7β-Methyl substituent is a structural locus associated with activity cliff for nepenthone analogues

With the purpose of identifying novel selective κ opioid receptor (KOR) antagonists as potential antidepressants from nepenthone analogues, starting from N-nor-N-cyclopropylmethyl-nepenthone (SLL-020ACP), a highly selective and potent KOR agonist, a series of 7β-methyl-nepenthone analogues was conceived, synthesized and assayed on opioid receptors based on the concept of hybridization. According to the pharmacological results, the functional reversal observed in orvinol analogues by introduction of 7β-methyl substituent could not be reproduced in nepenthone analogues. Alternatively, introduction of 7β-methyl substituent was associated with substantial loss of both subtype selectivity and potency but not efficacy for nepenthone analogues, which was not found in 7β-methyl orvinol analogues. Surprisingly, SLL-603, a 7β-methyl analogue of SLL-020ACP, was identified to be a KOR full agonist. The possible molecular mechanism for the heterogeneity in activity cliff was also investigated. In conclusion, 7β-methyl substituent was a structural locus associated with activity cliff and demonstrated as a pharmacological heterogeneity between nepenthone and orvinol analogues that warrants further investigations.     

Investigation of gastrin-releasing peptide as a mediator for 5'-guanidinonaltrindole-induced compulsive scratching in mice

Gastrin-releasing peptide (GRP) has been implicated in the itch-scratch cycle. We investigated if this gut-brain-skin peptide plays a role in the compulsive, hindleg scratching of the neck of mice by 5′-guanidinonaltrindole (GNTI), the kappa opioid receptor antagonist, and in the antipruritic activity of nalfurafine, the kappa opioid agonist. Previously, we showed that GNTI (0.03-1 mg/kg, s.c.) elicits dose-related scratching and that nalfurafine (0.001-0.02 mg/kg, s.c.) inhibits this behavior in mice. Utilizing immunohistochemistry, GRP positive nerve fibers were detected in mouse skin and superficial layer of the dorsal horn of the spinal cord as well as GRP positive cells in the dorsal root ganglion. Pretreating mice with either a pseudopeptide GRP receptor antagonist, RC-3095 (10-30 mg/kg, s.c. at −15 min), or a peptide GRP receptor antagonist, [d-Phe⁶]bombesin(6-13) methyl ester (2-100 nmol, i.t. at −10 min), did not suppress GNTI-induced scratching. However, pretreating mice with either antagonist inhibited scratching precipitated by the GRP receptor agonist, GRP_18-27 (2 nmol, i.t.). Pretreating mice with a muscarinic M₁ receptor agonist, McN-A-343 (1.5-15 μg/5 μl, i.t. at −10 min) antagonized GNTI-induced scratching. Norbinaltorphimine (20 mg/kg, i.p. at −18 to −20 h), a kappa opioid antagonist, countered the antiscratch activity of nalfurafine. We conclude that (a) the GRP receptor system does not mediate GNTI-induced scratching and (b) the kappa opioid system is involved, at least in part, in the scratch suppressing activity of nalfurafine.     

Effects of the neoclerodane Hardwickiic acid on the presynaptic opioid receptors which modulate noradrenaline and dopamine release in mouse central nervous system

We have comparatively investigated the effects of Hardwickiic acid and Salvinorin A on the K⁺-evoked overflow of [³H]noradrenaline ([³H]NA) and [³H]dopamine ([³H]DA) from mouse hippocampal and striatal nerve terminals, respectively. The K⁺-evoked overflow of [³H]DA was inhibited in presence of Salvinorin A (100 nM) but not in presence of Hardwickiic acid (100 nM). Hardwickiic acid (100 nM) mimicked Salvinorin A (100 nM) in facilitating K⁺-evoked hippocampal [³H]NA overflow and the two compounds were almost equipotent. Facilitation of [³H]NA overflow caused by 100 nM Hardwickiic acid was prevented by the κ-opioid receptor (KOR) antagonist norbinaltorphimine (norBNI, 100 nM) and by the selective δ-opioid receptor (DOR) antagonist naltrindole (100 nM), but was not altered by 100 nM D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), a selective μ-opioid receptor (MOR) antagonist. We conclude that Hardwickiic acid modulates hippocampal [³H]NA overflow evoked by a mild depolarizing stimulus by acting at presynaptic opioid receptor subtypes.