Melatonin inhibits the development of tolerance to U-50,488H analgesia via benzodiazepine-GABAAergic mechanisms

Melatonin, a primary secretory product of pineal gland, is known to produce many of its pharmacological actions via benzodiazepine-gamma-aminobutyric acidA (GABAA)ergic mechanisms. Recently, we showed that benzodiazepine-GABAAergic mechanisms play an important role in U-50,488H (U50) analgesia and its tolerance. Hence, in the present study, the effect of melatonin on U50 analgesia and its tolerance was investigated. Furthermore, the possible role of benzodiazepine-GABAAergic mechanisms in the actions of melatonin on U50 analgesia was investigated. All experiments were performed using the radiant tail-flick test for mice. Melatonin [0.2, 1 and 5 mg/kg, intraperitoneal (i.p.)] neither produced analgesia nor affected the acute U50 (40 mg/kg, i.p.) analgesia. Tolerance to U50 analgesia was induced by administering U50 (40 mg/kg, i.p.) twice daily over 6 days. Treatment with melatonin (1 and 5 mg/kg, i.p) 15 min prior to each dose of U50 inhibited the development of tolerance, whereas a low dose of melatonin (0.2 mg/kg, i.p.) did not. The inhibition of U50 tolerance by melatonin was reversed by the chronic treatment with flumazenil (0.1 mg/kg), a benzodiazepine receptor antagonist and picrotoxin (1 mg/kg), a GABAA-gated chloride channel blocker. Flumazenil and picrotoxin neither affected tail-flick latencies nor altered acute U50 analgesia and its tolerance. Interestingly, chronic 6-day melatonin treatment in a vehicle (U50-naive) group did not alter U50 analgesia measured on day 7. Together, these findings suggest that melatonin interferes with the neural mechanisms involved in the development of tolerance to U50 analgesia. The inhibition of U50 tolerance by melatonin was reversed by flumazenil and picrotoxin treatment, suggesting that benzodiazepine-GABAAergic mechanisms play an important role in the development of tolerance to U50 analgesia and that melatonin inhibits the development of U50 tolerance via benzodiazepine-GABAAergic mechanisms.     

Influences of electrical lesions of the dopaminergic system on morphine- and U-50,488H-induced analgesia in rats

The effects of electrical lesions of brain areas containing dopamine cell bodies and terminals on morphine analgesia were investigated and compared with those of a selective kappa-opioid agonist, U-50,488H. The analgesic effect of morphine 10 mg/kg IP was potentiated significantly in substantia nigra (SN)- or caudate-putamen-lesioned rats, but not by ventral tegmental area (VTA) or nucleus accumbens lesions. However, electrical lesions of neither SN nor VTA affected the analgesic activity of U-50,488H 32 mg/kg IP. Although the tolerance to morphine analgesia developed in all four of the lesioned groups as well as in sham-lesioned rats, a significant analgesic effect in the SN-lesioned group prevailed during chronic treatment for 14 days as compared with that of sham-lesioned rats. From these results, it is suggested that morphine analgesia is potentiated by dysfunction of the nigro-striatal dopaminergic system, but not by that of the mesolimbic dopaminergic system, the central dopaminergic system is not involved in the appearance of U-50,488H analgesia and is not basically related to the development of tolerance to morphine analgesia.     

A selective kappa-opioid agonist, U-50,488H, blocks the development of tolerance to morphine analgesia in rats

The development of tolerance to morphine analgesia was completely blocked by the coadministration of a selective kappa-opioid agonist, U-50,488H at doses of 3.2 or 10 mg/kg i.p. These doses of U-50,488H exerted no analgesic effect by themselves and did not affect the analgesia induced by 10 mg/kg of morphine. The analgesic effect of morphine was restored when 10 mg/kg of U-50,488H was coinjected in morphine-tolerant rats. These findings suggest that activation of the kappa-opioid system prevents the development of tolerance to morphine analgesia.     

Endothelin ETA receptor blockade potentiates morphine analgesia but does not affect gastrointestinal transit in mice

Development of analgesic tolerance and constipation remain a major clinical concern during long-term administration of morphine in pain management. Central endothelin mechanisms are involved in morphine analgesia and tolerance. The present study was conducted to investigate the effect of intracerebroventricular (i.c.v.) and peripheral administration of endothelin ET(A) receptor antagonist, BMS182874, and endothelin ET(B) receptor agonist, IRL1620, on morphine analgesia and changes in gastrointestinal transit in male Swiss Webster mice. Results indicate that morphine (6 mg/kg, s.c.) produced a significant increase in tail flick latency compared to control group. Pretreatment with BMS182874 (50 microg, i.c.v.) significantly enhanced morphine-induced analgesia, while IRL1620 (30 microg, i.c.v.) pretreatment did not affect tail-flick latency values. Changes in gastrointestinal transit were measured by percent of distance traveled by charcoal in the small intestine of gastrointestinal tract. Percent distance traveled in morphine (6 mg/kg, s.c.) treated mice (48.45+/-5.65%) was significantly lower (P<0.05) compared to control group (85.07+/-1.82%). Administration of BMS182874 centrally (50 mug, i.c.v.) or peripherally (10 mg/kg, i.p.) did not affect morphine-induced inhibition of gastrointestinal transit. Pretreatment with IRL1620 (30 microg, i.c.v., or 10 mg/kg, i.v.) also did not affect morphine-induced inhibition of gastrointestinal transit. This study demonstrates that endothelin ET(A) receptor antagonists delivered to the CNS enhance morphine analgesia without affecting gastrointestinal transit.     

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.     

Effect of kappa-opioid receptor agonists on morphine analgesia in morphine-naive and morphine-tolerant rats

The effect of i.p. administration of kappa-opioid receptor agonists, bremazocine, tifluadom and U-50,488H on morphine (8 mg/kg i.p.)-induced analgesia in morphine-naive and morphine tolerant male Sprague-Dawley rats was determined using the tail-flick test. The tolerance to morphine in the rats was induced by s.c., implantation of six morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of tolerance as evidenced by the decrease in the analgesic response to morphine when compared to placebo pellets implanted rats. Bremazocine (0.3, 1.0 and 3.0 mg/kg) and U-50,488H (16 mg/kg) antagonized morphine-induced analgesia in morphine-naive rats while tifluadom (8 and 16 mg/kg) potentiated the effect. In morphine-tolerant rats, bremazocine (3 mg/kg) and U-50,488H (16 mg/kg) potentiated morphine-induced analgesia. Tifluadom at any of the doses had no effect on morphine-induced analgesia in morphine-tolerant rats. These results provide evidence that different kappa-opioid agonists modify morphine-induced analgesia differentially in morphine-naive and morphine-tolerant rats.     

Role of sodium cromoglycate on analgesia,locomotor activity and opiate withdrawal in mice

The role of sodium cromoglycate (CRO) on analgesia, locomotor activity and morphine withdrawal in mice was studied in morphine-dependent and drugnaive mice. CRO (0.5, 1, 5, 10, 30, 50 and 100 mg/kg, SC) induces analgesia (hot plate), an effect blocked by previous administration of the opiate antagonist naloxone (1 mg/kg). Furthermore, CRO (30 mg/kg) potentiates morphine analgesia. In morphine-tolerant mice, moderate doses of CRO (0.5, 1, 5, 10 and 30 mg/kg) do not induce analgesia, which suggested the development of cross tolerance between CRO and morphine, whereas coadministration of CRO and morphine in morphine tolerant animals restored the sensitivity to morphine. Administration of CRO (10 and 30 mg/kg) induces an increase in spontaneous locomotor activity, and previous administration of naloxone (1 mg/kg) blocks this effect, whereas CRO (10 mg/kg) blocks morphine (10 mg/kg) and amphetamine (3 mg/kg)-induced hyperactivity. CRO (10, 50 and 100 mg/kg) induces a significant and dose-dependent reduction in the number of jumps (jumping up) during naloxone (1 mg/kg)-induced withdrawal in morphine-dependent mice. Finally, CRO (100 mg/kg) reduces the wet dog shake phenomenom during naloxone-induced withdrawal in morphine-dependent mice. These results suggest a possible stabilizing effect of CRO on the membranes of neurones that mediate analgesia, locomotor activity and opiate abstinence. Changes and inhibition of DA, NA and 5-HT release may also explain these effects.This work was supported by a grant from the CICYT (Program FAR 90/0543).     

Paradoxical effects of opioid antagonist naloxone on SSRI-induced analgesia and tolerance in mice

Selective serotonin reuptake inhibitors (SSRIs) have been used clinically as co-analgesics in various devastating painful conditions. Upon chronic treatment tolerance develops to their analgesic effect, which is often refractory to increasing dose. Although modulation of serotonergic pathways considerably explains their clinical efficacy, numerous reports nevertheless indicate the direct/indirect role of the opioidergic pathway in SSRI-induced analgesia. The present study was designed to investigate the effect, if any, of the opioid antagonist naloxone on SSRIs-induced analgesia and tolerance employing acetic acid-induced writhing assay. Two SSRIs, fluoxetine (FLX), and citalopram (CTP) were used in the study. Acute systemic (5-40 mg kg(-1) i.p.), or intrathecal (5-40 microg per mouse, i.t.) administration of fluoxetine or citalopram exhibited a dose-dependent and significant (p < 0.05) antinociceptive effect. Single systemic (2-5 mg kg(-1) i.p.) or intrathecal (1 microg per mouse, i.t.) administration of opioid antagonist naloxone blocked where as systemic ultra-low dose (10 ng/kg) or intrathecal (0.05 ng) naloxone potentiated the acute antinociceptive effect of both SSRIs (10 mg kg(-1) i.p. and 10 microg i.t.). Animals treated chronically over a 7-day period with SSRIs developed tolerance to their antinociceptive effect. Further, chronic administration of ultra-low dose of naloxone intrathecal (0.05 ng per mouse, i.t.) or systemic (10 ng kg(-1) i.p.) with fluoxetine or citalopram (10 microg i.t.; 5 mg kg(-1) i.p.) over a 7-day period reversed the tolerance to the antinociceptive effect of SSRIs. Thus, in ultra-low doses, naloxone paradoxically enhances SSRIs-induced analgesia and reverse tolerance through spinal and peripheral action. These effects of opioid antagonist naloxone on SSRIs-induced antinociception may have an implication in refractory cases upon chronic use of SSRIs as co-analgesics.     

Modification of morphine-induced analgesia, tolerance and dependence by bromocriptine

The effect of two doses of bromocriptine, a dopamine agonist, on morphine-induced analgesia, tolerance and dependence was investigated in mice. Bromocriptine at doses of 0.04 and 0.08 mg/kg did not affect the baseline tail flick latency of mice but potentiated the morphine analgesia. Pretreatment of mice with 5 mg/kg of sulpiride, a D-2 antagonist, not only blocked the effect of 0.08 mg/kg of bromocriptine but also antagonized the morphine analgesia. Control animals given daily injections of 10 mg/kg of morphine rapidly developed tolerance to the analgesic effect. A combined treatment of bromocriptine with morphine given daily suppressed the development of tolerance to morphine analgesia. However, development of tolerance to morphine analgesia was not significantly modified in the animals treated daily with bromocriptine (0.08 mg/kg) plus sulpiride (5 mg/kg). Acute dependence was induced by the administration of 100 mg/kg of morphine. The administration of bromocriptine 30 min before naloxone significantly decreased the ED₅₀ value for naloxone for inducing jumping in mice. Coadministration of sulpiride and bromocriptine attenuated the ability of bromocriptine to potentiate the withdrawal syndrome of morphine dependence. The results indicate that bromocriptine potentiates morphine analgesia, suppresses the development of tolerance to morphine analgesia but exacerbates opiate withdrawal signs in morphine-dependent mice. These effects of bromocriptine appear to be mediated via D-2 receptors.     

Dose-dependent effects of prolyl-leucyl-glycinamide on morphine-induced analgesia, tolerance and dependence

Prolyl-leucyl-glycinamide (PLG) at a low dose (10 ng/mouse) administered intracerebroventricularly (i.c.v.) did not affect morphine analgesia, but produced a greater increase in the ED50 of morphine-pretreated (100 mg/kg of morphine sulfate) mice as compared to control mice. PLG at doses of 10 and 100 micrograms/mouse antagonized morphine analgesia. Development of morphine tolerance was unaffected by 10 micrograms/mouse but antagonized by 100 micrograms/mouse of PLG. Development of morphine dependence was assessed by changes in body weight and temperature during naloxone-induced withdrawal. PLG (10 ng/mouse) potentiated, 10 micrograms/mouse had no effect and 100 micrograms/mouse antagonized development of morphine dependence. PLG at doses of 10 and 100 micrograms/mouse precipitated withdrawal in morphine-dependent mice. When mice were pretreated with 1.0 mg/kg naloxone i.p. 15 min before PLG, all doses of PLG had no effect on morphine analgesia, but potentiated the development of morphine tolerance and dependence. None of the doses of PLG altered whole brain levels of morphine. PLG did not alter the affinity of opioid receptors for etorphine or the maximal number of binding sites but PLG did exhibit a very weak affinity for opioid receptors. These results indicate that PLG potentiated development of morphine tolerance and dependence through a mechanism not involving opioid receptors. However, at very high doses it was a weak opioid receptor antagonist.     

Potentiation of kappa-opioid receptor agonist-induced analgesia and hypothermia by fluoxetine.

The effect of fluoxetine, a selective 5-HT reuptake inhibitor on the analgesic and hypothermic response of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methane sulphonate (U-50,488H) and (+/-)-trans-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzo[b] thiophene-4-acetamide (PD 117302), kappa-opioid receptor agonists, was determined in female Sprague-Dawley rats using the tail-flick method and telethermometer, respectively. Intraperitoneal injections of U-50,488H (U50) and PD 117302 (PD117) produced a dose-dependent analgesic and hypothermic response. Fluoxetine (10 mg/kg, i.p.) by itself did not produce an analgesic response. The analgesic response to U50 (10, 20, and 40 mg/kg, i.p.) and PD117 (7.5, 15, and 22.5 mg/kg, i.p.) was potentiated by fluoxetine injected intraperitoneally 60 min prior to the injection of kappa-opioid agonists. Similarly, the hypothermic response of U50 (20 and 40 mg/kg, i.p.) and PD117 (7.5, 15, and 22.5 mg/kg, i.p.) was potentiated by fluoxetine. The results indicate that selective kappa-opioid receptor agonists-induced analgesia and hypothermia is potentiated by fluoxetine, suggesting the role of extracellular 5-HT in the kappa-opioid receptor-mediated analgesia and hypothermia.     

The role of vasopressin on the effect of U-50,488 to block the development of morphine tolerance and physical dependence

U-50,488, a selective κ-opioid receptor agonist, has been reported to inhibit the development of antinociceptive tolerance to morphine in mice, rats and guinea pigs, but the mechanism involved in this action remains unknown. Since U-50,488 has been reported to supress the plasma vasopressin level, we investigated the role of vasopressin with U-50,488 in the male Sprague Dawley rat in this study. Animals (230–270 g) were chronically treated with morphine (10 mg/kg, i.p.) twice a day for 6 days in order to induce tolerance to antinociceptive effect measured by tail-flick test. Withdrawl symptoms were precipitated by naloxone (10 mg/kg, i.p.) on day 7. U-50,488 (i.p.) or AVP (i.p. or i.c.v.) or U-50,488 and AVP was (were) coadministered with chronic morphine to investigate their effects on morphine tolerance and dependence. We found that coadministration of 8 mg/kg U-50,488 (i.p.) with morphine almost completely block morphine tolerance and partially block withdrawal symptoms. In contrast, coadministration of AVP (0.3 μg/kg, i.p., or 0.01 μg, i.c.v.) with morphine and U-50,488, the effects of U-50,488 to block morphine tolerance and dependence were reversed. In addition, treatment of AVP antagonist (dPTyr(Me)AVP, 0.5 μg/kg, i.p. or 0.5 μg, i.c.v.) has the similar effect as U-50,488 to block morphine tolerance. In summary, the effect of U-50,488 to block morphine tolerance and dependence may relate to its inhibitory effect on AVP release. Received: 20 February 1996 / Accepted: 14 October 1996     

Acute and chronic effects of ginseng total saponin and amphetamine on fixed-interval performance in rats

The effect of ginseng total saponin (GTS) on amphetamine (AMPH)-induced disruption of fixed-interval (FI) responding in rats was examined. GTS (50 mg/kg) significantly improved the temporal responding impaired by 2 mg/kg of AMPH. A higher dose of 100 mg/kg GTS disrupted performance when given alone; this disruption was reversed by a low dose of AMPH (0.5 mg/kg) and tolerance developed to the effects of GTS with its repeated administration. Neurochemical analysis revealed that GTS (50 mg/kg) attenuated the increase in striatal dopamine caused by AMPH leading to the conclusion that brain dopamine may partially mediate the behavioral effects of GTS.     

The influence of dextromethorphan on morphine analgesia in Swiss Webster mice is sex-specific

NMDA (N-methyl-d-aspartate) antagonists are known to enhance the analgesic effects of opioids. However, virtually, all studies of this phenomenon have been done using male subjects. Here, the noncompetitive NMDA receptor antagonist dextromethorphan (DEX) was tested over a range of doses (10-200 microg intracerebroventricularly [i.c.v.]) in male and female Swiss Webster mice in combination with 5 mg/kg intraperitoneal (i.p.) morphine. Males exhibited enhanced morphine analgesia following either 100 or 200 microg DEX, but there was no evidence of DEX-mediated potentiation in females at any dose. Instead, DEX attenuated morphine analgesia in females. We also evaluated the effect of 100 microg i.c.v. DEX with different doses of morphine (1, 5 and 10 mg/kg). Again, DEX significantly enhanced morphine analgesia in male mice and attenuated it in females. Next, ovariectomized (OVX) female mice were compared to males following 5 mg/kg i.p. morphine and 100 microg i.c.v. DEX. Male and OVX females exhibited equivalent maximal levels of analgesia following administration of DEX. Morphine analgesia was not enhanced by DEX in sham-treated females and OVX mice with estradiol treatment (5 microg i.p. once daily for 7 days) also did not show DEX enhancement. These experiments demonstrate that the ability of NMDA receptor antagonists to potentiate morphine analgesia is modulated by an estrogen-sensitive mechanism and suggest that sex differences may play a critical role toward a more general understanding of the potentiation of opioid-induced analgesia through NMDA receptor antagonists.     

Disability of development of tolerance to morphine and U-50,488H, a selective kappa-opioid receptor agonist, in neuropathic pain model mice

We examined the analgesic and anti-allodynic effects of morphine and U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide methanesulfonate salt), a selective kappa-opioid receptor agonist, and the development of tolerance to their effects in neuropathic pain model mice induced by sciatic nerve ligation (SNL). In the tail-pinch method, morphine at 10 mg/kg, s.c. produced a weak analgesic effect in SNL mice; however, U-50,488H at 5 mg/kg, s.c. produced an analgesic effect equipotent to that in normal mice. In contrast, morphine produced an adequate analgesic effect when given either intracerebroventricularly (i.c.v.) or intrathecally (i.t.), but U-50,488H only produced analgesia when given i.t. Repeated administration of morphine (either i.c.v. or i.t.) or U-50,488H (either s.c. or i.t.), did not induce tolerance to the effect. In the static allodynia test with an application of von Frey filaments, both compounds given s.c. suppressed the allodynic effect, but in the dynamic allodynia test involving lightly stroking the plantar surface with a cotton bud, only U-50,488H produced an anti-allodynic effect. Repeated administrations of both compounds did not develop tolerance to these anti-allodynic effects. Thus, U-50,488H was found to be a highly effective at blocking hyperalgesia and allodynia in nerve injury, and these findings suggest that kappa-opioid receptor agonists are attractive pharmacological targets for the control of patients with neuropathic pain.     

Alternative transcripts of the GABA(A) receptor epsilon subunit in human and rat

The effect of 1,4-dihydropyridine (DHP) calcium channel blockers (CCBs), nimodipine (NIM) and lercanidipine (LDP) on the analgesic response of selective kappa-opioid receptor agonists, U50,488H, PD117,302 and U69,593 was determined in male Sprague-Dawley rats using the tail-flick test. The effect of NIM on development of tolerance to U50,488H-induced analgesia and the status of brain DHP-sensitive Ca(2+) channel (L-type) binding sites in both U50,488H-naive and tolerant rats was determined using the highly selective DHP radioligand, [(3)H]PN200-110. Tolerance was induced by injecting U50,488H (25 mg/kg, i.p.) twice daily for 4 days. Intraperitoneal (i.p.) injection of kappa-opioid receptor agonists produced a dose-dependent acute analgesic response. NIM (1 mg/kg; i.p.) and LDP (0.3 mg/kg; i.p.) used in the study produced no tail-flick analgesia. Administration of NIM and LDP (15 min prior) significantly potentiated the analgesia produced by three kappa-opioid receptor agonists. Tolerance developed completely to the analgesic effect induced by U50,488H (25 mg/kg, i.p.) administered on the 5th day. NIM (1 mg/kg, i.p.) twice daily for 4 days not only completely inhibited the development of tolerance to analgesic response but also significantly potentiated it (supersensitivity). There was a significant up-regulation of DHP binding sites (B(max): +41%) in whole brain membranes of tolerant rats when compared to vehicle treated naive rats, implicating increased influx of Ca(2+) through L-type channels in kappa-opioid tolerance. U50,488H (25 mg/kg, i.p.) and NIM (1 mg/kg, i.p.) twice daily for 4 days also resulted in an equivalent up-regulation of DHP binding sites (+36%) as that of U50,488H alone. These results strongly suggest a functional role of L-type Ca(2+) channels in the regulation of pain sensitivity, mechanism of kappa-opioid analgesia and expression of tolerance.     

Effects of U-50,488H on isolated atria from rats chronically treated with the κ-opioid agonist,U-50,488H

相似文献   

The role of nitrergic system in lidocaine-induced convulsion in the mouse

The effects of N-nitro-L-arginine-methyl ester (L-NAME) a nitric oxide (NO) synthase inhibitor and L-arginine, a NO precursor, were investigated on lidocaine-induced convulsions. In the first experiment, four groups of mice received physiological saline (0.9%), L-arginine (300 mg/kg, i.p.), L-NAME (100 mg/kg, i.p.) and diazepam (2 mg/kg), respectively. Thirty minutes after these injections, all mice received lidocaine (50 mg/kg, i.p.). In the second experiment, four groups of mice received similar treatment in the first experiment, and 30 min after these injections, all mice received a higher dose of lidocaine (80 mg/kg). L-NAME (100 mg/kg, i.p.) and diazepam (2 mg/kg) significantly decreased the incidence of lidocaine (50 mg/kg)-induced convulsions. In contrast, the L-arginine treatment increased the incidence of lidocaine (80 mg/kg, i.p.)-induced convulsions significantly. These results may suggest that NO is a proconvulsant mediator in lidocaine-induced convulsions.     

Inhibition of fatty acid amide hydrolase produces analgesia by multiple mechanisms

1 The reversible fatty acid amide hydrolase (FAAH) inhibitor OL135 reverses mechanical allodynia in the spinal nerve ligation (SNL) and mild thermal injury (MTI) models in the rat. The purpose of this study was to investigate the role of the cannabinoid and opioid systems in mediating this analgesic effect. 2 Elevated brain concentrations of anandamide (350 pmol g(-1) of tissue vs 60 pmol g(-1) in vehicle-treated controls) were found in brains of rats given OL135 (20 mg kg(-1)) i.p. 15 min prior to 20 mg kg(-1) i.p. anandamide. 3 Predosing rats with OL135 (2-60 mg kg(-1) i.p.) 30 min before administration of an irreversible FAAH inhibitor (URB597: 0.3 mg kg(-1) intracardiac) was found to protect brain FAAH from irreversible inactivation. The level of enzyme protection was correlated with the OL135 concentrations in the same brains. 4 OL135 (100 mg kg(-1) i.p.) reduced by 50% of the maximum possible efficacy (MPE) mechanical allodynia induced by MTI in FAAH(+/+)mice (von Frey filament measurement) 30 min after dosing, but was without effect in FAAH(-/-) mice. 5 OL135 given i.p. resulted in a dose-responsive reversal of mechanical allodynia in both MTI and SNL models in the rat with an ED(50) between 6 and 9 mg kg(-1). The plasma concentration at the ED(50) in both models was 0.7 microM (240 ng ml(-1)). 6 In the rat SNL model, coadministration of the selective CB(2) receptor antagonist SR144528 (5 mg kg(-1) i.p.), with 20 mg kg(-1) OL135 blocked the OL135-induced reversal of mechanical allodynia, but the selective CB(1) antagonist SR141716A (5 mg kg(-1) i.p.) was without effect. 7 In the rat MTI model neither SR141716A or SR144528 (both at 5 mg kg(-1) i.p.), or a combination of both antagonists coadministered with OL135 (20 mg kg(-1)) blocked reversal of mechanical allodynia assessed 30 min after dosing. 8 In both the MTI model and SNL models in rats, naloxone (1 mg kg(-1), i.p. 30 min after OL135) reversed the analgesia (to 15% of control levels in the MTI model, to zero in the SNL) produced by OL135.     

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.