WORLD JOURNAL OF ACUPUNCTURE-MOXIBUSTION

Vol.10 No.2,  June, 2000

Can Long-Term Potentiation Be 

Induced by Acupoint Stimulation?

by Wu Dingzong (吴定宗) Zhang Yu(张 煜) Wan Ping(万 平)*

(Shanghai University of Traditional Chinese Medicine, 200032,China

*East China Normal University, 200062, China)

Abstract Long-term potentiation (LTP ) is usually induced by direct brain stimulation. An attempt has been made to evoke LTP in dentate granule cells of hippocampus by acupoint stimulation in anesthetized rats. Assuming a gradual increasing course, LTP rose to 146% at the end of one hour. After applying such stimulation to the awake rats for six days (once everyday), their discriminative learning capacity in Y maze test markedly improved as compared with that of the control.

Key Words LTP  Acupoint stimulation  Y maze test

The classical method for induction of long-term potentiation (LTP) is to stimulate the perforant pathway and record LTP in dentate granule cells of hippocampal formation^[1]. Long-term changes in dentate evoked potential as induced by electroconvulsive shock seizure in the rats has been reported^[2]. While these might be merged into the same category: stimulating current is applied directly onto the brain. Whether LTP can be provoked by peripheral nerve or acupoint stimulation? The important physiological significance of LTP lies in its relevance to memory and learning; if LTP can be induced by peripheral stimulation, then it might provide a non-drug means to influence memory and learning. The purpose of this paper is to explore this possibility.

Materials and Methods 

In urethane-anesthetized male SD rats, weighing 200±40 g, a stimulating elect rode was guided to the perforant pathway to deliver an electrical stimulus with single pulse, its intensity being to induce 50% the amplitude of the maximal population spike (PS). This served as a test stimulus. A recording electrode was positioned in the granule cell layer of the dentate gyrus to search for PS. The potential recorded was the average of 16 times of stimulation-induced potentials: as performed by an instrument for recording evoked potential “Neuromatic 2000 C". The stimulating and recording electrodes were guided by stereotaxic apparatus according to Paxinos Atlas (stimulation: 7.5 mm posterior to the Bregma, 4.0 mm lateral to the midline and 2-4 mm under the dura; recording: 3.5 mm posterior, 2.0 mm lateral, 2-4 mm under. The depth of the electrodes varied according to occurrence of the maximal response). As for the conditioned stimuli, tetanic stimuli at 100 Hz were applied intermittently (delivered at every other second from DZ-22 Electro-acupuncture Apparatus) to the acupoints corresponding to Zusanli (ST 36) and Taichong(LR 3), where traverse the deep peroneal nerve and its branches. The total duration of stimulation was 5 min: the first 3 min and the last 2 min with intercalated pause of 3 min. The intensity of stimulation was about 5 mA (peak value of the pulses) with appearance of trembling of the lower limbs.

Awake rats received daily electro-acupuncture stimulation (once a day) applied to the same acupoints with the same intensity and frequency for 6 days; before and after such treatment Y maze tests were performed to investigate the discriminative learning capacity. The maze consisted of three radiating passages with bottom-grid for passing electric current, and a signal lamp being set at the inlet of each passage. As a correct response, rats ran to the safe area (without current) where illuminated a lamp so as to avoid electric shocks. In every test rats performed 20 trials, and each correct response would be recorded as 5 marks.

Results

Table 1 shows the amplitude of PS in mV and the corresponding percentage. The value of amplitude is calculated as a+b/2, where a is the amplitude of EPSP, and b , the amplitude of PS, as Chida reported^[3].

　Table 1. Changes of the Amplitude (mV, M±SD) of PS After Electro-acupuncture (EA) Stimulation

From Table 1, it can be seen that LTP can be induced in most rats by EA

Table 2 lists the marks of Y maze test in awake rats with EA (EA group) or without EA (Control group).

Table 2. Effect of EA on Rat's Discriminative Learning Capacity (marks)

From Table 2, it shows that after EA stimulation, the marks of Y maze test rises significantly in comparison with pre-EA (P<0.05), while those of Y maze test have no apparent changes in control group (P>0 .05).

The above-mentioned results showed that EA applied to certain acupoints might evoke LTP and also promote the discriminative learning capacity of rats in this study.

Discussion

It was found that in case of direct application of tetanic stimuli onto the brain, the amplitude of PS increases promptly to reach LTP level and maintains there after. While EA-induced LTP, assuming a gradual increasing course, presented a higher level at the end of 60 min. One of the typical characteristics of EA is its retarding onset and persisting action. The mean value of PS increment was of great variation. Among the 10 rats tested, 3 rats showed little PS increment after EA. The effect of EA applied to the acupoints usually reveals obvious individual variation, as shown in the observations of analgesic effect induced by EA.

Whether the increment of PS observed in our experiments is a real LTP? It has be en reported that LTP is established if the PS increment is over 20-30% and persists for 20-60 min after tetanic stimulation^[4,5]. The PS increment in this study was about half of the original value at the end of one hour after needling. Moreover, we observed that the influence of LTP induction led to a facilitating effect on rats' discriminative learning capacity. This is in line with Berger's report that promoting of discriminative conditioning of the rabbit nictitating membrane response was observed after LTP induction^[6].

The underlying mechanism of needling-induced LTP is obscure yet. It is highly possible that the deep peroneal nerve under the applied acupoints was stimulated. As for the pathway transmitting the evoked impulses from the acupoints to hippocampus is not clear, while it was reported that unit discharges of hippocampus could be elicited during EA applied to the acupoint (ST 36)^[7]. And a number of brain sites might be agitated by needling applied to this acupoint^[8]. Among these sites, some of them may be relevant to LTP induction, such as mesencephalic reticular formation^[9], septal area^[10], and median raphe nucleus^[11]. Induction of hippocampal LTP can be augmented by excitation of these sites. Activation of the raphe nucleus is known to induce release of 5-HT, as usually occurred in EA. Attenuation of LTP in the rats was observe d in case of 5-HT depletion by pretreatment of 5,7-DHT or pCPA^[12].

Thus, an approach to induce LTP by peripheral or acupoint stimulation becomes possible, yet much (such as other acupoints, nerves, stimulating frequency and strength, etc.) remains to be explored for a better effect.

References

1 T.V.P Bliss, et al. J. Physiol.(London), 1973, 232,331.

2 W.M. Burnham, et al. Brain Res. 1995, 698, 180.

3 N. Chida, et al. Brain Res. 1992, 593, 57.

4 H. Saito, et al. Europ. J. Pharmcol. 1991, 205, 303.

5 O. Ramires, et al. Neuros. Lett. 1988, 92, 275.

6 T. W. Berger, Science. 1984, 224, 627.

7 L.Z.Zhou, et al. Acta Physiol. Sin. 1981, 33,328.

8 G.W.Yu. In progress of Acupuncture Research. Z.P.Ji, Ed. People's Hea lth Press, Beijing, 1981, P 114.

9 V. Bloch, et al. J.Physiol.(London).1985, 360,215.

10 G.B.Robin son, et al. Brain Res. 1982, 249,162.

11 J.M.Klancnik, et al. Brain Res. 1991,557,236.

12 T.V.P.Bliss, et al. J. Physiol.(London)1983,334,475.