Effect of Prunus domestica L. (mirabelle) on learning and memory in mice.

BACKGROUND: Plums have been known to have various pharmacological activities. AIMS: The aim of the present study was to evaluate the effect of administration of hydro-alcoholic extract of plum, using passive avoidance task. SETTINGS AND
DESIGN: Mice were divided into four groups (n = 7 each) one control and three plum (75, 100, 150 mg/kg) groups.
MATERIALS AND METHODS: The control group received saline and plum groups received the extract by oral gavage for 7 days. The number of trials to acquisition, step through latency (STLr) in the retention test and the time spent in the dark compartment (TDC) during the retention test were measured. STATISTICAL ANALYSIS: Differences between groups were tested by one-way ANOVA with Tukey post-hoc test.
RESULTS: A significant difference was found in the number of trials to acquisition between the groups. The results also indicated in the retention test, administration of 75 and 100 mg/kg plum caused an increased STLr (compared with the untreated control group). The results also showed that the total time spent in TDC by the animals of the extract groups was lower than that of the control group.
CONCLUSIONS: Hydro-alcoholic extract of plum has a beneficial effect on learning and memory in passive avoidance task. It can be concluded that its antioxidant and antidyslipidemic activities may be involved in the obtained effects.

INTRODUCTION

Several studies have confirmed the positive effects of flavonoids found in fruits including anthocyanin and flavanols on learning and memory.[123] Antioxidants are known for their neurological action[4] and also flavonoids can exhibit neuroprotective action,[5] fruits that are rich in these compounds have been noted. Because of the numerous side-effects of the chemical drugs effective on the nervous system, there is a tendency to use the natural agents to treat the cognitive and learning deficits. Extracts of Ginkgo biloba,[6] Panax ginseng[7] and Dinh lang (Policias fruticosum L.) root extract,[8] have shown beneficial effects on cognition and memory. Prunus domestica L. subsp. Syriaca Janch. is one of the species of plum. It belongs to Rosacea family and prunus genus. Plums have been known to have various pharmacological activities and have also been used in folk Iranian medicine as sedatives, for neurasthenia and also for prevention of anemia,[9] aging (to prevent Alzheimer's disease),[10] hypercholesterolemia and cardiovascular diseases.[11] It has also been demonstrated that plums have anti-inflammatory[12] and anxiolytic effects.[13] Several studies have acknowledged the positive effects of plums in the improvement of blood lipid profile, osteoporosis, inhibition of low-density lipoprotein (LDL) oxidation and alleviation of constipation.[14] However, until now, there have not been any reports of its hydro-alcoholic extract effects on cognition. Studies have confirmed high levels of antioxidants in plums in comparison with the other fruits.[151617] With regard to the antioxidant and antidyslipidemic effects of plums, the aim of the present study was to evaluate the effect of short-term administration of (75, 100 and 150 mg/kg) hydro-alcoholic extract of plum, in mice using the passive avoidance test.

MATERIALS AND METHODS

A total of 28 male mice (20-30 g) were purchased from the breeding colony of Hamadan University of Medical Science and kept at 22 +2°C temperature and 12 h light/dark cycle. Animals were randomly divided into four groups (n = 7 each) one control and three plum (75, 100, 150 mg/kg) groups. Animals were allowed free access to drinking water and standard laboratory chow. The control group received saline and plum groups, received the extract, by oral gavage once a day for 7 days. Experiments were carried out in accordance with the criteria outlined in the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publication 86-23; revised 1985; http://www.oacu.od.nih.gov/regs/guide/guidex.htm) and were also approved by the Local Committee of Hamadan University of Medical Sciences.

Preparing the plant extract

In this study, the plum (1 kg) was authenticated by Dr. M. Ranjbar (Department of Biology, School of Basic Sciences, Bu-Ali Sina University, Hamedan, Iran). Then, peels of the plums were removed and they were pitted, the average yield was 750 g. After that, it was combined with 80% (v/v) ethanol and was kept in the refrigerator for a week. The combined extract was filtered through filter paper. Then, we used the rotary to do solvent extraction. The resulting filtrate was dried. The dried yield was weighting 40 g. Finally, it was dissolved in a proper amount of saline in order to prepare the doses of extract as needed.

Passive avoidance learning test

The passive avoidance test was used to evaluate learning and memory. The procedure was the same as in our previous studies.[1819] In general, the apparatus consists of lightened and darkened compartments (20 cm × 20 cm × 30 cm) connected to each other by guillotine door. Both floors are made of stainless steel rods (3 mm diameter) spaced 1 cm apart. The floor of dark compartment could be electrified by a shock generator. For habituation, mice were placed in the light compartment, after 5 s the door was opened. Mice enter the dark compartment (TDC) in accordance with their native preference to the dark environments. As soon as the mice entered TDC, the door was closed and after 30 s mice were transferred to their home cages. The habituation was repeated 2 min later. For acquisition, mice were placed in the light compartment 30 min later and latencies (step-through latency, STLa) for entering TDC were recorded. Then, after entering TDC, the door was closed and a mild shock (0.5 mA) was applied for 3 s. After 30 s mice were transferred to their home cage and this trial was repeated 2 min later. When the mice didn’t enter TDC for 120 s, this trial was terminated. We were recorded the number of trials to avoidance from shock. 24 h later, the retention test was carried out in which mice were placed in the light compartment and the time latency to TDC (STLr) and the time spent in TDC for 300 s were recorded. Retention test was terminated when the mice didn’t enter TDC within 300 s.

Statistical analysis

Differences between groups were tested by one-way ANOVA with Tukey post-hoc test. All results were shown as mean ± SEM. The level of P < 0.05 was considered significant.

RESULTS AND DISCUSSION

The results of this study indicated that there was no significant difference in the STLa (the time latency for entering TDC) in the first acquisition between any of the groups [P > 0.05, Figure 1a]. A significant difference was found in the number of trials to acquisition between the control group and extract-treated groups [P < 0.05, Figure 1b]. This effect was more significant in 75 mg/kg extract-treated group (1.14 ± 0.143) [P < 0.01, Figure 1b]. In the retention test, the results also indicated the 75 mg/kg and 100 mg/kg treated groups had increased STLr (STLr in the retention) (192.43 ± 50.94 and 197.14 ± 49.98 respectively) compared with the control group [P < 0.05, Figure 1c]. Administration of the animals by 150 mg/kg of extract did not significantly affect the (STLr) [P > 0.05, Figure 1c]. It also emerges from results that the total time spent in TDC by animals of extract-treated groups was lower than that of the control group [P < 0.05, Figure 1d]. In this case, 75 mg/kg dose of plum extract was more effective (51.57 ± 24.50) [P < 0.01, Figure 1d].

Figure 1

Effect of short-term administration of hydro-alcoholic extract of plum on the passive avoidance test, step-through latency (STLa) in the first acquisition trial. (a) The number of trials to acquisition. (b) STLr in the retention test. (c) The time spent in the dark compartment. (d) *Difference between control group (P < 0.05). **Difference between control group (P < 0.01)

Effect of plum administration on body weight

Table 1 demonstrates the body weight of the animals at the beginning and at the end of the experiment. There was no significant difference in body weight of the animals at the beginning and the end of the experiment.

Table 1

Body weight of different animal groups at the beginning and end of the study

The results of the present study show that treatment with hydro-alcoholic extract of plum has beneficial effects on learning and memory in passive avoidance test. The number of trials was decreased by administration of extract and the dose 75 mg/kg was more effective in decreasing the number of trials. In this study, we found that treatment with hydro-alcoholic extract of plum caused increased STLr and decreased TDC in treated groups. In fact, reducing the number of trials to avoid shock shows the improvement of memory. Increased STLr and decreased TDC also show a positive effect on memory.[2021] It has been shown that fruits and vegetables high in flavonoids are beneficial to neurological functioning.[3] The underlying mechanism seems to be their ability to affect memory processing through interacting with innate structures of the brain involved in memory.[3] Extracts of spinach, strawberry or blueberry were effective in reversing many neuronal function factors.[122] It is said that phytochemical-rich fruits exert their effect on cognitive improvement through their effect on synaptic plasticity.[3] It is also estimated that blueberry flavonoids can act to promote the efficiency of spatial memory through acting on dentate gyrus.[23] Correlation between cerebral blood flow (CBF) and cognitive function in humans has been proved and it is seen that patients with dementia have decreased CBF.[2425] Supplementation with flavonoids has been shown to increase CBF in humans.[26] It can be concluded that flavonoids can affect different neuronal parameters leading to alteration in cognitive performance.[27] Plums are a rich source of antioxidants such as phenolic acids, anthocyanins and other flavonoids.[16] Results of oxygen radical absorbing capacity (ORAC) assay revealed that plums and prunes compared with 100 common foods have high total antioxidant capacity. The total ORAC of plums reported as 59.2 μmol (Trolox equivalents) (TE)/g of edible portion. The results also indicate that the total ORAC of prunes was 3 times higher than the other dried fruits such as dates and figs.[17] Plums and prunes are shown to have large amounts of polyphenolic phytochemicals, with the total phenolic compounds between 282 and 922 mg/100 g of fruit.[28] Neochlorogenic acid (3-O-caffeoylquinic), chlorogenic acid (5-O-caffeoylquinic) and cryptochlorogenic acid (4-O-caffeoylquinic), caffeic acid, anthocyanins, flavanols and flavonoid are predominant phenolic compound in plums.[16] Chlorogenic acid is known as an antioxidant (with a high antioxidant capacity) and scavenger for oxygen reactive species.[29] All chlorogenic acid isomers show the antioxidant activity.[16] Quercetin 3-glucoside and 3-rutinoside are predominant flavonoids in plums.[30] It has shown that consumption of plum juice led to memory improvement in the Morris water maze. This seems consistent with our results.[31] The leading cause of cognition improvement due to acute dietary flavonoid-rich fruits studies is seemingly dependent on increased CBF.[32] Physiological doses of flavonoids in such studies has indicated their ability to stimulate neuronal signaling pathways, which are involved in cognition processing, hence, acute alterations in cognition, possibly, to a certain extent is contributed to their action on neurons directly.[32] With regard to such potent antioxidant properties and efficient compounds which are found in plum, it could be stated that efficacy of plum in this study on learning and memory to some extent is attributed to its phytochemicals.

Serum lipids are associated with cognition, but data are contradictive and inconsistent.[33] On one hand, some studies suggest that blood lipid profile has a role in brain function and total cholesterol (TC) increases the liquidity of cell membrane in brain neurons[33] and memory and high levels of TC are associated with better cognitive performance.[34] Some studies suggest that due to lipids being profluent in the brain, there is a significant correlation between serum cholesterol and memory and TC is important for brain activity.[33] On the other hand, elevated lipids are a risk factor for dementia[35] due to the fact that they promote atherosclerosis that is believed to be a risk factor for cognitive deficits.[36] These theories are supported by some evidences like statin being associated with reduction in risk of dementia and lipid-lowering agents reducing the risk of dementia and cognitive deficits.[37] Study with Murraya koenigii leaves ethanolic extract and Aegle marmelos indicate that their cholesterol-lowering effect contributed to memory improvement.[3839] Cholesterol-lowering effects of plums have been reported.[4041] Regular consumption of prunes has been reported to lower serum cholesterol levels since it contains a good amount of pectin.[42] It has been reported that consumption of 100 g prune/d for 30 days led to a reduction in both TC and LDL-cholesterol.[41] With regard to antidyslipidemic effects of plum, possibly might have been induced in the treatment period, it seems that they are attributed to learning and memory improvement observed in this study.

CONCLUSION

In this study, oral administration of hydro-alcoholic extract of plum had a beneficial effect on learning and memory in mice. This effect was more significant in 75 mg/kg dose. It can be concluded that its antioxidant and antidyslipidemic activities may be involved in the obtained effects.