Effects of systemic morphine on diffuse noxious inhibitory controls: role of the periaqueductal grey

Exploring the Relationship Between Endogenous Pain Modulation, Pain Intensity, and Depression in Patients Using Opioids for Chronic Low Back Pain

INTRODUCTION

Endogenous pain modulatory processes appear to play an important role in shaping pain-related outcomes, but we know relatively little about the influence of psychosocial factors on those pain modulatory processes. The primary objective of this study was to explore associations between endogenous pain modulation (ie, conditioned pain modulation, CPM; temporal summation, TS), chronic pain, and negative affective factors (ie, depression, anxiety symptoms) in a sample of participants with chronic low back pain (CLBP) treated with long-term daily opioids.

METHODS

Adults with opioid-treated CLBP (N=107) completed questionnaires assessing pain, pain symptoms, and psychological measures. CPM and TS were evaluated as predictors of pain intensity ratings (Brief Pain Inventory), with depression scores (Hospital Anxiety and Depression Scale, depression subscale) examined as potential moderators of those associations.

RESULTS

Moderation analyses demonstrated associations between CPM and back pain intensity ratings, moderated by depression symptom scores (B=-0.002, SE=0.0008, P<0.01) when controlling for daily opioid dose, with participants with higher depression scores showing a relatively stronger link between lower CPM and increased pain intensity ratings. Significant associations were observed between depression, pain intensity, and CPM-derived outcomes.

CONCLUSION

Our findings suggest that reduced pain-inhibitory capacity is associated with elevated self-reported pain intensity in adults with opioid-treated CLBP, particularly among those with higher severity of depression symptoms.

Developments in Understanding Diffuse Noxious Inhibitory Controls: Pharmacological Evidence from Pre-Clinical Research

Bulbospinal pathways regulate nociceptive processing, and inhibitory modulation of nociception can be achieved via the activity of diffuse noxious inhibitory controls (DNIC), a unique descending pathway activated upon application of a conditioning stimulus (CS). Numerous studies have investigated the effects of varied pharmacological systems on the expression status of a) DNIC (as measured in anaesthetised animals) and b) the descending control of nociception (DCN), a surrogate measure of DNIC-like effects in conscious animals. However, the complexity of the underlying circuitry that governs initiation of a top-down inhibitory response in reaction to a CS, coupled with the methodological limitations associated with using pharmacological tools for its study, has often obscured the exact role(s) of a given drug. In this literature review, we discuss the pharmacological manipulation interrogation strategies that have hitherto been used to examine the functionality of DNIC and DCN. Discreet administration of a substance in the spinal cord or brain is considered in the context of action on one of four hypothetical systems that underlie the functionality of DNIC/DCN, where interpreting the outcome is often complicated by overlapping qualities. Systemic pharmacological modulation of DNIC/DCN is also discussed despite the fact that the precise location of drug action(s) cannot be pinpointed. Chiefly, modulation of the noradrenergic, serotonergic and opioidergic transmission systems impacts DNIC/DCN in a manner that relates to drug class, route of administration and health/disease state implicated. The advent of increasingly sophisticated interrogation tools will expedite our full understanding of the circuitries that modulate naturally occurring pain-inhibiting pathways.

Effects of Transcranial Direct Current Stimulation Block Remifentanil-Induced Hyperalgesia: A Randomized, Double-Blind Clinical Trial

Background: Remifentanil-induced hyperalgesia (r-IH) involves an imbalance in the inhibitory and excitatory systems. As the transcranial Direct Current Stimulation (tDCS) modulates the thalamocortical synapses in a top-down manner, we hypothesized that the active (a)-t-DCS would be more effective than sham(s)-tDCS to prevent r-IH. We used an experimental paradigm to induce temporal summation of pain utilizing a repetitive cold test (rCOLDT) assessed by the Numerical Pain Score (NPS 0-10) and we evaluated the function of the descending pain modulatory system (DPMS) by the change on the NPS (0–10) during the conditioned pain modulation (CPM)-task (primary outcomes). We tested whether a-tDCS would be more effective than s-tDCS to improve pain perception assessed by the heat pain threshold (HPT) and the reaction time during the ice-water pain test (IPT) (secondary outcomes).

Methods: This double-blinded, factorial randomized trial included 48 healthy males, ages ranging 19–40 years. They were randomized into four equal groups: a-tDCS/saline, s-tDCS/saline, a-tDCS/remifentanil and s-tDCS/remifentanil. tDCS was applied over the primary motor cortex, during 20 min at 2 mA, which was introduced 10 min after starting remifentanil infusion at 0.06 μg⋅kg^-1⋅min^-1 or saline.

Results: An ANCOVA mixed model revealed that during the rCOLDT, there was a significant main effect on the NPS scores (F = 3.81; P = 0.01). The s-tDCS/remifentanil group presented larger pain scores during rCOLDT, [mean (SD) 5.49 (1.04)] and a-tDCS/remifentanil group had relative lower pain scores [4.15 (1.62)]; showing its blocking effect on r-IH. a-tDCS/saline and s-tDCS/saline groups showed lowest pain scores during rCOLDT, [3.11 (1.2)] and [3.15 (1.62)], respectively. The effect of sedation induced by remifentanil during the rCOLDT was not significant (F = 0.76; P = 0.38). Remifentanil groups showed positive scores in the NPS (0–10) during the CPM-task, that is, it produced a disengagement of the DPMS. Also, s-tDCS/Remifentanil compared to a-tDCS showed lower HPT and larger reaction-time during the IPT.

Conclusion: These findings suggest that effects of a-tDCS prevent the summation response induced by r-IH during rCOLDT and the a-tDCS blocked the disengagement of DPMS. Thereby, tDCS could be considered as a new approach to contra-regulate paradoxical mechanisms involved in the r-IH. Clinical trials identification: NCT02432677. URL:https://clinicaltrials.gov/.

Influence of Morphine and Naloxone on Pain Modulation in Rheumatoid Arthritis, Chronic Fatigue Syndrome/Fibromyalgia, and Controls: A Double-Blind, Randomized, Placebo-Controlled, Cross-Over Study

BACKGROUND

Impaired pain inhibitory and enhanced pain facilitatory mechanisms are repeatedly reported in patients with central sensitization pain. However, the exact effects of frequently prescribed opioids on central pain modulation are still unknown.

METHODS

A randomized, double-blind, placebo-controlled cross-over trial was carried out. Ten chronic fatigue syndrome (CFS)/fibromyalgia (FM) patients, 11 rheumatoid arthritis (RA) patients and 20 controls were randomly allocated to the experimental (10 mg morphine or 0.2 mg/mL Naloxone) and placebo (2 mL Aqua) group. Pressure pain thresholds (PPTs) and temporal summation at the Trapezius and Quadriceps were assessed by algometry. Conditioned pain modulation (CPM) efficacy and deep tissue pain pressure were assessed by adding ischemic occlusion at the opposite upper arm.

RESULTS

Deep tissue pain pressure was lower and temporal summation higher in CFS/FM (P = 0.002 respectively P = 0.010) and RA patients (P = 0.011 respectively P = 0.047) compared to controls at baseline. Morphine had only a positive effect on PPTs in both patient groups (P time = 0.034). Accordingly, PPTs increased after placebo (P time = 0.015), and no effects on the other pain parameters were objectified. There were no significant effects of naloxone nor nocebo on PPT, deep tissue pain, temporal summation or CPM in the control group.

CONCLUSIONS

This study revealed anti-hyperalgesia effects of morphine in CFS/FM and RA patients. Nevertheless, these effects were comparable to placebo. Besides, neither morphine nor naloxone influenced deep tissue pain, temporal summation or CPM. Therefore, these results suggest that the opioid system is not dominant in (enhanced) bottom-up sensitization (temporal summation) or (impaired) endogenous pain inhibition (CPM) in patients with CFS/FM or RA.

Bladder filling attenuates spinal cord nociceptive reflexes in humans

OBJECTIVE

To examine the viscerosomatic interaction between bladder afferents and somatic nociception we evaluated the effect of bladder filling on the nociceptive withdrawal reflex (NWR) in 21 healthy subjects.

METHODS

NWR was evoked in the lower and upper limbs by stimulating the sural and index finger digital nerves, respectively, while simultaneously recording EMG activity in the biceps femoris and biceps brachialis. NWR pain-related perception was quantified on a 10-point pain scale. Bladder filling was evaluated with suprapubic bladder sonography. Subjects were examined during empty bladder, medium and high level of bladder filling sessions.

RESULTS

NWR magnitude in both upper and lower limbs and perceived pain for the upper limb were significantly decreased at higher levels of bladder filling compared to empty bladder sessions.

CONCLUSIONS

Reduced NWR magnitude in both upper and lower limbs during bladder filling strongly indicates that bladder control and nociception share common modulatory descending pathways. Bladder afferents may activate these pathways to suppress the micturition reflex, but they may also inhibit spinal reflexes to maintain continence during pain stimuli.

SIGNIFICANCE

The effect of bladder filling on the NWR may represent a useful tool to investigate interactions between the neural pathways controlling the bladder and pain.

Sex differences in the stability of conditioned pain modulation (CPM) among patients with chronic pain

OBJECTIVES

To examine the temporal stability of conditioned pain modulation (CPM), formerly termed diffuse noxious inhibitory controls, among a sample of patients with chronic pain. The study also examined the factors that might be responsible for the stability of CPM.

DESIGN, SUBJECTS, AND METHODS

In this test-retest study, patients underwent a series of standardized psychophysical pain-testing procedures designed to assess CPM on two separate occasions (i.e., baseline and follow up). Patients also completed self-report measures of catastrophizing (Pain Catastrophizing Scale [PCS] and negative affect [NA]).

RESULTS

Overall, results provided evidence for the stability of CPM among patients with chronic pain. Results, however, revealed considerable sex differences in the stability of CPM. For women, results revealed a significant test-retest correlation between baseline and follow-up CPM scores. For men, however, the test-retest correlation between baseline and follow-up CPM scores was not significant. Results of a Fisher's Z-test revealed that the stability of CPM was significantly greater for women than for men. Follow-up analyses revealed that the difference between men and women in the stability of CPM could not be accounted for by any demographic (e.g., age) and/or psychological factors (PCS and NA).

CONCLUSIONS

Our findings suggest that CPM paradigms possess sufficient reliability to be incorporated into bedside clinical evaluation of patients with chronic pain, but only among women. The lack of CPM reproducibility/stability observed among men places limits on the potential use of CPM paradigms in clinical settings for the assessment of men's endogenous pain-inhibitory function.

Dysfunctional pain inhibition in patients with chronic whiplash-associated disorders: an experimental study

Inefficient endogenous pain inhibition, in particular impaired conditioned pain modulation (CPM), may disturb central pain processing in patients with chronic whiplash-associated disorders (WAD). Previous studies revealed that abnormal central pain processing is responsible for a wide range of symptoms in patients with chronic WAD. Hence, the present study aimed at examining the functioning of descending pain inhibitory pathways, and in particular CPM, in patients with chronic WAD. Thirty-five patients with chronic WAD and 31 healthy controls were subjected to an experiment evaluating CPM. CPM was induced by an inflated occlusion cuff and evaluated by comparing temporal summation (TS) of pressure pain prior to and during cuff inflation. Temporal summation was provoked by means of 10 consecutive pressure pulses at upper and lower limb location. Pain intensity of first, fifth, and 10th pressure pulse was rated. During heterotopic noxious conditioning stimulation, TS of pressure pain was significantly depleted among healthy controls. In contrast, TS was quite similar prior to and during cuff inflation in chronic WAD, providing evidence for dysfunctional CPM in patients with chronic WAD. The present study demonstrates a lack of endogenous pain inhibitory pathways, and in particularly CPM, in patients with chronic WAD, and hence provides additional evidence for the presence of central sensitization in chronic WAD.

Effects of dexmedetomidine on conditioned pain modulation in humans

BACKGROUND

Systemic administration of dexmedetomidine (DEX; selective α(2) -adrenoceptor agonist) is found to inhibit diffuse noxious inhibitory control in rats, now referred to as conditioned pain modulation (CPM) in humans. The present study was designed to investigate the effect of intravenous administration of DEX on CPM in humans.

METHODS

There were two sequential sessions in this double blind, randomized study. The first session was the control with normal saline infusion (N(1st), L(1st), H(1st)). During the second session, three types of agents were infused: normal saline (N(2nd)); a low plasma concentration of DEX (0.04 ng/mL; L(2nd)); and a high plasma concentration of DEX (0.08 ng/mL; H(2nd)). The amplitude of somatosensory evoked potentials (ampSEP)s and the visual analogue scale of tooth pain (VASt) induced by electrical tooth stimulation were evaluated with and without conditioning CO(2) laser stimulation of the hand. The inhibition rate (% inhibition) was calculated [= (1-[ampSEP or VASt with conditioning stimuli]/[ampSEP or VASt without conditioning stimuli]) × 100] to compare the magnitude of the DEX effects on CPM.

RESULTS

The inhibition rates of ampSEPs and VASt in Types N, L and H varied significantly, demonstrating a dose-dependent reduction of CPM effects of ampSEP and VASt during randomized DEX administration, consistent with results from animal studies.

CONCLUSIONS

The present study shows that systemic administration of an α(2) -adrenoceptor agonist (DEX), less than the clinical dose, inhibited CPM in humans. These results may provide some mechanistic insight into why many chronic pain patients show impaired CPM.

Blockade of opioid receptors in the medullary reticularis nucleus dorsalis, but not the rostral ventromedial medulla, prevents analgesia produced by diffuse noxious inhibitory control in rats with muscle inflammation

Diffuse Noxious Inhibitory Controls (DNIC) involves application of a noxious stimulus outside the testing site to produce analgesia. In human subjects with a variety of chronic pain conditions, DNIC is less effective; however, in animal studies, DNIC is more effective after tissue injury. While opioids are involved in DNIC analgesia, the pathways involved in this opioid-induced analgesia are not clear. The aim of the present study was to test the effectiveness of DNIC in inflammatory muscle pain, and to study which brainstem sites mediate DNIC- analgesia. Rats were injected with 3% carrageenan into their gastrocnemius muscle and responses to cutaneous and muscle stimuli were assessed before and after inflammation, and before and after DNIC induced by noxious heat applied to the tail (45 °C and 47 °C). Naloxone was administered systemically, into rostral ventromedial medulla (RVM), or bilaterally into the medullary reticularis nucleus dorsalis (MdD) prior to the DNIC-conditioning stimuli. DNIC produced a similar analgesic effect in both acute and the chronic phases of inflammation reducing both cutaneous and muscle sensitivity in a dose-dependent manner. Naloxone systemically or microinjected into the MdD prevented DNIC-analgesia, while naloxone into the RVM had no effect on DNIC analgesia. Thus, DNIC analgesia involves activation of opioid receptors in the MdD.

PERSPECTIVE

The current study shows that DNIC activates opioid receptors in the MdD, but not the RVM, to produce analgesia. These data are important for understanding clinical studies on DNIC as well as for potential treatment of chronic pain patients.

Treating pain with pain: supraspinal mechanisms of endogenous analgesia elicited by heterotopic noxious conditioning stimulation

While being exposed to an intensive tonic pain stimulus at one area of the body, another phasic pain stimulus applied to a remote site is perceived as less painful. The neurophysiological basis for this "pain inhibits pain" phenomenon has been presumed to be an activation of the spino-bulbo-spinal mechanism termed "diffuse noxious inhibitory controls." However, several additional mechanisms such as an activation of the descending pain control system may contribute to this observation. Here we investigated the underlying supraspinal mechanisms of "heterotopic noxious conditioning stimulations" (HNCS), representing this specific experimental constellation. We used functional magnetic resonance imaging and behavioral recordings in combination with a modified cold-pressor task and phasic painful stimuli, and investigated the contribution of endogenous opioids to this mechanism using the opioid antagonist naloxone in a double-blind crossover design. HNCS led to marked endogenous analgesia and this effect correlated positively with the perceived intensity of the tonic painful stimulus. Furthermore, HNCS was paralleled by reduced blood oxygen level dependent (BOLD) responses in classical pain-responsive regions. Conversely, HNCS led to tonic BOLD increases in subregions of the anterior cingulate cortex. The strength of functional coupling between the subgenual anterior cingulate cortex and key structures of the descending pain control system was enhanced during HNCS, which correlated positively with the individual endogenous analgesia during HNCS. These effects were in part reversed by naloxone, speaking for the contribution of endogenous opioid neurotransmission to this mechanism. Taken together, these results demonstrate a substantial contribution of higher-order brain regions to the phenomenon of hypoalgesia during HNCS. Functional magnetic resonance imaging shows how the human brain is involved in heterotopic noxious conditioning and reveals active supraspinal pain modulatory mechanisms during dual pain stimulation.

Cerebral and cerebrospinal processes underlying counterirritation analgesia

Pain is a complex experience involving extensive interactions between brain and spinal cord processes. Various interventions that modulate pain, such as the application of a competing noxious stimulus (counterirritation), are thought to involve cerebrospinal regulation through diffuse noxious inhibitory controls (DNICs). However, no study has yet examined the relation between brain and spinal cord activity during counterirritation analgesia in humans. This fMRI study investigates brain responses to phasic painful electrical stimulation administered to the sural nerve to evoke a spinal nociceptive response (RIII reflex) before, during and after counterirritation induced by the immersion of the left contralateral foot in cold water. Responses are compared with a control condition without counterirritation. As expected, counterirritation produced robust pain inhibition with residual analgesia persisting during the recovery period. In contrast, RIII reflex amplitude was significantly decreased by counterirritation only in a subset of subjects. Modulatory effects of counterirritation on pain perception and spinal nociception were paralleled by decreased shock-evoked activity in pain-related areas. Individual changes in shock-evoked brain activity were specifically related to analgesia in primary somatosensory cortex (SI), anterior cingulate cortex and amygdala, and to RIII modulation in supplementary motor area and orbitofrontal cortex (OFC). Moreover, sustained activation induced by the counterirritation stimulus in the OFC predicted shock-pain decrease while sustained activity in SI and the periaqueductal gray matter predicted RIII modulation. These results provide evidence for the implication of at least two partly separable neural mechanisms underlying the effects of counterirritation on pain and spinal nociception in humans.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Poderia a atividade física induzir analgesia em pacientes com dor crônica?

A dor crônica caracteriza-se pela persistência do sintoma além do período fisiológico de recuperação do tecido lesado. Essas dores causam incapacidade física e redução da performance cognitiva, reduzem a qualidade de vida e o bem-estar dos pacientes, cujo tratamento proposto contradiz o clássico binômio da terapia da dor aguda (repouso e fármacos). Para a dor crônica prescrevem-se exercícios físicos e sugerem-se tratamentos multidisciplinares. Embora a atividade física seja prescrita há mais de 20 anos, os mecanismos neurofisiológicos envolvidos ainda não são compreendidos. Descrevemos brevemente os mecanismos endógenos de controle da dor crônica e evidências da literatura científica que defendem o sistema opioide como mecanismo de ação na analgesia induzida pelo exercício em indivíduos sadios e atletas. Esse mecanismo também parece agir na população com dor crônica, embora haja controvérsias. Finalizamos o artigo com considerações clínicas para a prescrição do exercício para a população com dor crônica.

Oral opioid use alters DNIC but not cold pain perception in patients with chronic pain - new perspective of opioid-induced hyperalgesia

Opioids can elicit unexpected changes in pain sensitivity, known as opioid-induced hyperalgesia (OIH). The aim of this study was to explore whether OIH exists in patients with chronic pain treated with oral opioids (OP) versus non-opioid (NOP) analgesics. The sensitivity to cold pain and the magnitude of diffuse noxious inhibitory control (DNIC) were evaluated in 73 OP and 37 NOP treated patients. Pain threshold, intensity and tolerance in response to the cold pressor (1 degrees C) were measured. DNIC was tested by co-administrating conditioned heat stimulation (47 degrees C) to the left forearm and a conditioning stimulation of 12 degrees C for 30s to the right hand. The results showed no differences between the two groups in any of the cold pain measures. In contrast, the magnitude of DNIC was significantly larger in the NOP than in the OP treated patients (p=0.003). A gender based analysis showed a significant difference in DNIC between OP and NOP treated men only. However, a mixed model ANOVA demonstrated a significant effect of treatment (OP versus NOP) (F=5.928, p=0.017) rather than gender on DNIC. A regression analysis showed that opioid dosage and treatment duration had a significant negative effect on the magnitude of DNIC in OP treated men (beta=-2.175, p=0.036 and beta=-2.061, p=0.047, respectively). In conclusion, oral opioids usage for the treatment of chronic pain does not result in abnormal sensitivity to cold pain, but seems to alter pain modulation. The use of 'advanced' psychophysics tests such as evaluation of DNIC can help understanding the phenomenon of OIH.

Differential induction of c-fos expression in brain nuclei by noxious and non-noxious colonic distension: role of afferent C-fibers and 5-HT3 receptors

Experimental animal models have been established to gain insight into the pathogenesis and the mechanisms of visceral hyperalgesia in the irritable bowel syndrome (IBS). However, data about the mechanisms and pathways involved in the induction of neuronal activity in forebrain and midbrain structures by a physiological GI stimulus, like colonic distension (CD), in the range from non-noxious to noxious intensities are scarce. Thus, the effect of proximal CD with non-noxious (10 mmHg) and noxious (40 and 70 mmHg) stimulus intensities on neuronal activity in brain nuclei, as assessed by c-fos expression, was established. In additional studies, the role of vagal and non-vagal afferent sensory C-fibers and 5-HT(3) receptors in the mediation of visceral nociception was investigated in this experimental model at noxious colonic distension (70 mmHg). At CD, the number of c-Fos like immunoreactivity (c-FLI)-positive neurons increased pressure-dependently in the nucleus of the solitary tract (NTS), rostral ventrolateral medulla (RVLM), nucleus cuneiformis (NC), periaqueductal gray (PAG), and the amygdala (AM). In the dorsomedial (DMH) and ventromedial nucleus (VMH) of the hypothalamus, as well as in the thalamus (TH), neuronal activity was also increased after CD, but independently of stimulus intensities. A decrease of the CD-induced c-fos expression after sensory vagal denervation by perivagal capsaicin treatment was only observed in brainstem nuclei (NTS and RVLM). In all other activated brain nuclei examined, the CD-related induction of c-fos expression was diminished only after systemic neonatal capsaicin treatment. In the NTS and RVLM, a trend of decrease of c-fos expression was also observed after systemic neonatal capsaicin treatment. In order to assess the role of the 5-HT(3) receptor in CD-induced neuronal activation of brain nuclei, animals were pretreated with the 5-HT(3) receptor antagonist granisetron (1250 microg/kg, i.p. within 18 h before CD). Pretreatment with granisetron significantly reduced the number of c-FLI-positive cells/section in the NTS by 40%, but had no significant effect on the CD-induced c-fos expression in other brain areas. The data suggest that distinct afferent pathways and transmitters are involved in the transmission of nociceptive information from the colon to the brain nuclei activated by proximal colonic distension. Activation of NTS neurons at such a condition seems to be partially mediated via capsaicin-sensitive vagal afferents and 5-HT(3) receptors. In contrast, activation of brain nuclei in the di- and telencephalon by nociceptive mechanical stimulation of the proximal colon, as assessed by c-fos expression, is partially mediated by capsaicin-sensitive, non-vagal afferents, and independent of neurotransmission via 5-HT(3) receptors. The modulation of CD-induced c-fos expression exclusively in the NTS by granisetron points to a role of 5-HT(3) receptor antagonists in the modulation of vago-vagal sensomotoric reflexes rather than an influence on forebrain nuclei involved in nociception.

Neuropathy reduces viscero-somatic inhibition via segmental mechanisms in rats

The effect of an experimental neuropathy on the viscero-somatic inhibition was studied in lightly anesthetized rats. In controls, colo-rectal distension at noxious intensities produced a multisegmental prolongation of the withdrawal response induced by noxious stimulation of the skin. In rats with a spinal nerve-ligation induced neuropathy this viscero-somatic inhibition was significantly reduced within the neuropathic segment (the hindlimb) but not outside of it (the tail). Naloxone, an opioid antagonist, attenuated this viscero-somatic inhibition in controls and it did not restore the inhibition in neuropathic rats. The results indicate that somatic neuropathy produces a segmental attenuation of viscero-somatic inhibition and this attenuation cannot be explained by a nerve injury-induced release of endogenous opioids. The decreased inhibition of somatic signals may contribute to the hypersensitivity observed in neuropathic conditions.

[Neurophysiological bases of the counterirritation phenomenon:diffuse control inhibitors induced by nociceptive stimulation]

To define the counterirritation phenomenon, we might refer to the Hippocratic aphorism: 'If two sufferings take place at the same time, but at different points, the stronger one makes the weaker silent'. On the basis of this clinically common observation, often used advantageously by the patients themselves, a number of therapeutic methods have been developed which are grouped under the terms counterirritation or counterstimulation. This phenomenon has not been scientifically analysed until recent years. Experimental results gathered during the last decade have shown that counterirritation phenomena have a well-defined neural substrate both in animals and in man. In particular, they have proved not to rely on segmental mechanisms, but rather imply spino-bulbo-spinal loops involving ascending pathways in the anterolateral spinal columns, integration in the lower brain stem, and descending influences reaching dorsal horn neurons via the dorsolateral quadrant. The results also suggest that the study of counterirritation is essential for accessing the physiology of nociception and pain control. The very existence of the counterirritation phenomenon is the easiest demonstrable index of a specific system for pain modulation in man. Besides its scientific interest, the elucidation of its neurophysiological bases has clinical importance, in as much as it may ameliorate our understanding of certain pain syndromes and contribute to the development of new investigative and therapeutic procedures.

Paradoxical inhibition of nociceptive neurons in the dorsal horn of the rat spinal cord during a nociceptive hindlimb reflex

Nociceptive-specific and multireceptive neurons in the lumbar dorsal horn are excited by noxious stimuli applied to the hindpaw and inhibited by noxious stimuli applied to distant body regions. Given that at least a subset of these neurons are part of the circuit for nociceptive reflexes, inhibition of nociceptive-specific and multireceptive neurons should inhibit nociceptive reflexes. Unfortunately, previous attempts to test this hypothesis have been inconclusive because of methodological differences between electrophysiological and behavioral experiments. The present study overcame this problem by recording neural and reflex activity simultaneously. Rats were anesthetized with halothane and surgically prepared for single-unit recording from the lumbar dorsal horn. Hindpaw heat caused a burst of activity that reliably preceded hindpaw withdrawal in 10 nociceptive-specific and 17 multireceptive neurons. A distant noxious stimulus (tail in 50 degrees C water or ear pinch) inhibited the evoked activity of both nociceptive-specific and multireceptive neurons and simultaneously changed the topography of the hindpaw reflex from flexion to extension without altering reflex latency. The present data are consistent with previous reports of inhibition of nociceptive-specific and multireceptive neurons during application of a distant noxious stimulus. However, inhibition of nociceptive-specific and multireceptive neurons concomitant with a shift in the hindlimb reflex from flexion to extension suggests that these neurons are part of the circuit for flexor reflexes specifically. Presumably, lateral inhibition from the flexor to extensor circuit allows for the release of hindlimb extension when neurons in the flexion circuit are inhibited by a distant noxious stimulus. Such a system reduces the chance of injury by allowing for withdrawal reflexes to a single noxious stimulus and escape reactions, such as running and jumping, to multiple noxious stimuli.

Morphine microinjected into the nucleus raphe magnus does not block the activity of spinal trigeminal nucleus oralis convergent neurons in the rat

This study investigated the effects of morphine microinjection into the nucleus raphe magnus (RMg) on electrically evoked C-fiber activities of convergent neurons in the spinal trigeminal nucleus oralis (Sp5O), in halothane-anesthetized rats. Although the neurons could be depressed by systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible fashion, morphine microinjected into the RMg (2. 5 microgram or 5 microgram) neither depressed their C-fiber-evoked responses, nor the diffuse noxious inhibitory controls acting on them. It is concluded that the RMg is not involved in reinforcing descending inhibitory controls that are tonic or triggered by noxious stimuli acting on Sp5O convergent neurons.

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.

Intense peripheral electrical stimulation differentially inhibits tail vs. limb withdrawal reflexes in the rat

In an on-going study on mechanisms by which activation of sensory afferents regulates nociception, high-intensity, low-frequency electrical stimulation was applied to previously defined meridian and non-meridian points of the hindlimb or forelimb, and the effects measured on the withdrawal reflex of the tail or limb in the lightly anesthetized rat. Withdrawal was evoked by application of noxious radiant heat to the tip of the tail or to the plantar surface of a hindpaw or forepaw. Parameters of conditioning electrical stimulation were 2 ms pulses at 4 Hz for 20 min at 20 x threshold (20-30 mA) where threshold was the minimum intensity which evoked muscle twitch. In experiments on tail withdrawal, stimulation applied to meridian points fengshi (GB-31), femur-futu (ST-32) and zusanli (ST-36) of the hindlimb or to wai-kuan (TH-5) and hoku (LI-4) of the forelimb increased the latency of the withdrawal reflex to 70-100% of the maximum possible inhibition (MPI) during the stimulation. Inhibition persisted for more than 1 h after the end of stimulation. Bilateral stimulation of hindlimb meridian points evoked a greater inhibition during the stimulation (> 95% of the MPI); the inhibition persisted for 40 min. Stimulation of non-meridian sites in hindlimb or forelimb inhibited the withdrawal reflexes by 45-50% of the MPI during the stimulation only. Thus, the evoked inhibition has two components, a brief effect elicited by non-meridian point stimulation and a persistent post-stimulation effect produced only upon stimulation of meridian points. Stimulation produced little effect on nociceptive limb withdrawal reflexes. The results suggest that high-intensity, low-frequency electrical stimulation of meridian points produced a long-lasting, extrasegmental inhibition of the tail withdrawal but not of limb withdrawal reflexes. This differential inhibition may be due to differences in neuronal circuitry and CNS modulatory control mechanisms. The persistent inhibition appears to be dependent on the site of stimulation because it is not evoked by stimulation of sites outside of meridian points.

Descending antinociceptive mechanisms in the brainstem: their role in the animal's defensive system

The identification of specialized mechanisms in the mammalian brainstem that function to inhibit the rostral transmission of nociceptive (pain-related) information in the spinal cord led to an explosion of research into the neuroanatomical and neurochemical substrates of these antinociceptive systems. As outlined in the present paper, most attention was directed at those mechanisms in the periaqueductal grey (PAG) and rostral ventromedial medulla (RVM). However, comparatively little attention has been paid to the functional role of these mechanisms in animal behaviour. The purpose of the present paper is to review research into the behavioural significance of those antinociceptive mechanisms in the PAG and RVM. It is concluded that these mechanisms function as part of the animal's fear or defensive system, serving to make a threatened animal insensitive to noxious stimulation and thereby allowing that animal to engage in defensive responses instead of recuperative activities. Further, it is argued that the organization of these antinociceptive circuits reflects the animal's increasing capacity for early detection of danger. Specifically, nociception itself is held to signify the presence of immediate threat, and consequently, nociceptive input directly activates antinociceptive circuits at either the spinal level (during intense noxious stimulation) or RVM (following exposure to moderate noxious stimuli). In contrast, events that are themselves innocuous but which signal threat (either learned or innate danger signals) activate fear and defensive systems in the amygdala and PAG which engage the descending antinociceptive projections in the RVM.

Buprenorphine blocks diffuse noxious inhibitory controls in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve was recorded from the ipsilateral biceps femoris muscle in anaesthetised rats. Such reflex responses can be inhibited by applying noxious conditioning stimuli to heterotopic areas of the body. These inhibitory processes have been termed diffuse noxious inhibitory controls. The responses were recorded before, during and after the immersion of the tail in a thermoregulated waterbath (at 50 degrees C) for 1 min. The C-fibre reflex responses were depressed by a maximum of 71 +/- 3% at 45 s after the start of such conditioning stimuli. A dose of 3 mu g/kg buprenorphine completely blocked the inhibition and post-stimulus effects triggered by the heterotopic noxious stimuli. In the 0.3-3 mu g/kg range, buprenorphine increased, in a dose-dependent manner, the magnitude of the inhibition. These doses did not produce any changes in the C-fibre reflex itself. The results are discussed in terms of the mechanisms underlying the analgesic properties of buprenorphine.

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.

Audible and ultrasonic vocalization elicited by single electrical nociceptive stimuli to the tail in the rat

We describe audible and ultrasonic vocalization elicited in rats by a short electrical pulse applied to the tail. Three types of vocal emissions were recorded: (1) 'peep', characterized by a repartition of energy over a wide range (0-50 kHz) of frequencies without any clear structure; (2) 'chatters', characterized by an audible (frequencies in hearing range of humans) fundamental frequency (2.47 +/- 0.03 kHz) and harmonics; and (3) 'ultrasonic emissions', characterized by a succession of slightly modulated pulses with frequencies in the 20-35 kHz range. Peeps and chatters were never recorded before the application of the stimuli. Several different vocalization patterns were described in terms of these types of responses. Just after the stimulation, all the animals emitted a 1st peep, which was generally (61%) followed by a 2nd one. They appeared with reproducible latencies, durations and envelopes. The envelopes of the audible (peeps and chatters) responses were intensity-dependent. Experimental data (moving the stimulation site, lidocaine injection) indicated that the 1st and 2nd peeps were triggered by two different groups of peripheral fibres with mean conduction velocities of 7.3 +/- 0.8 and 0.7 +/- 0.1 m/sec, respectively. This suggested an involvement of A delta and C fibres. Morphine showed a naloxone-reversible and dose-dependent antinociceptive effect by decreasing the 1st and 2nd peep envelopes. It is concluded that a short stimulus applied to the tail triggers a complex behavioural repertoire. It is proposed that this model will be a useful tool for physiological and pharmacological studies of nociception.

Paradoxical analgesia induced by low doses of naloxone is not potentiated by complete inhibition of enkephalin degradation

The involvement of a presynaptic autoinhibition of endogenous enkephalin release has been suggested as a possible explanation for the paradoxical analgesia induced by low doses of naloxone. This hypothesis was investigated by using the systemically active mixed inhibitor of enkephalin degrading enzymes, RB 101. As already described, in both hot plate (55 +/- 0.5 degrees C) and acetic acid (0.6%) abdominal constriction tests in mice, subcutaneous administration of naloxone produced biphasic effects, with antinociceptive responses at very low doses (microgram range) and hyperalgesia at higher dose (mg range). However at concentrations producing an extracellular increase in enkephalin levels and subsequent analgesia, the mixed inhibitor prodrug of the enkephalin-metabolizing enzymes RB 101 (20 or 100 mg/kg i.v. and 5 or 10 mg/kg i.v., in the hot plate test and in the abdominal constriction test, respectively) did not potentiate the paradoxical analgesia induced by naloxone. These results are inconsistent with a negative autoregulation of endogenous enkephalin release and could suggest the involvement of the diffuse noxious inhibitory controls (DNIC). Indeed, the finding that low doses of RB 101 (1 mg/kg i.v. in the hot plate test, and 250 micrograms/kg i.v. in the abdominal constriction test) were able to induce hyperalgesic responses could indicate that the DNIC are tonically activated by endogenous enkephalins accounting for the antinociceptive responses elicited by low doses of naloxone.

Studies of brain structures involved in diffuse noxious inhibitory controls in the rat: the rostral ventromedial medulla

1. Previous electrophysiological, pharmacological and anatomical evidence had suggested a possible participation of rostral ventromedial medulla (RVM) in the supraspinal part of the loop underlying diffuse noxious inhibitory controls (DNICs). In order to test this hypothesis, two experimental series were performed during which DNICs were compared in control sham-operated rats and rats with lesions of the RVM and of an adjacent candidate for such a role, the nucleus gigantocellularis reticularis (Gi). 2. In the first experimental series, lesions were induced, in anaesthetized animals, by injections of quinolinic acid (0.3-0.8 microliter of a 360 nmol/microliter solution) into the RVM or Gi. In the control animals (n = 10), the vehicle alone (artificial cerebrospinal fluid) was injected. Histological lesion reconstructions were performed after each electrophysiological experiment. Three groups of animals were considered: in the first group (n = 5), the lesion was centered on the RVM, including the two caudal thirds of nucleus raphe magnus (NRM) and adjacent reticular areas; in the second group (n = 5), the lesions extended more rostrally and involved the rostral pole of NRM; in the third group (n = 5), the lesion extended more laterally and dorsally and included nucleus reticularis gigantocellularis pars alpha (GiA), nucleus reticularis paragigantocellularis lateralis (LPGi) and the Gi. In each case, all the neuronal cell bodies within the lesioned area were destroyed. 3. In the second experimental series, electrolytic lesions of the total rostrocaudal extent of the NRM (n = 5) were induced, in anesthetized animals, by passing cathodal current (5 mA, 8 s). In the control animals (n = 5), the electrode was lowered but current was not applied. 4. One week after lesioning, the animals were anaesthetized, paralysed, artificially ventilated and recordings were made from convergent neurones in trigeminal nucleus caudalis. These neurones gave responses due to activation of A and C fibres when percutaneous electrical stimuli were applied to their receptive fields. DNICs were triggered by immersion of each paw in a 50 degrees C water-bath. Both the general properties of the convergent neurones and the inhibitions of the C fibre-evoked responses produced by these heterotopic noxious stimuli were compared in the different groups of animals. 5. The sizes of receptive field, spontaneous activities, thresholds for C fibre-evoked responses and responses to C fibre activation were not different in the control and lesioned animals. The percentage inhibitions of the C fibre-evoked responses both during and in the 44s following the conditioning periods were also very similar in the different groups of animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphine and diffuse noxious inhibitory controls in the rat: effects of lesions of the rostral ventromedial medulla

The aim of this study was to investigate whether the rostral ventromedial medulla (RVM) participates in the lifting of diffuse noxious inhibitory controls (DNIC) by systemic morphine. The effects of morphine (1 mg/kg i.v.) on DNIC were compared in sham-operated rats and animals with electrolytic lesions of the RVM performed one or three weeks earlier. The C-fibre-evoked responses of spinal dorsal horn convergent neurones were similar in the sham-operated and lesioned animals. DNIC acting on these responses were also similar in these groups. DNIC were similarly reduced naloxone reversibly following morphine injections in sham-operated animals and animals tested one week after lesioning of the RVM. In contrast, DNIC were not significantly altered by morphine in animals tested three weeks after lesioning. The lesions were similar in both groups of animals. This time-dependent attenuation of the effects of morphine indicates that the RVM is not directly involved in the reduction of DNIC induced by systemic morphine. However, it is suggested that lesions in this region can induce a reorganization of brainstem opioidergic systems.