Effects Of U50 Research Articles

Cross-tolerance of associative and nonassociative morphine tolerance in the rat with mu- and kappa-specific opioids

The present study examined the cross-tolerance profiles of associatively and nonassociatively morphine-tolerant rats with analgesia produced by morphine and fentanyl (mu-receptor agonists) and U50,488H (a kappa-receptor agonist). Subjects were given a series of eight morphine injections either paired or unpaired with a distinctive environment and then tested for tolerance using the tail-flick method. Evidence was found that nonassociative morphine tolerance, which was produced using a 6-h interdose interval (IDI), was receptor-specific, i.e. cross-tolerant with analgesia produced by mu-specific, but not kappa-specific drugs. Nonassociative tolerance was characterized by a shift to the right in dose response curves of 0.32 log units in morphine-tested animals and 0.28 log units in fentanyl-tested animals. Conversely, associative morphine tolerance, which was produced using a 96-h IDI, evidenced a lack of receptor specificity by showing cross-tolerance to the analgesic effects of U50,488H. Associative tolerance was characterized by shifts of 0.42 log units in morphine-tested animals, 0.34 log units in fentanyl-tested animals, and 0.39 log units in U50,488H-tested animals. These results were interpreted as suggesting the mechanisms responsible for associative tolerance differ from those producing nonassociative tolerance. This conclusion is problematic for theories of learned tolerance that assume a unitary set of mechanisms subserving associative and nonassociative tolerance.

κ-Opioid agonist U50488 inhibits P-type Ca 2+ channels by two mechanisms

The effects of U50488, κ-opioid agonist on P-type Ca 2+ channels, were studied. U50488 inhibited depolarization-induced Ca 2+ uptake into rat brain synaptosomes, which was sensitive to ω-Agatoxin IVA (ω-AgaIVA; P-type Ca 2+ channel blocker) and inhibited P-type Ca 2+ channel currents recorded from rat cerebellar Purkinje neurons by the whole-cell patch clamp method. Dynorphin A also inhibited P-type Ca 2+ channel currents. The inhibition by U50488 was biphasic; high affinity component (21%, ICS, = 8.9 × 10 −8 M) and low affinity component (79%, IC 50 = 1.1 × 10 −5 M). At low concentrations of U50488 (10 −6 M), P-type Ca 2+ channel current inhibition was attenuated by norbinartorphimine (nor-BNI), κ-opioid antagonist, and by dialysis of cells with a pipette solution containing guanosine 5′- O-(2-thiodiphosphate) (GDP-βS). At high concentrations of U50488 (10 −5 M), P-type Ca 2+ channel current inhibition was frequency-dependent. Thus U50488-induced current inhibition is mediated by two mechanisms. Its high affinity component is produced by activation of κ-opioid receptors, whereas the low affinity component is due to its direct action on the P-type Ca 2+ channel.

A reappraisal of the role of the various opioid receptor subtypes in cell-mediated immunity.

Opioid peptides have been shown by several studies to modulate various parameters of the immune response, but scant experimental findings exist on the role played by specific opioid receptor subtypes in the control of immune mechanisms. This study focuses on the in vitro influences of [Trp4,Asn7]dermorphin, a mu-selective agonist, [D-Ala2]deltorphin I, a delta-selective agonist and U50,488, a kappa-selective agonist, on the proliferative response of splenocytes to concanavalin A (Con A). [Trp4,Asn7]dermorphin at low concentrations (10(-11P) and 10(-12) M) enhanced the proliferative response to Con A, whereas higher concentrations (10(-6) to 10(-7) M) inhibited it. Both effects were antagonized by naloxone. [D-Ala2]deltorphin I at very low concentrations (10(-12) to 10(-13) M) also produced a significant increase in the proliferative response of splenocytes to Con A. This effect was significantly antagonized by natrindole, a specific delta-receptor antagonist. Finally U50,488 at concentrations ranging from 10(-8) to 10(-9) M inhibited the proliferative response to Con A. The effects of U50,488 were mediated by the stimulation of the kappa-opioid receptors, since a preincubation of splenocytes with the selective antagonist norbinaltorphimine significantly reduced or abolished the U50,488-induced suppression of the mitotic response. In conclusion, our results clearly indicate that the different opioid receptor subtypes play a different role in the control of immune mechanisms and suggest that immunoenhancing effects of opioid peptides are very likely due to the stimulation of mu- and delta-receptors, whereas the immunosuppressive effects are mediated through the stimulation of kappa-opioid receptors.

A kappa opiate agonist, U50, 488H, enhances energy expenditure in rats

The effect of U50, 488H (a selective kappa opiate agonist) on oxygen consumption was measured in either resting and free-moving rats. In both states, U50, 488H provokes an increase in oxygen consumption. In resting rats, the increase occurs at lower doses than in free-moving rats. The explanation could be that in the free-moving rats the drug results in an increase in energy expenditure, partially compensated by a decrease in activity.

The effects of dynorphin a (1–13) and U50, 488H on free intracellular calcium in guinea pig cerebellar synaptosomes

The effects of the kappa-selective ligands dynorphin A (1–13) (DYN) and U50, 488H (U50) on free intracellular calcium were evaluated using synaptosomes prepared from the cerebellum of the guinea pig, an area with a high density of kappa receptors. DYN (10 μM) produced small nonsignificant decreases in basal free intracellular calcium (5–7%). U50 (10μM) produced significant 15–20% decreases in basal free intracellular calcium which were reversed by nor-BNI (1μM). When intracellular calcium levels were increased 8–10% by the administration of c-AMP or forskolin, DYN (10 μM) produced significant decreases in intracellular calcium of 10%. The effects of U50, 488H were not enhanced by increasing the synaptosomal levels of c-AMP. Neither DYN nor U50 (1 μM) significantly blocked the rise in free intracellular calcium induced by 50 mM KCl. When intracellular calcium concentrations were increased by the administration of 50 mM KCl prior to the administration of DYN or U50 (10 μM), the kappa ligands decreased intracellular calcium concentrations. These data indicate that DYN and U50 interact with kappa receptors resulting in a decrease in free intracellular calcium possibly via an enhancement of the efflux of calcium. The modulation of intracellular free calcium by the kappa opioids may be a mechanism by which these opioids produce their biological effects.

Opiate agonists activate feeding in Limax: Comparison of in vivo and in vitro effects.

The neural control system for feeding in the terrestrial mollusc Limax maximus is modulated by at least two major families of peptides. Sequence homology between one of the peptides known to modulate Limax feeding and some members of the opioid peptide family suggested that opioid peptides might also modulate Limax feeding. Experiments with the mu agonist morphine and the kappa agonist U50,488H showed that the probability of feeding, but not meal size, was increased by morphine injection into intact animals, whereas the length of feeding motor program responses elicited from the isolated lip-brain preparation of Limax was augmented by U50,488H. The behavioral effect of morphine was blocked by naltrexone injection, whereas the physiological effect of U50,488H was blocked by naloxone. Factors that influence the behavioral and electrophysiological effects of opioids on mollusc feeding are discussed.

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.

Effects of u50,488, a selective kappa agonist, on atypical mouse opiate systems

We have proposed that endogenous opioids play a critical role in the etiology of anorexia nervosa, mediating an auto-addiction, and that atypical opioid systems in mice may be representative of those in anorexia nervosa patients, in contrast to normal humans and rats. A biological predisposition to eating disorders may result from these atypical opioid systems. Definition of these systems as atypical is based on their responses to morphine, which are preferential for the mu receptor subtype. Three patterns have been described in four strains of mice: anorexia with hyperactivity (BALB/C and C57BL/J), anorexia without hyperactivity (DBA/J), and a biphasic curve (CF-1). The latter showed anorexia and hyperactivity at high doses but increased food intake without a change in motor activity at low doses. These patterns contrast to the increase in food intake and sedation in typical species, including rats and normal humans. In the present study, U50,488, a selective kappa agonist, increases food intake in all four mouse strains, as previously reported in rats. Thus, these two agonists have opposite effects on the atypical mouse systems, but similar effects on the typical rat system. The typical and atypical opioid systems respond oppositely to morphine but similarly to U50,488.

Opioid receptor ligands in the neonatal rat spinal cord: binding and in vitro depression of the nociceptive responses

1. Opioid receptors in the neonatal rat spinal cord have been characterized by measurements of ligand binding to crude membrane fractions and by functional tests on the nociceptive spinal response in a spinal cord-tail preparation in vitro. 2. There were high affinity binding sites for [3H]-[D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGOL), [3H]-U69593, and [3H]-ethylketocyclazocine (EKC) on spinal cord membranes from neonatal rats. Hill slopes for binding of [3H]-DAGOL and [3H]-U69593 were close to unity. The Hill slope for binding of [3H]-EKC was less than unity, even after its interactions at mu-receptors had been blocked with 100 nM unlabelled DAGOL. Binding sites for [3H]-[D-Pen2, D-Pen5]enkephalin (DPDPE) could not be detected. 3. In competition assays U50488 was as potent as PD117302 and U69593 in competition for either [3H]-U69593 or [3H]-EKC binding sites. Hill slopes for a range of competing ligands at [3H]-DAGOL or [3H]-U69593 sites were close to unity. Hill slopes for competition at [3H]-EKC sites were less than one. 4. In the spinal cord-tail preparation from neonatal rats, opioid receptor agonists depressed spinal nociceptive responses evoked by application of capsaicin or heat to the tail. The order of potency was DAGOL greater than U69593 = PD117302 greater than morphine greater than U50488 = [D-Pen2, L-Pen5]enkephalin (DPLPE). 5. The antagonist naloxone was about equally potent against DAGOL, morphine and DPLPE, and about ten times less potent against U69593 and PD117302. The effects of U50488 were much less sensitive to blockade by naloxone than the effects of PD11703 or U69593. The Kappa antagonist, nor-binaltorphimine was equipotent against all three Kappa agonists. 6. The absence of delta-binding sites, and the low potency and relatively high sensitivity to naloxone suggest that DPLPE could be working at mu-receptors in the neonatal rat spinal cord. 7. The binding assays show that U50488 has the same affinity as PD1 17302 and U69593 for Kappa-receptors, yet it was less effective in the depression of nociceptive responses. This may be because U50488 has a relatively low efficacy at Kappa-receptors. It is possible that at high concentrations U50488 activates receptors not affected by other Kappa-ligands. These additional receptors may be non-opioid receptors (hence the insensitivity to naloxone), or they could be a subtype of Kappa-opioid receptor.

Kappa-Opioid-Receptor-Mediated Regulation of α-Melanocyte-Stimulating Hormone Secretion and Tuberohypophysial Dopaminergic Neuronal Activity

The effects of the kappa-opioid receptor agonist trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene- acetamide methanesulfonate hydrate (U-50488) were examined on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion and the activity of tuberohypophysial dopamine (DA) neurons in the male rat. Tuberohypophysial DA neuronal activity was estimated by measuring: (1) the rate of DA synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) following inhibition of aromatic L-amino acid decarboxylase], and (2) DA metabolism [concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC)] in the intermediate lobe of the pituitary. U-50488 produced a dose- and time-dependent increase in plasma concentrations of alpha-MSH which was accompanied by a decrease in the accumulation of DOPA and in the intermediate lobe. The effects of U-50488 were blocked by pretreatment with the DA agonist apomorphine but not by the beta-adrenergic antagonist propranolol. The effects of U-50488 on plasma alpha-MSH concentrations and intermediate-lobe DOPA accumulation were blocked by pretreatment with the selective kappa-opioid receptor antagonist nor-binaltorphimine. These results indicate that U-50488, by acting on kappa-opioid receptors, inhibits the activity of intermediate-lobe tuberohypophysial DA neurons, and through this action increases the secretion of alpha-MSH from melanotrophs.