Quantified distribution of serotonin transporter and receptors during the postnatal development of the rat barrel field cortex

Learning with three factors: modulating Hebbian plasticity with errors

Synaptic plasticity is a central theme in neuroscience. A framework of three-factor learning rules provides a powerful abstraction, helping to navigate through the abundance of models of synaptic plasticity. It is well-known that the dopamine modulation of learning is related to reward, but theoretical models predict other functional roles of the modulatory third factor; it may encode errors for supervised learning, summary statistics of the population activity for unsupervised learning or attentional feedback. Specialized structures may be needed in order to generate and propagate third factors in the neural network.

Development of Cortical Maps: Perspectives From the Barrel Cortex

One approach to examining how higher sensory, motor, and cognitive faculties emerge in the neocortex is to elucidate the underlying wiring principles of the brain during development. The mammalian neocortex is a layered structure generated from a sheet of proliferating ventricular cells that progressively divide to form specific functional areas, such as the primary somatosensory (S1) and motor (M1) cortices. The basic wiring pattern in each of these functional areas is based on a similar framework, but is distinct in detail. Functional specialization in each area derives from a combination of molecular cues within the cortex and neuronal activity-dependent cues provided by innervating axons from the thalamus. One salient feature of neocortical development is the establishment of topographic maps in which neighboring neurons receive input relayed from neighboring sensory afferents. Barrels, which are prominent sensory units in the somatosensory cortex of rodents, have been examined in detail, and data suggest that the initial, gross formation of the barrel map relies on molecular cues, but the refinement of this topography depends on neuronal activity. Several excellent reviews have been published on the patterning and plasticity of the barrel cortex and the precise targeting of ventrobasal thalamic axons. In this review, the authors will focus on the formation and functional maturation of synapses between thalamocortical axons and cortical neurons, an event that coincides with the formation of the barrel map. They will briefly review cortical patterning and the initial targeting of thalamic axons, with an emphasis on recent findings. The rest of the review will be devoted to summarizing their understanding of the cellular and molecular mechanisms underlying thalamocortical synapse maturation and its role in barrel map formation.

Insights into the complex influence of 5-HT signaling on thalamocortical axonal system development

The topographic organization of the thalamocortical axons (TCAs) in the barrel field (BF) in the rodent primary somatosensory cortex results from a succession of temporally and spatially precise developmental events. Prenatally, growth and guidance mechanisms enable TCAs to navigate through the forebrain and reach the cortex. Postnatally, TCAs grow into the cortex, and the refinement of their terminal arborization pattern in layer IV creates barrel-like structures. The combined results of studies performed over the past 20 years clearly show that serotonin (5-hydroxytryptamine; 5-HT) signaling modulates these pre- and early postnatal developmental processes. In this context, 5-HT signaling can purposely be described as 'modulating' rather than 'controlling' because developmental alterations of 5-HT synthesis, uptake or degradation either have a dramatic, moderate or no effect at all on TCA pathway and BF formation. In this review we summarize and compare the outcomes of diverse pharmacological and genetic manipulations of 5-HT signaling on TCA pathway and BF formation, in an attempt to understand these discrepancies.

Effects of electrical stimulation of dorsal raphe nucleus on neuronal response properties of barrel cortex layer IV neurons following long-term sensory deprivation

OBJECTIVE

To evaluate the effect of electrical stimulation of dorsal raphe nucleus (DRN) on response properties of layer IV barrel cortex neurons following long-term sensory deprivation.

METHODS

Male Wistar rats were divided into sensory-deprived (SD) and control (unplucked) groups. In SD group, all vibrissae except the D2 vibrissa were plucked on postnatal day one, and kept plucked for a period of 60 d. After that, whisker regrowth was allowed for 8-10 d. The D2 principal whisker (PW) and the D1 adjacent whisker (AW) were either deflected singly or both deflected in a serial order that the AW was deflected 20 ms before PW deflection for assessing lateral inhibition, and neuronal responses were recorded from layer IV of the D2 barrel cortex. DRN was electrically stimulated at inter-stimulus intervals (ISIs) ranging from 0 to 800 ms before whisker deflection.

RESULTS

PW-evoked responses increased in the SD group with DRN electrical stimulation at ISIs of 50 ms and 100 ms, whereas AW-evoked responses increased at ISI of 800 ms in both groups. Whisker plucking before DRN stimulation could enhance the responsiveness of barrel cortex neurons to PW deflection and decrease the responsiveness to AW deflection. DRN electrical stimulation significantly reduced this difference only in PW-evoked responses between groups. Besides, no DRN stimulation-related changes in response latency were observed following PW or AW deflection in either group. Moreover, condition test (CT) ratio increased in SD rats, while DRN stimulation did not affect the CT ratio in either group. There was no obvious change in 5-HT(2A) receptor protein density in barrel cortex between SD and control groups.

CONCLUSION

These results suggest that DRN electrical stimulation can modulate information processing in the SD barrel cortex.

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.

Neonatal fluoxetine exposure affects the neuronal structure in the somatosensory cortex and somatosensory-related behaviors in adolescent rats

Selective serotonin reuptake inhibitor (SSRI)-type antidepressants are often prescribed to depressive pregnant women for their less adverse side effects. However, growing evidences have shown increased congenital malformations and poor neonatal adaptation in the perinatal SSRI-exposed human infants as well as animal pups. In this study, we examined the effects of early exposure of fluoxetine, the most popular SSRI-type antidepressant, on the developing somatosensory system. Physiological saline or fluoxetine (10 mg/kg) was subcutaneously injected into neonatal rats from P0 to P6. Somatosensory-related behaviors were examined in adolescence (P30-P35). Morphological features of the primary somatosensory cortex were checked at P7 and P35. The tactile and thermal perceptions as well as locomotor activity were affected by neonatal fluoxetine treatment. At the morphological level, the number of branch tips of thalamocortical afferents to the somatosensory cortex was reduced in the fluoxetine-treated rats. Furthermore, the spiny stellate neurons in the layer IV somatosensory cortex had reduced dendritic span and complexity with fewer branches, shorter dendritic length, and smaller dendritic field. The spine density of spiny stellate neurons was significantly reduced whereas the spine length of mushroom- and branched-type was increased. Taken together, these results indicate that neonatal fluoxetine administration has long-lasting effects on the function and structure in the somatosensory system. Sensory information processing may be disturbed in the neonatal fluoxetine-treated animals due to the structural deformation in the thalamocortical afferents and dendritic structures of the spiny stellate neurons in the layer IV somatosensory cortex.

Neonatal serotonin depletion alters behavioral responses to spatial change and novelty

Multiple brain disorders that show serotonergic imbalances have a developmental onset. Experimental models indicate a role for serotonin as a morphogen in brain development. To selectively study the effects of serotonin depletions on cortical structural development and subsequent behavior, we developed a mouse model in which a serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), is injected into the medial forebrain bundle (mfb) on the day of birth. Littermates with saline injections into the mfb and age matched mice served as controls. This study characterized the extent and duration of serotonergic denervation after the selective neonatal lesion and investigated effects on exploratory behavior, spatial learning and anxiety in mice of both sexes. We report significant decreases in the serotonergic (5-HT) innervation to cortex and hippocampus, but not to subcortical forebrain structures in 5,7-DHT-lesioned mice. The depletion of 5-HT fibers in cortical areas was long lasting in lesioned mice but autoradiographic binding to high affinity 5-HT transporters was only transiently reduced. Male but not female lesioned mice reduced their exploration significantly in response to spatial rearrangement and object novelty, suggesting increased anxiety in response to change but normal spatial cognition. Our data show that developmental disruptions in the serotonergic innervation of cortex and hippocampus are sufficient to induce permanent, sex specific, behavioral alterations. These results may have significant implications for understanding brain disorders presenting with cortical morphogenetic abnormalities and altered serotonin neurotransmission, such as autism, schizophrenia and affective disorders.

Modeling early cortical serotonergic deficits in autism

Autism is a developmental brain disorder characterized by deficits in social interaction, language and behavior. Brain imaging studies demonstrate increased cerebral cortical volumes and micro- and macro-scopic neuroanatomic changes in children with this disorder. Alterations in forebrain serotonergic function may underlie the neuroanatomic and behavioral features of autism. Serotonin is involved in neuronal growth and plasticity and these actions are likely mediated via serotonergic and glutamatergic receptors. Few animal models of autism have been described that replicate both etiology and pathophysiology. We report here on a selective serotonin (5-HT) depletion model of this disorder in neonatal mice that mimics neurochemical and structural changes in cortex and, in addition, displays a behavioral phenotype consistent with autism. Newborn male and female mice were depleted of forebrain 5-HT with injections of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the bilateral medial forebrain bundle (mfb). Behavioral testing of these animals as adults revealed alterations in social, sensory and stereotypic behaviors. Lesioned mice showed significantly increased cortical width. Serotonin immunocytochemistry showed a dramatic long-lasting depletion of 5-HT containing fibers in cerebral cortex until postnatal day (PND) 60. Autoradiographic binding to high affinity 5-HT transporters was significantly but transiently reduced in cerebral cortex of 5,7-DHT-depleted mice. AMPA glutamate receptor binding was decreased at PND 15. We hypothesize that increased cerebral cortical volume and sensorimotor, cognitive and social deficits observed in both 5-HT-depleted animals and in individuals with autism, may be the result of deficiencies in timely axonal pruning to key cerebral cortical areas.

Canadian Association of Neurosciences review: postnatal development of the mammalian neocortex: role of activity revisited

The mammalian neocortex is the largest structure in the brain, and plays a key role in brain function. A critical period for the development of the neocortex is the early postnatal life, when the majority of synapses are formed and when much of synaptic remodeling takes place. Early studies suggest that initial synaptic connections lack precision, and this rudimentary wiring pattern is refined by experience-related activity through selective elimination and consolidation. This view has been challenged by recent studies revealing the presence of a relatively precise pattern of connections before the onset of sensory experience. The recent data support a model in which specificity of neuronal connections is largely determined by genetic factors. Spontaneous activity is required for the formation of neural circuits, but whether it plays an instructive role is still controversial. Neurotransmitters including acetylcholine, serotonin, and gamma-Aminobutyric acid (GABA) may have key roles in the regulation of spontaneous activity, and in the maturation of synapses in the developing brain.

Transient synaptic zinc-positive thalamocortical terminals in the developing barrel cortex

In rat barrel cortex, layer 4 has a transiently high density of zinc-positive terminations from postnatal day (P)9 to P12 [P.W. Land & L. Shamalla-Hannah (2002)J. Comp. Neurol., 447, 43-56]. These terminations have been proposed to originate from cortico-cortical connections, but their exact origin is unknown. To determine their sources, we injected sodium selenite into the barrel cortex of two adult rats and 32 pups, from P5 to P28. As predicted, abundant zinc-positive cortically projecting neurons were visible around the injection sites and in distant cortical areas. From P9 to P13, however, neurons retrogradely labeled by zinc selenite occurred in the thalamus, in topographically appropriate regions of the ventroposterior medial (VPM) and posterior nuclei (Po). Because there are no previous reports of zinc-positive sensory thalamocortical connections, we sought corroboration of this unexpected finding by electron microscopy. This revealed a subset of boutons in layers 4 and 1, positive for both zinc and vesicular glutamate transporter 2, a protein used by thalamocortical terminations. Finally, in an additional nine rats, we carried out in situ hybridization for zinc transporter 3 mRNA. Moderate signal was detected in VPM and Po at P10, but this disappeared by P28. In contrast, a strong signal was apparent in the anterodorsal nucleus, which projects to limbic areas, and this persisted at P28. The timing of the transient zinc-positive terminations in the sensory thalamus roughly coincides with the onset of exploratory and whisking behavior in the middle of the second postnatal week; and this suggests zinc is important for activity-related refinement of circuitry.

Subbarrel patterns of thalamocortical innervation in rat somatosensory cortical barrels: Organization and postnatal development

Barrel hollows in the posteromedial barrel subfield of adult rat somatosensory cortex typically encompass two or three metabolically and structurally distinct regions, termed subbarrels. We used immunohistochemical staining for vesicular glutamate transporter 2 and the neuronal serotonin transporter, in conjunction with cytochrome oxidase (CO) histochemistry, to investigate the distribution of thalamocortical (TC) axon terminals in relation to subbarrel domains. We found, first, that CO-dark subbarrels are more intensely immunoreactive for thalamocortical terminals than the CO-light clefts that separate them. Second, during the first postnatal week, immunoreactivity for markers of TC terminals is relatively homogeneous throughout the barrel hollow; subbarrel patterns of distribution only become recognizable between P-8 and P-10. These observations extend previous findings that subbarrels denote barrel regions enriched in synaptic contacts. The data also indicate that allocation of TC terminals into subbarrel domains does not occur immediately upon thalamic axon ingrowth. Instead, refinement of TC arbors into subbarrels is a gradual process, the outcome of which is not manifest until the second week of postnatal life.

Multiple receptors mediate the trophic effects of serotonin on ventroposterior thalamic neurons in vitro

Serotonin (5-HT) exerts prominent morphogenetic roles during development. For example, somatosensory cortical barrel formation is altered in mouse models characterized by excessive extracellular 5-HT, suggesting that 5-HT affects development of thalamic afferents and/or neocortical target regions. The present study assessed 5-HT effects in primary cultures of fetal ventroposterior thalamic (VPT) neurons. 5-HT produces concentration-dependent trophic effects, with impressive 59% and 106% peak increases in total neurite length and number of branching points, respectively, at a dose of 30 microM 5-HT. The exposure of VPT neurons to specific 5-HT receptor agonists 8-OH-DPAT (5-HT(1A)), CGS-12066A (5-HT(1B)), DOI (5-HT(2A/2C)), and m-CPBG (5-HT(3)), enhances primary neurite length and number of branching points with rank-order potency 5-HT(1B) > 5-HT(2A/2C) = 5-HT(3) > 5-HT(1A) = vehicle. Trophic 5-HT effects on embryonic VPT neurons are thus much more prominent than previously reported, and can be mediated by multiple 5-HT receptor subtypes.

Inactivation of 5HT transport in mice: modeling altered 5HT homeostasis implicated in emotional dysfunction, affective disorders, and somatic syndromes

Animal models have not only become an essential tool for investigating the neurobiological function of genes that are involved in the etiopathogenesis of human behavioral and psychiatric disorders but are also fundamental in the development novel therapeutic strategies. As an example, inactivation of the serotonin (5HT) transporter (5Htt, Slc6a4) gene in mice expanded our view of adaptive 5HT uptake regulation and maintenance of 5HT homeostasis in the developing human brain and molecular processes underlying anxiety-related traits, as well as affective spectrum disorders including depression. 5Htt-deficient mice have been employed as a model complementary to direct studies of genetically complex traits and disorders, with important findings in biochemical, morphological, behavioral, and pharmacological areas. Based on growing evidence for a critical role of the 5HTT in the integration of synaptic connections in the rodent, nonhuman primate, and human brain during critical periods of development and adult life, more in-depth knowledge of the molecular mechanisms implicated in these fine-tuning processes is currently evolving. Moreover, demonstration of a joint influence of the 5HTT variation and environmental sources during early brain development advanced our understanding of the mechanism of genexgene and genexenvironment interactions in the developmental neurobiology of anxiety and depression. Lastly, imaging techniques, which become increasingly elaborate in displaying the genomic influence on brain system activation in response to environmental cues, have provided the means to bridge the gap between small effects of 5HTT variation and complex behavior, as well as psychopathological dimensions. The combination of elaborate genetic, epigenetic, imaging, and behavioral analyses will continue to generate new insight into 5HTT's role as a master control gene of emotion regulation.

Pharmacogenetics of the serotonin transporter

Response to psychopharmacologic drugs is genetically complex, results from an interplay of multiple genomic variations with environmental influences, and depends on the structure or functional expression of gene products, which are direct drug targets or indirectly modify the development and synaptic plasticity of neural networks critically involved in their effects. During brain development, the serotonin (5HT) system, which is commonly targeted by antidepressant, anxiolytic, and antipsychotic drugs, controls neuronal specification, differentiation, and phenotype maintenance. While formation and integration of these neural networks is dependent on the action of multiple proteins, converging lines of evidence indicate that genetically controlled variability in the expression of the 5HT transporter (5HTT) is critical to the development and plasticity of distinct neurocircuits. The most promising finding to date indicate an association between the response time as well as overall response to serotonin reuptake inhibitors (SSRIs) and a common polymorphism (5HTTLPR) within the transcriptional control region of the 5HTT gene (SLC6A4) in patients with depressive disorders. The formation and maturation of serotonergic and associated systems, in turn, are influencing the efficacy of serotonergic compounds in a variety of psychiatric conditions. Based on the notion that complex gene x gene and gene x environment interactions in the regulation of brain plasticity are presumed to contribute to individual differences in psychopharmacologic drug response, the concept of developmental psychopharmacogenetics is emerging.

Developmental disruption of serotonin transporter function impairs cerebral responses to whisker stimulation in mice

There is growing evidence that serotonin (5-hydroxtryptamine, 5-HT) has major influences on brain development in mammals. Genetic and pharmacological disruption of 5-HT signaling during early postnatal development in rodents causes neuroanatomical cortical abnormalities, including malformations in the somatosensory cortex. Possible functional consequences of this developmental perturbation by 5-HT are not yet understood. We have examined the effects of deletion of the 5-HT transporter (5-HTT) gene on somatosensory responses to sensory stimulation in mice. Local cerebral glucose utilization (lCMR(glc)) was measured by the quantitative 2-deoxy[(14)C]glucose method during unilateral whisker stimulation in awake adult mice. lCMR(glc) was increased by stimulation but to a markedly lesser extent in 5-HTT(-/-) mice than in 5-HTT(+/+) controls in each of four major stations in the whisker-to-barrel cortex pathway (the spinal and principal sensory trigeminal nuclei, the ventral posteromedial thalamic nucleus, and the barrel region of the somatosensory cortex). Lowering brain 5-HT levels by administration of the selective tryptophan hydroxylase inhibitor p-chlorophenylalanine on postnatal days 0 and 1 restored the metabolic responses to functional activation in the whisker-to-barrel cortex pathway in adult 5-HTT(-/-) mice. These results indicate that functional deficits in this pathway in 5-HTT(-/-) mice may be due to excessive postnatal 5-HT activity. With or without postnatal p-chlorophenylalanine treatment, 5-HTT(-/-) mice exhibited lower resting (unstimulated) lCMR(glc) than did 5-HTT(+/+) controls in the whisker-to-barrel cortex pathway and throughout the brain. These findings have implications for understanding the potential long-term consequences of genetic and pharmacological disruption of 5-HT neurotransmission on cerebral functions during critical periods of postnatal development.

Effects of prenatal alcohol exposure on the development of the vibrissal somatosensory cortical barrel network

We have previously shown that the serotonin (5-HT) and its thalamocortical afferents are compromised by prenatal alcohol exposure (PAE). The development of the sensory cortical barrels is regulated by 5-HT-rich thalamocortical afferents. Therefore, it is hypothesized that PAE will deleteriously affect the postnatal development of the cortical barrel formations. On embryonic day (E)7, C57BL/6 mice were grouped into: Alcohol (Alc), Pair-fed (PF), or Chow, and maintained on diet until E18. On postnatal day 7, cortices were stained with 5-HT for thalamocortical fibers, and a NeuN for identification of mature neurons. The area of the posterior medial barrel subfield (PMBSF), was measured as well as the number of NeuN+ neurons within the barrel patches. Though brain weight and brain volume were similar among the three groups, a significant reduction was seen in total area of the PMBSF, and in the average individual barrel area in the Alc group as compared to Chow. Furthermore, the volumes of the B, but not C row barrels were significantly reduced. Barrels were found missing in layer IV, specifically in the posterior aspects of the A, B, and straddler row in the Alc group. Cell counts demonstrated a nearly 50% reduction in NeuN+ neuron number in both rows. This reduction in size of the PMBSF and fewer neurons within these sensory barreloids may underlie a change in the development of the discriminatory sensitivity of the whiskers and serves as an excellent model for the study of a compromised sensory modality following PAE.

Genetic alterations of the murine serotonergic gene pathway: the neurodevelopmental basis of anxiety

The relative contribution of genetic and environmental factors in the configuration of behavioral differences is among the most prolonged and contentious controversies in intellectual history. Although current views emphasize the joint influence of genes and environmental sources during early brain development, the physiological complexities of multiple gene-gene and gene-environment interactions in the developmental neurobiology of fear and anxiety remain elusive. Variation in genes coding for proteins that control serotonin (5-hydroxytryptamine, 5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction as well as cellular pathways have been implicated in the genetics of anxiety and related disorders. This review selects anxiety and avoidance as paradigmatic traits and behaviors, and it focuses on mouse models that have been modified by deletion of genes coding for key players of serotonergic neurotransmission. In particular, pertinent approaches regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, and monoamine oxidase A and other genes related to 5-HT signaling will be discussed and major findings highlighted.

Fluoxetine treatment of prepubescent rats produces a selective functional reduction in the 5-HT2A receptor-mediated stimulation of oxytocin

Various childhood mood disorders are being treated with serotonin selective reuptake inhibitors (SSRIs) such as fluoxetine (Prozac(R)), yet limited data are available on their effects on serotonergic systems prior to maturation. This study investigated the effects of chronic fluoxetine treatment on 5-HT2A serotonin receptor-mediated neuroendocrine responses in young male rats. Prepubescent male rats were treated with saline or fluoxetine (10 mg/kg/day, i.p.) for 14 days, a treatment regimen producing maximal changes in postsynaptic 5-HT2A function in adults. Eighteen hours post-treatment, the rats received saline or increasing doses (0.5, 2.0, or 5.0 mg/kg, i.p.) of the 5-HT2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl ((+/-)-DOI). Trunk blood was obtained to determine changes in oxytocin, ACTH, corticosterone, and renin responses. Fluoxetine produced a small ( approximately 6%) but significant reduction in body weight gain, but no changes were observed in basal hormone levels. In both saline- and fluoxetine-treated rats, (+/-)-DOI increased plasma oxytocin levels in a dose-dependent manner. However, the magnitude of the oxytocin responses to all doses of (+/-)-DOI were markedly attenuated ( approximately 50%) in the fluoxetine-treated rats, indicating a functional reduction in the E(max) of 5-HT(2A) receptor-mediated oxytocin responses. In contrast, fluoxetine did not alter the (+/-)-DOI-induced increases in plasma ACTH, corticosterone, or renin. These data provide the first demonstration of selective neuroadaptive responses in 55-HT2A serotonin receptor function due to prepubescent treatment with fluoxetine. These data may be clinically relevant with respect to the use of selective serotonin reuptake inhibitors in children and adolescents.

Serotonergic regulation of membrane potential in developing rat prefrontal cortex: coordinated expression of 5-hydroxytryptamine (5-HT)1A, 5-HT2A, and 5-HT7 receptors

The developing prefrontal cortex receives a dense serotonergic innervation, yet little is known about the actions of serotonin [5-Hydroxytryptamine (5-HT)] in this region during development. Here, we examined the developmental regulation of 5-HT receptors controlling the excitability of pyramidal neurons of this region. Using whole-cell recordings in in vitro brain slices, we identified a dramatic shift in the effects of 5-HT on membrane potential during the postnatal developmental period. In slices derived from young animals [postnatal day (P) 6 to P19], administration of 5-HT elicits a robust depolarization of layer V pyramidal neurons, which gradually shifts to a hyperpolarization commencing during the third postnatal week. This progression is the result of coordinated changes in the function of 5-HT7 and 5-HT2A receptors, which mediate different aspects of the depolarization, and of 5-HT1A receptors, which signal the late developing hyperpolarization. The loss of the 5-HT7 receptor-mediated depolarization and the appearance of the 5-HT1A receptor-mediated hyperpolarization appears to reflect changes in receptor expression. In contrast, the decline in the 5-HT2A receptor depolarization with increasing age was associated with changes in the effectiveness with which these receptors could elicit a membrane depolarization, rather than loss of the receptors per se. Together, these results outline coordinated changes in the serotonergic regulation of cortical excitability at a time of extensive synaptic development and thus suggest a key role for these receptor subtypes in the postnatal development of the prefrontal cortex.

Selective serotonin reuptake inhibitor disrupts organization of thalamocortical somatosensory barrels during development

To further investigate the role of the transiently expressed serotonin (5-HT) transporter (5-HTT) in the development of thalamic fibers projecting to cortical barrels and the potential developmental changes in neuronal circuitry caused by a selective serotonin reuptake inhibitor (SSRI), paroxetine (5 mg/kg, twice daily, s.c.) or saline was administered to rat pups from postnatal day 0 (P0) to P8. Pups were perfused on P8 for 5-HT immunostaining (-im) to confirm the 5-HT uptake blockade, and 5-HTT-im and phospholipase C-beta1 (PLC-beta1)-im to label the thalamic afferents to barrels and barrel cells respectively. Paroxetine treatment completely blocked 5-HT uptake into the thalamocortical fibers as indicated by the negative 5-HT-im in cortical barrel areas. Organization of thalamic afferents to barrels, indicated by 5-HTT-im or PLC-beta1, was altered in paroxetine-treated pups in the following manners: (1) segregation of thalamocortical fibers was partially disrupted and thalamocortical fibers corresponding to anterior snouts and row A mystacial vibrissae were fused; (2) sizes of the unfused thalamocortical fiber patches related to the long caudal vibrissae in rows B, C, D and E were significantly decreased without changes in the brain weights and cortical areas representing these vibrissae; and (3) thalamocortical fibers corresponding to C4 and D4 vibrissae tended to be closer to each other along the arc while the relative positions of thalamocortical fibers related to the rest of the vibrissae were normal. Our study demonstrated that 5-HTT plays an important role in the refinement, but not the formation, of barrel-like clusters of thalamocortical fibers and that the development of neural circuitry in rodent somatosensory cortex was affected by exposure to a SSRI during thalamocortical synaptic formation.

Anxiety-related traits in mice with modified genes of the serotonergic pathway

The neurobiology of anxiety is complex, reflecting the cumulative physiological effects of multiple genes. These genes are interactive with each other and with the environment in which they are expressed. Variation in genes coding for proteins that control serotonin (5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction and cellular pathways have been implicated in the genetics of anxiety and related disorders. Here, we selected anxiety and avoidance as paradigmatic traits and behavior and cover both traditional studies with inbred murine strains and selected lines which have been modified by gene knockout technologies. The design of a mouse model partially or completely lacking a gene of interest during all stages of development (constitutive knockout) or in a spatio-temporal context (conditional knockout) is among the prime strategies directed at elucidating the role of genetic factors in fear and anxiety. In many cases, knockout mice have been able to confirm what has already been anticipated based on pharmacological studies. In other instances, knockout studies have changed views of the relevance of 5-HT homeostasis in brain development and plasticity as well as processes underlying emotional behavior. In this review, we discuss the pertinent literature regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, monoamine oxidase A and other components of the serotonergic pathway. Finally, we attempt to identify future directions of genetic manipulation in animal models to advance our understanding of brain dysregulation characteristic of anxiety disorders.

Transient expression of S-100beta immunostaining in developing thalamus and somatosensory cortex of rat

Serotonin is thought to affect the development of barrel fields in somatosensory cortex of rat and transient expression of the serotonin transporter has been reported in relevant thalamic (ventral posterior) and cortical (layer IV of parietal) regions in support of this. Much of the developmental role of serotonin is mediated by release of the neurotrophic protein S-100beta. The current work was thus undertaken to determine if S-100beta also shows a transient expression pattern in thalamus and barrel fields. Male Sprague-Dawley rats were examined immunocytochemically for S-100beta expression on postnatal days (PD) 1, 7, 15 and 22. Expression of S-100beta selectively peaked in the ventral posterior nucleus of the thalamus at PD 7, and in layer IV of the parietal cortex from PD 7 to 15, in a 'barrel-like' pattern. Our findings suggest that S-100beta could indeed be the mediator of serotonin's effects on barrel field formation.

Unique expression patterns of 5-HT2A and 5-HT2C receptors in the rat brain during postnatal development: Western blot and immunohistochemical analyses

Serotonin (5-HT) is recognized as a potential regulatory factor in neuronal development. Two subtypes of receptors for it, 5-HT2A and 5-HT2C, are distributed broadly in the rat brain, suggesting their role in a variety of brain functions. Here, we investigated the expression patterns of these 5-HT2 receptors in the rat brain during postnatal development by using Western blot and immunohistochemical analyses. By Western blot analysis, the expression of the 5-HT2A receptor was at a low level at postnatal day 3 (P3) and increased greatly during the first 3 postnatal weeks; whereas the 5-HT2C receptor was already expressed at a high level at P3, and its expression increased only slightly during postnatal development. Immunohistochemical analysis showed the different expression patterns of 5-HT2A and 5-HT2C receptor subtypes during postnatal development: the transient expression of the 5-HT2C receptor was observed in layer IV of the somatosensory, visual, and auditory cortices from P10 to P28, and in the thalamus, mainly in the ventral posterolateral and ventral posteromedial nuclei, from P7 to P21; however, the immunoreactivity of the 5-HT2A receptor was detectable slightly at P3, but thereafter the intensity of immunolabeling increased with postnatal development and at P21 reached the adult level and pattern. These results suggest that 5-HT2 receptors have potential significance in brain development, with a functional difference between 5-HT2A and 5-HT2C receptor subtypes.

Effects of elevated serotonin levels on patterns of GAP-43 expression during barrel development in rat somatosensory cortex

Elevating cortical serotonin (5-HT) in rats with clorgyline, a monoamine oxidase A (MAO(A)) inhibitor, from postnatal day (P-0) to P-6 delays the organization of thalamocortical afferent fibers into a vibrissae-related pattern in the somatosensory cortex (S-I). Despite continued elevation of cortical 5-HT through P-8, the thalamocortical fibers do form, albeit with some delay, a characteristic vibrissae pattern of barrels in layer IV of S-I by P-8. The growth-associated protein, GAP-43, is transiently expressed in developing S-I cortex of normal rats in a vibrissae related pattern until P-7. After P-7, GAP-43 expression is reduced in the barrel centers and increased in the septa. The present study evaluated the effect of elevated 5-HT levels on the distribution of GAP-43 immunoreactivity in S-I. We employed 5-HT immunocytochemistry and 1,1'-dioctadecyl-3,3,3",3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamic radiations to confirm a 'barrelless' phenotype in P-6 clorgyline-treated animals and a recovered barrel pattern in treated animals allowed to survive until P-8 and P-10. GAP-43 immunocytochemistry was used to evaluate the cortical distribution of this protein in similarly treated littermates. Continuous inhibition of MAO(A) from P-0 to P-6 resulted in a corresponding loss of the GAP-43 vibrissae-related pattern at P-6. Despite continued elevation of cortical 5-HT until P-8 and P-10, the characteristic vibrissae-complementary pattern of GAP-43 emerged with expression concentrated in the septa and rows. GAP-43 vibrissae-related thalamocortical axon pattern never appeared in the clorgyline-treated animals. Thus, while elevated 5-HT delays development of a vibrissae-related pattern of thalamocortical afferents, it does not appear to alter the time when a GAP-43 vibrissae-related complementary pattern emerges.

Variation of serotonergic gene expression: neurodevelopment and the complexity of response to psychopharmacologic drugs

Individual differences in drug effects and treatment response are relatively enduring, continuously distributed, as well as substantially heritable, and are therefore likely to result from an interplay of multiple genomic variations with environmental influences. As the etiology and pathogenesis of behavioral and psychiatric disorders is genetically complex, so is the response to drug treatment. Psychopharmacologic drug response depends on the structure and functional expression of gene products, which may be direct drug targets or may indirectly modify the development and synaptic plasticity of neural networks critically involved in drug response. While formation and integration of these neural networks is dependent on the action of manifold proteins, converging lines of evidence indicate that genetically controlled variability in the expression of genes critical to the development and plasticity of distinct neurocircuits influences a wide spectrum of quantitative traits including treatment response. During brain development, neurotransmitter systems (e.g. serotonergic system), which are frequently targeted by psychotropic drugs, control neuronal specification, differentiation, and phenotype maintenance. The formation and maturation of these neurotransmitter systems, in turn, is directed by an intrinsic genetic program. Based on the notion that complex gene-gene and gene environment interactions in the regulation of brain plasticity are presumed to contribute to interindividual differences in drug response, the concept of developmental psychopharmacogenetics is emerging. This review appraises prototypical genomic variation with impact on gene expression and complementary studies of genetic and environmental effects on brain development and synaptic plasticity in the mouse model. Although special emphasis is given to molecular mechanisms of neurodevelopmental genetics, relevant conceptual and methodological issues pertinent to the dissection of the psychopharmacogenetic-neurodevelopmental interface are also considered.

Barrel pattern formation requires serotonin uptake by thalamocortical afferents, and not vesicular monoamine release

Thalamocortical neurons innervating the barrel cortex in neonatal rodents transiently store serotonin (5-HT) in synaptic vesicles by expressing the plasma membrane serotonin transporter (5-HTT) and the vesicular monoamine transporter (VMAT2). 5-HTT knock-out (ko) mice reveal a nearly complete absence of 5-HT in the cerebral cortex by immunohistochemistry, and of barrels, both at P7 and adulthood. Quantitative electron microscopy reveals that 5-HTT ko affects neither the density of synapses nor the length of synaptic contacts in layer IV. VMAT2 ko mice, completely lacking activity-dependent vesicular release of monoamines including 5-HT, also show a complete lack of 5-HT in the cortex but display largely normal barrel fields, despite sometimes markedly reduced postnatal growth. Transient 5-HTT expression is thus required for barrel pattern formation, whereas activity-dependent vesicular 5-HT release is not.

Neither peripheral nerve input nor cortical NMDA receptor activity are necessary for recovery of a disrupted barrel pattern in rat somatosensory cortex

Elevating cortical serotonin (5-HT) in rats from postnatal day (P-) 0 to P-6 by administering the monoamine oxidase (MAO(A)) inhibitor, clorgyline, produces a dose-dependent spectrum of effects on rat somatosensory organization, ranging from enlarged with indistinct septa to a complete lack of vibrissae-related patterns. However, if clorgyline treatment is stopped on P-6, a qualitatively and quantitatively normal vibrissae-related pattern of thalamocortical afferents appears in somatosensory cortex (S-I) on P-10. We employed high performance liquid chromatography (HPLC), infraorbital nerve (ION) transection, N-methyl-D-aspartate (NMDA) receptor blockade, 1,1'-dioctadecyl-3,3,3"3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamic afferents, and CO histochemistry to determine whether peripheral nerve input and/or cortical NMDA receptor activity were required for the recovery of vibrissae-related patterns in clorgyline-treated animals. Clorgyline administration from P-0 to P-6 produced a 1589.4+/-53.3% increase in cortical 5-HT over control animals on P-6 and a 268.8+/-6.3% elevation over controls at P-10. Postnatal day 6 pups had significantly altered vibrissae-related patterns in S-I following 6 days of clorgyline treatment but by P-10, the characteristic vibrissae-related patterns were restored. Neither transection of the ION nor application of the NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (APV), to the cortices of P-6 pups that were treated with clorgyline from birth had any significant effect on the recovery of the vibrissae-related patterns by P-10. These results indicate that neither peripheral nerve input nor cortical NMDA receptor activity are necessary for the restoration of cortical vibrissae-related patterns in rats that have sustained transient elevations of 5-HT.

Excessive activation of serotonin (5-HT) 1B receptors disrupts the formation of sensory maps in monoamine oxidase a and 5-ht transporter knock-out mice

Deficiency in the monoamine degradation enzyme monoamine oxidase A (MAOA) or prenatal exposure to the monoamine uptake inhibitor cocaine alters behavior in humans and rodents, but the mechanisms are unclear. In MAOA knock-out mice, inhibiting serotonin synthesis during development can prevent abnormal segregation of axons in the retinogeniculate and somatosensory thalamocortical systems. To investigate this effect, we crossed MAOA knock-outs with mice lacking the serotonin transporter 5-HTT or the 5-HT1B receptor, two molecules present in developing sensory projections. Segregation was abnormal in 5-HTT knock-outs and MAOA/5-HTT double knock-outs but was normalized in MAOA/5-HT1B double knock-outs and MAOA/5-HTT/5-HT1B triple knock-outs. This demonstrates that the 5-HT1B receptor is a key factor in abnormal segregation of sensory projections and suggests that serotonergic drugs represent a risk for the development of these projections. We also found that the 5-HT1B receptor has an adverse developmental impact on beam-walking behavior in MAOA knock-outs. Finally, because the 5-HT1B receptor inhibits glutamate release, our results suggest that visual and somatosensory projections must release glutamate for proper segregation.

DOI-Induced activation of the cortex: dependence on 5-HT2A heteroceptors on thalamocortical glutamatergic neurons

Administration of the hallucinogenic 5-HT(2A/2C) agonist 1-[2, 5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) induces expression of Fos protein in the cerebral cortex. To understand the mechanisms subserving this action of DOI, we examined the consequences of pharmacological and surgical manipulations on DOI-elicited Fos expression in the somatosensory cortex of the rat. DOI dose-dependently increased cortical Fos expression. Pretreatment with the selective 5-HT(2A) antagonist MDL 100,907 completely blocked DOI-elicited Fos expression, but pretreatment with the 5-HT(2C) antagonist SB 206,553 did not modify DOI-elicited Fos expression. These data suggest that DOI acts through 5-HT(2A) receptors to increase cortical Fos expression. However, we found that DOI did not induce Fos in cortical 5-HT(2A) immunoreactive neurons but did increase expression in a band of neurons spanning superficial layer V to deep III, within the apical dendritic fields of layer V 5-HT(2A)-immunoreactive cells. This band of Fos immunoreactive neurons was in register with anterogradely labeled axons from the ventrobasal thalamus, which have previously been shown to be glutamatergic and express the 5-HT(2A) transcript. The effects of DOI were markedly reduced in animals pretreated with the AMPA/KA antagonist GYKI 52466, and lesions of the ventrobasal thalamus attenuated DOI-elicited Fos expression in the cortex. These data suggest that DOI activates 5-HT(2A) receptors on thalamocortical neurons and thereby increases glutamate release, which in turn drives Fos expression in cortical neurons through an AMPA receptor-dependent mechanism. These data cast new light on the mechanisms of action of hallucinogens.

Clorgyline treatment elevates cortical serotonin and temporarily disrupts the vibrissae-related pattern in rat somatosensory cortex

Manipulation of cortical serotonin (5-HT) levels in perinatal rodents produces significant alterations in the development of the layer IV cortical representation of the mystacial vibrissae. Monoamine oxidase A (MAO(A)) knockout mice have highly elevated cortical 5-HT and completely lack barrels in somatosensory cortex (S-I). The present study was undertaken to determine whether the effects on thalamocortical development seen in MAO(A) knockout mice can be replicated in perinatal rats treated with an MAO(A) inhibitor and, second, to determine whether these effects persist with continued treatment or after discontinuation of the drug. Littermates were injected with either clorgyline (5 mg/kg) or sterile saline five times daily. Clorgyline administration from birth to postnatal day (P) 6, 8, or 10 produced increases of 1,589.4 +/- 53.3%, 1660.2 +/- 43.1% and 1,700.5 +/- 84.5 %, respectively, in cortical 5-HT as compared with controls. Serotonin immunocytochemistry, 1,1;-dioctadecyl-3,3,3", 3;-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamocortical afferents and Nissl and cytochrome oxidase staining of layer IV cellular aggregates demonstrated that clorgyline treatment from P0 to P6 produced a complete absence of any segmentation of vibrissae-related patches in S-I. However, continued treatment until P8 or P10 did not prevent the appearance of these patches. Animals treated with clorgyline from birth to P6 and killed on P8 or P10 had increases of 546.8 +/- 33.2% and 268.8 +/- 6.3% in cortical 5-HT and they had qualitatively normal vibrissae-related patterns in S-I. These results indicate that clorgyline treatment produces a transient disruption of vibrissae-related patterns, despite the continued presence of elevated cortical 5-HT.

Selective facilitation of the serotonin(1B) receptor causes disorganization of thalamic afferents and barrels in somatosensory cortex of rat

Alteration of serotonin (5-HT) levels influences developing thalamocortical afferents (TCAs) in primary somatosensory cortex (SI) of rats and mice. The 5-HT(1B) receptor, present on TCAs during the first postnatal week, may be involved in these effects. The present study asked whether administration of 5-nonyloxytriptamine (NNT), a selective 5-HT(1B) receptor agonist, affects TCA organization in rat SI. Littermates were injected five times daily (5x/day), with either 0.1 mg/kg NNT or vehicle from birth to postnatal day 6 (P-6). Animals were killed on P-6, and their brains were processed for high-performance liquid chromatography (HPLC), cytochrome oxidase (CO) histochemistry, cresyl violet, or demonstration of TCAs by placement of 1,1'-dioctadecyl-3,3,3'' 3'-tetra-methylindocarbocyanine perchlorate (Di-I) on thalamocortical radiations. At P-6, NNT treatment decreased 5-HT levels slightly compared with controls, although this difference was not statistically significant. In NNT-treated rats, the Di-I-labeled vibrissae-related pattern showed a range of effects, from fusion of patches related to mystacial vibrissae in treated animals to a less distinct vibrissae-related pattern in SI barrelfield compared with controls. Staining for CO and Nissl stain in layer IV of SI showed a similar range of abnormalities. These results indicate that the agonist action of NNT at the 5-HT(1B) receptor causes TCA disorganization in rat barrel field cortex in the absence of elevated 5-HT.

The cortical vibrissae representation is normal in transgenic mice lacking the 5-HT(1B) receptor

Recent studies have suggested that 5-HT may modulate thalamocortical development in somatosensory cortex (S-I) of rats and mice, and that the 5-HT(1B) receptor may play a critical role in this process. Analysis of CO-stained sections through lamina IV of S-I in perinatal and adult 5-HT(1B) knockout mice revealed a normal vibrissae-related pattern, indicating that activation of the 5-HT(1B) receptor is not necessary for the normal development of the vibrissae representation in S-I.

Serotonin receptor activation enhances neurite outgrowth of thalamic neurones in rodents

Serotonin (5-HT) has been shown to influence the development of the rodent barrel field by affecting the patterning of thalamic axons in the somatic sensory cortex. To determine whether this is a direct effect on thalamocortical neurones, we analyzed primary thalamic cultures taken from E15 mouse embryos. We show that 5-HT enhances neurite outgrowth of thalamic neurones. The sodium channel blocker, TTX, blocks these effects, whereas the selective 5-HT1B agonist CGS-12066A maleate reproduced 5-HT's effect. Using PCR and immunocytochemistry, we found that 5-HT1B receptors are already expressed by thalamic neurones at E15, and that this expression is maintained in vitro. These results suggest that 5-HT-1B receptor activation directly affects the growth of thalamocortical axons.

Plasticity of 5-hydroxytryptamine(1B) receptors during postnatal development in the rat visual cortex

The distribution of 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptors in the visual cortex was studied by quantitative autoradiography during postnatal development. Overall, receptor densities increased throughout development, but exhibited regional rearrangements, particularly in the case of 5-hydroxytryptamine1B receptors. Neonatal treatment with 5,7-dihydroxytryptamine, which causes selective degeneration of serotoninergic neurons, had no effect on the density of 5-hydroxytryptamine1A receptors in the visual cortex. However, a transient increase in 5-hydroxytryptamine1B at postnatal days 10-12 was observed after this treatment, suggesting a regulation of postsynaptic receptors. Neonatal enucleation resulted in a marked increase in 5-hydroxytryptamine1B binding sites in all layers of the visual cortex by P16, whereas it had no effect upon 5-hydroxytryptamine1A binding sites. These results show that both receptor subtypes do not exhibit striking transient features in the visual cortex during postnatal development, but rather undergo discrete reorganizations. 5-Hydroxytryptamine1B receptors show changes in density after either neonatal degeneration of serotoninergic neurons or enucleation, indicating that the serotoninergic system involving this receptor subtype can exhibit some postnatal plasticity in the visual cortex.

Serotonin levels are abnormally elevated in the fetus of the monoamine oxidase-A-deficient transgenic mouse

Developmental changes in levels of serotonin, L-tryptophan and 5-hydroxyindol acetic acid (5-HIAA) were measured by high pressure liquid chromatography (HPLC) in the forebrain, brainstem and cervical cord of fetal, neonatal and adult mice from the wild strain C3H and the transgenic strain Tg8, created from the C3H line by the disruption of the gene encoding monoamine oxidase A. The results indicated that the absence of monoamine oxidase A activity in Tg8 mice results in abnormally high 5-hydroxytryptamine (5-HT) levels in all the central nervous structures and at all the studied developmental ages. Since serotonin levels were 4-5 times larger in Tg8 than in C3H mice at gestational day 20, comparing the central network function at birth of C3H and Tg8 neonates should shed some light on the role of serotonin in prenatal network maturation.

Postnatal development of dopamine and serotonin transporters in rat caudate-putamen and nucleus accumbens septi

Density of dopamine transporter (DAT) and serotonin transporter (5-HTT) membrane proteins in the caudate-putamen (CPu) and nucleus accumbens (NAc) of rat brain was assessed at seven ages at postnatal days (PD) 7-60, by in vitro quantitative autoradiography. Binding of [3H]GBR-12935 (to DAT) and [3H]paroxetine (to 5-HTT) increased steadily and very similarly, from low levels at PD-7 to maximal levels, to 6-7-fold higher density at PD-60 in both regions. These findings indicate that DAT and 5-HTT follow a synchronized course of development in rat CPu and NAc. In contrast to reported elimination of excessive receptors in CPu and NAc during maturation, there was no evidence of pruning of DAT or 5-HTT in these regions of rat forebrain.