1. Anticonvulsant Effects
An abnormal and transient neuronal synchronization in the brain causes epilepsy. This phenomenon alters the correct pattern of neuronal connections and is characterized by the peaks and troughs of electric discharge in the electroencephalograph (EEG) and may lead to various mental and physical symptoms associated with the origin of the disruption
[1]. Voltage-gated sodium channels (VGSCs) play an essential role in the generation and coordination of action potential in the central nervous system, which participates fundamentally in the pathophysiology of seizures; thus, these channels could be an important aim for pharmaco-therapy
[2]. The Fuzi total alkaloids (Aconiti Lateralis Radix Praeparata), an essential medicine in traditional Chinese medicine, significantly prolong the seizure latency time and decrease the mortality in pentylenetetrazole-induced mice
[3]. Additionally, anticonvulsant effects of several alkaloidal-rich fractions and also isolated DAs from
Delphinium and
Aconitum species have been investigated (
Table 1;
Figure 1). Ameri’s findings have greatly enhanced the knowledge of the anticonvulsant effects of DAs
[4]. Five distinct toxin binding sites were explored on voltage-dependent Na
+ channels. Aconitine (
1), batrachotoxin, and veratridine are tightly bound to site 2 in the α subunit of Na
+ channels which are localized in the transmembrane region
[5][6][7]. The voltage-dependent Na
+ channels are activated by
1, resulting in the depolarization of the presynaptic membrane
[8]. Compound
1 activates Na
+ channels at resting membrane potential permanently by blocking Na
+ channel inactivation and causing hyperpolarization of threshold activation due to sustained Na
+ influx; thereby, inexcitability occurs
[4]. The structurally related 3-acetylaconitine (
2) was shown to activate voltage-dependent Na
+ channels by a similar mechanism of action
[9]. In contrast to compounds
1 and
2, some other
Aconitum DAs exist, revealing inhibitory effects on the Na
+ channels; lappaconitine (
3), N-desacetyllappaconitine (
4), 6-benzoylheteratisine (
5), and 1-benzoylnapelline (
6) reduce the peak amplitude of the current and show an inhibitory effect on the Na
+ channel
[4]. While
3 could antagonize the suppressive effect of 1 on population spike in rat hippocampal slices which are attributed to their opposite mode of action; both
1 and
3 possess an anticonvulsant potential
[5][10][11]. Compounds
1 and
2 indirectly suppressed the uptake of [3H] noradrenaline via increasing sodium concentration in synaptosomes, while
3 and
4 do not indicate such an impact on [3H] noradrenaline uptake
[4][12]. Despite Ameri’s investigations, Voss et al. showed acute seizure-like activity induced by
1 [13]. Songorine (
7) might antagonize the GABA receptor in the rat brain via a different site of action different from the GABA recognition site
[14]. However, in an in vivo experiment,
7 acted as an agonist on D2 and GABA A receptors, which might be translated as a potential antiepileptic effect
[15]. The involvement of α-adrenoceptors and modulation of GABA receptors were reported for the antiepileptic activity of mesaconitine (
8) and Fuzi total alkaloid, respectively
[3][16].
Figure 1. The chemical structures of the anticonvulsant diterpene alkaloids.
Table 1. Anticonvulsant effects of fractions/constituents from Aconitum and Delphinium species.
2. Antagonizing α7 Nicotinic Acetylcholine Receptor
Nicotinic acetylcholine receptors (nAChRs) are cation-conducting ligand-gated receptors broadly expressed in the peripheral and central nervous systems and also in muscular cells. Seventeen nAChR subunits, comprising ten α, four β, γ, δ, and ε subunits have so far been identified in mammals where α7 homomeric nAChRs and heteromeric α4β2* nAChRs are predominant and expressed functionally in the brain. These receptors contribute to a range of neuronal functions from cognitive performance to neurotransmitter release (including glutamate, GABA, and dopamine) through altering intracellular Ca
2+ concentration. It has been clarified that the pathogenesis of several neurological disorders containing Alzheimer’s disease, schizophrenia, Parkinson’s disease, and depression as well as nicotine addiction are related to α7 and α4β2* receptor functions. Although many efforts have been devoted to developing a (partial) agonist of α7 nAChR targeting neurologic and psychiatric conditions, none has already been approved as medicine due to their undesirable side effects
[33][34][35]. Moreover, results from a meta-analysis indicated that curing the cognitive dysfunction in schizophrenia and Alzheimer’s disease with agonists of α7 nAChR cannot be considered a robust treatment
[36]. In 1986, Jennings et al. found that the insecticidal effect of
Delphinium plants is mainly due to the presence of methyllycaconitine (
16) (
Figure 2a), showing a high affinity to cholinergic receptors of insects
[37]. Indeed,
16 is a potent and highly selective α7 nAChR antagonist which acts similarly to the α-bungarotoxin protein of snake venom (
Figure 2b)
[38]. Despite former research, sporadic studies have recently focused on the beneficial role which low doses of
16 play in neurologic disorders (
Table 2); equivalent results were also reported for other nAChR antagonists, e.g., mecamylamine
[39]. Several hypotheses have been proposed to explain their mechanism of action.
Figure 2. (a) Chemical structure of methyllycaconitine; (b) Homomeric α7 nicotinic acetylcholine receptor.
Table 2. Beneficial effects of low doses of methyllycaconitine (16) on neurologic functions.
Based on the allosteric theory of ion channel functioning, nAChRs might have three different conformational states, resting, active, and desensitized, having the capability of altering their conformational states through spontaneous transitions. Ligand affinity highly depends on conformational states. On the other hand, α7 nAChRs possess five binding sites for interacting with agonists, where binding to two of them is necessary for activating the receptor and opening the channel. Binding an antagonist, e.g.,
16, to remaining binding sites may prevent or delay the desensitization of the receptor and affect the affinity of the receptor to other ligands. As a further explanation, the stoichiometry of the binding site with which the acetylcholine molecules (AChs) interact determines the degree of channel activation. The binding of two AChs to two consecutive binding sites leads to slow channel activation, while binding to two non-consecutive sites results in rapid activation and receptor desensitization. The binding of
16 impacts the binding pattern of ACh to sites. In case one, two, or three molecules of
16 connect to the binding sites, the probability of non-consecutive binding of ACh increases, whereas binding four and five molecules of
16 eliminates the possibility of simultaneous binding of two ACh molecules, which is essential for the activation of the receptor. The hypothesis explains the contradictory responses of α7 nAChR to lower and higher doses of
16 (
Figure 3)
[35].
Figure 3. Binding of methyllycaconitine could affect the binding pattern of ACh to binding sites and forms the three different conformational states. (a) The binding of two AChs to two consecutive binding sites leads to slow channel activation; (b) The binding of two AChs to two non-consecutive sites results in rapid activation and receptor desensitization; (c) Binding less than two AChs to binding sites causes a deactivation and the channel will be closed.
3. Analgesic Activity
Nociceptive pain develops in response to specific noxious stimulation of normal tissue in a normal somatosensory nervous system; in contrast, neuropathic pain comes from damage to the nerves or nervous system; there is also a third kind of pain called inflammatory pain, which may be described as increased sensitivity due to the inflammatory response associated with tissue damage
[44]. Herbal preparations containing
Aconitum have been widely used in traditional Chinese medicine to relieve rheumatoid arthritis
[45]. The effectiveness of
Aconitum-containing preparations for diabetic peripheral neuropathic pain was shown in a small clinical trial
[46]. Pharmacological in vivo studies confirmed the antinociceptive activity of various DAs and extracts from
Aconitum species (
Table 3;
Figure 4). Bulleyaconitine A (
17), a diterpenoid alkaloid isolated from the rhizomes of
A. bulleyanum, is structurally related to
1 and was approved in 1985 in China for the alleviation of chronic pain and is available as tablets, intramuscular injections, and soft gel capsules
[47]. Compound
17 showed more significant analgesic potency than morphine. The clinical utility of
17 in the three past decades in China has demonstrated the advantages of its application for relieving chronic pain without any serious side effects, contrary to what is observed in the case of the application of morphine or non-steroidal analgesics
[48]. Several studies have emphasized the positive central effect of
17 in the treatment of nociceptive pains
[34]. The analgesic effect of
17 could be antagonized by reserpine but not naloxone, which suggests an involvement of catecholamine in the
17 mechanisms of action
[49]. Compound
7 revealed a visceral antinociceptive effect by stimulating the release of dynorphin A, leading to the activation of presynaptic κ-opioid receptors in afferent neurons
[37]. Consistently, it was indicated that
17 can directly stimulate the dynorphin A expression in spinal microglia
[50]. On the other hand, Xie et al. suggested a preferable blocking effect on tetrodotoxin-sensitive Nav1.7 and Nav1.3 in dorsal root ganglion neurons as the mode of action of
17 [51]. Compound
17 could also inhibit Nav1.7 and Nav1.8 Na
+ currents in a use-dependent manner
[52]. Depressing of long-term potentiation at C-fiber synapses in the spinal dorsal horn and inhibition of transmitter release in paclitaxel-induced neuropathic pain were accounted for as well
[53].
Figure 4. Chemical structures of the analgesic DAs.
Another diterpenoid alkaloid, lappaconitine (
3) from
Aconitum species revealed antinociceptive effects in nociceptive test models. Compound
3 has been used for its analgesic properties in both traditional Chinese and Kampo medicine
[54]. Its analgesic potency is comparable to that of tramadol and morphine
[55][56]. Pretreatment with antagonists of adrenergic and serotoninergic systems could decrease the antinociceptive effects of
3, which might be mediated by β-adrenoceptors and 5-HT2 receptors in the brain and α-adrenoceptors and 5-HT receptors in the spinal cord
[57]. Moreover, when
3 was administered via subcutaneous or intracerebroventricular routes, it could inhibit behavioral response induced by substance P or somatostatin, while the intrathecal injection showed no effect. These observations provide some evidence to support the supra-spinal descending mechanism of action of
3 and suppressing the transmission of the nociceptive neuronal message
[58]. Inhibition of P2X(4) expression in microglia in the dorsal horn and P2X3 receptors in dorsal root ganglia neurons in the spinal cord were demonstrated as further plausible mechanisms of action
[59][60][61]. Li et al. indicated that inhibition of the Nav1.7 channel is involved as well
[62]. Similar to
17 stimulation of the expression of spinal microglial dynorphin A was suggested
[63]. Ono et al. indicated that the antinociceptive effect of
3 was not antagonized by naloxone in the hot plate and acetic acid-induced writing tests, contrary to what was observed in the tail pinch test. It was suggested that the action of
3 was only partially antagonized by naloxone, which explains the naloxone-resistant antinociceptive activity for
3 at the spinal sites
[64].
Mesaconitine (
8), one of the most important ingredients of
Aconitum species, also exhibits antinociceptive activity in animal models. Subcutaneous administration of
8 has shown much greater analgesic activity compared to that of morphine in animal models
[65]. Friese et al. categorized
Aconitum alkaloids into two groups based on their binding affinity to Na
+ channel epitope site 2. Alkaloids of the high-affinity group (
1 and
8) are supposed to act as Na
+ activators due to their increasing synaptosomal [Na
+]i and [Ca
2+]i effect. A higher antinociceptive action was shown in the case of the high-affinity group than the other, indicating the importance of Na
+ channels in the activity of
8 [66]. The analgesic action of
8 was not inhibited by levallorphan (an opiate antagonist) in an animal model. On the other hand,
8 could increase the release of endogenous norepinephrine from sympathetic nerve fibers
[65]. Application cyclic AMP and isoproterenol (a β receptor agonist) in combination with
8 enhanced its analgesic activity, while propranolol (a β receptor antagonist) suppressed it, which magnifies the role of the adrenergic system in the mode of action
[67]. Involvement of the nucleus reticularis gigantocellularis (NRGC), the nucleus reticularis paragigantocellularis (NRPG), the periaqueductal gray (PAG), and the lumbar enlargement in the antinociceptive action of
8 was shown in a rat model. Indeed,
8 might activate the inhibitory noradrenergic system in descending neurons from the NRPG, especially through β-receptor-mediated effects of noradrenaline
[57][58]. Moreover, it was also revealed that the nucleus raphe magnus (NRM) has a key role via the serotoninergic system in the mechanism of action
[68].
Isotalatizidine (
18) attenuates allodynia in neuropathic pain in a mice model. An increase in the expression of dynorphin A was shown, and its release from microglia was observed, which might be triggered by the activation of the ERK1/2 pathway and phosphorylation of CREB (cAMP response element-binding)
[69].
Bullatine A (
19) was shown to have a synergistic effect with morphine as it increases morphine analgesic action while counteracting morphine tolerance. Stimulation of spinal microglia and subsequent enhancing of the expression of dynorphin A were thought to have a role
[70][71].
Table 3. Antinociceptive effects of fractions/constituents from Aconitum and Delphinium species.
4. Antidementia Effect
A decrease in acetylcholine (ACh) in the central nervous system is associated with dementia. Several DAs possess an acetylcholinesterase (AChE) inhibitory effect (
Table 4;
Figure 5), and therefore, these compounds may theoretically act as antidementia agents. Some evidence has emphasized the role of K
+ channel blockers as drug candidates for the treatment of neurodegenerative disorders
[90]. Virtual screening from the Chinese natural product database resulted in the identification of four
Aconitum alkaloids, namely, songorine (
7), pyrochasmaconitine (
37), 14-benzoyl talatisamine (
12), and talatisamine (
13), as candidates for this effect; an electrophysiological assay of rat dissociated hippocampal neurons verified this assumption
[91]. All of the four alkaloids potently inhibited the delayed rectifier K
+ current, while
13 was slightly effective on Na
+ and Ca
2+ channels in rat hippocampal neurons
[90].
Figure 5. Chemical structures of diterpene alkaloids with antidementia activity.
Macroautophagy was previously considered one of the plausible mechanisms of the pathogenesis of Alzheimer’s disease (AD). Compound
16 protects the human neuroblastoma SH-SY5Y cell line against amyloid-β peptide-induced cytotoxicity. Compound
16 might prevent autophagy induced by amyloid-β through the mammalian target of the rapamycin (m-tor) pathway
[92]. Apetalrine B (
38), the semisynthetic DAs from its parent compound, aconorine (
A. apetalum), exhibits a neuroprotective effect in H
2O
2-treated SH-SY5Y cells by inhibiting cell apoptosis
[93]. Firstly,
7 was superior to piracetam (a nootropic agent) in the correction of the cholinergic abnormalities induced by scopolamine, which indicates its antiamnestic activity. Secondly,
7 might act as a cholinomimetic, antihypoxic, and cerebro-protective agent
[94][95]. Among the DAs from
A. anthoroideum, rotundifosine F (
39) possesses AChE inhibitory activity. Moreover, nominine (
40) represented a neuroprotective effect due to its protection against 1-methyl-4-phenylpyridinium (MPP+)-induced apoptosis in SH-SY5Y cells
[96]. Fuzi considerably induced growth of cell projection and cell proliferation in 0.4–0.8 mg/mL and 1.6–100 mg/mL, respectively, in the AD cell model. In addition, the involvement of GRIN1 and MAPK1 genes contributed to the antiAD mechanism of action
[97].
Table 4. Antidementia effect of fractions/constituents from Aconitum and Delphinium species.
5. Antidepressant Effects
Two antagonists of nicotinic acetylcholine receptor (nAChR) were demonstrated to induce antidepressant effects in the forced swim and tail suspension tests in mice, while agonists of nAChR did exhibit no similar activity. The observed effect was more potent in the case of the non-selective nAChR antagonist mecamylamine compared to that of
16 (an α7 nAChR selective antagonist)
[106]. Modulating serotonin sensitivity is considered a promising mechanism of action of DAs’ antidepressant activity in
Aconitum baicalense. Besides their positive results in the tail suspension test, these compounds showed an antiexudative effect in edema induced by serotonin in mice. These alkaloids were
7,
8,
32, and
31 in order from most to least potent
[107]. Antidepressant actions were also reported for chronic administration of Fuzi total alkaloids in normal and ovariectomized mice. An study designed to elucidate the mode of action discovered the potential roles of cAMP response element-binding (CREB) and brain-derived neurotrophic factor (BDNF) protein pathways
[108]. Moreover, the sex-dependent antidepressant effect of
16 was consistently reported in male mice, while female mice did not exhibit the same. Local administration of
16 in male mice hippocampus could reverse the depression-like effect which was induced by physostigmine
[109].
6. Miscellaneous Effects
Compound
7 has exhibited a more potent anxiolytic activity compared to phenazepam applying the Vogel conflict test in mice without a sedative effect. Anxiolytic activity of
7 was noted in a dose of 0.25 mg/kg, while a 10 times higher dose (2.5 mg/kg) showed an inferior activity
[110].
An ethanolic extract of
Delphinium denudatum has protected the brain against injury in a rat model of Parkinson’s disease. The expression of ipsilateral tyrosine hydroxylase was increased. The extract also prevented lipid peroxidation, depleted reduced glutathione in the substantia nigra, and attenuated the activity of superoxide dismutase and catalase in the striatum. Reduction in dopamine concentration was also reported in the striatum after the injection of 6-hydroxydopamine
[111].