Acupuncture is an ancient Chinese therapeutic technique, which is used to treat diseases by needling specific points on the surface of the human body, i.e. acupuncture points, with a history of more than 2000 years and a wide range of treatment. Acupuncture has been used in more than 160 countries and regions, including developed Western countries, and the World Health Organization (WHO) also recommends acupuncture for the treatment of 43 diseases. Among the many therapeutic effects of acupuncture, the analgesic effect of acupuncture at the relevant acupuncture points has attracted the most attention, and it is also the main area where modern western medicine has started to study the mechanism of acupuncture action.
1. Overview of acupuncture analgesia research
More than 30 years ago, the application of acupuncture in the West was limited to people of Asian descent, but with the visit of President Nixon to China in 1972, the uniqueness of acupuncture technology aroused a high degree of interest from the mainstream medical community in the West and opened the door to medical exchanges between the East and the West. In 1998, acupuncture became the most popular alternative medicine treatment among American physicians, and in 1999, the NIH established the National Center for Alternative Medicine.
From the 1970s, acupuncture-related research papers were gradually published in SCI-indexed academic journals, averaging 148 per year from 1973 to 1997, of which 98 were monographs, and from 1998 to 2009, the number increased to 700, of which 450 were monographs. Of the total 3975 acupuncture-related research papers published from 1991 to 2009, 41% (1647) were related to pain and analgesia. Therefore, to a certain extent, the research process of acupuncture analgesia mechanism is also a process of mutual integration and mutual reference of Eastern and Western medical theoretical systems.
2. The theoretical basis of Eastern and Western medicine for acupuncture analgesia
Compared with treatment, the philosophical basis of acupuncture in Chinese medicine emphasizes prevention. According to Chinese medicine, there are two mutually opposing and complementary forces in nature, namely yin and yang, and the interaction of these two forces can regulate the flow of key energy, or “qi”, in the body. When a person is in a healthy state, Yin and Yang are in balance and the flow of Qi is smooth and regular; when Yin and Yang are out of balance, it will interfere with Qi and lead to disease. Chinese medicine also believes that qi runs in a network of channels in the body that connects internal organs to the skin surface, these channels are called “meridians” and the 361 acupuncture points on the body are located on these meridians. Based on this theory, Chinese medicine believes that pain is caused by blockages in the meridians, and that when the “qi” flows smoothly through the acupuncture points, the pain is relieved naturally.
However, in modern scientific research, no such substance as “meridians” has been found to exist, that is, there is no anatomical basis. However, under the guidance of the theory of meridians in Chinese medicine, some diseases can be effectively treated, therefore, meridians may not be a substance, but more likely to be a functional assembly, which is the integrated performance of various physiological functions such as nerve, circulation, endocrine, immune, etc. For more than 40 years, many Chinese and foreign scholars have applied modern western science and technology to obtain a large number of anatomical, neurophysiological, biochemical and other evidence of acupuncture analgesia. In the past 40 years, many Chinese and foreign scholars have applied modern western science and technology to obtain a large amount of evidence on the anatomy, neurophysiology and biochemistry of acupuncture analgesia, and have initially constructed a theoretical framework of the mechanism of acupuncture analgesia, that is, acupuncture regulates the signal transmission of pain in the spinal cord and its perception in the brain through excitation of afferent nerves, and then produces analgesic effects. Many animal, human and imaging studies have been conducted under this theoretical framework.
3. Characteristics of acupuncture analgesia
Most of the current studies believe that the analgesic effect of acupuncture is mainly composed of two parts, namely the psycho-psychological component and the physiological effect, and the physiological effect of acupuncture analgesia depends on the selection of acupuncture points, whether or not it produces a sense of getting qi, the stimulation pattern, intensity, duration after acupuncture, and genetic individual variability.
3.1 Selection of acupuncture points
The selection of acupuncture points for pain caused by different diseases is very important and is directly related to the analgesic effect. Modern acupuncture researchers will usually be using selected points chosen by TCM acupuncturists based on TCM theory. There is a fundamental difference in the understanding of the nature of disease between TCM and Western medicine, i.e., Western medicine defines disease in a limited and quantitative manner, whereas TCM places more emphasis on a holistic view, considering disease to be the result of a dysfunction between the various functions of the organism and between the organism and the external environment. Therefore, based on the concept of restoring balance to the organism, the selection of acupuncture points should follow at least three principles: 1) In the meridian system, the selection of the correct meridian is more important than the selection of the the correct acupuncture point is more important; for example, the stomach meridian should be chosen to treat stomach-related diseases; (ii) the organ closer to the onset organ, such as the Golden Gate point (BL63) on the foot solar bladder meridian, is used to treat eye diseases, because the bladder meridian reaches the head and connects to the inner canthus of the eye. (iii) Where there is pain, the transit node is located, and this selects the A-Yi point, the place where pressure pain is felt by finger palpation.
In modern experimental studies of acupuncture, not many acupoints have actually been applied. According to statistics, among the studies related to acupuncture included in SCI (1899-2010), there are three acupoints that have been applied the most: 345 acupoints of the Hegu (LI4), 299 acupoints of the Foot Sanli (ST36), and 259 acupoints of the Neiguan (PC6). Other acupoints that have been studied more are the Baihui (mainly used for the treatment of psychiatric disorders), Lijiao acupoint (mainly for neck pain), and Guizhong acupoint (mainly for low back pain), which were used in 34, 10, and 8 studies, respectively.
2.2 Specificity of acupuncture points
In a comprehensive review of “acupuncture point specificity,” Zhang et al. found that in 12 clinical trials, half of the trials showed significant therapeutic effects for real acupoints compared with sham acupuncture groups of non-acupuncture points or unrelated acupuncture points. However, not all trials were with appropriate control groups. Five of the six trials with a low risk propensity showed no significant difference between the sham acupuncture group and the acupuncture group, and in a recent review, one author even concluded that the sham acupuncture group was as effective as the acupuncture group. From a neurophysiological point of view, it seems unreasonable that needling in any part of the body would produce the same effect. Since the distribution of nerves on the body surface is not homogeneous, it seems more reasonable that stimulation of certain areas should be able to produce different effects.
2. 3 Selection of non-acupuncture points
It seems more difficult to select an acupuncture point as a control group than to select a specific acupuncture point capable of producing a therapeutic effect. According to the theory of TCM, there are 14 “meridians” distributed throughout the body, similar to the main traffic arteries, and numerous “loos”, similar to the small paths that branch off from the main paths, so it is theoretically difficult to find a site that is not affected by meridians at all. Given that most of the meridians are concentrated in the front or back of the limbs and torso, the side of the torso and the shoulders seem to be more suitable as “non-acupuncture points”; another more general approach is to select a point a few millimeters or centimeters from the true point as a control group. Furthermore, in order to avoid the influence of another meridian, the midpoint of both meridians can be chosen as a non-acupuncture point control, which, in theory, has very little influence on both meridians.
2. 4 Getting the sense of qi
In clinical practice, TCM acupuncturists place special emphasis on the needle sensation, believing that the effect of acupuncture analgesia is only apparent when a special sensation occurs in the patient at the acupuncture site. This special sensation is described as soreness, swelling, numbness, and heaviness, and is called “qi sensation”. In contrast, the acupuncturist’s fingertips also produce a special sensation of being held, and the resistance to rotating or moving the needle body up and down increases.
2. 5 Cumulative effect
After giving acupuncture points, experiments on both animals and humans show that the pain threshold is gradually increased, suggesting that the analgesic effect of acupuncture is a gradual and cumulative process. Moreover, this analgesic effect persists for some time after the end of acupuncture. The pain model used in these experiments was acute pain caused by potassium ions, and the pain measurement points were selected at eight loci in the head, chest, back, abdomen and legs, and the analgesic effect was stable and maintained for 100 minutes. One of the studies on the Hegu point showed that the pain threshold gradually increased after acupuncture of the Hegu point, peaked after 20-40 minutes of acupuncture, and was maintained for about 30 minutes after the end of acupuncture.
2. 6 Tolerance phenomenon
In 1979, Tang et al. first found that there was no direct relationship between the duration of electroacupuncture and the analgesic effect. In further studies, it was found that acupuncture for 30 minutes could increase the pain threshold of rats by 89%, but if electroacupuncture stimulation was given continuously, the pain threshold would not increase further, but would gradually decrease and eventually return to normal levels, i.e., the tolerance phenomenon. This tolerance phenomenon of acupuncture analgesia may be related not only to the inactivation or down-regulation of central opioid peptide receptors, but also to the fact that acupuncture induces the release of opioid peptides along with anti-opioid peptide substances (such as cholecystokinin), and subsequently, Han et al. also demonstrated that intracardiac injection of anti- cholecystokinin serum could reverse the tolerance phenomenon of acupuncture analgesia, and acupuncture could still produce analgesia during 6 consecutive hours of acupuncture effect. Therefore, continuous electroacupuncture stimulation is not recommended. Reports suggest that 30 minutes is a reasonable duration of needling. As for the interval of acupuncture, it varies depending on the disease, with Liu et al. reporting that in inflammatory-related diseases, once a week is most effective, followed by twice a week, and five times a week has no therapeutic effect.
2.7 Individual variability
There is significant individual variability in acupuncture analgesia, and in a trial comparing the analgesic effects of three acupuncture modalities, it was found that both acupuncture and electroacupuncture stimulation were able to significantly elevate the pain threshold. Five of the 11 subjects had significant analgesic effects, and of these five, two had effects only on electroacupuncture stimulation, while three had effects only on manual needling. These results suggest that the effect of acupuncture analgesia is related not only to individual differences but also to the mode of acupuncture. In addition, this individual variability may be related to genetics.
2. 8 Frequency dependence
There is evidence that different frequencies of electroacupuncture point stimulation activate different neural pathways. in a study of arthritic rats, Sluka et al. found that low-frequency electroacupuncture stimulation (2 Hz) increased the release of 5-hydroxytryptamine in the spinal cord, whereas high-frequency acupuncture did not have this effect. On the other hand, high-frequency electroacupuncture stimulation (100 Hz) reduced the release of aspartate and glutamate in the dorsal horn of the spinal cord, whereas low-frequency did not have this effect. More interestingly, the analgesic effect of frequency seems to be correlated with the type of disease. Zhang et al. found that in a rat model of inflammatory response, 10 Hz, but not 100 Hz, was able to suppress inflammation by activating the hypothalamic-pituitary-adrenocortical axis (HPA). In a spinal stenosis-induced neuropathic pain, Sun et al. found that 2 Hz electroacupuncture for 30 minutes significantly inhibited hypersensitivity to cold stimulation for more than 24 hours, whereas 100 Hz electroacupuncture had no such effect. Currently, most studies show that low-frequency electroacupuncture has better analgesic effects than high-frequency electroacupuncture, but 100 Hz is more effective than 2 Hz in the treatment of muscle spasm caused by spinal cord trauma.
2.9 Stimulation intensity
Compared with frequency, the intensity of electroacupuncture does not seem to attract enough attention, one of the important reasons is that, unlike frequency, the intensity has little room for variation, i.e., it can only vary between sensory threshold and pain threshold, and this variation generally does not exceed 6 times (0,5-3mA), while the variation of frequency can reach 50-100 times (1 or 2-100Hz). Barlas et al. reported that in healthy volunteers without pathological pain, higher intensity stimulation was required to increase the pain threshold.Wang et al. compared the effects of two intensities of electroacupuncture stimulation on postoperative pain and found that transcutaneous electrical stimulation at 4-5 mA reduced the need for analgesics by approximately 34%, whereas 9-12 mA stimulation was able to reduce the amount of analgesics by 65%. And in inflammatory conditions, it seems that low-intensity stimulation is more beneficial.
2. 10 Mental factors
In clinical therapeutic interventions, the effects of drugs, physiology, and surgery are usually accompanied by the influence of psychological factors, and the process of acupuncture analgesia is also accompanied by the influence of psychological factors, which means that the physiological effects produced by acupuncture and the effects produced by patients’ expectations coexist, and it is difficult to distinguish the two effects. Kong et al. found that positive anticipation of the effects of acupuncture enhanced the analgesic effects of acupuncture, i.e., not only did subjective pain perception scores decrease, but also functional brain imaging in response to injurious stimuli was altered; they also found that anticipation produced analgesic effects in specific segments. In addition, the group found that although needling was able to induce functional changes in some specific brain regions, sham needling was able to produce analgesic effects similar to those of true needling. They hypothesized that anticipation activates some brain regions in the forebrain, which in turn affects the integration of pain in some subcortical brain structures, whereas acupuncture activates nerve conduction from the periphery to the center, which in turn inhibits the perception and integration of pain in the cortex, which means that the analgesia caused by mental factors and the nerve conduction pathways activated by acupuncture analgesia may be different.
3. Research on the mechanism of peripheral acupuncture points for acupuncture analgesia
In traditional acupuncture clinical practice, the analgesic efficacy of acupuncture is closely related to the sensation of “getting qi”, which includes the patient’s own characteristic “soreness, numbness, and distension”. This sense of “getting qi” includes the patient’s own characteristic sensations of “soreness, numbness, and distension” as well as the sense of resistance and pulling during the acupuncture process. Therefore, understanding how local changes occur at acupuncture points can help us understand the mechanism of acupuncture analgesia.
3.1 Physiological alterations within the muscle tissue of acupuncture points
In 1973, Shen et al. designed an experiment in which acupuncture needles were inserted into the L1-4 or the Sansili acupoints for stimulation and electromyography was recorded in the muscles below the acupuncture points. If procaine anesthetic was injected locally into the muscle under the acupuncture point, these sensations disappeared, while in patients with lumbar anesthesia, acupuncture at the foot three miles also failed to obtain the sense of “getting qi”, and the electromyography did not record the wave amplitude of muscle contraction. In 2002, Kawakita et al. suggested that the sensation of “getting qi” was mainly produced by multiple receptors on deep muscle tissue. One of them had no superficial skin sensation, but had the sensation of “getting qi” when the acupuncture needle was rotated in the acupuncture point, and the patient was able to respond autonomously; while the other patient, who had no superficial or deep sensation, had no “getting qi” sensation at all. The other patient, who was deprived of both deep and superficial sensation, had no “qi” sensation at all. This suggests that the activation of multiple receptors in deep tissues (mainly muscles) by acupuncture may be the origin of the “getting qi” sensation.
3.2 Physiological changes within the connective tissue of acupuncture points
In recent years, Langvin et al. have proposed a new hypothesis about the peri-acupuncture mechanisms that generate the “getting qi” sensation. They suggest that the mechanism of acupuncture treatment lies mainly in the continuous rotation of the acupuncture needle, which causes the surrounding connective tissue to wrap around the needle, and that the tissue wrapped around the needle transmits mechanical signals to the surrounding tissue during the movement of the needle. Another recent experiment also supports this idea that when there is a good analgesic effect of needling the foot Sanli point, the phenomenon of mast cell degranulation in the connective tissue is enhanced, and the analgesic effect of needling the foot Sanli is diminished if the mast cells are damaged by drugs. Therefore, mast cells in connective tissue play a very important role in the analgesic process of acupuncture.
In addition, Nanna Goldman et al. published a paper in Nature Neuroscience claiming that the analgesic effect of acupuncture is a local effect and that the mechanism of this local effect is mainly the release of adenosine around the acupuncture point. They found that acupuncture at the foot Sanli point significantly reduced inflammatory pain in the ipsilateral limb, but had no significant effect on pain in the contralateral limb. They also found that the analgesic effect of acupuncture was significantly reduced when adenosine synthesis was blocked or when adenosine A1 receptor antagonists were used; similarly, acupuncture had no analgesic effect on adenosine A1 receptor knockout mice. Therefore, they concluded that the mechanism of acupuncture analgesia mainly lies in inducing an increase in adenosine release around acupuncture points and playing a local analgesic effect through adenosine A1 receptors.
4. Research on the central mechanism of acupuncture analgesia
4,1 Neurophysiological studies
4,1,1 Spinal mechanisms of acupuncture analgesia
In clinical practice, TCM acupuncturists usually select different acupuncture points according to therapeutic needs, which is mainly based on the functional specificity of acupuncture points in meridian theory. One important principle is that if the pain occurs in the head, neck, or upper extremities, the acupoints of the upper extremities are generally used as the first choice for treatment; while the acupoints of the lower extremities are mainly used to treat sciatica and abdominal pain, which are consistent with the innervation of the spinal cord segments. The organism consists mainly of skin, muscles, bones, and internal organs, which are innervated by motor nerves, while sensory information enters the dorsal horn of the spinal cord via afferent nerves. Relevant neurophysiological studies support this view. Wu et al.20 found that when thermal injurious stimulation was given to the hind limbs of cats, the analgesic effect of selecting the foot San Li acupoint, which is innervated by the same spinal cord segment, was significantly better than that of the Hegu acupoint, which is not innervated by the same segment. Dai et al.21 found that acupuncture of the foot San Li could inhibit the expression of c-fos gene in the dorsal horn of the spinal cord induced by injurious stimulation. Thus, although for some pains the choice of acupuncture point for treatment is not in the same segment as the pain site, these findings suggest that the analgesic effect of acupuncture is accomplished, at least in part, by modulating the integration of spinal pain information. Furthermore, in studies of synaptic transmission in spinal cord neurons, it was found that acupuncture of acupuncture points such as foot San Li resulted in depolarization of presynaptic C afferent nerves, which led to a decrease in the release of neurotransmitters such as substance P and glutamate from nerve endings, and was also able to inhibit sympathetically mediated single-fiber nerve impulses in the dorsal horn of the spinal cord induced by injurious stimuli. Electroacupuncture of the foot and three li can also put the injurious neurons in the dorsal horn of the spinal cord in a state of prolonged membrane hyperpolarization, and thus the postsynaptic potentials can also be inhibited.22
4,1,1 Nerve conduction pathways activated by acupuncture
The nerve conduction of pain is mainly divided into upward and downward conduction pathways, and the upward conduction pathway is divided into the lateral spinal tract and the spinal thalamic tract There are mainly two major pain ascending pathways: in the lateral spinal tract and the spinal thalamic tract. The former originates from the superficial dorsal horn of the spinal cord and projects to the lateral parabrachial nucleus, connecting mainly to brain areas involved in processing the emotional component of pain; the latter originates from the superficial and deep dorsal horn of the spinal cord, projects to the thalamus, and connects to cortical areas involved in sensory discrimination and the emotional component of pain.23 Both clinical and basic studies have shown that the conduction pathways activated by acupuncture overlap with those of pain. As shown in Figure 2, neural excitations from pain sites and acupuncture points integrate information in the dorsal horn of the spinal cord and the mid-thalamus (e.g., parabrachial nucleus).
The endogenous downward inhibitory system of the central nervous system was an important discovery in the 20th century in understanding the mechanism of pain. The downward inhibitory system consists of many brain regions, including the ventral lateral part of the medulla oblongata (RVM), the periaqueductal gray matter (PAG), and the arcuate nucleus (Arc), and in fact, like opioid analgesia or analgesia induced by stimulation of brain regions, this system plays a key role in acupuncture analgesia 24.
4,2 Imaging studies
Research on the neurophysiological mechanisms of acupuncture analgesia has mainly focused on animal studies. In recent years, with the development of imaging techniques, due to their noninvasive nature, people have begun to apply these techniques directly to humans in order to reveal the central mechanisms of acupuncture analgesia. Functional magnetic resonance (fMRI) studies have shown the ability to modulate the limbic system and subcortical structures of the brain when acupuncture points such as Hegu, Foot Sanli, or Yanglingquan are acupuncture points. When the sensation of getting qi is produced, the periaqueductal gray matter and nucleus accumbens of the midbrain, insula, hypothalamus, nucleus ambiguus, and primary somatosensory-motor cortex are activated, while some brain regions show inactivation, such as part of the anterior rostral cingulate cortex, amygdala, and hippocampal complex. To clarify whether acupuncture at different points in the same spinal cord segment could produce different central responses, Zhang et al. compared the brain areas activated by acupuncture at Foot Sanli/Sanyinjiao with those activated by acupuncture at Yanglingquan/Shengshan and found that there were indeed differences, with the former activating mainly prefrontal brain areas and deactivating the hypothalamus, while the latter activating the hypothalamus and inhibiting motor brain areas. Thus, this demonstrates that stimulation of different brain regions in consenting spinal cord segments can produce different effects, a finding that supports the existence of acupoint specificity. Further studies have shown that stimulation of acupuncture points activates the hypothalamus and major somatosensory-motor areas, while inhibiting the forebrain cortex, whereas non-acupuncture point stimulation does not have this effect. When a vision-related acupoint is located on the lateral side of the foot, which is commonly used to treat eye-related disorders, fMRI shows that the visual cortex is activated when the acupoint is stimulated, whereas stimulation of non-acupoints does not have this effect.
5. Related neurotransmitters involved in acupuncture analgesia
As early as the 1970s, Han Jisheng et al. found that cerebrospinal fluid perfused into recipient rabbits after acupuncture could provide good analgesia in the recipient rabbits, strongly suggesting that chemical mediators play a very important role in acupuncture analgesia. For decades, numerous researchers have demonstrated in human and animal experiments that acupuncture analgesia is a complex physiological process involving many chemical transmitters and modulators. These neurotransmitters and modulators mainly include:
5,1 opioid peptides
In 1977, Mayer et al. found that opioid receptor-specific antagonists were able to partially reverse the effects of electroacupuncture stimulation for pulpalgia, and that patients with brain tumors as well as chronic pain treated with acupuncture analgesia could be found to have an increase in beta-endorphin-like substances in their cerebrospinal fluid. Naloxone attenuates the analgesic effect of acupuncture in cats and monkeys, and in addition, the analgesic effect of acupuncture is not very pronounced in opioid receptor-deficient CXBK mice. In contrast, the analgesic effect of acupuncture could be enhanced when peptidase inhibitors such as D-amino acid, D-phenylalanine, and bacitracin were used to protect opioid peptides from catabolism. In view of these results, the role of opioids in acupuncture analgesia has been extensively studied.
5,1,1 Peripheral opioid peptides
Numerous evidences fully demonstrate the ability of opioid substances to modulate peripheral inflammatory pain: local injection of opioid antagonists can eliminate the analgesic effect of acupuncture on CFA-induced inflammatory pain in rats, while local injection of β-endorphin antibodies or adrenocorticotropin-releasing factor can also reduce the analgesic effect of acupuncture. The above results strongly suggest that peripheral opioids play a role in modulating inflammatory pain during acupuncture analgesia.
5,1,2 Central opioid peptides
Numerous experiments have demonstrated that different frequencies of electroacupuncture stimulation induce the release of different classes of opioid substances. The results of radioimmunoassay of spinal cord perfusion fluid in rats showed an increase in the release of enkephalins at low frequency (about 2 Hz) stimulation and an increase in the release of prednisolone at high frequency (about 100 Hz) stimulation, and this result is also consistent with the results of human experiments. At the same time, different frequencies of electroacupuncture stimulation also activate different classes of opioid receptors: in the physiological state, low-frequency electroacupuncture stimulation activates μ- and δ-receptors, and high-frequency stimulation activates κ-receptors; while in pathological pain, electroacupuncture stimulation only activates μ- and δ-receptors, but not κ-receptors. Experiments on rats also demonstrated that low-frequency electroacupuncture stimulation to activate μ- and δ-receptors could obtain stronger and longer-lasting analgesic effects than high-frequency electroacupuncture stimulation. The analgesic effect of needling at different frequencies may also be mediated by brain nuclei expressing different opioid receptors; for example, destruction of the arcuate nucleus in rats can remove the analgesic effect of low-frequency electroacupuncture stimulation and has no effect on the effect of high-frequency stimulation, whereas the effect of high-frequency electroacupuncture stimulation can be affected when the parabrachial nucleus is destroyed and is not affected by the effect of low-frequency electroacupuncture stimulation. In addition, the experimental results of Zhu et al. 2004 demonstrated that microinjection of naloxone into the subcentral thalamic nucleus blocked the analgesic effect of high-frequency electroacupuncture stimulation, while microinjection of naloxone into the anterior parietal nucleus blocked the analgesic effect of low-frequency electroacupuncture stimulation.
Previous experimental studies have demonstrated that numerous brain regions and brain nuclei are involved in the analgesic process of acupuncture, such as the subcentral thalamic nucleus (Sm), caudate nucleus (Cd), septal area (Sp), volar nucleus (Ac), arcuate nucleus (Arc), midbrain conductance gray matter (PAG) and nucleus magnus of the middle suture (NRM), which express opioid peptides and opioid receptors.PAG is an important brain region for pain inhibition, and this region contains In contrast, peptidase inhibitors that block the degradation of endogenous opioid peptides are effective in enhancing and prolonging the analgesic effect of acupuncture. Similarly, blocking opioid receptors in the preoptic area, pineal gland, septum, caudate nucleus, amygdala and caudate nucleus also cleared the analgesic effect of acupuncture. he et al. demonstrated that the pain threshold of rabbits increased after electroacupuncture stimulation and that opioid substances in the caudate nucleus increased, while μ-opioid receptor blockers blocked the analgesic effect of electroacupuncture, while δ- and κ-blockers had no effect. The arch nucleus also contains a large number of β-endorphin neurons, and the analgesic effect of acupuncture was completely lost after arch nucleus destruction. The thalamic optic density expresses opioid receptors, and the analgesic effect of acupuncture can be blocked by microinjection of naloxone in this region, while the preoptic nucleus belongs to the limbic system, so it is speculated that electroacupuncture stimulation can activate opioid receptors on neurons in the preoptic nucleus of the thalamus and thus play a role in modulating nociceptive affect.
5,2 Cholecystokinin peptide (CCK-8)
CCK-8 is widely expressed in the spinal cord and various brain regions, and is involved in various physiological functions. When CCK-8 binds to its receptors, it can effectively inhibit the activity of opioids. Currently, several experiments have demonstrated the involvement of CCK-8 in acupuncture analgesia: Ko et al. 2006 showed that high-frequency electroacupuncture stimulation increased the mRNA expression of CCK receptors in the thalamus of rats; Huang et al. found that intrathecal injection of CCK-8 inhibited the analgesic effect of electroacupuncture, while injection of its receptor antagonist enhanced the analgesic effect; Zhou et al. If antisense oligonucleotides of CCK mRNA were injected into the lateral ventricle of electroacupuncture-insensitive rats, the CCK-8 content in their brains was reduced, and the rats could be changed from electroacupuncture- and morphine analgesia-insensitive to sensitive if the antisense oligonucleotides of CCK mRNA were injected into the lateral ventricle of electroacupuncture-insensitive rats. The rats could be converted from electroacupuncture and morphine analgesic insensitive to sensitive. In addition, Lee et al. demonstrated that the level of CCK-A receptor mRNA expression was significantly higher in rats with insignificant analgesic effect to acupuncture at the foot sanli point than in those with sensitivity. This shows that the amount of CCK release and the density of its receptor expression are closely related to the individual’s sensitivity to acupuncture analgesia.
5,3 5-Hydroxytryptamine (5-HT)
The central nervous system expresses a large amount of 5-HT and its receptors, which are closely associated with the regulation of injurious sensations. The analgesic effect of acupuncture is significantly reduced when the nucleus accumbens is destroyed or when the brain is depleted of 5-HT with 5,6-dimethyltryptamine (5,6-DHT) in various animal experiments. The 5-HT receptor blockers and receptor antagonists were able to almost completely eliminate the analgesic effect of acupuncture. These results suggest that the needling process may involve both 5-HT elevating and lowering pathways in the nucleus accumbens, thereby modulating the analgesic effect of needling.
In addition, recent studies have shown that there are multiple 5-HT receptors in the nervous system, with 5-HT2A and 5-HT3A receptors expressed mainly at the terminals of primary injurious sensory afferent nerve fibers, and 5-HT1A and 5-HT1B receptors expressed mainly at the dorsal horn neurons of the spinal cord that receive these nerve fiber terminals. Experimentally, acupuncture has been shown to affect the release of substance P (SP) by modulating the binding of 5-HT to 5-HT1A and 5HT3 receptors, thereby exerting an analgesic effect.
5,4 Norepinephrine (NA)
Norepinephrine is mainly found in norepinephrinergic neurons in the A1, A2, and A4-7 nuclei of the brainstem. The axons of these neurons can project bi-directionally to the forebrain via the ventral or dorsal tracts, and also to the spinal cord via the dorsolateral fasciculus to regulate nociception. A series of experiments demonstrated that electroacupuncture stimulation induced a decrease in norepinephrine-like substances in the rat brain during analgesia. And further experiments demonstrated that NA may have different effects at the spinal cord and supraspinal levels. When the intracerebroventricular injection of the NA precursor DOPS inhibited the analgesic effect of acupuncture, while the intrathecal administration of DOPS enhanced the analgesic effect of acupuncture instead. In addition, when the α2-adrenergic receptors expressed in the spinal cord were antagonized by its antagonist yohimbine, the analgesic effect of acupuncture was inhibited. In conclusion, NA acts to inhibit the analgesic effect of acupuncture in the brain, while promoting the analgesic effect in the spinal cord.
5,5 Glutamate and its receptors
Injury sensory afferent nerve fiber terminals contain a large number of excitatory amino acids, such as glutamate and aspartate, and there are also a large number of NMDA, AMPA/KA and glutamate metabotropic receptors in spinal dorsal horn surface neurons, which play an important role in transmitting injury information under physiological conditions as well as in central nociceptive sensitization under pathological conditions. Numerous experimental results demonstrate that blocking NMDA and AMPA/KA receptors can enhance the analgesic effect of acupuncture. In a rat spinal nerve ligation experiment, the sensitivity of NR1 NMDA receptors in the spinal cord was increased after ligation and decreased by acupuncture, and similarly, in a rat inflammatory pain model, acupuncture decreased the expression of NR1 and NR2 glutamate receptors in the spinal cord.
Pharmacological experiments also demonstrated that blocking NMDA receptors greatly improved the analgesic effect of acupuncture. The analgesic effect of low-dose NMDA receptor antagonist ketamine in combination with acupuncture was much higher than that of acupuncture alone, while the analgesic efficiency of glutamate receptor antagonist in combination with acupuncture was also greatly improved. Thus, the simultaneous use of NMDA and AMPA/KA receptor antagonists with acupuncture can have a synergistic effect on analgesia.
5,6 γ-aminobutyric acid (GABA)
GABA is an important inhibitory neurotransmitter in the central nervous system and is involved in a variety of physiological and pathological processes. It mainly includes three receptor subtypes: GABAa, BABAb and GABAc. It is currently believed that GABAa and BABAb type receptors are involved in the regulation of nociception, but their role in the process of acupuncture analgesia is not well understood.
5,7 Substance P (SP)
In addition to the above neurotransmitters, SP is also an important nociceptive signaling molecule and is a likely target for acupuncture analgesia. A variety of injurious stimuli are able to increase the release of substance P in the spinal cord, and Duan et al. showed that acupuncture for analgesia at the third mile of the foot was able to inhibit the release of substance P in the spinal cord, and in addition, acupuncture for pulpal pain was able to reduce substance P in the trigeminal nucleus. It has also been suggested that the process of acupuncture analgesia is not directly related to SP, such as Zhu et al. who experimentally demonstrated that low-intensity electroacupuncture stimulation of bilateral circumflex points had a significant analgesic effect without affecting the level of substance P in the spinal cord. On this basis the conjecture has been put forward that nociception itself can activate the endogenous opioid peptide system and thus inhibit SP release, while acupuncture enhances this process.
5,8 Other biologically active substances
Some other substances may also be involved in the process of acupuncture analgesia, such as angiotensin II, which promotes spinal angiotensin II release during high-frequency electroacupuncture stimulation, thereby impeding the analgesic effect of acupuncture; increased expression of growth hormone inhibitor in the spinal cord and dorsal root ganglia during acupuncture analgesia; microinjection of neurohypocretin in the central aqueduct enhances the effect of acupuncture analgesia, whereas activation of dopamine type 1 receptors can inhibit the analgesic effect of acupuncture.