Delayed kindling epileptogenesis and increased neurogenesis in adult rats housed in an enriched environment

Environmental risk factors such as stressful experiences have long been recognized to affect seizure susceptibility, but little attention has been paid to the potential effects of improving housing conditions. In this study, we investigated the influence of an enriched environment on epileptogenesis. Epileptic susceptibility was assessed in animals housed in an enriched environment either before and during (group I) or only during (group II) a kindling procedure and in animals placed in isolated conditions (group III). The kindling paradigm provides a reliable assessment of the capacity to develop seizures following repeated daily low-frequency electrical stimulations. As both enriched environment and seizures are known to interfere with hippocampal neurogenesis, the number of newly generated dentate cells was assessed before and after the kindling procedure to investigate in more detail the relationship between epileptogenesis and neurogenesis. We found that susceptibility to developing epilepsy differed in animals housed in complex enriched environments and in those housed in isolated conditions. Kindling epileptogenesis occurred significantly later in animals housed in enriched conditions throughout the procedure (group I) than in animals from groups II and III. We also demonstrated that cells generated during kindling survived for at least 42 days and that these cells were more numerous on both sides of the brain following environmental enrichment than in rats housed in isolated conditions. As similar values were obtained regardless of the duration of the period of enrichment, these cellular changes may not play a major role in delaying kindling development. We suggest that the increase response in neurogenesis following seizures may be an adaptative rather an epileptogenic response.     

Effects of enriched environment on morphine-induced reward in mice

Drug addiction and abuse have been extremely serious problems in our society. The effect of different environmental conditions on the susceptibility of human to drug addiction and abuse is still not fully understood. It was recently reported that enriched environment can trigger long-term modification in neural functions and might prevent the occurrence of pathogenic behaviors. Here we investigated the effects of enriched environment on morphine-induced reward in mice. We found that the enriched environment attenuated the acute morphine (10 mg/kg)-induced hyperlocomotion and repeated morphine (10 mg/kg)-induced behavioral sensitization. Moreover, the environmental condition blocked the conditioned place preference (CPP) of the mice induced by morphine (5 mg/kg). Associated with behavioral alterations, the expression of FosB-like proteins in the nucleus accumbens of the brain was down-regulated in the mice housed in the enriched environmental condition, but not in the standard environment. Taken together, these results indicate that enriched environmental condition leads to decrease in sensitivity of the mice to morphine-induced reward.     

Effects of undernourishment,recurrent seizures and enriched environment during early life in hippocampal morphology

It has been recently shown that enriched environment led to a significant benefit in learning and retention of visual–spatial memory, being able to reverse the cognitive impairment generated by undernourishment and recurrent seizures. We investigated the hippocampal morphological effects of recurrent seizures and undernourishment early in life in Wistar rats and the possible benefits produced by the enriched environment in these conditions. The morphological parameters stereologically evaluated were hippocampal volume, thickness of pyramidal stratum of the CA1 subfield and neuronal and glial densities in the same subfield. Male Wistar rats were divided into eight groups including nourished, nourished + enriched environment, nourished + recurrent seizures, nourished + recurrent seizures + enriched environment, undernourished, undernourished + enriched environment, undernourished + recurrent seizures and undernourished + recurrent seizures + enriched environment. Undernourishment model consisted in nutritional deprivation regimen from post-natal day 2 (P2) to P15. From P8 to P10, recurrent seizures group were induced by flurothyl three times per day. Enriched environment groups were exposed between P21 and P51. Our main findings were: (1) animals submitted to the enriched environment showed an increased hippocampal volume; (2) enriched environment promotes increases in the thickness of the pyramidal layer in hippocampal CA1 subfield in animals nourished and undernourished with recurrent seizures; (3) undernourishment during early development decreased neuronal density in CA1 and CA3 subfields. Our findings show that these three conditions induces important changes in hippocampal morphology, the most deleterious changes are induced by undernourishment and recurrent seizures, while more beneficial morphological changes are produced by enriched environment.     

Gender and endogenous levels of estradiol do not influence adult hippocampal neurogenesis in mice

In several species, including rat and vole, the proliferation of new neurons in the adult dentate gyrus (DG) subgranular zone (SGZ) is influenced by both gender and endogenous levels of the gonadotropic steroid hormone estradiol. However, little is known about how adult neurogenesis is regulated by these factors in the mouse. We report here that adult C57BL/6 mice do not have gender differences in hippocampal proliferation or neurogenesis. In addition, the production of new SGZ cells in female mice was not influenced by estrous cycle or after ovariectomy, suggesting that fluctuations in endogenous estradiol levels do not alter adult neurogenesis in the mouse. Both male and female mice had a greater number of BrdU-immunoreactive SGZ cells following chronic treatment with fluoxetine. This demonstrates a parallel proliferation response in both genders, and opens avenues for addressing the neurogenesis hypothesis of depression in female rodents. These findings underscore a distinct regulation of adult neurogenesis in mice vs. other rodents, and are discussed in regard to their implications for the study of adult hippocampal neurogenesis.     

Chronic stress followed by social isolation promotes depressive-like behaviour,alters microglial and astrocyte biology and reduces hippocampal neurogenesis in male mice

Unpredictable chronic mild stress (UCMS) is one of the most commonly used, robust and translatable models for studying the neurobiological basis of major depression. Although the model currently has multiple advantages, it does not entirely follow the trajectory of the disorder, whereby depressive symptomology can often present months after exposure to stress. Furthermore, patients with depression are more likely to withdraw in response to their stressful experience, or as a symptom of their depression, and, in turn, this withdrawal/isolation can further exacerbate the stressful experience and the depressive symptomology. Therefore, we investigated the effect(s) of 6 weeks of UCMS followed by another 6 weeks of social isolation (referred to as UCMSI), on behaviour, corticosterone stress responsivity, immune system functioning, and hippocampal neurogenesis, in young adult male mice. We found that UCMSI induced several behavioural changes resembling depression but did not induce peripheral inflammation. However, UCMSI animals showed increased microglial activation in the ventral dentate gyrus (DG) of the hippocampus and astrocyte activation in both the dorsal and ventral DG, with increased GFAP-positive cell immunoreactivity, GFAP-positive cell hypertrophy and process extension, and increased s100β-positive cell density. Moreover, UCMSI animals had significantly reduced neurogenesis in the DG and reduced levels of peripheral vascular endothelial growth factor (VEGF) – a trophic factor produced by astrocytes and that stimulates neurogenesis. Finally, UCMSI mice also had normal baseline corticosterone levels but a smaller increase in corticosterone following acute stress, that is, the Porsolt Swim Test. Our work gives clinically relevant insights into the role that microglial and astrocyte functioning, and hippocampal neurogenesis may play in the context of stress, social isolation and depression, offering a potentially new avenue for therapeutic target.     

Experience‐dependent regulation of adult hippocampal neurogenesis: Effects of long‐term stimulation and stimulus withdrawal

Exposure to an enriched environment has been shown to cause an increase in neurogenesis in the dentate gyrus of adult mice. In this study we examined how this experience‐dependent response in adult hippocampal neurogenesis of C57BL/6 mice is modulated under the conditions of long‐term stimulation and of withdrawal from the enriched environment. We found that a group which experienced withdrawal from the enriched environment 3 months earlier, had more than twice as many proliferating cells in the subgranular zone as controls and mice experiencing long‐term stimulation. We propose that the greater number of proliferating cells after withdrawal reflects a survival‐promoting effect on the dividing neuronal stem and progenitor cells during the earlier period of stimulation. No differences between the groups were observed in the number of surviving progeny or their phenotypes. Therefore, the existence of more dividing cells in the withdrawal group did not translate into a significant net increase in neurogenesis in the absence of continued stimulation. Similarly, the finding in the group experiencing long‐term stimulation showing no clear benefit over controls could be interpreted as a diminished efficiency of continued environmental stimuli to elicit a neurogenic response. Thus, we propose as a working hypothesis that: 1) stimulation early in life may preserve the neurogenic potential in the dentate gyrus, and 2) the novelty of complex stimuli rather than simply continued exposure to complex stimuli elicits the environmental effects on adult hippocampal neurogenesis. Hippocampus 1999;9:321–332. © 1999 Wiley‐Liss, Inc.     

Weaning age, social isolation, and gender, interact to determine adult explorative and social behavior, and dendritic and spine morphology in prefrontal cortex of rats

We tested the effect of weaning at 21 or 30 days, followed by individual or group housing, on explorative and social behavior in adult male and female rats, and in males, on dendritic length and spine density in prefrontal cortex. In the open field, rats weaned early were the most active, while those weaned late and group housed were the most explorative. In the social interaction test, behavior in adult females was relatively impervious to weaning age or rearing condition. Isolated males sought out social interaction, whereas, group-reared males tended to avoid it. Social behaviors in males weaned early or group-reared correlated with decreased dendrite length and spine density, whereas, non-social behaviors correlated with increased dendritic length. Such changes are consistent with neural pruning in the development of social behavior. Although our experimental manipulations were mild, and serve as standard rearing conditions in many laboratories, their effects on brain and behavior were marked, and differed by gender. Early rearing conditions may have few appreciable effects when studied in isolation, but their interactive effects on adult social behavior are significant and varied.     

Therapeutic effect of Yokukansan on social isolation-induced aggressive behavior of zinc-deficient and pair-fed mice

In patients with dementia including Alzheimer's disease, hallucinations, agitation/aggression and irritability are known to frequently occur and as distressing behavioral and psychological symptoms of dementia (BPSD). On the basis of the evidence on clinical efficacy and safety of Yokukansan, a traditional Japanese herbal medicine, on BPSD, in the present study, Yokukansan was examined in the therapeutic effects on social isolation-induced aggressive behavior of zinc-deficient and pair-fed mice. Yokukansan was p.o. administered for 7 days as a drinking water to isolated mice fed a zinc-deficient diet for 10 days, which exhibited aggressive behavior, and isolated pair-fed mice fed a control diet of the amount consumed by zinc-deficient mice for 10 days, which exhibited aggressive behavior. Aggressive behavior was evaluated by the resident-intruder test. Yokukansan (312 mg/kg/day) attenuated both aggressive behaviors of zinc-deficient and pair-fed mice. Because Yokukansan can suppress abnormal glutamatergic neuron activity, MK-801, an N-methyl-D-aspartate (NMDA) receptor blocker, and aminooxyacetic acid (AOAA), a γ-amino butyric acid (GABA) transaminase blocker, were also examined in the effects on social isolation-induced aggressive behavior. MK-801 (0.1 mg/kg) or AOAA (23 mg/kg) was i.p. injected into isolated aggressive mice. Thirty minutes later, the resident-intruder test was performed to evaluate the effect of the drugs. Both drugs attenuated aggressive behavior of zinc deficient mice, but not that of pair-fed mice. These results suggest that Yokukansan ameliorates social isolation-induced aggressive behavior of zinc-deficient and pair-fed mice through the action against glutamatergic neurotransmitter system and other neurotransmitter systems.     

Effects of intra-hippocampal infusion of WAY-100635 on plus-maze behavior in mice. Influence of site of injection and prior test experience

The positive profile of systemically-administered 5-HT(1A) receptor antagonists in several rodent models of anxiolytic activity suggests an important role for postsynaptic 5-HT(1A) receptor mechanisms in anxiety. To test this hypothesis, we investigated the effects of WAY-100635 microinfusions (0, 0.1, 1.0 or 3.0 microg in 0.2 microl) into the dorsal (DH) or ventral (VH) hippocampus on behaviours displayed by male Swiss-Webster mice in the elevated plus-maze. As prior experience is known to modify pharmacological responses in this test, the effects of intra-hippocampal infusions were examined both in maze-na?ve and maze-experienced subjects. Test videotapes were scored for conventional indices of anxiety (% open arm entries/time) and locomotor activity (closed arm entries), as well as a range of ethological measures (e.g. risk assessment). In maze-na?ve mice, intra-VH (but not intra-DH) infusions of WAY-100635 (3.0 microg but not lower doses) increased open arm exploration and reduced risk assessment. These effects were observed in the absence of significant changes in locomotor activity. In contrast, neither intra-VH nor intra-DH infusions of WAY-100635 altered the behaviour of maze-experienced mice. These findings suggest that postsynaptic 5-HT(1A) receptors in the ventral (but not dorsal) hippocampus play a significant role both in the mediation of plus-maze anxiety in mice and in experientially-induced alterations in responses to this test.     

Effects of isolation on activity,reactivity, excitability and aggressive behavior in two inbred strains of mice

In order to investigate mechanisms of isolation-induced aggressive behavior, inbred mice of the C57BL/6 and DBA/2 strains were individually housed over a period of 8 weeks. In the DBA/2 strain only, isolation was followed by a clear increase in activity (Animex), reactivity (reactions upon tactile body stimulation), excitability (duration of EEG desynchronization elicited by tactile stimulation of the thorax area under urethane anesthesia) and intermale aggression (biting and fighting responses). The use of inbred strains of mice proved to be a useful tool for the examination of the relationship between various parameters. It is concluded that there are no clear correlations between activity, reactivity and aggressive behavior and that the resulting aggressive responses in the DBA/2 strain are likely due to the increase of excitability.     

Effects of juvenile isolation and morphine treatment on social interactions and opioid receptors in adult rats: behavioural and autoradiographic studies

The consequences of juvenile isolation and morphine treatment during the isolation period on (social) behaviour and mu-, delta- and kappa-opioid receptors in adulthood were investigated by using a social interaction test and in vitro autoradiography in rats. Juvenile isolation reduced social exploration in adults. Morphine treatment counteracted this reduction in isolated rats, but decreased social exploration in nonisolated rats. Self-grooming and nonsocial exploration were enhanced after juvenile isolation. Morphine treatment had no effect on self-grooming, but suppressed nonsocial exploration in isolated rats. With respect to the opioid receptors, juvenile isolation resulted in regiospecific increases in mu-binding sites with a 58% increase in the basolateral amygdala and a 33% increase in the bed nucleus of stria terminalis. Morphine treatment in isolated rats reversed this upregulation in both areas. The number of delta-binding sites did not differ between the experimental groups. A general upregulation of kappa-binding sites was observed after juvenile isolation, predominantly in the cortical regions, the hippocampus and the substantia nigra. Morphine treatment did not affect the upregulation of kappa-receptors. The results show that juvenile isolation during the play period causes long-term effects on social and nonsocial behaviours and on the number of mu- and kappa- but not delta-opioid receptors in distinct brain areas. The number of mu-receptors in the basolateral amygdala appears to be negatively correlated with the amount of social exploration in adult rats.     

Effects of social defeat stress and fluoxetine treatment on neurogenesis and behavior in mice that lack zinc transporter 3 (ZnT3) and vesicular zinc

Depression is a leading cause of disability worldwide, in part because the available treatments are inadequate and do not work for many people. The neurobiology of depression, and the mechanism of action of common antidepressant drugs such as selective serotonin reuptake inhibitors (SSRIs), is not well understood. One mechanism thought to underlie the effects of these drugs is upregulation of adult hippocampal neurogenesis. Evidence indicates that vesicular zinc is required for modulation of adult hippocampal neurogenesis, at least under some circumstances. Vesicular zinc refers to zinc that is stored in the synaptic vesicles of certain neurons, including in the hippocampus, and released in response to neuronal activity. It can be eliminated from the brain by deletion of zinc transporter 3 (ZnT3), as is the case in ZnT3 knockout mice. Here, we examined the effects of repeated social defeat stress and subsequent chronic treatment with the SSRI fluoxetine on behavior and neurogenesis in ZnT3 knockout mice. We hypothesized that fluoxetine treatment would increase neurogenesis and reverse stress‐induced behavioral symptoms in wild type, but not ZnT3 knockout, mice. As anticipated, stress induced persistent depression‐like effects, including social avoidance and anxiety‐like behavior. Fluoxetine decreased social avoidance, though the effect was not specific to the stressed mice, but did not affect anxiety‐like behavior. Surprisingly, stress increased the survival of neurons born 1 day after the last episode of defeat stress. Fluoxetine treatment also increased cell survival, particularly in wild type mice, though it did not affect proliferation. Our results did not support our hypothesis that vesicular zinc is required for the behavioral benefits of fluoxetine treatment. As to whether vesicular zinc is required for the neurogenic effects of fluoxetine, our results were inconclusive, warranting further investigation into the role of vesicular zinc in adult hippocampal neurogenesis.     

Effects of perinatal exposure to low dose of bisphenol A on anxiety like behavior and dopamine metabolites in brain

Bisphenol A (BPA), an endocrine-disrupting chemical, is widely present in the environment. It has been reported that perinatal exposure to low doses of BPA that are less than the tolerable daily intake level (50 μg/kg/day) affects anxiety-like behavior and dopamine levels in the brain. Although the dopaminergic system in the brain is considered to be related to anxiety, no study has reported the effects of low-dose BPA exposure on the dopaminergic system in the brain and on anxiety-like behavior using the same methods of BPA exposure.