Antinociceptive Effect Of Morphine Research Articles

Amitriptyline preserves morphine’s antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine’s antinociceptive effect, with a 5.3-fold leftward shift of morphine’s dose–response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 μg/10 μl, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKCα, PKCβII, and PKCγ. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine’s antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

Comparative measurement of spinal CSF microdialysate concentrations and concomitant antinociception of morphine and morphine-6β-glucuronide in rats

Morphine-6β-glucuronide (M6G) is well known as a potent active metabolite in humans. To clarify concentration–antinociceptive effect relationships for morphine and M6G, we evaluated comparatively the pharmacokinetics and antinociceptive effects of morphine and M6G. The spinal CSF concentration and antinociception were simultaneously measured by using the combination of a microdialysis method and the formalin test in conscious rats after the s.c. administration of morphine (0.3–3 mg/kg) and M6G (0.1–3 mg/kg). The plasma concentration of M6G after s.c. administration was higher than that of morphine, as shown by the 2.1 times greater value of area under the concentration–time curve (AUC plasma). The spinal CSF concentrations of morphine and M6G increased dose-dependently. The AUC CSF of M6G was 1.6–1.8 times higher than that of morphine at each dose. Administration of morphine and M6G dose-dependently suppressed the flinching behavior induced by formalin injection. The ED 50 values for M6G were 3 times lower than those of morphine, although the spinal CSF concentration versus antinociceptive effect curves of morphine and M6G were very similar, with similar EC 50 values. These results suggest that the antinociceptive potencies of morphine and M6G, evaluated by simultaneous measurements of spinal CSF drug concentration and antinociception, are equivalent. Simultaneous measurement of spinal CSF concentration and antinociception by using microdialysis should be useful for elucidating the relationship between pharmacokinetics and pharmacodynamics of various opioids.

Different effects of morphine and morphine-6β-glucuronide on formalin-evoked spinal glutamate release in conscious and freely moving rats

The aim of this study was to investigate comparatively the role of spinal glutamate in the antinociceptive effect of morphine and morphine-6β-glucuronide (M6G). The glutamate concentration in the spinal microdialysates and flinching behavior were simultaneously measured in conscious and freely moving rats after the intraplanter injection of formalin. The subcutaneous administration of morphine (0.3–3 mg/kg) in these rats suppressed dose dependently both flinching behavior and spinal glutamate release induced by formalin. Similarly, the subcutaneous administration of M6G at doses of 0.1–3 mg/kg suppressed the formalin-induced flinching behavior in the dose-dependent manner, but it did not cause a dose-related inhibition of spinal glutamate release. The inhibitory effects of morphine on the formalin-induced flinching behavior and spinal glutamate release were markedly attenuated by repeated treatment with this drug for 5 days in rats. Thus, there was a significant ( P < 0.05) correlation between antinociception and inhibitory effect on glutamate release of morphine in rats. These results suggest a significant difference between morphine and M6G in the participation of spinal glutamate for the antinociceptive effect.

Possible involvement of protease‐activated receptor‐1 in the regulation of morphine‐induced dopamine release and hyperlocomotion by the tissue plasminogen activator‐plasmin system

We have previously demonstrated that tissue plasminogen activator (tPA)-plasmin system participates in the rewarding effect of morphine, by regulating dopamine release in the nucleus accumbens (NAc). However, it is unclear how plasmin increases the morphine-induced release of dopamine and hyperlocomotion. In the present study we investigated whether protease activated receptor-1 (PAR-1) is involved in the regulation of acute morphine-induced dopamine release by the tPA-plasmin system. Morphine significantly but transiently increased extracellular tPA activity in the NAc, which was completely blocked by naloxone. Microinjection of a PAR-1 antagonist, (tyr(-1))-thrombin receptor activating peptide 7, into the NAc significantly reduced morphine-induced dopamine release in the NAc and hyperlocomotion although the treatment had no effect on basal dopamine release and spontaneous locomotor activity. Furthermore, the PAR-1 antagonist blocked the ameliorating effect of plasmin on the defect of morphine-induced dopamine release in the NAc of tPA-deficient mice. In contrast, intracerebroventricular injection of the PAR-1 antagonist had no effect on the antinociceptive effects of morphine in mice. These results suggest that PAR-1 is a target for the tPA-plasmin system in the regulation of acute morphine-induced dopamine release in the NAc.

Pain sensitivity is altered in animals after subchronic ketamine treatment

Clinical observations have shown that pain sensitivity is altered in some schizophrenic patients. To study alterations in pain sensitivity, the ketamine model in schizophrenia research was employed. Rats were subchronically injected with the dissociative anaesthetic ketamine (Ket, ten injections of 30 mg/kg, one injection per day over a period of 10 days). Two weeks after treatment completion, the animals' pain sensitivity was assayed in the hot plate test and they were subjected to electrical stimulation of the tail root. In addition, the effect of morphine was studied. In group-housed animals, there was no difference between Ket-injected animals and control rats as measured in both nociceptive tests. In singly housed Ket-injected rats, pain threshold was increased in the electrical stimulation test. This suggests that stress due to single housing might be essential for modifications of pain sensitivity. Moreover, the antinociceptive effect of morphine was modified after single housing. Interestingly, the effect of morphine on locomotor activity was similar in both groups. In group-housed rats, mu receptor binding was unchanged in the frontal cortex, whereas Ket-injected animals had decreased levels in the hippocampus. In singly housed animals, mu receptor binding in Ket-injected rats increased in the frontal cortex and decreased in the hippocampus. (35)S-GTPgamma-S binding increased in the frontal cortex in both singly housed groups, but remained unchanged in the hippocampus. The data suggest that the ketamine model might be useful for studying altered pain sensitivity in schizophrenia. Moreover, the data suggest that modifications in mu opioid receptor binding contribute to this phenomenon.

Decreased morphine analgesia in rat overexpressing β-arrestin 2 at periaqueductal gray

β-arrestin 2 plays important physiological roles in regulating the function of the μ opioid receptor (μOR) in vivo. Periaqueductal gray (PAG) is a potential structure where morphine produces its antinociception, but it is unclear whether β-arrestin 2 plays its regulatory effect on morphine at PAG. In the present study β-arrestin 2 overexpression was induced by adenovirus at PAG of rats. Morphine was administrated in these rats through PAG microinjection and systemic administration. The antinociceptive effects of morphine were abolished in rats received microinjection of morphine at PAG and partially attenuated in rats received systemic administration of morphine. These findings support the notion that PAG is an important site where β-arrestin 2 plays its regulatory effects on morphine analgesia.

Morphine can produce analgesia via spinal kappa opioid receptors in the absence of mu opioid receptors

Previous studies have demonstrated the virtual lack of analgesia in mu opioid receptor knockout mice after systemic administration of morphine. Thus, it has been suggested that analgesic actions of morphine are produced via the mu opioid receptor, despite its ability to bind to kappa and delta receptors in vitro. However, it is not clear whether the results of these studies reflect the effect of morphine in the spinal cord. In the present study, we report study of the analgesic actions of spinally-administered morphine and other opioid receptor agonists in mu opioid receptor knockout and wild type mice. Morphine produced a dose-dependent antinociceptive effect in the tail flick test in the knockout mice, although higher doses were needed to produce antinociception than in wild type mice. The antinociceptive effect of morphine was completely blocked by naloxone (a non-selective opioid antagonist) and nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist), but not by naltrindole (a selective delta-opioid receptor antagonist). U-50,488H (a selective kappa-opioid receptor agonist) also produced a dose-dependent antinociceptive effect in knockout mice but presented lower analgesic potency in knockout mice than in wild type mice. Analgesic effects of [ d-Pen2, d-Pen5]enkephalin (DPDPE, a selective delta-opioid receptor agonist) were observed in wild type mice but abolished in knockout mice. SNC80 (a selective delta-opioid receptor agonist) was not antinociceptive even in wild type mice. The present study demonstrated that morphine can produce thermal antinociception via the kappa opioid receptor in the spinal cord in the absence of the mu opioid receptor. Lower potency of U50,488H in mu opioid receptor knockout mice suggests interaction between kappa and mu opioid receptors at the spinal level.

Persistent pain model reveals sex difference in morphine potency

Central or systemic administration of agonists directed at the mu or delta opiate receptors generally produce a greater degree of analgesia in males than in females. To date, most studies examining sex-based differences in opioid analgesia have used acute noxious stimuli (i.e., tail-flick and hot plate test); thus the potential dimorphic response of centrally acting opiates in the alleviation of persistent inflammatory pain is not well established. In the present study, right hind paw withdrawal latency (PWL) to radiant thermal stimuli was measured in intact male and cycling female Sprague-Dawley rats before and after unilateral hind paw injection of the inflammatory agent complete Freund's adjuvant (CFA). Control animals received intraplantar injection of saline. Twenty four hours after CFA or saline injection, animals received either saline or morphine bisulfate (0.5-15 mg/kg sc). Separate groups of control or inflamed animals were tested on their responsiveness to morphine at 7, 14, and 21 days post-CFA or saline. No sex differences were noted for baseline PWLs, and females displayed slightly less thermal hyperalgesia at 24 h post-CFA. At all morphine doses administered, both the antihyperalgesic effects of morphine in the inflamed animals and the antinociceptive effects of morphine in control animals were significantly greater in males compared with females. Similarly, in males, the antihyperalgesic effects of morphine increased significantly at 7-21 days post-CFA; no significant shift in morphine potency was noted for females. These studies demonstrate sex-based differences in the effects of morphine on thermal hyperalgesia in a model of persistent inflammatory pain.

Strain-specific Differences in Modulatory Effects of Morphine on Peritoneal Inflammation in Mice

It has been previously shown that local administration of a high dose of morphine together with a proinflammatory agent has anti-inflammatory effects in several (but not all) strains of mice. In the present paper, behavioural and cellular consequences of morphine co-injection are compared between several strains of mice with zymosan-induced peritonitis. Males of C57C3H, Swiss, Balb/c, C57BL/6, and CBA strains were ip injected with Zymosan (40 mg/kg) and/or morphine at 0, 5, 10, 20 mg/kg without or with naltrexone pretreatment. Early stages of Zymosan-induced peritonitis were connected with intraperitoneal accumulation of leukocytes (mainly PMNs), pain symptoms (body writhes of strain-specific frequency: C57C3H>CBA>Swiss>>Balb/c-C57BL/6), and sedation (significant in Swiss and Balb/c). Morphine co-injection abolished pain symptoms at all doses in every investigated strain, and restored locomotor activity or induced hyperlocomotion in a dose- and strain-specific manner. The highest dose of morphine inhibited intraperitoneal accumulation of peritoneal leukocytes (PTLs) including polymorphonuclear cells (PMNs) in all but the CBA strain of mice. Anti-inflammatory and anti-nociceptive effects of morphine were reversed in naltrexone pre-treated animals. The pre-incubation of Swiss but not CBA leukocytes with morphine inhibited cell chemotaxis towards zymosan-activated serum. In conclusion, morphine exerts various strain-specific effects on peritonitis.

Modulatory Role of Green Tea Extract on Antinociceptive Effect of Morphine in Diabetic Mice

Diabetic neuropathic pain is an important microvascular complication, and morphine has been demonstrated to be ineffective in this condition. Therefore the present study was designed to investigate the modulatory effect of green tea extract (GTE) on the decreased antinociceptive effect of morphine in diabetic mice. The tail withdrawal test was performed for measurement of the nociceptive threshold in both streptozotocin (STZ)-injected and control mice. Four weeks after administration of STZ, antinociception of morphine (5 mg/kg, s.c.) alone or in combination with GTE (25, 50, and 100 mg/kg, i.p.) was measured. Experimental diabetes markedly decreased the antinociceptive effect of morphine. The decrement in morphine response was significantly attenuated by GTE administration. When GTE (25 mg/kg) and a nitric oxide (NO) inhibitor, L-N(G)-nitroarginine methyl ester (L-NAME) (10 mg/kg, i.p), were co-administered along with morphine (5 mg/kg, s.c) in diabetic mice, the antinociceptive action of morphine was significantly increased as compared with the GTE + morphine-treated diabetic group, but the increased antinociceptive action was significantly attenuated by administration of an NO precursor, L-arginine (100 mg/kg, i.p), instead of L-NAME. Plasma nitrite concentrations were estimated using the Griess reagent. Diabetes significantly increased the plasma nitrite levels that were attenuated by GTE administration. When GTE (25 mg/kg) and L-NAME (10 mg/kg, i.p) were co-administered along with morphine (5 mg/kg, s.c) in diabetic mice, the plasma nitrite levels were significantly decreased as compared with the GTE + morphine alone-treated diabetic group, but the decreased plasma nitrite levels were significantly reversed by administration of L-arginine (100 mg/kg) instead of L-NAME. It may be concluded that increased NO formation may be responsible for the decreased antinociceptive effect of morphine in diabetic mice and that GTE restored the antinociceptive effect of morphine by inhibition of NO production. The results of the present study indicate the possibility of adding GTE as an adjuvant in the treatment of diabetic neuropathic pain.

Enhanced analgesic effect of morphine-nimodipine combination after intraspinal administration as compared to systemic administration in mice

Calcium plays an important role in the pathophysiology of pain. A number of studies have investigated the effect of L-type calcium channel blockers on the analgesic response of morphine. However, the results are conflicting. In the present study, the antinociceptive effect of morphine (2.5 microg) and nimodipine (1 microg) co-administered intraspinally in mice was observed using the tail flick test. It was compared to the analgesic effect of these drugs (morphine - 250 microg subcutaneously; nimodipine - 100 microg intraperitoneally) after systemic administration. Nimodipine is highly lipophilic and readily crosses the blood brain barrier. Addition of nimodipine to morphine potentiated the analgesic response of the latter when administered through the intraspinal route but not when administered through systemic route. It may be due to direct inhibitory effect of morphine and nimodipine on neurons of superficial laminae of the spinal cord after binding to mu -opioid receptors and L-type calcium channels respectively.

Morphine has an antinociceptive effect through activation of the okadaic-acid-sensitive Ser/Thr protein phosphatases PP2A and PP5 estimated by tail-pinch test in mice

Although the serine/threonine protein kinases involved in the pharmacological action of morphine are well recognized, the critical contribution of serine/threonine protein phosphatase (PP) has been appreciated on to a slight degree. We examined the involvement of subtypes of serine/threonine protein phosphatase (PP) in the antinociceptive effect of morphine in mice. The antinociceptive effect of subcutaneously administered morphine was attenuated by simultaneously intracerebroventricular (i.c.v.) or intrathecal (i.t.) injection of okadaic acid (OA), a PP inhibitor. To reveal which subtypes of PPs participated in the antinociceptive effect of morphine, mice received i.c.v. or i.t. injections of antisense oligodeoxynucleotide (AS-ODN) directed against either the PP2A or PP5 subtypes of PPs before assessment of morphine-induced antinociception. Pretreatment with AS-ODN against PP2A or PP5 via each route weakened the antinociceptive effect of morphine, accompanied by reduction of expression levels of PP in the periaqueductal gray (PAG) and the spinal cord. Subcutaneously administered morphine increased activity of OA-sensitive PPs in the PAG and the spinal cord in a dose-dependent manner; this was prevented by concurrent administration of naloxone. These results suggest that PP2A and PP5 are involved in the antinociceptive effect of morphine in mice.

Attenuation of morphine tolerance by intrathecal gabapentin is associated with suppression of morphine-evoked excitatory amino acid release in the rat spinal cord

This study was designed to investigate the effect of acute and chronic intrathecal (i.t.) injection of gabapentin (GBP) on the antinociceptive effect of morphine and tolerance development using a tail-flick latency test. Levels of excitatory amino acids (EAA) in i.t. CSF dialysates were also measured by high performance liquid chromatography. Male Wistar rats were implanted with either one or two i.t. catheters for drug injection or pump infusion and with a microdialysis probe for CSF dialysate collection. The effect of acute GBP (10 μg i.t.) injection on the morphine dose response was examined in both naïve rats and rats made tolerant by continuous infusion of morphine (15 μg/h i.t.) for 5 days. At such a low dose (10 μg i.t.), GBP did not enhance morphine's antinociception in naïve rats. In morphine-tolerant rats, however, acute GBP (10 μg i.t.) injection potentiated morphine's antinociception and yielded a 14.6-fold shift in morphine's dose–response curve. When GBP (10 μg/h i.t.) was co-infused with morphine (15 μg/h i.t.) to examine its effect on the development of morphine tolerance, GBP attenuated the development of morphine tolerance. The effect of GBP and morphine on CSF glutamate and aspartate levels was examined in naïve rats, and the effect of morphine challenge on CSF glutamate and aspartate levels was examined in rats previously infused for 5 days with morphine alone or morphine plus GBP. Acute injection of GBP (10 μg i.t.), morphine (50 μg i.t.), or GBP (10 μg i.t.) followed by morphine (50 μg i.t.) 30 min later had no significant effect on CSF EAA concentration in naïve rats; however, in tolerant rats, morphine challenge (50 μg i.t.) increased aspartate and glutamate levels to 221 ± 22% and 296 ± 43%, respectively, of those before morphine challenge, and this phenomenon was inhibited by GBP co-infusion. Our results show that GBP, at a dose without enhanced effect on morphine's antinociception in naïve rats, not only potentiates morphine's antinociceptive effect in morphine-tolerant rats but also attenuates the development of morphine tolerance. The mechanism of the effect of GBP on morphine tolerance might be via suppression of the EAA concentration in spinal CSF dialysate.

A Comparison of the Antinociceptive and Adverse Effects of the μ‐Opioid Agonist Morphine and the δ‐Opioid Agonist SNC80

delta-Opioid receptor agonists have been postulated to induce analgesia without the adverse effects commonly associated with mu-opioids e.g. morphine. In the present study, we evaluated the occurrence of antinociceptive and opioid-like side effects in rats (n=5-7) treated with a single dose of subcutaneous morphine (0.01 to 40 mg/kg) or SNC80 (0.63 to 80 mg/kg). The antinociceptive effects of morphine and SNC80 were compared using a range of nociceptive tests including the tail withdrawal test, the acetic acid-induced abdominal constriction (writhing) assay, the automated formalin test and a model of inflammation-induced thermal hyperalgesia. The adverse effects of both drugs were examined in animal models for gastrointestinal (GI) inhibition (charcoal test; ricinus oil test), respiratory depression (blood-gas analysis), motor disturbances (automated rotarod model) and abuse liability (drug discrimination learning). Morphine displayed significant antinociceptive and adverse effects in all the animal models employed. SNC80 exhibited a significant effect in the writhing test and limited efficacy in a model of inflammation-induced thermal hypersensitivity. A delay in the occurrence of diarrhoea and some limited increases in PCO(2) were observed with the higher doses of SNC80 (> or =40 mg/kg). In conclusion, the delta-opioid agonist SNC80 lacks both the analgesic efficacy and adverse effects of mu-opioids. However, the activity of SNC80 in the inflammatory model suggests delta-opioid agonists may be useful analgesics in the treatment of some forms of inflammatory pain.

Inhibition of neuronal nitric oxide synthase attenuates the development of morphine tolerance in rats

Our previous results have shown the involvement of nitric oxide in acute opioid desensitization of mu-opioid receptors in vitro. In the present study, we investigated the effect of repeated administration of 7-nitroindazole (7-NI; 30 mg/kg/12 h, i.p., 3 days), an inhibitor of neuronal nitric oxide synthase in vivo, on mu-opioid receptor tolerance induced by subchronic treatment with morphine in rats. The inhibitory effect of the opioid agonist Met5-enkephalin (ME) on the cell firing rate was evaluated by single-unit extracellular recordings of noradrenergic neurons in the locus coeruleus from brain slices, and the antinociceptive effect of morphine was measured by tail-flick techniques. In morphine-treated animals, concentration-effect curves for ME in the locus coeruleus were shifted by 5-fold to the right as compared to those in sham-treated animals, which confirmed the induction of mu-opioid receptor tolerance. However, tolerance to ME in morphine-treated rats was fully prevented by co-administration of 7-NI when compared to the vehicle-morphine group. Likewise, the antinociceptive effect of morphine was reduced in morphine-treated animals as compared to the sham group, whereas the antinociceptive tolerance was partially prevented by co-administration of 7-NI in morphine-treated rats (when compared to the vehicle-morphine group). Finally, 7-NI administration in sham-treated rats failed to change the effect induced by ME on the locus coeruleus or by morphine in the tail-flick test as compared to vehicle groups. These results demonstrate that subchronic administration of a neuronal inhibitor of nitric oxide synthase attenuates the development of morphine tolerance to the cellular and analgesic effects of mu-opioid receptor agonists.

Modification by the tissue plasminogen activator–plasmin system of morphine‐induced dopamine release and hyperlocomotion, but not anti‐nociceptive effect in mice

The extracellular serine protease tissue plasminogen activator (tPA) that converts plasminogen into plasmin is abundantly expressed throughout the central nervous system. We have recently demonstrated that the tPA-plasmin system participates in the rewarding and locomotor-stimulating effects of morphine by acutely regulating morphine-induced dopamine release in the nucleus accumbens (NAc). In the present study, we examined the effects of microinjections of plasminogen activator inhibitor-1 (PAI-1), tPA or plasmin into the NAc on morphine-induced dopamine release, hyperlocomotion and anti-nociceptive effects in ICR mice. A single morphine treatment resulted in an increase in protein levels of PAI-1 in the NAc. Microinjection of PAI-1 into the NAc dose-dependently reduced morphine-induced dopamine release and hyperlocomotion. In contrast, microinjection of tPA into the NAc significantly potentiated morphine-induced dopamine release and hyperlocomotion without affecting basal levels. Furthermore, microinjection of plasmin enhanced morphine-induced dopamine release, but did not modify the hyperlocomotion induced by morphine. The intracerebroventricular injection of PAI-1, tPA and plasmin at high doses had no effect on the anti-nociceptive effects of morphine. These results suggest that the tPA-plasmin system is involved in the regulation of morphine-induced dopamine release and dopamine-dependent behaviors but not the anti-nociceptive effects of morphine.

Intrathecal administration of doxepin attenuated development of formalin-induced pain in rats

The aim of the present research was to assess the influence of a tricyclic antidepressant doxepin administered intrathecally (i.t.) on the pain behavior in the formalin test (100 microl of 12% formalin was injected into the dorsal part of the hind paw under halotane anesthesia) in male Wistar rats. The influence of doxepin (62.5 microg i.t.) on the pain threshold and number of formalin-induced pain behaviors, as well as antinociceptive effect of morphine was studied. Doxepin significantly increased the nociceptive threshold in the paw pressure test, reduced formalin-induced pain behavior and potentiated morphine antinociceptive effect in formalin test. The obtained results indicate that analgesic effect of doxepin used before the injury is observable at the spinal level after intrathecal treatment, but not only after peripheral administration, which was shown in our previous study. The results of the present research demonstrated a possibility to modify the spinal nociceptive process by administration of doxepin before the formalin injection.

Environmental and intraperitoneal ethanol influences morphine antinociceptive effect in mice

The ability of acute environmental or intraperitoneal (i.p.) ethanol to influence morphine antinociceptive effect was studied in mice. In order to induce tolerance to morphine analgesia, mice received daily injections of 10mg/Kg morphine over a period of 10 days. Mice were divided into three groups: i.p. ethanol (E), environmental ethanol (E*), and control saline (M). During the induction of tolerance these groups were treated identically except on days 1 and 11. On these days, 10 minutes prior to morphine injection, mice received either i.p. ethanol (1g/Kg), environmental ethanol (a bottle of 10% ethanol placed next to the animals cage during the experiments), or an equivalent volume of saline. Analgesia was assessed using a standard hot plate protocol and dose-response cumulative curves for morphine analgesia were obtained on days 1 and 11. On day 1, both the i.p. and environmental administration of ethanol showed similar morphine-potentiation effects [Mean Effective Dose: ED 50 (M 1) = 4.5 mg/kg; ED 50 (E 1) = 2.4 mg/kg; ED 50 (E* 1) = 2.1 mg/kg]. On day 11, control group mice showed a reduction of morphine analgesia at test [ED 50 (M 11) = 14.1 mg/kg]. Mice receiving i.p. and environmental ethanol again showed a leftward shift in dose-response cumulative curves for morphine antinociception with respect to controls [ED 50 (E 11) = 9.1 mg/kg; ED 50 (E* 11) = 4.7 mg/kg]. I.p. ethanol administration at non-antinociceptive doses enhances the morphine antinociception effect similarly in tolerant and non-tolerant (naive) mice. The presence of environmental ethanol can also induce a similar pattern of increase in morphine antinociception effect.

Dextromethorphan differentially affects opioid antinociception in rats

Opioid drugs such as morphine and meperidine are widely used in clinical pain management, although they can cause some adverse effects. A number of studies indicate that N-methyl-D-aspartate (NMDA) receptors may play a role in the mechanism of morphine analgesia, tolerance and dependence. Being an antitussive with NMDA antagonist properties, dextromethorphan (DM) may have some therapeutic benefits when coadministered with morphine. In the present study, we investigated the effects of DM on the antinociceptive effects of different opioids. We also investigated the possible pharmacokinetic mechanisms involved. The antinociceptive effects of the mu-opioid receptor agonists morphine (5 mg kg(-1), s.c.), meperidine (25 mg kg(-1), s.c.) and codeine (25 mg kg(-1), s.c.), and the kappa-opioid agonists nalbuphine (8 mg kg(-1), s.c.) and U-50,488H (20 mg kg(-1), s.c.) were studied using the tail-flick test in male Sprague-Dawley rats. Coadministration of DM (20 mg kg(-1), i.p.) with these opioids was also performed and investigated. The pharmacokinetic effects of DM on morphine and codeine were examined, and the free concentration of morphine or codeine in serum was determined by HPLC.It was found that DM potentiated the antinociceptive effects of some mu-opioid agonists but not codeine or kappa-opioid agonists in rats. DM potentiated morphine's antinociceptive effect, and acutely increased the serum concentration of morphine. In contrast, DM attenuated the antinociceptive effect of codeine and decreased the serum concentration of its active metabolite (morphine). The pharmacokinetic interactions between DM and opioids may partially explain the differential effects of DM on the antinociception caused by opioids.

Increased nociceptive response in mice lacking the adenosine A 1 receptor

The role of the adenosine A 1 receptor in nociception was assessed using mice lacking the A 1 receptor (A 1R−/−) and in rats. Under normal conditions, the A 1R−/− mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A 1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A 1R−/− mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the μ-opioid ligand DAMGO in the spinal cord. A 1R−/− mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A 1R−/−mice. Peripheral nerve injury did not change the level of adenosine A 1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A 1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A 1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A 1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.