Katherine M. Nautiyal

Brain mast cells link the immune system to anxiety-like behavior

Mast cells are resident in the brain and contain numerous mediators, including neurotransmitters, cytokines, and chemokines, that are released in response to a variety of natural and pharmacological triggers. The number of mast cells in the brain fluctuates with stress and various behavioral and endocrine states. These properties suggest that mast cells are poised to influence neural systems underlying behavior. Using genetic and pharmacological loss-of-function models we performed a behavioral screen for arousal responses including emotionality, locomotor, and sensory components. We found that mast cell deficient KitW−sh/W−sh (sash−/−) mice had a greater anxiety-like phenotype than WT and heterozygote littermate control animals in the open field arena and elevated plus maze. Second, we show that blockade of brain, but not peripheral, mast cell activation increased anxiety-like behavior. Taken together, the data implicate brain mast cells in the modulation of anxiety-like behavior and provide evidence for the behavioral importance of neuroimmune links.

Serotonin of mast cell origin contributes to hippocampal function

In the central nervous system, serotonin, an important neurotransmitter and trophic factor, is synthesized by both mast cells and neurons. Mast cells, like other immune cells, are born in the bone marrow and migrate to many tissues. We show that they are resident in the mouse brain throughout development and adulthood. Measurements based on capillary electrophoresis with native fluorescence detection indicate that a significant contribution of serotonin to the hippocampal milieu is associated with mast cell activation. Compared with their littermates, mast cell‐deficient C57BL/6 KitW‐sh/W‐sh mice have profound deficits in hippocampus‐dependent spatial learning and memory and in hippocampal neurogenesis. These deficits are associated with a reduction in cell proliferation and in immature neurons in the dentate gyrus, but not in the subventricular zone – a neurogenic niche lacking mast cells. Chronic treatment with fluoxetine, a selective serotonin reuptake inhibitor, reverses the deficit in hippocampal neurogenesis in mast cell‐deficient mice. In summary, the present study demonstrates that mast cells are a source of serotonin, that mast cell‐deficient C57BL/6 KitW‐sh/W‐sh mice have disrupted hippocampus‐dependent behavior and neurogenesis, and that elevating serotonin in these mice, by treatment with fluoxetine, reverses these deficits. We conclude that mast cells contribute to behavioral and physiological functions of the hippocampus and note that they play a physiological role in neuroimmune interactions, even in the absence of inflammatory responses.

Distinct circuits underlie the effects of 5-HT1b receptors on aggression and impulsivity

Impulsive and aggressive behaviors are both modulated by serotonergic signaling, specifically through the serotonin 1B receptor (5-HT1BR). 5-HT1BR knockout mice show increased aggression and impulsivity, and 5-HT1BR polymorphisms are associated with aggression and drug addiction in humans. To dissect the mechanisms by which the 5-HT1BR affects these phenotypes, we developed a mouse model to spatially and temporally regulate 5-HT1BR expression. Our results demonstrate that forebrain 5-HT1B heteroreceptors expressed during an early postnatal period contribute to the development of the neural systems underlying adult aggression. However, distinct heteroreceptors acting during adulthood are involved in mediating impulsivity. Correlating with the impulsivity, dopamine in the nucleus accumbens is elevated in the absence of 5-HT1BRs and normalized following adult rescue of the receptor. Overall, these data show that while adolescent expression of 5-HT1BRs influences aggressive behavior, a distinct set of 5-HT1B receptors modulates impulsive behavior during adulthood.

Genetic approaches for understanding the role of serotonin receptors in mood and behavior.

Serotonin (5-hydroxytryptamine; 5-HT) is an ancient signaling molecule that has a conserved role in modulating mood and behavior. Integral to its pleiotropic actions is the existence of multiple receptors, expressed in distinct but often overlapping patterns within the brain and the periphery. The existence of ∼14 mammalian receptor subtypes, many of which possess similar pharmacological profiles, has made assigning functional roles for these receptors challenging. This challenge has been further compounded by the revelation that a single receptor can have several different functions depending upon where and when it is expressed and activated, that is, in brain versus periphery, or at different developmental time points. This review highlights the contribution of genetic techniques to dissect the specific function of distinct serotonin receptor populations across the life course, with an emphasis on the contribution of different serotonin 1A receptor populations to mood and behavior. Similar approaches hold the promise to elucidate the functional roles of other receptors, as well as the interaction of serotonin with other neuroendocrine modulators of mood and behavior.

Mast cells are necessary for the hypothermic response to LPS-induced sepsis

As central nervous system residents, mast cells contain many cytokines and are localized primarily near large blood vessels in the diencephalon and within the leptomeninges, making them candidates for immune to neural cross talk. Using mast cell-deficient Kit(W-sh/W-sh) mice, we assessed the role of these cells in the thermoregulatory component of the immune response to lipopolysaccharide (LPS). Kit(W-sh/W-sh) and wild-type (WT) mice differed in several respects in response to injection of a high dose of LPS (1 mg/kg ip). Core temperature (T(c)) of WT mice decreased by approximately 3 degrees C, whereas Kit(W-sh/W-sh) mice did not become hypothermic but instead exhibited pronounced low-frequency T(c) oscillations around their baseline temperature. In addition, Kit(W-sh/W-sh) mice had lower levels of whole brain TNF-alpha but no differences in IL-1beta, IL-6, IFN-gamma, or histamine compared with WT mice following injection of the high dose of LPS, consistent with the role of TNF-alpha in sepsis. Kit(W-sh/W-sh) mice had increased resistance to LPS, and some survived a dose of LPS that was lethal in littermate controls. In contrast, Kit(W-sh/W-sh) and WT mice were similar in other aspects, namely, in the hyperthermia following injection of TNF-alpha (1.5 microg icv), reduced nighttime T(c) and locomotor activity (to 1 mg/kg LPS), response to a low dose of LPS (10 microg/kg ip), and response to subcutaneous turpentine injection. These results indicate that mast cells play a role in the regulation of thermoregulatory responses and survival following sepsis induction and suggest a brain site of action.

Combining small-volume metabolomic and transcriptomic approaches for assessing brain chemistry.

The integration of disparate data types provides a more complete picture of complex biological systems. Here we combine small-volume metabolomic and transcriptomic platforms to determine subtle chemical changes and to link metabolites and genes to biochemical pathways. Capillary electrophoresis–mass spectrometry (CE–MS) and whole-genome gene expression arrays, aided by integrative pathway analysis, were utilized to survey metabolomic/transcriptomic hippocampal neurochemistry. We measured changes in individual hippocampi from the mast cell mutant mouse strain, C57BL/6 KitW-sh/W-sh. These mice have a naturally occurring mutation in the white spotting locus that causes reduced c-Kit receptor expression and an inability of mast cells to differentiate from their hematopoietic progenitors. Compared with their littermates, the mast cell-deficient mice have profound deficits in spatial learning, memory, and neurogenesis. A total of 18 distinct metabolites were identified in the hippocampus that discriminated between the C57BL/6 KitW-sh/W-sh and control mice. The combined analysis of metabolite and gene expression changes revealed a number of altered pathways. Importantly, results from both platforms indicated that multiple pathways are impacted, including amino acid metabolism, increasing the confidence in each approach. Because the CE–MS and expression profiling are both amenable to small-volume analysis, this integrated analysis is applicable to a range of volume-limited biological systems.

Serotonin receptors in depression: from A to B

The role of serotonin in major depressive disorder (MDD) is the focus of accumulating clinical and preclinical research. The results of these studies reflect the complexity of serotonin signaling through many receptors, in a large number of brain regions, and throughout the lifespan. The role of the serotonin transporter in MDD has been highlighted in gene by environment association studies as well as its role as a critical player in the mechanism of the most effective antidepressant treatments – selective serotonin reuptake inhibitors. While the majority of the 15 known receptors for serotonin have been implicated in depression or depressive-like behavior, the serotonin 1A (5-HT 1A) and 1B (5-HT 1B) receptors are among the most studied. Human brain imaging and genetic studies point to the involvement of 5-HT 1A and 5-HT 1B receptors in MDD and the response to antidepressant treatment. In rodents, the availability of tissue-specific and inducible knockout mouse lines has made possible the identification of the involvement of 5-HT 1A and 5-HT 1B receptors throughout development and in a cell-type specific manner. This, and other preclinical pharmacology work, shows that autoreceptor and heteroreceptor populations of these receptors have divergent roles in modulating depression-related behavior as well as responses to antidepressants and also have different functions during early postnatal development compared to during adulthood.

The Behavioral Effects of the Antidepressant Tianeptine Require the Mu-Opioid Receptor

Depression is a debilitating chronic illness that affects around 350 million people worldwide. Current treatments, such as selective serotonin reuptake inhibitors, are not ideal because only a fraction of patients achieve remission. Tianeptine is an effective antidepressant with a previously unknown mechanism of action. We recently reported that tianeptine is a full agonist at the mu opioid receptor (MOR). Here we demonstrate that the acute and chronic antidepressant-like behavioral effects of tianeptine in mice require MOR. Interestingly, while tianeptine also produces many opiate-like behavioral effects such as analgesia and reward, it does not lead to tolerance or withdrawal. Furthermore, the primary metabolite of tianeptine (MC5), which has a longer half-life, mimics the behavioral effects of tianeptine in a MOR-dependent fashion. These results point to the possibility that MOR and its downstream signaling cascades may be novel targets for antidepressant drug development.

Blood-borne donor mast cell precursors migrate to mast cell-rich brain regions in the adult mouse

Mast cells are hematopoietic immune cells located throughout the body, including within the brain. Reconstitution of mast cell deficient Kit(W-sh/W-sh) mice has proven valuable in determining peripheral mast cell function. Here we study the brain mast cell population using a novel method of blood transfusion for reconstitution. We show that blood transfusion results in mast cells of donor origin in the WT mouse, including in the brain where they are restricted to regions bearing host mast cells. In contrast, in Kit(W-sh/W-sh) mice, transfusion results in mast cells in the pinna of the ear, but not the brain.

A Lack of Serotonin 1B Autoreceptors Results in Decreased Anxiety and Depression-Related Behaviors.

The effects of serotonin (5-HT) on anxiety and depression are mediated by a number of 5-HT receptors, including autoreceptors that act to inhibit 5-HT release. While the majority of anxiety and depression-related research has focused on the 5-HT1A receptor, the 5-HT1B receptor has a lesser known role in modulating emotional behavior. 5-HT1B receptors are inhibitory GPCRs located on the presynaptic terminal of both serotonin and non-serotonin neurons, where they act to inhibit neurotransmitter release. The autoreceptor population located on the axon terminals of 5-HT neurons is a difficult population to study due to their diffuse localization throughout the brain that overlaps with 5-HT1B heteroreceptors (receptors located on non-serotonergic neurons). In order to study the contribution of 5-HT1B autoreceptors to anxiety and depression-related behaviors, we developed a genetic mouse model that allows for selective ablation of 5-HT1B autoreceptors. Mice lacking 5-HT1B autoreceptors displayed the expected increases in extracellular serotonin levels in the ventral hippocampus following administration of a selective serotonin reuptake inhibitor. In behavioral studies, they displayed decreased anxiety-like behavior in the open field and antidepressant-like effects in the forced swim and sucrose preference tests. These results suggest that strategies aimed at blocking 5-HT1B autoreceptors may be useful for the treatment of anxiety and depression.