E. David Leonardo

Neurogenesis-Dependent and -Independent Effects of Fluoxetine in an Animal Model of Anxiety/Depression

Understanding the physiopathology of affective disorders and their treatment relies on the availability of experimental models that accurately mimic aspects of the disease. Here we describe a mouse model of an anxiety/depressive-like state induced by chronic corticosterone treatment. Furthermore, chronic antidepressant treatment reversed the behavioral dysfunctions and the inhibition of hippocampal neurogenesis induced by corticosterone treatment. In corticosterone-treated mice where hippocampal neurogenesis is abolished by X-irradiation, the efficacy of fluoxetine is blocked in some, but not all, behavioral paradigms, suggesting both neurogenesis-dependent and -independent mechanisms of antidepressant action. Finally, we identified a number of candidate genes, the expression of which is decreased by chronic corticosterone and normalized by chronic fluoxetine treatment selectively in the hypothalamus. Importantly, mice deficient in one of these genes, beta-arrestin 2, displayed a reduced response to fluoxetine in multiple tasks, suggesting that beta-arrestin signaling is necessary for the antidepressant effects of fluoxetine.

Experience Dictates Stem Cell Fate in the Adult Hippocampus

Adult hippocampal neurogenesis has been implicated in cognitive and emotional processes, as well as in response to antidepressant treatment. However, little is known about how the adult stem cell lineage contributes to hippocampal structure and function and how this process is modulated by the animals experience. Here we perform an indelible lineage analysis and report that neural stem cells can produce expanding and persisting populations of not only neurons, but also stem cells in the adult hippocampus. Furthermore, the ratio of stem cells to neurons depends on experiences of the animal or the location of the stem cell. Surprisingly, social isolation facilitated accumulation of stem cells, but not neurons. These results show that neural stem cells accumulate in the adult hippocampus and that the stem cell-lineage relationship is under control of anatomic and experiential niches. Our findings suggest that, in the hippocampus, fate specification may act as a form of cellular plasticity for adapting to environmental changes.

Anxiety as a Developmental Disorder

There is increasing recognition that many psychiatric disorders including anxiety disorders are neurodevelopmental in their origins. Here, we review and integrate data from human studies and from animal models that point to a critical period during which neural circuits that mediate anxiety develop. We then postulate that this highly plastic critical period is a time of heightened responsiveness that is particularly susceptible to adverse events. We discuss these concepts in the context the current heightened interest in gene by environment interactions in psychiatric illness emphasizing the importance of the temporal relationship between gene action and environmental milieu.

Flexible Accelerated STOP Tetracycline Operator-Knockin (FAST): A Versatile and Efficient New Gene Modulating System

We created the Flexible Accelerated STOP Tetracycline Operator (tetO)-knockin (FAST) system, an efficient method for manipulating gene expression in vivo to rapidly screen animal models of disease. A single gene targeting event yields two distinct knockin mice-STOP-tetO and tetO knockin-that permit generation of multiple strains with variable expression patterns: 1) knockout, 2) Cre-mediated rescue, 3) tetracycline-controlled transcriptional activator (tTA)-mediated misexpression, 4) tetracycline-controlled transcriptional activator (tTA)-mediated overexpression, and 5) tetracycline-controlled transcriptional silencer (tTS)-mediated conditional knockout/knockdown. Using the FAST system, multiple gain-of-function and loss-of-function strains can therefore be generated on a time scale not previously achievable. These strains can then be screened for clinically relevant abnormalities. We demonstrate the flexibility and broad applicability of the FAST system by targeting several genes encoding proteins implicated in neuropsychiatric disorders: Mlc1, neuroligin 3, the serotonin 1A receptor, and the serotonin 1B receptor.

P5-HT 1A receptors in mood and anxiety: recent insights into autoreceptor versus heteroreceptor function

RationaleSerotonin (5-HT) neurotransmission is intimately linked to anxiety and depression and a diverse body of evidence supports the involvement of the main inhibitory serotonergic receptor, the serotonin-1A (5-HT1A) subtype, in both disorders.ObjectivesIn this review, we examine the function of 5-HT1A receptor subpopulations and re-interpret our understanding of their role in mental illness in light of new data, separating both spatial (autoreceptor versus heteroreceptor) and the temporal (developmental versus adult) roles of the endogenous 5-HT1A receptors, emphasizing their distinct actions in mediating anxiety and depression-like behaviors.ResultsIt is difficult to unambiguously distinguish the effects of different populations of the 5-HT1A receptors with traditional genetic animal models and pharmacological approaches. However, with the advent of novel genetic systems and subpopulation-selective pharmacological agents, direct evidence for the distinct roles of these populations in governing emotion-related behavior is emerging.ConclusionsThere is strong and growing evidence for a functional dissociation between auto- and heteroreceptor populations in mediating anxiety and depressive-like behaviors, respectively. Furthermore, while it is well established that 5-HT1A receptors act developmentally to establish normal anxiety-like behaviors, the developmental role of 5-HT1A heteroreceptors is less clear, and the specific mechanisms underlying the developmental role of each subpopulation are likely to be key elements determining mood control in adult subjects.

Developmental effects of serotonin 1A autoreceptors on anxiety and social behavior.

The serotonin 1A receptor (5-HT1A) has a major role in modulating the effects of serotonin on mood and behavior. Previous studies have shown that knockout of 5-HT1A selectively in the raphe leads to higher levels of anxiety during adulthood. However, it remains unclear whether this phenotype is due to variation in receptor levels specifically during development or throughout life. To test the hypothesis that developmental sensitivity may underlie the effects of 5-HT1A on anxiety, we used an inducible transgenic system to selectively suppress 5-HT1A levels in serotonergic raphe neurons from post-natal days (P) 14 to P30, with a maximal reduction of 40% at P21 and return to regular levels by P30. This developmental decrease in receptor levels has long-lasting consequences, increasing anxiety and decreasing social investigation in adulthood. In addition, post-natal knockdown of autoreceptors leads to long-term increases in the excitability of serotonergic neurons, which may represent a mechanism underlying the effects of post-natal receptor variation on behavior later in life. Finally, we also examined the interplay between receptor variation and juvenile exposure to stress (applied from P14 to P21). Similar to receptor knockdown, juvenile exposure to stress led to increased anxiety phenotypes but did not exacerbate 5-HT1A knockdown-mediated anxiety levels. This work indicates that the effects of 5-HT1A autoreceptors on anxiety and social behaviors are developmentally mediated and suggests that natural variations in the expression of 5-HT1A may act during development to influence individual anxiety levels and contribute to susceptibility to anxiety disorders.

Rethinking 5-HT1A Receptors: Emerging Modes of Inhibitory Feedback of Relevance to Emotion-Related Behavior

The complexities of the involvement of the serotonin transmitter system in numerous biological processes and psychiatric disorders is, to a substantial degree, attributable to the large number of serotonin receptor families and subtypes that have been identified and characterized for over four decades. Of these, the 5-HT(1A) receptor subtype, which was the first to be cloned and characterized, has received considerable attention based on its purported role in the etiology and treatment of mood and anxiety disorders. 5-HT(1A) receptors function both at presynaptic (autoreceptor) and postsynaptic (heteroreceptor) sites. Recent research has implicated distinct roles for these two populations of receptors in mediating emotion-related behavior. New concepts as to how 5-HT(1A) receptors function to control serotonergic tone throughout life were highlights of the proceedings of the 2012 Serotonin Club Meeting in Montpellier, France. Here, we review recent findings and current perspectives on functional aspects of 5-HT(1A) auto- and heteroreceptors with particular regard to their involvement in altered anxiety and mood states.

Is there a role for young hippocampal neurons in adaptation to stress

The hippocampus has been implicated in many cognitive and emotional behaviors and in the physiology of the stress response. Within the hippocampus, the dentate gyrus has been implicated in the detection of novelty. The dentate is also a major target for stress hormones and modulates the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Whether these functions of the dentate integrate or segregate remains unknown, as most investigations of its role in stress and learning are separate. Since the exciting discovery of adult neurogenesis in the dentate gyrus, adult-born neurons have been implicated in both novelty detection and the stress response. In this perspective we will discuss the literature that implicates the hippocampus, and potentially, adult-born neurons in these two functions. We will attempt to reconcile the seemingly contradictory behavioral results for the function of adult-born neurons. Finally, we will speculate that a key function of adult-born neurons within hippocampal function may be to modulate the stress response and perhaps assign stress salience to the sensory context.

Adolescent but not adult-born neurons are critical for susceptibility to chronic social defeat

Recent evidence implicates adult hippocampal neurogenesis in regulating behavioral and physiologic responses to stress. Hippocampal neurogenesis occurs across the lifespan, however the rate of cell birth is up to 300% higher in adolescent mice compared to adults. Adolescence is a sensitive period in development where emotional circuitry and stress reactivity undergo plasticity establishing life-long set points. Therefore neurogenesis occurring during adolescence may be particularly important for emotional behavior. However, little is known about the function of hippocampal neurons born during adolescence. In order to assess the contribution of neurons born in adolescence to the adult stress response and depression-related behavior, we transiently reduced cell proliferation either during adolescence, or during adulthood in GFAP-Tk mice. We found that the intervention in adolescence did not change adult baseline behavioral response in the forced swim test, sucrose preference test or social affiliation test, and did not change adult corticosterone responses to an acute stressor. However following chronic social defeat, adult mice with reduced adolescent neurogenesis showed a resilient phenotype. A similar transient reduction in adult neurogenesis did not affect depression-like behaviors or stress induced corticosterone. Our study demonstrates that hippocampal neurons born during adolescence, but not in adulthood are important to confer susceptibility to chronic social defeat.

5-HT1A Autoreceptors in the Dorsal Raphe Nucleus Convey Vulnerability to Compulsive Cocaine Seeking

Cocaine addiction and depression are comorbid disorders. Although it is well recognized that 5-hydroxytryptamine (5-HT; serotonin) plays a central role in depression, our understanding of its role in addiction is notably lacking. The 5-HT system in the brain is carefully controlled by a combined process of regulating 5-HT neuron firing through 5-HT autoreceptors, neurotransmitter release, enzymatic degradation, and reuptake by transporters. This study tests the hypothesis that activation of 5-HT1A autoreceptors, which would lessen 5-HT neuron firing, contributes to cocaine-seeking behaviors. Using 5-HT neuron-specific reduction of 5-HT1A autoreceptor gene expression in mice, we demonstrate that 5-HT1A autoreceptors are necessary for cocaine conditioned place preference. In addition, using designer receptors exclusively activated by designer drugs (DREADDs) technology, we found that stimulation of the serotonergic dorsal raphe nucleus (DRN) afferents to the nucleus accumbens (NAc) abolishes cocaine reward and promotes antidepressive-like behaviors. Finally, using a rat model of compulsive-like cocaine self-administration, we found that inhibition of dorsal raphe 5-HT1A autoreceptors attenuates cocaine self-administration in rats with 6 h extended access, but not 1 h access to the drug. Therefore, our findings suggest an important role for 5-HT1A autoreceptors, and thus DRNNAc 5-HT neuronal activity, in the etiology and vulnerability to cocaine reward and addiction. Moreover, our findings support a strategy for antagonizing 5-HT1A autoreceptors for treating cocaine addiction.