Salvinorin A analogues PR-37 and PR-38 attenuate compound 48/80-induced itch responses in mice

Abstract

Background and purpose: The opioid system plays a crucial role in several physiological processes in the CNS and in the periphery. It has also been shown that selective opioid receptor agonists exert potent inhibitory action on pruritus and pain. In this study we examined whether two analogues of Salvinorin A, PR-37 and PR-38, exhibit antipruritic properties in mice.

Experimental approach: To examine the antiscratch effect of PR-37 and PR-38 we used a mouse model of compound 48/80-induced pruritus. In order to elucidate the mechanism of action of tested compounds, specific antagonists of opioid and cannabinoid receptors were used. The effect of PR-37 on the CNS was assessed by measuring motor parameters and exploratory behaviours in mice.

Key results: PR-37 and PR-38, jnjected s.c., significantly reduced the number of compound 48/80-induced scratching behaviours in mice in a dose- and time-dependent manner. PR-38 was also active when orally administered. The antiscratch activity of PR-37 was blocked by the selective κ opioid receptor antagonist, nor-binaltorphimine, and that of PR-38 by the selective μ opioid receptor antagonist, β-funaltrexamine.

Conclusion and implications: In conclusion, a novel framework for the development of new antipruritic drugs derived from salvinorin A has been validated.

Figure 1

Pruritic effect of 48/80 compound in LA versus HA mice. Figure shows data for vehicle-treated LA and HA mice, and SA-treated LA mice (10 mg·kg⁻¹, s.c.). Data represent mean ± SEM of n = 6–10 animals. ***P < 0.001, as compared with the LA group.

Figure 2

The effect of PR-37, PR-38 and ICI 204 448 on compound 48/80-induced pruritus in mice. (A) Effect of s.c. administration of PR-37 (1–20 mg·kg⁻¹) on the number of scratches per 30 min. (B) Effect of s.c. administration of PR-38 (1–20 mg·kg⁻¹) on the number of scratches per 30 min. (C) Effect of s.c. administration of a reference drug ICI 204 448 (10 mg·kg⁻¹) on the number of scratches per 30 min. Data represent mean ± SEM of n = 6–10 animals. *P < 0.05, **P < 0.01, ***P < 0.001, as compared with the control group.

Figure 3

The time-course of the antiscratch activity of PR-37 and PR-38 after a single s.c. administration at the dose of 10 mg·kg⁻¹ in mice. (A) Time-course of the effect of PR-37 on the number of scratches per 30 min. The strongest effect occurs 90 min post-administration (B) Time-course of the effect of PR-38 on the number of scratches per 30 min. The strongest effect occurs 45 min post-administration. Data represent mean ± SEM of n = 8–10 mice for each experimental group. *P < 0.05, ***P < 0.001, as compared with the control group.

Figure 4

Antiscratch effect of s.c. administration (10 mg·kg⁻¹) of PR-37 alone or in the presence of opioid and cannabinoid receptor antagonists. (A) Antiscratch effect of PR-37 is blocked by KOR antagonist norBNI (10 mg·kg⁻¹, s.c.) administered 30 min prior to the test compound but not MOR, DOR and CB₁ receptor antagonists (β-FNA, NLTR and AM 251 respectively). (B) Antiscratch effect of PR-37 is blocked by the non-selective opioid antagonist NLX (1 mg·kg⁻¹, s.c.) and KOR antagonist norBNI (10 mg·kg⁻¹, s.c.) administered 120 min prior to the test compound. Data represent mean ± SEM of n = 6–10 mice for each experimental group. *P < 0.05, **P < 0.01, as compared with the control group. #P < 0.05, as compared with PR-37.

Figure 5

Antiscratch effect of s.c. administration (10 mg·kg⁻¹) of PR-38 alone or in the presence of opioid and cannabinoid receptor antagonists. (A) Antiscratch effect of PR-38 is blocked by MOR antagonist β-FNA (1 mg·kg⁻¹, s.c.) administered 30 min prior to the test compound, but not KOR, DOR and CB₁ receptor antagonists (norBNI, NLTR and AM 251 respectively). (B) Antiscratch effect of PR-38 is blocked by the non-selective opioid receptor antagonist NLX (1 mg·kg⁻¹, s.c.) and MOR antagonist β-FNA (1 mg·kg⁻¹, s.c.) administered 120 min prior to the test compound. Data represent mean ± SEM of n = 6–10 mice for each experimental group. *P < 0.05, **P < 0.01, as compared with the control group. #P < 0.05, as compared with PR-37 or PR-38.

Figure 6

Antiscratch activity of PR-37 (A) and PR-38 (B) after oral administration (10 mg·kg⁻¹) in mice. Data represent mean ± SEM of n = 6–8 mice for each experimental group. *P < 0.05, as compared with the control group.

Figure 7

Effect of PR-37 on the CNS. (A) Effect of PR-37 (10 mg·kg⁻¹, i.p.) on horizontal locomotor activity. (B) Effect of PR-37 (10 mg·kg⁻¹, i.p.) on vertical locomotor activity. (C) Effect of PR-37 (10 mg·kg⁻¹, i.p) on time spent in the opened and closed areas of elevated o-maze. (D–I) Catwalk automated gait analysis in mice after PR-38 administration (10 mg·kg⁻¹, i.p.) Depicted are mean ratios ± SEM (8–10 mice per experimental group) of pawprint intensity (D), print width (E), print area (F), max area (G), swing (H) and swing speed (I), separately, for front and hind paws. **P < 0.01, ***P < 0.001, as compared with the control group.