Exaggerated adrenomedullary response to immobilization in mice with targeted disruption of the serotonin transporter gene

Serotonin and the serotonin transporter in the adrenal gland

The adrenal glands are key components of the mammalian endocrine system, helping maintain physiological homeostasis and the coordinated response to stress. Each adrenal gland has two morphologically and functionally distinct regions, the outer cortex and inner medulla. The cortex is organized into three concentric zones which secrete steroid hormones, including aldosterone and cortisol. Neural crest-derived chromaffin cells in the medulla are innervated by preganglionic sympathetic neurons and secrete catecholamines (epinephrine, norepinephrine) and neuropeptides into the bloodstream, thereby functioning as the neuroendocrine arm of the sympathetic nervous system. In this article we review serotonin (5-HT) and the serotonin transporter (SERT; SLC6A4) in the adrenal gland. In the adrenal cortex, 5-HT, primarily sourced from resident mast cells, acts as a paracrine signal to stimulate aldosterone and cortisol secretion through 5-HT4/5-HT7 receptors. Medullary chromaffin cells contain a small amount of 5-HT due to SERT-mediated uptake and express 5-HT1A receptors which inhibit secretion. The atypical mechanism of the 5-HT1A receptors and interaction with SERT fine tune this autocrine pathway to control stress-evoked catecholamine secretion. Receptor-independent signaling by SERT/intracellular 5-HT modulates the amount and kinetics of transmitter release from single vesicle fusion events. SERT might also influence stress-evoked upregulation of tyrosine hydroxylase transcription. Transient signaling via 5-HT3 receptors during embryonic development can limit the number of chromaffin cells found in the mature adrenal gland. Together, this emerging evidence suggests that the adrenal medulla is a peripheral hub for serotonergic control of the sympathoadrenal stress response.

Serotonergic Modulation of Neurovascular Transmission: A Focus on Prejunctional 5-HT Receptors/Mechanisms

5-Hydroxytryptamine (5-HT), or serotonin, plays a crucial role as a neuromodulator and/or neurotransmitter of several nervous system functions. Its actions are complex, and depend on multiple factors, including the type of effector or receptor activated. Briefly, 5-HT can activate: (i) metabotropic (G-protein-coupled) receptors to promote inhibition (5-HT₁, 5-HT₅) or activation (5-HT₄, 5-HT₆, 5-HT₇) of adenylate cyclase, as well as activation (5-HT₂) of phospholipase C; and (ii) ionotropic receptor (5-HT₃), a ligand-gated Na⁺/K⁺ channel. Regarding blood pressure regulation (and beyond the intricacy of central 5-HT effects), this monoamine also exerts direct postjunctional (on vascular smooth muscle and endothelium) or indirect prejunctional (on autonomic and sensory perivascular nerves) effects. At the prejunctional level, 5-HT can facilitate or preclude the release of autonomic (e.g., noradrenaline and acetylcholine) or sensory (e.g., calcitonin gene-related peptide) neurotransmitters facilitating hypertensive or hypotensive effects. Hence, we cannot formulate a specific impact of 5-HT on blood pressure level, since an increase or decrease in neurotransmitter release would be favoured, depending on the type of prejunctional receptor involved. This review summarizes and discusses the current knowledge on the prejunctional mechanisms involved in blood pressure regulation by 5-HT and its impact on some vascular-related diseases.

Behavioral phenotype, intestinal microbiome, and brain neuronal activity of male serotonin transporter knockout mice

The serotonin transporter (5-HTT) plays a critical role in the regulation of serotonin neurotransmission. Mice genetically deficient in 5-HTT expression have been used to study the physiological functions of 5-HTT in the brain and have been proposed as a potential animal model for neuropsychiatric and neurodevelopmental disorders. Recent studies have provided evidence for a link between the gut-brain axis and mood disorders. However, the effects of 5-HTT deficiency on gut microbiota, brain function, and behavior remain to be fully characterized. Here we investigated the effects of 5-HTT deficiency on different types of behavior, the gut microbiome, and brain c-Fos expression as a marker of neuronal activation in response to the forced swim test for assessing depression-related behavior in male 5-HTT knockout mice. Behavioral analysis using a battery of 16 different tests showed that 5-HTT-/- mice exhibited markedly reduced locomotor activity, decreased pain sensitivity, reduced motor function, increased anxiety-like and depression-related behavior, altered social behavior in novel and familiar environments, normal working memory, enhanced spatial reference memory, and impaired fear memory compared to 5-HTT+/+ mice. 5-HTT+/- mice showed slightly reduced locomotor activity and impaired social behavior compared to 5-HTT+/+ mice. Analysis of 16S rRNA gene amplicons showed that 5-HTT-/- mice had altered gut microbiota abundances, such as a decrease in Allobaculum, Bifidobacterium, Clostridium sensu stricto, and Turicibacter, compared to 5-HTT+/+ mice. This study also showed that after exposure to the forced swim test, the number of c-Fos-positive cells was higher in the paraventricular thalamus and lateral hypothalamus and was lower in the prefrontal cortical regions, nucleus accumbens shell, dorsolateral septal nucleus, hippocampal regions, and ventromedial hypothalamus in 5-HTT-/- mice than in 5-HTT+/+ mice. These phenotypes of 5-HTT-/- mice partially recapitulate clinical observations in humans with major depressive disorder. The present findings indicate that 5-HTT-deficient mice serve as a good and valid animal model to study anxiety and depression with altered gut microbial composition and abnormal neuronal activity in the brain, highlighting the importance of 5-HTT in brain function and the mechanisms underlying the regulation of anxiety and depression.

Novel Antidepressant-Like Properties of the Iron Chelator Deferiprone in a Mouse Model of Depression

Depressed individuals who carry the short allele for the serotonin-transporter-linked promotor region of the gene are more vulnerable to stress and have reduced response to first-line antidepressants such as selective serotonin reuptake inhibitors. Since depression severity has been reported to correlate with brain iron levels, the present study aimed to characterise the potential antidepressant properties of the iron chelator deferiprone. Using the serotonin transporter knock-out (5-HTT KO) mouse model, we assessed the behavioural effects of acute deferiprone on the Porsolt swim test (PST) and novelty-suppressed feeding test (NSFT). Brain and blood iron levels were also measured following acute deferiprone. To determine the relevant brain regions activated by deferiprone, we then measured c-Fos expression and applied network-based analyses. We found that deferiprone reduced immobility time in the PST in 5-HTT KO mice and reduced latency to feed in the NSFT in both genotypes, suggesting potential antidepressant-like effects. There was no effect on brain or blood iron levels following deferiprone treatment, potentially indicating an acute iron-independent mechanism. Deferiprone reversed the increase in c-Fos expression induced by swim stress in 5-HTT KO mice in the lateral amygdala. Functional network analyses suggest that hub regions of activity in mice treated with deferiprone include the caudate putamen and prefrontal cortex. The PST-induced increase in network modularity in wild-type mice was not observed in 5-HTT KO mice. Altogether, our data show that the antidepressant-like effects of deferiprone could be acting via an iron-independent mechanism and that these therapeutic effects are underpinned by changes in neuronal activity in the lateral amygdala.

BDNF Overexpression in the Ventral Hippocampus Promotes Antidepressant- and Anxiolytic-Like Activity in Serotonin Transporter Knockout Rats

BDNF plays a pivotal role in neuroplasticity events, vulnerability and resilience to stress-related disorders, being decreased in depressive patients and increased after antidepressant treatment. BDNF was found to be reduced in patients carrying the human polymorphism in the serotonin transporter promoter region (5-HTTLPR). The serotonin knockout rat (SERT^-/-) is one of the animal models used to investigate the underlying molecular mechanisms of depression in humans. They present decreased BDNF levels, and anxiety- and depression-like behavior. To investigate whether upregulating BDNF would ameliorate the phenotype of SERT^-/- rats, we overexpressed BDNF locally into the ventral hippocampus and submitted the animals to behavioral testing. The results showed that BDNF overexpression in the vHIP of SERT^-/- rats promoted higher sucrose preference and sucrose intake; on the first day of the sucrose consumption test it decreased immobility time in the forced swim test and increased the time spent in the center of a novel environment. Furthermore, BDNF overexpression altered social behavior in SERT^-/- rats, which presented increased passive contact with test partner and decreased solitary behavior. Finally, it promoted decrease in plasma corticosterone levels 60 min after restraint stress. In conclusion, modulation of BDNF IV levels in the vHIP of SERT^-/- rats led to a positive behavioral outcome placing BDNF upregulation in the vHIP as a potential target to new therapeutic approaches to improve depressive symptoms.

Mechanisms of Shared Vulnerability to Post-traumatic Stress Disorder and Substance Use Disorders

Psychoactive substance use is a nearly universal human behavior, but a significant minority of people who use addictive substances will go on to develop an addictive disorder. Similarly, though ~90% of people experience traumatic events in their lifetime, only ~10% ever develop post-traumatic stress disorder (PTSD). Substance use disorders (SUD) and PTSD are highly comorbid, occurring in the same individual far more often than would be predicted by chance given the respective prevalence of each disorder. Some possible reasons that have been proposed for the relationship between PTSD and SUD are self-medication of anxiety with drugs or alcohol, increased exposure to traumatic events due to activities involved in acquiring illegal substances, or addictive substances altering the brain's stress response systems to make users more vulnerable to PTSD. Yet another possibility is that some people have an intrinsic vulnerability that predisposes them to both PTSD and SUD. In this review, we integrate clinical and animal data to explore these possible etiological links between SUD and PTSD, with an emphasis on interactions between dopaminergic, adrenocorticotropic, GABAergic, and glutamatergic neurobehavioral mechanisms that underlie different emotional learning styles.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Serotonin Transporter Deficiency is Associated with Dysbiosis and Changes in Metabolic Function of the Mouse Intestinal Microbiome

Serotonin transporter (SERT) plays a critical role in regulating extracellular availability of serotonin (5-HT) in the gut and brain. Mice with deletion of SERT develop metabolic syndrome as they age. Changes in the gut microbiota are being increasingly implicated in Metabolic Syndrome and Diabetes. To investigate the relationship between the gut microbiome and SERT, this study assessed the fecal and cecal microbiome profile of 11 to 12 week-old SERT+/+ and SERT-/- mice. Microbial DNA was isolated, processed for metagenomics shotgun sequencing, and taxonomic and functional profiles were assessed. 34 differentially abundant bacterial species were identified between SERT+/+ and SERT-/-. SERT-/- mice displayed higher abundances of Bacilli species including genera Lactobacillus, Streptococcus, Enterococcus, and Listeria. Furthermore, SERT-/- mice exhibited significantly lower abundances of Bifidobacterium species and Akkermansia muciniphilia. Bacterial community structure was altered in SERT-/- mice. Differential abundance of bacteria was correlated with changes in host gene expression. Bifidobacterium and Bacilli species exhibited significant associations with host genes involved in lipid metabolism pathways. Our results show that SERT deletion is associated with dysbiosis similar to that observed in obesity. This study contributes to the understanding as to how changes in gut microbiota are associated with metabolic phenotype seen in SERT deficiency.

Adrenal serotonin derives from accumulation by the antidepressant-sensitive serotonin transporter

Adrenal chromaffin cells comprise the neuroendocrine arm of the sympathetic nervous system and secrete catecholamines to coordinate the appropriate stress response. Deletion of the serotonin (5-HT) transporter (SERT) gene in mice (SERT^-/- mice) or pharmacological block of SERT function in rodents and humans augments this sympathoadrenal stress response (epinephrine secretion). The prevailing assumption is that loss of CNS SERT alters central drive to the peripheral sympathetic nervous system. Adrenal chromaffin cells also prominently express SERT where it might coordinate accumulation of 5-HT for reuse in the autocrine control of stress-evoked catecholamine secretion. To help test this hypothesis, we have generated a novel mouse model with selective excision of SERT in the peripheral sympathetic nervous system (SERT^ΔTH), generated by crossing floxed SERT mice with tyrosine hydroxylase Cre driver mice. SERT expression, assessed by western blot, was abolished in the adrenal gland but not perturbed in the CNS of SERT^ΔTH mice. SERT-mediated [³H] 5-HT uptake was unaltered in midbrain, hindbrain, and spinal cord synaptosomes, confirming transporter function was intact in the CNS. Endogenous midbrain and whole blood 5-HT homeostasis was unperturbed in SERT^ΔTH mice, contrasting with the depleted 5-HT content in SERT^-/- mice. Selective SERT excision reduced adrenal gland 5-HT content by ≈ 50% in SERT^ΔTH mice but had no effect on adrenal catecholamine content. This novel model confirms that SERT expressed in adrenal chromaffin cells is essential for maintaining wild-type levels of 5-HT and provides a powerful tool to help dissect the role of SERT in the sympathetic stress response.

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Serotonin and Serotonin Transporters in the Adrenal Medulla: A Potential Hub for Modulation of the Sympathetic Stress Response

Serotonin (5-HT) is an important neurotransmitter in the central nervous system where it modulates circuits involved in mood, cognition, movement, arousal, and autonomic function. The 5-HT transporter (SERT; SLC6A4) is a key regulator of 5-HT signaling, and genetic variations in SERT are associated with various disorders including depression, anxiety, and autism. This review focuses on the role of SERT in the sympathetic nervous system. Autonomic/sympathetic dysfunction is evident in patients with depression, anxiety, and other diseases linked to serotonergic signaling. Experimentally, loss of SERT function (SERT knockout mice or chronic pharmacological block) has been reported to augment the sympathetic stress response. Alterations to serotonergic signaling in the CNS and thus central drive to the peripheral sympathetic nervous system are presumed to underlie this augmentation. Although less widely recognized, SERT is robustly expressed in chromaffin cells of the adrenal medulla, the neuroendocrine arm of the sympathetic nervous system. Adrenal chromaffin cells do not synthesize 5-HT but accumulate small amounts by SERT-mediated uptake. Recent evidence demonstrated that 5-HT1A receptors inhibit catecholamine secretion from adrenal chromaffin cells via an atypical mechanism that does not involve modulation of cellular excitability or voltage-gated Ca2+ channels. This raises the possibility that the adrenal medulla is a previously unrecognized peripheral hub for serotonergic control of the sympathetic stress response. As a framework for future investigation, a model is proposed in which stress-evoked adrenal catecholamine secretion is fine-tuned by SERT-modulated autocrine 5-HT signaling.

The Serotonin Transporter and Early Life Stress: Translational Perspectives

The interaction between the serotonin transporter (SERT) linked polymorphic region (5-HTTLPR) and adverse early life stressing (ELS) events is associated with enhanced stress susceptibility and risk to develop mental disorders like major depression, anxiety, and aggressiveness. In particular, human short allele carriers are at increased risk. This 5-HTTLPR polymorphism is absent in the rodent SERT gene, but heterozygous SERT knockout rodents (SERT^+/−) show several similarities to the human S-allele carrier, therefore creating an animal model of the human situation. Many rodent studies investigated ELS interactions in SERT knockout rodents combined with ELS. However, underlying neuromolecular mechanisms of the (mal)adaptive responses to adversity displayed by SERT rodents remain to be elucidated. Here, we provide a comprehensive review including studies describing mechanisms underlying SERT variation × ELS interactions in rodents. Alterations at the level of translation and transcription but also epigenetic alterations considerably contribute to underlying mechanisms of SERT variation × ELS interactions. In particular, SERT^+/− rodents exposed to adverse early rearing environment may be of high translational and predictive value to the more stress sensitive human short-allele carrier, considering the similarity in neurochemical alterations. Therefore, SERT^+/− rodents are highly relevant in research that aims to unravel the complex psychopathology of mental disorders. So far, most studies fail to show solid evidence for increased vulnerability to develop affective-like behavior after ELS in SERT^+/− rodents. Several reasons may underlie these failures, e.g., (1) stressors used might not be optimal or severe enough to induce maladaptations, (2) effects in females are not sufficiently studied, and (3) few studies include both behavioral manifestations and molecular correlates of ELS-induced effects in SERT^+/− rodents. Of course, one should not exclude the (although unlikely) possibility of SERT^+/− rodents not being sensitive to ELS. In conclusion, future studies addressing ELS-induced effects in the SERT^+/− rodents should extensively study both long-term behavioral and (epi)genetic aspects in both sexes. Finally, further research is warranted using more severe stressors in animal models. From there on, we should be able to draw solid conclusions whether the SERT^+/− exposed to ELS is a suitable translational animal model for studying 5-HTTLPR polymorphism and stress interactions.

Serotonin-mediated modulation of acetylcholine-induced intracellular calcium responses in chromaffin cells isolated from the rat adrenal medulla

We examined serotonin (5-HT)-mediated modulation of acetylcholine (ACh)-induced intracellular Ca²⁺ ([Ca²⁺]_i) responses in rat adrenal chromaffin cells using calcium imaging. 5-HT did not induce any [Ca²⁺]_i response in clustered chromaffin cells. However, the magnitude of ACh-induced [Ca²⁺]_i increases in the same specimens was inhibited in the presence of 5-HT. ACh-induced [Ca²⁺]_i increases in chromaffin cells were also inhibited by the 5-HT1A receptor agonist, 8-hydroxy-2-(dipropylamino) tetralin hydrobromide, but were not changed by the 5-HT1B, 5-HT2, or 5-HT3 receptor agonists, CP93129, α-methyl-5-HT, or 1-(m-chlorophenyl) biguanide, respectively. RT-PCR analysis detected the expression of all 5-HT receptor subtype mRNAs, except for 5-HT5 receptors, in extracts of the adrenal medulla. Immunohistochemistry revealed that immunoreactivity for 5-HT1A receptor was located in the chromaffin cells immunoreactive for the biosynthetic enzyme for noradrenaline, dopamine β-hydroxylase. These results suggest that 5-HT inhibits ACh-induced excitability in adrenal chromaffin cells via the 5-HT1A receptor in order to reduce catecholamine release during preganglionic sympathetic stimuli.

Identification of quantitative trait loci and candidate genes for an anxiolytic-like response to ethanol in BXD recombinant inbred strains

Genetic differences in acute behavioral responses to ethanol contribute to the susceptibility to alcohol use disorder and the reduction of anxiety is a commonly reported motive underlying ethanol consumption among alcoholics. Therefore, we studied the genetic variance in anxiolytic-like responses to ethanol across the BXD recombinant inbred (RI) mouse panel using the light-dark transition model of anxiety. Strain-mean genetic mapping and a mixed-model quantitative trait loci (QTL) analysis replicated several previously published QTL for locomotor activity and identified several novel anxiety-related loci. Significant loci included a chromosome 11 saline anxiety-like QTL (Salanq1) and a chromosome 12 locus (Etanq1) influencing the anxiolytic-like response to ethanol. Etanq1 was successfully validated by studies with BXD advanced intercross strains and fine-mapped to a region comprising less than 3.5 Mb. Through integration of genome-wide mRNA expression profiles of the mesocorticolimbic reward circuit (prefrontal cortex, nucleus accumbens and ventral midbrain) across the BXD RI panel, we identified high priority candidate genes within Etanq1, the strongest of which was Ninein (Nin), a Gsk3β-interacting protein that is highly expressed in the brain.

An interplay between the serotonin transporter (SERT) and 5-HT receptors controls stimulus-secretion coupling in sympathoadrenal chromaffin cells

Adrenal chromaffin cells (ACCs), the neuroendocrine arm of the sympathetic nervous system, secrete catecholamines to mediate the physiological response to stress. Although ACCs do not synthesize 5-HT, they express the serotonin transporter (SERT). Genetic variations in SERT are linked to several CNS disorders but the role(s) of SERT/5-HT in ACCs has remained unclear. Adrenal glands from wild-type mice contained 5-HT at ≈ 750 fold lower abundance than adrenaline, and in SERT(-/-) mice this was reduced by ≈80% with no change in catecholamines. Carbon fibre amperometry showed that SERT modulated the ability of 5-HT1A receptors to inhibit exocytosis. 5-HT reduced the number of amperometric spikes (vesicular fusion events) evoked by KCl in SERT(-/-) cells and wild-type cells treated with escitalopram, a SERT antagonist. The 5-HT1A receptor antagonist WAY100635 blocked the inhibition by 5-HT which was mimicked by the 5-HT1A agonist 8-OH-DPAT but not the 5-HT1B agonist CP93129. There was no effect on voltage-gated Ca(2+) channels, K(+) channels, or intracellular [Ca(2+)] handling, showing the 5-HT receptors recruit an atypical inhibitory mechanism. Spike charge and kinetics were not altered by 5-HT receptors but were reduced in SERT(-/-) cells compared to wild-type cells. Our data reveal a novel role for SERT and suggest that adrenal chromaffin cells might be a previously unrecognized hub for serotonergic control of the sympathetic stress response.

Enhanced serotonin signaling increases intestinal neuroplasticity

BACKGROUND

The intestinal mucosa recovers from injury by accelerating enterocyte proliferation resulting in villus growth. A similar phenomenon is seen after massive bowel resection. Serotonin (5-HT) has been implicated as an important regulator of mucosal homeostasis by promoting growth in the epithelium. The impact of 5-HT on other components of growing villi is not known. We hypothesized that 5-HT-stimulated growth in the intestinal epithelium would be associated with growth in other components of the villus such as enteric neural axonal processes.

MATERIALS AND METHODS

Enteric serotonergic signaling is inactivated by the serotonin reuptake transporter, or SERT, molecule. Enhanced serotonin signaling was achieved via SERT knockout (SERTKO) and administration of selective serotonin reuptake inhibitors (SSRI) to wild-type mice (WT-SSRI). 5-HT synthesis inhibition was achieved with administration of 4-chloro-L-phenylalanine (PCPA). Intestinal segments from age-matched WT, SERTKO, WT-SSRI, and corresponding PCPA-treated animals were assessed via villus height, crypt depth, and crypt proliferation. Gap 43, a marker of neuroplasticity, was assessed via immunofluorescence and Western blot.

RESULTS

SERTKO and WT-SSRI mice had taller villi, deeper crypts, and increased enterocyte proliferation compared with WT mice. Gap 43 expression via immunofluorescence was significantly increased in SERTKO and WT-SSRI samples, as well as in Western blot analysis. PCPA-treated SERTKO and WT-SSRI animals demonstrated reversal of 5-HT-induced growth and Gap 43 expression.

CONCLUSIONS

Enhanced 5-HT signaling results in intestinal mucosal growth in both the epithelial cell compartment and the enteric nervous system. Furthermore, 5-HT synthesis inhibition resulted in reversal of effects, suggesting that 5-HT is a critically important regulator of intestinal mucosal growth and neuronal plasticity.

Serotonin transporter gene polymorphism modulates inflammatory cytokine responses during acute stress

Cytokines are important mediators of various stress-related modulations of immune function. A major genetic factor determining inter-individual differences in stress reactivity is polymorphisms of the serotonin (5-hydroxytryptamine, 5HT) transporter (5HTT) gene. A short (S) variant, compared with a long (L) variant, of the promoter region of the 5HTT gene-linked polymorphic region (5HTTLPR) has been related to emotional and stress hyper-reactivity. The present study examined whether the 5HTTLPR can modulate responses of inflammatory cytokines under acute stress. Nine Japanese male participants carrying two copies of the S alleles and nine Japanese males carrying S and L alleles underwent the Trier Social Stress Test (TSST). Inflammatory cytokines, endocrine parameters, heart rate and subjective stress were measured before, during and after the task. The participants carrying the SS alleles, but not those carrying the SL alleles, showed a significant increase of IL-1β immediately after TSST. This hyper-reactivity to acute stress in individuals with the SS alleles was also observed in their heart rate and cortisol levels. These results suggest that the S allele of the 5HTTLPR is consistently associated with stress reactivity in multi-level stress-related biological systems.

Improved Stress Control in Serotonin Transporter Knockout Rats: Involvement of the Prefrontal Cortex and Dorsal Raphe Nucleus

Variations in serotonin transporter (5-HTT) expression have been associated with altered sensitivity to stress. Since controllability is known to alter the impact of a stressor through differential activation of the medial prefrontal cortex (mPFC) and dorsal raphe nucleus (DRN), and that these regions are functionally affected by genetic 5-HTT down-regulation, we hypothesized that 5-HTT expression modulates the effect of controllability on stressor impact and coping. Here, we investigated the effects of a signaled stress controllability task or a yoked uncontrollable stressor on behavioral responding and mPFC and DRN activation. 5-HTT(-/-) rats proved better capable of acquiring the active avoidance task than 5-HTT(+/+) animals. Controllability determined DRN activation in 5-HTT(+/+), but not 5-HTT(-/-), rats, whereas controllability-related activation of the mPFC was independent of genotype. These findings suggest that serotonergic activation in the DRN is involved in stress coping in a 5-HTT expression dependent manner, whereas mPFC activation seems to be implicated in control over stress independently of 5-HTT expression. We speculate that alterations in serotonergic feedback in the DRN might be a potential mechanism driving this differential stress coping.

Perinatal vs genetic programming of serotonin states associated with anxiety

Large numbers of women undergo antidepressant treatment during pregnancy; however, long-term consequences for their offspring remain largely unknown. Rodents exposed to serotonin transporter (SERT)-inhibiting antidepressants during development show changes in adult emotion-like behavior. These changes have been equated with behavioral alterations arising from genetic reductions in SERT. Both models are highly relevant to humans yet they vary in their time frames of SERT disruption. We find that anxiety-related behavior and, importantly, underlying serotonin neurotransmission diverge between the two models. In mice, constitutive loss of SERT causes life-long increases in anxiety-related behavior and hyperserotonemia. Conversely, early exposure to the antidepressant escitalopram (ESC; Lexapro) results in decreased anxiety-related behavior beginning in adolescence, which is associated with adult serotonin system hypofunction in the ventral hippocampus. Adult behavioral changes resulting from early fluoxetine (Prozac) exposure were different from those of ESC and, although somewhat similar to SERT deficiency, were not associated with changes in hippocampal serotonin transmission in late adulthood. These findings reveal dissimilarities in adult behavior and neurotransmission arising from developmental exposure to different widely prescribed antidepressants that are not recapitulated by genetic SERT insufficiency. Moreover, they support a pivotal role for serotonergic modulation of anxiety-related behavior.

Benefits of adversity?! How life history affects the behavioral profile of mice varying in serotonin transporter genotype

Behavioral profiles are influenced by both positive and negative experiences as well as the genetic disposition. Traditionally, accumulating adversity over lifetime is considered to predict increased anxiety-like behavior (“allostatic load”). The alternative “mismatch hypothesis” suggests increased levels of anxiety if the early environment differs from the later-life environment. Thus, there is a need for a whole-life history approach to gain a deeper understanding of how behavioral profiles are shaped. The aim of this study was to elucidate the effects of life history on the behavioral profile of mice varying in serotonin transporter (5-HTT) genotype, an established mouse model of increased anxiety-like behavior. For this purpose, mice grew up under either adverse or beneficial conditions during early phases of life. In adulthood, they were further subdivided so as to face a situation that either matched or mismatched the condition experienced so far, resulting in four different life histories. Subsequently, mice were tested for their anxiety-like and exploratory behavior. The main results were: (1) Life history profoundly modulated the behavioral profile. Surprisingly, mice that experienced early beneficial and later escapable adverse conditions showed less anxiety-like and more exploratory behavior compared to mice of other life histories. (2) Genotype significantly influenced the behavioral profile, with homozygous 5-HTT knockout mice displaying highest levels of anxiety-like and lowest levels of exploratory behavior. Our findings concerning life history indicate that the absence of adversity does not necessarily cause lower levels of anxiety than accumulating adversity. Rather, some adversity may be beneficial, particularly when following positive events. Altogether, we conclude that for an understanding of behavioral profiles, it is not sufficient to look at experiences during single phases of life, but the whole life history has to be considered.

Ghrelin effects expression of several genes associated with depression-like behavior

Ghrelin (Ghr) is an orexigenic peptide that is being investigated for its potential role in development of anxiety-like behavior and modulation of depressive-like symptoms induced by bilateral olfactory bulbectomy (OB) in rodents. Olfactory bulbectomy is an animal model useful to study of depression and Ghr could be an alternative therapeutic tool in depression therapy. We studied the effects of intracerebroventricular (i.c.v.) Ghr administration on the expression of hypothalamic genes related to depression and mood (delta opioid receptor (DOR), mu opioid receptor (MOR) and kappa opioid receptor (KOR), lutropin-choriogonadotropic hormone receptor (LHCGR), serotonin transporter (SERT), interleukin 1 beta (IL-1b), vasopressin (AVP) and corticotrophin releasing hormone (CRH)) in OB animals, as well as changes in plasma levels of AVP, CRH and adenocorticotropic hormone (ACTH). We found that acute Ghr 0.3 nmol/μl administration increases gene expression of DOR, SERT and LHCGR in OB mice and decreased expression of IL-1b, suggesting that these genes could be involved in the antidepressant-like effects of Ghr. In addition, OB animals exhibit high AVP gene expression and elevated plasma concentrations of AVP and ACTH and acute Ghr 0.3 nmol/μl administration reduces AVP gene expression and the concentration of these hormones, suggesting that peptide-effects on depressive-like behavior could be mediated at least in part via AVP. In conclusion, this study provides new evidence about genes, receptors and hormones involved in the antidepressant mechanism/s induced by Ghr in OB animals.

Early life adversity and serotonin transporter gene variation interact at the level of the adrenal gland to affect the adult hypothalamo-pituitary-adrenal axis

The short allelic variant of the serotonin transporter (5-HTT) promoter-linked polymorphic region (5-HTTLPR) has been associated with the etiology of major depression by interaction with early life stress (ELS). Furthermore, 5-HTTLPR has been associated with abnormal functioning of the stress-responsive hypothalamo-pituitary-adrenal (HPA) axis. Here, we examined if, and at what level, the HPA-axis is affected in an animal model for ELS × 5-HTTLPR interactions. Heterozygous and homozygous 5-HTT knockout rats and their wild-type littermates were exposed daily at postnatal days 2-14 to 3 h of maternal separation. When grown to adulthood, plasma levels of adrenocorticotropic hormone (ACTH), and the major rat glucocorticoid, corticosterone (CORT), were measured. Furthermore, the gene expression of key HPA-axis players at the level of the hypothalamus, pituitary and adrenal glands was assessed. No 5-HTT genotype × ELS interaction effects on gene expression were observed at the level of the hypothalamus or pituitary. However, we found significant 5-HTT genotype × ELS interaction effects for plasma CORT levels and adrenal mRNA levels of the ACTH receptor, such that 5-HTT deficiency was associated under control conditions with increased, but after ELS with decreased basal HPA-axis activity. With the use of an in vitro adrenal assay, naïve 5-HTT knockout rats were furthermore shown to display increased adrenal ACTH sensitivity. Therefore, we conclude that basal HPA-axis activity is affected by the interaction of 5-HTT genotype and ELS, and is programmed, within the axis itself, predominantly at the level of the adrenal gland. This study therefore emphasizes the importance of the adrenal gland for HPA-related psychiatric disorders.

5-HTT deficiency affects neuroplasticity and increases stress sensitivity resulting in altered spatial learning performance in the Morris water maze but not in the Barnes maze

The purpose of this study was to evaluate whether spatial hippocampus-dependent learning is affected by the serotonergic system and stress. Therefore, 5-HTT knockout (-/-), heterozygous (+/-) and wildtype (+/+) mice were subjected to the Barnes maze (BM) and the Morris water maze (WM), the latter being discussed as more aversive. Additionally, immediate early gene (IEG) expression, hippocampal adult neurogenesis (aN), and blood plasma corticosterone were analyzed.

While the performance of 5-HTT-/- mice in the BM was undistinguishable from both other genotypes, they performed worse in the WM. However, in the course of the repeated WM trials 5-HTT-/- mice advanced to wildtype level. The experience of a single trial of either the WM or the BM resulted in increased plasma corticosterone levels in all genotypes. After several trials 5-HTT-/- mice exhibited higher corticosterone concentrations compared with both other genotypes in both tests. Corticosterone levels were highest in 5-HTT-/- mice tested in the WM indicating greater aversiveness of the WM and a greater stress sensitivity of 5-HTT deficient mice.

Quantitative immunohistochemistry in the hippocampus revealed increased cell counts positive for the IEG products cFos and Arc as well as for proliferation marker Ki67 and immature neuron marker NeuroD in 5-HTT-/- mice compared to 5-HTT+/+ mice, irrespective of the test. Most differences were found in the suprapyramidal blade of the dentate gyrus of the septal hippocampus. Ki67-immunohistochemistry revealed a genotype x environment interaction with 5-HTT genotype differences in naïve controls and WM experience exclusively yielding more Ki67-positive cells in 5-HTT+/+ mice. Moreover, in 5-HTT-/- mice we demonstrate that learning performance correlates with the extent of aN.

Overall, higher baseline IEG expression and increased an in the hippocampus of 5-HTT-/- mice together with increased stress sensitivity may constitute the neurobiological correlate of raised alertness, possibly impeding optimal learning performance in the more stressful WM.

Extracellular norepinephrine clearance by the norepinephrine transporter is required for skeletal homeostasis

Changes in bone remodeling induced by pharmacological and genetic manipulation of β-adrenergic receptor (βAR) signaling in osteoblasts support a role of sympathetic nerves in the regulation of bone remodeling. However, the contribution of endogenous sympathetic outflow and nerve-derived norepinephrine (NE) to bone remodeling under pathophysiological conditions remains unclear. We show here that differentiated osteoblasts, like neurons, express the norepinephrine transporter (NET), exhibit specific NE uptake activity via NET and can catabolize, but not generate, NE. Pharmacological blockade of NE transport by reboxetine induced bone loss in WT mice. Similarly, lack of NE reuptake in norepinephrine transporter (Net)-deficient mice led to reduced bone formation and increased bone resorption, resulting in suboptimal peak bone mass and mechanical properties associated with low sympathetic outflow and high plasma NE levels. Last, daily sympathetic activation induced by mild chronic stress was unable to induce bone loss, unless NET activity was blocked. These findings indicate that the control of endogenous NE release and reuptake by presynaptic neurons and osteoblasts is an important component of the complex homeostatic machinery by which the sympathetic nervous system controls bone remodeling. These findings also suggest that drugs antagonizing NET activity, used for the treatment of hyperactivity disorders, may have deleterious effects on bone accrual.

The serotonin transporter gene-linked polymorphic region (5-HTTLPR) and cortisol stress reactivity: a meta-analysis

Recent meta-analyses have stimulated an active debate on whether the serotonin transporter gene-linked polymorphic region (5-HTTLPR) is associated with an elevated vulnerability to psychiatric diseases upon exposure to environmental adversity. As a potential mechanism explaining genotype-dependent differences in stress sensitivity, altered stress-induced activation of the hypothalamus-pituitary-adrenal (HPA) axis has been investigated in several experimental studies, with most of these studies comprising small samples. We evaluated the association of 5-HTTLPR genotype and cortisol reactivity to acute psychosocial stress by applying a meta-analytical technique based on eleven relevant data sets (total N=1686), which were identified through a systematic literature search up to October 2011. This meta-analysis indicates a small (d=0.27), but significant association between 5-HTTLPR genotype and HPA-axis reactivity to acute psychosocial stress with homozygous carriers of the S allele displaying increased cortisol reactivity compared with individuals with the S/L and L/L genotype. The latter association was not further moderated by participants' age, sex or the type of stressor. Formal testing revealed no evidence for a substantial selection or publication bias. Our meta-analytical results are consistent with a wide variety of experimental studies indicating a significant association between 5-HTTLPR genotype and intermediate phenotypes related to stress sensitivity. Future studies are needed to clarify the consistency of this effect and to further explore whether altered HPA-axis stress reactivity reflects a potential biological mechanism conveying an elevated risk for the development of stress-related disorders in S allele carriers.

Development of autonomic dysfunction with intermittent hypoxia in a lean murine model

Intermittent hypoxia (IH) has been previously shown in a lean murine model to produce sustained hypertension and reverse the diurnal variation of blood glucose (BG). Concomitant glucose infusion attenuated the hypertension but exacerbated the BG fluctuations. In this study, cardiovascular variability analysis was employed to track the development of autonomic dysfunction in mice exposed to room air (IA) or IH, in combination with saline or glucose infusion. Baroreflex sensitivity was found to decrease in all animals, except in the control group. Low-frequency power of pulse interval spectrum, reflecting vagal activity, decreased more rapidly in glucose relative to saline while low-frequency power of blood pressure, reflecting sympathetic activity, decreased more slowly in IH relative to IA. Ultradian (≈ 12 h) rhythmicity was substantially suppressed in IH groups. These findings suggest that IH acted to increase sympathetic activity while glucose infusion led to reduced parasympathetic activity. The combination of IH and hyperglycemia leads to progressively adverse effects on autonomic control independent of obesity.

The role of the serotonergic and GABA system in translational approaches in drug discovery for anxiety disorders

There is ample evidence that genetic factors play an important role in anxiety disorders. In support, human genome-wide association studies have implicated several novel candidate genes. However, illumination of such genetic factors involved in anxiety disorders has not resulted in novel drugs over the past decades. A complicating factor is the heterogeneous classification of anxiety disorders in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) and diverging operationalization of anxiety used in preclinical and clinical studies. Currently, there is an increasing focus on the gene × environment (G × E) interaction in anxiety as genes do not operate in isolation and environmental factors have been found to significantly contribute to the development of anxiety disorders in at-risk individuals. Nevertheless, extensive research on G × E mechanisms in anxiety has not resulted in major breakthroughs in drug discovery. Modification of individual genes in rodent models has enabled the specific study of anxiety in preclinical studies. In this context, two extensively studied neurotransmitters involved in anxiety are the gamma-aminobutyric acid (GABA) and 5-HT (5-hydroxytryptamine) system. In this review, we illustrate the complex interplay between genes and environment in anxiety processes by reviewing preclinical and clinical studies on the serotonin transporter (5-HTT), 5-HT1A receptor, 5-HT2 receptor, and GABAA receptor. Even though targets from the serotonin and GABA system have yielded drugs with known anxiolytic efficacy, the relation between the genetic background of these targets and anxiety symptoms and development of anxiety disorders is largely unknown. The aim of this review is to show the vast complexity of genetic and environmental factors in anxiety disorders. In light of the difficulty with which common genetic variants are identified in anxiety disorders, animal models with translational validity may aid in elucidating the neurobiological background of these genes and their possible role in anxiety. We argue that, in addition to human genetic studies, translational models are essential to map anxiety-related genes and to enhance our understanding of anxiety disorders in order to develop potentially novel treatment strategies.

Dopamine synthesis and D3 receptor activation in pancreatic β-cells regulates insulin secretion and intracellular [Ca(2+)] oscillations

Pancreatic islets are critical for glucose homeostasis via the regulated secretion of insulin and other hormones. We propose a novel mechanism that regulates insulin secretion from β-cells within mouse pancreatic islets: a dopaminergic negative feedback acting on insulin secretion. We show that islets are a site of dopamine synthesis and accumulation outside the central nervous system. We show that both dopamine and its precursor l-dopa inhibit glucose-stimulated insulin secretion, and this inhibition correlates with a reduction in frequency of the intracellular [Ca(2+)] oscillations. We further show that the effects of dopamine are abolished by a specific antagonist of the dopamine receptor D3. Because the dopamine transporter and dopamine receptors are expressed in the islets, we propose that cosecretion of dopamine with insulin activates receptors on the β-cell surface. D3 receptor activation results in changes in intracellular [Ca(2+)] dynamics, which, in turn, lead to lowered insulin secretion. Because blocking dopaminergic negative feedback increases insulin secretion, expanding the knowledge of this pathway in β-cells might offer a potential new target for the treatment of type 2 diabetes.

Circadian rhythmicity in serotonin transporter knockout mice

AIMS

Serotonin transporter knockout (5-HTT KO) mice exhibit elevated basal extracellular serotonin, increased depressive-like behaviors and increased rapid eye movement sleep. Because abnormalities of circadian rhythms are associated with mood disorders, we tested the hypothesis that 5-HTT KO mice would have altered circadian rhythmicity.

MAIN METHODS

Homecage locomotor activity was recorded in wild-type (WT) and KO mice under a standard 12:12 light-dark cycle. After 4weeks of recording, mice received a one-hour light pulse at circadian time (CT) 14 and then were kept under constant darkness for 3weeks.

KEY FINDINGS

There were no significant differences in amplitude, period, acrophase or total home cage locomotor activity between WT and KO mice during the 12:12 light-dark cycle or during constant darkness. The mean phase delay to a CT 14 light pulse was significantly attenuated in KO compared to WT mice.

SIGNIFICANCE

Acute increases in serotonin have been reported to attenuate photic phase shifts. The current study demonstrates that this effect is maintained in the face of a lifelong absence of 5-HTT.

Reduced serotonin reuptake transporter (SERT) function causes insulin resistance and hepatic steatosis independent of food intake

Serotonin reuptake transporter (SERT) is a key regulator of serotonin neurotransmission and a major target of antidepressants. Antidepressants, such as selectively serotonin reuptake inhibitors (SSRIs), that block SERT function are known to affect food intake and body weight. Here, we provide genetic evidence that food intake and metabolism are regulated by separable mechanisms of SERT function. SERT-deficient mice ate less during both normal diet and high fat diet feeding. The reduced food intake was accompanied with markedly elevated plasma leptin levels. Despite reduced food intake, SERT-deficient mice exhibited glucose intolerance and insulin resistance, and progressively developed obesity and hepatic steatosis. Several lines of evidence indicate that the metabolic deficits of SERT-deficient mice are attributable to reduced insulin-sensitivity in peripheral tissues. First, SERT-deficient mice exhibited beta-cell hyperplasia and islet-mass expansion. Second, biochemical analyses revealed constitutively elevated JNK activity and diminished insulin-induced AKT activation in the liver of SERT-deficient mice. SERT-deficient mice exhibited hyper-JNK activity and hyperinsulinemia prior to the development of obesity. Third, enhancing AKT signaling by PTEN deficiency corrected glucose tolerance in SERT-deficient mice. These findings have potential implications for designing selective SERT drugs for weight control and the treatment of metabolic syndromes.

Pre-gestational stress reduces the ratio of 5-HIAA to 5-HT and the expression of 5-HT1A receptor and serotonin transporter in the brain of foetal rat

Background

Many studies have found that stress before or during pregnancy is linked to an increased incidence of behavioural disorders in offspring. However, few studies have investigated hypothalamic-pituitary-adrenal (HPA) axis activity and the serotonergic system as a consequence of pregestational stress. In the present study, we investigated the effect of pre-gestational stress on HPA axis activity in maternal rats and their foetuses and examined whether changes in HPA axis activity of maternal rats produced functional changes in the serotonergic system in the brain of foetuses.

Results

We used the behavioural tests to assess the model of chronic unpredictable stress (CUS) in maternal rats. We found the activity in the open field and sucrose consumption was lower for rats with CUS than for the controls. Body weight but not brain weight was higher for control foetuses than those from the CUS group. Serum corticosterone and corticotrophin-releasing hormone levels were significantly higher for mothers with CUS before pregnancy and their foetuses than for the controls. Levels of 5-hydroxytryptamine (5-HT) were higher in the hippocampus and hypothalamus of foetuses in the CUS group than in the controls, and 5-hydroxyindoleacetic acid (5-HIAA) levels were lower in the hippocampus in foetuses in the CUS group than in the control group. Levels of 5-HIAA in the hypothalamus did not differ between foetuses in the CUS group and in the control group. The ratio of 5-HIAA to 5-HT was significantly lower for foetuses in the CUS group than in the control group. Levels of 5-HT1A receptor were significantly lower in the foetal hippocampus in the CUS group than in the control group, with no significant difference in the hypothalamus. The levels of serotonin transporter (SERT) were lower in both the foetal hippocampus and foetal hypothalamus in the CUS group than in the control group.

Conclusions

Our data demonstrate that pre-gestational stress alters HPA axis activity in maternal rats and their foetuses, which is associated with functional changes in 5-HT activity (5-HT, 5-HIAA and ratio of 5-HIAA to 5-HT), as well as the levels of the 5-HT1A receptor and SERT in the hippocampus and hypothalamus of foetuses.

Differential serotonin transport is linked to the rh5-HTTLPR in peripheral blood cells

The human serotonin transporter (SERT) gene possesses a 43-base pair (bp) insertion-deletion promoter polymorphism, the h5-HTTLPR. Genotype at this locus correlates with variation in anxiety-related personality traits and risk for major depressive disorder in many studies. Yet, the complex effects of the h5-HTTLPR, in combination with closely associated single-nucleotide polymorphisms (SNPs), continue to be debated. Moreover, although SERT is of high clinical significance, transporter function in vivo remains difficult to assess. Rhesus express a promoter polymorphism related to the h5-HTTLPR. The rh5-HTTLPR has been linked to differences in stress-related behavior and cognitive flexibility, although allelic variations in serotonin uptake have not been investigated. We studied the serotonin system as it relates to the 5-HTTLPR in rhesus peripheral blood cells. Sequencing of the rh5-HTTLPR revealed a 23-bp insertion, which is somewhat longer than originally reported. Consistent with previous reports, no SNPs in the rh5-HTTLPR and surrounding genomic regions were detected in the individuals studied. Reductions in serotonin uptake rates, cell surface SERT binding, and 5-hydroxyindoleacetic acid/serotonin ratios, but not SERT mRNA levels, were associated with the rh5-HTTLPR short allele. Thus, serotonin uptake rates are differentiable with respect to the 5-HTTLPR in an easily accessible native peripheral tissue. In light of these findings, we foresee that primary blood cells, in combination with high sensitivity functional measurements enabled by chronoamperometry, will be important for investigating alterations in serotonin uptake associated with genetic variability and antidepressant responsiveness in humans.

Mapping functional brain activation using [14C]-iodoantipyrine in male serotonin transporter knockout mice

Background

Serotonin transporter knockout mice have been a powerful tool in understanding the role played by the serotonin transporter in modulating physiological function and behavior. However, little work has examined brain function in this mouse model. We tested the hypothesis that male knockout mice show exaggerated limbic activation during exposure to an emotional stressor, similar to human subjects with genetically reduced transcription of the serotonin transporter.

Methodology/Principal Findings

Functional brain mapping using [¹⁴C]-iodoantipyrine was performed during recall of a fear conditioned tone. Regional cerebral blood flow was analyzed by statistical parametric mapping from autoradiographs of the three-dimensionally reconstructed brains. During recall, knockout mice compared to wild-type mice showed increased freezing, increased regional cerebral blood flow of the amygdala, insula, and barrel field somatosensory cortex, decreased regional cerebral blood flow of the ventral hippocampus, and conditioning-dependent alterations in regional cerebral blood flow in the medial prefrontal cortex (prelimbic, infralimbic, and cingulate). Anxiety tests relying on sensorimotor exploration showed a small (open field) or paradoxical effect (marble burying) of loss of the serotonin transporter on anxiety behavior, which may reflect known abnormalities in the knockout animal's sensory system. Experiments evaluating whisker function showed that knockout mice displayed impaired whisker sensation in the spontaneous gap crossing task and appetitive gap cross training.

Conclusions

This study is the first to demonstrate altered functional activation in the serotonin transporter knockout mice of critical nodes of the fear conditioning circuit. Alterations in whisker sensation and functional activation of barrel field somatosensory cortex extend earlier reports of barrel field abnormalities, which may confound behavioral measures relying on sensorimotor exploration.

The anti-fibrinolytic SERPIN, plasminogen activator inhibitor 1 (PAI-1), is targeted to and released from catecholamine storage vesicles

Recent studies suggest a crucial role for plasminogen activator inhibitor-1 (PAI-1) in mediating stress-induced hypercoagulability and thrombosis. However, the mechanisms by which PAI-1 is released by stress are not well-delineated. Here, we examined catecholaminergic neurosecretory cells for expression, trafficking, and release of PAI-1. PAI-1 was prominently expressed in PC12 pheochromocytoma cells and bovine adrenomedullary chromaffin cells as detected by Northern blotting, Western blotting, and specific PAI-1 ELISA. Sucrose gradient fractionation studies and immunoelectron microscopy demonstrated localization of PAI-1 to catecholamine storage vesicles. Secretogogue stimulation resulted in corelease of PAI-1 with catecholamines. Parallel increases in plasma PAI-1 and catecholamines were observed in response to acute sympathoadrenal activation by restraint stress in mice in vivo. Reverse fibrin zymography demonstrated free PAI-1 in cellular releasates. Detection of high molecular weight complexes by Western blotting, consistent with PAI-1 complexed with t-PA, as well as bands consistent with cleaved PAI-1, suggested that active PAI-1 was present. Modulation of PAI-1 levels by incubating PC12 cells with anti-PAI-1 IgG caused a marked decrease in nicotine-mediated catecholamine release. In summary, PAI-1 is expressed in chromaffin cells, sorted into the regulated pathway of secretion (into catecholamine storage vesicles), and coreleased, by exocytosis, with catecholamines in response to secretogogues.

The winner and loser effect, serotonin transporter genotype, and the display of offensive aggression

Aggressive behaviour results from a complex interplay between genetic and environmental factors. Key modulators of aggression include the serotonergic system on the molecular level and experience in prior aggressive contests as an environmental factor. The aim of this study was to elucidate the effects of fighting experience on the display of offensive aggressive behaviour in adult male mice varying in serotonin transporter (5-HTT) genotype. 5-HTT +/+, 5-HTT +/- and 5-HTT -/- mice were given either a winning or a losing experience on each of three consecutive days and were subsequently observed for their offensive aggressive behaviour as residents against a docile intruder from the C3H strain in a resident-intruder paradigm. The main findings were: There was no significant difference between the amount of offensive aggressive behaviour displayed by the genotypes. Winners showed more engagement with the intruder, attacked him faster and exhibited overall higher aggression scores than losers. There was no significant genotype × social experience interaction: winning and losing had a similar effect on offensive aggressive behaviour in all three 5-HTT genotypes. We conclude that social experience in terms of having been a winner or having been a loser rather than the 5-HTT genotype determines the behaviour towards a docile intruder.

Impact of stress on cancer metastasis

The influence of psychosocial factors on the development and progression of cancer has been a longstanding hypothesis since ancient times. In fact, epidemiological and clinical studies over the past 30 years have provided strong evidence for links between chronic stress, depression and social isolation and cancer progression. By contrast, there is only limited evidence for the role of these behavioral factors in cancer initiation. Recent cellular and molecular studies have identified specific signaling pathways that impact cancer growth and metastasis. This article provides an overview of the relationship between psychosocial factors, specifically chronic stress, and cancer progression.

Away game or home match: the influence of venue and serotonin transporter genotype on the display of offensive aggression

Aggression can be modulated by both genetic and environmental factors. Here, we analyse how the serotonin transporter (5-HTT) genotype and the environmental situation in which a contest takes place shape the display of offensive aggression. Therefore, male wildtype, heterozygous, and homozygous 5-HTT knockout mice, which are known to differ in inborn levels of anxiety, were confronted three times with a docile opponent in one of three environmental situations: own territory, opponent's territory or neutral area. The main findings were: The frequency of approaching the contestant in order to gather information about him depended significantly on the venue but not on the genotype with lowest frequencies in the opponent's territory. The decision how quickly to attack the opponent was significantly influenced by the 5-HTT genotype but not by the venue: Homozygous 5-HTT knockout mice showed longest latencies. The sum of offensive aggression was significantly influenced by the 5-HTT genotype, the environmental situation, and a genotype by environment interaction. It is likely that, due to their varying genetic predisposition for anxiety, mice of the three genotypes were differentially affected by the aversiveness of the respective venue and the opponent's behaviour, which influenced their decision to display offensive aggression. As a consequence, the amount of aggression shown by homozygous 5-HTT knockout mice was influenced by the venue and the opponent's behaviour, whereas heterozygotes reacted only to the venue. Strikingly, wildtypes behaved always the same way, irrespective of venue and opponent.

Serotonin transporter knockout and repeated social defeat stress: impact on neuronal morphology and plasticity in limbic brain areas

Low expression of the human serotonin transporter (5-HTT) gene presumably interacts with stressful life events enhancing susceptibility for affective disorders. 5-Htt knockout (KO) mice display an anxious phenotype, and behavioural differences compared to wild-type (WT) mice are exacerbated after repeated loser experience in a resident-intruder stress paradigm. To assess whether genotype-dependent and stress-induced behavioural differences are reflected in alterations of neuronal morphology in limbic areas, we studied dendritic length and complexity of pyramidal neurons in the anterior cingulate and infralimbic cortices (CG, IL), hippocampus CA1 region, and of pyramidal neurons and interneurons in the lateral (La) and basolateral (BL) amygdaloid nuclei in Golgi-Cox-stained brains of male WT and 5-Htt KO control and loser mice. Spine density was analysed for IL apical and amygdaloid apical and basal pyramidal neuron dendrites. While group differences were absent for parameters analysed in CG, CA1 and amygdaloid interneurons, pyramidal neurons in the IL displayed tendencies to shorter and less spinous distal apical dendrites in 5-Htt KO controls, and to extended proximal dendrites in WT losers compared to WT controls. In contrast, spine density of several dendritic compartments of amygdaloid pyramids was significantly higher in 5-Htt KO mice compared to WT controls. While a tendency to increased spine density was observed in the same dendritic compartments in WT after stress, changes were lacking in stressed compared to control 5-Htt KO mice. Our findings indicate that disturbed 5-HT homeostasis results in alterations of limbic neuronal morphology, especially in higher spinogenesis in amygdaloid pyramidal neurons. Social stress leads to similar but less pronounced changes in the WT, and neuroplasticity upon stress is reduced in 5-Htt KO mice.

Cerebral metabolic responses to 5-HT2A/C receptor activation in mice with genetically modified serotonin transporter (SERT) expression

Variation in the human serotonin transporter gene (hSERT; 5-HTT) resulting in a life-long alteration in SERT function influences anxiety and the risk of developing affective disorders. The mechanisms underlying the influence of the hSERT gene on these phenotypes remain unclear but may involve altered 5-HT receptor function. Here we characterise the cerebral metabolic response to 5-HT(2A/C) receptor activation in two transgenic mouse models of altered SERT function, SERT knock-out (SERT KO) and hSERT over-expressing (hSERT OE) mice, to test the hypothesis that genetically mediated variability in SERT expression alters 5-HT(2A/C) function. We found that a constitutive increase in SERT expression (hSERT OE) enhanced, whereas a constitutive decrease in SERT expression (SERT KO) attenuated, 5-HT(2A/C) function. Therefore, altered 5-HT(2A/C) receptor functioning in response to hSERT gene variation may contribute to its influence on affective phenotypes.

Differential gene expression in mutant mice overexpressing or deficient in the serotonin transporter: a focus on urocortin 1

Transcriptome analyses were performed in the anterior raphe area of mutant mice deficient in the serotonin transporter (5-HTT KO) or overexpressing this protein (5-HTT TG), which exhibit opposite changes in anxiety-related behavior. Among genes with altered expression, the gene encoding the neuropeptide urocortin 1 was down-regulated in 5-HTT KO and up-regulated in 5-HTT TG mice. Expression of the gene encoding cocaine-and-amphetamine-related-peptide, which colocalizes with urocortin 1, was also increased in 5-HTT TG mutants. Real-time RT-PCR confirmed these data and immunoautoradiographic labeling showed that parallel changes in neuropeptide levels were confined to the non-preganglionic Edinger-Westphal nucleus. Thus, 5-HTT expression correlates with that of urocortin 1, suggesting that this peptide can be involved in the behavioral changes observed in 5-HTT mutant mice.

Consequences of serotonin transporter genotype and early adversity on behavioral profile - pathology or adaptation?

This review focuses on how behavioral profile is shaped by early adversity in individuals with varying serotonin transporter (5-HTT) genotype. In a recent study on 5-HTT knockout mice Heiming et al. (2009) simulated a 'dangerous environment' by confronting pregnant and lactating females with odor cues of unfamiliar males, indicating the risk of infant killing. Growing up in a dangerous environment induced increased anxiety-related behavior and decreased exploratory locomotion in the offspring, the effects being most pronounced in mice lacking 5-HTT expression. We argue that these alterations in behavioral profile represent adaptive maternal effects that help the individuals to cope with adversity. In principle, such effects of adversity on behavioral profile should not automatically be regarded as pathological. Rather and in accordance with modern evolutionary theory they may represent adaptations, although individuals with 5-HTT genotype induced susceptibility to adversity may be at risk of developing pathologies.

Adaptive modulation of behavioural profiles by social stress during early phases of life and adolescence

The development of individual behavioural profiles can be powerfully influenced by stressful social experiences. Using a comparative approach, we focus on the role of social stressors for the modulation of behavioural profile during early phases of life and adolescence. For gregarious species, the stability of the social environment in which the pregnant and lactating female lives is of major importance for foetal brain development and the behavioural profile of the offspring in later life. Social instability during these critical periods of development generally brings about a behavioural and neuroendocrine masculinisation in daughters and a less pronounced expression of male-typical traits in sons. Moreover, when mothers live in a socially threatening world during this time, anxiety-like behaviour of their offspring often is elevated in adulthood. These effects of the social environment are likely to be mediated by maternal hormones and/or maternal behaviour. In addition, they can be modulated significantly by offspring genotype. We favour the hypothesis that the behavioural effects of social stress during this phase of life are not necessarily "pathological" (nonadaptive) consequences or constraints of adverse social conditions. Rather, mothers could be adjusting the offspring to their environment in an adaptive way. Adolescence is another period in which behavioural development is particularly susceptible to social influences. There is some evidence that stressful social events experienced at this time alter and canalize behaviour in an adaptive fashion, so that earlier influences on behavioural profile development can be complemented and readjusted, if necessary, to meet current environmental conditions. In terms of underlying neuroendocrine mechanism, a central role for the interaction of testosterone and stress hormones is suggested. In summary, the modulation of behavioural profiles by social stress from the prenatal phase through adolescence appears to represent an effective mechanism for repeated and rapid adaptation.

Platelet serotonin transporter function and heart rate variability in patients with panic disorder

Many studies showed abnormal serotonin transporter (5-HTT) function and heart rate variability (HRV) in panic disorder patients. The present study investigated the relationship between HRV power spectral analysis findings and platelet serotonin uptake in panic disorder patients. Short-term HRV over 5 min and platelet serotonin transporter uptake parameters (V(max) and K(m)) were measured both in 45 patients with panic disorder and in 30 age-matched normal healthy control subjects. Low frequency power (LF) normalized unit (nu) and LF/high frequency power (HF) were significantly higher, whereas HF and HF nu were lower in the patient group than in the control group. V(max) and K(m) were all significantly lower (i.e., reflects decreased 5-HTT function) in patients with panic disorder than in normal controls. In the patient group, K(m) was negatively correlated with LF/HF and LF nu whereas no such correlations between them were found in the control group. By multivariate analysis based on multiple hierarchical linear regression, a low K(m) independently predicted an increased LF nu even after controlling for age, sex, and body mass index in the patient group. These results suggest that impaired 5-HTT function is closely related to dysregulation of autonomic nervous system in panic disorder.

Altered sleep homeostasis after restraint stress in 5-HTT knock-out male mice: a role for hypocretins

Restraint stress produces changes in the sleep pattern that are mainly characterized by a delayed increase in rapid eye movement sleep (REMS) amounts. Because the serotonin (5-HT) and the hypocretin (hcrt) systems that regulate REMS are interconnected, we used mutant mice deficient in the 5-HT transporter (5-HTT(-/-)) to examine the role of 5-HT and hcrt neurotransmissions in the sleep response to stress. In contrast to wild-type mice, restraint stress did not induce a delayed increase in REMS amounts in 5-HTT(-/-) mice, indicating impaired sleep homeostasis in mutants. However, pharmacological blockade of the hcrt type 1 receptor (hcrt-R1) before restraint stress restored the REMS increase in 5-HTT(-/-) mice. In line with this finding, 5-HTT(-/-) mutants displayed after restraint stress higher long-lasting activation of hypothalamic preprohcrt neurons than wild-type mice and elevated levels of the hcrt-1 peptide and the hcrt-R1 mRNA in the anterior raphe area. Thus, hypocretinergic neurotransmission was enhanced by stress in 5-HTT(-/-) mice. Furthermore, in 5-HTT(-/-) but not wild-type mice, hypothalamic levels of the 5-HT metabolite 5-hydroxyindole acetic acid significantly increased after restraint stress, indicating a marked enhancement of serotonergic neurotransmission in mutants. Altogether, our data show that increased serotonergic -and in turn hypocretinergic- neurotransmissions exert an inhibitory influence on stress-induced delayed REMS. We propose that the direct interactions between hcrt neurons in the hypothalamus and 5-HT neurons in the anterior raphe nuclei account, at least in part, for the adaptive sleep-wakefulness regulations triggered by acute stress.

Polymorphism of the serotonin transporter gene modulates brain and physiological responses to acute stress in Japanese men

A short (S) variant, compared to a long (L) variant, of the promoter region of the serotonin transporter gene-linked polymorphic region (5HTTLPR) has been related to emotional hyper-reactivity. We tested whether the 5HTTLPR could modulate acute stress responses in the brain and, the cardiovascular and neuroendocrine systems. Ten Japanese male participants carrying double copies of the S alleles and 10 Japanese males carrying S and L alleles conducted a mental arithmetic task, and their regional cerebral blood flow by (15)O positron emission tomography and cardiovascular and neuroendocrine parameters were measured. During the acute stress task, the participants with the SS alleles showed stronger reactivity in blood pressure and secretion of epinephrine, compared to the participants with the SL and LL alleles. Furthermore, the SS carriers showed greater activation in stress-related brain regions such as the hypothalamus, cerebellum, midbrain, and pulvinar compared to the SL and LL carriers during the acute stress task. The present findings indicated that the S allele of the 5HTTLPR is associated with greater brain and physiological reactivity to acute stress in Japanese men.

Global disturbances in autonomic function yield cardiovascular instability and hypertension in the chromogranin a null mouse

We reported previously that chromogranin A (Chga) knockout (KO) mice are hypertensive and hyperadrenergic. Here we sought to determine the basis of such alterations by probing physiological, biochemical, and pharmacological responses to perturbations of the autonomic nervous system. In the conscious state, KO mice had substantially elevated basal high blood pressure (BP) and heart rate (HR); immobilization stress caused increments in systolic BP and HR in both wild-type (WT) and KO mice, with higher maxima but blunted increments in the KO state. Catestatin (CST; CHGA(352-372)) selectively diminished stress-induced increments in BP and HR in KO mice, implicating CST as an antihypertensive peptide, even in stressful conditions. Heightened plasma catecholamines in KO mice returned to WT level after CST. Stress caused further increments in catecholamines in WT mice but no change in KO mice. KO mice displayed diminished baroreflex sensitivity in response to either phenylephrine or sodium nitroprusside, accounting for exaggerated pressor and depressor responses to these compounds; baroreceptor function was normalized by CST. To probe the relative roles of endogenous/basal sympathetic vs. parasympathetic tone in control of BP and HR, we used the muscarinic-cholinergic antagonist atropine or the beta-adrenergic antagonist propranolol; HR and BP responses to each antagonist were exaggerated in KO animals. We conclude that ablation of Chga expression results in global disturbances in autonomic function, both sympathetic and parasympathetic, that can be abrogated (or rescued), at least in part, by replacement of CST. The results point to mechanisms whereby CHGA and its CST fragment act to control cardiovascular homeostasis.

Living in a dangerous world: the shaping of behavioral profile by early environment and 5-HTT genotype

Anxiety and anxiety disorders are influenced by both, environmental and genetic factors. One genetic factor under scrutiny for anxiety disorders is the genetically encoded variation of the serotonin transporter (5-HTT). The aim of this study was to elucidate the effects of a threatening environment during early phases of life on anxiety-like (ANX) and exploratory behavior (EXP) in adult mice, varying in serotonin transporter (5-HTT) genotype. For this purpose, pregnant and lactating 5-HTT +/− dams were repeatedly exposed to olfactory cues of unfamiliar adult males by introducing small amounts of soiled bedding to their home cage. These stimuli signal the danger of infanticide and simulate a threatening environment. Control females were treated with neutral bedding. The offspring (5-HTT +/+, +/−, −/−) were examined for their ANX and EXP. The main results were: (1) a main effect of genotype existed, with 5-HTT −/− showing higher levels of ANX and lower levels of EXP than 5-HTT +/− and wildtypes. (2) When mothers had lived in a threatening environment, their offspring showed increased ANX and reduced EXP compared to controls. (3) These effects were most pronounced in 5-HTT −/− mice. By applying a new ecologically relevant paradigm we conclude: If 5-HTT +/− mothers live in a threatening environment during pregnancy and lactation, their offspring behavioral profile will, in principle, be shaped in an adaptive way preparing the young for an adverse environment. This process is, however, modulated by 5-HTT genotype, bearing the risk that individuals with impaired serotonergic neurotransmission (5-HTT −/−) will develop an exaggerated, potentially pathological level of anxiety from gene × environment interactions.

Sex-specific association between the 5-HTT gene-linked polymorphic region and basal cortisol secretion

OBJECTIVE

A key regulator of serotonergic neurotransmission is the serotonin transporter (5-HTT) and a common 5HTT gene promoter polymorphism, termed 5HTTLPR, is associated with phenotypes related to anxiety and depression. Furthermore, the serotonergic system influences hypothalamus-pituitary-adrenal (HPA) axis activity, which, in turn, is related to psychiatric diseases.

METHODS

To explore the association between the 5-HTTLPR and HPA axis regulation we performed a detailed endophenotyping in 216 healthy subjects (all 126 females used oral contraceptives).

RESULTS

While ACTH and cortisol responses to an established psychosocial stress paradigm (Trier Social Stress Test) were not found to be related to the 5-HTTLPR, we observed a significant and sex-specific association with the cortisol awakening response, which is a reliable marker of basal cortisol secretion, and with ACTH levels after dexamethasone administration. The supplementary inclusion of a 5-HTT A/G polymorphism (rs25531) in the analyses did not substantially modify our results.

CONCLUSION

These findings support the view that the 5-HTTLPR is associated with a major neuroendocrine stress system. It could be speculated that the sex-specific nature of this association contributes to the distinct gender differences in the vulnerability for depression.