Intraregional variation in expression of serotonin transporter messenger RNA by 5-hydroxytryptamine neurons

Chronic restraint stress induces serotonin transporter expression in the rat adrenal glands

Chronic restraint stress (CRS) magnifies restraint-induced corticosterone secretion through a mechanism involving increased adrenocortical 5-HT content and turnover. We analysed the impact of CRS on serotonin transporter (SERT) expression and distribution in rat adrenal glands. Male Wistar rats were submitted to CRS (20 min/day) or undisturbed control conditions for 14 days. Exposure to CRS induced a remarkable increase in SERT-like immunoreactivity in the adrenal cortex, which closely matched that of chromogranin A immunostaining, along with a significant increase in SERT protein and mRNA levels in whole adrenals as determined by immunohistochemistry, Western blot and RT-PCR assays, respectively; all these CRS-induced changes occurred almost exclusively in left adrenals. Closely similar results were obtained in animals that received a 14-day chronic corticosterone treatment. These results unravel an interesting association between chronic stress exposure and SERT expression in adrenocortical chromogranin A-positive cells, which seems to be a glucocorticoid-dependent phenomenon.

Serotonin-specific lesions of the dorsal raphe disrupt maternal aggression and caregiving in postpartum rats

The behavioral modifications associated with early motherhood, which include high aggression, caring for the young, and low anxiety, are all affected by acute pharmacological manipulation of serotonin signaling. However, the effects on all these behaviors of permanently disrupting serotonin signaling from one of its primary sources, the dorsal raphe nucleus (DR), have not been examined in detail. To address this, serotonin-specific lesions centered on the dorsomedial DR (DRdm; DR subregion strongly implicated in emotional behaviors) were induced at mid-pregnancy (day 15) or early postpartum (day 2) in rats using a saporin-conjugated neurotoxin targeting the serotonin transporter (Anti-SERT-SAP). Prepartum or postpartum Anti-SERT-SAP reduced DRdm serotonin immunoreactivity by ∼40-65%, and postpartum Anti-SERT-SAP also reduced it in the ventromedial and lateral wings of the DR, as well as in the median raphe. Serotonin-immunoreactive fibers were significantly reduced in the anterior hypothalamus, but not medial preoptic area, of lesioned dams. Pre- or postpartum lesions both greatly reduced maternal aggression, but while prepartum lesions did not affect later undisturbed maternal caregiving, the larger postpartum lesions prevented the postpartum decline in kyphotic nursing and reduced pup licking. Serotonin lesions did not affect pup retrieval, but the prepartum lesions temporarily increased maternal hovering over and licking the pups observed immediately after the disruptive retrieval tests. Dams' anxiety-like behaviors and litter weight gains were unaffected by the lesions. These findings suggest that DRdm serotonin projecting to the AH is particularly critical for maternal aggression, but that more widespread disruption of midbrain raphe serotonin is necessary to greatly impair maternal caregiving. Postpartum anxiety may rely more on other neurochemical systems or different midbrain serotonergic cell populations.

A Tph2GFP Reporter Stem Cell Line To Model in Vitro and in Vivo Serotonergic Neuron Development and Function

Modeling biological systems in vitro has contributed to clarification of complex mechanisms in simplified and controlled experimental conditions. Mouse embryonic stem (mES) cells can be successfully differentiated toward specific neuronal cell fates, thus representing an attractive tool to dissect, in vitro, mechanisms that underlie complex neuronal features. In this study, we generated and characterized a reporter mES cell line, called Tph2^GFP, in which the vital reporter GFP replaces the tryptophan hydroxylase 2 (Tph2) gene. Tph2^GFP mES cells selectively express GFP upon in vitro differentiation toward the serotonergic fate, they synthesize serotonin, possess excitable membranes, and show the typical morphological, morphometrical, and molecular features of in vivo serotonergic neurons. Thanks to the vital reporter GFP, we highlighted by time-lapse video microscopy several dynamic processes such as cell migration and axonal outgrowth in living cultures. Finally, we demonstrated that predifferentiated Tph2^GFP cells are able to terminally differentiate, integrate, and innervate the host brain when grafted in vivo. On the whole, the present study introduces the Tph2^GFP mES cell line as a useful tool allowing accurate developmental and dynamic studies and representing a reliable platform for the study of serotonergic neurons in health and disease.

Short-term fluoxetine treatment induces neuroendocrine and behavioral anxiogenic-like responses in adolescent male rats

Fluoxetine (FLX) is prescribed to treat depression and anxiety in adolescent patients. However, FLX has anxiogenic effects during the acute phase of treatment, and caution has been raised due to increased suicidal thinking and behavior. Herein, we sought to study in adolescent (35-day-old) male rats, the effects of short-term FLX treatment (10 mg/kg/day, i.p. for 3-4 days) on hypothalamic-pituitary-adrenal axis activity, serotonin (5-hidroxytriptamine, 5-HT) transporter (SERT) mRNA expression in the dorsal raphe nucleus (DRN), energy balance-related variables and behavioral profiles in the holeboard. Our results revealed that daily FLX administration increased plasma corticosterone (B) concentrations without affecting basal gene expression of corticotrophin releasing hormone in the hypothalamic paraventricular nucleus (PVN) nor of pro-opiomelanocortin in the anterior pituitary. However, FLX had significant effects increasing the mRNA expression of PVN arginine vasopressin (AVP) and reducing SERT mRNA levels in the dorsolateral subdivision of the DRN. In the holeboard, FLX-induced anxiety/emotionality-like behaviors. As expected, FLX treatment was endowed with anorectic effects and reduced body weight gain. Altogether, our study shows that short-term FLX treatment results in physiological, neuroendocrine and behavioral stress-like effects in adolescent male rats. More importantly, considering that the AVP- and 5-HTergic systems: (1) are intimately involved in regulation of the stress response; (2) are regulated by sex hormones and (3) are related to regulation of aggressive behaviors, our results highlight the potential significance of these systems mediating the anxiogenic/emotionality/stress-like responses of adolescent male rats to short-term FLX treatment.

Dietary levels of pure flavonoids improve spatial memory performance and increase hippocampal brain-derived neurotrophic factor

Evidence suggests that flavonoid-rich foods are capable of inducing improvements in memory and cognition in animals and humans. However, there is a lack of clarity concerning whether flavonoids are the causal agents in inducing such behavioral responses. Here we show that supplementation with pure anthocyanins or pure flavanols for 6 weeks, at levels similar to that found in blueberry (2% w/w), results in an enhancement of spatial memory in 18 month old rats. Pure flavanols and pure anthocyanins were observed to induce significant improvements in spatial working memory (p = 0.002 and p = 0.006 respectively), to a similar extent to that following blueberry supplementation (p = 0.002). These behavioral changes were paralleled by increases in hippocampal brain-derived neurotrophic factor (R = 0.46, p<0.01), suggesting a common mechanism for the enhancement of memory. However, unlike protein levels of BDNF, the regional enhancement of BDNF mRNA expression in the hippocampus appeared to be predominantly enhanced by anthocyanins. Our data support the claim that flavonoids are likely causal agents in mediating the cognitive effects of flavonoid-rich foods.

Effects of the antidepressant fluoxetine on the subcellular localization of 5-HT1A receptors and SERT

Serotonin (5-HT) 5-HT(1A) autoreceptors (5-HT(1A)autoR) and the plasmalemmal 5-HT transporter (SERT) are key elements in the regulation of central 5-HT function and its responsiveness to antidepressant drugs. Previous immuno-electron microscopic studies in rats have demonstrated an internalization of 5-HT(1A)autoR upon acute administration of the selective agonist 8-OH-DPAT or the selective serotonin reuptake inhibitor antidepressant fluoxetine. Interestingly, it was subsequently shown in cats as well as in humans that this internalization is detectable by positron emission tomography (PET) imaging with the 5-HT(1A) radioligand [(18)F]MPPF. Further immunocytochemical studies also revealed that, after chronic fluoxetine treatment, the 5-HT(1A)autoR, although present in normal density on the plasma membrane of 5-HT cell bodies and dendrites, do not internalize when challenged with 8-OH-DPAT. Resensitization requires several weeks after discontinuation of the chronic fluoxetine treatment. In contrast, the SERT internalizes in both the cell bodies and axon terminals of 5-HT neurons after chronic but not acute fluoxetine treatment. Moreover, the total amount of SERT immunoreactivity is then reduced, suggesting that SERT is not only internalized, but also degraded in the course of the treatment. Ongoing and future investigations prompted by these finding are briefly outlined by way of conclusion.

Chronic social defeat up-regulates expression of the serotonin transporter in rat dorsal raphe nucleus and projection regions in a glucocorticoid-dependent manner

Chronic stress and dysfunction of the serotonergic system in the brain have been considered two of the major risks for development of depression. In this study, adult Fischer 344 rats were subjected to a regimen of chronic social defeat (CSD). To mimic stressful conditions, some rats were not exposed to CSD, but instead treated with corticosterone (CORT) in oral solution while maintained in their home cage. Protein levels of the serotonin transporter (SERT) in the dorsal raphe nucleus (DRN), hippocampus, frontal cortex, and amygdala were examined by Western blotting or immunofluorescence staining. The results showed that CSD up-regulated SERT protein levels in the DRN, hippocampus, frontal cortex, and amygdala regions. This up-regulation was abolished or prevented by adrenalectomy, or treatment with antagonists of corticosteroid receptors mifepristone and spironolactone, alone or in combination. Similarly, up-regulated SERT protein levels in these brain regions were also observed in rats treated with oral CORT ingestion, which was analogously prevented by treatment with mifepristone and spironolactone. Furthermore, both CSD- and CORT-induced up-regulation of SERT protein levels in the DRN and three brain regions were attenuated by simultaneous treatment with fluoxetine, an antidepressant that specifically inhibits serotonin reuptake. The results indicate that up-regulation in SERT protein levels in the DRN and forebrain limbic structures caused by CSD regimen was mainly motivated by CORT through corticosteroid receptors. The present findings demonstrate that chronic stress is closely correlated with the serotonergic system by acting on the regulation of the SERT expression in the DRN and its projection regions, which may contribute to the development of depression.

Blueberry supplementation induces spatial memory improvements and region-specific regulation of hippocampal BDNF mRNA expression in young rats

RATIONALE

Flavonoid-rich foods have been shown to be able to reverse age-related cognitive deficits in memory and learning in both animals and humans. However, to date, there have been only a limited number of studies investigating the effects of flavonoid-rich foods on cognition in young/healthy animals.

OBJECTIVES

The aim of this study was to investigate the effects of a blueberry-rich diet in young animals using a spatial working memory paradigm, the delayed non-match task, using an eight-arm radial maze. Furthermore, the mechanisms underlying such behavioural effects were investigated.

RESULTS

We show that a 7-week supplementation with a blueberry diet (2 % w/w) improves the spatial memory performance of young rats (2 months old). Blueberry-fed animals also exhibited a faster rate of learning compared to those on the control diet. These behavioural outputs were accompanied by the activation of extracellular signal-related kinase (ERK1/2), increases in total cAMP-response element-binding protein (CREB) and elevated levels of pro- and mature brain-derived neurotrophic factor (BDNF) in the hippocampus. Changes in hippocampal CREB correlated well with memory performance. Further regional analysis of BDNF gene expression in the hippocampus revealed a specific increase in BDNF mRNA in the dentate gyrus and CA1 areas of hippocampi of blueberry-fed animals.

CONCLUSIONS

The present study suggests that consumption of flavonoid-rich blueberries has a positive impact on spatial learning performance in young healthy animals, and these improvements are linked to the activation of ERK-CREB-BDNF pathway in the hippocampus.

The integrated model of serotonin transporter gene variation (5HTTLPR) and the glial cell transporter in stress vulnerability and depression

The serotonin transporter gene (SLC6A4) promoter polymorphism (5HTTLPR) has been associated with individual stress responses such that individuals with childhood abuse history have higher rates of depression in later life if they are homozygous short (s/s) of the gene. It is hypothesized that these findings could be explained by an integrated model of a role of the glial cell transporter and a functional difference of 5HTTLPR in the capacity of absorbing serotonin from the synapse. A hypothetical integrated model of the SLC6A4 function and the role of glial cells are put forward to explain accumulating results of recent investigations exploring the relationship between the gene and the diverse mental activities including depression and stress response. A model based on SLC6A4 variation is proposed to explain individual differences in stress vulnerability/resilience. The role of the glial cell transporter surrounding the synapse is integrated in the model to understand the modulation of the neurotransmission. It is hypothesized that a synapse with less serotonin transporter contributes to unstable processing in neurotransmission as compared to a synapse with more serotonin transporter. As such, based on functional differences of 5HTTLPR in the expression of the serotonin transporter, it is asserted that individuals with the s/s genotype process neurotransmission differently and in a reactive way. This integrated model of 5HTTLPR and glial cells suggests that the efficacy of serotonin reuptake in the synapse may play a crucial role in variability of neurotransmission, which can lead to differences in the stress response and the pathophysiology of depression.

Role of serotonin transporter inhibition in the regulation of tryptophan hydroxylase in brainstem raphe nuclei: time course and regional specificity

Drugs that selectively inhibit the serotonin transporter (SERT) are widely prescribed for treatment of depression and a range of anxiety disorders. We studied the time course of changes in tryptophan hydroxylase (TPH) in four raphe nuclei after initiation of two different SERT inhibitors, citalopram and fluoxetine. In the first experiment, groups of Sprague-Dawley rats received daily meals of rice pudding either alone (n=9) or mixed with citalopram 5 mg/kg/day (n=27). Rats were sacrificed after 24 h, 7 days or 28 days of treatment. Sections of dorsal raphe nucleus (DRN), median raphe nucleus (MRN), raphe magnus nucleus (RMN) and caudal linear nucleus (CLN) were processed for TPH immunohistochemistry. Citalopram induced a significant reduction in DRN TPH-positive cell counts at 24 h (41%), 7 days (38%) and 28 days (52%). Similar reductions in TPH-positive cell counts were also observed at each timepoint in the MRN and in the RMN. In the MRN, citalopram resulted in significant reductions at 24 h (26%), 7 days (16%) and 28 days (23%). In the RMN, citalopram induced significant reductions of TPH-positive cell counts at 24 h (45%), 7 days (34%) and 28 days (43%). By contrast, no significant differences between control and treatment groups were observed in the CLN at any of the time points that we studied. To investigate whether these changes would occur with other SERT inhibitors, we conducted a second experiment, this time with a 28-day course of fluoxetine. As was observed with citalopram, fluoxetine induced significant reductions of TPH cell counts in the DRN (39%), MRN (38%) and RMN (41%), with no significant differences in the CLN. These results indicate that SERT inhibition can alter the regulation of TPH, the rate limiting enzyme for serotonin biosynthesis. This persistent and regionally specific downregulation of serotonin biosynthesis may account for some of the clinical withdrawal symptoms associated with drugs that inhibit SERT.

Chronic low dose ovine corticotropin releasing factor or urocortin II into the rostral dorsal raphe alters exploratory behavior and serotonergic gene expression in specific subregions of the dorsal raphe

Corticotropin releasing factor (CRF) family peptides play key roles in integrating neural responses to stress. Both major CRF receptors have been pharmacologically identified in the dorsal raphe nucleus (DRN), a stress sensitive and internally heterogeneous nucleus supplying many forebrain regions with serotonergic input. Despite the involvement of chronic stress and serotonergic dysfunction in human mood and anxiety disorders, little is known about the effects of chronic CRF receptor activation on the DRN. We infused ovine CRF (1 ng/h), urocortin II (UCNII, 1 ng/h), or vehicle alone into rat DRN over 6 days. During infusion, animals were allowed to freely explore an open field for 15 min on each of 2 days, with the addition of a novel object on the second day. Following behavioral testing, 5-HT1A, 5-HT1B, 5-HT transporter (SERT), and tryptophan hydroxylase-2 (Tph2) expression was examined through the DRN by in situ hybridization. Ovine CRF infusion resulted in significantly decreased novel object touches, climbs, as well as increased latency to first novel object contact. UCNII had a similar but less dramatic effect, decreasing only climbing behavior. Both ovine CRF and UCNII blunted the decrease in corner time expected on re-exposure to the open field. Both peptides also produced regionally specific changes in gene expression: 5-HT1A expression was increased 30% in the mid-rostral ventromedial DRN, while SERT was decreased by 30% in the mid-caudal shell dorsomedial DRN. There also appeared to be a shift in the relative level of Tph2 expression between the ventromedial and core dorsomedial DRN at the mid-rostral level. Changes in 5-HT1A, SERT, and relative Tph2 mRNA abundance were correlated with novel object exploration. These findings suggest chronic intra-DRN administration of CRF agonists decreases exploratory behavior, while producing subregionally limited changes in serotonergic gene expression. These studies may be relevant to mechanisms underlying behavioral changes after chronic stress.

Quantitative mapping of tryptophan hydroxylase-2, 5-HT1A, 5-HT1B, and serotonin transporter expression across the anteroposterior axis of the rat dorsal and median raphe nuclei

Depression and anxiety disorders are among the leading causes of morbidity, mortality, and disability in the United States. Impaired serotonin neurotransmission appears to be a central mechanism inducing depressive and anxiety symptoms. Most serotonergic innervation of the forebrain arises from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN). The DRN displays a complex internal morphology, with distinct subregions varying across the anteroposterior (A-P) axis. However, many studies have considered the DRN as a whole or used easily confused terminology to describe position. Given the large differences in receptor expression, electrophysiological properties, and connectivity between various subregions of the DRN, it appears probable that they have distinct functional roles in the regulation of behavior. To foster uniform definitions of locations within these nuclei, we have quantitatively mapped gene expression in DRN and MRN for tryptophan hydroxylase-2 (Tph2), the serotonin transporter, as well as 5-HT1A and 5-HT1B receptors. These quantitative studies revealed differences in the density of expression of each gene in the ventromedial, dorsomedial, and dorsolateral subnuclei of the DRN, as well as distinct variation in expression across the A-P axis. These findings provide additional evidence that subregions of the DRN are heterogeneous and need to be considered independently. In addition, a fine scale map of Tph2 expression suggests definitions for categorical divisions of the DRN across the A-P axis. These are based on distinct morphological patterns of Tph2 expression and may be more reflective of physiology than the classic terminology dividing the DRN into equal thirds.

Molecular correlates of emotional learning using genetically selected rat lines

The genetic contributions to active avoidance learning in rodents have been well established, yet the molecular basis for genetically selected line differences remains poorly understood. To identify candidate genes influencing this active avoidance paradigm, we utilized the bidirectionally selected Syracuse high- and low-avoidance (SHA and SLA) rat lines that markedly differ in their two-way active avoidance behavior. Rats were phenotyped, rested to allow recovery from testing stress and then hippocampi were dissected for gene expression profiling (Affymetrix U34A chips; approximately 7000 known genes), comparing SLA to SHA. Next, a subset of differentially expressed genes was confirmed by real-time PCR (RT-PCR) in hippocampi. Additional studies at the protein level were performed for some genes. Using triplicate arrays on pooled hippocampal samples, differentially expressed genes were identified by microarray suite 5.0 and robust multi-array average analyses. By RT-PCR analysis in hippocampi, eight genes were nominated as potential candidate genes consistent with the differential expression from the microarray data. Four genes, Veli1 (mlin-7B), SLC3a1, Ptpro and Ykt6p, showed higher expression in SHA hippocampi than SLA. Four genes, SLC6A4, Aldh1a4, Id3a and Cd74, showed higher expression in SLA hippocampi than SHA. The active avoidance behavioral difference between lines probably emerges from 'many small things'. These potential candidate genes generate hypotheses for future testing in human association and rodent studies. Differences in levels of a pleiotropic gene like Ptpro and SLC6A4 suggest that small differences over a lifespan may contribute to large behavioral differences.

Expression of serotonin 5-HT2C receptors in GABAergic cells of the anterior raphe nuclei

We have used double in situ hybridization to examine the cellular localization of 5-HT2C receptor mRNA in relation to serotonergic and GABAergic neurons in the anterior raphe nuclei of the rat. In the dorsal and median raphe nuclei 5-HT2C receptor mRNA was not detected in serotonergic cells identified as those expressing serotonin (5-HT) transporter mRNA. In contrast, 5-HT2C receptor mRNA was found in most GABAergic cells, recognized by the presence of glutamic acid decarboxylase mRNA. Such 5-HT2C receptor-positive GABAergic neurons were mainly located in the intermediolateral and lateral portions of the dorsal raphe and lateral part of the median raphe. The present data give anatomical support to a previous hypothesis that proposed a negative-feedback loop involving reciprocal connections between GABAergic interneurons bearing 5-HT2A/2C receptors and 5-HT neurons in the dorsal raphe and surrounding areas. According to this model, the excitation of GABAergic interneurons through these 5-HT2C (and also 5-HT2A) receptors would result in the suppression of 5-HT cell firing.

Wheel running alters serotonin (5-HT) transporter, 5-HT1A, 5-HT1B, and alpha 1b-adrenergic receptor mRNA in the rat raphe nuclei

BACKGROUND

Altered serotonergic (5-HT) neurotransmission is implicated in the antidepressant and anxiolytic properties of physical activity. In the current study, we investigated whether physical activity alters factors involved in the regulation of central 5-HT neural activity.

METHODS

In situ hybridization was used to quantify levels of 5-HT transporter (5-HTT), 5-HT(1A), 5-HT(1B), and alpha(1b)-adrenergic receptor (alpha(1b) ADR) messenger ribonucleic acids (mRNAs) in the dorsal (DRN) and median raphe (MR) nuclei of male Fischer rats after either sedentary housing or 3 days, 3 weeks, or 6 weeks of wheel running.

RESULTS

Wheel running produced a rapid and lasting reduction of 5-HT(1B) mRNA in the ventral DRN. Three weeks of wheel running decreased 5-HTT mRNA in the DRN and MR and increased alpha(1b) ADR mRNA in the DRN. After 6 weeks of wheel running, 5-HTT mRNA remained reduced, but alpha(1b) ADR mRNA returned to sedentary levels. Serotonin(1A) mRNA was increased in the MR and certain DRN subregions after 6 weeks only.

CONCLUSIONS

Data suggest that the central 5-HT system is sensitive to wheel running in a time-dependent manner. The observed changes in mRNA regulation in a subset of raphe nuclei might contribute to the stress resistance produced by wheel running and the antidepressant and anxiolytic effects of physical activity.

Rat brain serotonin neurones that express neuronal nitric oxide synthase have increased sensitivity to the substituted amphetamine serotonin toxins 3,4-methylenedioxymethamphetamine and p-chloroamphetamine

Substituted amphetamines such as p-chloroamphetamine and the abused drug methylenedioxymethamphetamine cause selective destruction of serotonin axons in rats, by unknown mechanisms. Since some serotonin neurones also express neuronal nitric oxide synthase, which has been implicated in neurotoxicity, the present study was undertaken to determine whether nitric oxide synthase expressing serotonin neurones are selectively vulnerable to methylenedioxymethamphetamine or p-chloroamphetamine. Using double-labeling immunocytochemistry and double in situ hybridization for nitric oxide synthase and the serotonin transporter, it was confirmed that about two thirds of serotonergic cell bodies in the dorsal raphé nucleus expressed nitric oxide synthase, however few if any serotonin transporter immunoreactive axons in striatum expressed nitric oxide synthase at detectable levels. Methylenedioxymethamphetamine (30 mg/kg) or p-chloroamphetamine (2 x 10 mg/kg) was administered to Sprague-Dawley rats, and 7 days after drug administration there were modest decreases in the levels of serotonin transporter protein in frontal cortex, and striatum using Western blotting, even though axonal loss could be clearly seen by immunostaining. p-Chloroamphetamine or methylenedioxymethamphetamine administration did not alter the level of nitric oxide synthase in striatum or frontal cortex, determined by Western blotting. Analysis of serotonin neuronal cell bodies 7 days after p-chloroamphetamine treatment, revealed a net down-regulation of serotonin transporter mRNA levels, and a profound change in expression of nitric oxide synthase, with 33% of serotonin transporter mRNA positive cells containing nitric oxide synthase mRNA, compared with 65% in control animals. Altogether these results support the hypothesis that serotonin neurones which express nitric oxide synthase are most vulnerable to substituted amphetamine toxicity, supporting the concept that the selective vulnerability of serotonin neurones has a molecular basis.

The Ets transcription factor Fev is specifically expressed in the human central serotonergic neurons

In the mouse and the rat brain, the Ets transcription factor pet-1 is exclusively expressed in the central serotonin (5-hydroxytryptaminergic: 5-HT) system. In pet-1 null mice, the defect of this factor induces early disruption of the 5-HT function, resulting in an increase in anxiety and aggression; thus indicating its pivotal role in this system. Here, we studied the expression of fev, the homologue of pet-1, in the human brain. We showed that this transcription factor is exclusively expressed in the midline part of the human brainstem containing raphe nuclei, which also specifically expressed 5-HT transporter (sert) and tryptophan hydroxylase (tph), two markers of the 5-HT neurotransmitter system. This clearly suggests that fev is expressed in human serotonergic neurons. While the deficiency of the central serotonergic signaling is a major factor involved in the development of some psychiatric disorders, Fev could be a good diagnosis marker as well as a good target for pharmacological treatments of these patients.

5-ht5BReceptor mRNA in the raphe nuclei: Coexpression with serotonin transporter

We used double-label in situ hybridization to examine the cellular localization of 5-ht(5B) receptor mRNA in relation to serotonin transporter mRNA in the rat dorsal raphe (DR) and central superior nucleus (CS, median raphe nucleus). 5-ht(5B) receptor mRNA hybridization signal was often found on serotonin transporter mRNA-positive neuron profiles. The degree of cellular colocalization of these mRNAs notably varied among the different regions of the raphe nuclei. In the DR, cell bodies showing 5-ht(5B) receptor mRNA expression were abundant in the medial portions of the nucleus, all of them being also labeled for serotonin transporter mRNA. In contrast, in the ventrolateral regions (lateral wings) of the DR, we observed serotonin transporter mRNA-positive cells, but they were devoid of 5-ht(5B) receptor mRNA signal. In the CS, the level of coexpression of 5-ht(5B) receptor mRNA with serotonin transporter mRNA was high in the intermediate portions of the nucleus; however, we were unable to detect specific 5-ht(5B) receptor mRNA hybridization signal in its caudal extent. Our results support the presence of 5-ht(5B) receptor in serotonergic neurons in the DR and CS, suggesting an autoreceptor role for this receptor subtype.

Electroconvulsive shock increases serotonin transporter in the rat frontal cortex

The antidepressive action of electroconvulsive shock (ECS) is thought to involve the alteration in serotonin (5-HT) neurotransmission, including the increase in 5-HT release and uptake. In our previous study, 5-HT transporter (5-HTT) mRNA expression was decreased after single and repetitive ECS in rat raphe nucleus. In the present study, we investigated the effects of single and repetitive ECS on the protein levels of 5-HTT in the frontal cortex, hippocampus and raphe nucleus of rat brain using quantitative Western blot analysis. Single ECS did not alter 5-HTT protein expression in any brain regions examined. Repetitive ECS stably increased 5-HTT protein in the frontal cortex, but not in the hippocampus and raphe nucleus. Because ECS is known to facilitate the release of neurotransmitters, our results suggest that the increased 5-HTT protein expression in the frontal cortex might be a compensatory change against the enhanced 5-HT release by ECS in presynaptic terminals.

GABAB receptor mRNA in the raphe nuclei: co-expression with serotonin transporter and glutamic acid decarboxylase

We have used double-label in situ hybridization techniques to examine the cellular localization of GABAB receptor mRNA in relation to serotonin transporter mRNA and glutamic acid decarboxylase mRNA in the rat dorsal raphe, median raphe and raphe magnus nuclei. The degree of cellular co-localization of these markers notably varied among the different nuclei. In the dorsal raphe, cell bodies showing GABAB receptor mRNA were very abundant, the 85% being also labelled for serotonin transporter mRNA, and a low proportion (5%) showing glutamic acid decarboxylase mRNA. In the median raphe, the level of co-expression of GABAB receptor mRNA with serotonin transporter mRNA was significantly lower. Some cells were also identified that contained GABAB receptor mRNA in the absence of either one of the other mRNA species studied. Our results support the presence of GABAB receptors in serotonergic as well as GABAergic neurones in the dorsal and median raphe, providing the anatomical basis for the reported dual inhibitory/disinhibitory effect of the GABAB agonist baclofen on serotonergic function.

Pet-1 ETS gene plays a critical role in 5-HT neuron development and is required for normal anxiety-like and aggressive behavior

The central serotonin (5-HT) neurotransmitter system is an important modulator of diverse physiological processes and behaviors; however, the transcriptional mechanisms controlling its development are largely unknown. The Pet-1 ETS factor is a precise marker of developing and adult 5-HT neurons and is expressed shortly before 5-HT appears in the hindbrain. Here we show that in mice lacking Pet-1, the majority of 5-HT neurons fail to differentiate. Remaining ones show deficient expression of genes required for 5-HT synthesis, uptake, and storage. Significantly, defective development of the 5-HT system is followed by heightened anxiety-like and aggressive behavior in adults. These findings indicate that Pet-1 is a critical determinant of 5-HT neuron identity and implicate a Pet-1-dependent program in serotonergic modulation of behavior.

Diurnal variation of c-Fos expression in subdivisions of the dorsal raphe nucleus of the Mongolian gerbil (Meriones unguiculatus)

Recent studies suggest that the dorsal raphe nucleus (DRN) of the brainstem contains several subdivisions that differ both anatomically and neurochemically. The present study examined whether variation of c-Fos expression across the 24-hour light-dark cycle may also be different in these subdivisions. Animals were kept on a 12:12 light-dark cycle, were perfused at seven different time points, and brain sections were processed by using c-Fos immunocytochemistry. At all coronal levels of the DRN, c-Fos expression reached a peak 1 hour after the light-dark transition (lights-off) and reached its lowest levels in the middle of the light period. In contrast to the light-dark transition, c-Fos levels did not change significantly after the dark-light transition (lights-on). One-way analysis of variance (ANOVA) revealed that the diurnal variation of c-Fos expression was highly significant in the caudal ventral DRN. Similar variation in c-Fos expression also was observed in the other DRN subdivisions, but this variation appeared to gradually diminish in the caudal-to-rostral and ventromedial-to-dorsomedial directions. Double-label immunocytochemistry revealed that, 1 hour after lights-off, only 11% of c-Fos-positive neurons in the caudal ventral DRN were serotonin (5-HT)-immunoreactive. These results suggest that DRN subdivisions may differ functionally with regard to the diurnal cycle, and that these differences may be reflected in the activity of nonserotonergic cells in the DRN.

Functional consequences of 5-HT transporter gene disruption on 5-HT(1a) receptor-mediated regulation of dorsal raphe and hippocampal cell activity

The consequences of the absence of 5-HT reuptake on the functional properties of 5-HT(1A) receptors were examined in the dorsal raphe nucleus and the hippocampus of knock-out mice lacking the serotonin transporter (5-HTT). Extracellular recordings showed that application of selective 5-HT reuptake inhibitors such as paroxetine and citalopram onto brainstem slices resulted in a concentration-dependent inhibition of 5-HT neuron firing in the dorsal raphe nucleus of wild-type 5-HTT+/+ mice, but not 5-HTT-/- mutants. By contrast, the 5-HT(1A) receptor agonists ipsapirone and 5-carboxamidotryptamine inhibited the discharge in both groups. However, the potency of these agonists was markedly decreased (by approximately 55- and approximately 6-fold, respectively) in 5-HTT-/- compared with 5-HTT+/+ animals. Similarly, intracellular recordings showed that the potency of 5-carboxamidotryptamine to hyperpolarize 5-HT neurons in the dorsal raphe nucleus was significantly lower in 5-HTT-/- than in 5-HTT+/+ animals. These data contrasted with those obtained with hippocampal slices in which 5-carboxamidotryptamine was equipotent to hyperpolarize CA1 pyramidal neurons in both mutant and wild-type mice. As expected from their mediation through 5-HT(1A) receptors, the effects of ipsapirone and 5-carboxamidotryptamine were competitively inhibited by the selective 5-HT(1A) antagonist WAY 100635 in both groups. These data showed that 5-HTT gene knock-out induced a marked desensitization of 5-HT(1A) autoreceptors in the dorsal raphe nucleus without altering postsynaptic 5-HT(1A) receptor functioning in the hippocampus. Similarities between these changes and those evoked by chronic treatment with 5-HT reuptake inhibitors emphasize the existence of regional differences in 5-HT(1A) receptor regulatory mechanisms.

Differential expression of the noradrenaline transporter in adrenergic chromaffin cells, ganglion cells and nerve fibres of the rat adrenal medulla

Expression of the noradrenaline transporter (NAT) was identified in various cell and fibre populations of the rat adrenal medulla, examined with immunohistochemistry and confocal microscopy. Immunoreactivity for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH), aromatic-L-amino-acid decarboxylase (AADC) and dopamine beta-hydroxylase (DBH) was present in all chromaffin cells, while phenylethanolamine N-methyltransferase (PNMT) was used to determine adrenergic chromaffin cell groups. Labelling with NAT antibody was predominantly cytoplasmic and colocalised with PNMT immunoreactivity. Noradrenergic chromaffin cells were not NAT immunoreactive. Additionally, NAT antibody labelling demonstrated clusters of ganglion cells (presumably Type I) and nerve fibres. Expression of TH, AADC, DBH, PNMT and NAT mRNA was examined using reverse transcription-polymerase chain reaction (RT-PCR) from adrenal medulla punches and single chromaffin cells, and results were consistent with those obtained with immunocytochemistry. Chromaffin cells and fibres labelled with antibodies against growth associated protein-43 (GAP-43) were not NAT immunoreactive, while ganglion cells were doubled labelled with the two antibodies. The presence of NAT in adrenergic chromaffin cells, and its absence from noradrenergic cells, suggests that the adrenergic cell type is primarily responsible for uptake of catecholamines in the adrenal medulla.

Acute stress decreases serotonin transporter mRNA in the raphe pontis but not in other raphe nuclei of the rat

In addition to elevated corticosterone levels, stress produces structural changes and neuronal damage especially in the hippocampus. In this line it has been shown, that in rats single or repeated immobilisation markedly reduces brain-derived neurotrophic factor (BDNF) mRNA levels in the hippocampal formation. Since this neurotrophin also controls the efficacy of serotonergic neurotransmission, the aim of the current study was to investigate the effect of acute immobilization stress on the expression of serotonin transporter (SERT) mRNA in the raphe nuclei as a parameter of serotonergic innervation. We have examined the expression of SERT mRNA and of BDNF mRNA in rats upon acute immobilisation by quantitative in situ hybridisation with a (35)S-labelled oligonucleotide probe. Elevated corticosterone levels in stressed animals confirmed as internal controls the effect of stress under our conditions. Acute stress led to a significant decrease of BDNF mRNA in the hippocampus and of SERT mRNA in the raphe pontis, but not in other raphe nuclei investigated. These data provide evidence for fast interactions between neurotrophins, corticosterone and serotonergic neurotransmission under stress conditions.

Localization of calcitonin receptor mRNA in the mouse brain: coexistence with serotonin transporter mRNA

To elucidate the sites of and mechanisms of analgesic effect of centrally injected calcitonin, we examined expression of calcitonin receptor mRNA in the mouse brain by in situ hybridization techniques. Calcitonin receptor mRNA was expressed in various brain regions, including the preoptic area, dorsomedial hypothalamic nucleus, lateral hypothalamic area, periaqueductal gray, dorsal raphe nucleus, locus coeruleus, lateral parabrachial nucleus, gigantocellular reticular nucleus alpha part, lateral paragigantocellular nucleus, raphe magnus nucleus and solitary tract nucleus, which are known to play important roles in pain modulation. In addition, a double in situ hybridization technique demonstrated the intense expression of calcitonin receptor mRNA on serotonergic neurons in some raphe nuclei and the lateral paragigantocellular nucleus, suggesting the involvement of central serotonergic pathways in analgesic effect of calcitonin.

Ultrastructural localization of the serotonin transporter in limbic and motor compartments of the nucleus accumbens

Extracellular levels of serotonin [5-hydroxytryptamine (5-HT)] in the nucleus accumbens (NAc) can influence both cognitive and motor functions involving extensive connections with the frontal cortex. The 5-HT levels reflect vesicular release and plasmalemmal reuptake through the serotonin transporter (SERT). We used electron microscopic immunocytochemistry to determine the sites for SERT activation in the limbic shell and motor-associated core of the rat NAc. Of the SERT-immunoreactive profiles in each region, >90% were serotonergic axons and axon terminals; the remainder were nonserotonergic dendrites and glia. Axonal SERT immunogold labeling was seen mainly at nonsynaptic sites on plasma membranes and often near 5-HT-containing large dense core vesicles (DCVs). SERT-labeled axonal profiles were larger and had a higher numerical density in the shell versus the core but showed no regional differences in their content of SERT immunogold particles. In contrast, immunoreactive dendrites had a lower numerical density in the shell than in the core. SERT labeling in dendrites was localized to segments of plasma membrane near synaptic contacts from unlabeled terminals and/or dendritic appositions. Our results suggest that in the NAc (1) reuptake into serotonergic axons is most efficient after exocytotic release from DCVs, and (2) increased 5-HT release without concomitant increase in SERT expression in individual axons may contribute to higher extracellular levels of serotonin in the shell versus the core. These findings also indicate that SERT may play a minor substrate-dependent role in serotonin uptake or channel activity in selective nonserotonergic neurons and glia in the NAc.