THE JOURNAL OF ALTERNATIVE AND COMPLEMENTARY MEDICINE Volume 3, Supplement 1, 1997, pp. S-lOl-S-108 Mary Ann Liebert, Inc.

Physiology of Acupuncture: Review of Thirty Years of Research

JI-SHENG HAN, M.D.

ABSTRACT

Acupuncture and electroacupuncture effects are mediated through a variety of neural and neurochemical mechanisms. Research done in the early 1970s first elucidated the mechanisms for the effect of acupuncture anesthesia. Further experiments showed that this effect can be transferred from one rabbit to another by cerebrospinal fluid (CSF) transfusion. Further in­vestigation explored the role of classic central neurotransmitters in the mediation of acupunc­ture analgesia, including catecholamines and serotonin. The availability of rat models for elec­troacupuncture, using the tail flick latency as a bioassay, allowed further experiments to be done to explain the basis for the effects. Differential release of central nervous system (CNS) opioid peptides by electroacupuncture has been noted, with 2-Hz electroacupuncture trig­gering release of enkephalins and j8-endorphins, and 100-Hz stimulation selectively increased the release of dynorphin in the spinal cord. A combination of both frequencies allows syn­ergistic interaction among the three endogenous opioid peptides and a powerful analgesic ef­fect. In addition, multiple acupuncture treatments with the optimal time spacing may result in an accumulation of electroacupuncture effect. A bimodal distribution of analgesic effect can be noted if a large group of rats is given electroacupuncture ("low responders" and "high responders"). The mechanism of low response is at least twofold: a low rate of release of opi­oid peptides in the CNS, and a high rate of release of CCK-8, which exerts potent anti-opi-oid effects. A newly discovered anti-opioid peptide is orphanin FQ, which has also been linked to negative feedback control of electroacupuncture stimulation. Further research will serve to elucidate the mechanisms of acupuncture treatment in opiate detoxification, and how different disease entities may respond differently to electroacupuncture.

BASIC PHENOMENON OF threshold as measured by the potassium ion-ACUPUNCTURE ANALGESIA IN tophoresis method (Research Group of

NORMAL HUMANS Acupuncture Anesthesia, 1973). The time course of slow onset (30-minute induction pe-

[anual twisting of a needle inserted into riod) and exponential decay (Tl/2 = 16 min-lone acupuncture point (Hegu, or LI 14 utes) suggested the involvement of a humoral

point) produced a marked increase in skin pain mechanism. M:

Neuroscience Research Center, Beijing Medical University, Beijing, China. „ . , ^ , , Dr Ti-Sheng Han has been active in the field of neuroscience research for several decades. His laboratory has been

very productive in elucidating mechanisms of acupuncture action, from "whole animal" physiology, to neuropep­tide rnetabolism, and most recently, looking at gene expression and regulation induced by electroacupuncture.

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ANIMAL MODEL OF ACUPUNCTURE plied at the leg points of the rat was still capa-ANALGESIA ble of inducing antinociception, using tail flick

as the index of nociception. Evidence was obtained showing that

acupuncture or electroacupuncture (EA, elec­trical stimulation administered via the inserted THE SITE SPECIFICITY OF needles) induces an increase in the tail flick la- AN ACUPOINT tency (TFL) in the rat (Ren and Han, 1979), and an increase in the escape response latency The site specificity of an acupoint should not (movement of the head away from the radiant be over stressed so far as analgesia is concerned, heat source) in the rabbit (Han et al., 1973). Ev- Nor is there any clear evidence to show that idence was also provided by the operant con- stimulation at a single acupoint can induce a ditioning model to show acupuncture analge- site-specific analgesia in remote areas. Among sia in primates. 10 different acupoints tested in human volun­

teers using potassium iontophoresis to induce experimental pain, Hegu (LI 14) was found to

INTENSITY OF STIMULATION be one of the most effective acupoints to pro­duce a general analgesic effect (Research Group

The EA-induced antinociceptive effect re- of Acupuncture Anesthesia, 1973), probably be-vealed in rats and rabbits showed an intensity- cause of a dense innervation of A/3 fibers in this response relation. The parameters of the elec- area (Lu, 1983). The gate control mechanism trical stimulation (frequency from 2 Hz to 100 must be involved in acupuncture induced seg-Hz, pulse width 0.3 ms, intensity 1-3 mA) at- mental analgesia, especially when acupuncture tached to the stainless-steel needle inserted into was given at a local tender point (the "Ashi" the acupoint are apt to induce an excitation of point, or "Ah yes" point). Aa,(3 fibers, a part of A5 fibers, as well as a small portion of C fibers. Further increase in the intensity of EA stimulation to involve more ACUPUNCTURE ANESTHESIA VERSUS C fibers, as in the case of diffuse noxious in- ACUPUNCTURE-ASSISTED ANESTHESIA hibitory control (DNIC) would certainly in­crease the potency of analgesia, but the pain The extent of analgesia that acupuncture or and stress elicited by these noxious stimuli EA can provide in experimental animals or in would prevent it from clinical use. It was re- humans is substantial, but is only partial. In the ported that using capsaicin to block the C fiber rat experiment, EA stimulation (intensity < 3 transmission in the sciatic nerve of the rat did mA) elicits an increase in tail flick latency to an not affect the EA analgesia to a significant ex- extent equivalent to 4 mg/kg (half of the full tent (Fan et al., 1986), suggesting that C fiber dose) of morphine. In surgical operations, the afferents may not be essential for the produc- use of EA in combination with general anes-tion of conventional EA analgesia. Thus, Aj8 thesia or epidural anesthesia reduced the con-and A5 fibers may be the most important com- sumption of anesthetics by 50% (Wang et al., ponent of the afferent fibers mediating 1994; Qu et al., 1996). The results suggest that acupuncture signals to CNS in order to pro- acupuncture or EA is capable of producing a duce an antinociceptive effect. This was sub- substantial analgesic effect, yet is not strong stantiated by a recent study using c-fos expres- enough to totally abolish the sharp surgically sion as an indicator of nociception in the rat evoked pain. In a recent article I recommended dorsal spinal cord (Zhang et al., 1994), show- the term acupuncture-assisted anesthesia ing that formalin-induced c-fos expression in (AAA) (Han, 1996), where acupuncture is used the superficial layers of the dorsal horn can be as an adjunct for drug anesthesia. This has been almost totally abolished by topical application shown to result in a marked reduction of the of capsaicin on the sciatic nerve. Under this side effects produced by the anesthetics in car-same situation of C fiber dysfunction, EA ap- diovascular, respiratory and other vital para-

REVIEW OF 30 YEARS OF ACUPUNCTURE RESEARCH S-103

meters as well as a substantial shortening of shifting automatically between each other, each postoperative hospitalization (Wang et al., lasting for 3 seconds (termed dense and dis-1994; Qu et al., 1996; Han, 1996) perse or DD mode), then all three kinds of opi­

oid peptides (enkephalins, endorphins, and dynorphins) can be released simultaneously. A

CEREBROSPINAL FLUID synergistic interaction among the three en-TRANSFUSION STUDY dogenous opioid peptides produces a most

powerful analgesic effect (Chen and Han, 1992; A cerebrospinal fluid transmission study Chen et al., 1994). Recent studies revealed that

was performed in 1972 and published in 1974 2-Hz and 100-Hz stimulation use different (Research Group of Acupuncture Anesthesia, nerve pathways for mediating their analgesic 1974). This revealed that the effect of acupunc- effects (Guo et al., 1996a, 1996b; Han and Wang, ture analgesia in one rabbit can be transferred 1992). These findings formed the basis for the to another rabbit by cerebrospinal fluid (CSF) invention of an electronic device termed Han's transfusions. This was the first scientific evi- Acupoint Nerve Stimulator (HANS), which dence to suggest a neurochemical mechanism generates optimized parameters for maximal mediating acupuncture analgesia. This finding activation of the endogenous analgesic sys-triggered a series of studies to explore the role tems. of central neurotransmitters in the mediation of acupuncture analgesia, among which the sero­tonin (Han et al., 1979; Xu et al., 1994b) and the TRANSCUTANEOUSLY APPLIED catecholamines (Han et al., 1979b) were the ELECTRIC STIMULATION CAN BE AS most thoroughly studied. In fact, chemicals in- EFFECTIVE AS ELECTROACUPUNCTURE tensifying serotonin availabihty at the synaptic IN PRODUCING ANALGESIC EFFECT cleft (eg, chloroimipramine) were shown to sig­nificantly potentiate acupuncture analgesia in It has been a general belief that EA stimula-surgical procedures such as dental extraction, tions are characterized by low frequency and

high intensity, whereas transcutaneous electri­cal nerve stimulations (TENS) are of high fre-

DIFFERENTIAL RELEASE OF CNS quency and low intensity. One of the reasons OPIOID PEPTIDES BY leading to this impression is that TENS was de-

ELECTROACUPUNCTURE OF signed based on the gate control theory, which DIFFERENT FREQUENCIES stressed the importance of using high-fre­

quency, low-intensity stimulation to activate One of the most important mechanisms of the subcutaneously located large fibers in an at-

EA analgesia is that EA stimulation accelerates tempt to produce segmental analgesia. Because the release of CNS opioid peptides that inter- we now have much better insight into the re­act with relevant opioid receptors to induce an lation between stimulation parameters and the antinociceptive effect. A key finding in this activation of various neurotransmitter systems field was that 2-Hz EA triggers the release of in the CNS, one should not rely on one set of enkephalins and j3-endorphin in the brain and stimulations, but rather, make use of all possi-spinal cord to interact with the /JL and 8 opioid ble stimulation parameters, trying to combine receptor in the CNS, whereas 100-Hz stimula- those based on gate control theory with those tion selectively increased the release of dynor- based on the theory of frequency-dependent phin in the spinal cord to interact with the K peptide release. Technically, it should make no opioid receptor in dorsal spinal cord (Han and difference whether the stimulation is applied Wang, 1992). This phenomenon originally re- via metal needles inserted into the tissue, or via vealed in rats and rabbits, has also been con- skin electrodes placed on the surface of the firmed in humans (Han et al., 1991). Further identified points. For the sake of simplicity and studies revealed that if low-frequency (2 Hz) easy handling, it is much more convenient to and high-frequency (100 Hz) stimulations are place the two skin electrodes across the acu-

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point in case the point is located at the limbs. The mechanisms for the development of EA This would allow the current to pass through tolerance are many-fold, two of them have been the deep structures where the acupoint is sup- clarified: (1) repeated EA accelerates the release posed to be located. In an experiment in rats, of opioid peptides that elicits a downregulation it was found that using a constant current out- of the gene expression of opioid receptors in put device (HANS), the antinociceptive effect identified brain areas (Wang et al., unpublished induced by transcutaneous stimulation was at data); (2) the release of large amount of opioid least as good as EA (Wang et al., 1992). The key peptide in the CNS triggers the release of an-point is that the stimulator should be designed other kind of neuropeptide, named cholecys-to guarantee a constant current output that tokinin octapeptide (CCK-8), to counteract the would keep the desired waveforms without be- opioid effect (Zhou et al., 1993a, 1993b). Indeed, ing distorted by the very high and complicated the development of tolerance to E A can be post-skin impedance. poned by the central administration of the CCK

receptor antagonist L-365260, or the antibody against CCK.

A DECREASE IN SENSITIVITY (TOLERANCE) TO

ELECTROACUPUNCTURE MAY LOW RESPONDERS VERSUS HIGH DEVELOP DURING PROLONGED RESPONDERS FOR

STIMULATION ELECTROACUPUNCTURE ANALGESIA

The optimal duration of EA stimulation has When a large group (>100) of rats is given a been found to be 30 minutes, which is the in- standard session of EA, one can easily find a duction period necessary for the full develop- bimodal distribution of the analgesic effect, ment of acupuncture analgesia in humans (Re- Cluster analysis revealed two distinct groups: search Group of Acupuncture Anesthesia, one showed an increase of TFL no more than 1973). On the other hand, stimulation lasting 50% (the low responders, LR), the other for more than 1 to 2 hours would inevitably re- showed an increase of TFL by 50% to 150% suit in a gradual decrease of the analgesic ef- (high responders, HR). This phenomenon is re­fect. This can be comparable to the develop- producible, at least within 2 days. What is in-ment of morphine tolerance when multiple teresting is that a LR to EA is also a LR to small injections were given at short time intervals, dose (3-4 mg/kg) of morphine, and vice versa hence the term acupuncture tolerance (Han et (Tang et al., 1997). al.,1981)- The mechanisms of being a LR is at least

An interesting finding was that rats made twofold: a low rate of release of opioid peptides tolerant to 2-Hz EA were still reactive to 100 in the CNS and a high rate of release of CCK-Hz, and vice versa. This is understandable be- 8 that is a very potent anti-opioid. An LR rat cause 2-Hz and 100-Hz EA analgesia are me- can be changed into an HR by injection of a diated by different types of opioid receptors, CCK antisense RNA into the brain to block the ie, activation of /x/5 opioid receptors by expression of the gene coding for CCK (Tang enkephalin and endorphin in low-frequency et al., 1997), or by the administration of a com-EA, and activation of K-opioid receptors by pound L-365260 characterized as an antagonist dynorphin in high-frequency EA (Chen and to the CCK-B receptor (Tang et al., 1996). On Han, 1992). In the HANS device, a 30-minute the other hand, a breed of rat called P77PMC auto-off mechanism has been installed to pre- rat, bred to be highly susceptible to audiogenic vent unintentional excessive prolongation of seizure, was found to be an HR to EA analge-the stimulation. For severe chronic pain or can- sia. These rats were found to contain high level cer pain patients who need multiple treat- of j8-endorphin and low level of CCK in the ments, it is advisable that the HANS be used brain. They can be changed into LRs by the cen-no more than 3 to 4 times (30 minutes for each tral administration of a vector containing CCK session) a day. cDNA that induces an overexpression of CCK

REVIEW OF 30 YEARS OF ACUPUNCTURE RESEARCH S-105

in the CNS (Zhang et al., 1992). Thus, a dy- calcium concentration by mobilization of in-namic balance between opioid peptides and tracellular calcium stores, whereas the opioid anti-opioid peptides in the CNS seems to be a effect lowers intracellular free calcium levels cardinal factor determining the effectiveness of (Wang et a l , 1992). EA analgesia.

It should be stressed, however, that CCK-8 is just one of the members of the family called MULTIPLE ACUPUNCTURE anti-opioid peptides. A new member was dis- TREATMENTS WITH PROPER TIME covered recently called orphanin FQ (OFQ), a SPACING MAY RESULT IN AN peptide with 17 amino acid residues. That OFQ ACCUMULATION OF may be functioning as another negative feed- ELECTROACUPUNCTURE EFFECT back control mechanism for EA analgesia is ev­idenced by our recent finding that blockade of Some acupuncturists claim that the thera-OFQ gene expression by central administration peutic effects produced by multiple acupunc-of OFQ antisense RNA produced a dramatic ture treatments once a week (QW) is better than augmentation of EA-induced analgesia (Tian, that of once a day (QD). In normal rats we have Xu, Grandy, Han, abstract for the 1996 Inter- compared the analgesic effect induced by EA national Narcotic Research Conference, Jul. administered QD, Q4D, and Q7D, and found 20-25, 1996, Long Beach, USA). that in the Q4D regime, the EA analgesia

showed a trend of gradual strengthening ac­companied with a gradual increase of the con-

MECHANISMS UNDERLYING THE centration of monoamines in the spinal per-ANTIOPIOID EFFECT OF CCK-8 fusate, whereas in the QD regime, there was a

gradual decrease of the analgesic effect, ie, de-Ample evidence has been obtained to show velopment of tolerance (Xu, unpublished data),

that CCK-8 forms a negative feedback control However, in rats with experimental arthritis, for opioid analgesia, ie, an increased level of the optimal time spacing for a best therapeutic opioids will trigger the gene transcription, pro- effect becomes different from that observed in tein synthesis, and ultimate release of the CCK normal rats, depending on the pathological peptide, forming a brake for excessive opioid model being used. This is an issue deserving analgesia (Han, 1995a). A series of studies were further investigation, then conducted to explore its molecular mech­anisms of action (Han, 1995b). (1) Cross-talk be­tween opioid and CCK receptors: CCK-8 was FROM ACUPUNCTURE ANALGESIA TO shown to decrease the number and to lower the HEROIN DETOXIFICATION affinity of opioid receptors, as evidenced by the decrease of Bmax and an increase of Kd in re- Because EA has been shown to release opi-ceptor-binding assays. (2) Patch-clamp study oid peptides in the CNS for pain control, theo-provided direct evidence to show that opioid retically it should be useful for the treatment of suppression of voltage-gated calcium current heroin addiction by releasing endogenous opi-can be reversed by CCK-8, indicating that opi- oids to replace the exogenous opiates. This was oid/CCK interaction takes place at the mem- tested in rats made dependent on morphine brane of one and the same neuron (Liu et al., (Han and Zhang, 1993). The results were en-1995; Xu et a l , 1996). (3) CCK-8 seems to in- couraging, and led to using HANS for the treat-duce an uncoupling of opioid receptors from ment of heroin addicts. their relevant G protein, thus interfering with In heroin addicts, HANS treatment (30 min-the transmembrane signal transduction in- utes per session, 1 to 3 sessions per day for 7 duced by opioid peptides (Zhang et a l , 1993)- days) produced a significant reduction of the (4) Activation by CCK-8 of the phosphoinosi- withdrawal syndrome. The DD (2/100 Hz) tide (PI) signaling system in the CNS (Zhang et wave was significantly more efficient than the a l , 1992), which increases the intracellular free constant low (2 Hz) or constant high (100 Hz)

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wave in ameliorating tachycardia, insomnia, and reducing craving. All three treatment groups using 2-, 100-, and 2/100-Hz stimula­tion, respectively, were equally effective in in­creasing the body weight as compared with the placebo group in which a gradual decrease of body weight was observed in the first week of drug abstinence (Han et al., 1994, 1995). After the detoxification period of 7 to 10 days, HANS can be used for the treatment of postdetoxifi-cation syndromes including insomnia and var­ious kinds of pain. An acceleration of the re­lease of endogenous opioid peptides in the CNS may account for the mechanisms under­lying HANS detoxification. It should be re­membered that 2/100 Hz DD waves releases not only enkephalins and j8-endorphin, which work in much the same way as morphine, but also dynorphin that may interact with K-opioid receptors resulting in a suppression of the ab­stinence syndrome (Wu et al., 1995).

SELECTION OF ELECTROACUPUNCTURE PARAMETERS

SHOULD BE TAILORED ACCORDING TO DISEASE CATEGORY AND

INDIVIDUAL CASES

DD waves have been shown to be the best choice for pain control for most cases, but it may not be the standard solution in all cases. This can be seen in the HANS treatment of pain resulting from muscle spasticity caused by spinal injuries (Han et al., 1994). Intrathecal in­jection of dynorphin has been shown to pro­duce analgesic effects in the dorsal horn and may produce paralytic effects in the ventral horn (Xue et al., 1995a, 1995b). In spinal spas­ticity, the anterior horn neurons are in a state of hyperexcitability. It was hypothesized that an increased release of dynorphin in the spinal cord by high-frequency (100 Hz) EA might be helpful to suppress the excitability of spinal motor neurons. This was tested in clinical cases of spinal spasticity. The best therapeutic effect was obtained by using HANS of 100 Hz; that of DD mode (2/100 Hz) was only half effective; 2-Hz EA was totally without therapeutic effect, and may even produce a negative effect. The therapeutic effect of 100-Hz EA became in­

creasingly prominent over 5 days and re­mained stable after 12 days (Han et al., 1994).

Another issue worth mentioning is that a LR rat at 2 Hz EA may not be a low responder at 100-Hz EA, and vice versa. Therefore, unless the best parameter for the treatment of an iden­tified case is known, the DD wave should be tried first.

CONCLUSION

The use of modern scientific methodology is absolutely essential for the clarification of the scientific basis of acupuncture therapy. Re­search on the physiology of acupuncture has been contributing to the development of neu­roscience from the molecular level to the be­havioral. Questions arising from clinical prac­tice are the precious resource for basic research on acupuncture mechanisms. High-quality sci­entific research will certainly pave the way for the acceptance and more popular use of acupuncture and related techniques for the ben­efit of patients suffering from chronic pain and many other functional disorders (Han, 1994).

ACKNOWLEDGMENT

This study was supported by the National Institute of Drug Abuse, USA, DA 03983, and by the National Natural Science Foundation of China.

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Address reprint requests to: Ji-Sheng Han, M.D.

Neuroscience Research Center Beijing Medical University

38 Xue-Yuan Road Beijing, China 100083