Fragile X syndrome, the most frequent form of hereditary mental retardation, is due to a mutation of the fragile X mental retardation 1 (FMR1) gene on the X chromosome. Like fragile X patients, FMR1-knockout (FMR1-KO) mice lack the normal fragile X mental retardation protein (FMRP) and show both cognitive alterations and an immature neuronal morphology. We reared FMR1-KO mice in a C57BL͞6 background in enriched environmental conditions to examine the possibility that experience-dependent stimulation alleviates their behavioral and neuronal abnormalities. FMR1-KO mice kept in standard cages were hyperactive, displayed an altered pattern of open field exploration, and did not show habituation. Quantitative morphological analyses revealed a reduction in basal dendrite length and branching together with more immatureappearing spines along apical dendrites of layer five pyramidal neurons in the visual cortex. Enrichment largely rescued these behavioral and neuronal abnormalities while increasing ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit 1 (GluR1) levels in both genotypes. Enrichment did not, however, affect FMRP levels in the WT mice. These data suggest that FMRP-independent pathways activating glutamatergic signaling are preserved in FMR1-KO mice and that they can be elicited by environmental stimulation.fragile X mental retardation protein ͉ mental retardation ͉ FMR1 gene ͉ AMPA receptor ͉ dendritic spines S everal genes associated with mental retardation have been mapped on the X chromosome and, among them is the fragile X mental retardation 1 (FMR1) gene. The fragile X mental retardation protein (FMRP) absence or mutation is responsible for the fragile X syndrome (FXS), which is the most common form of inherited mental retardation. Most of the individuals affected carry a trinucleotide repeat that, after methylation, leads to transcriptional silencing of the FMR1 gene (1). Patients with the FXS do not express FMRP and exhibit phenotypic traits ranging from severe (IQ 20) to moderate (IQ 60) mental retardation, defective attention, autistic behavior, and physical features including an elongated face, large ears, joint laxity, and macroorchidism (2-5).FMR1 is highly conserved between human and mouse, with a nucleotide and amino acid identity of 95% and 97%, respectively (6). The expression pattern of mouse FMR1 is similar to its human counterpart in both tissue specificity and timing (7). Interestingly, FMR1-knockout (FMR1-KO) mice, the mouse model for the FXS, lack the normal FMRP and show macroorchidism, hyperactivity, and mild learning deficits (8, 9) reminiscent of the human syndrome.One common brain feature of fragile X patients and of the mouse model for the syndrome is the presence of long and thin immature dendritic spines indicative of defective pruning during development (10)(11)(12)(13)(14). At the molecular level, it has been shown that protein synthesis triggered by the type I metabotropic glutamate receptor (mGluR1) agonist dihydroxyphenylglycine is dramati...

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The aim of this study was to investigate the role played by intra-accumbens N-methyl-D-aspartate (NMDA) receptors in spatial information encoding. For this purpose, the effect of local administration of both competitive (AP-5) and non-competitive (MK-801) NMDA antagonists was assessed in a task designed to estimate the ability of rodents to encode spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). The results show that both doses of MK-801 (0.15 and 0.3 microg/side) induced a selective impairment in the capability of mice to detect spatial novelty. A similar effect was obtained by injecting the low dose of the competitive antagonist AP-5 (0.1 microg/side), whereas the high dose (0.15 microg/side) abolished detection of both spatial and object novelty. Taken together, these results show that intra-accumbens injections of low doses of competitive and non-competitive NMDA antagonists can produce selective deficits in processing spatial information resembling those observed after hippocampal damage. Moreover, the fact that pharmacological treatments spare memory processes involved in habituation suggests that NMDA antagonists may interfere with the formation of spatial representations rather than producing memory deficits per se.