Extracellular Serotonin Levels Research Articles

Ketamine treatment involves medial prefrontal cortex serotonin to induce a rapid antidepressant-like activity in BALB/cJ mice

Unlike classic serotonergic antidepressant drugs, ketamine, an NMDA receptor antagonist, exhibits a rapid and persistent antidepressant (AD) activity, at sub-anaesthetic doses in treatment-resistant depressed patients and in preclinical studies in rodents. The mechanisms mediating this activity are unclear. Here, we assessed the role of the brain serotonergic system in the AD-like activity of an acute sub-anaesthetic ketamine dose. We compared ketamine and fluoxetine responses in several behavioral tests currently used to predict anxiolytic/antidepressant-like potential in rodents. We also measured their effects on extracellular serotonin levels [5-HT]ext in the medial prefrontal cortex (mPFCx) and brainstem dorsal raphe nucleus (DRN), a serotonergic nucleus involved in emotional behavior, and on 5-HT cell firing in the DRN in highly anxious BALB/cJ mice. Ketamine (10 mg/kg i.p.) had no anxiolytic-like effect, but displayed a long lasting AD-like activity, i.e., 24 h post-administration, compared to fluoxetine (18 mg/kg i.p.). Ketamine (144%) and fluoxetine (171%) increased mPFCx [5-HT]ext compared to vehicle. Ketamine-induced AD-like effect was abolished by a tryptophan hydroxylase inhibitor, para-chlorophenylalanine (PCPA) pointing out the role of the 5-HT system in its behavioral activity. Interestingly, increase in cortical [5-HT]ext following intra-mPFCx ketamine bilateral injection (0.25 μg/side) was correlated with its AD-like activity as measured on swimming duration in the FST in the same mice. Furthermore, pre-treatment with a selective AMPA receptor antagonist (intra-DRN NBQX) blunted the effects of intra-mPFCx ketamine on both the swimming duration in the FST and mPFCx [5-HT]ext suggesting that the AD-like activity of ketamine required activation of DRN AMPA receptors and recruited the prefrontal cortex/brainstem DRN neural circuit in BALB/c mice. These results confirm a key role of cortical 5-HT release in ketamine’s AD-like activity following the blockade of glutamatergic NMDA receptors. Tight interactions between mPFCx glutamatergic and serotonergic systems may explain the differences in this activity between ketamine and fluoxetine in vivo.This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’.

The serotonin reuptake inhibitor citalopram suppresses activity in the neonatal rat barrel cortex in vivo.

Inhibition of serotonin uptake, which causes an increase in extracellular serotonin levels, disrupts the development of thalamocortical barrel maps in neonatal rodents. Previous in vitro studies have suggested that the disruptive effect of excessive serotonin on barrel map formation involves a depression at thalamocortical synapses. However, the effects of serotonin uptake inhibitors on the early thalamocortical activity patterns in the developing barrel cortex in vivo remain largely unknown. Here, using extracellular recordings of the local field potentials and multiple unit activity (MUA) we explored the effects of the selective serotonin reuptake inhibitor (SSRI) citalopram (10-20mg/kg, intraperitoneally) on sensory evoked activity in the barrel cortex of neonatal (postnatal days P2-5) rats in vivo. We show that administration of citalopram suppresses the amplitude and prolongs the delay of the sensory evoked potentials, reduces the power and frequency of the early gamma oscillations, and suppresses sensory evoked and spontaneous neuronal firing. In the adolescent P21-29 animals, citalopram affected neither sensory evoked nor spontaneous activity in barrel cortex. We suggest that suppression of the early thalamocortical activity patterns contributes to the disruption of the barrel map development caused by SSRIs and other conditions elevating extracellular serotonin levels.

Antidepressants differentially affect striatal amphetamine-stimulated dopamine and serotonin release in rats with high and low novelty-oriented behaviour

In the studies of depression pathogenesis and antidepressant action, the monoaminergic hypothesis of depression has mainly focused on the serotonergic and noradrenergic mechanisms. However, dopaminergic neurotransmission is also linked to both depressive symptomatology as well as antidepressant effects. We have previously shown that persistent inter-individual differences in the rat behavioural activity in novel environments is associated with differences in the striatal extracellular levels of dopamine and serotonin, depressive-like behaviour and the expression of several depression-related genes. The aim of the current study was to investigate the relative potency of the tricyclic antidepressant imipramine, the selective serotonin re-uptake inhibitor fluoxetine, and the selective noradrenaline re-uptake inhibitor reboxetine (all drugs administered in the dose of 10mg/kg, i.p.) to enhance amphetamine-stimulated dopamine and serotonin release in the striatum using in vivo microdialysis in awake, freely-moving rats, categorized into high explorers (HE) and low explorers (LE) based on their spontaneous novelty-oriented behaviour. The basal extracellular dopamine and serotonin concentration in the striatum did not differ between the LE- and HE-rats. None of the antidepressants alone were able to modify baseline striatal dopamine levels, but the amphetamine-stimulated dopamine release was significantly higher in the HE-rats after acute and chronic imipramine (but not fluoxetine or reboxetine). Acute imipramine and fluoxetine, but not reboxetine, increased both the basal and amphetamine-stimulated levels of serotonin in the striatum. Again, the HE-rats had higher amphetamine-stimulated serotonin release after fluoxetine administration. These findings suggest that rats with depressive-like phenotype are less sensitive to the neurochemical effects of antidepressants in the striatum. These results may have relevance in understanding the neurobiological bases for inter-individual differences in antidepressant treatment response in humans and development of novel medicines.

Amphetamine Withdrawal Differentially Increases the Expression of Organic Cation Transporter 3 and Serotonin Transporter in Limbic Brain Regions.

Amphetamine withdrawal increases anxiety and stress sensitivity related to blunted ventral hippocampus (vHipp) and enhances the central nucleus of the amygdala (CeA) serotonin responses. Extracellular serotonin levels are regulated by the serotonin transporter (SERT) and organic cation transporter 3 (OCT3), and vHipp OCT3 expression is enhanced during 24 hours of amphetamine withdrawal, while SERT expression is unaltered. Here, we tested whether OCT3 and SERT expression in the CeA is also affected during acute withdrawal to explain opposing regional alterations in limbic serotonergic neurotransmission and if respective changes continued with two weeks of withdrawal. We also determined whether changes in transporter expression were confined to these regions. Male rats received amphetamine or saline for two weeks followed by 24 hours or two weeks of withdrawal, with transporter expression measured using Western immunoblot. OCT3 and SERT expression increased in the CeA at both withdrawal timepoints. In the vHipp, OCT3 expression increased only at 24 hours of withdrawal, with an equivalent pattern seen in the dorsomedial hypothalamus. No changes were evident in any other regions sampled. These regionally specific changes in limbic OCT3 and SERT expression may partially contribute to the serotonergic imbalance and negative affect during amphetamine withdrawal.

Commentary: Chronic SSRI stimulation of astrocytic 5-HT2B receptors change multiple gene expressions/editings and metabolism of glutamate, glucose and glycogen: a potential paradigm shift.

Working on the serotonin (5-hydroxytryptamine, 5-HT) 5-HT2B receptor since several years, we have read with high interest the review by Hertz et al., (Hertz et al., 2015). Previous studies from our group demonstrated that a direct injection in mouse raphe nucleus of the 5-HT2B agonist BW723C86 has the ability to increase extracellular levels of serotonin, which can be blocked by the selective 5-HT2B receptor antagonist RS127445 (Doly et al., 2008;Doly et al., 2009). We also reported that an acute injection of paroxetine 2 mg/kg in mice knocked out for the 5-HT2B receptor gene or in wild type mice injected with RS127445 (0.5 mg/kg) triggers a strong reduction in extracellular accumulation of 5-HT in hippocampus (Diaz et al., 2012). Following these observations, we showed that acute and chronic BW723C86 injection (3 mg/kg) can mimic the fluoxetine (3 mg/kg) and paroxetine (1 mg/kg) behavioral and biochemical antidepressant effects in mice (Diaz and Maroteaux, 2011;Diaz et al., 2012). Hertz and coworkers used some of our published data on mice to support that fluoxetine and other SSRIs are acting as direct 5-HT2B receptor agonists independently of the serotonin transporter (SERT), based on their work on astrocytes. These authors forgot other important informations provided in Diaz's paper (Diaz et al., 2012), ie the absence of antidepressant effects of either fluoxetine or BW723C86 in mice lacking either the serotonin transporter (knockout for SERT) or differentiated serotonin neurons (knockout for Pet1). These data (i) rule out that the antidepressant effects of the 5-HT2B agonist BW723C86 or of fluoxetine could be independent of SERT; (ii) they indicate that serotonin neurons expressing SERT (and 5-HT2B receptors) are necessary for the 5-HT2B receptor effects independently of other cell types; (iii) they rule out the possibility that SSRIs mediate antidepressant effects only by stimulating directly putative astrocytic 5-HT2B receptors, which should be intact in these two mutant mice (SERT KO and Pet1 KO). A previous work (Launay et al., 2006) demonstrated a 5-HT2B receptor-mediated control of SERT activity in primary neurons from raphe nuclei, in which the BW723C86/5-HT2B receptor coupling promotes phosphorylations of SERT that can easily explain our findings in mice by a direct regulation of SERT by 5-HT2B receptors in a cell autonomous manner.

Sex- and SERT-mediated differences in stimulated serotonin revealed by fast microdialysis.

In vivo microdialysis is widely used to investigate how neurotransmitter levels in the brain respond to biologically relevant challenges. Here, we combined recent improvements in the temporal resolution of online sampling and analysis for serotonin with a brief high-K(+) stimulus paradigm to study the dynamics of evoked release. We observed stimulated serotonin overflow with high-K(+) pulses as short as 1 min when determined with 2-min dialysate sampling in ventral striatum. Stimulated serotonin levels in female mice during the high estrogen period of the estrous cycle were similar to serotonin levels in male mice. By contrast, stimulated serotonin overflow during the low estrogen period in female mice was increased to levels similar to those in male mice with local serotonin transporter (SERT) inhibition. Stimulated serotonin levels in mice with constitutive loss of SERT were considerably higher yet, pointing to neuroadaptive potentiation of serotonin release. When combined with brief K(+) stimulation, fast microdialysis reveals dynamic changes in extracellular serotonin levels associated with normal hormonal cycles and pharmacologic vs genetic loss of SERT function.

Fluoxetine impairs insulin secretion without modifying extracellular serotonin levels in MIN6 β-cells.

Pancreatic β-cells synthetize and store Serotonin (5-Hydroxytriptamine, 5HT) which is co-released with insulin. It has been proposed that extracellular 5HT binds to specific cell surface receptors and modulate insulin secretion. On the other hand, Selective Serotonin Reuptake Inhibitor (SSRI) fluoxetine seems to reduce Glucose-Stimulated Insulin Secretion (GSIS). However, it is unknown whether this effect results from changes in extracellular 5HT concentration owed to the blockade of 5HT transporter (SERT) or from non-5HT dependent actions. The aims of this work were: 1) to quantify extracellular 5HT levels and GSIS in β-cell lines, 2) to determine whether extracellular 5HT levels and GSIS are changed by fluoxetine or 5-Hydroxytryptophan (5HTP, the immediate 5HT biosynthetic precursor), and 3) to quantify the expression of Slc6a4 gene (encoding SERT) in β-cell lines in relation to other genes involved in 5HT system. β-cell lines MIN6 and RINm5f were subjected to GSIS protocols, after treatment with fluoxetine, 5HTP or 5HT. Insulin and 5HT were quantified by ELISA and HPLC, respectively. Relative mRNA expression was quantified by RT-qPCR. MIN6 β-cells secretes 5HT in response to glucose, showing a sharp increase in 5HT release when cells were preloaded with 5HTP. Treatment with 5HT or fluoxetine reduces GSIS. Fluoxetine fails to further increases 5HTP-induced elevation of secreted 5HT. MIN6 β-cells express both isoforms of Tryptophan Hydroxylase (Tph1 and Tph2), and have high expression levels of L-Dopa decarboxylase (Ddc), both enzymes involved in 5HT biosynthetic pathway, but do not express the 5HT transporters Slc6a4 or Slc6a3 (the Dopamine-5HT transporter) genes. The inhibitory effect of fluoxetine on β-cell glucose stimulated insulin secretion is not mediated by blockage of 5HT transporter through SERT.

Use of selective serotonin reuptake inhibitors and bone mass in adolescents: An NHANES study

ContextSelective serotonin reuptake inhibitors (SSRIs) are commonly prescribed medications to treat depression and anxiety. SSRIs exert their effects by inhibiting the serotonin transporter and modulating extracellular serotonin levels, a neurotransmitter that has been shown to affect bone metabolism in animals. Studies in adults suggest a negative association between SSRI use and bone mineral density (BMD), greater rates of bone loss with SSRI use and increased risk of fractures. However, the results on bone mass have been inconsistent. Furthermore, there is a dearth of studies examining an association between SSRI use and bone mass in the pediatric and adolescent age group. ObjectiveTo investigate associations between SSRI use and bone mass in adolescents. DesignCross-sectional analysis of data from the 2005–2010 National Health and Nutrition Examination Study (NHANES). Participants4303 NHANES participants aged 12–20years. The mean age was 15.65±2.42years. Main outcomesTotal femur, femoral neck and lumbar spine bone mineral content (BMC) and BMD assessed via dual-energy X-ray absorptiometry (DXA). Results62 out of 4303 subjects used SSRIs. SSRI use was an independent predictor of bone mass after adjusting for age, gender, height and weight Z score, socioeconomic status, physical activity, serum cotinine level and race/ethnicity. After multivariable adjustment, total femur BMC was 8.8% lower among SSRI users versus non-users (mean difference 2.98g, SE±0.105g, p=0.0006), while total femur BMD was 6.1% lower (mean difference 0.06g/cm2, SE±0.002g/cm2, p=0.016). Femoral neck BMC and BMD and lumbar spine BMC were similarly negatively associated with SSRI use. Compared to nonusers, lumbar spine BMC was 7% lower among SSRI users (mean difference 0.97g, SE±0.048g, p=0.02) and BMD was 3.2% lower (mean difference 0.03g/cm2, SE±0.015g/cm2, p=0.09). Sub-analysis of those individuals treated for more than 6months yield similar results. Finally, the association of SSRIs with bone mass persisted after excluding individuals with Body Mass Index (BMI) less than 5th percentile thus accounting for the possible confounding effect of anorexia nervosa, which can be treated with SSRIs. ConclusionIn this NHANES study, adolescents treated with SSRIs had lower DXA measurements of the total femur and lumbar spine compared to SSRI non-users. These findings support the need for future prospective studies to examine the effects of SSRI use on bone mass in adolescents.

The Involvement of Norepinephrine in Behaviors Related to Psychostimulant Addiction.

Although it is generally accepted that the abuse-related effects of amphetamines and cocaine result from the activation of the brain dopaminergic (DA) system, the psychostimulants also alter other neurotransmitter systems. In particular, they increase extracellular levels of norepinephrine (NE) and serotonin by inhibiting respective plasma membrane transporters and/or inducing release. The present review will discuss the preclinical findings on the effects of the NE system modulation (lesions, pharmacological and genetic approaches) on behaviors (locomotor hyperactivity, behavioral sensitization, modification of intracranial self-stimulation, conditioned place preference, drug self-administration, extinction/reinstatement of drug seeking behavior) related to the psychostimulant addiction.

Effects of AT1 receptor antagonism on kainate-induced seizures and concomitant changes in hippocampal extracellular noradrenaline, serotonin, and dopamine levels in Wistar-Kyoto and spontaneously hypertensive rats

In the management of epilepsy, AT1 receptor antagonists have been suggested as an additional treatment strategy. A hyperactive brain angiotensin (Ang) II system and upregulated AT1 receptors are implicated in the cerebrovascular alterations in a genetic form of hypertension. Uncontrolled hypertension could also, in turn, be a risk factor for a seizure threshold decrease and development of epileptogenesis. The present study aimed to assess the effects of the selective AT1 receptor antagonist ZD7155 on kainic acid (KA)-induced status epilepticus (SE) development and accompanying changes in the hippocampal extracellular (EC) neurotransmitter levels of noradrenaline (NAD), serotonin (5-HT), and dopamine (DA) in spontaneously hypertensive rats (SHRs) and their parent strain Wistar-Kyoto (WKY) rats, since monoamines are well-known neurotransmitters involved in mechanisms of both epilepsy and hypertension. Status epilepticus was evoked in freely moving rats by a repetitive intraperitoneal (i.p.) administration of KA in subconvulsant doses. In the treatment group, ZD7155 (5mg/kg i.p.) was coadministered with the first KA injection. Spontaneously hypertensive rats exhibited higher susceptibility to SE than WKY rats, but the AT1 receptor antagonist did not alter the development of SE in SHRs or in WKY rats. In vivo microdialysis demonstrated significant KA-induced increases of the hippocampal NAD and DA levels in SHRs and of NAD, 5-HT, and DA in WKY rats. Although SHRs developed more severe seizures while receiving a lower dose of KA compared to WKY rats, AT1 receptor antagonism completely prevented all KA-induced increases of hippocampal monoamine levels in both rat strains without affecting seizure development per se. These results suggest a lack of direct relationship between KA-induced seizure susceptibility and adaptive changes of hippocampal NAD, 5-HT, and DA levels in the effects of ZD7155 in WKY rats and SHRs.

Olanzapine augments the effect of selective serotonin reuptake inhibitors by suppressing GABAergic inhibition via antagonism of 5-HT6 receptors in the dorsal raphe nucleus

The combination of the selective serotonin reuptake inhibitors (SSRIs) and atypical antipsychotic drugs shows better therapeutic efficacy than SSRI monotherapy in the treatment of depression. However, the underlying mechanisms responsible for the augmenting effects of olanzapine are not fully understood. Here, we report that olanzapine enhances the SSRI-induced increase in extracellular serotonin (5-HT) levels and antidepressant-like effects by inhibiting GABAergic neurons through 5-HT6 receptor antagonism in the dorsal raphe nucleus (DRN). In organotypic raphe slice cultures, treatment with olanzapine (1–100 μM) enhanced the increase in extracellular 5-HT levels in the presence of fluoxetine (10 μM) or citalopram (1 μM). The enhancing effect of olanzapine was not further augmented by the GABAA receptor antagonist bicuculline. Electrophysiological analysis revealed that olanzapine (50 μM) decreased the firing frequency of GABAergic neurons in acute DRN slices. Among many serotonergic agents, the 5-HT6 receptor antagonist SB399885 (1–100 μM) mimicked the effects of olanzapine by enhancing the SSRI-induced increase in extracellular 5-HT levels, which was not further augmented by bicuculline or olanzapine. SB399885 (50 μM) also decreased the firing frequency of GABAergic neurons in the DRN. In addition, an intraperitoneal administration of SB399885 (10 mg/kg) to mice significantly enhanced the antidepressant-like effect of a subeffective dose of citalopram (3 mg/kg) in the tail-suspension test. These results suggest that olanzapine decreases local inhibitory GABAergic tone in the DRN through antagonism of 5-HT6 receptors, thereby increasing the activity of at least part of serotonergic neurons, which may contribute to the augmentation of the efficacy of SSRIs.

Exposure to serotonin adversely affects oligodendrocyte development and myelination in vitro.

Serotonin (5-hydroxytryptamine, 5-HT) has been implicated to play critical roles in early neural development. Recent reports have suggested that perinatal exposure to selective serotonin reuptake inhibitors (SSRIs) resulted in cortical network miswiring, abnormal social behavior, callosal myelin malformation, as well as oligodendrocyte (OL) pathology in rats. To gain further insight into the cellular and molecular mechanisms underlying SSRIs-induced OL and myelin abnormalities, we investigated the effect of 5-HT exposure on OL development, cell death, and myelination in cell culture models. First, we showed that 5-HT receptor 1A and 2A subtypes were expressed in OL lineages, using immunocytochemistry, Western blot, as well as intracellular Ca(2+) measurement. We then assessed the effect of serotonin exposure on the lineage development, expression of myelin proteins, cell death, and myelination, in purified OL and neuron-OL myelination cultures. For pure OL cultures, our results showed that 5-HT exposure led to disturbance of OL development, as indicated by aberrant process outgrowth and reduced myelin proteins expression. At higher doses, such exposure triggered a development-dependent cell death, as immature OLs exhibited increasing susceptibility to 5-HT treatment compared to OL progenitor cells (OPC). We showed further that 5-HT-induced immature OL death wasmediated at least partially via 5-HT2A receptor, since cell death could be mimicked by 5-HT2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride, (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride, but atten-uated by pre-treatment with 5-HT2A receptor antagonist ritanserin. Utilizing a neuron-OL myelination co-culture model, our data showed that 5-HT exposure significantly reduced the number of myelinated internodes. In contrast to cell injury observed in pure OL cultures, 5-HT exposure did not lead to OL death or reduced OL density in neuron-OL co-cultures. However, abnormal patterns of contactin-associated protein (Caspr) clustering were observed at the sites of Node of Ranvier, suggesting that 5-HT exposure may affect other axon-derived factors for myelination. In summary, this is the first study to demonstrate that manipulation of serotonin levels affects OL development and myelination, which may contribute to altered neural connectivity noted in SSRIs-treated animals. The current in vitro study demonstrated that exposure to high level of serotonin (5-HT) led to aberrant oligodendrocyte (OL) development, cell injury, and myelination deficit. We propose that elevated extracellular serotonin levels in the fetal brain, such as upon the use of selective serotonin reuptake inhibitors (SSRIs) during pregnancy, may adversely affect OL development and/or myelination, thus contributing to altered neural connectivity seen in Autism Spectrum Disorders. OPC = oligodendrocyte progenitor cell.

Modulation of Extracellular Levels of 5-HT in the Caudate Putamen of Freely Moving Rats by High Frequency Stimulation of the Subthalamic Nucleus

Electrical high frequency stimulation (HFS) in the subthalamic nucleus (STN) has been shown to have a thera- peutic effect in several movement disorders. But, debilitating psychiatric effects like depression and suicidality are occa- sionally seen and might be caused by the changes in the serotoninergic activity. Previous studies could show that HFS of the STN results in inhibition of the serotonergic neurons originating in the dorsal raphe nucleus. The aim of this study was to characterize the effect of HFS (124 Hz, 0.5 mA) in the STN, on the extracellular levels of serotonin, dopamine and their metabolites HIAA, DOPAC and HVA in the caudate-putamen (CPu) in conscious and freely moving rats. Extracellular levels of the neurotransmitters and their metabolites were quantified using high performance liquid chromatography with electrochemical detection. Under HFS conditions, a significant reduction in the extracellular levels of serotonin was ob- served. Cessation of HFS showed a recovery back to basal levels. Dopamine levels were not affected, although significant increase of its metabolites DOPAC and HVA were measured. In the case of low frequency stimulation (LFS), levels of se- rotonin and its metabolite HIAA remained unchanged, while the levels of dopamine metabolites, DOPAC and HVA, showed a significant decline. These results demonstrate evidence for a strong linkage between HFS in the STN and reduc- tion of the levels of serotonin in the caudate-putamen, which is likely responsible for psychiatric side effects seen in Park- insonian patients who are treated with STN stimulation.

Effect of psilocin on extracellular dopamine and serotonin levels in the mesoaccumbens and mesocortical pathway in awake rats.

Psilocin (3-[2-(dimethylamino)ethyl]-1H-indol-4-ol) is a hallucinogenic component of the Mexican mushroom Psilocybe mexicana and a skeletal serotonin (5-HT) analogue. Psilocin is the active metabolite of psilocybin (3-[2-(dimethylamino)ethyl]-1H-indol-4-yl dihydrogen phosphate). In the present study, we examined the effects of systemically administered psilocin on extracellular dopamine and 5-HT concentrations in the ventral tegmental area (VTA), nucleus accumbens, and medial prefrontal cortex of the dopaminergic pathway in awake rats using in vivo microdialysis. Intraperitoneal administration of psilocin (5, 10 mg/kg) significantly increased extracellular dopamine levels in the nucleus accumbens. Psilocin did not affect the extracellular 5-HT level in the nucleus accumbens. Conversely, systemic administration of psilocin (10 mg/kg) significantly increased extracellular 5-HT levels in the medial prefrontal cortex of rats, but dopamine was decreased in this region. However, neither extracellular dopamine nor 5-HT levels in the VTA were altered by administration of psilocin. Behaviorally, psilocin significantly increased the number of head twitches. Thus, psilocin affects the dopaminergic system in the nucleus accumbens. In the serotonergic system, psilocin contribute to a crucial effect in the medial prefrontal cortex. The present data suggest that psilocin increased both the extracellular dopamine and 5-HT concentrations in the mesoaccumbens and/or mesocortical pathway.

Serotonergic, brain volume and attentional correlates of trait anxiety in primates.

Trait anxiety is a risk factor for the development and maintenance of affective disorders, and insights into the underlying brain mechanisms are vital for improving treatment and prevention strategies. Translational studies in non-human primates, where targeted neurochemical and genetic manipulations can be made, are critical in view of their close neuroanatomical similarity to humans in brain regions implicated in trait anxiety. Thus, we characterised the serotonergic and regional brain volume correlates of trait-like anxiety in the marmoset monkey. Low- and high-anxious animals were identified by behavioral responses to a human intruder (HI) that are known to be sensitive to anxiolytic drug treatment. Extracellular serotonin levels within the amygdala were measured with in vivo microdialysis, at baseline and in response to challenge with the selective serotonin reuptake inhibitor, citalopram. Regional brain volume was assessed by structural magnetic resonance imaging. Anxious individuals showed persistent, long-term fearful responses to both a HI and a model snake, alongside sustained attention (vigilance) to novel cues in a context associated with unpredictable threat. Neurally, high-anxious marmosets showed reduced amygdala serotonin levels, and smaller volumes in a closely connected prefrontal region, the dorsal anterior cingulate cortex. These findings highlight behavioral and neural similarities between trait-like anxiety in marmosets and humans, and set the stage for further investigation of the processes contributing to vulnerability and resilience to affective disorders.

Role of nitric oxide in the behavioral and neurochemical effects of IB-MECA in zebrafish.

The adenosine A3 receptor and the nitric oxide (NO) pathway regulate the function and localization of serotonin transporters (SERTs). These transporters regulate extracellular serotonin levels, which are correlated with defensive behavior. The purpose of this study was to understand the role of the A3AR on anxiety and arousal models in zebrafish, and whether this role is mediated by the nitrergic modulation of serotonin uptake. The effects of IB-MECA (0.01 and 0.1 mg/kg) were assessed in a series of behavioral tasks in adult zebrafish, as well as on extracellular serotonin levels in vivo and serotonin uptake in brain homogenates. Finally, the interaction between IB-MECA and drugs blocking voltage-dependent calcium channels (VDCCs), NO synthase, and SERT was analyzed. At the lowest dose, IB-MECA decreased bottom dwelling and scototaxis, while at the highest dose, it also decreased shoaling, startle probability, and melanophore responses. These effects were accompanied by an increase in brain extracellular serotonin levels. IB-MECA also concentration-dependently increased serotonin uptake in vitro. The effects of IB-MECA on extracellular 5-HT, scototaxis, and geotaxis were blocked by L-NAME, while only the effects on 5-HT and scototaxis were blocked by verapamil. In vitro, the increase in 5-HT uptake was dependent on VDCCs and NO. Finally, fluoxetine blocked the effect of IB-MECA on scototaxis, but not geotaxis. These results suggest that the effect of IB-MECA on scototaxis are mediated by a VDCC-NO-SERT pathway. While NO seems to mediate the effects of IB-MECA on geotaxis, neither VDCCs nor SERT seems to be involved in this process.

The antiallodynic effect of intrathecal tianeptine is exerted by increased serotonin and norepinephrine in the spinal dorsal horn

The purpose of this study was to validate the effects of tianeptine on serotonergic and noradrenergic neurotransmission in a rat model of neuropathic pain. Neuropathic pain was induced by ligating the L5 and L6 spinal nerves in male Sprague–Dawley rats, and mechanical allodynia was assessed using von Frey filaments. The effects of intrathecally administered tianeptine on mechanical allodynia were assessed. Dihydroergocristine or yohimbine, a serotonergic or α-2 adrenergic receptor antagonists, respectively, were intrathecally administered 10min before tianeptine to investigate its mechanism of action. Additionally microdialysis studies were performed to measure the extracellular levels of serotonin (5-HT) and norepinephrine (NE) in the spinal dorsal horn following tianeptine administration. Intrathecal tianeptine significantly increased the paw withdrawal thresholds in a dose-dependent manner and the antiallodynic effect was antagonized by dihydroergocristine and yohimbine. Microdialysis studies revealed that tianeptine increased the levels of 5-HT and NE in the spinal dorsal horn. These findings suggest that tianeptine may be effective for the management of neuropathic pain and that its analgesic mechanism is exerted by increased levels of 5-HT and NE in the synaptic cleft at the spinal level.

Chronic loss of noradrenergic tone produces β-arrestin2-mediated cocaine hypersensitivity and alters cellular D2 responses in the nucleus accumbens.

Cocaine blocks plasma membrane monoamine transporters and increases extracellular levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT). The addictive properties of cocaine are mediated primarily by DA, while NE and 5-HT play modulatory roles. Chronic inhibition of dopamine β-hydroxylase (DBH), which converts DA to NE, increases the aversive effects of cocaine and reduces cocaine use in humans, and produces behavioral hypersensitivity to cocaine and D2 agonism in rodents, but the underlying mechanism is unknown. We found a decrease in β-arrestin2 (βArr2) in the nucleus accumbens (NAc) following chronic genetic or pharmacological DBH inhibition, and overexpression of βArr2 in the NAc normalized cocaine-induced locomotion in DBH knockout (Dbh -/-) mice. The D2/3 agonist quinpirole decreased excitability in NAc medium spiny neurons (MSNs) from control, but not Dbh -/- animals, where instead there was a trend for an excitatory effect. The Gαi inhibitor NF023 abolished the quinpirole-induced decrease in excitability in control MSNs, but had no effect in Dbh -/- MSNs, whereas the Gαs inhibitor NF449 restored the ability of quinpirole to decrease excitability in Dbh -/- MSNs, but had no effect in control MSNs. These results suggest that chronic loss of noradrenergic tone alters behavioral responses to cocaine via decreases in βArr2 and cellular responses to D2/D3 activation, potentially via changes in D2-like receptor G-protein coupling in NAc MSNs.

Fluoxetine and WAY 100,635 dissociate increases in scototaxis and analgesia induced by conspecific alarm substance in zebrafish (Danio rerio Hamilton 1822)

Alarm reactions to a substance secreted by the damaged skin of conspecifics and closely-related species are increasingly being recognized as fear-like responses in fish. The neurochemical underpinnings of these effects are so far unknown; however, given the role of the serotonergic system on defensive behavior, it is possible that the alarm reaction is mediated by this monoamine. Exposure to conspecific alarm substance (CAS) increased anxiety-like behavior in the light/dark test in zebrafish and decreased nocifensive behavior. These effects were accompanied by increases in blood glucose, hemoglobin, epinephrine and norepinephrine levels, as well as extracellular levels of serotonin in the brain. Pretreatment with fluoxetine blocked the anxiogenic effects of CAS on the light/dark test as well as all physiological parameters and the increase in extracellular brain 5-HT, but not the reduction in nocifensive behavior. Conversely, pretreatment with the 5-HT1AR antagonist WAY 100635 blocked the effects on nocifensive behavior, but not the effects on anxiety-like behavior nor on physiological parameters. These results point to an important and complex role of the serotonergic system in the mediation of fear-potentiated behavior in the light/dark test and in fear-induced analgesia in zebrafish.

Ascorbic acid protects against anxiogenic-like effect induced by methylmercury in zebrafish: action on the serotonergic system.

To evaluate the protector effect of ascorbic acid (AA) against anxiogenic-like effect induced by methylmercury (MeHg) exposure, adult zebrafish were treated with AA (2 mg g(-1), intraperitoneal [i.p.]) before MeHg administration (1.0 μg g(-1), i.p.). Groups were tested for the light/dark preference as a behavioral model of anxiety, and the content of serotonin and its oxidized metabolite tryptamine-4,5-dione (T-4,5-D) in the brain was determined by high-performance liquid chromatography. MeHg has produced a marked anxiogenic profile in both tests, and this effect was accompanied by a decrease in the extracellular levels of serotonin, and an increase in the extracellular levels of T-4,5-D. Added to this, a marked increase in the formation of a marker of oxidative stress accompanied these parameters. Interestingly, the anxiogenic-like effect and biochemical alterations induced by MeHg were blocked by pretreatment with AA. These results for the first time demonstrated the potential protector action of AA in neurobehavioral and neurochemical alterations induced by methylmecury exposure demonstrating that zebrafish model could be used as an important tool for testing substances with neuroprotector actions.