The current study was performed to explore the effect of coconut oil supplementation on cognitive and non-cognitive functions and levels of the AD pathology markers, neurotrophic factors, cholinergic activity, synaptic transmission and oxidative stress in the D-GAL/AlCl3 administration-induced experimental AD model. Our results show that coconut oil (2 ml kg− 1) supplementation did not affect locomotor activity, but led to improvement of exploratory and anxiety-like behaviors and spatial learning and memory functions. Moreover, it is noteworthy that in the experimental AD model, coconut oil supplementation suppressed AD pathology markers and cholinergic activity, while improving synaptic transmission, neurotrophic factors, and oxidative stress.
Different animal models are employed to examine multiple aspects of AD. Transgenic animals are not the best study subjects for the more prevalent sporadic type of AD, because the familial type accounts for less than 1% of all AD cases [30]. Although it requires numerous invasive administrations of the chemical substance over a lengthy period of time, the use of some compounds, such as Aβ protein, D-GAL, and AlCl3, gives superior models to research sporadic AD type [31]. It has been claimed that the cognitive impairment and pathological abnormalities of clinical AD patients can be more accurately simulated by the AD model caused by D-GAL/AlCl3 [32]. In light of these considerations, a combination of D-GAL/AlCl3 was chosen in this study to induce a sporadic pattern of AD.
Regarding the body weight change, our data show that body weights of all groups increased over the study period and this increase was greater in the coconut oil supplemented groups. Coconut oil supplementation has been shown to increase, decrease, or not affect body weight. Consistent with our findings, Ströher et al. [28] reported that 21 days of coconut oil supplementation caused an increase in body weight of young rats. However, according to the Alves et al. [33] body weight decreased in spontaneously hypertensive rats after 4 weeks of coconut oil supplementation. Additionally, some studies [16,27,34] have reported that coconut oil does not cause any changes in body weight. We think that the saturated fatty acids in the coconut oil may increase food consumption, which in turn may raise body weight.
Consistent with previous studies showing that non-cognitive functions such as locomotor activity and exploratory behavior are negatively affected by AD [35–38], non-cognitive functions were decreased in the D-GAL/AlCl3 administered rats in this study as well. There can be two explanations for this. Firstly, it may be due to reduced movements of the animal in the test box, and secondly, it may be the result of decreased interest in objects. Contrary to our results, there are also studies [39–41] showing that locomotor activity and exploratory behavior are not different from healthy controls. These differences in outcomes can be related to variations in animal species, ages, and sexes, as well as the period between behavioral testing and variations in AD induction methods. Our current findings revealed that coconut oil produced a positive effect on our AD model by improving exploratory behavior but not locomotor activity. Consistent with our findings, coconut oil has been shown to improve non-cognitive symptoms [42,43].
Rats given D-GAL/AlCl3 treatment in this study spent more time in the closed arms and less time in the open arms of the EPM, and they defecated more frequently during the OF test, showing that they were more "anxious" than the healthy controls. It has been shown in both clinical and experimental studies that AD causes anxiety-like behaviors [24,37,44,45]. In the early stages of AD, anxiety may occur as a psychological response to the illness and because of difficulties in adapting to AD. In the late stages of AD, severe cognitive decline limits emotional reactions and expression. Finally, due to the severity of AD's effects on the person, anxiety is worse in early-onset AD [46]. The molecular mechanisms linking AD to anxiety are not fully understood. However, the increase in anxiety levels in this study can be attributed to Aβ protein, and tau accumulation and oxidative damage. The present findings showed that coconut oil supplementation resulted in reduction in anxiety levels measured in both OF and EPM tests. In line with our findings, it has been reported in various studies that coconut oil reduces the level of anxiety and this effect is due to the medium chain fatty acids in its content [42,43,47].
The MWM test, one of the most frequently used tests in behavioral research, measures spatial memory and learning capacity [48]. AD-induced cognitive decline has been reported in different experimental models such as transgenic animals [49], icv-STZ injection [50], hippocampal Aβ injection [40], and administration of D-GAL/AlCl3 [23,51], and in different behavioral testing devices such as novel object recognition [52], MWM [40,51,52] and Y maze [53], which is consistent with our findings. Our group has previously shown that D-GAL/AlCl3 exposure causes cognitive impairment at the same dose and exposure time as in the current investigation [24]. In the current study, a considerably high number of platform crossings, time spent in the platform zone, and time spent in the target quadrant during the MWM probe trial clearly demonstrated the neuroprotective impact of coconut oil. These findings are in line with those of other studies [17–19], which demonstrate that coconut oil can restore memory and spatial learning deficits brought on by experimental AD. In this study, in addition to cognitive functions, non-cognitive functions such as locomotor activity (total distance traveled and velocity) and anxiety level (thigmotactic behavior) were also evaluated in the MWM probe trail test. Consistent with the findings from the EPM and OF tests, coconut oil supplementation improved non-cognitive disturbances in addition to cognitive functions.
As one of the biochemical results of this study, there was an increase in hippocampal Aβ1−42 and MAPT levels in rats treated with D-GAL/AlCl3. It is well known that accumulation of Aβ1−42 and MAPT in brain regions serving memory and cognition such as the hippocampus strongly contributes to the development of AD [3]. Consistent with our results, numerous studies [23,24] have shown that administration of D-GAL/AlCl3 causes an increase in Aβ and tau protein levels. These two pathogenic characteristics of AD may be related. Aβ may trigger aberrant protein kinase activation, phosphorylate tau abnormally, and result in neuronal death [54,55]. Additionally, it has been established that phosphorylated tau can be used as a therapeutic target in AD and that this protein is inversely correlated with the severity of the disease [56]. In this study, administration of coconut oil reduced the hippocampal levels of Aβ1−42 and MAPT in rats treated with D-GAL/AlCl3. The beneficial effects of coconut oil on hippocampal Aβ and tau levels have been demonstrated in both in vivo [17–19] and in vitro [21,22,57] studies. The therapeutic impact of coconut oil in AD is demonstrated by a decrease in the hippocampal levels of Aβ and tau. The hydrogen of Aβ can be held by the hydroxyl group of phenolic substances, reducing the buildup of Aβ [58]. Ferulic acid, a phenolic substance found in coconut oil, prevents Aβ accumulation by reducing and binding to Aβ fibers, inhibiting the prolongation process [11].
ACh is a cholinergic neurotransmitter important for learning and memory [59]. According to the cholinergic theory of AD, cholinergic neuron and reduced ACh-mediated neurotransmission degeneration in the hippocampus are both factors in cognitive loss in AD [60]. Since AChE is the main enzyme that breaks down ACh in the synaptic cleft, it serves as a marker for the loss of cholinergic neurons in the brain [61]. Consistent with previous studies [23,39,52], in this study, a compromised cholinergic system was observed in rats given D-GAL/AlCl3, as indicated by increased AChE activity. In the current work, rats' hippocampal AChE activity was dramatically decreased by co-administration of coconut oil and D-GAL/AlCl3. In line with our findings, Attia et al. [19] showed that coconut oil consumption reduced the rise in hippocampal AChE activity induced by Al in young rats. Similarly, Rahim et al. [62] showed that supplementing young, healthy rats with coconut oil reduced AChE activity, which in turn improved cognitive skills. This is due to both the active polyphenol compounds in coconut oil, which can enhance cholinergic neurotransmission by modulating AChE activity, and the cytokinins (phytohormones) present, which are thought to have anti-aging properties [63–65].
Since increased AChE activity in AD terminates synaptic transmission by hydrolyzing ACh to acetate and choline [66], in this study we analyzed SYP levels, a marker of synaptic transmission. In the present study, D-GAL/AlCl3 administration induced a reduction in the hippocampal SYP levels. Consistent with our findings, both animal [67–69] and human [70] studies have showed that hippocampal SYP levels are decreased in AD disease. Furthermore, a relationship was found between decreased SYP in the hippocampus and the severity of cognitive impairment in AD [70,71]. In the present study, coconut oil administration restored hippocampal SYP levels. Consistent with our findings, an in vitro study [22] showed that coconut oil activates Akt and ERK, reducing the number of cells in which caspase is cleaved, thereby recovering SYP loss. Therefore, one of the molecular mechanisms of the positive effects of coconut oil on learning and memory impairment in AD may be its increase in SYP levels.
In the current investigation, treatment of D-GAL/AlCl3 led to an increase in hippocampal oxidative stress with higher levels of MDA and PC and lower SOD activity. According to the available data, oxidative stress contributes to the etiopathology of AD by generating mitochondrial dysfunction, increasing tau hyperphosphorylation and neurofibrillary tangle formation, boosting Aβ-mediated neurotoxicity, and promoting neuronal death and synaptic dysfunction [72–74]. According to some reports, the development of AD is a direct result of the brain's increased oxidative stress [75]. By boosting SOD activity and reducing the rise in MDA and PC levels, coconut oil supplementation changed the oxidative/antioxidative state of D-GAL/AlCl3-treated rats, demonstrating its capacity to scavenge free radicals and antioxidative activity. Consistent with our findings, coconut oil has been shown to reduce reactive oxygen species production in vitro [22], increase GSH levels in both the hippocampus and prefrontal cortex, and decrease MDA levels in the cortex, in vivo [17]. The presence of phenolic acids such as caffeic acid, syringic acid, p-coumaric acid, vanillic acid, and ferulic acid, which play a significant role in antioxidation, can be used to explain the antioxidant mechanism of action of coconut oil [76]. It is reported that the production of adenosine triphosphate in AD is inhibited due to oxidative damage to enzymes involved in glucose metabolism, which reduces the amount of energy available to the brain [77]. Also, the decreased level of the antioxidant enzymes reported in AD also inhibit the activity of the appropriate detoxification mechanism [78]. Since coconut oil is converted into ketone bodies by the liver and provides an important alternative energy source to the brain [76], this function may also contribute to its antioxidant effect. Thus, alleviation of D-GAL/AlCl3-induced behavioral alterations by coconut oil may be related to its antioxidant effect.
In this study, experimental AD model resulted in a reduction in hippocampal BDNF levels, while coconut oil supplementation tended to increase BDNF levels. BDNF is a member of the neurotrophin family, which is involved in processes such as synaptic plasticity, cell survival and energy metabolism [79]. Many studies have shown that BDNF levels are reduced in postmortem specimens from patients with AD [80] and in experimental animal models of AD [81]. In AD, Aβ accumulation attenuates cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), resulting in a decrease in BDNF levels [82]. As in this study, in addition to Aβ accumulation, increased AChE activity and oxidative stress may also cause decreased BDNF level. Consistent with our findings, decreased BDNF signaling causes impairment in spatial learning and memory [83]. Due to the medium chain triglycerides in coconut oil content, it modulates the energy metabolism of the brain and may provide neuroprotection through increased BDNF expression, reduced neuroinflammation, and enhanced neurotransmission [84]. However, to our knowledge, there are no studies investigating the effect of coconut oil supplementation on hippocampal BDNF levels in AD. Our study is the first research on this subject. Our findings showed that coconut oil tended to increase hippocampal BNDF levels in the experimental AD model. This increase in BDNF levels may have been mediated by decreased AChE activity and oxidative stress. Consistent with our findings, Attia et al. [19] reported that coconut oil increased serum BDNF levels in an experimental AD model. Taken together, these findings indicate that coconut oil administration stimulates upregulation of BDNF levels, followed by improvement in cognitive function and subsequent neuroprotection.