Neural stem cells exist not only in the developing mammalian nervous system but also in the adult nervous system of all mammalian organisms, including humans. Neural stem cells can also be derived from more primitive embryonic stem cells. The location of the adult stem cells and the brain regions to which their progeny migrate in order to differentiate remain unresolved, although the number of viable locations is limited in the adult. The mechanisms that regulate endogenous stem cells are poorly understood. Potential uses of stem cells in repair include transplantation to repair missing cells and the activation of endogenous cells to provide "self-repair. " Before the full potential of neural stem cells can be realized, we need to learn what controls their proliferation, as well as the various pathways of differentiation available to their daughter cells.

https://science.sciencemag.org/content/sci/287/5457/1433.full.pdf

Mammalian neural stem cells.

Exposure to an enriched environment increases neurogenesis in the dentate gyrus of adult rodents. Environmental enrichment, however, typically consists of many components, such as expanded learning opportunities, increased social interaction, more physical activity and larger housing. We attempted to separate components by assigning adult mice to various conditions: water-maze learning (learner), swim-time-yoked control (swimmer), voluntary wheel running (runner), and enriched (enriched) and standard housing (control) groups. Neither maze training nor yoked swimming had any effect on bromodeoxyuridine (BrdU)-positive cell number. However, running doubled the number of surviving newborn cells, in amounts similar to enrichment conditions. Our findings demonstrate that voluntary exercise is sufficient for enhanced neurogenesis in the adult mouse dentate gyrus.

http://www.neurogenesisresearch.com/Runing_and_Neurogenesis_Nat.pdf

Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus.

Childhood maltreatment has been linked to a variety of changes in brain structure and function and stress–responsive neurobiological systems. Epidemiological studies have documented the impact of childhood maltreatment on health and emotional well–being. After a brief review of the neurobiology of childhood trauma, we use the Adverse Childhood Experiences (ACE) Study as an epidemiological “case example” of the convergence between epidemiologic and neurobiological evidence of the effects of childhood trauma. The ACE Study included 17,337 adult HMO members and assessed 8 adverse childhood experiences (ACEs) including abuse, witnessing domestic violence, and serious household dysfunction. We used the number of ACEs (ACE score) as a measure of cumulative childhood stress and hypothesized a "doseresponse" relationship of the ACE score to 18 selected outcomes and to the total number of these outcomes (comorbidity). Based upon logistic regression analysis, the risk of every outcome in the affective, somatic, substance abuse, memory, sexual,and aggressionrelated domains increased in a graded fashion as the ACE score increased (P <0.001). The mean number of comorbid outcomes tripled across the range of the ACE score. The graded relationship of the ACE score to 18 different outcomes in multiple domains theoretically parallels the cumulative exposure of the developing brain to the stress response with resulting impairment in multiple brain structures and functions.

https://traumasensitiveschools.org/wp-content/uploads/2013/06/Anda-Article-Enduring-Effects-of-Abuse.pdf

The enduring effects of abuse and related adverse experiences in childhood: A convergence of evidence from neurobiology and epidemiology

Recent studies suggest that stress-induced atrophy and loss of hippocampal neurons may contribute to the pathophysiology of depression. The aim of this study was to investigate the effect of antidepressants on hippocampal neurogenesis in the adult rat, using the thymidine analog bromodeoxyuridine (BrdU) as a marker for dividing cells. Our studies demonstrate that chronic antidepressant treatment significantly increases the number of BrdU-labeled cells in the dentate gyrus and hilus of the hippocampus. Administration of several different classes of antidepressant, but not non-antidepressant, agents was found to increase BrdU-labeled cell number, indicating that this is a common and selective action of antidepressants. In addition, upregulation of the number of BrdU-labeled cells is observed after chronic, but not acute, treatment, consistent with the time course for the therapeutic action of antidepressants. Additional studies demonstrated that antidepressant treatment increases the proliferation of hippocampal cells and that these new cells mature and become neurons, as determined by triple labeling for BrdU and neuronal- or glial-specific markers. These findings raise the possibility that increased cell proliferation and increased neuronal number may be a mechanism by which antidepressant treatment overcomes the stress-induced atrophy and loss of hippocampal neurons and may contribute to the therapeutic actions of antidepressant treatment.

https://www.jneurosci.org/content/jneuro/20/24/9104.full.pdf

Chronic Antidepressant Treatment Increases Neurogenesis in Adult Rat Hippocampus

Mechanisms and functional implications of adult neurogenesis.

Thousands of hippocampal neurons are born in adulthood, suggesting that new cells could be important for hippocampal function. To determine whether hippocampus-dependent learning affects adult-generated neurons, we examined the fate of new cells labeled with the thymidine analog bromodeoxyuridine following specific behavioral tasks. Here we report that the number of adult-generated neurons doubles in the rat dentate gyrus in response to training on associative learning tasks that require the hippocampus. In contrast, training on associative learning tasks that do not require the hippocampus did not alter the number of new cells. These findings indicate that adult-generated hippocampal neurons are specifically affected by, and potentially involved in, associative memory formation.

Learning enhances adult neurogenesis in the hippocampal formation.

The vertebrate brain continues to produce new neurons throughout life. In the rat hippocampus, several thousand are produced each day, many of which die within weeks. Associative learning can enhance their survival; however, until now it was unknown whether new neurons are involved in memory formation. Here we show that a substantial reduction in the number of newly generated neurons in the adult rat impairs hippocampal-dependent trace conditioning, a task in which an animal must associate stimuli that are separated in time. A similar reduction did not affect learning when the same stimuli are not separated in time, a task that is hippocampal-independent. The reduction in neurogenesis did not induce death of mature hippocampal neurons or permanently alter neurophysiological properties of the CA1 region, such as long-term potentiation. Moreover, recovery of cell production was associated with the ability to acquire trace memories. These results indicate that newly generated neurons in the adult are not only affected by the formation of a hippocampal-dependent memory, but also participate in it.

Neurogenesis in the adult is involved in the formation of trace memories

The role of the hippocampus in trace conditioning: Temporal discontinuity or task difficulty?

Exposure to an acute stressful experience facilitates classical conditioning in male rats but impairs conditioning in female rats (T J Shors, C Lewczyk, M Paczynski, P R Mathew, & J Pickett, 1998; G E Wood & T J Shors, 1998) The authors report that these effects extend to performance on the hippocampal-dependent task of trace conditioning The stress-induced impairment of conditioning in females was evident immediately, 24 hr and 48 hr after stress, depending on the stage of estrus Moreover, the effect could be reactivated days later by reexposure to the stressful context Corticosterone levels correlated with overall performance in males but not in females Unlike the effect seen in males, adrenalectomy did not prevent the stress-induced effect on conditioning in females These data indicate that exposure to the same experience can have opposite effects on learning in males versus females and that these opposing effects are mediated by differing hormonal systems

Anna V. Beylin

Papers

Learning enhances adult neurogenesis in the hippocampal formation.

Neurogenesis in the adult is involved in the formation of trace memories

The role of the hippocampus in trace conditioning: Temporal discontinuity or task difficulty?

The contribution of adrenal and reproductive hormones to the opposing effects of stress on trace conditioning in males versus females.

Glucocorticoids are necessary for enhancing the acquisition of associative memories after acute stressful experience