AGING-ASSOCIATED COGNITIVE DECLINE IS REVERSED BY D-SERINE SUPPLEMENTATION

Abstract

Brain aging is a natural process that involves structural and functional changes that lead to cognitive decline, even in healthy subjects. This detriment has been associated with N-methyl-D-aspartate receptor (NMDAR) hypofunction due to a reduction in the brain levels of D-serine, the endogenous NMDAR co-agonist. However, it is not clear if D-serine supplementation could be used as an intervention to reduce or reverse age-related brain alterations. In the present work, we aimed to analyze the D-serine effect on aging-associated alterations in cellular and large-scale brain systems that could support cognitive flexibility in rats. We found that D-serine supplementation reverts the age-related decline in cognitive flexibility, frontal dendritic spine density, and partially restored large-scale functional connectivity without inducing nephrotoxicity; instead, D-serine restored the thickness of the renal epithelial cells that were affected by age. Our results suggest that D-serine could be used as a therapeutic target to reverse age-related brain alterations.SIGNIFICANT STATEMENTAge-related behavioral changes in cognitive performance occur as a physiological process of aging. Then, it is important to explore possible therapeutics to decrease, retard or reverse aging effects on the brain. NMDA receptor hypofunction contributes to the aging-associated cognitive decline. In the aged brain, there is a reduction in the brain levels of the NMDAR co-agonist, D-Serine. However, it is unclear if chronic D-serine supplementation could revert the age-detriment in brain functions. Our results show that D-serine supplementation reverts the age-associated decrease in cognitive flexibility, functional brain connectivity, and neuronal morphology. Our findings raise the possibility that restoring the brain levels of D-serine could be used as a therapeutic target to recover brain alterations associated with aging.

Keywords: Aging; Cognitive Flexibility; D-serine; Functional brain connectivity; fMRI.

Figure 1.

Aging-associated cognitive flexibility decline is prevented by D-serine supplementation. A, Behavioral task design during training sessions where the reward is delivered by pressing the lever ipsilateral (I) to the light. B, time course of correct trials during the training sessions. C, During the cognitive flexibility (CF) test the reward is delivered after pressing the lever contralateral to the light. D, Middle-aged and aged rats showed a significant increase in the number of perseverative errors compared with young rats. E, Middle-aged and aged rats supplemented with D-serine (300 mg/kg of weight) had significantly less perseverative errors during the evaluation of cognitive flexibility in comparison to those receiving vehicles. G, Young rats receiving D-serine (300 mg/kg of weight) did not show significant differences when compared with young control rats. F, Middle-aged and aged rats supplemented with a lower dose of D-serine (30 mg/kg of weight) had no differences in the perseverative errors during the evaluation of cognitive flexibility in comparison to those receiving vehicles. One-way ANOVA for multiple comparisons. Two-tailed t test for comparison between two groups; *p ≤ 0.05.

Figure 2.

Decreased brain functional connectivity by aging is restored by D-serine. A, Left, Coronal slices and axial view of the rat templates overlayed with five ROIs taken from Tohoku University and WHS atlases. Dorsolateral orbital cortex (ODL), frontal association cortex (FrA), cingulate cortex (Cing), striatum (STR), retrosplenial cortex (RScx). A brain network affected by age was identified using NBS; this network comprises FrA, Cing, and RScx cortices (right). Middle-aged (B) and aged rats (C) had less functional connectivity between FrA-RScx and FrA-Cing, respectively, compared with young rats. Middle-aged (D) and aged rats (E) that received D-serine restore the functional connectivity between FrA-RScx and FrA-Cing, respectively. Data are expressed as median ± IC 10% and 90%; *p ≤ 0.05.

Figure 3.

D-serine makes functional brain connectivity relevant for cognitive flexibility performance. A, Young, middle-aged and aged rats receiving vehicle did not show a correlation between the network functional connectivity and the perseverative errors. B, Chronic D-serine supplementation to middle-aged and aged rats had a negative correlation between the network functional connectivity and the perseverative errors. Middle age + D-serine, r² = 0.93 p = 0.0068; aged + D-serine, r² = 0.070 p = 0.0023.

Figure 4.

D-serine increases frontal neuron spines without affecting dendritic features. A, 3D reconstruction of a typical frontal neuron of an aged rat supplemented with D-serine. Morphologic features of frontal neuron dendrites of middle-aged (B) and aged rats (C) receiving vehicle or D-serine. D, Representative image and 3D reconstruction of dendritic segments of middle-aged and aged rats receiving vehicle (left) and supplemented with D-serine (right). Orange arrowheads indicate spines. Scale bar: 5 μm. Population density of frontal spines of middle-aged (E) and aged rats (F) in control conditions and supplemented with D-serine; two-tailed t test, *p ≤ 0.05.

Figure 5.

D-serine did not affect attentional components decreased with age. A, Behavioral task design during training and attention test sessions. A correct trial was counted when the rat pressed the level ipsilateral to the light. Reaction time was determined as the time occurring between the light was switched off and the ipsilateral lever was pressed. B, The number of correct trials significantly decreased (left) and reaction time significant increased (right) in middle-aged and aged rats compared with young rats. C, Correct trials (left) and reaction time (right) were not modified by D-serine supplementation in middle-aged and aged rats. Data are expressed as median ± IC 10% and 90%. One-way ANOVA; * p ≤ 0.05.

Figure 6.

D-serine increased the cell size of proximal tubules in the kidney. A, Representative images of kidney proximal straight tubules from young, middle-aged, aged, and aged rats supplemented with D-serine. The tissue is stained with Masson’s trichrome. Orange arrows indicate the diameter of the cells analyzed. B, Middle-aged and aged rats show a significant decrease in the size of tubular cells compared with young rats. Cell size was normalized in relation to the mean of young cells. C, D-serine increased the diameter of tubular cells in the proximal straight tubule. Data are expressed as mean + maximum and minimum values. Two-tailed t test; *p ≤ 0.05. Scale bar: 50 μm.