Characterization of chronic pain and somatosensory function in spinal cord injury subjects.

The pathophysiology of the chronic pain following spinal cord injury (SCI) is unclear. In order to study it's underlying mechanism we characterized the neurological profile of SCI subjects with (SCIP) and without (SCINP) chronic pain. Characterization comprised of thermal threshold testing for warmth, cold and heat pain and tactile sensibility testing of touch, graphesthesia and identification of speed of movement of touch stimuli on the skin. In addition, spontaneously painful areas were mapped in SCIP and evoked pathological pain--allodynia, hyperpathia and wind-up pain evaluated for both groups. Both SCIP and SCINP showed similar reductions in both thermal and tactile sensations. In both groups thermal sensations were significantly more impaired than tactile sensations. Chronic pain was present only in skin areas below the lesion with impaired or absent temperature and heat-pain sensibilities. Conversely, all the thermally impaired skin areas in SCIP were painful while painfree areas in the same subjects were normal. In contrast, chronic pain could be found in skin areas without any impairment in tactile sensibilities. Allodynia could only be elicited in SCIP and a significantly higher incidence of pathologically evoked pain (i.e. hyperpathia and wind-up pain) was seen in the chronic pain areas compared to SCINP. We conclude that damage to the spinothalamic tract (STT) is a necessary condition for the occurrence of chronic pain following SCI. However, STT lesion is not a sufficient condition since it could also be found in SCINP. The abnormal evoked pain seen in SCIP is probably due to neuronal hyperexcitability in these subjects. The fact that apparently identical sensory impairments manifest as chronic pain and hyperexcitability in one subject but not in another implies that either genetic predisposition or subtle differences in the nature of spinal injury determine the emergence of chronic pain following SCI.

Acute pain threshold in subjects with chronic pain following spinal cord injury.

Studies of pain perception in patients with chronic pain have yielded contradictory results. While several studies found that acute pain threshold is raised in chronic pain subjects, others showed that these subjects exhibit a decreased pain threshold compared to pain free subjects. The aim of this study was to further examine this topic by studying pain perception in subjects with chronic pain following partial or complete spinal cord injury (SCI). We found a significant elevation of heat-pain threshold (measured above the level of lesion) in complete SCI subjects with chronic pain (CSCIP) as opposed to complete SCI subjects without pain, incomplete SCI subjects with (ISCIP) and without chronic pain and normal controls. This elevation of pain threshold was completely reversed following a complete relief of the chronic pain by DREZ lesion. Moreover, the CSCIP exhibited significantly higher scores in the McGill pain questionnaire compared to ISCIP, indicative of a more intense chronic pain perceived by these subjects. In addition, the chronic pain below the level of spinal lesion, reported by CSCIP originated from a significantly larger body area than that of ISCIP. These results indicate that a critical level of chronic pain must be perceived in order to induce an elevation in acute pain threshold.

Antinociceptive effect induced by the combined administration of spinal morphine and systemic buprenorphine.

UNLABELLED: We evaluated the antinociceptive effect of combined spinal administration of morphine and systemic administration of buprenorphine. Experiments were performed on male Wistar rats. Nociception was measured using the tail immersion test. Buprenorphine was injected intraperitoneally (IP) and morphine was injected intrathecally (IT) via a catheter implanted in the subarachnoid space. Interaction of drugs was analyzed using a dose addition model. Both IT (1-5 microg) morphine and IP (50-500 microg/kg) buprenorphine increased the latencies of nociceptive responses in a dose-dependent manner. IT morphine (4 microg) and IP buprenorphine (100 microg/kg) produced 62.9+/-6.3 and 48.8+/-6.6 percent of the maximal possible effect (%MPE), respectively. The combined administration of 2 microg of IT morphine and 50 microg/kg IP buprenorphine produced a %MPE of 97.1+/-3.4. The analysis of drug interaction revealed that IT morphine interacted with IP buprenorphine in a supraadditive manner while producing a potent antinociceptive effect. IMPLICATIONS: The concurrent administration of spinal morphine and systemic buprenorphine produces an antinociceptive effect that is greater than what could have been predicted from individual dose-response curves. This mode of interaction allows maintenance at a significant level of analgesia with reduced doses of opioids, which minimizes the incidence of undesirable side effects.

Neuroanatomical aspects of mydriatic action of morphine in rats.

Morphine causes mydriasis in rats. In order to investigate whether this effect is due to direct inhibition of preganglionic pupilloconstrictor neurons in the Edinger-Westphal nucleus (EWN), we injected opiate agonists into the EWN in male albino Charles River rats. Bilateral stereotactic microinjections of morphine (10, 20, 30, 40 micrograms/side) inhibited spinal nociceptive reflexes and caused pronounced catalepsy, but had no effect on pupillary size. The powerful opiate agonist, fentanyl, also elicited analgesia and catalepsy, when given in doses of 5 and 10 micrograms/side, but no dose of fentanyl up to 10 micrograms/side induced mydriasis. Naloxone (10 micrograms/side), given into the EWN, effectively antagonized inhibition of the tail-flick response induced by subcutaneously administered morphine (30 micrograms/kg), but had no effect on the cataleptic and mydriatic actions of systemic morphine. These results indicate that, in the rat, morphine-induced mydriasis is not accounted for by a direct action on the EWN.

Strain differences in autotomy levels in mice: relation to spinal excitability.

The consequences of combined transection of the sciatic and femoral nerves were investigated in mice of the ICR and C3HEB strains. Whereas all the animals of the C3HEB strain showed very clear self mutilatory behavior of the denervated limb (autotomy) none of the ICR mice showed autotomy. Further tests, using the hot plate and tail flick methods, show that C3HEB mice were more sensitive to noxious thermal stimuli than ICR mice. Finally, spinalization at the lumbar level revealed a markedly higher level of spinal excitability in C3HEB mice as evident from a marked decrease in nociceptive thresholds in these animals. No such threshold decrease was observed in spinalized ICR mice. The results suggest that different levels of spinal excitability underlie the susceptibility for the emergence of autotomy in mice. It is proposed that such different levels of excitability may also underlie the susceptibility for the emergence of neuropathic pain.

Antinociception induced by simultaneous intrathecal and intraperitoneal administration of low doses of morphine.

The application of morphine simultaneously into the spinal cord and brain ventricles produces a supraadditive antinociceptive effect. In this study, we attempted to determine whether combined intrathecal (IT) and intraperitoneal (IP) administration of small doses of morphine also produces such a synergistic antinociceptive effect. The experiments were performed on male Wistar rats. Nociception was measured using the tail immersion test. For IT administration morphine was injected through a catheter implanted in the subarachnoid space. Combined administration of small doses of IT (1 microgram) and IP (1 mg/kg) morphine resulted in a strong, highly significant antinociceptive effect. This effect was not only much higher than that produced by separate administration of the same doses of morphine, but also much higher than the expected effect of the combination. These results demonstrate that low doses of IT and IP morphine interact in a supraadditive fashion to produce potent analgesia.

Tail-pinch induced analgesia and immobility: altered responses to noxious tail-pinch by prior pinch of the neck.

Noxious pinch to the scruff of the neck using a metal clip produces profound immobility and analgesia. Noxious pinch delivered to the tail fails to induce immobility and results in nociceptive behavior directed at the pinched tail. However, when administered shortly after neck-clip removal, noxious tail-pinch reinstated immobility without any nociceptive response. Prior neck-clip also enhanced the antinociception induced by the tail-pinch as measured by nociceptive response to a leg pinch. Immobility, as well as antinociception, decreased as the time interval between neck-clip removal and the tail-pinch application increased. Pharmacological manipulations which reduce nociception produced a similar alteration in the response to tail-pinch. Thus, following local injections antinociceptive doses of lidocaine to the base of the tail and systemic morphine administration tail-pinch produced marked immobility. Transection of the brain at the intercollicular level provides evidence for supraspinal involvement in post-neck pinch effects. Not only was the ability of prior neck-pinch to confer antinociceptive properties on tail-pinch abolished, but increased responsiveness to noxious tail-pinch was seen. We, therefore, propose that prior neck-pinch confers new stimulus properties on noxious pinch of other body regions resulting in an enhanced antinociceptive effect, which affects both remote regions and the site of stimulation, and the ability to induce immobility.

Different behavioral deficits are induced by anoxia/hypoxia in neonatal and senescent rats: blockade by MK-801.

Rats exposed on their first postnatal day to 100% nitrogen for 25 min developed hyperactivity and lower performance in passive avoidance task during development. Administration of MK-801 (0.5 mg/kg i.p.) 1 h before anoxia or (0.25 and 0.5 mg/kg) 1 h after completely reversed this behavioral impairment. Senescent rats (24-26 months) exposed to hypoxia (92% N2 + 8% O2) for 5 h failed in their performance in C.A.R., 30 days later. Pretreatment with MK-801 (1 mg/kg i.p.) completely reversed this impairment. These data suggest that activation of endogenous NMDA receptors produces different behavioral consequences in neonatal and senescent rats and that MK-801 administration close to exposure of animals to anoxia or hypoxia can prevent such damage, thus preventing behavioral impairments in postnatal as well as in senescent rats.

Neurotoxic effects of excitatory amino acids in the mouse spinal cord: quisqualate and kainate but not N-methyl-D-aspartate induce permanent neural damage.

Despite extensive evidence for the neurotoxic effects of excitatory amino acids (EAA) in the brain little is known about their neurotoxic action in the spinal cord. In this study we attempted to produce differential lesions of spinal neurons by pretreating mice, intrathecally, with high concentrations of the EAA: N-methyl-D-aspartate (NMDA), quisqualate and kainate. Pharmacological, behavioral and histological consequences were examined 1, 3, 7 and, in some cases, 30 days after pretreatment. A single, intrathecal, injection of high concentrations of quisqualate and kainate but not NMDA, resulted in damage to spinal cord neurons. The highest concentrations of these agonists produced, in some animals, a massive, non-selective destruction of neurons within the lumbar spinal cord, accompanied by complete paralysis of the hindlimbs. Pretreatment with lower concentrations of intrathecal kainate or quisqualate produced damage to spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal cord to EAA, as seen from the decrease in biting behavior elicited by intrathecal EAA. The neurotoxic effects of quisqualate were completely blocked by the quisqualate/kainate receptor antagonist glutamylaminomethylsulphonate (GAMS), but not the NMDA antagonist 2-amino-5-phosphovalerate. GAMS attenuated the effects of kainate only partially.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal antinociceptive effects of excitatory amino acid antagonists: quisqualate modulates the action of N-methyl-D-aspartate.

Blockade of N-methyl-D-aspartate (NMDA) receptors in the spinal cord of rodents has been shown to produce antinociceptive effects and motor impairment. To find out whether other receptors for excitatory amino acids (EAA) can influence spinal pathways utilizing the NMDA receptors we compared, in mice, the behavioral consequences of intrathecal injection of four EAA antagonists, 2-amino-5-phosphono valerate (APV), kynurenate, gamma-D-glutamyl glycine (DGG) and glutamylaminomethyl sulphonate (GAMS). The selectivity of these antagonists at different concentrations was evaluated behaviorally by assessing their ability to block the biting behavior elicited by intrathecal EAA agonists. Blockade of the NMDA receptor was necessary to elicit antinociceptive effects and motor impairment. Thus, APV produced antinociception at concentrations selective for the action of NMDA. The wide spectrum EAA antagonists, DGG and kynurenate, and the quisqualate/kainate antagonist, GAMS, all produced antinociception and motor impairment at concentrations which also blocked NMDA-induced bites. However, an inhibitory modulation of the action of NMDA by quisqualate-sensitive systems was also observed. Thus, high concentrations of APV (greater than 1 mM), which also blocked quisqualate-elicited bites, produced a surprising, sharp decrease in APV antinociception and motor impairment, effects which were reversed by quisqualate. Furthermore, quisqualate significantly inhibited NMDA-induced bites. Additional evidence for such an inhibitory-modulatory effect of quisqualate can be gathered from the antinociceptive potency of DGG. This antagonist, which blocks the action of both NMDA and quisqualate, was less potent as an antinociceptive agent than APV. No such discrepancy between the ability to inhibit the action of NMDA and to elicit antinociceptive effects and motor impairment was noted for either kynurenate or GAMS. Evidence is provided that these different profiles of action are due to the fact that DGG and high concentrations of APV act at different subpopulations of quisqualate receptors than do kynurenate and GAMS, and that the former subpopulation is involved in the modulation of the action of NMDA.

Involvement of N-methyl-D-aspartate receptors in nociception and motor control in the spinal cord of the mouse: behavioral, pharmacological and electrophysiological evidence.

The present study evaluated, using behavioral and electrophysiological methods, the involvement of the N-methyl-D-aspartate receptor in the processing of noxious information in the spinal cord of the mouse. The selectivity of the excitatory amino acid antagonists 2-amino-5-phosphonovalerate and glutamylaminomethylsulphonate was assessed behaviorally, using their ability to reverse the biting behavior elicited by intrathecal excitatory amino acid administration as a tool. 2-Amino-5-phosphonovalerate at concentrations up to 1 mM was shown to be selective for the N-methyl-D-aspartate receptor, while glutamylaminomethylsulphonate was selective for the kainate and quisqualate receptors at similar concentrations. At these concentrations, intrathecal administration of 2-amino-5-phosphonovalerate to awake mice produced significant analgesia on a battery of tests, as well as a dose-related motor impairment, while glutamylaminomethylsulphonate was without effect. Proof that 2-amino-5-phosphonovalerate exerts its effects via N-methyl-D-aspartate receptors is that glutamylaminomethylsulphonate, at concentrations which also block this receptor (greater than 1 mM), also produced analgesia and motor effects. Furthermore, N-methyl-D-aspartate, but not kainate or quisqualate, reversed the analgesic effects of 2-amino-5-phosphonovalerate. In fact, significant potentiation of analgesia could be seen with quisqualate. In accordance with the behavioral pharmacological data, topical application of 2-amino-5-phosphonovalerate onto the spinal cord of anesthetized mice significantly depressed the response of spinal sensory neurons to noxious mechanical and electrical stimulation, but did not affect the activity of neurons which showed no preferential reaction to noxious stimulation. Glutamylaminomethylsulphonate at non-analgesic concentrations was without effect. Based on these and other studies we conclude that N-methyl-D-aspartate receptor bearing interneurons participate in nociception, and that N-methyl-D-aspartate antagonists exert their analgesic and motor effect by changing the tone of spinal neural action in the spinal cord, rather than direct intervention in primary afferent transmission.

Clip-induced analgesia: noxious neck pinch suppresses spinal and mesencephalic neural responses to noxious peripheral stimulation.

Pinch of the nape of the neck, of mice, with a serrated clip, produces immobility and lack of responsiveness to noxious stimulation. In this study we attempted to determine whether clip application produces true blockade of nociception, independent of its immobilizing effect, and examined the level of the neuroaxis at which such an effect takes place. To this end nociception was measured using indices not requiring a motor response. Neck pinch eliminated the elevation of heart rate induced by noxious pinch of the tail without affecting heart rate by itself providing evidence for its analgesic effect. Direct evidence that neck pinch suppresses the transmission of noxious information is also provided. Neck pinch inhibits neural activity evoked by noxious peripheral stimulation while exerting minimal effects on the effects of nonnoxious stimuli. Thus, sensory evoked activity in the periaqueductal gray area, elicited by noxious electrical stimulation, but not innocuous stimuli, is inhibited by neck pinch. Similarly, neck pinch inhibits the response of spinal cord neurons to noxious but not nonnoxious stimulation. It, therefore, appears that neck pinch produces true analgesia by activating supraspinal systems which in turn acts to inhibit the transmission of nociception both at spinal and supraspinal levels.

Analgesic and hyperalgesic effects of midazolam: dependence on route of administration.

The effects of intraperitoneal (IP) and lumbar intrathecal (IT) midazolam (MID) on nociception was studied in 38 male albino rats using the noxious tale-flick and hot-plate tests. Four groups received IP 0.1, 1, and 10 mg/kg MID or an equal volume of its vehicle benzyl alcohol 0.1 mg in 1 ml saline, while the other three groups received IT 10 and 100 micrograms MID or 0.5 microgram benzyl alcohol in 5 microliter saline. The two higher doses of IP MID produced statistically significant decrease of tale-flick latencies (P less than 0.005 and 0.05 at 10 and 100 mg/kg MID, respectively). This hyperalgesic effect could be seen, although the animals appeared highly sedated with reduced motor activity and relatively unresponsive to non-noxious stimuli. In contrast, IT injections of 10 micrograms MID produced moderate but statistically significant prolongation of tail-flick latencies (P less than 0.05) without noticeable change in motor activity. This analgesic effect could not be observed with the higher dose of IT MID until an hour after its administration. The almost complete immobility and ataxia seen after the high doses of IP and IT MID (animals lying on their sides) precluded reliable hot plate testing in these animals. Apparently part of the high IT dose of MID diffused into the brain, as observed after high-dose IP administration. We therefore propose that the analgesic effect of midazolam stems from its action at the spinal level, while its sedative and hyperalgesic effects are a function of its supraspinal action.

Studies of peptidergic input to the lateral spinal nucleus.

Lesions were made to interrupt potential sources of peptidergic input to the lateral spinal nucleus (LSn) in rats. Rhizotomies and spinal transections, as well as lesions of the lateral funiculus, failed to reduce immunohistochemical staining for substance P, dynorphin, Met-enkephalin, somatostatin and FMRF-amide in the LSn at lumbar levels. Thus, all examined peptidergic afferent input to the LSn appears to originate locally within the spinal cord.

Behavioral classification of excitatory amino acid receptors in mouse spinal cord.

Intrathecal injections of excitatory amino acid (EAA) agonists to the spinal cord of mice produces behavioral activation manifest as biting and scratching of the hindquarters. The dose-response relationship of EAA (N-methyl-D-aspartate (NMDA), kainate, quisqualate and glutamate)-induced activation revealed qualitative and quantitative differences in their pattern of action, suggesting that these agonists act at distinct receptors. Evaluation of the blockade of EAA-induced bites by a series of antagonists: DL-2-amino-5-phosphonovalerate (APV), gamma-D-glutamyl glycine (DGG), kynurenate and glutamylaminomethylsulphonate (GAMS), indicated that selective activation of the NMDA, quisqualate and kainate receptors can be demonstrated using this behavior. The NMDA receptors could be subdivided on the basis of different sensitivity to kynurenate and APV. Antagonist-resistant components of both kainate and quisqualate action were also shown. Thus, the biting behavior induced by the administration of intrathecal EAA agonists can be used as a relatively selective behavioral tool for assessing the pharmacological profile of action of excitatory amino acid agonists and antagonists in the spinal cord.

Different endogenous analgesia systems are activated by noxious stimulation of different body regions.

Noxious pinch of the neck and the base of the tail can produce equipotent analgesia as measured by the tail flick method. However, noxious stimulation of the neck can suppress pain responsiveness both at the site of stimulation and at sites remote from the stimulated area while noxious stimulation of the tail produces analgesia only at sites remote from the stimulated area. Thus, neck pinched animals are immobile and completely unresponsive to the noxious pinch whereas pinch to the base of the tail, which results in tail flick suppression, causes vocalization and well organized biting behavior directed at the pinched area. The analgesia elicited by noxious stimulation applied to both body regions is eliminated by spinalization, the administration of intermediate doses of barbiturates (30 and 45 mg/kg) and transection at the midcollicular, but not more rostral, brain level. Concurrent with the elimination of the analgesic effect of noxious pinch on tail flick is the emergence of responses to noxious neck pinch with vocalization and intense motor reactions now elicited by noxious stimulation of the nape of the neck. These results indicate that different analgesic systems are activated by noxious tail and neck pinch both requiring the integrity of mesencephalic structures for their normal function. Furthermore, these systems can be distinguished by their ability to produce recurrent, inhibitory, supraspinal effects on nociceptive information originating at different body regions.

Is substance P a primary afferent neurotransmitter for nociceptive input? I. Analysis of pain-related behaviors resulting from intrathecal administration of substance P and 6 excitatory compounds.

Intrathecal (i.t.) injection of substance P (SP), capsaicin, kainic acid, picrotoxin, strychnine, morphine, and L-glutamic acid in rats induced rhythmic scratching movements with the hindlimbs, biting, and, with some of these compounds, vocalization and myoclonic twitches. Although biting was directed to the dermatome corresponding to the injection site, scratching was aimed at anterior dermatomes. Presumably painful chemical stimulation produced by cutaneous and subcutaneous application of capsaicin or acetic acid never elicited scratching. Vocalization was never elicited by SP. When vocalization occurred following i.t. picrotoxin and morphine, it was correlated with myoclonic twitches rather than with scratching and/or biting. These findings indicate that scratching (a) is not pain-related and, (b) when elicited by the i.t. administration of the compounds listed above, does not result from activation of nociceptive primary afferent synapses.

Clip-induced analgesia and immobility in the mouse: pharmacological characterization.

A pinch to the nape of the neck of mice, by application of a noxious clip, produces analgesia and immobility. Because both opiate and dopaminergic systems are usually implicated in analgesia and immobility, the pharmacological profile of clip-induced effects was compared to those elicited by the dopamine antagonist haloperidol, and by morphine. In addition, the effects of a series of pharmacological agents on clip-induced effects was examined. Haloperidol, but not morphine, produced immobility similar to that seen after application of the clip to the neck. Application of clip completely inhibited righting in all tests utilized. Haloperidol inhibited righting in all tests, except for inversion from a supine position. Righting from this position could also be inhibited in mice treated with haloperidol when a mild pinch, ineffective in a naive animal, was applied. Clip-induced immobility, but not analgesia, was reversed by amphetamine. Administration of the cholinergic antagonist scopolamine, but not methylscopolamine, reversed both the analgesia and immobility. Pinch-induced analgesia was as marked as that elicited by morphine but could not be reversed by the opiate antagonist naloxone. It is proposed that pinch-induced immobility is mediated by both dopaminergic and cholinergic systems. Additional unidentified systems are also involved. Analgesia, induced by a noxious pinch, can be dissociated pharmacologically from the immobilizing effect, is non-opiate in nature, and involves activation of central cholinergic synapses.

The antihypertensive effect of ethylcholine aziridinium (AF64A), a cholinergic neurotoxin, in spontaneously hypertensive rats, following administration into the posterior hypothalamus.

The aim of this study was to ascertain whether drug-induced cholinergic hypofunction in the posterior hypothalamus would affect the development and the maintenance of hypertension in hypertensive rats. Spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats were treated with AF64A, a neurotoxin which can irreversibly inhibit cholinergic transmission in vivo. AF64A or saline was injected bilaterally into the posterior hypothalamus of rats of two age groups: normotensive one month-old rats whose blood pressure was subsequently measured at the age of three months and hypertensive three month-old rats, whose blood pressure was measured four weeks later. In both age groups there was a significant fall in mean arterial blood pressure in SHR but not WKY rats. In SHR injected at the age of one month, there was a fall of at least 15.9 mm Hg, while in the rats injected at the age of three months there was a fall of 14.3 mm Hg. Heart rate in either strain was not affected. When AF64A was injected into the anterior hypothalamus of one month-old SHR, no antihypertensive effect was observed in these rats at the age of three months. These results show that cholinergic stimulation in the posterior hypothalamus may play a role in both the development and maintenance of hypertension in SHR.