Autoradiographic distribution of serotonin transporters and receptor subtypes in human brain

5-HT2A gene variants influence specific and different aspects of antidepressant response in Japanese and Italian mood disorder patients

The 5-HT2A receptor is a key modulator of the serotonin pathway. We previously observed a marginal association between 5-HT2A gene variants and antidepressant efficacy in Japanese and Italian population but in the opposite direction. In the present report, we hypothesize that discrepant findings on 5-HT2A gene variants could be due to both the effect of ethnicity and a possible specific effect on some symptom improvement. The sample comprised 203 patients affected by mood disorders and treated for major depression with paroxetine or fluvoxamine. The total depressive scores for all patients were analyzed in previous reports, but symptomatologic clusters were not examined previously. The 21-item Hamilton Rating Scale for Depression (HAM-D) was administered to evaluate depressive symptoms at baseline and bi-weekly over 6 weeks of treatment. All patients were genotyped for the 5-HT2A T102C polymorphism. Compared with patients with the 5-HT2A T and C variants, in the Japanese sample T allele carriers showed selective and slower score reductions than C allele carriers in delusion and activity symptoms; on the other hand, in the Italian sample, C allele carriers showed a slower and selective score reduction compared with T allele carriers in Somatic anxiety, while they did not differ from other patients on the other scores. Despite the limitations of the small sample size and modest significance levels, these findings suggest that response to SSRIs is not a unitary phenomenon and discrepant findings across ethnic groups may be due to differential effects of gene variants.

Association between triallelic polymorphism of the serotonin transporter and [18F]MPPF binding potential at 5-HT1A receptors in healthy subjects

Previous [(11)C]WAY100-635 PET studies have demonstrated that the short (S) and long (L) alleles of the serotonin transporter gene-linked polymorphic region (5-HTTLPR) were associated with distinct patterns of 5-HT(1A) receptor distribution in human. However, these studies reported discordant findings and did not take into account the recent description of two functional variants of the L allele (L(A)/L(G)). To further explore this issue, we investigated the triallelic functional polymorphism of the 5-HTTLPR in 38 healthy volunteers who underwent a [(18)F]MPPF PET study of 5-HT1A receptors. We used a simplified reference tissue model to generate parametric images of [(18)F]MPPF binding potential (BP(ND)), and compared these data among the different genotypes using statistical parametric mapping and region of interest of the raphe nuclei. Homozygote carriers of the S allele demonstrated greater [(18)F]MPPF BP(ND) than carriers of the L(A) allele, but this association was only found in women. Differences in [(18)F]MPPF BP(ND) between women with and without L(A) allele were observed over large clusters encompassing the right and left temporal lobes, cingulate and perisylvian regions, as well as the right precuneus and frontal dorso-lateral cortex, and the left orbitofrontal cortex. In contrast, no difference was found between groups in the raphe nuclei. The greater [(18)F]MPPF BP(ND) observed in women homozygote carriers of the S allele could either reflect a greater 5-HT1A receptor density or a lower extracellular concentration of 5-HT. Our data suggest that any future PET studies of 5-HT1A receptors should incorporate the 5-HTTLPR polymorphism status of the population studied.

Imaging the serotonin transporter using (123)I-ADAM in the human brain

The aim of this study was to examine the feasibility of (123)I-ADAM to image the serotonin transporter (SERT) in Asian (Taiwanese) subjects. Single photon emission computed tomography (SPECT) scans were performed on nine healthy volunteers who were s-allele carriers at the polymorphism within the serotonin transporter promoter region (SERTPR) after intravenous bolus injection of (123)I-ADAM. Quantification of (123)I-ADAM binding was performed using the ratio equilibrium method (REM) with specific uptake ratio (SUR) and a simplified reference tissue model (SRTM). Curve-fitting techniques were used to obtain the peak equilibrium point from 241 to 301 min (average 264+/-20 min) after injection of (123)I-ADAM for the midbrain and from 215 to 270 min (average 235+/-18 min) after injection of (123)I-ADAM for the striatum. Two sets of SUR were obtained by either curve fitting (estimated values) or integrated period from 240 to 270 min (observed values). The estimated values of SUR were 2.11+/-0.51 for the midbrain and 1.50+/-0.44 for the striatum, whereas the observed values were 2.11+/-0.83 for the midbrain and 1.24+/-0.31 for the striatum. The SRTM showed that the binding potential (BP) was 2.10+/-0.66 for the midbrain and 1.35+/-0.25 for the striatum. There was a good correlation between estimated SUR, observed SUR and SRTM in the midbrain but not in the striatum. The optimal scanning duration for both the midbrain and the striatum should be 220 to 280 min similar to that suggested by previous studies in Caucasians. However, due to the low signal-to-noise ratio in the striatum, (123)I-ADAM could be an ideal tracer for imaging SERT in the midbrain but not in the striatum.

Current considerations in the treatment of generalized anxiety disorder

Generalized anxiety disorder (GAD) is a chronic disorder that frequently co-occurs with a variety of co-morbidities in patients with somatic conditions and other mental disorders. GAD is highly prevalent and is one of the most common anxiety disorders seen by primary care physicians. The individual and societal cost associated with GAD is high and the marked level of impairment experienced by patients with this disorder is equivalent in magnitude to that reported in patients with major depressive disorder. Furthermore, patients with GAD are at risk of suicide or suicide attempts, and are frequent users of healthcare services. Thus, GAD is a serious and chronic condition that requires appropriate long-term treatment. The focus of acute treatment for patients with GAD is the improvement of symptoms, while the primary goal of long-term clinical management is remission, i.e. the complete resolution of both symptoms and functional impairment. The consensus across current treatment guidelines is that first-line treatment for patients with GAD should consist of an antidepressant, either a selective serotonin reuptake inhibitor (SSRI) such as sertraline, paroxetine or escitalopram, or a selective serotonin noradrenaline (norepinephrine) reuptake inhibitor (SNRI) such as venlafaxine or duloxetine. However, the SSRIs and SNRIs have efficacy limitations, such as lack of response in many patients, a 2- to 4-week delay before the onset of symptom relief, lack of full remission, and risk of relapse. In addition, there are troublesome adverse effects associated with both the SSRIs and SNRIs. Evidence from early clinical studies of the atypical antipsychotics in the treatment of anxiety and GAD indicate that they may have a potential role in the treatment of GAD, either as monotherapy or as augmentation to standard treatment.

Role of variation in the serotonin transporter protein gene (SLC6A4) in trait disturbances in the ventral anterior cingulate in bipolar disorder

Bipolar disorder (BD) is associated with abnormalities of the ventral anterior cingulate cortex (vACC) and its connection sites, including the amygdala, which are key components of a corticolimbic neural system that subserves emotional regulation. Decreased functional connectivity from the vACC to the amygdala in healthy individuals is associated with the short 's' allele--as opposed to the long 'l' allele--of a well-known serotonin transporter promoter polymorphism (5-HTTLPR, locus SLC6A4), as are features of BD. This study tests the hypothesis that the s allele influences dysfunction in the vACC-amygdala neural system in BD. A total of 30 euthymic individuals with BD (20 s carriers, 10 ll) and 48 healthy comparison (HC) participants (34 s, 14 ll) participated in an event-related functional magnetic resonance imaging scan while processing fearful, happy, or neutral faces. During fear and happy face processing, vACC activation was significantly lower in the BD compared to the HC group, and in s carriers compared to ll individuals within both the HC and BD groups, such that BD s carriers exhibited the greatest magnitude of vACC dysfunction. No significant differences were detected in amygdala activation. The findings suggest that the 5-HTTLPR s allele may contribute to a trait-related, genetically derived, neurobiological subgroup within BD characterized by prominent vACC dysfunction. Future treatment may be optimized for this BD subgroup by targeting the serotonergic system and the vACC.

Neuroimaging in human MDMA (Ecstasy) users

MDMA (3,4 methylenedioxymethamphetamine) has been used by millions of people worldwide as a recreational drug. The terms "MDMA" and "Ecstasy" are often used synonymously, but it is important to note that the purity of Ecstasy sold as MDMA is not certain. MDMA use is of public health concern, not so much because MDMA produces a common or severe dependence syndrome, but rather because rodent and nonhuman primate studies have indicated that MDMA (when administered at certain dosages and intervals) can cause long-lasting reductions in markers of brain serotonin (5-HT) that appear specific to fine-diameter axons arising largely from the dorsal raphe nucleus (DR). Given the popularity of MDMA, the potential for the drug to produce long-lasting or permanent 5-HT axon damage or loss, and the widespread role of 5-HT function in the brain, there is a great need for a better understanding of brain function in human users of this drug. To this end, neuropsychological, neuroendocrine, and neuroimaging studies have all suggested that human MDMA users may have long-lasting changes in brain function consistent with 5-HT toxicity. Data from animal models leads to testable hypotheses regarding MDMA's effects on the human brain. Because neuropsychological and neuroimaging findings have focused on the neocortex, a cortical model is developed to provide a context for designing and interpreting neuroimaging studies in MDMA users. Aspects of the model are supported by the available neuroimaging data, but there are controversial findings in some areas and most findings have not been replicated across different laboratories and using different modalities. This paper reviews existing findings in the context of a cortical model and suggests directions for future research.

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.

MAO A VNTR polymorphism and variation in human morphology: a VBM study

The X-linked monoamine oxidase A (MAO A) gene, coding for an enzyme especially involved in the serotonin catabolism, presents a well-characterized functional polymorphism (long and short variants) in the promoter region that alters the transcriptional activity of the gene and hence the function of the corresponding proteins. Using optimized voxel-based morphometry, we studied the effect of this functional polymorphism on brain morphology in normal individuals. Fifty-nine male healthy individuals (33 MAO A-high and 26 MAO A-low) were investigated. Voxel-based morphometry showed that the carriers of the long variant were significantly associated with loss of grey matter in orbitofrontal cortex, bilaterally. This study reveals pronounced genotype-related structural changes in a specific prefrontal region previously observed to mediate neurofunctional responses in behavioral tasks.

The effect of acute tryptophan depletion on the neural correlates of emotional processing in healthy volunteers

The processing of affective material is known to be modulated by serotonin (5-HT), but few studies have used neurophysiological measures to characterize the effect of changes in 5-HT on neural responses to emotional stimuli. We used functional magnetic resonance imaging to investigate the effect of acute tryptophan depletion, which reduces central 5-HT synthesis, on neural responses to emotionally valenced verbal stimuli. Though no participants experienced significant mood change, emotional information processing was substantially modified following 5-HT depletion. A behavioral bias toward positive stimuli was attenuated following depletion, which was accompanied by increased hemodynamic responses during the processing of emotional words in several subcortical structures. Inter-individual differences in tryptophan depletion-elicited anxiety correlated positively with the caudate bias toward negative stimuli. These data suggest that 5-HT may play an important role in mediating automatic negative attentional biases in major depression, as well as resilience against negative distracting stimuli in never-depressed individuals.

Transcriptional regulation at a HTR1A polymorphism associated with mental illness

The serotonin-1A (5-HT1A) receptor serves as a hub to regulate the activity and actions of the serotonin system, and is expressed both as a presynaptic autoreceptor on raphe neurons, and as a major postsynaptic receptor in hippocampal, cortical, and hypothalamic regions involved in mood, emotion and stress response. As such, the level of expression of 5-HT1A receptors is implicated in the development of anxiety and depression phenotypes. This review focuses on the C(-1019)G (rs6295) promoter polymorphism of the 5-HT1A receptor gene (HTR1A) and its effect on the activity of transcription factors that recognize the C-allele, including Deaf-1, Hes1 and Hes5; its effects on 5-HT1A receptor expression in pre- and postsynaptic areas; as well as its implication in early postnatal development and adult neurogenesis in the hippocampus and cortex. Although several studies have now replicated the association of the G-allele with depression, panic disorder, neuroticism, and reduced response to antidepressant or antipsychotic treatment, ethnic, disease and genetic heterogeneity among subjects in different studies may obscure such associations. Gene-gene interaction studies suggest that the 5-HT1A receptor G(-1019) allele is a risk allele which could be used as a marker for depression and related mood disorders. Finally, association of the G(-1019) allele with increased raphe 5-HT1A binding potential, increased amygdala reactivity to emotional stimuli, and reduced amygdala volume, particularly in disease states, suggests a functional role for the C(-1019)G site in 5-HT1A receptor dys-regulation and predisposition to mental illness.

Cytology and receptor architecture of human anterior cingulate cortex

Anterior cingulate cortex (ACC) is involved in emotion, mood, and autonomic regulation. Although a subgenual part of ACC (sACC) may be vulnerable in depression and area 25 is cytologically unique, there are no assessments that contrast this region with pregenual ACC (pACC). Thus, we undertook independent multimodal verifications of architectural differences among subregions and areas. Areas 24a and 24b have pregenual and subgenual components. The latter have a thin layer III. Area 24c has dorsal (pd24c) and ventral (pv24c) parts. Area pd24c has larger neurofilament-expressing neurons in layer Va, and neurons in Vb form aggregates in area pv24c. Area pd24c occupies both banks of the cingulate sulcus, with pv24c on the ventral bank. Layer III of pd24cd has many larger neurofilament-expressing neurons and a richer dendritic plexus. Area 32 has pregenual (p32) and subgenual (s32) components. Layer II in s32 is of particular note because it has a neuron-dense IIa and sparse IIb. Area 25 has anterior (25a) and posterior (25p) parts; 25p has the thinnest layer III in the cingulate gyrus. Area 25a contains significantly higher AMPA, kainate, NMDA, GABA(A), GABA(B), and alpha(1) densities than 25p. Area 33 continues around the genu and ventrally to encompass the full caudal extent of area 25. Subgenual ACC has significantly higher GABA(A), GABA(B), benzodiazepine (BZ), alpha(1), and 5-HT(1A) densities than pACC. GABA(B), BZ, and alpha(1) binding confirms the subdivision of area pd24c. In conclusion, ACC comprises two parts that are unique in terms of their cytoarchitecture and neurotransmitter receptor organization.

Dystrophic serotonergic axons in neurodegenerative diseases

Neurodegenerative diseases such as Parkinson's disease (PD), frontal lobe dementia (FLD) and diffuse Lewy-body dementia (DLBD) have diverse neuropathologic features. Here we report that serotonin fibers are dystrophic in the brains of individuals with these three diseases. In neuropathologically normal (control) brains (n=3), serotonin axons immunoreactive (IR) with antibodies against the serotonin transporter (5-HTT) protein were widely distributed in cortex (entorhinal and dorsolateral prefrontal), hippocampus and rostral brainstem. 5-HTT-IR fibers-of-passage appeared thick, smooth, and unbranched in medial forebrain bundle, medial lemniscus and cortex white matter. The terminal branches were fine, highly branched and varicose in substantia nigra, hippocampus and cortical gray matter. In the diseased brains, however, 5-HTT-IR fibers in the forebrain were reduced in number and were frequently bulbous, splayed, tightly clustered and enlarged. Morphometric analysis revealed significant differences in the size distribution of the 5-HTT-IR profiles in dorsolateral prefrontal area between neurodegenerative diseases and controls. Our observations provide direct morphologic evidence for degeneration of human serotonergic axons in the brains of patients with neurodegenerative diseases despite the limited size (n=3 slices for each region (3) from each brain (4), total slices was n=36) and the lack of extensive clinical characterization of the analyzed cohort. This is the first report of dystrophic 5-HTT-IR axons in postmortem human tissue.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Evaluation of the Serotonin Transporter Ligand 123I-ADAM for SPECT Studies on Humans

Imaging serotonin transporters in the living human brain is important in several fields, such as normal psychophysiology, mood disorders, eating disorders, and neurodegenerative disorders. The aim of this study was to compare different kinetic and semiquantitative methods for assessing serotonin transporters using (123)I-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM) in humans: an arterial plasma input model, simplified and Logan reference tissue models, and standardized uptake value ratios.

METHODS

Nine subjects were scanned with dynamic (123)I-ADAM SPECT (mean age, 31 y; range, 24-43 y), and metabolite-corrected arterial input was measured. Tissue reference models (simplified reference tissue model, Logan reference tissue model, and ratio method) were validated against the outcome of a 1-tissue-compartment model, and performance with decreasing scan length was evaluated. The specificity of (123)I-ADAM binding was investigated in a blocking experiment.

RESULTS

Binding estimates from the simplified reference tissue and Logan reference tissue models correlated tightly with full kinetic modeling when based on a 240- or 360-min dynamic acquisition (r = 0.99); however, there were slight underestimations (3%-5%), especially in high-binding regions. Application of the ratio method to data from 200 to 240 min overestimated specific binding (on average, by 10% +/- 28%) and correlated only moderately with estimates from the 1-tissue-compartment model (r = 0.94). With an acquisition time of 0-120 min, the Logan model still yielded an acceptable outcome when a fixed clearance rate constant (k2') from the cerebellum was applied. Intravenously injected citalopram was not associated with a decrease in cerebellar binding. A lipophilic metabolite that did not seem to bind specifically to serotonin transporter was seen in 2 of 7 subjects.

CONCLUSION

Serotonin transporter binding with (123)I-ADAM SPECT can be assessed with the Logan model based on a 120-min acquisition when a constant k2' is applied. This model, because it allows for more accurate and less biased binding estimates and thus reduces the required sample size, is advantageous over the ratio method used in clinical studies so far. A single blocking experiment supported the use of the cerebellum as a reference region.

Genetically dependent modulation of serotonergic inactivation in the human prefrontal cortex

Previous research suggests that genetic variations regulating serotonergic neurotransmission mediate individual differences in the neural network underlying impulsive and aggressive behaviour. Although with conflicting findings, the monoamine oxidase-A (MAOA) and the serotonin transporter (5HTT) gene polymorphisms have been associated with an increased risk to develop impulsive and aggressive behaviour. Double knock-out mice studies have also demonstrated that MAOA and 5HTT genes strongly interact in the metabolic pathway leading to the serotonergic inactivation; however, their potential interactive effect in human brain remains uninvestigated. We used blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to assess the independent and interactive effects of both MAOA and 5HTT polymorphisms on the brain activity elicited by a response inhibition task in healthy volunteers. Multivariate analysis demonstrated an individual effect of both MAOA and 5HTT polymorphisms and a strong allele-allele interaction in the anterior cingulate cortex (ACC), a key region implicated in cognitive control and in the pathophysiology of impulsive and aggressive behaviour. These findings suggest that the MAOAx5HTT allelic interaction exerts a significant modulation on the BOLD response associated with response inhibition and contribute to validate haplotype models as useful tools for a better understanding of the neurobiology underlying complex cognitive functions.

Serotonin differentially regulates short- and long-term prediction of rewards in the ventral and dorsal striatum

Background

The ability to select an action by considering both delays and amount of reward outcome is critical for maximizing long-term benefits. Although previous animal experiments on impulsivity have suggested a role of serotonin in behaviors requiring prediction of delayed rewards, the underlying neural mechanism is unclear.

Methodology/Principal Findings

To elucidate the role of serotonin in the evaluation of delayed rewards, we performed a functional brain imaging experiment in which subjects chose small-immediate or large-delayed liquid rewards under dietary regulation of tryptophan, a precursor of serotonin. A model-based analysis revealed that the activity of the ventral part of the striatum was correlated with reward prediction at shorter time scales, and this correlated activity was stronger at low serotonin levels. By contrast, the activity of the dorsal part of the striatum was correlated with reward prediction at longer time scales, and this correlated activity was stronger at high serotonin levels.

Conclusions/Significance

Our results suggest that serotonin controls the time scale of reward prediction by differentially regulating activities within the striatum.

Elevated serotonin transporter binding in major depressive disorder assessed using positron emission tomography and [11C]DASB; comparison with bipolar disorder

BACKGROUND

Altered serotonergic function is thought to play a role in the pathophysiology of major depressive episodes based upon evidence from neuroimaging, pharmacological, postmortem and genetic studies. It remains unclear, however, whether depressed samples that differ with respect to having shown a unipolar versus a bipolar illness course also would show distinct patterns of abnormalities within the serotonergic system. The current study compared serotonin transporter (5-HTT) binding between unipolar-depressives (MDD), bipolar-depressives (BD) and healthy-controls (HC) to assess whether the abnormalities in 5-HTT binding recently found in depressed subjects with BD extend to depressed subjects with MDD.

METHODS

The 5-HTT binding-potential (BP) measured using positron emission tomography (PET) and [(11)C]DASB was compared between unmedicated, depressed subjects with MDD (n = 18) or BD (n = 18) and HC (n = 34).

RESULTS

Relative to the healthy group both MDD and BD groups showed significantly increased 5-HTT BP in the thalamus (24%, 14%, respectively), insula (15%) and striatum (12%). The unipolar-depressives had elevated 5-HTT BP relative to both BD and HC groups in the vicinity of the periaqueductal gray (PAG, 20%, 22%, respectively). The bipolar-depressives had reduced 5-HTT BP relative to both HC and MDD groups in the vicinity of the pontine raphe nuclei. Depression-severity correlated negatively with 5-HTT BP in the thalamus in MDD-subjects.

CONCLUSIONS

The depressed phases of MDD and BD both were associated with elevated 5-HTT binding in the insula, thalamus and striatum, but showed distinct abnormalities in the brainstem. The latter findings conceivably could underlie differences in the patterns of illness symptoms and pharmacological sensitivity observed between MDD and BD.

Brain imaging correlates of depressive symptom severity and predictors of symptom improvement after antidepressant treatment

BACKGROUND

It would be therapeutically useful to predict clinical response to antidepressant drugs. We evaluated structural magnetic resonance imaging (MRI) and functional MRI (fMRI) data as predictors of symptom change in people with depression.

METHODS

Brain structure and function were measured with MRI in 17 patients with major depression immediately before 8 weeks treatment with fluoxetine 20 mg/day. For fMRI, patients were scanned during visual presentation of faces representing different intensities of sadness. Clinical response was measured by change in serial scores on the Hamilton Rating Scale for Depression. Symptom change scores (and baseline symptom severity) were regressed on structural and functional MRI data to map brain regions where grey matter volume, or activation by sad facial affect processing, was significantly associated with symptom change (or baseline severity).

RESULTS

Faster rates of symptom improvement were strongly associated with greater grey matter volume in anterior cingulate cortex, insula, and right temporo-parietal cortex. Patients with greater than median grey matter volume in this system had faster rates of improvement and significantly lower residual symptom scores after 8 weeks' treatment. Faster improvement was also predicted by greater functional activation of anterior cingulate cortex. Baseline symptom severity was negatively correlated with greater grey matter volume in dorsal prefrontal and anterior midcingulate regions anatomically distinct from the pregenual and subgenual cingulate regions predicting treatment response.

CONCLUSIONS

Structural MRI measurements of anterior cingulate cortex could provide a useful predictor of antidepressant treatment response.

Autoradiography reveals selective changes in serotonin binding in neocortex of patients with temporal lobe epilepsy

The main goal of the present study was to evaluate binding to serotonin in the neocortex surrounding the epileptic focus of patients with mesial temporal lobe epilepsy (MTLE). Binding to 5-HT, 5-HT(1A), 5-HT(4), 5-HT(7) receptors and serotonin transporter (5-HTT) in T1-T2 gyri of 15 patients with MTLE and their correlations with clinical data, neuronal count and volume were determined. Autopsy material acquired from subjects without epilepsy (n=6) was used as control. The neocortex from MTLE patients demonstrated decreased cell count in layers III-IV (21%). No significant changes were detected on the neuronal volume. Autoradiography experiments showed the following results: reduced 5-HT and 5-HT(1A) binding in layers I-II (24% and 92%, respectively); enhanced 5-HT(4) binding in layers V-VI (32%); no significant changes in 5-HT(7) binding; reduced 5-HTT binding in all layers (I-II, 90.3%; III-IV, 90.3%, V-VI, 86.9%). Significant correlations were found between binding to 5-HT(4) and 5-HT(7) receptors and age of seizure onset, duration of epilepsy and duration of antiepileptic treatment. The present results support an impaired serotoninergic transmission in the neocortex surrounding the epileptic focus of patients with MTLE, a situation that could be involved in the initiation and propagation of seizure activity.

Monoamine transporters and psychostimulant addiction

Psychostimulants are a broadly defined class of drugs that stimulate the central and peripheral nervous systems as their primary pharmacological effect. The abuse liability of psychostimulants is well established and represents a significant public health concern. An extensive literature documents the critical importance of monoamines (dopamine, serotonin and norepinephrine) in the behavioral pharmacology and addictive properties of psychostimulants. In particular, the dopamine transporter plays a primary role in the reinforcing and behavioral-stimulant effects of psychostimulants in animals and humans. Moreover, both serotonin and norepinephrine systems can reliably modulate the neurochemical and behavioral effects of psychostimulants. However, there is a growing body of evidence that highlights complex interactions among additional neurotransmitter systems. Cortical glutamatergic systems provide important regulation of dopamine function, and inhibitory amino acid gamma-aminobutyric acid (GABA) systems can modulate basal dopamine and glutamate release. Repeated exposure to psychostimulants can lead to robust and enduring changes in neurobiological substrates, including monoamines, and corresponding changes in sensitivity to acute drug effects on neurochemistry and behavior. Significant advances in the understanding of neurobiological mechanisms underlying psychostimulant abuse and dependence have guided pharmacological treatment strategies to improve clinical outcome. In particular, functional agonist treatments may be used effectively to stabilize monoamine neurochemistry, influence behavior and lead to long-term abstinence. However, additional clinical studies are required in order to identify safe and efficacious pharmacotherapies.

Architectonic distribution of the serotonin transporter within the orbitofrontal cortex of the vervet monkey

To elucidate the organization of the serotoninergic innervation within the orbitofrontal cortex (OFC), serotonin transporter (SERT) density was quantified by autoradiography using [(3)H]cyanoimipramine binding. In six adult vervet monkeys, 15 architectonic areas were delineated according to cytoarchitectonic (Nissl), myeloarchitectonic (Gallyas) and chemoarchitectonic (acetylcholinesterase) criteria to assess SERT distribution at two levels of organization: cortical area and cortical type. For cortical type, the 15 areas were evenly divided into three different categories primarily based upon the degree of granularization of layer IV: agranular, dysgranular, and granular. Within agranular and dysgranular, but not granular cortical types, SERT density was area-specific and progressively decreased in a medial to lateral gradient. Across cortical types, SERT density decreased in a caudal to rostral gradient: agranular>dysgranular>granular. A similar caudal to rostral gradient was seen when serotonin content was measured (using high performance liquid chromatography) in areas representative of each cortical type. Collectively, these results suggest that the serotoninergic innervation is organized according to both cortical type and area, and is thus structured to differentially modulate information processing within the OFC.

Imaging genetics for neuropsychiatric disorders

Many neuropsychiatric disorders of childhood and adolescence have a strong genetic component, and all present challenging questions about the neural abnormalities that underlie complex and unique behavioral and cognitive phenotypes. A useful research strategy in this setting is imaging genetics, a relatively new approach that combines genetic assessment with multimodal neuroimaging to discover neural systems linked to genetic abnormalities or variation. In this article, the authors review this strategy as applied to two areas. First, the authors present results on dissecting neural mechanisms underlying the complex neuropsychiatric phenotype of Williams syndrome. Second, they examine neural systems that are linked to candidate gene genetic variation that mediate risk for psychiatric disorders in a gene by environmental interaction. These data provide convergent evidence for neural circuitry mediating emotional regulation and social cognition under genetic control in humans.

Differences in Cortical Serotonergic Innervation among Humans, Chimpanzees, and Macaque Monkeys: A Comparative Study

In this study, we assess the possibility that the evolution of human intellectual capacities was supported by changes in the supply of serotonin to the frontal cortex. To this end, quantitative comparative analyses were performed among humans, chimpanzees, and macaques. Immunohistochemical methods were used to visualize serotonin transporter-immunoreactive (SERT-ir) axons within the cerebral cortex. Areas 9 and 32 were chosen for evaluation due to their roles in working memory and theory of mind, respectively. Primary motor cortex was also evaluated because it is not associated with higher cognitive functions. The findings revealed that humans do not display a quantitative increase in serotonin innervation. However, the results indicated region- and layer-specific differences among species in serotonergic innervation pattern. Compared with macaques, humans and chimpanzees together displayed a greater density of SERT-ir axons relative to neuron density in layers V/VI. This change was detected in cortical areas 9 and 32, but not in primary motor cortex. Further, morphological specializations, coils of axons, were observed in humans and chimpanzees that were absent in macaques. These features may represent a greater capacity for cortical plasticity exclusive to hominoids. Taken together, these results indicate a significant reorganization of cortical serotonergic transmission in humans and chimpanzees.

Synthesis and initial evaluation of [11C](R)-RWAY in monkey-a new, simply labeled antagonist radioligand for imaging brain 5-HT1A receptors with PET

PURPOSE

We aimed to fulfill a need for a radioligand that may be simply labeled with carbon-11 for effective positron emission tomography (PET) imaging of brain 5-HT(1A) receptors.

METHODS

Racemic RWAY (2,3,4,5,6,7-hexahydro-1-[4-[1-[4-(2-methoxyphenyl)piperazinyl]]-2-phenylbutyryl]-1H-azepine) has high affinity for 5-HT(1A) receptors. The enantiomers of RWAY and O-desmethyl-RWAY, synthesized from commercially available materials, were each labeled with carbon-11 by treating the respective O-desmethyl precursor with [(11)C]iodomethane, and injected into rhesus monkey for measurement of regional brain uptake. The 5-HT(1A) selectivity of (R)-[(11)C]RWAY was checked by administering WAY-100635, before and after radioligand administration. Radiometabolites of (R)-[(11)C]RWAY in blood and urine were analyzed by HPLC with partial elucidation of their structures by LC-MS-MS.

RESULTS

(R)-[(11)C]RWAY was a 5-HT(1A) receptor antagonist exhibiting high brain uptake with regional distribution consistent with specific binding to 5-HT(1A) receptors. The similar affinity, (S)-[(11)C]RWAY was a weak partial agonist at 5-HT(1A) receptors exhibiting similar brain peak uptake with much less 5-HT(1A) receptor-specific binding. The maximal ratio in receptor-rich cingulate gyrus to receptor-devoid cerebellum reached 6.4 at 87.5 min after injection of (R)-[(11)C]RWAY. After treatment with WAY-100635 before or after (R)-[(11)C]RWAY administration, radioactivity levels in 5-HT(1A) receptor-rich regions were reduced almost to that in cerebellum. Blood and urine radiometabolites were less lipophilic than parent and were not due to hydrolysis but to ring hydroxylations, oxidation, and dephenylation.

CONCLUSION

(R)-[(11)C]RWAY is simply prepared and an effective antagonist for imaging brain 5-HT(1A) receptors. This radioligand resists hydrolysis in vivo, gives less lipophilic radiometabolites, and warrants further PET studies in human subjects.

Reduced serotonin-1A receptor binding in social anxiety disorder

BACKGROUND

Results from studies in serotonin-1A (5-HT1A) knockout mice and previous positron emission tomography (PET) studies in humans imply a role for 5-HT1A receptors in normal state anxiety as well as in certain anxiety disorders. The objective of this study was to investigate 5-HT1A receptor binding potential (BP) in social anxiety disorder (SAD).

METHODS

Using PET and [carbonyl-11C]WAY-100635, we compared a homogeneous group of 12 unmedicated, male SAD patients with 18 healthy control subjects (HC). A multivariate ANOVA with all regional BP values as dependent variables, age and four radiochemical variables as covariates was performed.

RESULTS

We found a significantly lower 5-HT1A BP in several limbic and paralimbic areas but not in the hippocampus (p = .234) of SAD patients. The difference in 5-HT1A binding was most significant in the amygdala (-21.4%; p = .003). There was also a more than 20% lower 5-HT(1A) BP of SAD patients in the anterior cingulate cortex (p = .004), insula (p = .003), and dorsal raphe nuclei (p = .030).

CONCLUSIONS

The lower 5-HT1A binding in the amygdala and mesiofrontal areas of SAD patients is consistent with 1) preclinical findings of elevated anxiety in 5-HT1A knockout mice, 2) a previous PET study in healthy volunteers showing an inverse correlation between 5-HT1A BP and state anxiety, and 3) another human PET study in patients with panic disorder showing reduced 5-HT1A binding, thus corroborating the potential validity of 5-HT1A receptors as targets in the treatment of human anxiety disorders.

Serotonin and the Evaluation of Future Rewards: Theory, Experiments, and Possible Neural Mechanisms

The ability to select an action by considering both delays and amount of reward outcome is critical for survival and well-being of animals and humans. Previous animal experiments suggest a role of serotonin in action choice by modulating the evaluation of delayed rewards. It remains unclear, however, through which neural circuits, and through what receptors and intracellular mechanisms, serotonin affects the evaluation of delayed rewards. Here, we review experimental studies and computational theory of decisions under delayed rewards, and propose that serotonin controls the timescale of reward prediction by regulating neural activity in the basal ganglia.

GABAergic neurons immunoreactive for calcium binding proteins are reduced in the prefrontal cortex in major depression

Post-mortem morphometric studies report reductions in the average density and size of cortical neurons in the dorsolateral prefrontal cortex (dlPFC) and orbitofrontal cortex (ORB) in major depressive disorder (MDD). The contribution of specific neuronal phenotypes to this general pathology in depression is still unclear. Post-mortem sections from the dlPFC and ORB regions of 14 subjects with MDD and 11 controls were immunostained to visualize calbindin-immunoreactive (CB-IR) and parvalbumin-immunoreactive (PV-IR) presumptive GABAergic neurons. A three-dimensional cell counting probe was used to assess the cell packing density and size of CB-IR neurons in layers II+IIIa and PV-IR neurons in layers III-VI. The density of CB-IR neurons was significantly reduced by 50% in depression in the dlPFC and there was a trend toward reduction in the ORB. The size of CB-IR somata was significantly decreased (18%) in depression in the dlPFC with a trend toward reduction in the ORB. In contrast, there was no difference in the density of PV-IR neurons between the depressed and control groups in the dlPFC. The size of PV-IR neuronal soma was unchanged in depressed compared to control subjects in either dlPFC or ORB. In depression, subpopulations of GABAergic neurons may be affected differently in dlPFC and ORB. A significant reduction in the density and size of GABAergic interneurons immunoreactive for calcium binding proteins was found predominantly in the dlPFC region. These cellular changes are consistent with recent neuroimaging studies revealing a reduction in the cortical levels of GABA in depression.

Neuroimaging research in human MDMA users: a review

RATIONALE

Determining whether, under what circumstances, and to what extent 3,4-methylenedioxymethamphetamine (MDMA) exposure produces chronic changes in human brain function is a critical public health issue. MDMA is a widely used recreational drug commonly sold as "Ecstasy". Because findings from the animal literature have indicated that specific dosage regimens of MDMA can produce long-lasting alterations in serotonergic function, existing studies of MDMA effects in humans have examined brain serotonin (5-HT) transporters (5-HTT) and receptors or have examined brain structures or functions potentially affected by MDMA.

OBJECTIVES

The objectives of this review are to provide a background for interpreting human MDMA neuroimaging research, to examine existing neuroimaging data regarding the rationale for and limitations to human MDMA research, and to provide suggestions for improving the design and interpretation of future neuroimaging approaches.

RESULTS

Of the existing neuroimaging studies in human MDMA users, few experimental designs have been replicated across different research groups. Only investigations employing nuclear imaging methods to assay brain 5-HTT levels have been replicated across methods and research laboratories. These studies have found reduced levels of the 5-HTT in recently abstinent MDMA users with some evidence for normalization of 5-HTT levels with prolonged abstinence. However, the sensitivity of these methods is unknown.

CONCLUSIONS

The current state of neuroimaging in human MDMA users does not permit conclusions regarding the long-term effects of MDMA exposure. Future study designs might benefit from improved sample homogeneity, increased length of MDMA abstinence, longitudinal study design, test-retest measures, serotonergic specificity, and multimodal approaches.

Effects of acute tryptophan depletion on mood and facial emotion perception related brain activation and performance in healthy women with and without a family history of depression

The present study examined the effects of acute tryptophan (Trp) depletion (ATD), a well-recognized method to lower central serotonin (5-HT) metabolism, on brain activation during a facial emotion perception task. Brain activation was measured using fMRI, and healthy female volunteers with a positive family history of unipolar depression (FH+) were compared to healthy female volunteers without such a history (FH-). Participants viewed two morphed faces and were instructed to choose between the faces based either on the intensity of the emotional expression (direct task) or the gender of the face (incidental task). In the FH+ group, depletion led to the expected lowering of mood, which partly determined the effect of depletion on performance and brain activation. A stronger mood lowering effect was associated with less accurate performance on faces expressing a negative emotion in the incidental task and a stronger right amygdala response to fearful faces in comparison to happy faces. These results were explained in terms of a mood-induced bias leading to a stronger impact of the expressed negative emotion which subsequently leads to more interference in the incidental task and a stronger amygdala response. It was concluded that the effects of ATD on mood, performance, and brain activation in a facial emotion perception task depend on family history of depression. Performance and brain activation partly depend on the effect of ATD on mood.

A comparison of the effects of citalopram and moclobemide on resting brain perfusion in social anxiety disorder

INTRODUCTION

The serotonin specific reuptake inhibitor (SSRI) citalopram and the reversible mono-amine oxidase-A inhibitor (RIMA) moclobemide have both been used successfully for the treatment of social anxiety disorder (SAD). In this study we investigate the effects of these compounds on resting brain function using single photon emission computed tomography (SPECT).

METHODS

Subjects meeting DSM-IV criteria for SAD underwent regional cerebral blood flow (rCBF) SPECT using Tc-HMPAO at baseline and after 8 weeks of treatment with either citalopram or moclobemide. Using statistical parametric mapping brain SPECT studies were analysed to determine the effects of treatment on rCBF, to compare the effects of citalopram and moclobemide, and to detect correlations between changes in rCBF and clinical response.

RESULTS

Subjects received citalopram (n=17) or moclobemide (n=14) as therapy. Subjects in both treatment groups demonstrated a significant improvement of SAD symptoms as measured by the Liebowitz Social Anxiety Scale total score. All subjects demonstrated a decrease in rCBF in the insulae post therapy. Subjects receiving citalopram had decreased superior cingulate rCBF after therapy compared to those receiving moclobemide.

CONCLUSION

Both SSRI's and RIMA's decreased rCBF in the insulae during treatment of SAD; an effect that may be consistent with the role of these regions in processing internal somatic cues evoked by emotional stimuli. Citalopram had a greater effect on superior cingulate perfusion, an effect that is consistent with evidence of high levels of 5-HT transporters in this region.

Serotonin transporter binding in bipolar disorder assessed using [11C]DASB and positron emission tomography

BACKGROUND

Evidence from neuroimaging post-mortem, and genetic studies suggests that bipolar disorder (BD) is associated with abnormalities of the serotonin-transporter (5-HTT) system. Because of various limitations of these studies, however, it has remained unclear whether 5-HTT binding is abnormal in unmedicated BD-subjects. This study used PET and [(11)C]DASB, a radioligand that afforded higher sensitivity and specificity for the 5-HTT than previously available radioligands, to compare 5-HTT binding between BD and control subjects.

METHODS

The 5-HTT binding-potential (BP) was assessed in 18 currently-depressed, unmedicated BD-subjects and 37 healthy controls using PET and [(11)C]DASB.

RESULTS

In BD, the mean 5-HTT BP was increased in thalamus, dorsal cingulate cortex (DCC), medial prefrontal cortex and insula and decreased in the brainstem at the level of the pontine raphe-nuclei. Anxiety ratings correlated positively with 5-HTT BP in insula and DCC, and BP in these regions was higher in subjects manifesting pathological obsessions and compulsions relative to BD-subjects lacking such symptoms. Subjects with a history of suicide attempts showed reduced 5-HTT binding in the midbrain and increased binding in anterior cingulate cortex versus controls and to BD-subjects without attempts.

CONCLUSIONS

This is the first study to report abnormalities in 5-HTT binding in unmedicated BD-subjects. The direction of abnormality in the brainstem was opposite to that found in the cortex, thalamus, and striatum. Elevated 5-HTT binding in the cortex may be related to anxiety symptoms and syndromes associated with BD.

Anterior cingulate and subgenual prefrontal blood flow changes following tryptophan depletion in healthy males

In healthy humans, there is an apparent dissociation between cognitive and affective consequences of reduced brain serotonin (5-HT), as rapid tryptophan depletion (RTD) causes alterations in a consistent constellation of cognitive processes in the general absence of mood deterioration. This study aimed to investigate possible neural mechanisms underlying this relative dissociation by measuring the effects of reduced 5-HT on regional cerebral blood flow (rCBF). A total of 16 healthy, euthymic male subjects (mean age 39+/-9 years) without a personal or family history of affective disorder had mood ratings and single photon emission computed tomography scans with the rCBF tracer 99mTc-HMPAO under reduced 5-HT (RTD) and control conditions. Across individuals, modest positive and negative changes in subjective happiness associated with RTD were significantly correlated with change of rCBF in a cluster comprising subgenual (affective) anterior cingulate cortex (ACC) and associated regions (Brodmann's area (BA) 25, posterior BA11 and 47, caudate nucleus and ventral striatum; SPM99 p<0.05, corrected). The covariation was such that increasing sadness was associated with increased rCBF and vice versa. Independent of mood change, RTD was associated with reduced rCBF in the dorsal (cognitive) ACC (BA32; SPM99 p<0.05, corrected). The subgenual prefrontal cortex and dorsal ACC are important components of the ventral and dorsal neural systems that regulate and integrate affective and cognitive functions. The results therefore suggest that the dissociation between affective and cognitive consequences of RTD may possibly be attributable to differential effects of reduced 5-HT on these neural systems.

Topography of cerebral monoamine transporter availability in families with SCA2 mutations: a voxel-wise [123I]beta-CIT SPECT analysis

PURPOSE

The purpose of this study was to investigate the monoamine transporter status of dopamine, serotonin and norepinephrine throughout the brain in spinocerebellar ataxia type 2 (SCA2). To this end, nine patients were studied with [(123)I]beta-CIT SPECT.

METHODS

Data were compared with ten age-matched healthy control subjects and ten patients with young-onset Parkinson's disease (YOPD), matched for age. Parametric SPECT images of the specific-to-non-displaceable equilibrium partition coefficient (V (3)''), which is proportional to the receptor density (B (max)), were generated. In order to objectively localise focal changes in beta-CIT uptake throughout the brain volume without having to make an a priori hypothesis as to their location, statistical parametric mapping (SPM) was applied to SPECT images. Data clusters revealed by SPM, showing significant differences in V (3)'' values between groups, were transformed onto the individual V (3)'' image to obtain mean regional uptake values.

RESULTS

Both SCA2 and YOPD patients showed significant decreases in striatal [(123)I]beta-CIT SPECT uptake when compared with controls. However, in SCA2 patients, additional reductions in caudate/anterior putamen, midbrain and pons [(123)I]beta-CIT uptake were localised with SPM.

CONCLUSION

Voxel-wise analysis of [(123)I]beta-CIT SPECT revealed more widespread decline of monoamine transporter availability in SCA2 than in YOPD, reflecting differences in the underlying pathology. We suggest that the quantification of midbrain and pons [(123)I]beta-CIT signal is likely to improve the diagnostic accuracy in patients presenting with clinical features of both SCA2 and YOPD at initial investigation.

The serotonin transporter in rhesus monkey brain: comparison of DASB and citalopram binding sites

We have characterized the interaction of the serotonin transporter ligand [3H]-N,N-dimethyl-2-(2-amino-4-cyanophenylthio)-benzylamine (DASB) with rhesus monkey brain in vitro using tissue homogenate binding and autoradiographic mapping. [3H]-DASB, a tritiated version of the widely used [11C] positron emission tomography tracer, was found to selectively bind to a single population of sites with high affinity (K(d)=0.20+/-0.04 nM). The serotonin transporter density (B(max)) obtained for rhesus frontal cortex was found to be 66+/-8 fmol/mg protein using [3H]-DASB, similar to the B(max) value obtained using the reference radioligand [3H]-citalopram, a well-characterized and highly selective serotonin reuptake inhibitor (83+/-22 fmol/mg protein). Specific binding sites of both [3H]-DASB and [3H]-citalopram were similarly and nonuniformly distributed throughout the rhesus central nervous system, in a pattern consistent with serotonin transporter localization reported for human brain. Regional serotonin transporter densities, estimated from optical densities of the autoradiographic images, were well correlated between the two radioligands. Finally, DASB and fluoxetine showed dose-dependent full inhibition of [3H]-citalopram binding in a competition autoradiographic study, with K(i) values in close agreement with those obtained from rhesus brain homogenates. This side-by-side comparison of [3H]-DASB and [3H]-citalopram binding sites in rhesus tissue homogenates and in adjacent rhesus brain slices provides additional support for the use of [11C]-DASB to assess the availability and distribution of serotonin transporters in nonhuman primates.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Neural mechanisms of genetic risk for impulsivity and violence in humans

Neurobiological factors contributing to violence in humans remain poorly understood. One approach to this question is examining allelic variation in the X-linked monoamine oxidase A (MAOA) gene, previously associated with impulsive aggression in animals and humans. Here, we have studied the impact of a common functional polymorphism in MAOA on brain structure and function assessed with MRI in a large sample of healthy human volunteers. We show that the low expression variant, associated with increased risk of violent behavior, predicted pronounced limbic volume reductions and hyperresponsive amygdala during emotional arousal, with diminished reactivity of regulatory prefrontal regions, compared with the high expression allele. In men, the low expression allele is also associated with changes in orbitofrontal volume, amygdala and hippocampus hyperreactivity during aversive recall, and impaired cingulate activation during cognitive inhibition. Our data identify differences in limbic circuitry for emotion regulation and cognitive control that may be involved in the association of MAOA with impulsive aggression, suggest neural systems-level effects of X-inactivation in human brain, and point toward potential targets for a biological approach toward violence.

Brain SPECT imaging and whole-body biodistribution with [123I]ADAM — a serotonin transporter radiotracer in healthy human subjects

INTRODUCTION

[(123)I]-2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I]ADAM), a novel radiotracer, has promising application in the imaging of the serotonin transporter (SERT) in the human brain. In this study, the optimal scanning time for acquiring brain single photon emission computed tomography (SPECT) images was determined by performing dynamic SPECT studies at intervals from 0 to 6 h postinjection of [(123)I]ADAM. Additionally, radiation-absorbed doses were determined for three healthy human subjects using attenuation-corrected images.

METHODS

Twelve subjects were randomized into one of three study groups as follows: whole-body distribution imaging (n=3), dynamic SPECT imaging (n=3) and brain SPECT imaging (n=6). The radiation-absorbed dose was calculated using MIRDOSE 3.0 software with attenuation-corrected data. The specific binding (SB) ratio of the brain stem was measured from dynamic SPECT images to determine the optimal scanning time.

RESULTS

Dynamic SPECT images showed that the SB of the brain stem gradually increased to a maximum 4 h postinjection. Single photon emission computed tomography images at 4 h postinjection showed a high uptake of the radiotracer (SB) in the hypothalamus (1.40+/-0.12), brain stem (1.44+/-0.16), pons (1.13+/-0.14) and medial temporal lobe (0.59+/-0.10). The mean adult male value of effective dose was 3.37 x 10(-2) mSv/MBq with a 4.8-h urine-voiding interval. Initial high uptake in SERT-rich sites was demonstrated in the lung and brain. A prominent washout of the radiotracer from the lung further increased brain radioactivity that reached a peak value of 5.03% of injected dose 40 min postinjection.

CONCLUSIONS

[(123)I]ADAM is a promising radiotracer for SPECT imaging of SERT in humans with acceptable dosimetry and high uptake in SERT-rich regions. Brain SPECT images taken within 4 h following injection show optimal levels of radiotracer uptake in known SERT sites. However, dynamic changes in lung SERT distribution must be carefully evaluated.

Psychiatry as a clinical neuroscience discipline

One of the fundamental insights emerging from contemporary neuroscience is that mental illnesses are brain disorders. In contrast to classic neurological illnesses that involve discrete brain lesions, mental disorders need to be addressed as disorders of distributed brain systems with symptoms forged by developmental and social experiences. While genomics will be important for revealing risk, and cellular neuroscience should provide targets for novel treatments for these disorders, it is most likely that the tools of systems neuroscience will yield the biomarkers needed to revolutionize psychiatric diagnosis and treatment. This essay considers the discoveries that will be necessary over the next two decades to translate the promise of modern neuroscience into strategies for prevention and cures of mental disorders. To deliver on this spectacular new potential, clinical neuroscience must be integrated into the discipline of psychiatry, thereby transforming current psychiatric training, tools, and practices.

Regional expression of 5-HT1B receptor mRNA in the human brain

The regional mRNA expression pattern of 5-HT(1B) receptors has been extensively characterized in the rodent and guinea pig brain, but a detailed mapping of the 5-HT(1B) receptor mRNA expression in the human brain has not previously been performed. In the present study, the mRNA expression of 5-HT(1B) receptors was analyzed using in situ hybridization histochemistry and whole hemisphere sections of the human postmortem brain. The mRNA expression was compared with the autoradiographic distribution of 5-HT(1B) receptors. High levels of mRNA expression were found in the striatum, cortex, lateral geniculate nucleus, and raphe nuclei. The expression was higher in ventral than in dorsal striatal regions and was absent from the substantia nigra and pallidum, where high levels of 5-HT(1B) receptors were found. A layer-specific expression pattern was observed in cortical regions. The results extend previous knowledge about the localization of the 5-HT(1B) receptor in the human brain. This study provides evidence of a mismatch of the regional expression of 5-HT(1B) receptor mRNA and the 5-HT(1B) receptor distribution in human brain, similar to what has been demonstrated in other species. This is in line with the localization of this receptor subtype in nerve terminals. The results give support to species differences in the cortical mRNA expression pattern of this receptor subtype.

5-HTTLPR polymorphism impacts human cingulate-amygdala interactions: a genetic susceptibility mechanism for depression

Carriers of the short allele of a functional 5' promoter polymorphism of the serotonin transporter gene have increased anxiety-related temperamental traits, increased amygdala reactivity and elevated risk of depression. Here, we used multimodal neuroimaging in a large sample of healthy human subjects to elucidate neural mechanisms underlying this complex genetic association. Morphometrical analyses showed reduced gray matter volume in short-allele carriers in limbic regions critical for processing of negative emotion, particularly perigenual cingulate and amygdala. Functional analysis of those regions during perceptual processing of fearful stimuli demonstrated tight coupling as a feedback circuit implicated in the extinction of negative affect. Short-allele carriers showed relative uncoupling of this circuit. Furthermore, the magnitude of coupling inversely predicted almost 30% of variation in temperamental anxiety. These genotype-related alterations in anatomy and function of an amygdala-cingulate feedback circuit critical for emotion regulation implicate a developmental, systems-level mechanism underlying normal emotional reactivity and genetic susceptibility for depression.

Distribution of 5-HT7 receptors in the human brain: a preliminary autoradiographic study using []SB-269970

The distribution of the 5-HT7 receptor was analyzed using the selective 5-HT7 receptor antagonist radioligand [3H]SB-269970 and human brain whole hemisphere autoradiography. The results indicated that 5-HT7 receptors are most abundantly localized in the anterior thalamus and in the dentate gyrus. Other regions containing intermediate levels of 5-HT7 receptors included the hypothalamus, anterior cingulate gyrus, hippocampus, amygdala and certain brainstem nuclei. The distribution pattern obtained in this study supports the involvement of this receptor subtype in affective behavior and cognition.