Tolerance to effects of clonidine and morphine on sulfobromophthalein disposition in mice

Chronic treatment of mice with clonidine or morphine caused tolerance to the analgesic and thermoregulatory effects of these drugs. After chronic morphine, mice also became tolerant to the analgesic and thermoregulatory effects of clonidine. Cross tolerance to the hypothermic effect of morphine was demonstrated after chronic clonidine administration, but no diminution of morphine-induced analgesia could be shown. Morphine and clonidine acutely increased the retention of sulfobromophthalein (BSP) in plasma and liver. Chronic dosing with morphine or clonidine caused partial tolerance and cross-tolerance to the rise in hepatic BSP caused by an acute challenge with either agonist. However, both drugs elevated plasma BSP levels similarly in tolerant and non-tolerant mice. Thus, regimens which readily induced tolerance to the analgesic and hypothermic effects of morphine or clonidine were only partially effective in modifying the acute hepatobiliary effects of these drugs.     

Orexin receptor type-1 antagonist SB-334867 inhibits the development of morphine analgesic tolerance in rats

Herein the effect of orexin receptor type-1 antagonist SB-334867 on the development of tolerance to analgesic effects of morphine was studied in rats. To incite tolerance, morphine sulfate was injected intraperitoneally (i.p., 10mg/kg) once a day for 7 days. The tail flick test was used to evaluate antinociceptive effects of the morphine. A selective OxR1 receptor antagonist, SB-334867, was microinjected (i.c.v.) into the right cerebral ventricle (10 μg/10 μl) immediately before each morphine injection. Repeated morphine application resulted in tolerance to morphine analgesic effects as a decreasing trend during 7 days. Also, repeated administration of SB-334867 (i.c.v.) alone was without significant effect on the nociception as compared to control. Microinjection of SB-334867 prior to each morphine injection inhibited the development of tolerance, so that the analgesic effects of morphine were significantly higher in SB-334867 plus morphine treated rats than that of vehicle plus morphine treated ones on days 4-7. It is concluded that orexin receptor type-1 might be involved in the development of tolerance to morphine analgesic effects.     

Analgesic activity of spiradoline mesylate (U-62,066E), a kappa opioid agonist in mice

The present study was undertaken to evaluate the analgesic potency of spiradoline mesylate, a k(kappa) opioid agonist, in comparison with that of morphine, by hot plate, tail-pinch and acetic acid-induced writhing assay. The ED50 values of spiradoline in hot plate, tail-pinch and acetic acid-induced writhing assay were 0.46, 0.26 and 0.20 mg/kg, respectively. The analgesic potency of spiradoline was 1.5-7.0 times higher than that of morphine. Repeated treatment with spiradoline as well as morphine developed tolerance to the analgesic effect in hot plate assay. In mice developed tolerance to one analgesic, response to the other analgesic did not alter compared to saline-treated mice. Single administration of spiradoline (1.5 and 3 mg/kg, s.c.) did not inhibit morphine-induced analgesia. These results suggest that spiradoline has more potent analgesic activity than morphine, presumably mediated through stimulation of receptors different from morphine.     

Effect of methamphetamine on the development of acute morphine tolerance and dependence in mice

The effect of methamphetamine on morphine analgesia (tail-flick assay) was studied in non-tolerant mice and in mice made acutely tolerant to morphine following a single injection of 100 mg/kg morphine. The analgesic potency of morphine was increased in non-tolerant and tolerant mice to the same extent by 3.2 mg/kg methamphetamine (3.3 and 4.4 fold increases, respectively). In contrast, the ED50's for morphine analgesia and naloxone-precipitated jumping in mice pretreated with either 100 mg/kg morphine or both morphine and 3.2 mg/kg methamphetamine were not significantly different, indicating that methamphetamine had no effect on the development of acute morphine tolerance and dependence. Although methamphetamine had no effect on the development of acute tolerance to morphine, 4-day pretreatment with methamphetamine produced cross-tolerance to morphine analgesia. However, cross-tolerance to morphine was not accompanied by enchanced sensitivity to naloxone.     

Comparative effects of Pro-Leu-Gly-NH2 and cyclo(Leu-Gly) administered orally on the development of tolerance to the analgesic effect of morphine in the rat

Comparative effects of Pro-Leu-Gly-NH2 (MIF) and cyclo(Leu-Gly) (CLG) administered orally at different stages of chronic morphine treatment on the development of tolerance to the analgesic effect of morphine in the rat were determined. Male Sprague-Dawley rats were implanted with either 6 placebo or morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of a high degree of tolerance as evidenced by a decrease in the analgesic response to morphine. Administration of CLG (8 and 16 mg/kg/day) on day 5, 6 and 7 of implantation inhibited the development of tolerance to morphine but 4 and 32 mg/kg doses had no effect. Further, CLG (2 mg/kg/day for 7 days) inhibited the development of tolerance but higher doses (4 and 8 mg/kg) had no effect. MIF (26 and 52 mg/kg) administered orally on the last three days of the implantation schedule inhibited the development of tolerance to morphine. MIF (6.5 mg/kg/day for 7 days) inhibited the development of tolerance but the higher doses had no effect. Concurrent administration of MIF (6.5 mg/kg) and CLG (2 mg/kg) for seven days failed to inhibit the development of tolerance. A single dose of MIF or CLG administered a day before the assessment of tolerance did not affect the morphine tolerance. Thus, even after a significant degree of tolerance to morphine had developed, neuropeptides like MIF and CLG given orally, in appropriate doses, can inhibit development of tolerance to morphine and restore the analgesic effect of morphine.     

Tolerance to morphine-induced analgesia in mice: Magnetic fields function as environmental specific cues and reduce tolerance development

Mice receiving daily injection of morphine (10 mg/kg) developed tolerance to morphine-induced analgesia, such that after 5–7 days of treatment their thermal response (paw licking) latencies in the hot plate test were indistinguishable from those of control animals. Exposure to a rotating magnetic field for thirty minutes before the daily morphine administrations significantly reduced the development of tolerance. These magnetic exposure also significantly increased over 7–10 days the basal nociceptive thresholds and paw licking response latencies of saline treated mice. Control and sham exposed mice that were fully tolerant to the analgesic effects of morphine failed to show any tolerance to morphine-induced analgesia when exposed to the magnetic stimuli prior to injection. Likewise, the partial tolerance to morphine shown by mice exposed to the rotating magnetic field pre-injection environmental cues was eliminated when control or sham pre-injection cues lacking the magnetic stimuli were provided. In all cases tolerance to morphine-induced analgesia was evident in the subsequent re-test with the original cues. These results indicate that magnetic field exposure can reduce the development of tolerance to the analgesic effects of morphine. They also show that magnetic stimuli function as significant environmental cues for the development of tolerance to morphine-induced analgesia. This suggests that magnetic stimuli affect both the associative (classical conditioning) and non-associative (physiological, pharmacological) mechanisms involved in the development of opiate tolerance.     

Effects of fluoxetine and LY 367265 on tolerance to the analgesic effect of morphine in rats

Morphine is widely used to treat chronic pain, however its utility is hindered by the development of tolerance to its analgesic effects. The aim of this study was to investigate effects of fluoxetine, a specific serotonin (5-HT) reuptake inhibitor, and LY 367265, an inhibitor of the 5-HT transporter and 5-HT2A receptor antagonist, on tolerance induced to the analgesic effect of morphine in rats. The study was carried out on male Wistar Albino rats (weighing 170-190 g). To constitute morphine tolerance, animals received morphine (50 mg/kg; s.c.) once daily for 3 days. After last dose of morphine, injected on day 4, morphine tolerance was evaluated. The analgesic effects of fluoxetine (10 mg/ kg; i.p.), LY 367265 (3 mg/kg; i.p.) and morphine were considered at 30-min intervals by tail-flick and hot-plate tests. The results showed that fluoxetine and LY 367265 significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effects were obtained 30 min after administration of fluoxetine and 60 min after administration of LY 367265. In conclusion, we observed that co-injection of morphine with fluoxetine and LY 367265 increased the analgesic effects of morphine and delayed development of tolerance to morphine analgesia.     

Inability of cyclo (Leu-Gly) to facilitate the development of tolerance to and physical dependence on morphine in the rat

The effect of cyclo (Leu-Gly), an analog of melanotropin release inhibition factor on the development of tolerance to and physical dependence on morphine in the rat was investigated. Administration of cyclo (Leu-Gly) (1 μg/rat/day) prior to and during morphine pellet implantation failed to facilitate the development of tolerance to the analgesic and hypothermic effects of morphine. Similarly the development of dependence on morphine was not facilitated by cyclo (Leu-Gly) as evidenced by changes in body weight and body temperature observed during abrupt withdrawal of morphine. These studies do not lend support to the previous observations that cyclo (Leu-Gly) and other related peptides facilitate the development of tolerance to and physical dependence on morphine.     

Short-term tolerance to morphine: effects of diazepam, phenobarbital, and amphetamine

Short-term tolerance to morphine, which can be demonstrated in as little as 3 hours after a single administration of the opiate, was examined in animals chronically pretreated with diazepam, phenobarbital, or amphetamine. Tail-flick latency in mice and changes in plasma corticosterone in rats were the parameters tested in these experiments. Rats primed with either saline or morphine, 10 mg/kg, were injected 3 hours subsequently with morphine, 5 mg/kg. Those primed with saline showed the characteristic plasma corticosterone elevation following morphine, when serial blood samples were examined, whereas those previously treated with morphine did not. Mice were primed with saline or either of two doses of morphine, 30 or 100 mg/kg, 3.5 hours prior to estimation of tail-flick latency and ED50 determinations. Mice primed with either dose of morphine had significantly higher ED50's than those primed with saline. Chronic treatment with diazepam or amphetamine in either species did not significantly alter short-term tolerance development by either parameter. However, with phenobarbital pretreatment, the plasma corticosterone response was attenuated and short-term tolerance to morphine's analgesic effects did not occur. Further studies in morphine-pelleted mice showed that analgesic tolerance occurred similarly in all groups. This suggests that barbiturates may delay the process.     

Neuromodulatory effects of TRH upon swim and cholinergic analgesia

In addition to short-acting analgesic actions by itself and modulation of analgesic responses induced by endogenous opioids and neurotensin, central administration of thyrotropin-releasing hormone (TRH) potentiates footshock analgesia. The present study evaluated the effects of TRH upon the neurohormonally-mediated though nonopioid analgesia induced by swims in rats. Intracerebroventricular TRH (10 and 50 micrograms) dose-dependently potentiated swim (21, 15, 2 degrees C baths) analgesia on the tail-flick test, an effect which was not due to the hypothermic or basal pain threshold changes. Intravenous (8 mg/kg) TRH potentiated swim (21 degrees C) analgesia; the 600:1 difference in potency between routes strongly suggests central sites of neuromodulatory action. Intracerebroventricular diketopiperazine (50 micrograms), a TRH metabolite, and RX77368 (50 micrograms), a TRH analogue, also potentiated swim (21 degrees C) analgesia, effects also independent of hypothermia and basal reactivity to pain. Finally, given the excitatory interaction between TRH and acetylcholine as well as the cholinergic involvement in swim analgesia, intracerebroventricular TRH potentiated pilocarpine (10 mg/kg, IP) analgesia.     

Intragastric administration of cyclo(Leu-Gly) inhibits the development of tolerance to the analgesic effect of morphine in the rat

The effect of intragastric administration of cyclo(Leu-Gly), a cyclic dipeptide derived from melanotropin release inhibiting factor (Pro-Leu-Gly-NH2), on the development of tolerance to the analgesic effect of morphine in the rat was determined. The tolerance to morphine in the rat was induced by subcutaneous implantation of four morphine pellets during a 3-day period. The rats which served as controls were implanted with placebo pellets. The analgesic response to a challenge dose of morphine was determined by the tail-flick test. The tail-flick latencies were determined before and then every 30 min for 180 min. The analgesic response was computed by determining the area under the time-response curve. Implantation of morphine pellets resulted in the development of tolerance as evidenced by decreased analgesic response to morphine in morphine pellet implanted rats as compared to placebo pellet implanted rats. Chronic intragastric administration of cyclo(Leu-Gly) (4 to 16 mg/kg) inhibited the development of tolerance to morphine. A dose of 8 mg/kg of cyclo(Leu-Gly) completely blocked the tolerance to morphine. The study provides for the first time evidence that intragastric administration of a cyclic peptide can inhibit the development of tolerance to morphine, and that effective neuropeptides and their analogs can be developed as potential drugs to inhibit opiate-induced tolerance.     

The effects of pituitary adenylate cyclase-activating polypeptide on acute and chronic morphine actions in mice

The effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on pain sensitivity, on morphine analgesia, on morphine tolerance and withdrawal were investigated in mice. The heat-radiant tail-flick test was used to assess antinociceptive threshold. Intracerebroventricular (i.c.v.) administration of PACAP alone had no effect on pain sensitivity but in a dose of 500 ng, it significantly diminished the analgesic effect of a single dose of morphine (2.25 mg/kg, s.c.). PACAP (500 ng, i.c.v.) significantly increased the chronic tolerance to morphine and enhanced the naloxone (1 mg/kg, s.c.)-precipitated withdrawal jumping. Theophylline (1 mg/kg, i.p.) pretreatment significantly enhanced the effect of PACAP on morphine analgesia but the effects of PACAP on tolerance and withdrawal were unaffected upon theophylline administration. On the grounds of our previous studies with vasoactive intestinal polypeptide (VIP), it appears that different receptors are involved in the effects of PACAP in acute and chronic morphine actions. Our results indicate that PACAP-induced actions likely participate in acute and chronic effects of morphine and suggest a potential role of PACAP in opioid analgesia, tolerance and withdrawal.     

Structure activity relationship studies with hypothalamic peptide hormones. II. Effects of thyrotropin releasing hormone analogs on morphine-induced responses in mice

The effects of several analogs of thyroliberin (TRH), that have a chloro-acetyl substituent at the amino terminus, on locomotor depressant, locomotor stimulant, hyperthermic and hypothermic response to morphine were determined in the mouse. These compounds included N-(chloroacetyl)-L-phenylalanylpyrrolidine (ClAc-Phe-Pyrr), N-[m-(chloroacetyl)benzoyl]-L-phenylalanylpyrrolidine] (mClAcBz-Phe-Pyrr), N-[m-(chloroacetyl)benzoyl]-L-alanyl-L-phenylalanylpyrrolidine (mClAcBz-Ala-Phe-Pyrr), N-[p-(chloroacetyl)benzoyl]-L-alanyl-L-phenylalanyl-pyrrolidine (pClAcBz-Ala-Phe-Pyrr), N-(chloroacytyl)-L-alanyl-L-phenylalanyl-L-prolineamide(ClAc-Ala-Phe-Pro-NH₂), N-[m-(chloroacetyl)-benzoyl]-L-phenylalanyl-L-prolineamide (mClAcBz-Phe-Pro-NH₂), N-[p-(chloroacetyl)benzoyl]-L-phenylalanyl-L-prolineamide (pClAcBz-Phe-Pro-NH₂). Since TRH is metabolized to cyclo (His-Pro) and the latter is shown to possess TRH like activity, an analog cyclo (Phe-Pro) was also used. Administration of morphine to mice at 10 mg/kg ip produced hyperthermia and depression in locomotor activity, while at 80 mg/kg ip, hypothermia and stimulation in locomotor activity were observed. Intracerebral injection of the following peptides (10 μg each per mouse) administered 10 min prior to morphine injection antagonized locomotor depression, hyperthermia, locomotor stimulation and hypothermia induced by an appropriate dose of morphine: mClAcBz-Phe-Pyrr, pClAcBz-Ala-Phe-Pyrr, ClAcAla-Phe-Pro-NH₂, pClAcBz-Phe-Pro-NH₂, cyclo (Phe-Pro) and TRH. The compounds which had no effect on low dose or high dose morphine induced responses included pGlu-Phe-Pyrr, mClAcBz-Ala-Phe-Pyrr, and mClAcBz-Phe-Pro-NH₂. One compound, namely ClAc-Phe-Pyrr, antagonized morphine-induced locomotor stimulation and hypothermia but did not affect locomotor depression and hyperthermia produced by morphine. None of these peptides had any effect on the body temperature or the locomotor activity of normal mice. Many of the active compounds were previously shown to possess extremely weak or no activity in releasing thyrotropin from the pituitary. It is concluded that several of these analogs of TRH possess CNS activity in antagonizing morphine effects, and that a lack of relationship exists between the CNS and endocrine activity of these peptides.     

Inhibition of abstinence syndrome in opiate defendent mice by cyclo (His-Pro)

Intracerebral administration of cyclo (His-Pro), the postulated metabolite of thyroliberin (TRH, pGlu-His-Pro-NH₂) inhibited the naloxone induced withdrawal responses in morphine dependent mice. Mice were rendered dependent on morphine by the subcutaneous implantation of a pellet (containing 75 mg of morphine free base) for three days. Six hours after pellet removal, the naloxone ED₅₀ for the jumping response was found to be higher in mice injected with cyclo (His-Pro) compared with that of vehicle controls. Similarly, the hypothermic response observed following 50 μg/kg of naloxone given given 6 h after pellet removal or that seen with 100 μg/kg of naloxone given 24 h after pellet removal from morphine-dependent mice was inhibited by cyclo (His-Pro). Previously, we have shown similar results with TRH on the morphine abstinence syndrome. It appears, therefore, that cyclo (His-Pro) may be the active metabolite of TRH and analogs of cyclo (His-Pro) may be useful in blocking the symptoms of the opiate abstinence syndrome.     

NMDA receptor antagonism disrupts the development of morphine analgesic tolerance in male, but not female C57BL/6J mice

Multiple studies demonstrate that coadministration of N-methyl-D-aspartate (NMDA) receptor antagonists with the opioid agonist morphine attenuates the development of analgesic tolerance. Sex differences in the effects of noncompetitive, but not competitive NMDA receptor antagonists on acute morphine analgesia, have been reported in mice, yet the role of sex in modulation of morphine tolerance by NMDA receptor antagonists has yet to be addressed. Therefore, we tested whether there is a sex difference in the effect of NMDA receptor antagonists on the development of morphine analgesic tolerance in C57BL/6J mice. Acutely, at a dose required to affect morphine tolerance in male mice, the noncompetitive NMDA receptor antagonist dizocilpine (MK-801) prolonged morphine analgesia similarly in both sexes in the hot plate and tail withdrawal assays. In the hot plate assay, coadministration of MK-801 or the competitive antagonist 3-(2-carboxpiperazin-4-yl)propyl-1-phosphanoic acid (CPP) with morphine attenuated the development of tolerance in male mice, while having no effect in females. Like normal and sham females, ovariectomized mice were similarly insensitive to the attenuation of morphine tolerance by MK-801 in the hot plate assay. Surprisingly, in the tail withdrawal assay, MK-801 facilitated the development of morphine-induced hyperalgesia and tolerance in males but not females. The results demonstrate that male mice are more sensitive to modulation of nociception and morphine analgesia after repeated coadministration of NMDA receptor antagonists. Furthermore, the underlying mechanisms are likely to be different from those mediating the sex difference in the modulation of acute morphine analgesia that has previously been reported.     

The involvement of midbrain astrocyte in the development of morphine tolerance

Aims

Systemic administration of opiate analgesics such as morphine remains the most effective treatment for alleviating severe pain across a range of conditions including acute pain. However, chronic or repeated administration of opiate analgesics results in the development of analgesic tolerance. Glial cells such as microglia and astrocytes are known to release various inflammatory cytokines and neurotrophic factors leading to regulation of neuronal function. Recently, glial cells were reported to play important roles in the development of analgesic tolerance to morphine. Here, we focused on the involvement of midbrain glial cells, particularly astrocytes, in the development of analgesic tolerance to morphine.

Main methods

Mice were treated with morphine (10 mg/kg, s.c.) or vehicle once a day for 5 days. Pentoxifylline (an inhibitor of glial activation; 20 mg/kg, i.p. or 50 and 100 μg/mouse, i.c.v.) was administered 30 min before morphine treatment. Flavopiridol (a cyclin-dependent kinase inhibitor; 5 nmol/mouse, i.c.v.) was administered 10 min before and 10 h after morphine treatment. The analgesic effect of morphine was measured using the tail flick method.

Key findings

The development of analgesic tolerance to morphine was gradually observed during daily treatment of morphine for 5 days in mice. On days 1 and 3 after repeated morphine treatment, astrocyte marker glial fibrillary acidic protein expression levels were significantly increased, as determined by western blot analyses. These phenomena were significantly inhibited following pre-treatment with pentoxifylline or flavopiridol.

Significance

We demonstrated that midbrain astrocytes play an important role in the development of analgesic tolerance to morphine.     

Comparative characteristics of the discriminative and analgesic effects of morphine

Experiments on mice were made to study and compare the discriminative and analgesic effects of morphine. The time-course of tolerance to the drug effects was found to be different. The Schild method permitted one to determine significantly different characteristics of naloxone antagonism (pA2) as regards the analgesic and discriminative effects of morphine. The data obtained attest to the different mechanisms of the analgesic and discriminative effects of morphine.     

Attenuation of morphine tolerance and dependence by alpha-melanocyte stimulating hormone(alpha-MSH).

Repeated administration of morphine resulted in significant reduction of its analgesic potency. If 0.1 mg/kg α-MSH was coadministered, the tolerance development was attenuated, 1 mg/kg MIF (MSH release inhibiting factor), given simultaneously with morphine, did not affect tolerance. Injecting, however, MIF 1 hour prior to the daily opiate treatment resulted in accelerated development of tolerance supposedly by lowering the plasma α-MSH level at the time of morphine administration. Of the morphine abstinence symptoms the naloxone-induced jumping in morphine pretreated mice could not be modified either by α-MSH coadministration or by MIF pretreament, but the withdrawal body weight loss was found to be diminished by the former and increased by the latter peptide. The possible role of α-MSH in preventing the development of tolerance to the analgesic effect of endogenous opioid peptides is discussed.     

The effect of thyrotropin releasing hormone and morphine on gastrointestinal transit

The effect of thyrotropin releasing hormone (TRH) alone and in combination with morphine on the gastrointestinal transit was investigated by using the charcoal meal test in mice. The intraperitoneal (IP) administration of TRH decreased the transit when given in a dose of 1.0 mg/kg 10 min prior to the meal. The intracerebroventricular (ICV) administration of TRH (10 μg/mouse) also inhibited the transit when given just prior to the charcoal meal. Subcutaneous (SC) administration of morphine (5, 10 and 20 mg/kg) inhibited gastrointestinal transit in a dose dependent manner. When TRH (1, 3 and 10 mg/kg, IP as well as 0.3 μg, ICV) which had no effect on the transit by itself was combined with morphine (10 mg/kg, SC), an enhancement in the inhibition of the transit was observed. TRH-induced inhibition of the transit was antagonized by naloxone (0.1 mg/kg, SC). It is concluded that TRH inhibits gastrointestinal transit in the mouse possibly via the opiate receptor system.     

Ethanol-induced hypothermia. Cross tolerance with morphine

The development of tolerance to ethanol-induced hypothermia and hypnosis, and cross-tolerance with morphine was studied in mice and rats. Ethanol significantly decreased the body temperature in rats (3.0 and 3.2 g/kg) and in mice (3.5 and 4.0 g/kg). Chronic administration of ethanol resulted in the tolerance not only to ethanol hypothermia but also to hypothermic effects of morphine in examined animals. Implantation of morphine pellets caused the development of cross tolerance to ethanol-induced hypothermia in rats but not in mice. The hypnotic effect of ethanol was significantly shorter in chronic alcoholized rats but not in morphine-implanted rats. Neither chronic ethanol administration nor implantation of morphine pellets changed the duration of ethanol-induced hypnosis in mice. These results seem to support the hypothesis on the opiate-like mechanism of ethanol action.