Morphine-tolerant Rats Research Articles

Dynorphin-(1-13): antinociceptive action and its effects on morphine analgesia and acute tolerance.

Antinociceptive actions and effects of intracerebroventricular (i.c.v.) dynorphin-(1-13) (DYN) on morphine (MOR) analgesia and acute tolerance were studied in male Sprague-Dawley rats. Antinociceptive effect against hind paw pressure was produced by 30 micrograms of DYN, but not by 0.5-10 micrograms. Acetic acid writhing was inhibited dose-dependently by DYN at the doses of 2-30 micrograms, and the order of potency of the anti-writhing effect was beta-endorphin > MOR > DYN >> Met-enkephalin. The anti-writhing effect of DYN that was partially antagonized by naloxone at 10 mg/kg, s.c. in MOR tolerant rats was the same as that in MOR naive rats. The anti-writhing effect of i.c.v.-MOR was increased synergistically by DYN. Continuous s.c. (6 mg/kg/hr) and i.c.v. (7.5 micrograms/rat/hr) infusion of MOR produced antinociception against hind paw pressure, which reached maximum (MAX) and attenuated thereafter during MOR infusion for 6 hr. The attenuation of antinociception was also produced during MOR infusion combined with multiple i.c.v.-injection of DYN. The MAX and area under the antinociceptive curve during MOR infusion was not affected by multiple injection of DYN, i.e., no effect of i.c.v.-DYN on the development of acute MOR tolerance induced by s.c.- and i.c.v.-infusion was observed. In conclusion, the anti-writhing effect of i.c.v.-DYN might not be mediated via mu-receptors, although DYN increased the anti-writhing effect of i.c.v.-MOR synergistically and the development of acute tolerance to MOR (i.c.v., s.c.) was not affected by i.c.v.-DYN.

Dynorphin-(1–13): Antinociceptive Action and Its Effects on Morphine Analgesia and Acute Tolerance

Antinociceptive actions and effects of intracerebroventricular (i.c.v.) dynorphin-(1–13) (DYN) on morphine (MOR) analgesia and acute tolerance were studied in male Sprague-Dawley rats. Antinociceptive effect against hind paw pressure was produced by 30 μg of DYN, but not by 0.5-10 μg. Acetic acid writhing was inhibited dose-dependently by DYN at the doses of 2–30 μg, and the order of potency of the anti-writhing effect was β-endorphin > MOR > DYN ≫ Met-enkephalin. The anti-writhing effect of DYN that was partially antagonized by naloxone at 10 mg/kg, s.c. in MOR tolerant rats was the same as that in MOR naive rats. The anti-writhing effect of i.c.v.-MOR was increased synergistically by DYN. Continuous s.c. (6 mg/kg/hr) and i.c.v. (7.5 μg/rat/hr) infusion of MOR produced antinociception against hind paw pressure, which reached maximum (MAX) and attenuated thereafter during MOR infusion for 6 hr. The attenuation of antinociception was also produced during MOR infusion combined with multiple i.c.v.-injection of DYN. The MAX and area under the antinociceptive curve during MOR infusion was not affected by multiple injection of DYN, i.e., no effect of i.c.v.-DYN on the development of acute MOR tolerance induced by s.c.- and i.c.v.-infusion was observed. In conclusion, the anti-writhing effect of i.c.v.-DYN might not be mediated via mw-receptors, although DYN increased the anti-writhing effect of i.c.v.-MOR synergistically and the development of acute tolerance to MOR (i.c.v., s.c.) was not affected by i.c.v.-DYN.

IgG from neuropeptide FF antiserum reverses morphine tolerance in the rat

Previous studies suggest that neuropeptide FF (NPFF) plays a role in opiate dependence and subsequent abstinence syndrome. The present study assessed the role of NPFF in opiate tolerance. Third ventricular injection of IgG from NPFF antiserum restored the analgesic response to i.c.v. morphine in morphine-tolerant rats (radiant heat tail flick test). IgG from control serum failed to produce this effect. In opiate-naive rats, however, the same treatment with IgG from NPFF antiserum did not affect the analgesic response to i.c.v. morphine. Thus, immunoneutralization of NPFF appears to selectively restore morphine sensitivity in opiate-tolerant animals. These results support the hypothesis that endogenous NPFF contributes to opiate tolerance.

Delayed application of MK-801 attenuates development of morphine tolerance in rats

To investigate the possible involvement of enduring or delayed changes at the N-methyl- d-aspartic acid (NMDA) receptor in the mechanisms of morphine tolerance, rats were treated with the specific NMDA receptor antagonist, MK-801 (0.15 mg/kg) 2 h after morphine injection (20 mg/kg) during a 4-day induction period of tolerance. On the fifth day rats were injected only with morphine (15 mg/kg), and analgesia was assessed using the hot-plate test. Morphine tolerance was significantly reduced by MK-801. These findings suggest that long-lasting or delayed changes at the NMDA receptor underlie the development of morphine tolerance Moreover, because MK-801 was delivered 2 h after morphine and therefore could not serve as a cue for morphine administration, these findings indicate that the attenuating effect of MK-801 on the development of morphine tolerance is not attributable to state-dependent learning.

Enhanced development of morphine tolerance in rats treated with 2-deoxy-d-galactose

Rats treated subcutaneously for 6 days with morphine developed a weak tolerance which was characterized by a decrease in the analgesic action of the opioid. Under those experimental conditions a simultaneous intracerebroventricular application of 2-deoxy-D-galactose enhanced development of morphine tolerance, while other deoxy-sugars like 2-deoxy-D-glucose and 6-deoxy-D-galactose were ineffective. In contrast, no influence of 2-deoxy-D-galactose on a more enhanced morphine tolerance after a 11-day pretreatment with morphine was found. The results are discussed in the light of a rather specific interference of 2-deoxy-D-galactose with neuronal glycoprotein processing and related cellular mechanism underlying adaptive processes involved in the development of morphine tolerance.

Inhibition of the analgesia inhibitory system by D-phenylalanine and proglumide

Stimulation of a nonacupuncture point (NAPS) does not normally produce analgesia in the same way that stimulation of an acupuncture point does. However, NAPS did produce dexamethasone reversible analgesia after intraperitoneal treatment with D-phenylalanine (DPA) or proglumide, or after microinjection of these compounds into such parts of the analgesia inhibitory system (AIS) as the lateral centromedian nucleus of the thalamus and part of the posterior hypothalamus. Inhibition of acupuncture analgesia (AA), or of morphine analgesia (MA) by 0.5 mg/kg, IP, which is equivalent to AA after AIS lesion, and of potentials in the lateral periaqueductal central gray evoked by repetitive NAPS or stimulation of the AIS, were antagonized by DPA. Disappearance of AA and MA in morphine tolerant acupuncture responder and nonresponder rats was reversed to reproduce the same magnitude of analgesia after proglumide application. The reproduced AA and MA were antagonized by dexamethasone. These results indicate that DPA and proglumide antagonized the AIS and unmasked the dexamethasone reversible AA and MA.

Urocanic acid as a physiological anti-allergic agent

1.1. The possible role of pharmacokinetics of morphine in the development of tolerance to the analgesic and hyperthermic effects of morphine was studied in the rat.2.2. Male Sprague-Dawley rats were made tolerant to morphine by implanting 6 morphine pellets each containing 75 mg of morphine base for 7 days. The assessment of the degree of tolerance to morphine and pharmacokinetic parameters were done 72 hr after pellet removal.3.3. Tolerance developed to both the analgesic and hyperthemic effects of morphine as evidenced by decreased responses to morphine in morphine pellet implanted rats compared with placebo pellet implanted rats.4.4. The pharmacokinetic parametrs, AUC0→∞, Cmax, t12, k, MRT, Vss and Clt were determined after injecting 5 and 10 mg/kg doses of morphine intravenously to placebo and morphine pellet implanted rats and using a highly sensitive and specific RIA method to quantitate serum levels of morphine. For a 5 mg/kg dose of morphine, the AUC0→∞ and t12 in morphine pellet implanted rats were significantly higher than in placebo pellet implanted rats, but the k value was lower. The other pharmacokinetic parameters for morphine in the two treatment groups did not differ. For 10 mg/kg dose, the only change was an increase in the MRT in morphine tolerant rats when compared to nontolerant rats.5.5. The results establish that the development of tolerance to the analgesic and hyperthermic effects of morphine is not related to pharmacokinetics of morphine in serum but may be related to modification of receptor systems in the central nervous system.

Differential cross-tolerance to mu and kappa opioid agonists in morphine-tolerant rats responding under a schedule of food presentation

If different populations of opioid receptors mediate the actions of mu and kappa opioid agonists, then tolerance induced by the chronic administration of a mu agonist should confer cross-tolerance to other mu agonists but not necessarily to those compounds whose effects are mediated by the kappa receptor. This hypothesis was evaluated in the present investigation by examining the effects of the mu agonists morphine, l-methadone and fentanyl, the kappa agonists U50,488 and bremazocine, and the mixed kappa/mu agonist ethylketocyclazocine in rats responding under a fixed-ratio 30 schedule of food presentation before, during and after exposure to a regimen of chronic morphine administration. For comparison, naloxone was evaluated as a representative mu antagonist and the phenothiazine chlorpromazine as a control drug. During all phases of the experiment, each of these compounds produced dose-related decreases in rate of responding. During the daily administration of 40 mg/kg morphine, tolerance developed to the rate-decreasing effects of morphine, l-methadone and fentanyl, and an enhanced sensitivity to the effects of naloxone. In contrast to the effects obtained with these mu opioids, there was no evidence that chronic morphine administration produced tolerance or enhanced sensitivity to the rate-decreasing effects of U50,488, bremazocine, ethylketocyclazocine and chlorpromazine. The present findings demonstrate that the chronic administration of morphine results in the selective development of tolerance to other mu agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Butorphanol's efficacy at mu and kappa opioid receptors: Interences based on the schedule-controlled behavior of nontolerant and morphine-tolerant rats and on the responding of rats under a drug discrimination procedure

The purpose of the present investigation was to characterize the mu agonist and kappa antagonist effects of the mixed opioid agonist/antagonist butorphanol. To this end, the effects of butorphanol were examined: 1) alone and in combination with the kappa agonist bremazocine in nontolerant and morphine-tolerant rats responding under a fixed-ratio 30 (FR30) schedule of food presentation, and 2) in rats trained to discriminate 10 mg/kg morphine from saline. Prior to the induction of morphine tolerance, morphine, bremazocine and butorphanol produced dose-dependent decreases in rate of responding under the FR30. In these nontolerant rats, butorphanol failed to antagonize bremazocine's rate-decreasing effects. During the chronic morphine regimen, the dose-effect curve for morphine was shifted to the right of its prechronic position by approximately 0.9 log units, whereas the bremazocine curve was not altered substantially. The butorphanol dose-effect curve, in contrast, was shifted to the right and flattened such that doses which eliminated responding in nontolerant rats, as well as doses approximately 1.0 log unit higher, had no effect on responding. In these morphine-tolerant rats, butorphanol produced a dose-dependent antagonism of bremazocine's rate-decreasing effects. In rats trained to discriminate morphine from saline, butorphanol substituted completely for the morphine stimulus. Unlike morphine, which produced its stimulus effects only at doses that decreased rate of responding, butorphanol substituted for the morphine stimulus at doses that had little or no effect on rate of responding. The present investigation indicates that butorphanol acts as a kappa antagonist in the morphine-tolerant rat, an effect that is masked by butorphanol's rate-decreasing effects in the nontolerant rat. In addition, the profound degree of cross-tolerance conferred to butorphanol in morphine-tolerant rats and the finding that butorphanol substituted for the morphine stimulus at doses that had no effect on rate of responding suggest that butorphanol is an agonist at the mu receptor with intermediate efficacy.

Plasma β-endorphin and cortisol levels in morphine-tolerant rats and in naloxone-induced withdrawal

The relationship between morphine tolerance and pituitary-adrenocortical activity was examined. In rats made tolerant to morphine by implantation of morphine-containing pellets, there was a significant reduction in plasma levels of β-endorphin-like immunoreactivity (β-END-LI), whereas no significant changes in cortisol levels were seen. Naloxone treatment induced an increase in plasma β-END-LI and cortisol levels in morphine-tolerant animals. Additionally, acute morphine administration induced an increase in plasma levels of β-END-LI and cortisol, an effect which was prevented by naloxone. These results are consistent with an increased release of pro-opiomelanocortin-derived peptides after acute morphine and with a decreased release of these peptides in tolerant rats, and suggest that opioid peptides play an important role in the regulation of pituitary-adrenocortical function.

Effects of opioid agonists and antagonists in morphine-tolerant pigeons and rats responding under a schedule of food presentation

Effects of opioid agonists and antagonists in morphine-tolerant pigeons and rats responding under a schedule of food presentation

Modification of rat thermal responses to morphine by α-FMH suggests a role for neural histamine in morphine tolerance

In rats, morphine may either raise or lower body temperature depending on the dose. A morphine dose of 50 mg/kg, i.p., consistently produced a nearly maximal hypothermic response in non tolerant rats, whereas this dosage induced an elevation of body temperature in tolerant rats. In rats pretreated with alpha-fluoromethylhistidine (alpha-FMH), an irreversible inhibitor of histidine decarboxylase which induces a reduction in brain histamine synthesis, this morphine dose of 50 mg/kg, i.p. produced an elevation of rectal temperature resembling that observed in morphine-tolerant rats. To confirm the suggestion that hyperthermic effects of the higher dose of morphine in morphine-tolerant rats or in alpha-FMH-pretreated rats could be related to a possible involvement of mediators of fever, e.g. prostaglandins, animals were pretreated with acetylsalicylic acid (aspirin, Bayer) 30 mg/kg, i.p., 60 min before morphine. Results showed that acetylsalicylic acid prevented the hyperthermic response of morphine, resulting in a fall in body temperature. Since morphine releases histamine and alpha-FMH inhibits histamine synthesis, our data demonstrating that an inhibitor of prostaglandin-synthetase showed efficacy only in animals responding with fever to the higher dose of the opiate, suggests a physiological antagonism between histamine and prostaglandins on mechanisms underlying hyper/hypothermic responses to morphine.

Some effects of the opioid antagonist, naloxone, upon the rat's reactions to a heat stressor.

Eight experiments examined the apparently paradoxical analgesia that accrues when rats are repeatedly injected with an opiate antagonist, naloxone, and exposed to a heat stressor. Experiments 1 and 2 showed that such pairings came to enhance in a dose-dependent manner the latencies with which rats paw-licked in response to the stressor. Experiment 3 demonstrated that the latencies to paw-lick in saline-treated rats decreased with increases in the intensity of the heat, indicating that naloxone had not provoked long latencies to paw-lick by increasing the functional intensity of the stressor. Experiment 4 documented a role for conditioning processes in recruiting the naloxone-induced analgesia. Experiment 5 showed that the analgesic effect was due to the pairings of the drug and the heat stressor, as a history of exposure to naloxone in a distinctive environment did not render the animals analgesic when challenged with the drug and the stressor. Experiments 6 and 7 provided evidence that the conditioned analgesia that accrued from drug-stressor pairings was non-opioid in nature, as the analgesia was observed in morphine-tolerant rats and was not reversed by an administration of naloxone in advance of exposure to the conditioning context. Experiment 8 demonstrated that the administration of morphine in the context previously associated with naloxone-stressor pairings provoked a superanalgesia. Although analgesic on the paw-lick assay, naloxone-treated subjects did not appear to be insensitive to the heat or impaired motorically, as they persistently reared with short latencies. The results were discussed in terms of the collateral inhibition model of the endogenous pain control system, and some speculations were offered concerning the relation between paw-licking and rearing.

Antinociception in the rat induced by a cold environment

Rats placed in a cold environment (4 °C) for 2 h had a sustained increase in tail flick latency (TFL) as well as increase in tail pinch latency (TPch) that was often biphasic with an early peak response at 15 min and a later, often higher, peak at 2 h. Plasma beta-endorphin levels after a modest increase at 5 min (24%) declined throughout the remaining time in the cold. The long-acting opioid antagonist naltrexone had no effect on TFL increases but led to a greater in reases in TPch (P<0.04). In morphine-tolerant rats TFL response was the same as in controls but TPch increases were greater (P<0.04). Rats exposed to 2 h of cold for 17 or 18 consecutive days generally developed tolerance to the analgesia of cold, i.e. TFL and TPch increases were diminished; however, the response to morphine on day 18 was the same as in rats never exposed to cold. Adrenalectomy and hypophysectomy led to significantly smaller increases in TFL (P<0.02andP<0.001, respectively). The TPch response in contrast, was greater in adrenalectomized (P<0.001) and the same in hypophysectomized rats compared to sham controls. An opioid kappa receptor antagonist (Mr 1452) given prior to cold reduced both TFL and TPch response during the first hour. Thus the analgesia induced by cold appeared to shift from an early possibly kappa opioid to a later non-opioid form. The TFL effects seemed to be under hormonal influence while the TPch were not.

Stereospecific effects of a κ-opiate antagonist on the actions of morphine in morphine-tolerant rats

The effects of κ-opiate receptor antagonist, MR 2266 and its dextro isomer, MR 2267 on morphine-induced analgesia and changes in colonic temperature were determined in morphine-naive and morphine-tolerant male Sprague-Dawley rats. Intraperitoneal administration of morphine (8 mg/kg) produced analgesia and hyperthermia in morphine-naive rats. MR 2266 (0.3-3.0 mg/kg) antagonized morphine-induced analgesia and hyperthermia in morphine-naive rats but MR 2267 was inactive. Subcutaneous implantation of six morphine pellets during a 7 day period induced tolerance to the analgesic and hyperthermic effects of morphine in the rat. MR 2266 also antagonized morphine analgesia and hyperthermia in morphine-tolerant rats, however, MR 2267 had no effect. A high dose (3 mg/kg) of MR 2266 produced an intense hypothermic response in morphine-tolerant rats. Previously we have shown that κ-opiate receptor agonists antagonize morphine analgesia in morphine-naive rats but potentiate it in morphine-tolerant rats. The results of the present studies indicate that the antagonist of κ-opiate receptors, on the other hand, antagonize morphine effects in both morphine-naive and morphine-tolerant rats in a stereospecific manner.

Stress-induced potentiation of morphine-induced analgesia in morphine-tolerant rats

This study was designed to evaluate whether or not rats that were tolerant to the analgesic action of morphine were also tolerant to stress-induced potentiation of morphine-induced analgesia. Rats were trained to drink either solutions of morphine (0.5 mg/ml) or drug-free tap water on a limited access schedule (10 min every 6 hr). The daily intake of morphine averaged 46 mg/kg. Nontolerant and rats tolerant to morphine were tested for morphine-induced analgesia (tail-flick assay), while either unstressed or stressed (i.e. immobilized in Plexiglas cylinders). Morphine produced dose- and time-dependent increases in tail-flick latencies in all groups. Increased sensitivity to analgesia induced by morphine was evident for both nontolerant and tolerant, stressed rats, when compared to their unstressed counterparts. Stress-induced potentiation of morphine-induced analgesia was characterized by dose-related increases in the peak effect and duration of the effect. Stress potentiated the analgesic effect of morphine, comparably in nontolerant (1.7-fold) and tolerant (1.5-fold) rats. Differential tolerance to analgesia induced by morphine and to stress-induced potentiation of morphine-induced analgesia suggests that different mechanisms mediate these two effects.

Kappa antagonist properties of buprenorphine in non-tolerant and morphine-tolerant rats

Buprenorphine was evaluated for its ability to act as a kappa opioid antagonist in rats responding under a fixed-ratio 30 schedule of food presentation both before and after the induction of morphine tolerance. Before the induction of morphine tolerance, both buprenorphine and the selective kappa agonist bremazocine decreased rates of responding in a dose-dependent manner, and buprenorphine (0.03 and 0.3 mg/kg) failed to antagonize bremazocine's rate-decreasing effects. Following the induction of morphine tolerance, the bremazocine dose-effect curve was unaffected, but a profound cross-tolerance developed to buprenorphine. Furthermore, buprenorphine (0.03, 0.3 and 1.0 mg/kg) produced a dose-dependent antagonism of the rate-decreasing effects of bremazocine in the morphine-tolerant rats. These results support the hypothesis that buprenorphine has antagonist activity at kappa opioid receptors.

Locus ceruleus discharge characteristics of morphine-dependent rats: Effects of naltrexone

Spontaneous and sensory-evoked discharge was recorded from locus ceruleus (LC) neurons of halothane-anesthetized rats that were chronically administered morphine. LC spontaneous discharge rates of morphine-treated rats were comparable to those of rats chronically administered saline. Administration of 1.0 μg morphine (i.c.v.), a dose which completely inhibits LC discharge of morphine-naive rats, had no effect on LC spontaneous discharge of morphine-treated rats, demonstrating that opiate tolerance had developed. Naltrexone, 0.3 and 1.0 μg i.c.v., produced increases in LC spontaneous discharge rates that were 172 and 166% greater that baseline, respectively. Additionally, naltrexone disrupted LC discharge evoked by repeated sciatic nerve stimulation such that evoked discharge was decreased with respect to tonic discharge, and postactivation inhibition was attenuated. Naltrexone did not alter spontaneous or sensory-evoked LC discharge of rats chronically administered saline indicating that these neuronal effects are specific to opiate withdrawal. Pretreatment of rats with dexamethasone, or with an antagonists of corticotropin-releasing factor (CRF), a-helical CRF 9−41, did not attenuate the effects of naltrexone on LC discharge of morphine tolerant rats. The present study confirms other reports of LC activation associated with antagonist precipitated opiate withdrawal in vivo, and extends these observations by characterizing the disruptive effect of opiate withdrawal on the response of LC cells to phasically presented sensory stimuli, and demonstrating that the withdrawal response is not mediated by release of endogenous CRF.

Down-regulation of proopiomelanocortin synthesis and beta-endorphin utilization in hypothalamus of morphine-tolerant rats

We investigated regulation of the dynamic state of enkephalin and endorphin brain stores during morphine tolerance and dependence using cDNA hybridization and radioimmunoassay of the biologically active peptide(s) and their respective peptide precursors. Rats were made tolerant to morphine with the subcutaneous implantation of three morphine pellets (75 mg each) for a period of five days. Hypothalamic proopiomelanocortin (POMC) mRNA, POMC, and corticotropin-like intermediate lobe peptide content were decreased by 50% in morphine-dependent rats. However, beta-endorphin content remained unchanged. Enkephalin and proenkephalin mRNA content in various brain structures failed to change. A single injection of naltrexone (2 mg/kg) 1 hour before decapitation did not reverse the decrease in POMC mRNA and POMC content elicited by morphine. However, a slower, spontaneous withdrawal caused by removal of the pellets did reverse (after two days) the down-regulation of the hypothalamic POMC system. A single injection of morphine (10 mg/kg) failed to affect any parameter used to assess the dynamic state of opioid peptides.

Procedures that produce context-specific tolerance to morphine in rats also produce context-specific withdrawal.

Rats previously injected with morphine in the presence of a distinct environment (paired animals) were more tolerant to the analgesic effects of morphine in that environment than were rats previously injected with morphine in another environment (unpaired animals). When injected with saline instead of morphine in the distinct environment, paired animals were more reactive to pain (hyperalgesic) than unpaired animals, but no more reactive to pain than animals never given morphine. More important, the paired animals also exhibited more withdrawal symptoms (wet dog shakes, genital licking, circling, rearing, and defecation) during abstinence and naltrexone-precipitated withdrawal in the distinct environment than did the unpaired and saline animals. Thus, procedures that are capable of producing context-specific opiate tolerance are also capable of producing context-specific opiate withdrawal.