Serotonin may stimulate granule cell proliferation in the adult hippocampus, as observed in rats grafted with foetal raphe neurons

Dentate gyrus neurogenesis and depression

Major depressive disorder (MDD) is a debilitating and complex psychiatric disorder that involves multiple neural circuits and genetic and non-genetic risk factors. In the quest for elucidating the neurobiological basis of MDD, hippocampal neurogenesis has emerged as a candidate substrate, both for the etiology as well as treatment of MDD. This chapter critiques the advances made in the study of hippocampal neurogenesis as they relate to the neurogenic hypothesis of MDD. While an involvement of neurogenesis in the etiology of depression remains highly speculative, preclinical studies have revealed a novel and previously unrecognized role for hippocampal neurogenesis in mediating some of the behavioral effects of antidepressants. The implications of these findings are discussed to reevaluate the role of hippocampal neurogenesis in MDD.

Binge administration of 3,4-methylenedioxymethamphetamine (“ecstasy”) impairs the survival of neural precursors in adult rat dentate gyrus

3,4-Methylenedioxymethamphetamine (MDMA) is a potent stimulant and hallucinogenic drug whose ability to regulate neurogenesis in the adult has not been previously investigated. We used 5'-bromo-2-deoxyuridine (BrdU) and Ki-67 as mitotic markers, and doublecortin (DCX) as a marker of immature neurons, to study proliferation, survival and maturation of adult-generated cells in the dentate gyrus (DG) of the hippocampus following binge administration of MDMA (8 injections of 5 mg/kg at 6 h intervals). The results showed that MDMA treatment did not affect cytogenesis in the DG, but significantly decreased the survival rate of cells incorporated after 2 weeks to the granular layer of the DG by ca. 50%, and of those remaining in the subgranular layer by ca. 30%. Two weeks after exposure to MDMA the length of dendritic arbors and the number of dendritic branches of immature DCX+ neurons were nearly identical to those of control rats, as was the level of colocalization of BrdU with DCX. These results demonstrate that binge MDMA administration does not affect the proliferation rates of progenitor cells in the DG, but has deleterious effects on adult neurogenesis by impairing the short-term survival of vulnerable neural precursors.

Effects of repeated phencyclidine administration on adult hippocampal neurogenesis in the rat

Dysfunctional maturation of neural networks, particularly hippocampus-prefrontal networks, may be of particular interest in determining the pathophysiology of schizophrenia. Phencyclidine (PCP)-induced symptoms in humans appear to offer a more complete model of schizophrenia than do amphetamine-induced symptoms. This study investigated the effects of intermittent i.p. injections of PCP (7.5 mg/kg) on cell proliferation and survival of granule cells in the dentate gyrus of the rat brain using quantitative immunohistochemical techniques for 5-bromo-2'-deoxyuridine (BrdU)-positive cells. After repeated PCP injection for 14 days, mean scores for stereotyped behavior increased with the number of injections, while scores for ataxia and backpedaling as serotonergic behaviors gradually decreased. The number of BrdU-positive cells decreased by 23% in the subgranular zone of the dentate gyrus by 24 h after repeated injections. However, decreased levels of BrdU-positive cells returned to control levels within 1 week. Differentiation of newly formed cells was not influenced. Repeated PCP administration after BrdU injection did not exert any effects on survival of newly generated cells. These findings suggest that transient disturbances of cell proliferation in the dentate gyrus occur under PCP-related behavioral abnormalities. Whether disturbed cell proliferation would thus be closely implicated in the development of behavioral sensitization induced by PCP administration is unclear, but this would possibly result from adaptation to new pharmacological conditions under behavioral sensitization or stressful conditions of PCP-related abnormal behaviors. Further studies are required to elucidate the biological significance of hippocampal neurogenesis in the mechanisms underlying the development of cognitive dysfunctions and the psychosis of schizophrenia.

Decreased Hippocampal Neurogenesis Following Olfactory Bulbectomy is Reversed by Repeated Citalopram Administration

1. Whereas much progress has been made in the treatment of depression, the exact pathogenetic mechanisms of the disorder are still poorly understood. It has been proposed that one possible mechanism could be a decrease in adult hippocampal neurogenesis. 2. The olfactory bulbectomy (OB) in rats is widely accepted as an animal model of depression. In the present study, we investigated whether hippocampal neurogenesis is affected by an OB, and whether chronic citalopram, a serotonin selective reuptake inhibitor, counteracts OB-induced impairment of neurogenesis. 3. Our study shows that OB decreases proliferation of the neuronal precursors in the dentate gyrus and retards their differentiation into mature granule neurons. In OB rats, repeated administration of citalopram restores reduced proliferative activity and enhances the differentiation of precursors into mature calbindin-positive neurons. 4. The obtained data demonstrate that a citalopram-induced increase in neurogenesis in OB rats could be one possible mechanism by which antidepressants alleviate OB-induced depressive-like behavior.

Hippocampal neurogenesis: regulation by stress and antidepressants

Accumulating evidence implicates hippocampal neurogenesis in the pathophysiology of depression. Psychosocial stress reduces neurogenesis in rodents, whereas chronic treatment with antidepressants increases neurogenesis and blocks the effects of stress. The effects of stress and antidepressant treatment on hippocampal neurogenesis parallel behavioral changes in animal models. Moreover, ablating hippocampal neurogenesis renders antidepressants inactive in behavioral paradigms used to model antidepressant response and anxiety-like behavior in mice. In humans, monoamine-modulating antidepressants demonstrate clinical efficacy in treating depression and anxiety, which are often precipitated by psychosocial stress. This review examines the mounting evidence that stress and antidepressant treatment regulate neurogenesis in animals. Special attention is paid to the cellular and molecular mechanisms by which this regulation takes place. An analysis of current animal models used to study response to stress and antidepressants indicates the importance of modeling chronic treatment, which reflects both changes in neurogenesis and clinical response. Exploring responses of hippocampal neurogenesis to experimental challenges in appropriate animal models should delineate the role of adult-born neurons in hippocampal physiology. Focusing on neurogenic response to experimental paradigms of stress and antidepressant treatment is particularly interesting for understanding the pathophysiology of major depressive disorder.

Agomelatine, a new antidepressant, induces regional changes in hippocampal neurogenesis

BACKGROUND

Antidepressant treatments increase neural plasticity and adult neurogenesis, especially in the hippocampus. Here, we determined the effects of agomelatine (S-20098), a new antidepressant, on various phases of neurogenesis in the dentate gyrus of adult rat.

METHODS

Animals were injected with agomelatine for different time periods. Immunostaining for bromodeoxyuridine, neuron specific nuclear protein, and glial fibrillary acid protein, as well as for the highly polysialylated form of neuronal cell adhesion molecule and doublecortin, was used to detect changes in cell proliferation, neurogenesis, and survival. Cell death was estimated by terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling and cresyl violet staining.

RESULTS

Chronic (3 weeks) but not acute (4 hours) or subchronic (1 week) administration of agomelatine increased cell proliferation and neurogenesis in the ventral dentate gyrus, a region notably implicated in response to emotion, which is consistent with the antidepressant-anxiolytic properties of the drug. Extending agomelatine treatment over several weeks, however, increases survival of newly formed neurons in the entire dentate gyrus. Finally, agomelatine treatment does not affect mature granule cells.

CONCLUSIONS

This study shows that an antidepressant can affect differentially various stages of neurogenesis in the dorsal and ventral hippocampus. Altogether, these changes lead to a pronounced augmentation in the total number of new granule cells.

Environmental enrichment and voluntary exercise massively increase neurogenesis in the adult hippocampus via dissociable pathways

Environmental enrichment (EE) and voluntary exercise (VEx) have consistently been shown to increase adult hippocampal neurogenesis and improve spatial learning ability. Although it appears that these two manipulations are equivalent in this regard, evidence exists that EE and VEx affect different phases of the neurogenic process in distinct ways. We review the data suggesting that EE increases the likelihood of survival of new cells, whereas VEx increases the level of proliferation of progenitor cells. We then outline the factors that may mediate these relationships. Finally, we provide a model showing that VEx leads to the convergence of key somatic and cerebral factors in the dentate gyrus (DG) to induce cell proliferation. Although insufficient evidence exists to provide a similar model for EE, we suggest that EE-induced cell survival in the DG involves cortical restructuring as a means of promoting survival. We conclude that EE and VEx lead to an increase in overall hippocampal neurogenesis via dissociable pathways, and should therefore, be considered distinct interventions with regard to hippocampal plasticity and associated behaviors.

Decreased cell proliferation in the dentate gyrus of rats after repeated administration of cocaine

Cell proliferation in the dentate gyrus of hippocampus was assessed using in vivo labeling with 5-bromo-2'-deoxyuridine (BrdU) in adult rats that were administered cocaine (20 mg/kg) for 14 consecutive days. Rats showed increased stereotypy at a challenge dose of cocaine after 1 week of withdrawal, suggesting the acquisition of behavioral sensitization. Twenty-four hours after final injection of repetitive cocaine administration, a 26% decrease in BrdU-positive cells was observed, compared with control rats. However, this returned to control level within 1 week. No differences were observed in rats that received a single injection of cocaine. Differentiation of newly formed cells was not influenced. These data imply that the regulation of hippocampal cell proliferation by cocaine may be involved in the development of certain symptoms of addiction, such as cognitive impairment and acquisition of behavioral sensitization.

Selective serotonin depletion does not regulate hippocampal neurogenesis in the adult rat brain: differential effects of p-chlorophenylalanine and 5,7-dihydroxytryptamine

Serotonin is suggested to regulate adult hippocampal neurogenesis, and previous studies with serotonin depletion reported either a decrease or no change in adult hippocampal progenitor proliferation. We have addressed the effects of serotonin depletion on distinct aspects of adult hippocampal neurogenesis, namely the proliferation, survival and terminal differentiation of hippocampal progenitors. We used the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) or the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to deplete serotonin levels. 5,7-DHT selectively decreased hippocampal serotonin levels, while PCPA resulted in a significant decline in both serotonin and norepinephrine levels. We observed a robust decline in the proliferation and survival of adult hippocampal progenitors following PCPA treatment. This was supported by a decrease in the number of doublecortin-positive cells in the neurogenic niche in the hippocampus. In striking contrast, 5,7-DHT did not alter the proliferation or survival of adult hippocampal progenitors and did not alter the number of doublecortin-positive cells. The terminal differentiation of adult hippocampal progenitors was not altered by either PCPA or 5,7-DHT treatment. An acute increase in serotonin levels also did not influence adult hippocampal progenitor proliferation. These results suggest that selective serotonin depletion or an acute induction in serotonin levels does not regulate adult hippocampal neurogenesis, whereas treatment with PCPA that induces a decline in both serotonin and norepinephrine levels results in a significant decrease in adult hippocampal neurogenesis. Our results highlight the need for future studies to examine the role of other monoamines in both the effects of stress and antidepressants on adult hippocampal neurogenesis.

Effect of long-lasting serotonin depletion on environmental enrichment-induced neurogenesis in adult rat hippocampus and spatial learning

The dentate gyrus of the hippocampal formation produces new neurons throughout adulthood in mammalian species. Several experimental statuses and factors regulating to neurogenesis have been identified in the adult dentate gyrus. For example, exposure to an enriched environment enhances neurogenesis in the dentate gyrus and improves hippocampus-dependent spatial learning. Furthermore, serotonin is known to influence adult neurogenesis, and learning and memory. However, the effects of long-lasting depletion of serotonin over the developing period on neurogenesis have not been investigated. Thus, we examined the influence of long-lasting serotonin depletion on environmental enrichment-induced neurogenesis and spatial memory performance. As reported previously, environmental enrichment significantly increased new neurons in the dentate gyrus. However, there was no improvement of the spatial learning test in adult rats in standard and in environmental enrichment housings. Intracisternal administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine, on postnatal day 3 apparently reduced serotonin content in the adult hippocampus without regeneration. This experimental depletion of serotonin in the hippocampus of rats housed in an enriched environment had no effect on spatial memory performance, but produced significant decreases in the number of bromodeoxyuridine-labeled new cells in the dentate gyrus. These findings indicate that newly generated cells stimulated by environmental enrichment are not critical for improvements in hippocampus-dependent learning. Furthermore, numbers of bromodeoxyuridine-labeled cells in the dentate gyrus of 5,7-dihydroxytryptamine-injected rats did not differ between 1 day and 4 weeks after bromodeoxyuridine injection. These data suggest that survival of newly generated dentate gyrus cells remains relatively constant under long-lasting serotonin depletion.

Alcohol and adult neurogenesis: Roles in neurodegeneration and recovery in chronic alcoholism

The concept of "structural plasticity" has emerged as a potential mechanism in neurodegenerative and psychiatric diseases such as drug abuse, depression, and dementia. Chronic alcoholism is a progressive neurodegenerative disease while the person continues to abuse alcohol, though clinical and imaging studies show that some recovery may occur with abstinence. The neural plasticity observed in chronic alcoholism coupled with conflicting reports on alcohol-induced hippocampal neuropathology make this disease ripe for reconsideration in terms of the phenomenon of adult neurogenesis. This review describes opposing neurogenic processes that occur with alcohol intoxication and abstinence following alcohol dependence and how these opposing events relate to neurodegeneration and recovery from chronic alcoholism.

Dépression et neuroplasticité : implication des systèmes sérotoninergiques

Neuroplasticity contributes to both normal and pathological brain function. A recent hypothesis links depression to lack of adaptive responses to stress or other aversive stimuli, and effects of antidepressant treatments on adult neurogenesis are more and more extensively studied because of the structural changes involved in the pathophysiology of depression. Indeed, neuronal remodelling in hippocampal formation is associated with chronic stress and is reversed by antidepressant treatments in animals. Decrease in hippocampal volume has also been associated to cognitive deficits in patients with major depression. Interestingly, serotonergic (5-HT) systems play a major role both as antidepressants and by increasing hippocampal neurogenesis through various receptor subtypes. Recently, we have also demonstrated that agomelatine, a new antidepressant drug having serotonergic and melatonergic properties, can increase proliferation and survival of newly formed hippocampal cells. Although the mechanisms underlying such effects are still unknown, these data reinforce the view that changes in hippocampal neurogenesis might belong to the cellular correlates of mood disorders.

Adult neurogenesis and repair of the adult CNS with neural progenitors, precursors, and stem cells

Recent work in neuroscience has shown that the adult central nervous system contains neural progenitors, precursors, and stem cells that are capable of generating new neurons, astrocytes, and oligodendrocytes. While challenging previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them future possibilities for the development of novel neural repair strategies. The purpose of this review is to present current knowledge about constitutively occurring adult mammalian neurogenesis, to highlight the critical differences between "neurogenic" and "non-neurogenic" regions in the adult brain, and to describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system, and the dentate gyrus of the hippocampus. We also provide an overview of currently used models for studying neural precursors in vitro, mention some precursor transplantation models, and emphasize that, in this rapidly growing field of neuroscience, one must take caution with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims toward molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what the function might be of newly generated neurons in the adult brain and provide a summary of current thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.

St John's wort (Hypericum perforatum) diminishes cognitive impairment caused by the chronic restraint stress in rats

In this study we tested the hypothesis that St John's wort (Hypericum perforatum) may counteract stress-induced memory impairment. Object recognition test and Morris water maze were used to determine whether administration of H. perforatum (350 mg kg(-1) for 21 days), standardized to 0.3% hypericin content, protects against non-spatial and/or spatial memory impairments due to chronic restraint stress (2h daily for 21 days). A group of rats administered the exogenous corticosterone at the dose of 5 mg kg(-1) daily for 21 days, yielding its similar plasma levels as these observed in stress was run in parallel. In the first experiment all rats were tested for recognition memory in the object recognition test. On the following day, the animals were tested in open field and elevated "plus" maze to control for the contribution of respectively, motor and emotional effects of our treatments to the memory tests. In the second experiment, new group of stressed animals was tested for spatial memory in the water maze. We observed that H. perforatum prevented the deleterious effects of both chronic restraint stress and long-term corticosterone on learning and memory as measured in both, the object recognition and the water maze tests. The herb not only prevented stress- and corticosterone-induced memory impairments, but it significantly improved recognition memory (p<0.01) in comparison to control. These results suggest that H. perforatum has a potential to prevent stress memory disorders.

Serotonergic-like progenitor cells propagated from neural stem cells in vitro: survival with SERT protein expression following implantation into brains of mice lacking SERT

Neural stem cells (NSCs) obtained from the midbrain region of embryonic (E14) mice were initially cultured with basic fibroblast growth factor (bFGF), Sonic hedgehog, and FGF-8 in a serum-free N-2 culture medium to foster differentiation into a serotonergic-like phenotype. During the initial differentiating phase, these progenitor cells expressed En1, Pax3, and Pax5 mRNA. Subsequently, a single serotonin [5-hydroxytryptamine (5-HT)] and tryptophan hydroxylase-positive clone was isolated, which gave rise to cells that developed serotonergic properties. Sixty percent of these progenitor cells expressed the serotonin transporter (SERT), as indicated by specific ligand binding of [125I]-RTI-55. To further evaluate SERT functionality, we showed that these progenitor cells possessed specific [3H]-5-HT uptake activity. Implantation of the serotonergic-like progenitors into the hippocampus of adult mice genetically lacking SERT was followed by migration of these cells into adjacent brain regions, and survival of the cells at 8 weeks was accompanied by a gradual increase in density of SERT protein expression, which was not found in vehicle-injected, control mice. These findings suggest that this serotonergic-like NSC model will be a useful contribution to the development of cell biotechnology in regard to the expression of missing genes such as SERT in the adult brain.

Cell Proliferation in the Adult Hippocampal Formation of Rodents and its Modulation by Entorhinal and Fimbria–Fornix Afferents

New granule neurons are produced in the dentate gyrus (DG) of rodents throughout adult life. Recent studies have also reported adult neurogenesis in the cerebral cortex in normal animals or after brain injury. However, few of these studies focused on the hippocampal formation (HF), a cortical area involved in learning and memory in which extensive cell death occurs in neurodegenerative diseases. Thus, we studied cell proliferation in the HF of rodents and the intrinsic putative neurogenic potential of entorhinal cortex (EC) progenitors. We show that only the DG generates new neurons in the HF. In addition, neurospheres from the EC differentiate into neurons and glia in vitro and after transplantation in the adult DG. We also analyzed whether the absence of the synaptic input from the main hippocampal afferents induces neuronal generation in the HF outside the DG and/or regulates the proliferation of DG neuroprogenitors. We show that the denervation of the hippocampus does not induce neurogenesis in HF regions other than the DG. However, neuroprogenitor proliferation in the DG is reduced after fimbria-fornix lesions but not after entorhinal deafferentation, which supports the view that neuroprogenitor proliferation and/or differentiation in the DG are controlled from basal forebrain/septal neurons.

Ontogeny of 5-HT1A receptor expression in the developing hippocampus

Serotonin (5-HT) has long been implicated in a number of neurodevelopmental processes including neuronal cell division, migration, neurite outgrowth, and synapse formation. However, relatively little is known about how these effects are mediated during normal brain development in vivo and the identity of the receptor subtypes involved in mediating these effects. In recent years, a number of pharmacological studies have suggested a role for the serotonin 1A (5HT1A) receptor subtype in mediating the developmental effects of 5-HT in the hippocampus. These studies, however, have been difficult to interpret due to lack of information regarding the expression and distribution of 5HT1A in the developing brain and hippocampus in particular. In the current study, specific anti-5-HT1A antibodies, developed in our laboratory [F.C. Zhou, T.D. Patel, D. Swartz, Y. Xu, M.R. Kelley, Production and characterization of an anti-serotonin 1A receptor antibody which detects functional 5-HT1A binding sites, Brain Res Mol Brain Res, 69 (1999) 186-201], were utilized to map the ontogeny and distribution of the 5HT1A receptor protein in the developing rat hippocampus through embryonic and early postnatal life. This is the first such study of 5-HT1A expression in the developing rat brain. Our findings revealed that expression of the 5HT1A receptor emerges during the initial stages of embryonic hippocampal development. Remarkably, most if not all hippocampal neurons begin to express 5HT1A shortly upon completion of their terminal mitosis. We found that 5HT1A is initially concentrated around the cell bodies and later becomes more sparsely distributed along the dendrites after the neurons have matured. In addition to postmitotic neurons, we have observed that S100 and GFAP positive glia transiently express 5HT1A during early postnatal development of the hippocampus. These findings demonstrate that the 5-HT1A receptor is positioned to mediate developmental effects of serotonin in the hippocampus. Furthermore, the temporal patterns of expression suggest a role for 5-HT1A in postmitotic events such as neuronal migration, neurite outgrowth, and phenotypic differentiation.

Offer and demand: proliferation and survival of neurons in the dentate gyrus

The proliferation and survival of new cells in the dentate gyrus of mammals is a complex process that is subject to numerous influences, presenting a confusing picture. We suggest regarding these processes on the level of small networks, which can be simulated in silico and which illustrate in a nutshell the influences that proliferating cells exert on plasticity and the conditions they require for survival. Beyond the insights gained by this consideration, we review the available literature on factors that regulate cell proliferation and neurogenesis in the dentate gyrus in vivo. It turns out that the rate of cell proliferation and excitatory afferents via the perforant path interactively determine cell survival, such that the best network stability is achieved when either of the two is increased whereas concurrent activation of the two factors lowers cell survival rates. Consequently, the mitotic activity is regulated by systemic parameters in compliance with the hippocampal network's requirements. The resulting neurogenesis, in contrast, depends on local factors, i.e. the activity flow within the network. In the process of cell differentiation and survival, each cell's spectrum of afferent and efferent connections decides whether it will integrate into the network or undergo apoptosis, and it is the current neuronal activity which determines the synaptic spectrum. We believe that this framework will help explain the biology of dentate cell proliferation and provide a basis for future research hypotheses.

Association of human hippocampal neurochemistry, serotonin transporter genetic variation, and anxiety

The impact of the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) on anxiety-related behavior and related cerebral activation has facilitated the understanding of neurobiological mechanisms of anxiety. However, the influence of the 5-HTTLPR genotype on hippocampal neuronal development and neurochemistry, which is relevant to anxiety behavior, has not been investigated. In 38 healthy subjects, absolute concentrations of N-acetylaspartate (NAA) were measured as a main surrogate parameter for hippocampal neurochemistry on a 3-T scanner. A significantly lower hippocampal NAA concentration in s allele carriers was observed as compared to l/l genotype. Other metabolites (choline, creatine + phosphocreatine, glutamate) were unaffected by genotype. The hippocampal NAA concentration was negatively correlated with trait anxiety scores (STAI). Metabolites measured in the anterior cingulate cortex (reference region) were not associated with genotype. The results are in accordance with the recently reported relationship between hippocampal neuronal development and anxiety behavior in adult animals and show an association between human limbic neurochemistry and genetically driven serotonergic neurotransmission relevant to anxiety.

Adult neurogenesis: from precursors to network and physiology

The discovery that the adult mammalian brain creates new neurons from pools of stemlike cells was a breakthrough in neuroscience. Interestingly, this particular new form of structural brain plasticity seems specific to discrete brain regions, and most investigations concern the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampal formation (HF). Overall, two main lines of research have emerged over the last two decades: the first aims to understand the fundamental biological properties of neural stemlike cells (and their progeny) and the integration of the newly born neurons into preexisting networks, while the second focuses on understanding its relevance in brain functioning, which has been more extensively approached in the DG. Here, we propose an overview of the current knowledge on adult neurogenesis and its functional relevance for the adult brain. We first present an analysis of the methodological issues that have hampered progress in this field and describe the main neurogenic sites with their specificities. We will see that despite considerable progress, the levels of anatomic and functional integration of the newly born neurons within the host circuitry have yet to be elucidated. Then the intracellular mechanisms controlling neuronal fate are presented briefly, along with the extrinsic factors that regulate adult neurogenesis. We will see that a growing list of epigenetic factors that display a specificity of action depending on the neurogenic site under consideration has been identified. Finally, we review the progress accomplished in implicating neurogenesis in hippocampal functioning under physiological conditions and in the development of hippocampal-related pathologies such as epilepsy, mood disorders, and addiction. This constitutes a necessary step in promoting the development of therapeutic strategies.

Treadmill Training Effects in Different Age Groups following Middle Cerebral Artery Occlusion in Rats

BACKGROUND

Despite the increased understanding of treadmill training on stroke patients, its effects on different age groups are not clearly known. The present study presents such effects through a model of cerebral ischemia on young and old groups of rats.

OBJECTIVES

To investigate the effect of treadmill training on young and old rats after cerebral ischemia caused by middle cerebral artery occlusion (MCAO).

METHODS

Forty old (22-24 months of age) and 32 young (3-4 months of age) rats underwent the MCAO procedure for 60 min. Rats that survived the procedure were randomly assigned to a 1- or a 2-week treadmill training group, or a time-matched control group (n=6-8 for each group). The infarct volume was compared between the treadmill training and the control groups for both the young and old rats at 1 or 2 weeks.

RESULTS

After treadmill training for 1 week, the mean infarct volume was 7.26+/-0.49 and 9.51+/-0.84% for the young and old rats, respectively. The 1-week treadmill training effect was significant in the young rats (p=0.0207) but not in the old rats (p=0.0840). The mean infarct volume was 6.84+/-0.51 and 7.63+/-0.52% for the young and old rats, respectively, after the 2-week treadmill training. Both the young and old rat groups demonstrated a significant reduction in the infarct volume compared with that of the control group (p=0.021 for the young group and p=0.039 for the old group) after 2 weeks of treadmill training.

CONCLUSION

The present findings clearly demonstrate the different training effects of locomotor activity in reducing ischemic infarction in young and old rats. The delayed reduction in ischemic infarction in old rats was notable and may be attributable to the slow response of angiogenic and neurogenic mechanisms in the old rats.

Differential properties of dentate gyrus and CA1 neural precursors

In the present article we investigated the properties of CA1 and dentate gyrus cell precursors in adult rodents both in vivo and in vitro. Cell proliferation in situ was investigated by rating the number of cells incorporating BrdU after kainate-induced seizures. CA1 precursors displayed a greater proliferation capacity than dentate gyrus precursors. The majority of BrdU-labeled cells in CA1 expressed Nestin and Mash-1, two markers of neural precursors. BrdU-positive cells in the dentate gyrus expressed Nestin, but only a few expressed Mash-1. In animals pretreated with the antimitotic azacytidine, the capacity of kainate to enhance the proliferation was higher in CA1 than in the dentate gyrus. Differences in intrinsic progenitor cell activity could underlie these different expansion capacities. Thus, we compared the renewal- expansion and multipotency of dentate gyrus and CA1 precursors isolated in vitro. We found that the dissected CA1 region, including the periventricular zone, is enriched in neurosphere-forming cells (presumed stem cells), which respond to either EGF or FGF-2. Dentate gyrus contains fewer neurosphere-forming cells and none that respond to FGF-2 alone. Neurospheres generated from CA1 were multipotent and produced neurons, astrocytes, and oligodendrocytes, while dentate gyrus neurospheres mostly produced glial cells. The analysis of the effects of EGF on organotypic cultures of hippocampal slices depicted similar features: BrdU and Nestin immunoreactivities increased after EGF treatment in CA1 but not in the dentate gyrus. These results suggest that CA1 precursors are more stem-cell-like than granule cell precursors, which may represent a more restricted precursor cell.

Pregnenolone sulfate enhances neurogenesis and PSA-NCAM in young and aged hippocampus

Age-dependent cognitive impairments have been correlated with functional and structural modifications in the hippocampal formation. In particular, the brain endogenous steroid pregnenolone-sulfate (Preg-S) is a cognitive enhancer whose hippocampal levels have been linked physiologically to cognitive performance in senescent animals. However, the mechanism of its actions remains unknown. Because neurogenesis is sensitive to hormonal influences, we examined the effect of Preg-S on neurogenesis, a novel form of plasticity, in young and old rats. We demonstrate that in vivo infusion of Preg-S stimulates neurogenesis and the expression of the polysialylated forms of NCAM, PSA-NCAM, in the dentate gyrus of 3- and 20-month-old rats. These influences on hippocampal plasticity are mediated by the modulation of the gamma-aminobutyric acid receptor complex A (GABA(A)) receptors present on hippocampal neuroblasts. In vitro, Preg-S stimulates the division of adult-derived spheres suggesting a direct influence on progenitors. These data provide evidence that neurosteroids represent one of the local secreted signals controlling hippocampal neurogenesis. Thus, therapies which stimulate neurosteroidogenesis could preserve hippocampal plasticity and prevent the appearance of age-related cognitive disturbances.

Decreased neurogenesis after cholinergic forebrain lesion in the adult rat

Adult neurogenesis has been shown to be regulated by a multitude of extracellular cues, including hormones, growth factors, and neurotransmitters. The cholinergic system of the basal forebrain is one of the key transmitter systems for learning and memory. Because adult neurogenesis has been implicated in cognitive performance, the present work aims at defining the role of cholinergic input for adult neurogenesis by using an immunotoxic lesion approach. The immunotoxin 192IgG-saporin was infused into the lateral ventricle of adult rats to selectively lesion cholinergic neurons of the cholinergic basal forebrain (CBF), which project to the two main regions of adult neurogenesis: the dentate gyrus and the olfactory bulb. Five weeks after lesioning, neurogenesis, defined by the number of cells colocalized for bromodeoxyuridine (BrdU) and the neuronal nuclei marker NeuN, declined significantly in the granule cell layers of the dentate gyrus and olfactory bulb. Furthermore, immunotoxic lesions to the CBF led to increased numbers of apoptotic cells specifically in the subgranular zone, the progenitor region of the dentate gyrus, and within the periglomerular layer of the olfactory bulb. We propose that the cholinergic system plays a survival-promoting role for neuronal progenitors and immature neurons within regions of adult neurogenesis, similar to effects observed previously during brain development. As a working hypothesis, neuronal loss within the CBF system leads not only to cognitive deficits but may also alter on a cellular level the functionality of the dentate gyrus, which in turn may aggravate cognitive deficits.

Regulation of serotonin levels by multiple light-entrainable endogenous rhythms

This study examined whether serotonin levels in the brain of the American lobster, Homarus americanus, are under circadian control. Using high-performance liquid chromatography and semi-quantitative immunocytochemical methods, we measured serotonin levels in the brains of lobsters at six time points during a 24-h period. Lobsters were maintained for 2 weeks on a 12 h:12 h light:dark cycle followed by 3 days of constant darkness. Under these conditions, brain serotonin levels varied rhythmically,with a peak before subjective dusk and a trough before subjective dawn. This persistent circadian rhythm in constant darkness indicates that serotonin levels are controlled by an endogenous clock. Animals exposed to a shifted light cycle for &gt;10 days, followed by 3 days in constant darkness,demonstrate that this rhythm is light entrainable. Separate analyses of two pairs of large deutocerebral neuropils, the accessory and olfactory lobes,show that serotonin levels in these functionally distinct areas also exhibit circadian rhythms but that these rhythms are out of phase with one another. The olfactory and accessory lobe rhythms are also endogenous and light entrainable, suggesting the presence of multiple clock mechanisms regulating serotonin levels in different brain regions.

7-Nitroindazole upregulates phosphorylated cAMP response element binding protein, polysialylated-neural cell adhesion molecule and tryptophan hydroxylase expression in the adult rat hippocampus

We examined the effect of nitric oxide (NO) on the expression of phosphorylated cAMP response element binding (pCREB) protein and on the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), in the hippocampus of the adult rat, by administering 7-nitroindazole (7-NI) for 5 days, to block neuronal nitric oxide synthase (nNOS) activity. It was found that PSA-NCAM and pCREB immunoreactive cells were increased two-fold in the dentate gyrus by this treatment. In addition, the immunoreactivities of serotonin (5-HT, 5-hydroxytryptamine), known to be involved in neural plasticity, and of tryptophan hydroxylase immunoreactive-fibers in the dentate gyrus were both upregulated by 7-NI. Our results indicate that nNOS inhibition upregulates neural plasticity and increases 5-HT in the rat hippocampus, and suggest the existence of regulatory relationships between NO, neural plasticity and 5-HT.

Role of neurotrophic factors in the etiology and treatment of mood disorders

Basic research in rodents has demonstrated that exposure to stress decreases levels of brain-derived neurotrophic factor (BDNF) in brain regions associated with depression. In contrast, antidepressant treatment produces the opposite effect and blocks the effects of stress on BDNF. BDNF upregulation and possibly other neurotrophic/growth factors could reverse or block the atrophy and cell loss that has been observed in rodent stress models and in depressed patients. The morphological alterations observed in depressed patients could result from decreased size or number of glia and/or neurons and may include regulation of adult neurogenesis. This article reviews the primary work leading to a neurotrophic hypothesis of depression and antidepressant action and the cellular mechanisms and signal transduction pathways that underlie these effects.

Role of neurotrophic factors in the etiology and treatment of mood disorders

Basic research in rodents has demonstrated that exposure to stress decreases levels of brain-derived neurotrophic factor (BDNF) in brain regions associated with depression. In contrast, antidepressant treatment produces the opposite effect and blocks the effects of stress on BDNF. BDNF upregulation and possibly other neurotrophic/growth factors could reverse or block the atrophy and cell loss that has been observed in rodent stress models and in depressed patients. The morphological alterations observed in depressed patients could result from decreased size or number of glia and/or neurons and may include regulation of adult neurogenesis. This article reviews the primary work leading to a neurotrophic hypothesis of depression and antidepressant action and the cellular mechanisms and signal transduction pathways that underlie these effects.

Adult hippocampal neurogenesis and voluntary running activity: Circadian and dose-dependent effects

Running activity increases cell proliferation and neurogenesis in the dentate gyrus of adult mice. The present experiment was designed to investigate whether the effect of activity on adult neurogenesis is dependent on the time of day (circadian phase) and the amount of activity. Mice received restricted access to a running wheel (0, 1, or 3 hr) at one of three times of day: the middle of the light phase (i.e., when mice are normally inactive), dark onset (i.e., when mice begin their nocturnal activity), and the middle of the dark period (i.e., when mice are in the middle of their active period). Cell proliferation and net neurogenesis were assessed after incorporation of the thymidine analog bromodeoxyuridine (BrdU) and immunohistochemical detection of BrdU and neuronal markers. Running activity significantly increased cell proliferation, cell survival, and total number of new neurons only in animals with 3 hr of wheel access during the middle of the dark period. Although activity was positively correlated with increased neurogenesis at all time points, the effects were not statistically significant in animals with wheel access at the beginning of the dark period or during the middle of the light period. These data suggest that the influence of exercise on cell proliferation and neurogenesis is modulated by both circadian phase and the amount of daily exercise, thus providing new insight into the complex relationship between physiological and behavioral factors that can mediate adult neuroplasticity.

La neurogenèse dans le cerveau adulte et ses modifications en situations pathologiques

In the central nervous system, neuron generation continues throughout adulthood. This neurogenesis is involved in a continual adaptative process in relation with the environmental complexity. It decreases with age and is reduced in several types of experimental conditions mimicking stress, depression, addiction and neurodegenerative diseases. It may be enhanced after excitotoxic, ischemic or traumatic injuries suggesting a compensatory adaptative response. Available data on this new concept in the field of neurosciences and possible therapeutic relevance are reported.

Serotonin-induced increases in adult cell proliferation and neurogenesis are mediated through different and common 5-HT receptor subtypes in the dentate gyrus and the subventricular zone

Increase in serotonin (5-HT) transmission has profound antidepressant effects and has been associated with an increase in adult neurogenesis. The present study was aimed at screening the 5-HT receptor subtypes involved in the regulation of cell proliferation in the subgranular layer (SGL) of the dentate gyrus (DG) and the subventricular zone (SVZ) and to determine the long-term changes in adult neurogenesis. The 5-HT1A, 5-HT1B, and 5-HT2 receptor subtypes were chosen for their implication in depression and their location in/or next to these regions. Using systemic administration of various agonists and antagonists, we show that the activation of 5-HT1A heteroreceptors produces similar increases in the number of bromodeoxyuridine-labeled cells in the SGL and the SVZ (about 50% over control), whereas 5-HT2A and 5-HT2C receptor subtypes are selectively involved in the regulation of cell proliferation in each of these regions. The activation of 5-HT2C receptors, largely expressed by the choroid plexus, produces a 56% increase in the SVZ, while blockade of 5-HT2A receptors produces a 63% decrease in the number of proliferating cells in the SGL. In addition to the influence of 5-HT1B autoreceptors on 5-HT terminals in the hippocampus and ventricles, 5-HT1B heteroreceptors also regulate cell proliferation in the SGL. These data indicate that multiple receptor subtypes mediate the potent, partly selective of each neurogenic zone, stimulatory action of 5-HT on adult brain cell proliferation. Furthermore, both acute and chronic administration of selective 5-HT1A and 5-HT2C receptor agonists produce consistent increases in the number of newly formed neurons in the DG and/or olfactory bulb, underscoring the beneficial effects of 5-HT on adult neurogenesis.

Newly proliferated cells in the adult male amygdala are affected by gonadal steroid hormones

Gonadal steroid hormones play an important role in the proliferation, survival, and activation of neurons. The present study was performed to examine the effects of testosterone and its metabolites on newly proliferated cells in the amygdala of adult male meadow voles (Microtus pennsylvanicus). Treatment with testosterone propionate (TP) in castrated males resulted in plasma testosterone levels similar to males following mating. TP-treated males displayed a significant increase in the density of cells labeled with a cell proliferation marker (BrdU) in the amygdala. Treatment with estradiol benzoate (EB) exerted a similar effect as TP on the density of BrdU-labeled cells, whereas 5alpha-dihydrotestosterone (DHT) was ineffective. A larger proportion (approximately 44%) of the BrdU-labeled cells in the amygdala displayed a neuronal phenotype, and a lesser percentage (approximately 35%) displayed a glial progenitor phenotype; however, treatment effects were not found in either population of cells. Hormonal effects appeared to be site-specific as no group differences were found in the dentate gyrus of the hippocampus or ventromedial hypothalamus. Finally, a time course study indicated that BrdU-labeled cells in the amygdala are present as early as 30 min following an acute injection of BrdU. Together, these data suggest that gonadal steroid hormones influence the number of newly proliferated cells in the amygdala, most likely by acting through an estrogenic mechanism, and these effects may be exerted on locally proliferating progenitors within the amygdala.

The developmental role of serotonin: news from mouse molecular genetics

New genetic models that target the serotonin system show that transient alterations in serotonin homeostasis cause permanent changes to adult behaviour and modify the fine wiring of brain connections. These findings have revived a long-standing interest in the developmental role of serotonin. Molecular genetic approaches are now showing us that different serotonin receptors, acting at different developmental stages, modulate different developmental processes such as neurogenesis, apoptosis, axon branching and dendritogenesis. Our understanding of the specification of the serotonergic phenotype is improving. In addition, studies have revealed that serotonergic traits are dissociable, as there are populations of neurons that contain serotonin but do not synthesize it.

Increased P27KIP1 protein expression in the dentate gyrus of chronically stressed rats indicates G1 arrest involvement

Various chronic stress paradigms decrease new cell proliferation in the hippocampal dentate gyrus, yet the exact underlying mechanism is still unclear. In the first gap (G1) phase of the cell cycle, both stimulatory and inhibitory signals derived from the extracellular environment converge. Corticosteroids, which increase during stress and are well-known anti-mitotics, cause cells in vitro to arrest in the G1 phase. Following 3 weeks of unpredictable stress, we therefore expected a change in protein expression of various important G1 cell cycle regulators in the adult rat subgranular zone. Using quantitative immunocytochemistry, we show that particularly cyclin-dependent kinase inhibitor p27Kip1 expression is significantly increased. In addition, 3 weeks of recovery after stress normalized the numbers of p27Kip1-expressing cells, consistent with the recovered adult cell proliferation in these animals. P27Kip1-positive cells do not overlap with GFAP-staining and only to a limited extent with Ki-67-expressing cells. Numbers of cyclin E- and cyclin D1-expressing cells did not change after chronic stress. These results indicate that chronic stress causes cycling cells in the adult hippocampus to arrest in G1, thereby providing more mechanistic insight in the stress-induced decrease in cell proliferation.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

Implication of corticosteroid receptors in the regulation of hippocampal structural plasticity

The dentate gyrus is one of the few areas of the adult brain that continues to produce neurons and to express the embryonic polysialylated isoforms of neuronal cell adhesion molecules (PSA-NCAM). The stress hormone corticosterone exerts a complex modulation on neurogenesis and PSA-NCAM, and previous studies have shown that mature granule cells require corticosterone for their survival. Thus, the aim of our work was to investigate the respective role of the different corticosteroid receptors on these three parameters in adrenalectomized rats. It was found that treatment with a low dose of the mineralocorticoid receptor agonist, aldosterone, prevents only the adrenalectomy-induced increase in cell death. Treatment with a higher dose of aldosterone normalized cell proliferation whereas PSA-NCAM expression was normalized only by treatment with the glucocorticoid receptor agonist, RU 28362. It is concluded that stimulation of the mineralocorticoid receptor is sufficient to mediate the effects of corticosterone on neurogenesis and to protect mature cells from cell death whereas stimulation of the glucocorticoid receptor is necessary to modulate PSA-NCAM expression.

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.

Depressed new neurons--adult hippocampal neurogenesis and a cellular plasticity hypothesis of major depression

In a novel theory, a failure of adult hippocampal neurogenesis has been proposed to provide the biological and cellular basis of major depression. The as yet unresolved function of the new hippocampal neurons will have to be in the center of any attempt to prove this hypothesis. Only knowledge of normal functional relevance of new neurons will allow an assessment of their potential role in disturbed hippocampal function in depression; however, major depression is not primarily a hippocampal disorder. We therefore propose that consideration of the neurogenesis hypothesis of depression be the most prominent aspect of a more general cellular plasticity hypothesis.

Regulation of life-long neurogenesis in the decapod crustacean brain

This article provides an overview of our understanding of life-long neurogenesis in the decapod crustacean brain, where the proliferation of sensory and interneurons is controlled by many of the same factors as is neurogenesis in the mammalian brain. The relative simplicity, spatial organization and accessibility of the crustacean brain provide opportunities to examine specific neuronal pathways that regulate neurogenesis and the sequence of gene expression that leads to neuronal differentiation.

Neuropeptide Y is neuroproliferative for post-natal hippocampal precursor cells

New neurones are produced in the adult hippocampus throughout life and are necessary for certain types of hippocampal learning. Little, however, is known about the control of hippocampal neurogenesis. We used primary hippocampal cultures from early post-natal rats and neuropeptide Y Y1 receptor knockout mice as well as selective neuropeptide Y receptor antagonists and agonists to demonstrate that neuropeptide Y is proliferative for nestin-positive, sphere-forming hippocampal precursor cells and beta-tubulin-positive neuroblasts and that the neuroproliferative effect of neuropeptide Y is mediated via its Y1 receptor. Immunohistochemistry confirmed Y1 receptor staining on both nestin-positive cells and beta-tubulin-positive cells in culture and short pulse 5-bromo-2-deoxyuridine studies demonstrated that neuropeptide Y has a proliferative effect on both cell types. These studies suggest that the proliferation of hippocampal neuroblasts and precursor cells is increased by neuropeptide Y and, therefore, that hippocampal learning and memory may be modulated by neuropeptide Y-releasing interneurones.

S100B and response to treatment in major depression: a pilot study

S100B is a protein which exerts both detrimental and neurotrophic effects, depending on its concentration in brain tissue. An increase of S100B in micromolar concentrations is observed in traumatic brain conditions and is associated with poor outcome. Micromolar levels of extracellular S100B in vitro may have deleterious effects. However, in nanomolar concentrations S100B has multiple neurotrophic effects in vitro may in vivo be regarded as a hallmark of neuroprotective efforts. This pilot study addresses the hypothesis that S100B serum concentrations may be of predictive validity for the response to antidepressant treatment in patients with major depression. S100B plasma levels were determined in 25 patients with major depression and 25 matched healthy controls using an immunofluorimetric sandwich assay. S100B plasma levels were significantly higher in major depressive patients than in healthy controls and positively correlated with treatment response after 4 weeks of treatment. In a linear regression model, a significant predictive effect was found only for S100B and severity of depressive symptoms upon admission. These results suggest that neuroprotective functions of S100B counterbalance neurodegenerative mechanisms that are involved in the pathophysiology of major depression and in the response to antidepressant treatment.

Chronic AMPA receptor potentiator (LY451646) treatment increases cell proliferation in adult rat hippocampus

Stress-induced neuronal atrophy and death in the hippocampus may play an important role in the etiology of clinical depression. Conventional antidepressants can stimulate hippocampal neurogenesis after chronic administration. AMPA receptor potentiators (ARPs) such as LY392098 and LY451616 are active in both the forced swim test and the tail suspension test, two behavioral despair procedures widely used to predict antidepressant efficacy. Unlike traditional antidepressants, this group of compounds does not affect extracellular concentrations of biogenic amines. In this study, we investigated the effect of LY451646 on progenitor cell proliferation in adult rat hippocampus. Male Sprague-Dawley rats (n = 4-5 per group) received either single or chronic (21 days) doses of LY451646 (0.025-0.5 mg/kg). Bromodeoxyuridine (BrdU) injections and immunohistochemistry were performed 30 min and 24 h after the last drug injection, respectively. Results show that chronic LY451646 treatment increased progenitor cell proliferation (approximately 45%) in the dentate gyrus in a dose-dependent manner. This upregulation of BrdU labeling appeared as an increase in the number of cells arranged in clusters. Similarly, a significant increase in the number of cells in clusters was observed after a single injection of LY451646 (0.05 mg/kg), although the increase in total number of BrdU-positive cells (approximately 30%) did not reach statistical significance. This is the first in vivo study showing the modulation of progenitor cell proliferation by an ARP. These findings suggest that the antidepressant-like activity of ARPs in animals may be attributed, at least in part, to the regulation of progenitor cell proliferation in the hippocampus.

Differential influence of rearing conditions and methamphetamine on serotonin fibre maturation in the dentate gyrus of gerbils (Meriones unguiculatus)

Environmental experience and drugs are two parameters that affect the maturation of neurotransmitter systems. The influence of impoverished rearing (IR) versus enriched rearing (ER) was compared in conjunction with postnatal methamphetamine (MA) treatment. The densities of immunostained 5-HT fibres were quantified in septal and temporal regions of the hippocampal dentate gyrus (DG) in young adult gerbils. In the IR group, 5-HT fibre densities were significantly increased in the molecular, granular and polymorphic layers of the DG in the temporal plane. After postnatal MA treatment, the 5-HT fibre density in the ER group reached a level equivalent to that of the IR group in nearly all respects. Under IR conditions, the pharmacological intervention significantly increased the maturation of fibre densities in septal layers only in the right hemisphere with no significant alterations in the left hemisphere and in temporal regions of either hemisphere. According to our previous studies on hippocampal neurogenesis, adaptations of 5-HT fibre densities partly proved to be positively correlated to cell proliferation rates for each of the specific conditions. Thus, the induced MA sensitivity, caused by pharmacological intervention at day 14, was manifested as direct interaction of 5-HT fibre maturation and cell proliferation in dependence of environmental factors. Both IR and MA together give us a better understanding of raphe-hippocampal plasticity and offer new perspectives for pharmacological studies on the 5-HT participation in mental disorders.

Neural stem cells and alcohol

This article summarizes the proceedings of a symposium held at the 2002 Research Society on Alcoholism Meeting in San Francisco, California. The aim of this symposium was to review research on the effects of ethanol on neural stems cells and neurogenesis. Ethanol is known to alter neurogenesis during development; however, recent studies indicate that the brain forms new neurons from stem cells throughout life. Furthermore, stem cells can be transplanted into the brain, creating exciting new possibilities to study brain function. The symposium covered these research areas. Dr. Michael W. Miller reviewed knowledge on the effects of ethanol on stem cell proliferation and differentiation during development. Dr. Wu Ma described studies in culture indicating that (1) neural stem cells express functional muscarinic acetylcholine receptors (mAchR), (2) mAchR-mediated proliferation involves Ca signaling and mitogen-activated protein kinase phosphorylation, and (3) phosphoinositol-3 kinase is a downstream effector for mAchR-mediated cell proliferation via activation of Akt. Drs. Kim Nixon and Fulton T. Crews followed with in vivo studies on ethanol's effects on adult neural stem cell proliferation and differentiation. Dr. W. Michael Zawada described studies directed at dopamine neuron cell transplants into mammalian central nervous system. These studies clearly establish that ethanol has significant effects on stem cells.