Neurotrophic effects of ipsapirone and other 5-HT1A receptor agonists on septal cholinergic neurons in culture

Relationships between serotonin 1A receptor DNA methylation, self-reported history of childhood abuse and gray matter volume in major depression

BACKGROUND

Early life adversity is a risk factor for psychopathology and is associated with epigenetic alterations in the 5-HT1A receptor gene promoter. The 5-HT1A receptor mediates neurotrophic effects, which could affect brain structure and function. We examined relationships between self-reported early childhood abuse, 5-HT1A receptor promoter DNA methylation, and gray matter volume (GMV) in Major Depressive Disorder (MDD).

METHODS

Peripheral DNA methylation of 5-HT1A receptor promoter CpG sites -681 and -1007 was assayed in 50 individuals with MDD, including 18 with a history of childhood abuse. T1-weighted structural magnetic resonance imaging (MRI) was performed. Voxel-based morphometry (VBM) was quantified in amygdala, hippocampus, insula, occipital lobe, orbitofrontal cortex, temporal lobe, parietal lobe, and at the voxel level.

RESULTS

No relationship was observed between DNA methylation and history of childhood abuse. We observed regional heterogeneity comparing -681 CpG site methylation and GMV (p = 0.014), with a positive relationship to GMV in orbitofrontal cortex (p = 0.035). Childhood abuse history was associated with higher GMV considering all ROIs simultaneously (p < 0.01). In whole-brain analyses, childhood abuse history was positively correlated with GMV in multiple clusters, including insula and orbitofrontal cortex (pFWE = 0.005), and negatively in intracalcarine cortex (pFWE = 0.001).

LIMITATIONS

Small sample size, childhood trauma assessment instrument used, and assay of peripheral, rather than CNS, methylation.

CONCLUSIONS

These cross-sectional findings support hypotheses of 5-HT1A receptor-related neurotrophic effects, and of increased regional GMV as a potential regulatory mechanism in the setting of childhood abuse. Orbitofrontal cortex was uniquely associated with both childhood abuse history and 5-HT1A receptor methylation.

NLX-101, a cortical 5-HT1A receptor biased agonist, reverses scopolamine-induced deficit in the delayed non-matching to position model of cognition

NLX-101 is a selective, high efficacy, biased agonist at post-synaptic cortical 5-HT_1A receptors. We have previously shown that it opposes deficits produced by blockade of NMDA receptors and has pro-cognitive activity of its own. Based on the strong interaction between 5-HT_1A receptors and the central cholinergic system, we tested NLX-101 on scopolamine-induced impairment of cognition in a delayed non-matching to position (DNMTP) model. The cholinesterase inhibitor, tacrine, was used as a comparator. In operant chambers with two retractable levers, male rats were trained to press one randomly presented lever during a "sample" phase. Following a time delay of either 1, 5 or 10 s, both levers were then presented, the rat being required to press the correct lever (i.e. the one not previously presented) to receive a food pellet reward. Scopolamine (0.16 mg/kg i.p.) significantly impaired accuracy (i.e. choice of correct lever) at 5 and 10 s delays. In contrast, NLX-101 (0.04, 0.16, 0.63 mg/kg i.p.) did not worsen accuracy, except at 0.63 mg/kg. Moreover, NLX-101 (0.04 and 0.16 mg/kg) dose-dependently and significantly opposed scopolamine-induced impairment for 5 and 10 s delays, with near-total reversal at 10 s. The acetylcholinesterase inhibitor, tacrine, also opposed scopolamine-induced impairment but was less potent and efficacious, with a single significant effect at 2.5 mg/kg and 5 s delay only. The present data suggest that biased agonism at post-synaptic, cortical 5-HT_1A receptors could prove useful in neurological or neuropsychiatric pathologies characterized by cognitive deficits consecutive to a reduced central cholinergic tone.

Of rodents and humans: A comparative review of the neurobehavioral effects of early life SSRI exposure in preclinical and clinical research

Selective serotonin reuptake inhibitors (SSRIs) have been a mainstay pharmacological treatment for women experiencing depression during pregnancy and postpartum for the past 25 years. SSRIs act via blockade of the presynaptic serotonin transporter and result in a transient increase in synaptic serotonin. Long-lasting changes in cellular function such as serotonergic transmission, neurogenesis, and epigenetics, are thought to underlie the therapeutic benefits of SSRIs. In recent years, though, growing evidence in clinical and preclinical settings indicate that offspring exposed to SSRIs in utero or as neonates exhibit long-lasting behavioral adaptions. Clinically, children exposed to SSRIs in early life exhibit increased internalizing behavior reduced social behavior, and increased risk for depression in adolescence. Similarly, rodents exposed to SSRIs perinatally exhibit increased traits of anxiety- or depression-like behavior. Furthermore, certain individuals appear to be more susceptible to early life SSRI exposure than others, suggesting that perinatal SSRI exposure may pose greater risks for negative outcome within certain populations. Although SSRIs trigger a number of intracellular processes that likely contribute to their therapeutic effects, early life antidepressant exposure during critical neurodevelopmental periods may elicit lasting negative effects in offspring. In this review, we cover the basic development and structure of the serotonin system, how the system is affected by early life SSRI exposure, and the behavioral outcomes of perinatal SSRI exposure in both clinical and preclinical settings. We review recent evidence indicating that perinatal SSRI exposure perturbs the developing limbic system, including altered serotonergic transmission, neurogenesis, and epigenetic processes in the hippocampus, which may contribute to behavioral domains (e.g., sociability, cognition, anxiety, and behavioral despair) that are affected by perinatal SSRI treatment. Identifying the molecular mechanisms that underlie the deleterious behavioral effects of perinatal SSRI exposure may highlight biological mechanisms in the etiology of mood disorders. Moreover, because recent studies suggest that certain individuals may be more susceptible to the negative consequences of early life SSRI exposure than others, understanding mechanisms that drive such susceptibility could lead to individualized treatment strategies for depressed women who are or plan to become pregnant.

Effects of developmental hyperserotonemia on the morphology of rat dentate nuclear neurons

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social cognition, disordered communication, restricted interests and repetitive behaviors. Furthermore, abnormalities in basic motor control, skilled motor gestures, and motor learning, are common in ASD. These characteristics have been attributed to a possible defect in the pre- and postnatal development of specific neural networks including the dentate-thalamo-cortical pathway, which is involved in motor learning, automaticity of movements, and higher cognitive functions. The current study utilized custom diolistic labeling and unbiased stereology to characterize morphological alterations in neurons of the dentate nucleus of the cerebellum in developing rat pups exposed to abnormally high levels of the serotonergic agonist 5-methyloxytryptamine (5-MT) pre-and postnatally. Occurring in as many as 30% of autistic subjects, developmental hyperserotonemia (DHS) is the most consistent neurochemical finding reported in autism and has been implicated in the pathophysiology of ASD. This exposure produced dramatic changes in dendritic architecture and synaptic features. We observed changes in the dendritic branching morphology which did not lead to significant differences (p>0.5) in total dendritic length. Instead, DHS groups presented with dendritic trees that display changes in arborescence, that appear to be short reaching with elaborately branched segments, presenting with significantly fewer (p>0.001) dendritic spines and a decrease in numeric density when compared to age-matched controls. These negative changes may be implicated in the neuropathological and functional/behavioral changes observed in ASD, such as delays in motor learning, difficulties in automaticity of movements, and deficits in higher cognitive functions.

Cajal and brain plasticity: insights relevant to emerging concepts of mind

The main legacies of Cajal are his drawings of brain structure and their connections, and his ideas of brain plasticity, not only in the mature brain but also during development and after brain injury. As the 21st century begins, many scientists are asking an old question: "how does the brain express the mind?" Although most models of mind incorporate the brain connections produced by Cajal, his ideas of plasticity are largely ignored. The purpose of this chapter is to review how some of Cajal's ideas can be useful in understanding the expression of the mind. I have also introduced several concepts and facts not available during Cajal's life. I cover the concept of homeostasis, the global projections of the monoamine neurons, and the actions of "mind-expanding" drugs. The global projecting neurons, because their monoamine transmitters have such a long history, are considered 1st order systems. The point-to-point connections are considered 2nd order systems. Their importance in theories of functional localization studies is briefly reviewed. Finally, a new model is presented called "Plastic Homeostasis," which incorporates the plastic interactions between 1st and 2nd order neurons. It is hoped that this review will encourage others to study the ideas presented by Cajal when considering functions of the brain. The emerging models of the mind would be well served by a review of the theoretical writing of Cajal.

Cognitive dysfunction in neuropsychiatric disorders: selected serotonin receptor subtypes as therapeutic targets

The indolamine, serotonin (5-hydroxytryptamine-5-HT) was identified and initially characterized around the middle of the twentieth century and it is now known to participate in multiple physiologic processes in mammalians. As a neurotransmitter, 5-HT is well documented to play a significant role in the pathophysiology and treatment of a variety of psychiatric disorders including anxiety, depression, and schizophrenia. In addition, there is also some evidence to suggest that 5-HT function in the brain may be important (particularly in the behavioral disturbances) in various forms of dementia including Alzheimer's disease. While 5-HT is undoubtedly involved in cognitive function, its role in specific domains of cognition (attention, learning, and memory, etc.) is poorly understood. This understanding has been impeded to some extent by the many complex interactions between 5-HT neurons and other neuronal phenotypes, 5-HT receptor heterogeneity, and the conflicting results of some behavioral experiments in animals conducted to date. Through the combined use of modern molecular biology, transgenic animal models, and other more traditional research methods such as medicinal chemistry and classical pharmacology, a clearer picture of the role of serotonin and its receptor subtypes in mnemonic processes is beginning to emerge, however. Considerable data now support the argument that selective ligands at specific 5-HT receptor subtypes can serve as therapeutic agents designed to enhance cognitive function in psychiatric disorders such as schizophrenia as well as age-related neurodegenerative illnesses such as Alzheimer's disease. The purpose of this review is to provide a brief overview of these therapeutic targets within the 5-HT system and the pharmacologic approaches (including the most recently developed compounds) designed to enhance memory function.

Embryonic and postnatal development of the serotonergic raphe system and its target regions in 5-HT1A receptor deletion or overexpressing mouse mutants

The neurotransmitter 5-HT regulates early developmental processes in the CNS. In the present study we followed the embryonic and postnatal development of serotonergic raphe neurons and catecholaminergic target systems in the brain of 5-HT1A receptor knockout (KO) and overexpressing (OE) in comparison with wild-type (WT) mice from embryonic day (E) 12.5 to postnatal day (P) 15.5. Up to P15.5 no differences were apparent in the differentiation and distribution of serotonergic neurons in the raphe area as revealed by the equal number of serotonergic neurons in the dorsal raphe in all three genotypes. However, the establishment of serotonergic projections to the mesencephalic tegmentum and hypothalamus was delayed at E12.5 in KO and OE animals and projections to the cerebral cortex between E16.5 and E18.5 were delayed in OE mice. This delay was only transient and did not occur in other brain areas including septum, hippocampus and striatum. Moreover, OE mice caught up with WT and KO animals postnatally such that at P1.5 serotonergic innervation of the cortex was more extensive in the OE than in KO and WT mice. Tissue levels of 5-HT and of its main metabolite 5-hydroxyindoleacetic acid as well as 5-HT turnover were considerably higher in brains of OE mice and slightly elevated in KO mice in comparison with the WT, starting at E16.5 through P15.5. The initial differentiation of dopaminergic neurons and fibers in the substantia nigra at E12.5 was transiently delayed in KO and OE mice as compared with WT mice, but no abnormalities in noradrenergic development were apparent in later stages. The present data indicate that 5-HT1A receptor deficiency or overexpression is associated with increased 5-HT synthesis and turnover in the early postnatal period. However, they also show that effects of 5-HT1A KO or OE on the structural development of the serotonergic system are at best subtle and transient. They may nonetheless contribute to the establishment of increased or reduced anxiety-like behavior, respectively, in adult mice.

Monoamine receptors and immature cerebellum cytoarchitecture after cisplatin injury

The experimental model of cisplatin treatment provides the opportunity to identify the precise function of the neurotransmitters in some crucial events of brain development, and their interactions or modulatory roles. The serotonin and noradrenaline monoamines influence the formation of the cerebellar cortex circuitry. In this study we found changes in the expression of the serotonin and noradrenaline receptors after a single injection of cisplatin in 10-day-old rats. The growth of Pc dendrites was early altered in lobules VI-VIII of cerebellum vermis. In these lobules, at postnatal day (PD) 17, the cisplatin-induced increase of the serotoninergic receptor 5-HT2AR, a factor that inhibits Pc dendrite growth by acting post-synaptically, occurred in all cerebellar layers, suggesting also alteration of granule cell proliferation and migration. The decreased labelling of beta l adrenergic receptor (beta1AR) in the soma of some Pc at PD11 can be correlated with the altered expression of glutamate receptors and GAD65 (glutamic acid decarboxylase) of and on Pc we have previously described [Pisu, M.B., Guioli, S., Conforti, E., Bernocchi, G., 2003. Signal molecules and receptors in the differential development of cerebellum lobules. Acute effects of cisplatin on nitric oxide and glutamate system in Purkinje cell population. Dev. Brain Res. 145, 229-240; Pisu, M.B., Roda, E., Avella, D., Bernocchi, G., 2004. Developmental plasticity of rat cerebellar cortex after cisplatin injury: inhibitory synapses and differentiating Purkinje neurons. Neuroscience 129, 655-664]. Moreover, beta1AR seems to be the key factor in the cerebellar reorganization between PD17 and PD30. The expression of this receptor was maintained in the molecular layer (ML), in particular in the inhibitory interneurons, despite their different distributions. The labelling of 5-HT1AR in the ML areas lacking Pc dendrite branches could contribute to the recovery phase of the cerebellar cytoarchitecture in cisplatin-treated rats. In general these findings should be taken into consideration in therapeutic interventions for developmental CNS disorders with a morphological basis.

Effect of neonatal handling on serotonin 1A sub-type receptors in the rat hippocampus

Serotonin 1A sub-type receptors play an important role in the etiopathogenesis of depression, which is known to occur more often in females than males. Early experiences can be a predisposing factor for depression; however, the underlying cellular processes remain unknown. In an effort to address such issues, we employed neonatal handling, an experimental model of early experience, which has been previously shown to render females more vulnerable to display enhanced depression-like behavior in response to chronic stress, while it increases the ability of males to cope. In rat pre-pubertal (30 days of age) and adult (90 days) hippocampus, of both males and females, the effect of neonatal handling on serotonin 1A sub-type receptor mRNA and protein levels was determined by in situ hybridization and immunohistochemistry, respectively, while the number of binding sites was determined by in vitro autoradiography using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin as the ligand. Our results revealed a significant sex difference in serotonin 1A sub-type receptor mRNA, protein and binding sites, with females having higher levels than males. Handling resulted in statistically significant decreased numbers of cells positive for serotonin 1A sub-type receptor mRNA or protein, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites in the area 4 of Ammon's horn and dentate gyrus of both pre-pubertal males and females. In adult animals the number of serotonin 1A sub-type receptor mRNA positive cells was increased as a result of handling in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of males, while it was decreased only in the area 4 of Ammon's horn of females. Furthermore, the number of serotonin sub-type 1A receptor immunopositive cells, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites was increased in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of handled males, whereas it was decreased in these same brain areas in the handled females. We can thus infer that neonatal handling results in alterations in postsynaptic serotonergic neurotransmission, which may contribute to the sex dimorphic effects of handling as to the vulnerability toward depression-like behavior in response to chronic stressful stimuli.

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.

Basic science behind the catastrophic epilepsies

The major catastrophic epileptic syndromes of childhood include infantile spasms, Lennox-Gastaut syndrome, and the progressive myoclonus epilepsies (PMEs). Although each of these syndromes manifests in an age-specific manner and is defined by distinct electroclinical features, they are all refractory to medical therapy and are invariably associated with psychomotor deficits, and in the most severe cases, either epileptic encephalopathy or progressive neurodegeneration. While much has been written about the clinical features and natural history of the catastrophic epilepsies, very little is known about the underlying pathophysiology. Progress in our understanding and treatment of these conditions has been hampered by the lack of suitable animal models in which putative mechanisms and novel targets for intervention could be rigorously studied. Nevertheless, recent clinical and basic investigations have identified certain mechanisms that may be relevant to their pathogenesis. In this review, three major hypotheses regarding the pathophysiology of infantile spasms are highlighted: the corticotropin-releasing hormone (CRH) hypothesis, the N-methyl-D-aspartate (NMDA) hypothesis, and the serotonin-kynurenine hypothesis. One or more of these mechanisms may be relevant in part to later-onset catastrophic epilepsies since infantile spasms can persist into later childhood and, like Lennox-Gastaut syndrome, well into adulthood. There is a profound need to develop more relevant animal models of the developmental encephalopathic epilepsies to truly develop better therapeutic strategies for these catastrophic disorders.

The acetylcholine fiber density of the neocortex is altered by isolated rearing and early methamphetamine intoxication in rodents

Alterations in the cholinergic physiology of the brain were the first to be observed when research on environmental influences on postnatal brain development began 35 years ago. Since then, the effects of isolated rearing (IR) or early pharmacological insults have been shown not only on the physiology, but also the anatomy of a variety of transmitter systems. The cholinergic fiber density, however, still remained to be assessed. We therefore used a histochemical procedure to stain cholinergic fibers in the brains of young adult gerbils reared either in groups in enriched environments or isolated in standard makrolon cages. Half of the animals from each rearing condition had received a single high dose of methamphetamine on postnatal day 14. Fiber densities were measured by computerized image analysis in the medial and orbital prefrontal cortex (PFC), dysgranular and granular insular cortex, sensorimotor cortices, and the entorhinal cortex of both hemispheres. Isolation rearing increased the cholinergic fiber densities in the prefrontal cortices of the left hemisphere and in the entorhinal cortex of the right hemisphere by about 10%, with no effect in the respective contralateral side. The early methamphetamine intoxication showed no influence in prefrontal and entorhinal cortices, but diminished the acetylcholine (ACh) innervation of the forelimb area of cortex in both hemispheres in IR gerbils and of the left hemisphere in ER gerbils, and reduced the acetylcholine innervation in the hindlimb area in both sides in both rearing groups. These results demonstrate that (a) cholinergic fiber density is differentially regulated in different cortical areas and (b) the plasticity of the cholinergic system can only be understood in the interplay with other neuromodulatory innervations.

Regulation of dendrite formation of Purkinje cells by serotonin through serotonin1A and serotonin2A receptors in culture

Serotonergic fibers and receptors appear in the rat cerebellum during early postnatal development. In the present study, we investigated the actions of serotonin (5-HT) and its receptors in the dendrite formation of Purkinje cells in organotypic cultures of anterior and posterior lobes of the cerebellum at postnatal day 7. In anterior lobes after 4 days in vitro (4DIV), the dendritic areas and branchings of Purkinje cells were increased by the treatment of 2 microM 5-HT, but decreased by 20 microM 5-HT. In posterior lobes after 4DIV, the dendritic areas of Purkinje cells were increased by 5-HT (2, 20 and 200 microM). In contrast, 5-HT treatment decreased dendritic areas of Purkinje cells in both anterior and posterior lobes after 7DIV. Next, we determined the actions of specific 5-HT receptors in mediating the effects of 5-HT by treatment with selective 5-HT receptor agonists. In anterior lobes after 4DIV, dendritic areas of Purkinje cells were increased by a 5-HT1A receptor agonist (8-OH-DPAT), whereas decreased by a 5-HT2A receptor agonist (DOI). The present study suggested that the dendrite formation of Purkinje cells is promoted by 5-HT through 5-HT1A receptors, but inhibited by 5-HT through 5-HT2A receptors.

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.

Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation, and apoptosis

Evolutionarily, serotonin existed in plants even before the appearance of animals. Indeed, serotonin may be tied to the evolution of life itself, particularly through the role of tryptophan, its precursor molecule. Tryptophan is an indole-based, essential amino acid which is unique in its light-absorbing properties. In plants, tryptophan-based compounds capture light energy for use in metabolism of glucose and the generation of oxygen and reduced cofactors. Tryptophan, oxygen, and reduced cofactors combine to form serotonin. Serotonin-like molecules direct the growth of light-capturing structures towards the source of light. This morphogenic property also occurs in animal cells, in which serotonin alters the cytoskeleton of cells and thus influences the formation of contacts. In addition, serotonin regulates cell proliferation, migration and maturation in a variety of cell types, including lung, kidney, endothelial cells, mast cells, neurons and astrocytes). In brain, serotonin has interactions with seven families of receptors, numbering at least 14 distinct proteins. Of these, two receptors are important for the purposes of this review. These are the 5-HT1A and 5-HT2A receptors, which in fact have opposing functions in a variety of cellular and behavioral processes. The 5-HT1A receptor develops early in the CNS and is associated with secretion of S-100beta from astrocytes and reduction of c-AMP levels in neurons. These actions provide intracellular stability for the cytoskeleton and result in cell differentiation and cessation of proliferation. Clinically, 5-HT1A receptor drugs decrease brain activity and act as anxiolytics. The 5-HT2A receptor develops more slowly and is associated with glycogenolysis in astrocytes and increased Ca(++) availability in neurons. These actions destabilize the internal cytoskeleton and result in cell proliferation, synaptogenesis, and apoptosis. In humans, 5-HT2A receptor drugs produce hallucinations. The dynamic interactions between the 5-HT1A and 5-HT2A receptors and the cytoskeleton may provide important insights into the etiology of brain disorders and provide novel strategies for their treatment.

Serotonergic lesion of median raphe nucleus alters nerve growth factor content and vulnerability of cholinergic septohippocampal neurons in rat

About 45% of the serotonergic raphe neurons are reported to express nerve growth factor (NGF) receptors. We therefore investigated whether selective serotonergic lesions of the median or dorsal raphe nuclei are associated with changes in NGF protein levels of the brain and whether the loss of serotonergic function alters the vulnerability of cholinergic septohippocampal neurons. In adult rats the hippocampal NGF content changed in a biphasic way after lesion of the median raphe nucleus by 5,7-dihydroxytryptamine (5,7-DHT), with a significant increase after 2-3 weeks of up to 35%, followed by a significant reduction of 22% below control levels after 7 weeks, and a return to control levels within the following 4 weeks. By contrast, the decrease in hippocampal serotonin and 5-hydroxyindoleacetic acid remained throughout the observation period of 11 weeks, being still reduced to 15 and 30% of the control levels, respectively. In the frontal cortex the partial loss of the serotonergic innervation projecting from the median raphe was associated 5 weeks after 5,7-DHT injection with an increase in NGF protein of 39.7+/-9.6% (P<0.05), which remained elevated up to 11 weeks. At 9 weeks after 5,7-DHT, the lesion of the septohippocampal cholinergic neurons induced by the cholinotoxin ethylcholine aziridinium (AF64A) was exaggerated (P<0.05) as compared to AF64A-treated rats with intact serotonergic innervation. The present data indicate that a serotonergic lesion of the median raphe nucleus results in biphasic changes of NGF protein content and in a delayed increase in the vulnerability of septohippocampal cholinergic neurons.

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.

Molecular ontogeny of major neurotransmitter receptor systems in the mammalian central nervous system: norepinephrine, dopamine, serotonin, acetylcholine, and glycine

Neurotransmitter receptors are critical elements in intercellular signaling within the central nervous system and are divided into two major types based on their molecular structure and biophysical properties. The first are ionotropic receptors--ligand-gated ion channels that directly affect the membrane potential via passage of permeant ions (such as sodium and calcium) and mediate fast synaptic transmission. The second type are slower metabotropic receptors that are also ligand gated but depend on an interaction with guanine nucleotide-binding proteins and mediate signal transduction by activating second-messenger systems within the cell. In the past two decades, a wealth of information has emerged regarding the molecular biology and pharmacology of classic neurotransmitter receptors (including adrenergic, dopaminergic, serotonergic, cholinergic, glycine, gamma-aminobutyric acid [GABA(A)], and glutamate receptors). Further, the distribution of subunits comprising these receptors has been extensively studied. This review focuses on the molecular ontogeny of several of the major neurotransmitter receptor systems in the mammalian central nervous system, highlighting the role that some of these may play during brain development and in certain pathologic states.

Serotonin 5-HT(1A) receptor expression is selectively enhanced in the striosomal compartment of chronic parkinsonian monkeys

Cynomolgus monkeys (Macaca fascicularis) were chronically treated with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) until stable parkinsonism was reached. Two months later, monkeys were sacrificed and monoamine content was measured in different brain regions of the lesioned monkeys and of age-matched controls. 5-HT(1A) serotonin receptor density was measured in coronal sections labeled with [(3)H]8-OH-DPAT. As expected, dopamine was virtually nonexistent in the caudate nucleus and putamen of MPTP-treated monkeys. Serotonin levels were significantly reduced in different brain regions, particularly in the raphe nuclei. 5-HT(1A) receptor density of control animals was high in the hippocampus, notably in the CA1 field and also in the raphe nuclei, and much lower in the striatum, where 5-HT(1A) receptors showed a patchy distribution which corresponded to striosomes with poor calbindin immunostaining. 5-HT(1A) receptor density was reduced in hippocampal fields and in the raphe nuclei of parkinsonian monkeys. Conversely, in the severely lesioned striatal nuclei 5-HT(1A) receptor density was increased at caudal levels of the striatum, particularly in the putamen. The results tend to support the possibility of an increased synthesis of 5-HT(1A) receptors in brain regions with higher neuronal cell death. Upregulation of this 5-HT receptor subtype in the limbic compartment of the striatum may represent a compensatory event for the serotonergic dysfunction and associated mental disorders in neurodegenerative diseases such as Parkinson disease.

Further characterization of 5-HT1A receptors in the goldfish retina: role of cyclic AMP in the regulation of the in vitro outgrowth of retinal explants

The presence of serotonin 5-HT1A receptors and their physiological role were further characterized in the goldfish retina. The effects of the 5-HT6/7 receptor antagonists pimozide, fluphenazine and amoxapine, the 5-HT1A receptor antagonist WAY-100,135, and the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, on the 5-HT1A receptor agonist [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes, were evaluated. In addition, the effects of serotonin, 8-hydroxy-2-(di-n-propylamino)tetralin, WAY-100,135, the adenylate cyclase inhibitors SQ22536 and MDL12330A, and the cyclic AMP analog 8-bromoadenosine-3':5' cyclic monophosphate were also studied on neuritic outgrowth from retinal explants. WAY-100,135 but not 5-HT6/7 receptor antagonists inhibited [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline decreased [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding sites up to 70%, while receptor turnover was similar to that reported in other tissues. Serotonin and 8-hydroxy-2-(di-n-propylamino)tetralin stimulated cyclic AMP production, both ex vivo and in vitro, and these increases were related to inhibition of neuritic outgrowth. The inhibitory effect was reduced by SQ22536 and by WAY-100,135, and was mimicked by 8-bromoadenosine-3':5'cyclic monophosphate. This study supports previous findings about the role of serotonin as a regulator of axonal outgrowth during in vitro regeneration of the goldfish retina and demonstrates that this effect is mediated, at least in part, by 5-HT1A receptors through a mechanism which involves an increase of cyclic AMP levels.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Chronic social stress reduces dendritic arbors in CA3 of hippocampus and decreases binding to serotonin transporter sites

Male rats housed in mixed-sex groups in a visible burrow system (VBS) form a dominance hierarchy in which subordinate animals show stress-related changes in behavior, endocrine function and neurochemistry. Dominants also appear to be moderately stressed compared to controls, although these animals do not develop the more pronounced behavioral and physiological deficits seen in the subordinates. In the present study, we examined the effects of chronic psychosocial stress on the morphology of Golgi-impregnated CA3 pyramidal neurons. In addition, since serotonin has been implicated in the mechanisms mediating the dendritic remodeling seen with other chronic stress regimens, we used quantitative autoradiography to measure binding to the serotonin transporter (5HTT) in hippocampus and dorsal and median raphe. Chronic social stress led to a decrease in the number of branch points and total dendritic length in the apical dendritic trees of CA3 pyramidal neurons in dominant animals compared to unstressed controls; subordinates also had a decreased number of dendritic branch points. [(3)H]paroxetine binding to the 5HTT was decreased in Ammon's horn in both dominants and subordinates compared to controls, while 5HTT binding remained unchanged in dentate gyrus and raphe. The similarity of the changes in 5HTT binding and dendritic arborization between both groups of VBS animals, despite apparent differences in stressor severity, suggests that these changes may be part of the normal adaptive response to chronic social stress. The mechanisms underlying dendritic remodeling in CA3 pyramidal neurons are likely to involve stress-induced changes in glucocorticoids and in 5HT and other transmitters.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

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.

Production and characterization of an anti-serotonin 1A receptor antibody which detects functional 5-HT1A binding sites

We describe the production and characterization of a specific anti-5-HT1A receptor antibody made against a fusion protein consisting of glutathione-S-transferase (GST) coupled to a 75-amino acid sequence from the middle portion of the third intracellular loop (5-HT1A-m3i, serine253-arginine327) of the rat 5-HT1A receptor protein. This region was chosen to avoid putative phosphorylation and glycosylation sites and regions of known homology with other 5-HT receptors. Western blot analysis indicated that the polyclonal anti-5-HT1A-m3i antibody accurately recognized the fusion protein expressed in bacteria and labeled a prominent 67 kDa protein band in the hippocampus, cortex, brainstem, cerebellum and kidney with a density profile corresponding to the relative abundance of the 5-HT1A receptor in these tissues. No protein was detected in liver or muscle tissue preparations, and no protein bands were labeled in any of the above tissues following preabsorption of the antibody with the 5-HT1A-m3i fusion protein. Immunohistochemistry revealed prominent labeling in limbic structures including the hippocampus, amygdala, entorhinal cortex, and septum as well as in raphe nuclei. In the hippocampus, 5-HT1A-m3i labeling revealed a characteristic laminar pattern that coincided with that seen by autoradiographic binding of the 5-HT1A agonist [3H]-8-OH-DPAT in all strata of the hippocampal formation. In the dorsal and medial raphe nuclei, anti-5-HT1A-m3i antibodies labeled the somatodendritic membranes of 5-HT neurons, consistent with its role as an autoreceptor. The detailed matching of the anti-5-HT1A-m3i antibody with [3H]-8-OH-DPAT binding suggests that the antibody recognizes a functionally active form of the 5-HT1A receptor protein capable of binding 5-HT1A agonist ligands. These anti-5-HT1A antibodies may therefore be useful tools in localizing functional 5-HT1A receptors in specific regions of the brain as well as in studying the plasticity and ontogeny of the 5-HT1A receptor at the cellular and subcellular level.

Serotonergic basis of antipsychotic drug effects in schizophrenia

Recent attention has been focused on the involvement of serotonin (5-HT) in the pathophysiology of schizophrenia and its role in mediating antipsychotic drug effects. There are two reasons for the new emphasis: the tremendous success of the so-called "atypical" antipsychotic drugs (a common feature of which is their high affinity for specific 5-HT receptor subtypes); and the elucidation of a complex family of 5-HT receptors whose function and pharmacology is only beginning to be understood. This paper will review the evidence that pertains to the role of 5-HT in mediating antipsychotic drug effects. The interaction of dopamine and 5-HT systems will be reviewed, and the mechanisms of action of atypical antipsychotic drugs will be evaluated in this context. The impact of serotonin on neurodevelopment, and the involvement of serotonin in the psychotomimetic and psychotogenic properties of hallucinogens, will be discussed. Together, these facts will be placed into the context of changes in serotonergic function in schizophrenia.

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

The distribution of 5-HT1A receptor mRNA in the human brain was studied in neonatal, children and adult cases by means of in situ hybridization histochemistry, using an oligonucleotide derived from the coding region of the human receptor. A prenatal pattern of development was observed. The hippocampus, raphe nuclei and neocortex presented high levels of hybridization already at the fetal/neonatal stage, fully comparable to the adult. A high and transient hybridization signal was found in cerebellum. These results support a role for 5-HT1A receptors in the regulation of neural development.

Role of the 5-HT1A receptor in development of the neonatal rat brain: preliminary behavioral studies

Serotonin exerts an influence on the prenatal development of rat brain. However, later developmental times may be more applicable to the understanding of the role of serotonin in human developmental disorders. Therefore, the current study was undertaken to gain preliminary information on the postnatal effects of serotonin on rat brain development. As the 5-HT1A receptor has been shown to be involved in much of the developmental functions of serotonin, an agonist for this receptor, 8-hydroxy-DPAT (8-OH-DPAT), was used. Neonatal rat pups at three ages (postnatal days, PNDs) 3-10, 10-17 or 17-24) were injected daily with 1 mg/kg 8-OH-DPAT and evaluated for behavioral consequences. The youngest group showed accelerated incisor eruption and eye-opening, a possible consequence of 5-HT1A receptor interactions with epidermal growth factor (EGF). Behaviorally, the animals were more anxious. Animals treated from PND 10-17, showed no change in craniofacial development but showed greater behavioral maturity in measures of spontaneous alternation and activity in the open field. The oldest animals (PND 17-24) showed no behavioral alterations, suggesting that this time length is beyond the critical period for serotonin's influence in brain development.

The serotonin innervation of the basal forebrain shows a transient phase during development

The serotonergic innervation of the adult and developing basal forebrain nuclei of the rat was studied with immunocytochemical techniques at the light and electron microscopic levels. A substantial number of relatively thick serotonergic fibers with few varicosities and random orientation were observed at the time of birth. During the subsequent weeks, the serotonergic fibers increased in number and became thinner with many varicosities. They were also re-oriented, and around the end of the third postnatal week they exhibited the pattern of distribution and density seen in the adult. Electron microscopic analysis revealed that serotonin varicosities formed symmetrical or asymmetrical synapses mainly with dendritic shafts throughout postnatal life. Stereological extrapolation from single sections to the whole volume of varicosities showed that the percentage of serotonin varicosities engaged in synaptic junctions varied according to age. The proportion of labelled varicosities forming synapses increased from birth (21.3%) to the end of the second postnatal week (42.5%), then declined markedly in the following week (17.1%) before increasing again to an adult value of 46%. These findings suggest that the formation of synaptic connections by serotonin axons in the basal forebrain shows two distinct phases in postnatal development: exuberant synapses present in the first two weeks of life may be related to the involvement of serotonin in the maturation of this area, whereas synapses formed later in development may affect the functional state of basal forebrain projections to the neocortex and hippocampus. Thus, at these late stages of development and in the adult, serotonin may influence the activity of these forebrain structures both directly and indirectly.

Serotonin1A receptors are expressed by a subpopulation of cholinergic neurons in the rat medial septum and diagonal band of Broca--a double immunocytochemical study

The possible colocalization of 5-hydroxytryptamine1A receptors and choline acetyltransferase in the same neurons of the medial septum and diagonal band of Broca was investigated using double immunocytochemical techniques, either on the same section or on adjacent thin sections of the rat brain. The presence of both antigens in the same neurons was demonstrated at the light and electron microscopic levels. The proportion of cholinergic neurons that express 5-hydroxytryptamine1A receptors was similar in the different parts of the septal complex (around 25%). By contrast, the proportion of 5-hydroxytryptamine1A receptor-positive neurons also exhibiting choline acetyltransferase immunoreactivity was much higher (40-44%) in the dorsal and ventral groups of cholinergic cells, than in the intermediate group (18%). In line with the topographical distribution of cholinergic projections, this result points out the potential involvement of 5-hydroxytryptamine1A receptors in the control of the septohippocampal cholinergic projection by serotonin. This connection might be relevant to learning and memory, and in the appearance of age-dependent or neurodegenerative cognitive deficits, which have been shown to involve alterations in both the serotoninergic and the cholinergic systems.

Enhanced synaptophysin immunoreactivity in rat hippocampal culture by 5-HT 1A agonist, S100b, and corticosteroid receptor agonists

Serotonin (5-HT) has been shown to modulate brain maturation during development and adult plasticity. This effect in the whole animal may be due to activation of 5-HT1A receptors and a corresponding increases in S100b and corticosterone. Synaptophysin, an integral protein of the synaptic vesicle membrane that correlates with synaptic density and neurotransmitter release, is reduced by depletion of 5-HT in the cortex and hippocampus of the adult rat. Injections of a 5-HT1A agonist or dexamethasone can reverse the loss of synaptophysin immunoreactivity (IR). In this study we used morphometric analysis of synaptophysin-IR to study the effects of the 5-HT1A agonist, ipsapirone, and the neuronal extension factor, S100b on hippocampal neurons grown in a serum and steroid free media. Both compounds increased the synaptophysin-IR at doses previously established to be highly specific. Ipsapirone (10(-9)M) was more effective on neuronal cell bodies staining and S100b (10 ng/ml) was more effective in increasing the number of synaptophysin-IR varicosities on neuronal processes. In addition both types of corticosteroid receptor agonists, at previously established specific doses, Ru28362 (10(-8) M) and aldosterone (10(-9) M) produced smaller increases compared to control groups in both the cell body staining and the number of varicosities. The effect of these differentiating factors on the expression of synaptophysin-IR suggests multiple regulation sites for producing and maintaining pre-synaptic elements in the brain.

Lack of barrels in the somatosensory cortex of monoamine oxidase A-deficient mice: role of a serotonin excess during the critical period

In a transgenic mouse line (Tg8) deficient for the gene encoding monoamine oxidase A (MAOA), we show that the primary somatosensory cortex (S1) lacks the characteristic barrel-like clustering of layer IV neurons, whereas normal pattern formation exists in the thalamus and the trigeminal nuclei. No barrel-like patterns were visible with tenascin or serotonin immunostaining or with labeling of thalamocortical axons. An excess of brain serotonin during the critical period of barrel formation appears to have a causal role in these cortical abnormalities, since early administration of parachlorophenylalanine, an inhibitor of serotonin synthesis, in Tg8 pups restored the formation of barrels in S1, whereas inhibition of catecholamine synthesis did not. Transient inactivation of MAOA in normal newborns reproduced a barrelless phenotype in parts of S1.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).