Heather A. Cameron

Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus.

Knowing the rate of addition of new granule cells to the adult dentate gyrus is critical to understanding the function of adult neurogenesis. Despite the large number of studies of neurogenesis in the adult dentate gyrus, basic questions about the magnitude of this phenomenon have never been addressed. The S‐phase marker bromodeoxyuridine (BrdU) has been extensively used in recent studies of adult neurogenesis, but it has been carefully tested only in the embryonic brain. Here, we show that a high dose of BrdU (300 mg/kg) is a specific, quantitative, and nontoxic marker of dividing cells in the adult rat dentate gyrus, whereas lower doses label only a fraction of the S‐phase cells. By using this high dose of BrdU along with a second S‐phase marker, [3H]thymidine, we found that young adult rats have 9,400 dividing cells proliferating with a cell cycle time of 25 hours, which would generate 9,000 new cells each day, or more than 250,000 per month. Within 5–12 days of BrdU injection, a substantial pool of immature granule neurons, 50% of all BrdU‐labeled cells in the dentate gyrus, could be identified with neuron‐specific antibodies TuJ1 and TUC‐4. This number of new granule neurons generated each month is 6% of the total size of the granule cell population and 30–60% of the size of the afferent and efferent populations (West et al. [ 1991 ] Anat Rec 231:482–497; Mulders et al. [ 1997 ] J Comp Neurol 385:83–94). The large number of the adult‐generated granule cells supports the idea that these new neurons play an important role in hippocampal function. J. Comp. Neurol. 435:406–417, 2001. Published 2001 Wiley‐Liss, Inc.

Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat

In order to determine whether newly born cells in the dentate gyrus of the adult rat express the neuronal marker, neuron-specific enolase, or the glial marker, glial fibrillary acidic protein, we performed combined immunohistochemistry and autoradiography on brains from adult rats perfused at various times ranging from 1 h to four weeks following [3H]thymidine administration. Light-microscopic examination revealed a negligible number of [3H]thymidine-labeled cells showing neuron-specific enolase immunoreactivity during mitosis. However, by two weeks after [3H]thymidine administration, a significant increase in the density of [3H]thymidine-labeled neuron-specific enolase-immunoreactive cells was detected. Three weeks following [3H]thymidine injection the majority of [3H]thymidine-labeled cells (> 70%) were immunoreactive for the neuronal marker. At the four-week time-point, [3H]thymidine-labeled neuron-specific enolase-immunoreactive cells were indistinguishable from neighboring granule cells. In contrast, glial fibrillary acidic protein immunoreactivity was observed in a small but significant number of [3H]thymidine cells at the 1-h time-point and the proportion of labeled cells that were immunoreactive for this cell marker did not increase with time. [3H]Thymidine-labeled cells that were immunoreactive for glial fibrillary acidic protein typically showed morphologic characteristics of radial glia at all time-points. At the 1-h time-point, the majority of [3H]thymidine-labeled cells were observed in the hilus (> 60%) with the remainder being located in the granule cell layer. However, with a four-week survival-time most [3H]thymidine-labeled cells (> 85%) were located in the granule cell layer. The majority of newly born cells in the adult dentate gyrus differentiate into neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult neurogenesis is regulated by adrenal steroids in the dentate gyrus

The dentate gyrus of the rat produces new granule neurons well into adulthood. In the adult, newly born granule neurons migrate from the hilus to the granule cell layer, receive synaptic input, extend axons into the mossy fiber pathway, and express a neuronal marker. No previous studies have identified factors that regulate neuronal birth in the adult dentate gyrus. In order to determine whether glucocorticoids control neurogenesis in the adult dentate gyrus, the effects of adrenal steroid manipulations on neuronal birth were assessed using [3H]thymidine autoradiography and immunohistochemistry for the neuronal marker neuron specific enolase. Acute treatment with corticosterone produced a significant decrease in the density of [3H]thymidine-labeled cells in the hilus of the dentate gyrus. In contrast, removal of endogenous adrenal steroids stimulated increased neuronal birth; adrenalectomy resulted in a significant increase in the number of neuron specific enolase-immunoreactive [3H]thymidine labeled cells in the granule cell layer compared to sham operation. Replacement of corticosterone to adrenalectomized rats after [3H]thymidine injection did not substantially alter the increase in neurogenesis observed following adrenalectomy, even though this replacement protects cells from adrenalectomy-induced cell death. These results indicate that the rate of neurogenesis in the dentate gyrus of the adult rat is dependent upon the levels of circulating adrenal steroids.

Adult hippocampal neurogenesis buffers stress responses and depressive behaviour

Glucocorticoids are released in response to stressful experiences and serve many beneficial homeostatic functions. However, dysregulation of glucocorticoids is associated with cognitive impairments and depressive illness. In the hippocampus, a brain region densely populated with receptors for stress hormones, stress and glucocorticoids strongly inhibit adult neurogenesis. Decreased neurogenesis has been implicated in the pathogenesis of anxiety and depression, but direct evidence for this role is lacking. Here we show that adult-born hippocampal neurons are required for normal expression of the endocrine and behavioural components of the stress response. Using either transgenic or radiation methods to inhibit adult neurogenesis specifically, we find that glucocorticoid levels are slower to recover after moderate stress and are less suppressed by dexamethasone in neurogenesis-deficient mice than intact mice, consistent with a role for the hippocampus in regulation of the hypothalamic–pituitary–adrenal (HPA) axis. Relative to controls, neurogenesis-deficient mice also showed increased food avoidance in a novel environment after acute stress, increased behavioural despair in the forced swim test, and decreased sucrose preference, a measure of anhedonia. These findings identify a small subset of neurons within the dentate gyrus that are critical for hippocampal negative control of the HPA axis and support a direct role for adult neurogenesis in depressive illness.

Short-term and long-term survival of new neurons in the rat dentate gyrus.

New neurons continue to be generated in the dentate gyrus throughout adulthood. Previous studies have shown that a significant proportion of new granule cells labeled with the thymidine analogue bromodeoxyuridine (BrdU) are lost from the adult dentate gyrus within 2 weeks. How long this loss continues and the extent to which it represents cell death, as opposed to dilution of label, is unclear. To address these questions, adult rats were injected with BrdU, and BrdU labeling in the dentate gyrus was compared at several survival time points. Double labeling with BrdU and the cell cycle marker Ki‐67 showed that BrdU is detectable for up to 4 days in some cells that continue to divide, indicating that any decrease in the number of BrdU‐labeled cells after 4 days is likely to reflect cell death rather than BrdU dilution. Death of new cells in the granule cell layer occurred at a steady rate between 6 and 28 days after labeling, resulting in loss of 50% of BrdU‐labeled cells over this 22‐day period. New granule cells that survived this first month lived for at least 5 additional months. In contrast, 26% of the granule cells labeled with BrdU at the peak of dentate gyrus development on postnatal day (P) 6 died between 1 and 6 months after labeling. These findings suggest that granule cells born during adulthood that become integrated into circuits and survive to maturity are very stable and may permanently replace granule cells born during development. J. Comp. Neurol. 460:563–572, 2003. Published 2003 Wiley‐Liss, Inc.

Adrenal steroids and N-methyl-D-aspartate receptor activation regulate neurogenesis in the dentate gyrus of adult rats through a common pathway

Adrenal steroids and N-methyl-D-aspartate receptor activation have both been shown to regulate the rate of proliferation of granule neuron progenitor cells in the dentate gyrus of adult rats [Cameron H. A. and Gould E. (1994) Neuroscience 61, 203-209; Cameron H. A. et al. (1995) J. Neurosci. 15, 46874692]. Parallels between the actions of these two factors suggest that they may regulate cell division through a common pathway. This hypothesis was tested by altering both of the factors simultaneously and determining whether the effects were additive. The results of this study demonstrate that alterations in N-methyl-D-aspartate receptor activation block the effects of corticosterone level on cell proliferation; N-methyl-D-aspartate blocks the adrenalectomy-induced increase in [3H]thymidine-labelled cell density in the dentate gyrus, whereas the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (MK-801) prevents the corticosterone-induced decrease in proliferating cells. This finding suggests that adrenal steroids and N-methyl-D-aspartate receptor activation regulate granule cell production in the adult rat dentate gyrus through a common pathway and that N-methyl-D-aspartate receptor activation operates downstream of corticosterone in this pathway.

New GABAergic interneurons in the adult neocortex and striatum are generated from different precursors

Ongoing neurogenesis in the adult mammalian dentate gyrus and olfactory bulb is generally accepted, but its existence in other adult brain regions is highly controversial. We labeled newly born cells in adult rats with the S-phase marker bromodeoxyuridine (BrdU) and used neuronal markers to characterize new cells at different time points after cell division. In the neocortex and striatum, we found BrdU-labeled cells that expressed each of the eight neuronal markers. Their size as well as staining for γ-aminobutyric acid (GABA), glutamic acid decarboxylase 67, calretinin and/or calbindin, suggest that new neurons in both regions are GABAergic interneurons. BrdU and doublecortin-immunoreactive (BrdU+/DCX+) cells were seen within the striatum, suggesting migration of immature neurons from the subventricular zone. Surprisingly, no DCX+ cells were found within the neocortex. NG2 immunoreactivity in some new neocortical neurons suggested that they may instead be generated from the NG2+ precursors that reside within the cortex itself.

Tianeptine attenuates stress-induced morphological changes in the hippocampus

Repeated 6-h daily restraint stress over 21 days reduces length and number of branch points of hippocampal CA3c pyramidal dendrites in the hippocampal formation of adult male rats. This effect is mimicked by daily injections of 40 mg/kg corticosterone. Daily treatment with tianeptine (15 mg/kg) prior to stress sessions or the corticosterone treatment prevented these effects of stress or corticosterone, respectively. Tianeptine treatment did not prevent the effects of stress to increase adrenal/body weight ratio, nor did it prevent the effects of stress to decrease body weight gain, indicating that its actions are not mediated solely by effects on stress-induced secretion of corticosterone. Because tianeptine is known to enhance neural uptake of serotonin, these results suggest that the serotonergic system may be involved in modulating stress and corticosterone effects on dendritic morphology.

Adult-Born Hippocampal Neurons Are More Numerous, Faster Maturing, and More Involved in Behavior in Rats than in Mice

Neurons are born throughout adulthood in the hippocampus and show enhanced plasticity compared with mature neurons. However, there are conflicting reports on whether or not young neurons contribute to performance in behavioral tasks, and there is no clear relationship between the timing of maturation of young neurons and the duration of neurogenesis reduction in studies showing behavioral deficits. We asked whether these discrepancies could reflect differences in the properties of young neurons in mice and rats. We report that young neurons in adult rats show a mature neuronal marker profile and activity-induced immediate early gene expression 1–2 weeks earlier than those in mice. They are also twice as likely to escape cell death, and are 10 times more likely to be recruited into learning circuits. This comparison holds true in two different strains of mice, both of which show high rates of neurogenesis relative to other background strains. Differences in adult neurogenesis are not limited to the hippocampus, as the density of new neocortical neurons was 5 times greater in rats than in mice. Finally, in a test of function, we find that the contribution of young neurons to fear memory is much greater in rats than in mice. These results reveal substantial differences in new neuron plasticity and function between these two commonly studied rodent species.

Adult Neurogenesis, Mental Health, and Mental Illness: Hope or Hype?

Psychiatric and neurologic disorders take an enormous toll on society. Alleviating the devastating symptoms and consequences of neuropsychiatric disorders such as addiction, depression, epilepsy, and schizophrenia is a main force driving clinical and basic researchers alike. By elucidating these disease neuromechanisms, researchers hope to better define treatments and preventive therapies. Research suggests that regulation of adult hippocampal neurogenesis represents a promising approach to treating and perhaps preventing mental illness. Here we appraise the role of adult hippocampal neurogenesis in major psychiatric and neurologic disorders within the essential framework of recent progress made in understanding “normal” adult neurogenesis. Topics addressed include the following: the life cycle of an adult hippocampal stem cell and the implications for aging; links between learning and hippocampal neurogenesis; the reciprocal relationship between cocaine self-administration and adult hippocampal neurogenesis; the role of adult neurogenesis in an animal model of depression and response to antidepressant exposure; the impact of neonatal seizures on dentate gyrus neurogenesis; and the contribution of a schizophrenia-susceptibility gene to adult hippocampal neurogenesis. These topics are discussed in light of the regulation of adult neurogenesis, the relationship to normal neurogenesis in adulthood and aging, and, importantly, the manipulation of neurogenesis to promote mental health and treat mental illness.