Early localization of mRNA coding for 5-HT1A receptors in human brain during development

Rewiring of the Serotonin System in Major Depression

Serotonin is a key neurotransmitter that is implicated in a wide variety of behavioral and cognitive phenotypes. Originating in the raphe nuclei, 5-HT neurons project widely to innervate many brain regions implicated in the functions. During the development of the brain, as serotonin axons project and innervate brain regions, there is evidence that 5-HT plays key roles in wiring the developing brain, both by modulating 5-HT innervation and by influencing synaptic organization within corticolimbic structures. These actions are mediated by 14 different 5-HT receptors, with region- and cell-specific patterns of expression. More recently, the role of the 5-HT system in synaptic re-organization during adulthood has been suggested. The 5-HT neurons have the unusual capacity to regrow and reinnervate brain regions following insults such as brain injury, chronic stress, or altered development that result in disconnection of the 5-HT system and often cause depression, anxiety, and cognitive impairment. Chronic treatment with antidepressants that amplify 5-HT action, such as selective serotonin reuptake inhibitors (SSRIs), appears to accelerate the rewiring of the 5-HT system by mechanisms that may be critical to the behavioral and cognitive improvements induced in these models. In this review, we survey the possible 5-HT receptor mechanisms that could mediate 5-HT rewiring and assess the evidence that 5-HT-mediated brain rewiring is impacting recovery from mental illness. By amplifying 5-HT-induced rewiring processes using SSRIs and selective 5-HT agonists, more rapid and effective treatments for injury-induced mental illness or cognitive impairment may be achieved.

The development of monoaminergic neurotransmitter systems in childhood and adolescence

Brain maturation extends throughout adolescence well into early adulthood. Knowledge on developmental changes is crucial for age-appropriate pharmacotherapy. This article reviews data on maturational processes with a focus on the noradrenergic, dopaminergic, and serotonergic neurotransmitter systems.The literature was searched with a focus on studies in humans. However, since data in humans are limited animal studies were also included. All reviewed neurotransmitter systems show age-related development processes that differentiate child and adolescent brain function from those of adult brains. Unfortunately, the state of knowledge surrounding development-related changes remains sufficiently sparse, There is a high need for more studies on pediatric psychopharmacology and its biological underpinnings. Safety and efficacy of psychopharmacological medicines cannot be readily extrapolated from adults.

Early human brain development: Starring the subplate

This review summarizes early human brain development on the basis of neuroanatomical data and functional connectomics. It indicates that the most significant changes in the brain occur during the second half of gestation and the first three months post-term, in particular in the cortical subplate and cerebellum. As the transient subplate pairs a high rate of intricate developmental changes and interactions with clear functional activity, two phases of development are distinguished: a) the transient cortical subplate phase, ending at 3 months post-term when the permanent circuitries in the primary motor, somatosensory and visual cortices have replaced the subplate; and subsequently, b) the phase in which the permanent circuitries dominate. In the association areas the subplate dissolves in the remainder of the first postnatal year. During both phases developmental changes are paralleled by continuous reconfigurations in network activity. The reviewed literature also suggests that disruption of subplate development may play a pivotal role in developmental disorders, such as cerebral palsy, autism spectrum disorders, attention deficit hyperactivity disorder and schizophrenia.

Placental and fetal effects of antenatal exposure to antidepressants or untreated maternal depression

OBJECTIVE

To assess systematically the effects of antidepressants and untreated maternal depression on human placenta and the developing fetus.

METHODS

Pertinent medical literature information was identified using MEDLINE/PubMed, SCOPUS and EMBASE. Electronic searches, limited to human studies published in English, provided 21 studies reporting primary data on placental and fetal effects of antidepressant exposure or untreated gestational depression.

RESULTS

The impact of antidepressants and non-medicated maternal depression on placental functioning and fetal biochemical architecture seems to be demonstrated, although its clinical significance remains unclear. More robust data seem to indicate that exposure to either antidepressants or untreated maternal depression may induce epigenetic changes and interfere with the physiological fetal behavior. Two cases of iatrogenic fetal tachyarrhythmia have also been reported.

CONCLUSIONS

Future research should clarify the clinical relevance of the impact of antidepressant and untreated maternal depression exposure on placental functioning. Moreover, ultrasound studies investigating fetal responses to antidepressants or maternal depressive symptoms are mandatory. This assessment should be performed during the whole duration of gestational period, when different fetal behavioral patterns become progressively detectable. Analyses of biochemical and epigenetic modifications associated with maternal mood symptoms and antidepressant treatment should also be implemented.

Brain signaling systems in the Type 2 diabetes and metabolic syndrome: promising target to treat and prevent these diseases

The changes in the brain signaling systems play an important role in etiology and pathogenesis of Type 2 diabetes mellitus (T2DM) and metabolic syndrome (MS), being a possible cause of these diseases. Therefore, their restoration at the early stages of T2DM and MS can be regarded as a promising way to treat and prevent these diseases and their complications. The data on the functional state of the brain signaling systems regulated by insulin, IGF-1, leptin, dopamine, serotonin, melanocortins and glucagon-like peptide-1, in T2DM and MS, are analyzed. The pharmacological approaches to restoration of these systems and improvement of insulin sensitivity, energy expenditure, lipid metabolism, and to prevent diabetic complications are discussed.

Genetic and early environmental influences on the serotonin system: consequences for brain development and risk for psychopathology

BACKGROUND

Despite more than 60 years of research in the role of serotonin (5-HT) in psychopathology, many questions still remain. From a developmental perspective, studies have provided more insight into how 5-HT dysfunctions acquired in utero or early in life may modulate brain development. This paper discusses the relevance of the developmental role of 5-HT for the understanding of psychopathology. We review developmental milestones of the 5-HT system, how genetic and environmental 5-HT disturbances could affect brain development and the potential role of DNA methylation in 5-HT genes for brain development.

METHODS

Studies were identified using common databases (e.g., PubMed, Google Scholar) and reference lists.

RESULTS

Despite the widely supported view that the 5-HT system matures in early life, different 5-HT receptors, proteins and enzymes have different developmental patterns, and development is brain region-specific. A disruption in 5-HT homeostasis during development may lead to structural and functional changes in brain circuits that modulate emotional stress responses, including subcortical limbic and (pre)frontal areas. This may result in a predisposition to psychopathology. DNA methylation might be one of the underlying physiologic mechanisms.

LIMITATIONS

There is a need for prospective studies. The impact of stressors during adolescence on the 5-HT system is understudied. Questions regarding efficacy of drugs acting on 5-HT still remain.

CONCLUSION

A multidisciplinary and longitudinal approach in designing studies on the role of 5-HT in psychopathology might help to bring us closer to the understanding of the role of 5-HT in psychopathology.

Mother/offspring co-administration of the traditional herbal remedy yokukansan during the nursing period influences grooming and cerebellar serotonin levels in a rat model of neurodevelopmental disorders

Neurodevelopmental impairment in the serotonergic system may be involved in autism spectrum disorder. Yokukansan is a traditional herbal remedy for restlessness and agitation in children, and mother-infant co-administration (MICA) to both the child and the nursing mother is one of the recommended treatment approaches. Recent studies have revealed the neuropharmacological properties of Yokukansan (YKS), including its 5-HT1A (serotonin) receptor agonistic effects. We investigated the influence of YKS treatment on behavior in a novel environment and on brain monoamine metabolism during the nursing period in an animal model of neurodevelopmental disorders, prenatally BrdU (5-bromo-2'-deoxyuridine)-treated rats (BrdU-rats). YKS treatment did not influence locomotor activity in BrdU-rats but reduced grooming in open-field tests. YKS treatment without MICA disrupted the correlation between locomotor behaviors and rearing and altered levels of serotonin and its metabolite in the cerebellum. These effects were not observed in the group receiving YKS treatment with MICA. These data indicate a direct pharmacological effect of YKS on the development of grooming behavior and profound effects on cerebellar serotonin metabolism, which is thought to be influenced by nursing conditions.

5-HT(1A) receptor: an old target as a new attractive tool in drug discovery from central nervous system to cancer

The serotonin receptor subtype 5-HT(1A) was one of the first serotonin receptor subtypes pharmacologically characterized. This receptor subtype has long been object of intense research and is implicated in the pathogenesis and treatment of anxiety and depressive disorders. In recent years, new chemical entities targeting the 5-HT(1A) receptor (alone or in combination with other molecular targets) have been proposed for novel therapeutic uses in neuroprotection, cognitive impairment, Parkinson's disease, pain treatment, malignant carcinoid syndrome, and prostate cancer. This Perspective compares existing data on expression and signaling activity of the 5-HT(1A) receptor to a ligand with an intrinsic agonist or antagonist profile. Our purpose is also to make a complete overview, useful for underlining the features needed to select a specific pharmacological profile rather than another one. This aspect could be really interesting to consider and justify the 5-HT(1A) receptor as a new attractive target for drug discovery.

Rethinking 5-HT1A receptors: emerging modes of inhibitory feedback of relevance to emotion-related behavior

The complexities of the involvement of the serotonin transmitter system in numerous biological processes and psychiatric disorders is, to a substantial degree, attributable to the large number of serotonin receptor families and subtypes that have been identified and characterized for over four decades. Of these, the 5-HT(1A) receptor subtype, which was the first to be cloned and characterized, has received considerable attention based on its purported role in the etiology and treatment of mood and anxiety disorders. 5-HT(1A) receptors function both at presynaptic (autoreceptor) and postsynaptic (heteroreceptor) sites. Recent research has implicated distinct roles for these two populations of receptors in mediating emotion-related behavior. New concepts as to how 5-HT(1A) receptors function to control serotonergic tone throughout life were highlights of the proceedings of the 2012 Serotonin Club Meeting in Montpellier, France. Here, we review recent findings and current perspectives on functional aspects of 5-HT(1A) auto- and heteroreceptors with particular regard to their involvement in altered anxiety and mood states.

Serotonin 1A receptor (5-HT1A) of the sea lamprey: cDNA cloning and expression in the central nervous system

Serotonergic cells are among the earliest neurons to be born in the developing central nervous system and serotonin is known to regulate the development of the nervous system. One of the major targets of the activity of serotonergic cells is the serotonin 1A receptor (5-HT1A), an ancestral archetypical serotonin receptor. In this study, we cloned and characterized the 3D structure of the sea lamprey 5-HT1A, and studied the expression of its transcript in the central nervous system by means of in situ hybridization. In phylogenetic analyses, the sea lamprey 5-HT1A sequence clustered together with 5-HT1A sequences of vertebrates and emerged as an outgroup to all gnathostome sequences. In situ hybridization analysis during prolarval, larval and adult stages showed a widespread expression of the lamprey 5-ht1a transcript. In P1 prolarvae 5-ht1a mRNA expression was observed in diencephalic nuclei, the rhombencephalon and rostral spinal cord. At P2 prolarval stage the 5-ht1a expression extended to other brain areas including telencephalic regions. 5-ht1a expression in larvae was observed throughout almost all the main brain regions with the strongest expression in the olfactory bulbs, lateral pallium, striatum, preoptic region, habenula, prethalamus, thalamus, pretectum, hypothalamus, rhombencephalic reticular area, dorsal column nucleus and rostral spinal cord. In adults, the 5-ht1a transcript was also observed in cells of the subcommissural organ. Comparison of the expression of 5-ht1a between the sea lamprey and other vertebrates reveals a conserved pattern in most of the brain regions, likely reflecting the ancestral vertebrate condition.

Serotonin receptor expression in human prefrontal cortex: balancing excitation and inhibition across postnatal development

Serotonin and its receptors (HTRs) play critical roles in brain development and in the regulation of cognition, mood, and anxiety. HTRs are highly expressed in human prefrontal cortex and exert control over prefrontal excitability. The serotonin system is a key treatment target for several psychiatric disorders; however, the effectiveness of these drugs varies according to age. Despite strong evidence for developmental changes in prefrontal Htrs of rodents, the developmental regulation of HTR expression in human prefrontal cortex has not been examined. Using postmortem human prefrontal brain tissue from across postnatal life, we investigated the expression of key serotonin receptors with distinct inhibitory (HTR1A, HTR5A) and excitatory (HTR2A, HTR2C, HTR4, HTR6) effects on cortical neurons, including two receptors which appear to be expressed to a greater degree in inhibitory interneurons of cerebral cortex (HTR2C, HTR6). We found distinct developmental patterns of expression for each of these six HTRs, with profound changes in expression occurring early in postnatal development and also into adulthood. However, a collective look at these HTRs in terms of their likely neurophysiological effects and major cellular localization leads to a model that suggests developmental changes in expression of these individual HTRs may not perturb an overall balance between inhibitory and excitatory effects. Examining and understanding the healthy balance is critical to appreciate how abnormal expression of an individual HTR may create a window of vulnerability for the emergence of psychiatric illness.

Layer II/III of the prefrontal cortex: Inhibition by the serotonin 5-HT1A receptor in development and stress

The modulation of the prefrontal cortex by the neurotransmitter serotonin (5-HT) is thought to play a key role in determining adult anxiety levels. Layer II/III of the prefrontal cortex, which mediates communication across cortical regions, displays a high level of 5-HT(1A) receptor binding in normal individuals and a significantly lower level in patients with mood and anxiety disorders. Here, we examine how serotonin modulates pyramidal neurons in layer II/III of the rat prefrontal cortex throughout postnatal development and in adulthood. Using whole cell recordings in brain slices of the rat medial prefrontal cortex, we observed that serotonin directly inhibits layer II/III pyramidal neurons through 5-HT(1A) receptors across postnatal development (postnatal days 6-96). In adulthood, a sex difference in these currents emerges, consistent with human imaging studies of 5-HT(1A) receptor binding. We examined the effects of early life stress on the 5-HT(1A) receptor currents in layer II/III. Surprisingly, animals subjected to early life stress displayed significantly larger 5-HT(1A)-mediated outward currents throughout the third and fourth postnatal weeks after elevated 5-HT(1A) expression during the second postnatal week. Subsequent exposure to social isolation in adulthood resulted in the almost-complete elimination of 5-HT(1A) currents in layer II/III neurons suggesting an interaction between early life events and adult experiences. These data represent the first examination of functional 5-HT(1A) receptors in layer II/III of the prefrontal cortex during normal development as well as after stress.

Comparative analysis of serotonin receptor (HTR1A/HTR1B families) and transporter (slc6a4a/b) gene expression in the zebrafish brain

In this study we analyze 5-hydroxytryptamine [5-HT]; serotonin) signaling in zebrafish, an increasingly popular vertebrate disease model. We compare and contrast expression of the 5-HT transporter genes slc6a4a and slc6a4b, which identify 5-HT-producing neurons and three novel 5-HT receptors, htr1aa, htr1ab, and htr1bd. slc6a4a and slc6a4b are expressed in the raphe nuclei, retina, medulla oblongata, paraventricular organ, pretectal diencephalic complex, and caudal zone of the periventricular hypothalamus, in line with the expression profiles of homologues from other vertebrates. Our analysis of serotonin transporter (SERT)-encoding genes also identifies parallel genetic pathways used to build the 5-HT system in zebrafish. In cells in which 5-HT is synthesized by tph1, slc6a4b is used for re-uptake, whereas tph2-positive cells utilize slc6a4a. The receptors htr1aa, htr1ab, and htr1bd also show widespread expression in both the larval and adult brain. Receptor expression is seen in the superior raphe nucleus, retina, ventral telencephalon, optic tectum, thalamus, posterior tuberculum, cerebellum, hypothalamus, and reticular formation, thus implicating 5-HT signaling in several neural circuits. We also examine larval brains double-labeled with 5-HTergic and dopaminergic pathway-specific antibodies, to uncover the identity of some 5-HTergic target neurons. Furthermore, comparison of the expression of transporter and receptor genes also allows us to map sites of autoreceptor activity within the brain. We detect autoreceptor activity in the pretectal diencephalic cluster (htr1aa-, htr1ab-, htr1bd-, and slc6a4a-positive), superior raphe nucleus (htr1aa-, htr1ab-, and slc6a4a-positive), paraventricular organ (htr1aa-, htr1ab-, htr1bd-, and slc6a4b-positive), and the caudal zone of the periventricular hypothalamus (htr1ab- and slc6a4b-positive).

The stimulus effects of 8-OH-DPAT: evidence for a 5-HT2A receptor-mediated component

A previous investigation in our laboratory found that the stimulus effects of the 5-HT2A agonist, LSD, are potentiated by 5-HT1A receptor agonists including the prototypic agonist, 8-OH-DPAT. Also suggestive of behaviorally relevant interactions between 5-HT1A and 5-HT2A receptors are behavioral analyses of locomotor activity, head-twitch response, forepaw treading and production of the serotonin syndrome; in some instances effects are augmented, in other, diminished. These observations led us in the present investigation to test the hypothesis that stimulus control by 8-OH-DPAT [0.2 mg/kg; 15 min pretreatment time] is modulated by 5-HT2A ligands. Stimulus control was established with 8-OH-DPAT in a group of 10 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. As shown previously, stimulus control by 8-OH-DPAT and the generalization of 8-OH-DPAT to the 5-HT1A partial agonist, buspirone, was completely blocked by the selective 5-HT1A antagonist, WAY-100635. In contrast, antagonism by the selective 5-HT2A antagonist, M100907 [0.1 mg/kg; 30 min pretreatment time], of 8-OH-DPAT and of the generalization of 8-OH-DPAT to buspirone was statistically significant but less than complete. In light of our previous conclusions regarding the interactions of 5-HT1A agonists with LSD-induced stimulus control, the present data suggest that the interaction between 5-HT1A and 5-HT2A receptors is bidirectional in drug discrimination studies.

Membrane Organization and Function of the Serotonin1A Receptor

(1) The serotonin(1A) receptor is a G-protein coupled receptor involved in several cognitive, behavioral, and developmental functions. It binds the neurotransmitter serotonin and signals across the membrane through its interactions with heterotrimeric G-proteins. (2) Lipid-protein interactions in membranes play an important role in the assembly, stability, and function of membrane proteins. The role of membrane environment in serotonin(1A) receptor function is beginning to be addressed by exploring the consequences of lipid manipulations on the ligand binding and G-protein coupling of serotonin(1A) receptors, the ability to functionally solubilize the serotonin(1A) receptor, and the factors influencing the membrane organization of the serotonin(1A) receptor. (3) Recent developments involving the application of detergent-based and detergent-free approaches to understand the membrane organization of the serotonin(1A) receptor under conditions of ligand activation and modulation of membrane lipid content, with an emphasis on membrane cholesterol, are described.

5-HT1A-iCre, a new transgenic mouse line for genetic analyses of the serotonergic pathway

The 5-HT1A receptor not only plays an important role in brain physiology but it may be also implicated in the etiology of behavioral disorders such as pathological anxiety. To further define the role of 5-HT1A receptor-expressing neurons, we generated a transgenic mouse line expressing Cre recombinase in these cells. The 5-HT1A receptor open reading frame was substituted for that of Cre recombinase in a BAC containing the 5-HT1A receptor gene. In adult transgenic brain, Cre expression perfectly matched the distribution of 5-HT1A receptor mRNA. Additionally, Cre-mediated DNA recombination was restricted to neuronal populations that express the receptor, e.g., cerebral cortex, septum, hippocampus, dorsal raphe, thalamic, hypothalamic and amygdaloid nuclei, and spinal cord. Recombination occurred as early as E13 in trigeminal nerve, spinal ganglia and spinal cord. This transgenic line will allow the generation of conditional mutant mice that lack specific gene products along the serotonergic pathways and represents a unique tool for studying 5-HT1A-mediated serotonin signaling in the developing and adult brain.

Adolescent cortical development: a critical period of vulnerability for addiction

Cortical growth and remodeling continues from birth through youth and adolescence to stable adult levels changing slowly into senescence. There are critical periods of cortical development when specific experiences drive major synaptic rearrangements and learning that only occur during the critical period. For example, visual cortex is characterized by a critical period of plasticity involved in establishing visual acuity. Adolescence is defined by characteristic behaviors that include high levels of risk taking, exploration, novelty and sensation seeking, social interaction and play behaviors. In addition, adolescence is the final period of development of the adult during which talents, reasoning and complex adult behaviors mature. This maturation of behaviors corresponds with periods of marked changes in neurogenesis, cortical synaptic remodeling, neurotransmitter receptors and transporters, as well as major changes in hormones. Frontal cortical development is later in adolescence and likely contributes to refinement of reasoning, goal and priority setting, impulse control and evaluating long and short term rewards. Adolescent humans have high levels of binge drinking and experimentation with other drugs. This review presents findings supporting adolescence as a critical period of cortical development important for establishing life long adult characteristics that are disrupted by alcohol and drug use.

Ligand Binding and G-protein Coupling of the Serotonin1A Receptor in Cholesterol-enriched Hippocampal Membranes

The serotonin1A receptor is the most extensively studied member of the family of seven transmembrane domain G-protein coupled serotonin receptors. Since a large portion of such transmembrane receptors remains in contact with the membrane lipid environment, lipid–protein interactions assume importance in the structure-function analysis of such receptors. We have earlier reported the requirement of cholesterol for serotonin1A receptor function in native hippocampal membranes by specific depletion of cholesterol using methyl- β-cyclodextrin. In this paper, we monitored the serotonin1A receptor function in membranes that are enriched in cholesterol using a complex prepared from cholesterol and methyl-β-cyclodextrin. Our results indicate that ligand binding and receptor/G-protein interaction of the serotonin1A receptor do not exhibit significant difference in native and cholesterol-enriched hippocampal membranes indicating that further enrichment of cholesterol has little functional consequence on the serotonin1A receptor function. These results therefore provide new information on the effect of cholesterol enrichment on the hippocampal serotonin1A receptor function.

Ontogeny of the human central nervous system: what is happening when?

The present paper reviews current data on the structural development of the human nervous system. Focus is on the timing of ontogenetic events in the telencephalon. Neuronal proliferation and migration especially occur during the first half of gestation; the second half of gestation is the period of the existence of the functionally important transient structure 'subplate' and the major period of glial cell proliferation and programmed cell death. Axon and dendrite sprouting and synapse formation bloom during the last trimester of gestation and the first postnatal year. Major part of telencephalic myelination occurs during the first year after birth. Many developmental processes, such as myelination, synapse formation and synapse elimination continue throughout childhood and adolescence. Evidence is emerging that the peak of synapse elimination occurs between puberty and the onset of adulthood. Neurotransmitter systems are present from early foetal life onwards and their pre- and perinatal development is characterized by periods of transient overexpression. The latter is for instance true for the acetylcholinergic, catecholaminergic and glutamate systems. Thus, the development of the human brain is characterized by a protracted, neatly orchestrated chain of specific ontogenetic events. The continuous changes of the nervous system have consequences for vulnerability to adverse conditions, for diagnostics and for physiotherapeutical intervention.

Role of cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The modulatory role of cholesterol in the function of a number of membrane proteins is well established. This effect has been proposed to occur either due to a specific molecular interaction between cholesterol and membrane proteins or due to alterations in the membrane physical properties induced by the presence of cholesterol. The contemporary view regarding heterogeneity in cholesterol distribution in membrane domains that sequester certain types of membrane proteins while excluding others has further contributed to its significance in membrane protein function. The seven transmembrane domain G-protein coupled receptors (GPCRs) are among the largest protein families in mammals and represent approximately 2% of the total proteins coded by the human genome. Signal transduction events mediated by this class of proteins are the primary means by which cells communicate with and respond to their external environment. GPCRs therefore represent major targets for the development of novel drug candidates in all clinical areas. In view of their importance in cellular signaling, the interaction of cholesterol with such receptors represents an important determinant in functional studies of such receptors. This review focuses on the effect of cholesterol on the membrane organization and function of GPCRs from a variety of sources, with an emphasis on the more contemporary role of cholesterol in maintaining a domain-like organization of such receptors on the cell surface. Importantly, the recently reported role of cholesterol in the function and organization of the neuronal serotonin(1A) receptor, a representative of the GPCR family which is present endogenously in the hippocampal region of the brain, will be highlighted.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

Membrane Organization and Dynamics of the G-Protein-Coupled Serotonin1A Receptor Monitored Using Fluorescence-Based Approaches

The G-protein-coupled receptor (GPCR) superfamily represents one of the largest classes of molecules involved in signal transduction across the plasma membrane. Fluorescence-based approaches have provided valuable insights into GPCR functions such as receptor-receptor and receptor-ligand interactions, real-time assessment of signal transduction, receptor dynamics on the plasma membrane, and intracellular trafficking of receptors. This has largely been possible with the use of fluorescent probes such as the green fluorescent protein (GFP) from the jellyfish Aequoria victoria and its variants. We discuss the potential of fluorescence-based approaches in providing novel information on the membrane organization and dynamics of the G-protein-coupled serotonin1A receptor tagged to the enhanced yellow fluorescent protein (EYFP).

Cholesterol modulates the antagonist-binding function of hippocampal serotonin1A receptors

The serotonin1A receptor is the most extensively studied member of the family of seven transmembrane domain G-protein coupled serotonin receptors. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions such as sleep, mood, pain, addiction, locomotion, sexual activity, depression, anxiety, alcohol abuse, aggression and learning. Since a significant portion of the protein lies embedded in the membrane and the ligand-binding pocket is defined by the transmembrane stretches in such receptors, membrane composition and organization represent a crucial parameter in the structure-function analysis of G-protein coupled receptors. In this paper, we have monitored the role of membrane cholesterol in the ligand-binding function of the hippocampal serotonin1A receptor. Our results demonstrate that the reduction of membrane cholesterol significantly attenuates the antagonist-binding function of the serotonin1A receptor. Based on prior pharmacological knowledge regarding the requirements for the antagonist to bind the receptor, our results indicate that membrane cholesterol modulates receptor function independently of its ability to interact with G-proteins. These effects on ligand-binding function of the receptor are predominantly reversed upon cholesterol-replenishment of cholesterol-depleted membranes. When viewed in the light of our earlier results on the effect of cholesterol depletion on the serotonin1A receptor/G-protein interaction, these results comprehensively demonstrate the importance of cholesterol in the serotonin1A receptor function and form the basis for understanding lipid-protein interactions involving this important neuronal receptor.

Developmental expression of 5-HT 5A receptor mRNA in the rat brain

In the central nervous system, serotonin (5-HT) may function as a mitogen as well as a neurotransmitter; and its early appearance suggests a potential role in development. The present experiments were done to determine the localization of the mRNA coding for the 5-HT 5A receptor during development of the rat brain. 5-HT 5A gene transcription was assessed by in situ hybridization, from E18 and during postnatal (PN) development. An intense signal of 5-HT 5A mRNA was found in the cerebral cortex and olfactory nucleus at E18, PN0 and PN5. A sharp decrease at PN11 was followed by an increase until reaching the adult level in the cerebral cortex; whereas in the olfactory nucleus, transcription remained weak. In contrast, in the hippocampal formation the signal was weak in the CA1, CA2 and CA3 regions at E18 and PO; increased at P5 and then decreased at P11 before attaining the adult level. We conclude that the gene coding for the 5-HT 5A receptor is already active in the embryonic rat brain and is differentially expressed during development.

Ontogeny of 5-HT1A receptor expression in the developing hippocampus

Serotonin (5-HT) has long been implicated in a number of neurodevelopmental processes including neuronal cell division, migration, neurite outgrowth, and synapse formation. However, relatively little is known about how these effects are mediated during normal brain development in vivo and the identity of the receptor subtypes involved in mediating these effects. In recent years, a number of pharmacological studies have suggested a role for the serotonin 1A (5HT1A) receptor subtype in mediating the developmental effects of 5-HT in the hippocampus. These studies, however, have been difficult to interpret due to lack of information regarding the expression and distribution of 5HT1A in the developing brain and hippocampus in particular. In the current study, specific anti-5-HT1A antibodies, developed in our laboratory [F.C. Zhou, T.D. Patel, D. Swartz, Y. Xu, M.R. Kelley, Production and characterization of an anti-serotonin 1A receptor antibody which detects functional 5-HT1A binding sites, Brain Res Mol Brain Res, 69 (1999) 186-201], were utilized to map the ontogeny and distribution of the 5HT1A receptor protein in the developing rat hippocampus through embryonic and early postnatal life. This is the first such study of 5-HT1A expression in the developing rat brain. Our findings revealed that expression of the 5HT1A receptor emerges during the initial stages of embryonic hippocampal development. Remarkably, most if not all hippocampal neurons begin to express 5HT1A shortly upon completion of their terminal mitosis. We found that 5HT1A is initially concentrated around the cell bodies and later becomes more sparsely distributed along the dendrites after the neurons have matured. In addition to postmitotic neurons, we have observed that S100 and GFAP positive glia transiently express 5HT1A during early postnatal development of the hippocampus. These findings demonstrate that the 5-HT1A receptor is positioned to mediate developmental effects of serotonin in the hippocampus. Furthermore, the temporal patterns of expression suggest a role for 5-HT1A in postmitotic events such as neuronal migration, neurite outgrowth, and phenotypic differentiation.

Cholesterol modulates ligand binding and G-protein coupling to serotonin(1A) receptors from bovine hippocampus

The serotonin(1A) (5-HT(1A)) receptor is an important member of the superfamily of seven-transmembrane domain G-protein-coupled receptors. We have examined the modulatory role of cholesterol on the ligand binding activity and G-protein coupling of the bovine hippocampal 5-HT(1A) receptor by depleting cholesterol from native membranes using methyl-beta-cyclodextrin (MbetaCD). Removal of cholesterol from bovine hippocampal membranes using varying concentrations of MbetaCD results in a concentration-dependent reduction in specific binding of the agonist 8-OH-DPAT to 5-HT(1A) receptors. This is accompanied by alterations in binding affinity and sites obtained from analysis of binding data. Importantly, cholesterol depletion affected G-protein-coupling of the receptor as monitored by the GTP-gamma-S assay. The concomitant changes in membrane order were reported by changes in fluorescence polarization of membrane probes such as DPH and TMA-DPH, which are incorporated at different locations (depths) in the membrane. Replenishment of membranes with cholesterol led to recovery of ligand binding activity as well as membrane order to a considerable extent. Our results provide evidence, for the first time, that cholesterol is necessary for ligand binding and G-protein coupling of this important neurotransmitter receptor. These results could have significant implications in understanding the influence of the membrane lipid environment on the activity and signal transduction of other G-protein-coupled transmembrane receptors.

Developmental regulation of 5-HT1A receptor mRNA in the fetal limbic system: response to antenatal glucocorticoid

The developmental changes in 5-HT1A receptor mRNA expression associated with advancing gestational age were examined in the fetal guinea pig hippocampus and dentate gyrus (DG) by in situ hybridization. We found that 5-HT1A receptor mRNA was present in the hippocampal CA1 subfield and dentate gyrus (DG), and was significantly (P < 0.05) elevated in the DG during the period of rapid brain growth [gestational day (gd) 50; term = 70 days]. Glucocorticoids have been shown to alter 5-HT1A receptor mRNA expression in the adult, but nothing is known about their impact on the developing fetal brain. Expression of 5-HT1A receptor mRNA in the fetal hippocampus was measured following repeated maternal administration (gd40, 41, 50, 51, 60 and 61) of synthetic glucocorticoid (dexamethasone; 1 and 10 mg/kg). Levels of 5-HT1A receptor mRNA were significantly (P < 0.005) elevated in CA1 and DG following repeated exposure to high-dose glucocorticoid (10 mg/kg) in male, but not in female fetuses. Because fetal exposure to glucocorticoids programs hypothalamo-pituitary-adrenal (HPA) function, and hippocampal serotonin is known to influence glucocorticoid receptor (GR) expression, the glucocorticoid-mediated changes in 5-HT1A receptor mRNA may play a role in the programming of HPA function.

Positron emission tomographic analysis of dose-dependent NAD-299 binding to 5-hydroxytryptamine-1A receptors in the human brain

RATIONALE

The serotonin 5-HT(1A) receptor has been ascribed a putative role in the pathophysiology and drug treatment of depression. NAD-299 (generic name robalzotan) is a new potential antidepressant with high affinity and selectivity for the 5-HT(1A) receptor.

OBJECTIVES

The aim of this positron emission tomography (PET) study was to examine the extent and time-course of 5-HT(1A) occupancy by NAD-299 in the human brain, in relation to plasma concentration after escalating single oral doses.

METHODS

Five healthy male subjects received one or more single oral doses of NAD-299 (0.5, 2.5 and 10 mg) in aqueous solution under fasting conditions. Total and unbound (after ultrafiltration) plasma concentrations of NAD-299 were determined by liquid chromatography-mass spectrometry (LC-MC), over a tentative dosage interval of 8 h. Regional 5-HT(1A) receptor occupancy in brain was calculated by the simplified reference tissue model using the radioligand [ carbonyl-(11)C]WAY-100635.

RESULTS

After the 10 mg dose, occupancy was high in the raphe (62-85%) and neocortical regions (68-75%) at time for C(max), but had declined considerably (17-44%) at 7 h after dose intake.

CONCLUSIONS

This study confirmed that the new selective 5-HT(1A) antagonist NAD-299 occupies 5-HT(1A) receptors in the living human brain in a dose-dependent manner following oral dosage. The curvilinear relationship between NAD-299 drug concentration and 5-HT(1A) receptor occupancy was established and can be used for dose selection in subsequent clinical patient studies.

Developmental effects of prenatal cocaine exposure on 5-HT1A receptors in male and female rat offspring

Prenatal cocaine exposure results in behavioral abnormalities throughout development in rats, but little is known regarding the biological mechanisms underlying these abnormalities. Pregnant rats received subcutaneous twice-daily injections (1 ml/kg) of normal saline or 15 mg/kg of cocaine hydrochloride throughout gestation (gestation days 1-20). Following delivery, pups were placed with untreated surrogates. Male and female pups were killed on postnatal days 30, 60 or 120 for assessment of 5-HT(1A) receptor development in the forebrain, diencephalon, midbrain and pons using radiolabel immunocytochemistry. Findings revealed gender and age differences in developmental regulation of 5-HT(1A) receptors, indicating that male rats are more susceptible to long-term consequences of prenatal cocaine exposure in comparison to females. This study also demonstrates gender-specific development of serotonin (5-HT(1A)) receptors across postnatal ages, demonstrating a fundamentally different pattern of development of 5-HT(1A) receptors between males and females.

Neuronal instability: implications for Rett's syndrome

The maturational changes in the brain and spinal cord do not linearly proceed from immature in infants to mature in adults. Dendrites dynamically extend or retract as neurotrophic factors fluctuate. In certain cases mature neurons can be seen soon after birth, and in other cases immature neurons can be identified in the aged brain. Monoamine 'neurotransmitter'; such as serotonin (5-HT), dopamine and norepinephrine appear to function as Maintenance Growth Factors since they must be present in order to produce their maturational actions. Serotonin neurons contain TRK-B receptors and are sensitive to availability of the trophic factor, BDNF. 5-HT also functions by promoting the release of the glial extension factor, S-100beta. 5-HT and S-100beta can provide maturational signals to a variety of neurons, in both cortical and subcortical areas, and appear to be involved in regulating the maturation and release of acetylcholine and dopamine. We have shown that activation of the 5-HT1A receptor is particularly effective in inducing growth of stunted neurons. The mechanism of action of the 5-HT1A receptor involves both a direct inhibition on c-AMP and pCREB formation in postsynaptic neurons and a release of S-100beta from glial cells. Both these events are capable of stabilization and elaboration of the cytoskeleton of the neuron and inhibition of apoptosis. 5-HT1A receptors have been shown to effectively reverse stunted neurons and microencephaly produced in animal models of fetal alcohol syndrome and prenatal cocaine administration. I discuss the implications for regressive disorders such as Rett's syndrome and autism, and the feasibility of treatments with 5-HT1A agonists in children with developmental disorders.

Aminergic receptors during the development of the human brain: the contribution of in vitro imaging techniques

The development of the human brain is a complex process and, in this regard, the maturation of neurotransmitter systems and their receptors is of special interest. The study of these systems requires methodological approaches with powerful anatomical resolution. In this paper we review the application of visualization procedures to the fine localization, pattern of appearance and functional relevance of monoaminergic receptors in postmortem human brain samples corresponding to different stages of development (fetal, neonatal, infant). Data obtained by using mostly in vitro autoradiography but also in situ hybridization and, very recently, second messenger labeling, are discussed, including the methodological limitations inherent in working with inmature human tissue. From these studies, several conclusions were made. (1) It is possible to visualize, in the human brain with high resolution, the presence of neuroreceptors at early prenatal stages. (2) The anatomical distribution of monoaminergic receptors in the developing human brain is, in general terms, comparable to that found in the adult. (3) During the developmental process, some receptors, which are early and sometimes transiently expressed, play important thophic roles in the regulation of neuronal development: this is the case with the serotonin 5-HT1A receptors, which attain peak levels of hyperexpression over the hippocampus (dentate gyrus, dendritic areas of CA fields) and the raphe nuclei and show a transient expression in the cerebellum, around the 25 week of gestational age. (4) Different patterns of ontogenetic appearance for human receptors have been identified: dopamine D2-like (caudate, putamen, nigra) and 5-HT1A receptors are good examples of prenatal development, while 5-HT1B sites (basal ganglia, neocortex) present a mainly postnatal pattern of appearance. (5) Neurotransmitter receptors at human fetal stages are already functional from the point of view of transducing response.

Postnatal development of 5-HT(1A) receptor expression in rat somatic motoneurons

Prior work has established that hypoglossal motoneurons (HMs) change postnatally in their response to serotonin (5-HT), in part as a result of a decline in expression of 5-HT(1A) receptors. In the current study, two issues were addressed. First, using in situ hybridization we found that transient expression of 5-HT(1A) receptors occurs in other populations of brainstem (facial and trigeminal) and spinal (cervical and lumbar) motoneurons. Second, the participation of motoneuronal afferent (serotonergic) and efferent (neuromuscular) innervation in inducing and maintaining this decline in expression was investigated. Serotonergic innervation of the hypoglossal nucleus (nXII) was disrupted in neonatal rats by intra-cisternal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and 5-HT(1A) receptor mRNA levels in nXII from these rats were assayed at postnatal day 21. In spite of an almost complete loss of serotonergic fibers in the region, the postnatal decrease in 5-HT(1A) receptor expression by HMs still occurred. To test for potential regulation by target-derived factors or by nerve injury, receptor mRNA levels were assayed after unilateral transection of the hypoglossal nerve in adult rats. Though this treatment resulted in re-induction of developmentally transient expression of the p75 neurotrophin receptor, 5-HT(1A) receptor expression remained low. Thus, neonatal expression of 5-HT(1A) receptors appears to be common to somatic motoneurons, but we found no evidence for changes in serotonergic innervation in influencing this expression, nor did we find evidence for its regulation by peripheral factors.

In utero exposure to serotonergic drugs alters neonatal expression of 5-HT(1A) receptor transcripts: a quantitative RT-PCR study

In embryonic rat brain, serotonin (5-HT) acts as a differentiation signal for 5-HT neurons and their target cells during midgestation. Serotonin receptors expressed during this period include the 5-HT(1A) subtype, which may mediate some of these developmental effects. Using the highly sensitive method of competitive RT-PCR, we quantified the effects of maternal treatment with either p-chlorophenylalanine (pCPA; which depletes 5-HT in embryonic rat brain) or 5-methoxytryptamine (5-MT; a general 5-HT(1) /5-HT(2) agonist) from embryonic day E12-17 on expression of 5-HT(1A) receptor mRNA transcripts in brains of offspring at postnatal day 4 (PND 4). In offspring of both pCPA and 5-MT treated mothers, 5-HT(1A) transcripts were significantly reduced compared to vehicle controls, although effects of pCPA were greater than those of 5-MT. These results indicate that either under-stimulation of 5-HT(1A) receptors (due to pCPA-induced 5-HT depletion) or over-stimulation (by the agonist 5-MT) during prenatal development significantly reduced expression of 5-HT(1A) receptor transcripts in neonatal offspring. This may occur by disruption of 5-HT(1A) gene transcription or by post-transcriptional mechanisms (such as altered translation or turnover of mRNA). Whatever the mechanism, reductions in 5-HT(1A) receptor transcripts following in utero exposure to serotonergic drugs could significantly impact the number of 5-HT(1A) receptors expressed in neonatal rat brain. Whether such effects will persist into adulthood remains to be determined.