Quantitative autoradiography of 3H-paroxetine binding sites in rat brain

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Associations between serotonin transporter and behavioral traits and diagnoses related to anxiety

The role of the serotonin transporter promoter-linked polymorphism (5-HTTLPR) in psychiatric disease remains unclear. Behavioral traits could serve as alternative outcomes that are stable, precede psychopathology, and capture more sub-clinical variation. We test associations between 5-HTTLPR and (1) behavioral traits and (2) clinical diagnoses of anxiety and depression. Second and third generation participants (N=203, 34.2±13.8 years, 54% female) at high- or low- familial risk for depression (where risk was defined by the presence of major depression in the 1st generation) were assessed longitudinally using the Schedule for Affective Disorders and Schizophrenia-lifetime interview, Barratt Impulsiveness Scale-11, Buss-Perry Aggression Questionnaire, and the NEO-Five Factor Inventory. High (but not low)-risk offspring with two risk (short, s) alleles had higher impulsivity (+13%), hostility (+31%) and neuroticism (+23%). SS was associated higher rates of panic (OR=7.05 [2.44, 20.38], p=0.0003) and phobic (OR=2.68[1.04, 6.93], p=0.04), but not other disorders. Impulsivity accounted for 16% of associations between 5-HTTLPR and panic, and 52% of association between 5-HTTLPR and phobias. We show that 5-HTTLPR predicts higher impulsivity, hostility, and neuroticism, and that impulsivity could serve as a useful independent outcome or intermediary phenotype in genetic studies of anxiety.

Voxel-based morphometric brain comparison between healthy subjects and major depressive disorder patients in Japanese with the s/s genotype of 5-HTTLPR

Individuals with s/s genotype of serotonin transporter gene-linked promotor region (5-HTTLPR), which appear with a high frequency in Japanese, exhibit more diagnosable depression in relation to stressful life events than those with the s/l or l/l genotype. We prospectively investigated the brain volume changes in first-episode and medication naïve major depression disorder patients (MDD) with the s/s genotype in Japanese. We assessed the differences between 27 MDD with the s/s genotype and 44 healthy subjects (HS) with the same genotype using a whole-brain voxel-by-voxel statistical analysis of MRI. Gray matter volume in a brain region with significant clusters obtained via voxel-based morphometry analysis were measured and, as an exploratory analysis, evaluated for relationships to the subcategory scores (core, sleep, activity, psychic, somatic anxiety, delusion) of the Hamilton Depression Rating Scale (HAM-D) and the Social Readjustment Rating Scale (SRRS). The brain volume in the left insula lobe was significantly smaller in the MDD than in the HS. The left insula lobe volume correlated negatively with the “psychic” score of HAM-D and the SRRS. In a Japanese population with the s/s genotype, we found an atrophy of the insula in the MDD, which might be associated with “psychic” symptom and stress events.

Autoradiographic imaging and quantification of the high-affinity GHB binding sites in rodent brain using 3H-HOCPCA

GHB (γ-hydroxybutyric acid) is a compound endogenous to mammalian brain with high structural resemblance to GABA. GHB possesses nanomolar-micromolar affinity for a unique population of binding sites, but the exact nature of these remains elusive. In this study we utilized the highly selective GHB analogue, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) as a tritiated version (³H-HOCPCA) to radioactively label the specific GHB high-affinity binding site and gain further insight into the density, distribution and developmental profile of this protein. We show that, in low nanomolar concentrations, ³H-HOCPCA displays excellent signal-to-noise ratios using rodent brain autoradiography, which makes it a valuable ligand for anatomical quantification of native GHB binding site levels. Our data confirmed that ³H-HOCPCA labels only the high-affinity specific GHB binding site, found in high density in cortical and hippocampal regions. The experiments revealed markedly stronger binding at pH 6.0 (K_d 73.8 nM) compared to pH 7.4 (K_d 2312 nM), as previously reported for other GHB radioligands but similar B_max values. Using ³H-HOCPCA we analyzed the GHB binding protein profile during mouse brain development. Due to the high sensitivity of this radioligand, we were able to detect low levels of specific binding already at E15 in mouse brain, which increased progressively until adulthood. Collectively, we show that ³H-HOCPCA is a highly sensitive radioligand, offering advantages over the commonly used radioligand ³H-NCS-382, and thus a very suitable in vitro tool for qualitative and quantitative autoradiography of the GHB high-affinity site.

Serotonin modulates glutamatergic transmission in the rat olfactory tubercle

The olfactory tubercle (OT) is found in the brains of mammals that are highly dependent on their sense of smell. Its human analogue is the poorly understood anterior perforated substance. Previous work on rat brain slices identified two types of field potential responses from the OT. The association fibre (AF) pathway was sensitive to muscarinic modulation, whereas the lateral olfactory tract (LOT) fibre pathway was not. Here, we establish that serotonin (5-hydroxytryptamine; 5-HT) also inhibits field potential excitatory postsynaptic potentials (EPSPs) in the AF, but not in the LOT fibre, pathway. Parallel experiments with adenosine (ADO) excluded ADO mediation of the 5-HT effect. Exogenous 5-HT at 30 microm caused a long-lasting approximately 40% reduction in the amplitude of AF postsynaptic responses, without affecting the time-course of EPSP decline, indicating a fairly restricted disposition of the 5-HT receptors responsible. The 5-HT(1)-preferring, 5-HT(5)-preferring and 5-HT(7)-preferring agonist 5-carboxamidotryptamine caused similar inhibition at approximately 100 nm. The 5-HT(1A)-preferring ligand 8-hydroxy-di-n-propylamino-tetralin at 10 microm, and the 5-HT uptake inhibitor citalopram at 3 microm, caused inhibition of AF-stimulated field potential responses in the 5-10% range. Order-of-potency information suggested a receptor of the 5-HT(1B) or 5-HT(1D) subtype. The 5-HT(1D) agonist L-694,247 (1 microm) suppressed the AF response by approximately 10% when used on its own. After washing out of L-694,427, inhibition by 30 microm 5-HT was reduced to negligible levels. Allowing for a partial agonist action of L-694,427 and complex interactions of 5-HT receptors within the OT, these results support the presence of active 5-HT(1D)-type receptors in the principal cell layer of the OT.

The comparative distributions of the monoamine transporters in the rodent, monkey, and human amygdala

The monoamines in the amygdala modulate multiple aspects of emotional processing in the mammalian brain, and organic or pharmacological dysregulation of these systems can result in affective pathologies. Knowledge of the normal distribution of these neurotransmitters, therefore, is central to our understanding of both the normal processes regulated by the amygdala and the pathological conditions associated with monoaminergic dysregulation. The monoaminergic transporters have proven to be accurate and reliable markers of the distributions of their substrates. The purpose of this review was twofold: First, to briefly recount the functional relevance of dopamine, serotonin, and norepinephrine transmission in the amygdala, and second, to describe and compare the distributions of the monoamine transporters in the rodent, monkey, and human brain. The transporters were found to be heterogeneously distributed in the amygdala. The dopamine transporter (DAT) is consistently found to be extremely sparsely distributed, however the various accounts of its subregional topography are inconsistent, making any cross-species comparisons difficult. The serotonin transporter (SERT) had the greatest overall degree of labeling of the three markers, and was characterized by substantial inter-species variability in its relative distribution. The norepinephrine transporter (NET) was shown to possess an intermediate level of labeling, and like the SERT, its distribution is not consistent across the three species. The results of these comparisons indicate that caution should be exercised when using animal models to investigate the complex processes modulated by the monoamines in the amygdala, as their relative contributions to these functions may differ across species.

Serotonin and aggression

The neurotransmitter serotonin (5-HT) has been implicated in the modulation of aggression in animals and humans. A longstanding dogma that aggression and serotonergic activity are inversely related has to be abandoned in light of many new findings. Trait and state aggression are differentially regulated by the 5-HT system and different 5-HT receptors seem to be involved. Of the 14 different 5-HT receptors, the 5-HT(1B) receptor, particularly the postsynaptically located 5-HT(1B) heteroreceptor, plays a highly selective role in the modulation of offensive aggression. We are still far from understanding the complex role played by the serotonergic system in the modulation of a complex set of behaviors like aggression.

[11C]SMe-ADAM, an imaging agent for the brain serotonin transporter: synthesis, pharmacological characterization and microPET studies in rats

N,N-Dimethyl-2-(2-amino-4-methylthiophenylthio)benzylamine (SMe-ADAM, 1) is a highly potent and selective inhibitor of the serotonin transporter (SERT). This compound was labeled with carbon-11 by methylation of the S-desmethyl precursor 10 with [(11)C]methyl iodide to obtain the potential positron emission tomography (PET) radioligand [(11)C]SMe-ADAM. The radiochemical yield was 27 +/- 5%, and the specific radioactivity was 26-40 GBq/micromol at the end of synthesis. Ex vivo and in vivo biodistribution experiments in rats demonstrated a rapid accumulation of the radiotracer in brain regions known to be rich in SERT, such as the thalamus/hypothalamus region (3.59 +/- 0.41%ID/g at 5 min after injection). The specific uptake reached a thalamus to cerebellum ratio of 6.74 +/- 0.95 at 60 min postinjection. The [(11)C]SMe-ADAM uptake in the thalamus was significantly decreased by pretreatment with fluoxetine to 38 +/- 11% of the control value. Furthermore, no metabolites of [(11)C]SMe-ADAM could be detected in the SERT-rich regions of the rat brain. It is concluded that [(11)C]SMe-ADAM may be a suitable PET ligand for SERT imaging in the living brain.

5-HT1B receptors and aggression: A review

The serotonergic (5-HT) system in the brain is involved in the modulation of offensive aggressive behavior. The dogma that activity of the 5-HT system is inversely related to aggression is obsolete now. Research on the status of the 5-HT system before, during and after the execution of aggression is ongoing but has not yet led to a clear picture about the actual functional role of the 5-HT system, the more because state versus trait aggression seems to play a pivotal role in the outcome. Pharmacological challenges pinpoint 5-HT(1A) and 5-HT(1B) receptors as key players in the modulation of offensive aggression. This review emphasizes in particular the role of postsynaptic 5-HT(1B) (hetero) receptors as a premier site to modulate offensive aggression. Modulation of the firing and 5-HT release of the serotonergic neuron, via presynaptic 5-HT(1A) (auto) receptors, presynaptic 5-HT(1B) (auto) receptors and serotonergic transporters, may also have striking influences on aggression under certain conditions. Therefore, it is hypothesized that postsynaptic 5-HT(1B) (hetero) receptors directly influence the executive, consummatory phases of agonistic behavior, whereas presynaptic serotonergic feedback systems are particularly useful in the introductory (appetitive) phases of the agonistic behavioral complex.

Central serotonin depletion affects rat brain areas differently: a qualitative and quantitative comparison between different treatment schemes

Depletion of rat brain serotonin (5-hydroxytryptamin, 5-HT) has been widely used to study effects of serotonin and its interaction with other transmitter systems. Various treatment regimes for serotonin depletion have been applied, but the efficacy of these seems to vary considerably. So far, no studies have systematically examined and compared different approaches. The present work combines quantitative and qualitative measurements and compares six different treatment schemes for 5-HT depletion. Treatment outcome was evaluated by HPLC measurements of 5-HT and 5-HIAA concentrations, and by 5-HT and tyrosine hydroxylase immunocytochemistry. The schemes included repeated administration of fenfluramine (FEN) and/or p-chlorophenylalanine (pCPA). The most efficient treatment for rat brain 5-HT depletion was the combined treatment with one daily pCPA (200 mg/kg) injection for 3 days followed by one injection of d,l-FEN (20 mg/kg) on the fourth day, causing a 94.9% brain 5-HT depletion. Immunostaining revealed a distinct brain distribution of the remaining 5-HT, with an almost complete depletion of 5-HT in the cerebral cortex, hippocampus and thalamus, while a substantial amount of 5-HT still was left in the raphe nuclei, the medial forebrain bundle, and the medial eminence. FEN or pCPA treatment alone caused from 68.2 to 94.0% decrease in 5-HT levels. While the pattern of 5-HT depletion using pCPA alone was comparable to the one seen with the combined treatment, the 5-HT depletion using FEN alone showed a different pattern with 5-HT distributed in several brain regions.

Modulation of behavior and cortical motor activity in healthy subjects by a chronic administration of a serotonin enhancer

SSRIs are postulated to modulate motor behavior. A single dose of selective serotoninergic reuptake inhibitors (SSRIs) like fluoxetine, paroxetine, or fluvoxamine, has been shown to improve motor performance and efficiency of information processing for simple sensorimotor tasks in healthy subjects. At a cortical level, a single dose of SSRI was shown to induce a hyperactivation of the primary sensorimotor cortex (S1M1) involved in the movement (Loubinoux, I., Boulanouar, K., Ranjeva, J. P., Carel, C., Berry, I., Rascol, O., Celsis, P., and Chollet, F., 1999. Cerebral functional magnetic resonance imaging activation modulated by a single dose of the monoamine neurotransmission enhancers fluoxetine and fenozolone during hand sensorimotor tasks. J. Cereb. Blood Flow Metab. 19 1365--1375, Loubinoux, I., Pariente, J., Boulanouar, K., Carel, C., Manelfe, C., Rascol, O., Celsis, P., and Chollet, F., 2002. A Single Dose of Serotonin Neurotransmission Agonist Paroxetine Enhances Motor Output. A double-blind, placebo-controlled, fMRI study in healthy subjects. NeuroImage 15 26--36). Since SSRIs are usually given for several weeks, we assessed the behavioral and cerebral effects of a one-month chronic administration of paroxetine on a larger group. In a double-blind, placebo controlled and crossover study, 19 subjects received daily 20 mg paroxetine or placebo, respectively, over a period of 30 days separated by a wash-out period of 3 months. After each period, the subjects underwent an fMRI (active or passive movement, dexterity task, sensory discrimination task) and a behavioral evaluation. Concurrently, a TMS (transcranial magnetic stimulation) study was conducted (Gerdelat-Mas, A., Loubinoux, I., Tombari, D., Rascol, O., Chollet, F., Simonetta-Moreau, M., 2005. Chronic administration of selective serotonin re-uptake inhibitor (SSRI) paroxetine modulates human motor cortex excitability in healthy subjects. NeuroImage 27,314--322).

RESULTS

On the one hand, paroxetine improved motor performances at the finger tapping test (P=0.02) without affecting choice reaction time, strength and dexterity significantly. Subjects were also faster in processing the spatial incongruency between a stimulus and the motor response (P=0.04). In order to differentiate behavioral components, a principal component analysis was performed on all motor tests, and several characteristics were differentiated: strength, speed, skill, attention, and motor response coding. Paroxetine would improve the efficiency of motor response coding (MANOVA on the factors; factor 3, P=0.01). On the other hand, the chronic administration induced a significant hypoactivation of S1M1 whatever the task: motor or sensory, simple or complex (random effect analysis, P<0.05). The hypoactivation correlated with the improvement of performances at the finger tapping test (P<0.05) suggesting more efficiency in cerebral motor processing.

CONCLUSIONS

Our results showed a clear modulation of sensory and motor cerebral activation after a chronic paroxetine administration. An improvement in both behavior and cerebral efficiency was suggested. It could be hypothesized that monoamines, by an unspecific effect, may tune the response of pyramidal neurons to optimize performances.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Serotonin transporter gene and autism: a haplotype analysis in an Irish autistic population

The role of the serotonin transporter (5-HTT) in the development of neuropsychiatric disorders has been widely investigated. Two polymorphisms, an insertion/deletion in the promoter region and a 12 repeat allele in a variable nucleotide tandem repeat (VNTR) in intron 2, drive higher expression of the 5-HTT gene. Four studies have shown nominally significant excess transmission of alleles of the 5-HTT gene in autism, while three studies have reported no excess transmission. This present study investigates the role of 5-HTT in the genetically homogenous Irish population. In all, 84 families were genotyped for five polymorphisms (three SNPs, a VNTR and an in/del). The analysis of allele transmissions using the transmission disequilibrium test (TDT) was undertaken and indicated preferential transmission of the short promoter allele (TDT P-value=0.0334). Linkage disequilibrium between markers was calculated and haplotypes were assessed for excess transmission and odds ratios (ORs) to affected children. A number of haplotypes, especially those involving and surrounding SNP10, showed evidence of association. The ORs ranged from 1.2 to 2.4. The most significant haplotype associated with transmission to affected probands was the SNP10-VNTR-SNP18 haplotype (chi(2)=7.3023, P=0.0069, odds ratio=1.8). This haplotype included the 12 repeat allele of the VNTR, which is associated with increased expression and may play a subtle role in the early development of the brain in affected probands.

Amygdala volume reductions in pediatric patients with obsessive-compulsive disorder treated with paroxetine: preliminary findings

The amygdala is believed to be highly relevant to the pathophysiology of obsessive-compulsive disorder (OCD) given its prominent role in fear conditioning and because it is an important target of the serotonin reuptake inhibitors (SRIs), the pharmacotherapy of choice for OCD. In the present study, we measured in vivo volumetric changes in the amygdala in pediatric patients with OCD following 16 weeks of monotherapy with the selective SRI, paroxetine hydrochloride. Amygdala volumes were computed from contiguous 1.5 mm magnetic resonance (MR) images in 11 psychotropic drug-naive patients with OCD prior to and then following treatment. Eleven healthy pediatric comparison subjects also had baseline and follow-up scans, but none of these subjects received medication. Patients demonstrated significant asymmetry of the amygdala (L>R) prior to pharmacologic intervention in contrast to healthy comparison subjects who showed no asymmetry at the time of their baseline scan. Mixed model analyses using age and total brain volume as time varying covariates indicated that left amygdala volume decreased significantly in patients following treatment. The reduction in left amygdala volume in patients correlated significantly with higher paroxetine dosage at the time of the follow-up scan and total cumulative paroxetine exposure between the scans. No significant changes in either right or left amygdala volume were evident among healthy comparison subjects from the baseline to the follow-up scan. These preliminary findings suggest that abnormal asymmetry of the amygdala may play a role in the pathogenesis of OCD and that paroxetine treatment may be associated with a reduction in amygdala volume.

Glucocorticoid-regulated human serotonin transporter (5-HTT) expression is modulated by the 5-HTT gene-promotor-linked polymorphic region

Mood, emotion and cognition are modulated by serotonergic neurotransmission, while the physiological function of serotonergic synapses depends on serotonin reuptake, which is mediated by the serotonin transporter (5-HTT). Allelic variation of 5-HTT expression in humans is caused by a functional gene-promoter polymorphism with two predominant variant alleles, which are associated with variations in anxiety measures as previously reported. Here we report that administration of dexamethasone, a potent glucocorticosteroid hormone, results in an increase in 5-HTT expression in immortalized human B-lymphoblastoid cells, which express the human 5-HTT. Functional reporter gene assays as well as 5-HT uptake and inhibitor binding measures revealed a genotype-dependent dose-response to glucocorticosteroid administration, which was antagonized by RU 38486, a non-specific glucocorticosteroid hormone antagonist. The allele-specific differences after administration of dexamethasone depended on the repetitive GC-rich sequence located approximately 1.4 kb upstream of the 5-HTT gene transcription site because of absence of a significant steroid effect after transfecting a deletional mutant reporter gene construct, which lacks this repetitive promoter sequence. Our findings may contribute to explain the vulnerability to stress-related disorders in susceptible individuals, in whom further clinical studies should follow up on these in vitro findings.

Selective serotonin reuptake inhibitor paroxetine modulates motor behavior through practice. A double-blind, placebo-controlled, multi-dose study in healthy subjects

We hypothesized that selective serotonin reuptake inhibitors (SSRIs) could modulate motor activity in healthy subjects in a dose-dependent manner. The effects of a single dose of paroxetine were tested in a double-blind, placebo-controlled study. Six randomized and counterbalanced subjects performed behavioral tests in three sessions 1 week apart (E1, E2 and E3) at peak plasma concentration (5 h after drug intake). Each subject was given 20 mg or 60 mg of the drug, or a placebo. Tasks were the Nine Peg Hole test (three trials), Moede dexteritymeter (two trials), and compatible and incompatible reaction time tasks. The results show that at the first trials, performance did not differ after placebo or paroxetine intake. However, 20 and 60 mg of paroxetine improved performance significantly at the third trial of the Nine Peg Hole test and subjects receiving the drug performed 7% faster than those under placebo. An amount of 20 mg, but not 60 mg, of paroxetine improved dexterity significantly at the second trial of the Moede test and subjects performed 30% faster. Conversely, the drug did not affect reaction time for the compatible task and subjects were 11% slower under 20 mg with the incompatible task. Thus, paroxetine decreased the ability to inhibit automatism. Thus, it was concluded that a single dose of paroxetine improved motor performance through practice. But negative effects occurred on tasks including the inhibition of an automatism. Paroxetine enhanced brain motor output (motor activity in S1M1) [NeuroImage, 15 (2002) 26]. This S1M1 hyperactivation is likely to be responsible for the better performance. The brain effect and motor improvement were dose dependent. For both, 20 mg was the optimal dose.

Sibutramine does not decrease the number of 5-HT re-uptake sites in rat brain and, like fluoxetine, protects against the deficits produced by dexfenfluramine

The effect of sibutramine and dexfenfluramine on 5-HT re-uptake sites, labelled with [(3)H]paroxetine, have been determined in various rat brain regions. In addition, the ability of fluoxetine and sibutramine to protect against the changes in [(3)H]paroxetine binding produced by dexfenfluramine was examined. Sibutramine (9 mg/kg, p.o.) and dexfenfluramine (1, 3 and 10 mg/kg, p.o.) were administered twice daily (before 09.00 h and after 16.00 h) for four days, followed by a 14 day drug-free period. In the protection studies, fluoxetine (10 mg/kg, i.p.) and sibutramine (9 mg/kg, p.o.) were given 1 h prior to dexfenfluramine (10 mg/kg, p.o.) using the same dosing regimen as described above. Sibutramine (9 mg/kg, p.o.; three times its ED(50) to inhibit food intake at 2 h) had no significant effect on the number or affinity of 5-HT re-uptake sites the brain regions studied. In contrast, dexfenfluramine at an equivalent dose (3 mg/kg, p.o.) significantly decreased the number of 5-HT re-uptake sites in frontal cortex (by 35%), hippocampus (by 47%) and hypothalamus (by 27%). This effect was dose-dependent with marked decreases (by 58-84%) in the number of sites following 10 mg/kg, p.o. These effects were not associated with changes in binding affinity. Fluoxetine (10 mg/kg, i.p.) completely blocked the effect of dexfenfluramine (10 mg/kg, p.o.) without having any significant effect alone. Sibutramine (9 mg/kg, p.o.) also blocked the effects of dexfenfluramine, although the reversal was only partial in frontal cortex, hippocampus and hypothalamus. Thus sibutramine, unlike dexfenfluramine, does not alter brain 5-HT re-uptake sites. Furthermore, sibutramine and fluoxetine protect against the deficits in 5-HT re-uptake sites produced by dexfenfluramine. These data provide further evidence that sibutramine is a 5-HT re-uptake inhibitor and it does not have neurotoxic potential.

The distribution of serotonergic nerves in microencephalic rats treated prenatally with methylazoxymethanol

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM) induces microencephaly in the offspring. In the present study of these microencephalic rats (MAM rats) we used quantitative autoradiography to investigate [3H] paroxetine binding sites, which are a selective marker of serotonin (5-HT) transporters (5-HTT). The binding in the accumbens, cortex, hippocampus, and dorsolateral thalamus was significantly increased in MAM rats, compared to the control rats, while there was a significant decrease in the dorsal raphe nucleus of the MAM rats. The levels of 5-HTT mRNA in the dorsal raphe nuclei were analyzed by in situ hybridization, which revealed a significant decrease in 5-HTT mRNA-positive neurons in the MAM rats compared to the control rats. The results imply serotonergic hyperinnervation in the cerebral hemispheres of MAM rats, while a target-dependent secondary degeneration of 5-HT neurons might be induced in the dorsal raphe nuclei of MAM rats.

Paracrine neurotransmission in the CNS: involvement of 5-HT

While GABA and glutamate have an established synaptic function in the CNS, recent evidence suggests 5-HT neurotransmission is predominantly paracrine. As the amino-acid neurotransmitters interact with receptors that produce effects rapidly, electrophysiological approaches can be used to assess the time delay between transmitter release and the postsynaptic response directly. However, this approach cannot be used for studies of 5-HT-mediated neurotransmission, because the majority of its receptors react more slowly, so anatomical and voltammetrical approaches have been used to provide insight into 5-HT-mediated events. These studies have revealed that extrasynaptic receptors and transporters for 5-HT exist, and that 5-HT escapes readily from the synaptic cleft. Attenuation of 5-HT binding by 5-HT-receptor antagonists and 5-HT-uptake inhibitors does not affect the synaptic efflux elicited by transient stimuli, although the effects of such drugs are apparent at later time points. Once it is extrasynaptic, 5-HT has a concentration that is similar to those estimated to be optimal for receptor and transporter activation, and it can diffuse a few micrometers until removed by its transporter. These properties of 5-HT raise the possibility that it can act on receptors that are distant from its release site and function as a paracrine transmitter.

Serotonin transporters are located on the axons beyond the synaptic junctions: anatomical and functional evidence

The serotonin (5-HT) transporter (5-HTT) is known to play a role in depression and many 5-HT related diseases, and is the target site for drugs of abuse, such as cocaine, MDMA, and methamphetamine. The major role of the 5-HTT has long been considered to be to inactivate serotonin transmission through the elimination of serotonin at release sites. However, immunocytochemistry using an antibody against the N-terminal of the 5-HTT at the light microscopic (LM) level indicates that the 5-HTT is associated not only with 5-HT varicosities but also with axons. Electron microscopy (EM) reveals that the majority of the 5-HTTs exist on the axolemma outside the synaptic junctions. In studying whether axonal 5-HTTs are involved in the uptake of 5-HT, we found with autoradiography that [3H]citalopram bound to all major 5-HT fibers, not only in the terminal regions, but also in 5-HT axonal bundles such as the cingulum bundle and medial forebrain bundle. Furthermore, voltammetry recordings indicated that serotonin axonal bundles were actively engaged in high affinity serotonin uptake. The evidence indicates that 5-HTTs on 5-HT axons away from the synapse are likely to be functional in a manner similar to the terminal 5-HTT for serotonin uptake. It also suggests that the role of the 5-HTT may not only be for the termination of synaptic transmission, but also for the regulation of 5-HT through extrasynaptic (volume) transmission. Our findings may also impact the understanding of the sites of action of selective serotonin reuptake inhibitors and drug entry into serotonin neurons via the numerous axonal sites.

Quantitative evaluation of 5-hydroxytryptamine (serotonin) neuronal release and uptake: an investigation of extrasynaptic transmission

Whether neurotransmitters are restricted to the synaptic cleft (participating only in hard-wired neurotransmission) or diffuse to remote receptor sites (participating in what has been termed volume or paracrine transmission) depends on a number of factors. These include (1) the location of release sites with respect to the receptors, (2) the number of molecules released, (3) the diffusional rate away from the release site, determined by both the geometry near the release site as well as binding interactions, and (4) the removal of transmitter by the relevant transporter. Fast-scan cyclic voltammetry allows for the detection of extrasynaptic concentrations of many biogenic amines, permitting direct access to many of these parameters. In this study the hypothesis that 5-hydroxytryptamine (5-HT) transmission is primarily extrasynaptic in the substantia nigra reticulata, a terminal region with identified synaptic contacts, and the dorsal raphe nucleus, a somatodendritic region with rare synaptic incidence, was tested in brain slices prepared from the rat. Using carbon fiber microelectrodes, we found the concentration of 5-HT released per stimulus pulse in both regions to be identical when elicited by single pulse stimulations or trains at high frequency. 5-HT efflux elicited by a single stimulus pulse was unaffected by uptake inhibition or receptor antagonism. Thus, synaptic efflux is not restricted by binding to intrasynaptic receptors or transporters. The number of 5-HT molecules released per terminal was estimated in the substantia nigra reticulata and was considerably less than the number of 5-HT transporter and receptor sites, reinforcing the hypothesis that these sites are extrasynaptic. Furthermore, the detected extrasynaptic concentrations closely match the affinity for the predominant 5-HT receptor in each region. Although they do not disprove the existence of classical synaptic transmission, our results support the existence of paracrine neurotransmission in both serotonergic regions.

Cellular localization and expression of the serotonin transporter in mouse brain

The high-affinity serotonin (5-HT) transporter (5-HTT) plays an important role in the removal of extracellular serotonin, thereby modulating and terminating the action of this neurotransmitter at various pre- and post-synaptic serotonergic receptors and heteroreceptors. In order to characterize the anatomical distribution of the 5-HTT in mouse brain, in situ hybridization histochemistry using 35S-labeled riboprobes was performed. These results were compared with 5-HTT binding site distribution as evaluated by [125I]RTI-55 autoradiography. High levels of 5-HTT mRNA were detected in all brain stem raphe nuclei, with variations in labeling among the various subnuclei. Those brain areas known to possess serotonergic cell bodies stained intensely for both 5-HTT mRNA and 5-HTT binding sites. In contrast to previous findings in rat brain, the highest densities of 5-HTT sites were found in areas outside the raphe complex, particularly in the substantia nigra, globus pallidus, and superior colliculi.

Enhancement of [123I]beta-CIT binding in the striatum with clomipramine: is there a serotonin-dopamine interaction?

Many reports support the concept of serotonergic-dopaminergic interaction in the brain. However, at present, there are few methods to study this relationship in vivo. The purpose of this study was to investigate the effect of serotonin (5-HT) uptake inhibitor, clomipramine, on a dopamine (DA) transporter ligand, [123I]beta-CIT (RTI-55), in rat brain. Dose-dependent changes in [123I]beta-CIT specific binding induced by clomipramine were studied in the striatum (rich in DA transporter) and the hypothalamus (rich in 5-HT transporter). The changes in the time-activity curves of [123I]beta-CIT specific binding after clomipramine injection were also examined in these two regions. Using the cerebellum as the reference region, k3 and k4 values with and without clomipramine administration were estimated by a two-compartment kinetic analysis. Clomipramine inhibited [123I]beta-CIT specific binding in the hypothalamus, but enhanced its specific binding in the striatum in a dose-dependent manner. Kinetic analysis showed that k3 in the striatum was increased by 55%. In conclusion, enhancement of [123I]beta-CIT binding in the striatum after clomipramine administration indicated the possibility of 5-HT-DA interaction.

Modulation of the discriminative stimulus properties of cocaine: comparison of the effects of fluoxetine with 5-HT1A and 5-HT1B receptor agonists

The present investigation examined the ability of serotonin (5-HT) agonists to substitute for, or alter (i.e. enhance or antagonize), the discriminative stimulus properties of a moderately low dose of cocaine (5 mg/kg) utilizing a two-lever, water-reinforced FR 20 drug discrimination procedure in rats. In substitution tests, the 5-HT1A receptor partial agonists buspirone and gepirone, the 5-HT1A/B receptor agonist RU 24969 and the 5-HT1B/2C receptor agonist m-trifluoromethyl-phenylpiperazine (TFMPP) failed to substitute for the cocaine stimulus, although RU 24969 did engender a maximum of 72% cocaine-lever responding. Fluoxetine (4 mg/kg) engendered primarily saline-appropriate responding. In combination tests, a fixed dose of either fluoxetine (4 mg/kg), RU 24969 (0.5 mg/kg) or TFMPP (0.5 mg/kg) produced a leftward shift in the cocaine dose-response curve (0.313-5 mg/kg). In contrast, buspirone (2.5-20 mg/kg) resulted in a dose-dependent attenuation (approximately 60% reduction) of the cocaine stimulus. Moreover, a dose of 10 mg/kg of buspirone co-administered with various doses of cocaine (1.25-10 mg/kg) engendered a rightward shift in the cocaine dose-response curve. Gepirone in combination with cocaine neither enhanced nor antagonized the cocaine discriminative stimulus. Whereas 5-HT agonists do not fully substitute for cocaine, the present results demonstrate that 5-HT1B, but not 5-HT1A, receptor agonists can modulate the discriminative stimulus properties of cocaine in a manner similar to that observed following administration of the 5-HT reuptake inhibitor fluoxetine. The ability of buspirone, but not gepirone, to attenuate the cocaine stimulus probably reflects its dopamine (DA) D2 receptor antagonist properties and not its efficacy at 5-HT1A receptors.

An autoradiographic study of dextromethorphan high-affinity binding sites in rat brain: sodium-dependency and colocalization with paroxetine

1. The distribution and some pharmacological properties of centrally located dextromethorphan high-affinity binding sites were investigated by in vitro autoradiography. 2. Sodium chloride (50 mM) induced a 7 to 12 fold increase in dextromethorphan binding to rat brain in all areas tested. The effect of sodium was concentration-dependent with a higher dose (120 mM) exerting a smaller effect on binding. 3. [3H]-dextromethorphan binding in the presence of sodium was inhibited in the presence of the anticonvulsant phenytoin at a concentration of 100 microM, while the sigma ligand (+)-3-(-3-hydroxyphenyl)-N-(1-propyl)pipendine ((+)-PPP) had no effect on the binding, suggesting an interaction with the DM2 site. 4. The distribution of the sodium-dependent binding identified in this study correlated significantly with the distribution of the selective 5-HT uptake inhibitor [3H]-paroxetine, and paroxetine and dextromethorphan mutually displaced their binding at concentrations in the low nanomolar range. 5. These data show that dextromethorphan and paroxetine share a sodium-dependent high affinity binding site in rat brain, and suggest that dextromethorphan might interact, in the presence of sodium, with the 5-HT uptake mechanism in rat brain.

Polarized expression of the antidepressant-sensitive serotonin transporter in epinephrine-synthesizing chromaffin cells of the rat adrenal gland

Antidepressant-sensitive serotonin (5-hydroxytryptamine, 5HT) transporters (SERTs) clear the amine from extracellular spaces in the CNS and periphery as a mechanism for transmitter inactivation and recycling. Although it is known that SERTs are preferentially expressed on basolateral domains in transfected epithelial cells, details of the transporter's membrane localization in vivo are lacking. 5HT and 5HT receptors have been identified in the rodent adrenal gland. Using SERT antagonist autoradiography, we establish the presence of antidepressant-sensitive transport sites in the rat adrenal medulla. Immunofluorescence experiments using antibodies specific for the SERT COOH and NH2 termini, for 5HT, or for catecholamine biosynthetic enzymes suggest that SERT mediates intra-cellular 5HT accumulation by epinephrine-secreting chromaffin cells. Using confocal microscopy, we establish that SERT expression is nonuniformly distributed along the plasma membrane of chromaffin cells. Notably, SERT immunoreactivity is largely absent from plasma membranes bordering smooth muscle that surrounds vascular sinusoids. Rather, SERT is highly expressed in membranes adjoining other chromaffin cells, consistent with a role for 5HT and SERT in autocrine or paracrine control of chromaffin cell physiology. SNAP-25, a t-SNARE protein implicated in neurotransmitter release, was found to colocalize with SERT. In contrast, Na,K ATPase and NCAM are uniformly distributed along the entire perimeter of chromaffin cell membranes. These findings underscore a role for 5HT and SERT in adrenal physiology, reveal unrecognized polarity of chromaffin cell plasma membranes, and warrant a consideration of common targeting mechanisms localizing amine transporters near release sites.

Association of anxiety-related traits with a polymorphism in the serotonin transporter gene regulatory region

Transporter-facilitated uptake of serotonin (5-hydroxytryptamine or 5-HT) has been implicated in anxiety in humans and animal models and is the site of action of widely used uptake-inhibiting antidepressant and antianxiety drugs. Human 5-HT transporter (5-HTT) gene transcription is modulated by a common polymorphism in its upstream regulatory region. The short variant of the polymorphism reduces the transcriptional efficiency of the 5-HTT gene promoter, resulting in decreased 5-HTT expression and 5-HT uptake in lymphoblasts. Association studies in two independent samples totaling 505 individuals revealed that the 5-HTT polymorphism accounts for 3 to 4 percent of total variation and 7 to 9 percent of inherited variance in anxiety-related personality traits in individuals as well as sibships.

Effects of tianeptine, sertraline and clomipramine on brain serotonin metabolism: a voltammetric approach in the rat

Tianeptine is a substance enhancing the serotonir uptake while sertraline and clomipramine inhibit it. By means of 5-hydroxyin-doleacetic acid (5-HIAA) voltammetric measurements, this study investigated their influence on serotonin metabolism which depends mainly upon the activity of monoamine oxidase type A. After tianeptine injection the 5-HIAA signal increased by about 60%. This effect was maintained when the animals were pre-treated with MDL 72145 (an inhibitor of monoamine oxidase type B) but reduced when clorgyline (an inhibitor of monoamine oxidase type A) was administered after tianeptine. Administration of sertraline or clomipramine reduced the 5-HIAA signal by about 30-50%, whether the animals were pre-treated with MDL 72145 or not. It is to be concluded that tianeptine, sertraline and clomipramine can regulate the 5-HT fraction present in the synaptic cleft, not only by acting at the level of the serotoninergic neurons, but also by favoring or reducing the access of the amine to monoamine oxidase type A which is synthesized within non-serotoninergic neurons and glial cells.

Mapping of serotonin transporter messenger RNA-containing nerve cell populations in the cat brainstem

The anatomical distribution of nerve cells populations expressing serotonin transporter messenger RNA was investigated in the cat brain by means of in situ hybridization histochemistry. Formalin fixed coronal sections were hybridized with [35S]dATP 3' end-labeled oligoprobes complementary to three nucleotide sequences taken from the human and serotonin transporter. A strong hybridization signal was found in nerve cells populations exclusively localized within the brainstem. These positive cells mainly resided in the nuclei of the raphe, especially in the nuclei of the raphe dorsalis and raphe centralis superior. A small number of labeled cells was also observed in various areas including the dorsal part of the interpeduncular nucleus, in the midbrain, and the region ventrolateral to the inferior olive, the ventral midline and around the central canal, in the medulla oblongata. Overall, these data agree with the notion that in the cat, as previously suggested in the human and in the rat brain, the serotonin membrane transporter messenger RNA is predominantly expressed in areas known to contain serotonergic cell bodies.

Differential kinetics of [123I]beta-CIT binding to dopamine and serotonin transporters

Iodine-123-labelled 3beta-(4-iodophenyl)tropane-2beta-carboxylic acid ([123I]beta-CIT) labels both the dopamine transporter (DAT) and the serotonin transporter (5-HTT) and this ligand is able to clarify pathological changes in both dopaminergic and serotonergic systems. However, the differential kinetics of beta-CIT binding to DAT and 5-HTT has not been clarified fully. In this study we examined time-activity curves of [123I]beta-CIT in individual regions in the rat brain. Using cerebellum as the reference region, k3 and k4 values were estimated by a two-compartment kinetic analysis. In the striatum, the kinetics was slowest among all brain areas. In this area specific binding reached its peak 4 h after the injection. In the hypothalamus, specific binding reached its peak 1 h after the injection and its amount did not change until 4 h after the injection. In the occipital cortex, the binding and washout of the ligand were fastest among all brain regions. Estimated k3 values were 0.040+/-0.003 in the striatum, 0.019+/-0.002 in the hypothalamus and 0.082+/-0.011 in the occipital cortex (min-1, mean +/-SD). Estimated k4 values were 0.0034+/-0.0005 in the striatum, 0.0071+/-0.0009 in the hypothalamus and 0.083+/-0.013 in the occipital cortex (min-1, mean +/-SD). Therefore binding kinetics of [123i]beta-cit in the region rich in dat is apparently different from that in the region rich in 5-HTT. These results will provide fundamental data to image both DAT and 5-HTT in one series of examinations with [123I]beta-CIT.

Quantitative autoradiography of the serotonin transporter to assess the distribution of serotonergic projections from the dorsal raphe nucleus

The binding of 3H-CN-IMI to 5-HT uptake sites, as measured by quantitative autoradiography, was used as a marker of serotonergic neurons. Within the dorsal raphe nucleus the binding of 3H-CN-IMI was compared in adjacent coronal sections of rat brain to the binding of 3H-DPAT to 5-HT1A receptors, which have a known somatodendritic localization. The heterogeneous pattern of binding of these two radioligands within the dorsal raphe nucleus was similar and corresponded to the distribution of serotonergic cell bodies as visualized by 5-HT immunohistochemistry. Intracerebroventricular administration of 5,7-dihydroxytryptamine (5,7-DHT), which caused a dramatic loss of 5-HT immunoreactivity and 3H-DPAT binding to 5-HT1A receptors, resulted in a marked reduction of 3H-CN-IMI binding in this nucleus. Treatment of rats with a dose of para-chloroamphetamine (PCA) which has been reported to selectively lesion serotonergic processes arising from the dorsal raphe nucleus, while sparing serotonergic cell bodies and projections from the median raphe nucleus, did not alter the binding of 3H-DPAT or 3H-CN-IMI in the dorsal raphe nucleus; serotonergic cell bodies appeared morphologically unaffected. The lack of effect of PCA treatment on the binding of 3H-DPAT and 3H-CN-IMI is consistent with a somatodendritic localization of the 5-HT transporter in the dorsal raphe nucleus. PCA treatment appeared to produce a moderate loss of serotonergic innervation in serotonergic terminal field areas as visualized by serotonin immunohistochemistry. The reductions in 3H-CN-IMI binding observed in terminal field areas (24 to 69%) following treatment of rats with PCA did not reflect a marked differential innervation of forebrain areas by the dorsal and medial raphe nuclei as expected from previous biochemical studies, and were not entirely consistent with the findings of neuroanatomical studies using histochemical techniques. Site-specific injection of 5,7-DHT into the dorsal raphe nucleus produced an 80 +/- 11% reduction in the binding of 3H-CN-IMI in this nucleus, whereas the binding of 3H-CN-IMI in the median raphe nucleus was not reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic analysis of serotonin receptors and transporter in kindled rat brain

While serotonin (5-HT) has been shown to be anticonvulsant in several types of experimentally induced seizures, 5-HT receptor binding has not been investigated in the kindling model of epilepsy. The present study examined the effects of amygdala kindling on two 5-HT receptor subtypes and on the 5-HT transporter in rat brain. Kindling induced a persistent bilateral increase in 5-HT1A binding in the dentate gyrus, while 5-HT1B receptors increased only in a delayed fashion. Binding to the 5-HT transporter was transiently decreased in dentate gyrus. In cerebral cortex, binding of the three ligands was unchanged. Alterations in 5-HT receptors and the 5-HT transporter may endogenously modulate kindled seizures. Additionally, autoradiography of adenosine A1 receptors revealed no change for these receptors in any brain region.