ACEA-1328, an NMDA receptor antagonist, increases the potency of morphine and U50,488H in the tail flick test in mice

Characterization of a Potential KOR/DOR Dual Agonist with No Apparent Abuse Liability via a Complementary Structure-Activity Relationship Study on Nalfurafine Analogues

Discovery of analgesics void of abuse liability is critical to battle the opioid crisis in the United States. Among many strategies to achieve this goal, targeting more than one opioid receptor seems promising to minimize this unwanted side effect while achieving a reasonable therapeutic profile. In the process of understanding the structure-activity relationship of nalfurafine, we identified a potential analgesic agent, NMF, as a dual kappa opioid receptor/delta opioid receptor agonist with minimum abuse liability. Further characterizations, including primary in vitro ADMET studies (hERG toxicity, plasma protein binding, permeability, and hepatic metabolism), and in vivo pharmacodynamic and toxicity profiling (time course, abuse liability, tolerance, withdrawal, respiratory depression, body weight, and locomotor activity) further confirmed NMF as a promising drug candidate for future development.

Combination of paracetamol or ketamine with meperidine enhances antinociception

OBJECTIVES

Dealing with pain is one of the most important issues of medicine. All of the studies aim to find a drug or combination of drugs in order to have more effective analgesia and less side effects. In this study, we aimed to investigate the antinociceptive effects of combination of paracetamol or ketamine with meperidine.

METHODS

In this study, we evaluated the systemic antinociceptive effects of meperidine, paracetamol, and ketamine one by one with their combinations. We used 50 mice (weighing 25-30 g), which were divided into 5 groups with each group consisting of 10 mice. Meperidine was applied to animals with increasing doses and their tail flick latencies (TFL) were noted at 20, 40, 60, 90, 120, 180, and 240 min. The same protocol was repeated after the combination of meperidine with paracetamol or ketamine.

RESULTS

There was no analgesic effect on low doses of ketamine and paracetamol at TFL measurements. But the combination of low doses of these drugs with meperidine significantly increased TFL (p < 0.05).

CONCLUSION

It was observed that meperidine + ketamine and meperidine + paracetamol combinations have potent analgesic effect.

Current approaches with the glutamatergic system as targets in the treatment of neuropathic pain

Glutamate is the most widely distributed and a major excitatory neurotransmitter in the CNS. It has been found to play a critical role in various physiological functions in which increased glutamate or its subsequent stimulation is thought to have a role in pathophysiological mechanism of various CNS diseases like epilepsy, stroke, depression and pain. Early attempts to develop glutamatergic antagonists failed in clinical studies due to nonselective or competitive antagonism and have a lot of safety issues like loss of cognitive functions, psychomimetic effect and sedation. Neuropathic pain can be described as pain associated with damage or permanent alteration of the peripheral or central nervous system. At present, there are very few effective therapies for neuropathic pain. The current approach includes targeting specific or alternate binding sites of glutamate receptors, resulting in reduced CNS liabilities. Targeting the glutamatergic system shows a better efficacy and fewer side effects, compared with classical drugs for the treatment of neuropathic pain. This review discusses the various targets on glutamatergic system, which includes the receptors, transporters and enzymes, for the treatment of neuropathic pain and their advantages over classical glutamatergic antagonists. The review also highlights the newer drugs in clinical trials for neuropathic pain.

Modulation of cholestasis-induced antinociception in rats by two NMDA receptor antagonists: MK-801 and magnesium sulfate

Acute cholestasis is associated with increased activity of the endogenous opioid system that results to changes including analgesia. N-methyl-d-aspartate (NMDA) receptors are involved in the nociceptive pathway and play a major role in the development of morphine induced analgesia. The magnesium acts as a non-competitive NMDA receptor antagonist by blocking the NMDA receptor channel. Considering the reported antinociceptive effect of magnesium sulfate as a NMDA receptor antagonist and the existence of close functional links between NMDA receptor antagonists and magnesium with the opioid system, we studied the effect of acute and chronic administration of MK-801 as a NMDA antagonist and magnesium sulfate on modulation of nociception in an experimental model of elevated endogenous opioid tone, acute cholestasis, using the tail-flick paradigm. Cholestasis was induced by ligation of the main bile duct using two ligatures and then transsection of the duct at the midpoint between them. A significant increase (P<0.001) in nociception threshold was observed in bile duct ligated rats compared to unoperated and sham-operated animals. In acute treatment, MK-801 (0.1 mg/kg, b.i.d), but not magnesium (150 mg/kg magnesium sulfate, i.e. 30 mg/kg of Mg(+2), i.p., b.i.d.) increased antinociception in cholestatic rats compared to saline treated cholestatics (P<0.05). In chronic treatment, administration of MK-801 or magnesium sulfate for 7 consecutive days, increased tail-flick latency (P<0.05, P<0.01) in cholestatic animals compared to saline treated cholestatics. These data showed that NMDA receptor pathway is involved in modulation of cholestasis-induced antinociception in rats and that repeated dosages of magnesium sulfate similar to MK-801 is able to modulate nociception in cholestasis.

Modulation of morphine analgesia by site-specific N-methyl-D-aspartate receptor antagonists: dependence on sex, site of antagonism, morphine dose, and time

Pharmacological blockade of N-methyl-D-aspartate (NMDA) receptors can modulate morphine analgesia in experimental animals and humans. However, this literature is highly inconsistent, with NMDA receptor antagonists variously shown to potentiate, attenuate or produce no effect on morphine analgesic magnitude. A number of factors influencing this modulation have been proposed, but no one has examined such factors simultaneously, and all existing studies in mice were conducted exclusively in male subjects. Thus, the influence of systemic administration of site-specific NMDA receptor antagonists-including dextromethorphan, dextrorphan, MK-801, LY235959, L-701,324, and Ro 25-6981-on morphine analgesia (15-45 mg/kg; 15, 30 and 60 min post-injection) was studied in male and female mice using the 49 degrees C tail-withdrawal test. We found that oral and intraperitoneal dextromethorphan, a low-affinity non-competitive antagonist, dose-dependently potentiated low-dose morphine analgesia but attenuated high-dose morphine analgesia. Dextrorphan and MK-801 were found to potentiate low- but not high-dose morphine analgesia. The competitive glutamate-site antagonist, LY235959, and glycine-site antagonist, L-701,324, potentiated morphine analgesia at all doses. In contrast, the polyamine (NR2B) site antagonist, Ro 25-6981, attenuated morphine analgesia at all doses. Strikingly, the non-competitive antagonists produced no modulation of morphine analgesia whatsoever in female mice, whereas no sex differences were observed using competitive or NR2B antagonists. These findings indicate that NMDA modulation of morphine analgesia is critically influenced by sex, site of antagonism, morphine dose and time after injection. Our data suggest that NMDA antagonism via competitive or glycine site antagonism might result in more reliable clinical effects on morphine analgesia in both sexes.

Antagonists and agonists at the glycine site of the NMDA receptor for therapeutic interventions

For decades neuroreceptor research has focused on the development of NMDA glycine-site antagonists, after Johnson and Ascher found out in 1987 about the co-agonistic character of this achiral amino acid at the NMDA receptor. Contrary to the inhibitory glycine receptor (glycine(A)) the glycine binding site on the NMDA receptor (glycine(B)) is strychnine-insensitive. A great diversity of diseases showing a disturbed glutamate neurotransmission have been linked to the NMDA receptor. Glycine site antagonists have been investigated for acute diseases like stroke and head trauma as well as chronic ones like dementia and chronic pain.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Effects of ACEA-1328, a NMDA receptor/glycine site antagonist, on U50,488H-induced antinociception and tolerance

Previously, we have shown that inhibition of the glycine site associated with the N-methyl-D-aspartate (NMDA) receptor is another viable approach to blocking morphine tolerance. In the present study, we sought to investigate the involvement of the NMDA receptor/glycine site in kappa-opioid receptor-mediated antinociception and tolerance in CD-1 mice. In antinociception studies, mice were injected with 5-nitro-6,7-dimethyl-1,4-dihydro-2, 3-quinoxalinedione (ACEA-1328), a systemically bioavailable NMDA receptor/glycine site antagonist, or the vehicle (Bis-Tris, 0.2 M) and then immediately with trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid e methanesulfonate (U50,488H), a kappa-opioid receptor agonist. Thirty minutes later, mice were tested for changes in nociceptive responses in the tail flick assay. ACEA-1328, per se, prolonged tail flick latencies with an ED(50) of approximately 50 mg/kg. Concurrent administration of ACEA-1328, at doses that did not produce antinociception, with U50,488H increased the potency of U50,488H in a dose-dependent manner. In tolerance studies, mice were treated, either once a day for 9 days or twice daily for 4 days, with the vehicle or ACEA-1328. Immediately after the initial injection, mice then received an injection of saline or U50,488H. On the test day, mice were injected with U50,488H alone and tested for antinociception 30 min later. Chronic treatment with U50,488H by either method produced tolerance. Unlike the acute effect of the drug, chronic treatment with ACEA-1328 decreased the antinociceptive potency of U50,488H. Taken together, the data suggest that acute and chronic administration of ACEA-1328 differentially affected the antinociceptive effect of U50,488H. Furthermore, the decreased in the potency of U50,488H induced by chronic treatment with ACEA-1328 also confounded the interpretation of the tolerance data.

ACEA-1328, a NMDA receptor/glycine site antagonist, acutely potentiates antinociception and chronically attenuates tolerance induced by morphine

The effect of ACEA-1328, a competitive and systemically bioavailable NMDA receptor/glycine site antagonist, was studied on morphine-induced antinociception and tolerance in CD-1 mice using the tail flick test. To study the effect of acute administration of ACEA-1328 on morphine-induced antinociception, mice were injected with either ACEA-1328 (1, 5, and 10 mg kg(-1)) or Bis-Tris (0.2 m) immediately followed by an injection of morphine and tested for antinociception 30 min later. ACEA-1328 significantly increased the antinociceptive potency of morphine. To study the effect of chronic administration of ACEA-1328 on morphine-induced antinociception and tolerance, mice were treated, either once per day for 9 days or twice daily for 4 days, with ACEA-1328 or with the vehicle. Mice were then, within 1 min, injected daily with either morphine or saline. On the day of the test, mice were injected with only morphine and tested for antinociception 30 min later. In comparison to the acute effect of ACEA-1328, chronic treatment with the NMDA receptor/glycine site antagonist did not affect the antinociceptive potency of morphine. Chronic treatment with morphine, by both methods, produced a significant degree of tolerance. Concurrent administration of ACEA-1328 with the opioid analgesic completely blocked morphine tolerance. Our results demonstrate that acute, but not chronic, treatment with ACEA-1328 increased the antinociceptive potency of morphine. Furthermore, co-administration of the NMDA receptor antagonist with morphine abolished the development of tolerance. Overall, the data support a growing body of evidence showing that activation of the NMDA receptor plays a functional role in opioid-induced antinociception and tolerance.