**5. Effects of TRK-820 on drug dependence**

#### **5.1 Effects of TRK-820 in the conditioned place preference (CPP) test**

The agonists have a rewarding effect, which accounts for the abuse of morphine by humans. In animal models, the rewarding effects of agonists have been evaluated by the conditioned place preference (CPP) and self-administration paradigms (Di Chiara & North, 1992). In contrast to μ agonists, conventional κ agonists such as U-50,488H and U-69,593 generally lack

Opioid Kappa Receptor Selective Agonist TRK-820 (Nalfurafine Hydrochloride) 91

shifted the dose-response curve for cocaine (10 mg/kg, i.p.) to the right without changing the response rate. This attenuating effect of TRK-820 was completely reversed by pre-treatment

In humans, withdrawal from the chronic administration of opioids such as morphine results in characteristic behaviors, including anxiety, nausea, insomnia, hot and cold flashes, muscle aches, perspiration, and diarrhea. Such symptoms would pose clinical problems in patients receiving long-term treatment with opioids for pain relief. Rodents that are physically dependent on morphine elicit characteristic signs (jumping, wet dog shakes, rearing, diarrhea, ptosis, and forepaw tremor) when administrated naloxone. The withdrawal signs precipitated by naloxone are used as an index of the physical dependence on morphine. The effects of agonists TRK-820 and U-50,488H on the development of physical dependence on morphine were reported. Co-injection of TRK-820 (0.003-0.03 mg/kg, s.c.) during chronic morphine treatment dose-dependently suppressed naloxone-precipitated body weight loss, and the other withdrawal signs in morphine-dependent mice treated with TRK-820 (0.03 mg/kg, s.c.) were significantly fewer than those in untreated mice. In contrast to TRK-820, co-injection of U-50,488H (1.0-10 mg/kg, s.c.) did not inhibit naloxone-precipitated body

Nicotine withdrawal produces characteristic syndromes, including irritability, anxiety, depression, and craving for nicotine. Pre-treatment with TRK-820 (10 and 30 μg/kg, s.c.) or U-50,488H (0.01-1.0 mg/kg, s.c.) has been reported to decrease dose-dependently mecamylamine-precipitated nicotine-withdrawal aversion in nicotine-dependent rats (Ise et

We described in the previous sections some pharmacological properties of TRK-820 that are different from conventional agonists, arylacetamides such as U-50,488H and U-69,593: binding properties (section 2) and exhibition of no preferential and no aversive effect in the CPP paradigm (section 5). As described below, drug discrimination procedures indicate

Drug discrimination procedures have shown that the properties of TRK-820 differ from those of conventional agonists, such as U-50,488H. In the cross-substitution tests using rats, U-50,488H (1.0-3.0 mg/kg) substituted for the discriminative stimulus effects of TRK-820 (40 g/kg, i.p.), whereas TRK-820 (10-76 g/kg) did not completely substitute for those of U-50,488H (3.0 mg/kg, i.p.). E-2078 (0.3-3.0 mg/kg), but not R-84760 (0.01-0.3 mg/kg), substituted for the discriminative stimulus effects of both TRK-820 and U-50,488H. KT-90 (0.03-3.0 mg/kg), CI-977 (1-30 mg/kg), or ICI-199441 (3.0-56 mg/kg) substituted for the discriminative stimulus effects of U-50,488H, but not for those of TRK-820 (Mori et al., 2004).

**6. Comparison of pharmacological properties between TRK-820 and** 

with nor-BNI (10 mg/kg, s.c.) (Mori et al., 2002).

**5.3 Effects of TRK-820 on the morphine withdrawal response** 

weight loss and other withdrawal signs (Tsuji et al., 2000).

conclusive difference between TRK-820 and arylacetamides.

al., 2002).

**conventional agonists** 

**6.1 Discriminative tests** 

**5.4 The effect of TRK-820 on the nicotine-withdrawal response** 

a rewarding effect (Dykstra et al., 1997). However in the CPP test, animals avoid an environment associated with the administration of the agonists, indicating that these drugs have aversive effects (Barr et al., 1994; Funada et al., 1993). In contrast to conventional agonists, such as U-50,488H, TRK-820 (3.0-30 g/kg, s.c.) did not induce significant place aversion in mice at doses producing significant antinociception (Fig. 6) (Nagase, 2010). Notably, TRK-820 exhibited neither preferential nor aversive properties. Recently, the peroral administration of TRK-820 (5.0 g/day) was reported to show no signs of psychological or physical dependence in an open-labeled clinical trial for one year (Nagase & Fujii, 2011).

Fig. 6. The effect of TRK-820 in the CPP test. Reprinted with permission from Nagase, 2010.

#### **5.2 The effects of TRK-820 on morphine and cocaine-induced rewarding effects**

The mechanism of agonist-induced rewarding effects is outlined below. The activation of the receptor on -aminobutyric acid-containing interneurons is likely to disinhibit ventrotegmental area dopaminergic neurons, thereby increasing dopamine release in their terminal areas, including the nucleus accumbens (N.Acc). On the other hand, the activation of the receptor decreases dopamine release in the N.Acc (Di Chiara & Imperato, 1988; Spanagel et al., 1992). Therefore, agonists may be useful for treating morphine dependence. Indeed, the pretreatment with U-50,488H attenuated the morphine-induced place preference in mice (Funada et al., 1993). TRK-820 also significantly suppressed the place preference produced by morphine, and the effect of TRK-820 was antagonized by pre-treatment with nor-BNI (3.0 mg/kg, s.c.) in mice (Tsuji et al., 2001). In addition, TRK-820 was effective in reducing the rewarding effect produced by cocaine. TRK-820 (20 and 40 g/kg, i.p.), at doses producing no aversive or sedative effects, suppressed the rewarding effect of cocaine (4.0 mg/kg, i.p.) in rats (Mori et al., 2002). U-50,488H and U-69,593 exhibited similar effects as TRK-820 (Shippenberg et al., 1996; Suzuki et al., 1992). Drug discrimination procedures provide relevant information about neuropharmacological mechanisms underlying the subjective effects of abused drugs, including cocaine, methamphetamine, and opioids, in animals. Therefore, the procedures are potentially useful for identifying candidate therapeutics for the management of drug abuse (Schuster & Johanson, 1988). Pre-treatment with TRK-820 (10 and 20 g/kg, s.c.) significantly shifted the dose-response curve for cocaine (10 mg/kg, i.p.) to the right without changing the response rate. This attenuating effect of TRK-820 was completely reversed by pre-treatment with nor-BNI (10 mg/kg, s.c.) (Mori et al., 2002).

## **5.3 Effects of TRK-820 on the morphine withdrawal response**

In humans, withdrawal from the chronic administration of opioids such as morphine results in characteristic behaviors, including anxiety, nausea, insomnia, hot and cold flashes, muscle aches, perspiration, and diarrhea. Such symptoms would pose clinical problems in patients receiving long-term treatment with opioids for pain relief. Rodents that are physically dependent on morphine elicit characteristic signs (jumping, wet dog shakes, rearing, diarrhea, ptosis, and forepaw tremor) when administrated naloxone. The withdrawal signs precipitated by naloxone are used as an index of the physical dependence on morphine. The effects of agonists TRK-820 and U-50,488H on the development of physical dependence on morphine were reported. Co-injection of TRK-820 (0.003-0.03 mg/kg, s.c.) during chronic morphine treatment dose-dependently suppressed naloxone-precipitated body weight loss, and the other withdrawal signs in morphine-dependent mice treated with TRK-820 (0.03 mg/kg, s.c.) were significantly fewer than those in untreated mice. In contrast to TRK-820, co-injection of U-50,488H (1.0-10 mg/kg, s.c.) did not inhibit naloxone-precipitated body weight loss and other withdrawal signs (Tsuji et al., 2000).

#### **5.4 The effect of TRK-820 on the nicotine-withdrawal response**

Nicotine withdrawal produces characteristic syndromes, including irritability, anxiety, depression, and craving for nicotine. Pre-treatment with TRK-820 (10 and 30 μg/kg, s.c.) or U-50,488H (0.01-1.0 mg/kg, s.c.) has been reported to decrease dose-dependently mecamylamine-precipitated nicotine-withdrawal aversion in nicotine-dependent rats (Ise et al., 2002).
