Proglumide selectively potentiates supraspinal mu 1 opioid analgesia in mice

The cholecystokinin antagonist proglumide potentiates morphine analgesia. To understand more fully the opiate receptor subtypes involved with this effect, we investigated the effect of proglumide on spinal and supraspinal mu and spinal delta analgesia in mice. Proglumide alone had no effect on tailflick latencies, but increased, in a dose-dependent manner, tailflick latencies in morphine-tolerant mice. Proglumide also potentiated morphine analgesia in naive mice in a dose-dependent manner, with a maximal effect at 5-10 mg/kg. Proglumide both shifted the dose-response curve for morphine analgesia to the left and prolonged morphine's duration of action. Proglumide increased the sensitivity of supraspinal mu 1 receptor mechanisms of analgesia without influencing spinal mechanisms. Proglumide administered subcutaneously potentiated the analgesic actions of intracerebroventricular [D-Ala2,MePhe4,Gly(ol)5]enkephalin (DAGO; (mu 1), but not intrathecal DAGO (mu 2) or [D-Pen2,D-Pen5]enkephalin (DPDPE; delta). The selective mu 1 receptor antagonist naloxonazine blocked proglumide-enhanced morphine analgesia.     

Analgesic potency of TRIMU-5: a mixed mu 2 opioid receptor agonist/mu 1 opioid receptor antagonist.

TRIMU-5 (Tyr-D-Ala-Gly-NH-(CH2)2CH(CH3)2) is a potent enkephalin analog with analgesic actions. Detailed studies show high affinity for both mu 1 and mu 2 sites, with poor affinity for delta, kappa 1 and kappa 3 receptors. Of all the mu ligands examined in binding assays, TRIMU-5 was the most mu-selective. In mice, TRIMU-5 administered either intracerebroventricularly (i.c.v.) or intrathecally elicited analgesia which was readily reversed by the mu-selective antagonist beta-funaltrexamine (beta-FNA). However, the analgesia observed following i.c.v. injections differed from traditional mu ligands: (1) the dose of drug required for analgesic activity i.c.v. was 100-fold greater than those following intrathecal administration; (2) although sensitive to beta-FNA, the analgesia was not antagonized by naloxonazine; and (3) the analgesia was reversed by an opioid antagonist given intrathecally (i.t.) but not i.c.v. Thus, TRIMU-5 analgesia appeared to be mediated spinally through mu 2 receptors. TRIMU-5 did have supraspinal actions, inhibiting gastrointestinal transit, another mu 2 action. In binding studies TRIMU-5 had high affinity for mu 1 sites, but pharmacological studies revealed antagonist actions at this receptor. In mice, the analgesia produced by morphine given i.c.v. was antagonized by coinjection of a low TRIMU-5 dose which was inactive alone. Similarly, TRIMU-5 coadministered with morphine into the periaqueductal gray of rats reversed the analgesia seen with morphine alone. Thus, TRIMU-5 is a highly selective mixed mu 2 opioid receptor agonist/mu 1 opioid receptor antagonist.     

NMDA antagonist modulation of morphine antinociception in female vs. male rats

NMDA antagonists may be useful for their potential to increase or prolong opioid analgesia while attenuating the development of opioid tolerance and dependence. The purpose of this study was to determine whether there are sex differences in NMDA antagonist modulation of morphine antinociception. Adult female and male Sprague–Dawley rats were injected s.c. with saline or one dose of MK-801 (0.005, 0.01, 0.02, or 0.04 mg/kg), dextromethorphan (5, 10, or 20 mg/kg), or LY235959 (0.5, 1.0, or 2.0 mg/kg) in combination with saline or one dose of morphine (1.8, 3.2, or 5.6 mg/kg), and tested on the 50 °C hotplate and tail withdrawal assays 15–120 min post-injection. At the doses examined, only LY235959 produced any antinociception when administered alone. MK-801 attenuated morphine antinociception on both assays, but only at sporadic (inconsistent) dose-combinations. Dextromethorphan increased morphine antinociception on the hotplate but not tail withdrawal assay, at all three morphine doses in males, but only the higher morphine doses in females. In contrast, LY235959 modulated morphine antinociception on both assays; the lowest dose attenuated, and higher doses enhanced morphine antinociception, but the particular morphine doses and assay in which these effects occurred depended on the sex of the subject. Thus, all three NMDA antagonists modulated morphine antinociception in female and male rats, but the direction of this modulation depended on the particular antagonist examined, the nociceptive test, the dose of antagonist and of morphine, and time post-injection.     

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.     

Effects of nor-binaltorphimine on the development of analgesic tolerance to and physical dependence on morphine.

The effects of a highly selective kappa antagonist, nor-binaltorphimine (nor-BNI), on the development of tolerance to morphine analgesia and physical dependence on morphine were examined. Pretreatment with nor-BNI (5 mg/kg s.c.) 2 h prior to injection of morphine or a selective kappa agonist, U-50,488H, significantly antagonized the analgesic effect of U-50,488H, but not morphine analgesia in mice. The development of tolerance to morphine analgesia was significantly potentiated by pretreatment of mice with nor-BNI 2 h prior to morphine treatment during chronic morphine treatment for 5 days. Additionally, the pretreatment with nor-BNI during chronic treatment with the high dose of morphine for 5 days significantly potentiated the naloxone-induced body weight loss in morphine-dependent mice and rats. These findings suggest that inactivation of the kappa opioid system may potentiate the development of tolerance to morphine analgesia in mice and may aggravate the naloxone-precipitated body weight loss in morphine-dependent mice and rats.     

Effects of nefazodone on the immune system of mice.

Mice exposed to a chronic auditory stressor and treated with nefazodone (10 mg/kg/day s.c.), showed a reduction in stress-induced suppression of thymus and spleen cellularity, and in peripheral T-Iymphocyte population. The in vitro blastogenic response of spleen lymphoid cells to mitogen concanavalin A, the in vitro and in vivo activity of phagocytosis, both measured using the zymosan and carbon clearance tests, respectively, were also assessed and nefazodone was found to partially reverse the inhibitory effect of stress on those parameters. Nefazodone did not significantly affect those parameters in unstressed mice. In conclusion, this report provides evidence on the immunoprotective effects of this novel antidepressant drug against the adverse effects of stress in mice.     

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.     

Negative relationship between morphine analgesia and P-glycoprotein expression levels in the brain

It is known that opioid analgesics given systemically have limited distribution into the brain because of their interaction with P-glycoprotein (P-gp), an ATP-dependent efflux pump acting at the blood-brain barrier (BBB). We previously found that morphine and fentanyl showed higher analgesic potencies in P-gp-deficient mice compared with those in wild-type mice, suggesting that their analgesic effects are considerably dependent on P-gp expression. In this study, we focused on individual differences in the analgesic effectiveness of morphine, in cortical P-gp expression, and in basal P-gp ATPase activity in male ICR mice. We found that there were 3- to 10-fold differences between the magnitude of morphine analgesia (3 mg/kg, s.c.; tail-pinch method) in mice. Furthermore, there was a significant negative correlation between morphine's analgesic effects and individual P-gp expression in the cortex as estimated by western blot analysis. In addition, basal P-gp ATPase activities in isolated membrane preparations of brain capillary endothelial cells (BCECs) were negatively correlated with the magnitude of the analgesic effect of morphine. These results indicate that the individual differences in morphine analgesia may be due to some functional or quantitative differences in individual P-gp in BCECs, acting at the BBB.     

Pharmacological interaction of opiates with various classes of centrally acting dopaminergic drugs.

The comparative analgesic and sedative (narcosis potentiating) efficacy of mu and kappa opioids was studied as a function of time in rats and mice. The mu agonists, morphine and fentanyl, produced antinociceptive actions against both heat and chemical noxious agents, but the half-lives of their ED50s were longer in the writhing than in the hot plate test. The kappa agonist drugs, bremazocine, ethylketocyclazocine and pentazocine, proved to be inactive against heat nociception, and produced a potent, long-lasting analgesia in the acetic acid writhing test, similar to mu agonists. The combination of two mu agonists resulted in a synergistic interaction and a remarkable prolongation of antinociceptive action. When the kappa-drug bremazocine was coadministered with morphine, there was a significant prolongation of the duration of analgesic action, without any influence on the potency. The interactions of mu and kappa opioids with agonists and antagonists at dopamine receptors were also studied in narcosis. The time course of the naloxone-morphine antagonism in analgesiometric assays revealed similarities, when apparent pA2 values were estimated at the peak of agonist and antagonist activity, but it was different in the writhing test when the pA2 was determined 84 minutes after morphine administration (EDt1/2, half-life belonging to the ED50) while naloxone was given at its peak effect.     

Continuous morphine produces more tolerance than intermittent or acute treatment

Dosing protocol and analgesic efficacy have been proposed to be important determinants of the magnitude of opioid tolerance. The present study examined the effect of acute, intermittent and continuous treatment with the low analgesic efficacy agonist morphine on analgesic tolerance. Mice were implanted s.c. with a 25 mg morphine pellet for 1-7 days. Other mice were implanted s.c. with two 25 mg, or one 75 mg morphine pellet for 7 days. The release of morphine from subcutaneous implanted pellets was quantitated using a spectrophotometric assay. In other studies, mice were injected with morphine once (18.5-185 mg/kg/day; ≈ 10-100 times ED₅₀ for morphine analgesia) or once/day for 7 days. Controls were implanted with a placebo pellet or injected with saline. Analysis of drug release from a 25 mg pellet indicated that release was greatest during the first 24 h, declined and then remained relatively constant. The amount of morphine released over 7 days by a 75 mg pellet (23.9 mg) was more than that of a single 25 mg pellet (15.4 mg) but less than two 25 mg pellets (30.8 mg). Following treatment, morphine cumulative dose-response studies were conducted (tailflick). Continuous treatment with morphine using pellet implantation produced a dose-dependent shift in the morphine ED₅₀ by 3.3, 5.8 and 8.5 fold for one 25 mg pellet, one 75 mg pellet and two 25 mg pellets, respectively. Acute and intermittent morphine administration produced substantially less analgesic tolerance than continuous release of morphine by implant pellets. The maximum shift in the ED₅₀ was 1.6 for acute treatment and 2.7 for 7 day intermittent treatment; despite a larger total daily dose. The present results indicate that continuous treatment with morphine results in greater analgesic tolerance than acute or intermittent morphine treatment even at comparable daily doses. These results are consistent with the suggestion that intermittent dosing has reduced risk of producing opioid tolerance.     

The local monoaminergic dependency of spinal ketamine.

The effects of s.c. doses of naloxone, methysergide and phentolamine on ketamine-induced spinal analgesia were assessed to determine the involvement of opiate, serotonergic and noradrenergic components mediating ketamine's antinociceptive action. Ketamine administered intrathecally (i.t.) produced a significant elevation in tail-flick latency in rats. The spinal antinociceptive effects of ketamine were dose dependently reversed by methysergide (ID50 = 0.008 mg/kg s.c.), phentolamine (ID50 = 0.88 mg/kg s.c.) and naloxone (ID50 = 3.0 mg/kg s.c.). Unlike morphine, which remains analgesic and dependent on opiate interactions following bilateral lesions of the dorsolateral funiculus (DLF), ketamine analgesia was absent following DLF lesions. Thus, ketamine appears to produce an antinociceptive response which is dependent upon the neuronal activity of the descending pain-inhibitory pathways. The monoaminergic components comprising the descending pathways appear to be more prominent in the action of ketamine than they are in the spinal action of morphine. Furthermore, the spinal opioid receptors involved in ketamine's effect may be different from the mu subtype preferred by morphine.     

Analgesic synergy between topical opioids and topical non-steroidal anti-inflammatory drugs in the mouse model of thermal pain

The main aim of the study was to examine analgesic effects of the topical opioids and non-steroidal anti-inflammatory drugs (NSAIDs) in a radiant heat tail-flick nociception model. Also, we have tested whether the addition of lauric acid to propylene glycol improves skin permeation for the opioids and NSAIDs. We found that the addition of lauric acid to propylene glycol dramatically improves the penetration of the drugs, measured by the drug's ED(50). We observed a significant dose response shift to the left for all tested drugs. So, morphine's ED(50) was decreased by 19-fold. The duration of the analgesic activity of morphine dissolved in a combination of propylene glycol and lauric acid was much longer compared with the same dose of the drug dissolved in propylene glycol only. Methadone and hydrocodone also produced analgesic activity in this experimental paradigm. We then assessed potential interactions between opioids, ibuprofen and diclofenac using a fixed, low dose of each. The inclusion of either S-ibuprofen or diclofenac to a fixed, low dose of morphine raised the analgesic response from around 20% to 50% and 80%, respectively. Topical methadone and diclofenac alone produced analgesia in 30% of mice. The combination produced analgesia in 100% of mice (100% versus 60%, P<0.001) and the analgesic effect was observed for 90 min. Alone, topical methadone and S-ibuprofen produced analgesia in 25% and 30% of mice, respectively. The combination elicited analgesia in 100% of mice (100% versus 55%, P<0.001) and this analgesic effect lasted for 120 min. Our current findings support the supra-additive interaction of topical mu opioids, S-ibuprofen and diclofenac in a model of moderate to severe pain, radiant heat tail-flick assay.     

Enhancement of morphine analgesia by tricyclic antidepressants

Using a cat tail-flick analgetic testing procedure, enhancement of the antinociceptive action of morphine (0.25 mg/kg s.c.) was detected following pre-treatment (1 hour) with 10 mg/kg s.c. of either amitriptyline or nortriptyline. No analgesia was observed following s.c. administration of either antidepressant or saline alone. Both central (amine re-uptake blockade; anticholinergic) and peripheral (decreased hepatic biotransformation) actions of tricyclic antidepressants may have contributed to augmentation of morphine analgesia.     

Influence of prazosin and clonidine on morphine analgesia,tolerance and withdrawal in mice

Rapid development of tolerance and dependence limits the usefulness of morphine in long-term treatment. We examined the effects of clonidine (₂-adrenoceptor agonist) and prazosin (₁-adrenoceptor antagonist) on morphine analgesia, tolerance and withdrawal. Morphine tolerance was induced using a 3-day cumulative twice-daily dosing regimen with s.c. doses up to 120 mg/kg. Tolerance was assessed on day 4, as loss of the antinociceptive effect of a test dose of morphine (5 mg/kg). After 10 h, morphine withdrawal was precipitated with naloxone (1 mg/kg). Prazosin had no analgesic effect alone but dose-dependently potentiated morphine analgesia in morphine-naive mice. Another ₁-adrenoceptor antagonist, corynanthine, had similar effects. Prazosin also increased the analgesic potency of the morphine test dose in morphine-tolerant mice. Naloxone-precipitated vertical jumping was not affected, but weight loss was reduced by prazosin. Acutely administered clonidine potentiated morphine analgesia and alleviated opioid withdrawal signs, as expected. We conclude that in addition to the already established involvement of ₂-adrenoceptors in opioid actions, also ₁-adrenoceptors have significant modulatory role in opioid analgesia and withdrawal.     

Gabapentin enhances the analgesic response to morphine in acute model of pain in male rats

Whenever opioids as drug of choice result in inadequate analgesia, the combinational therapy would be the solution. In this study the co-administration of gabapentin with morphine is evaluated in acute model of pain. Therefore the antinociceptive effect of gabapentin (30 or 90 mg/kg, s.c.) and morphine (0.5, 1 or 3 mg/kg, s.c.) alone or in combination were measured by tail-flick test in intact adult male rats. Control rats received normal saline. Tail-flick latency time and Area Under Curve (AUC), as antinociception index were calculated for each groups. There was not any significant difference between the antinociceptive response of 0.5 mg/kg morphine and 30 mg/kg gabapentin as compared to controls, but co-administration of these subanalgesic doses increased significantly AUC as compared to morphine alone. The co-administration of gabapentin with analgesic doses of 1 and 3 mg/kg morphine had also increased significantly AUC. Therefore gabapentin enhanced the antinociceptive effect of both analgesic and subanalgesic doses of morphine in a dose dependent manner. In conclusion co-administration of gabapentin with low doses of morphine produced therapeutic analgesia which could have important clinical application.     

Kappa 3 receptors and levorphanol-induced analgesia.

Levorphanol is a widely used opiate analgesic. Although structurally related to morphine, levorphanol has high affinity for a number of receptor subtypes, including both kappa 1 and kappa 3. Prior reports had implicated a kappa component of levorphanol-induced antinociception. Evidence is now presented suggesting that levorphanol-induced analgesia is produced by a mixture of mu and kappa 3 mechanisms. Levorphanol was a potent analgesic in the tail-flick assay, when given systemically, spinally or supraspinally. Isobolographic analysis of the combined administration of levorphanol, spinally and supraspinally implied synergistic interactions. Naloxonazine reduced levorphanol-induced analgesia, implicating a role for mu1 receptors. The kappa 1 antagonist nor-binaltorphimine at a dose which reversed analgesia induced by U50,488H did not antagonize levorphanol-induced analgesia. Additional studies revealed no cross tolerance in either direction, between levorphanol with the kappa 1 analgesic U50,488H. Together, these results strongly argue against a role for kappa 1 receptors in levorphanol-induced analgesia. However, mice tolerant to the kappa 3 analgesic, naloxone benzoylhydrazone (NalBzoH), showed cross tolerance to levorphanol, implying a role of kappa 3 mechanisms in levorphanol-induced analgesia.     

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.     

Analgesia and decrement in operant performance in socially defeated mice: Selective cross-tolerance to morphine and antagonism by naltrexone

During a social confrontation between a resident and an intruder mouse, only the submissive or defeated intruder shows an opioid-mediated analgesia to which tolerance develops. We investigated the altered morphine responsiveness after different kinds of social experiences. Mice were assessed for performance of operant behavior under the control of a fixed ratio schedule of positive reinforcement as well as for the tail flick response to a heat stimulus before and after one or five consecutive social confrontations. The dose-effect curves for morphine's suppression of schedule-controlled behavior were closely similar before and after defeat in a single or in five social confrontations. However, the concurrently measured response to pain in the tail flick assay produced morphine dose-effect curves that were shifted to the right after defeat in one or five social confrontations. Four to six times higher doses of morphine were necessary to produce analgesia in mice that were defeated in five social confrontations. Naltrexone (1 mg/kg, ip) antagonized the suppressive effects of morphine (10 mg/kg, ip) on rate of responding and the analgesic effects. Naltrexone also blocked the development of analgesia in mice that were defeated for the first time in a social confrontation, but did not prevent the suppressive effects on rate of responding. Specific social experiences such as defeat in a social confrontation appear to alter endogenous opioid process that mediate analgesia; these processes differ from those that suppress positively reinforced behavior. The differential development of morphine tolerance to the analgesic effects, but not the rate-decreasing effects as well as the differential naltrexone antagonism of both effects may indicate the involvement of opioid and non-opioid mechanisms.     

Ketamine-induced potentiation of morphine analgesia in rat tail-flick test: role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels

Ketamine is known to improve opioid efficacy, reduce postoperative opioid requirement and oppose opioid associated pain hypersensitivity and tolerance. However, the mechanisms underlying these beneficial effects are not clear. This study investigated the effects of ketamine at a non-analgesic dose (30 mg/kg, i.p.) on analgesia induced by morphine (2.5, 5.0, 7.5 mg/kg, s.c.), using rat tail-flick test as an animal model of acute pain. Further, the role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels was examined on the potentiating effect of ketamine. Male rats received morphine alone at 5.0 and 7.5 but not at 2.5 mg/kg showed a dose-related increase in tail-flick latencies. The combination of morphine and ketamine resulted in dose-related increase in morphine analgesia, both on the intensity as well as on duration. The ketamine-induced potentiation of morphine (7.5 mg/kg) analgesia was unaffected by glibenclamide (3 mg/kg, s.c.) and only partially blocked by yohimbine (2 mg/kg, i.p.), but more completely abolished by naloxone (2 mg/kg, i.p.). Both morphine (5.0 mg/kg) and ketamine (30 mg/kg) alone did not evoke catalepsy in rats but on combination produced a synergistic effect, which was however, abolished by naloxone pretreatment. In the open-field test, while morphine (5.0 mg/kg) caused a depressant effect, ketamine (30 mg/kg) enhanced the locomotor activity. Nevertheless, in combination potentiated the morphine's depressant effect on locomotion, which was also antagonized by naloxone. These results indicate that ketamine at a non-analgesic dose can potentiate morphine analgesia, induce catalepsy and cause locomotor depression, possibly involving an opioid mechanism. This potentiation, although favorable in acute pain management, may have some adverse clinical implications.     

Effects of morphine in rats treated chronically with U-50,488 H, a kappa opioid receptor agonist

The pharmacological effects of morphine, namely analgesic, hyperthermic and cataleptic effects, were assessed in rats rendered tolerant to U-50,488H, a kappa opioid receptor agonist. Male Sprague-Dawley rats were injected intraperitoneally with U-50,488H (25 mg/kg) twice a day for four days. The rats which served as controls were injected similarly with the vehicle. Chronic administration of U-50,488H resulted in the development of tolerance to its analgesic and hypothermic effects, but not to its diuretic effect. The development of tolerance to the pharmacological effects of U-50,488H was associated with decreased binding of [3H]ethylketocyclazocine [( 3H]EKC) to brain and spinal cord membranes. The decreased binding of [3H]EKC in U-50,488H-treated rats was due to changes in the Bmax value; the Kd values remained unaltered. Intraperitoneal administration of morphine (8 mg/kg) to rats produced analgesia (as determined by the tail-flick test) and hyperthermia. A dose of 50 mg/kg of morphine produced cataleptic response. The intensity of analgesic, hyperthermic and cataleptic effects of morphine were unaltered in rats tolerant to U-50,488H. The development of tolerance to analgesic and hypothermic effects of U-50,488H were associated with down-regulation of brain and spinal cord kappa opioid receptors. Finally, U-50,488H does not confer cross-tolerance to morphine, a predominantly mu opioid receptor agonist.