Acute morphine tolerance in new born and young rats

The development of tolerance after two consecutive doses of morphine administered at 4 h intervals was studied in 16, 25 and 38 day-old Sprague-Dawley rats. Tolerance was measured by the diminution of the rats' response to morphine on a hot plate (56 C). The ages selected correspond to clear stages of the brain barrier permeability to morphine. A loss of sensitivity to morphine was observed according to age. Acute tolerance developed at all ages tested, except when higher doses were employed in 25 and 38 day-old rats. When acute tolerance had developed it persisted for a long time. It is concluded that the development of acute tolerance to morphine analgesia does not depend on the stage of development of the rat.Partially supported by grants from the Gildemeister Foundation, Santiago, Chile, and from the Fondo de Investigaciones Científicas, Universidad Católica de Chile, Santiago, Chile     

The effects of p-chlorophenylalanine on morphine analgesia,tolerance and dependence development in two strains of rats

The effects of p-chlorophenylalanine (p-CPA; 100 mg/kg/day for 3 days) on morphine analgesia and the development of tolerance and physical dependence were investigated in Sprague-Dawley (SD) and Fisher (F) strains of albino rats. Using a modified flinch-jump method to detect changes in reactivity to electric footshock, F strain rats were more reactive to the footshock than SD rats, but showed less relative increase in threshold (analgesia) than SD rats following various doses of morphine. Pretreatment with p-CPA attenuated significantly morphine analgesia in SD, but not F rats. In animals implanted subcutaneously with morphine pellets, p-CPA appeared to delay the development of tolerance to morphine in both strains of rats. Hyperalgesia and loss of body weight resulted from administration of naloxone to pellet-implanted rats and p-CPA pretreatment lessened these withdrawal effects significantly in SD rats only. These results emphasize the importance of strain differences in the study of morphine analgesia and development of tolerance and dependence. Assuming differences in the function of the serotonergic inhibitory system in the two strains of rats, these data provide general support for the involvement of brain 5-HT mechanisms in modulating, if not mediating the effects of morphine.     

The influence of learning on morphine analgesia and tolerance development in rats tested on the hot plate

The influence of learning on the development of tolerance to the analgesic effect of morphine in rats was examined employing the hot plate procedure. A tested-reinforced (Tr) group and its yoked-control, a tested-non-reinforced (Tnr) group, received identical exposure to the testing procedure; the Tr group was reinforced daily for its behavior on the heated plate whereas the Tnr group was reinforced only on the last day of the experiment. Paired statistical comparisons between these two groups on the last day of the experiment revealed that: 1. premorphine control reaction times on the heated plate were significantly lower in Tr than in Tnr animals; and 2. post-morphine increases in reaction time did not differ between Tr and Tnr animals. It was concluded that whereas some learning does occur in this testing procedure, learning does not influence the behavioral tolerance to morphine which develops in this analgesiometric method. An hypothesis which accommodates this behavioral tolerance and a mechanistic scheme is offered.     

Estrous cycle stage-dependent expression of acute tolerance to morphine analgesia in rats

Both baseline pain sensitivity and the response to antinociceptive treatment are sensitive to an animal's sex and estrous cycle stage. Sex differences are also observed in the development of antinociceptive tolerance induced by repetitive exposure to opiate drugs such as morphine. Conventional tolerance study protocols do not assess the impact of the estrous cycle stage. The present study aimed to compare the development of acute tolerance to morphine-induced antinociception in male and female (cycling and ovariectomized) Wistar rats using the tail-flick test. Acute tolerance was induced by two consecutive subcutaneous injections of morphine (10 mg/kg) or saline separated by an interval of 6 h. It was found that rats pretreated with morphine were tolerant to the second morphine dose. Tolerance was most pronounced in proestrous female rats and, to a lesser degree, in male rats. It was absent in ovariectomized rats as well as during the estrus, metestrus and diestrus phases. Thus, the estrous cycle exerts dramatic effects on the induction of acute tolerance to morphine-induced antinociception. These results suggest that pain management strategies can be optimized through the use of sex- and estrous cycle-specific techniques.     

Restraint alters the effects of morphine and heroin on core temperature in the rat

The importance of restraint in determining the magnitude of alteration in the rat's core temperature (Tc) after the administration of morphine sulphate (M) and heroin hydrochloride (H) was investigated. M, in doses of 5, 15 and 30 mg/kg, or H, in doses of 0.1, 1 and 5 mg/kg was administered IP to either the restrained or free-moving rats as Tc was measured. After the administration of 5 mg/kg of H or 30 mg/kg of M to the restrained rat, a marked hypothermia was observed which reached a maximum mean depth of 3.1 and 4.5 degrees C below the baseline Tc, respectively. Conversely, a mean increase in Tc of 1.5 and 1.9 degrees C occurred following the administration of these same doses of M and H in the unrestrained animal. Furthermore, the hypothermic effect of the highest dose of M was not observed when the third of 3 consecutive injections of M, administered at 48-hr intervals, was administered to the restrained rat. On the other hand, when M was repeatedly administered to the free-moving rat, the hyperthermic response was consistently observed. Pretreatment with naloxone hydrochloride (5 mg/kg IP) effectively blocked the opiate-induced hypothermia in the restrained animal, but a total dose of 10 mg/kg was necessary to completely block the hyperthermic response in the free-moving rat. Although the factor of restraint itself did not alter the rat's Tc, it did dramatically alter the action of M and H on the body temperature of the rat.     

Development and loss of tolerance to morphine in the rat

The development of a differential tolerance to morphine was investigated with respect to the mean effective dose, the threshold dose of tolerance, the degree of tolerance after a fixed dose, and the speed of tolerance loss. The mean effective doses, the threshold doses of tolerance, and the degree of tolerance differed considerably from effect to effect, whereas in all tests tolerance loss remained the same. The mean effective doses were not correlated to threshold doses of tolerance, degree of tolerance, or to the loss of tolerance, but a strong correlation exists between threshold doses of tolerance and degree of tolerance to all effects measured. Consequences of these results upon current theories of tolerance are discussed.     

The development of tolerance to morphine in the rat

Tolerance to various effects of morphine in the rat can be quantified by means of a shift of semilogarithmic dose-response curves. Tolerance to analgesia (hot plate, acetic acid writhing), catalepsy, and the tilted plane develops in a closely similar manner. Also, the stimulating effects of about 1 mg/kg morphine-HCl tested in an open-field procedure are somewhat less pronounced in chronically treated rats than in naive ones. There is no correlation between tolerance development and the acute ED₅₀ of different tests.     

Changes in the sensitivity of smooth muscle to neurotransmitter during acute morphine treatment and its association with analgesia

Acute in vivo administration of morphine to mice produced dose- and time-dependent subsensitivity to acetylcholine in ileum and supersensitivity to norepinephrine in vas deferens. These responses were specific to the agonist as potassium chloride-induced responses were not altered. A correlation was observed between the analgesia and the sensitivity changes produced by morphine. The sensitivity changes were mediated through opiate receptors as naloxone antagonised the effect of morphine.     

Some relations between tolerance and physical dependence to morphine in mice

Tolerance to morphine analgesia and precipatated physical dependence were studied in mice under different conditions. There was a gradual loss of tolerance during the continous absorption of morphine from a pellet. Tolerance was decreased by nalorphine during morphine absorption. An attenuated physical dependence was observed two or four days after a single dose of morphine. In animals previously treated with pellets of morphine, single doses of morphine induced less tolerance than in mice that had never been implanted with pellets; in both cases cycloheximide prevented development of tolerance. Tolerance persisted for more than 20 days after absorption of the morphine pellet. These results reinforce the hypothesis that tolerance and physical dependence are produced by a similar mechanism and that an inhibitory process of tolerance exists.     

Morphine and heroin effects on multiple fixed-interval schedule performance in rats

Five albino rats were trained to stability on a multiple fixed interval 60 sec-fixed interval 60 sec schedule in which one component ended with food pellet reinforcements and the other with saccharin solution reinforcements. Morphine sulfate in doses 3, 9, and 27 mg/kg i. p. and doses of heroin hydrochloride, 1, 3, and 9 mg/kg i. p., produced roughly comparable dose-related decreases in both rate of responding and index of curvature in both the food and saccharin components. A second experiment using 6 albino rats investigated the effects of repeated administration of equivalent doses of morphine sulfate (7.5 mg/kg) and heroin hydrochloride (3.0 mg/kg) on responding in the above multiple FI 60 sec-FI 60 sec schedule. Increases in rates of responding were noted following one or two injections. Drug effects on FI scalloping diminished after a few injections. The present studies report a morphine-heroin equivalency ratio consistent with that used to produce analgesia. No major behavioral differences were noted in the development of tolerance to the 2 drugs.     

A genetic analysis of the response to morphine in mice: Analgesia and running

Two progenitor strains, BALB/cBy and C57BL/6By, their reciprocal F₁ hybrids, and seven of their recombinant-inbred derived lines were used to examine the genetic basis of the response to thermal pain, and morphine analgesia at doses of 2.5, 5.0 and 10.0 mg/kg. Both the latency of response to thermal pain and the analgesic response differed significantly among the various strains tested. Strong genetic determinants appear to control their responses. Analyses of the data did not permit clarification regarding the linkage of these determinants. Photoelectric activity cages were used to test the running response of the same strains to 12.5, 25 and 40 mg/kg morphine sulfate. The genetic determinants for running activity were different from those for analgesia. There is clear evidence for two or more loci controlling the behavior at 60 and 75 min after injection, but not enough information to define the loci involved.The Jackson Laboratory is fully accredited by the American Association for Accreditation of Laboratory Animal Care.     

Modification of morphine analgesia and tolerance by flumazenil in male and female rats

This study assessed the effect of the central benzodiazepine receptor antagonist, 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester (flumazenil), on morphine-induced analgesia, locomotor effects, and development of tolerance in rats. The thermally evoked pain (tail flick) response was determined after acute and chronic intraperitoneal (i.p.) administration of morphine and flumazenil, alone and in combination. In acute studies, flumazenil induced weak analgesia unrelated to dose and sex, whereas morphine-induced analgesia was dependent on both dose and sex (male>female). Flumazenil dose-dependently enhanced the analgesic effect of morphine in female but not in male rats. Isobolographic analysis suggested synergism between flumazenil and morphine in female rats, but antagonism in male rats. Flumazenil-induced locomotor changes (alone and with morphine) were related to sex but not dose. Chronic coadministration of flumazenil with morphine enhanced analgesia and attenuated tolerance development in female rats. The findings suggest a possible role for flumazenil as an adjunct with opioids in acute and chronic pain therapy.     

Behavioral influences on tolerance to the effects of morphine on schedule-controlled behavior

Responding of pigeons was maintained under a multiple fixed interval, fixed ratio schedule of food delivery, and 10 mg/kg morphine was administered daily. Responding during both schedule components was initially decreased and measureable tolerance developed to this effect after four daily injections. However, the rate of tolerance development differed depending on whether or not presence of the drug conicided with performance during experimental sessions. Tolerance developed more rapidly when morphine was given before daily experimental sessions than when morphine was given daily but animals did not perform daily in experimental sessions. Tolerance to the rate-decreasing effects of morphine depended on relations between presence of the drug and exposure to experimental sessions.     

Effects of morphine and heroin on discrimination learning and consolidation in mice

Morphine and heroin were administered to mice learning to swim toward a light source (l procedure) or toward the dark (d procedure), in a Y water maze, under pre- and post-trial drug treatment conditions.In the pre-trial experiments a clear disrupting effect on performance with the two procedures followed administration of both drugs, but for the l procedure, performance never fell below the 50% level of correct choices.Analysis of the performance within each session demonstrated a disruption in the long term memory consolidation mechanism. The administration of naloxone, or alternatively, discontinuation of the treatment, was followed by a gradual improvement, in performance by the treated animals.In both procedures, a performance disruption also followed the administration of the drugs immediately after each experimental session.     

Development of tolerance to the stimulatory effect of morphine in dog intestine

Intestinal motor responses to morphine and 5-hydroxytryptamine (5-HT) were measured in dogs injected daily for 16 days with morphine (stabilized at 30 mg/kg i.v.) and compared to responses obtained in control animals. Intestinal contractions produced by 10 and 20 μg/kg morphine were reduced in the morphine-treated animals. Responses to 5-HT did not differ between the two groups. The contractor dose-response curve for morphine, but not for 5-HT or bethanechol, was shifted markedly to the right in intestinal segments isolated from morphine-treated dogs. It is concluded that tolerance to the intestinal stimulatory effect of morphine occurs in the dog.     

A survey of acute and chronic heroin dependence in ten inbred mouse strains: evidence of genetic correlation with morphine dependence

Heroin and morphine exposure can cause physical dependence, with symptoms manifesting during their withdrawal. Inter-individual differences in symptom frequency during morphine withdrawal are a common finding that, in rodents, is demonstrably attributable to genotype. However, it is not known whether inter-individual differences characterize heroin withdrawal, and whether such variation can be similarly influenced by genotype. Therefore, we injected mice of ten inbred strains with acute and chronic heroin doses and compared their jumping frequencies, a common index of withdrawal magnitude, during naloxone-precipitated withdrawal. The data revealed significant strain frequency differences (range after acute and chronic heroin injection: 0-104 and 0-142 jumps, respectively) and substantial heritability (h(2)=0.94 to 0.96), indicating that genetic variance is associated with heroin withdrawal. The rank order of strain sensitivity for acute and chronic heroin withdrawal jumping, and for the current heroin and previous morphine strain data, were significantly correlated (r=0.75-0.94), indicating their genetic and, ultimately, physiological commonality. These data suggest that the genetic liability to heroin dependence remains constant across a period of heroin intake, and that heroin and morphine dependence may benefit from common treatment strategies.     

Psychopharmacological evidence for increase in receptor sensitivity following chronic morphine treatment

The behavioral effects of cholinergic and adrenergic agents on fixed ratio responding (FR5) were examined in control rats and in rats chronically treated with morphine (5 mg/kg/day). Tolerance to the effects of morphine on total responses was observed, but not on rate of responding. Following tolerance development, the directly acting muscarinic agonist, pilocarpine, depressed the behavior of the morphine-treated animals to a significantly greater degree than that of the controls. Similarly, drugs which directly or indirectly stimulate alpha adrenergic and central dopaminergic receptors also affected the behavior of the morphine-treated rats to a significantly greater degree. One interpretation of these findings is that muscarinic cholinergic, alpha adrenergic, and central dopaminergic receptors become supersensitive to their respective neurotransmitters during chronic treatment with morphine. Such a change in receptor sensitivity could constitute a mechanism underlying the development of tolerance to morphine.This research was supported by Grant MH18788 from the National Institute of Mental Health to Roger W. Russell.     

Development of behavioral tolerance to morphine and methadone using the schedule-controlled behavior of the pigeon

A multiple fixed-ratio, fixed-interval schedule of food presentation was used to study the development of behavioral tolerance to daily injections of equipotent doses of morphine and methadone in the pigeon. There was evidence that tolerance was developing to the rate-decreasing effects of both drugs after a single injection. Tolerance to morphine developed more rapidly during the first week of injections than did tolerance to methadone. Tolerance to the depressant effects of morphine and methadone was less complete under the fixed-ratio component of the schedule than under the fixed-interval component. After repeated injections, increases in the rate of responding were observed in some birds. These increases depended on the bird, rather than on the narcotic. Thus, the development of tolerance was a function of the drug, of the individual bird, and of the schedule maintaining the behavior.     

Contribution of associative and nonassociative processes to the development of morphine tolerance

The contribution of associative and nonassociative processes to the development of tolerance to the analgesic effects of morphine in rats was investigated in two experiments. Associative contingencies were manipulated by administering a series of moderately high morphine doses (20 mg/kg) either explicitly paired or explicitly unpaired with a distinctive context. During distinctive context exposures, animals were placed for 60 min in plastic boxes located in a room adjacent to the colony room. The distinctiveness of this environment was enhanced by the presence of white noise and a pine scent. Nonassociative processes were manipulated by administering the morphine at either a very short (6 h) or relatively long (96 h) interdose-interval (IDI). Analgesia was measured on a tail-flick test. At the 96 h IDI, tolerance, as indexed by shifts in dose-response curves, was controlled primarily by associative processes. Associative control over tolerance at the long IDI was evident at an immediate test (experiment 1) and was retained for a 30 day interval (experiment 2). In contrast, tolerance that developed at the 6 h IDI was not influenced by associative contingencies at the immediate test (experiment 1) and showed no retention over a 30 day interval (experiment 2). These data suggest that tolerance that developed at the short IDI was nonassociative. Overall, the results indicate that conditions conducive to the development of non-associative tolerance disrupt the acquisition of associative tolerance. Hypotheses regarding the absence of associative effects at the short IDI are reviewed. Methodological implications of these results for evaluations of associative and nonassociative morphine tolerance are also discussed.     

Increased analgesic tolerance to acute morphine in fosB knock-out mice: a gender study

The proteins of Fos family are a potential candidate to link molecular mechanisms of morphine action with behavioural effects such as morphine-induced reward, dependence and tolerance. We used both male and female mice lacking fosB gene to study its contribution to morphine effects. Morphine analgesia (tail-flick test) and hypothermia were studied using morphine at cumulative doses in morphine-naïve and morphine-tolerant (tolerance induced by 24 h prior 100 mg/kg morphine administration) mice. FosB −/− mice, as compared to fosB +/+ mice, developed enhanced tolerance to morphine-induced analgesia. No effects of genotype or gender on tolerance to morphine-induced hypothermia were observed. These results suggest that fosB may be involved in the development of tolerance to morphine analgesia but not hypothermia. The gender study implicates that lack of FosB proteins in female fosB −/− mice enhanced morphine analgesic potency. In conclusion, we show that fosB gene is important to analgesia but not hypothermia phenotype indicating its role in morphine effects.