Zinc deficiency alters responsiveness to antidepressant drugs in mice

Zinc deficiency alters responsiveness to antidepressant drugs in mice Katarzyna M³yniec1,3, Bogus³awa Budziszewska1,3, Witold Reczyñski4, Urszula Doboszewska3, Andrzej Pilc3,5, Gabriel Nowak2,3

Correspondence:

Abstract: Background: There is some evidence coming from preclinical and clinical studies suggesting a relationship between dietary zinc intake and depressive symptoms. The aim of the study was to determine whether zinc deficiency alters the response to antidepressants with a different mechanism of action. We examine also whether these changes are related to activity of the hypothalamic-pituitaryadrenal HPA axis. Methods: Male CD-1 mice were assigned to groups according to diet and antidepressant administration. To evaluate animal behavior, the immobility time in the forced swim test (FST) and locomotor activity were measured. To determine serum zinc levels the flame atomic absorption spectroscopy (FAAS) was used. The serum corticosterone was determined by radioimmunoassay (RIA). Results: Antidepressants administered to zinc-deprived mice induced an altered response in the FST when compared to animals fed with an adequate diet. There were no changes in locomotor activity. Animals subjected to a zinc-deficient diet showed a significant reduction in serum zinc levels, which was normalized by antidepressant treatment. An increase in serum corticosterone concentrations in mice fed with a zinc-deficient diet and treated with antidepressants was observed, so it can be concluded that reduced levels of zinc contribute hyperactivation of the HPA axis. Conclusion: The results of this study suggest that a diet with a reduced zinc level alters antidepressant action, which is associated with a reduction in the serum zinc level and rise in the corticosterone level. The results of this study may indicate the involvement of zinc deficiency in the pathogenesis of depression. Key words: zinc deficiency, depression, antidepressants, zinc, stress, corticosterone

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Introduction The number of people suffering from depression is increasing worldwide. Patients suffering from depression demonstrate some deficit in serotonergic, noradrenergic and/or dopaminergic tone [17]. Using the monoaminergic theory as a basis, new antidepressants were found; e.g., tricyclic inhibitors of noradrenaline and serotonin uptake, monoamine oxidase inhibitors, selective serotonin or noradrenaline reuptake inhibitors [67]. The dark side of this therapy, besides side effects, is the time needed for therapeutic efficacy. Between 29 and 46% of depressed patients fail to respond to antidepressant treatment of an adequate dose and duration, and 19 to 34% of this population do not respond to the therapy at all [16]. Non-response to two or more antidepressants from different pharmacological classes is one of the various definitions of treatment-resistant depression (TRD) [2, 51]. Another definition is the inadequate response to at least one antidepressant trial of adequate doses and duration [15] or the failure of two monotherapy trials, one or more augmentation trials at a greater resistance [43]. According to the World Health Organization (WHO), current drug therapies for depression are sufficient in about 60% of patients. Approximately 50% of the world’s population is at risk of zinc deficiency [3]. There is some evidence coming from preclinical and clinical studies suggesting a relationship between dietary zinc intake and depressive symptoms [5, 29, 64, 68, 69]. Zinc shows antidepressant-like properties in preclinical test and models [6, 10, 20, 21, 36, 42, 52]. Recently, there has been emerging evidence that zinc deficiency can induce depressive-like behavior in mice [30, 31, 69] and rats [57, 64, 67]. For two decades, there has been growing evidence suggesting a link between zinc and treatment-resistant depression. Clinical studies have demonstrated that not only do depressed patients exhibit significantly lower serum zinc levels than the control subjects, but also that treatment-resistant patients display lower serum zinc concentration than those responsive to drug therapy [24, 26, 29, 46]. An association between low zinc status and high depression scores has also been observed [24, 37, 70]. Additionally, it was found that lower serum zinc level might be normalized after successful antidepressant treatment [26, 29, 46]. There are also clinical data suggesting that zinc supplementation may enhance antidepressant therapy in patients 580

with unipolar depression, particularly in those previously nonresponsive [34, 45]. The other evidence supporting the link between zinc intake and depressive symptoms comes from animal studies. It has been reported recently that zinc deficiency induced by a zinc-deficient diet evokes behavioral disturbances in animals, e.g., enhanced depression-like behavior was found after dietary zinc deprivation in the tail suspension test (TST) [31, 69] and the forced swim test (FST) [30, 63, 64, 68, 69] and was reversed by desipramine [69] while administration of fluoxetine did not reduce behavioral despair caused by zinc deprivation [64]. Some authors correlate zinc deficient-induced depression-like behavior with hyperactivation of the hypothalamicpituitary-adrenal (HPA) axis [30, 60, 68]. This may in turn be linked to the dyshomeostasis of glutamatergic system. Glucocorticoids potentiate glutamate accumulation and glutamate excitotoxicity [57]. Activation of glutamate receptors and hyperactivation of HPA axis often results in an increase in Ca levels and are associated with neuronal excitation. The increase in basal Ca levels was observed in zinc deficiency [62]. Study of Watanabe et al. [68] showed an increased serum glucocorticoid concentration in zinc deficiency, which may facilitate glutamate release and inhibit glutamate removal by glutamate transporters [68]. The enhanced exocytosis in zinc deficiency is a possible mechanism on abnormal increase in extracellular glutamate [58]. Zinc is an antagonist of the ionotropic glutamate NMDA receptor and exhibits antidepressant-like activity in rodent tests and models of depression [22, 35, 38, 55]. Patients suffering from TRD that have been treated with antagonists of the glutamatergic system or its modulators showed a surprisingly good response after a short period of time [1, 27, 71, 72]. Taken together, these findings suggest that glutamate modulators are involved in the treatment of refractory depression, and it can be hypothesized that direct targeting of NMDA receptor complex may bring rapid antidepressant effects [18, 19, 44]. The aim of this study was to evaluate the efficacy of antidepressants in mice subjected to a zinc deficient diet which induced zinc deficiency using FST. Moreover, we examined also serum corticosterone and zinc concentrations. This is the first study showing significant changes in response to antidepressants with a different mechanism of action under zinc deficient conditions.

Zinc deficiency alters responsiveness to antidepressants

Materials and Methods Animals

Three-week-old male CD-1 mice were housed under standard laboratory conditions with a natural daynight cycle, a temperature of 22 ± 2°C and the humidity at 55 ± 5% as well as access to food and water ad libitum. Each experimental group consisted of 5–7 animals. All of the procedures were conducted according to the National Institute of Health Animal Care and Use Committee guidelines, which were approved by the Ethical Committee of the Jagiellonian University Medical College, Kraków. Zinc deficient diet

Control (33.5 mg Zn/kg) and zinc deficient (0.2 mg Zn/kg) diets were purchased from MP Biomedicals (France) and administrated for 6 weeks. Mice were assigned to one of ten different groups according to the diet and drug administration (Fig. 1). Drug administration

Mice under the control or zinc deficiency diet received chronically (14 days), intraperitoneally imipramine (30 mg/kg, Sigma-Aldrich, USA), escitalopram (4 mg/kg, Lundbeck, Denmark), reboxetine (10 mg/kg, Ascent, UK), bupropion (15 mg/kg, Sigma-Aldrich, USA) or saline (NaCl). The doses of drugs were determined as the lowest active dose in the FST (Fig. 2). BEHAVIORAL TESTS Forced swim test (FST)

The studies were carried out on mice according to the method of Porsolt et al. [40] and Castagné et al. [4]. The FST I was performed after 4 weeks of diet, 30–45 min following the first administration of saline or antidepressants (imipramine 30 min, bupropion 30 min, escitalopram 40 min and reboxetine 45 min). The FST II was performed 2 weeks later (after 6 weeks of diet), following the last administration of chronic treatment. The animals were dropped individually into glass cylinders (height 25 cm, diameter 10 cm) containing 10 cm of water, maintained at 23–25°C. The mice

Fig. 1.

were left in the cylinder for 6 min. After the first 2 min, the total duration of immobility was measured during the following 4-min test. The mouse was judged to be immobile when it remained floating passively in the water.

Locomotor activity

Locomotor activity was performed on the 12 day of chronic drug administration of the mice and was measured with photoresistor actometers (circular cages, diameter 25 cm, two light beams). The animals were individually placed in an actometer and their activity was then measured between 2 and 6 min. The number of crossings of the light beams by the mice was then recorded as the locomotor activity. 581

Corticosterone assay

Corticosterone was extracted from the serum using ethanol and was measured by a radioimmunological method. The ethanol serum extracts were dried under a nitrogen stream, dissolved in 0.1 ml of 0.05 mM phosphate buffer, pH 7.0, containing 0.9% NaCl and 0.1% gelatin (Sigma Chemical Co.), and were incubated with a 0.1 ml solution of 1,2,6,7-[ H]-corticosterone (20,000 dpm/sample; Radiochemical Centre, Amersham, s.a. 85 Ci/mmol) and with a 0.1 ml solution of a corticosterone antibody (Chemicon) for 16 h at 4°C. Free and bound corticosterone was separated using dextran-coated charcol. The samples were incubated for 10 min at 4°C with 0.2 ml of 0.05% dextran (Dextran T 70, Pharmacia) and 0.5% charcol (activated, Sigma) suspension. After centrifugation at 1000 × g for 20 min, 0.3 ml of the supernatant was placed in a scintillator and the radioactivity was measured in a !-counter (Beckmann LS 335). The crossreactivities of the used antiserum with 11-dehydrocorticosterone and deoxycorticosterone were 0.67 and 1.5%, respectively, whereas those of other steroids were below 0.01%. The corticosterone content was calculated using a log-logit transformation. The assay sensitivity was 10 pg/tube. The intra- and inter-assay coefficients of the variation were lower than 5 and 8%, respectively. Zinc assay

Fig. 2.

A

B

C vs.

BLOOD SAMPLING AND BIOCHEMICAL ASSAY

Serum corticosterone levels were estimated 24 h after the FST at 08:00 a.m. The animals were then sacrificed under non-stressful conditions by rapid decapitation and the blood was collected. The serum was separated by centrifugation in a refrigerated centrifuge at 800 × g for 15 min and stored at –20°C until the assay. 582

Due to the low volumes, no sample pretreatment procedures were applied. The thawed samples were thoroughly mixed as these were not homogenous, and then analyzed directly by the means of the atomic absorption spectrometry (AAS) method. In some instances (the samples with the smallest volume), the electrothermal technique (ETAAS) was used, while for the samples with a higher volume, the flame technique (FAAS) was applied. For both techniques, the determination procedure and instrumental parameters were optimized to obtain the highest possible sensitivity. On the other hand, the procedure was prepared in a way to prevent any possible contamination of the sample with the analyte from the environment. Only quadruple distilled water was used for both the sample and preparation of the standard solutions. The detection limits of Zn determination in the FAAS and ETAAS techniques were 3.6 and 0.07 µg/l, respectively. Precision of the measurements

Zinc deficiency alters responsiveness to antidepressants

was less than 7% (RSD), and it was a direct result of inhomogeneity of the analyzed samples. The accuracy of measurements was checked by means of comparative analysis of a chosen digested sample via the FAAS and voltammetry (ASV) methods. The difference between the obtained results was less than 2%. To determine the Zn concentration, a Perkin Elmer Model 3110 (USA) spectrometer was used; flame analysis was made in an air-acetylene flame, HCL lamp, wavelength 213.9 nm, slit 0.7 nm. The electrothermal analysis was conducted by the use of a Perkin Elmer HGA 600 instrument, at the same spectral conditions, using a pyrolytic coated graphite furnace. Data analysis

The data obtained were evaluated by the Student t-test or one-way ANOVA (with post-hoc Tukey’s test for dose dependent experiments). All the results are presented as the mean ± SEM; p < 0.05 was considered to be statistically significant.

Results Forced swim test – dose response

The effect of acute administration of escitalopram (Esc), reboxetine (Reb) or bupropion (Bup) on the duration of immobility time in the FST in mice are shown in Figures 2A, 2B, 2C, respectively. The lowest active dose was 4 mg/kg for escitalopram [F (3, 25) = 4.092, p = 0.0171], 10 mg/kg for reboxetine [F (3, 28) = 5.006, p = 0.0066] and 15 mg/kg for bupropion [F (3, 24) = 79.90, p < 0.001]. The lowest dose for imipramine (30 mg/kg) was established previously in our laboratory [20]. Four weeks deficiency FST I

The effect of acute saline or antidepressants’ treatment on the duration of immobility time in the FST in mice subjected to control or zinc deficit diet administration for 4 weeks is shown in Figure 3. Acute saline treatment induced a significant 25% [t (12) = 8.650, p < 0.0001] increase in the immobility time in animals subjected to a 4-week zinc deficient diet when compared to the control mice (Fig. 3A). Acute treatment

Fig. 3.

A B D

C E

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with imipramine, escitalopram, reboxetine or bupropion induced a significant 37% [t (12) = 2.199, p = 0.0482], 20% [t (11) = 2.687, p = 0.0212], 22% [t (12) = 4.387, p = 0.0009], 39% [t (12) = 5.134, p = 0.0002] increase in the immobility time in animals subjected to a 4-week zinc deficient diet when compared to the control mice, respectively (Fig. 3B–E). Six weeks deficiency (FST II) Saline

The effects of chronic saline treatment on the duration of the immobility time in the FST mice subjected to a control or zinc deficit diet administration for 6 weeks is shown in Figure 4A. The 6-week zinc deficient diet administration induced a small (by 8%) [t (12) = 6.050, p < 0.0001] but significant increase in the immobility time in mice. The effects of chronic saline treatment on serum corticosterone concentration in mice subjected to a control or zinc deficit diet administration for 6 weeks is shown in Figure 4B. There were no significant differences in corticosterone concentration between the control and zinc deficient animals [t (8) = 0.9298, p = 0.3797]. Imipramine

The effects of chronic imipramine treatment on the duration of immobility time in the FST in mice subjected to control or zinc deficient diet administration for 6 weeks is shown in Figure 5A. Imipramine treatment to animals subjected to a 6-week zinc deficient diet induced a similar level of immobility time in both experimental conditions [t (8) = 3.241, p = 0.1186]. The effects of chronic imipramine treatment on serum corticosterone concentration in mice subjected to a control or zinc deficient diet administration for 6 weeks is shown in Figure 5B. A significant (118%) increase in corticosterone concentration was observed in mice after the 6 weeks of the zinc deficient diet and chronic imipramine administration [t (9) = 2.784, p = 0.0213] (Fig. 5B). Escitalopram

Fig. 4. A C

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B D

The effects of chronic escitalopram treatment on the duration of immobility time in the FST in mice subjected to a control or zinc deficient diet administration

Zinc deficiency alters responsiveness to antidepressants

Fig. 5.

Fig. 6. A B

C

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B

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for 6 weeks is shown in Figure 6A. Escitalopram treatment to animals subjected to a 6-week zinc deficient diet induced a significant increase of 17% [t (12) = 2.997, p = 0.0111] in the immobility time when compared to the control mice. The effects of chronic escitalopram treatment on the serum corticosterone concentration in mice subjected to control or zinc deficient diet administration for 6 weeks is shown in Figure 6B. A significant (349%) increase in corticosterone was observed in mice after 6-weeks of the zinc deficient diet and chronic escitalopram administration [t (7) = 3.656, p = 0.0081] (Fig. 6B). Reboxetine

The effects of chronic reboxetine treatment on the duration of the immobility time in the FST in mice subjected to a control or zinc deficient diet administration for 6 weeks is shown in Figure 7A. Reboxetine treatment to animals subjected to 6-weeks of a zinc deficient diet induced a similar level of immobility time in both experimental conditions [t (12) = 1.740, p = 0.1074]. The effects of chronic reboxetine treatment on serum corticosterone concentration in mice subjected to a control or zinc deficient diet administration for 6 weeks is shown in Figure 7B. A significant (932%) increase in corticosterone was observed in mice after 6-weeks of the zinc deficient diet and chronic reboxetine administration [t (10) = 6.136, p = 0.0001] (Fig. 7B). Bupropion

The effects of chronic bupropion treatment on the duration of the immobility time in the FST in mice subjected to control or zinc deficient diet administration for 6 weeks is shown in Figure 8A. Bupropion treatment to animals subjected to 6-weeks of a zinc deficient diet induced significant increase of 22% [t (12) = 3.061, p = 0.0099] in the immobility time when compared to the control mice. The effects of chronic bupropion treatment on serum corticosterone concentration in mice subjected to a control or zinc deficient diet administration for 6 weeks is shown in Figure 8B. A significant (339%) increase in corticosterone was observed in mice after 6-weeks of the zinc deficient diet and chronic bupropion administration [t (12) = 10.51, p < 0.0001] (Fig. 8B). 586

Fig. 7. A B

C

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Zinc deficiency alters responsiveness to antidepressants

Locomotor activity

The effects on spontaneous locomotor activity in mice are shown in Figures 4C-8C (for saline [t (12) = 0.2355, p = 0.8178], imipramine [t (12) = 0.1459, p = 0.8865], escitalopram [t (12) = 0.6612, p = 0.5210], reboxetine [t (12) = 1.459, p = 0.1703] and bupropion [t (12) = 0.9826, p = 0.3452]). There were no changes in locomotor activity between all of the tested groups. Serum zinc concentration

A zinc-deficient diet induced a 40% reduction in the serum zinc level when compared to the ZnA group (Fig. 4D, t (8) = 0.2826, p = 0.0387). There are no significant differences between groups ZnA and ZnD treated with antidepressants (imipramine, t (12) = 2.016, p = 0.0668; escitalopram, t (12) = 0.5734, p = 0.5770; reboxetine, t(12) = 0.9686, p = 0.3519 or bupropion, t (12) = 0.2121, p = 0.8356) in the serum zinc level (Figs. 5D, 6D, 7D, 8D).

Discussion

Fig. 8. A B

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In the present study, we examined the alterations in behavior, serum corticosterone and the zinc level induced by different antidepressants in mice subjected to zinc deficient conditions. The doses of antidepressants used were determined by dose-dependent experiments. A previous study by Whittle et al. in 2009 [69], examined activity (not efficacy) of the conventional antidepressant desipramine and Hypericum perforatum extract in the FST and TST in mice in the zinc deficiency model. Since the authors did not compare the antidepressant-like activity of these agents in both experimental conditions (control and deficiency), and examined activity only in the ZnD groups, they indicated the activity of antidepressants in this procedure (zinc deficiency) as a possible model of depression. The goal of our study was to examine the efficacy of antidepressants (representing different pharmacological mechanisms) in FST in mice subjected to a zinc adequate (ZnA) as control and zinc deficient (ZnD) diet. To achieve our goal we simultaneously tested the activity of antidepressants in both experimental groups (control and Zn deficiency) in the FST. Similarly, we applied this experimental schedule pre587

viously and examined the efficacy of acute treatment with imipramine and escitalopram in the TST in mice [31]. We demonstrated the reduction in the efficacy of these antidepressants in the activity in the ZnD group in this test. In this case the interpretation of the data was quite simple since there were no significant alterations in immobility time between the control and ZnD groups treated with the vehicle. The final conclusion of this study was that zinc deficiency induced a subsensitivity in efficacy to antidepressants in the TST (antidepressants’ resistance) [31]. A similar conclusion was drawn by Tassabehji et al. [64] from a study using the rat deficiency model. Three weeks of zinc deficiency did not alter immobility time in the FST, and fluoxetine did not evoke an antidepressantlike effect in these zinc-deficient rats. In the present study, however, there were significant (although modest, by 8%) enhancements in the immobility time between control and ZnD groups treated with the vehicle. Thus, the data interpretation is much more complicated. Comparisons between control and ZnD groups treated with antidepressants indicated a similar immobility time in the case of imipramine and reboxetine, and a statistically higher immobility time in the cases of escitalopram and bupropion. How should the data be interpreted/analyzed whilst keeping in mind the higher immobility time induced by the vehicle in ZnD? From a statistical point of view, a low (by 8%) but significant increase in immobility time by the vehicle in ZnD indicates that the effect (insignificant) of imipramine (12% increase) and reboxetine (8% increase) is enhanced by zinc deficiency. On the other side, the effect (significant) of escitalopram (17% increase) and bupropion (22% increase) is similar to the effect of the vehicle in the ZnD group. However, intuition, in spite of statistics, suggests that the effect of bupropion (and maybe also escitalopram) is reduced, while the effect of imipramine and reboxetine is similar in the ZnD group. This equivocal interpretation should be cleared by the other measurement(s) of depression-like behavior of animals subjected to a zinc deficient diet. Nevertheless, since our study observed a better response for imipramine (12% between the control and zinc deficient diet) and reboxetine (8% between the control and zinc deficient diet), this may indicate that activation of noradrenergic transmission reverses depression-like behavior induced by a zinc deficient diet. The responsiveness to serotonergic antidepressants (escitalopram) and dopa-

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minergic (bupropion) is weaker, which is similar to the suggestion by other authors [64]. However, these results are in disagreement with the finding of the involvement of the serotonergic and dopaminergic system in the antidepressant action of zinc. Szewczyk et al. [56] showed that the antidepressant-like effect of zinc was significantly antagonized by pretreatment with an inhibitor of serotonin synthesis, p-chlorophenylalanine (pCPA), 5HT-2 receptor antagonist, ritanserin or 5HT-1 receptor antagonist, WAY 1006335. The authors also observed an increase in the swimming, but not climbing parameter of the rat FST after zinc administration, which indicates involvement of the serotonin pathway [56]. Moreover, joint administration of zinc and fluoxetine, paroxetine, citalopram or bupropion (all in ineffective doses), yet not reboxetine, significantly decreased the immobility time in the FST and TST [10, 56]. Furthermore, there is strong evidence also suggesting the involvement of the dopaminergic system in the antidepressant properties of zinc. Both zinc [42] and bupropion [11, 12] modulates the L-arginine-nitric oxide pathway. However, the effects mentioned above were observed in healthy organisms and zinc was administrated as treatment. In the present study, mice received antidepressants under zinc deficient conditions, causing depression-like behavior. Based on this data it is possible that serotonergic and dopaminergic neurotransmission is involved in the antidepressant-like properties of zinc in healthy organisms. Nonetheless, a better response under zinc deficient condition can be obtained by the noradrenergic system. Depressive-like behavior observed in zinc deficiency is probably caused by an alteration in glutamatergic neurotransmission. The glutamate hypothesis of depression was proposed by Skolnick et al. [48] and modified later [47, 49]. Based on the antidepressantlike activity of NMDA/glutamate receptor antagonists a link between hyperactivity of the glutamate system and depression was indicated [19, 48, 49, 53, 65]. Additionally, glutamate receptor antagonists or its modulators showed antidepressant properties in treatment refractory depression [1, 9, 13, 14, 27, 28, 41, 44, 71, 72]. Hyperactivity of the glutamatergic system was also observed in animals after both a 2- [63] and 4-week [58, 61] zinc deficient diet. Extracellular glutamate accumulation in the hippocampus is potentiated by glucocorticoids [59], the main hormones of the hypothalamic-pituitary-adrenal (HPA) axis. It is thought that zinc deficiency can cause alterations of

Zinc deficiency alters responsiveness to antidepressants

the HPA axis in depression [30, 60, 66, 68]. During zinc deficiency, chronic alteration of corticosterone secretion and a decrease in synaptic zinc levels seems to be linked to the excitability of glutamatergic neurons in the hippocampus [58, 60]. In the present study, mice with zinc deficiency treated chronically with different antidepressants exhibit an increased serum corticosterone concentration when compared to control animals treated with the same drug. Cousins et al. [8] described that the main stress hormones, glucocorticoids, increase liver metallothionein and reduce serum zinc. This may be related to an increased activity of interleukin-6 (IL-6), which exerts direct effects on the central nervous system by stimulating the HPA axis activity [39]. Cytokines have been shown to stimulate the expression and release of the corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH), as well as cortisol; all of which have been found to be elevated in patients with depression. IL-6 might change the negative feedback, resulting in the down-regulation of corticosteroid receptors [53]. It was found that patients suffering from depression exhibit increased inflammatory cytokines, such as IL-6, which positively correlated with plasma cortisol [25]. Despite the decreased cortisol/corticosterone level after antidepressants, we received opposite results under zinc deficient conditions in our study, and this may result from the elevated IL-6 observed in zinc deficiency [23]. According to Pennix et al. [39], these direct effects may lead to neurochemical and behavioral changes that may induce depression. This was observed in both our previous study [30, 31] and the present study measured by both the FST and TST. In the present study, a 6-week zinc-deficient diet induced reduction in the serum zinc level, which is similar to other data [30]. While there were no differences in the serum zinc level between groups ZnA and ZnD treated with all examined antidepressants, this indicated that there is normalization by antidepressants in this measure. Previously, we demonstrated a dose-related effect of imipramine on serum zinc concentrations in rats (30 mg/kg no effect, 15 mg/kg reduction) [7, 33]. Citalopram, however, increased serum zinc in rats [33]. Chronic unpredictable stress (similar to zinc deficiency) induces a reduction in serum zinc concentration and this effect was antagonized by chronic treatment with imipramine plus zinc [7]. Thus, the value of serum zinc as a correlation to antidepressant-like behavior needs further studies.

S³u¿ewska et al. [50] described that fluoxetine administration to depressed patients reversed pathologically elevated IL-6. Reduction in IL-6 concentration may decrease activity of the HPA axis and reduce the metallothionein level. This may be the mechanism responsible for the increased serum zinc level after antidepressants in zinc deficiency, but it needs further study. It is well known that antidepressants prevent enhancement of glutamate release induced by stress [32]. Probably, decreased glucocorticoid levels inhibit excitation-induced glutamate and this may also explain the normalization of serum zinc concentration after each antidepressant. Normalization of zinc after successful antidepressant treatment was also observed in the clinical study [24, 46]. In the research, the patients’ non-response to conventional antidepressants showed a lower concentration of zinc (by 14%). The same authors published data exhibiting that zinc supplementation augments the efficacy of imipramine in non-response patients. TRD patients receiving zinc supplementation (to antidepressant therapy) significantly reduced depression scores, which suggests a link between drug resistance and disturbances in glutamatergic transmission [45]. Low serum zinc concentration was proposed as a state marker of depression (particularly treatment resistant) [26, 46]. Our research shows that a zinc deficient diet alters the efficacy of antidepressants.

Conclusions The present results indicate that zinc deficiency alters the efficacy of antidepressants which may be involved in treatment-resistant depression.

Acknowledgments:

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