(±)-U-50488 hydrochloride

(±)-U-50488 hydrochloride is a selective agonist for κ-opioid receptor [1].

The κ-opioid receptor (KOR) is a type of opioid receptor for opioid peptide dynorphin and controls addiction. Also, KOR plays an important role in stress, anxiety, anhedonia, depression and increased drug-seeking behavior.

(±)-U-50488 hydrochloride is a selective KOR agonist [1]. In isolated rat DRG neurons, U-50488 (0.3-40 μM) inhibited voltage-independent Ca2+ channel currents. In HeLa cells that didn’t express KOR, U-50488 (20 μM) blocked Ca2+ channels [2].

In rhesus monkeys, U-50488 exhibited potent antinociceptive activity and produced diuresis [1]. U-50488 enhanced contraction of the rabbit vas deferens induced by electrically with IC50 value of 26.5 nM. In mice, U-50488 impaired motor function with ED50 value of 15.3 mg/kg and reduced spontaneous activity [3]. In adult rats, U-50488 increased the threshold required to maintain self-stimulation responding, a depressive-like effect. While, males were significantly more sensitive than females to the threshold-increasing effects [4].

References:
[1].  Tang AH, Collins RJ. Behavioral effects of a novel kappa opioid analgesic, U-50488, in rats and rhesus monkeys. Psychopharmacology (Berl), 1985, 85(3): 309-314.
[2].  Hassan B, Ruiz-Velasco V. The κ-opioid receptor agonist U-50488 blocks Ca2+ channels in a voltage- and G protein-independent manner in sensory neurons. Reg Anesth Pain Med, 2013, 38(1): 21-27.
[3].  Lu SN, Ma SC, Zhang KG, et al. Comparison of pharmacological profile of selective kappa-opioid agonist K-II and U-50488. Yao Xue Xue Bao, 1991, 26(3): 171-174.
[4].  Russell SE, Rachlin AB, Smith KL, et al. Sex differences in sensitivity to the depressive-like effects of the kappa opioid receptor agonist U-50488 in rats. Biol Psychiatry, 2014, 76(3): 213-222.

Blockade of the development of morphine tolerance by U-50,488, an AVP antagonist or MK-801 in the rat hippocampal slice.[Pubmed:9517380]

Br J Pharmacol. 1998 Feb;123(4):625-30.

1. In this study, we investigated the effects of different drugs (a kappa-opioid receptor agonist U-50,488, a vasopressin receptor antagonist dPTyr(Me)AVP or an N-methyl-D-aspartate (NMDA) receptor antagonist MK-801) on the development of morphine tolerance in rat hippocampal slices. 2. Hippocampal slices (450 microm) of Sprague-Dawley rats (250-300 g) were used. Slices were continuously superfused with artificial CSF or drugs at 1 ml min(-1). Nichrome wire electrodes were placed in the Schaffer-collateral pathway and used to deliver biphasic 0.2 ms pulses of 5-30 V (0.033 Hz). A glass microelectrode was placed in the CA1 area to record population spikes. 3. When the slices were superfused with 10 microM morphine, the amplitude of population spikes increased 2-3 fold in 30-40 min. However, this effect of morphine decreased, i.e. tolerance developed after continuous superfusion of morphine for 2-6 h. 4. When either U-50,488 (200 nM) or dPTyr(Me) AVP (500 pM) or MK-801 (500 pM) was co-superfused with morphine (10 microM), it significantly blocked the development of morphine tolerance. Nor-BNI (a kappa-opioid receptor antagonist, 200 nM) significantly reversed the inhibitory effect of U-50,488 but not those of dPTyr(Me)AVP or MK-801 on the development of morphine tolerance. 5. These data indicate that kappa-opioid receptors, AVP receptors and NMDA receptors are all involved in the development of morphine tolerance. The suppression of kappa-opioid receptor activity after chronic morphine may occur before the activation of AVP receptors or NMDA receptors during the development of morphine tolerance.

U-50,488 blocks the development of morphine tolerance and dependence at a very low dose in guinea pigs.[Pubmed:8045272]

Eur J Pharmacol. 1994 May 2;256(3):281-6.

U-50,488, (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide hydrochloride), is a selective kappa-opioid receptor agonist. In this study, we found that U-50,488 antagonized morphine-induced antinociception in morphine-naive guinea pigs at doses which did not have any antinociceptive effect by themselves (0.01-3 mg/kg). On the other hand, U-50,488 (3 mg/kg) partially restored morphine-induced antinociception in morphine-tolerant guinea pigs (8 mg/kg/day i.p. morphine HCl for 6 days). Furthermore, the development of tolerance to morphine antinociception was completely blocked by coadministration of U-50,488 at a very low dose (0.003 mg/kg i.p.) which neither exerted an antinociceptive effect by itself nor affected the antinociception induced by 8 mg/kg of morphine HCl. The withdrawal signs induced by 8 mg/kg (i.p.) naloxone HCl on the 7th day were also depressed by coadministration of 0.003 mg/kg U-50,488 with morphine HCl (8 mg/kg i.p.) every day for 7 days. These effects of U-50,488 could be applied to humans to prevent morphine tolerance and dependence.

Activation of kappa opioid receptors by U50488H and morphine enhances the release of substance P from rat trigeminal nucleus slices.[Pubmed:7679733]

J Pharmacol Exp Ther. 1993 Feb;264(2):648-53.

The modulation of the release of substance P (SP) from sensory primary afferents by activation of kappa opioid receptors is not only equivocal, but also contradictory. Thus, in the present study, we have determined the effect of nanomolar concentrations of the highly selective kappa opioid receptor agonist trans-(+)-3,4-dichloro-N-methyl-N-[2-91- (pyrrolidinyl)cyclohexyl]benzacetamide methane sulphonate (U50488H), as well as micromolar concentrations of moderately mu-selective agonist morphine, on K(+)-evoked SP release from rat trigeminal nucleus caudalis slices. U50488H (10-30 nM) and morphine (10-30 microM) increased K(+)-evoked SP release without stimulating basal SP release. Both U50488H and morphine concentration-response curves were biphasic because the highest and the lowest concentrations of U50488H (100 nM) and morphine (3 microM) tested, respectively, inhibited SP release. Enhancement of K(+)-evoked SP release induced by 30 nM U50488H and 30 microM morphine was blocked by the opioid receptor antagonists naloxone (30 nM; nontype selective) and norbinaltorphimine (3 nM; kappa selective), but not by N,N diallyl Tyr-Aib-Aib-Phe-Leu (0.3 microM; delta selective), naloxonazine (1 nM; mu 1 selective) or beta-funaltrexamine (20 nM; mu selective). These findings indicate that activation of at least one population of kappa opioid receptors increases the release of SP from trigeminal nucleus caudalis. Excitatory presynaptic kappa opioid receptors on SP-containing primary afferents may be involved in the hyperalgesia of inflammatory processes, the "anti-analgesic" effect of dynorphin and the paradoxical "analgesia" produced by low doses of naloxone.

Cardiovascular actions of the kappa-agonist, U-50,488H, in the absence and presence of opioid receptor blockade.[Pubmed:1320979]

Br J Pharmacol. 1992 Mar;105(3):521-6.

1. The cardiovascular actions of U-50,488H, a kappa-receptor agonist, were studied in rat isolated perfused hearts, and in anaesthetized rats, over concentrations or doses generally above those required to produce kappa-receptor-mediated effects. 2. U-50,488H dose-dependently decreased left-ventricular peak systolic pressure and beating rate in vitro and reduced blood pressure and heart rate in vivo. 3. Over the concentration range of 1-30 microM in vitro, and the dose-range of 0.5-32 mumol kg-1 in vivo, U-50,488H prolonged the P-R, QRS and Q-T intervals of the ECG. 4. The effects of U-50,488H were not antagonized by an opioid receptor antagonist, naloxone (1 microM or 8 mumol kg-1). Similarly, the opioid receptor antagonist, MR 2266, at 8 mumol kg-1 did not significantly reduce the cardiovascular actions of U-50,488H in vivo. 5. The actions of U-50,488H on responses to electrical stimulation were also studied. Over the dose range of 0.5-32 mumol kg-1, U-50,488H altered thresholds and effective refractory period. It had a biphasic action on thresholds for induction of ventricular fibrillation. Thresholds were decreased at lower doses (0.5-4 mumol kg-1) but increased at higher doses (8-32 mumol kg-1). The effects of lower doses were blocked by naloxone. Effective refractory period and threshold pulse width only increased with dose. 6. In conclusion, U-50,488H at high concentration, had direct depressant actions on cardiac contractility, electrical excitability and the ECG. These depressant effects were not antagonized by the opioid receptor antagonists, naloxone and MR 2266, and probably do not involve opioid receptors. Furthermore, some of the observed effects were those expected to result from sodium channel blockade.