The novel kappa-opioid receptor agonist TRK-820 suppresses the rewarding and locomotor-enhancing effects of morphine in mice

The effects of the novel kappa-opioid receptor agonist TRK-820 on the rewarding and locomotor-enhancing effects of morphine were investigated in mice. Morphine (1-5 mg/kg, s.c.) caused a dose-related preference for the drug-associated place. In contrast, TRK-820 (0.003-0.03 mg/kg, s.c.) did not produce a significant preference for either compartment of the test box. In combination studies, co-injection of TRK-820 (0.01 and 0.03 mg/kg, s.c.) with morphine significantly suppressed the morphine (5 mg/kg, s.c.)-induced place preference, and this effect of TRK-820 was antagonized by pretreatment with nor-BN1 (3 mg/kg, s.c.), a selective kappa-opioid receptor antagonist. TRK-820 also suppressed morphine-induced hyperlocomotion, and this suppression was also blocked by nor-BNI. These results suggest that TRK-820 suppresses the rewarding and locomotor-enhancing effects of morphine through the activation of kappa-opioid receptors. Thus, we propose that TRK-820 may be useful for controlling pain while reducing undesirable side-effects.     

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

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 mu-opioid receptor agonist morphine or other kappa-opioid receptor agonists. In addition, with four other antinociceptive assays, low temperature hot plate (51 degrees 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 degrees 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 kappa-, but not mu- or delta-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 kappa-, but not mu- or delta-opiod receptors.     

Salvinorin A: a novel and highly selective kappa-opioid receptor agonist

kappa-opioid receptors (KORs) represent the principal site of action of dynorphin and related neuropeptides. Recently, Salvinorin A--a naturally occurring neoclerodane diterpene hallucinogen was identified to be a highly selective KOR agonist. In this brief review we summarize the known chemistry, pharmacology and biology of salvinorin A. Because salvinorin A profoundly alters human consciousness and perception, a study of how salvinorin A exerts its actions on KORs may yield novel insights into the molecular and cellular basis of uniquely human higher cortical functions.     

Inhibitory effects of TRK-820 on systemic skin scratching induced by morphine in rhesus monkeys

The inhibitory effects of kappa-opioid receptor agonists on systemic skin scratching induced by the intravenous administration of morphine, a micro-opioid receptor agonist, were investigated in rhesus monkeys. Intravenous pretreatment with kappa-opioid receptor agonists, either TRK-820 at 0.25 and 0.5 microg/kg or U-50488H at 64 and 128 microg/kg, inhibited systemic skin scratching induced by morphine at 1 mg/kg, i.v. in a dose-dependent manner. By the intragastric route, apparent inhibitory effects on morphine-induced systemic skin scratching were evident following pretreatment with TRK-820 at 4 microg/kg but not with U-50488H from 512 to 2048 microg/kg. These results suggest that TRK-820 produces antipruritic effects on i.v. morphine-induced systemic skin scratching and is more readily absorbed intragastrically than is U-50488H, resulting in high bioavailability in the intragastric route.     

The effect of mepyramine on the development of morphine tolerance and physical dependence in mice.

Latency relaxation (LR) as well as resting tension and twitch tension of frog toe muscles are studied in an isotonic solution (= 1 T) and in solutions made hypotonic by leaving out the appropriate amounts of NaCl and KCl (0.54 T and 0.76 T). In hypotonic solutions there is an increase in peak twitch tension as well as a decrease in the depth of the LR: the resting tension is increased at sarcomere lengths which are greater than 2.8 μm and is decreased at sarcomere lengths which are less than this value. The behaviour of twitch tension is discussed with respect to the influence of the sarcoplasmic ionic strength on the interaction between the contractile filaments. Concerning the decrease in both the LR and the resting tension, it is assumed that these effects are induced osmotically, the tension of the membranes of the longitudinal sarcoplasmic reticulum being the particular parameter which is influenced.     

Effect of methamphetamine on the development of acute morphine tolerance and dependence in mice

The effect of methamphetamine on morphine analgesia (tail-flick assay) was studied in non-tolerant mice and in mice made acutely tolerant to morphine following a single injection of 100 mg/kg morphine. The analgesic potency of morphine was increased in non-tolerant and tolerant mice to the same extent by 3.2 mg/kg methamphetamine (3.3 and 4.4 fold increases, respectively). In contrast, the ED50's for morphine analgesia and naloxone-precipitated jumping in mice pretreated with either 100 mg/kg morphine or both morphine and 3.2 mg/kg methamphetamine were not significantly different, indicating that methamphetamine had no effect on the development of acute morphine tolerance and dependence. Although methamphetamine had no effect on the development of acute tolerance to morphine, 4-day pretreatment with methamphetamine produced cross-tolerance to morphine analgesia. However, cross-tolerance to morphine was not accompanied by enchanced sensitivity to naloxone.     

A neuropeptide FF agonist blocks the acquisition of conditioned place preference to morphine in C57Bl/6J mice

Neuropeptide FF behaves as an opioid-modulating peptide that seems to be involved in morphine tolerance and physical dependence. Nevertheless, the effects of neuropeptide FF agonists on the rewarding properties of morphine remain unknown. C57BL6 mice were conditioned in an unbiased balanced paradigm of conditioned place preference to study the effect of i.c.v. injections of 1DMe (D-Tyr1(NMe)Phe3]NPFF), a stable agonist of the neuropeptide FF system, on the acquisition of place conditioning by morphine or alcohol (ethanol). Morphine (10 mg/kg, i.p.) or ethanol (2 g/kg, i.p.) induced a significant place preference. Injection of 1DMe (1-20 nmol), given 10 min before the i.p. injection of the reinforcing drug during conditioning, inhibited the rewarding effect of morphine but had no effect on the rewarding effect of ethanol. However, a single injection of 1DMe given just before place preference testing was unable to inhibit the rewarding effects of morphine. By itself, 1DMe was inactive but an aversive effect of this agonist could be evidenced if the experimental procedure was biased. These results suggest that neuropeptide FF, injected during conditioning, should influence the development of rewarding effects of morphine and reinforce the hypothesis of strong inhibitory interactions between neuropeptide FF and opioids.     

Differential properties between TRK-820 and U-50,488H on the discriminative stimulus effects in rats

It has been demonstrated that the newly synthesized kappa-opioid receptor agonist TRK-820, which has a unique structure that is different from those of other prototypical kappa-opioid receptor agonists such as U-50,488H, exert some behavioral effects that differ from those induced by U-50,488H. Therefore, the present study was designed to examine the possible difference between the discriminative stimulus effects of TRK-820 and U-50,488H in rats. Substitution tests with several kappa-opioid receptor agonists were initiated in rats trained to discriminate between TRK-820 (40 microg/kg) or U-50,488H (3.0 mg/kg) and saline. In the cross-substitution tests, U-50,488H substituted for the discriminative stimulus effects of TRK-820, whereas TRK-820 did not substitute completely for those of U-50,488H, indicating that the discriminative stimulus effects of TRK-820 and U-50,488H were somewhat different. In the substitution tests, E-2078, but not R-84760, substituted for the discriminative stimulus effects of both TRK-820 and U-50,488H. KT-90, CI-977 and ICI-199441 each substituted for the discriminative stimulus effects of U-50,488H, but not to those of TRK-820. These results imply that these kappa-opioid receptor agonists possess U-50,488H-like discriminative stimulus effects. Furthermore, that U-50,488H and the other kappa-opioid receptor agonists substituted for the discriminative stimulus effects of U-50,488H, produced aversive effects in rats. These findings suggest the possibility that unlike those of TRK-820, the cue of the discriminative stimulus effects of U-50,488H may be, at least in part, associated with its aversive effects.     

The influence of NMDA receptor agonist and antagonist on morphine state-dependent memory of passive avoidance in mice

The measurement of step-down latency in passive avoidance has been used to study memory in laboratory animals. The pre-training injection of 5 mg/kg morphine impaired memory, which was restored when 24 h later the same dose of the drug was administered. To explore the possible involvement of NMDA modulators on morphine-induced memory impairment, we have investigated the effects of intracerebroventricular (i.c.v.) administration of NMDA and the competitive NMDA antagonist, DL-AP5, on morphine-induced memory impairment or recall, on the test day. Morphine (5 mg/kg, s.c.) was administered 30 min before training to induce impairment of memory and 24 h later, 30 min before test to improve it. Pre-test administration of NMDA (0.00001, 0.0001 and 0.001 microg/mouse, i.c.v.) did not alter the retention latency compared to the saline-treated animals. But restored the memory impairment induced by pre-training morphine (5 mg/kg, s.c.). Pre-test administration of DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) by itself decreased the retention latencies. The same doses of DL-AP5 increased pre-training morphine-induced memory impairment. Co-administration of NMDA (0.0001 and 0.001 microg/mouse, i.c.v.) and morphine (5 mg/kg, s.c.) on the test day increased morphine memory improvement. Conversely, DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) inhibited morphine-induced memory recall. It is concluded that NMDA receptors may be involved, at least in part, in morphine state-dependent learning in mice.     

Changes in ribonuclease in the central nervous system of mice during morphine dependence development, withdrawal and naloxone administration

Ribonuclease has been studied in the whole brain homogenate of mice after a single dose of morphine (10 mg/kg), during the development of tolerance and dependence, during the course of withdrawal and naloxone administration. The enzyme increased dose dependently following the administration of morphine. Withdrawal caused a sudden fall in the enzyme with partial recovery by the 6th day of abstinence. Naloxone injections in normal, tolerant, dependent and deprived animals caused a reduction in the enzyme level. The morphine-induced increase in enzyme activity is suggested to be in direct correlation with a reduction in protein synthesis which can be ascribed to a disturbance of the translation processes.     

Increased CNS sensitivity to flurothyl as a measure of physical dependence in mice following morphine,phenobarbital, and ethanol treatment

Male C57BL/6 mice were administered either morphine, phenobarbital or ethanol in their drinking water in order to make them dependent. During withdrawal onset of myoclonus, clonus and tonus was evaluated with flurothyl, a convulsant inhalant. Morphine, phenobarbital and ethanol treated subjects displayed significantly lower latencies for myoclonic and clonic convulsive behavior when compared with their respective controls. The increased CNS excitability observed is characteristic of physical dependence and the flurothyl technique employed is discussed in light of its broad applicability to a number of agents characterized by their dependence producing properties.     

Differential effect of environmental temperature on morphine physical dependence and abstinence

1) Ambient temperature (T_a) significantly influenced the display of 4 of the 14 naloxone-precipitated withdrawal signs (nesting, flat posture, vocalization, dyspnea) in morphine-dependent, non-hibernating ground squirrels ($\text{Citellus}$$\text{lateralis}$).2) Analysis of variance performed on the six quantified signs revealed that T_a during withdrawal, but not during the development of physical dependence, was a significant factor in determining the expression of two signs (nesting and vocalization).3) The interaction between the influence of T_a during the periods of morphine administration and abstinence was a significant factor in determining the expression of nesting behavior, a finding that is consistent with the natural role of nesting as a behavioral thermoregulatory response.4) We conclude that environmental temperature modulates the expression of selected components of the naloxone-precipitated abstinence syndrome in $\text{C. lateralis}$ without exerting a measurable influence on the development of morphine physical dependence itself.     

Influence of adenosine analogs on morphine tolerance and dependence in mice

A number of adenosine agonists were investigated for possible actions on tolerance to morphine withdrawal in mice. The induction of tolerance to a sustained release preparation of morphine was assessed by measuring the analgesic effect induced by a test dose of the drug. The concomitant treatment with L- and D-phenylisopropyl adenosine, (L- and D-PIA), cyclopentyladenosine (CPA) or chloroadenosine (CADO) during the period of morphine absorption did not alter the induction of the process. In contrast cyclohexyladenosine (CHA) significantly decreased the intensity of tolerance. The administration of naloxone 30 hrs, after the priming dose of morphine induced an intense withdrawal reaction. The intensity of the abstinence syndrome was decreased by the administration of L-PIA, CHA or CADO; CPA and D-PIA were ineffective. These results suggest that adenosine analogs may interfere with the known morphine effects on calcium disposition in nerve terminals.     

Modulation of the neurotoxic effects of methamphetamine by the selective kappa-opioid receptor agonist U69593

Kappa-opioid receptor agonists prevent alterations in dopamine neurotransmission that occur in response to repeated cocaine administration. The present microdialysis study examined whether administration of the selective kappa-opioid receptor agonist U69593 with methamphetamine prevents alterations in dopamine levels produced by neurotoxic doses of methamphetamine. Swiss Webster mice were injected intraperitoneally with methamphetamine (10.0 mg/kg) or saline, four times in 1 day, at 2-h intervals. Prior to the first and third injection, they received U69593 (0.32 mg/kg s.c.) or vehicle. Microdialysis was conducted 3, 7, or 21 days later. Basal and K+-evoked (60 and 100 mM) dopamine overflow were reduced 3 days after methamphetamine administration. These effects were long-lasting in that they were still apparent 7 and 21 days after methamphetamine treatment. Intrastriatal (5.0 and 50 microM) or systemic (1.0-10.0 mg/kg) administration of methamphetamine increased dopamine concentrations in control animals. In mice preexposed to methamphetamine, methamphetamine-evoked dopamine overflow was reduced. In animals that had received methamphetamine with U69593, basal dopamine levels did not differ from those of vehicle-treated controls. U69593 treatment attenuated the decrease in K+-evoked dopamine produced by prior methamphetamine exposure. The reduction in methamphetamine-evoked dopamine levels was also attenuated. The administration of U69593 alone did not modify basal or stimulus-evoked dopamine levels. These data demonstrate that repeated methamphetamine administration reduces presynaptic dopamine neuronal function in mouse striatum and that co-administration of a selective kappa-opioid receptor agonist with methamphetamine attenuates these effects. U69593 treatment did not modify the hyperthermic effects of methamphetamine, indicating that this kappa-opioid receptor agonist selectively attenuates methamphetamine-induced alterations in dopamine neurotransmission.     

Inability of cyclo (Leu-Gly) to facilitate the development of tolerance to and physical dependence on morphine in the rat

The effect of cyclo (Leu-Gly), an analog of melanotropin release inhibition factor on the development of tolerance to and physical dependence on morphine in the rat was investigated. Administration of cyclo (Leu-Gly) (1 μg/rat/day) prior to and during morphine pellet implantation failed to facilitate the development of tolerance to the analgesic and hypothermic effects of morphine. Similarly the development of dependence on morphine was not facilitated by cyclo (Leu-Gly) as evidenced by changes in body weight and body temperature observed during abrupt withdrawal of morphine. These studies do not lend support to the previous observations that cyclo (Leu-Gly) and other related peptides facilitate the development of tolerance to and physical dependence on morphine.     

Mu-, delta-, and kappa-opioid receptor agonists selectively modulate sexual behaviors in the female rat: differential dependence on progesterone.

Previous studies suggested that opioid receptor agonists infused into the lateral ventricles can inhibit (through mu receptors) or facilitate (through delta receptors) the lordosis behavior of ovariectomized (OVX) rats treated with estrogen and a low dose of progesterone. The present study investigated the behavioral and hormonal specificity of those effects using more selective opioid receptor agonists. Sexually experienced OVX rats were implanted stereotaxically with guide cannulae aimed at the right lateral ventricle. One group of rats was treated with estradiol benzoate (EB, 10 micrograms) 48 hr and progesterone (P, 250 micrograms) 4 hr before testing, whereas the other group was treated with EB alone. Rats were infused with different doses of the selective mu-receptor agonist DAMGO, the selective delta-receptor agonist DPDPE, or the selective kappa-receptor agonist U50-488. The females were placed with a sexually vigorous male in a bilevel chamber (Mendelson and Gorzalka, 1987) for three tests of sexual behavior, beginning 15, 30, and 60 min after each infusion. DAMGO reduced lordosis quotients and magnitudes significantly in rats treated with EB and P, but not in rats treated with EB alone. In contrast, DPDPE and U50-488H increased lordosis quotients and magnitudes significantly in both steroid-treatment groups. Surprisingly, measures of proceptivity, rejection responses, and level changes were not affected significantly by mu or kappa agonists, although proceptivity and rejection responses were affected by DPDPE treatment. These results suggest that the effects of lateral ventricular infusions of opioid receptor agonists on the sexual behavior of female rats are relatively specific to lordosis behavior. Moreover, the facilitation of lordosis behavior by delta- or kappa-receptor agonists is independent of progesterone treatment, whereas the inhibitory effect of mu-receptor agonists on lordosis behavior may require the presence of progesterone.