The effects of intrathecal morphine and naltrexone on autotomy in sciatic nerve sectioned rats

Ultra-low dose naloxone restores the antinociceptive effect of morphine in pertussis toxin-treated rats by reversing the coupling of mu-opioid receptors from Gs-protein to coupling to Gi-protein

Pertussis toxin (PTX) treatment results in ADP-ribosylation of Gi-protein and thus in disruption of mu-opioid receptor signal transduction and loss of the antinociceptive effect of morphine. We have previously demonstrated that pretreatment with ultra-low dose naloxone preserves the antinociceptive effect of morphine in PTX-treated rats. The present study further examined the effect of ultra-low dose naloxone on mu-opioid receptor signaling in PTX-treated rats and the underlying mechanism. Male Wistar rats implanted with an intrathecal catheter received an intrathecal injection of saline or PTX (1 microg in 5 microl of saline), then, 4 days later, were pretreated by intrathecal injection with either saline or ultra-low dose naloxone (15 ng in 5 microl of saline), followed, 30 min later, by saline or morphine (10 microg in 5 microl of saline). Four days after PTX injection, thermal hyperalgesia was observed, together with increased coupling of excitatory Gs-protein to mu-opioid receptors in the spinal cord. Ultra-low dose naloxone pretreatment preserved the antinociceptive effect of morphine, and this effect was completely blocked by the mu-opioid receptor antagonist CTOP, but not by the kappa-opioid receptor antagonist nor-BNI or the delta-opioid receptor antagonist naltrindole. Moreover, a co-immunoprecipitation study showed that ultra-low dose naloxone restored mu-opioid receptor/Gi-protein coupling and inhibited the PTX-induced mu-opioid receptor/Gs-protein coupling. In addition to the anti-neuroinflammatory effect and glutamate transporter modulation previously observed in PTX-treated rats, the re-establishment of mu-opioid receptor Gi/Go-protein coupling is involved in the restoration of the antinociceptive effect of morphine by ultra-low dose naloxone pretreatment by normalizing the balance between the excitatory and inhibitory signaling pathways. These results show that ultra-low dose naloxone preserves the antinociceptive effect of morphine, suppresses spinal neuroinflammation, and reduces PTX-elevated excitatory Gs-coupled opioid receptors in PTX-treated rats. We suggest that ultra-low dose naloxone might be clinically valuable in pain management.

Spontaneous pain following spinal nerve injury in mice

Autotomy behavior is frequently observed in rats and mice in which the nerves of the hindlimb are severed, denervating the paw. This is the neuroma model of neuropathic pain. A large body of evidence suggests that this behavior reflects the presence of spontaneous dysesthesia and pain. In contrast, autotomy typically does not develop in partial nerve injury pain models, leading to the belief that these animals develop hypersensibility to applied stimuli (allodynia and hyperalgesia), but not spontaneous pain. We have modified the widely used Chung (spinal nerve ligation [SNL]) model of neuropathic pain in a way that retains the fundamental neural lesion, but eliminates nociceptive sensory cover of the paw. These animals performed autotomy. Moreover, the heritable across strains predisposition to spontaneous pain behavior in this new proximal denervation model (SNN) was highly correlated with pain phenotype in the neuroma model suggesting that the pain mechanism in the two models is the same. Relative reproducibility of strain predispositions across laboratories was verified. These data indicate that the neural substrate for spontaneous pain is present in the Chung-SNL model, and perhaps in the other partial nerve injury models as well, but that spontaneous pain is not expressed as autotomy in these models because there is protective nociceptive sensory cover.

The significance of intrathecal catheter location in rats

Although chronic intrathecal catheterization is a widely used method in rats, few calibration experiments have been performed. In this study, we investigated the correlation between the side position of the catheter tip and the side differences observed in the motor and sensory disturbances after intrathecal administration of lidocaine to a large number of rats. The existence of a sensory block was determined by the paw withdrawal test. The motor impairment was assessed by observing the complete clubbing of the hindpaw and measuring the hindpaw grip strength. After experimental use, we established the position of the catheter tip. The catheter tips were variously located in all directions of the transverse plane in the rat spinal subarachnoid space. Lidocaine administration (100 or 500 microg/5 microL; n = 264 and 112, respectively) led to dose-dependent motor and sensory disturbances. The effect of 100 microg of lidocaine exhibited side differences; i.e., the extents of both motor (r = 0.77) and sensory (r = 0.60 and r = 0.67 for the right and the left side, respectively) disturbances correlated significantly with the location of the catheter tip. Our data have shown that detection of the paralytic and/or antinociceptive effect of small-dose lidocaine before planned experiments is a simple and reliable method for prediction of the location of the catheter tip. We suggest that the position of the catheter might cause side differences in the drug effect, especially if small doses of drugs are administered and their effects are investigated on both sides.

Mapping a gene for neuropathic pain-related behavior following peripheral neurectomy in the mouse

Total hindpaw denervation in rodents elicits an abnormal behavior of licking, scratching and self-injury of the anesthetic limb ("autotomy"). Since the same denervation produces phantom limb pain and anesthesia dolorosa in humans, autotomy has been used as a model of human neuropathic pain. Autotomy is an inherited trait in rodents, attributable to a few genes of major effect. Here we used recombinant inbred (RI) mouse lines of the AXB-BXA RI set to map a gene for autotomy. Autotomy levels following unilateral sciatic and saphenous nerve section were scored daily for 36 days, using a standardized scale, in all 23 RI lines available for this set. We used a genetic map of 395 marker loci and a permutation-based statistical method for categorical data to assess the statistical significance of mapping results. We identified a marker on chromosome 15 with statistical support (P=0.0003) in the range considered significant for genome-wide scans in the mouse. Several genes located in this chromosomal region encode for neural functions related to neuropathic pain and may indicate targets for development of novel analgesics.

Morphine-induced in vivo release of spinal cholecystokinin is mediated by delta-opioid receptors--effect of peripheral axotomy

Morphine and other opioid agonists induce spinal in vivo release of cholecystokinin (CCK), a neuropeptide with anti-opioid properties. However, so far the opioid receptor subtype responsible for this effect has not been determined. In the present in vivo microdialysis study, the morphine-induced release of cholecystokinin-like immunoreactivity (CCK-LI) in the dorsal horn was completely blocked by the delta-opioid antagonist naltrindole (10 microM in the perfusion fluid). Neither the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP; 10 microM in the perfusion fluid), nor the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI); 10 microM in the perfusion fluid) had any significant effect in this respect. In addition, systemic administration of the delta-opioid receptor agonist BW373U86 (1 mg/kg, s.c.) and spinal administration of the delta(2)-opioid receptor agonist, Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide ([D-Ala(2)] deltorphin II) (1 microM in the perfusion fluid) induced a significant increase of the CCK-LI level. The effect of BW373U86 on spinal CCK-LI release was completely blocked by spinal administration of naltrindole. The mu-opioid receptor agonist [D-ala(2)-N-Me-Phe(4)-Gly(5)-ol]-enkephalin (DAMGO) (1 microM in the perfusion fluid or 1 mg/kg, s.c.) failed to alter the CCK-LI level. Peripheral nerve lesions have previously been shown to down-regulate mu- and delta-opioid receptors in the dorsal horn, to increase the gene-expression of CCK and CCK-receptor mRNA in dorsal root ganglion neurons and to alter the potassium-induced spinal CCK-LI release. After complete sciatic nerve transection, administration of the two selective delta-opioid receptor agonists induced a significant release of CCK-LI, which was comparable to controls. In contrast, neither systemic nor spinal administration of morphine and DAMGO altered the spinal CCK-LI release in axotomized animals. The present data indicate that the delta-opioid receptor mediates morphine-induced CCK-LI release in the spinal cord.

Distribution of intrathecal catheter positions in rat

BACKGROUND

Although drug administration through an intrathecal catheter is widely used in the study of spinal pharmacology, the catheter positions in transverse plane that may cause a limited spread of a solution remain unclear. In the first step to clarify this issue, the distribution of the intrathecal catheter position was investigated in rats.

METHODS

A polyethylene catheter (PE-10) was inserted intrathecally 8.5 cm through the atlanto-occipital membrane, aiming the tip of the catheter to the dorsal surface of the spinal cord. Three or four weeks after the implantation of the catheter, 83 rats were transcardiacally perfused with fixative solution. The catheter positions were investigated longitudinally and transversely by cutting the spinal cord segmentally through the intervertebral disk at different spinal levels.

RESULTS

Seventeen rats were excluded from further data analysis. Transversely, catheters were located in the left lateral subarachnoid space in 23 rats (35%), in the right lateral in 15 (23%), in the dorsal in 22 (33%), and in the ventral in 6 (9%). Longitudinally, catheter position was significantly higher in the ventral group (median, T9) than that in the dorsal group (T11/12) (P<0.01).

CONCLUSION

Chronic intrathecal catheters were variously located in the rat spinal subarachnoid space in the transverse plane, and lateral subarachnoid placement of the catheter (58%) was frequently observed, whereas dorsal subarachnoid placement occurred in 33%.

Effects of estrogen on autotomy in normal and ovariectomized rats

Gonadal hormones may modulate analgesia responses induced by acute stress in humans and rats. To evaluate the effects of gonadal hormones in modifying neuropathic pain, we measured autotomy changes following sciatic nerve resection in ovariectomized rats and in the presence of estrogen replacement. Two groups of female rats were subjected to ovariectomy and sham surgery. Each group was then divided into two subgroups receiving subcutaneously sesame oil with or without estradiol benzoate (5 microg/day/rat). All rats then underwent sciatic nerve resection in one hindlimb. Degree of self-mutilation was measured daily for 8 weeks. Estradiol treatment resulted in significantly lower autotomy scores in ovariectomized rats (3.6 +/- 0.6 vs. 5.5 +/- 0.3, p < 0.01) and in sham-operated rats (3.4 +/- 0.7 vs. 5.1 +/- 0.4, p < 0.05). The results of this study indicate that estrogen can modify the autotomy behavior, an indicator of neuropathic pain, in rats after nerve injury.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

Effect of peripheral axotomy on dorsal root ganglion neuron phenotype and autonomy behaviour in neuropeptide Y-deficient mice

The lumbar 5 (L5) dorsal root ganglia (DRGs) were studied in neuropeptide tyrosine (NPY)-deficient (-/-) and wild type (+/+) mice after unilateral sciatic nerve transection using in situ hybridization and immunohistochemistry. NPY, galanin and two NPY receptors (Y-Rs) were analyzed as well as self-mutilation behaviour (autotomy) and nociceptive thresholds. No difference between wild type and NPY-deficient mice was seen in the tail-flick or hot plate test. However, -/- mice showed a much stronger autotomy behaviour than wild type mice. NPY was not found in L5 DRGs in -/- mice, not even after axotomy. Galanin was upregulated to the same extent after axotomy in NPY-deficient and wild type mice. Y1- and Y2-R mRNAs were found mainly in small DRG neuron profiles. Both receptor mRNAs were downregulated after axotomy, to about the same extent in NPY-deficient as in wild type mice. In control and contralateral ganglia the mRNA levels of both receptors were lower in NPY-deficient mice than in wild type mice. The contralateral Y2-R mRNA levels did not reach control values in the NPY-deficient mice, as they did in the wild type mice. In both strains the Y1-R protein was decorating the somatic plasmalemma. The present results suggest that lack of NPY may cause exaggerated autotomy, a self-mutilation behaviour possibly related to pain sensation, in agreement with the described analgesic effect of NPY. Although significant differences in levels of Y1- and especially Y2-R mRNAs were observed between wild type and NPY-deficient mice, they were only moderate. These findings suggest that expression, regulation, localization and possible function of Y1- and Y2-Rs are not dependent on presence of the endogenous ligand. Also, deletion of NPY does not seem to influence the expression of the partly coexisting peptide galanin.

Systemic morphine in the prevention of allodynia in the rat spinal nerve ligation model of neuropathic pain

Peripheral nerve injury may lead to neuropathic pain that has been considered unresponsive to opioids. In animal models of neuropathic pain, there are previous data of both increased and decreased effect of opioids, but only limited information of the long-term effects of opioid treatment on the development of the symptoms of neuropathy. The possibility of preventing the development of signs of neuropathy with either a single pre-injury injection or chronic postinjury administration of morphine was studied in rats with unilateral peripheral neuropathy due to tight ligation of the L5 and L6 spinal nerves. These rats developed both mechanical and cold allodynia, but no thermal hyperalgesia. Neither subcutaneous morphine given as single injection (10 mg/kg) before the nerve injury nor chronic administration starting immediately after surgery using slow-release pellets (one, two or five pellets containing 75 mg of morphine, resulting in a total dose of 75, 150 or 375 mg) prevented the development of either mechanical or cold allodynia. No autotomy, signs of distress, altered social behaviour or morphine withdrawal was seen in any of the rats. The fact that neuropathic pain-like symptoms were not attenuated by any of the treatments studied could indicate that neither premedication nor postoperative pain management with systemic morphine is effective in preventing postoperative neuropathic pain. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.

Interferon-gamma receptors in nociceptive pathways: role in neuropathic pain-related behaviour

Interferon-gamma receptor (IFN-gamma R) immunoreactivity was observed in the superficial dorsal horn and lateral spinal nucleus in rat and mouse spinal cord. Dorsal rhizotomies did not reduce immunoreactivity in the rat. IFN-gamma R distribution overlapped with nitric oxide synthase-1 immunoreactivity. In wild-type mice, intrathecal injections of mouse IFN-gamma evoked biting behaviour, whereas mice with disruption of the functional gene for IFN-gamma R did not respond. Both types of mice had similar withdrawal thresholds to mechanical stimulation and reacted similarly to foot-pad carrageenan injections. In contrast to wild-type mice, IFN-gamma R knock-out mice did not show autotomy after sciatic nerve section. This study demonstrates a functional IFN-gamma R in spinal nociceptive pathways related to neuropathic pain.

Pharmacology and mechanisms of opioid analgesic activity

Opiates by an action at specific receptors can induce a highly selective alteration in the response of humans and animals to strong and otherwise aversive chemical, mechanical or thermal stimuli. Specific investigations in a variety of species from rodent to primate using microinjection techniques to examine the pharmacology of local drug action have shown potent antinociceptive actions to be mediated by a receptor specific action at a number of sites within the brain, including the periaqueductal gray (PAG: mu receptor), the rostral ventral medulla (mu/delta receptor) and the substantia nigra (mu receptor) and within the spinal dorsal horn (mu/delta/kappa receptor). Mechanistic studies have shown these actions in the different sites to be mediated by several discrete mechanisms. For example, in the PAG, the local opiate effect is likely mediated by the indirect activation of bulbospinal pathways, rostral projections to forebrain sites and by a local alteration in afferent input into the brainstem core. In the spinal cord, this effect is mediated by an action presynaptic to the primary afferent and by a post-synaptic effect to hyperpolarize projection neurons. In addition, it is now appreciated that mu and kappa receptors in the periphery can modulate the sensitized state of the small afferent terminal innervating inflamed tissue and exert an anti-hyperalgesic action. After systemic delivery of an opiate, it is thus clear that a wide array of central and peripheral systems serve to explain the powerful analgesic effect exerted by this class of agents.

aFGF, bFGF and NGF differentially regulate neuropeptide expression in dorsal root ganglia after axotomy and induce autotomy

Using immunohistochemistry and in situ hybridization the in vivo effects of acidic and basic fibroblast growth factor (aFGF, bFGF), and of nerve growth factor (NGF) on the expression of galanin, neuropeptide Y (NPY) and substance P in axotomized dorsal root ganglia (DRGs) were examined. Self-mutilation (autotomy), a supposed pain-related behavior, was investigated after growth factor treatment. One microgram of aFGF, bFGF or NGF was applied directly to the transected sciatic nerve via a capsule. In normal rats 3.2%, 0% and 17.5% of the neuron profiles in the DRGs contained galanin-, NPY- and substance P-like immunoreactivity (LI), respectively. Sciatic nerve transection induced a distinct increase in galanin- and NPY-LIs, but a downregulation of substance P-LI. Thus three days after axotomy 23.5%, 26.9% and 9.8% of the DRG neuron profiles showed immunoreactivity for galanin-, NPY- and substance P-LI, respectively. In vivo administration of aFGF counteracted the axotomy-induced increase in galanin and NPY, whereas bFGF only suppressed NPY upregulation. NGF reversed in the injury-induced decrease in substance P-LI, but had no significant effect on galanin- and NPY-LIs. These results were confirmed by monitoring the mRNA levels for these neuropeptides. Moreover, aFGF was found to induce autotomy in 60% of the rats 3 days after axotomy. NGF produced autotomy in about 30% of the rats. Taken together, the present results suggest (1) that aFGF, bFGF and NGF differentially regulate neuropeptide expression in vivo; (2) that FGFs can inhibit neuropeptide upregulation of some peptides after nerve injury; and (3) that aFGF and NGF may induce pain-related behavior.

Effects of cerebral cortical and striatal lesions on autotomy following peripheral neurectomy in rats

Peripheral nerve lesions have been reported to produce deafferentation pain and persistent changes at various levels of the central nervous system. Using autotomy in rats following leg denervation, as a model for deafferentation pain, we studied the effect of various cerebral lesions on this abnormal behavior. Under deep anesthesia, rats were subjected either to massive hemidecortication or to subtotal hemispherectomy (involving parts of basal ganglia and limbic areas), which was followed 1 week later by a denervation of the ipsilateral or contralateral leg. Hemidecortication significantly delayed autotomy from 7.8 +/- 2.8 to 25.6 +/- 2.1 days without reducing its incidence, whereas hemispherectomy abolished the incidence of autotomy in the contralaterally denervated leg and delayed its onset from 7.8 +/- 2.8 to 34 +/- 6.1 days and decreased its incidence (from 100% to 60%) in the ipsilaterally denervated leg. Chemical lesions of the neostriatum produced similar effects on autotomy to those produced by hemispherectomy. Hemispherectomized and striatum-lesioned rats that failed to elicit autotomy in the contralaterally denervated leg were subjected after 7 weeks to denervation of the ipsilateral leg. Sixty to seventy percent of these rats showed autotomy and in half of them the attack was directed simultaneously to both legs. These results suggest an involvement of the cerebral cortex and a stronger contribution of subcortical structures (like striatum and limbic system) in the processing of nociceptive information.

Sciatic nerve section induces mechanical hyperalgesia in skin adjacent to the deafferented region in rats: lack of correlation with autotomy behavior

We have studied the development of cutaneous hypersensitivity in the innervation area of the saphenous nerve and autotomy behavior in rats after unilateral sciatic nerve section. Hypersensitivity was assessed by stimulating the saphenous area with mechanical (von Frey hairs and analgesimeter), cold (immersion) or heat (immersion or radiant heat) stimuli 10 days after sciatic nerve section. We did not observe any hypersensitivity to thermal stimulation or weak mechanical stimulation produced by von Frey hairs. A significant decrease in withdrawal threshold to intense mechanical pressure produced by analgesimeter was observed, indicating the possible presence of mechanical hyperalgesia. Eleven of 17 rats had started autotomy at this time, but there was no correlation between the presence of autotomy and mechanical hyperalgesia. It is concluded that in our experimental setting, only mechanical hyperalgesia was present 10 days after axotomy. Moreover, autotomy behavior and adjacent hyperalgesia may be triggered by different mechanisms.

Inhibition by spinal morphine of the tail-flick response is attenuated in rats with nerve ligation injury

Nerve ligation injury in rats produces increased sensitivity and exaggerated responses to nociceptive stimuli (hyperalgesia) as well as nociceptive responses to normally innocuous stimuli (allodynia) analogous to clinical conditions of neuropathic pain. However, the effect of nerve injury on acute nociception has not been extensively studied. Nerve ligation injury was produced by unilateral ligation of the L5 and L6 spinal roots of the sciatic nerve of male Sprague-Dawley rats. Intrathecal (i.th.) catheters were inserted for spinal drug administration. Response to acute nociception was measured by determining the latency to a rapid flick of the tail (TF) after immersion into a 55 degrees C water bath before (control) and after i.th. morphine administration. No change in baseline response to the nociceptive stimulus was observed in either sham-operated or nerve-injured rats. In sham-operated rats, morphine produced dose-dependent antinociception with a 97 +/- 2.3% maximal possible effect (MPE) at a 60 microgram dose; in these controls A50 (95% CL) was 22 micrograms (17-30 micrograms). Morphine administered to rats with nerve injury also produced dose-dependent increase in TF latency, but an MPE of only 60 +/- 17% was obtained at 100 micrograms; higher doses elicited signs of behavioral toxicity. While it was not possible to produce a proper dose-response curve with i.th. morphine in animals with nerve injury, an estimation of the A50 showed approximately a four-fold loss of potency compared to sham-operated controls. Antinociception was readily reversed by naloxone (5 mg/kg, i.p.) in both groups. These data indicate that nerve ligation injury reduces the potency and efficacy of i.th. morphine. While the reasons for this loss of morphine activity in nerve injured animals are unknown, it is possible to speculate that (a) degeneration of primary afferents subsequent to nerve ligation injury might result in a loss of presynaptic opioid (mu?) receptors in the dorsal horn, thereby reducing the antinociceptive activity of morphine at the spinal level; (b) changes in the efficiency of post-receptor transduction may occur following nerve injury which can reduce opioid efficacy; (c) changes in levels of spinal neurotransmitters (e.g., cholecystokinin) may act to diminish opioid action; or (d) sustained afferent input from the site of the injury may be important in limiting the activity of opioids.

Up-regulation of cholecystokinin in primary sensory neurons is associated with morphine insensitivity in experimental neuropathic pain in the rat

We examined the distribution of mRNA for the peptide cholecystokinin (CCK) with in situ hybridization in adult rat lumbar dorsal root ganglia following unilateral section of the sciatic nerve, as well as the effect of systemic CI 988, a selective antagonist of the CCK type B receptor, applied alone or in combination with intrathecal (i.t.) morphine, on the self-mutilating behavior of rats (autotomy) after axotomy, a sign of neuropathic pain and/or dysesthesia. There was a dramatic increase in the number of neurons in dorsal root ganglia synthesizing the peptide cholecystokinin (CCK) after sciatic nerve section. Furthermore, the autotomy behavior of rats was significantly inhibited by chronic i.t. administration of morphine in conjunction with subcutaneous (s.c.) injection of CI 988. Neither i.t. morphine nor s.c. CI 988 alone produced a comparable effect on autotomy. Our results suggested that up-regulation of the mRNA for CCK in primary afferents after nerve injury may be related to the clinical phenomenon of opioid insensitivity. Thus, coadministration of CCK antagonists in combination with opioids may offer a new approach in treating neuropathic pain.

Regulation of opioid binding sites in the superficial dorsal horn of the rat spinal cord following loose ligation of the sciatic nerve: comparison with sciatic nerve section and lumbar dorsal rhizotomy

The aim of the present study was to quantify time-related modifications in mu and delta opioid binding sites in the superficial layers (laminae I and II) of the L4 lumbar segment in a rat model of mononeuropathy induced by loose ligation of the sciatic nerve. We have shown a 28% (P < 0.01) and 24% (P < 0.01) decrease in ipsi/contralateral side binding ratios for tritiated (Tyr*-D-Ala-Gly-NMe-Phe-Gly-ol) ([3H]DAMGO) and tritiated (Tyr*-D-Thr-Gly-Phe-Leu-Thr) ([3H]DTLET) respectively, at two weeks postlesion which correspond to the delay of maximal hyperalgesia and of maximal alteration of fine diameter primary afferent fibers. In contrast, no change in [3H]U.69593 specific binding could be detected at this postlesion delay. For longer survival delays (four, eight and 15 weeks postlesion), mu and delta binding ratios return towards control values (approximately equal to 1), probably reflecting the occurrence of a long-term neuroplasticity (i.e. a new equilibrium in the metabolism of primary neurons, or collateral sprouting from intact primary afferents) following loose nerve ligation. In addition, a comparison of the results obtained in this model with those measured after sciatic nerve section and lumbar dorsal rhizotomy was performed in order to compare the degree of loss in opioid binding sites in these three types of lesion. The section of the sciatic nerve induced at eight days postlesion an 18% (P < 0.01) and 28% (P < 0.01) decrease in binding ratio for [3H]DAMGO and [3H]DTLET, respectively. At two weeks postlesion the loss was 24% (P < 0.01) for the two ligands, and at longer delays (four and 12 weeks), a progressive recovery in binding ratio was observed. Thus, it appears that both sciatic nerve lesions we have studied result in mu and delta binding modifications which have similar intensity and similar time course from two to 12-15 weeks postlesion. In contrast, the unilateral rhizotomy of nine consecutive dorsal roots (T13-S2), which is known to induce a massive degeneration of fine diameter primary afferent fibers, is followed by a dramatic decrease in binding ratios for [3H]DAMGO (53%, P < 0.001) and [3H]DTLET (45%, P < 0.001) at two weeks postlesion. These data suggest that the more deprived the dorsal horn is of fine diameter primary afferent fibers, the more dramatic is the opioid binding loss in the ipsilateral side as compared to the contralateral side.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracerebroventricular salmon calcitonin reduces autotomy behavior in rats after dorsal rhizotomy

We studied the effects of intracerebroventricularly (ICV) administered salmon calcitonin (sCT) on the self-mutilation behavior (autotomy) of rats after dorsal rhizotomy. sCT was given 2 micrograms/rat ICV, on alternate days) from day 15 to day 40 after surgery and autotomy was scored until day 60. Autotomy scores were significantly reduced during treatment but once the ICV sCT was stopped mean values for the treated group gradually returned to control levels. These data support the concept that autotomy is a useful model of chronic pain, sensitive to centrally acting antinociceptive agents. In addition, these findings extend the knowledge of the antinociceptive profile of sCT and are in agreement with recent clinical observations.

Intrathecal administration of clonidine suppresses autotomy, a behavioral sign of chronic pain in rats after sciatic nerve section

The effect of chronic intrathecal (i.t.) administration of the alpha 2 adrenoceptor agonist clonidine on self-mutilation behavior (autotomy) after unilateral transection of the sciatic nerve was studied in rats. Rats injected with 10 micrograms clonidine twice daily for 21 days, starting 1 h before nerve section, autotomized significantly less than saline controls. However, in the few rats which received clonidine and autotomized, clonidine did not delay the onset of autotomy compared to saline controls. Termination of clonidine injection after 21 days resulted in increased autotomy within 1-4 days. No sedation or motor impairment was found after this dose of i.t. clonidine. It is suggested that clonidine may be useful in treating neuropathic pain in humans.

The effects of oral morphine and buprenorphine on autotomy following brachial nerve sections in rat

This study assessed the relationship between autotomy and opioid systems following brachial nerve sections in the rat. Morphine, buprenorphine and/or naloxone were self-administered orally to rats following nerve sections. Oral morphine and buprenorphine increased the severity of autotomy. Naloxone alone had no effect, but reversed oral morphine effect on autotomy. These results suggest that mu-receptor activation by morphine and buprenorphine can increase the severity of autotomy.

Behavioural evidence that systemic morphine may modulate a phasic pain-related behaviour in a rat model of peripheral mononeuropathy

In a model of peripheral mononeuropathy produced by 4 ligatures around the sciatic nerve, we investigated the effects of various i.v. doses of morphine on the vocalization thresholds elicited by paw pressure and compared the effects obtained with the same doses in normal rats. In neuropathic rats, morphine (0.1 and 0.3 mg/kg) produced a significant analgesic effect on the lesioned hind paw, maximum at 15 min post injection with a recovery at 20-25 min. For doses of 0.6 and 1 mg/kg, a modification of the kinetics was observed, with maximum effect at 20-30 min post injection and a recovery at 50-80 min. An analgesic effect was also observed on the unlesioned side, significantly less potent than that observed on the lesioned paw. The effect of 1 mg/kg morphine was almost totally reversed by a 0.1 mg/kg dose of systemic naloxone. The effects induced by the successive doses of morphine on the lesioned paw appeared higher than in normal rats (maximum vocalization thresholds (% of control) following 1 mg/kg morphine (N = 12) were 193.92 +/- 6.57% versus 154 +/- 3.5% in normal rats N = 3), whereas they were comparable to those obtained from the sham-operated paw. The present data clearly show that morphine induces potent antinociceptive effects in a rat model of neuropathy, which seems to contradict the classical view that neuropathic pain is opioid resistant. Some possible pathophysiological mechanisms are discussed.

The threshold for the depressive effect of intrathecal morphine on the spinal nociceptive flexor reflex is increased during autotomy after sciatic nerve section in rats

The effect of intrathecal (i.t.) morphine on the spinal nociceptive flexor reflex in doses ranging between 10 ng and 10 micrograms was studied in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves or in rats in which the sciatic nerve had been unilaterally sectioned. In rats with intact nerves the initial effect of i.t. morphine on the flexor reflex was a brief facilitation followed by depression. The threshold dose of morphine for reflex depression was 100 ng. In animals which did not develop autotomy after nerve section or in which autotomy had ceased for several days prior to the acute experiments, i.t. morphine had a similar depressive effect on the flexor reflex as in animals with intact nerves. However, in rats which were autotomizing at the time of the acute experiment, the threshold dose of the depressive effect of morphine was increased 3-5 fold. With higher doses of morphine (1-3 micrograms), similar depression of the reflex was found in all groups. The present results revealed a decreased sensitivity of spinal reflex mechanisms to low, but not high, doses of morphine after sciatic nerve section accompanied by autotomy. Nerve section per se did not alter opioid sensitivity. Thus, decreased effectiveness of morphine in this model for neuropathic pain may be partially due to a desensitization to the analgesic action of opioids in the spinal cord. Since after sciatic nerve section there is a differential sensitivity to the antinociceptive effect of i.t. morphine between autotomizing and non-autotomizing rats, it is further suggested that autotomy after peripheral nerve section in rats is a useful model for the study of neuropathic pain.

Electrophysiological evidence that morphine can exert an antinociceptive effect in a neuropathic state: a study in the ventrobasal thalamus of rats after moderate ligation of one sciatic nerve

This study was performed in rats with a mononeuropathy induced by loose ligatures around the common sciatic nerve 2 weeks before the recording session, and exhibiting clear alterations of several pain-related behaviours. Morphine injected intravenously (0.6 and 1 mg/kg) strongly depressed the ventrobasal thalamic neuronal responses to pinch applied to the lesioned or the non-lesioned hindpaw. The effect, comparable on the both sides, was dose-related and reversed by naloxone (0.1 mg/kg i.v.).

Modifications of social conflict-induced analgesic and activity responses in male mice receiving chronic opioid agonist and antagonist treatments

This study examined the effects of chronic (7 day) administrations of opioid agonists, via osmotic minipumps (20 micrograms/microliters/h, or 2 mg/kg/h for each agent) on: 1) nociception and activity, and 2) the analgesic and locomotor responses of subordinate male mice experiencing social conflict (aggression without defeat) and defeat in a "resident-intruder" paradigm. Chronic infusion of the mu opioid antagonist, naltrexone, resulted in a hypoanalgesic response and a decrease in basal locomotor activity on days 3-7 postimplantation which returned to the basal levels of saline-implanted control mice after termination of the infusions on day 9. Naltrexone reduced defeat-induced analgesia on the second day after implantation, but had no consistent effects on analgesia on test days 6 and 9 or on the aggression-induced (nondefeat) analgesia and increases in activity. The delta opioid antagonist ICI-154, 129, while having no significant effects on basal nociception or locomotor activity, augmented nondefeat-induced analgesia (day 2) and reduced the defeat-induced increases in activity (days 2 and 6). The mu agonist, levorphanol, resulted in a significant analgesia on the first two days after infusion, followed by the development of tolerance to the analgesic effects over days 3-7. On day 9, a hypoanalgesic response indicative of withdrawal was evident. Levorphanol also induced a marked decrease in locomotor activity over days 3-7 postimplantation, with no evidence of the development of tolerance or withdrawal following termination of infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

The 'tonic' pain-related behaviour seen in mononeuropathic rats is modulated by morphine and naloxone

This study investigated the sensitivity to pharmacological manipulations of a rating method, adapted from the formalin test, to measure the tonic component of the pain-related behaviour induced by creating a peripheral mononeuropathy with 4 loose ligatures around the common sciatic nerve. Although the adequacy of opioid substances in alleviating neuropathic pain is highly controversial, the effects of morphine (1 mg/kg i.v.) and naloxone (1 mg/and 3 micrograms/kg i.v.) were tested 1-2 weeks after the nerve ligatures were established, when pain-related behaviours were well developed. Morphine (1 mg/kg i.v.) induced a potent and prolonged decrease in the pain-rating score at week 2 after surgery. Either at week 1 or week 2, naloxone elicited a bidirectional dose-dependent action: a further increase in the pain-rating score with the high dose (1 mg/kg i.v.), and a paradoxical decrease in the score with the low dose of 3 micrograms/kg i.v. These effects are comparable to those already described in several rat models of inflammatory pain and, in the same model of neuropathy, using a phasic nociceptive test, the measure of the vocalization to paw pressure. A few differences in the effects of naloxone on tonic and phasic pain are noted and discussed.

The nature of opioid responsiveness and its implications for neuropathic pain: new hypotheses derived from studies of opioid infusions

In recent years, the observation that the response of patients to opioid drugs may be influenced by properties inherent in the pain or pain syndrome, such as its pathophysiology, has evolved into the belief that certain types of pain, e.g., neuropathic pains, may be unresponsive to these drugs. This concept has important implications for both clinical practice and basic understanding of opioid mechanisms. We critically evaluate opioid responsiveness, particularly as it relates to neuropathic pain, and propose a clinically relevant definition and a paradigm for its investigation. The paradigm is illustrated by analgesic responses to opioid infusion in 28 patients with neuropathic pains and by a detailed presentation of the pharmacokinetic and pharmacodynamic relationships in one of these patients, whose central pain responded promptly to an infusion of hydromorphone. From this analysis, we hypothesize that (1) opioid responsiveness in man can be defined by the degree of analgesia achieved during dose escalation to either intolerable side effects or the occurrence of 'complete' or 'adequate' analgesia; (2) opioid responsiveness is a continuum, rather than a quantal phenomenon; (3) opioid responsiveness is determined by a diverse group of patient characteristics and pain-related factors, as well as drug-selective effects; and (4) a neuropathic mechanism may reduce opioid responsiveness, but does not result in an inherent resistance to these drugs. Given the complexity of factors contributing to opioid responsiveness and the observation that outcome cannot be reliably predicted, opioids should not be withheld on the assumption that pain mechanism, or any other factor, precludes a favorable response. Both the clinical use of opioids and paradigms to investigate opioid responsiveness should include dose escalation to maximally tolerated levels and repeated monitoring of analgesia and other effects.

Subarachnoid spinal cord transplantation of adrenal medulla suppresses chronic neuropathic pain behavior in rats

Several weeks after transection of the sciatic and saphenous nerves, rats respond by self-injury of the denervated limb ('autotomy'). This behavior serves as a model of neuropathic pain. In this study we allografted fragments of rat adrenal medulla into the subarachnoid space of other rats, at lumbar spinal cord level, in an attempt to suppress autotomy behavior. The results show that autotomy was reduced by an average of 63% throughout the 8 week observation period. Catecholamine (CA) histofluorescent staining performed up to 120 days postoperatively (P0) revealed viable transplants in 75% of the rats. Transplant viability correlated with suppression of autotomy. This suggests that medullary chromaffin cells function as a local, long-lasting source of anti-nociceptive agents at the spinal segments which process input from the injured nerves.

Autotomy behavior in rats following peripheral deafferentation is suppressed by daily injections of amitriptyline, diazepam and saline

Amitriptyline, a tricyclic antidepressant (TCA), was effective in suppressing self-mutilation of a peripherally deafferented hind paw in rats ('autotomy'). This effect was not due to the drug's sedative effect, since locomotor activity was not lower in treated than untreated rats. Daily injections of normal saline also suppressed autotomy, but for a shorter period of time than amitriptyline. This effect was not apparent in diazepam-treated rats, suggesting that the saline injection delayed autotomy as a result of stress-induced anti-nociception. Since amitriptyline is effective in humans in alleviating certain chronic pain disorders, these results further corroborate the suggestion that autotomy is a model of chronic pain, sensitive to centrally acting analgesics and to some forms of stress.

Electrophysiological evidence for a tonic activity of the spinal cord intrinsic opioid systems in a chronic pain model

The aim of this electrophysiological investigation was to evaluate the activity of the spinal endogenous opioid systems in a chronic pain model, the arthritic rat. The activity of nociceptive non-specific dorsal horn neurons (n = 23) were recorded in 23 spinal unanesthetized decerebrated rats. Naloxone (1 mg/kg i.v.) induced a highly significant increase in the spontaneous firing rate of these neurons. This observation is in favor of a tonic activity of spinal opioid endogenous systems in such a disease. In addition, the same dose of naloxone facilitates the transmission of noxious messages at the spinal level as revealed by the large enhancement of the responses of these neurons to C-fiber stimulation. These results are in good agreement with behavioral data showing that such a relatively high dose of naloxone induced well-reproducible hyperalgesia and with some biochemical observations showing an increase of levels and biosynthesis of endogenous opioids in the spinal cord of the arthritic rat.

Autotomy in rats after nerve section compared with nerve degeneration following intraneural injection of Ricinus communis agglutinin I

Partial unilateral deafferentation of the hind limb of rats was carried out by section of the sciatic nerve or the intraneural injection of Ricinus communis agglutinin 1 (RCA I). The development of autotomy was observed over a 6 week period. The axotomized animals autotomized more than those injected with RCA I. A neuroma developed on the proximal stump of the axotomized nerves. Within 7 days the axons of the RCA I-injected nerve degenerated and the cell bodies in dorsal root ganglia L4 and L5 were destroyed. Since the RCA I-injected animals autotomized, it is concluded that purely central factors have a role in the generation of this abnormal behavior. As the axotomized animals autotomized more than the RCA I-treated ones it is further concluded that abnormal impulse activity arising from a neuroma may be an additional factor in causing autotomy.

Investigation of tolerance to chronic intrathecal morphine infusion in the rat

Rats received chronic subcutaneous or intrathecal infusions of either saline or 25 micrograms/microliter/hr or 50 micrograms/microliter/hr of morphine sulfate. During five days of infusion individual groups of rats were assessed on either the nociceptive tail flick or hot plate test. After the infusions, the analgesic effects of either subcutaneous or intrathecal morphine test doses were evaluated. Tolerance developed to the analgesic effect of both subcutaneous and intrathecal morphine infusions on the tail flick test. Subcutaneously infused rats were also tolerant to a subcutaneous morphine challenge on this test. However, intrathecally infused rats were not tolerant to either the subcutaneous or intrathecal challenge. In contrast to these results, rats tested on the hot plate were not analgesic in response to either subcutaneous or intrathecal morphine infusions. However, these rats were tolerant when challenged with either subcutaneous or intrathecal morphine. The results are discussed in terms of the relative contribution of spinal and supraspinal sites to opiate tolerance, and the possibility that tolerance does not develop to the antinociceptive effect of spinal morphine on spinally mediated reflexes.

Simple catheter preparation for permitting bolus intrathecal administration during chronic intrathecal infusion

This paper briefly describes a simple method for preparing a polyethylene "Y" catheter in which the stem of the "Y" is inserted into the intrathecal space, one arm of the "Y" is externalized for intrathecal injection and the other arm of the "Y" connected to an osmotic infusion pump. This simple preparation permits the chronic infusion of drug into the spinal space and without further surgical preparation, permits bolus injection of drugs through the same catheter.

Deafferentation and chronic pain in animals: an evaluation of evidence suggesting autotomy is related to pain

This paper examines evidence which suggests that the self-mutilation of deafferented limbs exhibited by laboratory animals is a response to pain or dysesthesia and is therefore an adequate model of chronic pain. Evidence from studies using physiological, pharmacological and behavioral methods provides strong support that autotomy reflects chronic pain. New evidence presented in this paper demonstrates that specific treatments can be used to manipulate the extent of autotomy, causing increases or decreases, as well as restricting it to specific parts of a denervated foot. This evidence argues that autotomy scores are an appropriate measure of the degree of pain or dysesthesia which results from the deafferentation of a limb.

Absence of ongoing activity in fibres arising from proximal nerve ends regenerating into mesothelial chambers

Morphological studies have indicated that proximal nerve ends of transected rat sciatic nerves regenerating into preformed mesothelial chambers show a different organization as compared to neuromas developed in contact with a muscle fascia. We have studied the physiological properties of nerve fibres arising from these types of preparations with reference to ongoing activity, response to mechanical stimulation and noradrenaline sensitivity. The study included also fibres arising from ligated and encapsulated neuromas. Fibres with ongoing activity arising from the neuroma could be found from neuromas in contact with a muscle fascia and also from ligated and encapsulated neuromas. This ongoing activity was enhanced by mechanical stimulation and i.v. infusion of noradrenaline. In contrast, fibres arising from proximal nerve ends in mesothelial chambers did not show ongoing activity. These silent fibres responded dynamically to light mechanical stimulation. Noradrenaline did not induce ongoing activity in these fibres.

Dysesthesias and self-mutilation in humans and subhumans: a review of clinical and experimental studies

The chronic deafferentation syndrome includes a complex pattern of abnormal self-directed behavior and a stress response. Subhuman self-mutilation is a secondary consequence of the chronic deafferentation syndrome. The evidence indicates that the chronic deafferentation syndrome in subhumans is a valid model for the induced and the spontaneous dysesthesias in humans. Objective criteria for the definition of subhuman dysesthesias have been derived from independent sources of evidence, in neurally intact subjects; those criteria are then found to match the subhuman syndrome of deafferentation. Support for the validity of the inference of subhuman dysesthesias derives from the parallels with the various facts of the human dysesthesias. The credibility of this argument is significantly strengthened by reports of morphological and excitatory physiological abnormalities, in central somatosensory structures, in response to deafferentation. There is no independent subhuman evidence in support of alternate interpretations of the deafferentation syndrome, and those interpretations seem to be inadequate in several aspects. Doubts concerning the validity of this animal model have been allayed by reports of dysesthesias in humans with spinal posterior rhizotomies or ganglionectomies, and also those with congenital analgesia. Moreover, the occurrence of this syndrome in hypoalgesic areas as a consequence of anterolateral cordotomy in monkeys, can best be interpreted as a reflection of dysesthesias. This syndrome is released by neuropathological or neurosurgical lesions in the peripheral or central nervous system; lesions which involve small caliber peripheral afferents or the spinothalamic tract. Variability in the release of this syndrome has been associated with several different factors. So far, the chronic syndrome is intractable. Evidence relates the abnormalities of this syndrome to pathophysiological foci in central relays of the somatosensory system, and suggests that the chronic abnormalities of this syndrome can be sustained at brain levels.

Centrocentral anastomosis in the prevention and treatment of painful terminal neuroma. An experimental study in the rat

In this experimental study, microsurgical centrocentral anastomosis was applied to an experimental model of painful terminal neuroma resulting from left sciatic nerve section in the rat. The anastomosis consisted of end-to-end suturing of the sciatic nerve fascicles to the tibial branch, with the interposition of a nerve graft taken from the same anastomosed fascicle. As a control parameter for the experiment, the autotomy which follows sciatic nerve section in the rat was evaluated. Autotomy is considered an objective indication of abnormal sensations that are provoked by the formation of a terminal neuroma. Histological study of the proximal stump of the sciatic nerve was also performed. The observation period was 10 weeks. The study demonstrates that centrocentral anastomosis reduces the size of the neuroma formation and the incidence of autotomy.