Antinociceptive Effects of N-Acyloctahydropyrido[3,2,1-ij][1,6]naphthyridine in Mice: Structure–Activity Relation Study of Matrine-Type Alkaloids Part II

N-Acyloctahydropyrido[3,2,1-ij][1,6]naphthyridines were synthesized as derivatives of matrine-type alkaloids, and the structure–activity relations were examined by the acetic acid-induced abdominal contraction test. The antinociceptive potencies of N-acyloctahydropyrido[3,2,1-ij][1,6]naphthyridines were significantly lower than those of (+)-matrine. The antinociceptive effects of N-benzyloctahydropyrido[3,2,1-ij][1,6]naphthyridines are approximately 5.6 to 6.5 times less than those of N-benzoyloctahydropyrido[3,2,1-ij][1,6]naphthyridine. These findings suggest that the amide group of matrine-type alkaloids is an essential functional group that influences antinociceptive potency. The antinociceptive effect of 4c was markedly antagonized by pretreatment with Naloxone, and that of 3c partially so.