Age-dependent decline in hippocampal neurogenesis is not altered by chronic treatment with fluoxetine

Ageing abolishes the effects of fluoxetine on neurogenesis

Depression constitutes a widespread condition observed in elderly patients. Recently, it was found that several drugs employed in therapies against depression stimulate hippocampal neurogenesis in young rodents and nonhuman primates. As the rate of neurogenesis is dramatically reduced during ageing, we examined the influences of ageing on neurogenic actions of antidepressants. We tested the impact of fluoxetine, a broadly used antidepressant, on hippocampal neurogenesis in mice of three different age groups (100, 200 and over 400 days of age). Proliferation and survival rate of newly generated cells, as well as the percentage of cells that acquired a neuronal phenotype were analyzed in the hippocampus of mice that received fluoxetine daily in a chronic manner. Surprisingly, the action of fluoxetine on neurogenesis was decreasing as a function of age and was only significant in young animals. Hence, fluoxetine increased survival and the frequency of neuronal marker expression in newly generated cells of the hippocampus in the young adult group (that is 100 days of age) only. No significant effects on neurogenesis could be detected in fluoxetine-treated adult and elderly mice (200 and over 400 days of age). The data indicate that the action of fluoxetine on neurogenesis is highly dependent on the age of the treated individual. Although the function of neurogenesis in the clinical manifestation of depression is currently a matter of speculation, this study clearly shows that the therapeutic effects of antidepressants in elderly patients are not mediated by neurogenesis modulation.

Prozac during puberty: distinctive effects on neurogenesis as a function of age and sex

Neurogenesis is a possible substrate through which antidepressants alleviate symptoms of depression. In adult male rodents and primates, chronic treatment with fluoxetine increases neurogenesis in the hippocampal formation. Little is known about the effects of the antidepressant on neurogenesis during puberty or in female animals at any age. Therefore we examined the effects of chronic fluoxetine treatment on cell proliferation and survival in male and female rats during puberty and adulthood. Adult and peri-pubescent male and female rats were treated chronically with fluoxetine (Prozac, 5 mg/kg) or saline. Subsequently rats received a single injection of 5-bromo-2'-deoxyuridine (BrdU; 200 mg/kg) to label DNA synthesis. Rats were sacrificed 2 h, 24 h, or 28 days after BrdU injection to examine cell proliferation, survival and cell fate. Fluoxetine increased cell proliferation in adult male rats but not in peri-pubescent males or female rats of any age or stage of the estrous cycle. Treatment did not alter the number of surviving cells in the male hippocampus but decreased survival in the female hippocampus. Thus, fluoxetine has distinctive effects on neurogenesis as a function of age and sex. Circulating levels of the stress hormone corticosterone were also examined. Treatment of female rats with fluoxetine during puberty decreased circulating levels of corticosterone in adults, even in the absence of the drug suggesting disruption of maturation of the hypothalamic-pituitary-adrenal axis.

Increased neurogenesis after hypoxic-ischemic encephalopathy in humans is age related

The leading cause of morbidity and mortality after successful resuscitation is hypoxic-ischemic encephalopathy (HIE), which results in neuronal loss within the neocortex and the hippocampal formation. This study focuses on the impact of HIE on adult neurogenesis in the human hippocampal dentate gyrus as a potential intrinsic regenerative mechanism in response to neuronal damage. Brain sections of 22 autopsy cases with HIE and of 19 age-matched controls without neuropathological abnormalities were investigated by means of immunohistochemistry. The densities of immature granule cells during axon guidance and outgrowth (assessed by TUC-4 immunohistochemistry) and of young calretinin-expressing postmitotic neurons were increased in the granule cell layer of cases who had suffered from HIE (P = 0.0002 and P = 0.0001, respectively). Similarly, the density of apoptotic granule cells, as detected by in situ tailing and morphological criteria, was increased in HIE (P = 0.014). In cases with HIE, the increase in the density of TUC-4-labeled cells inversely correlated with age (P = 0.027). In contrast, neither the density of proliferating nor that of apoptotic cells was substantially influenced by age within the control group. Taken together, both an increase in adult neurogenesis and in neuronal apoptosis was observed in the human dentate gyrus in response to HIE. The data suggest a decrease of adult neurogenesis in older-aged cases. Whether neurogenesis can contribute to recovery after HIE remains to be determined. The stimulation of adult neurogenesis may be less efficient in older victims of HIE.

Exercise and the brain: something to chew on

Evidence is accumulating that exercise has profound benefits for brain function. Physical activity improves learning and memory in humans and animals. Moreover, an active lifestyle might prevent or delay loss of cognitive function with aging or neurodegenerative disease. Recent research indicates that the effects of exercise on the brain can be enhanced by concurrent consumption of natural products such as omega fatty acids or plant polyphenols. The potential synergy between diet and exercise could involve common cellular pathways important for neurogenesis, cell survival, synaptic plasticity and vascular function. Optimal maintenance of brain health might depend on exercise and intake of natural products.

The 5-HT7 receptor is involved in allocentric spatial memory information processing

The hippocampus has been implicated in aspects of spatial memory. Its ability to generate new neurons has been suggested to play a role in memory formation. Hippocampal serotonin (5-HT) neurotransmission has also been proposed as a contributor to memory processing. Studies have shown that the 5-HT(7) receptor is present in the hippocampus in relatively high abundance. Thus the aim of the present study was to investigate the possible role of the 5-HT(7) receptor in spatial memory using 5-HT(7) receptor-deficient mice (5-HT(7)(-/-)). A hippocampus-associated spatial memory deficit in 5-HT(7)(-/-) mice was demonstrated using a novel location/novel object test. A similar reduction in novel location exploration was observed in C57BL/6J mice treated with the selective 5-HT(7) receptor antagonist SB-269970. These findings prompted an extended analysis using the Barnes maze demonstrating that 5-HT(7)(-/-) mice were less efficient in accommodating to changes in spatial arrangement than 5-HT(7)(+/+) mice. 5-HT(7)(-/-) mice had specific impairments in memory compilation required for resolving spatial tasks, which resulted in impaired allocentric spatial memory whereas egocentric spatial memory remained intact after the mice were forced to switch back from striatum-dependent egocentric to hippocampus-dependent allocentric memory. To further investigate the physiological bases underlining these behaviors we compared hippocampal neurogenesis in 5-HT(7)(+/+) and 5-HT(7)(-/-) mice employing BrdU immunohistochemistry. The rate of cell proliferation in the dentate gyrus was identical in the two genotypes. From the current data we conclude that the 5-HT(7)(-/-) mice performed by remembering a simple sequence of actions that resulted in successfully locating a hidden target in a static environment.