Different effects on [3H]noradrenaline uptake of the Aconitum alkaloids aconitine, 3-acetylaconitine, lappaconitine, and N-desacetyllappaconitine in rat hippocampus

The known, unknown, and the intriguing about members of a critically endangered traditional medicinal plant genus Aconitum

Humanity will always be indebted to plants. In the ongoing scientific era, the 'Herbal Revolution' has helped discover several valuable medicinal plants and associated novel secondary metabolites from the diverse unexplored ecosystems, treating several diseases via phytotherapy. The Aconitum genus comprises several economically-important poisonous mountainous medicinal plant species whose unique biodiversity is on the verge of extinction due to illegal human intervention triggered habitat loss, over-harvesting, and unrestricted trading. Owing to its vast diversity of diterpene alkaloids, most species are extensively used to treat several ailments in rural parts of the world. Irrespective of this, many unexplored and intriguing prospects exist to understand and utilize this critical plant for human benefit. This systematic review tries to fill this gap by compiling information from the sporadically available literature known for ~300 Aconitum spp. regarding its nomenclature and classification, endangerment, plant morphology, ploidy, secondary metabolites, drug pharmacokinetics, conservation, and omics-based computational studies. We also depicted the disparity in the studied model organisms for this diverse genus. The absence of genomic/metagenomic data is becoming a limiting factor in understanding its plant physiology, metabolic pathways, and plant-microbes interactions, and therefore must be promoted. Additionally, government support and public participation are crucial in establishing conservation protocols to save this plant from endangerment.

Neuropharmacological Potential of Diterpenoid Alkaloids

This study provides a narrative review of diterpenoid alkaloids (DAs), a family of extremely important natural products found predominantly in some species of Aconitum and Delphinium (Ranunculaceae). DAs have long been a focus of research attention due to their numerous intricate structures and diverse biological activities, especially in the central nervous system (CNS). These alkaloids originate through the amination reaction of tetra or pentacyclic diterpenoids, which are classified into three categories and 46 types based on the number of carbon atoms in the backbone structure and structural differences. The main chemical characteristics of DAs are their heterocyclic systems containing β-aminoethanol, methylamine, or ethylamine functionality. Although the role of tertiary nitrogen in ring A and the polycyclic complex structure are of great importance in drug-receptor affinity, in silico studies have emphasized the role of certain sidechains in C13, C14, and C8. DAs showed antiepileptic effects in preclinical studies mostly through Na⁺ channels. Aconitine (1) and 3-acetyl aconitine (2) can desensitize Na⁺ channels after persistent activation. Lappaconitine (3), N-deacetyllapaconitine (4), 6-benzoylheteratisine (5), and 1-benzoylnapelline (6) deactivate these channels. Methyllycaconitine (16), mainly found in Delphinium species, possesses an extreme affinity for the binding sites of α7 nicotinic acetylcholine receptors (nAChR) and contributes to a wide range of neurologic functions and the release of neurotransmitters. Several DAs such as bulleyaconitine A (17), (3), and mesaconitine (8) from Aconitum species have a drastic analgesic effect. Among them, compound 17 has been used in China for decades. Their effect is explained by increasing the release of dynorphin A, activating the inhibitory noradrenergic neurons in the β-adrenergic system, and preventing the transmission of pain messages by inactivating the Na⁺ channels that have been stressed. Acetylcholinesterase inhibitory, neuroprotective, antidepressant, and anxiolytic activities are other CNS effects that have been investigated for certain DAs. However, despite various CNS effects, recent advances in developing new drugs from DAs were insignificant due to their neurotoxicity.

Hybrides of Alkaloid Lappaconitine with Pyrimidine Motif on the Anthranilic Acid Moiety: Design, Synthesis, and Investigation of Antinociceptive Potency

Convenient and efficient routes to construct hybrid molecules containing diterpene alkaloid lappaconitine and pyrimidine fragments are reported. One route takes place via first converting of lappaconitine to 1-ethynyl-lappaconitine, followed by the Sonogashira cross-coupling-cyclocondensation sequences. The other involves the palladium-catalyzed carbonylative Sonogashira reaction of 5'-iodolappaconitine with aryl acetylene and Mo (CO)6 as the CO source in acetonitrile and subsequent cyclocondensation reaction of the generated alkynone with amidines. The reaction proceeded cleanly in the presence of the PdCl2-(1-Ad)2PBn∙HBr catalytic system. The protocol provides mild reaction conditions, high yields, and high atom and step-economy. Pharmacological screening of lappaconitine-pyrimidine hybrids for antinociceptive activity in vivo revealed that these compounds possessed high activity in experimental pain models, which was dependent on the nature of the substituent in the 2 and 6 positions of the pyrimidine nucleus. Docking studies were undertaken to gain insight into the possible binding mode of these compounds with the voltage-gated sodium channel 1.7. The moderate toxicity of the leading compound 12 (50% lethal dose (LD50) value was more than 600 mg/kg in vivo) and cytotoxicity to cancer cell lines in vitro encouraged the further design of therapeutically relevant analogues based on this novel type of lappaconitine-pyrimidine hybrids.

The chemistry and pharmacology of alkaloids and allied nitrogen compounds from Artemisia species: A review

Several reviews have been published on Artemisia's derived natural products, but it is the first attempt to review the chemistry and pharmacology of more than 80 alkaloids and allied nitrogen compounds obtained from various Artemisia species (covering the literature up to June 2018). The pharmacological potential and unique skeleton types of certain Artemisia's alkaloids provoke the importance of analyzing Artemisia species for bioactive alkaloids and allied nitrogen compounds. Among the various types of bioactive Artemisia's alkaloids, the main classes were the derivatives of rupestine (pyridine-sesquiterpene), lycoctonine (diterpene), pyrrolizidine, purines, polyamine, peptides, indole, piperidine, pyrrolidine, alkamides, and flavoalkaloids. The rupestine derivatives are Artemisia's characteristic alkaloids, whereas the rest are common alkaloids found in the family Asteraceae and chemotaxonomically links the genus Artemisia with the tribes Anthemideae. The most important biological activities of Artemisia's alkaloids are including hepatoprotective, local anesthetic, β-galactosidase, and antiparasitic activities; treatment of angina pectoris, opening blocked arteries, as a sleep-inducing agents and inhibition of HIV viral protease, CYP450, melanin biosynthesis, human carbonic anhydrase, [3H]-AEA metabolism, kinases, and DNA polymerase β¹ . Some of the important nitrogen metabolites of Artemisia include pellitorine, zeatin, tryptophan, rupestine, and aconitine analogs, which need to be optimized and commercialized further.

Inhibitory effects of lappaconitine on the neuronal isoforms of voltage-gated sodium channels

Lappaconitine (LA) has been widely used for postoperative and cancer pain control. LA exhibits excellent analgesic activity with a longer effective time than common local anesthetics such as tetracaine and bupivacaine. However, the mechanisms underlying the featured analgesic activity of LA remain largely unknown. Here, we report that LA is an inhibitor of voltage-gated sodium channel 1.7 (Nav1.7) stably expressed in human embryonic kidney (HEK293) cells. LA inhibited Nav1.7 in a voltage-dependent manner with an IC₅₀ value (with 95% confidence limits) of 27.67 (15.68-39.66) µmol/L when the cell was clamped at -70 mV. In comparison with the quick and reversible inhibition of Nav1.7 by tetracaine and bupivacaine, the inhibitory effect of LA was rather slow and irreversible. It took more than 10 min to achieve steady-state inhibition when LA (300 µmol/L) was administered. Unlike tetracaine and bupivacaine, LA affected neither the voltage-dependent activation nor the inactivation of the channels. Five residues in domain III and domain IV have been reported to be critical for the effects of the two local anesthetics on Nav channels. But our mutant study revealed that only two residues (F1737, N1742) located in domain IV were necessary for the inhibitory activity of LA. The slow onset, irreversibility, and lack of influence on channel activation and inactivation accompanied with the different molecular determinants suggest that LA may inhibit Nav1.7 channels in a manner different from local anesthetics. These results may help to understand the featured analgesic activity of LA, thus benefiting its application in the clinic and future drug development.

A review on phytochemistry, pharmacology and toxicology studies of Aconitum

Objectives

A number of species belonging to herbal genus Aconitum are well-known and popular for their medicinal benefits in Indian, Vietnamese, Korean, Japanese, Tibetan and Chinese systems of medicine. It is a valuable drug as well as an unpredictable toxic material. It is therefore imperative to understand and control the toxic potential of herbs from this genus. In this review, the ethnomedicinal, phytochemistry, pharmacology, structure activity relationship and toxicology studies of Aconitum were presented to add to knowledge for their safe application.

Key findings

A total of about 76 of all aconite species growing in China and surrounding far-east and Asian countries are used for various medical purposes. The main ingredients of aconite species are alkaloids, flavonoids, free fatty acids and polysaccharides. The tuberous roots of genus Aconitum are commonly applied for various diseases such as rheumatic fever, painful joints and some endocrinal disorders. It stimulates the tip of sensory nerve fibres. These tubers of Aconitum are used in the herbal medicines only after processing. There remain high toxicological risks of the improper medicinal applications of Aconitum. The cardio and neurotoxicities of this herb are potentially lethal. Many analytical methods have been reported for quantitatively and qualitatively characterization of Aconitum.

Summary

Aconitum is a plant of great importance both in traditional medicine in general and in TCM in particular. Much attention should be put on Aconitum because of its narrow therapeutic range. However, Aconitum's toxicity can be reduced using different techniques and then benefit from its pharmacological activities. New methods, approaches and techniques should be developed for chemical and toxicological analysis to improve its quality and safety.

Diterpenoid alkaloids

The lasting attention that researchers have devoted to diterpenoid alkaloids is due to their various bioactivities and toxicities, structural complexity, and intriguing chemistry. From 1998 to the end of 2008, more than 300 new diterpenoid alkaloids were isolated from Nature. This review focuses on their structural relationships, and investigations into their chemical reactions, synthesis, and biological activities. A table that lists the names, plant sources, and structural types is given along with 363 references.

Diterpenoid alkaloid derivatives as potential chemotherapeutic agents in American trypanosomiasis

The use of natural products for the treatment of protozoal infections (Leishmania and Trypanosoma spp.) is well known and has been documented since ancient times. We have already established an in vitro culture system using mammalian host cells (Vero) infected with Trypanosoma cruzi in which the time course of parasite growth is determined quantitatively. This system was used to screen anti-T. cruzi agents using two experimental models: simultaneous cell infection and compound addition or preincubation of the parasite with the test compound prior to cell infection. Among 64 diterpenoid alkaloids tested, including C19 and C20 skeletons, five C20 compounds were active on T. cruzi epimastigotes: azitine, isoazitine and 15,22-O-diacetyl-19-oxodihydroatisine had moderate effects on the parasite, while atisinium chloride and 13-oxocardiopetamine were potent T. cruzi epimastigote growth inhibitors with activity levels similar to that of benznidazole, used as the reference drug. Additionally, these compounds decreased the ability of metacyclic forms to invade mammalian cells, their intracellular replications and their transformation into trypomastigotes, with no toxicity to the host cell. These results suggest that these alkaloids are structural leads of clinically active compounds against T. cruzi and probably other members of the Trypanosomatidae.

Structural diversity and defensive properties of norditerpenoid alkaloids

We have tested the insect antifeedant and toxic activity of 43 norditerpenoid alkaloids on Spodoptera littoralis and Leptinotarsa decemlineata including eserine (physostigmine), anabasine, and atropine. Antifeedant effects of the test compounds were structure- and species-dependent. The most active antifeedants to L. decemlineata were 1,14-diacetylcardiopetaline (9) and 18-hydroxy- 14-O-methylgadesine (33), followed by 8-O-methylconsolarine (12), 14-O-acetyldelectinine (27), karakoline (7), cardiopetaline (8), 18-O-demethylpubescenine (13), 14-O-acetyldeltatsine (18), takaosamine (21), ajadine (24), and 8-O-methylcolumbianine (6) (EC50 < 1 microg/cm2). This insect showed a moderate response to atropine. S. littoralis had the strongest antifeedant response to 24, 18, 14-O-acetyldelcosine (19), and delphatine (29) (EC50 < 3 microg/cm2). None of the model substances affected the feeding behavior of this insect. The most toxic compound to L. decemlineata was aconitine (1), followed by cardiopetalidine (10) (% mortality > 60), 14-deacetylpubescenine (14), 18-O-benzoyl-18-O-demethyl-14-O-deacetylpubescenine (17), 14-O-acetyldelcosine (19), 14-deacetylajadine (25) and methyllycaconitine (30) (% mortality > 45). Orally injected S. littoralis larvae were negatively affected by 1, cardiopetaline (8), 10, 1,14-O-acetylcardiopetalidina (11), 12, 14, 1,18-O-diacetyl-19-oxo-gigactonine (41), olivimine (43), and eserine in varying degrees. Their antifeedant or insecticidal potencies did not parallel their reported nAChR binding activity, but did correlate with the agonist/antagonist insecticidal/antifeedant model proposed for nicotininc insecticides. A few compounds [14, tuguaconitine (38), 14-demethyldelboxine (40), 19, dehydrodelsoline (36), 18-O-demethylpubescenine (13), 41, 9, and delcosine (23)] had selective cytotoxic effects to ward insect-derived Sf9 cells. None were cytotoxic to mammalian CHO cells and none increased Trypanosoma cruzi mortality. The selective cytotoxic effects of some structures indicate that they can act on biological targets other than neuroreceptors.

In vitro activity of C20-diterpenoid alkaloid derivatives in promastigotes and intracellular amastigotes of Leishmania infantum

The in vitro anti-proliferative effects are described of several atisine-type diterpenoid alkaloids against the protozoan parasite Leishmania infantum, which causes human visceral leishmaniasis and canine leishmaniasis in the Mediterranean basin, as well as human cutaneous leishmaniasis throughout the Mediterranean region. From a total of 43 compounds tested, including C19- and C20-diterpene alkaloids from several chemical classes, only 15,22-O-diacetyl-19-oxo-dihydroatisine, azitine and isoazitine were highly active against cultures of the parasite (promastigote form) with IC50 values within the range of the reference drug pentamidine-isothionate (7.39-12.80 mg/L for the test compounds, 11.32 mg/L for the positive control). These compounds were not toxic to the host cell. When treated with a dosage of 5 microg/mL of the active compounds (half of their IC50), the promastigote forms lost 80% of their infection capacity and the multiplication of extracellular forms of L. infantum was severely affected. The study showed that atisine-type C20-diterpenoid alkaloids exhibited promising anti-leishmanial properties with strong molecular selectivity. These might have implications for other intracellular pathogens- or phylogenetically related parasites, such as Trypanosoma spp.

Interaction of the structurally related aconitum alkaloids, aconitine and 6-benzyolheteratisine, in the rat hippocampus

Aconitine is a highly toxic diterpenoid alkaloid occurring in plants of the Aconitum genus. Aconitine is known to shift the voltage-dependence of the voltage-dependent Na(+) channel towards hyperpolarized direction, thereby leading to a permanent activation of the channel. 6-benzoylheteratisine is a plant alkaloid which is structurally related with aconitine. The aim of the present study was to investigate the interaction of aconitine and 6-benzoylheteratisine in the rat hippocampus. The experiments were carried out as extracellular recordings of stimulus evoked population spikes and field excitatory postsynaptic potential (EPSP) in rat hippocampal slices. Aconitine (10-100 nM) exerted a concentration-dependent decrease in the amplitude of the orthodromic population spike. When aconitine was applied in presence of 6-benzoylheteratisine (3 microM), the concentration-response curve was shifted to the right. Furthermore, the complete suppression of the population spike evoked by 100 nM aconitine was reversed by 10 microM 6-benzoylheteratisine. The closely related alkaloid heteratisine (3 and 30 microM), however, was not capable to antagonize the aconitine action. 6-benzoylheteratisine shifted the input-output relationship of the presynaptic fiber spike as function of the stimulation intensity and the input-output relationship of the field EPSP as function of the presynaptic fiber spike to the right. Thus, electrophysiologically this alkaloid seems to inhibit predominantly the excitability of the afferent fibres and, in consequence, neurotransmission between Schaffer collaterals and the CA1 neurons, thereby suppressing the firing of the latter. Spontaneously occurring epileptiform activity in area CA3 elicited by omission of Mg(2+) and elevation of K(+) was attenuated by 6-benzoylheteratisine (1 and 10 microM). Patch clamp studies performed on cultured rat hippocampal pyramidal cells revealed an inhibitory action of 6-benzoylheteratisine on whole cell Na(+) currents. It is concluded that the inhibitory and antiepileptiform effect of ajacine and lappaconitine is mediated by an inhibition of the voltage-dependent Na(+) channel which might be important for filtering high frequency bursts of action potentials characteristic for epileptiform activity in the hippocampus. Thus, 6-benzoylheteratisine seems to be a naturally occurring antagonist of the Na(+) channel activator aconitine.

Antagonism of the aconitine-induced inexcitability by the structurally related Aconitum alkaloids, lappaconitine and ajacine

Aconitine, lappaconitine and ajacine are structurally related alkaloids occurring in several species of the Aconitum genus. While aconitine is known to activate the voltage-dependent sodium channel, lappaconitine has been reported to block this channel. To investigate a possible antagonism of the aconitine action on neuronal activity by lappaconitine and the closely related alkaloid ajacine, we have performed extracellular recordings of stimulus evoked population spikes and field excitatory postsynaptic potential (EPSP) in rat hippocampal slices. Aconitine (10-100 nM) diminished the amplitude of the orthodromic population spike in a concentration-dependent manner. When aconitine was applied in presence of 10 microM lappaconitine, the concentration-response curve was shifted to the right. Furthermore, the complete suppression of the population spike evoked by 100 nM aconitine was reversed by 10 microM lappaconitine. The action of lappaconitine was mimicked by ajacine, however, the latter alkaloid was less potent. Both lappaconitine and ajacine shifted the input-output relationship of the presynaptic fiber spike as function of the stimulation intensity and of the field EPSP as function of the presynaptic fiber spike to the right. After pharmacological isolation, the presynaptic fiber spike was decreased by both compounds in a frequency-dependent manner indicative for a use-dependent action. Thus, electrophysiologically these alkaloids seem to inhibit predominantly the excitability of the afferent fibres and, in consequence, neurotransmission between Schaffer collaterals and the CA1 neurons, thereby suppressing the firing of the latter. Ajacine and lappaconitine inhibited stimulus-triggered epileptiform population bursts in area CA1 elicited by omission of Mg(2+) as well as spontaneously occurring epileptiform discharges in area CA3 elicited by omission of Mg(2+) and elevation of K(+). It is concluded that the inhibitory and antiepileptiform effect of ajacine and lappaconitine is mediated by a frequency-dependent inhibition of the voltage-dependent sodium channel, thereby decreasing the excitability which might be important for filtering high frequency bursts of action potentials characteristic for epileptiform activity in the hippocampus. Moreover, these alkaloids are naturally occurring antagonists of the sodium channel activator aconitine.

Frequency- and structure-dependent inhibition of normal and epileptiform activity by 6-benzoyldeltamine in rat hippocampal slices

The present study investigated the effects of the Aconitum alkaloids 6-benzoyldeltamine and the structurally related eldeline on neuronal activity in rat hippocampal slices. 6-Benzoyldeltamine (1-30 microM) decreased the orthodromic field potentials recorded in area CA1 in a concentration-dependent manner. The inhibitory effect of eldeline (3-100 microM) was lower. The attenuation of the postsynaptic population spike was accompanied by a simultaneous decrease in the presynaptic fibre spike evoked by electrical stimulation of the Schaffer collaterals. The input-output relationship of the presynaptic fibre spike as function of the stimulation intensity, and for the postsynaptic population spike as function of the presynaptic fibre spike was shifted to the right. Thus, electrophysiologically, these alkaloids seem to inhibit predominantly the excitability of the afferent fibres and, in consequence, neurotransmission between Schaffer collaterals and the CAI neurons, thereby suppressing the firing of the latter. The inhibitory action of 6-benzoyldeltamine revealed use-dependence as obvious by an enhanced attenuation of the antidromic spike when stimulation frequency was increased. 6-Benzoyldeltamine inhibited stimulus-triggered epileptiform population bursts in area CA1 elicited by omission of Mg2+, as well as spontaneously occurring epileptiform discharges in area CA3 elicited by omission of Mg2+ and elevation of K+. Complete suppression of spontaneous activity was observed at 1 microM 6-benzoyldeltamine, which reduced the population spike only by about 20% of control. It is concluded that the inhibitory and antiepileptiform effect of 6-benzoyldeltamine is mediated by a frequency-dependent decrease in excitability, which might be important for filtering high frequency bursts of action potentials characteristic for epileptiform activity in the hippocampus.