Acute Morphine Tolerance Research Articles

Effects of memantine on estrogen-dependent acute tolerance to the morphine analgesia in female rats

Both human and animal studies suggest that there are sex differences in responding to noxious stimulation as well as in effects of opiate analgesic drugs. Development and/or expression of tolerance to opiate analgesia are also affected by the hormonal status of the experimental subjects. The present study aimed to compare acute tolerance to morphine in cycling and ovariectomized female rats and to evaluate the effects of N-methyl- d-aspartate (NMDA) receptor channel blocker memantine as well as 17-β-estradiol on tolerance development using the tail-flick test. Acute tolerance to morphine analgesia was observed as a substantial reduction in the response to a test dose of morphine (10 mg/kg) given 6 h after the tolerance-inducing dose (10 mg/kg). Significant acute tolerance was observed in proestrous female rats and was prevented by memantine (3 or 10 mg/kg) treatment. Ovariectomized rats did not demonstrate tolerance to morphine analgesic effects but chronic estradiol administration (5 μg/day, 5 days) reinstated induction of tolerance. Both estrogen receptor modulator tamoxifen (5 mg/kg/day, 5 days) and memantine (3 mg/kg/day, 5 days) prevented estradiol-induced tolerance in ovariectomized rats. Thus, estrogens were found to play a key role in induction of acute tolerance to morphine antinociception. Estradiol-induced acute morphine tolerance may have NMDA receptor-dependent mechanisms.

Nitrous Oxide Revisited

Although opioids are unsurpassed analgesics for surgery, they also induce an N-methyl-D-aspartate-dependent enhancement of postoperative hyperalgesia. Because nitrous oxide (N2O) has anti-N-methyl-D-aspartate properties, the purpose of this study was to evaluate nitrous oxide ability to prevent such an opioid-induced hyperalgesia in rats. First, preventive effects of 50/50% N2O-O2 on the development of delayed hyperalgesia observed after inflammatory pain (hind paw carrageenan injection on D0) were examined for several days. Second, the ability of nitrous oxide (10-40%) to limit opioid-induced hyperalgesia induced by fentanyl was evaluated in nonsuffering rats. Third, antihyperalgesic effects of various nitrous oxide concentrations (20-50%) were assessed in both inflammatory and incisional pain models in fentanyl-treated rats (4 x 100 microg/kg subcutaneously). Finally, the analgesic effect of a single dose of morphine was evaluated 24 h after fentanyl administration and nitrous oxide (D0) to assess its preventive effect on acute morphine tolerance in both nonsuffering and hind paw-incised rats. When applied on D0, nitrous oxide reduced delayed hyperalgesia induced by inflammation. Exposure to nitrous oxide on D0 also reduced opioid-induced hyperalgesia in nonsuffering rats in a dose-dependent manner. In fentanyl-treated rats with inflammatory or incisional pain, nitrous oxide strongly limited both magnitude and duration of hyperalgesia. Moreover, nitrous oxide exposure on D0 opposed development of acute tolerance to analgesic effects of morphine administered on D1 in both nonsuffering and incised fentanyl-treated rats. Nitrous oxide, an N-methyl-D-aspartate receptor antagonist, prevented the enhancement of pain sensitivity induced by both nociceptive inputs and fentanyl and opposed acute morphine tolerance. Results suggest that perioperative nitrous oxide use reduces exaggerated postoperative pain and morphine consumption.

The spinal nitric oxide involved in the inhibitory effect of midazolam on morphine-induced analgesia tolerance

Previous studies had shown that pretreatment with midazolam inhibited morphine-induced tolerance and dependence. The present study was to investigate the role of spinal nitric oxide (NO) in the inhibitory effect of midazolam on the development of morphine-induced analgesia tolerance. Subcutaneous injection of 100 mg/kg morphine to mice caused an acute morphine-induced analgesia tolerance model. To develop chronic morphine tolerance in mice, morphine was injected for three consecutive days (10, 20, 50 mg/kg sc on Day 1, 2, 3, respectively). In order to develop chronic tolerance model in rats, 10 mg/kg of morphine was given twice daily at 12 h intervals for 10 days. Midazolam was intraperitoneally injected 30 min prior to administration of morphine. Tail-flick test, hot-plate and formalin test were conducted to assess the nociceptive response. Immunocytochemistry, histochemistry and western blot were performed to determine the effect of midazolam on formalin-induced expression of Fos protein, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and nitric oxide synthase (NOS) in chronic morphine-tolerant rats, respectively. The results showed that pretreatment with midazolam significantly inhibited the development of acute and chronic morphine tolerance in mice, which could be partially reversed by intrathecal injection of NO precursor L-arginine (L-Arg). In chronic morphine-tolerant rats, pretreatment with midazolam significantly decreased the formalin-induced expression of Fos and Fos/NADPH-d double-labeled neurons in the contralateral spinal cord and NADPH-d positive neurons in the bilateral spinal cord. Both inducible NOS (iNOS) and neuronal NOS (nNOS) protein levels in the spinal cord were significantly increased after injection of formalin, which could be inhibited by pretreatment with midazolam. The above results suggested that the decrease of the activity and expression of NOS contributed to the inhibitory effect of midazolam on the development of morphine tolerance.

RGS-Rz and RGS9-2 proteins control mu-opioid receptor desensitisation in CNS: the role of activated Gαz subunits

Two consecutive icv administrations of analgesic doses of mu-opioid receptor agonists lead to a profound desensitisation of the latter receptors; a third dose produced less than 20% of the effect obtained with the first administration. Desensitisation was still effective 24 h later. Impairing the activity of Gαz but not Gαi2 subunits prevented tolerance developing after the administration of three consecutive doses of morphine. Further, the icv injection of Gαi2 subunits potentiated morphine analgesia and abolished acute tolerance, whereas icv-administered Gαz subunits produced a rapid and robust loss of the response to morphine. The RGSZ1 and RGSZ2 proteins selectively deactivate GαzGTP subunits, and their knockdown increased the effects produced by the first dose of morphine. However, impairing their activity also accelerated tachyphylaxis following successive doses of morphine, and facilitated the development of acute morphine tolerance. In contrast, inhibiting the RGS9-2 proteins, which bind to GαoGTP and GαiGTP but only weakly deactivates them, preserved the effects of consecutive morphine doses and abolished the generation of acute tolerance. Therefore, desensitisation of mu-opioid receptors can be achieved by reducing the responsiveness of post-receptor elements (via the possible action of activated Gαz subunits) and/or by depleting the pool of receptor-regulated G proteins that agonists need to propagate their effects, e.g., through the activity of RGS9-2 proteins.

Modulation by α-difluoromethyl-ornithine and aminoguanidine of pain threshold, morphine analgesia and tolerance

The effects of α-difluoromethyl-ornithine (DFMO) and aminoguanidine, which might influence the metabolism of endogenous agmatine, on pain threshold, morphine analgesia and tolerance were investigated in mice. In the mouse acetic acid writhing test, intracerebroventricular (i.c.v.) injection of DFMO or aninoguanidine significantly elevated the pain threshold as indicated by a decrease in the number of writhings. DFMO or aminoguanidine obviously increased the analgesic effect of morphine in the mouse acetic acid writhing test and the mouse heat radiation tail-flick assay. These effects of DFMO and aminoguanidine were antagonized by idazoxan (3 mg/kg, i.p.), which is a selective antagonist of the imidazoline receptor. In the mouse heat radiation tail-flick assay, aminoguanidine significantly prolonged the tail-flick latency of animals, suggesting that the pain threshold was elevated. Furthermore, both DFMO and aminoguanidine enhanced morphine analgesia and inhibited acute morphine tolerance in the mouse heat radiation tail-flick assay. Neither DFMO nor aminoguanidine inhibited the activity of nitric oxide synthase in different brain areas in mice in vivo. These results indicate that the substances involved in the metabolism of endogenous agmatine could modulate the pain threshold, morphine analgesia and tolerance, indicating the possible role of endogenous agmatine in the pharmacological effects of morphine.

Effects of nandrolone on acute morphine responses, tolerance and dependence in mice

Anabolic–androgenic steroid exposure has been proposed to present a risk factor for the misuse of other drugs of abuse. We now examined whether the exposure to the anabolic–androgenic steroid, nandrolone, would affect the acute morphine responses, tolerance and dependence in rodents. For this purpose, mice received nandrolone using pre-exposure (for 14 days before morphine experiments) or co-administration (1 h before each morphine injection) procedures. Nandrolone treatments increased the acute hypothermic effects of morphine without modifying its acute antinociceptive and locomotor effects. Nandrolone also attenuated the development of tolerance to morphine antinociception in the hot plate test, but did not affect tolerance to its hypothermic effects, nor the sensitisation to morphine locomotor responses. After nandrolone pre-exposure, we observed an attenuation of morphine-induced place preference and an increase in the somatic manifestations of naloxone-precipitated morphine withdrawal. These results indicate that anabolic–androgenic steroid consumption may induce adaptations in neurobiological systems implicated in the development of morphine dependence.

The role of ketamine in preventing fentanyl-induced hyperalgesia and subsequent acute morphine tolerance.

Perioperative opioids increase postoperative pain and morphine requirement, suggesting acute opioid tolerance. Furthermore, opioids elicit N-methyl-D-aspartate (NMDA)-dependent pain hypersensitivity. We investigated postfentanyl morphine analgesic effects and the consequences of NMDA-receptor antagonist (ketamine) pretreatment. The rat nociceptive threshold was measured by the paw-pressure vocalization test. Four fentanyl boluses (every 15 min) elicited a dose-dependent (a) increase followed by an immediate decrease of the nociceptive threshold and (b) reduction of the analgesic effect of a subsequent morphine administration (5 mg/kg): -15.8%, -46.6%, -85.1% (4 x 20, 4 x 60, 4 x 100 microg/kg of fentanyl, respectively). Ketamine pretreatment (10 mg/kg) increased the fentanyl analgesic effect (4 x 60 microg/kg), suppressed the immediate hyperalgesic phase, and restored the full effect of a subsequent morphine injection. Fentanyl also elicited a delayed dose-dependent long-lasting decrease of the nociceptive threshold (days) that was prevented by a single ketamine pretreatment before fentanyl. However, a morphine administration at the end of the fentanyl effects restored the long-lasting hyperalgesia. Repeated ketamine administrations were required to obtain a complete preventive effect. Although ketamine had no analgesic effect per se at the dose used herein, our results indicate that sustained NMDA-receptor blocking could be a fruitful therapy for improving postoperative morphine effectiveness. Fentanyl-induced analgesia is followed by early hyperalgesia (hours), acute tolerance to the analgesic effects of morphine, and long-lasting hyperalgesia (days). All these phenomena are totally prevented by repeated administrations of the NMDA-receptor antagonist, ketamine, simultaneously with fentanyl and morphine administration.

Management update: how will we have enough nurses?

The metabotropic glutamate 5 (mGlu5) receptor is involved in both pain processing and modulation of µ-opioid induced antinociception and antihyperalgesia. Systemic mGlu5 receptor antagonists 2-methyl-6-phenylethynylpyridine (MPEP) or 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine (MTEP) provide antihyperalgesic effects in various pain models, but few studies have shown their interaction with morphine in neuropathic pain models. The aim of this study is to compare the effects of systemic and intrathecal MPEP/MTEP on morphine efficacy and tolerance in rats with chronic neuropathic pain. L5-6 spinal nerve ligation (SNL) was used to establish neuropathic pain model in rats. The Von Frey test and the hot water tail flick test were employed as behavior tests. Low, medium and high doses of MPEP/MTEP were tested for their effect on both acute morphine efficacy and chronic morphine tolerance. SNL provides sustained neuropathic pain on the ipsilateral hind paw of rats. Both systemic and intrathecal MPEP/MTEP had antiallodynia effects and boosted morphine's efficacy in a dose-dependent manner in the Von Frey tests but not in the tail flick tests. In fact, high doses of MTEP and MPEP attenuated morphine's antinociceptive effect in the latter test. After intrathecal chronic co-administration with morphine, low-doses of MTEP/MPEP attenuated morphine tolerance in both tests. Systemically, only MTEP attenuated morphine tolerance, and only in the Von Frey tests, not in the tail flick tests, whereas MPEP had no effect on morphine tolerance in either tests. The therapeutic use of mGlu5 receptor antagonists may have distinct effects in different pain models.

Nocistatin, a peptide reversing acute and chronic morphine tolerance.

It has been reported that intracerebroventricular (i.c.v.) injection of nociception/orphanin FQ (OFQ) can antagonize morphine analgesia, whereas i.c.v. OFQ antibody can reverse morphine tolerance. Nocistatin (NST) is a recently characterized neuropeptide possessing an antagonizing effect on OFQ. Here we examine whether i.c.v. NST would result in a reversal of morphine tolerance. The results showed that: (1) i.c.v. NST at doses of 0.005, 0.05, 0.5, 5 or 50 ng per rat produced a bell-shaped dose-dependent reversal of chronic morphine tolerance, with maximum response at 0.5 ng. (2) Acute morphine tolerance could also be reversed, albeit partially, by i.c.v. NST at 0.5 ng. (3) The reversal of acute and chronic morphine tolerance by NST was completely abolished when NST (0.5 ng) was co-injected with (8 microg) OFQ. Since OFQ and NST are derived from the same preprohormone, the profile of its splicing in the CNS may play an important role in determining the effectiveness of morphine analgesia.

O-169 - Protein kinase C-sensitive acute morphine tolerance and receptor internalization through dynamin

O-169 - Protein kinase C-sensitive acute morphine tolerance and receptor internalization through dynamin

Surgical pain attenuates acute morphine tolerance in rats

Nociceptive stimuli, such as formalin-induced pain and adjuvant-induced arthritis, attenuate tolerance to morphine antinociception. In this study, we have explored the effect of upper and lower abdominal surgical pain on the prevention of acute tolerance to morphine antinociception in Sprague-Dawley rats. Group I received lower abdominal surgery (LAS) and i.v. morphine infusion; group II received LAS and i.v. saline infusion; group III received upper abdominal surgery (UAS) and i.v. morphine infusion; group IV received UAS and i.v. saline infusion; group V received i.v. morphine infusion; and group VI received i.v. saline infusion. The antinociceptive effects of morphine were measured by an infrared thermal tail flick test. We also measured plasma concentrations of morphine in rats receiving morphine infusions with or without surgical treatment. We found that acute tolerance to morphine antinociception developed after 2 h following i.v. infusion of morphine alone. However, both UAS and LAS significantly slowed the rate of development of acute tolerance to morphine. The area under the time-response curves (AUC) of groups I and III were mean 34,556 (SD 5607) and 32,548 (9783), respectively, which were significantly different from that of group V (18,759 (8225)) (P < 0.01). Also, there were no significant differences between groups I and III. There were no significant differences between groups for plasma morphine concentrations during the 8-h study (e.g. groups I, III and V: 179.9 (22.6), 182.7 (14.4) and 170.9 (15.8) ng ml-1 at 8 h, respectively) and we suggest that the appearance of acute morphine tolerance after morphine infusion is not pharmacokinetic in nature.

Endogenous orphanin FQ: evidence for a role in the modulation of electroacupuncture analgesia and the development of tolerance to analgesia produced by morphine and electroacupuncture.

1. Our previous work has demonstrated that exogenously administered orphanin FQ (OFQ) antagonizes morphine analgesia and electroacupuncture analgesia (EAA) in the brain and potentiates morphine analgesia and EAA in the spinal cord of the rat. In the present study we evaluated the role of endogenously released OFQ in the development of tolerance to morphine and electroacupuncture (EA) and the analgesia produced by electroacupuncture, by use of the IgG fraction of an anti-OFQ antibody (OFQ-Ab) microinjected into the rat central nervous system (CNS). 2. EAA was produced by stimulating rats at a frequency of 100 Hz. Rats were classified as either high responders (HR) or low responders (LR) based on the analgesic effects of EA. LRs could be converted into HRs by the intracerebroventricular (i.c.v.) microinjection of OFQ-Ab at both 1:1 and 1:10 dilutions but not 1:100. HRs could be changed into LRs by the intrathecal (i.t.) injection of OFQ-Ab at both 1:1 and 1:10 dilutions, but not 1:100. 3. Acute morphine tolerance was induced in rats by repeated subcutaneous (s.c.) injections of morphine (5 mg kg, every 2 h) for 16 h. When injected i.c.v. the OFQ-Ab (1:1 dilution) had no effect on the development of acute morphine tolerance. 4. Chronic morphine tolerance was produced in rats by repeated injection of morphine (5-60 mg kg, s.c., 3 x a day) for 6 days. I.c.v. injection of OFQ-Ab (1:1 dilution) reversed this type of morphine tolerance in rats by 50% (P < 0.01). 5. Acute tolerance to the analgesia produced by EA developed after 6 h of continuous (100 Hz, 3 mA) stimulation. This tolerance was almost completely reversed by the i.c.v. injection of OFQ-Ab (1:1 dilution) (P < 0.05). 6. Chronic tolerance to the analgesic effect of EA was produced by repeatedly administering increasing current (1, 2 and 3 mA, each lasting for 10 min, for a total of 30 min) at a frequency of 100 Hz once a day for 6 days. I.c.v. injection of OFQ-Ab (1:1 dilution) reversed this kind of tolerance by 50% (P < 0.01). 7. Together these results suggest that 100 Hz EA may enhance the release of endogenous OFQ in the CNS of the rat, which in turn may act to antagonize EA-produced analgesia in the brain but potentiate EA produced analgesia in the spinal cord. Therefore, OFQ appears to play an important role in the development of tolerance to the analgesic effects produced by EA. 8. The mechanisms underlying the development of acute morphine tolerance and chronic morphine tolerance appear to be different. Central OFQ may play an important role in the development of tolerance after chronic morphine administration.

Chronic naloxone-induced supersensitivity affects neither tolerance to nor physical dependence on morphine at hypothalamus-pituitary-adrenocortical axis

This study reports the endocrine effects of chronic μ-blockade induced by naloxone on morphine tolerance and withdrawal at hypothalamus-pituitary-adrenocortical (HPA) axis level. Naloxone (0.5 mg/kg/h) or vehicle (1 μl/h) were infused s.c. to Sprague-Dawley rats via osmotic minipumps for 7 days, concomitantly with morphine or placebo pellets for 7–8 days. In opiate-naive rats, the μ-preferring opioid agonist morphine (30 mg/kg) increased plasma corticosterone in a partial but significant naloxone-reversible manner. In vehicle-perfused rats, chronic morphine treatment produced tolerance to its neuroendocrine effect, while the development of morphine tolerance was antagonized in the naloxone-treated group. An enhancement of plasma corticosterone levels after acute morphine (30 mg/kg) occurred 24 h after removal of chronic naloxone treatment in vehicle-perfused rats, as a functional index of supersensitivity to the neuroendocrine effects of the μ agonist. By contrast, 24 h after naloxone removal, rats implanted with morphine pellets were significantly less sensitive to acute morphine (tolerance) than its control-placebo group. Substantial elevation of plasma corticosterone, accompanied by motor and behavioural signs, was observed after acute naloxone injection (1 mg/kg) to tolerant rats 24 h after naloxone-pumps removal, which indicates withdrawal. No endocrine, motor or behavioural signs appeared in the naloxone group with pumps in place. These results indicated that morphine desensitizes μ-opioid receptors that were probably upregulated by chronic naloxone in presence of chronic agonist administration, and suggest that opioid tolerance/dependence as well as opioid supersensitivity simultaneously and independently can occur at μ-opioid receptors mediating HPA function.

Effects of secretin on acute and chronic effects of morphine

The effects of intracerebroventricularly (ICV) administered secretin on the analgesic, tolerance-inducing, and dependence-inducing actions of morphine were investigated, in adult, male CFLP mice. Secretin administered doses ICV did not itself affect pain sensitivity in a heat-radiant tail flick test. However, it depressed the acute nociceptive effect of a single subcutaneous (SC) dose of morphine (4 mg/kg) after ICV (1 or 10 ng/ animal) secretin administration. A dose of 10 ng secretin facilitated the development of acute morphine tolerance. On the other hand, none of the doses applied had any influence on chronic morphine tolerance, where animals were implanted SC with a morphine- containing pellet and the pain sensitivity was measured 3 days later. Morphine withdrawal signs were also evaluated by injecting naloxone. In a 100-ng dose, secretin increased the latency of the withdrawal jumping response; the peptide did not modify the other abstinence signs. These data suggest that central secretin administration can modify the analgesic effect of morphine.

Implantation of AtT-20 or genetically modified AtT-20/hENK cells in mouse spinal cord induced antinociception and opioid tolerance

AtT-20 cells, which make and release beta-endorphin, or AtT-20/hENK cells, an AtT-20 cell line transfected with the human proenkephalin gene and secreting enkephalin as well as presumably beta-endorphin, were implanted in mouse spinal subarachnoid space. Cell implants did not affect the basal response to thermal nociceptive stimuli. Administration of isoproterenol, believed to stimulate secretion from these cells, produced antinociception in groups receiving AtT-20 or AtT-20/hENK cell implants but not in control groups receiving no cells. The antinociceptive effect of isoproterenol was dose related and could be blocked by the opioid antagonist naloxone. Implantation of these cells offers a novel approach for the study of tolerance. Mice receiving AtT-20 cell implants developed tolerance to beta-endorphin and the mu-opioid agonist DAMGO, whereas mice receiving genetically modified AtT-20/hENK cell implants developed tolerance to the delta-opioid agonist DPDPE. Genetically modified AtT-20/hENK cell implants, but not AtT-20 cell implants, reduced the development of acute morphine tolerance in the host mice. This finding is consistent with the suggestion that enkephalin alters development of opioid tolerance. These results suggest that opioid-releasing cells implanted around mouse spinal cord can produce antinociception and may provide an alternative therapy for chronic intractable pain.

Dynorphin a-(1–13)-morphine interactions: Quantitative and qualitative eeg properties differ in morphine-naive vs. Morphine-tolerant rats

The effects of dynorphin A-(1–13) on cumulative IV morphine-induced EEG and EEG power spectra were studied in naive and morphine-tolerant rats. Adult female Sprague-Dawley rats were implanted with cortical EEG electrodes and permanent indwelling ICV and IV cannulae. In naive rats, dynorphin A-(1–13) quantitatively decreased cumulative IV morphine-induced EEG spectral power as well as qualitatively shifting the relative distribution of spectral power to predominantly faster frequencies. In morphine-tolerant rats, the quantitative and qualitative EEG properties were identical to those in dynorphin A-(1–13) pretreated morphine-naive rats. Thus, dynorphin A-(1–13) pretreatment apparently produced instantaneous acute morphine tolerance. Furthermore, in morphine-tolerant rats, dynorphin A-(1–13) pretreatment quantitatively increased morphine-induced EEG power without qualitatively changing the relative distribution of EEG spectral power. This latter effect may be due to a summation of increased endogenous levels of dynorphin A-(1–13) associated with the development of morphine tolerance and the experimentally administered dynorphin A-(1–13). These results indicate that dynorphin-induced quantitative and qualitative EEG changes of morphine may reflect different underlying processes. That is, quantitative changes may reflect the number of receptors that are activated, while qualitative changes may reflect the nature of the receptor-effector coupling.

Administration of morphine before surgery may lead to acute morphine tolerance after surgery

Administration of morphine before surgery may lead to acute morphine tolerance after surgery

Effects of atrial natriuretic peptide on acute and chronic effects of morphine

Atrial natriuretic peptide (ANP) is known to participate in different vegetative functions. The aim of the present study was to investigate the influence of ANP on nociception itself, pain sensitivity to morphine, and the development of acute and chronic tolerance to morphine. Morphine withdrawal signs were also evaluated by injecting naloxone. In adult, male NMRI mice, ANP administered SC or ICV did not affect pain sensitivity itself in a heat-radiant tail-flick test. Peptide treatment, however, depressed the acute nociceptive effect of a single dose of morphine (4 mg/kg, SC) after both SC (20–200 ng/animal) and ICV (5, 10, 20, or 200 ng/animal) ANP administration. ANP given SC and ICV attenuated the development of acute morphine tolerance. Acute morphine tolerance was assessed by giving a bolus injection of morphine (60 mg/kg) 24 h before the pain sensitivity to a challenge dose of morphine (4 mg/kg) was measured. ICV treatment with ANP also blocked the development of chronic morphine tolerance, but did not affect the appearance of naloxone-precipitated withdrawal syndromes. ANP seems to act differently on the development of tolerance to and dependence upon morphine.

Maintenance of acute morphine tolerance in mice by selective blockage of κ opioid receptors with norbinaltorphimine

In this study we investigated the effect of the highly selective κ opioid antagonist, norbinaltorphimine (norBNI) on the development of tolerance to a single dose of morphine. Mice were pretreated with 100 mg/kg of morphine sulfate (morphine), s.c. and 2 h later, norBNI (20 mg/kg s.c.) was administered and various times after this pretreatment, antinociceptive ED 50 value of morphine was determined in the tail-flick assay. Twenty-four and 72 h after morphine injection, ED 50 values of morphine were significantly increased by about 2.5-fold from those of their control mice that received saline instead of the tolerance-inducing dose of morphine. In a second set of experiments, animals were pretreated similarly with morphine and norBNI and 72 h after morphine injection, various opioid agonists were applied by the i.c.v. or i.t. route to see whether or not any cross-tolerance had developed to these agonists. The ED 50 of i.c.v.-administered morphine was significantly greater than that of the non-pretreated controls. A small degree of cross-tolerance was observed with U-50,488H but not with DPDPE [D-Pen 2, D-Pen 5]enkephalin (DPDPE) at the supraspinal site. At the spinal site, tolerance to morphine was not observed. These results suggest that antagonism at κ opioid sites after morphine administration, modulates positively the development of opioid tolerance.

Modulation of acute morphine tolerance by corticotropin- releasing factor and dynorphin a in the mouse spinal cord

Previously, we have demonstrated that intrathecally (i.t.) administered corticotropin-releasing factor (CRF) in mice produces stimulus-specific antinociception and modulation of morphine-induced antinociception by mechanisms involving spinal kappa opioid receptors. Recently, we also have found that CRF releases immunoreactive dynorphin A, a putative endogenous kappa opioid receptor agonist, from superfused mice spinal cords in vitro. Dynorphin A administered intracerebroventricularlly (i.c.v.) to mice has been shown to modulate the expression of morphine tolerance. In the present study, the possible modulatory effects of i.t. administered CRF as well as dynorphin A on morphine tolerance were studied in an acute tolerance model. Subcutaneous administration of 100 mg/kg of morphine sulfate (MS) to mice caused an acute tolerance to morphine-induced antinociception. The antinociceptive ED 50 of MS was increased from 4.4 mg/kg (naive mice) to 17.9 mg/kg (4 hours after the injection of 100 mg/kg MS). To study the modulatory effects of spinally administered CRF and dynorphin A on the expression of morphine tolerance, CRF and dynorphin A were injected i.t. at 15 min and 5 min, respectively, before testing the tolerant mice by the tail-flick assay. The antinociceptive ED 50 of MS in tolerant mice was decreased to 8.8 mg/kg and 7.1 mg/kg, respectively, after i.t. administration of CRF (0.1 nmol) and dynorphin A (0.2 nmol). In contrast, 0.5 nmol of alpha-helical CRF (9–41), a CRF antagonist and 0.4 nmol of norbinaltorphimine, a highly selective kappa opioid receptor antagonist, when administered i.t. at 15 min before the tail-flick test in tolerant mice, increased the antinociceptive ED 50 of MS to 56.6 mg/kg and 88.8 mg/kg, respectively. These data confirmed the modulatory effect of dynorphin A on morphine tolerance and suggested that CRF, which releases dynorphin A in several central nervous system regions, also plays a modulatory role in the expression of morphine tolerance.