Scholarly article on topic 'A systematic review and meta-analysis of acupuncture for improving learning and memory ability in animals'

A systematic review and meta-analysis of acupuncture for improving learning and memory ability in animals Academic research paper on "Psychology"

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Academic research paper on topic "A systematic review and meta-analysis of acupuncture for improving learning and memory ability in animals"

Huang et al. BMC Complementary and Alternative Medicine (2016) 16:297 DOI 10.1186/s12906-016-1298-3

BMC Complementary and Alternative Medicine

A systematic review and meta-analysis of acupuncture for improving learning and memory ability in animals

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Kai-Yu Huang11, Shuang Liang11, Mei-Ling Yu1, Shu-Ping Fu1,2, Xia Chen1 and Sheng-Feng Lu1"

Abstract

Background: Memory loss is the most prominent symptoms of brain aging, but there is currently no evidence-based treatment strategy. Acupuncture has been widely used in China and the effectiveness for improving learning and memory has been mentioned in previous studies. We conducted this systematic review and meta-analysis to evaluate the effectiveness of acupuncture for improving learning and memory in animal experiments. Methods: We searched Pubmed, Embase, Ovid Medline(R), the China National Knowledge Infrastructure (CNKI), Chinese Science and Technology Periodical Database (VIP) and Wanfang data Information Site to collect studies published up to December 2015. Study quality for each included article was evaluated according to the CAMARADES 10-item checklist. Outcome measure is Morris water maze. A meta-analysis was conducted according to the Cochrane systematic review method by using RevMan 5.3 software.

Results: Forty-two studies involving 944 animals were included. The quality score of the studies ranged from 2 to 8, with a mean of 5.3. Meta-analysis results showed that 24 studies reported significant effect of acupuncture for decreasing escape latency (-3.00, 95 % CI: -3.78—2.23, P <0.00001), 14 studies reported significant effect of acupuncture for increasing frequency of cross platform (2.57, 95 % CI: 1.92 ~ 3.22, P <0.00001), and 7 studies reported significant effect of acupuncture for increasing time in target quadrant (2.00, 95 % CI: 1.10 ~ 2.91, P < 0.00001) compared with the control group.

Conclusions: These findings show acupuncture has a potential role in improving learning and memory ability in animal models, suggesting it as a candidate therapy for memory loss of aged brain.

Keywords: Acupuncture, Learning, Memory, Meta-analysis

Abbreviations: 4-VO, 4-vessel occlusion; 6-OHDA, 6-OH-dopamine; AD, Alzheimer's disease; CFS, Chronic fatigue syndrome; CI, Confidence interval; CMS, Chronic mild stimulation; CNKI, China National Knowledge Infrastructure; MCAO, Middle cerebral artery occlusion; MDA, Malondialdehyde; MMP, Matrix metalloproteinase; OVX, Ovariectomy; PD, Parkinson's disease; PTSD, Post-traumatic stress disorder; SD, Sprague-Dawley; SMD, Standard mean difference; SOD, Superoxide dismutase; SPS, Single prolonged stress; STZ, Streptozotocin; VD, Vascular dementia; VIP, Chinese Science and Technology Periodical Database; VPA, Sodium valproate; WD, Wilson disease

* Correspondence: lushengfeng@njucm.edu.cn +Equalcontributors

1The No.2 ClinicalMedical College, Nanjing University of Chinese Medicine, Nanjing 210023, China

2Key Laboratory of Acupuncture and Medicine Research of Ministry of Education, Nanjing University of Chinese Medicine, Nanjing 210023, China

(3 BioMed Central

© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Background

Learning and memory are the most basal and important higher nervous functions and closely related to each other [1]. Learning means accepting information from the outside environment and memory refers being able to use this information at a later date [2-4]. As a symptom, learning and memory impairment often appears together in some diseases, such as Alzheimer's disease, vascular dementia, diabetes, autism, and so on [5-8]. With the increasing of living pressure and changing in life style, learning and memory impairment as the important embodiment of brain dysfunction have become one of the most important factors that affect people's lives [1, 2]. The treatment based on western medical science for most learning and memory impairment only relieves the symptoms and delays the progression of disease [9, 10]. Moreover, it also has some side effects caused by long time treatment [11].

As a kind of economical and side-effect free natural remedies, acupuncture has been used in China widely for over 2000 years [12]. The effect of acupuncture on encephalopathy has been recognized internationally. More and more studies have been published to confirm the effectiveness of acupuncture for improving learning and memory [13-16]. However, to some extent, the small sample size makes it hard to draw firm conclusions.

Up to now, there have been no systematic reviews to analyze the effectiveness of acupuncture for improving learning and memory. Reviews based on animal data could make trails' planning more perfect, increase the odds of success of future clinical trials and assist to decide what is valuable in further research [17]. Additionally, animal experiment can make us better understand the mechanism of acupuncture on learning and memory and guide the future clinical study. Therefore, we conducted a systematic review and meta-analysis of the effectiveness of acupuncture for improving learning and memory in animal experiments to provide suggestions for future animal experiments and clinical trials.

Methods

Search strategy

The following electronic databases were searched: Pubmed, Embase, Ovid Medline(R), China National Knowledge Infrastructure (CNKI), Chinese Science and Technology Periodical Database (VIP) and Wanfang data Information Site. The publication time is from the inception of each database up to December 2015. The languages were limited to English and Chinese. Search terms consisted of two groups: intervention (acupuncture and other related terms) and object (learning and memory and other related terms). All searches were

limited to animals. We combined the results of all searches and then removed the duplicates. We also tried to get additional records identified through other sources.

Inclusion criteria

They were included if the following criteria were met:

(1)Subjects: Animal models of learning and memory impairment were included.

(2)Interventions: Acupuncture was the main therapy and only included manual acupuncture and electroacupuncture.

(3)Outcomes: Morris water maze test was the primary outcome to explore the effectiveness of acupuncture groups and the difference between control groups and acupuncture groups. The Morris maze test is arguably the preferred test for assessing learning and memory in basic research. As a classic test, it has been accepted and used widely in most related animals experiments.

(4)Language: Chinese and English articles.

Exclusion criteria

They were excluded if the following criteria were met:

(1) Scalp acupuncture, auricular acupuncture, moxibus-tion and other forms of acupuncture; (2) studies that included Chinese herbal medicine or Western medicine; (3) studies that compared different acupuncture techniques or different acupoints; (4) studies without control group; (5) duplicate publications.

Study selection and data extraction

According to the above design, one reviewer (KYH) searched those databases and listed the titles of all articles. Two evaluators (KYH and SL) assessed the eligibility of these articles and made decision on every research (inclusion or exclusion) independently. If they did not reach the same decision, the concerned articles were discussed with a third reviewer (SFL).

Two reviewers (KYH and SL) extracted data independently from each study. The database included: (1) basic information, including publication year, the first author's name and model of learning and memory impairment;

(2) individual data, including the number of animals, species and weight in acupuncture group and control group; (3) information on treatment, including timing and duration for treatment and method of treatment procedure; (4) the results of Morris water maze test. If outcomes were presented at different time points, we extracted data from the last time point. Differences of extracted data were solved after discussion with a third reviewer (SFL).

Quality assessment

We evaluated the methodological quality of the included studies by a ten-item scale [12]: (1) publication in a peer-reviewed journal; (2) statements describing control of temperature; (3) random allocation to treatment or control; (4) blinded building of model; (5) assessment whether building model is successful; (6) blinded assessment of outcome;(7) use of anesthetic without significant intrinsic neuroprotective activity; (8) sample size calculation; (9) compliance with animal welfare regulations; (10) declared any potential conflict of interest. Each item of the ten-item scale was attributed to one point. Based on this, each study had a quality score from zero to ten. The higher the score is, the better the article's quality is.

Two reviewers (KYH and SL) extracted data independently and assessed study quality. Disagreements were solved after discussion with a third reviewer (SFL).

Statistical analysis

Some results of Morris water maze test including escape latency, frequency of cross platform and time in target quadrant were considered as continuous data. Standard mean difference (SMD) was given, which was an estimate of the combined effect sizes. Publication bias was assessed with a funnel plot. Moreover, to explore the impact of factors affecting the outcome measures, we analyzed the specific subgroups based on escape latency and frequency of cross platform for the following factors: manual acupuncture and elec-troacupuncture, articles published or unpublished, species of animals, different ways to make Alzheimer's disease (AD) model and different ways to make vascular dementia (VD) model.

The meta-analysis was performed with RevMan 5.3 software. The confidence interval (CI) was established at 95 %, and P values of less than 0.05 were considered statistically significant. For the assessment of heterogeneity, the I2 statistic and x2 distribution were used.

Results

Study inclusion

Initially, 1421 records were searched from six databases. After removing duplicates, the records were decreased to 875. Based on titles and abstracts of records, we excluded 584 papers with reasons, such as not an animal experiment, case report or review, not related to learning and memory, and so on. The 291 remaining articles were downloaded for further selection. Due to republications, not using the Morris water maze test, comparing with other forms of acupuncture or Chinese herbs, and so on, 250 articles

were excluded. Eventually, 42 studies were included [18-59]. The flow diagram of the study selection process is shown in Fig. 1.

Study characteristics

The 42 included studies involved 944 rats. The total animal number in control groups is 470 and the number in acupuncture groups is 474. 36 studies of all have mentioned specific weight of rats. The rats' weight ranged from 160 to 320 g in 29 studies. The rats' weight was around 20 g in 3studies and more than 320 g in 4 studies. The age of animals was different and mentioned concretely in 20 studies. It ranged from new-born to 24-month-old. 1 study used new-born rats; 10 studies used 2-4 months old rats; 6 studies used 6-9 months old rats; 3 studies used aged rats (more than 12 months old). Different subtests of the Morris water maze test were used in these studies: 41 studies with 912 animals reported data as escape latency, 18 studies with 406 animals reported data as frequency of cross platform and 12 studies using 255 animals reported data as time in target quadrant. The rat species included Sprague-Daw-ley (SD) rats, Wistar rats and AKR rats. Eighteen out of the 42 studies (42.9 %, n = 377) were AD models. Fourteen studies (33.3 %, n = 309) were VD models. And the 10 remaining studies (23.8 %) used other models. The main characteristics of the 42 studies are shown in Table 1.

Description of acupuncture regime

Varied acupuncture techniques were used in terms of selection of acupuncture-points, manipulation or stimulation methods (Table 1). The most commonly used acu-points, which have been used by four or more studies, were GV20 (baihui), ST36 (zusanli), GV14 (dazhui), BL23 (shenshu), BL17 (geshu) and CV17 (danzhong). The frequency of acupuncture was mostly once per day. Animals received acupuncture treatment 1 to 30 min per session. The course of acupuncture treatment ranged from 7 to 60 days. The average duration of acupuncture was 22.4 days (SD = 10.8). 13 studies used manual acupuncture, and the rest 29 studies chose elec-troacupuncture. 5 of 13 studies stated detailed operating methods of manual acupuncture. The operating method of manual acupuncture was mostly twirling reinforcing. 28 of 29 studies stated detailed operating parameters of electroacupuncture and only one ignored related descriptions. 20 of 28 studies used continuous waves. The frequency of continuous wave is from 0.5Hz to 150Hz. The current density of continuous wave is from 0.5 mA to 30 mA. The rest 8 studies used disperse-dense waves. The frequency of disperse wave is from 1Hz to 80Hz and the frequency of dense

Fig. 1 Flow diagram of the study selection process for this systematic review and meta-analysis

wave is from 15Hz to 100Hz. The current density of continuous wave is from 1 mA to 3 mA.

Description of control interventions

Eighteen of included studies used some interventions in control groups (Table 1). Control interventions consisted of western medicine and sham acupuncture. Western medicine was adopted in 9 studies and 9 experiments used sham acupuncture. Medication was administered for similar treatment duration as acupuncture. Types of control medication consisted of donepezil (3 studies), nimodipine (3 studies), fluoxetine (1 study), madopar (1 study) and piracetam (1 study). Puncturing points lateral to acupoints is the way of sham acupuncture in 8 experiments. Not using electroacupuncture apparatus is the other way of sham acupuncture in 1 experiment which adopted electroacupuncture as the intervention way.

Study quality and publication bias

The score of the study quality was ranged from 2 to 8 out of a total 10 points. Concretely, one study got 2 points; three studies got 3; twelve studies got 4; eleven studies got 5; seven studies got 6; three studies got 7 and five studies got 8 points. Five studies were not published

because they were Master's or Ph.D thesis. Twenty-seven studies mentioned control of temperature, including control of the room or water temperature. Nineteen studies adopted blinded building of model and seventeen mentioned assessment whether building model was successful. Random allocation to control group or acupuncture group and blinded assessment of outcome were described in 42 and 12 studies. No study reported inducing significant intrinsic neuroprotective activity because of anesthetic. Only one study described the sample size calculation. Fourteen studies reported statement of potential conflict of interests and eleven reported compliance with animal welfare regulations. The study quality and publication bias evaluation are shown in Table 2.

Effectiveness

Forty-one studies reported the impact of acupuncture on decreasing escape latency compared with the control group (p < 0.05 or p < 0.01). Twenty-four of them provided detailed data to show significant effectiveness of acupuncture compared with the control group (n = 494, SMD -3.00, 95 % CI: -3.78 ~ -2.23, P < 0.00001; heterogeneity x2 = 185.09, I2 = 88 %, Fig. 2). The remaining seventeen studies did not provide detailed data and just showed the data demonstrated in a graphical form.

Species (Nc/Na) Weight(g) Age (month) Model

Bao 2014 [18] SD Rats (12/12) 200±20 NR

PTSD (CMS)

Zeng 2008 [19] Wlstar Rats (10/10) 250+ 10 NR

AD (D-gal, NaN02)

Chen 2015 [20] SD Rats (13/14) 230±20 NR

VD (4-VO)

Chen 2006 [21] Wlstar Rats (34/34) NR New-born HI BD (closed space)

Dal 2015 [22] AKR Rats (10/10) 24.0 ±3.5 6

AD (SAMP8)

Hon 2013 [23] SD Rats (10/10) 220±20 NR

PTSD(SPS)

Gao 2012 [24] SD Rats (16/16) 400-500 20-24 AD(aged rats)

Huang 2010 [25] Wlstar Rats (10/10) 200±20 NR

PD (6-0 H DA)

Jl 2011 [26] SD Rats (10/10) 220-260 NR

Jla 2011 [27] SD Rats (20/20) 220±20 NR

VD (4-VO) VD (2-VO)

Wang 2012 [28] Wlstar Rats (10/10) 300± 10 NR

AD(STZ)

Lin 2008 [29] SD Rats (10/10) 620 ±80 12

VD (2-VO)

Acupuncture (acupoints)

Control intervention Outcome index

P value

Electroacupuncture 20 min/d for 21 d, Fluoxetine continuous waves of 2Hz of frequency and current density of 2 mA (GV20, GV29).

Electroacupuncture 20 min/d for 60d, Donepezi disperse- dense waves of 80/100 Hz of frequency (LI4, LR3).

Electroacupuncture 30 min/d for 7d, NR disperse- dense waves of 1/20 Hz of frequency (GV24, GV20).

Manual acupuncture 20 min/d for NR

10d (GV20, GV14).

Manual acupuncture 10 min/d for 28d NR (GV20, SP10, BL17, BL23).

Electroacupuncture 30 min/d for 7d, continuous waves of 2Hz of frequency and current density of 1 mA (GV20, ST36).

Electroacupuncture 30 min/d for 21 d, disperse- dense waves of 2/100 Hz of frequency and intensity of 2-4 V (GV20, KI1).

Electroacupuncture 30 min/d for 24d, continuous waves of 100Hz of frequency and current density of 0.5 mA (GV16.LR3).

Manual acupuncture 30 min/d for 30d (GV20, CV17, CV6, BL17, SP6).

Electroacupuncture 20 min/d for 14d, continuous waves of 2Hz of frequency and intensity of 3 V (GV20, GV14).

Electroacupuncture for 28d (GV20, GV14, ST36).

Electroacupuncture 20 min/d for 30d, continuous waves of 2Hz of frequency and current density of 1-2 mA (GV20, GV14.BL23).

Madopar

1. escape latency

2. total swimming distance

1. escape latency

1. escape latency

2.total swimming distance frequency of cross platfo

1. escape latency

2. percentage of time in target quadrant

3. percentage of swimming distance in target quadrant

1. escape latency

2. frequency of cross platform

3. time in target quadrant

1. escape latency

. swimming time

Sham acupuncture

1. escape latency

1. escape latency

2. frequency of cross platform

1. escape latency

2. frequency of cross platform

1. escape latency

2. frequency of cross platform

3. time in target quadrant

1. escape latency

2. time in target quadrant

1. P< 0.01

2. P < 0.01

1. P< 0.01

1. P < 0.05

2. P < 0.05

3. P < 0.05

1P < 0.05

2. P < 0.05

3. P < 0.05

1. P < 0.05

2. P < 0.05

3. P<0.05

1. P < 0.05

. P < 0.05

1. P < 0.01

1. P< 0.01

2. P < 0.01

1. P < 0.05

2. P < 0.05

1. P < 0.05

2. P < 0.05

3. P < 0.05

1. P < 0.05

2. P < 0.05

Luo 2007 [30]

Ma 2009 [31]

Zhang 2014 [32] Niu 2009 [33]

Su 2013 [34]

Tan 2014 [35]

Tang 2014 [36]

Wang 2013 [37] Wang 2009 [38] Hong 2014 [39] Xu 2006 [40]

Xu 2007 [41]

SD Rats (14/14) 200 ±20

Wistar Rats (13/13) 200-250

AKR Rats (10/10) NR

SD Rats (10/10) 300 ±20

SD Rats (12/12) 200-250

Wistar Rats (8/8) 250 ±50

SD Rats (10/10) 160-200

SD Rats (10/10) 200 ±20

SD Rats (12/13) 240 ±20

Wistar Rats (10/10) 300-350

SD Rats (13/14) 200-220

SD Rats (8/10) 180-200

NR VD (4-VO)

NR Diabete (STZ)

4 AD (SAMP8)

NR VD (4-VO)

NR AD(D-gal, Aß1- 42)

2 VD (MCAO)

NR AD (D-gal, NaN02)

3 VD (2-VO)

NR Autism (VPA)

2 VD (4-VO)

NR AD (D-gal)

Electroacupuncture 20 min/d for 15d, continuous waves of 150Hz of frequency and current density of 1 mA (GV20, BL17, BL20, BL23).

Electroacupuncture 15 min/d for 14d,

continuous waves of 0.5Hz of frequency and current density of 30 mA (GV20, GV14).

Manual acupuncture 30 mln/d for 30d, twisting 10 s a time (GV20,ST36).

Electroacupuncture 10 mln/d for42d, disperse- dense waves of 80/100 Hz of frequency and current density of 1-3 mA (GV29, LI20).

Electroacupuncture 15 mln/d for 28d, continuous waves of 35Hz of frequency and Intensity of 2 V (GV20, KB, ST36).

Electroacupuncture 15 mln/d for 21 d, continuous waves of 16Hz of frequency and current density of 1 m A (GV20, GV14).

Electroacupuncture 20 mln/d for 45d, continuous waves of 3-4Hz of frequency and current density of 4-5 mA (ST36, BL23).

Manual acupuncture 10 mln/d for 30d (LI4, LR3, ST36).

Manual acupuncture 10 mln/d for 30d (GV20, BL17, CV6, SP6, CV17).

Manual acupuncture 1 mln/d for 30d (GV1).

Electroacupuncture 20 mln/d for 20d, continuous waves of 150Hz of frequency and current density of 20 mA (GV20, GV14).

Electroacupuncture 20 mln/d for 21 d, continuous waves of 3Hz cf frequency and current density of 1 mA (GV20, ST36).

Nimodipine 1. escape latency 1. P< 0.01

NR 1. escape latency 1. P < 0.05

NR 1. escape latency 1. P < 0.05

2. frequency of cross platform 2. P < 0.05

3. time In target quadrant 3. P < 0.05

NR 1. escape latency 1. P< 0.01

2. frequency of cross platform 2. P < 0.01

NR 1. escape latency 1. P < 0.05

2. frequency of cross platform 2. P < 0.05

3. time in target quadrant 3. P < 0.05

NR 1. escape latency 1. P < 0.05

2. frequency of cross platform 2. P < 0.05

3. percentage of time In target 3. P<0.05

quadrant

Sham acupuncture 1. escape latency 1-P< 0.01

2. frequency of cross platform 2. P< 0.01

Donepezi 1. escape latency 1. P< 0.01

2. frequency of cross platform 2. P< 0.01

Piracetam 1. escape latency 1. P< 0.01

2. frequency of cross platform 2. P < 0.05

Sham acupuncture 1. escape latency 1. P < 0.05

2. average speed 2. P < 0.05

Nimodipine 1. escape latency 1. P< 0.01

2. frequency of cross platform 2. P< 0.01

NR 1. escape latency 1. P < 0.01

2. percentage of swimming 2. P< 0.01

distance In target quadrant 3. P< 0.01

3. percentage of time in target

quadrant

Table 1 Characteristics of the included studies (Continued) Yi 2014 [42] SD Rats (12/12) 200±34 4

AD (A|325—35)

Yu 2014 [43] Wistar Rats (10/10) 200-250 NR

WD (CuS04)

Feng 2013 [44] AKR Rats (10/9) 29-35 9 AD (SAMP8)

Li 2013 [45] Wistar Rats (10/10) 200-250 4 AD(STZ)

Wang 2013 [46] AKR Rats (10/10) NR 8

Zheng 2009 [47] Wistar Rats (8/7) 212 ± 15 2 Li 2012 [48] AKR Rats (15/15) NR 7.5

AD (SAMP8) VD (2-VO) AD (SAMP8)

Li 2014 [49] C57BL/6 Rats (6/6) NR 2

AD (APP/PS1)

Lee 2014 [50] SD Rats (7/7) 220-240 NR

AD(SCO)

Zhu 2013 [51] SD Rats (6/6) 432±30 12

VD (2-VO)

Lu 2014 [52] SD Rats (

200-250 NR

Ethano

Li 2012 [53] SD Rats (10/10) 250 ±30 3

VD (MCAO)

Guo 2015 [54]

SD Rats (10/10)

250-300 NR

AD (A(31 -40)

Electroacupuncture 30 min/d for 12d, disperse- dense waves of 2/30 Hz of frequency and current density of

1 mA (GV29, LI20).

Electroacupuncture 15 min/d for 7d, continuous waves of 2Hz of frequency and current density of 1 mA (HT7).

Manual acupuncture 20 min/d for 28d, twisting 25 times/s for 60 s(GV20, KI1).

Electroacupuncture for 28d, continuous waves of 30Hz of frequency and intensity of 2 V (BL23, KB, ST36, GV20, GV14).

Manual acupuncture for 15d (CV17, CV12, CV6, SP10,ST36).

Electroacupuncture for 28d, continuous waves of 2Hz of frequency (GV20, KI3).

Manual acupuncture for 15d, twisting

2 times/s for 30 s (CV17, CV12, CV6, SP10, ST36).

Electroacupuncture 30 min/d for 20d, disperse- dense waves of 2/15 Hz of frequency and current density of 1 mA (GV20).

Manual acupuncture for 15d (GV20).

Electroacupuncture 20 min/d for 30d, continuous waves of 4Hz of frequency and current density of 2 mA (GV20, GV14, BL23).

Electroacupuncture 20 min/d for 30d, continuous waves of 2Hz of frequency and current density of 1.5-2 mA (ST36).

Electroacupuncture 30 min/d for 14d, disperse- dense waves of 2/30 Hz of frequency and current density of 2 mA (GV20, GV14).

Electroacupuncture 30 min/d for 24d, continuous waves of 20Hz of frequency and current density of less than 2 mA (GV20, BL23).

NR 1. escape latency 1.P<0.05

2. frequency of cross platform 2. P<0.05

3. time in target quadrant 3. P<0.05

NR 1. escape latency 1. P < 0.01

NR 1. escape latency 1.P<0.05

2. frequency of cross platform 2. P<0.05

Donepezil 1. escape latency 1.P<0.05

2. frequency of cross platform 2. P<0.05

Sham acupuncture NR

Sham acupuncture

1. escape latency 1. P < 0.01

2. time in target quadrant 2. P<0.01

1. escape latency 1.P<0.05

1. escape latency 1.P<0.05

2. time in target quadrant 2. P<0.05

. escape latency

. P < 0.05

Sham acupuncture

1. escape latency 1.P<0.05

2. swimming speed 2. P<0.05

3. percentages of time in a 3. P<0.05 probe trial

1. escape latency 1.P<0.05

Sham acupuncture 1. escape latency 1.P<0.05

2. time in target quadrant 2. P<0.05

. escape latency

. P<0.01

Sham acupuncture 1. escape latency 1. P < 0.01

2. time in target quadrant 2. P<0.01

3. frequency of cross platform 3. P<0.01

Jiang 2015 [55] AKR Rats (10/10) NR 7.5 AD (SAMP8) Electroacupuncture for 14d, continuous waves of 2Hz of frequency,current density of 0.6 mA and intensity of 2 V (GV20,GV26, GV29). NR 1. escape latency 2. percentages of time in target quadrant 1. P <0.01 2. P <0.01

Shao 2008 [56] SD Rats (8/9) 180-220 NR VD (4-VO) Electroacupuncture 20 min/d for 15d, continuous waves of 150Hz of frequency and current density of 1-2 mA (GV20, BL17, BL20, BL23). Nimodipine 1. escape latency 2. time in target quadrant 1. P <0.01 2. P <0.01

Liu 2013 [57] SD Rats (12/12) 200±20 NR CFS Manualacupuncture 20 min/d for 21d, twirling reinforcing (ST36). NR 1. escape latency 2. frequency of cross platform 1. P < 0.05 2. P < 0.05

Li 2015 [58] Wistar Rats (11/11) 320-360 NR VD(micro- emboli) Manualacupuncture for 12d, twisting 2 times/s for 30 s (ST36). Sham acupuncture 1. escape latency 1. P <0.01

Lu 2008 [59] AKR Rats (12/12) 20 ±2 8 AD (SAMP8) Electroacupuncture 20 min/d for 7d, disperse- dense waves of 2/100 Hz of frequency and intensity of 2-4 V (GV20, KI1). NR 1. escape latency 2. time in target quadrant 1. P < 0.05 2. P < 0.05

Nc animal number in control group, Na animal number in acupuncture group, PTSD post-traumatic stress disorder, CMS chronic mild stimulation, NR not report, AD Alzheimer's disease, VD vascular dementia, 4-VO 4- vessel occlusion, SPS single prolonged stress, PD Parkinson's disease, 6-OHDA 6-OH-dopamine, STZ streptozotocin, CFS chronic fatigue syndrome, MCAO middle cerebral artery occlusion, OVX ovariectomy, VPA sodium valproate, WD Wilson disease

Table 2 Risk of bias of included studies

Study (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) Total

Bao 2014 [18] V V V V V V 6

Zeng 2008 [19] V V V V 4

Chen 2015 [20] V V V V V V V V 8

Chen 2006 [21] V V V V V 5

Dai 2015 [22] V V V V V 5

Hou 2013 [23] V V V V V 5

Gao 2012 [24] V V V V 4

Huang 2010 [25] V V V V 4

Ji 2011 [26] V V V V 4

Jia 2011 [27] V V V V 4

Wang 2012 [28] V V V V V V 6

Lin 2008 [29] V V V V 4

Luo 2007 [30] V V 2

Ma 2009 [31] V V V V V 5

Zhang 2014 [32] V V V V V 5

Niu 2009 [33] V V V V 4

Su 2013 [34] V V V 3

Tan 2014 [35] V V V V V V V 7

Tang 2014 [36] V V V 3

Wang 2013 [37] V V V V 4

Wang 2009 [38] V V V V 4

Hong 2014 [39] V V V V V V 6

Xu 2006 [40] V V V V 4

Xu 2007 [41] V V V V V 5

Yi 2014 [42] V V V V V V 6

Yu 2014 [43] V V V V V 5

Feng 2013 [44] V V V V V 5

Li 2013 [45] V V V V 4

Wang 2013 [46] V V V V V V 6

Zheng 2009 [47] V V V 3

Li 2012 [48] V V V V V V V V 8

Li 2014 [49] V V V V V V V 7

Lee 2014 [50] V V V V V V V V 8

Zhu 2013 [51] V V V V V V 6

Lu 2014 [52] V V V V V V V V 8

Li 2012 [53] V V V V V 5

Guo 2015 [54] V V V V V V 6

Jiang 2015 [55] V V V V V V V 7

Shao 2008 [56] V V V V V 5

Liu 2013 [57] V V V V V 5

Li 2015 [58] V V V V V V V V 8

Lu 2008 [59] V V V V 4

(1) publication in a peer-reviewed journal; (2) statements describing control of temperature; (3) random allocation to treatment or control; (4) blinded building of model; (5) assessment whether building model is successful; (6) blinded assessment of outcome;(7) use of anesthetic without significant intrinsic neuroprotective activity; (8) sample size calculation; (9) compliance with animal welfare regulations; (10) declared any potential conflict of interest

Eighteen studies reported the impact of acupuncture on increasing frequency of cross platform compared with the control group (p < 0.05 or p < 0.01). Fourteen of them provided detailed data to show significant effectiveness of acupuncture compared with the control group (n = 317, SMD 2.57, 95 % CI: 1.92 ~ 3.22, P < 0.00001; heterogeneity X = 52.81, I2 = 75 %, Fig. 3). The remaining four studies just showed the data demonstrated in the graphical form and failed for meta-analysis.

Eleven studies reported the impact of acupuncture on increasing time in target quadrant compared with the control group (p < 0.05 or p < 0.01). Seven of them provided detailed data to show significant effectiveness of acupuncture compared with the control group (n = 149, SMD 2.00, 95 % CI: 1.10 ~ 2.91, P < 0.00001; heterogeneity x2 = 28.18, I2 = 79 %, Fig. 4). The remaining five studies just showed the data demonstrated in the graphical form.

Signaling pathways

Several different signaling pathways were investigated to gain a better understanding of the underlying mechanism of acupuncture in the amelioration of learning and memory impairment. 39 of 42 included studies got detailed descriptions about possible mechanisms. It can be found that reduction nerve apoptosis and necrosis and suppression of oxidative stress are the main signaling pathways. A summary of proposed mechanism is shown in Table 3.

Assessment of bias

The funnel plot was approximately symmetric for the effect of acupuncture on escape latency, frequency of cross platform and time in target quadrant (Fig. 5). Nevertheless, publication bias was still considered to be potential.

Affecting factors of outcome indexes

In the subgroup analysis of escape latency, the efficacy of manual acupuncture was better than electroacupunc-ture (SMD -4.09, 95 % CI: -6.24 ~ -1.95, Fig. 6a). Compared with published articles, unpublished articles showed more obvious changes of escape latency caused by acupuncture treatment (SMD -6.89, 95 % CI: -8.68 —5.29, Fig. 6b). According to different varieties of experimental animals, SD rats were more sensitive to acupuncture than Wister rats and AKR rats (SMD -3.83, 95 % CI: -5.21 ~ -2.45, Fig. 6c). AD and VD are the common diseases causing learning and memory impairment. After treatment, escape latency was further improved in AD animal model (SMD -3.23, 95 % CI: -4.48 ~ -1.99, Fig. 6d). There are many ways making AD and VD animal models.

Study or Subgroup

Acupuncture Mean SD Total

Control Mean SD Total Weight

St (I, Mean Difference IV. Random. 95% CI

Std. Mean Difference IV. Rando il. 95% CI

1.1.1 Escape latency

Xu 2006

Xu 2007

Lu 2008

Shao 2008

Zeng 2008

Zheng 2009

Wang 2009

Ma 2009

Huang 2010

Ji 2011

Wang 2012

Zhu2013

Feng 2013

Su 2013

Li 2013

Wang2013

Wang 2013#

Tan 2014

Tang 2014

Yi 2014

Zhang 2014

Jiang 2015

Chen 2015

Dai 2015

Subtotal (95% CI)

Heterogeneity: Tau= =

Test for overall effect:

21.75 21.63 14 42.65 33.23 13 5.0% -0.73 [-1.51, 0.05] 2006

6.87 4.72 10 9.16 5.32 8 4.9% -0.44 [-1.38, 0.51] 2007

47.45 2.72 12 64.18 3.46 12 4.1% -5.19 [-6.98,-3.40] 2008

13.47 7.83 9 101.57 19.12 8 3.5% -5.86 [-8.31,-3.42] 2008

92.86 9.38 10 106.98 10.77 10 4.9% -1.34 [-2.33,-0.35] 2008

38.57 11.19 7 53 9.71 8 4.7% -1.30 [-2.45,-0.15] 2009

8.39 3.39 13 105.19 18.64 12 3.6% -7.13 [-9.43,-4.84] 2009

39.05 7.93 13 70.85 8.62 13 4.6% -3.72 [-5.06,-2.38] 2009

83.5 8.2 10 107.3 14.1 10 4.8% -1.98 [-3.09,-0.86] 2010

43.67 10.38 10 86.91 14.62 10 4.5% -3.27 [-4.70,-1.84] 2011

33.24 1.44 10 39.76 0.65 10 3.8% -5.59[-7.71,-3.47] 2012

10.28 4.27 6 25.78 13.02 6 4.6% -1.48[-2.82,-0.14] 2013

13.39 6.03 9 48.22 20.65 10 4.7% -2.13 [-3.31,-0.95] 2013

21.93 0.97 12 41.69 1.26 12 1.5% -16.97[-22.28,-11.66] 2013 <

37.69 5.42 10 52.73 7.26 10 4.7% -2.25 [-3.42,-1.08] 2013

21.57 13.4 10 109.82 11.37 10 3.4% -6.80 [-9.31,-4.29] 2013

48.34 15.65 10 73.3 21.74 10 4.9% -1.26 [-2.24,-0.28] 2013

79.31 6.21 8 95.07 10.43 8 4.7% -1.74 [-2.94,-0.54] 2014

19.02 4.79 10 42.95 4.05 10 3.9% -5.17 [-7.16,-3.18] 2014

13.06 5.69 12 28.99 9.61 12 4.9% -1.95 [-2.95,-0.95] 2014

25.66 0.11 10 41.74 0.12 10 0.0% -133.79 [-180.07,-87.51] 2014 4

47.1 10.78 10 53.67 8.28 10 4.9% -0.65 [-1.56, 0.25] 2015

27.04 12.72 14 50.3 8.32 13 4.9% -2.08[-3.05,-1.12] 2015

41.16 5.29 10 57.31 4.27 10 4.5% -3.221-4.63,-1.80] 2015

249 245 100.0% -3.00 [-3.78, -2.23]

2.94; Chi== Z= 7.60 (P

185.09, df= 23 (P < 0.00001); P : 0.00001)

Total (95% CI) 249 245 100.0%

Heterogeneity: Tau= = 2.94; Chi'= 185.09, df= 23 (P < 0.00001); P= 88% Test for overall effect: 7= 7.60 (P < 0.00001) Test for subgroup differences: Not applicable

Fig. 2 Pooled estimate of decreasing escape latency with acupuncture

-3.001-3.78, -2.23]

-20 -10 0 10 Favours [Acupuncture] Favours [control]

By analyzing different ways causing AD models, we found that AD models caused by D-gal and A|3 were more sensitive to acupuncture than other ways (SMD -16.97, 95 % CI: -22.28—11.66, Fig. 6e). By analyzing different ways causing VD models, we found that VD models caused by 2-VO were more sensitive to acupuncture

than other ways (SMD -3.12, 95 % CI: -5.96 ~ -0.28, Fig. 6f). Different weight and age of rats were included in these studies. We found that rats whose weight ranges from 18 to 35 g (SMD -3.4, 95 % CI: -5.05 ~ -1.75, Fig. 6g) and rats whose age ranges from 2 to 4 months old (SMD -2.86, 95 % CI: -4.43 ~ -1.29,

Study or Subgroup

Control Mean SD

Acupuncture Total Mean SD Total Weight

Std. Mean Difference

IV. Random. 95% CI Year

Std. Mean Difference IV. Random. 95% CI

2.1.1 Frequency of cross platform

Xu 2006 Jia 2011 Ji 2011 Wang 2012 Wang 2013 Feng 2013 Li 2013 Su 2013 Zhang 2014 Yi 2014 Tan 2014 Tang 2014 Chen 2015 Dal 2015

Subtotal (95% CI) Heterogeneity: Tau2 = 1.05; Chi2 = 52.81, df = 13 (P < 0.00001); I2 = Test for overall effect: Z = 7.79 (P < 0.00001)

5.95 2.13 14 2.31 1.84 13 8.6% 1.77 [0.86, 2.68] 2006

2.6 0.62 20 1.3 0.35 20 8.8% 2.53 [1.68, 3.38] 2011

12.38 3.67 10 5.49 1.56 10 7.7% 2.34 [1.15, 3.53] 2011

6.26 0.98 10 3.11 1.12 10 7.2% 2.87 [1.54, 4.19] 2012

2.3 1.16 10 0.7 0.67 10 8.2% 1.62 [0.58, 2.66] 2013

5.22 2.05 9 2.3 0.82 10 7.9% 1.83 [0.71, 2.94] 2013

4.98 1.27 10 2.53 1.08 10 7.9% 1.99 [0.88, 3.11] 2013

4.41 1.06 12 1.54 0.82 12 7.6% 2.92 [1.72, 4.13] 2013

1.23 0.13 10 0.23 0.13 10 3.7% 7.37 [4.67, 10.06] 2014

9.4 4.47 12 4.35 1.65 12 8.6% 1.45 [0.53, 2.36] 2014

4.63 0.54 8 1.88 0.23 8 3.7% 6.26 [3.58, 8.95] 2014

8.53 0.83 10 2.35 0.44 10 2.9% 8.91 [5.71, 12.11] 2014

2.75 1.13 14 0.96 0.66 13 8.6% 1.86 [0.93, 2.78] 2015

6.5 1.58 10 4.2 2.15 10 8.4% 1.17 [0.20, 2.13] 2015

159 158 100.0% 2.57 [1.92, 3.22]

2.57 [1.92, 3.22]

Total (95% CI) 159 158 100.0%

Heterogeneity: Tau2 = 1.05; Chi2 = 52.81, df = 13 (P < 0.00001); I2 = 75% Test for overall effect: Z = 7.79 (P < 0.00001) Test for subgroup differences: Not applicable

Fig. 3 Pooled estimate of increasing frequency of cross platform with acupuncture

-10 0 10 20 Favours [Control] Favours [Acupuncture]

Acupuncture

Std. Mean Difference

Std. Mean Difference

Studv or Subarouo Mean SD Total Mean SD Total Weiaht IV. Random. 95% CI Year IV. Randc m. 95% CI

3.1.1 Time in target quadrant

Lu 2008 39.55 5.47 12 30.27 6.12 12 15.8% 1.54 [0.61, 2.48] 2008 *

Shao 2008 6.97 1.56 9 2.45 0.85 8 11.9% 3.36 [1.75, 4.96] 2008

Wang 2012 7.16 6.69 10 4.1 2.25 10 16.0% 0.59 [-0.31, 1.49] 2012

Wang 2013 18.64 5.32 10 13.54 2.35 10 15.6% 1.19 [0.22, 2.16] 2013 ■

Su 2013 4.41 1.06 12 1.54 0.82 12 14.2% 2.92 [1.72, 4.13] 2013

Yl2014 39.01 19.32 12 23.92 7.74 12 16.2% 0.99 [0.13, 1.85] 2014

Dai 2015 21.82 3.85 10 4.98 2.65 10 10.3% 4.88 [2.98, 6.78] 2015

Subtotal (95% CI) 75 74 100.0% 2.00 [1.10, 2.91]

Heterogeneity: Tau2 = 1.13; Chi2 = 28.18, df = 6 (P < 0.0001); I2 = 79%

Test for overall effect: Z = 4.33 (P < 0.0001 )

Total (95% CI) ■i AO- Ok 2 — OO 75 O At — a /D -- n nnrn 74 100.0% 12 — 7(10/ 2.00 [1.10, 2.91] I- -1- -1-1

Test for overall effect: Z = 4.33 (P < 0.0001 ) Test for subgroup differences: Not applicable

Fig. 4 Pooled estimate of Increasing time In target quadrant with acupuncture

-5 0 5

Favours [Control] Favours [Acupuncture]

Fig. 6h) were more sensitive to acupuncture for improvement of frequency of escape latency.

In the subgroup analysis of frequency of cross platform, the electroacupuncture was more effective than manual acupuncture (SMD 2.72, 95 % CI: 1.91 ~ -3.52, Fig. 7a). Compared with unpublished articles, published articles showed more obvious changes of frequency of cross platform caused by acupuncture treatment (SMD 2.74, 95 % CI: 1.96 ~ 3.53, Fig. 7b). According to different varieties of experimental animals, Wister rats were more sensitive to acupuncture than SD rats and AKR rats (SMD3.31, 95 % CI: 1.47 ~ 5.16, Fig. 7c). Compared with AD models, VD models got more improvement of frequency of cross platform (SMD 2.42, 95 % CI: 1.60 ~ 3.24, Fig. 7d). By analyzing different ways causing AD models, we found that SAMP8 models were more sensitive to acupuncture (SMD 3.04, 95 % CI: 0.65 ~ 5.42, Fig. 7e). By analyzing different ways causing VD models, we found that MCAO models were more sensitive (SMD 6.26, 95 % CI: 3.58 ~ 8.95, Fig. 7f). Different weight and age of rats were included in these studies. We found that rats whose weight ranges from 160 to 320 g (SMD 2.52, 95 % CI: 1.86 ~ 3.17, Fig. 7g) and rats whose age ranges from 2 to 4 months (SMD4.06, 95 % CI: 2.30 ~ 5.83, Fig. 7h) were more sensitive to acupuncture for improvement of frequency of cross platform.

In the subgroup analysis of time in target quadrant, the efficacy of manual acupuncture was better (SMD 2.94, 95 % CI: -0.67 ~ 6.55, Fig. 8a). There were no unpublished articles using time in target quadrant as outcome indexes. According to different varieties of experimental animals, SD rats were more sensitive to acupuncture than Wister rats and AKR rats (SMD2.33, 95 % CI: 0.77 ~ 3.89, Fig. 8b). Compared

with AD models, VD models got more improvement of time in target quadrant (SMD 3.36, 95 % CI: 1.75 ~ 4.96, Fig. 8c). By analyzing different ways making models of AD, we found that AD models caused by D-gal and A|3 were more sensitive to acupuncture (SMD 2.92, 95 % CI: 1.72 ~ 4.13, Fig. 8d). 4-VO was the only way to be included causing VD models and using time in target quadrant as one of outcome indexes. Different weight and age of rats were included in these studies. We found that rats whose weight ranges from 18 to 35 g (SMD 3.10, 95 % CI: -0.16 ~ 6.37, Fig. 7e) and rats whose age ranges from 6 to 9 months old (SMD 2.31, 95 % CI: 0.65 ~ 3.98, Fig. 7f) were more sensitive to acupuncture for improvement of time in target quadrant.

These results were mostly inconsistent in three subgroup analyses. It may be caused by studies of low quality, publication bias or other reasons.

Discussion

To our knowledge, this is the first systematic review and meta-analysis to explore the efficacy of acupuncture for improving learning and memory in animal experiments with the results of Morris water maze test as the outcome measures. This systematic review and meta-analysis indicated that acupuncture could reduce time of escape latency, decrease frequency of cross platform and increase the time in target quadrant in animal model of learning and memory impairment. It suggests that acupuncture has a potential role in improving learning and memory impairment in animal models.

This review made a more detailed description of the acupuncture treatment procedure, including acu-point selection, stimulation methods, and treatment duration. The variation in the acupuncture protocol

Table 3 Summary of proposed mechanisms

Study Findings or proposed mechanisms

Bao 2014 [18] • Reduced apoptosis of hippocampalneurons

• Promoted restoration of damaged nerve cells

Chen 2015 [20] • Reduced Nogo-A and NgR

Chen 2006 [21] • Increased GABA

• Reduced Glu

Dai 2015 [22] • Increased NEP

Hou 2013 [23] • Increased MR

• Reduced GR

Gao 2012 [24] • Increased DA, 5-HT and NE

Huang 2010 [25] • Increased BDNF

Ji 2011 [26] • Reduced MDA

• Increased SOD

Jia 2011 [27] • Increased Syp, PKC, NMDAR and PKC mRNA

• Reduced mGluRs

Wang 2012 [28] • Increased NOS

• Reduced MAO

Lin 2008 [29] • Increased NMDAR-2BmRNA

Luo 2007 [30] • Decreased NO

Ma 2009 [31] • Increased CTGF protein and mRNA

Zhang 2014 [32] • Increased InsR mRNA

Niu 2009 [33] • Increased SS and AVP

Su 2013 [34] • Reduced MDA, P53 and P21

• Increased SOD

Tan 2014 [35] • Increased GAP-43 and c-fos

Tang 2014 [36] • Increased CHAT protein

Wang 2013 [37] • Reduced GSK-3p

Wang 2009 [38] • Reduced ET

• Increased CGRP

Hong 2014 [39] • Increased CX43, CX32 and CX36

Xu 2006 [40] • Increased Bcl-2 protein

• Reduced Bax protein and mRNA

Xu 2007 [41] • Reduced IL-1 p and TNF-a

Yi 2014 [42] • Increased SOD, PKA and pCREB

• Reduced MDA and c-fos

Feng 2013 [44] • Increased GluR2

Li 2013 [45] • Reduced MMP-2 and MMP-9

Zheng 2009 [47] • Increased ChAT

• Reduced TchE

Li 2012 [48] • Reduced neuron loss

Li 2014 [49] • Increased BDNF

Lee 2014 [50] • Stimulated cholinergic enzyme activities

• Regulated BDNF and CREB expression

Zhu 2013 [51] • Up-regulated mTOR and eIF4E

Lu 2014 [52] • Increased Fos expression

Li 2012 [53] • increased Bcl-2mRNA

• decreased caspase-3

Guo 2015 [54] • Down-regulated Notch1 and Hes1 mRNA

Jiang 2015 [55] • Increased the levelof uptake rate of glucose

Shao 2008 [56] • Regulated the amount of AVP, SS, and p-EP

Table 3 Summary of proposed mechanisms (Continued)

Liu 2013 [57] • Reduced MDA

• Increased SOD

Li 2015 [58] • Increased the pyramidalneuron number

• Decreased the number of astrocytes

Lu 2008 [59] • Increased NCAM and ST8SiaII/IVmRNA

Nogo-A neurite growth inhibitor-A, NgR neurite growth inhibitor receptor, GABA Y-aminobutyric acid, Glu glutamic acid, NEP neutral endopeptidase, MR mineralocorticoid receptor, GR glucocorticoid receptor, DA dopamine, 5-HT 5-hydroxytryptamine, NE norepinephrine, BDNF brain-derived neurotrophic factor, MDA malondialdehyde, SOD superoxide dismutase, SYP synaptophysin, PKC Protein kinase C, NMDAR N-methyl-D-aspartate receptor, mGluRs metabolism glutanic acid acceptor, NOS nitric oxide synthase, MAO monoamine oxidase, NO nitric oxide, CTGF connective tissue growth factor, insR insulin receptor, SS somatostatin, AVP arginine vasopressin, GAP-43 Growth Associated Protein-43, CHAT choline acetyl transferase, GSK-3j glycogen synthase kinase-3P, ET endothelia, CGRP calcitonin gene-related peptide, CX connexin, !L interleukin, TNF tumor necrosis factor, pCREB phosphorylated cAMP-response element binding, MMP metal matrix proteinase, ChAT choline acetyltransferase, TchE total cholinesterase, CREB cAMP response element-binding protein, mTOR mammalian target of rapamycin, e!F4E eukaryotic translation initiation factor 4E, NCAM neural cell adhesion molecule

might contribute to the heterogeneity in treatment outcome between studies. The most common acu-points in acupuncture treatment for improving learning and memory were GV20 (baihui), ST36 (zusanli), GV14 (dazhui), BL23 (shenshu), BL17 (geshu) and CV17 (danzhong). GV20 and GV14 are Governor Vessel acupoints. BL23 and BL17 are Bladder Meridian of Foot-Taiyang acupoints. This result suggests the possible importance of Governor Vessel and Bladder Meridian of Foot-Taiyang for learning and memory. About one-third of included studies used manual acupuncture, and two-third used electroacupuncture. We have found that continuous waves, 2-4Hz of frequency and 1-2 mA of current density are the most commonly used stimulus parameters of electroacupuncture and twirling slowly is commonly used in manual acupuncture groups. Thus it can be seen that mild stimulation which means the reinforcing method in Traditional Chinese Medicine theory is adopted widely for improving learning and memory.

In the acupuncture study on animal model, setting sham acupuncture as control is very important and difficult. The importance of sham acupuncture is that it can help to clear the effectiveness of acupuncture after excluding placebo effects. While popular sham ways are mainly skin penetration. Except for placebo effects, it can also produce biological effects. Therefore, the true effect of acupuncture may be underestimated when compared to sham acupuncture [60].

Acupuncture has been known as an effective therapy for learning and memory impairment appearing in Alzheimer's disease, vascular dementia, and so on

Fig. 5 Funnelplot for effectiveness of acupuncture on escape latency, frequency of cross platform and time in target quadrant

[5-8]. The mechanism of acupuncture on improving learning and memory ability remains unclear. It is recognized that learning and memory are associated with cerebral cortex and hippocampus closely [40]. Modern medical research suggests that acupuncture may reduce nerve apoptosis and necrosis to protect cortex and hippocampus neurons through different aspects, which include decreasing the level of cytokines in hippocampus [41], adjusting the neurotrophic factors and cholinergic system [42, 47], inhibiting the expression of matrix metalloproteina-se(MMP) [45], and so on. Additionally, it has been suggested that acupuncture can increase the activity of superoxide dismutase (SOD) and decrease the level of malondialdehyde (MDA) in brain to improve the antioxidant capacity and reduce brain tissue damage caused by free radical [58].

There are some limitations to this review. Firstly, our search only included Chinese and English articles and excluded those articles published in other languages. At the same time, we only included manual acupuncture and electroacupuncture and excluded some special acupuncture therapies, such as scalp acupuncture, auricular acupuncture, abdominal acupuncture, and so on. It may cause selective bias. Secondly, the total number of studies and the total sample size were too small for reliable. We have performed comprehensive literature search (six databases) and conducted extensive searches through other sources. But no more studies were found. Thirdly, articles which reported negative results may not be popular to publish so that the effectiveness of published articles would be better than those

unpublished. Therefore, the effectiveness of acupuncture for improving learning and memory may be overstated. Fourthly, the quality of included studies was very low, so that it had significant impacts on the outcomes of the meta-analysis.

Based on the above limitations, more other language articles and special acupuncture treatment means should be included in the future systematic. Meanwhile, control of temperature, random allocation to treatment or control, blinded building of model, assessment successful model building and blinded assessment of outcome should be pay attention to in the future animal or clinical studies. Not only positive results, but also negative should be reported in the future animal or clinical studies.

In addition, some implications are also brought out after analyzing affecting factors of outcome indexes. Manual acupuncture showed more effective to escape latency and time in target quadrant than electroacu-puncture. And they had roughly the same impact on frequency of cross platform. But in the present study, only 13 out of 42 studies performed manual acupuncture and the rest performed electroacupuncture. Because electroacupuncture is easier to control, standard and objectively measure than manual acupuncture [12], so it is used widely in clinical and experimental researches. Therefore, it remains unknown which means of acupuncture is more effective and convenient for people's learning and memory impairment. Other impact factors do not show any universal regularity through histogram analysis. It need be solved by higher quality studies, lower publication bias, and so on.

Fig. 6 Subgroup analysis according to escape latency. a The effect of manualacupuncture and electroacupuncture on the estimate of improvement in escape latency. b The impact of published articles compared with unpublished articles on the estimate of improvement in escape latency. c The type of strain on the estimate of improvement in escape latency. d The sensitiveness of AD modelcompared with VD modelon the estimate of improvement in escape latency. e The different ways making AD modelon the estimate of improvement in escape latency. f The different ways making VD modelon the estimate of improvement in escape latency. g The different weights on the estimate of improvement in escape latency. h The different age on the estimate of improvement in escape latency

Fig. 7 (See legend on next page.)

(See figure on previous page.)

Fig. 7 Subgroup analysis according to frequency of cross platform. a The effect of manualacupuncture and electroacupuncture on the estimate of improvement in frequency of cross platform. b The impact of published articles compared with unpublished articles on the estimate of improvement in frequency of cross platform. c The type of strain on the estimate of improvement in frequency of cross platform. d The sensitiveness of AD modelcompared with VD modelon the estimate of improvement in frequency of cross platform. e The different ways making AD modelon the estimate of improvement in frequency of cross platform. f The different ways making VD modelon the estimate of improvement in frequency of cross platform. g The different weights on the estimate of improvement in frequency of cross platform. h The different age on the estimate of improvement in frequency of cross platform

Fig. 8 Subgroup analysis according to time in target quadrant. a The effect of manualacupuncture and electroacupuncture on the estimate of improvement in time in target quadrant. b The type of strain on the estimate of improvement in time in target quadrant. c The sensitiveness of AD modelcompared with VD modelon the estimate of improvement in time in target quadrant. d The different ways making AD modelon the estimate of improvement in time in target quadrant. e The different weights on the estimate of improvement in time in target quadrant. f The different age on the estimate of improvement in time in target quadrant

Conclusion

In animal model, acupuncture has a potential role in improving learning and memory ability. But it is still ambiguous that which stimulating mode (manual acupuncture or electroacupuncture) is more effective. Low quality of studies and larvaceous publication bias may reduce persuasiveness of positive results and should be solved in the future.

Acknowledgements

We would like to express our gratitude and thanks to Antoine Grellet (Nanjing University of Chinese Medicine) and Xin Cao (Department of Pharmacology, Faculty of Medicine, Toho University) for editing of English.

Funding

This work was supported by grants from the National Natural Science Foundation of China (No.81202743, 81574062, 81303019, 81403478). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Availability of data and materials

The datasets supporting the conclusions of this article are included within the article.

Authors' contributions

Conceived and designed the experiments: KYH SL SFL. Performed the experiments: KYH SL. Analyzed the data: MLY SFL SPF. Contributed reagents/ materials/analysis tools: MLY SFL. Wrote the paper: KYH SL XC. Revised the manuscript: SFL SPF XC. Agreed with the manuscript's results and conclusions: KYH SL MLY SPF XC SFL. All authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

Consent for publication

Not applicable.

Ethics approval and consent to participate

Not applicable.

Received: 2 March 2016 Accepted: 17 August 2016 Published online: 19 August 2016

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