Effects of morphine on mechanical allodynia in a rat model of central neuropathic pain:

Intrathecal administration of nociceptin/orphanin FQ receptor agonists in rats: A strategy to relieve chemotherapy-induced neuropathic hypersensitivity

Oxaliplatin and paclitaxel are considered central components in the treatment of colorectal and breast cancer, respectively. The development of neuropathy during chronic treatment represents the major dose-limiting side effect that leads to discontinuation or interruption of therapies. The management of neuropathy is a challenge to individuate innovative therapeutic strategies based on new targets and correct routes of administration. We evaluated the hypersensitivity reliever effect of different opioid receptor agonists in rat models of oxaliplatin and paclitaxel-induced neuropathy. Compounds were spinally infused by intrathecal catheter. In oxaliplatin-treated rats, 0.3 nmol morphine induced the reversion of the mechanical hypersensitivity (Paw-pressure test), nociceptin/orphanin FQ (N/OFQ; 0.3-3 nmol) significantly increased the pain threshold without reaching the values of the control animals. The N/OFQ peptide (NOP) receptor full agonist UFP-112 reverted pain threshold alterations at lower dosage (0.1 nmol) vs morphine and N/OFQ, the partial agonist UFP-113 (0.1-1 nmol) was similar to N/OFQ. The higher efficacy of morphine vs N/OFQ was highlighted also in paclitaxel-treated rats. The mechanical hypersensitivity was fully reverted by 0.1 nmol UFP-112 and UFP-113. In conclusion, intrathecal μ opioid peptide (MOP) and NOP receptor agonists relieved chemotherapy-induced neuropathic pain. The synthetic peptides showed valuable potency and efficacy suggesting the NOP system as an exploitable target.

Antagonism of stimulation-produced analgesia by naloxone and N-methyl-D-aspartate: role of opioid and N-methyl-D-aspartate receptors

The present study aims to investigate the influence of electrical stimulation of periaqueductal gray (PAG) following peripheral nerve injury and its modulation by naloxone and N-methyl-D-aspartate (NMDA). Chronic neuropathic pain was induced by chronic constriction injury of the sciatic nerve, and subsequently a cannula was implanted in the PAG area for the purpose of electrical stimulation and intra-PAG drug administration. Intra-PAG administration of morphine, ketamine, and their combination were found to elicit antinociceptive response on hot-plate test. Electrical stimulation of PAG was also observed to demonstrate decreased pain response on hot-plate test, and this effect was reversed by the administration of naloxone, NMDA, and their combination, when injected into the PAG area. These findings suggest that apart from the opioid receptors, probably NMDA receptors also have a role to play in stimulation-produced analgesia.

The animal model of spinal cord injury as an experimental pain model

Pain, which remains largely unsolved, is one of the most crucial problems for spinal cord injury patients. Due to sensory problems, as well as motor dysfunctions, spinal cord injury research has proven to be complex and difficult. Furthermore, many types of pain are associated with spinal cord injury, such as neuropathic, visceral, and musculoskeletal pain. Many animal models of spinal cord injury exist to emulate clinical situations, which could help to determine common mechanisms of pathology. However, results can be easily misunderstood and falsely interpreted. Therefore, it is important to fully understand the symptoms of human spinal cord injury, as well as the various spinal cord injury models and the possible pathologies. The present paper summarizes results from animal models of spinal cord injury, as well as the most effective use of these models.

Spinal administration of a delta opioid receptor agonist attenuates hyperalgesia and allodynia in a rat model of neuropathic pain

Neuropathic (NP) pain is a debilitating chronic pain disorder considered by some to be inherently resistant to therapy with traditional analgesics. Indeed, micro opioid receptor (OR) agonists show reduced therapeutic benefit and their long term use is hindered by the high incidence of adverse effects. However, pharmacological and physiological evidence increasingly suggests a role for deltaOR agonists in modulating NP pain symptoms. In this study, we examined the antihyperalgesic and antiallodynic effects of the spinally administered deltaOR agonist, d-[Ala(2), Glu(4)]deltorphin II (deltorphin II), as well as the changes in deltaOR expression, in rats following chronic constriction injury (CCI) of the sciatic nerve. Rats with CCI exhibited cold hyperalgesia and mechanical allodynia over a 14-day testing period. Intrathecal administration of deltorphin II reversed cold hyperalgesia on day 14 and dose-dependently attenuated mechanical allodynia. The effects of deltorphin II were mediated via activation of the deltaOR as the effect was antagonized by co-treatment with the delta-selective antagonist, naltrindole. Western blotting experiments revealed no changes in deltaOR protein in the dorsal spinal cord following CCI. Taken together, these data demonstrate the antihyperalgesic and antiallodynic effectiveness of a spinally administered deltaOR agonist following peripheral nerve injury and support further investigation of deltaORs as potential therapeutic targets in the treatment of NP pain.

μ- but not δ- and κ-opioid receptors in the ventrolateral orbital cortex mediate opioid-induced antiallodynia in a rat neuropathic pain model

Previous studies have indicated that the ventrolateral orbital cortex (VLO) is involved in opioid-mediated antinociception in the tail flick test and formalin test. The aim of the current study was to examine the effect of opioids microinjected into the VLO on allodynia in the rat L5/L6 spinal nerve ligation (SNL) model of neuropathic pain and determine the roles of different subtypes of opioid receptors in this effect. The allodynia was assessed by both mechanical (von Frey filaments) and cold plate (4 degrees C) stimuli. Morphine (1.0, 2.5, and 5.0 microg) microinjected into the VLO contralateral to the nerve ligation dose-dependently depressed the mechanical and cold allodynia and these effects were reversed by nonselective opioid receptor antagonist naloxone (1.0 microg) administrated into the same site. Microinjection of endomorphin-1 (5.0 microg), a highly selective mu-opioid receptor agonist, and [D-Ala2, D-Leu5]-enkephalin (DADLE, 10 microg), a delta-/mu-opioid receptor agonist, also depressed the allodynia, and the effects of both drugs were blocked by selective mu-receptor antagonist beta-funaltrexamine (beta-FNA, 3.75 microg), but the effects of DADLE were not influenced by the selective delta-receptor antagonist naltrindole (5.0 microg). Microinjection of U-62066 (100 microg), a kappa-opioid receptor agonist, into the VLO had no effect on the allodynia. These results suggest that the VLO is involved in opioid-induced antiallodynia and mu- but not delta- and kappa-opioid receptor mediates these effects in the rat with neuropathic pain.

The roles of different subtypes of opioid receptors in mediating the nucleus submedius opioid-evoked antiallodynia in a neuropathic pain model of rats

Previous studies have indicated that thalamic nucleus submedius is involved in opioid-mediated antinociception in tail flick test and formalin test. The current study examined the effects of opioids microinjected into the thalamic nucleus submedius on the allodynia developed in neuropathic pain model rats, and determined the roles of different subtypes of opioid receptors in the thalamic nucleus submedius opioid-evoked antiallodynia. The allodynic behaviors induced by L5/L6 spinal nerve ligation were assessed by mechanical (von Frey filaments) and cold (4 degrees C plate) stimuli. Morphine (1.0, 2.5, and 5.0 microg) microinjected into the thalamic nucleus submedius contralateral to the nerve injury paw produced a dose-dependent inhibition of the mechanical and cold allodynia, and these effects were reversed by microinjection of the non-selective opioid receptor antagonist naloxone (1.0 microg) into the same site. Microinjection of endomorphin-1 (5.0 microg), a highly selective mu-opioid receptor agonist, and [D-Ala2, D-Leu5]-enkephalin (10 microg), a delta-/mu-opioid receptor agonist, also inhibited the allodynic behaviors, and these effects were blocked by selective mu-opioid receptor antagonist beta-funaltrexamine hydrochloride (3.75 microg). However, the [D-Ala2, D-Leu5]-enkephalin-evoked antiallodynic effects were not influenced by the selective delta-opioid receptor antagonist naltrindole (5.0 microg). Microinjection of the selective kappa-receptor agonist spiradoline mesylate salt (100 microg) into the thalamic nucleus submedius failed to alter the allodynia induced by spinal nerve ligation. These results suggest that the thalamic nucleus submedius is involved in opioid-evoked antiallodynia which is mediated by mu- but not delta- and kappa-opioid receptor in the neuropathic pain model rats.

Comparative actions of the opioid analgesics morphine, methadone and codeine in rat models of peripheral and central neuropathic pain

Controversy persists in relation to the analgesic efficacy of opioids in neuropathic pain. In the present study the effects of acute, subcutaneous administration of the mu-opioid receptor agonists morphine, methadone and codeine were examined in rat models of peripheral and central neuropathic pain. In the spared nerve injury (SNI) and chronic constriction injury (CCI) models of peripheral neuropathic pain, both morphine (6mg/kg) and methadone (3mg/kg) attenuated mechanical allodynia, mechanical hyperalgesia and cold allodynia for up to 1.5h post-injection (P<0.05); codeine (30mg/kg) minimally alleviated mechanical hypersensitivity in SNI, but not CCI rats. When administered to rats with photochemically-induced spinal cord injury (SCI), morphine (2 and 6mg/kg) and methadone (0.5-3mg/kg) robustly attenuated mechanical and cold allodynia for at least 2h post-injection (P<0.05). Codeine (10 and 30mg/kg) also attenuated mechanical and cold allodynia in this model for at least 3h after injection. The magnitude of opioid-mediated antinociception was similar between SNI, SCI and non-injured rats as measured in the tail flick test. At antinociceptive doses, no motor impairment as determined by the rotarod test was observed. The therapeutic window (based on antiallodynia versus ataxia) obtained for codeine, was vastly superior to that obtained with morphine or methadone in SNI and SCI rats. Furthermore, the therapeutic window for codeine in SCI rats was 4-fold greater than in SNI rats. Our results further support the efficacy of mu-opioid receptor agonists in alleviating signs of neuropathic pain in animal models of peripheral and especially central nerve injury.

Pharmacological evaluation of the selective spinal nerve ligation model of neuropathic pain in the rat

Rodent models of neuropathic pain are used to investigate the underlying mechanisms of pain associated with damage to peripheral nerves and to evaluate the efficacy of novel compounds. However, few models have been adequately characterized and the validity of many models remains unclear. The present experiment examined the activity of known anti-allodynic compounds in the L5 spinal nerve ligation (SNL) model of peripheral mononeuropathy in the rat, a modified version of the L5/L6 SNL model [S.H. Kim, J.M. Chung, An experimental model for peripheral neuropathy produced by segmental spinal nerve ligation in the rat, Pain 50 (1992) 355-363]. Tactile sensitivity was measured 7-21 days post-surgery using von Frey monofilaments before and following treatment with gabapentin (30, 60 and 120 mg/kg), morphine (1, 3 and 6 mg/kg), amitriptyline (1.5, 3 and 10 mg/kg), fluoxetine (3, 10 and 30 mg/kg), WIN55,212-2 (0.5, 1 and 2.5 mg/kg), indomethacin (1 and 5 mg/kg) or U-50,488H (3 and 6 mg/kg). Compared to sham-operated control animals, L5 SNL animals displayed significant tactile allodynia in the ipsilateral hindpaw that was completely reversed by treatment with gabapentin, morphine, and WIN55,212-2, partially reversed by amitriptyline and fluoxetine, and unaffected by U-50,488H or indomethacin. The robust effects of the non-selective cannabinoid receptor agonist WIN55,212-2 and morphine support reports in the literature that systemic cannabinoid receptor agonists and opioids are active in neuropathic pain. These results suggest that the L5 SNL model can be utilized to determine the anti-allodynic activity of novel compounds.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Ketamine potentiates the effect of electroacupuncture on mechanical allodynia in a rat model of neuropathic pain

Mu-opioid agonists and N-methyl-d-aspartate (NMDA) receptor antagonists have been shown to attenuate mechanical allodynia in neuropathic pain models. We have previously reported that 2Hz electroacupuncture (EA) produced analgesia via releasing endogenous opioid peptides (i.e. beta-endorphin and endomorphin) and the activated micro-opioid receptors. The present study aimed to examine whether ketamine, an NMDA receptor antagonist, can enhance the anti-allodynic effects induced by 2Hz EA in a rat model of neuropathic pain following spinal nerve ligation (SNL). The results are as follows: (1) EA itself or i.p. injection of ketamine reduced mechanical allodynia (i.e. increase in withdrawal threshold). (2) Although injection of ketamine at a low dose (1.0mg/kg) alone did not influence mechanical withdrawal threshold, combination of ketamine at this dose with EA produced more potent anti-allodynic effect than that induced by EA alone. (3) The anti-allodynic effect of EA combined with ketamine could be reversed by i.p. injection of naloxone (2.0 mg/kg). These results suggested that ketamine potentiate the anti-allodynic of EA in rats with spinal nerve ligation, and endogenous opioid system is likely to be involved in this process.