Folic acid administration produces an antidepressant-like effect in mice: Evidence for the involvement of the serotonergic and noradrenergic systems

Abstract

Clinical studies have shown that folic acid plays a role in the pathophysiology of depression. However, very few studies have investigated its effect in behavioral models of depression. Hence, this study tested its effect in the forced swimming test (FST) and the tail suspension test (TST), two models predictive of antidepressant activity, in mice. Folic acid administered by oral route (p.o.) produced a reduction in the immobility time in the FST (50–100 mg/kg) and in the TST (10–50 mg/kg). The administration of folic acid by i.c.v. route also reduced the immobility time in the FST (10 nmol/site) and in the TST (1–10 nmol/site). Both folic acid administered by oral and i.c.v. route produced no psychostimulant effect, which indicates that its antidepressant-like effect is specific. Pretreatment of mice with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, i.p., an inhibitor of serotonin (5-HT) synthesis, for 4 consecutive days), ketanserin (5 mg/kg, i.p., a 5-HT_2A/2C receptor antagonist), prazosin (1 mg/kg, i.p., an α₁-adrenoceptor antagonist) or yohimbine (1 mg/kg, i.p., an α₂-adrenoceptor antagonist) prevented the anti-immobility effect of folic acid (50 mg/kg, p.o.) in the FST. Moreover, the pretreatment of mice with WAY100635 (0.1 mg/kg, s.c., a selective 5-HT_1A receptor antagonist) blocked the decrease in immobility time in the FST elicited by folic acid (50 mg/kg, p.o.), but produced a synergistic effect with a subeffective dose of folic acid (10 mg/kg, p.o.). In addition, a subeffective dose of folic acid (10 mg/kg, p.o.) produced a synergistic antidepressant-like effect with fluoxetine (10 mg/kg, p.o.) in the FST. Overall, the results firstly indicate that folic acid produced an antidepressant-like effect in FST and in TST and that this effect appears to be mediated by an interaction with the serotonergic (5-HT_1A and 5-HT_2A/2C receptors) and noradrenergic (α₁- and α₂-adrenoceptors) systems.

Introduction

Folic acid (folate) is a water-soluble B-vitamin whose biologically active form is tetrahydrofolic acid (THF), which participates in the transfer of 1-carbon units (such as methyl, methylene, and formyl groups) to the essential substrates involved in the synthesis of DNA, RNA, and proteins (Bailey and Gregory, 1999).

Ingested folic acid can be converted to its physiological forms. This process is initiated by dihydrofolate reductase in a two-step reaction; the first step, conversion to dihydrofolate (DHF) is a slow and rate-limiting step (Wagner, 1995). In the second, more rapid, step dihydrofolate is further reduced to THF. THF can then be converted into additional physiological folates including 5-methyl-THF, the form that is normally found in the circulation and in tissues (Bailey and Gregory, 1999, Ramaekers, 2004). 5-Methyl-THF is also replenished by the conversion of folinic acid (5-formyltetrahydrofolate), an active metabolite of folic acid. Because de novo folate synthesis is not present in the CNS, it depends on adequate folate transport across the blood-brain barrier. Within neurons, part of the folate pool will be catabolized by oxidation to dihydrofolates and folic acid, which can be reconverted to THF by dihydrofolate reductase (Ramaekers, 2004).

Folic acid plays a role in the methylation of homocysteine providing the methyl group for the conversion of methionine to S-adenosylmethionine (SAM), which itself has been shown to influence 5-HT metabolism. It also has an important role in the synthesis of tetrahydrobiopterin, an essential co-factor for the hydroxylation of phenylalanine and tryptophan, rate-limiting steps in the synthesis of dopamine (DA), noradrenaline (NA) and 5-HT (Mattson and Shea, 2003, Taylor et al., 2004).

Several clinical studies have shown the association between depressive disorder and low folic acid levels (Reynolds, 2002, Paul et al., 2004, Abou-Saleh and Coppen, 2006). Additionally, a deficiency of folate causes elevated homocysteine concentrations, which may contribute to the pathogenesis of major depression (Bottiglieri, 2005, Coppen and Bolander-Gouaille, 2005). Moreover, patients diagnosed with major depressive disorder tend to have lower concentrations of serum or red cell folate than healthy control subjects (Reynolds, 2002, Morris et al., 2003, Coppen and Bolander-Gouaille, 2005) and antidepressant therapy is shown to be accompanied by an increase of folate in the erythrocytes (Levitt et al., 1998). Moreover, it has been shown that supplementing antidepressant medication with folic acid enhances the therapeutic effect (Coppen and Bailey, 2000, Abou-Saleh and Coppen, 2006).

According to the monoaminergic hypothesis of depression, the major neurochemical process in depression is the impairment of monoaminergic neurotransmission and the concomitant decrease of extracellular concentration of NA and/or 5-HT (Schildkraut, 1965). The theory was originally based on the observations that most antidepressants were inhibitors of transporters of 5-HT and NA in the presynaptic nerve ending, therefore, enhancing the monoaminergic neurotransmission (Glowinski and Axelrod, 1964, Ross and Renyi, 1969). This action by antidepressants is one of the cornerstones of the monoaminergic hypothesis of depression (Maas, 1975).

The serotonergic system is closely implicated in the pathogenesis of depression and in the mechanism of action of the antidepressant. Most of the prescribed antidepressants directly affect 5-HT turnover in the brain (Kreiss and Lucki, 1995), inhibit 5-HT reuptake and also interact with 5-HT_1A and 5-HT₂ receptors (Cryan et al., 2005). Newer antidepressants (selective serotonin reuptake inhibitors; SSRIs) are generally more selective and more potent than the older compounds at blocking transport of 5-HT over NA (Richelson, 2001).

Evidence for a role of NA in depression is well established, since most of the antidepressants until the 1980s were shown to be more potent at blocking uptake of NA than at blocking uptake of 5-HT (Richelson and Pfenning, 1984). Many antidepressants, such as doxepin, amitriptyline and nefazodone, have significant α₁-affinity and the antidepressant mirtazapine enhances neurotransmission by directly blocking presynaptic α₂-adrenoceptors (Richelson, 2001).

Given that depression is associated with low levels of folic acid, which is involved in the biosynthesis of monoamines that, in turn, exerts a significant role in the pathogenesis of depression, it is reasonable to hypothesize that folic acid administration produces antidepressant-like effects which are dependent on the serotonergic and noradrenergic systems. Considering that lack of preclinical studies dealing with the antidepressant-like effects of folic acid, this study therefore sought to investigate the effect of this vitamin in the forced swimming test (FST) and the tail suspension test (TST) in mice and the involvement of the serotonergic and noradrenergic systems in its antidepressant-like effect.