Involvement of supraspinal epsilon and mu opioid receptors in inhibition of the tail-flick response induced by etorphine in the mouse.

Etorphine, a potent opioid agonist, has been reported to bind to both mu and epsilon opioid receptors. The present studies were designed to determine what types of opioid receptors and neurotransmitters for descending pain control systems were involved in antinociception induced by etorphine in mice. Morphine, a typical mu opioid receptor agonist, and beta-endorphin, an epsilon opioid receptor agonist, were used for comparison. Antinociceptive response induced by etorphine (20 ng) given i.c.v was blocked by i.c.v administration of D-Phe-Cys-Tyr-D-Tyr-Orn-Thr-Pen-Thr-NH2 (CTOP, 25 ng) and beta-endorphin-(1-27) [beta-EP-(1-27)] (6 micrograms), but not ICI 174,864 (ICI, 5 micrograms) or norbinaltorphimine (N-BNI, 5 micrograms). The antinociception induced by i.c.v. etorphine was also antagonized by the i.c.v. pretreatment of beta-funaltrexamine (beta-FNA, 50 ng, 24 hr). Intracerebroventricular administration of beta-EP-(1-27) (3 micrograms) caused a further attenuation of the i.c.v. etorphine-induced antinociception in mice pretreated with beta-FNA. The antinociceptive response induced by morphine (2 micrograms) given i.c.v. was blocked by i.c.v. administration of CTOP (25 ng) or beta-FNA (50 ng), but not beta-EP-(1-27) (6 micrograms), ICI (5 micrograms) or N-BNI (5 micrograms). These results indicate that the antinociception induced by etorphine given i.c.v. is mediated by the stimulation of both mu and epsilon opioid receptors whereas the antinociception induced by morphine given i.c.v. is mediated by the stimulation of mu, but not epsilon opioid receptors at supraspinal sites.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of epsilon and kappa opioid receptors in inhibition of the tail-flick response induced by bremazocine in the mouse.

Bremazocine, a benzomorphan, has been reported to have kappa, mu and epsilon opioid receptor binding activities. The present studies were then designed to determine what types of opioid receptors and neurotransmitters were involved in inhibiting the tail-flick response induced by bremazocine in male ICR mice. U50, 488H, a prototypic kappa agonist, was used for comparison. Bremazocine, at doses from 0.1 to 1 microgram given i.c.v., dose-dependently inhibited the tail-flick response. The paw-licking hot plate response, even at high doses of bremazocine, was not completely inhibited. The inhibition of the tail-flick response induced by bremazocine (1 microgram) given i.c.v. was blocked by i.c.v. coadministration of beta-endorphin-(1-27) (3 and 6 micrograms), an epsilon opioid receptor antagonist and norbinaltorphimine (4 micrograms), a kappa opioid receptor antagonist. On the other hand, the inhibition induced by i.c.v. U50,488H (40 micrograms) was blocked by i.c.v. norbinaltorphimine, but not beta-endorphin-(1-27). D-Phe-Cys-Tyr-D-Try-Orn-Thr-Pen-Thr-NH2 (CTOP; 0.5 microgram) and beta-funaltrexamine (beta-FNA; 2.5 micrograms), selective mu opioid receptor antagonists, and ICI 174,864 (10 micrograms), a delta-opioid receptor antagonist, which blocked the effects induced by DAMGO (16 ng) and DPDPE (20 micrograms), respectively, did not block inhibition of the tail-flick response induced by bremazocine (1 microgram) given i.c.v. The inhibition of the tail-flick response induced by i.t. administration of bremazocine (1 microgram) was blocked by i.t. coadministration of norbinaltorphimine but not CTOP, ICI 174,864, or beta-endorphin-(1-27).(ABSTRACT TRUNCATED AT 250 WORDS)     

Reversible and irreversible binding of beta-funaltrexamine to mu, delta and kappa opioid receptors in guinea pig brain membranes

The effect of beta-funaltrexamine (beta-FNA), an irreversible mu receptor blocker in isolated tissue bioassays, on mu, kappa and delta opioid receptor binding and the binding of beta-[3H]FNA were determined in guinea pig brain membranes. beta-FNA inhibited the binding of mu, kappa and delta opioid ligands to their receptors with Ki values of 2.2, 14 and 78 nM, respectively. Pretreatment of brain membranes with beta-FNA (less than 2 microM) followed by extensive washing inhibited mu binding and to a lesser degree delta binding, without changing kappa binding. The extent of the irreversible inhibition was dependent on the concentration of beta-FNA, and this inhibition on mu binding could be observed with as little as 1 nM beta-FNA. The irreversible inhibition of mu binding by beta-FNA pretreatment was due to a decrease in the number of binding sites with little change in Kd, and was more pronounced in the presence of increasing concentrations of NaCl. Specific binding of beta-[3H]FNA to opioid receptors was demonstrated. The rate of specific binding with 2 nM beta-[3H]FNA was rapid in the initial 10 min and did not reach maximum in 90 min. The dissociation of bound beta-[3H]FNA (5 nM added) by the addition of excess unlabeled naloxone reached maximum at 30 min with approximately 35% of specifically bound beta-[3H]FNA remaining. Mu opioids were most effective in preventing specific binding of beta-[3H]FNA when added before beta-[3H]FNA. Opioids added 1 hr after 2 nM beta-[3H]FNA could displace maximally only 70 to 75% of specific binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of vagal transmission by cardiac sympathetic nerve stimulation in the dog: possible involvement of opioid receptor

The inhibitory effect of cardiac sympathetic nerve stimulation (SNS) on bradycardic response to vagal nerve stimulation was examined in anesthetized dogs pretreated with atenolol or propranolol to prevent tachycardia caused by SNS or norepinephrine infusion. The amplitudes of vagal bradycardia (0.5, 1, 2, and 4 Hz) during 10 and 30 Hz of SNS were significantly smaller than those during the resting state. The inhibitory effect of 10 Hz of SNS was neither affected by prazosin (10, 30 and 100 micrograms/kg i.v.) nor by yohimbine (10, 30 and 100 micrograms/kg i.v.). The SNS-induced inhibition of vagal bradycardia was inhibited by naloxone (0.3 and 1 mg/kg i.v.) or by naltrexone (0.3 and 1 mg/kg i.v.). Vagal bradycardia was unaffected by intracoronary infusion of norepinephrine (3 micrograms/min) into the right coronary artery, whereas it was effectively inhibited by leu-enkephalin (10, 30 and 100 micrograms/kg i.v.). Bradycardia induced by intracoronary injection of methacholine (0.3, 1 and 3 micrograms) was unaffected by SNS. These results suggest that a presynaptic alpha adrenoceptor mechanism is not involved in the SNS-induced inhibition of vagal bradycardia in the dog, and suggest further that an opioid receptor mechanism may be responsible for the inhibition.     

Effects of opioid agonists on sympathetic and parasympathetic transmission to the dog heart

Effects of leu- and met-enkephalin, pentazocine and morphine on negative or positive chronotropic response to vagal nerve stimulation or cardiac sympathetic nerve stimulation were examined in anesthetized dogs in order to determine whether opioid receptors modulate vagal and sympathetic transmission. Leu- and met-enkephalin (10-100 micrograms/kg i.v.) and pentazocine (100-1000 micrograms/kg i.v.) inhibited bradycardic response to vagal nerve stimulation (1-4 Hz) in a dose-dependent manner. Morphine (300 and 1000 micrograms/kg i.v.) did not affect vagal bradycardia. The inhibitory effect of leu-enkephalin (30 micrograms/kg) and pentazocine (300 micrograms/kg) was effectively antagonized by naloxone (1000 micrograms/kg i.v.). Bradycardic response to intracoronary injection of methacholine (0.1, 0.3 and 1 microgram) into the right coronary artery was unaffected by leu-enkephalin (30 micrograms/kg). On the other hand, leu-enkephalin and pentazocine did not modify tachycardic response to sympathetic nerve stimulation (1-8 Hz). Morphine attenuated sympathetic tachycardia only slightly. These results suggest that presynaptic opioid receptors, probably delta type, are present in the vagus nerves, and that the activation of opioid receptors inhibit vagal transmission to the dog heart. In contrast, the presence of opioid receptors in the cardiac sympathetic nerves is not evident.     

Selective antagonism by naltrindole of the antinociceptive effects of the delta opioid agonist cyclic[D-penicillamine2-D-penicillamine5]enkephalin in the rat.

Spinal delta opioid receptors have been proposed to mediate antinociception in the rat on the basis of 1) the efficacy of a small number of agonists; 2) the lack of effect of mu-selective antagonists; and 3) the lack of cross-tolerance with mu-selective agonists. However, direct evidence to support or refute this postulate has not been obtained in the rat due to a lack of suitable delta-selective antagonists. The present study characterized the ability of Naltrindole (NTI, 17-cyclopropylmethyl-6,7-dehydro-4,5 alpha-epoxy-3,14-dihydroxy-6,7-2',3'-indolomorphinan), a recently discovered delta-selective antagonist, to antagonize the antinocieption produced by intrathecal (i.t.) administration of the prototypic delta-selective agonist cyclic[D-penicillamine2-D-penicillamine5]enkephalin (DPDPE) or the mu-selective agonists morphine and [D-Ala2,MePhe4,Gly-ol5] enkephalin (DAMGO) in the rat. Intrathecal coadministration of NTI with DPDPE significantly antagonized the increase in tail-flick latency (TFL) and hot-plate latency (HPL) produced by DPDPE. In the absence of NTI, the ED50 values and 95% CL of DPDPE in the tail-flick and hot-plate tests were 2.8 (1.1-4.7) and 19.5 (13.3-33.7) micrograms, respectively. In the presence of 10 micrograms of NTI, the ED50 value of DPDPE in the tail-flick test was unchanged and was increased by 2-fold in the hot-plate test to 35.9 (26.2-60.1) micrograms. In the presence of 30 micrograms of NTI, the ED50 value of DPDPE in the tail-flick test was increased by 5-fold to 14.5 (8.5-24.9) micrograms and its antinociceptive effect in the hot-plate test was antagonized completely.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of beta-funaltrexamine in normal and morphine-dependent rhesus monkeys: observational studies

The behavioral effects of the opioid receptor alkylating agent beta-funaltrexamine (beta-FNA) were assessed in normal (drug-naive) and morphine-dependent rhesus monkeys. In normal monkeys, beta-FNA (10 mg/kg s.c.) produced muscle relaxation and stupor, which could be reversed by the opioid antagonist Win 44,441. Given as a 48-hr pretreatment, beta-FNA antagonized the behavioral effects of acute morphine, but not those of two kappa agonists, ethylketazocine and Mr 2033 (UM 1072). In morphine-dependent monkeys, beta-FNA (10 mg/kg, s.c. and 0.003 mg i.c.v.) precipitated severe abstinence which lasted for 3 days. beta-FNA was more than 13,000 times more potent in precipitating withdrawal after i.c.v. than s.c. administration, whereas naltrexone and Win 44,441 were equipotent by these routes. Deprivation-induced abstinence (14 hr) and withdrawal of similar severity precipitated by naltrexone, Win 44,441 or naloxonazine were suppressed completely by 17.5 mg/kg of morphine. In contrast, 320 mg/kg of morphine failed to suppress completely a withdrawal syndrome of the same severity elicited by s.c. or i.c.v. beta-FNA. These data are consistent with the view that beta-FNA has reversible opioid agonist and insurmountable mu selective antagonist activity in the rhesus monkey.     

Central and peripheral opioid modulation of gastric relaxation induced by feeding in dogs

The influence of i.v. vs. i.c.v. administration of [( D-Ala2, MetPhe4, Gly-ol5]enkephalin (DAMGO) and morphine), kappa U 50488 and ethylketocyclazocine and delta ([D-Pen2, D-Pen5]enkephalin (DPDPE)] opioid agonists on gastric relaxation induced by a standard meal was evaluated in conscious dogs with strain-gauge transducers implanted on the gastric fundus and antrum. Under control conditions, the amplitude of the gastric relaxation in response to feeding was 2.46 +/- 0.23 g. Given i.v. 10 min before feeding, both U 50488 (10 micrograms/kg) and ethylketocyclazocine (10 micrograms/kg) significantly (P less than .01) reduced the amplitude of the gastric relaxation by 57 and 68%, respectively, whereas DAMGO and morphine (10 micrograms/kg i.v.) increased markedly the response to feeding by 67 and 51%, respectively. In contrast, DPDPE had no effect on the gastric relaxation induced by feeding. Previous administration of naloxone (0.3 mg/kg i.v.) or MR 2266 (0.3 mg/kg i.v.) blocked the effect of U 50488 on gastric relaxation with no effect per se on the amplitude of response; naloxone also blocked the increase in gastric relaxation induced by DAMGO. When administered i.c.v. (0.1 microgram/kg) DAMGO induced a significant (P less than .05) increase in the amplitude of gastric relaxation whereas U 50488 and DPDPE (0.1 and 1 microgram/kg i.c.v.) had no effect. The effect of i.c.v. DAMGO on gastric relaxation was unaffected by a previous i.v. administration of SR 58002C (1 mg/kg). Truncal vagotomy blocked the increase in gastric relaxation induced by DAMGO (10 micrograms/kg i.v.), but did not change the effect of U 50488 (10 micrograms/kg i.v.) on gastric relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of the receptor selectivities of opioid drugs by investigating the block of their effect on urine output by beta-funaltrexamine

The effects of a series of opioid drugs on urine output in the water-loaded rat were studied and also the block of those effects by the irreversible opioid receptor antagonist, beta-funaltrexamine (beta-FNA). Fentanyl, d-propoxyphene, profadol, bromadoline, buprenorphine and nalbuphine produced only a decrease in urine output, which was antagonised by pretreatment with beta-FNA, 40 mg/kg s.c., 24 hr beforehand. These drugs were thus characterized as selective mu receptor agonists. U-50,488, tifluadom, Mr2034, proxorphan, ethylketocyclazocine and butorphanol all produced an initial decrease in urine output, which was antagonized by beta-FNA, and therefore probably mu receptor mediated, followed by a beta-FNA insensitive diuretic effect, which was probably kappa receptor mediated. For U-50,488, tifluadom, Mr2034 and proxorphan the threshold dose for increasing urine output was lower than that for decreasing it, suggesting that these four compounds are kappa-selective agonists. For ethylketocyclazocine and butorphanol, the threshold doses for producing both effects were similar, suggesting that these two drugs are non-selective agonists. SKF 10,047 produced a diuretic effect at low dose-levels, which may be kappa receptor mediated, and a beta-FNA insensitive decrease in urine output at higher dose-levels, which may suggest a sigma receptor mediated effect.     

Alteration of thermoregulatory set point with opioid agonists

This paper focuses on the behavioral thermoregulatory effects of i.c.v. administration of [D-Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO), cyclic [D-Pen2,D-Pen5]enkephalin (DPDPE) and U-50,488H, selective agonists for the mu, delta and kappa opioid receptors, respectively. Rats were tested in a thermally graded tunnel (thermocline) which allowed for simultaneous measurement of body temperature, ambient temperature selection and general ambulatory activity levels. DAMGO (0.3 micrograms) caused an increase in body temperature which was facilitated by the selection of a warm ambient temperature. Both DPDPE (30 micrograms) and U-50,488H (100 micrograms) caused decreases in body temperature which were accompanied by a selection of a cool ambient temperature. In each case there was evidence for a regulated change in body temperature, with DAMGO increasing thermoregulatory set point and DPDPE and U-50,488H decreasing set point. DAMGO and U-50,488H produced a depression of activity levels for the first 15 min after injection. DAMGO and DPDPE produced increases in activity levels which peaked after body temperature had returned toward base-line levels. These data characterize further the differentiable profiles of physiological effects produced by these three compounds. The central modulation of the control of body temperature by these opioid receptor agonists may reflect a role of endogenous opioids in thermoregulatory control.     

Intrathecal beta-funaltrexamine antagonizes intracerebroventricular beta-endorphin- but not morphine-induced analgesia in mice

We have reported previously that beta-endorphin and morphine administered supraspinally produce analgesia by activating different descending pain inhibitory systems in rats. The descending system activated by beta-endorphin involves a spinal endorphinergic system whereas the descending system activated by morphine does not. To determine if this differential action of intraventricular beta-endorphin and morphine also occurs in mice, the effects of pretreatment with intrathecal (i.t.) and i.c.v. beta-funaltrexamine (beta-FNA) on analgesic response induced by i.c.v. and i.t. beta-endorphin and morphine were studied in mice. beta-FNA (2.5 micrograms) was injected i.t. or i.c.v. 24 hr before beta-endorphin or morphine administration and hot-plate and tail-flick responses were measured. Intrathecal beta-FNA attenuated i.c.v. beta-endorphin- but not i.c.v. morphine-induced analgesia. On the other hand, i.t. beta-FNA blocked both i.t. beta-endorphin- and morphine-induced analgesia, but was more effective in blocking the effects of i.t. morphine than beta-endorphin. At the supraspinal sites, beta-FNA administered i.c.v. was found to antagonize i.c.v. morphine-induced analgesia but not i.c.v. beta-endorphin-induced analgesia. The present results in mice are consistent with previous studies in rats and indicate that beta-endorphin and morphine activate different supraspinal opioid receptors. Also, analgesia produced by these two opioids resulted from activation of different descending pain inhibitory systems. The spinal endorphinergic system was involved in the production of i.c.v. beta-endorphin-, but not morphine-induced analgesia.     

Assessment of relative intrinsic activity of mu-opioid analgesics in vivo by using beta-funaltrexamine

Morphine is the prototypic mu-opioid analgesic; however, in certain situations in vitro, morphine behaves as a partial agonist. To assess the relative intrinsic activity of morphine and three other mu-opioid analgesics in vivo, beta-funaltrexamine (beta-FNA), an irreversible antagonist selective for the mu receptor, was used to reduce the effective receptor reserve. By using a stereotaxic device, 1.25 to 20 micrograms of beta-FNA was infused into the lateral ventricle of rats. Twenty-four hours later, animals were tested in the tail-flick assay with cumulative doses of morphine, levorphanol, methadone or fentanyl. Pretreatment with 2.5 micrograms of beta-FNA induced parallel rightward shifts of both the morphine and levorphanol dose-effect curves and 5.0 micrograms of beta-FNA reduced the maximum analgesic effect of these agonists. Methadone surmounted the antagonism of 5.0 micrograms of beta-FNA; 10 micrograms was required to reduce the maximum analgesic effect of methadone. Fentanyl overcame the blockade induced by both 5.0 and 10 micrograms of beta-FNA. Only with a pretreatment dose of 20 micrograms of beta-FNA was the maximum analgesic effect of fentanyl reduced. Thus, when a certain proportion of mu receptors is inactivated, i.e., with 5.0 micrograms of beta-FNA, fentanyl and methadone have the capacity to surmount the blockade, whereas morphine and levorphanol do not. This suggests that fentanyl and methadone have higher intrinsic efficacies than do morphine and levorphanol. Thus, a strategy used widely in vitro was applied successfully in vivo to assess relative intrinsic activities of a series of mu-opioid agonists.     

Inverse agonism and neutral antagonism at wild-type and constitutively active mutant delta opioid receptors

The delta opioid receptor modulates nociceptive and emotional behaviors. This receptor has been shown to exhibit measurable spontaneous activity. Progress in understanding the biological relevance of this activity has been slow, partly due to limited characterization of compounds with intrinsic negative activity. Here, we have used constitutively active mutant (CAM) delta receptors in two different functional assays, guanosine 5'-O-(3-thio)triphosphate binding and a reporter gene assay, to test potential inverse agonism of 15 delta opioid compounds, originally described as antagonists. These include the classical antagonists naloxone, naltrindole, 7-benzylidene-naltrexone, and naltriben, a new set of naltrindole derivatives, H-Tyr-Tic-Phe-Phe-OH (TIPP) and H-Tyr-TicPsi[CH2N]Cha-Phe-OH [TICP(Psi)], as well as three 2',6'-dimethyltyrosine-1,2,3,4-tetrahydroquinoline-3-carboxylate (Dmt-Tic) peptides. A reference agonist, SNC 80 [(+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide], and inverse agonist, ICI 174864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu), were also included. In a screen using wild-type and CAM M262T delta receptors, naltrindole (NTI) and close derivatives were mostly inactive, and TIPP behaved as an agonist, whereas Dmt-Tic-OH and N,N(CH3)2-Dmt-Tic-NH2 showed inverse agonism. The two latter compounds showed negative activity across 27 CAM receptors, suggesting that this activity was independent from the activation mechanism. These two compounds also exhibited nanomolar potencies in dose-response experiments performed on wild-type, M262T, Y308H, and C328R CAM receptors. TICP(Psi) exhibited strong inverse agonism at the Y308H receptor. We conclude that the stable N,N(CH3)2-Dmt-Tic-NH2 compound represents a useful tool to explore the spontaneous activity of delta receptors, and NTI and novel derivatives behave as neutral antagonists.     

Naloxazone treatment in the guinea pig ileum in vitro reveals second functional opioid receptor site

Guinea pig ilea were incubated in vitro with naloxazone, an irreversible opioid receptor antagonist, in an attempt to determine a dissociation constant (KA) value for the opioid agonist, BW 942C, by the method of partial receptor alkylation. However, concentrations of naloxazone up to 3 X 10(-4) M (120 min incubation-60 min washout) did not depress the maximal response or the slope of the concentration-response curve for BW 942C and, therefore, did not allow calculation of KA value. To analyze the residual activity of BW 942C, tissues were incubated with naloxone for 60 min after naloxazone treatment. Schild analysis of naloxone antagonism after 3 X 10(-6) M naloxazone produced a pKB of 8.0 +/- 0.06 and a slope of 0.88 +/- 0.03, suggesting simple competitive antagonism at a single site. In the absence of naloxazone treatment, naloxone 10(-8) to 10(-5) M antagonized the effects of BW 942C yielding a pKB value and slope of 8.34 +/- 0.08 and 1.0 +/- 0.04, respectively. Schild analysis of naloxone antagonism after 10(-4) M naloxazone yielded a pA2 of 6.23 +/- 0.20 and a slope of 0.55 +/- 0.08. These values were not consistent with simple competitive antagonism at a single receptor site. Modeled curves showed that the data for naloxone antagonism after 10(-4) M naloxazone were consistent with action at two receptor sites with naloxone pKB values of approximately 8.3 (experimentally determined) and approximately 6.0. An alternative explanation of altered affinity for BW 942C and naloxone at a single site produced by naloxazone treatment cannot be excluded.     

极小剂量纳洛酮增强电针的镇痛作用但不影响电针对吗啡戒断症状的抑制作用

目的:纳洛酮(naloxone,NX)是一种阿片受体拮抗剂,应用于大鼠,在1～10mg/kg范围内能对抗吗啡镇痛和电针(electroacupuncture,EA)镇痛。但有人报道,极小剂量NX可加强吗啡镇痛。本工作观察极小剂量NX是否能加强大鼠电针镇痛,及电针对大鼠吗啡戒断的抑制作用。方法:选用成年雄性SD大鼠,分两批进行实验。第一批以辐射热甩尾阈(TFL)作为大鼠痛阈指标,检测腹腔注射(i.p.)NX10ng/kg对2Hz EA和100Hz EA镇痛效应的影响。第二批在慢性吗啡依赖大鼠模型上,以体重丢失作为吗啡戒断反应指标,观察i.p NX 10 ng/kg对 2Hz EA和 100Hz EA抑制 NX(lmg/kg,i.p.)诱发的体重丢失的影响。结果:(1)EA之前10分钟 i.p. NX 10 ng/kg,不仅能显著增强2Hz EA的镇痛作用(P<0.05),而且可使其镇痛效应延长至120分钟;同样剂量NX对100 Hz EA的镇痛效应无明显影响(P>0.05)。(2)EA之前10分钟i.p. NX 10 ng/kg,对两种频率EA抑制吗啡戒断大鼠体重丢失的效应均无显著影响。结论:极小剂量的NX能增强和延长2Hz EA的镇痛作用,该发现可能具有临床应用意义。极小剂量的NX未能影响EA抑制吗啡戒断大鼠体重丢失的作用,这可能与吗啡戒断状态下阿片受体的功能改变有关。     

Role of renal nerves in excretory responses to exogenous and endogenous opioid peptides

The present study was designed to investigate opioid peptide-mediated changes in renal function in conscious Sprague-Dawley rats after administration of the native opioid agonist methionine enkephalin (ME), its synthetic analog D-Ala2-methionine enkephalinamide (DALA) and the opioid antagonist naloxone. Intravenous infusion of DALA (25 micrograms/kg/min) and ME (75 micrograms/kg/min) produced no changes in mean arterial pressure, heart rate, glomerular filtration rate or effective renal plasma flow in rats with intact or bilaterally denervated kidneys. In contrast, i.v. infusion of these opioid agonists produced differing effects on the renal excretion of water and sodium; DALA produced an increase in urinary flow rate and sodium excretion and ME produced a decrease in these parameters. Changes in renal sympathetic nerve activity were not involved in producing these effects as supported by measurements of renal sympathetic nerve activity and the finding that prior bilateral renal denervation did not alter the renal responses to either agonist. The renal excretory responses to both DALA and ME infusion were prevented by pretreatment with the opioid receptor antagonist naloxone, thus suggesting an opioid receptor-mediated effect of both agonists. Intravenous bolus injections of naloxone alone produced a dose-dependent diuresis and natriuresis without producing changes in systemic or renal hemodynamics or renal sympathetic nerve activity. These studies, therefore, provide evidence that the administration of opioid receptor agonists and antagonists produce changes in the renal excretion of water and sodium via an action on renal tubular reabsorptive mechanisms which are independent of changes in systemic or renal hemodynamics or renal sympathetic nerve activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Systemic administration of (Tyr-D-Ser(O-tert-butyl)-Gly-Phe-Leu-Thr(O-tert-butyl), a highly selective delta opioid agonist, induces mu receptor-mediated analgesia in mice

The mu opioid receptors are unquestionably implicated both in supraspinal and spinal analgesia, but there is some controversy about the role of delta receptors in the control of pain at the supraspinal level. This could be due, at least in part, to the local or i.c.v. administration of the opioid agonists. It was therefore interesting to reassess the overall contribution of mu and delta opioid receptors in modulating nociceptive thermal stimuli in the hot plate-test in mice after i.v. injections of DAMGO (Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol) and BUBU (Tyr-D-Ser(O-tert-butyl)-Gly-Phe-Leu-Thr(O-tert-butyl), two highly selective mu and delta receptor agonists, respectively, whose passage into the brain has been demonstrated recently. Both agonists induced dose-dependent, short-lasting (less than 30 min), antinociceptive responses that peaked 5 min after the administration of DAMGO and 10 min after the administration of BUBU. At these times, DAMGO [ED50: 1.26 mumols (0.65 mg)/kg] was 34 times more potent than BUBU [ED50: 42.5 mumols (34 mg)/kg] in the jump response and 13 times more potent in the paw lick. Apparent pA2 values of naloxone (0.004-0.1 mg/kg s.c.) antagonism for DAMGO and BUBU did not differ significantly, 6.95 +/- 0.054 and 7.28 +/- 0.030 for paw lick tests and 7.11 +/- 0.045 and 7.25 +/- 0.027 for jump tests, respectively. The slopes of the pA2 plots were close to the theoretical -1 value for competitive antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential antagonism of delta opioid agonists by naltrindole and its benzofuran analog (NTB) in mice: evidence for delta opioid receptor subtypes.

In this study naltrindole (NTI) and its benzofuran derivative (NTB) were studied for their antagonist activity against various delta opioid receptor agonists in the tail-flick antinociceptive assay in mice. The antinociceptive ED50 of i.c.v. administered DSLET [(D-Ser2, Leu5, Thr6)enkephalin] was shifted about 4-fold by either s.c. NTB or i.c.v. NTI injection. On the other hand, the antinociceptive ED50 of i.c.v. administered DPDPE [(D-Pen2,D-Pen5)enkephalin] was shifted 1.4- and 1.8-fold with s.c. NTB and i.c.v. NTI administration, respectively, which were significantly lower than the shifts observed with DSLET. NTB did not alter the antinociceptive action of i.c.v. administered [(D-Ala2,D-Leu5)enkephalin], morphine sulfate, [(D-Ala2,MePhe4,Gly-ol5)enkephalin] or U-50,488H (trans(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]benzeneacetamide). At spinal sites, the antinociceptive ED50 of intrathecal (i.t.) administered DSLET was increased by 12.5-fold by s.c. NTB injection, whereas that of DPDPE was unaffected. NTB injection at this site also did not alter the antinociceptive action of i.t. administered [D-Ala2, D-Leu5]enkephalin, [(D-Ala2,MePhe4,Gly-ol5)enkephalin] or morphine sulfate. Pretreatment of animals with beta-funaltrexamine caused a large increase in the capacity of NTB to antagonize the antinociceptive activity of i.t. administered DSLET with little change in that of i.t. administered DPDPE. When cross-tolerance between DSLET and DPDPE was studied by i.c.v. injection of a single large dose of either DSLET or DPDPE 24 hr before the antinociceptive assay, there was no development of cross-tolerance between the two peptides. Based on these results, it was concluded that the antinociceptive action of DSLET and DPDPE may be mediated by different receptors, possibly delta opioid subtypes.     

Inhibition by opioid agonists and enhancement by antagonists of the release of catecholamines from the dog adrenal gland in response to splanchnic nerve stimulation: evidence for the functional role of opioid receptors

The aim of the present study is to examine how opioid agonists and antagonists modify the splanchnic nerve stimulation (SNS)-induced release of catecholamines from the dog adrenal gland in vivo, in an attempt to elucidate whether opioid receptors play a functional role in controlling catecholamine release. Output of epinephrine (EPI) and norepinephrine (NE) was determined from adrenal venous blood by using high-performance liquid chromatography with electrochemical detection. SNS (0.3, 1 and 3 Hz) produced increases in both EPI and NE output in a frequency-dependent manner. Leu-enkephalin (10-100 micrograms/kg i.v.) and morphine (10-100 micrograms/kg i.v.) attenuated the increase in EPI and NE output induced by 1 or 3 Hz of SNS without affecting the basal catecholamine output. A 25 to 40% reduction of the SNS-induced increase in catecholamine output was observed after the treatment with 100 micrograms/kg of leu-enkephalin or morphine. The increase in EPI and NE output induced by 1 and 3 Hz of SNS was enhanced markedly by naloxone (10-1000 micrograms/kg i.v.) and by naltrexone (10-1000 micrograms/kg i.v.). The SNS-induced increase in catecholamine output doubled after treatment with 100 and 1000 micrograms/kg of naloxone or naltrexone. Basal catecholamine output and the increase in output induced by 1 Hz of SNS were unaffected by naloxone or naltrexone. These results suggest that endogenously released opioid peptides inhibit the release of catecholamines by activating opioid receptors in the adrenal gland of the dog.     

Activity of mu- and delta-selective opioid agonists in the guinea pig ileum preparation: differentiation into peptide and nonpeptide classes with beta-funaltrexamine

Previous studies have demonstrated that pretreatment of guinea pig longitudinal muscle-myenteric plexus ileal preparations with the highly selective noncompetitive mu antagonist beta-funaltrexamine (beta-FNA) causes an increase in the Ke value for the interaction of morphine with naloxone, suggesting that beta-FNA inactivates those receptors at which morphine interacts in the guinea pig ileum. The effect is selective for mu receptors since beta-FNA has no effect upon the interaction of naloxone with the kappa agonist nalorphine. In the present study, it was found that although beta-FNA attenuated the effects of morphine and other morphine-like agonists at mu receptors in the guinea pig longitudinal muscle-myenteric plexus ileal preparation, the mu-mediated actions of delta-selective peptide agonists and mu-selective peptide agonists were not completely attenuated by beta-FNA pretreatment. These data suggest that morphine-like mu agonists and other mu-selective and delta-selective peptide agonists in the guinea-pig ileum preparation either interact with similar opioid receptors but in a distinguishable manner or interact with different populations of opioid receptors or the peptides studied had greater intrinsic activity than the nonpeptides.