Takashi Endoh

Antipruritic activity of the κ-opioid receptor agonist, TRK-820

The effects of the κ-opioid receptor agonist, TRK-820, (−)-17-(cyclopropylmethyl)-3, 14β-dihydroxy-4, 5α-epoxy-6β-[N-methyl-trans-3-(3-furyl) acrylamido] morphinan hydrochloride, on the itch sensation were compared with those of histamine H1 receptor antagonists, using the mouse pruritogen-induced scratching model. Peroral administration of TRK-820 reduced the numbers of substance P- or histamine-induced scratches dose dependently. No obvious suppression of the spontaneous locomotor activity was observed at the doses used for the experiments, indicating that the inhibition of scratches was not due to the effect on general behavior. Furthermore, the scratching inhibitory activity of TRK-820 was dose dependently antagonized by the specific κ-opioid receptor antagonist, nor-binaltorphimine, suggesting that the inhibitory activity was mediated via κ-opioid receptors. Histamine H1 receptor antagonists, chlorpheniramine and ketotifen, did not inhibit substance P-induced scratches, or did so only partially. Both antihistamines inhibited the histamine-induced scratches completely. These results suggest that TRK-820 has antipruritic activity which is mediated by κ-opioid receptors, and is effective in both antihistamine-sensitive and -resistant pruritus.

Potent antinociceptive effects of TRK-820, a novel κ-opioid receptor agonist

Abstract TRK-820, a new type of 4,5-epoxymorphinan derivative, was investigated in vivo for antinociceptive activities and its selectivity on various opioid receptors in mice. Trk-820 given s.c. or p.o. Was found to be 351- and 796-fold more potent than U50,488H with acetic acid-induced abdominal constriction test. The duration of the antinociceptive effect produced by TRK-820 was longer than that produced by μ-opioid receptor agonist morphine or other κ-opioid receptor agonists. In addition, with four other antinociceptive assays, low temperature hot plate (51 °C), thermal tail flick, mechanical tail pressure and tail pinch tests, TRK-820 was also found to be 68- to 328-fold more potent than U-50488H, and 41- to 349-fold more potent than morphine in producing antinociception, as comparing the weight of the different compound. However, TRK-820 was less active in inhibiting the high temperature (55 ° C) hot plate response. The antinociceptive effects produced by TRK-820 were inhibited by nor-BNI, but not by naloxone or naltrindole (NTI) with the abdominal constriction test, indicating that the antinociception is selectively mediated by the stimulation of κ-, but not μ- or δ- opioid receptors. Co-administration of TRK-820 with morphine slightly enhanced the antinociception induced by morphine in the mouse hot plate test. On the other hand, pentazocine significantly reduced the morphine-induced antinociception. Trk-820 produced sedation at doses, which are much higher than the doses for producing antinociception. These results indicate that the potent antinociception induced by TRK-820 is mediated via the stimulation of κ-, but not μ- or δ-opiod receptors.

Characterization of the antinociceptive effects of TRK-820 in the rat

We have already reported that TRK-820, (-)-17-cyclopropylmethyl-3, 14b-dihydroxy-4, 5a-epoxy-6b-[N-methyl-trans-3-(3-furyl)acrylamido]morphinan hydrochloride, a new selective kappa-opioid receptor agonist, has affinity for kappa-subtype opioid receptors other than the kappa(1)-opioid receptor. It would be of interest to examine whether the different kappa-opioid receptor subtype properties of TRK-820 participate in its antinociceptive action in the inflamed paw test and the formalin test. TRK-820 produced a potent antinociceptive effect, which was inhibited by the selective kappa-opioid receptor antagonist nor-binaltorphimine, but not by the mu-opioid receptor antagonist naloxone in the mechanical paw pressure test. TRK-820 also produced a potent antinociceptive effect in rats with adjuvant-induced arthritis. TRK-820 and morphine, a prototype mu-opioid receptor agonist, were equally effective in inhibiting the nociceptive responses in the arthritic rats and in the normal rats, while ICI-199441, 2-(3, 4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-(1-pyrrolidinyl)ethyl]- acetamide, a kappa-opioid receptor agonist, was about 5-fold less potent in the arthritic rats than in the normal rats. In the formalin test TRK-820 had a very similar antinociceptive potency to that of ICI-199441, unlike in the arthritic rats in which TRK-820 was 2.5 times more potent than ICI-199441. It is concluded that TRK-820 produced a potent antinociceptive action via the stimulation of kappa-opioid receptors in rats. TRK-820 has a unique antinociceptive profile different from that of the other kappa-opioid receptor agonists such as ICI-199441 in arthritic rats.

Effects of a highly selective nonpeptide δ opioid receptor agonist, TAN - 67, on morphine - induced antinociception in mice

The effects of a potent and highly selective nonpeptide delta opioid receptor agonist, 2- methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha- octahydroquinolino [2,3,3,-g] isoquinoline (TAN-67), on morphine-induced antinociception were examined using the warm-plate (51 degrees C) method. When a peptide delta 1 opioid receptor agonist, [D-Pen2, Pen5]enkephalin (DPDPE), was co-administered with i.c.v. morphine, low-dose morphine-induced antinociception was significantly increased. In contrast, i.c.v. co-administration of a peptide delta 2 opioid receptor agonist, [D-Ala2]deltorphin II (DELT), with morphine did not affect the morphine-induced antinociception. When morphine and TAN-67 were co-administered i.c.v., low-dose morphine-induced antinociception was significantly increased. Moreover, when TAN-67 and morphine were co-administered s.c., the morphine dose-response curve shifted to the left and the ED50 value of morphine decreased. These effects DPDPE and TAN-67 were antagonized by the delta opioid receptor antagonist naltrindole (NTI) and the delta 1 opioid receptor antagonist 7-benzylidenenaltrexone (BNTX) not by the delta 2 opioid receptor antagonist naltriben (NTB). Moreover, the mu opioid receptor antagonist beta-FNA also antagonized the effects of DPDPE and TAN-67. These results suggest that the effect of TAN-67 may result from the activation of central delta 1 opioid receptors, since the effect of TAN-67 was antagonized by NTI and BNTX, but not NTB. Furthermore, since pretreatment with beta-FNA also antagonized the effects of both DPDPE and TAN-67, a beta-FNA-sensitive site, i.e. a mu-delta complex site, may play an important role in the modulation of morphine-induced antinociception.

Effect of repeated administration of TRK-820, a κ-opioid receptor agonist, on tolerance to its antinociceptive and sedative actions

Repeated administration of micro-opioid receptor agonist, morphine induces tolerance not only to the antinociceptive effect but also to other pharmacological effects, resulting in shortened working duration and decreased efficacy. But less is known about kappa-opioid agonist-induced tolerance. The tolerance-development potency of kappa-opioid receptor agonists with a focus on TRK-820 was characterized. After five administrations of kappa-opioid receptor agonists, TRK-820 (0.1-0.8 mg/kg), U-50,488H (10-80 mg/kg) and ICI-199,441 (0.025-0.2 mg/kg) subcutaneously over 3 days, tolerance to the antinociceptive effects, assessed by an acetic acid-induced abdominal constriction test, developed in a repeated dose-dependent manner. The tolerance-development potency of TRK-820 was the least among these kappa-opioid receptor agonists. Similarly, TRK-820 and U-50,488H induced tolerance to their sedative effects as judged by a wheel-running test in mice. Greater tolerance was developed to the sedative effect than to the antinociceptive effect in both compounds. After repeated administration, the number of kappa-opioid receptors in the mouse brain was reduced by U-50,488H (80 mg/kg) but not by TRK-820 (0.4 mg/kg). There was no change of the affinity by the treatment with both compounds. These results demonstrated that the kappa-opioid receptor agonists developed tolerance both to the antinociceptive and the sedative effects, though the tolerance to the sedative effect developed more readily than tolerance to the antinociceptive effect. The difference in the potency for down-regulating the kappa-opioid receptors in the brain may account for the tolerance-development potency of the compounds.

The modulatory effect of (+)-TAN-67 on the antinociceptive effects of the nociceptin/orphanin FQ in mice

To clarify the pharmacological properties of (+)2-Methyl-4aalpha-(3-hydroxyphenyl)-1, 2, 3, 4, 4a, 5, 12, 12aalpha-octahydro-quinolino[2, 3, 3-g]isoquinoline ((+)-TAN-67), the effect of (+)-TAN-67 on the antinociception induced by the intrathecal (i.t.) administration of nociceptin/orphanin FQ was studied in mice using the tail-flick test and the formalin test. I.t. administration of (+)-TAN-67, at doses of 1 to 10 ng, facilitated the tail-flick response in a dose-dependent manner in mice. In addition, i.t. administration of (+)-TAN-67 (1 to 10 ng) in mice produced a marked pain-like aversive responses. I.t. pretreatment with D-Pro(9)-[spiro-gamma-lactam]-Leu(10)-Trp(11)-physalaemin(1-11) (GR82334, 0.1-1.0 nmol), a potent and selective tachykinin NK(1) receptor antagonist, dose-dependently blocked the reduction of the tail-flick response induced by (+)-TAN-67. Furthermore, (+)-TAN-67-induced facilitation of the tail-flick response was abolished in capsaicin-treated mice. On the other hand, (+)-TAN-67-induced flinching responses were dose-dependently and significantly reduced by i.t. pretreatment with GR82334 (0.1-1.0 nmol). The duration of i.t. (+)-TAN-67-induced flinching responses was significantly reduced in capsaicin-treated mice as compared with naive mice. I.t. administration of nociceptin/orphanin FQ (1-10 nmol) dose-dependently increased the tail-flick latency. I.t. administration of nociceptin/orphanin FQ (0.1-1.0 nmol) significantly and dose-dependently reduced the first-phase nociceptive response, but not the second-phase nociceptive response. I.t. pretreatment with (+)-TAN-67 (0.3-3.0 microg) for 30 min dose-dependently attenuated the antinociception induced by i.t. nociceptin (10 nmol) in the tail-flick test. Furthermore, the antinociceptive effect of nociceptin/orphanin FQ (1 nmol, i.t.) on the first-phase response in the formalin test was dose-dependently attenuated by s.c. pretreatment with (+)-TAN-67 (0.3-3.0 microg). (+)-TAN-67 (0.3-3.0 microg, i.t.), by itself, did not facilitate the tail-flick response or produce apparent behavioral changes. It is possible that (+)-TAN-67 has an antagonistic effect on nociceptin/orphanin FQ-induced antinociception.

Effect of beraprost sodium (BPS) on the postischemic neuropathological changes and stroke index after left carotid artery occlusion in gerbils

We investigated the effect of beraprost sodium (BPS), a new prostacyclin analog, on behavioral and neuropathological changes induced by a 10-min occlusion of the left carotid artery in gerbils. Gerbils were treated orally with BPS (1-100 micrograms/kg) 30 min before occlusion. Pathological evaluation of neural damage in the CA1 region of the hippocampus was performed 7 d after the ischemic insults. In the symptomatic group, in which the stroke index score was > 10, symptomatic behaviors, such as head cocked, splayed out hind limb, circling, and various similar behaviors, were observed. Pathologically, almost all CA1 neurons were destroyed 7 d after ischemia in the symptomatic group. BPS improved the stroke index during ischemia and neuropathological changes 7 d later, with statistical significant improvement occurring at a dose of 100 micrograms/kg.

Effect of beraprost sodium (BPS), a prostacyclin analog, and dizocilpine (MK-801) on repeated ischemia-induced chronic cortical atrophy in gerbils

We investigated the effect of beraprost sodium (BPS), a new prostacyclin analog, and dizocilpine (MK-801) on repeated ischemia-induced cerebral atrophy and chronic cortical neuronal loss in gerbils. The left common carotid artery of gerbils was repeatedly occluded (for 10, 7, 7, and 7 min) at intervals of 24 h. The thickness of the cerebral cortex of the ischemic hemisphere diminished with increasing time of reperfusion after an ischemic insult. The animals were given BPS (1-100 micrograms/kg, po) or MK-801 (3-300 micrograms/kg, sc) after the first ischemic insult, and then twice daily for 4 wk. Increases in the amount of neuronal loss and acidophilic neurons, and progressive atrophy were observed with increasing time of reperfusion in the cerebral cortex of the ischemic hemisphere. Cortical sections revealed no astrocytes positive for glial fibrillary acidic protein (GFAP), whereas the hippocampal CA1 area showed neuronal loss accompanied by GFAP-positive astrocytes. In control animals at 4 wk survival, the area ratio (area of ischemic cortex/area of opposite cortex) and the cortical neurons ratio (number of neurons in ischemic cortex/number of neurons in opposite cortex) were 89.8 +/- 3.0% and 74.6 +/- 3.4%, respectively. BPS was found to inhibit atrophy and chronic cortical neuronal loss in the ischemic hemisphere in a dose-dependent manner, whereas MK-801 showed no inhibitory effects at any dose tested. These results may suggest that the nature of neuronal degeneration differs between the cortical and hippocampal areas, that cortical neuronal degeneration might not involve glutamate pathways with NMDA receptors in this model, and that prostacyclin could play an essential role in prevention of ischemia-induced progressive neuronal loss.