Mode of antinociceptive and toxic action of alkaloids of Aconitum spec.

Abstract

Extracts of the plant Aconitum spec. are used in traditional Chinese medicine predominantly as anti-inflammatory and analgesic agents, the latter allegedly equally potent as morphine but without any habit-forming potential. As the only pharmacologically active compounds, the C19 diterpenoid alkaloid aconitine, and some of its derivatives, have been proven to be antinociceptive in different analgesic assays, but the mode of action is unknown. To elucidate the mode of action, ten aconitine-like derivatives were investigated with respect to their affinity for voltage-dependent Na+ channels, the action on synaptosomal Na+ and Ca2+ homoeostasis and their antinociceptive, arrhythmogenic and acute toxic properties. Since aconitine is known to bind to site II of Na+ channels, we determined the affinity of the aconitine-like derivatives in vitro to synaptosomal membranes by the [3H]-batrachotoxinin-binding assay and their properties on intrasynaptosomal concentrations of free Na+ and Ca2+ ([Na+]i and [Ca2+]i), both the latter determined fluorometrically with SBFI and Fura-2 respectively. Furthermore, the alkaloids' arrhythmogenic potential was investigated in guinea-pig isolated atria and the antinociceptive action on formalin-induced hyperalgesia and the acute toxic action estimated in mice. The results show that the alkaloids could be divided into at least three groups. The first is characterized by a high affinity to the site II of Na+ channels (Ki about 1.2 microM), the ability to enhance [Na+]i and [Ca2+]i (EC50 about 3 microM), a strong arrhythmogenic action that starts at about 30 nM, an antinociceptive effect (ED50 about 0.06 mg/kg) and high acute toxicity (LD50 values about 0.15 mg/kg). To this group belong aconitine, 3-acetylaconitine and hypaconitine. The second group, with lappaconitine as the only member, has an affinity to Na+ channels an order of magnitude lower (Ki = 11.5 microM), less acute toxicity (LD50 about 5 mg/kg), and a two orders of magnitude lower antinociceptive action (ED50 about 2.8 mg/kg) and lower cardiotoxicity (bradycardia observed at 3 microM). Additionally, lappaconitine suppresses the increase in [Ca2+]i of aconitine-stimulated synaptosomes and increases the excitation threshold of left atria, indicating an inhibition of Na+ channels. The other derivatives, i.e. delcorine, desoxydelcorine, karakoline, lappaconidine, lappaconine and lycoctonine, belong to the third group, which has hardly any effects. They have a low affinity to Na+ channels with Ki values in the millimolar range, show no effect on synaptosomal [Na+]i and [Ca2+]i, no arrhythmogenic potential up to 100 microM, no antinociceptive activity and low toxicity with LD50 values greater than 50 mg/kg. For the investigated alkaloids we suggest two different antinociceptive-like modes of action. Aconitine, hypaconitine and 3-acetylaconitine may induce a block of neuronal conduction by a permanent cell depolarisation, whereas lappaconitine might act like local anaesthetics. However, because of the low LD50/ED30 quotients of 2-6, the...

Determination of aconitine in body fluids by LC-MS-MS

“…Predominantly in traditional Chinese and Japanese medical preparations, aconite tubers (Tubera aconiti) and their processed products are used for their pharmaceutical properties including anti-inflammatory, analgesic and cardiotonic effects [3,4,5,6,7]. These effects can be attributed to the presence of C20 diterpene and C19 norditerpene alkaloids which are found in all of the tissues of the plant, but especially in the tubers.…”

Natural Products as a Source for Novel Analgesic Compounds

“…In contrast, lappaconitine has lower affinity for Na + channel and thereafter has lesser antinociceptive and lesser cardiotoxic activity, acting as a local anaesthetic. Other alkaloids with lower Na + channel affinity such as lappaconidine and oxydelcorine have no antinociceptive effect [37]. Despite the large number of alkaloids isolated and identified from Aconitum sp.…”

Naturally occurring antinociceptive substances from plants

“…Several Aconitum alkaloids such as aconitine, mesaconitine, 3-acetylaconitine, hypaconitine, heteratisine, songorine and lappacotine have revealed antinociceptive effects in thermal, mechanical and chemical nociceptive tests in animals (Friese et al, 1997;Gutser et al, 1998;Ono and Satoh, 1988). These alkaloids are characterized as binding, with different affinities, to site II of Na channels.…”