This study examined the possible effects of continuous dietary folic acid supplementation (beginning on day 22) on body weight, food intake, markers of metabolism, brain function, oxidative stress markers, antioxidant status, inflammatory parameters and neurotransmitter levels in adult mice. The results of the study revealed an increase in body weight, and improvement in antioxidant status; memory scores (in the radial arm and Y- maze), acetylcholine levels, brain derived neurotropic factor and interleukin levels; while a reduction in food intake, acetylcholinesterase activity, and levels of glucose, insulin and leptin were also observed.
It also showed open field behavioural changes that were suggestive of central inhibition (decreased horizontal locomotion, rearing and self grooming behaviours); while in the elevated plus maze, an anxiolytic behaviour was observed.
The continuous supplementation of folic acid from day 22 till adulthood (day 92) was associated with an increase in body weight (5 and 10 mg) and a reduction in food intake, blood glucose, insulin and leptin levels at the three concentrations of folic acid. This would suggest that folic acid supplementation beginning in the prepubertal period possibly increases feed efficiency, while also reducing the risk of developing dysmetabolism in adulthood. Effects of folic acid on metabolic parameters such as weight, food intake, blood glucose levels, lipid levels and metabolic hormones have been reported severally [11, 49–53]; and there have been reports suggesting that a possible relationship exists between folic acid metabolism and the maintenance of metabolism in the body. While the results of this study showed that folic acid supplementation beginning in the prepubertal period had the ability to increase body weight in rats, corroborating an earlier study using adult mice from our laboratory [11], it also demonstrated that this increase in body weight was not accompanied by derangement of metabolism markers such as glucose, insulin and leptin. This also corroborates the result of the study that evaluated the effect of folic acid supplementation in broiler chickens and reported that supplementation of folic acid at 5 mg.kg was associated with increased average body weight, and reduced adiposity [54]. The absence of metabolic derangement (as observed in this study) or adiposity following folate supplementations differentiates the body weight increase observed in this study from that reported with folate deficiency in humans, or with the administration folate deficient diet in animals [52, 53, 55]. Results of this study also suggest that folic acid has the ability to increase feed conversion efficiency, supporting a number of studies that had also reported improvement in body weight and feed conversion efficiency following folic supplemented diet [54, 56].
In the last few decades, folic acid has been shown to be an essential micronutrient necessary for optimal brain functioning. It is important in the synthesis of nucleotides, myelin and neurotransmitters. It is also crucial in methylation reactions, and ensuring that homocysteine is kept at non toxic levels [57]. More important is the association of low maternal folic acid intake during pregnancy with an increased risk of neural tube defects and other neurodevelopmental disorders including schizophrenia and autism spectrum disorders. In this study, folic acid supplementation was associated with a decrease in horizontal locomotion, rearing and self-grooming; with no significant effect on catalepsy. The results of this study contrast the results of an earlier study from our laboratory that examined the effect of folic acid supplemented diet fed at 25, and 50 mg/kg of feed in adult rats [11], and reported an increase in locomotor activity, rearing and self grooming behaviours in healthy adult rats. In this study, folic acid supplementation beginning in the prepubertal period was associated with an overall central inhibitory effect with a concentration dependent decrease in locomotor activity, rearing, and grooming behaviours. Locomotor activity, rearing and self grooming are brain behaviours that are modulated or regulated at multiple brain regions under the influence of several neurotransmitters such as glutamate, dopamine, gamma amino butyric acid and serotonin [58–66]. Also, in this study, we observed a decrease in brain levels of dopamine (a crucial neurotransmitter in the modulation of locomotor and self grooming behaviours) with increasing concentrations of folic acid (Table 3), suggesting that commencing dietary fortification of folic acid supplementation (at the higher concentrations) in the prepubertal period was associated with a decrease in dopamine levels in adulthood. While there is a dearth of scientific literature assessing the effects of folic acid on ligand/ receptor interactions in the brain, studies have shown that folic acid supplementation in adult rodents was associated with an increase in the level of dopamine and norepinephrine neurotransmitter in the brain [67]. Also, the relationship between brain neurotransmitter activity and neurobehavioural modulation has been studied severally. Reports have shown that dopaminergic transmission can modulate locomotor activity via ascending fibres that project to the basal ganglia and finally to locomotor networks in the brain. A decreased dopaminergic tone has been linked to a decrease in locomotor activity [68] while an inhibition of dopaminergic D1 receptors has been linked to a decrease in self-grooming. Also, the suppression of dopaminergic transmission in the nucleus accumbens results in hypolocomotion, while suppression of caudate-putamen dopaminergic transmission results in decreased rearing activity [64]. Therefore, the decrease in brain dopamine levels observed in this study could possible account for the central inhibitory response observed in the open field. However, other neurotransmitters could also be responsible for the central inhibitory effect observed in this study. Gamma amino butyric acid (GABA) is regarded as the most common inhibitory neurotransmitter in the brain, with its effect crucial to the regulation of central pattern generators and modulation of neuromuscular junction/ higher brain processing centres [66]. Another mechanism through which folic acid supplementation could result in central inhibitory effect would be through its effect on S-adenosylmethionine (SAM). Studies have shown that folic acid is important in one carbon metabolism as well as in the production of S-adenosyl methionine [69] which is a universal methyl donor required for various reactions and has been shown to result in hypolocomotion (horizontal and vertical) and decreased self-grooming. Also, SAM can be remethylated to methionine which is also associated with central inhibitory effects [60]. Motor coordination (measured as catalepsy score on the bar test) was however not altered by folic acid supplementation. Although, a 2012 study by Shooshtari et al [70] carried out in adult male rats had observed that oral administration of FA at 5 and 10 mg/kg but not at 15 mg/kg improved motor coordination in the rotarod.
Several clinical and preclinical studies have demonstrated the important roles played by folic acid and folate metabolism in memory processes [70–76], anxiety related behaviours, and depression [71, 77]. In this study dietary folic acid supplementation improved spatial working memory, decreased anxiety and depression-like behaviours. This supports the results of a number of studies that had reported the benefits of folic acid supplementation on memory processes, anxiety and ameliorating depression particularly in adults, the aged or following peripartum folic acid supplementation [67, 73, 78, 79]. There have been suggestions that these beneficial effects of FA supplementation could be attributed to its effects on brain derived neurotropic factor (oan increase was observed in this study), its antioxidant effects, its ability to reduce lipid peroxidation (which was also observed in this study) and its ability to increase levels of serotonin, and the expression of glutamate receptor 1 [67, 77]. Folic acid’s effects on memory have also been linked to its ability to decrease the activity of acetylcholinesterase and by extension increase acetylcholine levels in the brain [80] which was also observed in this study (Table 3) .
The antioxidant, antiinflammatory and free radical scavenging properties of folic acid is not in dispute, decades of research continue to affirm this fact [81–83]. Also, the results of this study demonstrated that at the concentrations administered, the commencement of dietary folic acid supplementation in the prepubertal folic improved antioxidant status, IL-10 and IL-6 levels in adult mice while also decreasing levels of lipid peroxidation levels and TNF-α supporting the results of a number of in vitro and in vivo studies that had reported similar effects [67, 84]. There have been reports that folic acid’s ability to increased total antioxidant capacity and decrease malondialdehyde levels could be linked to its effects on homocysteine concentration, although levels of homocysteine were not measured in this study. Also the antiinflammatory effects of folic acid have been linked to its ability to cause epigenetic regulation in cells [85]. These antiinflammatory effects have also been associated with a reduction in anxiety and improvement in memory and mood [85] which was also observed in this study. Supplementation with folic acid increased acetylcholine levels and decreased acetylcholinesterase activity corroborating the results of a few other studies [86]. The reduction in acetylcholinesterase activity with a corresponding increase in brain levels of acetylcholine could be responsible for the improvement in memory scores observed in this study.