The Antidepressant-Like Effect of Lactate in an Animal Model of Menopausal Depression

Abstract

Background: This study aimed to investigate the antidepressant-like effect of lactate and elucidate its mechanisms in ovariectomized rats with repeated stress.

Methods: Two experiments were conducted on female rats in which all groups, except normal, were ovariectomized and underwent immobilization for 14 days. Lactate was administered orally (100, 250, and 500 mg/kg) for 14 consecutive days, and the rats' cutaneous body temperature was measured during the same period. Depression-like behavior in rats was assessed by the tail suspension test (TST) and forced swimming test (FST). Furthermore, enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry were conducted to evaluate the changes that occurred in the neurotransmitter levels and activity.

Results: The lactate 100 and 250 groups had reduced time spent immobile in TST and FST and decreased peripheral body temperature. In ELISA tests, the lactate 250 group expressed elevated levels of serotonin and dopamine in many brain areas. Tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), and protein kinase C (PKC) immunoreactive cells showed increased density and cell counts in lactate administered groups.

Conclusion: Results indicated that lactate has an antidepressant effect that is achieved by activation of PKC and upregulation of TH and TPH expression, which eventually leads to enhanced serotonin and dopamine levels in the menopausal rat's brain.

Keywords: depression; lactate; neurotransmitters; ovariectomy; protein kinase C; serotonin and dopamine; tryptophan hydroxylase; tyrosine hydroxylase.

Figure 1

An experimental schedule for ovariectomy, oral lactate administration, and behavioral tests. Sprague–Dawley (SD) female rats were received and allowed to acclimatize themselves for one week. Consequently, all rats, except the normal group, were subjected to ovariectomy and allowed one week for post-surgery recovery. Therefore, from the 1^st day of treatment for two continuous weeks, lactate groups received their doses, and then, after 30 min, immobilization began. During the lactate receiving period, cutaneous body temperature was measured on the 2nd, 5th, 12th, and last day of treatment. All rats were subjected to a tail suspension test (TST) and forced swimming test (FST) on the 29th and 30th day, respectively. After the behavioral testing, the rats were sacrificed and brain tissues were immediately collected for analysis.

Figure 2

Effect of oral lactate administration on ovariectomized rats with stress in tail suspension tests (TST). Oral lactate administration (100 and 250) and estradiol decreased immobility time significantly compared to the control group. Values are presented as mean ± standard error of the mean (SEM). *** show the difference between the normal and control groups, while ### show significant differences between the control group and treatment groups. *** and ### p < 0.001. The p value of the test was < 0.001 by one-way ANOVA, followed by a post hoc LSD test.

Figure 3

Oral lactate administration reduced immobility time in FST following ovariectomy and repeated stress. One-way ANOVA comparisons revealed a remarkable decrease in the time spent immobile in forced swimming tests in lactate treated groups (100 and 250) and the estradiol group compared to control rats. Values are presented as mean ± SEM. *** show differences between the normal and control groups. ### show significant differences between the control group and treatment groups. *** and ### p < 0.001. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test.

Figure 4

Oral lactate administration induced a reduction in cutaneous body temperature in rats with ovariectomy surgery (OVX) and stress. Control groups exhibited a gradual increase in cutaneous temperature; however, the estradiol and all lactate groups showed a notable decline in the cutaneous body temperature, especially in the last day of treatment. Values are presented as mean ± SEM. *, ** and *** show differences between the normal and control groups, while # and ### show significant differences between the control group and treatment groups. # p < 0.05 and ### p < 0.001. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test.

Figure 5

Effect of oral lactate administration on serotonin (5-HT) and dopamine (DA) concentrations in different brain areas in ovariectomized rats with stress. The levels of 5-HT concentration (nanogram per milligram) in the hypothalamus (a), hippocampus (b), and prefrontal cortex (c) changed after repeated lactate administration. The levels of DA concentration (nanogram per milligram) in the lactate 250 group in the hypothalamus (d), hippocampus (e), and prefrontal cortex (f) (n = 6). Values are presented as mean ± SEM. * show differences between the normal and control groups, while ### show significant differences between the control and treatment groups. ### p < 0.001. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test.

Figure 6

Tyrosine hydroxylase positive (TH+) cell expression in the locus coeruleus (LC) after oral lactate administration. (a) Oral lactate administration increased the mean TH- immunoreactive (TH-IR) cell density in ovariectomized rat with repeated stress in comparison to the control group. Values are presented as mean ± SEM. ** show differences between the normal and control groups. ### show significant differences between the control group and treatment group. ### p < 0.001. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test (n = 6). (b) Representative images of TH+ cell expression in all groups in the LC region (magnification 100×). TH+ cell density was considerably increased in both lactate treated groups; however, it was denser in the lactate 100 group compared to the lactate 250 group.

Figure 7

Oral lactate administration enhanced tryptophan hydroxylase (TPH) expression in the hypothalamic nuclei. (a) The decreased number of TPH+ cells in the control group was notably upregulated in the lactate 100 and 250 groups. (b,c) Representative images of TPH+ cell expression in the hypothalamic nuclei (the scale bars represent 100 and 50 μm, respectively). PVN = paraventricular hypothalamic nucleus, AN = anterior hypothalamic nucleus, VMH = ventromedial hypothalamic nucleus, Arc = arcuate nucleus. Values are presented as mean ± SEM. *** show differences between the normal and control groups, while # and ## show significant differences between the control group and treatment groups. # p < 0.05 and ## p < 0.01. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test (n = 6).

Figure 8

Changes occurred in TPH expression in the peduncular part of the lateral hypothalamic (PLH) following oral lactate administration. (a) The mean TPH-IR cell density was significantly increased in both the lactate 100 and 250 groups in comparison to the control group. Values are presented as mean ± SEM. *** show differences between the normal and control groups, ### show significant differences between the control group and treatment groups. ### p < 0.001. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test (n = 6). (b) Representative images of TPH+ cell expression in lateral hypothalamic region (magnification 40×).

Figure 9

Oral lactate administration regulated protein kinase C (PKC) expression in the hypothalamic area. (a) Both the lactate 100 and 250 groups significantly upregulated the mean PKC-IR cell density in the hypothalamus (n = 6). (b) Representative images of all groups for PKC+ cell expression in hypothalamic region. PKC density was markedly increased in both lactate groups compared to the control group (the scale bar represents 200 µm). Values are presented as mean ± SEM. ** show differences between the normal and control groups, while ## and ### show significant differences between the control group and treatment groups. ## p < 0.01 and ### p < 0.001. The p-value of the test was <0.001 by one-way ANOVA, followed by a post hoc LSD test.