Characterization of the membrane-associating domain of the sperm adhesive protein, bindin

Male Swiss-Webster mice were rendered tolerant to morphine by subcutaneous implantation of a morphine pellet, each containing 75 mg morphine base, for 3 days. Mice implanted with placebo pellets served as controls. A high degree of tolerance to the analgesic effect of morphine developed as evidenced by decreased analgesic response to various doses of morphine. A selective kappa-opiate agonist, U-50,488H (8, 16 and 32 mg/kg, i.p.) produced dose-dependent analgesic and hypothermic effects in mice implanted with placebo pellets. A significant decrease in the analgesic and hypothermic effects of U-50,488H was observed in morphine tolerant mice as compared to placebo-treated mice. Mice were rendered tolerant to U-50,488H by injecting the drug (25 mg/kg, i.p.) twice daily for 4 days. Vehicle injected mice served as controls. Tolerance to the analgesic and hypothermic effects of U-50,488H in mice injected chronically with the drug was evidenced by the decreases in the intensity of these responses when compared to those observed in vehicle injected controls. Morphine produced a dose-dependent analgesic and hypothermic effects in mice injected chronically with vehicle but the intensity of these effects was significantly lower in mice injected chronically with U-50,488H. These results indicate that a substantial tolerance to analgesic and hypothermic effects of U-50,488H develops in morphine tolerant mice. The effect of chronic injections of U-50,488H on the binding of [3H]ethylketocyclazocine (EKC) and [3H]D-Ala2,MePhe4,Gly-ol5-enkephalin (DAMGO) to whole brain and spinal cord kappa- and mu-opiate receptors was determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of 7-nitroindazole on tolerance to morphine, U-50,488H and [D-Pen2, D-Pen5] enkephalin in mice

The effects of 7-nitroindazole (7-NI), an inhibitor of the neuronal nitric oxide synthase (nNOS) which does not increase blood pressure, on tolerance to the antinociceptive activity of mu-(morphine), kappa-(U-50,488H) and delta-([D-Pen2, D-Pen5]enkephalin, DPDPE) opioid receptor agonists were determined in mice. Male Swiss-Webster mice were made tolerant by twice daily injections of morphine (20 mg/kg, s.c.), U-50,488H (25 mg/kg, i.p.) or DPDPE (20 micrograms/mouse, i.c.v.) for 4 days. When tested on day 5, tolerance to their antinociceptive activity was evidenced by decreased response in chronic drug treated mice in comparison to vehicle-injected mice. Concurrent administration of 7-NI (20, 40 or 80 mg/kg, i.p.) with DPDPE did not modify the development of tolerance to the antinociceptive action of DPDPE. However, 7-NI (40 or 80 mg/kg, i.p.) inhibited the development of tolerance to the antinociceptive activity of morphine and U-50,488H but the lower dose of 7-NI (20 mg/kg, i.p.) was not effective. Chronic administration of 7-NI by itself did not modify the acute response to morphine, U-50,488H or DPDPE. It is concluded that a specific inhibitor of nNOS can inhibit tolerance to the antinociceptive activity of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Opioid antagonist profile of SC nor-binaltorphimine in the formalin paw assay

The antinociceptive effects of mu and kappa agonists were examined after the systemic administration of the opioid antagonists nor-binaltorphimine (nor-BNI) and naloxone in the late response or tonic nociceptive phase of the mouse formalin assay. Initially, SC morphine (ED50, 0.97 mg/kg), racemic U-50488H (ED50, 0.79 mg/kg), (-)U-50488 (ED50, 0.41 mg/kg), and another agonist PD 117,302 (ED50, 0.28 mg/kg) were found to produce graded increases in the level of antinociception as measured by this procedure; naloxone, administered immediately before morphine and U-50488H, antagonized their antinociceptive actions. The effects of morphine and U-50488H then were evaluated 10 min to 96 h after the administration of nor-BNI. Subcutaneous nor-BNI at 30.0 mg/kg, but not at 3.0 or 10.0 mg/kg, attenuated the antinociceptive effects of morphine and U-50488H when the interval separating nor-BNI and the agonists was kept constant at 1 h. Time-course analysis of the effects of combinations of nor-BNI with morphine led to irregular findings: 10.0 mg/kg of nor-BNI lessened the effects of morphine (2.0 mg/kg) if the dosing interval was 10 min, whereas 30.0 mg/kg of nor-BNI attenuated the effects of morphine (2.0 mg/kg) if the dosing interval was 1 or 4 h; 10.0 mg/kg of nor-BNI also diminished the antinociceptive effects of U-50488H (1.7 mg/kg) only if the interval spacing the two drugs was 24 h. In comparison, a threefold higher dose of nor-BNI (30.0 mg/kg) reduced the effects of U-50488H (1.7 mg/kg) if the interval was 1 h or more. In these latter experiments, the antagonist effects of SC nor-BNI (30.0 mg/kg) were evident up to 96 h posttreatment. These results show that the mu opioid antagonist activity of nor-BNI is variable and that the kappa opioid antagonist selectivity of nor-BNI is a function of dose and treatment interval and is long-lasting even after systemic administration.     

Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

The effects of acute and chronic administration of cocaine on the antinociception and tolerance to the antinociceptive actions of mu-(morphine), kappa-(U-50,488H), and delta-([D-Pen2,D-Pen5]enkephalin; DPDPE), opioid receptor agonists were determined in male Swiss-Webster mice. Intraperitoneal injection of 40 mg/kg of cocaine by itself produced weak antinociceptive response as measured by the tail-fick test but the lower doses were ineffective. Administration of morphine (10 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (10 microg/mouse, ICV) produced antinociception in mice. Cocaine (20 mg/kg) potentiated the antinociceptive action of morphine and DPDPE but had no effect on U-50,488H-induced antinociception. Administration of morphine (20 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (20 microg/mouse, ICV) twice a day for 4 days resulted in the development of tolerance to their antinociceptive actions. Tolerance to the antinociceptive actions of morphine and U-50,488H was inhibited by concurrent treatment with 20 or 40 mg/kg doses of cocaine; however, tolerance to the antinociceptive action of DPDPE was not modified by cocaine. It is concluded that cocaine selectively potentiates the antinociceptive action of mu- and delta- but not of the kappa-opioid receptor agonist. On the other hand, cocaine inhibits the development of tolerance to the antinociceptive actions of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

The competitive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor antagonist LY293558 attenuates and reverses analgesic tolerance to morphine but not to delta or kappa opioids

Antagonists of the NMDA type of excitatory amino acid (EAA) receptor attenuate or reverse the development of tolerance to the analgesic effects of the mu opioid agonist morphine, the delta-1 opioid agonist DPDPE but not the kappa-1 agonist U50,488H or the kappa-3 agonist naloxone benzoylhydrazone. The role of the AMPA subtype of EAA receptor in analgesic tolerance was examined using LY293558, a selective competitive antagonist that is active after systemic administration. Administration of morphine, DPDPE, or U50,488H three times daily for 3 days according to an escalating dosing schedule resulted in analgesic tolerance as indicated by an increase in analgesic ED50 values using the tail-flick test in mice. Analgesic tolerance was attenuated when mice received a continuous subcutaneous infusion of LY293558 at doses of 30, 45 or 60 mg/kg/24 hr via an osmotic pump concurrent with the morphine treatment. Continuous subcutaneous infusion of LY293558 (45 mg/kg/24 hr) also reversed established morphine tolerance. In contrast, continuous subcutaneous infusion of the highest dose of LY293558 (60 mg/kg/24 hr) was ineffective in preventing the development of analgesic tolerance to DPDPE or U50,488H. Continuous subcutaneous infusion of LY293558 (60 mg/kg/24 hr) for 3 days protected mice from generalized convulsions produced by the selective AMPA agonist ATPA, indicating that the dosage of LY293558 that attenuated morphine tolerance was effective as an antagonist at AMPA receptors. These results demonstrate that AMPA receptors may play a role in the development and maintenance of morphine, but not DPDPE or U50,488H, analgesic tolerance.     

Establishing morphine and U-50,488H as discriminative stimuli in a three-choice assay with pigeons.

Pigeons were trained in a 3-choice assay to discriminate among injections of 5.6 mg/kg U-50,488H, 5.6 mg/kg morphine, and vehicle solution. In dose-response tests, subjects rarely responded on the U-50,488H-appropriate key when morphine was administered or on the morphine-appropriate key when they received U-50,488H. A high dose of naltrexone (1.0 mg/kg) completely blocked the morphine cue but failed to block completely the U-50,488H cue. In generalization tests, d-amphetamine primarily engendered saline-appropriate responding. Ethylketazocine produced mixed results, in that moderate doses produced responding on both the morphine- and U-50,488H-appropriate keys, but 3.2 mg/kg engendered primarily morphine-appropriate responding. These results demonstrate the feasibility, but not necessarily the value, of 3-choice discrimination procedures involving mu and kappa agonists and vehicle. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of U-50,488H on scopolamine-, mecamylamine- and dizocilpine-induced learning and memory impairment in rats

The role of kappa opioid receptor agonists in learning and memory is controversial. In the present study, the effects of U-50,488H on scopolamine-, mecamylamine- and dizocilpine-induced learning and memory impairments in rats were investigated. Scopolamine (3.3 mumol/kg s.c.), a muscarinic cholinergic antagonist, and mecamylamine (40 mumol/kg s.c.), a nicotinic cholinergic antagonist, significantly impaired learning and memory in rats in a step-through type passive avoidance test. Administration of U-50,488H (0.17 or 0.51 mumol/kg s.c.) 25 min before the acquisition trial reversed the impairment of learning and memory induced by scopolamine and mecamylamine. Although low doses of scopolamine (0.17 mumol/kg) and mecamylamine (12 mumol/kg) had no effect, concurrent administration of both antagonists induced impairment of learning and memory. Scopolamine significantly increased acetylcholine release in the hippocampus as determined by in vivo brain microdialysis. On the other hand, mecamylamine significantly decreased acetylcholine release. U-50,488H completely blocked the decrease in acetylcholine release induced by mecamylamine, whereas it only partially blocked the increase of acetylcholine induced by scopolamine. On the other hand, an endogenous kappa opioid receptor agonist, dynorphin A (1-13), did not block the increase in acetylcholine release induced by scopolamine. The antagonistic effect of U-50,488H was abolished by pretreatment with nor-binaltorphimine (4.9 nmol/rat i.c.v.), a selective kappa opioid receptor antagonist. U-50,488H did not affect the impairment of learning and memory induced by the blockade of NMDA receptors by dizocilpine ((+)-MK-801). These results suggest that U-50,488H reverses the impairment of learning and memory induced by the blockade of cholinergic transmission and abolishes the decrease of acetylcholine release induced by mecamylamine via the kappa receptor-mediated opioid neuronal system.     

Dopaminergic modulation of kappa opioid-mediated ultrasonic vocalization, antinociception, and locomotor activity in the preweanling rat.

The purpose of this study was to determine whether dopamine (DA) systems modulate kappa opioid-mediated ultrasonic vocalizations (USVs), antinociception, and locomotion in young rats. Seventeen-day-old rats were injected with the kappa agonist U-50,488 (0.0-7.5 mg/kg) and saline, the D?-like receptor agonist R(-)-propylnorapomorphine (NPA; 0. 1 or 1.0 mg/kg), the indirect DA agonist cocaine (10 or 20 mg/kg), or the DA antagonist flupenthixol (0.25 or 0.5 mg/kg). USVs and locomotion were measured for 6 min, with antinociception being assessed with a tail-flick test. Kappa receptor stimulation produced analgesia and increased USVs and locomotion. U-50,488-induced analgesia was potentiated by NPA, whereas U-50,488-induced USVs were attenuated by both DA agonists. NPA and flupenthixol depressed U-50,488's locomotor effects. These results show that DA systems interact with kappa opioid systems to modulate USVs, antinociception, and locomotion in preweanling rats. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Genesis and significance of hypertension in autosomal dominant polycystic kidney disease

The aim of the present study was to determine whether U-50,488H and U-62,066E, kappa-opioid receptor agonists cause a neuroprotective action against hypoxia/hypoglycemia-induced reduction in 2-deoxyglucose (2-DG) uptake of hippocampal slices from U-50,488H-tolerant rats. Both U-50,488H and U-62,066E exhibited an attenuating effect on hypoxia/hypoglycemia-induced reduction in 2-DG uptake of hippocampal slices. Hypoxia/hypoglycemia-induced deficit of 2-DG uptake was prevented by cotreatment with naloxone, an opioid receptor antagonist, but potentiated by cotreatment with morphine, a mu-opioid receptor agonist. Chronic administration of U-50,488H resulted in the development of tolerance to the analgesic effect as well as the neuroprotective effect whereas this treatment affected neither basal- nor hypoxia/hypoglycemia-induced decreases in 2-DG uptake. Chronic administration of U-50,488H did not modify naloxone-induced attenuation of 2-DG uptake deficit but slightly potentiated the morphine-induced exacerbation. These findings suggest that the tolerance to kappa-opioid receptors does not affect the mu-opioid receptor-mediated neuroprotective or neurotoxic action.     

Expression of interstitial collagenase (matrix metalloproteinase-1) is related to the activity of human endometriotic lesions

The effects of the competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, LY235959, were determined on the analgesic and hypothermic effects as well as on the development of tolerance to these effects of U-50,488H, a kappa-opioid receptor agonist in mice and rats. In the mouse, a single injection of LY235959 given 10 min prior to U-50,488H did not modify the analgesic action of the latter. Similarly, chronic administration of LY235959 twice a day for 4 days did not modify U-50,488H-induced analgesia in mice. Repeated pretreatment of mice with LY235959 dose-dependently attenuated the development of tolerance to the analgesic actions of U-50,488H. In the rat, LY235959 by itself produced a significant analgesia and prior treatment of rats with LY235959 enhanced the analgesic action of U-50,488H. Similar effects were seen with the hypothermic action. Pretreatment of rats with LY235959 attenuated the development of tolerance to the analgesic but not to the hypothermic action of U-50,488H. These results provide evidence that LY235959 produces differential actions on nociception and thermic responses by itself and when given acutely with U-50,488H in mice and rats. However, when the animals are pretreated with LY235959, similar inhibitory effects are observed on the development of tolerance to the analgesic action of U-50,488H in both the species. These studies demonstrate an involvement of the NMDA receptor in the development of kappa-opioid tolerance and suggest that the biochemical consequences of an opioid's interaction with the opioid receptor are not the only factors that contribute to the acute and chronic actions of opioid analgesic drugs.     

Assessment of the discriminative stimulus effects of cocaine in the rat: lack of interaction with opioids

The present study examined the effects of several opioid agonists and antagonists in rats trained to discriminate cocaine (10 mg/kg) from saline in a two-lever, food-reinforced, discrimination task. Neither fentanyl, a mu agonist, nor the delta agonist BW 373U86 elicited cocaine-appropriate responding. Although pretreatment with fentanyl failed to alter the discriminative stimulus effects of low doses of cocaine, cocaine reversed the rate-suppressant effects of fentanyl. Although the kappa agonist U50,488H decreased response rates, it did not substitute for cocaine. Injection of U50,488H in combination with the training dose of cocaine (10 mg/kg) reversed the rate-suppressant effects of U50,488H but failed to affect the cocaine cue. Administration of U50,488H (3 mg/kg), in conjunction with several doses of cocaine, did not shift the cocaine dose-response curve. Naltrindole and naltrexone, delta and mu antagonists respectively, did not block the effects of cocaine. Further, naltrindole did not substitute for the cocaine cue. Complete generalization was observed to the dopamine uptake inhibitor bupropion (30 mg/kg). These results suggest that fentanyl and U50,488H, at doses that purportedly influence mesolimbic dopamine levels, do not alter the discriminative stimulus effects of cocaine. Moreover, activation of delta receptors and blockade of mu and delta receptors are similarly ineffective.     

Influence of chronic opioid treatment on low Km GTPase activity in rat brain: evidence for the involvement of G-proteins in opioid tolerance

To investigate G protein function during the initial state of opioid tolerance, low Km GTPase activity was measured following chronic treatment with morphine (mu agonist) and butorphanol (mu/delta/kappa mixed agonist) in rats. Chronic opioid administration (20 mg/kg, IP) was performed once a day for 7 consecutive days. Under these conditions, antinociceptive tolerance to morphine but not butorphanol was developed. Chronic morphine treatment enhanced basal low Km GTPase activity in the pons/medulla, but not in the cortex and midbrain. On the other hand, chronic butorphanol treatment had no effect on basal low Km GTPase activity. These results suggest that chronic in vivo treatment of rats with mu agonists leads to an increase in the hydrolysis of GTP to GDP, by a basal low Km GTPase activity of G-proteins in the pons/medulla and that an enhancement of GTPase activity in this specific area may contribute to the development of antinociceptive tolerance to mu agonists.     

Alteration of calcitonin gene related peptide and its receptor binding sites during the development of tolerance to mu and delta opioids

Calcitonin gene related peptide (CGRP), one of the most abundant peptides in the spinal cord, is localized in primary afferents and released following nociceptive stimuli. Its colocalization and corelease with substance P, a well-known nociceptive neuropeptide, support the importance of CGRP in pain mechanisms. However, its distinctive function in that regard remains to be fully established. Recently, we reported that increases in CGRP-like immunostaining and decrements in specific 125I-labelled human CGRP alpha ([125I]hCGRP alpha) binding sites in the spinal cord were correlated with the development of tolerance to the spinal antinociceptive action of a mu opioid agonist, morphine. The goal of the present study was to investigate whether the development of tolerance to other classes of opioids, namely, delta and kappa agonists, can also alter CGRP-like immunostaining and receptors in the rat spinal cord. The antinociceptive effects of all opioids were monitored by the tail-immersion test. Tolerance to their antinociceptive properties was induced by the infusion for 7 days of mu (morphine sulfate, 7.5 micrograms/h), delta D([D-Pen2,D-Pen5]enkephalin (DPDPE), 2.0 micrograms/h), and kappa (U-50488H, 10.0 micrograms/h) related agonists at the spinal level (L4), using osmotic minipumps. We confirmed that rats chronically treated with morphine showed significant decreases in [125I]CGRP alpha binding in laminae I, II, and III of the L4 spinal cord, while CGRP-like immunostaining was increased in these same laminae. Similar effects were observed following a treatment with the delta agonist, DPDPE, while the kappa agonist, U-50488H, apparently only slightly decreased [125I]CGRP alpha] binding in lamina II. Binding in other laminae and CGRP-like immunostaining were not affected. These results suggest a specific interaction between spinal CGRP systems and the development of tolerance to the spinal antinociceptive effects of mu- and delta-related agonists.     

Majonoside-R2, a major constituent of Vietnamese ginseng, attenuates opioid-induced antinociception

The effects of majonoside-R2 on antinociceptive responses caused by the mu-opioid receptor agonist morphine and the selective kappa-opioid receptor agonist U-50, 488H were examined by the tail-pinch test in mice. Intraperitoneal (IP) or intracerebroventricular (ICV) injection of majonoside-R2 (3.1-6.2 mg/kg, IP or 5-10 micrograms/mouse, ICV) and diazepam (0.1-0.5 mg/kg, IP or 0.5-1.0 microgram/mouse, ICV), as well as an opioid receptor antagonist naloxone (2 mg/kg, IP or 5 micrograms/mouse, ICV), dose-dependently attenuated the antinociception caused by subcutaneously administered morphine and U-50,488H. Moreover, when co-administered ICV or intrathecally (IT) with morphine (4 micrograms/mouse) or U-50,488H (60 micrograms/mouse), majonoside-R2 (5-20 micrograms/mouse) also exhibited antagonism against the antinociceptive action of these opioid receptor agonists in the tail-pinch test. The inhibitory effects of majonoside-R2 (10 micrograms/mouse, ICV) and diazepam (1 microgram/mouse, ICV) were reversed by flumazenil (2.5 micrograms/mouse, ICV), a selective benzodiazepine receptor antagonist, and picrotoxin (0.25 microgram/mouse, ICV), a GABA-gated chloride channel blocker. These results suggest that majonoside-R2 attenuates the opioid-induced antinociception by acting at the spinal and supraspinal levels, and that the GABAA receptor complex at the supraspinal level is involved in the effect of ICV administered majonoside-R2.     

Differential effects of specific delta and kappa opioid receptor antagonists on the bidirectional dose-dependent effect of systemic naloxone in arthritic rats, an experimental model of persistent pain

In an attempt to determine the opioid receptor class(es) which underly the two opposing effects of naloxone in models of persistent pain, we tested the action of the selective delta antagonist naltrindole, and that of the kappa antagonist MR-2266 on the bidirectional effect of systemic naloxone in arthritic rats. As a nociceptive test, we used the measure of the vocalization thresholds to paw pressure. The antagonists were administered at a dose (1 mg/kg i.v. naltrindole, 0.2 mg/kg i.v. MR-2266), without action per se but which prevents the analgesic effect of the delta agonist DTLET (3 mg/kg, i.v.) or the kappa agonist U-69,593 (1.5 mg/kg, i.v.) respectively, and does not influence the effect of morphine (1 mg/kg i.v.) or the mu agonist DAMGO (2 mg/kg, i.v.) in these animals. In arthritic rats injected with the delta antagonist, the paradoxical antinociceptive effect produced by 3 micrograms/kg i.v. naloxone was not significantly modified (maximal vocalization thresholds (% of control) were 146 +/- 9% versus 161 +/- 7% in the control group). By contrast, the hyperalgesic effect produced by 1 mg/kg i.v. naloxone was significantly reduced (maximal vocalization thresholds were 87 +/- 4% versus 69 +/- 5% in the control group). In rats injected with the kappa antagonist, the antinociceptive effect of the low dose of naloxone was almost abolished (mean vocalization thresholds were 115 +/- 3% versus 169 +/- 7%) whereas the hyperalgesic effect of naloxone 1 mg/kg i.v. was not significantly modified (mean vocalization thresholds = 70 +/- 3% and 65 +/- 3%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Butorphanol-mediated antinociception in mice: partial agonist effects and mu receptor involvement

In the present experiments, we characterized the agonist and antagonist effects of butorphanol in mice. In the mouse radiant-heat tail-flick test, the mu agonists morphine and fentanyl and the kappa agonist U50,488H were fully effective as analgesics, whereas butorphanol was partially effective (producing 82% of maximal possible analgesic effect). Naltrexone was approximately equipotent in antagonizing the effects of morphine, fentanyl and butorphanol; in vivo apparent pA2 values for these naltrexone/agonist interactions were 7.5 (unconstrained). Naltrexone was approximately 10 times less potent in antagonizing the effect of U50,488H (average apparent pK(B) = 6.7). The selective mu antagonist beta-funaltrexamine (0.1-1.0 mg/kg) antagonized the effects of butorphanol in a dose-dependent insurmountable manner. Pretreatment with nor-binaltorphimine (32 mg/kg), a kappa-selective antagonist, did not reliably antagonize butorphanol, and naltrindole (20 and 32 mg/kg), a delta-selective antagonist, failed to antagonize the effects of butorphanol. Low doses of butorphanol (1.0, 1.8 or 3.2 mg/kg) caused parallel, rightward shifts in the dose-effect curve for morphine and parallel leftward shifts in the dose-effect curve for U50,488H. Taken together, the results of the present study suggest that butorphanol is a partial agonist in the mouse radiant-heat tail-flick test and that activity at mu receptors accounts for the majority of its antinociceptive effects.     

Noninvasive differential diagnosis between chronic pulmonary thromboembolism and primary pulmonary hypertension by means of Doppler ultrasound measurement

We examined the development of tolerance to the antiallodynic effect of chronic intrathecal (IT) administration of the adenosine analog R-phenylisopropyladenosine (R-PIA) in a rat model of central pain after ischemic spinal cord injury. After 10 days of IT R-PIA treatment, the effect of IT morphine was also assessed to examine whether cross-tolerance between R-PIA and morphine was present. IT R-PIA completely alleviated allodynia-like behaviors to mechanical and cold stimuli in spinally injured rats. The anti-allodynic effect of R-PIA was maintained for 6-7 days with twice-daily administration and was reduced thereafter, particularly with respect to cold allodynia. IT morphine alleviated mechanical and cold allodynia in rats rendered tolerant to R-PIA to a degree comparable to that in R-PIA-naive (control) rats, which indicates that the anti-allodynic property of R-PIA is independent of the mechanisms by which morphine acts. The possibility of using agonists of adenosine receptors in treating refractory pain in patients with spinal cord injury is discussed. IMPLICATIONS: There is often no satisfactory treatment for chronic pain after spinal cord injury. Our study suggests such pain can be treated with a spinal injection of R-phenylisopropyladenosine in rats. Reduced effect to R-phenylisopropyladenosine was noted with repeated administrations. However, there was no cross-tolerance to morphine.     

Opioid receptors altered binding nature in guinea-pig brain following the development of morphine dependence

Morphine is well known to produce tolerance and dependence. The mechanisms for these phenomena are not clearly understood and there are a number of conflicting reports that chronic morphine administration decreases, increases, or leaves unchanged the number of opioid binding sites. We examined the potency of MScontin (oral controlled-release preparation of morphine) to induce morphine dependence and also determined the change of mu, delta and kappa opioid receptor types in brain homogenates obtained from morphine-dependent guinea-pigs. 1. Guinea-pigs were implanted subcutaneously with MScontin (300 mg.kg-1) and naloxone was employed to precipitate jumping behavior of withdrawal symptoms at various times. The highest degree of physical dependence was observed on the 2nd day after implantation. Therefore, this period was chosen to investigate opioid receptor binding assay. 2. Two days after implantation, the binding of 3H-DAGO (mu agonist), 3H-DPDPE (delta agonist) and 3H-U69593 (kappa agonist) to brain membranes prepared from morphine dependent and control guinea-pigs was determined. Scatchard plot of the saturation binding data revealed an increase in Bmax values (maximum specific binding) and no change in the Kd values (equilibrium dissociation constants) of 3H-opioid ligand bindings obtained from morphine-dependent animals as compared to controls. These results indicate that brain mu, delta and kappa opioid receptors are up-regulated in morphine dependent guinea-pigs.     

Effects of chronic U50,488H treatment on binding and mechanical responses of the rat hearts

The effects of chronic injection of U50,488H (trans-3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeacetamidel++ +), a selective kappa opioid agonist, on the properties of the binding sites of tritiated U69593 [(5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro (4,5)dec-8-yl)benzeneacetamide], another selective kappa opioid agonist, and mechanical responses to U50,488H of the heart were studied. Rats received injection twice a day with U50,488H for 4 days. Binding studies on the crude membrane homogenates revealed that there was no change in maximum binding, but a significant increase in Kd after the treatment, indicating that the number of kappa binding sites remained unchanged whereas the affinity of the binding sites to kappa-agonist decreased. The study on the mechanical responses to U50,488H in the isolated perfused heart preparation showed that although the agonist at 10(-6) M caused MR2266 reversible reductions in heart rate and force of contraction as well as ventricular ectopic beat in the heart of rats in the control group, its effects were absent in the U50,488H-treated group, indicating the development of tolerance to the mechanical effects of U50,488H on the heart. The results indicate that the development of tolerance to the mechanical effects of a kappa-agonist after chronic treatment with the agonist was not accompanied by down-regulation, but only a slight and significant reduction in affinity of kappa binding sites in the rat heart.     

Clocinnamox antagonism of the antinociceptive effects of mu opioids in squirrel monkeys

The opioid agonists morphine, etorphine, buprenorphine and U50,488 were examined alone and in combination with the insurmountable opioid antagonist clocinnamox (C-CAM) in squirrel monkeys responding under a schedule of shock titration. In this procedure, shock intensity increased every 15 sec from 0.01 to 2.0 mA in 30 increments. Five lever presses during any given 15-sec shock period produced a 15-sec timeout, after which shock resumed at the next lower intensity. When given alone, each of these agonists increased the median intensity at which the monkeys maintained shock [median shock level (MSL)]. At the highest dose examined alone, each agonist produced maximal increases in MSL and, except buprenorphine, decreased response rates. C-CAM dose-dependently antagonized the effects of morphine, etorphine and buprenorphine on MSL. In the presence of the higher C-CAM doses, etorphine, morphine and buprenorphine did not produce maximal effects on MSL. The effects of U50,488 were not systematically altered when tested in combination with the highest C-CAM dose. In general, C-CAM was more potent and the duration of antagonism was slightly longer against buprenorphine than against morphine and etorphine. Quantitative analysis of these data according to an extended model of yielded the following apparent affinity and efficacy estimates, respectively: etorphine (0. 085 mg/kg, 117); morphine (49 mg/kg, 24) and buprenorphine (0.62 mg/kg, 7.1). Determination of the individual q values over time indicated that the receptor population recovers more quickly after C-CAM antagonism of etorphine than from C-CAM antagonism of either morphine or buprenorphine. These data suggest that C-CAM functions as a long-lasting antagonist of mu opioid agonist actions in a shock titration procedure and yields estimates of relative intrinsic efficacy with the rank order of etorphine > morphine > buprenorphine.