Norepinephrine-deficient mice lack responses to antidepressant drugs, including selective serotonin reuptake inhibitors

The selective norepinephrine reuptake inhibitor reboxetine promotes late-stage fracture healing in mice

Impaired fracture healing is of high clinical relevance, as up to 15% of patients with long-bone fractures display non-unions. Fracture patients also include individuals treated with selective norepinephrine reuptake inhibitors (SNRI). As SNRI were previously shown to negatively affect bone homeostasis, it remained unclear whether patients with SNRI are at risk of impaired bone healing. Here, we show that daily treatment with the SNRI reboxetine reduces trabecular bone mass in the spine but increases cortical thickness and osteoblast numbers in the femoral midshaft. Most importantly, reboxetine does not impair bone regeneration in a standardized murine fracture model, and even improves callus bridging and biomechanical stability at late healing stages. In sum, reboxetine affects bone remodeling in a site-specific manner. Treatment does not interfere with the early and intermediate stages of bone regeneration and improves healing outcomes of the late-stage fracture callus in mice.

Local extracellular K+ in cortex regulates norepinephrine levels, network state, and behavioral output

Extracellular potassium concentration ([K+]e) is known to increase as a function of arousal. [K+]e is also a potent modulator of transmitter release. Yet, it is not known whether [K+]e is involved in the neuromodulator release associated with behavioral transitions. We here show that manipulating [K+]e controls the local release of monoaminergic neuromodulators, including norepinephrine (NE), serotonin, and dopamine. Imposing a [K+]e increase is adequate to boost local NE levels, and conversely, lowering [K+]e can attenuate local NE. Electroencephalography analysis and behavioral assays revealed that manipulation of cortical [K+]e was sufficient to alter the sleep-wake cycle and behavior of mice. These observations point to the concept that NE levels in the cortex are not solely determined by subcortical release, but that local [K+]e dynamics have a strong impact on cortical NE. Thus, cortical [K+]e is an underappreciated regulator of behavioral transitions.

Selective serotonin reuptake inhibitors suppress sharp wave ripples in the ventral hippocampus

Biased memory processing contributes to the development and exacerbation of depression, and thus could represent a potential therapeutic target for stress-induced mental disorders. Synchronized spikes in hippocampal neurons, corresponding to sharp wave ripples (SWRs), may play a crucial role in memory reactivation. In this study, we showed that the frequency of SWRs increased in the ventral hippocampus, but not in the dorsal hippocampus, after stress exposure. Administration of the selective serotonin reuptake inhibitors (SSRIs) fluoxetine and fluvoxamine inhibited the generation of ventral hippocampal SWRs and reduced locomotor activity and local field potential power in the gamma bands. These results suggest that the antidepressant effects of SSRIs may be mediated by the suppression of ventral hippocampal SWRs.

Selective Serotonin Reuptake Inhibitors within Cells: Temporal Resolution in Cytoplasm, Endoplasmic Reticulum, and Membrane

Selective serotonin reuptake inhibitors (SSRIs) are the most prescribed treatment for individuals experiencing major depressive disorder. The therapeutic mechanisms that take place before, during, or after SSRIs bind the serotonin transporter (SERT) are poorly understood, partially because no studies exist on the cellular and subcellular pharmacokinetic properties of SSRIs in living cells. We studied escitalopram and fluoxetine using new intensity-based, drug-sensing fluorescent reporters targeted to the plasma membrane, cytoplasm, or endoplasmic reticulum (ER) of cultured neurons and mammalian cell lines. We also used chemical detection of drug within cells and phospholipid membranes. The drugs attain equilibrium in neuronal cytoplasm and ER at approximately the same concentration as the externally applied solution, with time constants of a few s (escitalopram) or 200-300 s (fluoxetine). Simultaneously, the drugs accumulate within lipid membranes by ≥18-fold (escitalopram) or 180-fold (fluoxetine), and possibly by much larger factors. Both drugs leave cytoplasm, lumen, and membranes just as quickly during washout. We synthesized membrane-impermeant quaternary amine derivatives of the two SSRIs. The quaternary derivatives are substantially excluded from membrane, cytoplasm, and ER for >2.4 h. They inhibit SERT transport-associated currents sixfold or 11-fold less potently than the SSRIs (escitalopram or fluoxetine derivative, respectively), providing useful probes for distinguishing compartmentalized SSRI effects. Although our measurements are orders of magnitude faster than the therapeutic lag of SSRIs, these data suggest that SSRI-SERT interactions within organelles or membranes may play roles during either the therapeutic effects or the antidepressant discontinuation syndrome.SIGNIFICANCE STATEMENT Selective serotonin reuptake inhibitors stabilize mood in several disorders. In general, these drugs bind to SERT, which clears serotonin from CNS and peripheral tissues. SERT ligands are effective and relatively safe; primary care practitioners often prescribe them. However, they have several side effects and require 2-6 weeks of continuous administration until they act effectively. How they work remains perplexing, contrasting with earlier assumptions that the therapeutic mechanism involves SERT inhibition followed by increased extracellular serotonin levels. This study establishes that two SERT ligands, fluoxetine and escitalopram, enter neurons within minutes, while simultaneously accumulating in many membranes. Such knowledge will motivate future research, hopefully revealing where and how SERT ligands engage their therapeutic target(s).

Genetic loss of norepinephrine does not alter adult hippocampal neurogenesis in dopamine beta-hydroxylase deficient mice

Norepinephrine (NE), and specific adrenoceptors, have been reported to influence distinct aspects of adult hippocampal neurogenesis, including latent stem cell activation, progenitor proliferation, and differentiation. These findings are predominantly based on the use of pharmacological approaches in both in vitro and in vivo systems. Here, we sought to assess the consequences of genetic ablation of NE on adult hippocampal neurogenesis, by examining dopamine β hydroxylase knockout (Dbh -/-) mice, which lack NE from birth. We find that Dbh -/- mice exhibit no difference in adult hippocampal progenitor proliferation and survival. Further, the number of immature newborn neurons, labeled using stage-specific developmental markers within the hippocampal neurogenic niche, was also unaltered in Dbh -/- mice. In contrast, the noradrenergic neurotoxin DSP-4, which had previously been shown to reduce adult hippocampal neurogenesis in rats, also resulted in a decline in hippocampal progenitor proliferation in C57/Bl6N mice. These findings indicate that pharmacological lesioning of noradrenergic afferents in adulthood, but not the complete genetic loss of NE from birth, impairs adult hippocampal neurogenesis in mice.

Neuropharmacological characteristics of antidepressant action of a new 3-substituted thietane-1,1-dioxide derivative

Introduction: Due to severe burden of depressive disorders and a low rate of remission in patients receiving antidepressant therapy, there is an urgent need for developing novel agents with antidepressant action and a fundamentally new mechanism of action. 3-ethoxythietane-1,1-dioxide (N-199/1) is a new molecule that showed significant antidepressant properties when administered intraperitoneally once or repeatedly. The aim of the present study was to investigate the mechanism of action of N-199/1, using reserpine test.

Materials and methods: N-199/1 (2 mg/kg and 4.86 mg/kg) and the reference drugs (imipramine and fluoxetine) were administered once intraperitoneally to outbred male mice 4 h (Experiment 1) and 18 h (Experiment 2) after a single intraperitoneal injection of reserpine (2.5 mg/kg). The severity of reserpine-induced symptoms (hypothermia, ptosis and akinesia) was assessed.

Results and discussion: N-199/1 potentiated reserpine-induced hypothermia at both doses and reduced ptosis at a dose of 2 mg/kg when administered 4 h after reserpine. N-199/1 increased the duration of reserpine akinesia at a dose of 2 mg/kg when administered 18 h after reserpine and at a dose of 4.86 mg/kg when administered 4 h after reserpine. The effect of N-199/1 resembled the effect of fluoxetine and was dose-dependent.

Conclusion: Based on the results obtained, it can be assumed that the antidepressant action of N-199/1 is due to its serotonin-positive properties, and probably the blockade of serotonin 5HT2A/2C receptors and/or α2-adrenergic receptors. The effect of N-199/1 is dose-dependent and resembles the effect of fluoxetine.

Graphical abstract:

TRPA1 involvement in depression- and anxiety-like behaviors in a progressive multiple sclerosis model in mice

Progressive multiple sclerosis (PMS) is a neurological disease associated with the development of depression and anxiety, but treatments available are unsatisfactory. The transient receptor potential ankyrin 1 (TRPA1) is a cationic channel activated by reactive compounds, and the blockage of this receptor can reduce depression- and anxiety-like behaviors in naive mice. Thus, we investigated the role of TRPA1 in depression- and anxiety-like behaviors in a PMS model in mice. PMS model was induced in C57BL/6 female mice by the experimental autoimmune encephalomyelitis (EAE). Nine days after the PMS-EAE induction, behavioral tests (tail suspension and elevated plus maze tests) were performed to verify the effects of sertraline (positive control), selective TRPA1 antagonist (A-967,079), and antioxidants (α-lipoic acid and apocynin). The prefrontal cortex and hippocampus were collected to evaluate biochemical and inflammatory markers. PMS-EAE induction did not cause locomotor changes but triggered depression- and anxiety-like behaviors, which were reversed by sertraline, A-967,079, α-lipoic acid, or apocynin treatments. The neuroinflammatory markers (AIF1, GFAP, IL-1β, IL-17, and TNF-α) were increased in mice's hippocampus. Moreover, this model did not alter TRPA1 RNA expression levels in the hippocampus but decrease TRPA1 levels in the prefrontal cortex. Moreover, PMS-EAE induced an increase in NADPH oxidase and superoxide dismutase activities and TRPA1 endogenous agonist levels (hydrogen peroxide and 4-hydroxynonenal). TRPA1 plays a fundamental role in depression- and anxiety-like behaviors in a PMS-EAE model; thus, it could be a possible pharmacological target for treating these symptoms in PMS.

Co-released norepinephrine and galanin act on different timescales to promote stress-induced anxiety-like behavior

Both the noradrenergic and galaninergic systems have been implicated in stress-related neuropsychiatric disorders, and these two neuromodulators are co-released from the stress-responsive locus coeruleus (LC); however, the individual contributions of LC-derived norepinephrine (NE) and galanin to behavioral stress responses are unclear. Here we aimed to disentangle the functional roles of co-released NE and galanin in stress-induced behavior. We used foot shock, optogenetics, and behavioral pharmacology in wild-type (WT) mice and mice lacking either NE (Dbh^-/-) or galanin (Gal^cKO-Dbh) specifically in noradrenergic neurons to isolate the roles of these co-transmitters in regulating anxiety-like behavior in the elevated zero maze (EZM) either immediately or 24 h following stress. Foot shock and optogenetic LC stimulation produced immediate anxiety-like behavior in WT mice, and the effects of foot shock persisted for 24 h. NE-deficient mice were resistant to the anxiogenic effects of acute stress and optogenetic LC stimulation, while mice lacking noradrenergic-derived galanin displayed typical increases in anxiety-like behavior. However, when tested 24 h after foot shock, both Dbh^-/- and Gal^cKO-Dbh mice lacked normal expression of anxiety-like behavior. Pharmacological rescue of NE, but not galanin, in knockout mice during EZM testing was anxiogenic. In contrast, restoring galanin, but not NE, signaling during foot shock normalized stress-induced anxiety-like behavior 24 h later. These results indicate that NE and noradrenergic-derived galanin play complementary, but distinguishable roles in behavioral responses to stress. NE is required for the expression of acute stress-induced anxiety, while noradrenergic-derived galanin mediates the development of more persistent responses following a stressor.

Brain estrogen alters the effects of the antidepressant sertraline in middle-aged female and male mice

Estrogens are important in regulating mood, especially for females. However, whether tissue-specific estrogen, such as brain estrogen, contributes to the effects of antidepressant treatment has not been determined. The present study used middle-aged aromatase gene knockout (Ar^-/-) mice or overexpression (Thy1-Ar; hGFAP-Ar) mice as brain estrogen models to investigate whether brain estrogen synthesis alters the anti-depressive behaviors of sertraline treatment. Our results showed that depletion of brain estrogen increased depressive-like behavior in females, and elevated brain estrogen reduced depression-like behavior, regardless of sex. These genotype-related behaviors correlated with alterations of monoamine metabolism in the hippocampus (HPC) and the prefrontal cortex (PFC). We also demonstrated that male and female Ar^-/- mice exhibited an attenuation of sertraline-induced anti-depressive behaviors compared to wild-type (WT) mice. The present data suggest that brain estrogen alters depressive-like behaviors and changes the effectiveness of antidepressants in middle-aged mice, regardless of sex.

Glibenclamide restores dopaminergic reward circuitry in obese mice through interscauplar brown adipose tissue

BACKGROUND

Obesity, a critical feature in metabolic disorders, is associated with medical depression. Recent evidence reveals that brown adipose tissue (BAT) activity may contribute to mood disorders, Adenosine triphosphate (ATP)-sensitive K⁺ (K_ATP) channels regulate BAT sympathetic nerve activity. However, the mechanism through which BAT activity affects mood control remains unknown. We hypothesized the BAT is involved in depressive-like symptoms regulation by trafficking K_ATP channels.

METHODS

Eight-week-old male B6 mice fed with a high-fat diet (HFD) for 12 weeks exhibited characteristics of metabolic disorders, including hyperglycemia, hyperinsulinemia, and hyperlipidemia, as well as depressive symptoms. In this study, we surgically removed interscapular BAT in mice, and these mice exhibited immobility in the forced swim test and less preference for sugar water compared with other mice. To delineate the role of K_ATP channels in BAT activity regulation, we implanted a miniosmotic pump containing glibenclamide (GB), a K_ATP channel blocker, into the interscapular BAT of HFD-fed mice.

RESULTS

GB infusion improved glucose homeostasis, insulin sensitivity, and depressive-like symptoms. K_ATP channel expression was lower in HFD-fed mice than in chow-fed mice. Notably, GB infusion in HFD-fed mice restored K_ATP channel expression.

CONCLUSION

K_ATP channels are functionally expressed in BAT, and inhibiting BAT-K_ATP channels improves metabolic syndromes and reduces depressive symptoms through beta-3-adrenergic receptor-mediated protein kinase A signaling.

Animal models of major depression: drawbacks and challenges

Major depression is a leading contributor to the global burden of disease. This situation is mainly related to the chronicity and/or recurrence of the disorder, and to poor response to antidepressant therapy. Progress in this area requires valid animal models. Current models are based either on manipulating the environment to which rodents are exposed (during the developmental period or adulthood) or biological underpinnings (i.e. gene deletion or overexpression of candidate genes, targeted lesions of brain areas, optogenetic control of specific neuronal populations, etc.). These manipulations can alter specific behavioural and biological outcomes that can be related to different symptomatic and pathophysiological dimensions of major depression. However, animal models of major depression display substantial shortcomings that contribute to the lack of innovative pharmacological approaches in recent decades and which hamper our capabilities to investigate treatment-resistant depression. Here, we discuss the validity of these models, review putative models of treatment-resistant depression, major depression subtypes and recurrent depression. Furthermore, we identify future challenges regarding new paradigms such as those proposing dimensional rather than categorical approaches to depression.

Pharmacological Treatment of Depression in Alzheimer's Disease: A Challenging Task

Besides the memory impairment, Alzheimer’s disease (AD) is often complicated by neuropsychiatric symptoms also known as behavioral and psychological symptoms of dementia, which occur in one-third of patients at an early stage of the disease. Although the relationship between depressive disorders and AD is debated, the question if depression is a prodromal symptom preceding cognitive deficits or an independent risk factor for AD is still unclear. Moreover, there is growing evidence reporting that conventional antidepressants are not effective in depression associated with AD and, therefore, there is an urgent need to understand the neurobiological mechanism underlying the resistance to the antidepressants. Another important question that remains to be addressed is whether the antidepressant treatment is able to modulate the levels of amyloid-β peptide (Aβ), which is a key pathological hallmark in AD. The present review summarizes the present knowledge on the link between depression and AD with a focus on the resistance of antidepressant therapies in AD patients. Finally, we have briefly outlined the preclinical and clinical evidences behind the possible mechanisms by which antidepressants modulate Aβ pathology. To our opinion, understanding the cellular processes that regulate Aβ levels may provide greater insight into the disease pathogenesis and might be helpful in designing novel selective and effective therapy against depression in AD.

Sequential Photoperiodic Programing of Serotonin Neurons, Signaling and Behaviors During Prenatal and Postnatal Development

Early life stimuli during critical developmental time frames have been linked to increased risk for neurodevelopmental disorders later in life. The serotonergic system of the brain is implicated in mood disorders and is impacted by the duration of daylight, or photoperiod. Here we sought to investigate sensitive periods of prenatal and postnatal development for photoperiodic programming of DRN serotonin neurons, midbrain serotonin and metabolite levels along with affective behaviors in adolescence (P30) or adulthood (P50). To address these questions we restricted the interval of exposure to prenatal development (E0-P0) for Long summer-like photoperiods (LD 16:8), or Short winter-like photoperiods (LD 8:16) with postnatal development and maturation then occurring under the opposing photoperiod. Prenatal exposure alone to Long photoperiods was sufficient to fully program increased excitability of DRN serotonin neurons into adolescence and adulthood, similar to maintained exposure to Long photoperiods throughout development. Interestingly, Long photoperiod exposure can elevate serotonin and its’ corresponding metabolite levels along with reducing affective behavior, which appear to have both pre and postnatal origins. Thus, exposure to Long photoperiods prenatally programs increased DRN serotonin neuronal excitability, but this step is insufficient to program serotonin signaling and affective behavior. Continuing influence of Long photoperiods during postnatal development then modulates serotonergic content and has protective effects for depressive-like behavior. Photoperiodic programing of serotonin function in mice appears to be a sequential process with programing of neuronal excitability as a first step occurring prenatally, while programing of circuit level serotonin signaling and behavior extends into the postnatal period.

Molecular aspects of depression: A review from neurobiology to treatment

Major depressive disorder (MDD), also known as unipolar depression, is one of the leading causes of disability and disease worldwide. The signs and symptoms are low self‑esteem, anhedonia, feeling of worthlessness, sense of rejection and guilt, suicidal thoughts, among others. This review focuses on studies with molecular-based approaches involving MDD to obtain an integrated, more detailed and comprehensive view of the brain changes produced by this disorder and its treatment and how the Central Nervous System (CNS) produces neuroplasticity to orchestrate adaptive defensive behaviors. This article integrates affective neuroscience, psychopharmacology, neuroanatomy and molecular biology data. In addition, there are two problems with current MDD treatments, namely: 1) Low rates of responsiveness to antidepressants and too slow onset of therapeutic effect; 2) Increased stress vulnerability and autonomy, which reduces the responses of currently available treatments. In the present review, we encourage the prospection of new bioactive agents for the development of treatments with post-transduction mechanisms, neurogenesis and pharmacogenetics inducers that bring greater benefits, with reduced risks and maximized access to patients, stimulating the field of research on mood disorders in order to use the potential of preclinical studies. For this purpose, improved animal models that incorporate the molecular and anatomical tools currently available can be applied. Besides, we encourage the study of drugs that do not present "classical application" as antidepressants, (e.g., the dissociative anesthetic ketamine and dextromethorphan) and drugs that have dual action mechanisms since they represent potential targets for novel drug development more useful for the treatment of MDD.

Downregulation of the Central Noradrenergic System by Toxoplasma gondii Infection

Toxoplasma gondii is associated with physiological effects in the host. Dysregulation of catecholamines in the central nervous system has previously been observed in chronically infected animals. In the study described here, the noradrenergic system was found to be suppressed with decreased levels of norepinephrine (NE) in brains of infected animals and in infected human and rat neural cells in vitro The mechanism responsible for the NE suppression was found to be downregulation of dopamine β-hydroxylase (DBH) gene expression, encoding the enzyme that synthesizes norepinephrine from dopamine, with downregulation observed in vitro and in infected brain tissue, particularly in the dorsal locus coeruleus/pons region. The downregulation was sex specific, with males expressing reduced DBH mRNA levels whereas females were unchanged. Rather, DBH expression correlated with estrogen receptor in the female rat brains for this estrogen-regulated gene. DBH silencing was not a general response of neurons to infection, as human cytomegalovirus did not downregulate DBH expression. The noradrenergic-linked behaviors of sociability and arousal were altered in chronically infected animals, with a high correlation between DBH expression and infection intensity. A decrease in DBH expression in noradrenergic neurons can elevate dopamine levels, which provides a possible explanation for mixed observations of changes in this neurotransmitter with infection. Decreased NE is consistent with the loss of coordination and motor impairments associated with toxoplasmosis. Further, the altered norepinephrine synthesis observed here may, in part, explain behavioral effects of infection and associations with mental illness.

Influence of citicoline on citalopram-induced antidepressant activity in depressive-like symptoms in male mice

Depression is associated with significant functional disabilities. Application of new drugs which could enhance the effectiveness of antidepressants drug and reduce side effects of their long-term use seems necessary. Citicoline is used as an effective chemical agent for improving the symptoms of some neurodegenerative diseases. Therefore, in this survey, the application of citicoline as an adjuvant drug was evaluated in mice model of depression. A total of 180 adult NMRI male albino mice were used in this study. All groups were exposed to chronic unexpected mild stress (CUMS) followed by treatment with various doses of citalopram or/and citicoline or saline for 21 days. Sucrose preference (SP), open field (OF), and forced swimming test (FST) were applied to evaluate depression symptoms in the groups. The results indicated that only citicoline at the 5 mg/kg dose had shifted its status from being noneffective to become significantly effective in the co-administered group. The means of SP, OFT, and FST of the treatment groups were significantly different in favor of co-administered group compared with the other groups as well as the control group. Based on the results, it can be concluded that administration of citicoline, as an adjuvant drug, in combination with citalopram, enhanced the effectiveness of selective serotonin reuptake inhibitors (SSRI) drugs for depression treatment.

Selective Knockdown of TASK3 Potassium Channel in Monoamine Neurons: a New Therapeutic Approach for Depression

Current pharmacological treatments for major depressive disorder (MDD) are severely compromised by both slow action and limited efficacy. RNAi strategies have been used to evoke antidepressant-like effects faster than classical drugs. Using small interfering RNA (siRNA), we herein show that TASK3 potassium channel knockdown in monoamine neurons induces antidepressant-like responses in mice. TASK3-siRNAs were conjugated to cell-specific ligands, sertraline (Ser) or reboxetine (Reb), to promote their selective accumulation in serotonin (5-HT) and norepinephrine (NE) neurons, respectively, after intranasal delivery. Following neuronal internalization of conjugated TASK3-siRNAs, reduced TASK3 mRNA and protein levels were found in the brainstem 5-HT and NE cell groups. Moreover, Ser-TASK3-siRNA induced robust antidepressant-like behaviors, enhanced the hippocampal plasticity, and potentiated the fluoxetine-induced increase on extracellular 5-HT. Similar responses, yet of lower magnitude, were detected for Reb-TASK3-siRNA. These findings provide substantial support for TASK3 as a potential target, and RNAi-based strategies as a novel therapeutic approach to treat MDD.

Impact of Novel Antidepressants on Cardiac 123I-Metaiodobenzylguanidine Uptake: Experimental Studies on SK-N-SH Cells and Healthy Rabbits

¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) has independent prognostic value for risk stratification among heart failure patients, but the use of concomitant medication should not affect its quantitative information. We evaluated whether the 4 classes of antidepressants currently most prescribed as first-line treatment for major depressive disorder (MDD) have the potential to alter ¹²³I-MIBG imaging results. Methods: The inhibition effect of desipramine, escitalopram, venlafaxine, and bupropion on ¹³¹I-MIBG uptake was assessed by in vitro uptake assays using human neuroblastoma SK-N-SH cells. The half-maximal inhibitory concentration of tracer uptake was determined from dose-response curves. To evaluate the effect of intravenous pretreatment with desipramine (1.5 mg/kg) and escitalopram (2.5 or 15 mg/kg) on ¹²³I-MIBG cardiac uptake, in vivo planar ¹²³I-MIBG scanning of healthy New Zealand White rabbits was performed. Results: The half-maximal inhibitory concentrations of desipramine, escitalopram, venlafaxine, and bupropion on ¹³¹I-MIBG cellular uptake were 11.9 nM, 7.5 μM, 4.92 μM, and 12.9 μM, respectively. At the maximum serum concentration (as derived by previous clinical trials), the inhibition rates of ¹³¹I-MIBG uptake were 90.6% for desipramine, 25.5% for venlafaxine, 11.7% for bupropion, and 0.72% for escitalopram. A low inhibition rate for escitalopram in the cell uptake study triggered investigation of an in vivo rabbit model: with a dosage considerably higher than used in clinical practice, the noninhibitory effect of escitalopram was confirmed. Furthermore, pretreatment with desipramine markedly reduced cardiac ¹²³I-MIBG uptake. Conclusion: In the present in vitro binding assay and in vivo rabbit study, the selective serotonin reuptake inhibitor escitalopram had no major impact on neuronal cardiac ¹²³I-MIBG uptake within therapeutic dose ranges, whereas other types of first-line antidepressants for MDD treatment led to a significant decrease. These preliminary results warrant further confirmatory clinical trials regarding the reliability of cardiac ¹²³I-MIBG imaging, in particular, if the patient's neuropsychiatric status would not tolerate withdrawal of a potentially norepinephrine-interfering antidepressant.

Antidepressant-like activity of methyl jasmonate involves modulation of monoaminergic pathways in mice

PURPOSE

The efficacy of current antidepressant drugs has been compromised by adverse effects, low remission and delay onset of action necessitating the search for alternative agents. Methyl jasmonate (MJ), a bioactive compound isolated from Jasminum grandiflorum has been shown to demonstrate antidepressant activity but its mechanism of action remains unknown. Thus, the role of monoaminergic systems in the antidepression-like activity of MJ was investigated in this study.

MATERIALS AND METHODS

Mice were given i.p. injection of MJ (5, 10 and 20mg/kg), imipramine (10mg/kg) and vehicle (10mL/kg) 30min before the forced swim test (FST) and tail suspension test (TST) were carried out. The involvement of monoaminergic systems in the anti-depressant-like effect of MJ (20mg/kg) was evaluated using p-chlorophenylalanine (pCPA), metergoline, yohimbine, prazosin, sulpiride and haloperidol in the TST.

RESULTS

MJ significantly decrease the duration of immobility in the FST and TST relative to control suggesting antidepressant-like property. However, pretreatment with yohimbine (1mg/kg, i.p., an α₂-adrenergic receptor antagonist) or prazosin (62.5μg/kg, i.p., an α₁-adrenoceptor antagonist) attenuated the antidepressant-like activity of MJ. Also, pCPA; an inhibitor of serotonin biosynthesis (100mg/kg, i.p) or metergoline (4mg/kg, i.p., 5-HT₂ receptor antagonist) reversed the anti-immobility effect of MJ. Sulpiride (50mg/kg, i.p., a D₂ receptor antagonist) or haloperidol (0.2mg/kg, i.p., a dopamine receptor antagonist) reversed the anti-immobility effect of MJ.

CONCLUSION

The results of this study suggest that serotonergic, noradrenergic and dopaminergic systems may play a role in the antidepressant-like activity of MJ.

Enduring attenuation of norepinephrine synaptic availability and augmentation of the pharmacological and behavioral effects of desipramine by repeated immobilization stress

Here we provide evidence that repeated immobilization stress (RIS) in rats induces a persistent increase in noradrenergic activity in the anterior aspects of the anterolateral bed nucleus of the stria terminalis (alBNST). This increase in noradrenergic activity results from both enhanced synthesis and reuptake of norepinephrine (NE). It leads to a decrease in the synaptic availability of NE, which elicits an augmented noradrenergic response to the inhibitors of NE reuptake (NRIs), such as desipramine (DMI), an antidepressant. The enduring depression-like behavior and the augmentation of the climbing behavior seen in repeatedly stressed rats following subchronic administration of DMI in the forced swimming test (FST) might be explained by a dysregulation of noradrenergic transmission observed in alBNST. Taken together, we propose that dysregulation of noradrenergic transmission such as the one described in the present work may represent a mechanism underlying major depressive disorders (MDD) with melancholic features in humans.

The Impact of Housing Temperature-Induced Chronic Stress on Preclinical Mouse Tumor Models and Therapeutic Responses: An Important Role for the Nervous System

In the last 10-15 years, there has been a recognition that the catecholamines (norepinephrine, NE, and epinephrine, Epi) released by the sympathetic nervous system under stressful conditions promote tumor growth through a variety of mechanisms. Tumors recruit autonomic nerves during their development and NE is then released locally in the tumor microenvironment (TME). Acting through adrenergic receptors present on a variety of cells in the TME, NE and Epi induce proliferation, resistance to apoptosis, epithelial to mesenchymal transition, metastasis of tumor cells, angiogenesis, and inflammation in the TME. These pre-clinical studies have been conducted in mouse models whose care and housing parameters are outlined in "The Guide for the Care and Use of Laboratory Animals [1]. In particular, the Guide mandates that mice be housed at standardized sub-thermoneutral temperatures; however, this causes a state of chronic cold-stress and elevated levels of NE. Although mice are able to maintain a normal body temperature when kept at these cool temperatures, it is becoming clear that this cold-stress is sufficient to activate physiological changes which affect experimental outcomes. We find that when mice are housed under standard, sub-thermoneutral temperatures (~22 °C, ST), tumor growth is significantly greater than when mice are housed at thermoneutrality (~30 °C TT). We also find that the anti-tumor immune response is suppressed at ST and this immunosuppression can be reversed by housing mice at TT or by administration of propranolol (a β-adrenergic receptor antagonist) to mice housed at ST. Furthermore, at ST tumors are more resistant to therapy and can also be sensitized to cytotoxic therapies by housing mice at TT or by treating mice with propranolol. The implications of these observations are particularly relevant to the way in which experiments conducted in preclinical models are interpreted and the findings implemented in the clinic. It may be that the disappointing failure of many new therapies to fulfill their promise in the clinic is related to an incomplete preclinical assessment in mouse models. Further, an expanded understanding of the efficacy of a therapy alone or in combination obtained by testing under a wider range of conditions would better predict how patients, who are under various levels of stress, might respond in a clinical setting. This may be particularly important to consider since we now appreciate that long term outcome of many therapies depends on eliciting an immune response.It is clear that the outcome of metabolic experiments, immunological investigations and therapeutic efficacy testing in tumors of mice housed at ST is restricted and expanding these experiments to include results obtained at TT may provide us with valuable information that would otherwise be overlooked.

Chronic citalopram administration desensitizes prefrontal cortex but not somatodendritic α2-adrenoceptors in rat brain

Selective serotonin reuptake inhibitors (SSRIs) regulate brain noradrenergic neurotransmission both at somatodendritic and nerve terminal areas. Previous studies have demonstrated that noradrenaline (NA) reuptake inhibitors are able to desensitize α₂-adrenoceptor-mediated responses. The present study was undertaken to elucidate the effects of repeated treatment with the SSRI citalopram on the α₂-adrenoceptor sensitivity in locus coeruleus (LC) and prefrontal cortex (PFC), by using in vivo microdialysis and electrophysiological techniques, and in vitro stimulation of [³⁵S]GTPγS binding autoradiography. Repeated, but not acute, treatment with citalopram (5 mg/kg, i.p., 14 days) increased extracellular NA concentration selectively in PFC. The α₂-adrenoceptor agonist clonidine (0.3 mg/kg, i.p.), administered to saline-treated animals (1 ml/kg i.p., 14 days) induced NA decrease in LC (E_max = -44 ± 4%; p < 0.001) and in PFC (E_max = -61 ± 5%, p < 0.001). In citalopram chronically-treated rats, clonidine administration exerted a lower decrease of NA (E_max = -25 ± 7%; p < 0.001) in PFC whereas the effect in LC was not different to controls (E_max = -36 ± 4%). Clonidine administration (0.625-20 μg/kg, i.v.) evoked a dose-dependent decrease of the firing activity of LC noradrenergic neurons in both citalopram- (ED₅₀ = 3.2 ± 0.4 μg/kg) and saline-treated groups (ED₅₀ = 2.6 ± 0.5 μg/kg). No significant differences between groups were found in ED₅₀ values. The α₂-adrenoceptor agonist UK14304 stimulated specific [³⁵S]GTPγS binding in brain sections containing LC (144 ± 14%) and PFC (194 ± 32%) of saline-treated animals. In citalopram-treated animals, this increase did not differ from controls in LC (146 ± 22%) but was lower in PFC (141 ± 8%; p < 0.05). Taken together, long-term citalopram treatment induces a desensitization of α₂-adrenoceptors acting as axon terminal autoreceptors in PFC without changes in somatodendritic α₂-adrenoceptor sensitivity.

Organization of the locus coeruleus-norepinephrine system

The release of the neurotransmitter norepinephrine throughout the mammalian brain is important for modulating attention, arousal, and cognition during many behaviors. Furthermore, disruption of norepinephrine-mediated signaling is strongly associated with several psychiatric and neurodegenerative disorders in humans, emphasizing the clinical importance of this system. Most of the norepinephrine released in the brain is supplied by a very small, bilateral nucleus in the brainstem called the locus coeruleus. The goal of this minireview is to emphasize the complexity of the locus coeruleus beyond its primary definition as a norepinephrine-producing nucleus. Several recent studies utilizing innovative technologies highlight how the locus coeruleus-norepinephrine system can now be targeted with increased accuracy and resolution, in order to better understand its role in modulating diverse behaviors.

Mood and metabolism: Anhedonia as a clinical target in Type 2 diabetes

Epidemiological evidence suggests a bidirectional relationship between depression and Type 2 diabetes mellitus. In Type 2 diabetes, depression affects behavioural factors such as diet and physical activity that promote positive energy balance and influence diabetes outcomes. Examinations of depressive symptoms by dimension have suggested that anhedonia, the inability to anticipate, seek, choose and enjoy reward, may be of particular clinical importance. Structural and functional brain changes in Type 2 diabetes distributed throughout the principally dopaminergic reward circuitry suggest a neurobiological basis for motivational and decisional aspects of anhedonia. Interrelated neuroendocrine, bio-energetic, oxidative and inflammatory changes suggest mechanisms underlying neuronal damage and dopaminergic deficits. A consequential shift in effort-related reward choices and their effects on energy expenditure, self-care and eating behaviours is suggested to affect Type 2 diabetes outcomes. The clinical implications for screening and psychopharmacology of depressive symptoms in people with Type 2 diabetes are discussed.

Human Bacterial Artificial Chromosome (BAC) Transgenesis Fully Rescues Noradrenergic Function in Dopamine β-Hydroxylase Knockout Mice

Dopamine β-hydroxylase (DBH) converts dopamine (DA) to norepinephrine (NE) in noradrenergic/adrenergic cells. DBH deficiency prevents NE production and causes sympathetic failure, hypotension and ptosis in humans and mice; DBH knockout (Dbh -/-) mice reveal other NE deficiency phenotypes including embryonic lethality, delayed growth, and behavioral defects. Furthermore, a single nucleotide polymorphism (SNP) in the human DBH gene promoter (-970C>T; rs1611115) is associated with variation in serum DBH activity and with several neurological- and neuropsychiatric-related disorders, although its impact on DBH expression is controversial. Phenotypes associated with DBH deficiency are typically treated with L-3,4-dihydroxyphenylserine (DOPS), which can be converted to NE by aromatic acid decarboxylase (AADC) in the absence of DBH. In this study, we generated transgenic mice carrying a human bacterial artificial chromosome (BAC) encompassing the DBH coding locus as well as ~45 kb of upstream and ~107 kb of downstream sequence to address two issues. First, we characterized the neuroanatomical, neurochemical, physiological, and behavioral transgenic rescue of DBH deficiency by crossing the BAC onto a Dbh -/- background. Second, we compared human DBH mRNA abundance between transgenic lines carrying either a "C" or a "T" at position -970. The BAC transgene drove human DBH mRNA expression in a pattern indistinguishable from the endogenous gene, restored normal catecholamine levels to the peripheral organs and brain of Dbh -/- mice, and fully rescued embryonic lethality, delayed growth, ptosis, reduced exploratory activity, and seizure susceptibility. In some cases, transgenic rescue was superior to DOPS. However, allelic variation at the rs1611115 SNP had no impact on mRNA levels in any tissue. These results indicate that the human BAC contains all of the genetic information required for tissue-specific, functional expression of DBH and can rescue all measured Dbh deficiency phenotypes, but did not reveal an impact of the rs11115 variant on DBH expression in mice.

Fluoxetine Facilitates Fear Extinction Through Amygdala Endocannabinoids

Pharmacologically elevating brain endocannabinoids (eCBs) share anxiolytic and fear extinction-facilitating properties with classical therapeutics, including the selective serotonin reuptake inhibitor, fluoxetine. There are also known functional interactions between the eCB and serotonin systems and preliminary evidence that antidepressants cause alterations in brain eCBs. However, the potential role of eCBs in mediating the facilitatory effects of fluoxetine on fear extinction has not been established. Here, to test for a possible mechanistic contribution of eCBs to fluoxetine's proextinction effects, we integrated biochemical, electrophysiological, pharmacological, and behavioral techniques, using the extinction-impaired 129S1/Sv1mJ mouse strain. Chronic fluoxetine treatment produced a significant and selective increase in levels of anandamide in the BLA, and an associated decrease in activity of the anandamide-catabolizing enzyme, fatty acid amide hydrolase. Slice electrophysiological recordings showed that fluoxetine-induced increases in anandamide were associated with the amplification of eCB-mediated tonic constraint of inhibitory, but not excitatory, transmission in the BLA. Behaviorally, chronic fluoxetine facilitated extinction retrieval in a manner that was prevented by systemic or BLA-specific blockade of CB1 receptors. In contrast to fluoxetine, citalopram treatment did not increase BLA eCBs or facilitate extinction. Taken together, these findings reveal a novel, obligatory role for amygdala eCBs in the proextinction effects of a major pharmacotherapy for trauma- and stressor-related disorders and anxiety disorders.

Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system

Aside from its roles in as a classical neurotransmitter involved in regulation of behavior, noradrenaline (NA) has other functions in the CNS. This includes restricting the development of neuroinflammatory activation, providing neurotrophic support to neurons, and providing neuroprotection against oxidative stress. In recent years, it has become evident that disruption of physiological NA levels or signaling is a contributing factor to a variety of neurological diseases and conditions including Alzheimer's disease (AD) and Multiple Sclerosis. The basis for dysregulation in these diseases is, in many cases, due to damage occurring to noradrenergic neurons present in the locus coeruleus (LC), the major source of NA in the CNS. LC damage is present in AD, multiple sclerosis, and a large number of other diseases and conditions. Studies using animal models have shown that experimentally induced lesion of LC neurons exacerbates neuropathology while treatments to compensate for NA depletion, or to reduce LC neuronal damage, provide benefit. In this review, we will summarize the anti-inflammatory and neuroprotective actions of NA, summarize examples of how LC damage worsens disease, and discuss several approaches taken to treat or prevent reductions in NA levels and LC neuronal damage. Further understanding of these events will be of value for the development of treatments for AD, multiple sclerosis, and other diseases and conditions having a neuroinflammatory component. The classical neurotransmitter noradrenaline (NA) has critical roles in modulating behaviors including those involved in sleep, anxiety, and depression. However, NA can also elicit anti-inflammatory responses in glial cells, can increase neuronal viability by inducing neurotrophic factor expression, and can reduce neuronal damage due to oxidative stress by scavenging free radicals. NA is primarily produced by tyrosine hydroxylase (TH) expressing neurons in the locus coeruleus (LC), a relatively small brainstem nucleus near the IVth ventricle which sends projections throughout the brain and spinal cord. It has been known for close to 50 years that LC neurons are lost during normal aging, and that loss is exacerbated in neurological diseases including Parkinson's disease and Alzheimer's disease. LC neuronal damage and glial activation has now been documented in a variety of other neurological conditions and diseases, however, the causes of LC damage and cell loss remain largely unknown. A number of approaches have been developed to address the loss of NA and increased inflammation associated with LC damage, and several methods are being explored to directly minimize the extent of LC neuronal cell loss or function. In this review, we will summarize some of the consequences of LC loss, consider several factors that likely contribute to that loss, and discuss various ways that have been used to increase NA or to reduce LC damage. This article is part of the 60th Anniversary special issue.

Specificity and impact of adrenergic projections to the midbrain dopamine system

Dopamine (DA) is a neuromodulator that regulates different brain circuits involved in cognitive functions, motor coordination, and emotions. Dysregulation of DA is associated with many neurological and psychiatric disorders such as Parkinson's disease and substance abuse. Several lines of research have shown that the midbrain DA system is regulated by the central adrenergic system. This review focuses on adrenergic interactions with midbrain DA neurons. It discusses the current neuroanatomy including source of adrenergic innervation, type of synapses, and adrenoceptors expression. It also discusses adrenergic regulation of DA cell activity and neurotransmitter release. Finally, it reviews several neurological and psychiatric disorders where changes in adrenergic system are associated with dysregulation of the midbrain DA system. This article is part of a Special Issue entitled SI: Noradrenergic System.

Vesicular monoamine transporter 2 (Vmat2) knockdown elicits anxiety-like behavior in zebrafish

Vesicular monoamine transporter 2 (Vmat2) is widely distributed in the central nervous system, and responsible for uptaking transmitters into the vesicles. However, whether Vmat2-deficiency is related to the anxiety is rarely investigated, especially in zebrafish. Here, we reported Vmat2 heterzygous mutant zebrafish displayed anxiety-like behavior. The mutants spent less time in the top area and took longer latency to the top in the novel tank test. Consistently, they showed dark avoidance in the light/dark box test, with longer duration in the light zone and increased number of crossing between the two zones. Monoamine concentration analysis showed that the levels of monoamine neurotransmitters including dopamine (DA), 5-hydroxy tryptamine (5-HT) and norepinephrine (NE), as well as their metabolites were decreased in VMAT mutants. Taken together, these findings suggest that Vmat2 heterzygous mutant zebrafish may serve as a new model of anxiety, which may be related with the low level of DA, 5-HT and NE.

Treatment-resistant depression: are animal models of depression fit for purpose?

BACKGROUND

Resistance to antidepressant drug treatment remains a major health problem. Animal models of depression are efficient in detecting effective treatments but have done little to increase the reach of antidepressant drugs. This may be because most animal models of depression target the reversal of stress-induced behavioural change, whereas treatment-resistant depression is typically associated with risk factors that predispose to the precipitation of depressive episodes by relatively low levels of stress. Therefore, the search for treatments for resistant depression may require models that incorporate predisposing factors leading to heightened stress responsiveness.

METHOD

Using a diathesis-stress framework, we review developmental, genetic and genomic models against four criteria: (i) increased sensitivity to stress precipitation of a depressive behavioural phenotype, (ii) resistance to chronic treatment with conventional antidepressants, (iii) a good response to novel modes of antidepressant treatment (e.g. ketamine; deep brain stimulation) that are reported to be effective in treatment-resistant depression and (iv) a parallel to a known clinical risk factor.

RESULTS

We identify 18 models that may have some potential. All require further validation. Currently, the most promising are the Wistar-Kyoto (WKY) and congenital learned helplessness (cLH) rat strains, the high anxiety behaviour (HAB) mouse strain and the CB1 receptor knockout and OCT2 null mutant mouse strains.

CONCLUSION

Further development is needed to validate models of antidepressant resistance that are fit for purpose. The criteria used in this review may provide a helpful framework to guide research in this area.

Cognitive Function Related to the Sirh11/Zcchc16 Gene Acquired from an LTR Retrotransposon in Eutherians

Gene targeting of mouse Sushi-ichi-relatedretrotransposonhomologue11/Zinc fingerCCHCdomain-containing16 (Sirh11/Zcchc16) causes abnormal behaviors related to cognition, including attention, impulsivity and working memory. Sirh11/Zcchc16 encodes a CCHC type of zinc-finger protein that exhibits high homology to an LTR retrotransposon Gag protein. Upon microdialysis analysis of the prefrontal cortex region, the recovery rate of noradrenaline (NA) was reduced compared with dopamine (DA) after perfusion of high potassium-containing artificial cerebrospinal fluid in knockout (KO) mice. These data indicate that Sirh11/Zcchc16 is involved in cognitive function in the brain, possibly via the noradrenergic system, in the contemporary mouse developmental systems. Interestingly, it is highly conserved in three out of the four major groups of the eutherians, euarchontoglires, laurasiatheria and afrotheria, but is heavily mutated in xenarthran species such as the sloth and armadillo, suggesting that it has contributed to brain evolution in the three major eutherian lineages, including humans and mice. Sirh11/Zcchc16 is the first SIRH gene to be involved in brain function, instead of just the placenta, as seen in the case of Peg10, Peg11/Rtl1 and Sirh7/Ldoc1.

Retrotransposon-derived DNA sequences occupy approximately 40% of the mammalian genome, compared with only 1.5% of protein coding genes. They have been commonly considered “junk DNA” and even potentially harmful for host organisms. However, a series of knockout (KO) mouse analyses demonstrated that at least some of the LTR retrotransposon- and retrovirus-derived sequences play essential roles in the current mammalian developmental system as endogenous genes, such as Peg10, Peg11/Rtl1, Sirh7/Ldoc1, SYNCYTINs and FEMATRIN-1, which are active in multiple aspects of placental function. Here we demonstrate that another LTR retrotransposon-derived gene, Sirh11/Zcchc16, plays an important role in cognitive function in the brain. Sirh11/Zcchc16 KO mice exhibit abnormal behaviors related to cognition, including attention, impulsivity and working memory, possibly due to the locus coeruleus-noradrenaline (LC-NA) system, suggesting that human SIRH11/ZCCHC16 may be involved in X-linked intellectual disability and/or attention-deficit/hyperactivity disorder. Comparative genome analysis demonstrates that SIRH11/ZCCHC16 was acquired in a common eutherian ancestor, suggesting that it contributed to eutherian brain evolution because it confers a critically important advantage in the competition that occurs in daily life. This study provides further insight into the impact of LTR retrotransposon-derived genes on mammalian evolution.

Depressive-like behavior observed with a minimal loss of locus coeruleus (LC) neurons following administration of 6-hydroxydopamine is associated with electrophysiological changes and reversed with precursors of norepinephrine

Depression is a common co-morbid condition most often observed in subjects with mild cognitive impairment (MCI) and during the early stages of Alzheimer's disease (AD). Dysfunction of the central noradrenergic nervous system is an important component in depression. In AD, locus coeruleus (LC) noradrenergic neurons are significantly reduced pathologically and the reduction of LC neurons is hypothesized to begin very early in the progression of the disorder; however, it is not known if dysfunction of the noradrenergic system due to early LC neuronal loss is involved in mediating depression in early AD. Therefore, the purpose of this study was to determine in an animal model if a loss of noradrenergic LC neurons results in depressive-like behavior. The LC noradrenergic neuronal population was reduced by the bilateral administration of the neurotoxin 6-hydroxydopamine (6-OHDA) directly into the LC. Forced swim test (FST) was performed three weeks after the administration of 6-OHDA (5, 10 and 14 μg/μl), animals administered the 5 μg/μl of 6-OHDA demonstrated a significant increase in immobility, indicating depressive-like behavior. This increase in immobility at the 5 μg/μl dose was observed with a minimal loss of LC noradrenergic neurons as compared to LC neuronal loss observed at 10 and 14 μg/μl dose. A significant positive correlation between the number of surviving LC neurons after 6-OHDA and FST immobile time was observed, suggesting that in animals with a minimal loss of LC neurons (or a greater number of surviving LC neurons) following 6-OHDA demonstrated depressive-like behavior. As the 6-OHDA-induced loss of LC neurons is increased, the time spent immobile is reduced. Depressive-like behavior was also observed with the 5 μg/μl dose of 6-OHDA with a second behavior test, sucrose consumption. FST increased immobility following 6-OHDA (5 μg/μl) was reversed by the administration of a single dose of L-1-3-4-dihydroxyphenylalanine (DOPA) or l-threo-3,4-dihydroxyphenylserine (DOPS) prior to behavioral assessment. Surviving LC neurons 3 weeks after 6-OHDA (5 μg/μl) demonstrated compensatory changes of increased firing frequency, a more irregular firing pattern, and a higher percentage of cells firing in bursts. These results indicate that depressive-like behavior in mice is observed following the administration of 6-OHDA and the loss of LC noradrenergic neurons; however, the depressive-like behavior correlates positively with the number of surviving LC neurons with 6-OHDA administration. This data suggests the depression observed in MCI subjects and in the early stages of AD may due to the hypothesized early, minimal loss of LC neurons with remaining LC neurons being more active than normal.

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.

The VMAT-2 inhibitor tetrabenazine alters effort-related decision making as measured by the T-maze barrier choice task: reversal with the adenosine A2A antagonist MSX-3 and the catecholamine uptake blocker bupropion

RATIONALE

Depressed people show effort-related motivational symptoms, such as anergia, retardation, lassitude, and fatigue. Animal tests can model these motivational symptoms, and the present studies characterized the effort-related effects of the vesicular monoamine transport (VMAT-2) inhibitor tetrabenazine. Tetrabenazine produces depressive symptoms in humans and, at low doses, preferentially depletes dopamine.

OBJECTIVES

The current studies investigated the effects of tetrabenazine on effort-based decision making using the T-maze barrier task.

METHODS

Rats were tested in a T-maze in which the choice arms of the maze contain different reinforcement densities, and under some conditions, a vertical barrier was placed in the high-density arm to provide an effort-related challenge. The first experiment assessed the effects of tetrabenazine under different maze conditions: a barrier in the arm with 4 food pellets and 2 pellets in the no barrier arm (4-2 barrier), 4 pellets in one arm and 2 pellets in the other with no barrier in either arm (no barrier), and 4 pellets in the barrier arm with no pellets in the other (4-0 barrier).

RESULTS

Tetrabenazine (0.25-0.75 mg/kg IP) decreased selection of the high cost/high reward arm when the barrier was present, but had no effect on choice under the no barrier and 4-0 barrier conditions. The effects of tetrabenazine on barrier climbing in the 4-2 condition were reversed by the adenosine A2A antagonist MSX-3 and the catecholamine uptake inhibitor and antidepressant bupropion.

CONCLUSIONS

These studies have implications for the development of animal models of the motivational symptoms of depression and other disorders.

The forced swim test as a model of depressive-like behavior

The goal of the present protocol is to describe the forced swim test (FST), which is one of the most commonly used assays for the study of depressive-like behavior in rodents. The FST is based on the assumption that when placing an animal in a container filled with water, it will first make efforts to escape but eventually will exhibit immobility that may be considered to reflect a measure of behavioral despair. This test has been extensively used because it involves the exposure of the animals to stress, which was shown to have a role in the tendency for major depression. Additionally, the FST has been shown to share some of the factors that are influenced or altered by depression in humans, including changes in food consumption, sleep abnormalities and drug-withdrawal-induced anhedonia. The main advantages of this procedure are that it is relatively easy to perform and that its results are easily and quickly analyzed. Moreover, its sensitivity to a broad range of antidepressant drugs that makes it a suitable screening test is one of the most important features leading to its high predictive validity. Despite its appeal, this model has a number of disadvantages. First, the issue of chronic augmentation is problematic in this test because in real life patients need to be treated for at least several weeks before they experience any relief from their symptoms. Last, due to the aversiveness of the FST, it is important to take into account possible influences it might have on brain structure/function if brain analyses are to be carried out following this procedure.

Serotonin versus catecholamine deficiency: behavioral and neural effects of experimental depletion in remitted depression

Despite immense efforts into development of new antidepressant drugs, the increases of serotoninergic and catecholaminergic neurotransmission have remained the two major pharmacodynamic principles of current drug treatments for depression. Consequently, psychopathological or biological markers that predict response to drugs that selectively increase serotonin and/or catecholamine neurotransmission hold the potential to optimize the prescriber's selection among currently available treatment options. The aim of this study was to elucidate the differential symptomatology and neurophysiology in response to reductions in serotonergic versus catecholaminergic neurotransmission in subjects at high risk of depression recurrence. Using identical neuroimaging procedures with [(18)F] fluorodeoxyglucose positron emission tomography after tryptophan depletion (TD) and catecholamine depletion (CD), subjects with remitted depression were compared with healthy controls in a double-blind, randomized, crossover design. Although TD induced significantly more depressed mood, sadness and hopelessness than CD, CD induced more inactivity, concentration difficulties, lassitude and somatic anxiety than TD. CD specifically increased glucose metabolism in the bilateral ventral striatum and decreased glucose metabolism in the bilateral orbitofrontal cortex, whereas TD specifically increased metabolism in the right prefrontal cortex and the posterior cingulate cortex. Although we found direct associations between changes in brain metabolism and induced depressive symptoms following CD, the relationship between neural activity and symptoms was less clear after TD. In conclusion, this study showed that serotonin and catecholamines have common and differential roles in the pathophysiology of depression.

Systemic lipopolysaccharide-mediated alteration of cortical neuromodulation involves increases in monoamine oxidase-A and acetylcholinesterase activity

BACKGROUND

Lipopolysaccharide (LPS)-mediated sickness behaviour is known to be a result of increased inflammatory cytokines in the brain. Inflammatory cytokines have been shown to mediate increases in brain excitation by loss of GABAA-mediated inhibition through receptor internalization or inactivation. Inflammatory pathways, reactive oxygen species and stress are also known to increase monoamine oxidase-A (MAO-A) and acetylcholinesterase (ACh-E) activity. Given that neuromodulator actions on neural circuits largely depend on inhibitory pathways and are sensitive to alteration in corresponding catalytic enzyme activities, we assessed the impact of systemic LPS on neuromodulator-mediated shaping of a simple cortical network.

METHODS

Extracellular field recordings of evoked postsynaptic potentials in adult mouse somatosensory cortical slices were used to evaluate effects of a single systemic LPS challenge on neuromodulator function 1 week later. Neuromodulators were administered transiently as a bolus (100 μl) to the bath perfusate immediately upstream of the recording site to mimic phasic release of neuromodulators and enable assessment of response temporal dynamics.

RESULTS

Systemic LPS administration resulted in loss of both spontaneous and evoked inhibition as well as alterations in the temporal dynamics of neuromodulator effects on a paired-pulse paradigm. The effects on neuromodulator temporal dynamics were sensitive to the Monoamine oxidase-A (MAO-A) antagonist clorgyline (for norepinephrine and serotonin) and the ACh-E inhibitor donepezil (for acetylcholine). This is consistent with significant increases in total MAO and ACh-E activity found in hemi-brain samples from the LPS-treated group, supporting the notion that systemic LPS administration may lead to longer-lasting changes in inhibitory network function and enzyme (MAO/ACh-E) activity responsible for reduced neuromodulator actions.

CONCLUSIONS

Given the significant role of neuromodulators in behavioural state and cognitive processes, it is possible that an inflammatory-mediated change in neuromodulator action plays a role in LPS-induced cognitive effects and could help define the link between infection and neuropsychiatric/degenerative conditions.

Mesolimbic Dopamine and the Regulation of Motivated Behavior

It has been known for some time that nucleus accumbens dopamine (DA) is involved in aspects of motivation , but theoretical approaches to understanding the functions of DA have continued to evolve based upon emerging data and novel concepts. Although it has become traditional to label DA neurons as "reward" neurons, the actual findings are more complicated than that, because they indicate that DA neurons can respond to a variety of motivationally significant stimuli. Moreover, it is important to distinguish between aspects of motivation that are differentially affected by dopaminergic manipulations. Studies that involve nucleus accumbens DA antagonism or depletion indicate that accumbens DA does not mediate primary food motivation or appetite. Nevertheless, DA is involved in appetitive and aversive motivational processes including behavioral activation , exertion of effort, sustained task engagement, and Pavlovian-to-instrumental transfer. Interference with accumbens DA transmission affects instrumental behavior in a manner that interacts with the response requirements of the task and also shifts effort-related choice behavior, biasing animals toward low-effort alternatives. Dysfunctions of mesolimbic DA may contribute to motivational symptoms seen in various psychopathologies, including depression , schizophrenia, parkinsonism, and other disorders.

The interaction of escitalopram and R-citalopram at the human serotonin transporter investigated in the mouse

RATIONALE

Escitalopram appears to be a superior antidepressant to racemic citalopram. It has been hypothesized that binding of R-citalopram to the serotonin transporter (SERT) antagonizes escitalopram binding to and inhibition of the SERT, there by curtailing the elevation of extracellular 5-hydroxytryptamine (5-HTExt), and hence anti-depressant efficacy. Further, it has been suggested that a putative allosteric binding site is important for binding of escitalopram to the primary, orthosteric, site, and for R-citalopram's inhibition here of.

OBJECTIVES

Primary: Investigate at the human (h)SERT, at clinical relevant doses, whether R-citalopram antagonizes escitalopram-induced 5-HTExt elevation. Secondary: Investigate whether abolishing the putative allosteric site affects escitalopram-induced 5-HTExt elevation and/or modulates the effect of R-citalopram.

METHODS

Recombinant generation of hSERT transgenic mice; in vivo microdialysis; SERT binding; pharmacokinetics; 5-HT sensitive behaviors (tail suspension, marble burying).

RESULTS

We generated mice expressing either the wild-type human SERT (hSERT(WT)) or hSERT carrying amino acid substitutions (A505V, L506F, I507L, S574T and I575T) collectively abolishing the putative allosteric site (hSERT(ALI/VFL+SI/TT)). One mg/kg escitalopram yielded clinical relevant plasma levels and brain levels consistent with therapeutic SERT occupancy. The hSERT mice showed normal basal 5-HTExt levels. Escitalopram-induced 5-HTExt elevation was not decreased by R-citalopram co-treatment and was unaffected by loss of the allosteric site. The behavioral effects of the clinically relevant escitalopram dose were small and tended to be enhanced by R-citalopram co-administration.

CONCLUSIONS

We find no evidence that R-citalopram directly antagonizes escitalopram or that the putative allosteric site is important for hSERT inhibition by escitalopram.

Rodent models of treatment-resistant depression

Major depression is a prevalent and debilitating disorder and a substantial proportion of patients fail to reach remission following standard antidepressant pharmacological treatment. Limited efficacy with currently available antidepressant drugs highlights the need to develop more effective medications for treatment- resistant patients and emphasizes the importance of developing better preclinical models that focus on treatment- resistant populations. This review discusses methods to adapt and refine rodent behavioral models that are predictive of antidepressant efficacy to identify populations that show reduced responsiveness or are resistant to traditional antidepressants. Methods include separating antidepressant responders from non-responders, administering treatments that render animals resistant to traditional pharmacological treatments, and identifying genetic models that show antidepressant resistance. This review also examines pharmacological and non-pharmacological treatments regimes that have been effective in refractory patients and how some of these approaches have been used to validate animal models of treatment-resistant depression. The goals in developing rodent models of treatment-resistant depression are to understand the neurobiological mechanisms involved in antidepressant resistance and to develop valid models to test novel therapies that would be effective in patients that do not respond to traditional monoaminergic antidepressants.

Mice genetically depleted of brain serotonin do not display a depression-like behavioral phenotype

Reductions in function within the serotonin (5HT) neuronal system have long been proposed as etiological factors in depression. Selective serotonin reuptake inhibitors (SSRIs) are the most common treatment for depression, and their therapeutic effect is generally attributed to their ability to increase the synaptic levels of 5HT. Tryptophan hydroxylase 2 (TPH2) is the initial and rate-limiting enzyme in the biosynthetic pathway of 5HT in the CNS, and losses in its catalytic activity lead to reductions in 5HT production and release. The time differential between the onset of 5HT reuptake inhibition by SSRIs (minutes) and onset of their antidepressant efficacy (weeks to months), when considered with their overall poor therapeutic effectiveness, has cast some doubt on the role of 5HT in depression. Mice lacking the gene for TPH2 are genetically depleted of brain 5HT and were tested for a depression-like behavioral phenotype using a battery of valid tests for affective-like disorders in animals. The behavior of TPH2(-/-) mice on the sucrose preference test, tail suspension test, and forced swim test and their responses in the unpredictable chronic mild stress and learned helplessness paradigms was the same as wild-type controls. While TPH2(-/-) mice as a group were not responsive to SSRIs, a subset responded to treatment with SSRIs in the same manner as wild-type controls with significant reductions in immobility time on the tail suspension test, indicative of antidepressant drug effects. The behavioral phenotype of the TPH2(-/-) mouse questions the role of 5HT in depression. Furthermore, the TPH2(-/-) mouse may serve as a useful model in the search for new medications that have therapeutic targets for depression that are outside of the 5HT neuronal system.

Lgr4 protein deficiency induces ataxia-like phenotype in mice and impairs long term depression at cerebellar parallel fiber-Purkinje cell synapses

Cerebellar dysfunction causes ataxia characterized by loss of balance and coordination. Until now, the molecular and neuronal mechanisms of several types of inherited cerebellar ataxia have not been completely clarified. Here, we report that leucine-rich G protein-coupled receptor 4 (Lgr4/Gpr48) is highly expressed in Purkinje cells (PCs) in the cerebellum. Deficiency of Lgr4 leads to an ataxia-like phenotype in mice. Histologically, no obvious morphological changes were observed in the cerebellum of Lgr4 mutant mice. However, the number of PCs was slightly but significantly reduced in Lgr4(-/-) mice. In addition, in vitro electrophysiological analysis showed an impaired long term depression (LTD) at parallel fiber-PC (PF-PC) synapses in Lgr4(-/-) mice. Consistently, immunostaining experiments showed that the level of phosphorylated cAMP-responsive element-binding protein (Creb) was significantly decreased in Lgr4(-/-) PCs. Furthermore, treatment with forskolin, an adenylyl cyclase agonist, rescued phospho-Creb in PCs and reversed the impairment in PF-PC LTD in Lgr4(-/-) cerebellar slices, indicating that Lgr4 is an upstream regulator of Creb signaling, which is underlying PF-PC LTD. Together, our findings demonstrate for first time an important role for Lgr4 in motor coordination and cerebellar synaptic plasticity and provide a potential therapeutic target for certain types of inherited cerebellar ataxia.

Altered neuronal activity and differential sensitivity to acute antidepressants of locus coeruleus and dorsal raphe nucleus in Wistar Kyoto rats: a comparative study with Sprague Dawley and Wistar rats

The Wistar Kyoto rat (WKY) has been proposed as an animal model of depression. The noradrenergic nucleus, locus coeruleus (LC) and the serotonergic nucleus, dorsal raphe (DRN) have been widely implicated in the ethiopathology of this disease. Thus, the goal of the present study was to investigate in vivo the electrophysiological properties of LC and DRN neurons from WKY rats, using single-unit extracellular techniques. Wistar (Wis) and Sprague Dawley (SD) rats were used as control strains. In the LC from WKY rats the basal firing rate was higher than that obtained in the Wis and SD strain, and burst firing activity also was greater compared to that in Wis strain but not in SD. The sensitivity of LC neurons to the inhibitory effect of the α2-adrenoceptor agonist, clonidine and the antidepressant reboxetine was lower in WKY rats compared to Wis, but not SD. Regarding DRN neurons, in WKY rats burst activity was lower than that obtained in Wis and SD rats, although no differences were observed in other firing parameters. Interestingly, while the sensitivity of DRN neurons to the inhibitory effect of the 5-HT1A receptor agonist, 8-OH-DPAT was lower in the WKY strain, the antidepressant fluoxetine had a greater inhibitory potency in this rat strain compared to that recorded in the Wis group. Overall, these results point out important electrophysiological differences regarding noradrenergic and serotonergic systems between Wis and WKY rats, supporting the utility of the WKY rat as an important tool in the research of cellular basis of depression.

Evidence for involvement of the monoaminergic system in antidepressant-like activity of an ethanol extract of Boerhaavia diffusa and its isolated constituent, punarnavine, in mice

CONTEXT

Boerhaavia diffusa Linn. (Nyctaginaceae) roots possess potent antioxidant, antistress, and anticonvulsant activities. It is used as a medicinal plant in Ayurvedic and natural herbal medicines.

OBJECTIVE

To evaluate the effect of Boerhaavia diffusa root ethanol extract and its active constituent, punarnavine, on depression in Swiss albino mice.

MATERIALS AND METHODS

Ethanol extract (50, 100, and 200 mg/kg, p.o.) and punarnavine (20 and 40 mg/kg, p.o.) were separately administered to 22 and 17 groups of mice, respectively, for 14 successive days followed by testing in the tail suspension and forced swim tests (FST). About 2% w/v gum acacia and double distilled water were used as controls for the extract and punarnavine, respectively.

RESULTS

Antidepressant-like effect of the lowest dose (50 mg/kg) of the extract and lower dose (20 mg/kg) of punarnavine were found to be comparable to fluoxetine. The ED50 value of the ethanol extract was 26.30 mg/kg (FST) and 33.11 mg/kg (tail suspension test); and of punarnavine was 15.14 mg/kg (FST) and 17.38 mg/kg (tail suspension test). The drugs did not show any significant effect on locomotor activities of mice. Prazosin (α1-adrenoceptor antagonist), sulpiride (selective D2-receptor antagonist), para-chlorophenylalanine) (p-CPA) (tryptophan hydroxylase inhibitor), and baclofen (GABAB agonist) significantly attenuated the extract and punarnavine induced-antidepressant-like effect in the tail suspension test. The extract and punarnavine also significantly reduced mouse brain monoamine oxidase (MAO)-A levels, but there was no significant effect on plasma corticosterone levels.

CONCLUSION

Ethanol extract of Boerhaavia diffusa and punarnavine produced an antidepressant-like effect in mice probably through interaction with monoaminergic and GABAergic systems.

Blockade of the high-affinity noradrenaline transporter (NET) by the selective 5-HT reuptake inhibitor escitalopram: an in vivo microdialysis study in mice

BACKGROUND AND PURPOSE

Escitalopram, the S(+)-enantiomer of citalopram is the most selective 5-HT reuptake inhibitor approved. Although all 5-HT selective reuptake inhibitors (SSRIs) increase extracellular levels of 5-HT ([5-HT](ext)). some also enhance, to a lesser extent, extracellular levels of noradrenaline ([NA](ext)). However, the mechanisms by which SSRIs activate noradrenergic transmission in the brain remain to be determined.

EXPERIMENTAL APPROACH

This study examined the effects of escitalopram, on both [5-HT](ext) and [NA](ext) in the frontal cortex (FCx) of freely moving wild-type (WT) and mutant mice lacking the 5-HT transporter (SERT(-/-)) by using intracerebral microdialysis. We explored the possibilities that escitalopram enhances [NA](ext), either by a direct mechanism involving the inhibition of the low- or high-affinity noradrenaline transporters, or by an indirect mechanism promoted by [5-HT](ext) elevation. The forced swim test (FST) was used to investigate whether enhancing cortical [5-HT](ext) and/or [NA](ext) affected the antidepressant-like activity of escitalopram.

KEY RESULTS

In WT mice, a single systemic administration of escitalopram produced a significant increase in cortical [5-HT](ext) and [NA](ext). As expected, escitalopram failed to increase cortical [5-HT](ext) in SERT(-/-) mice, whereas its neurochemical effects on [NA](ext) persisted in these mutants. In WT mice subjected to the FST, escitalopram increased swimming parameters without affecting climbing behaviour. Finally, escitalopram, at relevant concentrations, failed to inhibit cortical noradrenaline and 5-HT uptake mediated by low-affinity monoamine transporters.

CONCLUSIONS AND IMPLICATIONS

These experiments suggest that escitalopram enhances, although moderately, cortical [NA](ext) in vivo by a direct mechanism involving the inhibition of the high-affinity noradrenaline transporter (NET).