Receptor selectivity of icv morphine in the rat cold water tail-flick test

Antagonism of the mu-delta opioid receptor heterodimer enhances opioid antinociception by activating Src and calcium/calmodulin-dependent protein kinase II signaling

ABSTRACT

The opioid receptors are important regulators of pain, reward, and addiction. Limited evidence suggests the mu and delta opioid receptors form a heterodimer (MDOR), which may act as a negative feedback brake on opioid-induced analgesia. However, evidence for the MDOR in vivo is indirect and limited, and there are few selective tools available. We recently published the first MDOR-selective antagonist, D24M, allowing us to test the role of the MDOR in mice. We thus cotreated CD-1 mice with D24M and opioids in tail flick, paw incision, and chemotherapy-induced peripheral neuropathy pain models. D24M treatment enhanced oxymorphone antinociception in all models by 54.7% to 628%. This enhancement could not be replicated with the mu and delta selective antagonists CTAP, naltrindole, and naloxonazine, and D24M had a mild transient effect in the rotarod test, suggesting this increase is selective to the MDOR. However, D24M had no effect on morphine or buprenorphine, suggesting that only specific opioids interact with the MDOR. To find a mechanism, we performed phosphoproteomic analysis on brainstems of mice. We found that the kinases Src and CaMKII were repressed by oxymorphone, which was restored by D24M. We were able to confirm the role of Src and CaMKII in D24M-enhanced antinociception using small molecule inhibitors (KN93 and Src-I1). Together, these results provide direct in vivo evidence that the MDOR acts as an opioid negative feedback brake, which occurs through the repression of Src and CaMKII signal transduction. These results further suggest that MDOR antagonism could be a means to improve clinical opioid therapy.

Route of administration affects the ability of naltrexone to reduce amphetamine-potentiated brain stimulation reward in rats

Opioid receptor antagonism has been shown to attenuate behavioral and neurochemical effects of amphetamine in humans and rodents. The effects of acute (oral or subcutaneous) or extended-release naltrexone (XR-NTX) were tested on the reward-enhancing effects of amphetamine using the intracranial self-stimulation (ICSS) paradigm. Acute exposure to drugs of abuse reduces the locus of rise (LOR) in the ICSS procedure, reflecting enhanced brain stimulation reward (BSR). Rats were treated once a day with naltrexone orally (PO; 5.0 mg/kg) or subcutaneously (SC; 0.5 mg/kg) for four consecutive days and tested with D-amphetamine (0.5 mg/kg, intraperitoneal) in the ICSS paradigm 30 minutes later on days 1 and 4. Separate groups of rats received XR-NTX (50 mg/kg, SC) or placebo microspheres (similar mass to XR-NTX, SC) on day 0 and tested with D-amphetamine in the ICSS paradigm on days 4, 14, 21, 28 and 41 after administration. Naltrexone plasma concentrations were determined for each amphetamine testing session using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). In rats pretreated with naltrexone acutely, amphetamine-potentiated BSR did not differ from vehicle-pretreated rats on either day 1 or day 4 (25-30% decrease in LOR). In XR-NTX-pretreated rats, amphetamine-potentiated BSR was reduced by 64 and 70% on days 4 and 14, respectively, compared to placebo microsphere-treated controls. This effect dissipated by day 21. Naltrexone plasma concentrations were comparable across all treatment groups (14-30 ng/ml) on days 1, 4 and 14. In summary, an extended-release formulation of naltrexone results in significant attenuation of psychostimulant-enhanced BSR that is not observed with acute naltrexone.

Pharmacological selectivity of CTAP in a warm water tail-withdrawal antinociception assay in rats

RATIONALE

To facilitate in vivo characterization of the mu antagonist Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), the present study characterized CTAP selectivity in vivo.

OBJECTIVES

CTAP, the classical antagonist naltrexone, the kappa-selective antagonist nor-binaltorphimine (BNI), and the delta-selective antagonist naltrindole were compared as antagonists of representative mu, kappa, and delta agonists in a warm water tail-withdrawal assay.

MATERIALS AND METHODS

Male Sprague-Dawley rats were pretreated with CTAP (0.01 to 10.0 microg, i.c.v.), naltrexone (0.1 to 10 mg/kg s.c.; 0.1 to 10 microg i.c.v.), nor-BNI (1 mg/kg s.c.), or naltrindole (0.01 to 1 microg, i.c.v.) and tested with cumulative doses of agonist in 50 or 55 degrees C tail-withdrawal assays.

RESULTS

At 55 degrees C, morphine and DAMGO produced dose-dependent antinociceptive effects that were antagonized by CTAP or naltrexone (s.c. or i.c.v.) in a surmountable, dose-dependent manner. Neither kappa agonists (bremazocine, spiradoline, U69,593; all s.c.) nor the delta agonist DPDPE (i.c.v.) produced antinociception at 55 degrees C, but all produced full antinociception at 50 degrees C. CTAP did not antagonize effects of spiradoline, U69,593, or DPDPE, whereas nor-BNI produced insurmountable antagonism of effects of kappa agonists, and naltrindole produced surmountable antagonism of effects of DPDPE. Apparent pA (2) estimates for naltrexone, CTAP, and naltrindole agreed with published estimates, although Schild slopes diverged from predictions for simple competitive antagonism.

CONCLUSIONS

CTAP produces dose-dependent antagonism selective for mu-agonist effects in a standard 55 degrees C tail withdrawal antinociceptive assay.

The chemokine CX3CL1/fractalkine interferes with the antinociceptive effect induced by opioid agonists in the periaqueductal grey of rats

We have reported that there is heterologous interaction between the mu, delta or kappa opioid receptors and the receptors for the chemokines CCL5/RANTES or CXCL12/SDF-1 in the regulation of antinociception in rats. CX3CL1/fractalkine, a chemokine that exclusively binds to CX3CR1, has been found to affect morphine analgesia and tolerance in the spinal cord. The purpose of the present study was to see if the interaction between the chemokine CX3CL1/fractalkine receptor and mu, delta or kappa opioid receptors occurs in the periaqueductal grey (PAG) of adult male S-D rats. The cold-water tail-flick (CWT) test was used to measure antinociception. The results showed that intra-PAG injection of 100 ng CX3CL1/fractalkine 30 min before administration of 400 ng DAMGO, 100 ng DPDPE or 20 microg dynorphin significantly reduced the antinociception induced by each of these peptides. These results demonstrate that activation of the CX3CL1 receptor diminishes the effect of mu, delta and kappa opioid agonists on their receptors in the PAG of rats.

In vivo pharmacological resultant analysis reveals noncompetitive interactions between opioid antagonists in the rat tail-withdrawal assay

BACKGROUND AND PURPOSE

Pharmacological resultant analysis is a technique that can detect secondary effects of competitive antagonists in vitro. The utility of pharmacological resultant analysis as a potential tool for the investigation of antagonist interactions in vivo was examined in the present study using two opioid antagonists, naltrexone and CTAP.

EXPERIMENTAL APPROACH

Using the experimental design of pharmacological resultant analysis, the well-characterized opioid antagonist naltrexone was examined in the presence of multiple doses of CTAP to block the antinociceptive effects of morphine in the rat warm-water (55(o)C), tail-withdrawal assay.

KEY RESULTS

Alone, all doses of naltrexone, CTAP, and CTOP examined blocked the antinociceptive effects of morphine. In the presence of fixed doses of 1 or 10 microg CTAP, increasing doses of naltrexone produced dose-dependent shifts to the right in the morphine dose-response curve. However, a lower dose of naltrexone in combination with 1 or 10 mug CTAP failed to alter the morphine dose-response curve. In the presence of a fixed dose of 0.1 mg kg(-1) naltrexone, CTAP doses produced irregular shifts to the right in the morphine dose-response curves.

CONCLUSIONS AND IMPLICATIONS

Resultant analysis was applied and an apparent pK(C) value for CTAP was found to be one log unit higher than the apparent pA(2) value for CTAP, evidence that CTAP may have secondary actions or that a signal transducer function may be altered by the combinations of these antagonists. Taken together, these data suggest pharmacological resultant analysis can reveal novel interactions between antagonists in vivo.

Opioid antagonists differ according to negative intrinsic efficacy in a mouse model of acute dependence

The purpose of the present study is to compare the capacity of opioid antagonists to elicit withdrawal jumping in mice following two acute pretreatment doses of the opioid agonist morphine. Antagonists that precipitate vigorous withdrawal jumping across both morphine treatment doses are hypothesized to be strong inverse agonists at the mu-opioid receptor, whereas antagonists that elicit withdrawal jumping in mice treated with the high but not the low dose of morphine are hypothesized to be weak inverse agonists. Male, Swiss-Webster mice (15-30 g) were acutely treated with 56 or 180 mg kg(-1) morphine 4 h prior to injection with naloxone, naltrexone, diprenorphine, nalorphine, or naloxonazine. Vertical jumping, paw tremors, and weight loss were recorded. Naloxone, naltrexone, and diprenorphine produced withdrawal jumping after 56 and 180 mg kg(-1)morphine pretreatment. Nalorphine and naloxonazine produced moderate withdrawal jumping after 180 mg kg(-1) morphine pretreatment, but failed to elicit significant withdrawal jumping after 56 mg kg(-1) morphine pretreatment. Nalorphine and naloxonazine blocked the withdrawal jumping produced by naloxone. All antagonists produced paw tremors and weight loss although these effects were generally not dose-dependent. Taken together, these findings reveal a rank order of negative intrinsic efficacy for these opioid antagonists as follows: naloxone=naltrexone> or =diprenorphine>nalorphine=naloxonazine. Furthermore, the observation that nalorphine and naloxonazine blocked the naloxone-induced withdrawal jumping provides additional evidence that nalorphine and naloxonazine are weaker inverse agonists than naloxone.

Potency differences for D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 as an antagonist of peptide and alkaloid micro-agonists in an antinociception assay

D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) is a peptide antagonist that demonstrates potent and selective affinity for micro-opioid receptors in radioligand binding assays and in vitro bioassays. However, previous studies indicate that CTAP may possess unusual pharmacology under certain conditions. Therefore, CTAP was evaluated as an antagonist of the antinociceptive effects of a range of structurally diverse high- and low-efficacy peptide and alkaloid opioid agonists and compared with the traditional antagonist naltrexone. Male Sprague-Dawley rats (N = 227) were loosely restrained and the latency for tail withdrawal from 55 degrees C water was measured. Morphine s.c. and i.c.v., buprenorphine s.c., etorphine s.c. and i.c.v., [N-Me-Phe3,D-Pro4]-morphiceptin and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin produced antinociceptive effects. CTAP was at least 10-fold more potent than naltrexone as an antagonist of the antinociceptive effects of all five agonists. High doses of CTAP produced a noncompetitive antagonism of etorphine s.c. and morphine s.c. suggesting that CTAP may interact with additional opioid receptors in vivo or produce insurmountable antagonism at these doses. CTAP was approximately 300-fold more potent as an antagonist of DAMGO than the other agonists, indicating that CTAP may distinguish some peptide agonists such as DAMGO from other agonists based on binding interactions within the micro-opioid receptor or pharmacodynamic properties of these peptides. Naltrexone, however, administered by either s.c. or i.c.v. routes of administration was approximately equipotent as an antagonist of the antinociceptive effects of most agonists. Taken together, these data indicate that the peptide antagonist CTAP possesses a unique pharmacology unlike traditional opioid antagonists such as naltrexone

Suppression of ethanol responding by centrally administered CTOP and naltrindole in AA and Wistar rats

BACKGROUND

Both mu- and delta-opioid receptors have been implicated in the reinforcing actions of ethanol. However, selective opioid receptor antagonists have not altered ethanol intake in all rodent strains consistently, which suggests that genotype may modulate their suppressive effects. Therefore, we tested the effects of the selective mu-antagonist D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and the selective delta-antagonist naltrindole in both high-drinking AA (Alko, Alcohol) and heterogeneous Wistar rats.

METHODS

AA and Wistar rats were trained to respond for ethanol (10% w/v) in a two-lever operant condition by using a saccharin fading procedure. After stable baseline responding was established, rats were implanted stereotaxically either with a guide cannula above the lateral ventricle or with bilateral cannulas above the nucleus accumbens, basolateral amygdala, or ventral tegmental area. After postoperative recovery, AA and Wistar animals were tested after intracerebroventricular microinjections of either CTOP (0-3 microg) or naltrindole (0-30 microg) or subcutaneous injections of naloxone (0-1 g/kg), which was used as a reference antagonist. Effects of intracerebral microinjections of CTOP and naltrindole (both 0-500 ng) were tested only in Wistar rats.

RESULTS

Subcutaneous naloxone and intracerebroventricular CTOP and naltrindole suppressed ethanol self-administration in a similar manner in AA and Wistar rats. Cumulative response patterns indicated that naloxone and naltrindole had no effect on the initiation of responding but suppressed it later during the session, whereas CTOP also affected initiation. In Wistar rats, naltrindole microinjections into both the nucleus accumbens and basolateral amygdala decreased ethanol responding, whereas CTOP was effective only in the amygdala. Injections of these antagonists into the ventral tegmental area had little effect on ethanol intake.

CONCLUSIONS

The results confirm previous results which showed that both mu- and delta-opioid receptors are involved in the regulation of ethanol self-administration and indicate that genetic differences between AA and Wistar rats produced by selection do not modify the effects of opioid antagonists. The nucleus accumbens and the basolateral amygdala may be important central sites for the mediation of their suppressive effects.

Differential actions of spinal analgesics on mono-versus polysynaptic Adelta-fibre-evoked field potentials in superficial spinal dorsal horn in vitro

Processing of nociceptive information can be modulated at various levels in spinal cord that may range from changes of neurotransmitter release from primary afferent Adelta- or C-fibres to excitability changes of spinal interneurones or motoneurones. The site and mechanism of action of spinal analgesics has been assessed with a number of in vivo and in vitro methods with sometimes conflicting results. Here, we have used transverse spinal cord slices with attached dorsal roots to simultaneously record mono- and polysynaptic Adelta-fibre-evoked field potentials in superficial spinal dorsal horn. Two classical spinal analgesics, morphine and clonidine, and the metabotropic glutamate receptor agonist (IS,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) differentially affected mono- and polysynaptic Adelta-fibre-evoked transmission in spinal dorsal horn. Polysynaptic responses were dose-dependently inhibited while the monosynaptic response remained unaffected. These results suggest that spinal analgesics may preferentially affect polysynaptic but not monosynaptic Adelta-fibre-evoked responses in superficial spinal dorsal horn.

Effects of opioid receptor antagonists on the effects of i.v. morphine on carrageenin evoked c-Fos expression in the superficial dorsal horn of the rat spinal cord

This study performed in freely moving rats evaluated the ability of specific opioid receptor antagonists to reverse the inhibitory effects of morphine on carrageenin-induced c-Fos expression in the spinal cord. Our study focused on the superficial dorsal horn (laminae I-II), which is the main termination site of nociceptive primary afferent fibers and is rich in opioid receptors. In order to replicate clinical routes of administration, all agents were administered intravenously (i.v.). As previously demonstrated, pre-administered i.v. morphine (3 mg/kg) produced a marked decrease (58+/-5%) in the number of Fos-LI neurones measured at 2 h after intraplantar (i.pl.) carrageenin (6 mg/150 microl) and yet was without influence on peripheral oedema. This decrease in c-Fos expression was completely blocked by combined administration of morphine with the mu-opioid receptor antagonist, [D-Phe-Cys-Tyr-D-Orn-Thr-Pen-Thr-NH2] (CTOP-1+1 mg/kg). Naltrindole (NTI-1+1 mg/kg), a delta-opioid receptor antagonist partially blocked the effects of systemic morphine, so that the inhibitory effects of morphine after NTI injection are now 40+/-4%. However, this effect of NTI was weak since the depressive effects of morphine were still highly significant (p<0.001). In contrast, nor-binaltorphimine (nor-BNI-1+1 mg/kg), a kappa-opioid receptor antagonist, had no significant effect on the effects of morphine. These results indicate the major contribution of mu-opioid receptors to the antinociceptive effects of systemic morphine at the level of the superficial dorsal horn. The observed effect of NTI is not necessarily related to a direct action of morphine on delta-opioid receptors and some possible actions of this antagonist are discussed.

Dietary modulation of mu and kappa opioid receptor-mediated analgesia

Research has demonstrated that intake of palatable carbohydrates and fats enhanced morphine-induced analgesia (MIA) in Sprague-Dawley rats. To determine if the effects of palatable foods on nociceptive responses would generalize to other strains of animals and other opioid agonists, the present experiments investigated whether intake of palatable foods would: a) alter MIA in Long-Evans rats, and b) alter analgesia produced by drugs acting at kappa opioid receptors. In experiment 1, adult male Long-Evans rats were fed Purina chow alone or chow and either a 32% sucrose solution, a 0.15% saccharin solution, or hydrogenated vegetable fat. Using a tail-flick apparatus, nociceptive responses, measured as percent maximal possible effect (%MPE), were examined after morphine administration [0.0, 1.0, 3.0, and 6.0 mg/kg subcutaneously (SC)]. %MPEs varied directly as a function of dose and were significantly greater for rats fed chow and either sucrose or fat than for rats fed chow alone or chow and saccharin. Experiment 2 compared the analgesic effect of the kappa opioid receptor agonist U50,488H (0, 5.0, 10.0, and 20.0 mg/kg SC) in rats fed chow alone or chow and a 32% sucrose solution. Administration of U50,488H led to analgesia. However, %MPEs did not vary directly as a function of dose. %MPEs of rats fed chow and sucrose were significantly greater than those of rats fed chow alone after injections of 10.0 and 20.0 mg/kg U50,488H. Experiment 3 compared the analgesic effect of U50,488H (5.0, 10.0, 15.0, and 20.0 mg/kg SC) in rats fed chow alone or chow and either a 0.15% saccharin solution or hydrogenated vegetable fat. Administration of U50,488H led to analgesia. However, %MPEs did not vary directly as a function of dose or as a function of diet. %MPEs of rats fed chow and fat were significantly greater than those of rats fed chow alone after injection of 5.0 mg/kg U50,488H.

Interaction of cholecystokinin and somatostatin with a selective mu-opioid agonist and mu- and kappa-antagonists in thermoregulation

We examined the effects of intracerebroventricular (i.c.v.) injections of cholecystokinin-octapeptide (CCK-8) and somatostatin (SST) and the interactions of these neuropeptides with the selective opioid antagonists, CTAP (mu) and nor-BNI (kappa) and the mu-agonist, PL017, on body temperature (Tb) of the rat at normal ambient temperature (21 +/- 0.5 degrees C). CCK-8 produced short-lasting (15-60 min), dose-related increases in Tb in a dose range of 20 to 900 ng but did not change the Tb at lower doses (0.1-2 ng). Lower doses of SST (1 and 2 micrograms) produced hyperthermia (30-60 min) and a higher dose of SST (10 micrograms) caused hypothermia (30-45 min). PL017 (1 microgram, i.c.v.), alone and in combination with CCK-8, produced hyperthermia. The CCK-8 (300 ng)-induced hyperthermia was blocked by pretreatment of rats with CTAP (1 microgram, i.c.v.), suggesting that the higher doses of CCK-8 increase Tb through the interaction with mu-receptors or the enhancement of release of endogenous opioids acting on the mu-receptor. The hyperthermia elicited by a lower dose of SST (1 microgram) was prevented by pretreatment with CTAP but not with nor-BNI (1 microgram, i.c.v.). Pretreatment with nor-BNI blocked the higher dose (10 micrograms) of SST-induced hypothermia. PL017 or CTAP did not prevent the hypothermic effect of that dose of SST. These results indicate that a lower dose of SST (1 microgram) stimulates the mu-receptor (directly or indirectly) and a higher dose (10 micrograms) interacts with the kappa-receptor in regulation of Tb. Thus, the effects of both CCK-8 and SST on Tb appear to involve the endogenous opioid system.

Naltrexone, naltrindole, and CTOP block cocaine-induced sensitization to seizures and death

I.c.v. injection for 9 days of either naltexone (NTX) (5, 10, 20, 40 micrograms/rat) or a selective mu peptide (CTOP) (0.125, 0.25, 0.5, 1, 3, 6 micrograms/rat) or delta (naltrindole) (NLT) (5, 10, 20 micrograms/rat) subtype opioid receptor antagonist affected sensitization to cocaine (COC) (50 mg/kg, i.p.) administered 10 min after. NTX (5 and 40 micrograms/rat), NLT (10 and 20 micrograms/rat), and the peptide CTOP (0.25-0.5 microgram/rat) attenuated seizure parameters (percent of animals showing seizures, mean score and latency) in a day-related manner. The DD50 (days to reach 50% of death) value for COC was 2.69, whereas it was 9.67 and 7.27 for NTX 5 and 40 micrograms/rat, 8.59 for NLT (10 micrograms/rat), and 6.11, 5.95, and 4.30 for CTOP (0.25, 0.5, and 1 microgram/rat respectively). These findings suggest a concurrent involvement of mu- and delta-opioid receptor subtype in COC-induced sensitization to toxic effects.

Intracerebroventricular treatment with an antisense oligodeoxynucleotide to kappa-opioid receptors inhibited kappa-agonist-induced analgesia in rats

In vivo treatment with an antisense (AS) phosphorothioate oligodeoxynucleotide (oligo) to the rat kappa-opioid receptor selectively inhibited kappa-mediated analgesia in the rat cold-water tail-flick test. Intracerebroventricular (i.c.v.) AS oligo significantly inhibited the analgesic effect of i.c.v. spiradoline, but not that of mu- or delta-opioid agonists. The dose-effect curve for s.c. spiradoline was shifted to the right after AS, but not missense or sense oligo treatment. Thus, AS oligos provide another technique with which to selectively manipulate opioid receptors and further support the role of non-mu opioid receptors in mediating analgesia in rats.