Reduced serine racemase expression contributes to age-related deficits in hippocampal cognitive function.
To gain insight into the contribution of d-serine to impaired cognitive aging, we compared the metabolic pathway and content of the amino acid as well as d-serine-dependent synaptic transmission and plasticity in the hippocampus of young and old rats of the Wistar and Lou/C/Jall strains. Wistar rats display cognitive impairments with aging that are not found in the latter strain, which is therefore considered a model of healthy aging. Both mRNA and protein levels of serine racemase, the d-serine synthesizing enzyme, were decreased in the hippocampus but not in the cerebral cortex or cerebellum of aged Wistar rats, whereas the expression of d-amino acid oxidase, which degrades the amino acid, was not affected. Consequently, hippocampal levels of endogenous d-serine were significantly lower. In contrast, serine racemase expression and d-serine levels were not altered in the hippocampus of aged Lou/C/Jall rats. Ex vivo electrophysiological recordings in hippocampal slices showed a marked reduction in N-methyl-d-aspartate-receptor (NMDA-R)-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) in the CA1 area of aged Wistar rats, which were restored by exogenous d-serine. In contrast, NMDA-R activation, LTP induction and responses to d-serine were not altered in aged Lou/C/Jall rats. These results further strengthen the notion that the serine racemase-dependent pathway is a prime target of hippocampus-dependent cognitive deficits with aging. Understanding the processes that specifically affect serine racemase during aging could thus provide key insights into the treatment of memory deficits in the elderly.
Turpin
F R
FR
Centre de Psychiatrie et Neurosciences, INSERM, UMR 894, Université Paris Descartes, Faculté de Médecine René Descartes, Paris, F-75014, France. fabrice.turpin@inserm.fr
Potier
B
B
Dulong
J R
JR
Sinet
P-M
PM
Alliot
J
J
Oliet
S H R
SH
Dutar
P
P
Epelbaum
J
J
Mothet
J-P
JP
Billard
J-M
JM
D000375:Aging / Q000235:genetics / Q000502:physiology*; D000818:Animals; D003072:Cognition Disorders / Q000201:enzymology / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic; D006624:Hippocampus / Q000201:enzymology / Q000502:physiology*; D008297:Male; D008569:Memory Disorders / Q000201:enzymology* / Q000235:genetics / Q000503:physiopathology*; D019998:Racemases and Epimerases / Q000037:antagonists & inhibitors* / Q000096:biosynthesis* / Q000235:genetics; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D000099300:Serine Racemase
D019998:Racemases and Epimerases; D000099300:Serine Racemase