Progress on Active Analgesic Components and Mechanisms of Commonly Used Traditional Chinese Medicines: A Comprehensive Review

Many clinical diseases are accompanied by the symptoms of pain, and the degree of pain is closely related to the patients’ suffering. Therefore, effectively relieving pain has become one of the vital concerns of clinical treatment and analgesic drug research. Non-opioid drugs are mainly used for the clinical treatment of mild to moderate pain, whereas opioid drugs are mainly used for treating moderate to severe pain. However, opioid drugs easily elicit adverse reactions, such as gastrointestinal discomfort, addiction, dependence, and so on. Traditional Chinese medicine and its active ingredients have unique advantages in the treatment of pain for quite a long time, and many analgesic drugs directly or indirectly were isolatiedfrom Chinese medicine or natural products, such as Liu Suan Yan Hu Suo Yi Su Pian and aspirin. With the development and modernization of research on herbal medicine more and more studies have been conducted on the active ingredients and mechanisms of traditional Chinese medicine analgesics. However, no review has been done on analgesic active components and their mechanisms. In this paper, 81 active components with clear chemical structure and definite analgesic effects in vivo and in vitro of traditional Chinese medicine and mechanisms of action reported in recent literatures are reviewed and summarized to provide reference for clinical analgesia and analgesics research. _______________________________________________________________________________________


INTRODUCTION
The International Association for the Study of Pain revised the definition of pain as follows: Pain is a mutually recognizable somatic experience that reflects a person's apprehension of threat to their bodily or existential integrity (1). Acute pain, which occurs as a response to a specific injury, has a biological importance and self-limiting character. By contrast, chronic pain is a disease state that may outlast the usual duration of recovery if accompanied with a disease or injury (2). Pain not only affects the quality of life and work efficiency, but also creates a huge financial burden, with a report showing that chronic pain alone costs as much as $635 billion a year in the United States (3). Pharmacotherapy is often used for pain relief.
Non-opioid drugs are used in the treatment of mild to moderate pain, whereas non-opioid drugs are used for moderate to severe pain treatment (4). However, for most patients with pain, the analgesic effect of opioid drugs is limited, and their long-term use can lead to abuse, addiction, and overuse, with the latest data showing that about 16,000 overdose deaths associated with prescription opioids are reported each year, and the incidence of iatrogenic opioid dependence or abuse was 4.7% of those prescribed with opioids for pain (5-7). Traditional Chinese medicine (TCM) has been used in the treatment of pain for more than one thousand years, and scientists studying herbal medicines have found more than 800 kinds of TCM to be effective in relieving pain, while also creating several monomeric compounds to develop novel analgesic drugs. TCM has the advantages of good efficacy, is non-addictive, has less adverse reactions, and has an abundant supply, which attracted the attention of clinicians in the field of pain treatment. _________________________________________ With the increasingly progress of medicine and technology and the better understanding of natural medicine, the interest and acceptance of TCM have been increasing, and its role in health has been widely recognized in the world (8-10). In recent years, the exploration of TCM in analgesia has become one of the hotspots of modern Chinese medicine research.
In modern pharmacological study, plenty of compounds with significant analgesic activity were isolated from TCM, including alkaloids, flavonoids, terpenoids, aromatic compounds, coumarins, aliphaticnatural products, and lignans. In this paper, the analgesic effect and mechanism of these compounds are reviewed, to provide a reference for the development and utilization of TCM for analgesia.

ANALGESICS BASED ON THE COMPOUNDS' PHYTOCHEMISTRY
Many classic analgesics, such as morphine, codeine, and aspirin are isolated from natural products. Therefore, the study on the analgesic effect and mechanisms of active components and the discoveries of new analgesic drugs from plants provide grounds for innovative research on analgesic drugs.
Upon a literature survey we identified a total of 81 compounds with significant analgesic activities, including 40 alkaloids, 15 flavonoids, 10 terpenoids, 5 aromatic compounds, 8 coumarins, 2 aliphaticnatural products, and 1 lignan. A compound may exist in a variety of herbal medicine, and we have summarized all its sources (Table 1). The structures of these compounds and their analgesic activities are shown in Figure. 1

Terpenoids
Terpenoids are the most structurally diverse class of plant natural products biosynthesized from isoprene units with a general formula of (C5H8)n. Terpenecompounds are the main components of volatile oils, also known as essential oils, and have wide physiological activities, such as expectorant, relieving cough, transpiration and analgesia. A total of 10 terpenoids (56-65) have been reported to show analgesic activities.

Aromatic compounds
Aromatic compounds are often used in cosmetics, consumer goods, and in the medical field. These compounds are widely found in Chinese herbal medicines, such as Paconia lactiflora, Saposhnikouia diuaricata, Cinnamomum japonicum, and so on, and some aromatic compounds (66-70) in TCM show good analgesic effect.

Aliphaticnatural products
Aliphatic naturalproducts are an important class of compounds in the organic chemistry industry. They can be used in spices and in the preparation of some drugs according to their properties. Two aliphatic natural products, such as fumaric acid (79) isolated from Corydalis yanhusuo and embelin (80) isolated from Embelia oblongifolia, Ardisia crenata, and Embelia ribes, also showed good analgesic activities.

Lignans
Lignan is a class of secondary metabolite that is usually in the form of a glycoside and consists of two phenyl-propanoid molecules connected by 8-8′carbon atoms. Liriodendrin (81) was the only lignan reported to have significant analgesic activity.

ANALGESIC ACTIVITIES
The analgesic activities of monomeric compounds isolated from TCM in vivo were studied. Experimental animal models of acute and chronic pains are the two major types of pain model for studying analgesic effects. Studies of acute pain model include chemical stimuli, such as acetic acid, capsaicin, glutamate, acetylcholine, and pbenzoquinone-writhing test, formalin test, and physical stimulation, such as hot or cold-plate test, tail-flick test, electrical stimulation test, Randall-Selitto test, tail immersion test, Hargreaves test, and the mechanical von Frey test. Studies have shown that chronic pain are induced by chronic constriction injury (CCI), spared nerve injury(SNI), spinal cord injury (SCI), and partial sciatic nerve ligation (PSNL). Other types include paclitaxel-induced neuropathic pain, diabetic neuropathic pain, complete Freund's adjuvant (CFA)-induced pain, LPS-induced inflammation pain, and cancer pain. The experimental studies on the analgesia of active compounds are summarized in Table 2.

ANALGESIC MECHANISM
Modern studies have shown that compounds isolated from TCM play a key role in the regulation of painrelated signaling pathways (10). The mechanisms of analgesia could be roughly divided into the central nervous system to exert a central analgesic effect and through the peripheral nervous system to relieve pain by reducing the secretion of analgesic substance, alleviating the accumulation of local analgesic substances, increasing the release of peripheral endogenous analgesic matter, and regulating c-Fos genes. Moreover, some compounds relieve pain by inhibiting the process of inflammation.
Activate opiate receptors and increase opioid peptide levels Opioid peptides (β-endorphin, encephalin, and dynorphin) are widely distributed in the hypothalamus, brain, and spinal cord. They bind to opioid receptors, μ (mu or MOP) receptor, δ (delta or DOP) receptor, and κ (kappa or KOP) receptor, and reduce the release of nociceptive substances and produce strong analgesic effects.
p-Cymene, an aromatic monocyclic monoterpene, could found in the volatile oils of more than 100 plants and naturally occurs in over 200 foods (12). The analgesic effects of p-cymene in tail-flick test was antagonized by naloxone (nonselective antagonist of opioid receptors), naltrindole (δ-opioid receptor antagonist), nor-BNI (κ-opioid receptor antagonist), and CTOP (μ-opioid receptor antagonist). These suggested that the antihyperalgesia of p-cymene maybe related to the opioid system (13). Naloxone and nor-NBI could antagonize the antinociceptive effect of menthol, which is a natural cooling product of peppermint and is widely used as an analgesic agent for pain conditions, such as sports injuries and arthritis, but CTOP, 7-benzylidenenal-trexone (δ1 antagonist) and naltriben (δ2 antagonist) could not. These suggest that menthol mediates analgesic properties through the selective activation of ҡ opioid receptors (14, 15).
Capsaicin, the main ingredient in hot pepper, has an analgesic action in humans. Capsaicin significantly could increase the proopiomelanocortin mRNA levels in the arcuate nucleus of rats, suggesting that the analgesic effect of capsaicin couldbe associated with the increased activities of cerebral opioid systems (16). Quercetin which could be widely found in flowers, leaves, and fruits of plants and found in more than 100 Chinese herbs, could relieve cancer pain and diabetic neuropathic pain via the opioid-dependent analgesic pathways (17). Curcumin, a yellow pigment extracted from zingiberaceous plants, has a wide range of therapeutic effects, such as anti-inflammatory, antibacterial, anti-viral, anti-fungal, anti-tumor, antispasmodic, and hepatoprotective effects. Curcumin could relieve diabetic peripheral neuropathic pain, and its mechanism could be related to the opioid system (18).
The analgesic effect of scotanamine B, an amide alkaloid from Scopolia tangutica, was recorded using the tail-flick assay and was reversed by naloxone. Scotanamine B displayed agonist activity at the μ receptor with an EC50 value of 7.3 Μm. Naloxone could also antagonize analgesic effects of govaniadine in the hot-plate test and the inhibition of paeoniflorin on bee venom-induced persistent spontaneous nociception (19-21). At the same time, naltrexone partially blocked analgesic effect in thehot-plate test of hesperidin (hesperetin-7rhamnoglucoside), which is the main flavonoid in sweet oranges and lemons with anti-inflammatory, sedative, and analgesic effects. Although the analgesic effect of hesperidin is mediated through opioid mechanisms, it does not directly bind to and activate μ opioid receptors and has no effect on the inward GIRK1/2 currents (22, 23).
Bullatine A, a C20-diterpenoid alkaloid, is one of the main active components of Aconitum brachypodum. Bullatine A could reduce pain hypersensitivity in the rat model of neuropathic pain, inflammatory pain, diabetic neuropathic pain, or bone cancer pain but could not block the acute nociceptive response effectively under normal conditions. This alkaloid specifically stimulates the expression of dynorphin A in spinal microglia in vivo and in the cultured primary microglia in vitro. The stimulatory effect of bullatine A was completely inhibited by the microglial inhibitor minocycline, and its spinal anti-allodynic effects were completely blocked by intrathecal injection of minocycline, the specific dynorphin A antiserum, and the selective ҡopioid receptor antagonist. Thus, the expression of spinal microglial dynorphin A could be stimulated to mediate bullatine A anti-nociception under the pain hypersensitivity conditions (24).

Affect cholinergic nerve system
The two types of cholinergic receptors are muscarinic (M) receptors and nicotinic (N) receptors. M cholinergic receptors are divided into five subtypes, M1-M5 and M2/M4 receptors are involved in mediating muscarinic analgesia. In addition, the agonists of nicotinic and muscarinic acetylcholine receptors are being evaluated as candidate analgesics and may represent a new therapeutic strategy for the treatment of pain (25).
Previous studies suggested that huperzine A, a natural plant alkaloid isolated from Huperzia serrata, is a potent analgesic with few side effects. In 2015, results of the conditioned place preference behavioral assay showed that huperzine A could attenuate mechanical allodynia induced by peripheral nerve injury, and this effect was blocked by the M-receptor antagonist atropine. Furthermore, ambenonium chloride, a competitive inhibitor of acetylcholinesterase, also increased paw-withdrawal threshold, but failed to induce place preference in conditioned place preference. Therefore, acetylcholinesterase in both the peripheral and central nervous systems were involved in the regulation of mechanical allodynia by huperzine A (27).

Affect central catecholaminergic system
Central catecholaminergic systems mainly include norepinephrine (NE), adrenaline (Adr), dopamine (DA), and so on, which are not only neurotransmitters, but also can cause pain directly and participate in pain and analgesia. In the peripheral nervous system, they stimulate sensory nerve endings and cause pain as pronociceptives through second messenger action on the local and paracrine signaling. In the central nervous system, they have inhibitory and excitatory effects on the neurons. However, the inhibition effect is the main factor resulted in pain threshold is raised.
Mesaconitine is a principal alkaloid from Aconitum carmichaeli and Aconitum kusnezoffii. The analgesic effect of mesaconitine was decreased by α-methyl-p-tyrosine, 6-hydroxydopamine, diethyldithiocarbamate, disulfiram, and reserpine, and was increased by methamphetamine and norepinephrine. In addition, mesaconitine promoted the α-methyl-p-tyrosine-induced decrease in norepinephrine levels in hippocampus, medulla oblongata plus pons, and spinal cord. Thus, the analgesic activity mediated by mesaconitine is closely related to responses involving thecentral catecholaminergic system, in particular, the noradrenergic system (28). Mesaconitine increases the excitabilities in rat hippocampal pyramidal cells by enhancing the extraneuronal noradrenaline level through inhibition of noradrenaline uptake (29). Results of tail-immersion and hot-plate tests showed that pretreatment with 6-hydroxydopamine (50 μg, i.c.v. or 20 μg, i.t.) or 5,7-dihydroxytryptamine (80 μg, i.c.v. or 20 μg, i.t.) couldclearly reduce lappaconitine, an important bisnorditerpenoid alkaloid. Meanwhile, the analgesic effect of lappaconitine (i.c.v.) could be reduced by pretreatment with β-adrenergic antagonist timolol. The i.t. administration of lappaconitine induces a strong analgesic effect, which can be reduced by pretreatment with α-adrenergic antagonist phenoxybenzamine. These data suggested that central norepinephrine influences the analgesic effect of lappaconitine and these pathways are mediated by the expression of β-adrenoceptors and α-adrenoceptors in the spinal cord (30).
Paeoniflorin and albiflorin have obvious analgesic effect in tail-pressure test in diabetic mice shown by measuring the struggling behavior as an index of threshold. Yohimbine, an α2-adrenoceptor antagonist, could abolish completely the antinociceptive effect of paeoniflorin, and albiflorin increased noradrenaline release and activatedα2adrenoceptor to modulate spinal nociceptive transmission in diabetic neuropathy (31).
Moreover, levo-tetrahydropalmatine, a tetrahydroprotoberberine isoquinoline alkaloid, is a primary active constituent from the genus Corydalis. Levo-tetrahydropalmatine does not influence motor function but exerts antihyperalgesic effects that can be abolished by a dopamine D1 receptor antagonist SCH23390 (0.02 mg/kg). Thus, this alkaloid improves mechanical hyperalgesia by enhancing dopaminergic transmission mediated by dopamine D1 receptor (32).
Increase the content of 5-hydroxytryptamine (5-HT) 5-HT is one of the main neurotransmitters involved in the descending control of pain or emotion in the central nervous system and it works by binding with specific 5-HT receptors (33). The analgesic effect of lappaconitine was reduced by pretreatment with ketanserin, a 5-HT2 antagonist, and mianserin, a 5-HT1 antagonist. Thus, the analgesic mechanism of lappaconitine may be related to the 5-HT2 receptor in the brain and the 5-HT1 receptor in the spinal cord (30).
Inhibite neurotransmitter γ-aminobutyric acid (GABA) GABA, the major inhibitory neurotransmitter in the central nervous system, has three types of receptors, namely, GABAA, GABAB, and GABAC. The dysfunction or deficiency of GABAergic system is associated with epilepsy, pain, and anxiety (36). Recent studies have demonstrated in a rat model of neuropathic pain that the GABAA receptor antagonist bicuculine dose-dependently blocked the antinociceptive effects of sinomenine, a main bioactive ingredient in Sinomenium acutum, which is well known to have anti-rheumatism and immunomodulatory effects (37).
Chrysin (5, 7-dihydroxyflavone) is a natural flavone commonly found in many plants with a wide range of biological activities, including antioxidant, anti-allergic, anxiolytic, and vasorelaxant activities. Tail-immersion tests also show that chrysin has significant analgesic activity, which can be significantly and dose-dependently suppressed by pretreatment with flumazenil, a specific antagonist for benzodiazepine sites associated with GABAA receptors and with bicuculline, a GABAA receptor antagonist. These results indicate that the analgesic effect of chrysin is acted on GABAA receptors (38).
Another study showed that the oxymatrinecarbenoxolone sodium inclusion compound had obvious analgesic effect in the hot-plate test, tail immersion test, acetic acid induced abdominal constriction, and formalin-induced pain, and simultaneously increased the GABAA α1 receptor expression in the spinal cord, the cerebral cortex, and the hippocampal region of mice (39).

Regulate ion channels
Calcium ion (Ca 2+ ) overload and Na + currents are instrumental in the etiology of pain and neuropathy. Ca 2+ enters into cells by different ways including cation channels, chemical channels, and voltage gated calcium channels. Furthermore, the two types of neuronal voltage-gated Na + channels are tetrodotoxin-sensitive and tetrodotoxin-resistant. Studies show that Na + channels have demonstrated a great involvement in inflammatory pain and in pain sensation (36,40).
Rhoifoline A, a benzodihydropyridine alkaloid obtained from Zanthoxylum nitidum, could inhibit the chemical nociception induced by acetic acid and formalin, the thermal nociception in the hot-plate test and tail-flick test. The analgesic effect of rhoifoline A was significantly antagonized by nimodipine, a blocker of L-type Ca 2+ channels in the hot-plate test. Therefore, the analgesic mechanism of rhoifoline A possibly involved L-type Ca 2+ channels (41).
Hyperin, an important natural product, has antiinflammatory, antispasmodic, diuretic, antitussive, antihypertensive, cholesterol-lowering, central analgesic, cardio-cerebral vascular protection, and other physiological activities. Low or high-calcium can significantly strengthen or antagonize the inhibitory effect of hyperin on nerve dischargeinduced histamine and potassium chloride. A23187, which promotes the influx of Ca 2+ , could antagonize the effect of hyperin. These findings suggest that the analgesia of hyperin is closely related to the reduction of Ca 2+ influx of sensory nerve endings (42). Sinomenine suppressed formalin-induced pain behavior only in the first phase, but not the second phase. Sinomenine also significantly reduced voltage gated sodium currents in a dose-dependent manner (IC50=2.3±0.2 mM), suggesting that sinomenine has a peripheral analgesic effect (43).
Oxymatrine, a natural quinolizidine alkaloid, is the main basic constituent derived from the root of Sophora flavescens and the seeds of Sophora alopecuroides. The inhibition of voltage-activated K + channel plays an important role in the analgesic effect of oxymatrine. Meanwhile, oxymatrine could decrease Ca2 + in cultured dorsal root ganalia neurons, decrease protein expression levels of Cav2.2 in the brain tissue, and increase protein expression levels of Cav2.2 in dorsal root ganalia tissues (44, 45).
Tetrandrine is a bisbenzylisoquinoline alkaloid, which mainly exists in Menispermaceae. Previous studies have shown that it possessed antiarrhythmic, antihypertensive, cardioprotective, antitumorigenic, anti-inflammatory, and analgesic effects (47). The analgesic effect of tetrandrine could be significantly antagonized by CaCl2 through intracerebroventricular or intraperitoneal administration. On the contrary, EGTA could enhance the analgesic effect of tetrandrine, suggesting that the analgesic mechanism of tetrandrine might be related to calcium antagonists (47).
A study showed that menthol decreased inflammatory pain induced by CFA in a dosedependent manner, and formalin-induced spontaneous nocifensive behavior. Menthol blocked voltage-gated sodium channels and voltage-gated calcium channels in a voltage-, state-, and usedependent manner. Furthermore, repetitive firing, action potential amplitude, and neuronal excitability were decreased by menthol. At the same time, spontaneous synaptic transmission of cultured superficial dorsal horn neurons was blocked by menthol. Menthol has central analgesic effect on inflammatory pain, which may be related to the blocking of voltage-gated Na + /Ca 2+ (48).
Studies showed that the natural coumarin osthole has a variety of pharmacological effects, such as anti-tumor, anti-convulsant, antiinflammatory, osteogenic, anti-hepatitis, neuroprotective, and analgesic activities, and has a obvious effect on nucleus pulposus-evoked pain by inhibiting overexpression of acid-sensitive ion channel 3 in rat dorsal root ganglion and inhibiting the activation of extracellular signal-regulated kinase in rats (49, 50) Inhibit of glutamate receptor Glutamate receptors, which are the major excitatory neurotransmitter receptors in the brain and play an important role in analgesia, has different subtypes of glutamate receptors each of which can be divided into several subtypes, such as NMDA receptors (GluN1 to GluN3), AMPA receptors (GluA1 to GluA4), kainate receptors (GluK1 to GluK5), and mGlu receptors (mGluR1 to mGluR8). Almost all types of glutamate receptors are involved in the formation of hyperalgesia (51).
Gelsemine is the main active ingredient of Gelsemium elegans and shows significant analgesic activity in various chronic pains models, such as formalin-induced tonic pain, spinal nerve ligationinduced painful neuropathy, and bone cancerinduced mechanical allodynia. Strychnine, the glycine receptor (GlyR) antagonist, could relieve the analgesic effect of gelsemine with an apparent ID50 value of 3.8 μg (52). The analgesic action of gelsemine in neuropathic pain was prevented by gene ablation of the GlyR α3 subunitnearly and completely, but not GlyR α1, indicating that gelsemine generates analgesic effect via spinal α3 glycine receptors. Gelsemine directly regulates recombinant and native glycine receptors and play conformational specificity and subunit selectivity effects (53).
Oxymatrine could lower the threshold of CCI mice in mechanical allodynia and thermal hyperalgesia test. At the same time, the mean IOD of NR2B in the dorsal horn and expression levels of NR2B, p-ERK, and p-CREB protein in the chronic neuropathic pain model were decreased by oxymatrine. Thus, the regulation of NMDA NR2B receptor-ERK/CREB signaling may be the targets for the antinociceptive effects of oxymatrine (56).
In addition, recent reports confirmed that paeoniflorin exerted central analgesic effect through adenosine A1 receptor by inhibiting colorectal distention-evoked glutamate release and the NMDA receptor dependent extracellular signal-regulated protein kinase (p-ERK) signaling in rats with neonatal maternal separation-induced visceral hyperalgesia (57).
Affect transient receptor potential (TRP) Transient Receptor Potential (TRP) family of ion channels expressed on nociceptors, the TRP superfamily contains 28 channels with 7 different subgroups, and TRPA1, TRPV1 and TRPV4 have been associated with pain transmission of sensory neurons, including DRG (40).
Hyperpolarization-activated inward currents are blocked by capsaicin via TRPV1 in the rat dorsal root ganglion neurons. The analgesic effects of capsaicin reportedly cause the densensitization of nociceptive neurons owing to depletion of painrelated substances (58). Apart from this, studies have shown that the aporphinic alkaloid dicentrine could attenuate spontaneous nociception and mechanical cold hypersensitivity in inflammation pain model probably via a TRPA1-dependent pathway (59).
Borneol, also known as bingpian or longnao in Chinese, is a bicyclic monoterpene compound and a time-honored herb from Cinnamomum tree in TCM, has been used for more than 2,000 years in clinical applications. The TRPM8 channel was identified as a molecular target for borneol in the pain mouse models induced by CFA, capsaicin, and formalin, showing that topical borneol-induced analgesia was almost exclusively mediated by TRPM8 (60).
1, 8-cineole, a terpene oxide and a TRPM8 agonist, is the main component of most eucalyptus oil (75%), rosemary (40%), and many other essential oils. The sensory irritation tests in vivo showed that 1, 8-cineole conferred an analgesic effect due to its inhibition of TRPA1 (61).
Additionally, L-menthol could effectively alleviate pain behavior in many pain models, such as chemical stimuli (capsaicin, acrolein, acetic acid), noxious heat and inflammation pain induced by CFA. The genetic deletion of TRPM8 completely abolished the analgesia caused by L-menthol in all these models, while other analgesics (acetaminophen) remained effective. When mice were treated with AMG2850, a selective TRPM8 inhibitor, the analgesia effect of L-menthol disappeared. Consequently, TRPM8 is the principal mediator of menthol-induced analgesia of acute and inflammatory pain (14). TRPV1 receptor was reported to be involved in the analgesic effect of hesperidin and curcuminoid in inflammatory pain model (62, 63).

Influence adenosine system
The adenosine receptor system is promising for pain treatment. Adenosine receptors are widely distributed not only in the spinal cord and brain areas involved in pain transmission but also in peripheral sensory afferents or adjacent cells.
Incarvillateine mainly exists in Incarvillea sinensis, a traditional Chinese medicine used to treat rheumatism and bruises and is very effective for pain and inflammation. Incarvillateine dose-dependently attenuated acetic acid-induced writhing, thermal hyperalgesia of CFA inflammatory pain, and mechanical allodynia of SNI or paclitaxel-induced neuropathic pain. Incarvillateine-induced analgesia was attenuated by theophylline (nonselective adenosine antagonist), 1, 3-dipropyl-8cyclopentylxanthine (A1 agonist), and 3, 7-dimethyl-1-propargylxanthine (A2 agonist). These findings showed that the analgesic mechanism of incarvillateine may be related to the adenosine system in inflammatory and neuropathic pain models (64).
Levo-tetrahydropalmatine has remarkable analgesic effect and as an analgesic has been used clinically for more than 40 years. Treatment with levo-tetrahydropalmatine suppressed the increase of mechanical allodynia and spinal phosphorylation of the NMDA receptor NR1 subunit expression in CCI mice model. Intrathecal treatment with levotetrahydropalmatine combined with the Sig-1R antagonist, BD1047, synergistically blocked mechanical allodynia. Intrathecal pretreatment with naloxone, a non-selective opioid receptor antagonist, did not affect levo-tetrahydropalmatine. These results show that analgesic effect of levotetrahydropalmatine modulates spinal Sig-1R activation (65).
Paeoniflorin significantly attenuates paclitaxelinduced allodynia, suppresses saphenous nerve firing, and inhibits demyelination in the plantar nerve. Moreover, paeoniflorin down-regulates the paclitaxel-induced expression of CHOP in cultured Schwann cells, thereby inhibits ER stress. Adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-diprooylxanrhine could inhibit attenuation of mechanical allodynia caused by paclitaxel and down-regulate CHOP levels in cell cultures induced by paeoniflorin. Hence, adenosine A1 receptor plays an important role in the analgesic effect of paeoniflorin (66).

Affect purinergic receptors
Adenosine triphosphate is thought to play a critical role in nociceptive transmission or pain signals. Adenosine triphosphate is implicated in peripheral pain signaling by acting on P2X receptors. The seven P2X subtypes are widely expressed in many tissues (67).
P2X7 receptors have an important role in immune and pain response, and bullatine A as a potent P2X7 antagonist could dose-dependently inhibit ATP-induced upregulation of P2X7 mRNA, but had no obvious effect on P2X4 mRNA level in BV-2 cells. P2X pathways may be a possible mechanism for the analgesia of bullatine A (68).
Inhibit c-Fos gene expression c-Fos has been the subject of study in relation to the pathophysiology of pain as a possible tool to aid in its understanding. In recent years, c-Fos has been used as a useful instrument for studying natural products with analgesic profile (69).
Gelsemine is effective for the treatment of neuropathic pain. After treatment with gelsemine, the mechanical thresholds and thermal latencies were prolonged in PSNL mice. A previous immunohistochemical study had shown that PSNL upregulated c-Fos expression in the neurons of the anterior cingulate cortex while gelsemine decreasedc-Fos expression by 58% (70). Besides, intrathecal pretreatment of paeoniflorin was effective in the management of bee venom-induced pain via suppression of spinal c-Fos expression in both superficial (lamina I-II) and deep (lamina IV-VI) layers of the L4-5 dorsal spine (21).
Inhibit cyclooxygenase (COX) activity and reduce the secretion of pain-causing substances Some compounds can reduce production of cytokine substances, including interleukin (IL), tumor necrosis factor (TNF), prostaglandins, and other peripheral pain-causing substances to produce an analgesic effect. NO is a kind of important information transmitting substance and neurotransmitter. Increasing the level of cyclic guanosine monophosphate (cGMP) in target cells by NO-cGMP pathway is one of the ways in which NO participates in analgesia. Nitric oxide synthase (NOS) is a key enzyme in the synthesis of NO, and its activity changes directly regulate the amount and biological effects of NO. Berberine has significant analgesic effect, and findings on visceral hypersensitivity in rats show that berberine decrease visceral hypersensitivity by increasing NO levels. In addition, quercetin could attenuate oxaliplatininduced chronic painful peripheral neuropathy, and the mechanism is related to NO and peroxynitrite (71, 72). Lycopene, a natural pigment is recognized by the Food and Agriculture Organization of the United Nations and the World Health Organization as a class A nutrient, and it is also the hotspot in the functional food, medicine, and cosmetics industries. Lycopene can reduce diabetic neuropathic pain by inhibiting the release of NO and TNF-α (73,74).
Tetrandrine exerted strong antinociceptive effects on LPS-induced hyperalgesia in mice by inhibiting IKKβ phosphorylation, which reduced the production of important pain mediators, such as PGE2 and COX-2, via the IKKβ/IҡB/NF-ҡB pathway (46). Meanwhile, the analgesic mechanism of tetrandrine may be through the inhibition of IL-6 production in blood and reduction of TNF-α level in plasma of endotoxin-induced mice (75).
Paeoniflorin and albiflorin could attenuate neuropathic pain by inhibiting the activation of p38 mitogen-activated protein kinase (p38 MAPK) pathway in spinal microglia and subsequently upregulating pro-inflammatory cytokines IL-1β and TNF-α in rats model induced by CCI (77).
Myrtenol is the main component of aromatic plant essential oil, and research shows that it could inhibit the writhing reaction of mice induced by acetic acid, but it had no significant effect on the licking time of mice in hot-plate test. The analgesic effect is also shown in the formalin experiment, but only in the second stage. Myrtenol reduces nociception in mice by inhibiting the release of inflammatory mediators, cell migration, and receptor signaling pathways involved in pain transmission (78).
Paeonol inhibits the production of NO, PGE2, and IL-6 induced by LPS via prevention of ERK activation in RAW264.7 macrophages. Meanwhile, paeonol decreased protein expression of iNOS and COX-2 and production of pro-inflammatory cytokines, such as TNF-α and IL-1β, NO and PGE2, and increased production of IL-10 in rat paw exudates of rat model of carrageenan-evoked thermal hyperalgesia (50, 79).
Caffeic acid, a main representative of natural phenolic compounds, is widely distributed in medicinal plants, such as Polygonum aviculare, Mentha canadaensis, Ligusticum chuanxiong, Slauia miltiorrhiza, Taraxacum mongolicum, and Artemisia capillaris (80, 81). Caffeic acid reduced neutrophil, free radical, and nitric oxide-mediated hypernociception as evident from the reduction in myeloperoxidase, malondialdehyde, and nitrite levels, respectively, in rat model induced by carrageenan and lipopolysaccharide (LPS)-induced mechanical hyperalgesia (80). 7-Hydroxycoumarin, also known as umbelliferone, is a coumarin found in a variety of edible fruits and plants. 7-Hydroxycoumarin has obvious analgesic effect in animal models of CFAinduced hyperalgesia by inhibiting release of TNF-α and IL-1βand the production of PGE2, directly acting as hyperalgesic mediator (82).

Other analgesic mechanisms
Geniposide, an acyclic enone glycoside compound, is one of the main active ingredients of dumplings of Gardenia jasminoides and Eucommia ulmoides. Geniposide completely protects against hydrogen peroxide-induced oxidative damage in PC12 and HEK293 cells that express rat and human GLP-1Rs but not in HEK293T cells that do not express GLP-1Rs. The orthosteric GLP-1R antagonist exendin (9-39) right-shifts the concentration response curve of geniposide without changing the maximal protection, with identical pA2 values in both cell lines. Subcutaneous and oral administration of geniposide blocks the formalin-induced tonic response, with respective maximum inhibitory rates of 72% and 68% and ED50 values of 13.1 and 52.7 mg/kg. Intrathecal geniposide induces dose-dependent antinociception, which is completely blocked by spinal exendin (9-39), siRNA/GLP-1R, and cyclic AMP/PKA pathway inhibitors. These data illustrate that geniposide exerts its analgesic effect via the spinal GLP-1 receptors (84).
Caffeic acid potent anti-hyperglycemic inhibits thiobarbituric acid reactive substances and elevates antioxidant enzyme and attenuates alpha-amylase and alpha-glucosidase in alloxan-induced diabetic mice (85). Its elevation in serum insulin levels and its antioxidant potential might be responsible for its analgesic effect properties. Lycopene weakened neuropathic pain in mice with partial sciatic nerve ligation by up-regulating the expression of spinal astrocytic connexin 43 (86).
Pre-administration (i.t.) of PK11195, an antagonist of translocator protein (18 kDa) partly reversed the analgesic effects of gelsemine. The analgesic mechanism of gelsemine might involve inhibition of spinal neuroinflammation and activation of translocator protein on postoperative pain rat model. Gelsemine could also regulate the biosynthesis of iso-leucine neurosteroids, which are potential therapeutic drugs that play a key role in analgesia in the spinal cord (76, 87).
Ellagic acid is widely distributed in in fruits and nuts, such as blueberries, blackberries, raspberries, strawberries, pomegranates, and walnuts. It has antitumor, anti-inflammatory, anti-oxidative, antidiabetic neuropathy effects, and it can inhibit PGE2 synthesis and has other pharmacological effects. Ellagic acid has peripheral and central analgesic effects, which involve the opioid receptors in the systemic and peripheral and L-arginine-NO-cGMP-ATP-sensitive K + channel pathway (88-90).

DISCUSSION
In spite of traditional Chinese medicine has a long history in pain treatment, its active ingredients and mechanism of analgesic action is not clear. However, monomeric compounds in traditional Chinese medicine have potential applications in pain treatment. Among the compounds reviewed in this paper, some alkaloids: trilobine, palmatine, tertrandrine, berberine, govaniadine, sinomenine, gelsemine, tetrahydropalmatine, and various aconitines showed significant analgesic activities, and analgesic mechanism of them have been deeply investigated, which may be the candidate compounds for new analgesics. Then, of the 81 compounds, only tetrahydropalmatine is used as an analgesic in clinical practice. The analgesic effects of the remaining compounds were studied by animal experiments. The purpose of animal experiments is to replicate various human pains on other animals to reveal the activity of the compounds, but the exploration of their clinical effects cannot be limited to animal experiments. Therefore, we deem that research on clinical trials of these analgesic compounds should be strengthened to develop new analgesics.
In addition, the analgesic mechanisms of most compounds are related to glutamate receptor, central catecholaminergic system, the transient receptor potential family of ion channels, and the NO-cGMP and NF-ҡB pathway are also focus on the research of anti-inflammatory and analgesic drugs.
Through literature investigation, we found that only positive results of some experiments have been reported and the negative results were simply mentioned or omitted directly, so we can't summarize the negative results very well in this paper. Here, we strongly appeal to the researchers to show the negative results to the readers bravely, because the negative results are also valuable for scientific research. Sharing negative results can reduce the likelihood of peer-researcher repetition, and encourage other researchers to explore new research methods.

CONCLUSION
All in all, the study on analgesic activity of traditional Chinese medicine and natural products is more in-depth, but further researches are needed in the development of these compounds into new drugs with significant analgesic activity, which are more efficient and less toxic. Besides, there are abundant supplies of medicinal herbs from a variety of plants in China, in this case, how to use these plant resources sustainably and develop antiinflammatory and analgesic drugs with remarkable activity is one of the directions worthy of efforts in the future.