Agonist-induced phosphoinositide hydrolysis in choroid plexus

Molecular and structural insights into the 5-HT2C receptor as a therapeutic target for substance use disorders

Substance use disorder (SUD) is a chronic condition, with maintained abuse of a substance leading to physiological and psychological alterations and often changes in cognitive and social behaviours. Current therapies include psychotherapy coupled with medication; however, high relapse rates reveal the shortcomings of these therapies. The signalling, expression profile, and neurological function of the serotonin 2C receptor (5-HT2C receptor) make it a candidate of interest for the treatment of SUD. Recently, psychedelics, which broadly act at 5-HT2 receptors, have indicated potential for the treatment of SUD, implicating the 5-HT2C receptor. The modern psychedelic movement has rekindled interest in the 5-HT2C receptor, resulting in many new studies, especially structural analyses. This review explores the structural, molecular and cellular mechanisms governing 5-HT2C receptor function in the context of SUD. This provides the basis of the preclinical and clinical evidence for their role in SUD and highlights the potential for future exploration.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Tracking Calcium Dynamics and Immune Surveillance at the Choroid Plexus Blood-Cerebrospinal Fluid Interface

The choroid plexus (ChP) epithelium is a source of secreted signaling factors in cerebrospinal fluid (CSF) and a key barrier between blood and brain. Here, we develop imaging tools to interrogate these functions in adult lateral ventricle ChP in whole-mount explants and in awake mice. By imaging epithelial cells in intact ChP explants, we observed calcium activity and secretory events that increased in frequency following delivery of serotonergic agonists. Using chronic two-photon imaging in awake mice, we observed spontaneous subcellular calcium events as well as strong agonist-evoked calcium activation and cytoplasmic secretion into CSF. Three-dimensional imaging of motility and mobility of multiple types of ChP immune cells at baseline and following immune challenge or focal injury revealed a range of surveillance and defensive behaviors. Together, these tools should help illuminate the diverse functions of this understudied body-brain interface.

A short history of the 5-HT2C receptor: from the choroid plexus to depression, obesity and addiction treatment

This paper is a personal account on the discovery and characterization of the 5-HT_2C receptor (first known as the 5-HT_1C receptor) over 30 years ago and how it translated into a number of unsuspected features for a G protein-coupled receptor (GPCR) and a diversity of clinical applications. The 5-HT_2C receptor is one of the most intriguing members of the GPCR superfamily. Initially referred to as 5-HT_1CR, the 5-HT_2CR was discovered while studying the pharmacological features and the distribution of [³H]mesulergine-labelled sites, primarily in the brain using radioligand binding and slice autoradiography. Mesulergine (SDZ CU-085), was, at the time, best defined as a ligand with serotonergic and dopaminergic properties. Autoradiographic studies showed remarkably strong [³H]mesulergine-labelling to the rat choroid plexus. [³H]mesulergine-labelled sites had pharmacological properties different from, at the time, known or purported 5-HT receptors. In spite of similarities with 5-HT₂ binding, the new binding site was called 5-HT_1C because of its very high affinity for 5-HT itself. Within the following 10 years, the 5-HT_1CR (later named 5-HT_2C) was extensively characterised pharmacologically, anatomically and functionally: it was one of the first 5-HT receptors to be sequenced and cloned. The 5-HT_2CR is a GPCR, with a very complex gene structure. It constitutes a rarity in the GPCR family: many 5-HT_2CR variants exist, especially in humans, due to RNA editing, in addition to a few 5-HT_2CR splice variants. Intense research led to therapeutically active 5-HT_2C receptor ligands, both antagonists (or inverse agonists) and agonists: keeping in mind that a number of antidepressants and antipsychotics are 5-HT_2CR antagonists/inverse agonists. Agomelatine, a 5-HT_2CR antagonist is registered for the treatment of major depression. The agonist Lorcaserin is registered for the treatment of aspects of obesity and has further potential in addiction, especially nicotine/ smoking. There is good evidence that the 5-HT_2CR is involved in spinal cord injury-induced spasms of the lower limbs, which can be treated with 5-HT_2CR antagonists/inverse agonists such as cyproheptadine or SB206553. The 5-HT_2CR may play a role in schizophrenia and epilepsy. Vabicaserin, a 5-HT_2CR agonist has been in development for the treatment of schizophrenia and obesity, but was stopped. As is common, there is potential for further indications for 5-HT_2CR ligands, as suggested by a number of preclinical and/or genome-wide association studies (GWAS) on depression, suicide, sexual dysfunction, addictions and obesity. The 5-HT_2CR is clearly affected by a number of established antidepressants/antipsychotics and may be one of the culprits in antipsychotic-induced weight gain.

Serotonin-2C and -2a receptor co-expression on cells in the rat medial prefrontal cortex

Neural function within the medial prefrontal cortex (mPFC) regulates normal cognition, attention and impulse control, implicating neuroregulatory abnormalities within this region in mental dysfunction related to schizophrenia, depression and drug abuse. Both serotonin-2A (5-HT2A) and -2C (5-HT2C) receptors are known to be important in neuropsychiatric drug action and are distributed throughout the mPFC. However, their interactive role in serotonergic cortical regulation is poorly understood. While the main signal transduction mechanism for both receptors is stimulation of phosphoinositide production, they can have opposite effects downstream. 5-HT2A versus 5-HT2C receptor activation oppositely regulates behavior and can oppositely affect neurochemical release within the mPFC. These distinct receptor effects could be caused by their differential cellular distribution within the cortex and/or other areas. It is known that both receptors are located on GABAergic and pyramidal cells within the mPFC, but it is not clear whether they are expressed on the same or different cells. The present work employed immunofluorescence with confocal microscopy to examine this in layers V-VI of the prelimbic mPFC. The majority of GABA cells in the deep prelimbic mPFC expressed 5-HT2C receptor immunoreactivity. Furthermore, most cells expressing 5-HT2C receptor immunoreactivity notably co-expressed 5-HT2A receptors. However, 27% of 5-HT2C receptor immunoreactive cells were not GABAergic, indicating that a population of prelimbic pyramidal projection cells could express the 5-HT2C receptor. Indeed, some cells with 5-HT2C and 5-HT2A receptor co-labeling had a pyramidal shape and were expressed in the typical layered fashion of pyramidal cells. This indirectly demonstrates that 5-HT2C and 5-HT2A receptors may be commonly co-expressed on GABAergic cells within the deep layers of the prelimbic mPFC and perhaps co-localized on a small population of local pyramidal projection cells. Thus a complex interplay of cortical 5-HT2A and 5-HT2C receptor mechanisms exists, which if altered, could modulate efferent brain systems implicated in mental illness.

Impact of RNA editing on functions of the serotonin 2C receptor in vivo

Transcripts encoding 5-HT(2C) receptors are modified posttranscriptionally by RNA editing, generating up to 24 protein isoforms. In recombinant cells, the fully edited isoform, 5-HT(2C-VGV), exhibits blunted G-protein coupling and reduced constitutive activity. The present studies examine the signal transduction properties of 5-HT(2C-VGV) receptors in brain to determine the in vivo consequences of altered editing. Using mice solely expressing the 5-HT(2C-VGV) receptor (VGV/Y), we demonstrate reduced G-protein coupling efficiency and high-affinity agonist binding of brain 5-HT(2C-VGV) receptors. However, enhanced behavioral sensitivity to a 5-HT(2C) receptor agonist was also seen in mice expressing 5-HT(2C-VGV) receptors, an unexpected finding given the blunted G-protein coupling. In addition, mice expressing 5-HT(2C-VGV) receptors had greater sensitivity to a 5-HT(2C) inverse agonist/antagonist enhancement of dopamine turnover relative to wild-type mice. These behavioral and biochemical results are most likely explained by increases in 5-HT(2C) receptor binding sites in the brains of mice solely expressing 5-HT(2C-VGV) receptors. We conclude that 5-HT(2C-VGV) receptor signaling in brain is blunted, but this deficiency is masked by a marked increase in 5-HT(2C) receptor binding site density in mice solely expressing the VGV isoform. These findings suggest that RNA editing may regulate the density of 5-HT(2C) receptor binding sites in brain. We further caution that the pattern of 5-HT(2C) receptor RNA isoforms may not reflect the pattern of protein isoforms, and hence the inferred overall function of the receptor.

Current status of inverse agonism at serotonin2A (5-HT2A) and 5-HT2C receptors

Contemporary receptor theory was developed to account for the existence of constitutive activity, as defined by the presence of receptor signaling in the absence of any ligand. Thus, ligands acting at a constitutively active receptor, can act as agonists, antagonists, and inverse agonists. In vitro studies have also revealed the complexity of ligand/receptor interactions including agonist-directed stimulus trafficking, a finding that has led to multi-active state models of receptor function. Studies with a variety of cell types have established that the serotonin 5-HT(2A) and 5-HT(2C) receptors also demonstrate constitutive activity and inverse agonism. However, until recently, there has been no evidence to suggest that these receptors also demonstrate constitutive activity and hence reveal inverse agonist properties of ligands in vivo. This paper describes our current knowledge of constitutive activity in vitro and then examines the evidence for constitutive activity in vivo. Both the serotonin 5-HT(2A) and 5-HT(2C) receptors are involved in a number of physiological and behavioral functions and are the targets for treatment of schizophrenia, anxiety, weight control, Parkinsonism, and other disorders. The existence of constitutive activity at these receptors in vivo, along with the possibility of inverse agonism, provides new avenues for drug development.

Pharmacologic mechanisms of serotonergic regulation of dopamine neurotransmission

The neurotransmitter dopamine (DA) has a long association with normal functions such as motor control, cognition, and reward, as well as a number of syndromes including drug abuse, schizophrenia, and Parkinson's disease. Studies show that serotonin (5-HT) acts through several 5-HT receptors in the brain to modulate DA neurons in all 3 major dopaminergic pathways. There are at least fourteen 5-HT receptor subtypes, many of which have been shown to play some role in mediating 5-HT/DA interactions. Several subtypes, including the 5-HT1A, 5-HT1B, 5-HT2A, 5-HT3 and 5-HT4 receptors, act to facilitate DA release, while the 5-HT2C receptor mediates an inhibitory effect of 5-HT on DA release. Most 5-HT receptor subtypes only modulate DA release when 5-HT and/or DA neurons are stimulated, but the 5-HT2C receptor, characterized by high levels of constitutive activity, inhibits tonic as well as evoked DA release. This review summarizes the anatomical evidence for the presence of each 5-HT receptor subtype in dopaminergic regions of the brain and the neuropharmacological evidence demonstrating regulation of each DA pathway. The relevance of 5-HT receptor modulation of DA systems to the development of therapeutics used to treat schizophrenia, depression, and drug abuse is discussed. Lastly, areas are highlighted in which future research would be maximally beneficial to the treatment of these disorders.

Agonist-induced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1

Multiple PDZ domain protein 1 (MUPP1), a putative scaffolding protein containing 13 PSD-95, Dlg, ZO-1 (PDZ) domains, was identified by a yeast two-hybrid screen as a serotonin2C receptor (5-HT2C R)-interacting protein (Ullmer, C., Schmuck, K., Figge, A., and Lubbert, H. (1998) FEBS Lett. 424, 63-68). MUPP1 PDZ domain 10 (PDZ 10) associates with Ser458-Ser-Val at the carboxyl-terminal tail of the 5-HT2C R. Both Ser458 and Ser459 are phosphorylated upon serotonin stimulation of the receptor (Backstrom, J. R., Price, R. D., Reasoner, D. T., and Sanders-Bush, E. (2000) J. Biol. Chem. 275, 23620-23626). To investigate whether phosphorylation of these serines in the receptor regulates MUPP1 interaction, we used several approaches. First, we substituted the serines in the receptor carboxyl tail with aspartates to mimic phosphorylation (S458D, S459D, or S458D/S459D). Pull-down assays demonstrated that Asp mutations at Ser458 significantly decreased receptor tail interaction with PDZ 10. Next, serotonin treatment of 5-HT2C R/3T3 cells resulted in a dose-dependent reduction of receptor interaction with PDZ 10. Effects of serotonin on receptor-PDZ 10 binding could be blocked by pretreatment with a receptor antagonist. Alkaline phosphatase treatment reverses the effect of serotonin, indicating that agonist-induced phosphorylation at Ser458 resulted in a loss of MUPP1 association and also revealed a significant amount of basal phosphorylation of the receptor. We conclude that 5-HT2C R interaction with MUPP1 is dynamically regulated by phosphorylation at Ser458.

Imaging brain phospholipase A2 activation in awake rats in response to the 5-HT2A/2C agonist (+/-)2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI)

Incorporation coefficients k(*) of intravenously injected [(3)H]arachidonic acid from blood into brain reflect the release from phospholipids of arachidonic acid by receptor-initiated activation of phospholipase A(2) (PLA(2)). In unanesthetized adult rats, 2.5 mg/kg intraperitoneally (i.p.) (+/-)2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), which is a 5-HT(2A/2C) receptor agonist, has been reported to produce the behavioral changes of what is known as the 5-HT(2) syndrome, but only a few small regional decrements in brain glucose metabolism. In this study, 2.5 mg/kg i.p. DOI, when administered to unanesthetized rats, produced widespread and significant increases, of the order of 60%, in k(*) for arachidonate, particularly in neocortical brain regions reported to have high densities of 5-HT(2A) receptors. The increases could be entirely blocked by chronic pretreatment with mianserin, a 5-HT(2) receptor antagonist. The results suggest that the 5-HT(2) syndrome involves widespread brain activation of PLA(2) via 5-HT(2A) receptors, leading to the release of the second messenger, arachidonic acid. Chronic mianserin, a 5-HT(2) antagonist, prevents this activation.

In vitro autoradiography of serotonin 5-HT(2A/2C) receptor-activated G protein: guanosine-5'-(gamma-[(35)S]thio)triphosphate binding in rat brain

Agonist activation of G protein-coupled receptors induces an increase in the binding of guanosine 5'-(gamma-[(35)S]thio)triphosphate ([(35)S]GTPgammaS); this increase in binding has been used as a tool to investigate receptor interaction with the heterotrimer guanine nucleotide-binding regulatory protein (G protein). The present study uses agonist-stimulated [(35)S]GTPgammaS binding to characterize serotonin 5-HT(2A/2C) receptors in rat brain membrane fractions and demonstrate the anatomical localization of the receptors by in vitro autoradiography on slide-mounted sections. The stimulatory effect of the agonist [1-(2,5-dimethoxy-4-iodophenyl)]-2 aminopropane (DOI) is compared to that of serotonin (5-HT). Autoradiography revealed a similar localization of DOI- and 5-HT-stimulated binding of [(35)S]GTPgammaS in distinct areas of prefrontal and parietal cortex, consistent with previously reported 5-HT(2A) receptor distribution. Specific binding was demonstrated in the frontal and parietal cortex, medial prefrontal, and cingular and orbital-insular areas as well as in the hippocampal formation, septal areas, the nucleus accumbens, and the choroid plexus. MDL 100105, a specific 5-HT(2A) antagonist, and ketanserin, an antagonist of 5-HT(2A/2C) receptors, blocked DOI stimulation in all labeled areas, whereas 5-HT stimulation was only partially blocked (70-80%). A small but significant inhibition was observed with the specific antagonist of 5-HT(2C/2B), SB 206553. This autoradiographic technique provides a useful tool for measuring in situ changes in specific receptor-Gq protein coupling in anatomically discrete brain regions, under physiological and pathological conditions.

Immunohistochemical localisation of the 5-HT2C receptor protein in the rat CNS

5-HT2C receptor mRNA has a widespread distribution in the human and rat CNS but the absence of a specific high affinity ligand has made autoradiographic localisation of the receptor difficult. By raising polyclonal antibodies against the rat 5-HT2C receptor protein this study reports the immunohistochemical distribution of this receptor in the rat CNS. A sephadex purified 5-HT2C antiserum visualised a single immunopositive band (54 kDa) in Western blots of membranes prepared from several rat brain regions and caused intense membrane immunofluorescence in HEK 293 cells transfected with h5-HT2C cDNA, which were both attenuated by incubation with the antigenic peptide sequence (200-300 microM). 5-HT2C-like immunoreactivity was located on neurones throughout the CNS. The most abundant 5-HT2C-like immunoreactive cell bodies were in the anterior olfactory nucleus, medial and intercalated amygdaloid nuclei, hippocampus layers CA1 to CA3, laterodorsal and lateral geniculate thalamic nuclei, caudate-putamen and several areas of the cortex (including piriform and frontal), consistent with this receptor being located postsynaptic to serotonergic neurones. Immunopositive neurones were also found in the dorsal raphé, suggesting that 5-HT2C receptors may be on some serotonergic neurones. The overall distribution of 5-HT2C-like immunoreactivity complements previous findings with conventional radioligands and agrees well with reported levels of 5-HT2C receptor mRNA.

Distribution of the 5-hydroxytryptamine2C receptor protein in adult rat brain and spinal cord determined using a receptor-directed antibody: effect of 5,7-dihydroxytryptamine

A synthetic peptide, corresponding to the N-terminal decapeptide (+Y11C12) of the rat 5-hydroxytryptamine2C (5-HT2C) receptor protein was used to produce a sheep polyclonal antiserum. Western blot analysis showed that the resultant antibody G241 recognised two membrane proteins, one (55 kDa) approximating the molecular mass of the 5-HT2C receptor (52 kDa) and a second (63 kDa), which may be a glycosylated form of the receptor protein. HEK 293 cells transfected with human 5-HT2C cDNA displayed intense cell surface immunoreactivity with the 5-HT2C antiserum, which was completely prevented by incubating the antibody with the synthetic 5-HT2C peptide (10 microM), whilst neither non-immune serum nor untransfected cells displayed any immunoreactivity. A radioimmunoassay was developed to quantify the regional distribution of 5-HT2C-like immunoreactivity (LI) in the adult rat brain. The choroid plexus contained five-fold higher levels of 5-HT2C-LI than any brain region but high levels were found in the frontal cortex, septum, hypothalamus, and striatum, intermediate levels in the thalamus and midbrain, and lower levels in brainstem, cerebellum, and spinal cord. In rat cortical membranes, the B(max) value from [3H]-mesulergine binding was ten-fold lower than 5-HT2C-LI levels determined by radioimmunoassay, which may reflect measurement of internalised receptor protein by radioimmunoassay which is not detected with conventional 5-HT2C ligands. Ten days after depletion of 5-HT with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), there was a significant increase in 5-HT2C-LI in the choroid plexus and the ventral cervical spinal cord, suggesting that receptors therein are located post-synaptic to destroyed serotonergic nerve terminals. In contrast, the significant reduction in 5-HT2C-LI observed in the midbrain, brainstem, and dorsal thoracic spinal cord following 5,7-DHT implies that 5-HT2C receptors may be located on 5-HT nerve terminals in these regions.

[3H]8-OH-DPAT labels a 5-HT site coupled to inhibition of phosphoinositide hydrolysis in the dorsal raphe

The present study was undertaken to compare the properties of the [3H]8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) binding site in the dorsal raphe nucleus with the hippocampal 5-HT1A receptor. In both tissues inclusion of 1 mM Mg2+ enhanced specific [3H]8-OH-DPAT binding, while 1 mM GTP decreased radioligand binding. [3H]8-OH-DPAT appears to bind to a single population of binding sites in both the hippocampus and the dorsal raphe nucleus, although the K(d) for the radioligand at the dorsal raphe site was five times that observed at the hippocampal 5-HT1A receptor. Similarly, although 5-HT and selective 5-HT1A receptor ligands displayed high affinity for the [3H]8-OH-DPAT binding site in the dorsal raphe nucleus, the affinity at the dorsal raphe site was less than that observed at the hippocampal 5-HT1A receptor. 8-OH-DPAT inhibited forskolin-stimulated adenylyl cyclase activity in the hippocampus, but did not alter enzyme activity in the dorsal raphe nucleus. Conversely, 8-OH-DPAT inhibited the accumulation of [3H]inositol phosphates in the dorsal raphe nucleus, but not in the hippocampus. An inhibition of phosphoinositide hydrolysis in the dorsal raphe nucleus also was found with the putative 5-HT1A receptor selective ligands, flesinoxan and gepirone. However, addition of another putative 5HT1A receptor selective ligand, buspirone, did not alter the generation of [3H]inositol phosphates, but blocked the inhibitory effect of 8-OH-DPAT on phosphoinositide hydrolysis. These studies demonstrate that the 8-OH-DPAT binding site in the dorsal raphe nucleus displays a binding profile which is similar to the hippocampal 5-HT1A receptor, but unlike this 5-HT1A receptor the binding site in the dorsal raphe nucleus is negatively coupled to phosphoinositide turnover.

Functional reconstitution in situ of 5-hydroxytryptamine2c (5HT2c) receptors with alphaq and inverse agonism of 5HT2c receptor antagonists

Membranes prepared after infection of Sf9 cells with recombinant baculovirus containing the rat 5HT2c receptor DNA, but not after infection with wild-type virus, expressed high affinity binding sites for 125I-lysergic acid diethylamide and [3H]mesulergine. The receptor site density reached an optimum of 50-70 pmol/mg membrane protein at 60 h postinfection. Extraction of peripheral membrane proteins from the postnuclear membrane fraction with 6 M urea depleted GTPgammaS-binding 4-fold without decreasing 5HT2c receptor binding activity. Urea-extracted Sf9 membranes expressing the 5HT2c receptor catalyzed the activation of squid retinal alphaq but not bovine retinal alphat or bovine alphao/alphai. Productive interaction of 5HT2c receptors with squid alphaq was enhanced by the addition of betagamma dimers prepared from either bovine brain or bovine rod outer segment discs. While the addition of serotonin increased 5HT2c receptor-catalyzed GTPgammaS binding to alphaq, the unoccupied receptor was also catalytically active. The 5HT2c receptor antagonists, mesulergine, mianserin, and ketanserin competitively inhibited 5HT activation of the receptor with predicted rank-order affinities; and mianserin and ketanserin markedly inhibited basal 5HT2c receptor activity. Interestingly, this "inverse agonist" efficacy did not correlate with antagonist affinity for the 5HT2c receptor. Baculoviral expression of the 5HT2c receptor and urea extraction of postnuclear Sf9 cell membranes have provided a high density of in situ, uncoupled, G-protein-linked receptor useful for reconstitution with purified G-protein subunits. This has allowed for independent manipulation of receptor and G-protein chemical concentrations and has revealed that a G-protein-linked receptor can possess a significant basal catalytic activity and that antagonist compounds can act as inverse agonists of this basal activity at the level of receptor activation of G-proteins.

Effects of ibogaine and noribogaine on phosphoinositide hydrolysis

The effects of the antiaddictive compound, ibogaine, and its primary metabolite, noribogaine (12-hydroxyibogamine), on phosphoinositide hydrolysis were investigated. Although ibogaine did not alter phosphoinositide turnover in either striatal or hippocampal slices, noribogaine elicited a concentration-dependent increase in the generation of [3H]inositol phosphates. This stimulation was not altered by inclusion of tetrodotoxin, cadmium or omega-conotoxin indicating that the increased production of [3H]inositol phosphates was not secondary to a release of one or more neurotransmitters. The present study indicates a stimulation of phosphoinositide hydrolysis by noribogaine may be involved in the behavioral effects of ibogaine.

Interactions of selective serotonin reuptake inhibitors with the serotonin 5-HT2c receptor

Interactions of the selective serotonin reuptake inhibitors (SSRIs) citalopram, fluoxetine and its main metabolite norfluoxetine, and the tricyclic antidepressant (TCA) imipramine with the rat serotonin 5-HT2C receptor in a clonal cell line and in the rat choroid plexus were investigated by radioligand binding and phosphoinositide (PI) hydrolysis assays. For comparison, the affinities of a variety of other antidepressants of different chemical classes for the cloned rat 5-HT2C and 5-HT2A receptors were also determined by radioligand binding assays. Fluoxetine displayed relatively high affinity for the 5-HT2C receptor in the choroid plexus, with a Ki value for inhibition of [3H]mesulergine binding of 55.4 nM. The Ki values for imipramine, norfluoxetine and citalopram were 136 nM, 203 nM, and 298 nM, respectively. Similar rank order of potency was detected in PI hydrolysis assays, which showed that these drugs are antagonists at the 5-HT2C receptor without exhibiting inverse agonist activity. [3H]Ketanserin (5-HT2A) binding assays revealed that the SSRIs fluoxetine, norfluoxetine and citalopram show 10- to 23-fold selectivity for the 5-HT2C receptor in vitro, whereas the TCA imipramine does not. Many other TCAs also had high to intermediate affinity for both 5-HT2A and 5-HT2C receptors. The present data provide evidence that fluoxetine, norfluoxetine and citalopram, along with many other antidepressant compounds, interact directly with the 5-HT2C receptor.

The interaction of clozapine with the meta-chlorophenylpiperazine (mCPP) discriminative stimulus

The psychotropic effects of the 5-HT2C agonist mCPP in human subjects are blocked by the atypical antipsychotic clozapine, but not by typical antipsychotics. An understanding of the mechanistic basis for the interaction of clozapine and mCPP would provide further insight into the basis for its unique therapeutic effects in humans. Drug-induced stimulus control provides an animal model for the subjective effects of psychotropic agents in humans. In the present study, the interaction of the atypical antipsychotic clozapine and the typical antipsychotic fluphenazine with the mCPP-stimulus were defined. Neither drug antagonized the stimulus effects of mCPP in vivo. In contrast, clozapine fully antagonized the mCPP-stimulated phosphoinositide turnover at the 5-HT2C receptor in vitro. The present data indicate that the paradigm of mCPP-induced stimulus control does not facilitate the differentiation of atypical and typical antipsychotic activities.

5-HT2C receptor-mediated phosphoinositide turnover and the stimulus effects of m-chlorophenylpiperazine

The present study was designed to investigate the hypothesis that agonist interactions at 5-HT2C receptors mediate the discriminative stimulus properties of m-chlorophenylpiperazine (mCPP). Three structural classes of compounds have been described to stimulate increases in phosphoinositide (PI) hydrolysis at the 5-HT2C receptor site: phenylpiperazines, phenylalkylamines, and indolamines. Four representative phenylpiperazines, mCPP, TFMPP, MK-212 and quipazine, one phenylalkylamine, (-)DOM, and one indolamine, LSD, were employed in the present study. The efficacies of these compounds were defined (1) in vitro, with respect to their abilities to stimulate increases in PI hydrolysis in the choroid plexus, and (2) in vivo with respect to their abilities to substitute for the mCPP discriminative stimulus. In vitro intrinsic activity at the 5-HT2C site was expressed as a fraction of the maximal PI hydrolysis response elicited by serotonin (5-HT). MK-212 (fractional efficacy = 1.1) and (-)DOM (0.77) were full agonists, while mCPP (0.72), LSD (0.27), quipazine (0.24), and TFMPP (0.22) were partial agonists with respect to the stimulation of PI hydrolysis at the 5-HT2C receptor. In vivo, each of the phenylpiperazines fully substituted for the mCPP stimulus, while (-)DOM (75%), and LSD (67%) elicited only partial substitution. While compounds with agonist activity at the 5-HT2C receptor in vitro substitute for the mCPP stimulus in vivo, no clear relationship exists between in vitro intrinsic activity at the 5-HT2C receptor with respect to the stimulation of PI turnover and maximal substitution for the mCPP stimulus in vivo. The present data suggest that mCPP elicits a compound stimulus which is mediated by agonist interactions at the 5-HT2C receptor and possibly additional interactions with 5-HT2A, 5-HT3, and/or 5-HT1B receptors.

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects of m-chlorophenylpiperazine

m-Chlorophenylpiperazine (mCPP), a major metabolite of the atypical antidepressant trazadone, has been observed to produce marked physiological and behavioral effects in both humans and animals. These effects have been attributed to the interaction of mCPP with serotonergic receptors. The present study was designed to characterize those interactions of mCPP with central serotonergic receptors which mediate mCPP-induced stimulus control. A series of serotonergic antagonists (mesulergine, pizotyline, ketanserin, spiperone, risperidone, ritanserin, metergoline, pirenpirone, and LY53857) was tested for the ability to block the mCPP stimulus. The affinity of these antagonists for 5-HT2A and 5-HT2C receptors was then correlated with maximal percent inhibition of the mCPP stimulus. Kd at the 5-HT2C receptor was inversely proportional (r = -0.75, P < 0.05), and Kd at the 5-HT2A receptor directly proportional (r = +0.67, P < 0.05) to the maximal percent inhibition of the mCPP stimulus. The 5-HT2C selectivity ratio [Kd(5-HT2A)/Kd(5-HT2C)] of the antagonists was directly proportional (r = +0.86, P < 0.01) to maximal percent inhibition of the mCPP stimulus. A multiple regressions analysis indicated that 81% of the variance in the ability of a given antagonist to block the mCPP stimulus could be predicted on the basis of its affinity for 5-HT2A and 5-HT2C receptors. It is concluded that the stimulus effects of mCPP are mediated predominantly by a combination of agonist activity at 5-HT2C receptors and antagonist activity at 5-HT2A receptors.

Na+,K(+)-ATPase in the choroid plexus. Regulation by serotonin/protein kinase C pathway

In the choroid plexus, the ion pump Na+,K(+)-ATPase regulates the production of cerebrospinal fluid. We now report that incubation of choroid plexus with an activator of protein kinase C, phorbol 12,13-dibutyrate, strongly stimulates the phosphorylation of Na+,K(+)-ATPase and inhibits its activity. Similar effects were obtained with serotonin, which in the choroid plexus stimulates phosphoinositide turnover, thereby activating protein kinase C. Serotonin (10 microM) increased by about 10-fold the amount of phosphorylated Na+,K(+)-ATPase and significantly reduced its activity. Two-dimensional peptide mapping showed comigration of Na+,K(+)-ATPase phosphorylated by either phorbol 12,13-dibutyrate or serotonin in intact cells and by protein kinase C in vitro. These results demonstrate that first messengers can regulate the activity of Na+,K(+)-ATPase through a mechanism involving protein phosphorylation. Moreover, they provide a plausible mechanism for the demonstrated ability of serotonin to decrease cerebrospinal fluid production.

Effect of pertussis toxin on the response of rat medial prefrontal cortex cells to the iontophoresis of serotonin receptor agonists

In this study, we examined the response of spontaneously active as well as quiescent cells (L-glutamate-activated) in the rat medial prefrontal cortex (mPFc) to the iontophoresis of 2-methylserotonin (2-Me-5-HT, 5-HT3 receptor agonist), (+/-)-2,5-dimethoxy-(4-iodo-phenyl)-2-aminopropane (DOI, 5-HT2A,2C receptor agonist), 8-hydroxy-N,N-di-propylamino tetralin (8-OH-DPAT, 5-HT1A receptor agonist) and gamma-aminobutyric acid (GABA, a non-selective GABA receptor agonist) after the intracerebral administration of pertussis toxin, an inactivator of the Gi/o protein. This was accomplished using the techniques of extracellular single cell recording and iontophoresis. The administration of pertussis toxin (0.5 microgram, 24 hours before the experiment) into the mPFc did not alter the response of mPFc cells to the iontophoresis of DOI, 2-Me-5HT or GABA compared to saline treated controls. However, the response of mPFc cells to the iontophoresis of 8-OH-DPAT was significantly attenuated in the animals pretreated with pertussis toxin compared to controls. These results suggest that the 5-HT1A but not 5-HT2A,2C or 5-HT3 receptor is coupled to the Gi/o protein.

Biochemical characterization of the mechanisms involved in the 5-hydroxytryptamine-induced increase in rat atrial rate

Several possible mechanisms for 5-hydroxytryptamine (5-HT)-induced tachycardia in rat have been suggested: an activation of 5-HT1C or 5-HT2 receptors, an indirect sympathomimetic effect or a mechanism independent of 5-HT2 receptor stimulation. The aim of this study was to investigate the involvement of these mechanisms in the 5-HT-induced increase in rat atrial rate using biochemical methods. Indeed, the 5-HT1C and 5-HT2 receptors are linked to phosphoinositide hydrolysis and the noradrenaline (NA) released by 5-HT can stimulate the beta 1-adrenergic receptors linked to adenylate cyclase stimulation. The effect of varying concentrations of 5-HT on inositol phospholipid hydrolysis and adenylate cyclase activity of the rat isolated atria were measured. 5-HT (2 microM) did not modify total inositol phosphate (IP) production, while 5-HT 10 and 50 microM increased it 2-fold. The 5-HT2 antagonist ketanserin (1 microM) abolished IP accumulation induced by 5-HT microM), which indicates that this accumulation is 5-HT2 and not 5-HT1C receptor-mediated. Moreover, cyclic AMP (cAMP) formation was enhanced by 5-HT (5, 10, 20 and 50 microM). When atria were incubated 10 min with the beta-adrenergic receptor antagonist nadolol (1 microM), the increase in the cAMP level induced by 5-HT, whatever its concentration (10, 20 or 50 microM), was inhibited. Treating rats with reserpine (2.5 mg/kg, i.p., 48 and 24 hr before experimentation), which caused NA depletion in the heart, seemed to reduce the stimulating effect of 5-HT 10 and 50 microM on adenylate cyclase activity. Thus, the 5-HT-induced increase in cAMP is indirectly due to the activation of the beta-adrenergic receptors by the NA released by 5-HT. It is concluded that 5-HT stimulates both phosphoinositide turnover and adenylate cyclase activity in the rat isolated atria by activation of 5-HT2 receptors and by an indirect sympathomimetic effect.

Distribution of the serotonin 5-HT2 receptor family mRNAs: comparison between 5-HT2A and 5-HT2C receptors

Because of their similarities, serotonin 5-HT2, 5-HT1C, and the recently described 5-HT2F receptors have been classified as members of the 5-HT2 receptor family, and they have been renamed 5-HT2A, 5-HT2C and 5-HT2B, respectively. The regional distribution and cellular localization of mRNA coding for the members of 5-HT2 receptor family were investigated in consecutive tissue sections from the rat brain by in situ hybridization histochemistry. No evidence for the expression of 5-HT2B receptor was found. High levels of 5-HT2A (formerly 5-HT2) receptor mRNA were observed only in few areas, as the frontal cortex, piriform cortex, ventro-caudal part of CA3, medial mammillary nucleus, the pontine nuclei and the motor cranial nerve nuclei in the brainstem, and the ventral horn of the spinal cord. The distribution of 5-HT2A receptor mRNA is generally in good agreement with that of the corresponding binding sites, although discrepancies were sometimes observed. 5-HT2C (formerly 5-HT1C) mRNA was present at very high levels in the choroid plexuses. However, very high levels were also seen in many other brain regions, as the retrosplenial, piriform and entorhinal cortex, anterior olfactory nucleus, lateral septal nucleus, subthalamic nucleus, amygdala, subiculum and ventral part of CA3, lateral habenula, substantia nigra pars compacta, several brainstem nuclei and the whole grey matter of the spinal cord. These results confirm and extend previous observations that 5-HT2C receptor mRNA is present in many brain areas in addition to those autoradiographically shown to have the corresponding binding sites and that 5-HT2C receptor subtype is a principal 5-HT receptor in the brain. From the comparison between their distributions, 5-HT2A and 5-HT2C receptor mRNAs appeared to be expressed in distinct but overlapping sets of brain regions. Both mRNAs coexisted at high levels in the anterior olfactory nucleus, piriform cortex, endopiriform nucleus, claustrum, pyramidal cell layer of the ventral part of CA3, taenia tecta, substantia nigra pars compacta, and several brainstem nuclei. In other regions both mRNAs were present but with different distributions, as the caudate-putamen. These results are also discussed in relation to the physiological meaning of the existence of two so similar receptor subtypes in the brain.

Receptors for 5-hydroxytryptamine: current perspectives on classification and nomenclature

With the increasing number of 5-HT receptors recently identified, using molecular biology techniques, the classification of 5-HT receptors is under review. An integrated approach is proposed to include operational and transductional as well as structural criteria for definitive receptor characterization. On this basis the existence of as many as seven classes of 5-HT receptor are recognized although only the 5-HT1, 5-HT2 and 5-HT3 receptor classes are well defined.

Disruption of potential alpha-helix in the G loop of the guinea pig 5-hydroxytryptamine2 receptor does not prevent receptor coupling to phosphoinositide hydrolysis

Heterogeneity of the 5-hydroxytryptamine2 (5-HT2) receptor across species has been implicated in several pharmacological and physiological studies. Although 5-HT2 receptors in the rat have been linked to increases in phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5-HT2 receptors with PI hydrolysis, the second messenger generally linked with 5-HT2 receptors. In the present study, we have taken a molecular and biochemical approach to determining whether species differences in brain 5-HT2 receptors exist between rat and guinea pig. First, we isolated partial cortical 5-HT2 receptor cDNA clones that encompassed the third intracellular loop, a receptor area putatively important in receptor-effector coupling. The amino acid sequences deduced from the cDNA clones for rat and guinea pig brain 5-HT2 receptor were 97% homologous. However, the guinea pig 5-HT2 receptor had two tandem substitutions that disrupted a potential alpha helix in the region of the third cytoplasmic loop, which theoretically could alter the intracellular coupling of the guinea pig cortical 5-HT2 receptor. Because of these molecular differences, we examined further the pharmacological activation of the brain 5-HT2 receptor from guinea pig. 5-HT and the 5-HT2 receptor agonist alpha-methyl-5-HT increased PI hydrolysis in guinea pig cortical slices whereas the 5-HT1C receptor agonist 5-methyltryptamine was significantly less potent. In addition, the 5-HT2 receptor antagonists LY53857, ketanserin, and spiperone blocked 5-HT-stimulated PI hydrolysis. These pharmacological data suggested that activation of the 5-HT2 receptor in guinea pig cortical slices was associated with PI hydrolysis. Thus, although areas of the guinea pig brain 5-HT2 receptor that influence receptor-effector coupling were different from the rat, such differences were not critical to receptor-effector coupling because, as in the rat, guinea pig brain 5-HT2 receptors were also coupled to PI hydrolysis.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Effect of endothelin on production of cerebrospinal fluid in rabbits

BACKGROUND AND PURPOSE

Endothelin is a potent vasoconstrictor in several tissues, including the choroid plexus. The goal of this study was to determine whether endothelin affects the production of cerebrospinal fluid.

METHODS

Ventriculocisternal perfusion was used to measure the production of cerebrospinal fluid in anesthetized rabbits. Changes in production of cerebrospinal fluid were examined in response to vehicle, intravenous endothelin (alone and in the presence of indomethacin), and intraventricular endothelin.

RESULTS

Under control conditions, the reduction in production of cerebrospinal fluid in response to endothelin administered intravenously was only modestly greater than that during infusion of vehicle. Because endothelin releases cyclooxygenase products that attenuate the direct effects of endothelin in several tissues, effects of endothelin on the production of cerebrospinal fluid were also examined after inhibition of cyclooxygenase. Production of cerebrospinal fluid in response to 1 micrograms/kg i.v. endothelin was reduced more in animals treated with indomethacin than in untreated animals (-34 +/- 7% [mean +/- SEM] versus -14 +/- 6%, p less than 0.05). Thus, effects of endothelin on the production of cerebrospinal fluid are attenuated by cyclooxygenase products. Finally, responses to intraventricular endothelin were examined. Intraventricular endothelin produced a modest, but significant, reduction in the production of cerebrospinal fluid.

CONCLUSIONS

In summary, endothelin may play a role in regulation of the brain fluid balance by affecting the rate of production of cerebrospinal fluid, and this effect is modulated by cyclooxygenase products.

Behavioural pharmacology of 5-HT3 receptor ligands

Extensive studies have ascribed a role for the central 5-HT3 receptor in the modulation of behaviour. Much of the work stems from the actions of potent and selective 5-HT3 receptor antagonists; these agents reduce mesolimbic dopamine initiated hyperactivity, release suppressed behaviour, reduce the reinforcing properties and withdrawal symptoms of drugs of abuse, enhance cognitive performance and modulate appetite. This article reviews the preclinical and clinical evidence implicating the 5-HT3 receptor in these indications and discusses the potential neurochemical mechanisms underlying the behavioural changes.

Centrally active 5-HT receptor agonists and antagonists

Eleven subtypes of central 5-HT receptor have so far been postulated, four of which have been cloned (5-HT1A, 5-HT1C, 5-HT1D and 5-HT2) and a fifth (the 5-HT3 receptor) purified. The present review discusses the agonists and antagonists which act at these subtypes with respect to their degree of selectivity and in vivo potency. Selective agonists exist for the 5-HT1A, 5-HT1B and 5-HT3 receptors and selective antagonists for the 5-HT2 and 5-HT3 receptors.

5-HT1C receptors mediate phosphoinositide turnover activation in the immature rat hippocampus

The activation of phosphoinositide turnover in rat cerebral cortex and choroid plexus is triggered by the stimulation of 5-HT2 and 5-HT1C receptors, respectively. To characterize the 5-HT receptor subtype mediating the activation of phosphoinositide turnover in the hippocampus, the potency of several 5-HT agonists and antagonists on total [3H]inositol phosphate formation has been compared in the hippocampus, cerebral cortex and choroid plexus of immature rats. 5-HT, alpha-methyl-5-HT, quipazine, MK-212, mCPP (m-chlorophenylpiperazine) and TFMPP (m-trifluoromethylphenylpiperazine) are less potent and efficient in stimulating phosphoinositide turnover in the hippocampus and cerebral cortex than in the choroid plexus. However, for a number of 5-HT receptor antagonists (ketanserin, spiperone, ritanserin, pizotifen, cyproheptadine, mesulergine, mianserin, methiothepin, methysergide) there is a good correlation (r = 0.82) between their antagonistic potency in the hippocampus and choroid plexus while such correlation is not observed for the hippocampus and cerebral cortex. The specific 5-HT2 receptor antagonist spiperone only partially antagonizes (37% inhibition at 1 microM) the stimulation by 5-HT of phosphoinositide turnover in the hippocampus. These results suggest that in the immature rat hippocampus the activation of phosphoinositide turnover by 5-HT is mainly mediated by the 5-HT1C receptor subtype.

The antimigraine drugs ergotamine and dihydroergotamine are potent 5-HT1C receptor agonists in piglet choroid plexus

1. Fozard & Gray (1989) proposed that migraine is mediated by stimulation of 5-HT1C receptors. We have examined the interaction of two effective anti-migraine agents, ergotamine and dihydroergotamine (DHE), with these receptors. Binding (inhibition of labelling by [3H]-mesulergine) and agonist activity (phosphoinositide hydrolysis) were measured in piglet choroid plexus, a tissue rich in 5-HT1C receptors. 2. The pKD for [3H]-mesulergine binding was 8.4. Ergotamine and DHE both inhibited [3H]-mesulergine binding with a pKD of 7.1. This was similar to the potency of m-chlorophenylpiperazine (m-CPP) (pKD 7.4) and rather less than that of 5-hydroxytryptamine (5-HT) (pKD 8.1). 3. Both ergotamine and DHE were full agonists (pEC50S 7.5 and 7.6 respectively) with potencies similar to that of 5-HT (pEC50 7.7) and greater than that of m-CPP (pEC50 7.1). Mesulergine 10(-7) M produced near-parallel rightward shifts of the concentration-response curves for all these agents of 1.8-2.2 log units, consistent with an action of the agonists at the same receptor. 4. There was no effect of prazosin, spiperone, mepyramine or atropine on the phosphoinositide hydrolysis induced by ergotamine, ruling out an action via alpha 1-adrenoceptors, 5-HT2, histamine H1, or muscarinic receptors. 5. It is concluded that, together with 5-HT, ergotamine and DHE are the most potent 5-HT1C agonists reported so far. These findings do not support the theory that 5-HT1C receptor activation causes migraine.

Characterization of the 5-HT receptor subtypes involved in the motor behaviours produced by intrathecal administration of 5-HT agonists in rats

1. The motor behavioural effects of intrathecal injections of 5-hydroxytryptamine (5-HT) and a variety of 5-HT receptor agonists were examined in adult Wistar rats to establish; (a) which 5-HT receptor subtype/s elicit each behaviour and (b) whether these receptors are located within the spinal cord. 2. Intrathecal injection of 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) or 2,5-dimethoxy-alpha,4-dimethylbenzene ethamine hydrochloride (DOM) produced dose-related back muscle contractions (BMC) and wet dog shakes (WDS) which were both markedly attenuated by intraperitoneal pretreatment with either ritanserin (1 mg kg-1), ketanserin (0.16 mg kg-1) or mianserin (0.6 mg kg-1) indicating the involvement of 5-HT2 receptors in both these motor behaviours. Both fluoxetine (1-20 mg kg-1, i.p.) and high doses of 5-HT (50 micrograms) following fluoxetine (5 mg kg-1, i.p.) also elicited BMC, further confirming the involvement of 5-HT in this behaviour. 3. Intrathecal 5-carboxamidotryptamine (5-CT) evoked a marked wet-dog shake response without producing any BMC. Intrathecal pretreatment with 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) enhanced, while in contrast 2-methyl-5-HT pretreatment attenuated, 5-HT agonist-induced BMC without affecting WDS. These data suggest that the spinal 5-HT2 receptors mediating BMC are positively modulated by 5-HT1A but negatively influenced by 5-HT3 receptor activation and may be of a different subtype to the supra-spinal 5-HT2 receptors which elicit WDS. 4. A contrast, reciprocal forepaw treading, lateral head weaving, flat body posture and Straub-tail were evoked by 5-MeODMT, 8-OH-DPAT or 5-CT but not by DOI or DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. Ritanserin (1 mg kg- 1, i.p.) or ketanserin (0.16mgkg-1, i.p.) pretreatment reduced the reciprocal forepaw treading induced by high intrathecal doses of either 5-MeODMT (25.pg) or 5-CT (50,ug) suggesting that this behaviour may be facilitated by 5-HT2 receptor activation. 5. Intrathecal injection of 5-HT (0.05-50pg, after systemic fluoxetine, 5mg kg 1, i.p.), or 1-(3-chlorophenyl) piperazine (mCPP) produced dose-related forepaw-licking and grooming, neither of which were attenuated by ketanserin (0.16 mgkg-1, i.p.) pretreatment suggesting these behaviours may be mediated by 5-HT1c receptors. In contrast, 2-methyl-5-HT (50 and 100pg) produced sideward tail-flicks, not evoked by any other 5-HT agonist and could therefore be mediated by spinal 5-HT3 receptor activation. 6. These data provide behavioural evidence for the existence of spinal 5-HT2 receptors which produce a novel motor behaviour, BMC. Ligand binding studies and dose-response studies with a range of selective 5-HT antagonists are required to establish whether BMC and WDS are mediated by different subtypes of 5-HT2 receptors.

Serotonin receptor heterogeneity and the role of potassium channels in neuronal excitability

Intracellular recordings in vitro from a variety of central neuronal types have shown both inhibition and excitation to be modulatory consequences of serotonin (5-HT) receptor activation. These responses can be seen in isolation or in some cases (e.g. hippocampal pyramidal cells) as a complex biphasic combination of hyperpolarisation followed by depolarisation, suggesting overall control of neuronal excitability may be dependent on the interaction between activation of more than one post-synaptic receptor and/or mechanism. Our studies have confirmed the 5-HT evoked depolarisation of rat facial motorneurones (FM's) and the hyperpolarisation seen in presumed serotonergic neurones of the dorsal raphe nucleus (DRN) to be the result of opposite effects on K+ ion permeability. Suppression of a resting K+ conductance leads to depolarisation while activation leads to hyperpolarisation. The same mechanisms appear to be responsible for the 5-HT evoked responses in hippocampal pyramidal cells but in addition there is also a suppression of a Ca++ dependent K+ conductance responsible for the long spike after hyperpolarisation (AHP). Data from the hippocampus and DRN indicate the 5-HT induced hyperpolarisation to be sensitive to Pertussis Toxin (PTX) and irreversibly mimicked by GTP gamma S, a non-hydrolysable analogue of GTP, suggesting the involvement of a G protein in K+ channel activation. The mechanism of K+ channel closure is less clear as it is unaffected by PTX or activation of adenylate cyclase, however there is indirect evidence that the phosphoinositide pathway may be involved from the cloned 5-HT1C receptor which also closes a K+ channel in cell lines. The results show that hyperpolarisation evoked by 5-HT in the hippocampus and DRN to be mimicked and blocked by 5-HT1A agonists and antagonists. However, the depolarisations in the hippocampus and FM's are mediated by site-dependent receptors with profiles which do not fit into the current 5-HT receptor subtype classification.

5-HT receptors: subtypes and second messengers

Our knowledge about 5-HT (serotonin, 5-hydroxytryptamine) receptors has gained significantly over the recent few years. The discovery of selective ligands and the use of new techniques have led to a significant increase in the number of recognised receptors subtypes. The present status of awareness is largely related to the use of radioligand binding studies, autoradiography, second messenger analysis and more recently, molecular biological techniques. Three main families of 5-HT receptors, of which subtypes have been described, are now accepted. This heterogeneity is further substantiated by the cloning of the cDNA's of three different 5-HT receptors. This article reviews some of the recent developments which led to the characterisation of 5-HT receptor subtypes.

Choroid plexus epithelial cells in primary culture: a model of 5HT1C receptor activation by hallucinogenic drugs

Behavioral, electrophysiological and biochemical evidence suggest that the 5HT2 receptor plays a role in the action of hallucinogenic agents. Considering the structural and functional similarities between the 5HT2 and 5HT1C receptors, we hypothesized that the 5HT1C receptor may also be an important site of action of hallucinogens. The present manuscript evaluates this hypothesis by examining the properties of hallucinogens in the phenalkylamine and indolealkylamine classes at 5HT1C receptors. Epithelial cells isolated from the rat choroid plexus have a high density of 5HT1C receptors linked to phosphoinositide hydrolysis. Comparison of the actions of drugs in cultured cells and whole choroid plexus confirmed that the cell culture system can serve as an in vitro model of 5HT1C receptor activation. 2,5-Dimethoxy-4-bromoamphetamine (DOB), 2,5-dimethoxy-4-methylamphetamine (DOM), 2,5-dimethoxy-4-iodoamphetamine (DOI) and 3,4-methylenedioxyamphetamine (MDA) were evaluated. The rank order of potency to activate 5HT1C receptors [(-)DOB greater than (+/-) DOI greater than (+)DOB greater than (-)DOM much greater than (-)MDA greater than (+) MDA] was consistent with the rank order of effective behavioral doses in rats and humans. The indolealkylamine hallucinogen, 5-methoxy-N,N-dimethyltryptamine was also a 5HT1C receptor agonist, as is the primary amine, 5-methoxytryptamine. These data, combined with previous studies showing that (+)LSD potently activates 5HT1C receptors, suggest that future investigations of the mechanism of action of hallucinogens should consider the role of 5HT1C receptors in addition to the more commonly investigated 5HT2 receptors.

The serotonin 5-HT1D receptor: a progress review

Most of the known neurotransmitters interact with more than one type of receptor. Some of them even dispose of receptor subtypes to exert their actions. Serotonin, far from being an exception to that, possesses at least 3 classes of receptors, which have all been reported to be heterogeneous, although convincing data only exist for the 5-HT1 class. This name has been proposed in 1979, two years before the introduction of 'A' and 'B' in the nomenclature to account for the observed heterogeneity of these sites. The 5-HT1C receptor subtype was first described in 1984 and the last member of the family, named 5-HT1D, was characterized in 1987. The pharmacological profiles, the signal transducing systems and the anatomical localizations, both at the regional and cellular levels, of all these subtypes have been investigated and possible functions have been proposed for each of them. Moreover, last and most definitive demonstration of the subtype individuality, the gene or complementary DNA coding for the 5-HT1A and 5-HT1C (and 5-HT2) receptors have been cloned and sequenced. Such data are still missing for 5-HT1D (and 5-HT1B) receptors, but will certainly be provided in the next few years. However and waiting for this decisive clue, the characterization of the 5-HT1D subtype leaves no doubt concerning its significance as a function 5-HT receptor. This review will concentrate on the characteristics of this subtype of 5-HT receptor.

Net production of cerebrospinal fluid is decreased by SCH-23390

A high density of binding sites for the ligands 3H-SCH-23390 and 3H-SKF-83566 has been found in the choroid plexus. Although these sites have similar pharmacology to D1 dopamine receptors, the high-affinity component of 3H-SCH-23390 binding in the choroid plexus has been identified as the 5-HT1c subtype of serotonin receptor. We investigated the possible role of these receptors in modulating the production of cerebrospinal fluid (CSF) in rats. (R) SCH-23390 produced up to a 50% decrease in net CSF production, compared to saline. This compound is a partial agonist at 5-HT1c serotonin receptors, and an antagonist at D1 dopamine receptors. The (S) enantiomer of SCH-23390 (SCH-23388) was ineffective. Drugs interacting with receptors for neurotransmitters in the choroid plexus may hold promise for the treatment of patients with increased intracranial pressure, including those with mass lesions, head trauma, acute or chronic hydrocephalus, or pseudotumor cerebri.

5-Hydroxytryptamine receptor agonists influence calcium-stimulated adenylate cyclase activity in the cerebral cortex and hippocampus of the rat

The effects of 5-hydroxytryptamine (5-HT) receptor agonists on calcium (Ca2+)-stimulated adenylate cyclase activity in the hippocampus and cerebral cortex of the rat were studied. In the presence of Ca2+ (1.5 microM), 5-HT dose dependently inhibited adenylate cyclase activity (EC50 = 10 +/- 2 nM). The inhibitory effect of 5-HT on Ca2(+)-stimulated adenylate cyclase was antagonized by spiperone (KB = 2 +/- 0.8 nM). The rank order of potency of 5-HT agonists to inhibit Ca2(+)-stimulated adenylate cyclase in the hippocampus was: 5-carboxamidotryptamine (5-CT) greater than 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) greater than 5-hydroxytryptamine (5-HT) = 5-methoxytryptamine (5-OCH3-T) greater than trifluoromethylphenylpiperazine (TFMPP) greater than m-chlorophenylpiperazine (mCPP). 2-Methyl-5-hydroxytryptamine (2-CH3-5-HT) did not exert an effect on Ca2(+)-stimulated enzyme activity. In the cerebral cortex 5-HT exerted a biphasic stimulatory effect on adenylate cyclase activity in the absence of Ca2+ (EC50 = 0.2 +/- 0.04 nM and 10 +/- 3 microM), whereas 8-OH-DPAT, 5-CT and 2-CH3-5-HT exerted a monophasic effect. In the presence of Ca2+ (1.5 microM), low concentrations of 5-HT, 8-OH-DPAT, 5-CT and 2-CH3-5-HT potentiated adenylate cyclase activity, whereas higher concentrations, except 2-CH3-5-HT, inhibited the enzyme activity. We propose that the 5-HT receptor mediating inhibition of Ca2(+)-stimulated adenylate cyclase in the rat hippocampus corresponds to the 5-HT1A subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution and cellular localization of the serotonin 1C receptor mRNA in the rodent brain examined by in situ hybridization histochemistry. Comparison with receptor binding distribution

The regional distribution and cellular localization of mRNA coding for the serotonin 1C receptor were investigated in tissue sections of mouse and rat brain by in situ hybridization histochemistry. Several 32P-labelled riboprobes derived from mouse genomic clones were used. The serotonin 1C receptor binding sites were visualized autoradiographically and quantified using [3H]mesulergine as ligand, in the presence of spiperone to block serotonin 1C receptors. Strong hybridization signal was observed in the choroid plexus of all brain ventricles. High levels of hybridization were also seen in the anterior olfactory nucleus, pyriform cortex, amygdala, some thalamic nuclei, especially the lateral habenula, the CA3 area of the hippocampal formation, the cingulate cortex, some components of the basal ganglia and associated areas, particularly the nucleus subthalamicus and the substantia nigra. The midbrain and brainstem showed moderate levels of hybridization. The distribution of the serotonin 1C receptor mRNA corresponded well to that of the serotonin 1C receptors. The highest levels of serotonin 1C receptor binding were observed in the choroid plexus. In addition, significant levels of the serotonin 1C receptor binding were seen in the anterior olfactory nucleus, pyriform cortex, nucleus accumbens, ventral aspects of the striatum, paratenial and paracentral thalamic nuclei, amygdaloid body and substantia nigra pars reticulata. The cingulate and retrosplenial cortices as well as the caudal aspects of the hippocampus (CA3) were also labelled. Binding in brainstem and medulla was low and homogeneously distributed. No significant binding was seen in the habenular and subthalamic nuclei. Similar findings were obtained in rat brain. These results demonstrate that, in addition to their enrichment in the choroid plexus, the serotonin 1C receptor mRNA and binding sites are heterogeneously distributed in the rodent brain and thus could be involved in the regulation of many different brain functions. The combination of in situ hybridization histochemistry with receptor autoradiography opens the possibility of examining the regulation of the serotonin 1C receptor synthesis after pharmacological or physiological alterations.

In vivo determination of 5-hydroxytryptamine receptor-stimulated phosphoinositide turnover in rat brain

Phosphoinositide turnover stimulated by 5-hydroxytryptamine (5-HT) receptors in the intact rat brain was studied using an in vivo method. Phosphoinositides in the rat brain were prelabeled with [3H]inositol injected into the lateral cerebral ventricles. The rats were killed by microwave irradiation after 48 h and the contents in the frontal cortex of 3H-inositol phosphates, [3H]inositol-1-monophosphate [( 3H]IP1), [3H]inositol-1,4-bisphosphate [( 3H]IP2), and a mixture of [3H]inositol-1,4,5-trisphosphate and [3H]inositol-1,3,4-trisphosphate [( 3H]IP3) were assayed by HPLC. Lithium treatment (10 mEq/kg, i.p., 2 h before) increased the content of [3H]IP1 and [3H]IP2. 5-Methoxy-N,N-dimethyltryptamine (5-MeODMT) and quipazine, 5-HT agonists, significantly increased the amount of 3H-inositol phosphates under lithium pretreatment. The response to 5-MeODMT was inhibited by ritanserin, a 5-HT2 antagonist, but not by (-)-propranolol, a 5-HT1 antagonist. These results suggest that phosphoinositide turnover in the rat frontal cortex in vivo is stimulated by 5-HT2 receptor activation. It is considered that this method will be useful for measurement of 5-HT2 receptor-stimulated phosphoinositide turnover in vivo to examine the in vivo effects of various psychotropic drugs such as antidepressants.

Postnatal development of 5-HT1 receptors: [3H]5-HT binding sites and 5-HT induced adenylate cyclase activations in rat brain cortex

The postnatal development of the 5-HT1 receptor system was studied in young rat brain cortex from birth to adulthood (14 successive ages). The high-affinity binding of [3H]5-HT was low at birth but developed markedly between the 8th and the 15th day postnatally. The basal adenylate cyclase activity produced 50 pmoles cAMP/mg protein/min at birth and increased from the 8th to the 15th day. 5-HT could stimulate the adenylate cyclase activity in adult rat brain cortex with two different affinity constants: Km = 1 nM and Km = 0.5 microM; these low- and high-affinity constants presumably correspond to 5-HT1A and 5-HT1non-A.non-B.non-C (5-HT1D) respectively. These two activities developed parallelly from the 14-15th to the 28th day. The 8-hydroxy-2-(di-n-propylamino-tetralin) (8-OH-DPAT)-induced activity described a curve similar to the one that corresponded to 10 microM 5-HT. These results establish that 5-HT1A and 5-HT1non-A.non-B.non-C receptors mainly develop during the synaptogenesis.

The 5-hydroxytryptamine 5-HT1D receptor subtype is negatively coupled to adenylate cyclase in calf substantia nigra

1) The possibility was explored that the recently defined 5-HT1D binding sites could be negatively coupled to adenylate cyclase in calf substantia nigra. 2) 5-HT inhibited forskolin-stimulated adenylate cyclase activity in a concentration-dependent manner (EC50 value = 24.0 nmol/l, Emax = 22.7% inhibition) in the presence of GTP (10 mumol/l), which was required for this inhibitory effect. 3) The following 5-HT receptor agonists inhibited adenylate cyclase activity (in decreasing order of potency): 5-carboxamidotryptamine greater than 5-HT greater than 5-methoxytryptamine greater than 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole (RU 24969) greater than or equal to N,N-dipropyl-5-carboxamidotryptamine greater than 8-hydroxy-2(di-n-propylamino)-tetralin (8-OH-DPAT) greater than buspirone greater than ipsapirone; the latter two compounds apparently behaved as partial agonists. 4) Other compounds displaying agonist activity in this system were: metergoline greater than methysergide greater than or equal to rauwolscine greater than or equal to cyanopindolol greater than or equal to yohimbine greater than (+/-)-4(3-tert-butyl-amino-2-hydroxypropoxy)-indol-2 carbonic acid isopropylester (21-009) greater than corynanthine. 5) Methiothepin, mianserin and spiperone displaced the concentration-effect curve of 5-HT to the right without depressing the Emax value. The same held true for the partial agonists ipsapirone, buspirone and corynanthine. 6) The rank order of potency of agonists as well as of antagonists in this system was in full agreement with their affinities at 5-HT1D binding site. A highly significant correlation was found between both parameters (r = 0.94, P = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of 5-hydroxytryptamine on the rate of cerebrospinal fluid production in rabbit

Infusion of 5-hydroxytryptamine in mock cerebrospinal fluid of rabbits lowered cerebrospinal fluid production dose-dependently by nearly 30%. The effect was amplified in the presence of the monoamine oxidase inhibitor, nialamide. The 5-hydroxytryptamine-induced inhibition was partly counteracted by ketanserin, and further addition of practolol completely blocked the reduction in cerebrospinal fluid formation, suggesting that both 5-hydroxytryptamine receptors and beta 1-adrenergic receptors were involved in the response mediated by both the secretory epithelium and the vascular bed of the plexus.