Effects of the kappa-opioid agonist U50,488 on parturition in rats.

1. The effects of the kappa-opioid agonist U50,488 on parturition were studied in the rat. 2. Given directly after the birth of the second pup U50,488 (5 mg or 10 mg kg-1, i.p.) delayed the birth of the subsequent 4 pups by ca. 100 min, acting like morphine (10 mg kg-1, i.p.). In controls given the vehicle i.p., the birth of the 4 pups after treatment took 45.4 +/- 4.6 min. The effects of U50,488 could be prevented by simultaneous naloxone injection (10 mg kg-1). Injection of either U50,488 or morphine at 1 mg kg-1, i.v. also significantly delayed parturition. The effects of U50,488 but not of morphine were fully prevented by preinjection with nor-binaltorphimine (0.5 mg kg-1, i.v.) showing selective kappa-opioid receptor-mediated inhibition by U50,488 of established parturition. 3. In rats with an indwelling jugular venous cannula, i.v. injection of U50,488 (5 mg kg-1) after the birth of the second pup slowed parturition in a similar way to i.p. injection and significantly reduced blood plasma oxytocin concentration measured by radioimmunoassay compared with vehicle-injected controls. 4. Bolus i.v. injections of oxytocin (4 mu once per 5 min) significantly reduced the delay in parturition caused by i.v. U50,488, but continuous i.v. infusion of oxytocin (4 mu 5 min-1) was less effective. 5. Since i.v. oxytocin did not immediately reverse the effects of U50,488 on parturition, direct effects of U50,488 on isometric uterine contractions in vitro were sought.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antidiuretic effect of bremazocine and U-50,488 in rats after alpha 2-adrenoceptor blockade.

The role of alpha 2-adrenoceptors and kappa-opioid receptors in urination was studied in rats. In water-loaded rats (40 mL kg-1 p.o.) the kappa-opioid agonist bremazocine (0.05 0.2 mg kg-1 i.p.) induced a dose-related diuretic response in the second hour after administration, but had no effect in the first hour. When rats were pretreated with the alpha 2-adrenoceptor antagonist idazoxan (1 mg kg-1 s.c.), bremazocine induced a dose-related antidiuretic response in the first hour; thereafter the rats showed an increase of urination similar to that with bremazocine alone. The antidiuretic effect of bremazocine was dependent on the dose of idazoxan with maximal response after 1-3 mg kg-1. Similar results were obtained with bremazocine in the presence of yohimbine (1 mg kg-1 s.c.). The antidiuretic profile of bremazocine after idazoxan was shared by U-50,488 (2.5-10 mg kg-1 i.p.), although this compound alone at the high dose reduces urine output in the first hour. The antidiuresis induced by bremazocine in the presence of idazoxan in water-loaded rats was completely antagonized by 10 but not 2 mg kg-1 i.p. of the opioid antagonist naloxone. Thus, kappa-opioid agonists, in addition to their diuretic effect, also produce an antidiuretic response which may be mediated by alpha 2-adrenoceptors.     

κ受体激动对去甲肾上腺素诱导的心肌肥大的抑制作用

目的　利用体外培养的乳鼠心肌细胞 ,观测κ阿片肽受体激动对去甲肾上腺素 (norepinephrine ,NE)诱导的心肌肥大的抑制作用 ,并探讨作用机制。方法　对培养乳鼠心肌细胞分组给药 48h后 ,用Lowrys法测心肌细胞的蛋白质含量 ;用消化分离法 ,通过目镜标尺测心肌细胞体积 ;用镜下计数法测心肌细胞的搏动频率。结果　κ阿片肽受体激动剂U5 0 ,488H(简称U5 0 ) (0 1～ 10 μmol·L-1)对培养心肌细胞的蛋白质含量具有抑制作用并呈剂量依赖性 ;对NE诱导的心肌细胞蛋白含量增加、体积增大具有抑制作用 ;同时观察到U5 0 ,488H(1μmol·L-1)具有抑制心肌细胞搏动频率的作用。U5 0 ,488H的上述作用均可被κ阿片肽受体拮抗剂Nor BNI(1μmol·L-1)拮抗。结论　U5 0 ,488H对NE诱导的心肌肥大具有抑制作用 ,这种作用是通过激动κ阿片肽受体发挥的。     

Role of protein kinase C-epsilon in the development of kappa-opioid receptor tolerance to U50,488H in rat ventricular myocytes

The role of protein kinase C-epsilon (PKC-epsilon) in the development of kappa-opioid receptor (kappa-OR) tolerance to the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) (U50,488H), the selective agonist of kappa-OR, was determined in rat ventricular myocytes. Incubation of ventricular myocytes with 1 microM U50,488H for 24 h significantly attenuated the inhibitory effects of 30 microM U50,488H on the electrically-induced [Ca(2+)](i) transient and forskolin-stimulated cyclic AMP accumulation, indicating the development of tolerance to the kappa-OR agonist. Chronic treatment of ventricular myocytes with U50,488H also induced translocation of PKC-epsilon to the particulate fraction. On the other hand, administration of 30 microM U50,488H for 10 min induced translocation of PKC-alpha to the particulate fraction in na?ve ventricular myocytes, but not in cells pretreated with 1 microM U50,488H for 24 h. In ventricular myocytes incubated for 24 h with 1 microM U50,488H together with 1 microM chelerythrine or 1 microM GF109203X, PKC inhibitors, or 0.1 microM epsilonV1-2 peptide, a selective inhibitor of PKC-epsilon, 30 microM U50,488H still produced the inhibitory effect on the electrically-induced [Ca(2+)](i) transient as it did in na?ve ventricular myocytes. Chronic treatment of ventricular myocytes with U50,488H and chelerythrine also attenuated the development of tolerance to acute U50,488H on cyclic AMP accumulation. Cells exposed to chelerythrine, GF109203X, or epsilonV1-2 peptide alone did not show an altered [Ca(2+)](i) response to U50,488H. These results indicate that activation of PKC-epsilon is a critical step in the development of tolerance in the kappa-OR.     

Kappa-opioid-receptor agonists modulate the renal excretion of water and electrolytes in anaesthetized rats.

1. Subcutaneous injection of the kappa-opioid agonists U50,488 (10 mg kg-1) and tifluadom (3.5 mg kg-1) into Inactin-anaesthetized, saline-infused rats was associated with a diuresis, antinatriuresis and antikaliuresis which lasted for up to 2 h. A high (5 mg kg-1), but not low (0.1 mg kg-1), dose of naloxone blocked the renal effects of U50,488. 2. U50,488 administration in anaesthetized, vasopressin-deficient Brattleboro DI rats was associated with an attenuated diuresis, though the antinatriuretic response remained intact. 3. The diuretic action of U50,488 was associated with an increase in glomerular filtration rate while fractional fluid reabsorption remained steady. In contrast, fractional sodium and potassium reabsorption were increased. 4. These data suggest that kappa-opioid agonists alter renal handling of both water and electrolytes. This appears to be mediated by two separate mechanisms: increased fluid loss largely reflects altered glomerular events while the fall in electrolyte excretion results from altered tubular handling.     

Administration of mu-, kappa- or delta2-receptor agonists via osmotic minipumps suppresses murine splenic antibody responses

Previously, our laboratory has shown that morphine given by implantation of a 75-mg slow-release pellet for 48 h suppresses murine splenic antibody responses to sheep red blood cells (SRBCs) in a plaque-forming cell (PFC) assay. However, the use of slow-release pellets for such studies is limited, as these pellets are only available in fixed doses and similar pellets for kappa and delta agonists have not been developed. In the present study, we investigated the feasibility of administering opioids via Alzet osmotic minipumps to assess their immunomodulatory effects. Groups of mice received minipumps dispensing morphine sulfate, which has primary activity at the mu opioid receptor; U50,488H, which is a kappa-selective agonist; deltorphin II, which is a delta2-selective agonist; or DPDPE, which has greater selectivity for delta1 than delta, receptors. Morphine, U50,488H and deltorphin II were all immunosuppressive, with biphasic dose-response curves exhibiting maximal (approximately 50%) suppression of the PFC response at doses of 0.5 to 2 mg/kg/day 48 h after pump implantation. Further, immunosuppression by morphine sulfate, U50,488H or deltorphin II was blocked by simultaneous implantation of a minipump administering the opioid receptor-selective antagonists CTAP (1 mg/kg/day), nor-binaltorphimine (5 mg/kg/day), or naltriben (3 mg/kg/day), respectively. DPDPE was inactive at doses lower than 10 mg/kg/day. We conclude that osmotic minipumps are a practical and useful way of administering opioids to study their effects on the immune system, and give further evidence that immunosuppression induced in vivo by opioid agonists is mediated not only via mu, but also via kappa and delta2 opioid receptors.     

Morphine,d-Pen2,d-Pen5 enkephalin and U50,488H differentially affect the locomotor activity and behaviours induced by quinpirole in guinea-pigs

The effects of morphined-Pen²,d-Pen⁵ enkephalin (DPDPE) and U50,488H on the behavioural syndrome elicited by the dopamine (DA) D-2 agonist quinpirole, were investigated. Morphine (1, 5 and 15 mg/kg SC) and morphine administered intracerebroventricularly (ICV) (2×5 µl, 10^–3 M; total dose = 10 nmol) produced piloerection and sedation. DPDPE-ICV (2×5 µl and 2×10 µl, 10–³ M; total doses = 10 and 20 nmol) produced piloerection and sedation similar to morphine. U50,488H (1 mg/kg SC) induced locomotor activity and some stereotyped behaviour, whereas U50,488H (5 and 10 mg/kg SC) induced muscle rigidity and dystonic-like movements. The locomotor and behavioural response elicited by quinpirole (3 mg/kg IP) was attenuated in guinea-pigs pretreated with morphine (1, 5 and 15 mg/kg SC), morphine-ICV (2 × 5 µl, 10–³ M), and DPDPE-ICV (2 × 5 µl and 2 × 10 µl, 10^–3 M). These effects were reversed by naloxone (15 mg/kg SC). U50,488H (1 mg/kg SC) increased the quinpirole-induced locomotor activity, whereas U50,488H (5 and 10 mg/kg SC) decreased the locomotor activity and stereotyped behaviours produced by quinpirole. These results indicate that the gross behavioural effects of mu, delta and kappa opioids differ in guinea-pigs compared to other rodent species, and suggest differential involvement of these opioid receptor subtypes with DA D-2 receptor-mediated activity.     

Chronic administration of U50,488H fails to produce hypothalamo-pituitary-adrenal axis tolerance in neonatal rats.

The present study investigated the effect of chronic administration of a kappa opioid receptor agonist on the function of kappa and mu opioid, serotonergic and cholinergic regulation of secretion from the hypothalamo-pituitary-adrenal axis in neonatal rats. After chronic treatment with saline or U50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]- benzeneacetamide methane sulfonate), a kappa opioid receptor agonist and subsequent pharmacological challenge, corticosterone (CS) in serum was determined. Kappa tolerance did not develop in pups treated on postnatal days 5-9 with increasing doses of U50,488H (0.5-2.5 mg/kg). When the rats were treated with the same chronic regimen of U50,488H at different stages of development from birth through weaning, only weanling rats became tolerant to U50,488H. In the absence of measurable kappa tolerance, the responses of corticosterone in serum to morphine, quipazine, a serotonin receptor agonist and physostigmine, an inhibitor of acetylcholinesterase, were attenuated in neonatal rats, treated with U50,488H. These studies suggest that kappa tolerance is more difficult to induce in developing rats than in adults and that regulation of the function of the hypothalamo-pituitary-adrenal axis by other neurotransmitter systems is altered by treatment with kappa opioid receptor agonists, even in the apparent absence of tolerance.     

Serotonin involvement in the discriminative stimulus effects of κ opioids in pigeons

The purpose of the present study was to examine the role of serotonin (5HT) in the discriminative stimulus effects of opioids. Pigeons were trained to discriminate 5.6 mg/kg of the opioid, U50,488, from water. During substitution tests, both U50,488 and another opioid, spiradoline, produced >80% responding on the U50,488-appropriate key. In contrast, the nonopioid compound, phencyclidine and several serotonergic compounds failed to substitute for the U50,488 discriminative stimulus across a wide range of doses. During combination tests, the selective 5HT_1A agonist, 8-OH-DPAT (0.001–3.2 mg/kg), dose-dependently attenuated the discriminative stimulus effects of 5.6 mg/kg U50,488 and 3.2 mg/kg spiradoline. This effect was reversed by the 5HT_1A antagonist, NAN-190 (0.01–1 mg/kg), in a dose-dependent manner. Buspirone (0.01–10 mg/kg), a 5HT_1A partial agonist, also attenuated the discriminative stimulus effects of the training dose of U50,488 but ipsapirone, another 5HT_1A partial agonist, did not. Ketanserin, a 5HT₂ antagonist, and MDL72222, a 5HT₃ antagonist, attenuated the effects of U50,488, whereas the 5HT_1B,1C agonist, mCPP, and the 5HT₂ agonist, DOI, did not. Depletion of 5HT withp-CPA also attenuated U50,488's discriminative stimulus effects. Taken together, the results suggest that serotonin release is an important component in the discriminative stimulus effects produced by opioids; however, the effects of DOI and mCPP alone suggest that activation of post-synaptic 5HT receptors is not sufficient to produce the full spectrum of opioid discriminative stimulus effects.     

Discriminative stimulus effects of spiradoline,a kappa-opioid agonist

This study represents the initial step in assessing the discriminative effects of spiradoline (U62,066), a potent congener of the selective kappa-opioid agonist, U50,488. Separate groups of rats were trained to discriminate between SC injections of saline and either 3.0 mg/kg spiradoline or 3.0 mg/kg morphine in a discrete-trial shock-avoidance/escape procedure. Spiradoline-trained rats generalized completely to U50,488 (ED₅₀s for spiradoline and U50,488 were 0.66 and 8.71 mg/kg, respectively), but selected the choice lever appropriate for saline in generalization tests with graded doses of morphine, phencyclidine, and agonist-antagonist opioids with varying degrees of kappa activity, ethylketocyclazocine, nalorphine, and butorphanol. Morphine-trained rats did not generalize to spiradoline. Naltrexone (0.01 or 0.1 mg/kg) blocked surmountably the discriminative effects of both spiradoline and morphine, but was approximately 10-fold less potent against spiradoline. These results indicate that the discriminative effects of spiradoline are mediated by kappa-opioid receptors; meaningful mu-opioid and PCP/sigma components of action were not in evidence. The potency and apparent pharmacological selectivity of spiradoline suggest the potential value of this drug for studying kappa-opioid-mediated stimulus control of behavior.     

kappa-opioid receptor stimulation inhibits cardiac hypertrophy induced by beta1-adrenoceptor stimulation in the rat

To test the hypothesis that kappa-opioid receptor stimulation inhibits cardiac hypertrophy induced by beta1-adrenoceptor stimulation, we determined the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on cardiac hypertrophy induced by isoprenaline, a selective beta-adrenoceptor agonist, in neonatal ventricular myocytes upon blockade of beta2-adrenoceptor. Hypertrophy of cardiomyocytes was determined by increases in (i) total protein content; (ii) [3H]leucine incorporation; and iii) cell size. 10 micromol/l isoprenaline increased all three parameters. The effects were abolished by 2 micromol/l propranolol, a beta-adrenergic receptor antagonist, or 300 nmol/l CGP20712A, a beta1-adrenoceptor antagonist, but not by 100 nmol/l ICI118,551, a beta2-adrenoceptor antagonist. The effects were also abolished by Rp-cAMPs 100 micromol/l, a protein kinase A inhibitor and not by pertussis toxin 5 mg/l. The effects of isoprenaline in the presence or absence of ICI118,551 were also abolished by 1 micromol/l U50,488H. The inhibitory effects of U50,488H were abolished by 1 micromol/l nor-binaltorphimine, a selective kappa-opioid receptor antagonist. U50,488H also abolished the increases in the amplitude and frequency of the spontaneous intracellular Ca2+ transient induced by 10 micromol/l isoprenaline in the presence or absence of ICI118,551, an effect also abolished by nor-binaltorphimine. In conclusion the results show that kappa-opioid receptor stimulation abolished both the cardiac hypertrophy and enhanced amplitude and frequency of the spontaneous intracellular Ca2+ transient induced by beta1-adrenoceptor stimulation.     

Serotonin involvement in the discriminative stimulus effects of mu and kappa opioids in rats

The role of serotonin (5-HT) in the discriminative stimulus effects of opioids was examined using a two-lever, food-reinforced drug discrimination procedure. The effects of the 5-HT(1A) full agonist 8-OH-DPAT, the 5-HT(1A) partial agonist buspirone and the 5-HT(2) antagonist ketanserin were evaluated in rats trained to discriminate the mu opioid agonist morphine, or the kappa opioid agonist U50, 488 from saline. In rats trained to discriminate 5.6mg/kg of morphine from saline, morphine dose-dependently substituted (produced >/= 80% morphine-appropriate responding) for the morphine stimulus. In contrast, U50,488, 8-OH-DPAT and ketanserin did not substitute for morphine, and buspirone produced only a small degree of substitution (approx. 40% morphine-appropriate responding). When administered in combination with morphine, 8-OH-DPAT, but not buspirone and ketanserin, attenuated the discriminative stimulus effects of higher doses of morphine. In rats trained to discriminate 5.6mg/kg of U50, 488 from saline, U50, 488 dose-dependently substituted for the U50, 488 stimulus. When administered alone, 8-OH-DPAT and buspirone partially substituted (produced between 40% and 79% U50, 488-appropriate responding) for the U50,488 stimulus, whereas morphine and ketanserin did not substitute for U50,488. The opioid antagonist naltrexone failed to antagonize the effects of 8-OH-DPAT and buspirone suggesting that the effects of these drugs in U50,488-trained rats were not mediated by opioid receptors. When administered in combination with U50,488, 8-OH-DPAT, but not buspirone or ketanserin, attenuated the discriminative stimulus effects of the training dose of U50,488. These results suggest that the 5-HT system is involved in the discriminative stimulus effects of both morphine and U50,488, although the exact nature of this 5-HT involvement is not clear.     

Effects of U50,488H on transient outward and ultra-rapid delayed rectifier K+ currents in young human atrial myocytes

The effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on transient outward K+ current (Ito1) and ultra-rapid delayed rectifier K+ current (IKur) in young human atrial myocytes were evaluated with a whole-cell patch-clamp technique. At +10 mV, U50,488H decreased Ito1 in a concentration-dependent manner (IC50=12.4+/-3.5 microM), while at +50 mV, U50,488H produced biphasic effects on Ito1-increasing and decreasing the current at 1-3 and 10-30 microM, respectively. U50,488H at 10 microM shifted the midpoint (V0.5) of Ito1 activation in a depolarizing direction by approximately 5 mV, accelerated the inactivation, and slowed the recovery from inactivation of Ito1. In addition, U50,488H inhibited IKur in a concentration-dependent manner (IC50=3.3+/-0.6 microM). The effects of U50,488H on the two types of K+ currents were not antagonized by either 5 microM nor-binaltorphimine or 300 nM naloxone. These results indicate that U50,488H affects both Ito1 and IKur in young human atrial myocytes in an opioid receptor-independent manner.     

The site of anti-arthritic action of the kappa-opioid, U-50, 488H, in adjuvant arthritis: importance of local administration.

1. Currently available pharmacological therapies treat arthritis inadequately. We have previously found that the kappa (kappa)-opioid, U-50,488H (trans-(+/-)- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]- benzene-acetamide methane sulphonate), possesses anti-arthritic effects. In light of the finding that opioid receptors in the periphery are upregulated during inflammation, kappa-opioids may represent a novel therapy for arthritis. The primary aim and unique feature of the present study is to investigate whether opioids exert their anti-arthritic effects in the periphery. Thus, the dose-effect relationship of a kappa-opioid agonist, U-50,488H was compared after both local and distant administration. Further, we tested whether the anti-arthritic effects of this drug are stereospecific and receptor-mediated by use of opioid antagonists. 2. Using an adjuvant model of arthritis in male Lewis rats, arthritis was judged by oedema, radiography and histological changes in the contralateral ankle of the hind limb. Treatment with (+/-)-U-50,488H for 3 days during disease onset and 3 days during established disease significantly attenuated arthritis, but the effects of (+/-)-U-50,488H on radiology and histology varied according to treatment time. Administration of (+/-)-U-50,488H during disease onset had a more marked effect on radiography, suggesting that treatment with that drug should be started early to prevent progressive joint destruction. Further, it was found that (+/-)-U-50,488H, administered for 3 days during the disease onset, either by direct subcutaneous injection into the inflamed paw or at a more distant site into the back of the neck, dose-dependently attenuated arthritic damage as measured by an index which pooled all three variables. More importantly however, (+/-)-U-50,488H was approximately fourfold more potent as an 'anti-arthritic' agent after local compared to distant subcutaneous injection (ED50; local vs distant: 5.8 +/- 1.6 vs 19.5 +/- 0.8 mg kg-1). 3. Equivalent doses of the (-)-enantiomer (20 mg kg-1day-1) and the racemate (+/-) of U-50,488H (40 mg kg-1day-1), elicited a similar attenuation of arthritic parameters while the (+/-)-enantiomer exacerbated arthritis, suggesting that the anti-arthritic activity lies solely with the (-)-enantiomer. 4. Both the peripherally selective antagonist, naloxone methiodide, and the kappa-selective antagonist, MR2266 ((-)-5,9 alpha-diethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphan), were able to reverse fully the peripheral anti-arthritic effects of U-50,488H, indicating that it exerts its effects through peripheral kappa-opioid receptors. 5. Taken together, these results not only confirm our previous findings that demonstrate anti-arthritic effects of U-50,488H but they indicate that the opioid attenuation of experimental arthritis is mediated via peripheral kappa-receptors in the arthritic joint. Peripherally acting kappa-opioid agonists should lead to new therapies for arthritis.     

The function of calcineurin and ERK1/2 signal in the antihypertrophic effects of kappa-opioid receptor stimulation on myocardial hypertrophy induced by isoprenaline

The aim of the present study, performed in an in vitro model of cardiac hypertrophy, was to examine the possible function of calcineurin and ERK1/2 in the inhibitory effects of kappa-opioid receptor stimulation on Ca2+ transients and myocardial hypertrophy induced by beta1-adrenoceptor stimulation. We determined the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on the enhancement of spontaneous Ca2+ transients and the induction of hypertrophy by isoprenaline, a beta-adrenoceptor agonist, in cultured neonatal ventricular myocytes. Total protein content, [3H]leucine incorporation and cell size were used as indices of hypertrophy; calcineurin activity and phospho-ERK1/2 level were determined by immunoblotting. Isoprenaline (10 micromol x L(-1)) increased all the three indices of hypertrophy, Ca2+ transients, calcineurin activity and the level of phospho-ERK1/2. The effects of isoprenaline were abolished by 1 micromol x L(-1) U50,488H in the absence but not in the presence of nor-binaltorphimine, a kappa-opioid receptor antagonist. The inhibitory effects of U50,488H were reproduced by cyclosporine-A, an inhibitor of calcineurin, U0126, the inhibitor of ERK1/2 and verapamil, a L-type Ca2+ channel antagonist. In addition, suppression of calcineurin activity by cyclosporine-A was associated with modest suppression of ERK1/2 phosphorylation. Meanwhile, suppression of ERK1/2 phosphorylation by U0126 was associated with modest suppression of calcineurin activity. In conclusion, the inhibitory effects of kappa-opioid receptor stimulation involved calcineurin and ERK1/2, and the two signaling pathways showed interaction in the mechanism of antihypertrophic effects afforded by kappa-opioid receptor stimulation.     

Effects of U50,488H on locomotor activity in the hamster

Two experiments investigated the effects of the specific kappa opiate agonist, U50,488 on locomotor activity in the golden Syrian hamster. In Experiment 1, the effects of U50,488 were found to be dose-related, with a 1 mg/kg dose eliciting hyperactivity and a 10 mg/kg dose eliciting hypoactivity. In Experiment 2, the dual effects of U50,488 on locomotor activity were shown to be naloxone (1 mg/kg) reversible. It is suggested that the effects of U50,488 on activity are consistent with the reported dual opposing influences of kappa agonists in the substantia nigra pars compacta and pars reticulata.     

Beta-lactam antibiotic prevents tolerance to the hypothermic effect of a kappa opioid receptor agonist

Beta-lactam antibiotics are the only clinically approved drugs which directly increase glutamate uptake. They activate the glutamate transporter subtype 1 (GLT-1), the protein responsible for 90% of glutamate uptake in the mammalian brain. The capacity of GLT-1 to clear extracellular glutamate suggests that glutamate transporter activators be explored for therapeutic approaches to clinical conditions caused by increased glutamatergic transmission. One of the most common drug effects mediated by increased glutamatergic signaling is opioid tolerance. Therefore, we tested the hypothesis that a beta-lactam antibiotic (ceftriaxone), by increasing glutamate uptake, prevents tolerance to hypothermia induced by a kappa opioid receptor agonist (U-50,488H). A single injection of U-50,488H (20mg/kg, s.c.) caused significant hypothermia in rats. Tolerance to the hypothermic effect of U50,488H was induced by injecting U50,488H (20mg/kg) twice daily for 7days. Pretreatment with ceftriaxone (200mg/kg, i.p.) for 7days did not alter the acute hypothermic response to U50,488H (20mg/kg) but did prevent tolerance to U50,488H-induced hypothermia. Central administration of dl-threo-beta-benzyloxyaspartic acid (TBOA) (0.2mumol, i.c.v.), a glutamate transporter inhibitor, abolished the effect of ceftriaxone. These results identify a functional interaction between ceftriaxone and U50,488H in vivo and provide pharmacological evidence that a beta-lactam antibiotic abolishes tolerance to hypothermia induced by a kappa opioid receptor agonist.     

Aversive properties of theκ opioid agonist U50,488 in the week-old rat pup

Opioids have been shown to produce analgesia and to be reinforcing during the first week of life in the rat. Opioids also have analgesic actions in both the infant and adult, but can be aversive in the mature animal. We examined the aversive effects of the opioid agonist U50,488 during the first postnatal week in the rat pup in three ways. In the first experiment, U50,488, injected peripherally (1.0–30.0 mg/kg), was paired with an odor and pups were tested 8 h later for positional preference for avoidance of that odor. This task is similar to conditioned preference/aversion tests used with adult animals. Both 3- and 7-day-old pups learned to avoid the odor adulterated side at the two higher doses. When exposed to odors previously associated with U50,488, pups at both ages decreased locomotor activity. In a second experiment, acute treatment with U50,488 increased ultrasonic distress vocalizations (USV) equally at 3 and 7 days of age, increased locomotor activity, and decreased rectal temperature. Neither of the latter two effects was correlated with the increase in USV production. The third experiment showed that conditioned odor cues increased USV 8 h later in 3- and 7-day-old pups at 1.0–10.0 mg/kg without changes in activity or rectal temperature. The results from these studies suggest that U50,488 can produce aversions in the neonatal rat pup as it does in the adult.Supported by U.S.P.H.S. grant DA-06600     

Effects of the kappa opioid agonist U50,488 and the kappa opioid antagonist nor-binaltorphimine on choice between cocaine and food in rhesus monkeys

Rationale Selective kappa opioid receptor agonists usually decrease cocaine self-administration in procedures that use rate-based measures of reinforcement; however, the rate-altering effects of kappa agonists complicate interpretation of these findings.Objectives To evaluate the effects of the selective kappa agonist U50,488 and the selective kappa antagonist nor-binaltorphimine (nor-BNI) on concurrent choice between cocaine and food in rhesus monkeys. The concurrent-choice procedure provides a rate-independent measure of the relative reinforcing effects of cocaine in comparison with food.Methods Four rhesus monkeys were trained to respond under a concurrent-choice schedule for food (1-g pellets) or cocaine (0–0.1 mg/kg per injection). Saline and increasing doses of U50,488 (0.0032–0.1 mg/kg per h) were administered by pseudo-continuous i.v. infusion (one infusion every 20 min) during sequential 3-day blocks. In a separate experiment, monkeys were treated with nor-BNI (3.2 mg/kg, i.v.), and cocaine choice was re-determined during pseudo-continuous infusion with saline or U50,488 (0.1 mg/kg per h).Results During saline treatment, cocaine maintained a dose-dependent and monotonic increase in cocaine choice. Monkeys responded primarily for food when low cocaine doses were available (0–0.01 mg/kg per injection) and primarily for cocaine when higher cocaine doses were available (0.032–0.1 mg/kg per injection). U50,488 produced a dose-dependent increase in cocaine choice, manifested as leftward shifts in the cocaine-choice, dose–effect curve. U50,488 also dose-dependently decreased overall response rates. Nor-BNI did not alter cocaine choice, but it attenuated the effects of U50,488.Conclusions These results suggest that continuous treatment with U50,488 produces a kappa receptor-mediated increase in the relative reinforcing effects of cocaine in comparison with food.     

kappa-Opioid receptor stimulation inhibits augmentation of Ca(2+) transient and hypertrophy induced by isoprenaline in neonatal rat ventricular myocytes - Role of CaMKIIdelta(B)

We aimed to further define the pathway mediating the inhibitory effects of kappa-opioid receptor stimulation on Ca(2+) transients and hypertrophic responses to beta(1)-adrenoceptor stimulation. We determined the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on the enhancement of spontaneous Ca(2+) transients and the induction of hypertrophy by isoprenaline, a beta-adrenoceptor agonist, in cultured neonatal ventricular myocytes. The results were compared with those found with KN93, a selective Ca(2+)/calmodulin-dependent kinase (CaMKII) inhibitor, propranolol, a beta-adrenoceptor antagonist, and verapamil, a L-type Ca(2+) channel antagonist. Hypertrophy of cardiomyocytes was characterized by increases in (i) total protein content; (ii) cell size; and (iii) [(3)H]leucine incorporation. 10 micromol/l isoprenaline increased all three parameters. We also determined the expression of nuclear CaMKIIdelta in response to U50,488H in the presence or absence of isoprenaline. To determine whether the effects of U50,488H were receptor-mediated, its effects were also measured following blockade of the kappa-opioid receptor with nor-binaltorphimine. kappa-Opioid receptor stimulation suppressed the stimulatory effect of isoprenaline on Ca(2+) transients and cardiac hypertrophy, as did KN93, propranalol and verapamil. U50,488H also suppressed the expression of nuclear CaMKIIdelta(B) in the presence, but not in the absence of isoprenaline. These results suggest that the inhibitory effect of kappa-opioid receptor stimulation on beta(1)-adrenoceptor stimulation may also involve CaMKIIdelta.