Differential regulation of the human kappa opioid receptor by agonists: etorphine and levorphanol reduced dynorphin A- and U50,488H-induced internalization and phosphorylation

We previously observed that (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488H) promoted internalization and phosphorylation of the FLAG-tagged human kappa opioid receptor (FLAG-hkor) stably expressed in Chinese hamster ovary (CHO) cells. In this study, we compared regulation of the FLAG-hkor expressed in CHO cells by U50,488H, dynorphin A, etorphine, and levorphanol, which were potent full agonists as determined by stimulation of guanosine 5'-O-(3-[(35)S]thio)triphosphate binding. Using fluorescence flow cytometry, we found that dynorphin A(1-17), like U50,488H, promoted internalization of the FLAG-hkor in a time- and dose-dependent manner. The antagonists naloxone and norbinaltorphimine, having no effect on FLAG-hkor internalization, effectively blocked dynorphin A(1-17)- and U50,488H-induced internalization. Interestingly, the full agonists etorphine and levorphanol did not cause internalization of the FLAG-hkor but significantly reduced dynorphin A(1-17)- and U50,488H-induced internalization in a dose-dependent manner. Immunofluorescence staining of FLAG-hkor yielded similar results. Dynorphin A(1-17) and U50,488H enhanced phosphorylation of FLAG-hkor to a greater extent than etorphine, but levorphanol did not increase FLAG-hkor phosphorylation. Etorphine or levorphanol decreased dynorphin- or U50,488H-induced phosphorylation. It is likely that conformations of the hkor required for phosphorylation and initiation of internalization are different from those for activation of G proteins. We also examined whether the four agonists had differential effects on superactivation of adenylate cyclase. Pretreatment with U50,488H, dynorphin A(1-17), or etorphine enhanced forskolin-stimulated adenylate cyclase activity to approximately 200 to 250% of the control, whereas levorphanol pretreatment did not result in significant adenylate cyclase superactivation. Thus, the degree of superactivation caused by an agonist is unrelated to its ability to promote internalization of the hkor.     

Ligands regulate cell surface level of the human kappa opioid receptor by activation-induced down-regulation and pharmacological chaperone-mediated enhancement: differential effects of nonpeptide and peptide agonists

Two peptide agonists, eight nonpeptide agonists, and five nonpeptide antagonists were evaluated for their capacity to regulate FLAG (DYKDDDDK)-tagged human kappa opioid receptors (hKORs) stably expressed in Chinese hamster ovary cells after incubation for 4 h with a ligand at a concentration approximately 1000-fold of its EC(50) (agonist) or K(i) (antagonist) value. Dynorphins A and B decreased the fully glycosylated mature form (55-kDa) of FLAG-hKOR by 70%, whereas nonpeptide full agonists [2-(3,4-dichlorophenyl)-N-methyl-N-[(2R)-2-pyrrolidin-1-ylcyclohexyl-]acetamide (U50,488H), 17-cyclopropylmethyl-3,14-dihydroxy-4,5-epoxy-6-[N-methyl-trans-3-(3-furyl) acrylamido] morphinan hydrochloride (TRK-820), ethylketocyclazocine, bremazocine, asimadoline, and (RS)-[3-[1-[[(3,4-dichlorophenyl)acetyl]-methylamino]-2-(1-pyrrolidinyl)ethyl]phenoxy] acetic acid hydrochloride (ICI 204,448) caused 10-30% decreases. In contrast, pentazocine (partial agonist) and etorphine (full agonist) up-regulated by approximately 15 and 25%, respectively. The antagonists naloxone and norbinaltorphimine also significantly increased the 55-kDa receptor, whereas selective mu, delta, and D(1) receptor antagonists had no effect. Naloxone up-regulated the receptor concentration- and time-dependently and enhanced the receptor maturation extent, without affecting its turnover. Treatment with brefeldin A (BFA), which disrupts Golgi, resulted in generation of a 51-kDa form that resided intracellularly. Naloxone up-regulated the new species, indicating that its action site is in the endoplasmic reticulum as a pharmacological chaperone. After treatment with BFA, all nonpeptide agonists up-regulated the 51-kDa form, whereas dynorphins A and B did not, indicating that nonpeptide agonists act as pharmacological chaperones, but peptide agonists do not. BFA treatment enhanced down-regulation of the cell surface receptor induced by nonpeptide agonists, but not that by peptide agonists, and unmasked etorphine- and pentazocine-mediated receptor down-regulation. These results demonstrate that ligands have dual effects on receptor levels: enhancement by chaperone-like effects and agonist-promoted down-regulation, and the net effect reflects the algebraic sum of the two.     

Rotational behavior mediated by dopaminergic and nondopaminergic mechanisms after intranigral microinjection of specific mu, delta and kappa opioid agonists

The regulation of motor behavior by mu, delta and kappa opiate receptors in the substantia nigra was examined. Unilateral microinjections of specific mu (DAGO), delta (DPDPE) and kappa (U-50,488H) ligands into the substantia nigra pars reticulata of rats produced dose-dependent contralateral turning. The opiate antagonist naloxone blocked these effects, suggesting that the circling was mediated through opiate receptors. The involvement of midbrain dopaminergic systems in this behavior was tested in two ways. Unilateral 6-hydroxydopamine lesions of the medial forebrain bundle decreased the circling produced by DPDPE and DAGO but increased the circling produced by U-50,488H. In contrast, activating dopaminergic systems with systemic injections of amphetamine increased the circling produced by DAGO and DPDPE but had no effect on the circling produced by U-50,488H. These findings suggest that kappa opioids exert opposite effects on locomotion: motor activation through the SNR and motor inhibition through actions in the SNC. Furthermore, the data suggest that the actions of kappa opioids in the SNC are opposite to those produced by mu and delta opioids.     

Antagonism of the analgesic effects of mu and kappa opioid agonists in the squirrel monkey

The effects of several mu and kappa opioid agonists were examined alone and in combination with the opioid antagonist quadazocine in squirrel monkeys responding under a schedule of shock titration. Under this procedure, shock was scheduled to increase once every 15 sec from 0.01 to 2.0 mA in 30 steps. Five responses on a lever during the 15-sec shock period terminated the shock for 15 sec, after which the shock resumed at the next lower intensity. The intensity below which the monkeys maintained the shock 50% of the time (median shock level) and the rate of responding in the presence of shock were determined under control conditions and after administration of the mu agonists, l-methadone and fentanyl and the kappa agonists, bremazocine, ethylketocyclazocine, ketocyclazocine and U50,488. When examined alone, intermediate doses of mu and kappa agonists increased median shock level. At the highest doses of these compounds responding was eliminated and shock rose to its peak intensity. When the mu and kappa agonists were examined in combination with quadazocine, dose-effect curves for median shock level and for rate of responding were shifted to the right in a dose-dependent fashion. A comparison of the pA2 values for quadazocine on median shock level and on rate of responding revealed similar values for the two measures; however, pA2 values differed depending on the agonist examined. That is, the pA2 values for quadazocine in combination with l-methadone and fentanyl on median shock level were 7.43 and 7.61, respectively; whereas the pA2 value for quadazocine in combination with bremazocine and U50,488 were 6.53 and 6.43, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactions between the discriminative stimulus effects of mu and kappa opioid agonists in the squirrel monkey.

A series of mu and kappa opioid agonists with varying degrees of selectivity were evaluated for their agonist and antagonist effects in squirrel monkeys trained to discriminate either the selective mu agonist fentanyl or the selective kappa agonist U50,488 from water. In the fentanyl-trained monkeys, fentanyl, as well as the less selective mu agonists buprenorphine and (-)-metazocine, produced dose-dependent and complete substitution for the training stimulus. U50,488 produced neither agonist nor antagonist effects in the fentanyl-trained monkeys, but the less selective kappa agonists bremazocine and tifluadom generally produced either agonist or antagonist effects, depending on the monkey tested. In the U50,488-trained monkeys, U50,488, bremazocine and tifluadom all produced a dose-dependent and complete substitution for the training stimulus. Fentanyl produced neither agonist nor antagonist effects in the U50,488-trained monkeys, but buprenorphine and (-)-metazocine antagonized the discriminative stimulus effects of U50,488. The inability of the selective mu agonist fentanyl and the selective kappa agonist U50,488 to antagonize each other's discriminative stimulus effects suggests that the stimulus effects mediated by mu and kappa opioid receptors in squirrel monkeys do not interact with a common biologic substrate. Rather, these results suggest that the stimulus effects mediated by mu and kappa receptors function independently of one another. Interactions involving the less selective mu agonists buprenorphine and (-)-metazocine, or the less selective kappa agonists bremazocine and tifluadom, can be explained on the basis of the low receptor selectivity of these drugs.     

Reinforcing effect of alfentanil is mediated by mu opioid receptors: apparent pA2 analysis

Apparent pA2 analysis was used to determine whether the short-duration opioid agonist, alfentanil, acts at mu receptors in the positive reinforcement of operant behavior in the rhesus monkey. In test sessions a red light signaled the availability of alfentanil injections. If a monkey pressed a response lever 30 times, a specific dose of alfentanil was injected i.v., and the red light was extinguished for 10 min. This cycle could be repeated for up to 130 min, the maximum length of a session. Between successive test sessions at least three maintenance sessions were held; in these sessions injections of 0.32 mg/kg/injection of codeine were made available. The dose of alfentanil was changed from one test session to the next, and dose-dependent changes in rates of responding resulted. Rates reached 3.05 responses/sec at 0.010 mg/kg/injection, the highest dose tested. The opioid antagonist, quadazocine, produced dose-dependent, parallel shifts to the right in the alfentanil dose-response curve. In Schild Plot analysis the regression line fit to the antagonism data had a slope of -1.1; the apparent pA2 value for quadazocine was 7.6. This value was close to apparent pA2 values obtained with mu agonists in studies of other behavioral effects of opioids, but distinct from values obtained with kappa agonists in those studies. Thus, it is likely that mu receptors mediate the positive reinforcing effect of alfentanil.     

Functional effects of systemically administered agonists and antagonists of mu,delta, and kappa opioid receptor subtypes on body temperature in mice

We have investigated the roles of peripheral and central mu, delta, and kappa opioid receptors and their subtypes in opioid-induced hypothermia in mice. Measuring rectal temperature after i.p. injection, opioid agonists [morphine, fentanyl, SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)3-methoybenzyl]-N,N-diethylbenzamide), U50,488H ((trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide), and loperamide)] were tested alone or with opioid antagonists [naloxone, beta-funaltrexamine, naloxonazine, naltrindole, 7-benzylidenenaltrexone (BNTX), naltriben, nor-binaltorphimine, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (DIPPA), and methyl-naltrexone] given 15 min after the agonist. All agonists produced dose-related hypothermia, although at low doses, morphine and U50,488H produced hyperthermia. The effects of morphine and fentanyl were antagonized by naloxone and by the mu(1) antagonist naloxonazine. The delta(2) antagonist naltriben potentiated the hypothermic effect of mu agonists. SNC80-induced hypothermia was blocked by the delta antagonist naltrindole but not by the delta(1) antagonist BNTX. Depending on the dose, the delta(2) antagonist naltriben produced either a potentiation or an attenuation of the effect of SNC80. U50,488H-induced hypothermia was antagonized by the kappa antagonist nor-binaltorphimine but not by acute treatment with the irreversible kappa antagonist DIPPA. The peripherally acting opioid loperamide produced hypothermia that could be blocked by several mu-, delta-, or kappa-selective antagonists as well as the peripherally acting antagonist methyl-naltrexone. Methyl-naltrexone produced a weak potentiation of morphine-, fentanyl-, and U50,488H-induced hypothermia, whereas a significant attenuation of SNC80-induced hypothermia was observed. In conclusion, at high doses, morphine- and fentanyl-induced hypothermia may involve composite action on mu, kappa, and possibly delta opioid receptors after initial activation. In the mediation of delta opioid-induced hypothermia, no clear selectivity between the delta(1) and delta(2) subtypes was defined. The studies provide new evidence that maintenance of the initial effects of agonist/receptor activation vary with the agonist and the receptor. The existence of both central and peripheral components of opioid-induced hypothermia is also emphasized.     

5-HT1A-sensitive adenylyl cyclase of rodent hippocampal neurons: effects of antidepressant treatments and chronic stimulation with agonists

The effects of chronic treatment with desimipramine (a tricyclic antidepressant), fluoxetine [a specific 5-hydroxytryptamine (5-HT) uptake inhibitor], clorgyline (a specific monoamine oxydase inhibitor of A type), ipsapirone (a specific 5-HT1A receptor agonist) as well as electroconvulsive shock treatment were investigated on rat hippocampal 5-HT1A receptors negatively coupled to adenylyl cyclase. Drugs were injected intraperitoneally in rats for 2 or 3 weeks, and biochemical determinations were made 4 to 72 hr after the final dose. Chronic treatments with desimipramine, ipsapirone and fluoxetine did not induce any change in the 5-HT1A-induced inhibition of the adenylyl cyclase activity. In contrast, chronic treatment with clorgyline and electroconvulsive shock treatment induced a slight but significant reduction of 5-HT's ability to inhibit hippocampal adenylyl cyclase. This indicates that, at least in hippocampal neurons, the 5-HT1A receptor coupled to adenylyl cyclase is not easily desensitized. This was verified in vitro on murine hippocampal neurons in culture, by measuring the effects of intense stimulation (1 and 2 hours), with 5-HT, ipsapirone and 8-hydroxy-2-(di-n-propylamino)tetralin. Indeed, such stimulations did not significantly affect the 5-HT1A receptor-induced inhibition of cAMP production in these hippocampal neurons in culture. Our results indicate that it is not the post-synaptic 5-HT1A receptor of hippocampus that is modified during antidepressant treatments, at least at the level of its coupling to adenylyl cyclase.     

Discriminative stimulus properties of U50,488 and morphine: effects of training dose on stimulus substitution patterns produced by mu and kappa opioid agonists

By using a two-lever drug discrimination task, four groups of rats were trained to discriminate either a low (3.0 mg/kg) and a high (5.6 mg/kg) training dose of the kappa opioid agonist U50,488 [trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolindinyl)cyclohexyl] benzeneacetamine methanesulfonate hydrate] or a low (3.0 mg/kg) and a high (10 mg/kg) training dose of the mu opioid agonist morphine from water. The stimulus effects of the high training dose of U50,488 were shared by the kappa agonists bremazocine and ethylketocyclazocine (i.e., these drugs produced at least 80% drug-appropriate responding), but not by the mu agonists morphine, fentanyl and l-methadone or the nonopioid compounds d-amphetamine, pentobarbital and phencyclidine. Conversely, the stimulus effects of the high training dose of morphine were shared by other mu agonists, but not by the kappa agonists or the nonopioid compounds examined. Similarities in the stimulus effects of morphine and U50,488 occurred, however, when mu and kappa agonists were examined in rats trained to discriminate relatively low training doses of morphine or U50,488 from water. At the low training dose of morphine, complete substitution was obtained with the mu agonists tested as well as the kappa agonist ketocyclazocine. In these rats, intermediate (approximately 70% drug-appropriate responding) levels of substitution were obtained with the kappa agonists bremazocine and ethyylketocyclazocine. Similarly, at the low training dose of U50,488 both the mu and kappa agonists examined substituted completely. Asymmetrical substitution occurred between U50,488 and morphine at the low training doses, with morphine substituting completely for the low training dose of U50,488 and U50,488 failing to substitute for the low training dose of morphine. The rank order of potency for naloxone as an antagonist of the stimulus effects of morphine and U50,488 was; 3.0 mg/kg of morphine greater than 10 mg/kg of morphine greater than 3.0 mg/kg of U50,488 = 5.6 mg/kg of U50,488. The present results indicate that training dose is an important determinant of the different levels of cross-substitution obtained between mu and kappa agonists, and that a greater pharmacological specificity of drug-induced discriminative stimuli can be obtained when relatively high training doses of mu and kappa opioid agonists are used to establish the discrimination.     

Behavioral effects of beta adrenergic agonists and antidepressant drugs after down-regulation of beta-2 adrenergic receptors by clenbuterol

Acute treatment with the centrally active beta-2 adrenergic agonist clenbuterol reduced response rate and increased reinforcement rate of rats responding under a differential-reinforcement-of-low-rate (DRL) 72-sec schedule in a dose-dependent manner (ED50 value of about 0.1 mg/kg). With repeated treatment, rapid tolerance developed to this effect of clenbuterol. Redetermination of the dose-response function for clenbuterol, following 2 weeks of repeated daily administration, showed that clenbuterol no longer affected DRL behavior at doses up to 3 mg/kg. Interestingly, tolerance developed to clenbuterol even when it was administered after each daily session. This suggests that behavioral factors did not contribute, in an appreciable manner, to the development of tolerance to clenbuterol, and that neuropharmacological changes were sufficient for tolerance development. Such an interpretation is supported by the finding that the density of beta-2 adrenergic receptors in the cerebral cortices and cerebella of rats receiving the same repeated-treatment regimen was reduced with a time course similar to the loss of behavioral responsiveness. The effects of two additional beta-2 selective agonists, SOM-1122 and zinterol, on DRL behavior also were attenuated after repeated treatment with clenbuterol. By contrast, the effects of the beta-1 selective agonists dobutamine and prenalterol and the antidepressants desipramine, phenelzine and fluoxetine on DRL behavior were unaltered after repeated treatment with clenbuterol. These findings suggest functional independence of the beta adrenergic receptor subtypes and further suggest that, consistent with neuropharmacological data, the behavioral effects of the antidepressants do not depend on functionally responsive beta-2 adrenergic receptors.     

Antagonist-induced up-regulation of the putative epsilon opioid receptor in rat brain: comparison with kappa, mu and delta opioid receptors

We reported previously that in the presence of appropriate blocking agents [3H]ethylketocyclazocine selectively labels a non-mu, non-delta, non-kappa binding site with opioid receptor characteristics. We now report that chronic treatment with the opiate antagonist naltrexone dramatically increases the number of these binding sites in rat brain, as well as the number of mu, delta and kappa receptors. This finding further supports the concept that the [3H]ethylketocyclazocine site is an opioid receptor and previously reported evidence indicating that it might be the beta-endorphin-specific epsilon receptor that has been hypothesized to exist for some time. The affinity of naltrexone for the putative epsilon site in vivo appears to be similar to its affinity for mu and delta receptors, since a single 30-mg naltrexone pellet was sufficient to induce a maximal rate of up-regulation for all three types of sites. In contrast, a maximal rate of kappa receptor up-regulation requires at least four naltrexone pellets and, therefore, the affinity of naltrexone for this site in vivo appears to be considerably lower than for the other sites. Unexpectedly, we found that the naltrexone-induced increase in binding to all four types of receptors continued to increase through 60 days of naltrexone exposure, the longest treatment period examined, and that the increases in receptor binding were linear with exposure time. This finding is contradictory to the generally held view that antagonist-induced opioid receptor up-regulation in brain increases asymptotically, leveling off after a relatively brief treatment period.     

Actions of mu, delta and kappa opioid agonists and antagonists on mouse primary afferent neurons in culture

The effects of selective mu, delta and kappa opioid agonists and antagonists were studied on somatic calcium-dependent action potentials recorded from mouse dorsal root ganglion (DRG) neurons grown in dissociated cell culture. The mu selective agonist, PL 017, and the delta selective agonist, [D-Pen2, L-Pen5] enkephalin (DPLPE), reduced action potential duration of a subpopulation (21/56) of DRG neurons. Leucine-enkephalin reduced action potential duration of all neurons sensitive to PL 017 or DPLPE, whereas 85% of neurons responding to leucine-enkephalin responded to either PL 017 or DPLPE. Only 15% of neurons responded to both PL 017 and DPLPE. There was no significant difference in the response to PL 017 or DPLPE when compared to leucine-enkephalin. In another experiment, the kappa selective agonist dynorphin A (DYN A), PL 017 and DPLPE reduced action potential duration of a subpopulation (15/67) of DRG neurons. There was a heterogeneous response among neurons to PL 017, DPLPE and DYN A inasmuch as 21.4% of neurons responded to all three agonists, 35.7% responded to PL 017 and DYN A, 35.7% responded only to PL 017 and 7.1% responded only to DYN A. Responses to the mu selective agonist PL 017 were antagonized by the reversible opioid antagonist naloxone and the selective mu antagonist SMS 201-995 in a concentration-dependent fashion. Responses to PL 017 were not altered by the selective delta antagonist ICI 174864. Responses to PL 017 were reduced by the irreversible, selective mu antagonists beta-funaltrexamine and naloxonazine in a concentration-dependent fashion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Helicobacter pylori-induced apoptosis in T cells is mediated by the mitochondrial pathway independent of death receptors

BACKGROUND: Chronic infection with Helicobacter pylori is related to the pathogenesis of the noncardia carcinoma of the stomach. In this study we investigated the mechanisms of H. pylori-induced apoptosis in T lymphocytes, which could explain a mechanism of immune evasion facilitating chronic inflammation of the mucosa and gastric carcinogenesis. MATERIALS AND METHODS: The supernatant of H. pylori culture was used to study the mechanism of apoptosis induction in human leukaemia T cell lines Jurkat and CEM and in primary T cells. The cytotoxin associated gene A (CagA) and vacuolating cytotoxin A (Vac A) positive bacterial strain H. pylori 60190 (CagA(+), VacA(+)) and as a control the less toxic H. pylori strain Tx30a (CagA(-), VacA(-)) were used to produce the supernatant. Cell death was determined by DNA fragmentation and protein expression by Western blot. RESULTS: H. pylori 60190-induced apoptosis was neither blocked by inhibition of the death ligands TRAIL (TNF-related apoptosis-inducing ligand), CD95L/FasL and TNF-alpha (tumour necrosis factor-a) in wild type Jurkat cells nor in FADD(def) (Fas-associated death domain protein) and caspase-8(def) subclones of the Jurkat cell line. Yet, the pancaspase inhibitor zVAD-fmk could inhibit up to 90% of H. pylori-induced apoptosis. Stable transfection of Jurkat wild type cells with Bcl-x(L and) Bcl-2 resulted in marked reduction of H. pylori-induced apoptosis, showing that the mitochondrial pathway is the key regulator. This is supported by the finding that surviving primary human lymphocytes upregulate Bcl-2 when exposed to H. pylori supernatant. CONCLUSIONS: H. pylori-induced apoptosis of T cells is mediated by the mitochondrial pathway and could create a local environment that facilitates life-long infection by immune evasion.     

Tonic opioid inhibition of the pressor region of the rostral ventrolateral medulla of rabbits is mediated by delta receptors

From previous observations, we have suggested that an endogenous opioid input tonically inhibits the pressor neurons of the rostral ventrolateral medulla (RVLM) of rabbits. In the present studies, the specific opioid receptor subtype(s) which may be activated by such an endogenous innervation were examined using local administration of selective antagonists. The RVLM pressor region of chloralose-anesthetized and artificially ventilated rabbits was functionally identified by local administration of L-glutamate (5 nmol). Selective blockade of neither mu nor kappa receptors in the RVLM after bilateral injections of beta-funaltrexamine (300-900 pmol) or nor-binaltorphimine (1 nmol), respectively, had any effect on either mean arterial pressure or heart rate. Delta receptors were blocked with ICI 174,864 (30-300 pmol). After the highest dose, there was a significant pressor effect (+32 +/- 6 mm Hg, mean +/- S.E.), which was of immediate onset and rapid time course (approximately 15 min), and which was accompanied by a bradycardia. In contrast, vehicle injections or injection of an inactive analog (ICI 178,173) had no effects. These results, together with previous pharmacological and anatomical evidence, suggest that there exists an enkephalinergic input to the RVLM that tonically inhibits the presympathetic pressor neurons via activation of delta-opioid receptors.     

Electrophysiologic studies on the spinal antinociceptive action of kappa opioid agonists in the adult and 21-day-old rat

The spinal antinociceptive actions of the selective kappa opioid receptor agonists U50488H and U69593 were investigated in anesthetized adult rats and 21-day-old rat pups. Single unit extracellular recordings were made of dorsal horn neurons responding to both innocuous and noxious peripheral stimuli. Mixed effects on neuronal responses were seen after intrathecal administration of lower doses of the kappa agonists but with higher doses selective inhibitions of C fiber-evoked responses were produced by either U50488H or U69593 in both the adult (ED50 420 and 250 micrograms, respectively) and pup (ED50 63 and 13 micrograms, respectively). In the adult intrathecal U50488H similarly inhibited the more prolonged nociceptive response evoked by s.c. formalin. Intravenous U50488H (4 mg/kg) also produced a rapid and selective inhibition of nociceptive responses in the adult rat. Intrathecal administration of the nonselective opiate antagonist naloxone reduced the inhibitions mediated by U50488H and U69593 in the pup and the adult. However, intrathecal norbinaltorphimine, a selective kappa antagonist, only prevented the action of the kappa agonists in the pup and not in the adult. Kappa receptors are reported to be sparse in the adult rat spinal cord so developmental changes in this receptor may be responsible for the differential action of norbinaltorphimine in the rat pup and adult.     

Roles of central and peripheral mu, delta and kappa opioid receptors in the mediation of gastric acid secretory effects in the rat

The opioid receptors involved in the mediation of gastric acid secretory effects were studied in the pylorus-ligated rat. The effects of i.c.v. and i.v. administration of morphine and mu ([D-Ala2, NMePhe4, Gly5-ol]enkephalin and Tyr-Pro-NMePheD-Pro-NH2)-, delta ([D-Pen2,D-Pen5]enkephalin)- and kappa-selective [trans-3,4-dichloro-N-methyl-N-[2-91-pyrrolidinyl)-cyclohexyl]- benzeneacetamide methanesulfonate (U-50,488H), dynorphin-(1-9), dynorphin-(1-17), nalorphine, alpha-neoendorphin and ethyl-ketocyclazocine) opioid receptor agonists on gastric volume and acid output were examined. Morphine, [D-Ala2, NMePhe4, Gly5-ol]enkephalin and Tyr-Pro-NMePhe-D-Pro-NH2 decreased gastric acid secretion more potently after i.c.v. than after i.v. administration. The inhibitory effect of i.v. administered morphine on gastric acid secretion was not blocked by the quaternary opioid antagonist naltrexone methylbromide when given s.c. However, when naltrexone methylbromide was administered i.c.v., it blocked completely the effects of i.c.v. morphine and partially antagonized the effects of i.v. morphine, indicating a central site of action for morphine. The delta-selective agonist [D-Pen2,D-Pen5]enkephalin did not alter gastric acid secretion after i.c.v. or i.v. administration. The kappa-selective opioid agonist U-50,488H produced a dose-dependent increase in gastric acid secretion after i.v. but not i.c.v. administration. The other kappa-selective agonists tested did not produce a significant increase in gastric acid secretion after i.c.v. or i.v. administration. The increase in gastric acid secretion produced by U-40,488H was blocked by pretreatment with the opioid receptor antagonist naloxone, the nonselective muscarinic receptor antagonist atropine and the M1 selective muscarinic receptor antagonist pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Discriminative stimulus effects of mu and kappa opioids in the pigeon: analysis of the effects of full and partial mu and kappa agonists

Pigeons were trained to discriminate a dose of either 0.01 mg/kg of bremazocine or 0.05 mg/kg of fentanyl from water using a two-key drug discrimination procedure. During tests of substitution, the selective kappa-opioid agonists bremazocine, U50, 488 and tifluadom substituted for the bremazocine stimulus, whereas the less selective kappa-opioid agonists ethylketocyclazocine, levallorphan, proxorphan and nalorphine substituted for the fentanyl stimulus. The full mu-opioid agonists fentanyl, morphine, I-methadone and levorphanol, as well as the partial agonists nalbuphine, butorphanol and buprenorphine, substituted for the fentanyl stimulus. Compounds with partial-opioid agonist effects, namely nalbuphine, butorphanol, buprenorphine, proxorphan, levallorphan and nalorphine, produced 50% fentanyl-appropriate responding at doses 25 to 369.2 times smaller than the doses required to decrease response rates to 50% of control values. In contrast, the full mu-opioid agonists fentanyl, morphine, I-methadone and levorphanol produced 50% fentanyl-appropriate responding at doses only 1.3 to 10.9 times smaller than those required to decrease response rates by 50%. During tests of antagonism, both naloxone and Mr2266 produced a dose-dependent attenuation of the stimulus effects of bremazocine and fentanyl, whereas beta-funaltrexamine antagonized the stimulus effects of fentanyl but not bremazocine. Although bremazocine has been reported to have mu-opioid antagonist effects, it failed to antagonize the stimulus effects of the training dose of fentanyl. The present investigation establishes further that pigeons can discriminate selective kappa-opioid agonists from mu-opioid agonists and that in pigeons the classification of numerous opioid compounds on the basis of their kappa-like or mu-like stimulus effects differ from those in rat and monkey. In addition, under the drug discrimination procedure the actions of compounds classified as partial-opioid agonists can be differentiated from those of full mu-opioid agonists on the basis of the ratio of the dose required to engender fentanyl-like stimulus effects to the dose required to reduce response rates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cannabinoid agonist signal transduction in rat brain: comparison of cannabinoid agonists in receptor binding, G-protein activation, and adenylyl cyclase inhibition

To investigate differences in agonist affinity, potency, and efficacy across rat brain regions, five representative cannabinoid compounds were investigated in membranes from three different rat brain regions for their ability to maximally stimulate [(35)S]guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) binding and bind to cannabinoid receptors (measured by inhibition of [(3)H]antagonist binding) under identical assay conditions. In all three brain regions, the rank order of potency for the stimulation of [(35)S]GTPgammaS binding and the inhibition of [(3)H]SR141716A binding for these compounds were identical, with CP55940 approximately levonantradol > WIN55212-2 >/= Delta(9)-tetrahydrocannabinol (Delta(9)-THC) > methanandamide. The rank order of efficacy was not related to potency, and relative maximal agonist effects varied across regions. Receptor binding fit to a three-site model for most agonists, stimulation of [(35)S]GTPgammaS binding fit to a two-site model for all agonists, and high-affinity receptor binding did not appear to produce any stimulation of [(35)S]GTPgammaS binding. WIN55212-2, methanandamide, and Delta(9)-THC also were assayed for the inhibition of adenylyl cyclase in cerebellar membranes. The rank orders of potency and efficacy were similar to those for [(35)S]GTPgammaS binding, but the efficacies and potencies of methanandamide and Delta(9)-THC compared with WIN55212-2 were higher for adenylyl cyclase inhibition, implying receptor/G-protein reserve.     

Differential coupling of somatostatin1 receptors to adenylyl cyclase in the rat striatum vs. the pituitary and other regions of the rat brain.

Subtypes of somatostatin (SRIF) receptors are expressed in the rat brain and may mediate the diverse actions of SRIF. In the present study we show that subtypes of SRIF receptors in different regions of the rat brain are differentially sensitive to the cyclic hexapeptide SRIF analog, MK 678. SRIF1 receptors are sensitive to MK 678 and found in high density in the cortex, hippocampus and striatum, as well as in the anterior pituitary. The pituitary appears to express only the SRIF1 receptor. The cortex, hippocampus and striatum also express SRIF2, or MK 678-insensitive, receptors. The proportion of SRIF1 receptors varies in different brain regions. In the cortex and hippocampus, SRIF1 receptors comprise approximately 50% of the total SRIF receptor population, whereas in the striatum SRIF1 receptors comprise the majority (86%) of SRIF receptors. SRIF1 receptors in the pituitary, cortex and hippocampus mediate, at least in part, the ability of SRIF to inhibit forskolin-stimulated adenylyl cyclase activity as MK 678 produced significant inhibition of activity in these tissues. However, in the striatum, MK 678 had no significant effect on forskolin-stimulated adenylyl cyclase activity, despite a significant inhibition produced by SRIF. The specific labeling of these receptors in the striatum by [125I]MK 678 is abolished in the presence of high concentrations of the nonhydrolyzable GTP analog, GTP gamma S, suggesting that SRIF1 receptors in this brain region are coupled to G proteins. The SRIF1 receptors in the striatum may be coupled via G proteins to cellular transducing systems other than adenylyl cyclase.