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Ascorbic acid supplementation could affect passive avoidance learning and memory in rat
Brain Research Bulletin 76 (2008) 109–113
Research report
Ascorbic acid supplementation could affect passive avoidance learning and memory in rat Siamak Shahidi a,∗ , Alireza Komaki a , Minoo Mahmoodi b , Nazanin Atrvash a , Marzieh Ghodrati a a
Department of Physiology, Hamadan University of Medical Sciences, Hamedan, Iran Department of Biology, Islamic Azad University, Hamedan Branch, Hamedan, Iran
b
Received 6 October 2007; received in revised form 19 December 2007; accepted 10 January 2008 Available online 4 February 2008
Abstract Ascorbic acid (vitamin C) is required for health and, in particular, its supplementation has beneficial effects in some pathological conditions. There are conflicting reports regarding the usefulness of ascorbic acid in the treatment of dementia. In this study, we investigated the effects of acute, short- and long-term pre-training administration of ascorbic acid (60,120 mg/kg) on passive avoidance learning (PAL) and memory in rats. Retention test was done 24 h after training. The results showed that acute injection of ascorbic acid had no significant effect on PAL. On the other hand, both in the short- and long-term ascorbic acid treated groups trials to acquisition were less than control group. Also, ascorbic acid prolonged the stepthrough latency (STL) and decreased the time spent in the dark compartment in retention test. Thus, it can be concluded that short- and long-term supplementation with ascorbic acid has facilitatory effects on acquisition and retrieval processes of passive avoidance learning and memory in rats. © 2008 Elsevier Inc. All rights reserved. Keywords: Ascorbic acid; Passive avoidance learning; Rat
1. Introduction Ascorbic acid (vitamin C) is an essential micronutrient required for normal metabolic functioning of the body [14,17]. It is a six-carbon lactone [26] which acts as an electron donor and therefore a reducing agent [4]. Also, it is a cofactor for several enzymes involved in the biosynthesis of collagen, carnitine, and neurotransmitters [5,37]. Many biochemical, clinical and epidemiological studies have indicated that ascorbic acid may be benefit in chronic diseases such as cardiovascular disease, cancer and cataract [12,14,38]. There is an observational study in elderly individuals that has indicated that ascorbic acid may have beneficial effect on the development of Alzheimer dementia [19]. Plasma vitamin C levels were lower in subjects with dementia compared to controls [7], and it was suggested that vitamin C might protect against cognitive impairment [27].
Also it has been reported that supplementation with vitamin C may be a novel therapeutic strategy to the cognitive dysfunction associated with hyperprolinemia type II [11]. Monteiro et al. suggested that vitamins E and C can prevent cognitive impairments in post-menopausal women [23]. On the other hand, it has reported that use of vitamin C did not delay the incidence of dementia or Alzheimer dementia. In addition, it has suggested that there is no significant correlation between ascorbic acid concentrations (intake) and cognitive function [13,34]. Thus, there are few and conflicting reports regarding the usefulness of ascorbic acid in the treatment of dementia and cognitive dysfunction. In addition, there is no report concerning the role of ascorbic acid on learning and memory in healthy subjects. The aim of present study was to determine the effects of acute, short- and long-term supplementation with ascorbic acid on passive avoidance learning (PAL) task, which is considered to be a simple test of learning and memory, in normal young rats. 2. Materials and methods
∗ Corresponding author at: Department of Physiology, School of Medicine, Hamadan University of Medical Sciences, Hamedan, Iran. Tel.: +98 811 8276296; fax: +98 811 8276299. E-mail address: [email protected] (S. Shahidi).
0361-9230/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2008.01.003
2.1. Animals Adult male Wistar rats weighing 200–250 g were obtained from the breeding colony of the Iran Pasteur Institute, Tehran. They were housed three per plexiglas
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S. Shahidi et al. / Brain Research Bulletin 76 (2008) 109–113
cage. Experiments were conducted in the afternoon (13:00–16:00 h). Food and water were available ad libitum. Lights were on from 7:00 to 19:00 h. All procedures for the treatment of animals were approved by the research committee of the Hamadan University of Medical Sciences and were done in accordance with the Guide for Care and Use of Laboratory Animals published by the United States National Institutes of Health (NIH Publication No. 85-23, revised 1985).
the rat was taken from the dark compartment into their home cage. Then after 2 min, the procedure was repeated. The rat received a foot-shock each time it reentered the dark and had placed all 4 paws in the dark compartment. Training was terminated when the rat remained in the light compartment for 120 consecutive seconds. The number of trials (entries into the dark chamber) was recorded.
2.4. Retention test 2.2. Passive avoidance apparatus The apparatus and procedure was basically the same as our previous studies [20,35]. Briefly, the step-through passive avoidance apparatus consisted of a lighted chamber (20 cm × 20 cm × 30 cm) made of transparent plastic and a dark chamber whose walls were made of dark opaque plastic (20 cm × 20 cm × 30 cm). The floor of both chambers was made of stainless steel rods (3 mm diameter) spaced 1 cm apart. The floor of the dark chamber could be electrified using a shock generator (Behbood Pardaz Co. Iran). A rectangular opening (6 cm × 8 cm) was located between the two chambers and could be closed by an opaque guillotine door.
2.3. Training First, all experimental groups were given two trials to habituate them to the apparatus. For these trials, the rats were placed in a lighted compartment of the apparatus facing away from the door and 5 s later the guillotine door was raised. The rat has native preference to the dark environment. Upon the rat entering the dark compartment, the door was closed and after 30 s the rats were taken from the dark compartment into their home cage. The habituation trial was repeated after 30 min and followed after the same interval by the first acquisition trial. The entrance latency to the dark compartment (step-through latency, STL) was recorded when the animal had placed all 4 paws in the dark compartment. After the animal had spontaneously entered the dark compartment, the guillotine door was lowered and a mild electrical shock (0.5 mA) was applied for 3 s. After 30 s,
The retention test was performed 24 h after the PAL acquisition trial. The rat was placed in the lighted chamber as in PAL training and 5 s later, the guillotine door was raised, and the step-through latency and the time spent in the dark compartment (TDC) were recorded up to 300 s. If the rat did not enter the dark compartment within 300 s, the retention test was terminated and a ceiling score of 300 s was assigned.
2.5. Experimental protocol Three experiments were done. In each experiment, rats were divided into three separate groups: a saline treated, a 60 mg/kg ascorbic acid treated group and a 120 mg/kg ascorbic acid (Merck, U.S.A.) treated group. In the first experiment, the rat received a single acute intraperitoneal (i.p.) injection of 60 mg/kg ascorbic acid (n = 8), 120 mg/kg ascorbic acid (n = 8) or saline (n = 7) 60 min before training and retention. In the second experiment in order to verify the effects of short-term i.p. injection of ascorbic acid, rats have been received saline (n = 12), ascorbic acid (60 mg/kg, n = 12) or 120 mg/kg ascorbic acid (n = 11) for 12 days. PAL training was started on the 13th day and retention test was done on the 14th day. Injection of saline or ascorbic acid was continued 60 min before the training and retention test. Therefore, these animals get 14 days of injections of ascorbic acid. In the third experiment, saline (n = 10) or ascorbic acid (60 mg/kg, n = 11) or 120 mg/kg ascorbic acid (n = 11) orally administrated using gavages needle for 30 days. In the long-term treatment, instead of i.p. injection, oral administrations
Fig. 1. Effects of short-term administration of ascorbic acid on PAL. The effect of short-term i.p. injection of ascorbic acid on the step-through latency (STL) in the first acquisition trial (A), number of trials to acquisition (B), STL in the retention test (C) and the time spent in the dark compartment in the retention test (TDC) (D) between saline (n = 12), 60 mg/kg ascorbic acid (n = 12) and 120 mg/kg ascorbic acid (n = 11) treated groups. * P < 0.05, ** P < 0.01 and *** P < 0.001 compare to saline group. Each column and bar represents mean ± S.E.M.
S. Shahidi et al. / Brain Research Bulletin 76 (2008) 109–113
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of solutions were done to prevent tissue damages due to multiple injections. Then, PAL training and retention was done in the next 2 consecutive days. Saline or ascorbic acid administration was continued 60 min before training and retention test.
2.6. Data analysis In each experiment, the data were analyzed using ANOVA (one-way analysis of variance) followed by the Tukey test for multiple comparisons. All results are shown as the mean ± S.E.M. The level P < 0.05 was considered as statistically significant.
3. Results 3.1. Experiment 1: effects of acute administration of ascorbic acid on PAL There was no significant difference between the number of trials to acquisition (F(2,20) = 0.61, P > 0.05) in the three experimental groups. Also, in the retention test there was no significant difference of STL (F(2,20) = 3.03, P > 0.05) and TDC (F(2,20) = 0.44, P > 0.05) between groups (data not shown). 3.2. Experiment 2: effects of short-term administration of ascorbic acid on PAL There was no significant difference between the weight of rats in the three experimental groups (F(2,32) = 0.82, P > 0.05). Oneway ANOVA indicated that there was no significant difference in the STL of first acquisition trial between groups (F(2,32) = 1.07, P > 0.05) and thus confirmed their uniformity (Fig. 1A). The number of trials to acquisition was significantly different (F(2,32) = 5.93, P > 0.05). The number of trials to acquisition of ascorbic acid treated groups was less than saline group. There was no difference between acquisition trials of 60 and 120 mg/kg ascorbic acid treated groups (Fig. 1B). Statistical analysis indicated that there were significant differences in step-through latency (F(2,32) = 10.29, P < 0.05) and time spent in the dark compartment (F(2,32) = 11.02, P < 0.05) between experimental groups in the retention test (Fig. 1C and D). There was no significant difference between these parameters in the 60 and 120 mg/kg ascorbic acid treated groups. 3.3. Experiment 3: effects of long-term ascorbic acid supplementation on PAL One-way ANOVA indicated that the number of trials to acquisition of ascorbic acid treated groups was significantly less than saline group (F(2,29) = 6.57, P < 0.01) (Fig. 2A). In addition, the results of retention test showed that STL of saline treated rats is significantly more than ascorbic acid treated groups (F(2,29) = 7.12, P < 0.01) and TDC of saline treated rats is less than ascorbic treated groups (F(2,29) = 7.35, P < 0.01) (Fig. 2B and C). 4. Discussion The findings of the present study are: The results of experiment 1 showed that acute administration of ascorbic acid
Fig. 2. Effects of long-term administration of ascorbic acid on PAL. The effect of long-term oral administration of ascorbic acid on the number of trials to acquisition (A), step-through latency (STL) in the retention test (B) and the time spent in the dark compartment in the retention test (TDC) (C) between saline (n = 10), 60 mg/kg ascorbic acid (n = 11) and 120 mg/kg ascorbic acid (n = 11) treated groups. * P < 0.05, ** P < 0.01 and *** P < 0.001 compare to saline group. Each column and bar represents mean ± S.E.M.
had no significant effects on PAL. The results of experiment 2 showed that short-term injection of ascorbic acid improved learning process of passive avoidance. Also retention of memory increased with ascorbic acid. The results of experiment 3 showed that long-term oral administration of ascorbic acid facilitate passive avoidance learning and memory. The effect of ascorbic acid on PAL was not dose dependent. Long-term oral supplementation and short-term i.p. injection of ascorbic acid had the same cognitive effects. Ascorbic acid had no significant effect on the native behavior of rats to the dark compartment. To prevent tissue damage due to repeated injections of ascorbic acid, it was administered orally in experiment 3. Ascorbic acid
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is a water-soluble vitamin which is well absorbed when given orally and its bioavailability following an oral dose is more than 85%. Thus it seems the effect of oral versus i.p. administration of ascorbic acid (in experiments 1 and 2) has not significant effect on ascorbic acid plasma concentration. In addition, in each of three experiments the results of ascorbic acid treated rats were compared with its control group. It seems that if ascorbic acid administrated at least for a short time period, it could improve acquisition and retention of passive avoidance task. Thus, it has facilitatory effects on passive avoidance learning and memory in healthy rats. The results of this study were similar to other studies which believed that ascorbic acid could affect learning and memory [8,11,19,23,28,31]. They have been reported that, ascorbic acid could reduce the risk of dementia caused by aging [2,19,28], or prevent memory impairment due to the scopolamine [28], hyperprolinemia [11], ovarectomy [23], methylmalonic acid [29], and homocyctein [31]. On the other hand in the present study, we tested the effects of ascorbic acid in the simple model of learning and memory without any memory impairment induction. Arzi et al. has reported that chronic oral supplementation of vitamin C (300 mg/kg, 60 days) improves step-down avoidance learning of aged but not young mice [2]. Therefore, it seems that non-acute administration of ascorbic acid has an enhancing effect on learning and memory in addition to its effects on memory deficits prevention. Ascorbic acid is a potent antioxidant [15,22,33] which is highly concentrated in the central nervous system [3,9]. In most studies, researchers believed that ascorbic acid prevents memory deficit by its antioxidant effect [6,8,31]. Thus the results of this study may be due to the antioxidant action of ascorbic acid. On the other hand, ascorbic acid has a modulatory action on brain neurotransmitter like cholinergic, serotonergic and dopaminergic system [6,9,21]. These neurotransmitter systems are important in learning and memory processes [24]. For example, Lee et al. believed that supplementation with vitamin C might be useful in maintaining brain acetylcholinesterase activity at the normal level and serotonin concentration for some extent under the condition to induce dementia by scopolamine administration [21]. Also, it has reported that local applications of ascorbic acid enhanced the response of neurons to dopamine [10,16,30,36] and glutamate [32]. Glutamate is a neurotransmitter which has a critical role in learning and memory processing [1]. Therefore it is possible that part of the ascorbic acid effects is due to its neurotransmitter modulator functions. A report revealed that ascorbic acid can potentiate hippocampal evoked potential activity [18]. The hippocampus is one of the brain structures that is involved in many kinds of learning and memory like PAL [25]. In addition, it seems that ascorbic acid could affect passive avoidance learning and memory by changing the hippocampal neuronal activity. As ascorbic acid affect hippocampus, it may change other hippocampal learning and memory tasks such as spatial learning and memory. Finally, we can conclude that in addition to the many beneficial effects of ascorbic acid, it can be considered to be a memory enhancer in healthy subjects.
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Research report
Ascorbic acid supplementation could affect passive avoidance learning and memory in rat Siamak Shahidi a,∗ , Alireza Komaki a , Minoo Mahmoodi b , Nazanin Atrvash a , Marzieh Ghodrati a a
Department of Physiology, Hamadan University of Medical Sciences, Hamedan, Iran Department of Biology, Islamic Azad University, Hamedan Branch, Hamedan, Iran
b
Received 6 October 2007; received in revised form 19 December 2007; accepted 10 January 2008 Available online 4 February 2008
Abstract Ascorbic acid (vitamin C) is required for health and, in particular, its supplementation has beneficial effects in some pathological conditions. There are conflicting reports regarding the usefulness of ascorbic acid in the treatment of dementia. In this study, we investigated the effects of acute, short- and long-term pre-training administration of ascorbic acid (60,120 mg/kg) on passive avoidance learning (PAL) and memory in rats. Retention test was done 24 h after training. The results showed that acute injection of ascorbic acid had no significant effect on PAL. On the other hand, both in the short- and long-term ascorbic acid treated groups trials to acquisition were less than control group. Also, ascorbic acid prolonged the stepthrough latency (STL) and decreased the time spent in the dark compartment in retention test. Thus, it can be concluded that short- and long-term supplementation with ascorbic acid has facilitatory effects on acquisition and retrieval processes of passive avoidance learning and memory in rats. © 2008 Elsevier Inc. All rights reserved. Keywords: Ascorbic acid; Passive avoidance learning; Rat
1. Introduction Ascorbic acid (vitamin C) is an essential micronutrient required for normal metabolic functioning of the body [14,17]. It is a six-carbon lactone [26] which acts as an electron donor and therefore a reducing agent [4]. Also, it is a cofactor for several enzymes involved in the biosynthesis of collagen, carnitine, and neurotransmitters [5,37]. Many biochemical, clinical and epidemiological studies have indicated that ascorbic acid may be benefit in chronic diseases such as cardiovascular disease, cancer and cataract [12,14,38]. There is an observational study in elderly individuals that has indicated that ascorbic acid may have beneficial effect on the development of Alzheimer dementia [19]. Plasma vitamin C levels were lower in subjects with dementia compared to controls [7], and it was suggested that vitamin C might protect against cognitive impairment [27].
Also it has been reported that supplementation with vitamin C may be a novel therapeutic strategy to the cognitive dysfunction associated with hyperprolinemia type II [11]. Monteiro et al. suggested that vitamins E and C can prevent cognitive impairments in post-menopausal women [23]. On the other hand, it has reported that use of vitamin C did not delay the incidence of dementia or Alzheimer dementia. In addition, it has suggested that there is no significant correlation between ascorbic acid concentrations (intake) and cognitive function [13,34]. Thus, there are few and conflicting reports regarding the usefulness of ascorbic acid in the treatment of dementia and cognitive dysfunction. In addition, there is no report concerning the role of ascorbic acid on learning and memory in healthy subjects. The aim of present study was to determine the effects of acute, short- and long-term supplementation with ascorbic acid on passive avoidance learning (PAL) task, which is considered to be a simple test of learning and memory, in normal young rats. 2. Materials and methods
∗ Corresponding author at: Department of Physiology, School of Medicine, Hamadan University of Medical Sciences, Hamedan, Iran. Tel.: +98 811 8276296; fax: +98 811 8276299. E-mail address: [email protected] (S. Shahidi).
0361-9230/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2008.01.003
2.1. Animals Adult male Wistar rats weighing 200–250 g were obtained from the breeding colony of the Iran Pasteur Institute, Tehran. They were housed three per plexiglas
110
S. Shahidi et al. / Brain Research Bulletin 76 (2008) 109–113
cage. Experiments were conducted in the afternoon (13:00–16:00 h). Food and water were available ad libitum. Lights were on from 7:00 to 19:00 h. All procedures for the treatment of animals were approved by the research committee of the Hamadan University of Medical Sciences and were done in accordance with the Guide for Care and Use of Laboratory Animals published by the United States National Institutes of Health (NIH Publication No. 85-23, revised 1985).
the rat was taken from the dark compartment into their home cage. Then after 2 min, the procedure was repeated. The rat received a foot-shock each time it reentered the dark and had placed all 4 paws in the dark compartment. Training was terminated when the rat remained in the light compartment for 120 consecutive seconds. The number of trials (entries into the dark chamber) was recorded.
2.4. Retention test 2.2. Passive avoidance apparatus The apparatus and procedure was basically the same as our previous studies [20,35]. Briefly, the step-through passive avoidance apparatus consisted of a lighted chamber (20 cm × 20 cm × 30 cm) made of transparent plastic and a dark chamber whose walls were made of dark opaque plastic (20 cm × 20 cm × 30 cm). The floor of both chambers was made of stainless steel rods (3 mm diameter) spaced 1 cm apart. The floor of the dark chamber could be electrified using a shock generator (Behbood Pardaz Co. Iran). A rectangular opening (6 cm × 8 cm) was located between the two chambers and could be closed by an opaque guillotine door.
2.3. Training First, all experimental groups were given two trials to habituate them to the apparatus. For these trials, the rats were placed in a lighted compartment of the apparatus facing away from the door and 5 s later the guillotine door was raised. The rat has native preference to the dark environment. Upon the rat entering the dark compartment, the door was closed and after 30 s the rats were taken from the dark compartment into their home cage. The habituation trial was repeated after 30 min and followed after the same interval by the first acquisition trial. The entrance latency to the dark compartment (step-through latency, STL) was recorded when the animal had placed all 4 paws in the dark compartment. After the animal had spontaneously entered the dark compartment, the guillotine door was lowered and a mild electrical shock (0.5 mA) was applied for 3 s. After 30 s,
The retention test was performed 24 h after the PAL acquisition trial. The rat was placed in the lighted chamber as in PAL training and 5 s later, the guillotine door was raised, and the step-through latency and the time spent in the dark compartment (TDC) were recorded up to 300 s. If the rat did not enter the dark compartment within 300 s, the retention test was terminated and a ceiling score of 300 s was assigned.
2.5. Experimental protocol Three experiments were done. In each experiment, rats were divided into three separate groups: a saline treated, a 60 mg/kg ascorbic acid treated group and a 120 mg/kg ascorbic acid (Merck, U.S.A.) treated group. In the first experiment, the rat received a single acute intraperitoneal (i.p.) injection of 60 mg/kg ascorbic acid (n = 8), 120 mg/kg ascorbic acid (n = 8) or saline (n = 7) 60 min before training and retention. In the second experiment in order to verify the effects of short-term i.p. injection of ascorbic acid, rats have been received saline (n = 12), ascorbic acid (60 mg/kg, n = 12) or 120 mg/kg ascorbic acid (n = 11) for 12 days. PAL training was started on the 13th day and retention test was done on the 14th day. Injection of saline or ascorbic acid was continued 60 min before the training and retention test. Therefore, these animals get 14 days of injections of ascorbic acid. In the third experiment, saline (n = 10) or ascorbic acid (60 mg/kg, n = 11) or 120 mg/kg ascorbic acid (n = 11) orally administrated using gavages needle for 30 days. In the long-term treatment, instead of i.p. injection, oral administrations
Fig. 1. Effects of short-term administration of ascorbic acid on PAL. The effect of short-term i.p. injection of ascorbic acid on the step-through latency (STL) in the first acquisition trial (A), number of trials to acquisition (B), STL in the retention test (C) and the time spent in the dark compartment in the retention test (TDC) (D) between saline (n = 12), 60 mg/kg ascorbic acid (n = 12) and 120 mg/kg ascorbic acid (n = 11) treated groups. * P < 0.05, ** P < 0.01 and *** P < 0.001 compare to saline group. Each column and bar represents mean ± S.E.M.
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of solutions were done to prevent tissue damages due to multiple injections. Then, PAL training and retention was done in the next 2 consecutive days. Saline or ascorbic acid administration was continued 60 min before training and retention test.
2.6. Data analysis In each experiment, the data were analyzed using ANOVA (one-way analysis of variance) followed by the Tukey test for multiple comparisons. All results are shown as the mean ± S.E.M. The level P < 0.05 was considered as statistically significant.
3. Results 3.1. Experiment 1: effects of acute administration of ascorbic acid on PAL There was no significant difference between the number of trials to acquisition (F(2,20) = 0.61, P > 0.05) in the three experimental groups. Also, in the retention test there was no significant difference of STL (F(2,20) = 3.03, P > 0.05) and TDC (F(2,20) = 0.44, P > 0.05) between groups (data not shown). 3.2. Experiment 2: effects of short-term administration of ascorbic acid on PAL There was no significant difference between the weight of rats in the three experimental groups (F(2,32) = 0.82, P > 0.05). Oneway ANOVA indicated that there was no significant difference in the STL of first acquisition trial between groups (F(2,32) = 1.07, P > 0.05) and thus confirmed their uniformity (Fig. 1A). The number of trials to acquisition was significantly different (F(2,32) = 5.93, P > 0.05). The number of trials to acquisition of ascorbic acid treated groups was less than saline group. There was no difference between acquisition trials of 60 and 120 mg/kg ascorbic acid treated groups (Fig. 1B). Statistical analysis indicated that there were significant differences in step-through latency (F(2,32) = 10.29, P < 0.05) and time spent in the dark compartment (F(2,32) = 11.02, P < 0.05) between experimental groups in the retention test (Fig. 1C and D). There was no significant difference between these parameters in the 60 and 120 mg/kg ascorbic acid treated groups. 3.3. Experiment 3: effects of long-term ascorbic acid supplementation on PAL One-way ANOVA indicated that the number of trials to acquisition of ascorbic acid treated groups was significantly less than saline group (F(2,29) = 6.57, P < 0.01) (Fig. 2A). In addition, the results of retention test showed that STL of saline treated rats is significantly more than ascorbic acid treated groups (F(2,29) = 7.12, P < 0.01) and TDC of saline treated rats is less than ascorbic treated groups (F(2,29) = 7.35, P < 0.01) (Fig. 2B and C). 4. Discussion The findings of the present study are: The results of experiment 1 showed that acute administration of ascorbic acid
Fig. 2. Effects of long-term administration of ascorbic acid on PAL. The effect of long-term oral administration of ascorbic acid on the number of trials to acquisition (A), step-through latency (STL) in the retention test (B) and the time spent in the dark compartment in the retention test (TDC) (C) between saline (n = 10), 60 mg/kg ascorbic acid (n = 11) and 120 mg/kg ascorbic acid (n = 11) treated groups. * P < 0.05, ** P < 0.01 and *** P < 0.001 compare to saline group. Each column and bar represents mean ± S.E.M.
had no significant effects on PAL. The results of experiment 2 showed that short-term injection of ascorbic acid improved learning process of passive avoidance. Also retention of memory increased with ascorbic acid. The results of experiment 3 showed that long-term oral administration of ascorbic acid facilitate passive avoidance learning and memory. The effect of ascorbic acid on PAL was not dose dependent. Long-term oral supplementation and short-term i.p. injection of ascorbic acid had the same cognitive effects. Ascorbic acid had no significant effect on the native behavior of rats to the dark compartment. To prevent tissue damage due to repeated injections of ascorbic acid, it was administered orally in experiment 3. Ascorbic acid
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is a water-soluble vitamin which is well absorbed when given orally and its bioavailability following an oral dose is more than 85%. Thus it seems the effect of oral versus i.p. administration of ascorbic acid (in experiments 1 and 2) has not significant effect on ascorbic acid plasma concentration. In addition, in each of three experiments the results of ascorbic acid treated rats were compared with its control group. It seems that if ascorbic acid administrated at least for a short time period, it could improve acquisition and retention of passive avoidance task. Thus, it has facilitatory effects on passive avoidance learning and memory in healthy rats. The results of this study were similar to other studies which believed that ascorbic acid could affect learning and memory [8,11,19,23,28,31]. They have been reported that, ascorbic acid could reduce the risk of dementia caused by aging [2,19,28], or prevent memory impairment due to the scopolamine [28], hyperprolinemia [11], ovarectomy [23], methylmalonic acid [29], and homocyctein [31]. On the other hand in the present study, we tested the effects of ascorbic acid in the simple model of learning and memory without any memory impairment induction. Arzi et al. has reported that chronic oral supplementation of vitamin C (300 mg/kg, 60 days) improves step-down avoidance learning of aged but not young mice [2]. Therefore, it seems that non-acute administration of ascorbic acid has an enhancing effect on learning and memory in addition to its effects on memory deficits prevention. Ascorbic acid is a potent antioxidant [15,22,33] which is highly concentrated in the central nervous system [3,9]. In most studies, researchers believed that ascorbic acid prevents memory deficit by its antioxidant effect [6,8,31]. Thus the results of this study may be due to the antioxidant action of ascorbic acid. On the other hand, ascorbic acid has a modulatory action on brain neurotransmitter like cholinergic, serotonergic and dopaminergic system [6,9,21]. These neurotransmitter systems are important in learning and memory processes [24]. For example, Lee et al. believed that supplementation with vitamin C might be useful in maintaining brain acetylcholinesterase activity at the normal level and serotonin concentration for some extent under the condition to induce dementia by scopolamine administration [21]. Also, it has reported that local applications of ascorbic acid enhanced the response of neurons to dopamine [10,16,30,36] and glutamate [32]. Glutamate is a neurotransmitter which has a critical role in learning and memory processing [1]. Therefore it is possible that part of the ascorbic acid effects is due to its neurotransmitter modulator functions. A report revealed that ascorbic acid can potentiate hippocampal evoked potential activity [18]. The hippocampus is one of the brain structures that is involved in many kinds of learning and memory like PAL [25]. In addition, it seems that ascorbic acid could affect passive avoidance learning and memory by changing the hippocampal neuronal activity. As ascorbic acid affect hippocampus, it may change other hippocampal learning and memory tasks such as spatial learning and memory. Finally, we can conclude that in addition to the many beneficial effects of ascorbic acid, it can be considered to be a memory enhancer in healthy subjects.
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