Brain-derived neurotrophic factor (BDNF) regulates both shortterm synaptic functions and activity-dependent synaptic plasticity such as long-term potentiation. In the present study, we investigated the role of BDNF in the spatial reference and working memory in a radial arm maze test. The radial arm maze training resulted in a significant increase in the BDNF mRNA expression in the hippocampus, although the expression in the frontal cortex did not change. When spatial learning was inhibited by treatment with 7-nitroindazole, an inhibitor of brain nitric oxide synthase, the increase in the hippocampal BDNF mRNA did not occur. To clarify the causal relation between BDNF mRNA expression and spatial memory formation, we examined the effects of antisense BDNF treatment on spatial learning and memory. A continuous intracerebroventricular infusion of antisense BDNF oligonucleotide resulted in an impairment of spatial learning, although the sense oligonucleotide had no effect. Treatment with antisense, but not sense, BDNF oligonucleotide was associated with a significant reduction of BDNF mRNA and protein levels in the hippocampus. Furthermore, treatment with antisense BDNF oligonucleotide in rats, which had previously acquired spatial memory by an extensive training, impaired both reference and working memory. There were no differences in locomotor activity, food consumption, and body weight between the antisense and sense oligonucleotide-treated rats. These results suggest that BDNF plays an important role not only in the formation, but also in the retention and/or recall, of spatial memory.

It is well established that neurogenesis in the dentate gyrus slows with aging, but it is unclear whether this change is due to slowing of the cell cycle, as occurs during development, or to loss of precursor cells. In the current study, we find that the cell cycle time of granule cell precursors in middle-aged male rats is not significantly different from that in young adults. The size of the precursor pool, however, was 3-4 times smaller in the middle-aged rats, as determined using both cumulative bromodeoxyuridine (BrdU) labeling as well as labeling with the endogenous marker of cell proliferation, proliferating cell nuclear antigen (PCNA). Loss of precursor cells was much greater in the granule cell layer than in the hilus, suggesting that dividing cells in the hilus belong to a distinct population, most likely glial progenitors, that are less affected by aging than neuronal precursors. BrdU-labeled precursor cells and young neurons, labeled with doublecortin, appeared to be lost equally from rostral and caudal, as well as suprapyramidal and infrapyramidal, subregions of the granule cell layer. However, doublecortin staining did show large parts of the caudal granule cell layer with few if any young neurons at both ages. Taken together, these findings indicate that precursor cells are not distributed evenly within the dentate gyrus in adulthood but that precursors are lost from throughout the dentate gyrus in old age with no concomitant change in the cell cycle time.

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Amyloid β‐peptide (Aβ) plays a critical role in the development of Alzheimer's disease. However, the molecular mechanisms of Aβ‐induced brain damage in vivo remain to be elucidated. Here, we investigated whether overproduction of nitric oxide (NO) catalyzed by inducible NO synthase (iNOS) is involved in Aβ‐induced brain dysfunction. Chronic intracerebroventricular infusion of Aβ1‐40 induced iNOS mRNA expression in the hippocampus on days 3 and 5 after the infusion. An accumulation of NO metabolites was observed, and the peak correlated with expression of iNOS mRNA. Measurement of NOS activities revealed an increase in Ca2+‐independent, but not Ca2+‐dependent, activity. Immunohistochemistry identified numerous iNOS‐immunoreactive microglia and astrocytes in the dentate gyrus and to a lesser extent in the CA1 subfield of the hippocampus. Daily treatment with the iNOS inhibitor aminoguanidine (AG, 100 mg/kg/day, i.p.) or S‐methylisothiourea (10 mg/kg/day, i.p.) during Aβ infusion prevented an impairment of nicotine‐evoked acetylcholine release induced by Aβ, whereas the neuronal NOS inhibitor 7‐nitroindazole (30 mg/kg/day, i.p.) had no effect. Daily treatment with AG also ameliorated the impairment of spatial learning of Aβ‐infused rats in a radial arm maze. Our findings suggest that overproduction of NO catalyzed by iNOS is responsible for Aβ‐induced brain dysfunction.