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

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

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

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

Ultrasonic vocalizations of preweanling rats: involvement of both alpha(2)-adrenoceptor and kappa-opioid receptor systems

Stimulation of alpha(2)-adrenoceptors and kappa-opioid receptors increases the ultrasonic vocalizations of preweanling rats. The purpose of the present study was to determine whether alpha(2)-adrenoceptors and kappa-opioid receptors modulate ultrasonic vocalization production via a common mechanism. To that end, 11-day-old rats were injected with the alpha(2)-adrenoceptor antagonist yohimbine (0, 0.5, or 1.0 mg/kg, i.p.) or the kappa-opioid receptor antagonist nor-binaltorphimine (0, 5, or 10 mg/kg, i.p.). After 15 min, the same rats were injected with saline, the alpha(2)-adrenoceptor agonist clonidine (0.25 mg/kg, i.p.), or the kappa-opioid receptor agonist trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate (U-50,488; 2.5 mg/kg, i.p.). Results showed that both clonidine and U-50,488 increased the ultrasonic vocalizations of preweanling rats. Not surprisingly, clonidine-induced ultrasonic vocalizations were blocked by yohimbine, while U-50,488-induced vocalizations were blocked by nor-binaltorphimine. Importantly, yohimbine also attenuated the vocalizations produced by U-50,488, whereas nor-binaltorphimine did not alter clonidine-induced ultrasonic vocalizations. Thus, it appears that alpha(2)-adrenoceptor and kappa-opioid receptor stimulation increases ultrasonic vocalization production via a common mechanism. It is likely that the kappa-opioid receptors responsible for modulating ultrasonic vocalizations are located "upstream" from the alpha(2)-adrenoceptors.     

Role of Ca2+ channels on the hypothermic response produced by activation of kappa-opioid receptors

The effect of nimodipine (NIM) and lercanidipine (LER) 1,4-dihydropyridine (DHP) calcium channel blockers (CCBs) on the hypothermic response of selective kappa-opioid receptor agonists U50,488H (U50), PD117,302 (PD) and U69,593 (U69) was determined in rats by recording colonic temperature using digital telethermometer. Intraperitoneal (i.p.) injections of U50 (7.5, 15, 22.5 and 40 mg/kg), PD (7.5, 15 and 22.5 mg/kg) and U69 (5 and 20 mg/kg) produced a dose-dependent hypothermic response. However, higher doses of U50 (60 and 80 mg/kg) produced hypothermia, which is less when compared to that produced by 22.5-mg/kg dose of U50. NIM (1 mg/kg i.p.; 15 min prior) and LER (0.3 mg/kg i.p.; 15 min prior) did not produce any change in basal colonic temperature. Treatment of NIM and LER potentiated the U50 (7.5, 15, 22.5 and 40 mg/kg)-induced hypothermic effect. NIM did not potentiate hypothermia produced by U50 (60 mg/kg). On the other hand, PD (7.5, 15 and 22.5 mg/kg)- and U69 (5 and 20 mg/kg)-induced hypothermia was unaffected by the pretreatment of either NIM or LER. This differential modulation of kappa-opioid agonist-induced hypothermia by CCBs suggest that there may be two mechanisms, Ca(2+)-sensitive and Ca(2+)-insensitive, involved in kappa-opioid agonist-induced hypothermic response.     

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

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

The kappa agonists PD117302 and U50488 produce a biphasic effect on 24 hour food intake in the rat

The effects of the selective kappa opioid receptor agonists PD117302 [(+/-)trans-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl-]benzo-[b]- thiophene-4-acetamide] and U50488 (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide) were investigated on food intake in non-deprived rats over a 24 hr period following subcutaneous administration. At doses of 2.5 mg/kg and 5 mg/kg, both PD117302 and U50488 initially increased food intake. Between 6-24 hr, however, PD117302 (2.5-10 mg/kg) and U50488 (2.5 mg/kg and 5 mg/kg) caused a reduction in food intake. The largest doses of PD117302 and U50488 also reduced total 24 hr food intake. The reduction in food intake produced by PD117302 (5 mg/kg) was selective, since the intake of water was not affected. It is concluded that the kappa agonists PD117302 and U50488 produce a biphasic effect on 24 hr food intake, with an initial hyperphagia followed by a decrease in food intake.     

The novel kappa-opioid receptor agonist TRK-820 has no affect on the development of antinociceptive tolerance to morphine in mice

The effects of the novel kappa-opioid receptor agonist 17-cyclopropylmethyl-3,14beta-dihydroxy-4, 5alpha-epoxy-6beta-[N-methyl-trans-3-(3-furyl)acrylamido+ ++]morphinan hydrochloride (TRK-820) on the development of antinociceptive tolerance to morphine were investigated in mice and compared with those of trans-3,4-dichloro-N-(2-(1-pyrrolidinyl)-cyclohexyl) benzenacetamide methane sulfonate hydrochloride (U-50,488H), a well-defined exogenous kappa-opioid receptor agonist. Morphine (1. 25-20 mg/kg, s.c.) produced a dose-related antinociceptive effect in the 51 degrees C warm-plate test. Daily treatment with morphine (10 mg/kg, s.c.) resulted in the development of antinociceptive tolerance. The development of antinociceptive tolerance to morphine was dose-dependently suppressed by the co-administration of U-50, 488H (1-10 mg/kg, s.c.) with morphine, but not TRK-820 (0.003-0.03 mg/kg, s.c.). These results suggest that TRK-820-sensitive kappa-opioid receptor subtypes may not be involved in modulating the development of antinociceptive tolerance to morphine.     

Buprenorphine is a potent kappa-opioid receptor antagonist in pigeons and mice

Buprenorphine was studied for its antagonist activity against the specific kappa-opioid agonist U-50,488H in pigeons responding under a multiple schedule of grain presentation and in mice in an antinociception test. U-50,488H decreased rates of responding of pigeons over the dose range (2.5-20 mg/kg i.m.). In the presence of 0.32 mg/kg of buprenorphine, the U-50,488H dose-effect curve was shifted to the right approximately two-fold. Buprenorphine alone (0.01-0.08 mg/kg s.c.) inhibited in mice the abdominal stretching induced by i.p. acetic acid. beta-Funaltrexamine pretreatment blocked the mu-like agonist analgesic effect of buprenorphine and revealed an antagonist action of buprenorphine against 2.5 mg/kg of U-50,488H over the same dose range that it produced antinociception at the mu-receptor. Thus, buprenorphine is a potent kappa-opioid receptor antagonist, producing the kappa-antagonist activity over the same dose range that it produces its mu-mediated partial agonist activity.     

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

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

Diltiazem enhances the analgesic but not the respiratory depressant effects of morphine in rhesus monkeys

There is evidence that blockade of Ca(2+) channels can modify the analgesia and respiratory depression produced by opioid drugs. The interaction between Ca(2+) channel blockade and drug-induced analgesia and respiratory depression was examined by administration of the L-type Ca(2+) channel blocker diltiazem together with various analgesic drugs. The antinociceptive effects of the drugs were evaluated using a warm-water (50 degrees C) tail-withdrawal assay in rhesus monkeys, and the respiratory depressant effects were evaluated using a pressure-displacement plethysmograph. Pretreatment with diltiazem (10-40 mg/kg, i.m.) 30 min before administration of morphine (0.3 to 10 mg/kg) or heroin (0.03 to 1.0 mg/kg) produced a dose-dependent potentiation of the opioid-induced analgesia. The analgesic potency of morphine and heroin was increased by approximately 0.5 log unit in the presence of 40 mg/kg diltiazem. However, diltiazem failed to alter the analgesic potencies of the mu-opioid receptor agonists, fentanyl, etonitazene, nalbuphine, the kappa-opioid receptor agonist, U-50,488 [(trans)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide ], or the non-opioid, clonidine. Respiratory frequency, minute volume, and tidal volume were suppressed by morphine, heroin, and fentanyl, but these effects were not modified by pretreatment with diltiazem (40 mg/kg). These results suggest that diltiazem selectively potentiates morphine- and heroin-induced analgesia without modifying the effects of these opioids on respiration.     

Different roles of mu-, delta- and kappa-opioid receptors in ethanol-associated place preference in rats exposed to conditioned fear stress

The present study was designed to investigate the role of the endogenous opioid system in the development of ethanol-induced place preference in rats exposed to conditioned fear stress (exposure to an environment paired previously with electric foot shock), using the conditioned place preference paradigm. The administration of ethanol (300 mg/kg, i.p.) with conditioned fear stress induced significant place preference. Naloxone (1 and 3 mg/kg, s.c.), a non-selective opioid receptor antagonist, significantly attenuated this ethanol-induced place preference. Moreover, the selective mu-opioid receptor antagonist beta-funaltrexamine (3 and 10 mg/kg, i.p.) and the selective delta-opioid receptor antagonist naltrindole (1 and 3 mg/kg, s.c.) significantly attenuated ethanol-induced place preference. In contrast, the selective kappa-opioid receptor antagonist nor-binaltorphimine (3 mg/kg, i.p.) significantly enhanced ethanol-induced place preference. Furthermore, 75 mg/kg ethanol (which tended to produce place preference) combined with the mu-opioid receptor agonist morphine (0.1 mg/kg, s.c.) or the selective delta-opioid receptor agonist 2-methyl-4aalpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aalpha- octahydroquinolino [2,3,3,-g] isoquinoline (TAN-67; 20 mg/kg, s.c.), at doses which alone did not produce place preference, produced significant place preference. However, co-administration of the selective kappa-opioid receptor agonist trans-3,4-dichloro-N-(2-(1-pyrrolidinyl)cyclohexyl)benzenacetamide methanesulfonate (U50,488H; 0.3 and 1 mg/kg, s.c.) with ethanol (300 mg/kg, i.p.) dose dependently attenuated ethanol-induced place preference. Moreover, conditioned fear stress shifted the response curve for the aversive effect of U50,488H to the left. These results suggest that mu- and delta-opioid receptors may play critical roles in the rewarding mechanism of ethanol, and that kappa-opioid receptors may modulate the development of the rewarding effect of ethanol under psychological stress.     

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.     

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

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

Abstinence from U-50,488H, a kappa-opiate receptor agonist, decreases the binding of [3H]DPAT to 5-HT1A receptors in the hypothalamus of the rat.

The effect of trans-( ± )-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzene-acetamide methane sulfonate (U-50,488H), a κ-opiate agonist-induced tolerance and abstinence on 5-HT_1A receptors was determined in regions of the brain and spinal cord of the rat. The administration of U-50,488H (25 mg/kg, i.p., twice daily) to male Sprague-Dawley rats for 4 days resulted in the development of almost complete tolerance to its analgesic and hypothermic effects. On day 5, the animals were divided into tolerant and abstinent groups and sacrificed. The brain and spinal cord were excised from all groups of rats and the brain was dissected into 6 regions, namely, amygdala, hypothalamus, striatum, midbrain, pons + medulla and cortex. The 5-HT_1A receptors were characterized by using [³H]8-hydroxy-2-(di-n-propylamino)tetralin ([³H]DPAT) as the ligand. The binding constants (B_max and K_dvalues) of [³H]DPAT in regions of the brain and spinal cord of rats tolerant to U-50,488H and vehicle did not differ. However, the B_max value of [³H]DPAT in the hypothalamus of U-50,488H-abstinent rats was decreased but the K_d value did not change. In the other regions of the brain and spinal cord of U-50,488H-abstinent rats, the B_max and K_d values of [³H]DPAT were unaffected. Subcutaneous administration of DPAT produced hypothermic response in vehicle- and U-50,488H-treated rats. The intensity of this effect was more marked in U-50,488H-abstinent group. It is concluded that 5-HT_1A receptors are down-regulated in the hypothalamus of U-50,488H-abstinent rats but the hypothermic response to 5-HT_1A agonist is intensified.     

Interaction of kappa receptor agonists with Ca2+ channel antagonists in the modulation of hypothermia

Administration of the opiate U-50,488H (3-20 mg/kg s.c.), a selective kappa receptor agonist, produced a dose-dependent decrease of rectal temperature in rats. This hypothermic effect of U-50,488H was accompanied by an enhanced activity of Ca2+/Mg2+ ATPase in crude synaptosomal (P2) fractions obtained from hypothalamus but not from cortex or cerebellum. Mg2+ ATPase activity in these regions was not altered by U-50,488H (15 mg/kg s.c.). Naloxone (5 mg/kg) partially and MR2266 (5 mg/kg) completely reversed the temperature and enzyme changes. Pretreatment with the calcium channel blockers nimodipine (1 mg/kg s.c.), diltiazem (10 mg/kg s.c.) and verapamil (2.5 mg/kg s.c.) potentiated the hypothermic effect of U-50,488H as well as the stimulation of Ca2+/Mg2+ ATPase in hypothalamus. These observations suggest that kappa agonists may produce opiate receptor mediated hypothermia through changes in intracellular Ca2+ levels in the hypothalamus.     

Non-competitive antagonism of N-methyl-d-aspartate receptor inhibits tolerance to the analgesic action of u-50,488H,a κ-opiate receptor agonist in the rat

• 1.1. The effect of dizocilpine (MK-801), a non-competitive inhibitor of N-methyl-d-aspartate (NMDA) receptor on the development of tolerance to the analgesic and hypothermic actions of U-50,488H, a highly selective κ-opiate receptor agonist, was determined in the rat.
• 2.2. Male Sprague-Dawley rats were made tolerant to the pharmacological actions of U-50,488H by twice daily intraperitoneal (i.p.) injections of the drug (25 mg/kg) for 4 days.
• 3.3. Multiple injections of U-50,488H resulted in the development of tolerance to its analgesic and hypothermic actions in the rat. MK-801 injected 10 min before each injection of U-50,488H, dosedependently inhibited the development of tolerance to the analgesic action but the tolerance to the hypothermic action of U-50,488H was unaffected.
• 4.4. Multiple injections of U-50,488H decreased the body weight gain. MK-801 dose-dependently decreased the gain in body weight further.
• 5.5. The results indicate that non-competitive antagonism of the NMDA receptor by MK-801 can selectively inhibit the tolerance to the analgesic action of U-50,488H in the rat, however, such an effect was associated with significant decreases in normal gain in body weight.

     

Effect of repeated administration of U-50,488H, a kappa opioid receptor agonist, on central 5-HT1 and 5-HT2 receptors in the rat

The effect of repeated administration of U-50,488H, a kappa-opioid receptor agonist, on the development of tolerance to its analgesic effect and on the 5-HT1 and 5-HT2 receptors in the cerebral cortex and spinal cord of the rat were determined. Male Sprague-Dawley rats were injected twice daily with U-50,488H, (25 mg/kg, i.p.) or its vehicle for 4 days. The assessment of tolerance to the analgesic effect and biochemical determinations were made on day 5. Repeated administration of U-50,488H resulted in the development of tolerance to its analgesic effect. The 5-HT1 and 5-HT2 receptors were characterized by using 3H-5-hydroxytryptamine (3H-5-HT) and 3H-spiperone as the ligands and unlabeled 5-hydroxytryptamine (5-HT) and ketanserin, respectively, to determine the nonspecific binding. In the spinal cord 3H-5-HT bound to 5-HT1 receptors at a single high-affinity site with a Bmax value of 41.3 +/- 9.6 fmol/mg protein and a Kd value of 22.6 +/- 7.0 nmol/l. 3H-Spiperone bound to 5-HT2 receptors in the spinal cord with a Bmax value of 16.1 +/- 3.8 fmol/mg protein and a Kd value of 0.36 +/- 0.15 nmol/l. Repeated administration of U-50,488H to rats did not affect spinal cord 5-HT1 and 5-HT2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

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

Functional interaction among opioid receptor types: up-regulation of mu- and delta-opioid receptor functions after repeated stimulation of kappa-opioid receptors

It has been widely accepted that repeated administration of kappa-opioid receptor agonists leads to the development of antinociceptive tolerance. The present study was designed to investigate the effect of repeated administration of a selective kappa-opioid receptor agonist (1S-trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride ((-)U-50,488H) on the mu- and delta-opioid receptor agonist-induced antinociception and G-protein activation in mice. The mice were injected either subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) pretreated with saline or (-)U-50,488H once a day for seven consecutive days. Two hours after the last injection, the mice were challenged by either mu- or delta-opioid receptor agonist for the antinociceptive assay. Repeated treatment with (-)U-50,488H (s.c. or i.c.v.) significantly enhanced antinociceptive effect of both mu-opioid receptor agonist (morphine) and delta-opioid receptor agonists ([d-Ala2]deltorphin (DELT) and (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dime thyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80) compared to saline-treated groups. Under these conditions, repeated s.c. injection of (-)U-50,488H significantly enhanced both mu- and delta-opioid receptor agonist-stimulated [35S]GTPgammaS binding in the membrane of the thalamus. On the contrary, either repeated administration of morphine (s.c. or i.c.v.) or SNC-80 failed to affect the kappa-opioid receptor agonist-induced antinociception and G-protein activation. Taken together, these results suggest that repeated stimulation of kappa-opioid receptor markedly increases the functional mu- and delta-opioid receptors, whereas repeated stimulation of either mu- or delta-opioid receptor had no direct effect on kappa-opioidergic function in mice.     

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.