Clinical exploration of nociception with the use of reflexologic techniques]

In the first part of this report a methodology is described which allows an objective and specific exploration of experimental pain in man by using some electrophysiological features of cutaneous reflexes. This method can be summarized as follows: in normal and trained volunteers, we studied simultaneously the recruitment curves of the nociceptive flexion reflex of a knee-flexor muscle (biceps femoris muscle) and that of pain sensation elicited by electrical stimulation of the ipsilateral sural nerve at the ankle. In this procedure, we found that the reflex threshold (Tr) was closely related to that of pain threshold (Tp) around a similar value (10 mA). In the same way, both the threshold of the maximal recruitment of the reflex (Tmr) and that of tolerance to pain (Tpt) were found to be close to 33 mA. These values are reliably reproducible in one subject from one session to the other, and in all subjects with minimal inter-individual variations and without any significant inter-sex difference. These close relationships between Tr and Tp and between Tmr and Tpt respectively constituted the basic ground for the elaboration of the methodology for investigating objectively the human nociceptive reactions. In the second part of the paper, this methodology is applied for studying the spinal mechanisms of morphine analgesia when the drug is given either by intravenous route in normal subjects and in paraplegic patients or administered epidurally in patients with acute postoperative pain. On the one hand, the resulting data strongly validate the model since they show that pain and nociceptive reflex are similarly depressed by morphine in a dose-response fashion. On the other hand, data also show that the spinal level is one of the main important sites of the mechanisms of morphine-induced analgesia since this drug is found to strongly depress selectively the nociceptive transmission directly at the spinal level. Finally, this method is applied for investigating the nociceptive reactions in patients affected either with a pathological lack of pain sensation or, by contrast, in patients complaining of acute or chronic pain from various origins. Since the nociceptive flexion reflex can be considered as a specific and objective physiological correlate of a pain sensation, it can be successfully employed as a useful tool for investigating some aspects of the human nociceptive reactions in both experimental and pathological situations.     

Comparative study of the nociceptive reflex and late components of the evoked somatosensory potential during stimulation of the sural nerve in healthy subjects

A study was carried out with 10 normal volunteers in order to find a correlation between nociceptive flexion reflexes from the biceps femoris muscle and the amplitude of the late component (N150-P220) of the vertex evoked potential elicited by sural nerve stimulations at various intensities randomly delivered. The range of stimulus varied from the perception threshold (usually 1 mA) to 2 times the reflex threshold. This latter parameter was usually found between 8 and 11 mA. While the nociceptive flexion reflex increased linearly as a function of stimulation intensity, the amplitude of N150-P220 was maximum at pain threshold and remained in plateau at this level or even slightly decreased as a function of stimulus intensity. No significant correlation was found between the recruitment curve of the nociceptive reflex and that of the late component of the evoked potential as a function of stimulus intensity. Functional implications of these data are discussed.     

Study of pain thresholds by recording flexor reflexes in thalamic syndromes

Both thresholds of nociceptive flexion reflex and pain sensation were studied in 6 normal subjects and in 6 patients with typical thalamic pain. In these patients, on the painful side, these thresholds were found increased (98 p. 100; 89 p. 100 respectively) compared to the normal side. Values obtained in this latter did not significantly differ from those observed in normal subjects. After 8 days of indalpine treatment, the nociceptive reflex threshold was furthered increased in the painful side while the pain threshold was not modified by this drug. In the normal side, changes observed after indalpine were similar to that obtained in normal subjects. All the indalpine-induced modifications were reversed by naloxone in both patients and normals. These results are discussed in the context of the possible mechanisms of thalamic hyperpathia.     

Differential effects of specific δ and κ opioid receptor antagonists on the bidirectional dose-dependent effect of systemic naloxone in arthritic rats, an experimental model of persistent pain

In an attempt to determine the opioid receptor class(es) which underly the two opposing effects of naloxone in models of persistent pain, we tested the action of the selective δ antagonist naltrindole, and that of the κ antagonist MR-2266 on the bidirectional effect of systemic naloxone in arthritic rats. As a nociceptive test, we used the measure of the vocalization thresholds to paw pressure. The antagonists were administered at a dose (1 mg/kg i.v. naltrindole, 0.2 mg/kg i.v. MR-2266), without action per se, but which prevents the analgesic effect of the δ agonist DTLET (3 mg/kg, i.v.) or the κ agonist U-69, 593 (1.5 mg/kg, i.v.) respectively, and does not influence the effect of morphine (1 mg/kg i.v.) or the μ agonist DAMGO (2 mg/kg, i.v.) in these animals. In arthritic rats injected with the δ antagonist, the paradoxical antinociceptive effect produced by 3 μg/kg i.v. naloxone was not significantly modified (maximal vocalization thresholds (% of control) were 146 ± 9% versus 161 ± 7% in the control group). By contrast, the hyperalgesic effect produced by 1 mg/kg i.v. naloxone was significantly reduced (maximal vocalization thresholds were87 ± 4% versus 69 ± 5% in the control group). In rats injected with the κ antagonist, the antinociceptive effect of the low dose of naloxone was almost abolished (mean vocalization thresholds were 115 ± 3% versuss 169 ± 7%) whereas the hyperalgesic effect of naloxone 1 mg/kg i.v. was not significantly modified (mean vocalization thresholds =70 ± 3% and 65 ± 3%, respectively). Based on these results, the possible role of each receptor subtype in the putative control exerted by endogenous opioid substances on nociceptive messages in pathological conditions is discussed.     

Effects of pain, morphine and naloxone on the duration of animal hypnosis

Previous research has shown that animal hypnosis (tonic immobility) in the rabbit may be elicited in a condition of prolonged nociceptive stimulation. These experiments show that long-lasting irritative pain, produced within 15 min of formalin injection, potentiates the duration of hypnosis. Morphine, in the absence of painful stimuli, also potentiates hypnosis duration and this effect is antagonized by naloxone. Naloxone reduces hypnosis duration, but only at high doses (15 mg/kg). In a condition of irritative pain, the potentiation of hypnosis duration is abolished by naloxone (5 mg/kg). Hypnosis response is abolished in 6 out of 7 morphine-tolerant rabbits, but prolonged pain restores the response. The hypothesis that an opioid mechanism may be activated during animal hypnosis is discussed.     

Systemic morphine reduces the wind-up of trigeminal nociceptive neurons.

We assessed the effects of intravenous morphine on the wind-up of nociceptive neurons of the spinal trigeminal nucleus oralis (Sp5O). Extracellular recordings of Sp5O nociceptive convergent neurons were performed in intact halothane-anesthetized rats. Wind-up of C-fiber-evoked responses was elicited by repetitive electrical stimulation (train of 16 shocks, 0.66 Hz) of their receptive field at C-fiber intensity (3 times the threshold). Wind-up was tested for its sensitivity to morphine (6 mg/kg,i.v.), and the specificity of the effects was verified with naloxone (0.4 mg/kg, i.v.). Nineteen convergent neurons displaying wind-up were recorded. Morphine reduced the wind-up of all but one. In five cases, notwithstanding a reduced wind-up, the neuronal response evoked by the first stimulus in the train (initial input) was unexpectedly increased. Naloxone always antagonized morphine inhibitory effects on the wind-up. When administered systemically, morphine reduced the wind-up of trigeminal nociceptive neurons. This inhibitory effect occurred independently of morphine's ability to affect the initial C-fiber-evoked input. Our findings support the idea that systemic morphine probably blocks wind-up by acting at opioid receptors located postsynaptically to nociceptive primary afferents.     

Dissociation between pain and the nociceptive blink reflex during psychological arousal.

OBJECTIVE: To investigate the effect of psychological arousal on pain ratings and the R2 component of the electrically evoked blink reflex to a 'pure' noiciceptive stimulus. METHODS: Pain ratings and R2 to a noiciceptive stimulus (pulse width 0.3ms, 2mA, delivered from a concentric electrode attached to the supraorbital region of the forehead) were investigated in 16 healthy participants before and during a serial subtraction task, and in 16 control participants who sat quietly during nociceptive stimulation. RESULTS: Pain ratings decreased whereas R2 amplitude increased during the serial subtraction task. CONCLUSIONS: Supra-spinal rather than spinal mechanisms inhibited pain perception during psychological arousal. Moreover, psychological arousal facilitated the R2 component of the blink reflex to a nociception-specific stimulus. SIGNIFICANCE: Supra-spinal influences need to be considered during clinical evaluation of the trigeminal nociceptive blink reflex.     

Responses of caudal brain stem neurons to vaginal and somatosensory stimulation in the rat and evidence of genital-nociceptive interactions.

In rabbits lightly anesthetized with pentobarbital, the effects of injecting morphine and its antagonist, nalorphine, directly into the cerebral circulation via a carotid artery cannulation, on the jaw-opening reflex elicited by intradental stimulation, were studied. Using averaged electromyogram signals from the digastric muscle as the index, a single dose of morphine (3 mg/kg) was found to produce only a transient depression of the jaw-opening reflex. After a second, cumulative injection of morphine (3 mg/kg), the jaw-opening reflex was sufficiently inhibited for at least 30 min. The morphine-induced analgesia could be reversed by nalorphine, indicating that such action was truly pharmacological. The likelihood of morphine acting on some central sites and promoting the release of neurotransmitters which produce inhibition of transmission of nociceptive information from the dental pulp is discussed in the light of recent physiological and pharmacological linkage between morphine-triggered and central stimulation-induced analgesia.     

Physiological characteristics of pressure immobility. Effects of morphine,naloxone and pain

This study is an attempt to detect the most important modifications of physiological parameters occurring during pressure immobility in rabbits and to compare them with those recorded during animal hypnosis. Like the latter, pressure immobility is characterized by the development of high voltage slow waves in the EEG, reduction in frequency and amount of rhythmic slow activity in the hippocampus (RSA) and depression of spinal polysynaptic reflexes. Systolic and diastolic blood pressures are not modified. Duration of two types of immobility is positively correlated within individuals. Treatment by a single dose of morphine (1 mg/kg) potentiates the duration and this effect is antagonized by naloxone (1 mg/kg). Repeated morphine injection up to tolerance reduces duration. Pressure immobility may also be produced under persistent nociceptive stimulation and is characterized by the development of high voltage slow waves in the EEG, as is typical in the absence of pain. Naloxone, (5 mg/kg) injected in a condition of persistent noxious stimulation, reduces immobility duration. In contrast to animal hypnosis, the duration of pressure immobility is neither potentiated by pain nor reduced by naloxone (1,5 or 20 mg/kg). It is suggested that the two immobilities are controlled by several mechanisms, some similar, some different.     

Nociceptive threshold and Parkinson disease]

It has now become possible to measure pain thresholds in man by the threshold or amplitude of the leg flexion nociceptive reflex (RIII reflex). These parameters accurately reflect the pain levels perceived by the patients. The aim of the present study was to assess pain thresholds in Parkinsonian patients, using the RIII reflex. Painful phenomena are often mentioned in Parkinson disease, but the perceived pain threshold level can be difficult to assess because of the depressive symptoms which are often associated with Parkinson disease. In 8 cases out of 10, the pain threshold was found to be higher in patients with Parkinson disease than in a control population of the same age: in 2 cases, naloxone injection led to recovery of the normal pain threshold level.     

A new method to increase nociception specificity of the human blink reflex.

OBJECTIVE: The medullary R2 response of the blink reflex can be elicited by innocuous and noxious stimuli. The purpose of this study was to elicit a nociception specific R2 response with a new surface electrode. METHODS: In 10 healthy subjects the blink reflex was elicited using a standard (10-15 mA) and a new concentric surface electrode type (0.6-1.6 mA) which produces a pin-prick-like pain. RESULTS: After topical local anaesthesia with lignocaine/prilocaine R1 was unchanged, R2 was attenuated by 12% after standard stimulation but was almost abolished (-91%) with the new electrode type. CONCLUSION: Stimulation with low stimulus intensities but electrode-dependent high current density allows preferential depolarization of superficial nociceptive A-delta fibres. This new method is less traumatic than others and is useful in the study of trigeminal nociception.     

Use of animal models of clinical pain

For a better understanding of clinical pain, several groups involved in the study of basic pain mechanisms have proposed the use of various experimental models close to clinical situations. They are based either on neurogenic or inflammatory processes. Data obtained with three of these models will be developed in the paper: rats rendered arthritic by Freund's adjuvant injection into the tail, rats with an intraplantar injection of carrageenin in one hind-paw, rats with a moderate ligature of one common sciatic nerve. The various pharmacological approaches revealed dramatic changes of the analgesic effects of morphine and other opioid substances, and a spectacular modification of the endogenous opioid reactivity. A further enhancement of the initial hyperalgesia was observed with high doses (1-3 mg/kg iv) of naloxone (known as an antagonist of morphine), contrasting with the paradoxical analgesia induced with the low dose (peaking up for 3 micrograms/kg iv). Electrophysiological studies emphasized dramatic changes of neuronal responsiveness in structures involved in the transmission of the nociceptive messages. In each of these models, electrophysiological data provide new insights on the physiopathological mechanisms of the related clinical pain.     

Comparison of trigeminal and spinal modulation of pain and nociception

Modulation of pain and nociception by noxious counterstimulation, also called "diffuse noxious inhibitory controls" or DNIC-like effect, is often used in studies of pain disorders. It can be elicited in the trigeminal and spinal innervation areas, but no study has previously compared effects in both innervation areas. Therefore, we performed a study comparing DNIC-like effects on the nociceptive flexion reflex (NFR) and the nociceptive blink reflex as well as the respective pain sensations. In 50 healthy volunteers, the blink reflex elicited with a concentric electrode and the NFR were recorded before and after immersion of the contralateral hand in cold water. Responses were recorded as the subjective pain sensation and the reflex size. The cold water immersion of the contralateral hand elicited a reduction of both subjective pain sensation and reflex amplitude following the stimulation of both reflexes. However, there were no strong correlations between the individual reductions of both subjective pain sensation and reflex amplitude for both reflexes, and neither when results of the two reflexes were compared with each other. The dissociation between DNIC-like effects on pain and on nociception, which had been found previously already for the NFR, implies that both effects need to be studied separately.     

Stimulation of the periaqueductal gray subdues sensitized pain in morphine- and meperidine-dependent rats

Because increased tolerance of narcotics is marked by progressive deactivation of the descending antinociceptive system, a question was raised whether stimulation of the periaqueductal gray matter (PAG) would have any electroanalgetic effect in animals adapted to increasing doses of narcotics. The daily dose of morphine (10 mg/kg) administered to rats was increased on alternate days by 10 mg/kg to 100 mg/kg/day. To another group, the daily dose of meperidine was increased from 15 mg/kg by 15 mg/kg to 150 mg/kg/day. Electrophysiological experiments were conducted under chloralose and urethane anesthesia 16 h after the last injection of morphine or meperidine. Spike potentials evoked from individual neurons of the nucleus ventralis posterolateralis by single-pulse stimulation of the sciatic nerve were accumulated in poststimulus time histograms. For nociceptive neurons the histograms were characterized by a short-latency activity peak and at least two late (270 and 420 ms) peaks. For non-nociceptive neurons the histograms had no late activity peaks. In control rats, stimulation of the PAG (400 ms at 70/s) prior to each sciatic nerve pulse reorganized the late activity peaks of the nociceptive neurons: a single late peak occurred during the 280 to 400 ms poststimulus interval, indicating suppression of pain by electroanalgesia. In rats adapted to morphine or meperidine, intracarotid infusion of naloxone lowered the nociceptive threshold. Stimulation of the PAG reorganized the late peaks but only if the sciatic nerve stimulation was not increased. At the voltage used to stimulate the sciatic nerve in control animals, two separate late peaks appeared, which were subdued by PAG stimulation after intracarotid infusion of 5-hydroxytryptophan (5-HTP). These results affirmed previous findings that electroanalgesia is induced by activity in an ascending and a descending pathway, both originating from the PAG. Since the function of the descending pathway is impaired by repeated administration of narcotics, only the pathway ascending to the somesthetic thalamus can be activated to mask pain, unless 5-HTP is injected. The latter renews the functional capacity of the descending pathway and thus reinstates the full capacity of electroanalgesia.     

The bidirectional dose-dependent effect of systemic naloxone is also related to the intensity and duration of pain-related disorders: a study in a rat model of peripheral mononeuropathy

In an experimental model of mononeuropathy in the rat, created by 4 ligatures around the sciatic nerve, i.v. naloxone 1 week after surgery induces bidirectional effects (antinociceptive effects at very low doses, hyperalgesic effects with high doses). Using the same nociceptive test (vocalization thresholds to paw pressure), the activity of the same doses of naloxone (3 micrograms/kg, and 1 mg/kg) was investigated 2 weeks after sciatic ligation, when the behavioural pain-related disorders are at a maximum. Three micrograms/kg naloxone produced a significant antinociceptive effect on the lesioned and non-lesioned paw, which was clearly related to the degree as well as to the duration of pain-related signs in the rat. By contrast, the high dose of naloxone did not induce a mean significant effect when tested on either paw; however, it elicited a potent hyperalgesic effect in those rats which had recovered from hyperalgesia at this 2 week time point after the sciatic injury.     

Evidence for opiate-activated NMDA processes masking opiate analgesia in rats

The acute interaction between opioid receptors and N-methyl-D-aspartate (NMDA) receptors on nociception was examined in rats using tail-flick and paw-pressure vocalisation tests. When injected at various times (1 to 6 h) after morphine (5 to 20 mg/kg, i.v.) or fentanyl (4x40 microgram/kg, i.v.), the opioid receptor antagonist naloxone (1 mg/kg, s.c.) not only abolished the opiate-induced increase in nociceptive threshold, but also reduced it below the basal value (hyperalgesia). The noncompetitive NMDA receptor antagonist MK-801 (0.15 or 0.30 mg/kg, s.c.) prevented the naloxone-precipitated hyperalgesia and enhanced the antinociceptive effects of morphine (7.5 mg/kg, i.v.) and fentanyl (4x40 microgram/kg, i.v.). These results indicate that the antinociceptive effects of morphine and fentanyl, two opiate analgesics widely used in humans in the management of pain, are blunted by concomitant NMDA-dependent opposing effects which are only revealed when the predominant antinociceptive effect is sharply blocked by naloxone. This study provides new rationale for beneficial adjunction of NMDA receptor antagonists with opiates for relieving pain by preventing pain facilitatory processes triggered by opiate treatment per se.     

Abolition by naloxone of the inhibitory effect of peripheral electrical stimulation on the blink reflex]

The effects of a low frequency (2 c/sec) peripheral stimulation (electro-acupuncture, EA) on the nociceptive (R2) response of the blink reflex elicited by supra-orbital nerve stimulation (0.1 msec, 1 shock/8 sec) were studied in 10 healthy subjects. EA stimulation produced a very significant inhibition of the reflex in 8 subjects. Double-blind injection of naloxone (0.8 mg) reversed this inhibition while no signiificant change was observed with placebo. These results suggest that EA stimulation induces the release of endogenous opiates.     

Operant conditioning of somatosensory evoked potential (SEP) amplitude in rats. I. Specific changes in SEP amplitude and a naloxone-reversible somatotopically specific change in facial nociception

The aim of this experiment was to investigate possible endogenous opioid modulation of innocuous somatosensory activity. Somatosensory activity was measured by recording cortical somatosensory evoked potential (SEP) and reflex movement amplitude evoked by innocuous electrical stimulation of the spinal trigeminal tract in awake rats. Putative endogenous opioid activity was blocked using the opiate antagonist naloxone (1 mg/kg). The amplitude of midlatency SEP components (14-50 ms latency) increased following administration of naloxone and repeated stimulus presentations. The amplitude of these components decreased following administration of the opiate agonist morphine (3 mg/kg). An early cortical component (10 ms latency) habituated following the administration of saline but did not habituate following naloxone. Naloxone also enhanced habituation of the late SEP components (60-120 ms latency) and reflex movement evoked at higher stimulus intensities. Morphine decreased the amplitude of the early cortical component but had no consistent effect on the amplitude of the late SEP components.     

Behavioural evidence for a bidirectional effect of systemic naloxone in a model of experimental neuropathy in the rat

In animal models of inflammatory pain, we have demonstrated that the opioid antagonist naloxone induces a paradoxical analgesic effect at very low systemic doses, and a hyperalgesic effect at high doses. We have therefore proposed, that opioid systems are modified in these animals with persistent pain. The aim of the present study was to investigate the activity of naloxone through another model of pain in the rat due to a peripheral neuropathy of the sciatic nerve. The neuropathy was created by 4 ligatures around the sciatic nerve. We analyzed the effects of i.v. naloxone (3 and 10 micrograms/kg, 1 mg/kg) on the vocalization thresholds to paw pressure 8 days after the sciatic ligation. Three and 10 micrograms/kg naloxone produced a significant paradoxical antinociceptive effect on responses from the affected paw (with a mean increase of about 50 and 30% of the preinjection values, respectively) and also from the non-affected paw, although the effect was less potent. By contrast, 1 mg/kg naloxone elicited a significant hyperalgesia on responses from the affected and non-affected paw. The effects of the microdoses, but not those of the high dose, were clearly related to the vocalization thresholds measured for each rat just before injection. This study clearly shows that naloxone induces bidirectional effects in a rat model of neuropathic pain, which contradicts the current statement that neuropathic pain is opioid-resistant. The present results also suggest that these effects are not related to inflammatory processes, and may be due to modifications of opioid systems in these animals with persistent pain.     

Failure to produce a non-opioid foot shock-induced antinociception in rats

Brief continuous foot shock reportedly produces a naloxone-insensitive and thus non-opioid form of antinociception. In the present study, current intensity and duration of foot shock were varied: lower current intensities (0.5 or 1 mA) failed to produce a significant increase in tail flick (TF) latency, while current intensities of 3 mA and 6 mA applied for 2 or 3 min produced significant and long-lasting inhibition of the nociceptive TF reflex. Naloxone pretreatment attenuated significantly the antinociception developed at 3 mA but failed to affect that produced at 6 mA. It was noted, however, that higher current intensities damage the tail and the antinociceptive efficacy of footshock was reevaluated under conditions when the tail of the animal was not allowed to contact the electrified grid during foot shock. A significant short-lasting antinociception was produced only at the 6 mA current intensity. This antinociception could be attenuated by naloxone pretreatment, developed tolerance over time (8 days) and exhibited cross-tolerance with morphine, thus characterizing it as opioid in nature. These results raise the question to what extent damage to the tail contributes to the non-opioid foot shock-induced antinociception assessed using the nociceptive TF reflex.