Involvement of hypothalamic pituitary adrenal axis on the effects of nifedipine in the development of morphine tolerance in rats

It has been shown that nifedipine, as a calcium channel blocker, can attenuate the development of tolerance to the antinociceptive effect of morphine; however, the role of HPA axis on this action has not been elucidated. We examined the effect of nifedipine on morphine analgesic tolerance in intact and adrenalectomized (ADX) rats and on HPA activity induced by morphine. Adult male rats were rendered tolerant to morphine by daily injection of morphine (15 mg/kg i.p.) for 8 days. To determine the effect of nifedipine on the development of morphine tolerance, nifedipine (1, 2 and 5 mg/kg i.p.) was injected concomitant with morphine. The tail-flick test was used to assess the nociceptive threshold, before and 30 min after morphine administration in days 1, 3, 5 and 8. Our results showed that despite the demonstration of tolerance in both ADX and sham operated rats, nifedipine in ADX rats prevented morphine tolerance development at a lower dose (2 mg/kg) than in sham operated rats, however corticosterone replacement prevented nifedipine effect in ADX rats. Acute administration of morphine produced significant increase in plasma corticosterone level, and with repeated injection, a tolerance to this neurosecretory effect was developed. Nifedipine (5 mg/kg) attenuated the acute effect of morphine, but could not block its neurosecretory tolerance.     

Desensitization of mu-opioid receptors does not modify the analgesia induced by an enkephalinase inhibitor.

Acetorphan, an enkephalinase inhibitor, or morphine was injected in mice which had received saline or morphine (32 mg/kg s.c. twice a day on 8 consecutive days) chronically. In the hot-plate test, the analgesia (increase in jump latency) induced by morphine (2 mg/kg i.p.) or by the mu selective opioid agonist, [D-Ala2,N-Me-Phe4, Gly5-ol]enkephalin (DAGO) (1.5, 3 or 6 ng/mouse i.c.v.), was significant in the saline group but was strongly decreased in morphine-pretreated mice. In contrast the analgesic effect of acetorphan (5 mg/kg i.v.) or of the delta selective opioid agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.75, 1.5 or 3 micrograms/mouse i.c.v.) was similar in both groups. These results suggest that the enkephalins protected by acetorphan act on the delta receptor site to produce antinociception.     

Intrathecal [Met5]enkephalin antibody blocks analgesia induced by intracerebroventricular beta-endorphin but not morphine in mice

Intrathecal (i.t.) injection of antibody directed to [Met5]enkephalin antagonized intracerebroventricularly (i.c.v.) administered beta-endorphin-induced inhibition of the tail-flick response but not the hot-plate response. [Met5]enkephalin antibody injected i.t., however, did not affect inhibition of either the tail-flick and hot-plate response induced by morphine given i.c.v. The antibodies to [Leu5]enkephalin, dynorphin A-(1-13) and beta-endorphin even at a high dose injected i.t. did not affect the inhibition induced by i.c.v. administered beta-endorphin or morphine either in the tail-flick and hot-plate test. Our results with antibodies support the results of previous studies that beta-endorphin but not morphine produces its analgesic action by selectively releasing [Met5]enkephalin.     

Zimelidine attenuates the development of tolerance to morphine-induced antinociception

Objectives:

The aim of this study was to investigate effect of zimelidine (a serotonin reuptake inhibitor) on morphine-induced tolerance in rats.

Materials and Methods:

Male Wistar albino rats weighing 160–180 g were used in these experiments (n=72). A 3-day cumulative dosing regimen was used for the induction of morphine tolerance. To constitute of morphine tolerance, animals received morphine twice daily for 3 days. After the last dose morphine was injected on the fourth day, morphine tolerance was evaluated. The analgesic effects of zimelidine (15 mg/kg; i.p.) and morphine (5 mg/kg) were considered at 30-min time intervals (0, 30, 60, 90 and 120 min) by tail-flick and hot-plate analgesiometer (n=6 in each experimental group).

Results:

The results showed that zimelidine significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effect of zimelidine was obtained at the 60 minutes measurements in the zimelidine group and at the 30 minutes measurements in the morphine tolerant group by the tail-flick and hot-plate tests. Administration of zimelidine with morphine showed additive analgesic effect.

Conclusion:

In conclusion, our results show that zimelidine reduces the development of tolerance to morphine-induced antinociception in rats.KEY WORDS: Antinociception, hot-plate test, morphine, tail-flick test, zimelidine     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

The effect of gabapentin on antinociceptive action of analgesics

The effect of gabapentin at a dose of 10 mg/kg (single administration) and at 3 mg/kg/day (administered for 10 days) on the analgesic action of morphine (10 mg/kg), metamizol (500 mg/kg) and indomethacin (10 mg/kg and 1.4 mg/kg) in mice was assessed on the basis of hot-plate and tail-flick tests. All the drugs were administered intraperitoneally (i.p.). Gabapentin was administered to mice 30 min before the administration of analgesics. The animals reactions to noxious stimuli were measured 60, 90 and 120 min after gabapentin administration. Gabapentin administered in a single dose, as well as in multiple ones, was found to cause antinociception, especially evident in the hot-plate test. The single dose of gabapentin enhances the analgesic effect of morphine in the hot-plate test, whereas in the tail-flick test it demonstrates an opposite effect. Gabapentin in a single dose does not affect significantly the effects of metamizol and indomethacin, whereas multiple doses decrease the action of these drugs. Gabapentin abolishes the tolerance of antinociceptive effect of morphine.     

Nifedipine potentiates antinociceptive effects of morphine in rats by decreasing hypothalamic pituitary adrenal axis activity

It has been shown that nifedipine, as a calcium channel blocker can potentiate the antinociceptive effect of morphine; however, the role of Hypothalamic-Pituitary-Adrenal (HPA) axis on this action has not been elucidated. We examined the effect of nifedipine on morphine-induced analgesia in intact and adrenalectomized (ADX) rats and on HPA activity induced by morphine. To determine the effect of nifedipine on morphine analgesia, nifedipine (2 mg/kg i.p.) that had no antinociceptive effect, was injected concomitant with sub-effective dose of morphine (1 and 2 mg/kg). The tail-flick test was used to assess the nociceptive threshold, before and 15, 30, 60, 90, 120 and 180 min after drug administration. Our results showed that, nifedipine could potentiate the antinociceptive effect of morphine and this effect of nifedipine in ADX was greater than sham operated rats which, was reversed by corticosterone replacement. Nifedipine has an inhibitory effect on morphine -induced corticosterone secretion. Thus, the data indicate that the mechanism underlying the potentiation of morphine analgesia by nifedipine involves mediation, at least in part, by attenuating the effect of morphine on HPA axis.     

Nifedipine suppresses morphine-induced thermal hyperalgesia: evidence for the role of corticosterone

It has been shown that systemic administration of morphine induced a hyperalgesic response at an extremely low dose. We have examined the effect of nifedipine, as a calcium channel blocker, on morphine-induced hyperalgesia in intact and adrenalectomized rats and on hypothalamic-pituitary-adrenal axis activity induced by ultra-low dose of morphine. To determine the effect of nifedipine on hyperalgesic effect of morphine, nifedipine (2 mg/kg i.p. and 10 microg i.t.) that had no nociceptive effect, was injected concomitant with morphine (1 microg/kg i.p. and 0.01 microg i.t. respectively). The tail-flick test was used to assess the nociceptive threshold, before and 30, 60, 120, 180, 240 and 300 min after drug administration. The data showed that low dose morphine systemic administration could produce hyperalgesic effect in adrenalectomized rats equivalent to sham-operated animals while intrathecal injection of morphine only elicited hyperalgesia in sham-operated animals. Nifedipine could block morphine-induced hyperalgesia in sham and adrenalectomized rats and even a mild analgesic effect was observed in the adrenalectomized group which was reversed by corticosterone replacement. Systemic administration of low dose morphine produced significant increase in plasma level of corticosterone. Nifedipine has an inhibitory effect on morphine-induced corticosterone secretion. Thus, the data indicate that dihydropyridine calcium channels are involved in ultra-low dose morphine-induced hyperalgesia and that both the pattern of morphine hyperalgesia and the blockage of it by nifedipine are modulated by manipulation of the hypothalamic pituitary adrenal axis.     

Are glycineB sites involved in the development of morphine tolerance?

Numerous data have indicated that competitive and non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists attenuate the development of tolerance to the analgesic effect of morphine. This study extends these findings on the effects of glycine(B) site antagonist, L-701.324. Tolerance to the analgesic effect of morphine was measured in hot-plate test in Wistar rats. For 9 days, animals were first injected with vehicle or glycine(B) receptor antagonist, L-701.324 (2.5 and 5 mg/kg, po). The non-competitive NMDA receptor antagonist, MK-801 (0.05 or 0.1 mg/kg, ip) was used as a reference compound. The injection of L-701.324, MK-801 or saline was followed, 20 min later, by the injection of morphine (10 mg/kg, sc). Hot-plate latencies were determined 20 min after the second injection on odd-numbered days. The results indicated that chronic administration of glycine(B) site antagonist, L-701.324 decreased the analgesic effect of morphine and they may suggest that this substance at both used doses increased the development of morphine tolerance, whereas non-competitive NMDA antagonist, MK-801 at the dose of 0.1 mg/kg potentiated the analgesic effect of morphine and attenuated the development of morphine tolerance.     

Effect of sumatriptan in different models of pain in rats

The effect of sumatriptan in two standard algesimetric tests and in a model of cephalalgia was evaluated in rats. The pain threshold was measured by the hot-plate and the writhing tests; cephalalgia was produced by injecting bradykinin (10 μg in a volume of 10 μl) into a common carotid artery. Sumatriptan was subcutaneously (s.c.) injected at the doses of 4, 8, 24 or 42 mg/kg; morphine (5 or 10 mg/kg s.c.) and indomethacin (5 or 10 mg/kg s.c) were used as standard analgesic drugs. Sumatriptan had no analgesic activity either in the hot-plate test or in the writhing test. On the other hand, at 24 and 42 mg/kg it dose-dependently reduced the response to the intracarotid injection of bradykinin (vocalization and tachypnea), this effect being prevented by the 5-HT_1B receptor antagonist, isamoltane. The 5-HT_1D receptor antagonist BRL15572 prevented the effect of sumatriptan on bradykinin-induced tachypnea, but not the effect of sumatriptan on bradykinin-induced vocalization. These data demonstrate that sumatriptan is significantly effective in a reliable animal model of cephalalgia, while having no systemic analgesic activity.     

GABAA-antagonists and baclofen analgesia

1. Intraperitoneal (i.p.) injection of different doses of baclofen (5, 7.5 and 10 mg/kg) induced analgesia in tail-flick test. The effect was dose-dependent. 2. The antinociception induced by baclofen (10 mg/kg, i.p.) was decreased in animals pretreated with bicuculline (1.5 mg/kg, i.p., 30 min), but not with naloxone (1.5 mg/kg, i.p., 30 min). 3. In picrotoxin (1 mg/kg, i.p., 15 min) pretreated mice, baclofen (5 mg/kg, i.p.) showed a significant analgesic effect. 4. Morphine (6 mg/kg, s.c.) induced analgesia which was antagonized by naloxone pretreatment (1.5 mg/kg, i.p.), while bicuculline or picrotoxin did not alter the morphine response. 5. These data suggest that a part of analgesic effect of baclofen may be mediated through GABAA receptor sites, and differs from that of morphine.     

Adrenalectomy potentiates the antinociceptive effects of calcium channel blockers

Calcium channel blockers can modulate the nociceptive threshold. However, the underlying mechanism(s), especially the role of hypothalamic-pituitary-adrenal (HPA) axis, on this effect has not yet been clarified. In the present study we investigated the analgesic effect of verapamil, diltiazem and nimodipine in intact and adrenalectomized (ADX) male rats and also measured the effect of these drugs on HPA function. The tail-flick and hot-plate tests were used to assess the nociceptive threshold before and 15, 30, 60 and 120 min after drug administration. Corticosterone level was measured by radioimmunoassay as a marker of HPA function. Our results showed that these drugs could elicit antinociceptive effects which were more prominent in the hot-plate than in the tail-flick tests. Following the exclusion of adrenal glands these drugs showed stronger analgesic effects. Acute administration of verapamil, diltiazem and nimodipine produced significant decrease in plasma corticosterone level that was more prominent by nimodipine. In conclusion, the results of our study show that the HPA function has an important role in the antinociceptive effect of calcium channel blockers.     

Intrathecal administration of thiorphan, bestatin, desipramine and fluoxetine differentially potentiate the antinociceptive effects induced by beta-endorphin and morphine, administered intracerebroventricularly

The effects of the intrathecal injection of thiorphan (an inhibitor of enkephalinase inhibitor), bestatin (an inhibitor of aminopeptidase), desipramine (an inhibitor of the uptake of noradrenaline) and fluoxetine (an inhibitor of the uptake of serotonin) on the antinociception induced by beta-endorphin and morphine, administered intracerebroventricularly, were studied in male ICR mice. Antinociceptive effects were assessed by the tail-flick and hot-plate tests. Thiorphan (16 micrograms) and bestatin (16 micrograms), injected intrathecally, potentiated inhibition of the tail-flick response, induced by beta-endorphin but not by morphine administered intracerebroventricularly, whereas desipramine (6 micrograms) and fluoxetine (6 micrograms), injected intrathecally potentiated inhibition of the tail-flick response induced by morphine, but not by beta-endorphin, administered intracerebroventricularly. Thiorphan, bestatin, desipramine or fluoxetine, given intrathecally, did not antagonize inhibition of the hot-plate response, induced by beta-endorphin or morphine administered intracerebroventricularly. The results indicate that inhibition of the tail-flick response, induced by beta-endorphin administered intracerebroventricularly, is mediated by the opioid system, but not by noradrenergic and serotonergic systems in the spinal cord. On the other hand, the inhibition of the tail-flick response, induced by morphine given intracerebroventricularly, is mediated by noradrenergic and serotonergic systems, but not by the opioid system in the spinal cord. The lack of effect of enzyme inhibitors and inhibitors of the uptake of biogenic amines intrathecally on beta-endorphin- and morphine-induced inhibition of the hot-plate response, is due to the supraspinal nature of the nociceptive hot-plate response. The present results further support the hypothesis, proposed previously, that intracerebroventricularly injected beta-endorphin and morphine elicit antinociception by activating different descending inhibitory systems.     

The effect of adenosine receptor agonists on analgesic effects of morphine

The effect of adenosine, S-phenylisopropyladenosine (S-PIA) and dipyridamole (an adenosine reuptake inhibitor) on the analgesic action of morphine in mice and rats was investigated in the hot-plate (56 degrees C) and tail immersion (52 degrees C) tests. Adenosine, 50 and 100 mg/kg, induced analgesia in mice and rats in the hot-plate test and potentiated the action of morphine (particularly in mice). The analgesic effects of adenosine were completely abolished by caffeine (10 mg/kg in mice and rats), and partially inhibited by naloxone (1 mg/kg, only in mice). S-PIA given alone (0.6 mg/kg) produced in mice some analgesic effect in the hot-plate test: the effect was abolished by caffeine and partially by naloxone. The effect of S-PIA on the action of morphine depends on the dose and the animal species. Dipyridamole alone did not affect the reactivity of animals in tests for analgesia, but potentiated the action of morphine.     

Gastroprotective action of glucocorticoid hormones in rats with desensitization of capsaicin-sensitive sensory neurons

The ability of glucocorticoid hormones to protect gastric mucosa during desensitization of capsaicin-sensitive afferent neurons has been investigated in rats. Functional ablation of the afferent neurons was performed by pre-treatment with neurotoxic doses of capsaicin (100 mg/kg s.c.). After 1 week of recovery, capsaicin-desensitized, as well as control rats were adrenalectomized or shamoperated. Seven days later, indomethacin at an ulcerogenic dose (35 mg/kg s.c.) was given to each group of rats. One half of adrenalectomized capsaicin-pre-treated rats were injected by corticosterone for replacement (4 mg/kg s.c., 15 min before indomethacin). Gastric lesions, plasma corticosterone and blood glucose levels were estimated 4 h after indomethacin administration. Indomethacin caused gastric erosions that were aggravated by adrenalectomy or desensitization of capsaicinsensitive afferent neurons approximately with the same extension. Combination of adrenalectomy with the sensory desensitization profoundly potentiated the effect of sensory desensitization alone on indomethacin-induced gastric erosions: the mean gastric erosion area was increased approximately 10-fold. Corticosterone replacement completely prevented this profound effect of adrenalectomy. The results suggest a pivotal role of glucocorticoid hormones in the maintenance of gastric mucosal integrity in the case of impaired gastroprotective mechanisms provided by PGs and capsaicin-sensitive sensory neurons.     

Influence of glycine on morphine-induced antinociception in mice

The effects of glycine on morphine-induced antinociception were investigated in mice, using a cutaneous thermal test (hot-plate), a visceral chemical test (acetylcholine writhing test), and a locomotor activity test. When glycine (200 mg/kg p.o.) and morphine (5 mg/kg s.c.) were given together during the first 30 min, glycine first antagonized the morphine-induced antinociception then this was followed by a synergistic effect. The two-phase influence of glycine on morphine-induced antinociception may be due to the interaction of glycine with different receptors.     

Influence of tiagabine on the antinociceptive action of morphine, metamizole and indomethacin in mice

The influence of tiagabine at a dose of 3.2 mg/kg (single administration) and at a dose of 1.2 mg/kg (multiple administration - 10 days) on the antinociceptive effect of morphine (10 mg/kg), metamizole (500 mg/kg) and indomethacin (10 mg/kg - single dose and 1.4 mg/kg - multiple doses) was investigated in mice using the hot-plate and tail-flick tests. All drugs were injected intraperitoneally. Tiagabine was administered to mice 30 min before the analgesic drugs. Measurement of the reaction to a noxious stimulus was performed 60, 90 and 120 min after administration of tiagabine. The study was further conducted for 10 days with repeated drug doses. Tiagabine and morphine administered in single doses demonstrate an additive antinociceptive effect in the hot-plate test and a slightly synergistic effect in the tail-flick test. A single administration of tiagabine slightly increased the antinoceptive action of metamizole and indomethacin in both tests, but that effect is less pronounced than the antinociceptive action of tiagabine alone. Repeated administration of tiagabine with morphine abolishes the tolerance to morphine analgesia. Both single and repeated administration of tiagabine alone exerted the antinociceptive effect in the hot-plate test.     

Effect of diazepam and midazolam on the antinociceptive effect of morphine, metamizol and indomethacin in mice

The influence of midazolam and diazepam on antinociceptive effect of morphine (10 mg/kg), metamizol (500 mg/kg) and indomethacin (10 mg/kg) was investigated in a mouse model using the tail-flick and hot-plate tests. All drugs were injected intraperitoneally. Benzodiazepines were administered to mice 30 min before applying the analgesic drugs. Measurement of nociception was performed within 2 h after benzodiazepine administration. Diazepam at doses of 0.25 mg/kg and 2.5 mg/kg injected with morphine was found to decrease the antinociceptive effect of morphine. Similarly, diazepam decreased the antinociceptive effect of metamizol (only in the tail-flick test) and indomethacin. Midazolam used at doses of 1.25 mg/kg and 2.5 mg/kg decreased the antinociceptive effect of morphine, metamizol (only in the tail-flick test) and indomethacin.     

Vasopressin and adrenalectomy-induced sensitivity to morphine

Arginine vasopressin, vasopressin antiserum and a specific vasopressin pressor antagonist were injected intracerebroventricularly into adrenalectomized rats before morphine-induced antinociception was tested. In these experiments we have exploited previous findings which showed that the antinociceptive effect of opioids was potentiated after adrenalectomy; rats that were adrenalectomized in the morning under basal resting conditions of the pituitary-adrenal system displayed significantly higher response latencies after morphine administration than rats adrenalectomized in the evening. These effects were measured 7 days after adrenalectomy. The same conditions were used in this study. Both, the vasopressin antiserum and the vasopressin antagonist abolished the morning adrenalectomy-induced hypersensitivity to centrally injected morphine and were not effective when administered to rats that had been adrenalectomized in the evening. The reverse was observed after intraventricular administration of vasopressin. The peptide significantly raised the sensitivity to morphine-induced antinociception of rats that had been adrenalectomized in the evening whereas it did not affect antinociception in animals that had been adrenalectomized in the morning. Vasopressin levels determined by radioimmunoassay in the cerebrospinal fluid were significantly higher in adrenalectomized animals. We propose that vasopressin is a critical neuropeptide factor involved in the adrenalectomy-induced hypersensitivity to morphine antinociception.     

Influence of antineoplastic drugs on morphine analgesia and on morphine tolerance

The possible influence of cisplatin, methotrexate, adriamycin and vincristine on thermal pain threshold, morphine analgesia and development of morphine tolerance was investigated in mice. In the hot-plate test, the nociceptive threshold was not affected by acute or repeated administration of any of the antineoplastic drugs used. The analgesic activity of morphine was significantly reduced by pretreatment with cisplatin, intraperitoneally (i.p.) injected at the dose of 2 mg/kg. In contrast, methotrexate, subcutaneously (s.c.) injected at the dose of 1 and 5 mg/kg, adriamycin (1 and 3 mg/kg s.c.), vincristine (0.25 and 0.5 mg/kg i.p.) and a lower dose of cisplatin (1 mg/kg i.p.) had no effect. The development of tolerance to morphine analgesia was delayed by adriamycin but was not influenced by the other antineoplastic drugs used. These data show that, of the four antineoplastic agents used in this study, cisplatin may interfere in the mechanism of action of morphine, and that adriamycin may delay the development of opiate tolerance.