The distribution of serotonergic fibers in the macaque monkey amygdala: An immunohistochemical study using antisera to 5-hydroxytryptamine

Functional neuroanatomy of monoaminergic systems in the basolateral nuclear complex of the amygdala: Neuronal targets, receptors, and circuits

This review discusses neuroanatomical aspects of the three main monoaminergic systems innervating the basolateral nuclear complex (BNC) of the amygdala (serotonergic, noradrenergic, and dopaminergic systems). It mainly focuses on immunohistochemical (IHC) and in situ hybridization (ISH) studies that have analyzed the relationship of specific monoaminergic inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of the monoaminergic modulation of BNC circuitry. First, light and electron microscopic IHC investigations identifying the main BNC neuronal subpopulations and characterizing their local circuitry, including connections with discrete PN compartments and other INs, are reviewed. Then, the relationships of each of the three monoaminergic systems to distinct PN and IN cell types, are examined in detail. For each system, the neuronal targets and their receptor expression are discussed. In addition, pertinent electrophysiological investigations are discussed. The last section of the review compares and contrasts various aspects of each of the three monoaminergic systems. It is concluded that the large number of different receptors, each with a distinct mode of action, expressed by distinct cell types with different connections and functions, should offer innumerable ways to subtlety regulate the activity of the BNC by therapeutic drugs in psychiatric diseases in which there are alterations of BNC monoaminergic modulatory systems, such as in anxiety disorders, depression, and drug addiction. It is suggested that an important area for future studies is to investigate how the three systems interact in concert at the neuronal and neuronal network levels.

Anatomical and molecular features of the amygdalohippocampal transition area and its role in social and emotional behavior processes

The amygdalohippocampal transition area (AHi) has emerged as a critical nucleus of sociosexual behaviors such as mating, parenting, and aggression. The AHi has been overlooked in rodent and human amygdala studies until recently. The AHi is hypothesized to play a role in metabolic and cognitive functions as well as social behaviors based on its connectivity and molecular composition. The AHi is small nucleus rich in neuropeptide and hormone receptors and is contiguous with the ventral subiculum of the hippocampus-hence its designation as a "transition area". Literature focused on the AHi can be difficult to interpret because of changing nomenclature and conflation with neighboring nuclei. Here we summarize what is currently known about AHi structure and development, connections throughout the brain, molecular composition, and functional significance. We aim to delineate current knowledge regarding the AHi, identify potential functions with supporting evidence, and ultimately make clear the importance of the AHi in sociosexual function.

Hof P. Understanding Emotions: Origins and Roles of the Amygdala

Emotions arise from activations of specialized neuronal populations in several parts of the cerebral cortex, notably the anterior cingulate, insula, ventromedial prefrontal, and subcortical structures, such as the amygdala, ventral striatum, putamen, caudate nucleus, and ventral tegmental area. Feelings are conscious, emotional experiences of these activations that contribute to neuronal networks mediating thoughts, language, and behavior, thus enhancing the ability to predict, learn, and reappraise stimuli and situations in the environment based on previous experiences. Contemporary theories of emotion converge around the key role of the amygdala as the central subcortical emotional brain structure that constantly evaluates and integrates a variety of sensory information from the surroundings and assigns them appropriate values of emotional dimensions, such as valence, intensity, and approachability. The amygdala participates in the regulation of autonomic and endocrine functions, decision-making and adaptations of instinctive and motivational behaviors to changes in the environment through implicit associative learning, changes in short- and long-term synaptic plasticity, and activation of the fight-or-flight response via efferent projections from its central nucleus to cortical and subcortical structures.

A comparison of cell density and serotonergic innervation of the amygdala among four macaque species

The genus Macaca is an ideal model for investigating the biological basis of primate social behavior from an evolutionary perspective. A significant amount of behavioral diversity has been reported among the macaque species, but little is known about the neural substrates that support this variation. The present study compared neural cell density and serotonergic innervation of the amygdala among four macaque species using histological and immunohistochemical methods. The species examined included rhesus (Macaca mulatta), Japanese (M. fuscata), pigtailed (M. nemestrina), and moor macaques (M. maura). We anticipated that the more aggressive rhesus and Japanese macaques would have lower serotonergic innervation within the amygdala compared to the more affiliative pigtailed and moor macaques. In contrast to our prediction, pigtailed macaques had higher serotonergic innervation than Japanese and moor macaques in the basal and central amygdala nuclei when controlling for neuron density. Our analysis of neural cell populations revealed that Japanese macaques possess significantly higher neuron and glia densities relative to the other three species, however we observed no glia-to-neuron ratio differences among species. The results of this study revealed serotonergic innervation and cell density differences among closely related macaque species, which may play a role in modulating subtle differences in emotional processing and species-typical social styles.

Serotonergic innervation of the human amygdala and evolutionary implications

OBJECTIVES

The serotonergic system is involved in the regulation of socio-emotional behavior and heavily innervates the amygdala, a key structure of social brain circuitry. We quantified serotonergic axon density of the four major nuclei of the amygdala in humans, and examined our results in light of previously published data sets in chimpanzees and bonobos.

MATERIALS AND METHODS

Formalin-fixed postmortem tissue sections of the amygdala from six humans were stained for serotonin transporter (SERT) utilizing immunohistochemistry. SERT-immunoreactive (ir) axon fiber density in the lateral, basal, accessory basal, and central nuclei of the amygdala was quantified using unbiased stereology. Nonparametric statistical analyses were employed to examine differences in SERT-ir axon density between amygdaloid nuclei within humans, as well as differences between humans and previously published data in chimpanzees and bonobos.

RESULTS

Humans displayed a unique pattern of serotonergic innervation of the amygdala, and SERT-ir axon density was significantly greater in the central nucleus compared to the lateral nucleus. SERT-ir axon density was significantly greater in humans compared to chimpanzees in the basal, accessory basal, and central nuclei. SERT-ir axon density was greater in humans compared to bonobos in the accessory basal and central nuclei.

CONCLUSIONS

The human pattern of SERT-ir axon distribution in the amygdala complements the redistribution of neurons in the amygdala in human evolution. The present findings suggest that differential serotonergic modulation of cognitive and autonomic pathways in the amygdala in humans, bonobos, and chimpanzees may contribute to species-level differences in social behavior.

Amygdala growth from youth to adulthood in the macaque monkey

Emerging evidence suggests that the human amygdala undergoes extensive growth through adolescence, coinciding with the acquisition of complex socioemotional learning. Our objective was to longitudinally map volumetric growth of the nonhuman primate amygdala in a controlled, naturalistic social environment from birth to adulthood. Magnetic resonance images were collected at five time-points in 24 male and female rhesus macaques from 6 months to adulthood at 5 years. We then compared amygdala growth to other brain regions, including newly collected isocortical gray and white matter volumes, and previously published data on the same cohort. We found that amygdala volume increases by nearly 50% from age 6 months to 5 years. This dramatic growth is in contrast to overall brain and hippocampal volume, which peak near 3 years, white matter, which slows from 3 to 5 years, and isocortical gray, which has a net decrease. Similar to isocortical gray and hippocampal volumes, amygdala volume is ~8% larger in males than females. Rate of growth does not differ by sex. Although the underlying neurobiological substrate for protracted amygdala growth into adulthood is unclear, we propose it may be due in part to the unique cellular development of immature neurons in paralaminar nucleus that mature in size and connectivity with age. Prolonged amygdala maturation raises the possibility that environmental and genetic perturbations that disrupt this trajectory may contribute to the emergence of psychiatric disorders, such as anxiety, depression, schizophrenia, and autism; all in which the amygdala is strongly implicated.

Neuroprotective efficacy of P7C3 compounds in primate hippocampus

There is a critical need for translating basic science discoveries into new therapeutics for patients suffering from difficult to treat neuropsychiatric and neurodegenerative conditions. Previously, a target-agnostic in vivo screen in mice identified P7C3 aminopropyl carbazole as capable of enhancing the net magnitude of postnatal neurogenesis by protecting young neurons from death. Subsequently, neuroprotective efficacy of P7C3 compounds in a broad spectrum of preclinical rodent models has also been observed. An important next step in translating this work to patients is to determine whether P7C3 compounds exhibit similar efficacy in primates. Adult male rhesus monkeys received daily oral P7C3-A20 or vehicle for 38 weeks. During weeks 2-11, monkeys received weekly injection of 5'-bromo-2-deoxyuridine (BrdU) to label newborn cells, the majority of which would normally die over the following 27 weeks. BrdU+ cells were quantified using unbiased stereology. Separately in mice, the proneurogenic efficacy of P7C3-A20 was compared to that of NSI-189, a proneurogenic drug currently in clinical trials for patients with major depression. Orally-administered P7C3-A20 provided sustained plasma exposure, was well-tolerated, and elevated the survival of hippocampal BrdU+ cells in nonhuman primates without adverse central or peripheral tissue effects. In mice, NSI-189 was shown to be pro-proliferative, and P7C3-A20 elevated the net magnitude of hippocampal neurogenesis to a greater degree than NSI-189 through its distinct mechanism of promoting neuronal survival. This pilot study provides evidence that P7C3-A20 safely protects neurons in nonhuman primates, suggesting that the neuroprotective efficacy of P7C3 compounds is likely to translate to humans as well.

RDoC-based categorization of amygdala functions and its implications in autism

Confusion endures as to the exact role of the amygdala in relation to autism. To help resolve this we turned to the NIMH's Research Domain Criteria (RDoC) which provides a classification schema that identifies different categories of behaviors that can turn pathologic in mental health disorders, e.g. autism. While RDoC incorporates all the known neurobiological substrates for each domain, this review will focus primarily on the amygdala. We first consider the amygdala from an anatomical, historical, and developmental perspective. Next, we examine the different domains and constructs of RDoC that the amygdala is involved in: Negative Valence Systems, Positive Valence Systems, Cognitive Systems, Social Processes, and Arousal and Regulatory Systems. Then the evidence for a dysfunctional amygdala in autism is presented with a focus on alterations in development, prenatal valproic acid exposure as a model for ASD, and changes in the oxytocin system therein. Finally, a synthesis of RDoC, the amygdala, and autism is offered, emphasizing the task of disambiguation and suggestions for future research.

A comparative analysis of the physiological properties of neurons in the anterolateral bed nucleus of the stria terminalis in the Mus musculus, Rattus norvegicus, and Macaca mulatta

The anterolateral group of the bed nucleus of the stria terminalis (BNST_ALG ) is a critical modulator of a variety of rodent and primate behaviors spanning anxiety behavior and drug addiction. Three distinct neuronal cell types have been previously defined in the rat BNST_ALG based on differences in the voltage-response to hyperpolarizing and depolarizing current injection. Differences in genetic expression profile between these three cell types suggest electrophysiological cell type may be an indicator for functional differences in the circuit of the rat BNST_ALG . Although the behavioral role of the BNST is conserved across species, it is unknown if the same electrophysiological cell types exist in the BNST_ALG of the mouse and nonhuman primate. Here, we used whole-cell patch clamp electrophysiology and neuronal reconstructions of biocytin-filled neurons to compare and contrast the electrophysiological and morphological properties of neurons in the BNST_ALG from the mouse, rat, and rhesus macaque. We provide evidence that the BNST_ALG of all three species contains neurons that match the three defined cell types found in the rat; however, there are intriguing differences in the relative frequency of these cell types as well as electrophysiological and morphological properties of the BNST_ALG neurons across species. This study suggests that the overall landscape of the BNST_ALG in the primate and mouse may be similar to that of the rat in some aspects but perhaps significantly different in others.

Pattern of distribution of serotonergic fibers to the amygdala and extended amygdala in the rat

As is well recognized, serotonergic (5-HT) fibers distribute widely throughout the forebrain, including the amygdala. Although a few reports have examined the 5-HT innervation of select nuclei of the amygdala in the rat, no previous report has described overall 5-HT projections to the amygdala in the rat. Using immunostaining for the serotonin transporter, SERT, we describe the complete pattern of distribution of 5-HT fibers to the amygdala (proper) and to the extended amygdala in the rat. Based on its ontogenetic origins, the amygdala was subdivided into two major parts, pallial and subpallial components, with the pallial component further divided into superficial and deep nuclei (Olucha-Bordonau et al. 2015). SERT⁺ fibers were shown to distributed moderately to densely to the deep and cortical pallial nuclei, but, by contrast, lightly to the subpallial nuclei. Specifically, 1) of the deep pallial nuclei, the lateral, basolateral, and basomedial nuclei contained a very dense concentration of 5-HT fibers; 2) of the cortical pallial nuclei, the anterior cortical and amygdala-cortical transition zone rostrally and the posteromedial and posterolateral nuclei caudally contained a moderate concentration of 5-HT fibers; and 3) of the subpallial nuclei, the anterior nuclei and the rostral part of the medial (Me) nuclei contained a moderate concentration of 5-HT fibers, whereas caudal regions of Me as well as the central nuclei and the intercalated nuclei contained a sparse/light concentration of 5-HT fibers. With regard to the extended amygdala (primarily the bed nucleus of stria terminalis; BST), on the whole, the BST contained moderate numbers of 5-HT fibers, spread fairly uniformly throughout BST. The findings are discussed with respect to a critical serotonergic influence on the amygdala, particularly on the basal complex, and on the extended amygdala in the control of states of fear and anxiety. J. Comp. Neurol. 525:116-139, 2017. © 2016 Wiley Periodicals, Inc.

Genetic variation in the tryptophan hydroxylase 2 gene moderates depressive symptom trajectories and remission over 8 weeks of escitalopram treatment

The serotonin system plays an important role in the pathogenesis of major depressive disorder (MDD) and genetic variations in serotonin-related genes affect the efficacy of antidepressants. The aim of this study was to investigate the relationship between genotypic variation in six candidate serotonergic genes (ADCY9, HTR1B, GNB3, HTR2A, TPH2, SLC6A4) and depressive and anxiety symptom severity trajectories as well as remission following escitalopram treatment. A total of 166 Chinese patients with MDD were treated with escitalopram (open-label) for 8 weeks. TPH2 rs4570625 GG carriers were more likely to achieve depressive and anxiety symptom remission compared with T-allele carriers. At the trend level (P(corrected)=0.05), depressive symptom severity trajectories were moderated by TPH2 rs4570625. Patients with the GT or the GG genotype showed more favorable depressive symptom severity trajectories compared with TT genotype carriers. Polymorphisms in ADCY9, HTR1B, and HTR2A were nominally associated with symptom remission, but did not withstand correction for multiple comparisons. The HTTLPR polymorphism was not included in our final analysis because of a high percentage of missing data. These results suggested that genotypic variation in TPH2 may moderate the therapeutic response to esciatlopram among Chinese patients with MDD.

Relationship of the serotonin transporter gene promoter polymorphism (5-HTTLPR) genotype and serotonin transporter binding to neural processing of negative emotional stimuli

BACKGROUND

The lower-expressing (S') alleles of the serotonin transporter (5-HTT) gene promoter polymorphism (5-HTTLPR) are linked to mood and anxiety related psychopathology. However, the specific neural mechanism through which these alleles may influence emotional and cognitive processing remains unknown. We examined the relationship between both 5-HTTLPR genotype and in vivo 5-HTT binding quantified via PET with amygdala reactivity to emotionally negative stimuli. We hypothesized that 5-HTT binding in both raphe nuclei (RN) and amygdala would be inversely correlated with amygdala reactivity, and that number of S' alleles would correlate positively with amygdala reactivity.

METHODS

In medication-free patients with current major depressive disorder (MDD; N=21), we determined 5-HTTLPR genotype, employed functional magnetic resonance imaging (fMRI) to examine amygdala responses to negative emotional stimuli, and used positron emission tomography with [(11)C]DASB to examine 5-HTT binding.

RESULTS

[(11)C]DASB binding in RN and amygdala was inversely correlated with amygdala response to negative stimuli. 5-HTTLPR S' alleles were not associated with amygdala response to negative emotional stimuli.

LIMITATIONS

Primary limitations are small sample size and lack of control group.

CONCLUSIONS

Consistent with findings in healthy volunteers, 5-HTT binding is associated with amygdala reactivity to emotional stimuli in MDD. 5-HTT binding may be a stronger predictor of emotional processing in MDD as compared with 5-HTTLPR genotype.

Genetic variations in the serotonergic system contribute to amygdala volume in humans

The amygdala plays a critical role in emotion processing and psychiatric disorders associated with emotion dysfunction. Accumulating evidence suggests that amygdala structure is modulated by serotonin-related genes. However, there is a gap between the small contributions of single loci (less than 1%) and the reported 63–65% heritability of amygdala structure. To understand the “missing heritability,” we systematically explored the contribution of serotonin genes on amygdala structure at the gene set level. The present study of 417 healthy Chinese volunteers examined 129 representative polymorphisms in genes from multiple biological mechanisms in the regulation of serotonin neurotransmission. A system-level approach using multiple regression analyses identified that nine SNPs collectively accounted for approximately 8% of the variance in amygdala volume. Permutation analyses showed that the probability of obtaining these findings by chance was low (p = 0.043, permuted for 1000 times). Findings showed that serotonin genes contribute moderately to individual differences in amygdala volume in a healthy Chinese sample. These results indicate that the system-level approach can help us to understand the genetic basis of a complex trait such as amygdala structure.

Activation of 5-HT₁A receptors in the medial subdivision of the central nucleus of the amygdala produces anxiolytic effects in a rat model of Parkinson's disease

Although the medial subdivision of the central nucleus of the amygdala (CeM) and serotonin-1A (5-HT1A) receptors are involved in the regulation of anxiety, their roles in Parkinson's disease (PD)-associated anxiety are still unknown. Here we assessed the importance of CeM 5-HT1A receptors for anxiety in rats with unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle (MFB). The lesion induced anxiety-like behaviors, increased the firing rate and burst-firing pattern of CeM γ-aminobutyric acid (GABA) neurons, as well as decreased dopamine (DA) levels in the striatum, medial prefrontal cortex (mPFC), amygdala and ventral part of hippocampus (vHip). Intra-CeM injection of the selective 5-HT1A receptor agonist 8-OH-DPAT produced anxiolytic effects in the lesioned rats, and decreased the firing rate of CeM GABAergic neurons in two groups of rats. Compared to sham-operated rats, the duration of the inhibitory effect on the firing rate of GABAergic neurons was shortened in the lesioned rats. The injection increased DA levels in the mPFC and amygdala in two groups of rats and the vHip in the lesioned rats, and increased 5-HT level in the lesioned rats, whereas it decreased NA levels in the mPFC in two groups of rats and the vHip in the lesioned rats. Moreover, the mean density of 5-HT1A receptor and GABA double-labeled neurons in the CeM was reduced after the lesioning. These results suggest that activation of CeM 5-HT1A receptor produces anxiolytic effects in the 6-OHDA-lesioned rats, which involves decreased firing rate of the GABAergic neurons, and changed monoamine levels in the limbic and limbic-related brain regions.

Neuronal localization of the 5-HT2 receptor family in the amygdaloid complex

The amygdaloid complex (or amygdala), a heterogeneous structure located in the medial portion of the temporal lobe, is composed of deep, superficial, and “remaining” nuclei. This structure is involved in the generation of emotional behavior, in the formation of emotional memories and in the modulation of the consolidation of explicit memories for emotionally arousing events. The serotoninergic fibers originating in the dorsal and medial raphe nuclei are critically involved in amygdalar functions. Serotonin (5-hydroxytryptamine, 5-HT) regulates amygdalar activity through the activation of the 5-HT₂ receptor family, which includes three receptor subtypes: 5-HT_2A, 5-HT_2B, and 5-HT_2C. The distribution and the functional activity of the 5-HT₂ receptor family has been studied more extensively than that of the 5-HT_2A receptor subtypes, especially in the deep nuclei. In these nuclei, the 5-HT_2A receptor is expressed on both pyramidal and non-pyramidal neurons, and could play a critical role in the formation of emotional memories. However, the exact role of the 5-HT_2A receptor subtypes, as well as that of the 5-HT_2B and 5-HT_2C receptor subtypes, in the modulation of the amygdalar microcircuits requires additional study. The present review reports data concerning the distribution and the functional roles of the 5-HT₂ receptor family in the amygdala.

Neurons in monkey dorsal raphe nucleus code beginning and progress of step-by-step schedule, reward expectation, and amount of reward outcome in the reward schedule task

The dorsal raphe nucleus is the major source of serotonin in the brain. It is connected to brain regions related to reward processing, and the neurons show activity related to predicted reward outcome. Clinical observations also suggest that it is important in maintaining alertness and its apparent role in addiction seems to be related to reward processing. Here, we examined whether the neurons in dorsal raphe carry signals about reward outcome and task progress during multitrial schedules. We recorded from 98 single neurons in dorsal raphe of two monkeys. The monkeys perform one, two, or three visual discrimination trials (schedule), obtaining one, two, or three drops of liquid. In the valid cue condition, the length and brightness of a visual cue indicated schedule progress and reward amount, respectively. In the random cue condition, the visual cue was randomly presented with respect to schedule length and reward amount. We found information encoded about (1) schedule onset, (2) reward expectation, (3) reward outcome, and (4) reward amount in the mean firing rates. Information theoretic analysis showed that the temporal variation of the neuronal responses contained additional information related to the progress of the schedule toward the reward rather than only discriminating schedule onset or reward/no reward. When considered in light of all that is known about the raphe in anatomy, physiology, and behavior, the rich encoding about both task progress and predicted reward outcome makes the raphe a strong candidate for providing signals throughout the brain to coordinate persistent goal-seeking behavior.

Functional anatomy of 5-HT2A receptors in the amygdala and hippocampal complex: relevance to memory functions

The amygdaloid complex and hippocampal region contribute to emotional activities, learning, and memory. Mounting evidence suggests a primary role for serotonin (5-HT) in the physiological basis of memory and its pathogenesis by modulating directly the activity of these two areas and their cross-talk. Indeed, both the amygdala and the hippocampus receive remarkably dense serotoninergic inputs from the dorsal and median raphe nuclei. Anatomical, behavioral and electrophysiological evidence indicates the 5-HT2A receptor as one of the principal postsynaptic targets mediating 5-HT effects. In fact, the 5-HT2A receptor is the most abundant 5-HT receptor expressed in these brain structures and is expressed on both amygdalar and hippocampal pyramidal glutamatergic neurons as well as on γ-aminobutyric acid (GABA)-containing interneurons. 5-HT2A receptors on GABAergic interneurons stimulate GABA release, and thereby have an important role in regulating network activity and neural oscillations in the amygdala and hippocampal region. This review will focus on the distribution and physiological functions of the 5-HT2A receptor in the amygdala and hippocampal region. Taken together the results discussed here suggest that 5-HT2A receptor may be a potential therapeutic target for those disorders related to hippocampal and amygdala dysfunction.

Serotonin modulates outward potassium currents in mouse olfactory receptor neurons

Monoaminergic neurotransmitter 5-hydroxytryptamine (5-HT), also known as serotonin, plays important roles in modulating the function of the olfactory system. However, thus far, the knowledge about 5-HT and its receptors in olfactory receptor neurons (ORNs) and their physiological role have not been fully characterized. In the present study, reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed the presence of 5-HT(1A) and 5-HT(1B) receptor subtypes in mouse olfactory epithelium at the mRNA level. With subtype selective antibodies and standard immunohistochemical techniques, both receptor subtypes were found to be positively labeled. To further elucidate the molecular mechanisms of 5-HT act on the peripheral olfactory transduction, the whole-cell patch clamp techniques were used on freshly isolated ORNs. We found that 5-HT decreased the magnitude of outward K(+) current in a dose-dependent manner and these inhibitory effects were markedly attenuated by the 5-HT(1A) receptor blocker WAY-100635 and the 5-HT(1B) receptor antagonist GR55562. These data suggested that 5-HT may play a role in the modulation of peripheral olfactory signals by regulating outward potassium currents, both 5-HT(1A) and 5-HT(1B) receptors were involved in this regulation.

Serotonin circuits and anxiety: what can invertebrates teach us?

Fear, a reaction to a threatening situation, is a broadly adaptive feature crucial to the survival and reproductive fitness of individual organisms. By contrast, anxiety is an inappropriate behavioral response often to a perceived, not real, threat. Functional imaging, biochemical analysis, and lesion studies with humans have identified the HPA axis and the amygdala as key neuroanatomical regions driving both fear and anxiety. Abnormalities in these biological systems lead to misregulated fear and anxiety behaviors such as panic attacks and post-traumatic stress disorders. These behaviors are often treated by increasing serotonin levels at synapses, suggesting a role for serotonin signaling in ameliorating both fear and anxiety. Interestingly, serotonin signaling is highly conserved between mammals and invertebrates. We propose that genetically tractable invertebrate models organisms, such as Drosophila melanogaster and Caenorhabditis elegans, are ideally suited to unravel the complexity of the serotonin signaling pathways. These model systems possess well-defined neuroanatomies and robust serotonin-mediated behavior and should reveal insights into how serotonin can modulate human cognitive functions.

Evidence for coordinated functional activity within the extended amygdala of non-human and human primates

Neuroanatomists posit that the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST) comprise two major nodes of a macrostructural forebrain entity termed the extended amygdala. The extended amygdala is thought to play a critical role in adaptive motivational behavior and is implicated in the pathophysiology of maladaptive fear and anxiety. Resting functional connectivity of the Ce was examined in 107 young anesthetized rhesus monkeys and 105 young humans using standard resting-state functional magnetic resonance imaging (fMRI) methods to assess temporal correlations across the brain. The data expand the neuroanatomical concept of the extended amygdala by finding, in both species, highly significant functional coupling between the Ce and the BST. These results support the use of in vivo functional imaging methods in nonhuman and human primates to probe the functional anatomy of major brain networks such as the extended amygdala.

Understanding behavioral effects of early life stress using the reactive scope and allostatic load models

The mechanisms through which early life stress leads to psychopathology are thought to involve allostatic load, the "wear and tear" an organism is subjected to as a consequence of sustained elevated levels of glucocorticoids caused by repeated/prolonged stress activations. The allostatic load model described this phenomenon, but has been criticized as inadequate to explain alterations associated with early adverse experience in some systems, including behavior, which cannot be entirely explained from an energy balance perspective. The reactive scope model has been more recently proposed and focuses less on energy balance and more on dynamic ranges of physiological and behavioral mediators. In this review we examine the mechanisms underlying the behavioral consequences of early life stress in the context of both these models. We focus on adverse experiences that involve mother-infant relationship disruption, and dissect those mechanisms involving maternal care as a regulator of development of neural circuits that control emotional and social behaviors in the offspring. We also discuss the evolutionary purpose of the plasticity in behavioral development, which has a clear adaptive value in a changing environment.

Where and what is the paralaminar nucleus? A review on a unique and frequently overlooked area of the primate amygdala

The primate amygdala is composed of multiple subnuclei that play distinct roles in amygdala function. While some nuclei have been areas of focused investigation, others remain virtually unknown. One of the more obscure regions of the amygdala is the paralaminar nucleus (PL). The PL in humans and non-human primates is relatively expanded compared to lower species. Long considered to be part of the basal nucleus, the PL has several interesting features that make it unique. These features include a dense concentration of small cells, high concentrations of receptors for corticotropin releasing hormone and benzodiazepines, and dense innervation of serotonergic fibers. More recently, high concentrations of immature-appearing cells have been noted in the primate PL, suggesting special mechanisms of neural plasticity. Following a brief overview of amygdala structure and function, this review will provide an introduction to the history, embryology, anatomical connectivity, immunohistochemical and cytoarchitectural properties of the PL. Our conclusion is that the PL is a unique subregion of the amygdala that may yield important clues about the normal growth and function of the amygdala, particularly in higher species.

Male predominance in autism: neuroendocrine influences on arousal and social anxiety

We offer a neurobiologic theory based on animal work that helps account for the conspicuous male predominance in autism spectrum disorders (ASD). In young male animals, testosterone (TST) binds to androgen receptors (AR) in brainstem neurons responsible for enhancing brain arousal. As a consequence, arousal-related neurotransmitters bombard the amygdala hypersensitized by TST acting though AR. Arousal-related inputs are known to prime amygdaloid mechanisms for fear and anxiety, with resultant social avoidance. We hypothesize that similar mechanisms contribute to autism's male predominance and to its defining impaired social skills. The theory rests on two key interacting factors: the molecular effects of TST in genetically vulnerable boys in combination with environmental stresses they experienced in utero, neonatally, or during the first years. We postulate that higher TST levels and, therefore, higher amounts of arousal-related inputs to the amygdala sensitize these genetically vulnerable male infants to very early stresses. In sharp contrast to boys, girls not only do not have high levels of TST-facilitated arousal-causing inputs to the amygdala but they also enjoy the protection afforded by estrogenic hormones, oxytocin, and the oxytocin receptor. This theory suggests that novel technologies applied to the molecular endocrinology of TST's actions through AR will offer new avenues of enquiry into ASD. Since the high male preponderance in autism is important yet understudied, we offer our theory, which is based on detailed neurobehavioral research with animals, to stimulate basic and clinical research in animals and humans and hopefully help develop novel more effective medical treatments for autism.

Distribution of 5-HT2A receptor immunoreactivity in the rat amygdaloid complex and colocalization with γ-aminobutyric acid

The 5-HT2A receptor (5-HT2Ar) is located in a variety of excitatory and inhibitory neurons in many regions of the central nervous system and is a major target for atypical antipsychotic drugs. In the present study, an immunoperoxidase experiment was used to investigate the distribution of 5-HT2Ar immunoreactivity in the rat amygdaloid complex. In the basolateral amygdala, the colocalization of 5-HT2Ar with inhibitory transmitter γ-aminobutyric acid (GABA) was studied using double-immunofluorescence confocal microscopy. The staining pattern obtained was colchicine-sensitive. In fact, pretreatment with colchicine increased the number of 5-HT2Ar-immunoreactive somata. Accordingly, with the exception of the intercalated nuclei, the amygdaloid complex of colchicine-injected rats exhibited a high density of 5-HT2Ar-IR somata. Morphological analyses indicated that 5-HT2Ar was located on both excitatory and inhibitory neurons in the rat amygdaloid complex. In addition, double-immunofluorescence observations revealed that the great majority of GABA-immunoreactive neurons in the basolateral amygdala exhibited 5-HT2Ar immunoreactivity (66.3%-70.6% depending on the nucleus). These data help to clarify the complex role of the 5-HT2Ar in the amygdaloid complex suggesting that this receptor can regulate amygdaloid activity by acting on different neuronal populations.

Preliminary evidence of the short allele of the serotonin transporter gene predicting poor response to cognitive behavior therapy in posttraumatic stress disorder

OBJECTIVE

This study was intended to assess the extent to which the low-expression alleles of the serotonin transporter gene promoter predict poor response to cognitive behavior therapy in patients with posttraumatic stress disorder (PTSD).

METHOD

Forty-five patients with PTSD underwent an 8-week exposure-based cognitive behavior therapy program and provided mouth swabs or saliva samples to extract genomic DNA and classify individuals according to four allelic forms (S(A), S(G), L(A), L(G)) of the 5-HTT-linked polymorphic region (5-HTTLPR). We determined whether the 5-HTTLPR genotype predicted change in PTSD severity following treatment (n = 45) and 6 months later (n = 42).

RESULTS

After controlling for pretreatment PTSD severity and number of treatment sessions, the 5-HTTLPR low-expression genotype group (S or L(G) allele carriers) displayed more severe PTSD 6 months following treatment relative to other patients.

CONCLUSIONS

This study suggests a genetic contribution to treatment outcome following cognitive behavior therapy and implicates the serotonergic system in response to exposure-based treatments in PTSD.

Serotonin transporter availability in the amygdala and bed nucleus of the stria terminalis predicts anxious temperament and brain glucose metabolic activity

The serotonin transporter (5-HTT) plays a critical role in regulating serotonergic neurotransmission and is implicated in the pathophysiology of anxiety and affective disorders. Positron emission tomography scans using [(11)C]DASB [(11)C]-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile] to measure 5-HTT availability (an index of receptor density and binding) were performed in 34 rhesus monkeys in which the relationship between regional brain glucose metabolism and anxious temperament was previously established. 5-HTT availability in the amygdalohippocampal area and bed nucleus of the stria terminalis correlated positively with individual differences in a behavioral and neuroendocrine composite of anxious temperament. 5-HTT availability also correlated positively with stress-induced metabolic activity within these regions. Collectively, these findings suggest that serotonergic modulation of neuronal excitability in the neural circuitry associated with anxiety mediates the developmental risk for affect-related psychopathology.

Amygdala projections to central amygdaloid nucleus subdivisions and transition zones in the primate

In rats and primates, the central nucleus of the amygdala (CeN) is most known for its role in responses to fear stimuli. Recent evidence also shows that the CeN is required for directing attention and behaviors when the salience of competing stimuli is in flux. To examine how information flows through this key output region of the primate amygdala, we first placed small injections of retrograde tracers into the subdivisions of the central nucleus in Old world primates, and examined inputs from specific amygdaloid nuclei. The amygdalostriatal area and interstitial nucleus of the posterior limb of the anterior commissure (IPAC) were distinguished from the CeN using histochemical markers, and projections to these regions were also described. As expected, the basal nucleus and accessory basal nucleus are the main afferent connections of the central nucleus and transition zones. The medial subdivision of the central nucleus (CeM) receives a significantly stronger input from all regions compared to the lateral core subdivision (CeLcn). The corticoamygdaloid transition zone (a zone of confluence of the medial parvicellular basal nucleus, paralaminar nucleus, and the sulcal periamygdaloid cortex) provides the main input to the CeLcn. The IPAC and amygdalostriatal area can be divided in medial and lateral subregions, and receive input from the basal and accessory basal nucleus, with differential inputs according to subdivision. The piriform cortex and lateral nucleus, two important sensory interfaces, send projections to the transition zones. In sum, the CeM receives broad inputs from the entire amygdala, whereas the CeLcn receives more restricted inputs from the relatively undifferentiated corticoamygdaloid transition region. Like the CeN, the transition zones receive most of their input from the basal nucleus and accessory basal nucleus, however, inputs from the piriform cortex and lateral nucleus, and a lack of input from the parvicellular accessory basal nucleus, are distinguishing afferent features.

A link between serotonin-related gene polymorphisms, amygdala activity, and placebo-induced relief from social anxiety

Placebo may yield beneficial effects that are indistinguishable from those of active medication, but the factors underlying proneness to respond to placebo are widely unknown. Here, we used functional neuroimaging to examine neural correlates of anxiety reduction resulting from sustained placebo treatment under randomized double-blind conditions, in patients with social anxiety disorder. Brain activity was assessed during a stressful public speaking task by means of positron emission tomography before and after an 8 week treatment period. Patients were genotyped with respect to the serotonin transporter-linked polymorphic region (5-HTTLPR) and the G-703T polymorphism in the tryptophan hydroxylase-2 (TPH2) gene promoter. Results showed that placebo response was accompanied by reduced stress-related activity in the amygdala, a brain region crucial for emotional processing. However, attenuated amygdala activity was demonstrable only in subjects who were homozygous for the long allele of the 5-HTTLPR or the G variant of the TPH2 G-703T polymorphism, and not in carriers of short or T alleles. Moreover, the TPH2 polymorphism was a significant predictor of clinical placebo response, homozygosity for the G allele being associated with greater improvement in anxiety symptoms. Path analysis supported that the genetic effect on symptomatic improvement with placebo is mediated by its effect on amygdala activity. Hence, our study shows, for the first time, evidence of a link between genetically controlled serotonergic modulation of amygdala activity and placebo-induced anxiety relief.

A developmental neurobiological model of motivated behavior: anatomy, connectivity and ontogeny of the triadic nodes

Adolescence is the transition period that prepares individuals for fulfilling their role as adults. Most conspicuous in this transition period is the peak level of risk-taking behaviors that characterize adolescent motivated behavior. Significant neural remodeling contributes to this change. This review focuses on the functional neuroanatomy underlying motivated behavior, and how ontogenic changes can explain the typical behavioral patterns in adolescence. To help model these changes and provide testable hypotheses, a neural systems-based theory is presented. In short, the Triadic Model proposes that motivated behavior is governed by a carefully orchestrated articulation among three systems, approach, avoidance and regulatory. These three systems map to distinct, but overlapping, neural circuits, whose representatives are the striatum, the amygdala and the medial prefrontal cortex. Each of these system-representatives will be described from a functional anatomy perspective that includes a review of their connectivity and what is known of their ontogenic changes.

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.

Affect is a form of cognition: A neurobiological analysis

In this paper, we suggest that affect meets the traditional definition of "cognition" such that the affect-cognition distinction is phenomenological, rather than ontological. We review how the affect-cognition distinction is not respected in the human brain, and discuss the neural mechanisms by which affect influences sensory processing. As a result of this sensory modulation, affect performs several basic "cognitive" functions. Affect appears to be necessary for normal conscious experience, language fluency, and memory. Finally, we suggest that understanding the differences between affect and cognition will require systematic study of how the phenomenological distinction characterising the two comes about, and why such a distinction is functional.

From anxiety to autism: spectrum of abnormal social behaviors modeled by progressive disruption of inhibitory neuronal function in the basolateral amygdala in Wistar rats

RATIONALE

Social behaviors are disrupted in several psychiatric disorders. The amygdala is a key brain region involved in social behaviors, and amygdala pathology has been implicated in disease states ranging from social anxiety disorder to autism.

OBJECTIVE

To test the effects of progressive disruption of the inhibitory function within the basolateral nucleus of the amygdala (BLA) on conspecific social interaction in rats and investigate functional networks from the ventral medial prefrontal cortex (mPFCv) to the BLA.

MATERIALS AND METHODS

BLA inhibitory tone was disrupted by priming it with the stress-peptide corticotrophin releasing factor (CRF) receptor agonist urocortin 1 (Ucn 1, 6 fmol), or by selective lesioning of a subset of BLA-GABAergic interneurons containing neurokinin 1 receptors using the targeted toxin SSP-Saporin. The effects of the disruption of GABAergic tone in the BLA were examined using a repeated exposure and habituation paradigm of social interaction (SI/h). Lesions and selectivity of lesions were confirmed postmortem. Additionally, effects of stimulating mPFCv on cFos activity in interneurons of the BLA were examined.

RESULTS

Rats primed with Ucn 1 showed persistent social inhibition, which could be overcome with habituation, putatively modeling social anxiety. Rats with a selective lesioning of a subset of GABAergic interneurons in the BLA exhibited persistent social inhibition that was not reversed by SI/h paradigm. We also demonstrate selective functional inputs to this subset of interneurons when mPFCv was activated.

CONCLUSIONS

These models with different gradations of disrupted BLA inhibition could help to study social dysfunction in disorders ranging from social anxiety to autism spectrum disorders.

Serotonin-immunoreactive axon terminals innervate pyramidal cells and interneurons in the rat basolateral amygdala

The basolateral nuclear complex of the amygdala (BLC) receives a dense serotonergic innervation that appears to play a critical role in the regulation of mood and anxiety. However, little is known about how serotonergic inputs interface with different neuronal subpopulations in this region. To address this question, dual-labeling immunohistochemical techniques were used at the light and electron microscopic levels to examine inputs from serotonin-immunoreactive (5-HT+) terminals to different neuronal subpopulations in the rat BLC. Pyramidal cells were labeled by using antibodies to calcium/calmodulin-dependent protein kinase II, whereas different interneuronal subpopulations were labeled by using antibodies to a variety of interneuronal markers including parvalbumin (PV), vasoactive intestinal peptide (VIP), calretinin, calbindin, cholecystokinin, and somatostatin. The BLC exhibited a dense innervation by thin 5-HT+ axons. Electron microscopic examination of the anterior basolateral nucleus (BLa) revealed that 5-HT+ axon terminals contained clusters of small synaptic vesicles and a smaller number of larger dense-core vesicles. Serial section reconstruction of 5-HT+ terminals demonstrated that 76% of these terminals formed synaptic junctions. The great majority of these synapses were symmetrical. The main targets of 5-HT+ terminals were spines and distal dendrites of pyramidal cells. However, in light microscopic preparations it was common to observe apparent contacts between 5-HT+ terminals and all subpopulations of BLC interneurons. Electron microscopic analysis of the BLa in sections dual-labeled for 5-HT/PV and 5-HT/VIP revealed that many of these contacts were synapses. These findings suggest that serotonergic axon terminals differentially innervate several neuronal subpopulations in the BLC.

Serotonin transporter gene promoter polymorphism predicts SSRI response in generalized social anxiety disorder

OBJECTIVES

To determine whether variation in the serotonin transporter gene promoter (5HTTLPR) influences the efficacy of selective serotonin reuptake inhibitors (SSRIs) in generalized social anxiety disorder (GSAD).

METHODS

Consecutive series of N=32 patients with DSM-IV GSAD for whom DNA and standardized outcome data from a 12-week SSRI trial were available. After ensuring that neither clinical response [clinical global impression of change scale (CGI-C)] nor 5HTTLPR genotype was confounded by ethnicity or sex, we determined whether the number of copies (0, 1, or 2) of hi-risk alleles using either the diallelic L-S system or the triallelic La-Lg-S system, predicted response and change in Liebowitz social anxiety scale (LSAS) and brief social phobia scale (BSPS) scores during SSRI treatment.

RESULTS

Twenty-one patients (66%) were responders to SSRI (i.e., CGI-C much or very much improved). A trend was seen for a linear association between 5HTTLPR genotype and likelihood of response to SSRI: diallelic classification L/L 7/8 (88%), L/S 12/18 (67%), S/S 2/6 (33%), p=0.051; triallelic classification L'/L' 4/5 (80%), L'/S' 14/19 (74%), S'/S' 3/8 (38%), p=0.093. Reduction in LSAS (and BSPS) scores during SSRI treatment was significantly (p<0.02) associated with 5HTTLPR genotype using either the diallelic or triallelic classification.

CONCLUSIONS

Variation in a functional polymorphism known to influence serotonin reuptake is associated with SSRI response in patients with GSAD. Independent replication in larger samples is required before the predictive utility of this information can be confirmed and generalized to clinical settings.

The distribution of dopamine D1 receptor and μ-opioid receptor 1 receptor immunoreactivities in the amygdala and interstitial nucleus of the posterior limb of the anterior commissure: Relationships to tyrosine hydroxylase and opioid peptide terminal systems

Mismatches between dopamine innervation and dopamine D1 receptor (D1) distribution have previously been demonstrated in the intercalated cell masses of the rat amygdala. Here the distribution of enkephalin and beta-endorphin immunoreactive (IR) nerve terminals with respect to their mu-opioid receptors is examined in the intercalated cell masses, along with a further immunohistochemical analysis of the dopamine/D1 mismatches. A similar analysis is also made within the extended amygdala. A spatial mismatch in distribution patterns was found between the mu-opioid receptor-1 immunoreactivity and enkephalin IR in the main intercalated island of the amygdala. Discrete cell patches of dopamine D1 receptor and mu-opioid receptor-1 IR were also identified in a distinct region of the extended amygdala, the interstitial nucleus of the posterior limb of the anterior commissure, medial division (IPACM), which displayed sparse tyrosine hydroxylase or enkephalin/beta-endorphin IR nerve terminals. Furthermore, distinct regions of the main intercalated island that showed dopamine/D1 receptor matches (the rostral and rostrolateral parts) were associated with strong dopamine and cyclic AMP regulated phosphoprotein, 32 kDa-IR in several D1 IR neuronal cell bodies and dendrites, whereas this was not the case for the dopamine/D1 mismatch areas (the rostromedial and caudal parts) of the main intercalated island. The lack of correlation between the terminal/receptor distribution patterns suggests a role for volume transmission for mu-opioid receptor- and dopamine D1 receptor-mediated transmission in distinct regions of the amygdala and extended amygdala. This may have implications for amygdaloid function, where slow long lasting responses may develop as a result of volume transmission operating in opioid peptide and dopaminergic communication.