Development and characterization of monoclonal antibodies specific to the serotonin 5-HT2A receptor

Exposure to low environmental concentrations of manganese, lead, and cadmium alters the serotonin system of blue mussels

Serotonin plays a crucial role in mussel survival and reproduction. Although the serotonin system can be affected by metals, the effects of environmental concentrations of metals such as manganese (Mn), lead (Pd), and cadmium (Cd) have never been studied in blue mussels. The present study aimed to determine the effects of exposure to Mn, Pb, or Cd on serotonin levels, monoamine oxidase (MAO) activity, and serotonin transporter (SERT) levels in the blue mussel Mytilus edulis. Mussels were exposed in vivo to increasing and environmentally relevant doses of Mn (10-1000 nM; 0.5-50 μg/L), Pb (0.01-10 nM; 0.002-2 μg/L), or Cd (0.01-10 nM; 0.001-1 μg/L) for 28 d. Serotonin levels, MAO activity, and SERT expression were analyzed in the mussel mantle. Expression of SERT protein was significantly decreased, by up to 81%, following Mn, Pb, or Cd exposure. The activity of MAO in females was almost 2-fold higher, versus males, in nonexposed control mussels. In mussels exposed to 0.1 nM of Pb (0.02 μg/L), MAO activity was increased in males and decreased in females. In Cd-exposed mussels, a sex-dependent, inverted nonmonotonic pattern of MAO activity was observed. These results clearly indicate that low environmental concentrations of Mn, Pb, and Cd affect the serotonin system in blue mussels. Environ Toxicol Chem 2018;37:192-200. © 2017 SETAC.

Evidence for the differential co-localization of neurokinin-1 receptors with 5-HT receptor subtypes in rat forebrain

Studies suggest that like selective 5-hydroxytryptamine (5-HT; serotonin) reuptake inhibitors, antagonists at neurokinin-1 receptors (NK(1)Rs) may have antidepressant and anxiolytic properties. NK(1)Rs are present in 5-HT innervated forebrain regions which may provide a common point of interaction between these two transmitter systems. This study aimed to investigate for cellular co-localization between NK(1)Rs and 5-HT receptor subtypes in mood-related brain regions in the rat forebrain. With experiments using fluorescence immunocytochemistry, double-labelling methods demonstrated a high degree of co-localization between NK(1)Rs and 5-HT(1A) receptors in most regions examined. Co-localization was highest in the medial septum (88% NK(1)R expressing cells were 5-HT(1A) receptor-positive) and hippocampal regions (e.g. dentate gyrus, 65%), followed by the lateral/basolateral amygdala (35%) and medial prefrontal cortex (31%). In contrast, co-localization between NK(1)Rs and 5-HT(2A) receptors was infrequent (< 8%) in most areas examined except for the hippocampus (e.g. CA3, 43%). Overall co-localization between NK(1)Rs and 5-HT(1A) receptors was much greater than that between NK(1)Rs and 5-HT(2A) receptors. Thus, these experiments demonstrate a high degree of co-localization between NK(1)Rs and 5-HT(1A) receptors in cortical and limbic regions of the rat forebrain. These findings suggest a novel site of interaction between NK(1)R antagonists and the 5-HT system.

Neuronal localization of 5-HT2A receptor immunoreactivity in the rat hippocampal region

The 5-HT2A receptor subtype (5-HT2Ar) plays an important role in the modulation of the hippocampal region activity and it has been associated with learning and memory processes. In the present study, the 5-HT2Ar was immunohistochemically localized in the rat hippocampal region, which includes the hippocampal formation and the parahippocampal region. In the hippocampal formation (dentate gyrus, hippocampus proper and subiculum) and entorhinal cortex, the colocalization of the 5-HT2Ar with the inhibitory transmitter γ-aminobutyric acid (GABA) was studied using double immunofluorescence confocal microscopy. The patterns of immunostaining were very different in non-injected and colchicine-injected rats. In untreated rats, the immunoreactivity could be attributed especially to neuropil. Interestingly, in non-injected rats, the 5-HT2Ar immunoreactivity was located in the mossy fibers, suggesting that serotonin acts presynaptically via this receptor subtype directly on glutamate axons. Pretreatment with colchicine increased the number of 5-HT2Ar-immunoreactive somata. Morphological and double immunofluorescence analyses indicated that the 5-HT2Ar was located on both the excitatory and the inhibitory neurons of the rat hippocampal region. The results of the present study suggest that the 5-HT2Ar could participate in the hippocampal neurotransmission by acting on different neuronal populations.

Contribution of 5-HT to locomotion - the paradox of Pet-1(-/-) mice

Serotonin (5-HT) plays a critical role in locomotor pattern generation by modulating the rhythm and the coordinations. Pet-1, a transcription factor selectively expressed in the raphe nuclei, controls the differentiation of 5-HT neurons. Surprisingly, inactivation of Pet-1 (Pet-1(-/-) mice) that causes a 70% reduction in the number of 5-HT-positive neurons in the raphe does not impair locomotion in adult mice. The goal of the present study was to investigate the operation of the locomotor central pattern generator (CPG) in neonatal Pet-1(-/-) mice. We first confirmed, by means of immunohistochemistry, that there is a marked reduction of 5-HT innervation in the lumbar spinal cord of Pet-1(-/-) mice. Fictive locomotion was induced in the in vitro neonatal mouse spinal cord preparation by bath application of N-methyl-d,l-Aspartate (NMA) alone or together with dopamine and 5-HT. A locomotor pattern characterized by left-right and flexor-extensor alternations was observed in both conditions. Increasing the concentration of 5-HT from 0.5 to 5 μm impaired the pattern in Pet-1(-/-) mice. We tested the role of endogenous 5-HT in the NMA-induced fictive locomotion. Application of 5-HT(2) or 5-HT(7) receptor antagonists affected the NMA-induced fictive locomotion in both heterozygous and homozygous mice although the effects were weaker in the latter strain. This may be, at least partly, explained by the reduced expression of 5-HT(2A) R as observed by means of immunohistochemistry. These results suggest that compensatory mechanisms take place in Pet-1(-/-) mice that make locomotion less dependent upon 5-HT.

Serotonergic projections and serotonin receptor expression in the reticular nucleus of the thalamus in the rat

The reticular nucleus (RT) of the thalamus, a thin sheet of GABAergic neurons located between the external medullary lamina and the internal capsule of the thalamus, has functionally distinct afferent and efferent connections with thalamic nuclei, the neocortex, the basal forebrain and the brainstem. RT is critically positioned to rhythmically pace thalamocortical networks leading to the generation of spindle activity during the early phases of sleep and during absence (spike-wave) seizures. Serotonin, acting on 5-HT(1A) receptors on parvalbumin-containing cells of RT, has been implicated in this rhythmicity. However, the precise source(s) of 5-HT afferents to the RT remains to be determined. In the present study, we injected the retrograde tracer, Fluorogold, into dorsal and ventral regions of RT to determine the origins of raphe input to RT. We further characterized the distribution of 5-HT fibers to RT by using immunohistochemistry for 5-HT and for the 5HT transporter (SERT) detection. Finally, we described the presence of the two major postsynaptic 5-HT receptors in RT, 5-HT(1A) and 5-HT(2A) receptors. Our results show that the dorsal raphe nucleus and the supralemniscal nucleus (B9) of the midbrain are the principal sources of raphe projections to RT. In addition, serotonergic fibers (5-HT and SERT positive) were richly distributed throughout RT, and 5-HT(1A) and 5-HT(2A) receptors were highly expressed on RT neurons and dendrites. These findings suggest a significant 5-HT modulatory influence on GABAergic neurons of RT in the control of rhythmical (or spindle) activity in thalamocortical systems directly associated with sleep and possibly with absence seizures.

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

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

Htr2a Gene and 5-HT(2A) Receptor Expression in the Cerebral Cortex Studied Using Genetically Modified Mice

Serotonin receptors of the 5-HT_2A subtype are robustly expressed in the cerebral cortex where they have been implicated in the pathophysiology and therapeutics of mental disorders and the actions of hallucinogens. Much less is known, however, about the specific cell types expressing 5-HT_2A receptors in cortex. In the current study we use immunohistochemical and electrophysiological approaches in genetically modified mice to address the expression of the Htr2a gene and 5-HT_2A receptors in cortex. We first use an EGFP-expressing BAC transgenic mice and identify three main Htr2A gene expressing neuronal populations in cortex. The largest of these cell populations corresponds to layer V pyramidal cells of the anterior cortex, followed by GABAergic interneurons of the middle layers, and non-pyramidal cells of the subplate/Layer VIb. We then use 5-HT_2A receptor knockout mice to identify an antibody capable of localizing 5-HT_2A receptors in brain and use it to map these receptors. We find strong laminar expression of 5-HT_2A receptors in cortex, especially along a diffuse band overlaying layer Va. This band exhibits a strong anteroposterior gradient that closely matches the localization of Htr2A expressing pyramidal cells of layer V. Finally we use electrophysiological and immunohistochemical approaches to show that most, but not all, GABAergic interneurons of the middle layers are parvalbumin expressing Fast-spiking interneurons and that these cells are depolarized and excited by serotonin, most likely through the activation of 5-HT_2A receptors. These results clarify and extend our understanding of the cellular distribution of 5-HT_2A receptors in the cerebral cortex.

Different serotonergic expression in nevomelanocytic tumors

The neuromediator serotonin (5-hydroxytryptamine; 5-HT) has been proposed to play a role in tumor progression. Thus, the aim of the present investigation was to determine whether alterations in the serotonergic system occur in nevomelanocytic tumors. For this purpose, paraffin-embedded biopsies of superficial spreading malignant melanoma (SSM), dysplastic compound nevi (DN) and benign compound nevi (BCN) were characterized with regard to their expression of 5-HT, the 5-HT1A and 5-HT2A receptors, and the serotonin transporter protein (SERT), by immunohistochemical analysis. Melanocytes in the region surrounding the tumor were found to express both the 5-HT1A and 5-HT2A receptors. Tumor cells that immunostained positively for the different serotonergic markers were observed in the suprabasal epidermis of DN tissue and, to an even greater extent, in the case of SSM. Furthermore, some of these latter cells expressed both 5-HT1AR and 5-HT2AR. The level of expression of 5-HT1AR at the junctional area was lower for SSM than for DN or BCN. As the degree of atypia increased, the intensity of tumor cell staining in the dermis for 5-HT1AR and SERT declined. Vessel immunoreactivity for 5-HT2A was more intense in SSM than in BCN tissue. Round-to-dendritic cells that expressed both SERT and 5-HT1AR were seen to infiltrate into the dermal region of the tumor, this infiltration being more evident in the case of DN and SSM. These latter cells were also tryptase-positive, indicating that they are mast cells. Thus, alterations in serotonergic system may be involved in nevomelanocytic tumors and mast cells may play an important role in this connection.

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

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

Evidence for distinct antagonist-revealed functional states of 5-hydroxytryptamine(2A) receptor homodimers

The serotonin (5-hydroxytryptamine; 5-HT) 2A receptor is a cell surface class A G protein-coupled receptor that regulates a multitude of physiological functions of the body and is a target for antipsychotic drugs. Here we found by means of fluorescence resonance energy transfer and immunoprecipitation studies that the 5-HT(2A)-receptor homodimerized in live cells, which we linked with its antagonist-dependent fingerprint in both binding and receptor signaling. Some antagonists, like the atypical antipsychotics clozapine and risperidone, differentiate themselves from others, like the typical antipsychotic haloperidol, antagonizing these 5-HT(2A) receptor-mediated functions in a pathway-specific manner, explained here by a new model of multiple active interconvertible conformations at dimeric receptors.

Characterization of serotonin receptors in pregnant human myometrium

The monoamine, 5-hydroxytryptamine (5-HT), stimulates contraction of human uterine smooth muscle (myometrium), but the receptor subtypes involved have not been characterized. We studied the effects of a range of 5-HT receptor subtype-selective agonists and antagonists in isolated strips of myometrium obtained at the time of caesarean section. The 5-HT(1A) receptor agonist, 8-hydroxy-2-dipropylaminotetralin, produced an increase in contractions that was highly variable, of low potency, and was not significantly inhibited by the 5-HT(1A) antagonist WAY100635 [[O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide]. The 5-HT(2) receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), produced a strong, consistent, and concentration-dependent stimulation of contractions (pEC(50) = 7.60 +/- 0.10, n = 5). The 5-HT(2A) receptor antagonist, ketanserin [3-[2-[4-(4-fluoro benzoyl)-piperidin-1-yl]ethyl]-1H-quinazoline-2,4-dione], caused a parallel shift in the response to alpha-Me-5-HT, with a pK(B) value consistent with its known affinity for the 5-HT(2A) receptor (pK(B) = 8.47 +/- 0.16, n = 5), but it had no effect on the response to oxytocin. The 5-HT(2B) and 5-HT(2C) receptor agonists, BW723C86 [(alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine)] and Ro-60-01-75 [(S)-2-(6-chloro-5-fluoro-indol-1-yl)-1-methyl-ethylamine fumarate], produced inconsistent responses at potencies that were lower than expected for activation of their cognate receptors. The response to alpha-Me-5-HT was unaffected by the 5-HT(2B) and 5-HT(2C) receptor antagonists, SB204741 [(N-(1-methyl-1H-indolyl-5-yl)-N-(3-methyl-5-isothiazolyl)urea)] and RS102221 [8-[5-(2,4-dimethoxy-5-(4-trifluoromethyl phenylsulphonamido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione]. The 5-HT(1B/1D) receptor agonist, sumatriptan [1-[3-(2-dimethylaminoethyl)-1H-indol-5-yl]-N-methyl-methanesulfonamide], the 5-HT(4) agonist, cisapride [4-amino-5-chloro-N-[1-[3-(4-fluorophenoxy)propyl]-3-methoxy-4-piperidyl]-2-methoxy-benzamide], and the 5-HT(7) agonist, AS19 [(2S)-(+)-5-(1,3,5-trimethylpyrazol-4-yl)-2-(dimethylamino)tetralin], all had no effect on myometrial contractility. 5-HT(2A) receptor mRNA and immunoreactivity were identified using reverse transcriptase-polymerase chain reaction, Western blotting, and immunohistochemistry. Specific binding of [(3)H]ketanserin was demonstrated. This study provides strong evidence for the expression of contractile 5-HT(2A) receptors in pregnant human myometrium, and this receptor is a potential target for novel uterotonic therapies.

c-Src tyrosine kinase, a critical component for 5-HT2A receptor-mediated contraction in rat aorta

Serotonin (5-hydroxytryptamine, 5-HT) receptors (5-HTRs) play critical roles in brain and cardiovascular functions. In the vasculature, 5-HT induces potent vasoconstrictions, which in aorta are mainly mediated by activation of the 5-HT(2A)R subtype. We previously proposed that one signalling mechanism of 5-HT-induced vasoconstriction could be c-Src, a member of the Src tyrosine kinase family. We now provide evidence for a central role of c-Src in 5-HT(2A)R-mediated contraction. Inhibition of Src kinase activity with 10 mum 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) prior to contraction resulted in approximately 90-99% inhibition of contractions induced by 5-HT or by alpha-methyl-5-HT (5-HT(2)R agonist). In contrast, PP2 pretreatment only partly inhibited contractions induced by angiotensin II and the thromboxane A(2) mimetic, U46619, and had no significant action on phenylephrine-induced contractions. 5-Hydroxytryptamine increased Src kinase activity and PP2-sensitive tyrosine-phosphorylated proteins. As expected for c-Src identity, PP2 pretreatment inhibited 5-HT-induced contraction with an IC(50) of approximately 1 mum. Ketanserin (10 nm), a 5-HT(2A) antagonist, but not antagonists of 5-HT(2B)R (100 nm SB204741) or 5-HT(2C)R (20 nm RS102221), prevented 5-HT-induced contractions, mimicking PP2 and implicating 5-HT(2A)R as the major receptor subtype coupled to c-Src. In HEK 293T cells, c-Src and 5-HT(2A)R were reciprocally co-immunoprecipitated and co-localized at the cell periphery. Finally, 5-HT-induced Src activity was unaffected by inhibition of Rho kinase, supporting a role of c-Src upstream of Rho kinase. Together, the results highlight c-Src activation as one of the early and pivotal mechanisms in 5-HT(2A)R contractile signalling in aorta.

Neuroimmune mechanisms in patients with atopic dermatitis during chronic stress

Objective

To identify pathoaetiological neuroimmune mechanisms in patients with atopic dermatitis (AD) and chronic stress, focusing at nerve density, sensory neuropeptides, and the serotonergic system.

Methods

Eleven patients with AD with histories of stress worsening were included. Biopsies from involved and non-involved skin were processed for immunohistochemistry. Salivary cortisol test was done as a marker for chronic stress.

Results

There were more acanthosis and fewer nerve fibres in epidermis and papillary dermis of involved compared with non-involved skin. Whereas there was no significant change in the number of substance P and calcitonin gene-related peptide–positive nerve fibres between the involved and non-involved skin, there was an increase in the epidermal fraction of 5-hydroxtrytamine 1A (5-HT1A) receptor and serotonin transporter protein (SERT) immunoreactivity in the involved skin. The number of 5-HT2AR, CD3-positive cells, and SERT-positive cells, most of them being CD3 positive, was increased in involved skin. There was an increase in mast cells in the involved skin, and these cells were often located close to the basement membrane. There was a strong tendency to a correlation between 5-HT2AR positive cells in the papillary dermis of involved skin and low cortisol ratios, being an indicator of chronic stress.

Conclusion

A changed innervation and modulation of the serotonergic system are indicated in chronic atopic eczema also during chronic stress.

Colocalisation of serotonin2A receptors with the glutamate receptor subunits NR1 and GluR2 in the dentate gyrus: an ultrastructural study of a modulatory role

The serotonin(2A) receptor (5-HT(2A)R) is implicated in many neurological disorders and has a role in cognitive processes, reliant upon hippocampal glutamate receptors. Recent studies show that 5-HT(2A)R agonists and/or antagonists can influence cognitive function, suggesting a critical hippocampal role for these receptors, yet their cellular and subcellular distribution within this region has not been comprehensively analysed. Here, we have conducted an electron microscopic examination of 5-HT(2A)R distribution with the glutamate N-methyl-D-aspartate (NMDA) and amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor subunits NR1 and GluR2 in the hippocampal dentate gyrus (DG) in order to investigate whether 5-HT(2A)R location is compatible with a modulatory role over NMDA and/or AMPA receptor mediated neurotransmission. Of 5-HT(2A)R positive profiles, 56% were dendrites and 16% were dendritic spines. Labelling was both cytoplasmic and membranous. Spinous labelling was more frequently membranous at peri- and extra-synaptic sites, though was also associated with synaptic specialisations. Profiles displaying colocalisation of immunoreactivity for 5-HT(2A)Rs with NR1 or GluR2 were predominantly dendrites, representing 11% and 8% of 5-HT(2A)R positive profiles, respectively. Additionally, 12% of 5-HT(2A)R labelled profiles also displayed immunoreactivity for gamma-aminobutyric acid (GABA). These data indicate most 5-HT(2A)Rs are expressed on granule cell projections, with a smaller subpopulation expressed on GABAergic interneurons.

Identification of a key amino acid of the human 5-HT(2B) serotonin receptor important for sarpogrelate binding

Based on radio-ligand binding and molecular modeling studies, sarpogrelate shows a moderate selectivity for 5-HT(2B) versus 5-HT(2A) receptors. To confirm the modeling data of sarpogrelate to 5-HT(2B) receptors predicting interaction of sarpogrelate towards Asp135 in helix 3 of 5-HT(2B) receptors, we constructed and characterized the mutation of this residue by site-directed mutagenesis. The Asp135Ala mutant did not exhibit any affinity for [(3)H]rauwolscine. Therefore, it was not possible to find sarpogrelate affinity to the mutant using [(3)H]rauwolscine. The mutation also abolished agonist-stimulated inositol phosphates formation. These results provide evidence that Asp135 is important for the interaction between 5-HT(2B) receptors and sarpogrelate.

5-HT2A receptors are differentially expressed in bullfrog and rat retinas: a comparative study

Expression of 5-hydroxytryptamine (5-HT) 2A receptor (5-HT2A) was studied in bullfrog and rat retinas by immunocytochemistry. In the bullfrog retina, 5-HT2A-immunoreactivity was observed in both the outer and inner plexiform layers (OPL and IPL). Double labeling experiments further showed that 5-HT2A was expressed in Müller cells stained by GFAP. Labeling for 5-HT2A was strong in the somata and endfeet and relatively weak in the major processes and fine branchets of Müller cells. In contrast, 5-HT2A immunoreactivity was hardly detected in the rat retina, and no rat Müller cells were labeled. Furthermore, immunocytochemical assay demonstrated that labeling for 5-HT was present in amacrine cells and displaced amacrine cells in the inner retina of bullfrog, but not in the rat retina. These results suggest that 5-HT may modulate retinal information processing via activating 5-HT2A expressed in neuronal and glial elements in bullfrog, but that such modulation is unlikely to occur in rat.

Neuronal localization of 5-HT type 2A receptor immunoreactivity in the rat basolateral amygdala

Although it is well established that there are alterations in type 2A 5-HT receptors (5-HT2ARs) in the basolateral nuclear complex of the amygdala (BLC) in several neuropsychiatric disorders, very little is known about the neuronal localization of these receptors in this brain region. Single-labeling and dual-labeling immunohistochemical techniques were utilized in the rat to address this question. Three different 5-HT2AR antibodies were used, each producing distinct but overlapping patterns of immunostaining. Two of three 5-HT2AR antibodies mainly stained pyramidal projection neurons in the BLC. The third antibody only stained pyramidal cells in the dorsolateral subdivision of the lateral amygdalar nucleus. With one of the antibodies, the most intensely stained neurons were a population of large nonpyramidal neurons whose morphology and distribution closely resembled those shown in previous studies to project to the mediodorsal thalamic nucleus (MD). This was confirmed in the present study using a technique that combined 5-HT2AR immunohistochemistry with fluorogold retrograde tract-tracing. Two of three 5-HT2AR antibodies stained large numbers of parvalbumin-containing interneurons in the BLC. One of these two antibodies also stained a subpopulation of somatostatin-containing neurons. None of the 5-HT2AR antibodies stained significant numbers of the other two main interneuronal subpopulations, the large cholecystokinin-positive neurons or the small interneurons that exhibit extensive colocalization of calretinin and cholecystokinin. Since each of the three antibodies was raised against a distinct immunizing antigen, they may recognize different conformations of 5-HT2AR in different neuronal domains. The expression of 5-HT2ARs in pyramidal cells and parvalbumin-positive interneurons in the BLC is consistent with the results of previous electrophysiological studies, and suggests that 5-HT may produce excitation of several neuronal populations in the BLC via 5-HT2ARs.

Changes in the serotonergic system in the main olfactory bulb of rats unilaterally deprived from birth to adulthood

The serotonergic system plays a key role in the modulation of olfactory processing. The present study examined the plastic response of this centrifugal system after unilateral naris occlusion, analysing both serotonergic afferents and receptors in the main olfactory bulb. After 60 days of sensory deprivation, the serotonergic system exhibited adaptive changes. Olfactory deprivation caused a general increase in the number of fibres immunopositive for serotonin but not of those immunopositive for the serotonin transporter. HPLC data revealed an increase in serotonin levels but not in those of its major metabolite, 5-hydroxyindole acetic acid, resulting in a decrease in the 5-hydroxyindole acetic acid/serotonin ratio. These changes were observed not only in the deprived but also in the contralateral olfactory bulb. Double serotonin-tyrosine hydroxylase immunolabelling revealed that the glomerular regions of the deprived olfactory bulb with a high serotonergic fibre density showed a strong reduction in tyrosine hydroxylase. Finally, the serotonin(2A) receptor distribution density and the number of juxtaglomerular cells immunopositive for serotonin(2A) receptor remained unaltered after olfactory deprivation. Environmental stimulation modulated the serotonergic afferents to the olfactory bulb. Our results indicate the presence of a bilateral accumulation of serotonin in the serotonergic axon network, with no changes in serotonin(2A) receptor density after unilateral olfactory deprivation.

Identification of amino acid residues important for sarpogrelate binding to the human 5-hydroxytryptamine2A serotonin receptor

The purpose of the present study was to examine 5-hydroxytryptamine (5-HT)(2A)-receptor sarpogrelate interactions by site-directed mutagenesis. Based on molecular modeling studies, aspartic acid (Asp)155[3.32] and tryptophan (Trp)151[3.28] in transmembrane helix (TMH) III and Trp336[6.48] in TMH VI of the 5-HT(2A) receptor were found to interact with sarpogrelate. All of these residues were mutated to alanine (Ala). The Asp3.32Ala mutant did not exhibit any affinity for [(3)H]ketanserin, whereas the Trp3.28Ala mutant showed a markedly decrease in the binding affinity for [(3)H]ketanserin (K(d) >10,000 nM). Therefore, it was not possible to find any sarpogrelate affinity to the mutants using [(3)H]ketanserin. The mutation also abolished agonist-stimulated formation of [(3)H]inositol phosphates (IP) in both cases. On the other hand, the Trp6.48Ala mutant showed reduced binding affinity for [(3)H]ketanserin (K(d) 2.0 nM vs 0.8 nM for the wild-type receptor) and had reduced affinity for sarpogrelate (pK(i) value of 5.71 vs 9.06 for the wild-type receptor). The Trp6.48Ala mutant also showed the greatest decrease in sensitivity to sarpogrelate (pK(b) value 8.81 for wild-type and 5.11 for mutant) in inhibiting agonist-stimulated IP formation. These results provide direct evidence that Asp3.32, Trp3.28, and less importantly, Trp6.48 are responsible for the interaction between the 5-HT(2A) receptor and sarpogrelate. In addition, these results support the findings obtained from molecular modeling studies.

Expression of serotonergic receptors in psoriatic skin

Psoriasis appears to be influenced by stress, which causes release of adrenal hormones. Serotonin, or hormonal actions on serotonin and serotonin receptors, may have a role in psoriasis. Distribution of serotonin receptors was studied in involved and noninvolved skin in patients with psoriasis and compared to normal skin, by using immunohistochemistry and antibodies to 5-HT1A, 5-HT2A and 5-HT3 receptors (R). There was a decreased (P<0.001) number of 5-HT1AR positive cells, the majority being tryptase positive, in involved and noninvolved psoriatic papillary dermis, compared to normal skin. 5-HTlAR expression was also found in the upper part of the epidermis, on vessel walls and on melanocytes. 5-HT2AR expressing papillary mononuclear cells, CD3 positive, were increased (P<0.001 and P<0.01, respectively) in involved and noninvolved psoriatic skin, compared to normal skin, an increase (P<0.01) also being found in the involved compared to noninvolved skin. Expression of 5-HT3R could be found in the basal epidermal layer of noninvolved but not in the involved skin of psoriasis, where it was only found in the acrosyringium. The present findings are compatible with the 5-HT1A and 5-HT2A receptors having antagonistic functions, and raise the possibility of using receptor specific drugs in the treatment of psoriasis.

The 5-HT2A serotoninergic receptor is expressed in the MCF-7 human breast cancer cell line and reveals a mitogenic effect of serotonin

Serotonin (5-hydroxytryptamine, 5-HT) has been described as a mitogen in a variety of cell types and carcinomas. It exerts its mitogenic effect by interacting with a wide range of 5-HT receptor types. Certain studies suggest that some selective serotonin re-uptake inhibitors promote breast cancer in animals and humans. This study attempts to clarify the role of serotonin in promoting the growth of neoplastic mammary cells. Expression of the 5-HT(2A) serotoninergic receptor subtype in MCF-7 cells was determined by RT-PCR, Western blotting, and immunofluorescence analysis. The mitogenic effect of 5-HT on MCF-7 cells was determined by means of the MTT proliferation assay. We have demonstrated that the 5-HT(2A) receptor subtype is fully expressed in the MCF-7 human breast cancer cell line, in terms of encoding mRNA and receptor protein. Automated sequencing has confirmed that the 5-HT(2A) receptor present in this cell line is identical to the 5-HT(2A) receptor found in human platelets and in human cerebral cortex. Furthermore, this receptor was found by immunofluorescence to be on the plasma membrane. MTT proliferation assays revealed that 5-HT and DOI, a selective 5-HT(2A) receptor subtype agonist, stimulated MCF-7 cell. These results indicate that 5-HT plays a mitogenic role in neoplastic mammary cells. Our data also indicate that 5-HT exerts this positive growth effect on MCF-7 cells through, in part, the 5-HT(2A) receptor subtype, which is fully expressed in this cell line.

Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Serotonin (5-HT) is an ancient chemical that plays a crucial functional role in almost every living organism. It regulates platelet aggregation, activation of immune cells, and contraction of stomach and intestinal muscles. In addition, serotonin acts as a neurotransmitter in the brain and the peripheral nervous system. These activities are initiated by the binding of serotonin to 15 or more receptors that are pharmacologically classified into seven groups, 5-HT1 through 5-HT7. Each group is further divided into subgroups of receptors that are homologous but are encoded by discrete genes. With the exception of the 5-HT3 receptor--a cation channel--all of the others are G protein-coupled receptors that potentially activate or inhibit a large number of biochemical cascades. This review will endeavor to compare and contrast such signaling pathways with special attention to their tissue-specific occurrence, their possible role in immediate effects on covalent modification of other proteins, and relatively slower effects on gene expression, physiology and behavior.

Effects of donepezil, nicotine and haloperidol on the central serotonergic system in mice: implications for Tourette's syndrome

We have previously reported that acute and chronic donepezil and nicotine administration significantly attenuate DOI-induced head twitch response (HTR) in mice. This behavior, primarily mediated by stimulation of 5-HT2A receptors, has been proposed to model tic symptoms seen in Tourette's syndrome (TS). Haloperidol, a drug widely used to treat symptoms of TS, has also been reported to reduce DOI-induced head shakes in rodents when administered acutely. These findings suggest an inhibitory interaction of these drugs with 5-HT2A receptors. To test this hypothesis, we evaluated the effects of chronic donepezil, nicotine and haloperidol on expression levels of 5-HT2A mRNA and 5-HT2A receptor density in select brain regions. Initially, we established a dose-response relationship for the acute and chronic haloperidol and DOI-induced HTR. Male ICR mice were treated twice daily with donepezil (0.1 mg/kg), nicotine (0.5 mg/kg), and once daily with haloperidol (0.4 mg/kg) for 14 days and were sacrificed 16-18 h after the last injection. These drug regimens were chosen because of their significant effects on DOI-induced HTR. Donepezil significantly increased 5-HT2A mRNA level, but not the receptor density in the striatum. In the midbrain, donepezil significantly decreased the receptor density without affecting the 5-HT2A mRNA level. In the frontal cortex, only haloperidol significantly reduced the 5-HT2A receptor density. The cortex was the only area where donepezil, nicotine and haloperidol significantly reduced the 5-HT2A receptor density. The results suggest that the anti-tic properties of donepezil, nicotine and haloperidol in this paradigm might be due to antagonism of cortical 5-HT2A receptors. Thus, further investigation of involvement of cortical 5-HT2A receptors in TS as well as evaluation of selective 5-HT2A receptor antagonists in this disorder is warranted.

Implication of 5-HT2A subtype receptors in DOI activity in the four-plates test-retest paradigm in mice

The four-plates test (FPT) is an animal model of anxiety which allows the detection of anxiolytic effect not only of benzodiazepines (BZDs) but also of other non-BZDs anxiolytic compounds such as antidepressants (ADs). Furthermore, DOI, a 5-HT(2A/2C) agonist, has been shown to exert an anxiolytic-like effect in this model. Retesting mice in animal models of anxiety (test-retest paradigm) induces an anxiogenic-like and a loss of anxiolytic-like effects in response to BZDs and ADs. On the contrary, DOI has been reported to oppose the fear potentiation induced by trial 1 in the FPT. Despite DOI is considered as one of the most selective 5-HT(2A) available, it acts as agonist at all three 5-HT(2) receptor subtypes (5-HT(2A), 5-HT(2B) and 5-HT(2C)). The aim of this study was thus to investigate in the FPT test-retest paradigm, which 5-HT(2) receptor subtype(s) was involved in the DOI-induced effect in experienced mice. The effect of DOI (0.25-4 mg/kg) and the agonists, 5-HT(2B), BW 723C86 (1-16 mg/kg) and 5-HT(2C), RO 60-0175 (0.25-4 mg/kg) have also been studied. Then, antagonism studies were conducted combinating the 5-HT(2A) receptor antagonist SR 46349B, the 5-HT(2B/2C) receptor antagonist SB 206553 or the selective 5-HT(2C) receptor antagonist RS 10-2221 (at the doses of 0.1 and 1 mg/kg) with the DOI (1 mg/kg). Our study shows that the BW 723C86 had no effect on retesting mice, whereas it exerted an anxiolytic-like effect in naive mice. By contrast to DOI, the RO 60-0175 had no effect neither in naive nor experienced mice. Furthermore, only the SR 46349B antagonized the DOI-induced anti-punishment effect. Diazepam included as a positive control also increased in each case the number of punished passages in naive mice. Our findings altogether also suggest that DOI exerts its anxiolytic-like effect in the FPT test-retest paradigm through 5-HT(2A) receptors.

Expression of the 5-HT2A Serotoninergic Receptor in Human Placenta and Choriocarcinoma Cells: Mitogenic Implications of Serotonin

Serotonin (5-hydroxytryptamine, 5-HT) has diverse physiological functions and acts as a mitogen in a variety of cell types, including bovine placental cells. It exerts its mitogenic effect by interacting with a wide range of 5-HT receptor types. Previous studies have reported the presence of 5-HT(2) binding sites in human placental trophoblastic cells, but this has never been confirmed at the molecular level. In this study, we demonstrated that the 5-HT(2A) receptor subtype is fully expressed in the human choriocarcinoma cell lines JEG-3 and BeWo as well as in normal human placental tissue. DNA sequencing has confirmed that the 5-HT(2A) receptor present in these cell lines and tissues is identical to the human 5-HT(2A) receptor found in platelets and in the cerebral cortex. This receptor was localized by immunofluorescence on the plasma membrane, in JEG-3 and BeWo cells. Furthermore, MTT proliferation assays revealed a positive effect of 5-HT on the proliferation of JEG-3 and BeWo cells. These results suggest that 5-HT constitutes a potent mitogen for neoplastic placental cells.

Expression of serotonin2A receptors in Purkinje cells of the developing rat cerebellum

Previous physiological and pharmacological studies have shown that the serotonin2A (5-HT2A) receptor is involved in cerebellar functions. However, the expression of 5-HT2A receptors in the developing cerebellum has not been elucidated to date. In the present immunohistochemical study, we examined developmental changes of the distribution of 5-HT2A receptors in Purkinje cells of the rat cerebellum from embryonic day 18 (E18) to postnatal day 21 (P21). The weak immunoreaction to 5-HT2A receptors was found in the deep cerebellar nuclei on E19. In the cerebellar cortex of the hemisphere and the posterior vermis, somata of Purkinje cells became weakly immunoreactive on P0. With the dendritic elongation and arborization, the immunoreaction appeared in the proximal parts of Purkinje cell dendrites. Distal parts of the dendrites became immunoreactive after P12, and were strongly immunolabeled by P21. The present study may provide a structural basis to investigate the roles of 5-HT2A receptors during the cerebellar development.

5-HT1A receptor mRNA and immunoreactivity in the rat medial septum/diagonal band of Broca—relationships to GABAergic and cholinergic neurons

Activation of 5-HT1A receptors results in a variety of physiological responses, depending on their localization on neurons with different phenotypes in the brain. This study investigated the localization of 5-HT1A receptor mRNA and 5-HT1A receptor immunoreactivity in cell bodies of the rat septal complex using in situ hybridization and immunohistochemistry. In adjacent sections of the medial septum/diagonal band of Broca (MSDB), the distribution of cell bodies expressing 5-HT1A receptor mRNA was closely related to cells labeled with oligonucleotide probes to GAD (glutamic acid decarboxylase), VAChT (vesicular acetylcholine transporter) or parvalbumin mRNA. Using antiserum to GAD and antibodies to GABA, 5-HT1A receptor immunoreactivity was demonstrated in a majority of GABAergic cells in the MSDB. 5-HT1A receptor-immunoreactive GABAergic cells in the MSDB were also demonstrated to contain the calcium-binding protein parvalbumin, a marker for septohippocampal projecting GABAergic neurons. In the lateral septum, 5-HT1A receptor immunoreactivity was colocalized with the calcium-binding protein calbindin D-28k, a marker for septal GABAergic somatospiny neurons. 5-HT1A receptor immunoreactivity was also detected in a subpopulation of VAChT-containing cholinergic neurons of the MSDB. In MSDB neurons, colocalization of 5-HT1A and 5-HT2A receptor immunoreactivities was demonstrated. These observations suggest that serotonin via 5-HT1A receptors may represent an important modulator of hippocampal transmission important for cognitive and emotional functions through actions on both GABAergic and cholinergic neurons of the rat septal complex. In addition, 5-HT may exert its effects in the MSDB via cells expressing both 5-HT1A and 5-HT2A receptors.

The atypical antipsychotic drug clozapine enhances chronic PCP-induced regulation of prefrontal cortex 5-HT2A receptors

The ability of antipsychotic drugs to affect 5-HT(2A) receptor function has been widely suggested to contribute to their therapeutic properties. We have compared the ability of the antipsychotic drugs clozapine and haloperidol, alone and in combination with chronic phencyclidine (PCP), to modulate 5-HT(2A) receptor binding and mRNA. Acute (i.p. 45 min) and chronic (21-day) clozapine (osmotic minipump (OMP); 20 mg/kg/day) produced widespread decreases in 5-HT(2A) receptor binding (-60%-80%), measured using [(3)H]ketanserin autoradiography. Conversely, 5-HT(2A) mRNA levels, determined using in-situ hybridisation, were modestly increased by chronic clozapine treatment (+10%-30%). Chronic PCP treatment, at a dose (2.58 mg/kg i.p. intermittently for 28 days) that reproduces many of the neurochemical deficits of schizophrenia, decreased 5-HT(2A) receptor binding in the prefrontal cortex (PFC; -16%), consistent with the changes in post-mortem brain tissue from schizophrenic patients. Combined chronic PCP (i.p.) and clozapine (OMP) treatment down-regulated 5-HT(2A) receptor binding in many areas, similar to the effects of clozapine treatment alone and clozapine further enhanced the effects of PCP in the prefrontal cortex. In contrast 5-HT(2A) mRNA was not altered. Haloperidol treatment alone (1 mg/kg/day; OMP) and in combination with PCP (i.p.), generally produced no changes in 5-HT(2A) receptor protein or mRNA. Hence chronic PCP treatment, as employed here, mimics the decreased 5-HT(2A) receptor binding observed in the PFC of schizophrenic patients. Clozapine's enhancement of the natural response of PCP to down-regulate PFC 5-HT(2A) receptors may contribute to it's improved therapeutic profile against negative symptoms and cognitive deficits.

The 5-HT receptor cell is a new member of secondary mesenchyme cell descendants and forms a major blastocoelar network in sea urchin larvae

A gene encoding the serotonin (5-hydroxytryptamine, 5-HT) receptor (5-HT-hpr) was identified from the sea urchin, Hemicentrotus pulcherrimus. Partial amino acid sequence deduced from the cDNA showed strong similarity to Aplysia californica 5-HT2 receptor. Immunoblotting analysis of this 5-HT-hpr protein (5-HT-hpr) with an antibody raised against a deduced peptide showed two bands. Their relative molecular masses were 69 and 53 kDa, respectively. The larger band alone disappeared after N-glycopeptidase F digestion, indicating the protein was N-glycosylated. Immunolocalization analysis showed that cells expressing the 5-HT-hpr (SRC) first appeared near the tip of the archenteron in 33-h post-fertilization (33 hpf) prism larvae. Their cell number doubled in 2 h, and 5-HT-hpr protein expression increased further without cell proliferation. SRC spread ventrally on the basal surface of the oral ectoderm in 36 hpf prism larvae, and then clockwise on the ventral ectoderm to the posterior region to complete formation of the SRC network in 48 hpf early plutei. The SRC network was comprised of 7 main tracts: 4 spicule system-associated tracts and 3 spicule system-independent tracts. The network extended short fibers to the larval body surface through the ectoderm, implicating a signal transmission system that receives exogenous signal. Double-stain immunohistochemistry with antibodies to primary mesenchyme cells showed that SRC were not stained by the antibody. In embryos deprived of secondary mesenchyme cell (SMC) by microsurgery, the number of SRC decreased considerably. These two data indicate that SRC are SMC descendants, adding a new member to the SMC lineage.

Postnatal appearance of 5-HT2A receptors on fast flexor and slow extensor rat motor neurons

Motor neurons to the slowly contracting extensor soleus muscle in behaving rats begin to fire tonically in the 2nd week after birth. In the adult, tonic firing becomes predominant and appears to arise from plateau potentials under monoaminergic control. In the present work, motor neurons to slowly contracting extensor soleus and rapidly contracting extensor digitorum longus, a physiological flexor muscle, were retrogradely labeled with fluorescent dextran and examined for immunoreactivity to 5-HT(2A) receptors in 1 and 2 week old and adult rats. No reactivity was detected at 1 week. At 2 weeks, reactivity was detected on 67% slowly contracting extensor soleus (16 of 24) and 19% extensor digitorum longus (11 of 57) motor neurons. In the adult, the intensity of staining was higher and the percentage of labeled motor neurons 79 for slowly contracting extensor soleus (34 of 43) and 31 for extensor digitorum longus (11 of 35). On slowly contracting extensor soleus motor neurons, labeling appeared more often on soma and dendrites than on dendrites only, whereas on extensor digitorum longus motor neurons, labeling appeared more often on dendrites only. These results are consistent with the hypothesis that serotonergic innervation contributes to the appearance and subsequent increase in tonic firing of rat slowly contracting extensor soleus motor neurons in postnatal development.

Chemical identity of 5-HT2A receptor immunoreactive neurons of the rat septal complex and dorsal hippocampus

Brain 5-HT2A receptors have been implicated in various behavioural and physiological processes including hippocampus-dependent learning and memory. To clarify the cellular localization and chemical identity of 5-HT2A receptor-immunoreactive (-ir) neurons in the rat septal complex and dorsal hippocampus, an immunofluorescence histochemical study was performed using a monoclonal antibody to the 5-HT2A receptor. Pretreatment with colchicine increased the number of 5-HT2A receptor-ir cell bodies, indicating that the 5-HT2A receptor protein undergoes microtubule-dependent anterograde transport in axons and dendrites. 5-HT2A receptor immunoreactivity was detected in septal cholinergic neurons, identified with an antiserum to the vesicular acetylcholine transporter (VAChT), and in GABAergic cell bodies in the medial septum/diagonal band of Broca, identified with antisera to glutamic acid decarboxylase (GAD) and the calcium-binding protein parvalbumin. In the dorsal hippocampus, 5-HT2A receptor immunoreactivity was demonstrated in cells located in the pyramidal cell layer (CA1-3) throughout the Ammon's horn and in the granular cell layer of the dentate gyrus. Furthermore, 5-HT2A receptor immunoreactivity was present in most hippocampal interneurons identified by the presence of GAD65, parvalbumin, calbindin D-28k, somatostatin and neuropeptide Y. In contrast, 5-HT2A receptor immunoreactivity was present in only a few interneurons containing cholecystokinin and calretinin immunoreactivity. The results suggest that serotonin acting on 5-HT2A receptors can modulate hippocampal functions via direct actions on hippocampal glutamatergic principal cells and indirectly via actions on hippocampal interneurons with different phenotypes as well as GABAergic and cholinergic septohippocampal neurons.

Neuronal and astrocytic responses involving the serotonergic system in human spongiform encephalopathies

The relationships between the degree of cortical prion protein (PrP) deposition, tissue vacuolation and astrocytosis were studied in the frontal cortex of 27 cases of human spongiform encephalopathy, encompassing 13 cases of sporadic Creutzfeldt-Jakob disease (sCJD), four cases of familial CJD (fCJD) (one owing to E200K mutation, one owing to 144 bp insertion, one owing to P102L mutation and one owing to A117V mutation), five cases of iatrogenic CJD (iCJD) owing to growth hormone therapy and five cases of variant CJD (vCJD). The size and number of tryptophan hydroxylase (TPH) positive cells in the dorsal raphe were determined as an index of the function of the brain's serotonergic system. The amount of PrP deposited in frontal cortex in vCJD was significantly greater than that in both sCJD and iCJD, which did not differ significantly from each other. The extent of grey matter deposition of PrP correlated with that of white matter deposition. Deposition of PrP as plaques was greater in cases of sCJD bearing valine at codon 129 of PrP gene, especially when homozygous. However, all cases of vCJD displayed florid plaque formation yet these were homozygous for methionine at codon 129. Prion protein deposition as plaques was greater in cases of sCJD with 2A PrP isotype than those with 1 PrP isotype, similar to that seen in cases of vCJD all of which are 2B PrP isotype. There were no significant differences in the extent of astrocytosis between the different aetiological groups, in either grey or white matter, as visualized with glial fibrillary acidic protein (GFAP) or 5HT-2A receptor (5HT-2AR) immunostaining, although there was a strong correlation between the severity of 5HT-2AR and GFAP reactions within both grey and white matter. The extent of PrP deposition within the grey, but not white, matter correlated with the degree of astrocytosis for both GFAP and 5HT-2AR and the extent of tissue vacuolation in grey and white matter, although the latter did not correlate with degree of astrocytosis for either GFAP or 5HT-2AR. Astrocytes may be responding directly to the presence of PrP within the tissue, rather than the vacuolar damage to neurones. Although S100beta immunoreactivity was present in astrocytes in control cases, no S100beta staining was seen in astrocytes in either grey or white matter in most CJD cases. There were no differences in the number of TPH-positive cells between CJD and control cases, although the mean TPH-positive cell size was significantly greater, and cells were more intensely stained, in CJD compared to controls, suggesting a pathological overactivity of the brain's serotonergic system in CJD. This may result in excessive release of 5HT within the brain triggering increased 5HT-2AR expression within activated astrocytes leading to release and depletion of S100beta protein from such cells. The clinical symptoms of fluctuating attention and arousal could be mediated, at least in part, by such alterations in function of the serotonergic system.

Expression of MUPP1 protein in mouse brain

Localizing cell surface receptors to specific subcellular sites can be crucial for proper functioning. PDZ proteins apparently play central roles in such protein localizations. 5-HT(2C) receptors have previously been shown to interact with MUPP1, a multi PDZ domain protein, in heterologous systems and in rat choroid plexus. We now report the generation and characterization of two independent MUPP1 antisera, which recognise distinct areas of the mouse brain in agreement with previous in-situ hybridization studies. Our results indicate that MUPP1 immunoreactivity co-localizes with 5-HT(2A) or 5-HT(2C) receptor expression in all regions of the mouse brain, including the choroid plexus where 5-HT(2C) receptors are highly enriched.

Ultrastructural localization of serotonin 2A and N-methyl-D-aspartate receptors in somata and dendrites of single neurons within rat dorsal motor nucleus of the vagus

Both glutamate and serotonin are potent modulators of autonomic functions involving the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMNV) at the level of the area postrema. Moreover, many of the dendrites in this NTS region express both N-methyl-D-aspartate (NMDA) and serotonin (5HT) 2A receptors, and some of these dendrites may arise from the adjacent DMNV. Thus, single neurons in DMNV may also express both receptors. To test this hypothesis, we used electron microscopic immunocytochemistry for dual localization of the essential R1 subunit of the NMDA receptor (NR1) and the 5HT2A receptor in rat intermediate DMNV, a region serving mainly gastrointestinal functions. Gold particles representing NR1 and peroxidase reaction product for 5HT2A receptors were seen in the cytoplasm, as well as on distinct segments of the plasma membrane of many dendrites. Of the NR1-labeled dendrites, 31% (254/814) also contained 5HT2A immunoreactivity; among the 5HT2A-labeled dendrites, 52% (254/485) expressed NR1. The 5HT2A labeling was also present in numerous small unmyelinated axons, axon terminals, and glial processes. These profiles were largely without NR1 immunoreactivity, although NR1 was detected in some of the dendrites postsynaptic to 5HT2A-labeled terminals. Our results suggest that calcium entry through NMDA channels and 5HT2A receptor activation may dramatically affect postsynaptic excitability of single neurons in the DMNV. In addition, the findings also indicate that the 5HT2A receptor is strategically positioned for involvement in modulation of the presynaptic release of neurotransmitters affecting the postsynaptic activity of DMNV neurons responsive to NMDA activation.

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.

Similar ultrastructural distribution of the 5-HT(2A) serotonin receptor and microtubule-associated protein MAP1A in cortical dendrites of adult rat

As visualized by light and electron microscopic immunocytochemistry, the distribution of the neuronal serotonin-2A (5-HT(2A)) receptor is mainly intracellular throughout adult rat brain. This localization is particularly striking in the pyramidal cells of cerebral cortex, the dendrites of which are intensely immunoreactive, but without any labeling of their spines. In view of recent yeast two-hybrid and biochemical results suggesting an association of 5-HT(2A) receptors with the cytoskeletal microtubule-associated protein MAP1A, the respective subcellular distributions of the receptors and of MAP1A were compared by quantitative electron microscopic immunocytochemistry in dendrites of adult rat frontoparietal cortex. Counts of silver-intensified immunogold particles revealed a higher density of 5-HT(2A) receptors in smaller rather than larger dendrites, and an apportionment between pre-defined compartments representing the plasma membrane and the cytoplasm that was proportional to the relative surface area of these compartments. MAP1A immunoreactivity also predominated in smaller versus larger dendrites, but with a slightly lower proportion of labeling in the plasma membrane versus cytoplasmic compartment. The co-localization of 5-HT(2A) receptors and MAP1A protein in the same dendrites could be demonstrated in double immunolabeling experiments. These results confirmed the predominantly somato-dendritic, intracellular localization of 5-HT(2A) receptors in cerebral cortex, showed their higher concentration in distal as opposed to proximal dendrites, and suggested their potential association to the cytoskeleton in cortical neurons in vivo. Such a distribution of 5-HT(2A) receptors reinforces our earlier hypothesis that 5-HT(2A) receptors participate in intraneuronal signaling processes involving the cytoskeleton, and raises the possibility that their activation could be dependent upon that of another co-localized, plasma membrane-bound, 5-HT receptor.

5-HT2A receptor subtype in the peripheral branch of sensory fibers is involved in the potentiation of inflammatory pain in rats

One of the major serotonin (5-HT) receptor subtypes expressed in the rat dorsal root ganglion (DRG) neurons is the 5-HT2A receptor. We have previously shown that 5-HT2A receptors in the peripheral sensory terminals are responsible for 5-HT-induced pain and hyperalgesia. In the present study, we characterized neurons expressing 5-HT2A receptors in the rat DRG neurons by means of in situ hybridization, immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and behavioral tests. In situ hybridization on consecutive sections revealed that 5-HT2A receptor mRNA is colocalized with calcitonin-gene related peptide (CGRP) mRNA (100/104; 96.2%) but not with c-Ret mRNA (1/115; 0.9%). Signals for 5-HT2A receptor mRNA were found in 9.4 +/- 2.2% of normal DRG (L5) neurons, most of which were small to medium in size. Four days of complete Freund's adjuvant-induced inflammation of the hindpaw doubled the incidence of 5-HT2A receptor mRNA-expressing neurons to 19.3 +/- 2.8%. The level of 5-HT2A receptor mRNA in DRGs of normal and various pathological conditions was then determined by RT-PCR. The level was up-regulated by peripheral inflammation, but not by axotomy or chronic constriction of the peripheral nerve. Systemic administration of 5-HT2A receptor antagonist (Sarpogrelate HCI) produced analgesic effects on thermal hyperalgesia caused by peripheral inflammation, but failed to attenuate thermal hyperalgesia in chronic constriction injury model. These findings suggest that 5-HT2A receptors are mainly expressed in CGRP-synthesizing small DRG neurons and may be involved in the potentiation of inflammatory pain in the periphery.

Localization of 5-HT(2A) receptors on dopamine cells in subnuclei of the midbrain A10 cell group

Considerable evidence suggests that a dysfunction of the dopamine and serotonin (5-hydroxytryptamine or 5-HT) neurotransmitter systems contributes to a diverse range of pathological conditions including schizophrenia, depression and drug abuse. Recent electrophysiological and behavioral studies suggest that 5-HT modulates dopaminergic neurons in the ventral tegmental area via activation of 5-HT(2A) receptors. It is currently unknown if 5-HT(2A) receptors mediate their actions on dopaminergic neurons in the ventral tegmental area via direct or indirect mechanisms. This study investigated whether 5-HT(2A) receptors were localized on dopamine cells within the A10 dopamine subnuclei of the rat, including the ventral tegmental area. We discovered that 5-HT(2A) receptor-like immunoreactivity colocalized with tyrosine hydroxylase, a marker for dopamine neurons, throughout the A10 dopamine cell population. Colocalization was most prominent in rostral and mid A10 regions, including the paranigral, parabrachial, and interfascicular subnuclei. Though more rare, non-dopaminergic neurons also expressed 5-HT(2A) receptor immunoreactivity in the ventral tegmental area. Additionally, although a dense population of 5-HT(2A) immunoreactive cells was observed in the rostral dorsal raphe nucleus, rarely were these cells immunoreactive for tyrosine hydroxylase. The linear raphe A10 dopamine subdivisions also displayed a low degree of 5-HT(2A) receptor and tyrosine hydroxylase colocalization. These findings provide an anatomical basis for the physiological modulation of dopamine neurons in the rostral ventral tegmental area either directly, by 5-HT(2A) receptors localized on dopamine cells, or indirectly, through a non-dopaminergic mechanism. Interestingly, 5-HT(2A) receptors were expressed on dopamine neurons in several A10 subnuclei that project to mesolimbic forebrain regions implicated in drug addiction, and recent evidence indicates that ventral tegmental area 5-HT(2A) receptor activation may modulate reward-related behavior in rodents. 5-HT(2A) receptors were also expressed on dopamine cells in A10 subnuclei that project to forebrain areas that have been implicated in schizophrenia, and atypical antipsychotic drugs have high affinities for 5-HT(2A) receptors. Thus, findings in this study could have important implications for understanding 5-HT and dopamine circuitry dysfunction in schizophrenia.

Co-localization of 5-HT 2A -receptor- and GABA-immunoreactivity in neurones in the periaqueductal grey matter of the rat

The dorsal half of the midbrain periaqueductal grey matter (dPAG) functions as a midbrain aversive system. Serotonin exerts anti-aversive effects in the dPAG that are mediated by actions at 5-hydroxytryptamine (5-HT)(1A)- and 5-HT(2A)-receptors. Since at the cellular level, 5-HT(2A)-receptor activation has been shown to evoke excitatory responses in the dPAG, it is possible that anti-aversive 5-HT(2A)-mediated effects are mediated indirectly by activation of inhibitory gamma-aminobutyric acid (GABA)ergic interneurones. In rats, immunoreactivity for 5-HT(2A)-receptors was present on the soma and dendrites of neurones throughout the PAG. Co-localization studies revealed that the majority (>90%) of 5-HT(2A)-receptor-labelled cells also showed immunoreactivity for GABA. These findings may provide an anatomical substrate for 5-HT(2A)-mediated anti-aversive effects in the dPAG.

Differential distribution of 5HT2A and NMDA receptors in single cells within the rat medial nucleus of the solitary tract

Activation of serotonin (5-hydroxytryptamine; 5HT) receptors of the 2A subtype (5HT2A) in the intermediate portion of the medial nucleus tractus solitarius (mNTS) produces marked hypotension and bradycardia. This portion of the mNTS receives major input from glutamatergic baroreceptor afferents. Thus, the cardiorespiratory effects of 5HT2A agonists may be attributed, in part, to interactions involving the glutamatergic target neurons, some of which express N-methyl-D-aspartate (NMDA) glutamate receptors. To determine the functional sites for activation of 5HT2A receptors and their relationship to NMDA receptors in this region, we used electron microscopic immunocytochemistry for the localization of antipeptide antisera selectively recognizing each receptor protein in the intermediate mNTS in rat brain. Of 1,052 5HT2A-labeled profiles, 38% were dendrites and dendritic spines, 27% were unmyelinated axons, 14% were axon terminals, and 11% were glial processes. These 5HT2A-labeled profiles frequently contained NR1 gold particles with dendrites comprising 68% of the total dual-labeled profiles. In dendrites, the 5HT2A immunoreactivity was localized to cytoplasmic organelles or discretely distributed on synaptic or extrasynaptic segments of the plasma membrane. In contrast, NR1 immunoreactivity was prominently localized to postsynaptic junctions and these were distinct from the 5HT2A receptor labeling when coexpressed in the same dendrites. Dendrites containing both receptors composed 56% (224/399) of the total 5HT2A-labeled dendrites and 34% (224/659) of the total NR1-labeled dendrites. Our results provide the first ultrastructural evidence that in the intermediate mNTS, 5HT2A receptor agonists may affect the postsynaptic excitability of many of the same neurons that show NMDA-evoked responses to glutamate.

Expression of serotonin 5-HT(2A) receptors in the human cerebellum and alterations in schizophrenia

The occurrence of human cerebellar serotonin 5-HT(2A) receptors (5-HT(2A)R) is equivocal and their status in schizophrenia unknown. Using a range of techniques, we investigated cerebellar 5-HT(2A)R expression in 16 healthy subjects and 16 subjects with schizophrenia. Immunocytochemistry with a monoclonal antibody showed labelling of Purkinje cell bodies and dendrites, as well as putative astrocytes. Western blots showed a major band at approximately 45 kDa. Receptor autoradiography and homogenate binding with [(3)H]ketanserin revealed cerebellar 5-HT(2A)R binding sites present at levels approximately a third of that in prefrontal cortex. 5-HT(2A)R mRNA was detected by reverse transcriptase-polymerase chain reaction, with higher relative levels in men than women. Several aspects of 5-HT(2A)R expression were altered in schizophrenia. 5-HT(2A)R immunoreactivity in Purkinje cells was partially redistributed from soma to dendrites and was increased in white matter. 5-HT(2A)R mRNA was decreased in the male patients. 5-HT(2A)R measured by dot blots and [(3)H]ketanserin binding (B(max) and K(d)) were not significantly altered in schizophrenia. These data show that 5-HT(2A)R gene products (mRNA, protein, binding sites) are expressed in the human cerebellum at nonnegligible levels; this bears upon 5-HT(2A)R imaging studies which use the cerebellum as a reference region. 5-HT(2A)R expression is altered in schizophrenia; the shift of 5-HT(2A)R from soma to dendrites is noteworthy since atypical antipsychotics have the opposite effect. Finally, the results emphasise that expression of a receptor gene is a mutifaceted process. Measurement of multiple parameters is necessary to give a clear picture of the normal situation and to show the profile of alterations in a disease.

Relationships of 5-hydroxytryptamine immunoreactive terminal-like varicosities to 5-hydroxytryptamine-2A receptor-immunoreactive neuronal processes in the rat forebrain

The distributions of 5-hydroxytryptamine (5-HT)-immunoreactive (IR) varicosities and 5-hydroxytryptamine-2A receptor (5-HT2A)-IR neuronal structures in the rat brain have previously been described individually. Using double labeling immunocytochemistry, the relationships between 5-HT2A-IR and 5-HT-IR elements in the forebrain of male rats has been studied at the light microscopic level. In neocortical regions (frontal, parietal and retrosplenial cortex), the strongest 5-HT2A-IR was found in the apical dendrites of pyramidal cells in layers III-V, while 5-HT-IR terminal-like varicosities were present in all layers but most prominently in the outer layers. In other forebrain regions, the olfactory bulb, the hippocampal formation, and the islands of Calleja and Calleja magna, localized discrepancies were present between the 5-HT2A-IR neuronal profiles and the 5-HT-IR terminal-like varicosities. Hardly any additional juxtapositions between the 5-HT2A-IR neuronal profiles and 5-HT-IR terminal-like varicosities were revealed when the intraneuronal level of 5-HT was increased by monoamine oxidase inhibitor pretreatment (nialamide, 250 mg/kg, 3 h). Thus, in most forebrain regions, there were overall few juxtapositions between 5-HT terminal-like varicosities and 5-HT2A-IR neuronal structures. This observation suggests that 5-HT2A receptor mediated 5-HT transmission in the rat forebrain is mainly a volume transmission process mediated via short distance diffusion in the extra-cellular space.

Distribution of serotonin 5-HT(2A) receptors in afferents of the rat striatum

Treatment with conventional antipsychotic drugs (APDs) is accompanied by extrapyramidal side effects (EPS), which are thought to be due to striatal dopamine D(2) receptor blockade. In contrast, treatment with atypical APDs is marked by a low incidence or absence of EPS. The reduced motor side effect liability of atypical APDs has been attributed to a high serotonin 5-HT(2A) receptor affinity coupled with a relatively low D(2) affinity. Despite the high density of 5-HT(2A) binding sites in the striatum, there are few detectable 5-HT(2A) mRNA-expressing neurons in the striatum. This suggests that most striatal 5-HT(2A) receptors are heteroceptors located on afferent axons. A combined retrograde tracer-immunohistochemistry method was used to determine the sites of origin of striatal 5-HT(2A)-like immunoreactive axons. 5-HT(2A)-like immunoreactive neurons in both the cortex and globus pallidus were retrogradely labeled from the striatum; very few nigrostriatal or thalamostriatal neurons expressed 5-HT(2A)-like immunoreactivity. Within the striatum, parvalbumin-containing interneurons displayed 5-HT(2A) immunolabeling; these neurons are the targets of cortical and pallidal projections. Our data indicate that cortico- and pallido-striatal neurons are the major source of 5-HT(2A) receptor binding in the striatum, and suggest that cortico- and pallido-striatal neurons are strategically positioned to reduce the motor side effects that accompany striatal D(2) receptor blockade or are seen in parkinsonism.

Serotonin 2A receptor-like immunoreactivity is detected in astrocytes but not in oligodendrocytes of rat spinal cord

The distribution of the serotonin 2A (5-HT2A) receptor in glial cells in the white matter of rat spinal cord was immunohistochemically examined with specific antibodies against the 5-HT2A receptor. 5-HT2A receptor-like immunoreactivity was detected in astrocytes that were identified by an antibody against the glial fibrillary acidic protein. In contrast, 5-HT2A receptor-like immunoreactivity was not observed in oligodendrocytes.

Expression of 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors in the mouse embryo

Expression patterns of 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors during mouse embryogenesis were investigated using highly specific monoclonal antibodies. Differential and overlapping spatio-temporal patterns of 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptor immunoreactivity were observed during active phases of morphogenesis of a variety of embryonic tissues, including neuroepithelia of brain and spinal cord, notochord, somites, cranial neural crest, craniofacial mesenchyme and epithelia, heart myocardium and endocardial cushions, tooth germs, whisker follicles, cartilage and striated muscle. The functional significance of these receptors was tested by exposing headfold stage mouse embryos to different subtype-selective 5-HT(2) receptor antagonists for 2 days in whole embryo culture. The most potent was the pan 5-HT(2) receptor antagonist ritanserin, which has high affinity for the 5-HT(2B) receptor. Ritanserin caused 100% malformed embryos at a dose of 1 microM. The 5-HT(2A/2C) receptor antagonist mianserin also caused a significant number of malformed embryos, but only when used at a 10 fold higher dose (10 microM). Ketanserin, which primarily targets 5-HT(2A) receptors, did not cause a significant number of malformed embryos at any dose tested. Together with previous evidence that 5-HT acts as an important morphoregulatory signal during mouse embryogenesis, present evidence for the early and continued expression of functional 5-HT(2) receptors throughout gestation raises the possibility that psychotropic drugs taken during pregnancy could interfere with developmental actions of 5-HT during prenatal development of neural and non-neural tissues.

Ultrastructural localization of the serotonin 2A receptor in dopaminergic neurons in the ventral tegmental area

Serotonin (5-hydroxytryptamine, 5-HT) 2A receptor antagonists are clinically effective antipsychotics that may differentially target mesocortical and mesolimbic dopaminergic neurons having partially segregated distribution in the parabrachial (PB) and paranigral (PN) ventral tegmental area (VTA). We examined the ultrastructural immunocytochemical localization of the 5-HT2A receptor in these subdivisions of rat VTA, to determine (1) the functional sites for receptor activation, and (2) cellular associations between the receptor and dopaminergic neurons identified by their content of tyrosine hydroxylase (TH). The mean area density of neuronal profiles containing 5-HT2A receptor labeling was not significantly different in the PB and PN VTA. In each region approximately 44% of the 5-HT2A labeled profiles were dendrites while the remainder were mainly axons. Dendritic 5-HT2A-immunoreactivity was often localized to membranous cytoplasmic organelles resembling smooth endoplasmic reticulum, and to more rarely to segments of the plasma membrane beneath contacts from unlabeled axon terminals. 5-HT2A labeling was also seen within the cytoplasm of a few axon initial segments and many small unmyelinated axons. Approximately 40% of the 5-HT2A-labeled dendritic profiles contained TH in either PB or PN VTA. Our results suggest that 5-HT2A receptors in VTA are largely cytoplasmic and play an equally important role in modulating dopaminergic neurons in PB and PN VTA. These results also implicate 5-HT2A receptors in the postsynaptic activation of non-dopaminergic neurons and possibly the presynaptic release from terminals of axons originating in, or passing through, these regions.

N-methyl-D-aspartate (NMDA) receptors in the ventral tegmental area: subcellular distribution and colocalization with 5-hydroxytryptamine(2A) receptors

Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype have been implicated in behavioral sensitization to psychostimulants and in psychotic behaviors involving excitation of ventral tegmental area (VTA) dopaminergic neurons. Antagonists of serotonin (5-hydroxytryptamine, 5-HT) receptors of the 5-HT(2A) subtype are potent antipsychotics that attenuate these NMDA-evoked responses. We examined the electron microscopic immunocytochemical localization of antisera against the NMDA R1 subunit (NMDAR1) and 5-HT(2A) receptors to determine potential sites for their dual activation in the rat paranigral and parabrachial VTA subdivisions that are distinguished, in part, by their respective striatolimbic and cortical projections. In both regions, NMDAR1 immunoreactivity was localized mainly to the cytoplasm of somata and dendrites, and was only occasionally seen near or within excitatory-type asymmetric synapses. Many of the NMDAR1-labeled somata and dendrites also expressed 5-HT(2A) receptors, having a similar, but largely non-overlapping, neuronal distribution. The mean area density of NMDAR1 and dually labeled dendritic profiles was significantly greater in the paranigral than in the parabrachial VTA. NMDAR1 was also present in small axons showing a similar regional difference in area density. No regional difference in area density was seen in dendrites or small axons containing only 5-HT(2A) receptors. Our results indicate that NMDA and 5-HT(2A) receptors in the VTA are transiently expressed on synaptic plasma membranes of single neurons showing widespread cytoplasmic distributions of each of the receptors. They also suggest a major role for NMDA receptors in modulating the output of paranigral neurons and the release of transmitters from axons passing through this region.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

Serotonergic systems, their phylogeny and ontogeny have been thoroughly described up to the ultrastructural level, thanks to the multiplicity of methodological approaches. They have often been referred to as a 'Rosetta stone', as several features first described for serotonin neurons or paraneurons have been then extended to other neurotransmitter systems: coexistence with neuropeptides or even a canonical neurotransmitter (GABA), volume transmission, regrowth after lesioning, and characterization of multiple receptor subtypes. This review deals with the contributions of neuroanatomical approaches for studying serotoninergic systems, and focuses on recent advances concerning the topological relationships between serotonergic innervation, receptors and target cells. This aspect is particularly important with regard to the possibility for serotonin to act through classical synaptic transmission and/or non-junctional transmission. Serotonin then can selectively regulate different neuronal systems through the activation of distinct receptor subtypes, which in turn can be linked to different transduction pathways. Neurocytochemical approaches constitute unique tools to analyse both anatomical and functional characteristics of complex neuronal systems.

Human midbrain dopamine neurons express serotonin 2A receptor: an immunohistochemical demonstration

We demonstrated intense serotonin (5-HT) 2A receptor immunoreactivity in the human ventral tegmental area (VTA) using by a recently raised antibody against 5-HT2A receptor. The substantia nigra (SN) neurons also showed 5-HT2A receptor immunoreactivity. Double immunohistochemistry of 5-HT2A receptor and tyrosine hydroxylase (TH) revealed many neurons doubly labeled by 5-HT2A receptor and TH in the VTA and SN. It is suggested that activity of human midbrain dopaminergic neurons might be strongly regulated via 5-HT2A receptors at the level of their originating nuclei.