A Transgenic Rat for Specifically Inhibiting Adult Neurogenesis

A complex relation between levels of adult hippocampal neurogenesis and expression of the immature neuron marker doublecortin

We investigated the mechanisms underlying the effects of the antidepressant fluoxetine on behavior and adult hippocampal neurogenesis (AHN). After confirming our earlier report that the signaling molecule β-arrestin-2 (β-Arr2) is required for the antidepressant-like effects of fluoxetine, we found that the effects of fluoxetine on proliferation of neural progenitors and survival of adult-born granule cells are absent in the β-Arr2 knockout (KO) mice. To our surprise, fluoxetine induced a dramatic upregulation of the number of doublecortin (DCX)-expressing cells in the β-Arr2 KO mice, indicating that this marker can be increased even though AHN is not. We discovered two other conditions where a complex relationship occurs between the number of DCX-expressing cells compared to levels of AHN: a chronic antidepressant model where DCX is upregulated and an inflammation model where DCX is downregulated. We concluded that assessing the number of DCX-expressing cells alone to quantify levels of AHN can be complex and that caution should be applied when label retention techniques are unavailable.

Rewarded Maze Training Increases Approach Behavior in Rats Through Neurogenesis-Dependent Growth of Ventral Hippocampus-Prelimbic Circuits

Background

Learning complex navigation routes increases hippocampal volume in humans, but it is not clear whether this growth impacts behaviors outside the learning situation or what cellular mechanisms are involved.

Methods

We trained rats with pharmacogenetic suppression of adult neurogenesis and littermate controls in 3 mazes over 3 weeks and tested novelty approach behavior several days after maze exposure. We then measured hippocampus and prelimbic cortex volumes using magnetic resonance imaging and assessed neuronal and astrocyte morphology. Finally, we investigated the activation and behavioral role of the ventral CA1 (vCA1)-to-prelimbic pathway using immediate-early genes and DREADDs (designer receptors exclusively activated by designer drugs).

Results

Maze training led to volume increase of both the vCA1 region of the hippocampus and the prelimbic region of the neocortex compared with rats that followed fixed paths. Growth was also apparent in individual neurons and astrocytes in these 2 regions, and behavioral testing showed increased novelty approach in maze-trained rats in 2 different tests. Suppressing adult neurogenesis prevented the effects on structure and approach behavior after maze training without affecting maze learning itself. The vCA1 neurons projecting to the prelimbic area were more activated by novelty in maze-trained animals, and suppression of this pathway decreased approach behavior.

Conclusions

Rewarded navigational learning experiences induce volumetric and morphologic growth in the vCA1 and prelimbic cortex and enhance activation of the circuit connecting these 2 regions. Both the structural and behavioral effects of maze training require ongoing adult neurogenesis, suggesting a role for new neurons in experience-driven increases in novelty exploration.

Progenitor Cells Play a Role in Reinstatement of Ethanol Seeking in Adult Male and Female Ethanol Dependent Rats

Female and male glial fibrillary acidic protein-thymidine kinase (GFAP-TK) transgenic rats were made ethanol dependent via a six-week chronic intermittent ethanol vapor (CIE) and ethanol drinking (ED) procedure. During the last week of CIE, a subset of male and female TK rats was fed valcyte to ablate dividing progenitor cells and continued the diet until the end of this study. Following week six, all CIE rats experienced two weeks of forced abstinence from CIE-ED, after which they experienced relapse to drinking, extinction, and reinstatement of ethanol seeking sessions. CIE increased ED in female and male rats, with females having higher ethanol consumption during CIE and relapse sessions compared with males. In both sexes, valcyte reduced the levels of Ki-67-labeled progenitor cells in the subgranular zone of the dentate gyrus and did not alter the levels in the medial prefrontal cortex (mPFC). Valcyte increased ED during relapse, increased lever responses during extinction and, interestingly, enhanced latency to extinguish ethanol-seeking behaviors in males. Valcyte reduced the reinstatement of ethanol-seeking behaviors triggered by ethanol cues in females and males. Reduced seeking by valcyte was associated with the normalization of cytokines and chemokines in plasma isolated from trunk blood, indicating a role for progenitor cells in peripheral inflammatory responses. Reduced seeking by valcyte was associated with increases in tight junction protein claudin-5 and oligodendrogenesis in the dentate gyrus and reduction in microglial activity in the dentate gyrus and mPFC in females and males, demonstrating a role for progenitor cells in the dentate gyrus in dependence-induced endothelial and microglial dysfunction. These data suggest that progenitor cells born during withdrawal and abstinence from CIE in the dentate gyrus are aberrant and could play a role in strengthening ethanol memories triggered by ethanol cues via central and peripheral immune responses.

Hippocampal neurogenesis promotes effortful responding but does not regulate effort-based choice

A fundamental trait of depression is low motivation. Hippocampal neurogenesis has been associated with motivational deficits but detailed evidence on how it regulates human-relevant behavioral traits is still missing. We used the hGFAP-TK rat model to deplete actively dividing neural stem cells in the rat hippocampus. Use of the effort-discounting operant task allowed us to identify specific and detailed deficits in motivation behavior. In this task, rats are given a choice between small and large food rewards, where 2-20 lever presses are required to obtain the large reward (four sugar pellets) versus one press to receive the smaller reward (two sugar pellets). We found that depleting adult neurogenesis did not affect effort-based choice or general motivation to complete the task. However, lack of adult neurogenesis reduced the pressing rate and thus increased time to complete the required presses to obtain a reward. In summary, the present study finds that adult hippocampal neurogenesis specifically reduces response vigor to obtain rewards and thus deepens our understanding in how neurogenesis shapes depression.

Inhibition of Hippocampal Neurogenesis Starting in Adolescence Increases Anxiodepressive Behaviors Amid Stress

Stressors during the adolescent period can affect development of the brain and have long-lasting impacts on behavior. Specifically, adolescent stress impairs hippocampal neurogenesis and can increase risk for anxiety, depression, and a dysregulated stress response in adulthood. In order to model the functional effects of reduced hippocampal neurogenesis during adolescence, a transgenic neurogenesis ablation rat model was used to suppress neurogenesis during the adolescent period and test anxiodepressive behaviors and stress physiology during adulthood. Wildtype and transgenic (TK) rats were given valganciclovir during the first two weeks of adolescence (4-6 weeks old) to knock down neurogenesis in TK rats. Starting in young adulthood (13 weeks old), blood was sampled for corticosterone at several time points following acute restraint stress to measure negative feedback of the stress response, and rats were tested on a battery of anxiodepressive tests at baseline and following acute restraint stress. Although TK rats had large reductions in both cell proliferation during adolescence, as measured by bromodeoxyuridine (BrdU), and ongoing neurogenesis in adulthood (by doublecortin), resulting in decreased volume of the dentate gyrus, negative feedback of the stress response following acute restraint was similar across all rats. Despite similar stress responses, TK rats showed higher anxiety-like behavior at baseline. In addition, only TK rats had increased depressive-like behavior when tested after acute stress. Together, these results suggest that long-term neurogenesis ablation starting in adolescence produces hippocampal atrophy and increases behavioral caution and despair amid stressful environments.

Suppression of adult cytogenesis in the rat brain leads to sex-differentiated disruption of the HPA axis activity

OBJECTIVES

The action of stress hormones, mainly glucocorticoids, starts and coordinates the systemic response to stressful events. The HPA axis activity is predicated on information processing and modulation by upstream centres, such as the hippocampus where adult-born neurons (hABN) have been reported to be an important component in the processing and integration of new information. Still, it remains unclear whether and how hABN regulates HPA axis activity and CORT production, particularly when considering sex differences.

MATERIALS AND METHODS

Using both sexes of a transgenic rat model of cytogenesis ablation (GFAP-Tk rat model), we examined the endocrinological and behavioural effects of disrupting the generation of new astrocytes and neurons within the hippocampal dentate gyrus (DG).

RESULTS

Our results show that GFAP-Tk male rats present a heightened acute stress response. In contrast, GFAP-Tk female rats have increased corticosterone secretion at nadir, a heightened, yet delayed, response to an acute stress stimulus, accompanied by neuronal hypertrophy in the basal lateral amygdala and increased expression of the glucocorticoid receptors in the ventral DG.

CONCLUSIONS

Our results reveal that hABN regulation of the HPA axis response is sex-differentiated.

Hippocampal cytogenesis abrogation impairs inter-regional communication between the hippocampus and prefrontal cortex and promotes the time-dependent manifestation of emotional and cognitive deficits

Impaired ability to generate new cells in the adult brain has been linked to deficits in multiple emotional and cognitive behavioral domains. However, the mechanisms by which abrogation of adult neural stem cells (NSCs) impacts on brain function remains controversial. We used a transgenic rat line, the GFAP-Tk, to selectively eliminate NSCs and assess repercussions on different behavioral domains. To assess the functional importance of newborn cells in specific developmental stages, two parallel experimental timeframes were adopted: a short- and a long-term timeline, 1 and 4 weeks after the abrogation protocol, respectively. We conducted in vivo electrophysiology to assess the effects of cytogenesis abrogation on the functional properties of the hippocampus and prefrontal cortex, and on their intercommunication. Adult brain cytogenesis abrogation promoted a time-specific installation of behavioral deficits. While the lack of newborn immature hippocampal neuronal and glial cells elicited a behavioral phenotype restricted to hyperanxiety and cognitive rigidity, specific abrogation of mature new neuronal and glial cells promoted the long-term manifestation of a more complex behavioral profile encompassing alterations in anxiety and hedonic behaviors, along with deficits in multiple cognitive modalities. More so, abrogation of 4 to 7-week-old cells resulted in impaired electrophysiological synchrony of neural theta oscillations between the dorsal hippocampus and the medial prefrontal cortex, which are likely to contribute to the described long-term cognitive alterations. Hence, this work provides insight on how newborn neurons and astrocytes display different functional roles throughout different maturation stages, and establishes common ground to reconcile contrasting results that have marked this field.

Hippocampal neurogenesis promotes preference for future rewards

Adult hippocampal neurogenesis has been implicated in a number of disorders where reward processing is disrupted but whether new neurons regulate specific aspects of reward-related decision making remains unclear. Given the role of the hippocampus in future-oriented cognition, here we tested whether adult neurogenesis regulates preference for future, advantageous rewards in a delay discounting paradigm for rats. Indeed, blocking neurogenesis caused a profound aversion for delayed rewards, and biased choice behavior toward immediately available, but smaller, rewards. Consistent with a role for the ventral hippocampus in impulsive decision making and future-thinking, neurogenesis-deficient animals displayed reduced activity in the ventral hippocampus. In intact animals, delay-based decision making restructured dendrites and spines in adult-born neurons and specifically activated adult-born neurons in the ventral dentate gyrus, relative to dorsal activation in rats that chose between immediately-available rewards. Putative developmentally-born cells, located in the superficial granule cell layer, did not display task-specific activity. These findings identify a novel and specific role for neurogenesis in decisions about future rewards, thereby implicating newborn neurons in disorders where short-sighted gains are preferred at the expense of long-term health.

Functional Activation of Newborn Neurons Following Alcohol-Induced Reactive Neurogenesis

Abstinence after alcohol dependence leads to structural and functional recovery in many regions of the brain, especially the hippocampus. Significant increases in neural stem cell (NSC) proliferation and subsequent "reactive neurogenesis" coincides with structural recovery in hippocampal dentate gyrus (DG). However, whether these reactively born neurons are integrated appropriately into neural circuits remains unknown. Therefore, adult male rats were exposed to a binge model of alcohol dependence. On day 7 of abstinence, the peak of reactive NSC proliferation, rats were injected with bromodeoxyuridine (BrdU) to label dividing cells. After six weeks, rats underwent Morris Water Maze (MWM) training then were sacrificed ninety minutes after the final training session. Using fluorescent immunohistochemistry for c-Fos (neuronal activation), BrdU, and Neuronal Nuclei (NeuN), we investigated whether neurons born during reactive neurogenesis were incorporated into a newly learned MWM neuronal ensemble. Prior alcohol exposure increased the number of BrdU+ cells and newborn neurons (BrdU+/NeuN+ cells) in the DG versus controls. However, prior ethanol exposure had no significant impact on MWM-induced c-Fos expression. Despite increased BrdU+ neurons, no difference in the number of activated newborn neurons (BrdU+/c-Fos+/NeuN+) was observed. These data suggest that neurons born during alcohol-induced reactive neurogenesis are functionally integrated into hippocampal circuitry.

Recovery of Hippocampal-Dependent Learning Despite Blunting Reactive Adult Neurogenesis After Alcohol Dependence

Background:

The excessive alcohol drinking that occurs in alcohol use disorder (AUD) causes neurodegeneration in regions such as the hippocampus, though recovery may occur after a period of abstinence. Mechanisms of recovery are not clear, though reactive neurogenesis has been observed in the hippocampal dentate gyrus following alcohol dependence and correlates to recovery of granule cell number.

Objective:

We investigated the role of neurons born during reactive neurogenesis in the recovery of hippocampal learning behavior after 4-day binge alcohol exposure, a model of an AUD. We hypothesized that reducing reactive neurogenesis would impair functional recovery.

Methods:

Adult male rats were subjected to 4-day binge alcohol exposure and two approaches were tested to blunt reactive adult neurogenesis, acute doses of alcohol or the chemotherapy drug, temozolomide (TMZ).

Results:

Acute 5 g/kg doses of EtOH gavaged T6 and T7 days post binge did not inhibit significantly the number of Bromodeoxyuridine-positive (BrdU+) proliferating cells in EtOH animals receiving 5 g/kg EtOH versus controls. A single cycle of TMZ inhibited reactive proliferation (BrdU+ cells) and neurogenesis (NeuroD+ cells) to that of controls. However, despite this blunting of reactive neurogenesis to basal levels, EtOH-TMZ rats were not impaired in their recovery of acquisition of the Morris water maze (MWM), learning similarly to all other groups 35 days after 4-day binge exposure.

Conclusions:

These studies show that TMZ is effective in decreasing reactive proliferation/neurogenesis following 4-day binge EtOH exposure, and baseline levels of adult neurogenesis are sufficient to allow recovery of hippocampal function.

Adult neurogenesis alters response to an aversive distractor in a labyrinth maze without affecting spatial learning or memory

Adult neurogenesis has been implicated in learning and memory of complex spatial environments. However, new neurons also play a role in nonmnemonic behavior, including the stress response and attention shifting. Many commonly used spatial tasks are very simple, and unsuitable for detecting neurogenesis effects, or are aversively motivated, making it difficult to dissociate effects on spatial learning and memory from effects on stress. We have therefore created a novel complex spatial environment, the flex maze, to enable reward-mediated testing of spatial learning in a flexibly configurable labyrinth. Using a pharmacogenetic method to completely inhibit neurogenesis in adulthood, we found that rats lacking new neurons (TK rats) and wild type controls completed and remembered most mazes equally well. However, control rats were slower to complete peppermint-scented mazes than other mazes, while neurogenesis-deficient rats showed no effect of mint on maze behavior, completing these mazes significantly faster than control rats. Additional testing found that wild type and TK rats showed similar detection of, avoidance of, and glucocorticoid response to the mint odor. These results suggest that spatial learning and memory in a labyrinth task is unaffected by the loss of new neurons, but that these cells affect the ability of an aversive stimulus to distract rats from completing the maze.

Voluntary wheel running during adolescence distinctly alters running output in adulthood in male and female rats

Adult female rats show greater running output compared with age-matched male rats, and the midbrain dopaminergic system may account for behavioral differences in running output. However, it is unknown if the lower running output in adult males can be regulated by wheel running experience during adolescence, and whether wheel running experience during adolescence will diminish the sex differences in running output during adulthood. We therefore determined and compared the exercise output in adult male and female rats that either had initiated voluntary wheel running only during adulthood or during adolescence. Our results demonstrate that running output in adult males were significantly higher when running was initiated during adolescence, and this higher running output was not significantly different from females. Running output did not differ during adulthood in females when wheel running was initiated during adolescence or during adulthood. Higher running output in females was associated with reduced expression of tyrosine hydroxylase and hyperactivation of calcium/calmodulin-dependent protein kinase II (CaMKII) in the dorsal striatum. Notably, running during adolescence-induced higher exercise output in adult males was associated with hyperactivation of CaMKII in the dorsal striatum, indicating a mechanistic role for CaMKII in running output. Together, the present results indicate sexually dimorphic adaptive biochemical changes in the dorsal striatum in rats that had escalated running activity, and highlight the importance of including sex as a biological variable in exploring neuroplasticity changes that predict enhanced exercise output in a voluntary physical activity paradigm.

Adult neurogenesis promotes efficient, nonspecific search strategies in a spatial alternation water maze task

Goal-directed navigation requires learning strategies that are efficient and minimize costs. In some cases it may be desirable to flexibly adjust behavioral responses depending on the cues that vary from one episode to the next. In others, successful navigation might be achieved with inflexible, habit-like responses that reduce cognitive load. Adult neurogenesis is believed to contribute to the spatial processing functions of the hippocampus, particularly when behavioral flexibility is required. However, little is known about the role of neurogenesis in spatial navigation when goals are unpredictable or change repeatedly according to certain rules. We hypothesized that neurogenesis is necessary in a spatial navigation task that involves different patterns of reinforcement. Intact and neurogenesis-deficient rats were trained to escape to one of two possible platform locations in a spatial water maze. The platform either repeated in the same location for all trials in a day, alternated between two locations across trials, or randomly moved between the two locations. Neurogenesis selectively enhanced escape performance in the alternating condition, but not by improving platform choice accuracy. Instead, neurogenesis-intact rats made fewer search errors and developed an efficient habit-like strategy where they consistently swam to a preferred location. If the platform was not present, they proceeded to the other possible location. In contrast, neurogenesis-deficient rats were indecisive and navigationally less-efficient. Thus, in conditions where goals follow a predictable spatiotemporal pattern, adult neurogenesis promotes the adoption of navigation strategies that are spatially nonspecific but, nonetheless, accurate and efficient.

A role for hippocampal adult neurogenesis in shifting attention toward novel stimuli

New granule neurons are born in the dentate gyrus region of the hippocampus throughout life. Behavioral effects of slowing or stopping this ongoing neurogenesis are generally observed only in complex cognitive tasks involving high levels of cue or memory interference or in tests of emotion presented after stress exposure. Here, we tested the role of new neurons in naïve rats in a simple, one-trial orienting task previously shown to be affected by hippocampal lesions. Using a pharmacogenetic method to inhibit adult neurogenesis, we found that loss of new neurons decreased orienting toward a novel auditory cue. Rats lacking new neurons showed this change in orienting only when they were drinking from a water bottle and not when they were exploring an empty arena, suggesting that the deficit is not in the ability to orient to a novel sound but in shifting of attention toward a second stimulus. Orienting was reduced to the same extent after 4 or 8 weeks of neurogenesis reduction but was not detectably altered after 2 or 3 weeks of treatment, suggesting that new neurons must mature for approximately a month before functioning in this behavior. These findings demonstrate that adult-born neurons affect behavior in a simple attention reorienting task in naïve animals with no prior stress or task-related learning.

A balanced evaluation of the evidence for adult neurogenesis in humans: implication for neuropsychiatric disorders

There is a widespread belief that neurogenesis exists in adult human brain, especially in the dentate gyrus, and it is to be maintained and, if possible, augmented with different stimuli including exercise and certain drugs. Here, we examine the evidence for adult human neurogenesis and note important limitations of the methodologies used to study it. A balanced review of the literature and evaluation of the data indicate that adult neurogenesis in human brain is improbable. In fact, in several high-quality recent studies in adult human brain, unlike in adult brains of other species, neurogenesis was not detectable. These findings suggest that the human brain requires a permanent set of neurons to maintain acquired knowledge for decades, which is essential for complex high cognitive functions unique to humans. Thus, stimulation and/or injection of neural stem cells into human brains may not only disrupt brain homeostatic systems, but also disturb normal neuronal circuits. We propose that the focus of research should be the preservation of brain neurons by prevention of damage, not replacement.

A combination of running and memantine increases neurogenesis and reduces activation of developmentally-born dentate granule neurons in rats

During hippocampal-dependent memory formation, sensory signals from the neocortex converge in the dentate gyrus. It is generally believed that the dentate gyrus decorrelates inputs in order to minimize interference between codes for similar experiences, often referred to as pattern separation. The proportion of dentate neurons that are activated by experience is therefore likely to impact how memories are stored and separated. Emerging evidence from mouse models suggests that adult-born neurons can both increase and decrease activity levels in the dentate gyrus. However, the conditions that determine the direction of this modulation, and whether it occurs in other species, remains unclear. Furthermore, since the dentate gyrus is composed of a heterogeneous population of cells that are born throughout life, newborn neurons may not modulate all cells equally. We aimed to investigate whether adult neurogenesis in rats regulates activity in dentate gyrus neurons that are born at the peak of early postnatal development. Adult neurogenesis was increased by subjecting rats to an alternating running and memantine treatment schedule, and it was decreased with a transgenic GFAP-TK rat model. Activity was measured by Fos expression in BrdU⁺ cells after rats explored a novel environment. Running+memantine treatment increased adult neurogenesis by only 17%, but completely blocked experience-dependent Fos expression. In contrast, GFAP-TK rats had a 68% reduction in adult neurogenesis but normal experience-dependent Fos expression. The inconsistent relationship between neurogenesis and Fos expression suggests that neurogenesis does not regulate DG activity during exploration of a novel environment. Nonetheless, running and memantine may benefit disorders where there is elevated activity in the dentate gyrus, such as anxiety and age-related memory impairments.

New neurons restore structural and behavioral abnormalities in a rat model of PTSD

Post-traumatic stress disorder (PTSD) has been associated with anxiety, memory impairments, enhanced fear, and hippocampal volume loss, although the relationship between these changes remain unknown. Single-prolonged stress (SPS) is a model for PTSD combining three forms of stress (restraint, swim, and anesthesia) in a single session that results in prolonged behavioral effects. Using pharmacogenetic ablation of adult neurogenesis in rats, we investigated the role of new neurons in the hippocampus in the long-lasting structural and behavioral effects of SPS. Two weeks after SPS, stressed rats displayed increased anxiety-like behavior and decreased preference for objects in novel locations regardless of the presence or absence of new neurons. Chronic stress produced by daily restraint for 2 or 6 hr produced similar behavioral effects that were also independent of ongoing neurogenesis. At a longer recovery time point, 1 month after SPS, rats with intact neurogenesis had normalized, showing control levels of anxiety-like behavior. However, GFAP-TK rats, which lacked new neurons, continued to show elevated anxiety-like behavior and enhanced serum corticosterone response to anxiogenic experience. Volume loss in ventral CA1 region of the hippocampus paralleled increases in anxiety-like behavior, occurring in all rats exposed to SPS at the early time point and only rats lacking adult neurogenesis at the later time point. In chronic stress experiments, volume loss occurred broadly throughout the dentate gyrus and CA1 after 6-hr daily stress but was not apparent in any hippocampal subregion after 2-hr daily stress. No effect of SPS was seen on cell proliferation in the dentate gyrus, but the survival of young neurons born a week after stress was decreased. Together, these data suggest that new neurons are important for recovery of normal behavior and hippocampal structure following a strong acute stress and point to the ventral CA1 region as a potential key mediator of stress-induced anxiety-like behavior.

Regulation of Adult Neurogenesis by Non-coding RNAs: Implications for Substance Use Disorders

The discovery of non-coding RNAs (ncRNAs)has been one of the central findings from early genomic sequencing studies. Not only was the presence of these genes unknown previously, it was the staggering disproportionate share of the genome that was predicted to be encoded by ncRNAs that was truly significant in genomic research. Over the years the function of various classes of these ncRNAs has been revealed. One of the first and enduring regulatory programs associated with these factors was development. In the neurosciences, the discovery of adult derived populations of dividing cells within the brain was equally substantial. The brain was hypothesized to be plastic only in its neuronal connectivity, but the discovery of the generation of new neurons was a novel mechanism of neuronal and behavioral plasticity. The process of adult neurogenesis resembles early neuronal development and has been found to share many parallels in the proper stages of specified genetic programs. Adult neurogenesis has also been found to play a role in learning and memory involved in particular hippocampal-dependent behaviors. Substance use disorders (SUDs) are an example of a behavioral condition that is associated with and possibly driven by hippocampal alterations. Our laboratory has determined that hippocampal adult neurogenesis is necessary for a rodent model of methamphetamine relapse. Due to the previous research on ncRNAs in development and in other brain regions involved in SUDs, we posit that ncRNAs may play a role in adult neurogenesis associated with this disorder. This review will cover the regulatory mechanisms of various classes of ncRNAs on the coordinated genetic program associated with adult neurogenesis with a special focus on how these programs could be dysregulated in SUDs.

Hippocampal neural progenitor cells play a distinct role in fear memory retrieval in male and female CIE rats

Adult male and female GFAP-TK transgenic rats experienced six weeks of chronic intermittent ethanol vapor inhalation (CIE). During the last week of CIE, a subset of male and female TK rats were fed with Valcyte to ablate neural progenitor cells (NPCs). Seventy-two hours after CIE cessation, all CIE and age-matched ethanol naïve controls experienced auditory trace fear conditioning (TFC). Twenty-four hours later all animals were tested for cue-mediated retrieval in the fear context. Adult male CIE rats showed a significant burst in NPCs paralleled by reduction in fear retrieval compared to naïve controls and Valcyte treated CIE rats. Adult female CIE rats did not show a burst in NPCs and showed similar fear retrieval compared to naïve controls and Valcyte treated CIE rats, indicating that CIE-mediated impairment in fear memory and its regulation by NPCs was sex dependent. Valcyte significantly reduced Ki-67 and NeuroD labeled cells in the dentate gyrus (DG) in both sexes, demonstrating a role for NPCs in reduced fear retrieval in males. Valcyte prevented adaptations in GluN2A receptor expression and synaptoporin density in the DG in males, indicating that NPCs contributed to alterations in plasticity-related proteins and mossy fiber projections that were associated with reduced fear retrieval. These data suggest that DG NPCs born during withdrawal and early abstinence from CIE are aberrant, and could play a role in weakening long-term memory consolidation dependent on the hippocampus.

The role of hippocampal adult neurogenesis in methamphetamine addiction

One of the consequences of chronic methamphetamine (Meth) abuse and Meth addiction is impaired hippocampal function which plays a critical role in enhanced propensity for relapse. This impairment is predicted by alterations in hippocampal neurogenesis, structural- and functional-plasticity of granule cell neurons (GCNs), and expression of plasticity-related proteins in the dentate gyrus. This review will elaborate on the effects of Meth in animal models during different stages of addiction-like behavior on proliferation, differentiation, maturation, and survival of newly born neural progenitor cells. We will then discuss evidence for the contribution of adult neurogenesis in context-driven Meth-seeking behavior in animal models. These findings from interdisciplinary studies suggest that a subset of newly born GCNs contribute to context-driven Meth-seeking in Meth addicted animals.

New Neurons in the Dentate Gyrus Promote Reinstatement of Methamphetamine Seeking

Addictive drugs effect the brain reward circuitry by altering functional plasticity of neurons governing the circuits. Relapse is an inherent problem in addicted subjects and is associated with neuroplasticity changes in several brain regions including the hippocampus. Recent studies have begun to determine the functional significance of adult neurogenesis in the dentate gyrus of the hippocampus, where new neurons in the granule cell layer are continuously generated to replace dying or diseased cells. One of the many negative consequences of chronic methamphetamine (METH) abuse and METH addiction in rodent and nonhuman primate models is a decrease in neural progenitor cells in the dentate gyrus and reduced neurogenesis in the granule cell layer during METH exposure. However, the number of progenitors rebound during withdrawal and abstinence from METH and the functional significance of enhanced survival of the progenitors during abstinence on the propensity for relapse was recently investigated by Galinato et al. A rat model of METH addiction in concert with a pharmacogenetic approach of ablating neural progenitor cells revealed that neurogenesis during abstinence promoted a relapse to METH-seeking behavior. Biochemical and electrophysiology studies demonstrated that an increase in neurogenesis during abstinence correlated with increases in plasticity-related proteins associated with learning and memory in the dentate gyrus and enhanced spontaneous activity and reduced neuronal excitability of granule cell neurons. Based on these findings, we discuss the putative molecular mechanisms that could drive aberrant neurogenesis during abstinence. We also indicate forebrain-dentate gyrus circuits that could assist with aberrant neurogenesis and drive a relapse into METH-seeking behavior in METH-addicted animals.

Intact memory for local and distal cues in male and female rats that lack adult neurogenesis

The dentate gyrus is essential for remembering the fine details of experiences that comprise episodic memory. Dentate gyrus granule cells receive highly-processed sensory information and are hypothesized to perform a pattern separation function, whereby similar sensory inputs are transformed into orthogonal neural representations. Behaviorally, this is believed to enable distinct memory for highly interfering stimuli. Since the dentate gyrus is comprised of a large number of adult-born neurons, which have unique synaptic wiring and neurophysiological firing patterns, it has been proposed that neurogenesis may contribute to this process in unique ways. Some behavioral evidence exists to support this role, whereby neurogenesis-deficient rodents are impaired at discriminating the fine visuospatial details of experiences. However, the extent to which newborn neurons contribute to dentate gyrus-dependent learning tasks is unclear. Furthermore, since most studies of dentate gyrus function are conducted in male rats, little is known about how females perform in similar situations, and whether there might be sex differences in the function of adult neurogenesis. To address these issues, we examined spatial discrimination memory in transgenic male and female rats that lacked adult neurogenesis. The first task probed memory for the position of local objects in an open field, assessed by behavioral responses to novel object locations. The second task examined memory for distal environmental cues. All rats were able to successfully discriminate local and distal cue changes. Males and females also performed comparably, although females displayed higher levels of rearing and locomotion. Collectively, our results indicate that rats are capable of learning about local and distal cues in the absence of adult neurogenesis.

Adult neurogenesis affects motivation to obtain weak, but not strong, reward in operant tasks

Decreased motivation to seek rewards is a key feature of mood disorders that correlates with severity and treatment outcome. This anhedonia, or apathy, likely reflects impairment in reward circuitry, but the specific neuronal populations controlling motivation are unclear. Granule neurons generated in the adult hippocampus have been implicated in mood disorders, but are not generally considered as part of reward circuits. We investigated a possible role of these new neurons in motivation to work for food and sucrose rewards in operant conditioning tasks using GFAP-TK pharmacogenetic ablation of adult neurogenesis in both rats and mice. Rats and mice lacking adult neurogenesis showed normal lever press responding during fixed ratio training, reward devaluation, and Pavlovian Instrumental Transfer, suggesting no impairment in learning. However, on an exponentially progressive ratio schedule, or when regular chow was freely available in the testing chamber, TK rats and mice showed less effort to gain sucrose tablets. When working for balanced food tablets, which rats and mice of both genotypes strongly preferred over sucrose, the genotype effects on behavior were lost. This decrease in effort under conditions of low reward suggests that loss of adult neurogenesis decreases motivation to seek reward in a manner that may model behavioral apathy.

Neurogenesis during Abstinence Is Necessary for Context-Driven Methamphetamine-Related Memory

Abstinence from methamphetamine addiction enhances proliferation and differentiation of neural progenitors and increases adult neurogenesis in the dentate gyrus (DG). We hypothesized that neurogenesis during abstinence contributes to context-driven drug-seeking behaviors. To test this hypothesis, the pharmacogenetic rat model (GFAP-TK rats) was used to conditionally and specifically ablate neurogenesis in the DG. Male GFAP-TK rats were trained to self-administer methamphetamine or sucrose and were administered the antiviral drug valganciclovir (Valcyte) to produce apoptosis of actively dividing GFAP type 1 stem-like cells to inhibit neurogenesis during abstinence. Hippocampus tissue was stained for Ki-67, NeuroD, and DCX to measure levels of neural progenitors and immature neurons, and was stained for synaptoporin to determine alterations in mossy fiber tracts. DG-enriched tissue punches were probed for CaMKII to measure alterations in plasticity-related proteins. Whole-cell patch-clamp recordings were performed in acute brain slices from methamphetamine naive (controls) and methamphetamine experienced animals (+/-Valcyte). Spontaneous EPSCs and intrinsic excitability were recorded from granule cell neurons (GCNs). Reinstatement of methamphetamine seeking enhanced autophosphorylation of CaMKII, reduced mossy fiber density, and induced hyperexcitability of GCNs. Inhibition of neurogenesis during abstinence prevented context-driven methamphetamine seeking, and these effects correlated with reduced autophosphorylation of CaMKII, increased mossy fiber density, and reduced the excitability of GCNs. Context-driven sucrose seeking was unaffected. Together, the loss-of-neurogenesis data demonstrate that neurogenesis during abstinence assists with methamphetamine context-driven memory in rats, and that neurogenesis during abstinence is essential for the expression of synaptic proteins and plasticity promoting context-driven drug memory.SIGNIFICANCE STATEMENT Our work uncovers a mechanistic relationship between neurogenesis in the dentate gyrus and drug seeking. We report that the suppression of excessive neurogenesis during abstinence from methamphetamine addiction by a confirmed phamacogenetic approach blocked context-driven methamphetamine reinstatement and prevented maladaptive changes in expression and activation of synaptic proteins and basal synaptic function associated with learning and memory in the dentate gyrus. Our study is the first to demonstrate an interesting and dysfunctional role of adult hippocampal neurogenesis during abstinence to drug-seeking behavior in animals self-administering escalating amounts of methamphetamine. Together, these results support a direct role for the importance of adult neurogenesis during abstinence in compulsive-like drug reinstatement.

Hormonal Regulation of Hippocampal Neurogenesis: Implications for Depression and Exercise

Adult hippocampal neurogenesis exists in all mammalian species, including humans, and although there has been considerable research investigating the function and regulation of neurogenesis, there remain many open questions surrounding the complexity of this phenomenon. This stems partially from the fact that neurogenesis is a multistage process that involves proliferation, differentiation, migration, survival, and eventual integration of new cells into the existing hippocampal circuitry, each of which can be independently influenced. The function of adult neurogenesis in the hippocampus is related to stress regulation, behavioral efficacy of antidepressants, long-term spatial memory, forgetting, and pattern separation. Steroid hormones influence the regulation of hippocampal neurogenesis, stress regulation, and cognition and differently in males and females. In this chapter, we will briefly tap into the complex network of steroid hormone modulation of neurogenesis in the hippocampus with specific emphasis on stress, testosterone, and estrogen. We examine the possible role of neurogenesis in the etiology of depression and influencing treatment by examining the influence of both pharmacological (selective serotonin reuptake inhibitors, tricyclic antidepressants) treatments and non-pharmacological (exercise) remedies.

Stress and Loss of Adult Neurogenesis Differentially Reduce Hippocampal Volume

BACKGROUND

Hippocampal volume loss is a hallmark of clinical depression. Chronic stress produces volume loss in the hippocampus in humans and atrophy of CA3 pyramidal cells and suppression of adult neurogenesis in rodents.

METHODS

To investigate the relationship between decreased adult neurogenesis and stress-induced changes in hippocampal structure and volume, we compared the effects of chronic unpredictable restraint stress and inhibition of neurogenesis in a rat pharmacogenetic model.

RESULTS

Chronic unpredictable restraint stress over 4 weeks decreased total hippocampal volume, reflecting loss of volume in all hippocampal subfields and in both dorsal and ventral hippocampus. In contrast, complete inhibition of adult neurogenesis for 4 weeks led to volume reduction only in the dentate gyrus. With prolonged inhibition of neurogenesis for 8 or 16 weeks, volume loss spread to the CA3 region, but not CA1. Combining stress and inhibition of adult neurogenesis did not have additive effects on the magnitude of volume loss but did produce a volume reduction throughout the hippocampus. One month of chronic unpredictable restraint stress and inhibition of adult neurogenesis led to atrophy of pyramidal cell apical dendrites in dorsal CA3 and to neuronal reorganization in ventral CA3. Stress also significantly affected granule cell dendrites.

CONCLUSIONS

The findings suggest that adult neurogenesis is required to maintain hippocampal volume but is not responsible for stress-induced volume loss.

Nuclear deterrents: Intrinsic regulators of IL-1β-induced effects on hippocampal neurogenesis

Hippocampal neurogenesis, the process by which new neurons are born and develop into the host circuitry, begins during embryonic development and persists throughout adulthood. Over the last decade considerable insights have been made into the role of hippocampal neurogenesis in cognitive function and the cellular mechanisms behind this process. Additionally, an increasing amount of evidence exists on the impact of environmental factors, such as stress and neuroinflammation on hippocampal neurogenesis and subsequent impairments in cognition. Elevated expression of the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus is established as a significant contributor to the neuronal demise evident in many neurological and psychiatric disorders and is now known to negatively regulate hippocampal neurogenesis. In order to prevent the deleterious effects of IL-1β on neurogenesis it is necessary to identify signalling pathways and regulators of neurogenesis within neural progenitor cells that can interact with IL-1β. Nuclear receptors are ligand regulated transcription factors that are involved in modulating a large number of cellular processes including neurogenesis. In this review we focus on the signalling mechanisms of specific nuclear receptors involved in regulating neurogenesis (glucocorticoid receptors, peroxisome proliferator activated receptors, estrogen receptors, and nuclear receptor subfamily 2 group E member 1 (NR2E1 or TLX)). We propose that these nuclear receptors could be targeted to inhibit neuroinflammatory signalling pathways associated with IL-1β. We discuss their potential to be therapeutic targets for neuroinflammatory disorders affecting hippocampal neurogenesis and associated cognitive function.

Axonal ribosomes and mRNAs associate with fragile X granules in adult rodent and human brains

Local mRNA translation in growing axons allows for rapid and precise regulation of protein expression in response to extrinsic stimuli. However, the role of local translation in mature CNS axons is unknown. Such a mechanism requires the presence of translational machinery and associated mRNAs in circuit-integrated brain axons. Here we use a combination of genetic, quantitative imaging and super-resolution microscopy approaches to show that mature axons in the mammalian brain contain ribosomes, the translational regulator FMRP and a subset of FMRP mRNA targets. This axonal translational machinery is associated with Fragile X granules (FXGs), which are restricted to axons in a stereotyped subset of brain circuits. FXGs and associated axonal translational machinery are present in hippocampus in humans as old as 57 years. This FXG-associated axonal translational machinery is present in adult rats, even when adult neurogenesis is blocked. In contrast, in mouse this machinery is only observed in juvenile hippocampal axons. This differential developmental expression was specific to the hippocampus, as both mice and rats exhibit FXGs in mature axons in the adult olfactory system. Experiments in Fmr1 null mice show that FMRP regulates axonal protein expression but is not required for axonal transport of ribosomes or its target mRNAs. Axonal translational machinery is thus a feature of adult CNS neurons. Regulation of this machinery by FMRP could support complex behaviours in humans throughout life.

Lasting Adaptations in Social Behavior Produced by Social Disruption and Inhibition of Adult Neurogenesis

Research on social instability has focused on its detrimental consequences, but most people are resilient and respond by invoking various coping strategies. To investigate cellular processes underlying such strategies, a dominance hierarchy of rats was formed and then destabilized. Regardless of social position, rats from disrupted hierarchies had fewer new neurons in the hippocampus compared with rats from control cages and those from stable hierarchies. Social disruption produced a preference for familiar over novel conspecifics, a change that did not involve global memory impairments or increased anxiety. Using the neuropeptide oxytocin as a tool to increase neurogenesis in the hippocampus of disrupted rats restored preference for novel conspecifics to predisruption levels. Conversely, reducing the number of new neurons by limited inhibition of adult neurogenesis in naive transgenic GFAP-thymidine kinase rats resulted in social behavior similar to disrupted rats. Together, these results provide novel mechanistic evidence that social disruption shapes behavior in a potentially adaptive way, possibly by reducing adult neurogenesis in the hippocampus.

SIGNIFICANCE STATEMENT

To investigate cellular processes underlying adaptation to social instability, a dominance hierarchy of rats was formed and then destabilized. Regardless of social position, rats from disrupted hierarchies had fewer new neurons in the hippocampus compared with rats from control cages and those from stable hierarchies. Unexpectedly, these changes were accompanied by changes in social strategies without evidence of impairments in cognition or anxiety regulation. Restoring adult neurogenesis in disrupted rats using oxytocin and conditionally suppressing the production of new neurons in socially naive GFAP-thymidine kinase rats showed that loss of 6-week-old neurons may be responsible for adaptive changes in social behavior.

Magnetic resonance imaging of odorant activity-dependent migration of neural precursor cells and olfactory bulb growth

Neural progenitors or neuroblasts are produced by precursor cells in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) to the olfactory bulbs (OB) throughout life. In the OB, these adult born neurons either die or replace existing olfactory interneurons, playing a critical role in the stabilization of OB circuitry. Although several aspects of the addition of new neurons into the OB have been studied, it is unclear whether long-distance activity from the OB can regulate the influx of migrating neuroblasts along the RMS. In this study, iron oxide-assisted MRI was used to track the migration of neuroblasts in combination with reversible naris occlusion to manipulate odorant-induced activity. It was found that decreasing olfactory activity led to a decrease in the rate of neuroblast migration along the RMS. Removal of the naris occlusion led to an increase in migratory rate back to control levels, indicating that olfactory activity has regulatory function on neuroblast migration in the RMS. Blocking odorant activity also led to an arrest in OB growth and re-opening the block led to a rapid re-growth returning the bulb size to control levels. Furthermore, pharmacogenetic elimination of the neuroblasts demonstrated that they were required for re-growth of the bulb following sensory deprivation. Together, these results show that sensory activity, neural migration and OB growth are tightly coupled in an interdependent manner.