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 Table of Contents  
Year : 2022  |  Volume : 9  |  Issue : 2  |  Page : 51-57

The effects of flavonoids in curcumin on neurobehavioral deficits in insulin-resistant rats

1 Department of Anatomy, Faculty of Basic Medical Sciences, College of Health Sciences, Osun State University, Osogbo, Osun, Nigeria
2 Department of Anatomy, Faculty of Basic Medical Sciences, College of Health Sciences, University of Ilorin, Ilorin, Kwara, Nigeria

Date of Submission24-Jun-2022
Date of Acceptance28-Jul-2022
Date of Web Publication01-Sep-2022

Correspondence Address:
Abdullahi Abiodun Mohammed
Department of Anatomy, Faculty of Basic Medical Sciences, College of Health Sciences, Osun State University, Osogbo, Osun
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jnbs.jnbs_17_22

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Background: Diabetes mellitus is a risk factor for dementia, particularly Alzheimer's disease (AD). In a Wistar rat model, we studied Alzheimer-like symptoms using a high-fat diet (HFD) and streptozotocin (STZ) to replicate insulin resistance and the resulting neurobehavioral abnormalities. Curcumin, a flavonoid in turmeric, was studied for its potential therapeutic effects. Aim: This study sought to look at the exploratory, discriminatory, and spatial cognitive indices in rats. Materials and Methods: Thirty-six male Wistar rats were randomized into six groups and given the following treatments: olive oil only for control; curcumin only for the curcumin group; HFD and three doses STZ for the diabetic rats; HFD, three doses STZ, and concurrent treatment with curcumin for the protective group; pretreatment with curcumin, then HFD and three doses STZ for a preventive group; and HFD, three doses STZ, and curcumin for a therapeutic group. Subsequently, line and center line crossing frequency assessed rats' exploratory activities; rearing frequency data assessed novel environment behavior. The novel object recognition test and Morris water maze test assessed discrimination and spatial memory. Data were analyzed using one-way analysis of variance and Tukey's post hoc test. P < 0.05 was considered statistically significant. Results: Our findings revealed that insulin resistance prolonged escape latency of untreated diabetic rats; contrariwise, curcumin significantly reduced escape latency, increased difference score in novel object recognition paradigm, and increased explorative activities. Conclusion: Oral curcumin improves exploratory activity, discriminating memory, and spatial memory in male Wistar rats with AD-like neurobehavioral impairments. Patients with neurobehavioral abnormalities and comorbid insulin resistance may benefit from the flavonoids in curcumin.

Keywords: Alzheimer's disease, curcumin, insulin resistance, metabolic disorder, neurobehavioral deficits

How to cite this article:
Mohammed AA, Akinola OB. The effects of flavonoids in curcumin on neurobehavioral deficits in insulin-resistant rats. J Neurobehav Sci 2022;9:51-7

How to cite this URL:
Mohammed AA, Akinola OB. The effects of flavonoids in curcumin on neurobehavioral deficits in insulin-resistant rats. J Neurobehav Sci [serial online] 2022 [cited 2023 Feb 6];9:51-7. Available from: http://www.jnbsjournal.com/text.asp?2022/9/2/51/355252

  Introduction Top

Diabetes mellitus is a serious problem of insulin control that is endemic. It is a huge public health concern.[1] According to estimates, there are around 171 million diabetics globally (3% of the population). The World Health Organization projected that 1.5 million fatalities were caused by type 2 diabetes mellitus (T2DM) in 2021 and that there will be 300 million diabetics globally by 2025.[2] Estimation of T2DM in Nigeria was reported[3] to be 5.77%, which translates to about 11.2 million Nigerians; they also found the highest regional frequency in the South–South region and the lowest regional prevalence in Nigeria's North-Western region. This epidemic is exacerbated by the fact that T2D is becoming more common in young people, defying the popular belief that it only affects persons over the age of 30 years.[4]

Poorly managed diabetes leads to complications and adverse damage in various organs in the body, notably in the heart, kidney, liver, eye, and blood vessels, where diabetic complications have been well documented and widely studied. In addition, are the less reported nervous complications with varying degrees of neurobehavioral deficits ranging from mild cognitive impairment to more adverse neurodegenerative diseases such as dementia.

Dementia is a brain disorder defined by a loss of intellectual ability severe enough to impede with either occupational functioning or normal social activities.[5] It is one of the several neurological consequences of poorly treated diabetes.[5] A report from several studies suggests that patients with diabetes have a 50%–75% increased risk of developing Alzheimer's disease (AD), and 80% of AD patients have either T2DM or impaired fasting blood glucose.[6] The Alzheimer's Association 2021 reported that one out of three seniors in America dies with AD or another dementia; AD kills more people than breast and prostate cancers combined; AD and dementias death in the US increased by 16% during the COVID-19 pandemic. The financial burden of AD and other dementias is about $355 billion and this figure is projected to rise to $1.1 trillion by 2050 if the solution is not proffered to this public health concern.[7]

Mitochondria dysfunction, oxidative stress, inflammation, dysregulated immunology, poor neuronal–glial communication, and an increase in neurotoxic chemicals are all factors that cause neuronal death in AD.[8] Amyloid-beta (Aβ) pathology characterized by the aggregation of insoluble (Aβ plaques) and tau pathology characterized by the formation of neurofibrillary tangles (NFTs) due to hyperphosphorylation of tau protein are the pathological hallmarks of AD.[8] According to mounting evidence, abnormal forms of tau protein damage neuronal structure and, as a result, neuronal function, resulting in neuronal cell death. According to growing evidence, aberrant forms of tau protein harm neuronal structure and function, leading to neuronal cell death. Tau proteins typically associate with tubulin to stabilize microtubules and aid vesicular transport. NFTs are hyperphosphorylated and aggregated forms of tau proteins. Tau becomes insoluble and loses its affinity for microtubules when it is hyperphosphorylated, resulting in neurodegeneration.[8]

Insulin resistance enhances age-related memory deficits and is a risk factor for AD, according to numerous studies. However, the biochemical and cellular connection between insulin resistance and AD is uncertain. In the same way that poor insulin function has been increasingly documented in T2D, decreased brain insulin levels/action may be a relationship between the two disorders.[1]

The role of metabolic abnormalities in the etiology of AD has been investigated. Glucose balancing is the most important of them, as it is involved in energy maintenance, energy utilization, neurogenesis, neuronal survival, and synaptic plasticity, all of which are essential for learning and memory. In the insulin-resistant condition, cellular sensitivity to insulin is diminished, resulting in hyperinsulinemia, which impairs insulin signaling and contributes to the etiology of AD. Neuroinflammation, oxidative stress, accumulation and aggregation of Aβ, and cell death are all signs of the steady but progressive neurodegenerative changes characterized in AD.[9] Treatments that control insulin resistance have been shown to reduce Aβ buildup in the brain of both human and animal models. As a result, therapeutic techniques that efficiently regulate insulin resistance will be of considerable assistance in AD, perhaps opening up a window of opportunity for the long-awaited permanent solution to the problem.[9]

The flavonoid; curcumin, which is found in the commonly consumed turmeric, has been shown to many positive effects.[10] It is thought to act as a neuroprotective agent by regulating neuronal insulin signaling and glucose metabolism.[10] Curcumin poses as a good candidate in our quest to identify a good, safe, readily available and affordable therapy to neurobehavioural defects in the insulin resistant state of T2DM, hence our study sought to evaluate the effects of curcumin on exploratory, discriminatory, and spatial cognitive indices in an insulin resistant rat model.

  Materials and Methods Top

The postgraduate ethical review committee of the university granted ethical approval with the number (UERC/ASN/2016/654) for this study. A total of 36 adult male Wistar rats (Rattus norvegicus) weighing 170 g ± 30 and of about 15 weeks old were employed in this investigation. The rats were kept in cages with a 12-h light/dark cycle and normal room temperature/humidity, as well as free access to regular rat pellets, high-fat diet (HFD), and water. The rats were put into six groups, each having six rats.

Experimental design

After acclimatization, the animal models received the following treatments; control group rats received 1 ml olive oil; the curcumin group received 200 mg/kg BW curcumin; the diabetic/insulin-resistant model administered HFD for 60 days, then three doses of 40 mg/kg BW of streptozotocin (STZ); the protective group were administered HFD for 60 days, then exposed to three doses of 40 mg/kg BW of STZ as well as a concurrent treatment with 200 mg/kg BW curcumin all within 60 days; preventive group rats received pretreatment of 200 mg/kg BW of curcumin, followed by HFD for 60 days and three doses of 40 mg/kg BW of STZ; the therapeutic group rats were administered HFD and exposed to three doses of 40 mg/kg BW of STZ within 60 days followed by treatment with 200 mg/kg BW of curcumin for 21 days; All STZ treatments were done via the intraperitoneal route, whereas curcumin and HFD were administered orally.

After all treatments, the rats were fasted overnight, blood was taken from their tail veins, and the fasting blood glucose level was measured with a digital glucometer (Accu-Chek, Roche, Belgium). Diabetic rats were included in the study if their fasting blood glucose levels were <200 mg/mol.

Rats were subjected to memory tests 24 h after treatment completion in each group, exploratory behaviors were assessed using line crossing frequency and center line crossing frequency, and novel environment behavior was assessed using rearing frequency, all of which are components of the open-field test conducted according to the methods of Gould et al.[11] The novel object recognition was evaluated according to the methods described by Lueptow,[12] and the Morris water maze test as described by Bromley-Brits et al.[13] was used to assess discriminating and spatial memory. Plasma glucose was measured with blood glucose meters using the glucose oxidase method and insulin concentrations in the blood were estimated using enzyme-linked immunosorbent kit (eBioscience, Inc., USA) according to the manufacturer's instructions.

Statistical analysis

The data were evaluated using a one-way analysis of variance; afterward, Tukey's post hoc test was conducted and the values were presented as mean ± SEM. P < 0.05 was considered statistically significant. Tables and bar charts with error bars were used to display the mean and standard error of the mean, respectively.

  Results Top

The results from our study revealed [Table 1] a significant increase in the weight and weight difference of rats in the diabetic and protective group relative to the olive oil group; curcumin reduced the rate of body weight change in rats that received a pretreatment of it and after the model was created (P ≤ 0.05).

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Blood glucose levels

[Figure 1] revealed a significant increase in blood glucose in diabetic rats, the protective group, and the preventive group (225.3 ± 45.59, 200.8 ± 31.76, and 223.3 ± 51.41), whereas a significant decrease compared to the diabetic rats was recorded in the therapeutic group with (154.0 ± 51.86) (P ≤ 0.05).
Figure 1: Effect of curcumin on the blood glucose levels in HFD + STZ-induced insulin resistance. Fasting blood glucose levels showed a remarkable reduction in the therapeutic group when compared to the untreated diabetic rats. “$” compared to the STZ + HFD group, “#” compared to the Cur group, and “*” compared to the olive oil group. STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin data were mean ± SEM (P < 0.05) n = 6

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Homeostatic model assessment–insulin resistance values across the groups

[Figure 2] showed that olive oil and curcumin only group recorded (5.33±0.48 and 5.85±0.53 respectively), a significant increase was recorded in diabetic rats with (14.89±1.01), while a significant reduction relative to the diabetic group observed in the therapeutic group (7.20±1.48) (P ≤ 0.05).
Figure 2: Effect of curcumin on homeostatic model assessment of insulin resistance (HOMA-IR) in HFD + STZ-induced Wistar rat model HOMA-IR index was highest in the untreated diabetic rats relative to the controls and the curcumin treated with significantly lower index values. “$” compared to the STZ + HFD group, STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin, data were mean ± SEM (P < 0.05) n = 6

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Line and center line crossing frequency

[Figure 3] reveals that Control rats present with line crossing frequency 45.00±5.29. There was a significantly lower exploration in the diabetic rats; however, therapeutic group rats recorded increased explorative activities with line crossing 33.00±9.64. Similarly a significant increase in center line crossing was observed [Figure 4] in the Protective and Therapeutic group rats, while a significant decrease in center line crossing was observed in the untreated diabetic rats compared to the controls and therapeutic group rats (P < 0.05).
Figure 3: Line crossing frequency. The frequency of line crossing was reduced in the diabetic rats relative to control. Concurrent, pre-, and posttreatment with curcumin increased the line crossing frequency in curcumin-treated rats compared to the untreated diabetic rats. “$” compared to the STZ + HFD group. STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin, data were mean ± SEM (P < 0.05) n = 6

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Figure 4: Center-line crossing. Center-line crossing frequency significantly increased in rats that took curcumin intervention relative to the untreated diabetic rats which recorded decreased explorative activities when compared to the control. “$” compared to the STZ + HFD group. STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin, data were mean ± SEM (P < 0.05) n = 6

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Rearing frequency

[Figure 5] indicated that STZ+HFD reduced the rearing frequency in the rat model, olive oil treated control rats recorded a rearing frequency 14.00±1.00, curcumin treated control presents with 12.67±1.53, while a significant increase relative to diabetic group was recorded in therapeutic group rats 12.67±1.15 (P < 0.05).
Figure 5: Rearing frequency. The frequency of rearing was observed to have decreased in the untreated diabetic/insulin-resistant rats relative to the control and therapeutic group rats. “$” compared to the STZ + HFD group, “!” compared to the Cur + STZ + HFD group and “@” compared to concurrent STZ + HFD + Cur group, STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin, data were mean ± SEM (P < 0.05) n = 6

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Escape latency

In [Figure 6], it was observed that the diabetic rats showed significantly increased escape latency (41.30 ± 9.04) compared to control rats (22.46 ± 9.80). Conversely, curcumin treatment significantly reduced the escape latency (P < 0.05).
Figure 6: Escape latency. Effects of curcumin on spatial memory were evaluated, STZ + HFD treated rats showed weaker spatial memory performance relative to the control and the curcumin-treated rats. “$” compared to the STZ + HFD group. STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin, data were mean ± SEM (P < 0.05) n = 6

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Difference score in novel object recognition test across the groups

[Figure 7] indicated that control rats recorded 11.33±3.57; a significant decrease in difference score relative to the controls was recorded in the untreated diabetic rats -4.32±1.528. Protective and Preventive group rats recorded 5.00±1.03 and 6.47±4.22 respectively, while a significant increase was recorded in the therapeutic group 11.32±4.51 (P < 0.05).
Figure 7: Difference score. The untreated diabetic rats recorded a negative figure difference score. “$” compared to the STZ + HFD group. STZ = Streptozotocin, HFD = High-fat diet, Cur = Curcumin, data were mean ± SEM (P < 0.05) n = 6

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  Discussion Top

Insulin resistance in diabetes mellitus is associated with a marked increase in blood glucose and a compensatory increase in insulin secretion but is devoid of insulin sensitivity and action. This increase in secretion leads to a cascade of events that prompt the promotion of metabolic imbalance and obesity. Obesity is consistent with the observed increase in body weight of our untreated diabetic/insulin resistant model; this finding is consistent with earlier reports from the work of Akbari and colleagues.[14]

Observable behavioral changes were investigated in this study employing various memory paradigms. STZ and HFD reduced exploratory activities of rats in an open field test where line crossing frequency was measured without curcumin intervention. Compared to the control and curcumin-treated diabetic rats, there was a decrease in the frequency of line crossing in the untreated diabetic rats [Figure 3]. The data from this study on center line crossing [Figure 4] indicate a significant decrease in center line crossing in STZ and HFD exposed rats compared to the control and diabetic rats treated with curcumin.

While contrasting the rearing frequency of rats across the treatment groups, [Figure 5] showed that rats treated with olive oil only, curcumin only, and STZ, HFD start, followed by curcumin had a high frequency of rearing. This novel environment exploratory behavior in rodents depicts comfort and willingness to explore a new environment, while the untreated diabetic rats had reduced rearing frequency which suggests behavioral deficits caused by the deleterious effects of STZ and HFD.

Our evaluation revealed that spatial memory in untreated diabetic rats was impaired [Figure 6], contrary to the observed in the control and curcumin treated model where spatial memory was intact and no impairment; their shorter escape latencies compared to STZ and HFD exposed rats gives credence to this. These findings confirm the findings of Johansson and co-researchers,[8] who reported depression, and disturbances in drive and emotions in a translational study of mild behavioral impairment with a marked increase in tau deposition.[8]

Our study reported a link between recognition memory and insulin resistance, as indicated in [Figure 7] which shows rats from different groups had similar exploratory inclinations toward the same objects during the training phase. However, relative to the STZ and HFD exposed rats, the control and curcumin-treated rats identified the novel place and item from the familiar one in the testing phase.

Our findings suggested that curcumin administration improved spatial memory and cognitive function in diabetic rats with HFD and STZ-induced neuronal loss compared to untreated diabetic rats with HFD and STZ-induced neuronal loss, as earlier reported.[1]

In similarity to documented literature, our recent findings confirm that amyloid pathology and tau hyperphosphorylation in the AD model caused neuronal death and subsequent loss of synaptic plasticity, which could explain the reduction in recognition memory and poor cognition seen in untreated diabetic rats. The findings of our study show that curcumin is helpful in improving memory and correcting cognitive impairment and neurodegeneration in the insulin-resistant/hyperglycemic state as earlier reported by Ghorbani et al.[15]

Our findings showed that curcumin was able to enhance exploration and ameliorated the deficits initiated by STZ and HFD. These results build on evidence earlier reported by Yow and colleagues[15] which indicated that curcumin exerted ameliorating effects on anxiety and restored non-spatial recognition memory impairment induced by kainic acid. Hence, curcumin improved the reported deficits in a rat model of chemo-convulsant-induced epilepsy.[16]

Isik et al.[17] similarly conveyed the viewpoint that curcumin could halt oxidative injury and cognitive decline connected to aging by modulating the activities of various molecular targets, such as transcription factors, different enzymes, cell cycle proteins, receptors, and various adhesion particles.[15] This demonstrates that Alzheimer-like impairments and insulin resistance resulted in a significant decrease in exploratory and motor activities in rats, while curcumin treatment was able to mitigate the negative effects and restore normal behavioral characteristics. These findings are similar to those reported by Yow et al.,[16] where curcumin was found to improve locomotor and exploratory activities in a rat model of epilepsy-induced with kainic acid.[16]

Curcumin was able to assist in keeping spatial and nonspatial memory intact in the animals that had curcumin intervention. This finding is similar to that of Yow et al. and Isik et al.,[16],[17] where curcumin was able to exert the same memory retention effect in models of depression and epilepsy, respectively.

STZ-treated animals in the test phase traveled a substantially greater distance to the platform than controls, according to previous reports. The untreated diabetic model traveled a more confusing path to the escape platform[17] and spent much fewer seconds in the target quadrant with the escape platform than the model from the other groups. However, curcumin-treated models searched the escape platform for a significantly longer time in the target quadrant with the escape platform than STZ-treated animals.[17] Summarily, Choudhary and others reported that Intracerebroventricular administration of STZ caused a decline in spatial memory retention in their studied rat model, and similar to our findings they concluded that curcumin was able to alleviate the deficiencies.[18]

  Conclusion Top

Curcumin played an ameliorative role against neurobehavioral deficits caused by streptozotocin and a HFD in the insulin-resistant animal models. Curcumin was able to promote memory retention and reduced escape latency, increased discriminative memory, and explorative deficits observed in the untreated diabetic/insulin-resistant rats.

Patient informed consent

There is no need for patient informed consent.

Ethics committee approval

The authors confirm that this research work was done in compliance with approval from the University ethical review committee. The postgraduate ethical review committee of the University of Ilorin granted ethical approval for this study with the number (UERC/ASN/2016/654).

Financial support and sponsorship

No funding was received.

Conflicts of interest

There are no conflicts of interest.

Author contribution subject and rate

  • Abdullahi Abiodun Mohammed (50%): Design the research, data collection, analyses, and wrote the whole manuscript.
  • Oluwole Busayo Akinola (50%): Supervised the research, Design the research, contributed with comments on research design and manuscript.

  References Top

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World Health Organization, Diabetes Facts, Fact Sheet. Available from: http://www.who.int/news room/factsheets/details/diabetes. [Last accessed on 2021 May 02].  Back to cited text no. 2
Uloko AE, Musa BM, Ramalan MA, Gezawa ID, Puepet FH, Uloko AT, et al. Prevalence and risk factors for diabetes mellitus in Nigeria: A systematic review and meta-analysis. Diabetes Ther 2018;9:1307-16. doi: 10.1007/s13300-018-0441-1.  Back to cited text no. 3
Carvalho C, Cardoso S, Correia SC, Santos RX, Santos MS, Baldeiras I, et al. Metabolic alterations induced by sucrose intake and Alzheimer's disease promote similar brain mitochondrial abnormalities. Diabetes 2012;61:1234-42. doi: 10.2337/db11-1186.  Back to cited text no. 4
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Alzheimer's Association. 2021 Alzheimer's Disease Facts and Figures. Available form: http://www.alz.org/facts-figures. [Last accessed on 2021 Jul 15].  Back to cited text no. 7
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van Praag H, Fleshner M, Schwartz MW, Mattson MP. Exercise, energy intake, glucose homeostasis, and the brain. J Neurosci 2014;34:15139-49. doi: 10.1523/JNEUROSCI. 2814-14.2014.  Back to cited text no. 9
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Gould TD, Dao DT, Kovacsics CE. The open field test. In: Gould T, editor. Mood and Anxiety Related Phenotypes in Mice (Neuromethods). Vol. 42. Totowa, NJ: Humana Press; 2009. doi: 10.1007/978-1-60761-303-9_1.  Back to cited text no. 11
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Ghorbani Z, Hekmatdoost A, Mirmiran P. Anti-hyperglycemic and insulin sensitizer effects of turmeric and its principle constituent curcumin. Int J Endocrinol Metab 2014;12:e18081. doi: 10.5812/ijem. 18081.  Back to cited text no. 15
Yow Y, Ahmad H, Azmi N, Bakry M. The effect of curcumin on anxiety and recognition memory in Kainate model of epileptic rats. Indian J Pharma Sci 2017;79:267-76. doi: 10.4172/pharmaceutical-sciences. 1000225.  Back to cited text no. 16
Isik AT, Celik T, Ulusoy G, Ongoru O, Elibol B, Doruk H, et al. Curcumin ameliorates impaired insulin/IGF signalling and memory deficit in a streptozotocin-treated rat model. Age (Dordr) 2009;31:39-49. doi: 10.1007/s11357-008-9078-8.  Back to cited text no. 17
Choudhary KM, Mishra A, Poroikov VV, Goel RK. Ameliorative effect of curcumin on seizure severity, depression like behavior, learning and memory deficit in post-pentylenetetrazole-kindled mice. Eur J Pharmacol 2013;704:33-40. doi: 10.1016/j.ejphar. 2013.02.012.  Back to cited text no. 18


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

  [Table 1]


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