Morphine-induced Analgesia In Mice Research Articles

Effects of Aminoadamantane Derivatives on Morphine-Induced Analgesia in Mice

The effects of low-affinity NMDA-receptor antagonists amantadine (1-aminoadamantane hydrochloride) and hemantane [N-(2-adamantyl)hexamethyleneimine hydrochloride] on morphine-induced analgesia in C57Bl/6 mice were studied. Amantadine (10 and 20 mg/kg, i.p.) per se did not affect the latent period of the response in the hot-plate test while hemantane (10 and 20 mg/kg, i.p.) increased dose-dependently pain thresholds 180 and 240 min after administration. Morphine (20 mg/kg, s.c.) showed a time—effect dependence (30 – 120 min). The aminoadamantanes were administered 90 min after the opioid to assess their effects on morphine-induced antinociception. The responses of the animals were recorded for the next 2.5 h. The aminoadamantanes potentiated and extended the analgesic activity of morphine in the order of efficacy amantadine < hemantane. The results indicated that the aminoadamantanes had different capabilities to cause delayed analgesia and modulated opioid antinociceptive activity at the supraspinal level.

Yin-and-yang bifurcation of opioidergic circuits for descending analgesia at the midbrain of the mouse

In the descending analgesia pathway, opioids are known to disinhibit the projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), leading to suppression of pain signals at the spinal cord level. The locus coeruleus (LC) has been proposed to engage in the descending pathway through noradrenergic inputs to the spinal cord. Nevertheless, how the LC is integrated in the descending analgesia circuit has remained unknown. Here, we show that the opioidergic analgesia pathway is bifurcated in structure and function at the PAG. A knockout as well as a PAG-specific knockdown of phospholipase C β4 (PLCβ4), a signaling molecule for G protein-coupled receptors, enhanced swim stress-induced and morphine-induced analgesia in mice. Immunostaining after simultaneous retrograde labeling from the RVM and the LC revealed two mutually exclusive neuronal populations at the PAG, each projecting either to the LC or the RVM, with PLCβ4 expression only in the PAG-LC projecting cells that provide a direct synaptic input to LC-spinal cord (SC) projection neurons. The PAG-LC projection neurons in wild-type mice turned quiescent in response to opiates, but remained active in the PLCβ4 mutant, suggesting a possibility that an increased adrenergic function induced by the persistent PAG-LC activity underlies the enhanced opioid analgesia in the mutant. Indeed, the enhanced analgesia in the mutant was reversed by blocking α2-noradrenergic receptors. These findings indicate that opioids suppress descending analgesia through the PAG-LC pathway, while enhancing it through the PAG-RVM pathway, i.e., two distinct pathways with opposing effects on opioid analgesia. These results point to a therapeutic target in pain control.

Activation of TREK-1 by morphine results in analgesia without adverse side effects

Morphine is the gold-standard pain reliever for severe acute or chronic pain but it also produces adverse side effects that can alter the quality of life of patients and, in some rare cases, jeopardize the vital prognosis. Morphine elicits both therapeutic and adverse effects primarily through the same μ opioid receptor subtype, which makes it difficult to separate the two types of effects. Here we show that beneficial and deleterious effects of morphine are mediated through different signalling pathways downstream from μ opioid receptor. We demonstrate that the TREK-1 K(+) channel is a crucial contributor of morphine-induced analgesia in mice, while it is not involved in morphine-induced constipation, respiratory depression and dependence-three main adverse effects of opioid analgesic therapy. These observations suggest that direct activation of the TREK-1 K(+) channel, acting downstream from the μ opioid receptor, might have strong analgesic effects without opioid-like adverse effects.

Contribution of acid sphingomyelinase in the periaqueductal gray region to morphine-induced analgesia in mice

Opioids are the most widely used drugs for long-term pain management, but their use is limited by the development of antinociceptive tolerance. The present study investigated the role of ceramide production through acid sphingomyelinase (ASM) activation in the periaqueductal gray region, a brain region implicated in opioid analgesia and tolerance. Morphine treatment was found, using immunohistochemistry, to increase ASM expression and intracellular ceramide in the periaqueductal gray 30 min after an acute injection (10 mg/kg). The effects of acute morphine treatment on ASM expression and ceramide generation in the periaqueductal gray region were completely blocked by pretreatment with naloxone and by silencing the ASM gene by plasmid-mediated transfection of ASM shRNA. In chronic morphine pellet-implanted mice, ASM expression and ceramide generation in the periaqueductal gray region were also significantly increased. Functionally, selective silencing of the ASM gene by local ASM shRNA transfection reduced the analgesic response to acute morphine, but the data on the effect of ASM shRNA on the development of antinociceptive tolerance were inconclusive. These data provide evidence that ASM activation and ceramide generation in the periaqueductal gray region play a major role in the antinociceptive mechanism of morphine.

Stress‐Induced Analgesia and Morphine Responses Are Changed in Catechol‐O‐methyltransferase‐Deficient Male Mice

Catechol-O-methyltransferase (COMT) polymorphisms modulate pain and opioid analgesia in human beings. It is not clear how the effects of COMT are mediated and only few relevant animal studies have been performed. Here, we used old male Comt gene knock-out mice as an animal model to study the effects of COMT deficiency on nociception that was assessed by the hot plate and tail flick tests. Stress-induced analgesia was achieved by forced swim. Morphine antinociception was measured after 10 mg/kg of morphine subcutaneously. Morphine tolerance was produced with subcutaneous morphine pellets and withdrawal provoked with subcutaneous naloxone. In the hot plate test, morphine-induced antinociception was significantly greater in the COMT knock-out mice, compared to the wild-type mice. This may be due to increased availability of opioid receptors as suggested by previous human studies. In the tail flick test, opioid-mediated stress-induced analgesia was absent and morphine-induced analgesia was decreased in COMT knock-out mice. In the hot plate test, stress-induced analgesia developed to all mice regardless of the COMT genotype. There were no differences between the genotypes in the baseline nociceptive thresholds, morphine tolerance and withdrawal. Our findings show, for the first time, the importance of COMT activity in stress- and morphine-induced analgesia in mice. COMT activity seems to take part in the modulation of nociception not only in the brain, as suggested earlier, but also at the spinal/peripheral level.

Agmatine-morphine interaction on nociception in mice.

The aim of this study was to investigate the possible participation of nitric oxide in the agmatine-mediated potentiation of morphine-induced analgesia in mice. Agmatine and L-NAME (a nitric oxide synthesis inhibitor) enhanced morphine-induced analgesia in the tail flick test, but not in the hot plate test. L-NAME did not block the agmatine-induced potentiation of morphine effect. Our results indicate that agmatine potentiates morphine-induced spinal but not supraspinal analgesia, and that this effect is not mediated by a nitric oxide-dependent mechanism.

Interaction between Calcium Channel Blockers and Sweetening Agents on Morphine-Induced Analgesia in Mice by Formalin Test

1. Calcium is known to be an important ion in the modulation of nociception and inflammation. Previous research has shown that mice drinking sweet-tasting solutions such as sucrose, saccharin and aspartame exhibit significant changes in morphine-induced analgesia in both phases of the formalin test. 2. In this study, the role of calcium channel blockers on the effectivity of a 12-day regimen of different sweetening agents (sucrose 32%, saccharin 0.08% and aspartame 0.16%) on the alteration of the morphine response has been investigated. 3. Male albino mice weighing 20–27 g were used for experiments. Animals were given 12 days to adapt to dietary conditions. Animals were given morphine (1.5, 3, 6, 9 mg/kg) subcutaneously 30 min before observation. Nifedipine (5 mg/kg), verapamil (5 mg/kg) and diltiazem (10 mg/kg) were administered intraperitoneally 20 min before morphine injection. 4. Recording of the early phase started immediately and lasted for 10 min after formalin injection. Recording of the late response started 20 min after formalin injection and lasted for 10 min. 5. Calcium channel blockers potentiated the antinociceptive effects of sweetening agents and diminished the antagonistic effects of these compounds on morphine-induced analgesia in the early and late phases of the formalin test. 6. It is proposed that calcium has a role for the interactive effects of sweetening agents and morphine on pain sensitivity.

Effect of Granisetron on Morphine-Induced Analgesia in Mice by Formalin Test

1. In this investigation, the influence of a selective 5HT 3 antagonist, granisetron, on morphine-induced antinociception in the formalin test has been studied. There is evidence that the 5HT 3 receptor system takes part in analgesic response. 2. Male albino mice weighing 22–27 g were used in the experiments. All drugs were administered 30 min before formalin injection. Comparison between groups was made by analysis of variance and then Newman-Keuls test. 3. Different doses of morphine (1.5–9.0 mg/kg) subcutaneously induced antinociception in both phases of the formalin test. 4. Coadministration of granisetron (0.1–10.0 mg/kg) intraperitoneally had no effect on morphine analgesia, but granisetron injection alone induced dose-dependent analgesia in both phases of the for- malin test. 5. Results of this study suggest a role for a new selective 5HT 3 antagonist in analgesia.

Effects of Sweetening Agents on Morphine-Induced Analgesia in Mice by Formalin Test

1. There is evidence that sweet-tasting substances such as sucrose and saccharin can interact with endogenous opioid systems. Further evidence showed that feeding mice different concentrations of sucrose and saccharin alter the latency in the tail-flick test. 2. In the current study, the effects of a 12-day regimen of different sweetening agents [sucrose (32%), saccharin (0.08%) and aspartame (0.16%)] on morphine-induced analgesia with the formalin test were investigated. 3. Male albino mice (20–27 g) were used for the experiments. Animals were given 12 days to adapt to dietary conditions. Animals were first given saline or morphine subcutaneously (1.5, 3.0, 6.0, or 9.0 mg/kg) 30 min before the observation period. The recording of the early phase started immediately and lasted for 10 min. The recording of the late response started 20 min after formalin injection and lasted for 10 min. Statistical analysis was performed by using analysis of variance followed by Newman-Keuls test, and P≤0.05 was considered significant. 4. Sucrose and aspartame increased morphine analgesia in the early phase, but saccharin had no effect on the early phase. On the other hand, saccharin and sucrose decreased the effect of morphine in the late phase, but aspartame increased the effect of morphine-induced analgesia. 5. In conclusion, the present data provide further evidence for an important role for dietary variables in determining the effects of exogenous opioids on pain sensitivity.

RU 486 reduces morphine-induced analgesia in mice, but steroid receptors are not involved.

The effects of subcutaneous pretreatment with the glucocorticoid/progesterone receptor antagonist RU 486 on the antinociceptive action of peripherally administered morphine were evaluated by the hot-plate test in mice. The steroid significantly reduced the analgesic effect of the opiate. Neither dexamethasone nor progesterone modified the effects of RU 486 on morphine-induced analgesia. Therefore, the present data indicate that the modulatory effect of RU 486 on morphine-induced analgesia does not involve the binding of this drug to classical steroid receptors.

Nitric oxide synthase inhibition attenuates tolerance to morphine but not to [D-Ala2, Glu4] deltorphin II, a delta 2-opioid receptor agonist in mice.

The effects of NG-nitro-L-arginine (L-NNA) and NG-monomethyl-L-arginine (L-NMMA), two potent inhibitors of nitric oxide synthase (NOS) on the development of tolerance to the analgesic action of [D-Ala2, Glu4] deltorphin II (deltorphin II), a delta 2-opioid receptor agonist, and morphine, a mu-opioid receptor agonist, were determined in mice. Male Swiss-Webster mice were rendered tolerant to deltorphin II by twice daily ICV injections of the drug for 4 days. Tolerance to morphine was induced by twice daily s.c. injections of the drug for 4 days. Multiple injections of deltorphin II (20 micrograms/mouse) or morphine (15 mg/kg) resulted in the development of tolerance to their analgesic action as evidenced by decreases in the response in comparison to mice injected with vehicle. Concurrent administration of L-NNA or L-NMMA (2,4, or 8 mg/kg, i.p.) had no effect on the development of tolerance to the analgesic action of deltorphin II. However, the same doses of L-NNA or L-NMMA inhibited the development of tolerance to the analgesic action of morphine. Acute treatment with L-NNA or L-NMMA did not modify deltorphin II- or morphine-induced analgesia in mice. It is concluded that NOS inhibition attenuates tolerance to the analgesic action of morphine but not to that of deltorphin II, a delta 2-opioid receptor agonist, in the mouse.

High affinity ibogaine binding to a mu opioid agonist site

The naturally occurring indole alkaloid ibogaine is of interest because of its reported ability to block drug seeking behavior for extended periods. The compound also potentiates morphine-induced analgesia in mice and reduces certain naltrexone-precipitated withdrawal signs in morphine-dependent rats. Although these results might suggest ibogaine interaction with opioid receptors, previous receptor binding studies (Brain Res. 571:242–247, 1980) found that ibogaine had a K i value of only 2 μM for the kappa opioid receptor and was virtually inactive in blocking mu and delta receptor binding ( K i >100 μM). The present investigation of ibogaine interaction with the mu opioid receptor from mouse forebrain labeled with [3H]-naloxone, however, yielded significantly more potent mu opioid K i values. LIGAND analysis indicated that the data were best fit by a two site binding model, with K i values of about 130 nM and 4 μM, reflecting ibogaine recognition of different agonist affinity states of the receptor. Inclusion of 100 mM NaCl in the assay to induce the agonist low affinity state of the receptor, reduced ibogaine's inhibition of [ 3H]-naloxone binding. These results suggest that ibogaine is an agonist at the mu opioid receptor with a K i value of about 130 nM, potentially explaining ibogaine's antinociceptive effects as well as its reported reduction of opioid withdrawal symptoms and attenuation of drug seeking behavior.

The effect of lithium on morphine-induced analgesia in mice

1. 1. The effects of acute and chronic lithium (Li +) treatments on the antinociception caused by morphine were studied in mice using the tail-flick test. 2. 2. Subcutaneous injection of morphine (10 mg/kg) caused significant antinociception. 3. 3. Acute Li + administration (0.05, 0.1, 0.3, 1, 5 and 10 mg/kg, i.p.) alone had no significant antinociceptive effect but changed morphine analgesia; low doses of Li + (0.1, 0.3 and 1 mg/kg) were found to decrease the antinociception induced by morphine whereas higher doses of the drug (10 mg/kg) potentiated this effect. 4. 4. The 6 day administration of Li + with a serum level of 0.528 mM decreased the antinociceptive effect of morphine. 5. 5. The effect of Li + on morphine-induced analgesia persisted for 96 hr in spite of the fact that Li + drinking was discontinued (the serum Li + level decreased from 0.528 to 0.022 mM). 6. 6. It has been reported that Li + might change both the binding of opioids to their receptors and biosynthesis or release of endogenous opioids. There is also a considerable body of evidence which indicates that both Li + and morphine affect phosphoinositide turnover, intraceullular calcium content and cyclic AMP level. The interaction of two drugs may conceivably take place through these systems. 7. 7. These data suggest that the biological effects of Li + may exist at very much lower serum Li + levels than the commonly accepted therapeutic range.

Effect of prolonged administration of clonidine on [ 3H]PN 200-110 and [ 125I]ϵ-conotoxin binding in mouse brain

The effect of chronic exposure to clonidine or morphine on clonidine- and morphine-induced analgesia in mice was examined. Binding of L- or N-type calcium channel antagonist to cortical membrane fractions was also compared between these groups of mice. A decrease in the analgesic effect of clonidine and morphine was observed following prolonged administration of clonidine or morphine. Binding of [ 3H]PN 200-110, an L-type calcium channel antagonist, decreased following prolonged administration of clonidine whereas it increased after morphine treatment. On the other hand, a significant increase of [ 125I]ϵ-conotoxin, an N-type calcium channel antagonist, binding was observed after chronic clonidine or morphine treatment. These results will be discussed in relation with the possible development of cross-tolerance between clonidine and morphine through the change in calcium channels, more specifically in N-type channels.

Peripheral administration of taurine antagonizes morphine-induced analgesia in mice

1. A single i.p. dose of taurine (25 mg/mouse) given to mice 3 hr before an i.p. injection of morphine (0.1 mg/mouse) decreased the analgesic response of the animals to morphine. 2. Neither a lower dose of taurine nor the same dose of another amino acid was effective. 3. The analgesic response to morphine was also reduced by inclusion of taurine in the drinking water. 4. The results of the present study indicate that peripherally administered taurine antagonized morphine-induced analgesia, similar to a previous report that taurine administered centrally, diminished the analgesic response to a centrally injected opioid peptide.

An ATP-dependent potassium channel blocker antagonizes morphine analgesia

We showed that glibenclamide, a blocker of ATP-dependent potassium channels in the CNS, antagonizes morphine-induced analgesia in mice

Differential mechanisms mediating β-endorphin- and morphine-induced analgesia in mice

Effects of yohimbine, methysergide and naloxone given intrathecally (i.t.) and naloxone given intracerebroventricularly (i.c.v.) on inhibition of the tail-flick and hot-plate response induced by β-endorphin and morphine given i.c.v. were studied in male ICR mice. Yohimbine (1.5 and 15 μg) and methysergide (1.5 and 15 μg) injected i.t. antagonized inhibition of the tail-flick response induced by morphine but not β-endorphin administered i.c.v. On the other hand, naloxone (20 ng) injected i.t. antagonized inhibition of the tail-flick response induced by i.c.v. administered β-endorphin but not morphine. Yohimbine and methysergide given i.t. did not antagonize inhibition of the hot-plate response induced by morphine nor did naloxone given i.t. antagonized i.c.v. β-endorphin-induced inhibition of the hot-plate response. Naloxone given i.c.v. was more effective in antagonizing morphine-induced inhibition of the tail-flick and hot-plate response than inhibition induced by β-endorphin given i.c.v. Naloxone at doses (0.1 and 1 μg) which effectively reversed inhibition of the tail-flick response to i.c.v. morphine was not effective in reversing the i.c.v. β-endorphin-induced inhibition of the tail-flick response. Our results indicate that β-endorphin and morphine produce analgesia by stimulating separate types of opioid receptors, ϵ- for β-endorphin and μ- for morphine, and activate separate descending pain modulatory control systems. The supraspinal ϵ system stimulated by β-endorphin is mediated by activation of spinal opioid receptors whereas the supraspinal μ system stimulated by morphine is mediated by activation of spinal α 2-adreceptors and serotonin receptors for the production of analgesia.

Antagonism by nalmefene of systemic and intrathecal morphine-induced analgesia in mice.

Nalmefene is an orally active opiate antagonist structurally related to naloxone and naltrexone. In this study using two different strains of mice (Swiss Cox and ICR), the antagonist activity of nalmefene given subcutaneously (sc) was quantified by determination of the apparent pA2 values against the antinociceptive activity (tail flick and hot plate tests) of morphine given sc or intrathecally (lumbar spinal cord). The apparent pA2 values (constrained to a slope of -1) were 8.06 (7.79-8.33) in Swiss Cox mice and 7.81 (7.62-8.00) in ICR mice in the tail flick test with sc morphine. These values were larger than the corresponding value for naloxone in ICR mice, 7.35 (7.10-7.60). The hot plate test provided similar results: the apparent pA2 values for nalmefene with sc morphine were 8.14 (7.89-8.39) in Swiss Cox mice and 7.81 (7.65-7.97) in ICR mice, values which were different from naloxone 7.33 (7.23-7.42) in ICR mice. Apparent pA2 values for nalmefene with intrathecal morphine were not different from those for naloxone in the tail flick test. Thus, these sets of results suggest that it may be worthwhile to further determine whether systemic nalmefene might possibly possess an advantage over naloxone in antagonizing systemic side effects of morphine arising from local spinal morphine administration.

Attenuation of morphine-induced analgesia in mice by exposure to magnetic resonance imaging: Separate effects of the static, radiofrequency and time-varying magnetic fields

Exposure of adult male mice to a magnetic resonance imaging (MRI) procedure has been shown to abolish the nocturnal analgesic responses observed following treatment with morphine. The field component(s) responsible for this inhibitory effect were examined by exposing mice to either the static, time-varying or rf magnetic field components associated with an MRI procedure. In the middle of the night portion of their day-night cycle, mice were exposed for 23.2 min to one of the above field components, intraperitoneally injected with morphine sulphate (10 mg/kg) and then exposed to the field conditions for another 23.2 min, after which analgesic responses were determined. Analgesia was quantitated by determining the length of time mice were content to be on a hot surface (50°C) before they showed discomfort by licking their paws. It was observed that the time-varying magnetic field completely abolished, the rf field significantly reduced, while the static field component (0.15 T) had no evident effect on morphine-induced analgesia. These results indicate that the time-varying, and to a lesser extent the rf, fields associated with the MRI procedure inhibit morphine-induced analgesia in mice. These data also raise the possibility that exposure in humans to some of the magnetic field components associated with MRI may have clinically relevant effects on the actions of narcotic drugs such as morphine.

Failure of coffee to inhibit the pharmacodynamic activity of morphine in vivo.

High doses of caffeine-containing as well as decaffeinated instant coffee neither inhibited morphine-induced analgesia in mice nor the morphine-induced fall of blood pressure, heart rate and respiratory rate in rats. On the contrary, caffeine-containing coffee even enhanced the analgesic effects of morphine in mice. Coffee thus does not exhibit opiate-antagonizing activity in the whole organism in vivo. The very weak morphine-antagonistic efficacy of coffee powder in the myenteric plexus-longitudinal muscle preparation from the guinea pig ileum is of no practical importance.