrathecal (i.t.) administration of lorcaserin alone induced antinociceptive TFR behavior, an effect that was not prevented by the opioid receptor antagonist naloxone. This route of administration (i.t.) also led to a significant augmentation of oxycodone-induced antinociception. Lorcaserin (s.c.) did not alter the brain or blood concentrations of oxycodone, which suggests that its adjunctive effects on opioid-induced antinociception do not depend upon changes in opioid metabolism. Together, these data indicate that lorcaserin-mediated activation of the 5-HT_2CR may represent a new pharmacological approach to augment opioid-induced antinociception. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

[Spinal serotonergic receptor is involved in descending inhibition of cardiac nociception by the lateral reticular nucleus in rats]

OBJECTIVE

To investigate the role of the lateral reticular nucleus (LRN) in descending inhibition of cardiac nociception in rats and the involvement of spinal serotonergic receptors in the descending inhibition.

METHODS

Male SD rats were randomly divided into 5 groups, namely bradykinin (BK) group, BK + glutamate group, BK + methysergide group, BK + glutamate + methysergide group, and BK + glutamate + vehicle group. The rats received glutamate microinjection in the LRN combined with the intrathecal injection of serotonergic receptor antagonist methysergide, and the changes in the cardiacsomatic motor reflex induced by intrapericardial BK injection were monitored by observing electromyogram (EMG) responses of the dorsal spinotrapezius muscle; c-Fos expression in the spinal dorsal horn was also tested.

RESULTS

Compared with BK group, intra-LRN glutamate administration produced a significant inhibitory effect on intrapericardial BK-induced EMG in a dose-dependent manner, and c-Fos expression was significantly decreased in the spinal dorsal horn (P<0.05). Compared with BK + glutamate group, intrathecal administration of methysergide significantly attenuated the inhibitory effect of chemical stimulation of the LRN on intrapericardial BK-induced EMG and increased c-Fos expression in the spinal dorsal horn (P<0.05). Intrathecal administration of the vehicle did not produce any effect on EMG or c-Fos expression (P>0.05).

CONCLUSION

The serotonergic receptors in the spinal cord are involved in LRN-mediated descending inhibition of cardiac nociception in rats.

Response Properties of Nucleus Reticularis Lateralis Neurons After Electroacupuncture Stimulation in Rats

A descending inhibitory mechanism from the periaqueductal gray (PAG) to the spinal cord through the nucleus raphe magnus (NRM) is strongly involved in endogenous analgesic system produced by acupuncture stimulation. In addition to the PAG to NRM system which descends in the medial pathway of the brain stem, the nucleus reticularis lateralis (NRL) situated in the lateral part of the brain stem is reported to play an important role in modulating centrifugal antinociceptive action. In the present study, to clarify the role of NRL in acupuncture analgesia, we investigated the response properties of NRL neurons to acupuncture stimulation. The majority of NRM-projecting NRL neurons were inhibited by electroacupuncture stimulation. This effect was antagonized by ionophoretic application of naloxone, indicating that endogenous opioids act directly onto these NRL neurons. By contrast, about half of spinal projecting NRL neurons were excited by electroacupuncture stimulation, suggesting that part of the NRL neurons may modulate pain transmission directly at the spinal level.

Lesions of the caudal ventrolateral medulla block the hypertension-induced inhibition of noxious-evoked c-fos expression in the rat spinal cord

The effect of lesioning the lateral portion of the caudal ventrolateral medullary reticular formation (VLMIat) on the noxious-evoked expression of the c-fos proto-oncogene in spinal neurons, was studied in short-term hypertensive rats. Occlusion of the renal artery for 96 h in unlesioned animals induced a 52% increase in blood pressure (BP) and a 66% decrease in the number of Fos-immunoreactive (Fos-IR) spinal cells following noxious cutaneous stimulation, as compared to values in normotensive controls. Lesioning the VLMIat in hypertensive rats by unilateral quinolinic acid (QA) injection (0.3 microl of a 180 nmol/microl solution) 24 h before noxious stimulation, prevented the Fos-IR cell decrease. In normotensive rats, lesioning the VLMIat produced no changes in c-fos expression. To investigate the role played by the VLMIat in cardiovascular control, BP and heart rate (HR) were measured during local injections of QA or glutamate (0.5 microl of a 100 nmol/microl solution) to normotensive animals. Injections of QA produced an immediate rise in BP and HR which reached maximal values (18 and 14% increase, respectively) 5 min after the administration onset, then returning gradually to baseline levels. Glutamate injections resulted in an immediate decrease of the same values, which reached 29 and 39%, respectively, 4 min after the beginning of injection, after which they decreased to baseline levels. These results suggest that VLMIat neurons inhibit nociceptive spinal neurons in response to rises in blood pressure, while exerting negative control of cardiovascular parameters. It is suggested that the VLMIat is involved in the genesis of hypoalgesia during hypertension.

c-fos and CRF receptor gene transcription in the brain of acetic acid-induced somato-visceral pain in rats

We aimed to characterize neuronal and corticotrophin-releasing (CRF) pathways in a model of somato-visceral pain in rats. Male rats received an intraperitoneal (i.p.) injection of either vehicle (controls) or acetic acid (AA) and were sacrificed 1, 2, 3, 4, or 6 h later. Coronal frozen sections of the brain were cut and mRNAs encoding the rat c-fos, CRF(1), CRF(2 alpha,beta) receptors were assayed by in situ hybridisation histochemistry. Localization of these transcripts within CRF-immunoreactive (i.r.) neurons of the paraventricular nucleus (PVN) of the hypothalamus was also determined. AA i.p. induced c-fos mRNA expression in brain nuclei involved in the autonomic, behavioural and neuroendocrine response to pain. Some of these nuclei are involved in the control of digestive motility, as represented by the PVN, locus coeruleus and nucleus tractus solitarius. CRF pathways, in particular in the PVN, are activated in this model. Indeed, a robust signal of c-fos and CRF(1) transcripts was observed in the PVN and numerous CRF-i.r. neurons expressed c-fos or CRF(1) transcripts in the PVN of AA-treated animals. In contrast, no expression of CRF(2) transcripts was observed in the PVN either in basal conditions or after AA i.p. These data argue for an activation of CRF pathways within the PVN in this model of somato-visceral pain. The use of CRF antagonists, particularly of the CRF(1) type, should have an interest in somato-visceral pain pathology.

The caudal ventrolateral medulla as an important inhibitory modulator of pain transmission in the spinal cord

The caudal ventrolateral medulla (VLM) has emerged during the last decade as one of the main components of the endogenous pain control system. Profound and long-lasting analgesia is produced by mild stimulation of the VLM. The VLMlat, the reticular formation located between the spinal trigeminal nucleus and the lateral reticular nucleus (LRt), appears to play a major role in that antinociceptive action. The projections to spinal cord laminae involved in nociceptive transmission originate exclusively in the VLMlat. The VLMlat participates in a disynaptic pathway involving spinally projecting pontine A5 noradrenergic neurons, which appears to convey alpha(2)-adrenoreceptor-mediated analgesia produced from the VLM. Neurons in the VLMlat and in lamina I are reciprocally connected by a closed loop that is likely to mediate feedback control of supraspinal nociceptive transmission. On the other hand, the LRt, which is targeted by ventral (lamina VII) and deep dorsal (laminae IV to V) horn inputs, projects to the premotor lamina VII. Nociceptive input ascending from the cord and increases in blood pressure are discussed as possible physiologic triggers of the analgesia produced by the VLM. The overall role of the VLM as a center for integration of nociceptive, cardiovascular, and motor functions is discussed. The putative therapeutic benefits of manipulating the VLM for the control of chronic pain are envisaged.

Postsynaptic dorsal column and cuneate neurons in raccoon: comparison of response properties and cross-correlation analysis

The responses of 111 postsynaptic dorsal column (PSDC) neurons in the cervical spinal cord and 51 cuneate neurons with receptive fields on the glabrous skin of the forepaw were studied in anesthetized raccoons using extracellular recording techniques. The PSDC neurons had larger receptive fields than the cuneate neurons, but in both groups the fields never extended onto hairy skin. PSDC and cuneate neurons had approximately the same mean latency to electrical stimulation of the receptive field, but PSDC neurons had significantly lower thresholds. The majority of both PSDC and cuneate neurons also responded to electrical stimulation of an adjacent digit, even though they did not respond to mechanical stimulation of that digit. Cross-correlation analysis of the activity of 51 pairs of PSDC and cuneate neurons recorded simultaneously revealed a significant interaction in 26 pairs during spontaneous activity. In 20 of these neuron pairs, the probability that the cuneate neuron would fire was greater after the PSDC neuron had fired (suggesting a spinocuneate interaction), five pairs showed an interaction in the opposite (cuneospinal) direction, and one pair had a significant inhibitory interaction. These interactions occurred more often when the receptive fields of the two neurons were overlapping than when their fields were on adjacent digits. Frequency response analysis revealed greater coherence for those pairs showing a spinocuneate interaction than for those with a cuneospinal interaction. These results support the hypothesis that the PSDC system exerts a tonic facilitatory effect on cuneate neurons and that there may be some somatotopic organization to the interactions. However, the similar response latencies of the two groups of neurons makes it unlikely that PSDC neurons could contribute to the rapid initial processing of cutaneous information by the cuneate nucleus.

GABAergic modulation of descending inhibitory systems from the rostral ventromedial medulla (RVM). Dose-response analysis of nociception and neurological deficits

We have examined the effects of muscimol and bicuculline microinjected in the rostral ventromedial medulla (RVM) on motor function and on nociception in three pain tests. In Exp. 1 microinjection of muscimol (6.25-400 ng in 1 microl) in the RVM dose-dependently decreased pain threshold of rats and the ED(50) for muscimol was the same in both the hot plate and tail immersion pain tests. In the hot plate test, but not in the tail immersion test, paw withdrawal latencies increased again with high doses of muscimol (75-400 ng). High doses also produced catalepsy. Exp. 2 examined the effects of muscimol (50 ng) and bicuculline (50 ng) over a range of formalin concentrations (0.25-4%) in the formalin test. Muscimol increased responsiveness to formalin and reduced the slope of the formalin dose-response relation. Bicuculline decreased responses to formalin and reduced the slope of the formalin dose-response relation. It is suggested that RVM cells with inhibitory projections to the dorsal horn are not subject to strong GABAergic influence under mild noxious stimulation. RVM cells are thus active, and spinal dorsal horn relay neurons are inhibited. On the other hand, intense noxious peripheral stimulation may stimulate the release of GABA onto RVM cells, which in turn shuts off descending inhibitory fibers to allow transmission of nociceptor input through the dorsal horn.

Fedotozine, a kappa-opioid agonist, prevents spinal and supra-spinal Fos expression induced by a noxious visceral stimulus in the rat

Fedotozine, a kappa opioid agonist, reverses digestive ileus caused by acetic acid (AA)-induced visceral pain in rats. The aims of this study were: to map, in conscious rats, central pathways activated by AA using Fos as a marker of neuronal activation; to characterize primary afferent fibres involved in this activation; and to investigate the effect of fedotozine on AA-induced Fos expression. AA (0.6%; 10 mL kg-1) was injected i.p. in conscious rats either untreated; pretreated 14 days before with capsaicin; pretreated 20 min previously with fedotozine; or pretreated 2 h prior to fedotozine with the kappa-antagonist nor-binaltorphimine (nor-BNI). Controls received the vehicle alone. 60 min after injection of AA, rats were processed for Fos immunohistochemistry. Visceral pain was assessed by counting abdominal cramps. AA induced Fos in the thoraco-lumbar spinal cord (laminae I, V, VII and X) and numerous brain structures such as the nucleus tractus solitarius, and paraventricular nucleus (PVN) of the hypothalamus, whereas almost no Fos labelling was observed in controls. Capsaicin pretreatment blocked AA-induced Fos in all structures tested. Fedotozine significantly decreased AA-induced abdominal cramps and Fos immunoreactivity in the spinal cord and PVN, this effect being reversed by nor-BNI pretreatment. AA induces Fos in the spinal cord and numerous brain nucuei, some of which are involved in the control of digestive motility in rats. This effect is mediated through capsaicin-sensitive afferent fibres and prevented by fedotozine most likely through a peripheral action on visceral afferents.

Neurokinin 1 receptor expression by neurons in laminae I, III and IV of the rat spinal dorsal horn that project to the brainstem

Large neurons in laminae III and IV of the spinal cord which express the neurokinin 1 receptor and have dendrites that enter the superficial laminae are a major target for substance P (SP)-containing (nociceptive) primary afferents. Although some of these neurons project to the thalamus, we know little about other possible projection targets. The main aim of this study was to determine whether all cells of this type are projection neurons and to provide information about brainstem sites to which they project. Injections of cholera toxin B subunit were made into four brainstem areas that receive input from the spinal cord, and the proportion of cells of this type in the L4 spinal segment that were retrogradely labelled was determined in each case. The results suggest that most of these cells (>90%) project to the contralateral lateral reticular nucleus (or to a nearby region), while many (>60%) send axons to the lateral parabrachial area and some to the dorsal part of the caudal medulla. However, few of these cells project to the periaqueductal grey matter. As lamina I neurons with the neurokinin 1 receptor appear to be important in the generation of hyperalgesia, we also examined projection neurons in this lamina and found that for each injection site the great majority possessed the receptor. These results demonstrate that dorsal horn neurons which express the neurokinin 1 receptor contribute to several ascending pathways that are thought to be important in pain mechanisms.

Supraspinal metabolic activity changes in the rat during adjuvant monoarthritis

Pain is a multi-dimensional experience including sensory-discriminative and affective-motivational components. The attribution of such components to a corresponding cerebral neuronal substrate in the brain refers to conclusions drawn from electrical brain stimulation, lesion studies, topographic mappings and metabolic imaging. Increases in neuronal metabolic activity in supraspinal brain regions, suggested to be involved in the central processing of pain, have previously been shown in various animal studies. The present investigation is the first to describe supraspinal structures which show increased metabolic activity during ongoing monoarthritic pain at multiple time-points. Experimental chronic monoarthritis of a hindlimb induced by complete Freund's adjuvant is one of the most used models in studies of neuronal plasticity associated with chronic pain. Such animals show typical symptoms of hyperalgesia and allodynia for a prolonged period. Metabolic activity changes in supraspinal brain regions during monoarthritis were assessed using the quantitative [14C]-2deoxyglucose technique at two, four, 14 days of the disease and, furthermore, in a group of 14-day monoarthritic rats which were mechanically stimulated by repeated extensions of the inflamed joint. Local glucose utilization was determined ipsi- and contralateral to the arthritic hindpaw in more than 50 brain regions at various supraspinal levels, and compared with saline-injected controls. At two and 14 days of monoarthritis significant bilateral increases in glucose utilization were seen in many brain structures, including brainstem, thalamic, limbic and cortical regions. Within the brainstem, animals with 14-day monoarthritis showed a higher number of regions with increased metabolic activity compared with two days. No differences between ipsi- and contralateral sides were detected in any of the experimental groups. Average increases ranged from 20 to 40% compared with controls and maximum values were detected in specific brain regions, such as the anterior pretectal nucleus, the anterior cingulate cortex and the nucleus accumbens. Interestingly, at four days of monoarthritis, the glucose utilization values were in the control range in almost all regions studied. Moreover, in monoarthritic rats receiving an additional noxious mechanical stimulation, the rates of glucose utilization were also comparable to controls in all brain areas investigated. Such patterns of brain metabolic activity agreed with concomitant changes in the lumbar spinal cord, described in the accompanying report. The present data show that a large array of supraspinal structures displays elevated metabolic activity during painful monoarthritis, with a non-linear profile for the time-points investigated. This observation most probably reflects mechanisms of transmission and modulation of nociceptive input arising from the monoarthritis and accompanying its development.

Fluorescent double-label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat

Following injections of WGA-HRP into either the spinal cord or periaqueductal gray, labeled neurons were observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double-labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double-labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15-18 mu in their greatest transverse diameter. No double-labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN-spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions.

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.

Spinohypothalamic tract neurons in the cervical enlargement of rats: locations of antidromically identified ascending axons and their collateral branches in the contralateral brain

Antidromic activation was used to determine the locations of ascending spinohypothalamic tract (SHT) axons and their collateral projections within C1, medulla, pons, midbrain, and caudal thalamus. Sixty-four neurons in the cervical enlargement were antidromically activated initially by stimulation within the contralateral hypothalamus. All but one of the examined SHT neurons responded either preferentially or specifically to noxious mechanical stimuli. A total of 239 low-threshold points was classified as originating from 64 ascending (or parent) SHT axons. Within C1, 38 ascending SHT axons were antidromically activated. These were located primarily in the dorsal half of the lateral funiculus. Within the medulla, the 29 examined ascending SHT axons were located ventrolaterally, within or adjacent to the lateral reticular nucleus or nucleus ambiguus. Within the pons, the 25 examined ascending SHT axons were located primarily surrounding the facial nucleus and the superior olivary complex. Within the caudal midbrain, the 23 examined SHT ascending axons coursed dorsally in a position adjacent to the lateral lemniscus. Within the anterior midbrain, SHT axons traveled rostrally near the brachium of the inferior colliculus. Within the posterior thalamus, all 17 examined SHT axons coursed rostrally through the posterior nucleus of thalamus. A total of 114 low-threshold points was classified as collateral branch points. Sixteen collateral branches were seen in C1; these were located primarily int he deep dorsal horn. Forty-five collateral branches were located in the medulla. These were primarily in or near the medullary reticular nucleus, nucleus ambiguus, lateral reticular nucleus, parvocellular reticular nucleus, gigantocellular reticular nucleus, cuneate nucleus, and the nucleus of the solitary tract. Twentysix collateral branches from SHT axons were located in the pons. These were in the pontine reticular nucleus caudalis, gigantocellular reticular nucleus, parvocellular reticular nucleus, and superior olivary complex. Twenty-three collateral branches were located in the midbrain. These were in or near the mesencephalic reticular nucleus, brachium of the inferior colliculus, cuneiform nucleus, superior colliculus, central gray, and substantia nigra. Int he caudal thalamus, two branches were in the posterior thalamic nucleus and two were in the medial geniculate. These results indicate that SHT axons ascend toward the hypothalamus in a clearly circumscribed projection in the lateral brain stem and posterior thalamus. In addition, large numbers of collaterals from SHT axons appears to project to a variety of targets in C1, the medulla, pons, midbrain, and caudal thalamus. Through its widespread collateral projections, the SHT appears to be capable of providing nociceptive input to many areas that are involved in the production of multifaceted responses to noxious stimuli.

The ventrolateral medulla of the rat is connected with the spinal cord dorsal horn by an indirect descending pathway relayed in the A5 noradrenergic cell group

The pathway conveying the descending inhibitory noradrenergic input elicited from the caudal ventrolateral medulla (VLM) onto the spinal cord dorsal horn was studied in the rat. Retrograde labeling with cholera toxin subunit B (CTb) injected into the dorsal horn was combined with immunostaining for dopamine-beta-hydroxylase (DBH) in the VLM and other brainstem nuclei containing noradrenergic cells. CTb-labeled neurons occurred in the lateral part of the VLM (VLMlat), located ventrolaterally to the DBH-immunoreactive cells of the A1 noradrenergic cell group. Neuronal profiles stained for CTb and DBH (double labeled) occurred in the A5 (31%), A6 (57%), and A7 (12%) noradrenergic cell groups. To ascertain whether noradrenergic cells targeting the spinal cord in those groups received projections from the VLMlat, this area was injected with the anterograde tracer biotinylated dextran amine (BDA). Labeled terminal fibers with boutons en passant were apposed to numerous double-stained neurons in the A5 cell group. Similar appositions occurred in small amounts in the ventral subcoerulear component of the A6. Correlated light and electron microscopic analyses of the labeled appositions revealed that the BDA-labeled axonal boutons contained spherical vesicles and were presynaptic at asymmetrical contacts to somata and dendritic profiles of the double-stained A5 neurons. These data indicate the occurrence of an indirect dysynaptic pathway connecting the VLM to the spinal cord, with a relay in the A5 cells. This pathway may convey the antinociceptive effects mediated by alpha 2-adrenoreceptors, which have been previously observed in the spinal cord following VLM stimulation.

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)

Central delay of the laser-activated rat tail-flick reflex

The latency of the heat-activated rat tail-flick (TF) reflex is dependent upon 4 variables, none of which has previously been determined: activation of cutaneous nociceptors (TN); afferent conduction to the dorsal horn (TA); conduction within the central nervous system (CNS) (central delay); and conduction from the ventral horn (VH) to, and activation of, tail muscles (TE). Using a CO2 infrared laser (10 W, 45 msec) to produce synchronous activation of tail-skin nociceptors, TF latency (EMG response) was measured in 10 awake rats. Based on shifts in response latency from points of stimulation near the tip and base of the tail, conduction velocity in the afferent limb of the reflex was estimated to be 0.76 +/- 0.11 m/sec. This indicates that the response is mediated by C fibers. The rats were then anesthetized with pentobarbital and multiple-unit activity and evoked potentials (EPs) were recorded from the superficial dorsal horn at spinal segments S3-CO1 during laser or high-intensity electrical (10 mA, 1 msec) stimulation of the tail. Unit activity and EPs elicited by both stimuli consisted of two distinct components, corresponding to activation of A and C fibers. The difference in latency between laser and electrical evoked activity indicated that 60.00 +/- 7.33 msec was required for activation of nociceptors by the laser. Electrical stimulation of the VH at S3-CO1 in 3 rats produced a TF (EMG) response in 4 msec. Central delay, calculated as total TF time minus (TN+TA+TE), was 82.3 +/- 13.08 msec. This represents the time frame during which modulation of the reflex by an intrinsic, pain-activated, supraspinal system could occur.

Rostral ventrolateral medulla descending neurons excited by nucleus tractus solitarii inputs

Neurons in the rostral ventrolateral medulla (RVLM) were electrophysiologically characterized and anatomically identified using an intracellular recording technique in vivo. Of 49 neurons recorded, 7 were antidromically activated from the dorsolateral funiculus in the thoracic spinal cord, with axonal conduction velocities ranging from 16.6 to 55.0 m/s. The RVLM-spinal neurons were spontaneously active and non-bursting. Additionally, they demonstrated a flat post-R-wave histogram and a flat average of the neuronal membrane potential triggered by the pulsatile arterial pressure. Therefore their activity was not related to cardiac rhythm. Electrical stimulation of the nucleus tractus solitarii (NTS) at the level of the obex evoked monosynaptic excitatory postsynaptic potential (EPSP) on 3 RVLM-spinal neurons; median latency was 1.5 ms. The recorded neurons, intracellularly labeled with horseradish peroxidase (HRP) or biocytin, were located in the rostral pole of the RVLM, between 0.3 and 0.7 mm from the ventral medullary surface and in many cases close to the neurons containing phenylethanolamine-N-methyltransferase (PNMT). These findings are discussed in relation to the physiological role in cardiovascular and nociceptive functional regulation played by the neurons analyzed in this study.

Role of norepinephrine in the interaction between the lateral reticular nucleus and the nucleus raphe magnus: an electrophysiological and behavioral study

We have previously demonstrated that the nucleus raphe magnus (NRM) sends a predominantly inhibitory projection to the lateral reticular nucleus (LRN); however, the pharmacology of this pathway is not known. The purpose of this study was to examine the role of norepinephrine in the NRM-LRN system using both electrophysiological and behavioral techniques. Sixty-nine LRN cells were recorded extracellularly. Cells were tested for their response to noxious and innocuous peripheral stimulation applied to the dorsal body surface. The majority of cells were classified as wide dynamic range, with inhibition being the predominant response; receptive fields were located primarily on the tail and hind limbs. The effect of excitatory amino acid glutamate (GLU) administration into NRM (GLU-NRM) was tested on all 69 cells. GLU-NRM inhibited 55 of 69 LRN cells tested; 7 cells were excited and 7 cells did not respond. Thirty-nine LRN cells were tested for their response to norepinephrine (NE) iontophoretically applied in LRN (NE-LRN). Two distinct types of effects were noted. In 9 cells, both NE-LRN and GLU-NRM produced a strong inhibition, with the magnitude of effect between the 2 drugs significantly correlated. In a second group of cells (n = 12), GLU-NRM produced an inhibitory effect while NE-LRN had no effect on the cells' baseline firing rate. However, when the 2 drugs were applied simultaneously, NE-LRN blocked the inhibitory effects of NRM stimulation. The effect of the alpha 2-receptor antagonist yohimbine (YOH) on NRM-evoked responses was tested in 30 LRN cells. The majority of these cells were inhibited by GLU-NRM. Similar to the dichotomous effect noted by NE-LRN, YOH applied iontophoretically in LRN (YOH-LRN) had two predominant effects on NRM-produced inhibition. In 14 of 27 cells, YOH-LRN significantly potentiated the inhibitory effects of NRM stimulation by increasing the duration of the inhibitory epoch an average of 100 sec. In 7 of 27 cells, YOH directly applied in LRN partially antagonized NRM-evoked inhibition. In a second series of experiments, microinjection cannulas were placed within NRM and LRN in order to determine the effect of blocking alpha 2-receptor activity within LRN on NRM stimulation-produced analgesia in an intact animal. Administration of D,L-homocysteic acid in NRM resulted in a significant increase in baseline tail-flick latency of approximately 140%. Pretreatment with YOH (3 micrograms in 0.5 microliter) in LRN resulted in a significant potentiation of this analgesic effect.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Electrophysiological characterization of the projection from the nucleus raphe magnus to the lateral reticular nucleus: possible role of an excitatory amino acid in synaptic activation

Numerous studies have shown that the lateral reticular nucleus (LRN), located in the caudal ventrolateral medulla, is an important nuclear region in the descending analgesia system. Activation of this brainstem region, either electrically or chemically, results in a reduction in nociceptive threshold. In addition, destruction of LRN abolishes the tonic descending inhibition present on dorsal horn neurons. Recent neuroanatomical tracing studies have shown that the nucleus raphe magnus (NRM), long implicated in nociception, sends direct projections to LRN; however, no information exists regarding the physiological characteristics of this pathway, nor its role in the endogenous descending analgesia system. The purpose of this study was to physiologically characterize the synaptic influence(s) of projections from the NRM to the LRN using electrophysiological recording, electrical and chemical stimulation, and iontophoretic techniques. Sixty-one percent of LRN neurons responded to single pulse stimulation of NRM; 52% of the responsive cells were excited and 48% were inhibited. The mean latency to onset of excitation was 4.9 +/- 1.2 ms. High frequency (100 Hz) electrical stimulation of NRM influenced 69/102 neurons; 52% (36/69) were excited, while 48% (33/69) were inhibited. Microinjection of glutamate into NRM significantly modified the discharge of 83% (93/112) of LRN cells tested; of these, 71% were inhibited, while 29% were excited. In 35 cells the effects of the excitatory amino acid antagonist kynurenic acid (KYN) were studied. In 75% of the cells excited by glutamate administration into the NRM (18/24), KYN partially antagonized this response. In 11 LRN cells inhibited by NRM chemical stimulation, KYN had no effect on this inhibition. Overall, 95% of the LRN cells responsive to NRM stimulation were also responsive to noxious peripheral stimulation, indicating that these cells are receiving ascending information from the spinal cord regarding somatosensory stimulation as well as receiving descending input from the NRM. It is concluded that LRN neurons are highly responsive to both noxious peripheral stimulation and NRM efferent activation, and that this region plays a significant role as an integrator for both ascending and descending information.

Effect of systemic morphine on neurons in the lateral reticular nucleus area of the rat

The present study is undertaken to investigate the effect of systemic morphine on neurons in the lateral reticular nucleus (LRN) using extracellular recording techniques. The spontaneous activities of 64 neurons in the LRN area were tested with morphine (3-5 mg/kg, IV). Morphine excited 23 and inhibited 28 neurons tested, and 13 neurons were not affected. Of the 28 neurons inhibited, 20 were identified as nociceptive and the remaining 6 were nonnociceptive. Of the 23 neurons excited by morphine, 18 were nociceptive and 5 were nonociceptive. Systemic naloxone (0.3-0.5 mg/kg) significantly reversed the morphine effect in 15 out of 19 neurons excited and 19 out of 20 neurons inhibited by morphine. Thirteen out of 64 neurons were further identified as reticulospinal neurons, of which four were excited and four were inhibited by morphine. The remaining five were not affected. The results demonstrate that a similar proportion of neurons in the LRN area were either excited or inhibited by systemic morphine, and the majority of them are nociceptive neurons. It is suggested that different types of neurons in the LRN area may have different functions in morphine analgesia.

Involvement of the subnucleus reticularis dorsalis in diffuse noxious inhibitory controls in the rat

Several lines of evidence have suggested the participation of the caudal medulla, including the subnucleus reticularis dorsalis (SRD), in the supraspinally mediated diffuse noxious inhibitory controls (DNICs). To test this hypothesis directly, DNICs acting on spinal convergent neurones were compared in sham-operated rats and rats with unilateral quinolinic acid-induced lesions of the SRD. Inhibitions produced by heterotopic noxious stimuli (i.e. DNIC), of the C-fibre-evoked responses of the convergent neurones were significantly reduced in the lesioned animals. This depression of DNIC, by 40-45%, was similar no matter where the conditioning stimuli were applied and was not significantly different for neurones recorded ipsilaterally to the lesion from those recorded contralaterally. These results suggest that the SRD is one of the supraspinal relay(s) in the circuitry underlying DNIC in the rat.

Selective blockade by yohimbine of descending spinal inhibition from lateral reticular nucleus but not from locus coeruleus in rats

The present study was undertaken to compare the effects of the alpha 2-adrenoceptor antagonist yohimbine on inhibition of C-fiber-evoked responses of dorsal horn neurons produced by electrical stimulation of the lateral reticular nucleus (LRN) and the Locus coeruleus (LC) in the rat. In the majority of neurons, C-fiber-evoked responses were significantly inhibited by 53.84 +/- 5.02% and 57.63 +/- 5.79% of control by LRN and LC stimulation, respectively, whereas in less than half of the neurons, A-fiber-evoked responses were reduced by 20.99 +/- 6.06% and 21.78 +/- 4.48% of control, respectively. After systemic or spinal administration of yohimbine, LC-induced inhibition of C-fiber-evoked responses was not affected. In contrast, LRN-induced inhibition was markedly attenuated by yohimbine. The results suggest that alpha 2-adrenoceptors may be involved in mediation of inhibition of spinal nociception induced by stimulation of LRN but not by LC.

A re-examination of the spino-reticulo-diencephalic pathway in the cat

One commonly accepted idea is that affective aspects of pain sensation are derived from a flow of information from the spinal cord through the reticular formation to the intralaminar thalamus and subthalamus. Little is known, however, about the extent to which spinoreticular terminations and reticulodiencephalic neuronal cell bodies overlap. This study used a combination of anterograde and retrograde tracing techniques to compare these distributions in the cat. Whereas spinoreticular terminations were concentrated caudally and laterally, neurons projecting to intralaminar thalamus and subthalamus were concentrated rostrally and medially. Thus, information conveyed from the spinal cord to the reticular formation appears to have direct access to intralaminar thalamus and subthalamus only by way of a few widely scattered neurons. When considered with the results of others, these results encourage less emphasis on a putative spino-reticulo-diencephalic pathway for pain. Rather, the reticular formation's role in pain is more likely to involve its full complement of interconnected descending and ascending connections.

Vagal afferent modulation of nociception

Chemical, electrical or physiological activation of cardiopulmonary vagal (cervical, thoracic or cardiac), diaphragmatic vagal (DVAG) or subdiaphragmatic vagal (SDVAG) afferents can result in either facilitation or inhibition of nociception in some species. In the rat, these effects depend upon vagal afferent input to the NTS and subsequent CNS relays, primarily in the NRM and ventral LC/SC, although specific relay nuclei vary as a function of the vagal challenge stimulus. Spinal pathways and neurotransmitters have been identified for vagally mediated effects on nociception and consistently implicate the involvement of descending 5-HT and noradrenergic systems, as well as intrinsic spinal opioid receptors. Species differences may exist with respect to both the effects of DVAG and SDVAG afferents on nociception and the efficacy of vagal afferents to modulate nociception. However, it is also possible that such differences reflect the modality of noxious input (e.g., visceral versus cutaneous), the type of neuronal activity investigated (e.g., resting versus noxious-evoked), spinal location of recording (e.g., thoracic versus lumbosacral) and/or parameters of stimulation. It is also possible that activation of some vagal afferents is aversive, but whether this contributes to changes in nociception produced by vagal activation has not clearly been established. Finally, the vagal-nociceptive networks described in this review provide a fertile area for future study. These networks can provide an understanding of physiological and pathophysiological peripheral events that affect nociception.

The antinociceptive activity of excitatory amino acids in the rat brainstem: an anatomical and pharmacological analysis

Rats were stereotaxically implanted with microinjection cannulae aimed at sites ranging caudally from the lower medulla and rostrally to the diencephalon and received microinjections of the excitatory amino acid: L-glutamate 30 nmol/0.5 microliters. The subsequent spontaneous behavioral response and the effect on the thermal noxious-evoked tail flick (TF) and hot plate (HP) responses was recorded. From 331 brain sites mapped with glutamate, an elevation of tail flick and hot plate response latencies was observed in 59 cases and in 34 of these sites the antinociceptive activity was preceded by a shortlasting aversion characterized by vocalization and running. The glutamate-sensitive sites at which TF and HP response latencies were elevated were exclusively distributed in the medullary reticular formation (MRF) and the mesencephalic periaqueductal gray matter (PAG). The aversive and antinociceptive activity of glutamate was dose-dependent and mimicked by the excitatory amino acid (EAA) receptor agonists N-methyl-D-aspartate + (NMDA) kainate and less so quisqualate. The EAA receptor antagonists MK-801 and AP-5, but not glutamyl-amino-methyl-sulfonic acid, antagonized in a dose-dependent fashion both the aversive and antinociceptive responses evoked from the PAG. It is suggested that NMDA receptor-linked neurons in the PAG activate both nociceptive and antinociceptive systems.

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.

c-FOS-like immunoreactivity in rat brainstem neurons following noxious chemical stimulation of the nasal mucosa

It has previously been shown that noxious and non-noxious peripheral stimuli induce c-fos expression in spinal dorsal horn neurons. In the present study we have examined the expression of c-fos in brainstem neurons following noxious chemical stimulation of the respiratory region of the nasal mucosa. In urethane-anaesthetized rats we injected mustard oil or applied CO2 pulses to the right nasal cavity. In control animals we applied paraffin oil or a continuous flow of air. A further group of control animals was anaesthetized and not subjected to any experimental treatment. Two hours after the first stimulus the rats were perfused with 4% phosphate-buffered paraformaldehyde. Brainstem sections were incubated with primary antiserum against the FOS protein and processed according to the ABC method. Only the mustard oil-treated rats had obvious signs of rhinitis and displayed FOS-positive cells in laminae I and II of the subnucleus caudalis and in the subnucleus interpolaris of the trigeminal brainstem nuclear complex as well as in the medullary lateral reticular nucleus. These areas are known to be involved in the processing of nociceptive information. Although CO2 pulses applied to the nasal mucosa are known to evoke pain sensations in man we did not observe any FOS-positive neurons in trigeminal and reticular brainstem areas of CO2-treated rats. This lack of c-fos expression probably results from the fact that unlike mustard oil, CO2 did not induce any apparent inflammatory reactions. In all animals c-fos expression was found in the nucleus of the solitary tract and in the area postrema. Staining in these areas might partly result from factors related to anaesthesia, changed respiration parameters and stress. Since the mustard oil-treated rats displayed the highest levels of immunoreactivity in the nucleus of the solitary tract and in the area postrema, additional effects specifically related to nociceptive input are very likely.

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.

Involvement of solitary tract nucleus in control of nociceptive transmission in cat spinal cord neurons

In cats anesthetized with Nembutal and immobilized with Flaxedil, extracellular recordings were made from dorsal horn neurons and lamina X neurons in the lumbar spinal cord. The nociceptive responses of these neurons elicited by peripheral nerve stimulation were significantly inhibited by stimulation of the nucleus tractus solitarius (NTS) at low intensity without any noticeable cardiovascular reaction. As usual, the late response or C-response was found to be preferentially inhibited by NTS stimulation as compared with the early response or A-response. The effective current intensity for NTS stimulation-produced inhibition ranged from 80 microA to 200 microA. Stronger inhibition was induced when the stimulating site was within or in the immediate vicinity of the NTS. There was no significant difference in the efficacy of the NTS stimulation-produced inhibition of nociceptive response between dorsal horn neurons and lamina X neurons. A similar inhibitory effect was elicited by microinjection of monosodium glutamate into the NTS area. The results demonstrate that the NTS may be involved in the control of nociceptive transmission at the spinal cord level.

Responses of reticulospinal neurons in the lateral reticular nucleus area of the rat to cutaneous noxious and non-noxious stimulation

Response properties of reticulospinal neurons in the lateral reticular nucleus (LRN) area to natural cutaneous stimulation were investigated systematically in 45 urethane-anesthetized rats by using extracellular recording techniques. A total of 64 neurons were tested with peripheral stimuli, of which 19 were responsive only to noxious stimuli; 7 responsive to both noxious and non-noxious stimuli; 4 responsive only to non-noxious stimuli; and 34 not responsive to any cutaneous stimuli. Both the noxious and non-noxious receptive fields were large and bilateral. Among the neurons responding to noxious stimuli, the majority (72%) was excited. This study provides evidence that some reticulospinal neurons in the rat LRN area are involved in the mechanisms of nociception.

Response properties and descending control of rat dorsal horn neurons with deep receptive fields

The study was designed to obtain information on the spinal processing of input from receptors in deep somatic tissues (muscle, tendon, joint). In anaesthetized rats, the impulse activity of single dorsal horn cells was recorded extracellularly. In a pilot series, the proportion of neurons responding to mechanical stimulation of deep tissues was determined: 46.7% had receptive fields in the skin only, 35.5% could only be driven from deep tissues (deep cells), and 17.7% possessed a convergent input from both skin and deep tissues (cutaneous-deep cells). In each category, neurons with low and high mechanical thresholds were encountered. Experiments employing a reversible cold block of the spinal cord showed that deep cells with high threshold were subject to a stronger descending inhibition than low-threshold deep cells. In cutaneous-deep neurons the input combination high-threshold cutaneous and high-threshold deep was the most frequent one (48.7% of the cutaneous-deep cells). In these presumably nociceptive cells the descending inhibition had a differential action in that the input from deep tissues was more strongly affected than was the cutaneous input to the same neuron. The recording sites of the neurons with deep input were located in the superficial dorsal horn and in and around lamina V. The results suggest that in the rat a considerable proportion of dorsal horn cells receives input from deep nociceptors and that this input is controlled by descending pathways in a rather selective way.

Medullary neurons with projections to lamina X of the rat as demonstrated by retrograde labeling after HRP microelectrophoresis

Brainstem neurons were retrogradely labeled with microelectrophoresis of HRP or WGA-HRP into lamina X of the cervical or lumbar cord of rats. The results reveal that lamina X of the lumbar cord receives bulbar projections originating mainly within the nucleus raphe magnus and the nucleus reticularis paragigantocellularis (including the medial or alpha-ventral part and lateral part) and that lamina X of the cervical cord receives projections from similar but more extensive regions in the lower brainstem. These findings provide a neuroanatomical substrate for medullary descending modulation of nociceptive transmission in lamina X.

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.

Descending influences on the cutaneous receptive fields of postsynaptic dorsal column neurones in the cat

1. The influence of activity in descending systems on the cutaneous receptive field properties of postsynaptic dorsal column (PSDC) neurones has been investigated in chloralose-anaesthetized cats. The main aim of the study was to determine whether the receptive field boundaries of PSDC neurones are under the control of systems descending from the brain. 2. Single-unit recordings were made from the ascending axons of PSDC units in the dorsal columns. Receptive fields were analysed using light tactile and noxious mechanical and thermal stimuli, both before and during a reversible block of spinal conduction produced by cooling the cord rostral of the recording site. 3. The light tactile excitatory fields of PSDC neurones were largely unaffected by the cold-block procedure. 4. In contrast, both the sensitivity of PSDC neurones to noxious stimuli and the area of skin from which they could be effectively excited by such stimuli were found to be profoundly modified by interruption of descending activity. Two-thirds of the units excited by noxious pinch responded more vigorously in the cold-blocked state and one-half from an expanded area of skin. Responses to noxious radiant heat were similarly modified. 5. Inhibition evoked in PSDC neurones, whether by light tactile or noxious stimuli, involved predominantly segmental mechanisms since it remained effective in the cold-blocked state. 6. It is concluded that neurones of the PSDC system are amongst those dorsal horn neurones with receptive field geometries which may be modified by activity in descending systems.

Cortical influences on neurons of the lateral reticular nucleus responding to noxious stimuli

The effect of frontoparietal sensorimotor (FPSM) cortex stimulation on both the spontaneous and the noxious evoked activity of neurons in the lateral reticular nucleus (LRN) was tested in barbiturate-anesthetized rats. Ninety-three LRN neurons that responded to a noxious heat stimulus (HS) were recorded (72% antidromically fired from the cerebellum). Of these, 66 neurons altered their spontaneous firing rates in response to cortical stimulation. Two patterns of responses were found: either an excitation followed by a suppression of spontaneous activity (52 neurons), or a pure suppression of spontaneous activity lasting 50-400 msec (14 neurons). In 46 of these neurons, it was found that cortical stimulation reduced HS-evoked activity to near the baseline level. Furthermore, it was found that when applied after a prolonged cortical stimulation, the HS was ineffective. It is concluded that FPSM cortex can influence nociceptive information in LRN neurons that respond to its stimulation, possibly interfering with the mechanisms underlying stimulation-produced analgesia (SPA). In this context, it is proposed that the cortex can modulate the activity of LRN neurons that activate, through local loops, a descending antinociceptive system and also a separate projection system to the cerebellum.

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.

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.

Descending phenylethanolamine-N-methyltransferase projections to the monkey spinal cord: an immunohistochemical double labeling study

In the present study, we determined that a population of spinally projecting neurons in the monkey brainstem also contained the enzyme phenylethanolamine-N-methyltransferase (PNMT). Following bilateral placements of horseradish peroxidase (HRP) in the cervical spinal cord, brainstem sections containing retrogradely labeled cells were immunohistochemically stained for PNMT. Single labeled PNMT-positive cells were found in a distinctive pattern in the dorsomedial and ventrolateral medulla. A population of double labeled cells was observed in the latter group only. This population was dispersed among other single labeled HRP and single labeled PNMT neurons. Possible functional roles of descending PNMT cells include involvement in sympathetic control of cardiovascular mechanisms and/or tonic descending inhibition of dorsal horn neurons.

Dissociation of antinociceptive from cardiovascular effects of stimulation in the lateral reticular nucleus in the rat

The lateral reticular nucleus (LRN) in the caudal ventrolateral medulla has been implicated in the regulation of spinal nociceptive transmission and hemodynamics. Experiments were undertaken to examine the relationship between inhibition of the tail flick reflex and cardiovascular effects produced by electrical stimulation in the LRN in rats lightly anesthetized with pentobarbital. Intensity- and frequency-dependent increases in mean arterial pressure and vascular resistance in the hindquarter, mesenteric, renal and caudal arterial beds were observed. Inhibition of the tail flick reflex, however, occurred at intensities of electrical stimulation which produced no significant changes in mean arterial pressure or vascular resistance in any of the arterial beds studied. Selective stimulation of cell bodies in the LRN by microinjection of glutamate similarly inhibited the tail flick reflex but produced significant reductions in mean arterial pressure, without substantially affecting regional vascular resistances. These results suggest that the antinociceptive and depressor effects of stimulation in the LRN are mediated by activation of cell bodies, while pressor effects produced by focal electrical stimulation are mediated by activation of fibers of passage. The descending inhibition produced by stimulation in the LRN is independent of stimulation-produced cardiovascular responses.

Effect of spinal norepinephrine depletion on descending inhibition of the tail flick reflex from the locus coeruleus and lateral reticular nucleus in the rat

The lateral reticular nucleus (LRN) and locus coeruleus-subcoeruleus (LC/SC), brainstem structures which overlap the A1 and A6 noradrenergic nuclei respectively, have been implicated in descending modulation of spinal nociceptive transmission. The present studies were designed to examine the role of norepinephrine (NE) in the mediation of inhibition of the nociceptive tail flick reflex produced by focal electrical stimulation in the LRN and LC/SC. Spinal NE was depleted by intrathecal administration of 6-hydroxydopamine (6-OHDA; 20 micrograms) and the threshold electrical stimulation in the LRN and the LC/SC necessary to inhibit the tail flick reflex in lightly pentobarbital-anesthetized rats was determined 9 and 14 days later. Despite a significant depletion (greater than 85%) of lumbar spinal cord NE content, there was no significant change in the tail flick inhibitory stimulation thresholds in the LRN or LC/SC. NE depletion did, however, potentiate the elevation in the inhibitory stimulation threshold in the LRN produced by intrathecal administration of the alpha 2-adrenoceptor antagonist, yohimbine, suggesting that upregulation of spinal adrenoceptors had occurred following 6-OHDA treatment. Adrenoceptor up-regulation was examined quantitatively by characterizing the dose-dependent antinociceptive potency of the selective alpha 2-adrenoceptor agonist clonidine 3, 7, 10, and 14 days following 6-OHDA administration, and analysis of [3H]rauwolscine binding to lumbar spinal cord 9 days following administration of the neurotoxin. The development of supersensitivity, defined as the leftward parallel shift of the dose-response curves for clonidine administered intrathecally, corresponded to the time course of NE depletion following 6-OHDA treatment on the days tested. Binding of [3H]rauwolscine to lumbar spinal cord revealed an elevation in the estimated Bmax without a change in the estimated Kd of the high affinity binding component 9 days following 6-OHDA administration. This study demonstrates that spinal adrenoceptor denervation supersensitivity develops rapidly following intrathecal administration of 6-OHDA and compensates for the selective destruction of spinal noradrenergic nerve terminals. Thus, the absence of effect of NE depletion on the tail flick inhibitory stimulation threshold in the LRN and the LC/SC does not argue against the hypothesis that spinopetal NE-containing neurons in these brainstem loci are involved in modulation of spinal nociceptive transmission.