A review of central 5-HT receptors and their function

Abnormal neurochemical asymmetry in the temporal lobe of schizophrenia

Neuroanatomical asymmetries are known to be present in the human brain, and loss of reversal of these asymmetries, particularly through changes in the left temporal lobe, have been found in the brains of patients with schizophrenia. In addition to disturbed neuroanatomical asymmetries, disturbed neurochemical asymmetries have also been reported in the brains of patients with schizophrenia. However, in the temporal lobe, the laterality of most of these neurochemical changes has not been specifically evaluated. Few neurochemical studies have addressed left-right differences in the superior temporal gyrus (STG). A deteriorated serotonin2A receptor-G protein qalpha (Gqalpha)-phosphoinositide-specific phospholipase C beta1(PLC beta1) cascade has been found in the left, but not right, STG of patients with schizophrenia. Not only neuroanatomical but also neurochemical evidence supports the loss or reversal of normal asymmetry of the temporal lobe in schizophrenia, which might be due to a disruption of the neurodevelopmental processes involved in hemispheric lateralization.

Influence of serotonin receptor antagonists on substance P and serotonin release evoked by tooth pulp stimulation with electro-acupuncture in the trigeminal nucleus cudalis of the rabbit

We studied the effect of NAN-190 (5-HT(1A) antagonist), ketanserin (5-HT(2) antagonist) and ICS 205-930 (5-HT(3) antagonist) on tooth pulp stimulation (TPS)-induced 5-HT release and substance P (SP) release in the superficial layers of the trigeminal nucleus caudalis (SpVc-I,II) in the presence or absence of electro-acupuncture (EAP). TPS slightly increased 5-HT release and significantly increased SP release. In combination with EAP, TPS-induced 5-HT release was remarkably enhanced, whereas SP release was significantly suppressed. Pretreatment with NAN-190 (3.5 mg/kg, i.v.) significantly enhanced the increase in TPS-induced 5-HT release in the presence of EAP. On the other hand, the increase of 5-HT release induced following TPS in the presence of EAP was inhibited by pretreatment with ketanserin (2.5 mg/kg, i.v.) and ICS 205-930 (1 mg/kg, i.v.). When NAN-190 was pre-treated in the animals combined TPS and EAP, the amount of SP release was significantly reduced compared with the absence of this drug. On the other hand, pretreatment with ketanserin and ICS 205-930 reversed the inhibitory effect of EAP on the TPS-generated SP release, especially ICS 205-930, which remarkably enhanced TPS-induced SP release compared with the absence of this drug. On the basis of the obtained results, we concluded that NAN-190 and ICS 205-930 act on EAP-induced analgesia positively and suppressively, respectively, by regulation of TPS-generated SP release through activation of their subtype receptors. On the other hand, ketanserin does not affect TPS-induced 5-HT release and SP release in the presence of EAP.

Characterization of 5-HT(₁A,B) and 5-HT(₂A,C) serotonin receptor binding

This unit describes assays for measuring the binding of radioligands to two major types of receptors for 5-hydroxytryptamine (5-HT or serotonin), 5-HT₁ and 5-HT₂ receptors, in homogenates of brain tissue or cloned into cells in culture. The specific receptor subtypes covered are 5-HT(₁A), 5-HT(₁B), 5-HT(₂A), and 5-HT(₂C). In addition, methodology for using quantitative autoradiography to measure radioligand binding to serotonin receptors in brain slices is described. Protocols are provided for characterization of both saturation and competition binding assays, and instructions for data analysis of these assays is also described. In addition, methodology is provided for the quantification (image analysis) of radioligand binding in brain tissue sections to determine receptor density, preparation of rat brain sections for quantitative autoradiography, and thionin staining of thaw-mounted tissue sections to define certain brain regions.

Rostral ventral medulla 5-HT1A receptors selectively inhibit the somatosympathetic reflex

The role of the 5-hydroxytryptamine (5-HT1A) receptors in the rostral ventrolateral medulla (RVLM) on somatosympathetic, baroreceptor, and chemoreceptor reflexes was examined in anesthetized rats. Microinjection of the selective 5-HT1A agonist 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) decreased arterial blood pressure and splanchnic sympathetic nerve activity (SNA). Electrical stimulation of the hindlimb evoked early and late excitatory sympathetic responses. Bilateral microinjection in the RVLM of 8-OH-DPAT markedly attenuated both the early and late responses. This potent inhibition of the somatosympathetic reflex persisted even after SNA and arterial blood pressure returned to preinjection levels. Preinjection of the selective 5-HT1A antagonist NAN-190 in the RVLM blocked the sympathoinhibitory effect of 8-OH-DPAT and attenuated the inhibitory effect on the somatosympathetic reflex. 8-OH-DPAT injected in the RVLM did not affect baroreceptor or chemoreceptor reflexes. Our findings suggest that activation of 5-HT1A receptors in the RVLM exerts a potent, selective inhibition on the somatosympathetic reflex.

5-HT1A and 5-HT2 receptors differentially regulate the excitability of 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro

We have used intracellular recording techniques to examine the effects of 5-hydroxytryptamine (5-HT, serotonin) on 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro. Bath-applied 5-HT (30-300 microM) had two opposing effects on the membrane excitability of these cells, reflecting the activation of distinct 5-HT receptor subtypes. As demonstrated previously in the rat, 5-HT evoked a hyperpolarization and inhibition of 5-HT neurones, which appeared to involve the activation of an inwardly rectifying K(+) conductance. This hyperpolarizing response was blocked by the 5-HT(1A) receptor-selective antagonist WAY-100635 (30-100 nM). In the presence of WAY-100635, 5-HT induced a previously unreported depolarizing, excitatory response of these cells, which was often associated with an increase in the apparent input resistance of the neurone, likely due to the suppression of a K(+) conductance. Like the hyperpolarizing response to 5-HT, this depolarization could be recorded in the presence of the Na(+) channel blocker tetrodotoxin. In addition, the response was not significantly attenuated by the alpha(1)-adrenoceptor antagonist prazosin (500 nM), indicating that it is not due to the release of noradrenaline, or to the direct activation of alpha(1)-adrenoceptors by 5-HT. The 5-HT(3) receptor antagonist granisetron (1 microM) and the 5-HT(4) receptor antagonist SB 204070 (100 nM) failed to reduce the depolarizing response to 5-HT; however, ketanserin (100 nM), mesulergine (100 nM) and lysergic acid diethylamide (1 microM) significantly reduced or abolished the depolarization, indicating that this effect of 5-HT is mediated by 5-HT(2) receptors.

Activation of 5-HT(1B/1D) receptors in the mesolimbic dopamine system increases dopamine release from the nucleus accumbens: a microdialysis study

This study was designed to investigate the role of 5-hydroxytryptamine (5-HT)(1B) receptors located in the ventral tegmental area and nucleus accumbens in the modulation of accumbal dopaminergic transmission. The selective 5-HT(1B) receptor agonist CP 93129 [3-(1,2,5,6-tetrahydro-4-pyridyl)pyrrolo[3,2-b]pyrid-5-one] was administered into the ventral tegmental area or nucleus accumbens of freely moving Sprague-Dawley rats via retrograde microdialysis. The effects of intra-accumbal and intra-tegmental CP 93129 on extracellular dopamine levels in the nucleus accumbens were measured using one- and dual-probe microdialysis, respectively. For dual-probe microdialysis, one probe was in the ventral tegmental area for drug administration and the other in the ipsilateral nucleus accumbens for dopamine measurement. The results show that infusion of CP 93129 (2, 5 and 10 microM) into the nucleus accumbens increased local dopamine levels in a concentration-related manner. Infusion of CP 93129 (10 and 20 microM) into the ventral tegmental area also increased dopamine levels in the ipsilateral nucleus accumbens. The increased dopamine release in the nucleus accumbens produced by intra-accumbal or intra-tegmental CP 93129 was antagonized by co-infusion of cyanopindolol (5 microM), a 5-HT(1B/1A) receptor antagonist, but not by WAY-100635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridinyl-cyclohexanecarboxamide] (5 microM), a highly selective 5-HT(1A) receptor antagonist. In addition, augmentations of dopamine release in the nucleus accumbens induced by intra-accumbal CP 93129 were sensitive to Na(+) channel blockade with tetrodotoxin. These results are not in opposition to the concept that 5-HT(1B) receptors within the ventral tegmental area and nucleus accumbens are all involved in the modulation of dopamine release in the terminal area of the mesolimbic dopamine system.

Distribution of serotonin 2A, 2C and 3 receptor mRNA in spinal cord and medulla oblongata

It is known that 5-HT receptors have significant roles in nociceptive and motor functions. We have compared the cellular localization of the mRNAs encoding serotonin 5-HT(2A,) 5-HT(2C,) 5-HT(3) receptor subtypes within different levels of the rat spinal cord and medulla. In the spinal cord, 5-HT(2C) receptor mRNA is expressed at high levels in most of the gray matter, except for lamina II. In contrast, 5-HT(2A) receptor mRNA is expressed exclusively in lamina IX. 5-HT(3) receptor mRNA has a low level and diffuse pattern of expression increasing towards the ventral horn. In both gray and white matter, there is a characteristic presence of a few highly stained cells. For each subtype, the expression pattern is similar in all four levels of the spinal cord. In the medulla, 5-HT(2C) receptor mRNA is at high levels in many nuclei including the hypoglossal nucleus, the gigantocellular reticular nucleus alpha and the parvocellular reticular nucleus alpha, the spinal nucleus of the trigeminal tract, the facial, and the dorsal medullary reticular field. Moderate to low levels of expression are seen in the spinal vestibular nucleus, the vagus, the solitary nuclei and the raphe. 5-HT(2A) receptor is expressed at high levels in some nuclei such as the hypoglossal nucleus, the intercalate nucleus, the inferior olive and the lateral reticular nucleus. Moderate to low levels of expression are seen in the facial, the medial vestibular nuclei, the nucleus ambiguous, the vagus, and the gigantocellular reticular nucleus. 5-HT(3) receptor mRNA is present at low levels in most of the nuclei examined, with a few scattered strongly labeled cells. The results show a distinct distribution of the three subtypes of receptors supporting their physiological roles and will help to understand the mechanisms of nociception and motor function.

Subcellular distribution of 5-HT(1B) and 5-HT(7) receptors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), a circadian oscillator, receives glutamatergic afferents from the retina and serotonergic (5-HT) afferents from the median raphe. 5-HT(1B) and 5-HT(7) receptor agonists inhibit the effects of light on SCN circadian activity. Electron microscopic (EM) immunocytochemical procedures were used to determine the subcellular localization of 5-HT(1B) and 5-HT(7) receptors in the SCN. 5-HT(1B) receptor immunostaining was associated with the plasma membrane of thin unmyelinated axons, preterminal axons, and terminals of optic and nonoptic origin. 5-HT(1B) receptor immunostaining in terminals was almost never observed at the synaptic active zone. To a much lesser extent, 5-HT(1B) immunoreaction product was noted in dendrites and somata of SCN neurons. 5-HT(7) receptor immunoreactivity in gamma-aminobutyric acid (GABA), vasoactive intestinal polypeptide (VIP), and vasopressin (VP) neuronal elements in the SCN was examined by using double-label procedures. 5-HT(7) receptor immunoreaction product was often observed in GABA-, VIP-, and VP-immunoreactive dendrites as postsynaptic receptors and in axonal terminals as presynaptic receptors. 5-HT(7) receptor immunoreactivity in terminals and dendrites was often associated with the plasma membrane but very seldom at the active zone. In GABA-, VIP-, and VP-immunoreactive perikarya, 5-HT(7) receptor immunoreaction product was distributed throughout the cytoplasm often in association with the endoplasmic reticulum and the Golgi complex. The distribution of 5-HT(1B) receptors in presynaptic afferent terminals and postsynaptic SCN processes, as well as the distribution of 5-HT(7) receptors in both pre- and postsynaptic GABA, VIP, and VP SCN processes, suggests that serotonin plays a significant role in the regulation of circadian rhythms by modulating SCN synaptic activity.

Effects of 8-OHDPAT administration into the dorsal raphe nucleus and dorsal hippocampus on fear behavior and regional brain monoamines distribution in rats

The effects of R(+)-8-hydroxy-dipropylaminotetralin (8-OHDPAT) administration into the dorsal raphe nucleus (DRN) or bilaterally into the dorsal hippocampus (HIP) on fear behavior in a modified version of the light-dark transitions test and regional brain monoamines (NA, DA, 5-HT) and their metabolites (MHPG, DOPAC, 5-HIAA) in the hypothalamus, midbrain central gray matter, amygdala, hippocampus and pons were examined. The experiments were performed on 36 male, 3-month old Wistar rats. Administration of 8-OHDPAT (200 ng) into the DRN reduced time out from the illuminated part of the chamber and time of motionless behavior in the illuminated part, increased the number of returns from the dark to illuminated part and number of head dipping from the dark to illuminated part without effect on time of motionless behavior in the dark part and on time of locomotor activity in the illuminated as well as in dark part of the chamber. HPLC analysis showed reduction of 5-HT content in the midbrain and amygdala, reduction of 5-HIAA content in pons, increased 5-HIAA/5-HT ratio in the hippocampus and increased DOPAC/DA ratio in the hypothalamus, midbrain, hippocampus and pons without affecting the MHPG/NA ratio and NA content. The administration of 8-OHDPAT (100 ng per site) into the HIP reduced time out from the illuminated part of chamber, time of locomotor activity in the illuminated part and head dipping from the dark to illuminated part without effect on the number of returns from the dark to illuminated part, time of locomotor activity in the dark part and time of motionless in the illuminated as well as in the dark part of chamber. HPLC analysis showed reduction of NA content in the hypothalamus, amygdala and pons, increased the MHPG content in all the investigated structures, increased MHPG/NA ratio in all the investigated structures except the hypothalamus. Dopamine content decreased in the hypothalamus and amygdala, and DOPAC/DA ratio increased in the amygdala and hippocampus. Concentrations of 5-HT, 5-HIAA and 5-HIAA/5-HT ratio were unchanged. The results obtained indicate that 8-OHDPAT acting on the pre-synaptic 5-HT1A receptors decreases fear behavior and acting on 5-HT1A post-synaptic receptors increases fear behavior in the light-dark transitions test. The neurochemical base of anxiolytic and anxiogenic effects evoked by 8-OHDPAT is being discussed.

Structure and function of invertebrate 5-HT receptors: a review

Over the last decade, knowledge of invertebrate serotonin receptors has expanded greatly. The first 5-HT receptor from Drosophila was cloned 10 years ago, and subsequently, 11 additional receptor genes have been cloned from Drosophila, molluscs (Lymnaea and Aplysia) and nematodes (Caenorhabditis and Ascaris). Information has also accumulated from physiological and biochemical studies that have used vertebrate serotonergic ligands to characterize endogenous invertebrate receptors. Although the endogenous receptors are often classified according to mammalian-based categories, in many cases the pharmacological properties of vertebrate and invertebrate receptors differ significantly and the actual identity of the latter is questionable. By providing information on the gene structure and amino acid sequence, molecular cloning studies offer a more definitive way to identify and classify invertebrate 5-HT receptors. This review summarizes information on the pharmacological and transductional properties of cloned invertebrate 5-HT receptors, and considers recent studies of endogenous receptors in the light of this new data.

Pharmacological characterization of serotonin receptor subtypes modulating primary afferent input to deep dorsal horn neurons in the neonatal rat

Spinal cord slices and whole-cell patch clamp recordings were used to investigate the effects of serotonergic receptor ligands on dorsal root-evoked synaptic responses in deep dorsal horn (DDH) neurons of the neonatal rat at postnatal days (P) 3 - 6 and P10 - 14. Bath applied 5-hydroxytryptamine (5-HT) potently depressed synaptic responses in most neurons. Similarly, the 5-HT(1/7) receptor agonist, 5-carboxamidotryptamine (5-CT) depressed synaptic responses. This action was probably mediated by 5-HT(1A) receptor activation, since it occurred in the presence of the 5-HT(7) receptor antagonist clozapine and was not observed in the presence of NAN-190, a 5-HT(1A) receptor antagonist. In the absence of any agonist, 5-HT(1A) receptor antagonists often facilitated synaptic responses, suggesting that there is sufficient endogenous 5-HT to tonically activate 5-HT(1A) receptors. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), the 5-HT(1A/7) receptor agonist, facilitated synaptic responses, an action probably mediated by 5-HT(7) receptors, since the facilitation could be reversed by subsequent application of the 5-HT(7) receptor antagonist clozapine. Agonists for the 5-HT(1B), 5-HT(2) and 5-HT(3) receptors exerted only modest modulatory actions. A pharmacological analysis of the depression evoked by 5-HT suggested an action partly mediated by 5-HT(1A) receptor activation, since antagonism of the 5-HT(1A) receptor with NAN-190 or WAY-100635 partly reversed 5-HT-evoked depression. In comparison, 5-HT(7) receptor activation could account for much of the 5-HT-evoked facilitation. We conclude that 5-HT is capable of modulating sensory input onto DDH neurons via several receptor subtypes, producing both facilitatory and depressant actions. Also, the actions of most receptor ligands on the evoked responses were similar within the first 2 postnatal weeks.

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

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

Effects of MDL 73005 on water-maze performances and locomotor activity in scopolamine-treated rats

The stimulation of 5-HT1A receptors in the raphe or their blockade in the hippocampus can reduce cognitive deficits induced by blockade of muscarinic receptors in the hippocampus. We investigated the effects of MDL 73005 (8-[2-(2,3-dihydro-1,4-benzodioxin-2-ylmethylamino) ethyl]-8-azaspiro[4,5] decane-7,9-dione methyl sulphonate), an agonist at 5-HT1A somatodendritic autoreceptors and an antagonist at postsynaptic 5-HT1A receptors in rats treated systemically with scopolamine. Spatial memory was assessed in a water maze using protocols testing reference and working memory. Home cage locomotor activity was also determined. Working memory and locomotor activity were evaluated before and after para-chlorophenylalanine (pCPA) treatment. Scopolamine produced a weak impairment of reference memory at 0.5 mg/kg, and a more pronounced impairment of working memory at 0.25 and 0.5 mg/kg. MDL 73005 alone (2 mg/kg, i.p.) had no effect, but prevented the memory impairments induced by 0.25 mg/kg of scopolamine. Scopolamine induced hyperlocomotion. MDL 73005 alone did not affect locomotor activity, but exacerbated the hyperlocomotion induced by 0.5 mg/kg of scopolamine. pCPA did not abolish the effects of MDL 73005, suggesting that these effects were not due to an action at presynaptic receptors, or even that they involved receptors other than serotonergic ones (e.g., D2). In conclusion, MDL 73005 is able to antagonise moderate spatial memory dysfunctions induced by systemic muscarinic blockade.

Molecular dynamics of buspirone analogues interacting with the 5-HT1A and 5-HT2A serotonin receptors

Three-dimensional (3-D) models of the human serotonin 5-HT1A and 5-HT2A receptors were constructed, energy refined, and used to study the interactions with a series of buspirone analogues. For both receptors, the calculations showed that the main interactions of the ligand imide moieties were with amino acids in transmembrane helix (TMH) 2 and 7, while the main interactions of the ligand aromatic moieties were with amino acids in TMH5, 6 and 7. Differences in binding site architecture in the region of highly conserved serine and tyrosine residues in TMH7 gave slightly different binding modes of the buspirone analogues at the 5-HT1A and 5-HT2A receptors. Molecular dynamics simulations of receptor-ligand interactions indicated that the buspirone analogues did not alter the interhelical hydrogen bonding patterns upon binding to the 5-HT2A receptor, while interhelical hydrogen bonds were broken and others were formed upon ligand binding to the 5-HT1A receptor. The ligand-induced changes in interhelical hydrogen bonding patterns of the 5-HT1A receptor were followed by rigid body movements of TMH2, 4 and 6 relative to each other and to the other TMHs, which may reflect the structural conversion into an active receptor structure.

The anxiolytic-like effect of MCI-225, a selective NA reuptake inhibitor with 5-HT3 receptor antagonism

We have previously reported that MCI-225, a selective noradrenaline (NA) reuptake inhibitor with serotonin (5-HT)3 receptor antagonism, shows antidepressant-like properties in experiments using rodents. In this study, we investigated the effect of MCI-225 in anxiety models in comparison with diazepam, ondansetron, maprotiline, imipramine, and trazodone. In social interaction (SI) test in rats, MCI-225 (10 and 30 mg/kg, p.o.), diazepam (1-10 mg/kg, p.o.), and a selective 5-HT3 receptor antagonist ondansetron (1 mg/kg, p.o.) significantly increas

Regulation of phospholipase Cbeta activity by muscarinic acetylcholine and 5-HT(2) receptors in crude and synaptosomal membranes from human cerebral cortex

Stimulation of phospholipase Cbeta by receptor agonists and G proteins has been characterized in crude cerebral membrane preparations, but little is known about their presynaptic localizations and little information is currently available for human brain tissue. The characteristics of phosphoplipase C transmembrane signaling were studied in crude and synaptosomal plasma membranes from postmortem human prefrontal cortex by measuring the hydrolysis of exogenous [(3)H]phosphatidylinositol4,5bisphosphate(PIP(2)) and the immunoreactive levels of phospholipase C (PLC) and G(alphaq/11) proteins. Regulation of PLC activity by Ca(2+) and the 5-HT(2) receptor agonist 5-methyltryptamine, but not by guanosine 5'-O-[3-thiotriphosphate] and the muscarinic acetylcholine receptor agonist carbachol were different between crude and synaptosomal membranes. KCl (20 mM) stimulation was absent in both preparations. Levels of G(alphaq/11)-protein subunits differed between preparations. The functional inhibition carried out with pirenzepine in crude membranes in order to reverse the carbachol-induced PLC stimulation indicates the existence of a component (53%) of the response that is activated by the M(1) muscarinic acetylcholine receptor subtype, and another component (47%) probably mediated by the M(3) muscarinic acetylcholine receptor subtype. In synaptosomal plasma membranes an increased inhibition of carbachol-induced PLC activation through M(1) was found. The PLC activation by 5-methyltryptamine (ketanserin-sensitive in crude membranes) was absent in synaptosomal plasma membranes suggesting the lack of activity mediated by 5-HT(2)-serotonin receptors.

Comparison of [(18)F]altanserin and [(18)F]deuteroaltanserin for PET imaging of serotonin(2A) receptors in baboon brain: pharmacological studies

The regional distribution in brain, distribution volumes, and pharmacological specificity of the PET 5-HT(2A) receptor radiotracer [(18)F]deuteroaltanserin were evaluated and compared to those of its non-deuterated derivative [(18)F]altanserin. Both radiotracers were administered to baboons by bolus plus constant infusion and PET images were acquired up to 8 h. The time-activity curves for both tracers stabilized between 4 and 6 h. The ratio of total and free parent to metabolites was not significantly different between radiotracers; nevertheless, total cortical R(T) (equilibrium ratio of specific to nondisplaceable brain uptake) was significantly higher (34-78%) for [(18)F]deuteroaltanserin than for [(18)F]altanserin. In contrast, the binding potential (Bmax/K(D)) was similar between radiotracers. [(18)F]Deuteroaltanserin cortical activity was displaced by the 5-HT(2A) receptor antagonist SR 46349B but was not altered by changes in endogenous 5-HT induced by fenfluramine. These findings suggest that [(18)F]deuteroaltanserin is essentially equivalent to [(18)F]altanserin for 5-HT(2A) receptor imaging in the baboon.

Distribution of 5-ht(5A), 5-ht(5B), 5-ht(6) and 5-HT(7) receptor mRNAs in the rat brain

The precise involvement of 5-ht(5A), 5-ht(5B), 5-ht(6) and 5-HT(7) receptors in the pleiotropic actions of 5-HT remain incompletely known. To gain insights into their physiological function(s), localization of mRNAs encoding these subtypes was carried out using in situ hybridization on rat brain sections. Localization was heterogeneous. For example, 5-ht(5A) mRNA was widely expressed while 5-ht(5B) mRNA was predominantly expressed in habenula, hippocampus and inferior olive. 5-ht(6) mRNA was abundant in olfactory tubercles and caudate putamen, and highest levels of 5-HT(7) mRNA were observed in multiple thalamic nuclei. These data suggest that these receptors may have distinct functional roles within the serotonergic system.

Ligand induced conformational states of the 5-HT(1A) receptor

It has been shown that G-protein coupled receptors have seven transmembrane alpha-helices, but the structural changes occurring in a G-protein coupled receptor as a response on agonist stimulus and the molecular events leading to blockade of the signal transduction by antagonists are not well understood. In the present study, the AMBER 5.0 force field was used for comparative molecular dynamics simulations of a 5-HT(1A) receptor model in the absence of ligand, in complex with a 5-HT(1A) receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT], in complex with a selective 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl ]-2-phenylpropanamide [(S)-WAY100135], and in complex with the partial agonist, buspirone. In the simulations, the agonist induced larger conformational changes into transmembrane helix 3 and 6 than into the other helices, while the main conformational differences between the agonist bound receptor and the antagonist bound receptor were in transmembrane helix 5 and 6. During the simulations, all the three ligands constrained the helical movements compared to those observed in the receptor without any ligand.

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

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

Neurochemical and electrophysiological evidence that 5-HT4 receptors exert a state-dependent facilitatory control in vivo on nigrostriatal, but not mesoaccumbal, dopaminergic function

In this study we investigated, using in vivo microdialysis and single unit recordings, the role of serotonin4 (5-HT4) receptors in the control of nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity. In freely moving rats, the 5-HT4 antagonist GR 125487 (1 mg/kg, i.p.), without effect on its own, significantly reduced the enhancement of striatal DA outflow induced by 0.01 (-35%) and 0.1 (-66%), but not 1 mg/kg, s.c. haloperidol (HAL). Intrastriatal infusion of GR 125487 (1 microM) had no influence on basal DA outflow, but attenuated (-49%) the effect of 0.01 mg/kg HAL. Systemic administration of GR 125487 modified neither basal nor 0.01 mg/kg HAL-stimulated accumbal DA outflow. In halothane-anaesthetized rats, 1 or 10 mg/kg GR 125487, without effect by itself, failed to modify the changes in accumbal and striatal DA outflow elicited by electrical stimulation (300 microA, 1 ms, 20 Hz, 15 min) of the dorsal raphe nucleus. Finally, GR 125487 (444 microg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra or the ventral tegmental area, reduced HAL-stimulated (1--300 microg/kg, i.v.) impulse flow of nigrostriatal DA neurons only. These results indicate that 5-HT4 receptors exert a facilitatory control on both striatal DA release and nigral DA neuron impulse flow only when nigrostriatal DA transmission is under activated conditions. Furthermore, they indicate that the striatum constitutes a major site for the expression of the control exerted by 5-HT4 receptors on DA release. In contrast, 5-HT4 receptors have no influence on mesoaccumbal DA activity in either basal or activated conditions.

The 5-HT1A receptor in schizophrenia: a promising target for novel atypical neuroleptics?

Increasing attention is being directed towards the role of the serotonergic system in the neurochemistry of schizophrenia and antipsychotic drug treatment. This review considers the 5-HT1A receptor in this context. In patients with schizophrenia, the majority of post-mortem studies have reported increases in 5-HT1A receptor density in the prefrontal cortex in the approximate range 15-80%. Although the pathophysiological significance of this finding is unclear, given the location of a major proportion of these receptors on pyramidal cells, it may reflect an abnormal glutamatergic network. In terms of drug treatment, 5-HT1A agonists clearly display anticataleptic activity in rats. In addition, 5-HT1A agonists consistently increase dopamine release in the prefrontal cortex in rodents, which is an effect that might be predicted to improve negative symptoms. 5-HT1A agonists augment classical neuroleptics in some rat models of antipsychotic action and may be capable of modulating the glutamatergic network therapeutically. Despite the encouraging preclinical data, there is a paucity of clinical studies of 5-HT1A agonist augmentation of neuroleptics in the treatment of schizophrenia. However, the clinical relevance may be clarified by the atypical antipsychotic drugs clozapine, quetiapine and ziprasidone which combine D2 receptor antagonism and 5-HT1A agonism. In conclusion, given the increased prefrontal 5-HT1A receptor density in the illness, and the anticataleptic activity of 5-HT1A agonists combined with their ability to evoke prefrontal dopamine release, there is now a sufficient rationale to examine thoroughly the role of the 5-HT1A receptor in schizophrenia and antipsychotic drug treatment.

Could the 5-HT1B receptor inverse agonism affect learning consolidation?

Diverse evidence indicates that, the 5-HT system might play a role in learning and memory, since it occurs in brain areas mediating such processes and 5-HT drugs modulate them. Hence in this work, in order to explore further 5-HT involvement on learning and memory 5-HT1B receptors' role is investigated. Evidence indicates that SB-224289 (a 5-HT1B receptor inverse agonist) post-training injection facilitated learning consolidation in an associative autoshaping learning task, this effect was partially reversed by GR 127935 (a 5-HT1B/1D receptor antagonist), but unaffected by MDL 100907 (a 5-HT2A receptor antagonist) or ketanserin (a 5-HT1D/2A/7 receptor antagonist) at low doses. Moreover, SB-224289 antagonized the learning deficit produced by TFMPP (a 5-HT1A/1B/1D/2A/2C receptor agonist), GR 46611 (a 5-HT1A/1B/1D receptor agonist), mCPP (a 5-HT2A/2C/3/7 receptor agonist/antagonist) or GR 127935 (at low dose). SB-224289 did not alter the 8-OH-DPAT (a 5-HT1A/7 receptor agonist) learning facilitatory effect. SB-224289 eliminated the deficit learning produced by the anticholinergic muscarinic scopolamine or the glutamatergic antagonist dizocilpine. Administration of both, GR 127935 (5mg/kg) plus ketanserin (0.01 mg/kg) did not modify learning consolidation; nevertheless, when ketanserin dose was increased (0.1-1.0mg/kg) and SB-224289 dose was maintained constant, a learning facilitation effect was observed. Notably, SB-224289 at 1.0mg/kg potentiated a subeffective dose of the 5-HT1B/1D receptor agonist/antagonist mixed GR 127935, which facilitated learning consolidation and this effect was abolished by ketanserin at a higher dose. Collectively, the data confirm and extend the earlier findings with GR 127935 and the effects of non-selective 5-HT(1B) receptor agonists. Clearly 5-HT1B agonists induced a learning deficit which can be reversed with SB-224289. Perhaps more importantly, SB-224289 enhances learning consolidation when given alone and can reverse the deficits induced by both cholinergic and glutamatergic antagonist. Hence, 5-HT1B receptor inverse agonists or antagonists could represent drugs for the treatment of learning and memory dysfunctions.

Laminar distribution of cholinergic- and serotonergic-dependent plasticity within kitten visual cortex

Both cholinergic and serotonergic modulatory projections to mammalian striate cortex have been demonstrated to be involved in the regulation of postnatal plasticity, and a striking alteration in the number and intracortical distribution of cholinergic and serotonergic receptors takes place during the critical period for cortical plasticity. As well, agonists of cholinergic and serotonergic receptors have been demonstrated to facilitate induction of long-term synaptic plasticity in visual cortical slices supporting their involvement in the control of activity-dependent plasticity. We recorded field potentials from layers 4 and 2/3 in visual cortex slices of 60--80 day old kittens after white matter stimulation, before and after a period of high frequency stimulation (HFS), in the absence or presence of either cholinergic or serotonergic agonists. At these ages, the HFS protocol alone almost never induced long-term changes of synaptic plasticity in either layers 2/3 or 4. In layer 2/3, agonist stimulation of m1 receptors facilitated induction of long-term potentiation (LTP) with HFS stimulation, while the activation of serotonergic receptors had only a modest effect. By contrast, a strong serotonin-dependent LTP facilitation and insignificant muscarinic effects were observed after HFS within layer 4. The results show that receptor-dependent laminar stratification of synaptic modifiability occurs in the cortex at these ages. This plasticity may underly a control system gating the experience-dependent changes of synaptic organization within developing visual cortex.

First tricyclic oximino derivatives as 5-HT3 ligands

The design and synthesis of a new type of 5-HT3 ligand with subnanomolar affinity are described. The O-dialkylaminoethyloximinothienopyrrolizine structure was deduced from molecular modeling studies by replacement of an amidine moiety by an oximino one.

Dominant role of the cytosolic C-terminal domain of the rat 5-HT1B receptor in axonal-apical targeting

The 5-HT(1A) and 5-HT(1B) receptors for serotonin exhibit a different membrane localization to either soma and dendrites (5-HT(1A)R) or axons and terminals (5-HT(1B)R) of neurons in the CNS. The mechanisms responsible for their differential targeting were investigated previously by transfecting various 5-HT(1A)R/5-HT(1B)R chimeras in the epithelial Lilly pork kidney (LLC-PK1) cell line. This first study suggested that a specific targeting signal is located in the C-terminal portion (comprising the last two transmembrane and the cytoplasmic C-terminal domains) of the 5-HT(1A)and/or 5-HT(1B) receptors. In the present study, the role of the cytosolic C-terminal tail of the receptors was further investigated by transfecting truncated receptors and 5-HT(1A)R/5-HT(1B)R chimeras in both the epithelial LLC-PK1 cells and rat hippocampal neurons in primary culture. Confocal microscopic analysis of immunofluorescence with specific anti-5-HTR antibodies and anti-microtubule-associated protein 2 or anti-neurofilament 200k antibodies showed that substitution of the cytosolic C-terminal tail of the 5-HT(1B)R in the 5-HT(1A)R addressed the resulting chimera to the axon of neurons and to the apical domain of LLC-PK1 cells. Therefore, the short tail of the 5-HT(1B)R presents an apical targeting signal that can also act as an axonal targeting signal. In addition, a domain within the third intracytoplasmic loop of the 5-HT(1B)R, responsible for its Golgi sequestration in LLC-PK1 cells, appeared to act as another axonal targeting signal in hippocampal neurons.

Genetic diversity of the human serotonin receptor 1B (HTR1B) gene

We systematically and comprehensively investigated polymorphisms of the HTR1B gene as well as their linkage disequilibrium and ancestral relationships. We have detected the following polymorphisms in our sample via denaturing gradient gel electrophoresis, database comparisons, and/or previously published assays: G-511T, T-261G, -182INS/DEL-181, A-161T, C129T, T371G, T655C, C705T, G861C, A1099G, G1120A, and A1180G. The results of the intermarker analyses showed strong linkage disequilibrium between the C129T and the G861C polymorphisms and revealed four common haplotypes: ancestral (via chimpanzee comparisons), 129T/861C, -161T, and -182DEL-181. The results of association tests with schizophrenia were negative, although A-161T had a nominal P = 0.04 via ASPEX/sib_tdt. The expressed missense substitutions, Phe124Cys, Phe219Leu, Ile367Val, and Glu374Lys, could potentially affect ligand binding or interaction with G proteins and thus modify drug response in carriers of these variants. On average, the human cSNPs and differences among other primates clustered in the more thermodynamically unstable regions of the mRNA, which suggests that the evolutionary survival of nucleotide sequence variation may be influenced by the mRNA structure of this gene.

The 5-HT3 antagonist tropisetron (ICS 205-930) is a potent and selective alpha7 nicotinic receptor partial agonist

The 5-HT3 receptor antagonist tropisetron (ICS 205-930) was found to be a potent and selective partial agonist at alpha7 nicotinic receptors. Two other 5-HT3 receptor antagonists, ondansetron and LY-278,584, were found to lack high affinity at the alpha7 nicotinic receptor. Quinuclidine analogues (1 and 2) of tropisetron were also found to be potent and selective partial agonists at alpha7 nicotinic receptors.

Effect of different 5-hydroxytryptamine receptor subtype antagonists on the micturition reflex in rats

OBJECTIVE

To evaluate the effects of antagonists of different subfamilies of 5-hydroxytryptamine (5-HT) receptors on bladder function in anaesthetized and conscious rats. MATEERIALS AND METHODS: The urinary bladder of female anaesthetized rats was catheterized urethrally and filled with physiological saline until spontaneous bladder contractions occurred. Infravesical pressure was measured by a pressure transducer and displayed continuously on a chart recorder. The time of bladder quiescence (to the disappearance of rhythmic contractions) after injection with different compounds tested was recorded. Conscious rats underwent cystometry with chronically (infravesical) implanted catheters to continuously record bladder capacity (evaluated as amount of saline infused between voiding cycles) and maximal voiding pressure. The affinity for the human recombinant serotoninergic 5-HT1A subtype (inhibition of specific binding of [3H]8-hydroxy-2-(di-n-propylamino) tetralin) and the effects on the [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding in HeLa cells was also evaluated.

RESULTS

Among the compounds tested, only 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethyl-piperazine (p-MPPI) and methiothepin showed nanomolar affinity for the 5-HT1A receptors, the former being a neutral antagonist and the latter an inverse agonist in the [35S]GTPgammaS binding model. Intravenous injection of low doses of p-MPPI and methiothepin induced a dose-dependent disappearance of isovolumic bladder contractions in anaesthetized rats (> 10 min). At the highest doses, the dose-response curves were bell-shaped. The amplitude of bladder contractions was not markedly altered. The tested antagonists of 5-HT2, 5-HT3, 5-HT4, and 5-HT6 serotoninergic subtypes were poorly active or inactive in the model. Similarly, these compounds were inactive on cystometry in conscious rats, whereas p-MPPI and methiothepin induced a consistent increase in bladder capacity. Methiothepin also decreased the voiding pressure, whereas p-MPPI did not affect this variable.

CONCLUSIONS

These findings confirm that only selective 5-HT1A receptor antagonists have favourable effects on the bladder, inducing an increase in bladder capacity with no derangement of bladder contractility.

Serotonergic modulation of retinal input to the mouse suprachiasmatic nucleus mediated by 5-HT1B and 5-HT7 receptors

Serotonin (5-HT) and 5-HT receptor agonists can modify the response of the mammalian suprachiasmatic nucleus (SCN) to light. It remains uncertain which 5-HT receptor subtypes mediate these effects. The effects of 5-HT receptor activation on optic nerve-mediated input to SCN neurons were examined using whole-cell patch-clamp recordings in horizontal slices of ventral hypothalamus from the male mouse. The hypothesis that 5-HT reduces the effect of retinohypothalamic tract (RHT) input to the SCN by acting at 5-HT1B receptors was tested first. As previously described in the hamster, a mixed 5-HT(1A/1B) receptor agonist, 1-[3-(trifluoromethyl)phenyl]-piperazine hydrochloride (TFMPP), reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by selectively stimulating the optic nerve of wild-type mice. The agonist was negligibly effective in a 5-HT1B receptor knockout mouse, suggesting minimal contribution of 5-HT1A receptors to the TFMPP-induced reduction in the amplitude of the optic nerve-evoked EPSC. We next tested the hypothesis that 5-HT also reduces RHT input to the SCN via activation of 5-HT7 receptors. The mixed 5-HT(1A/7) receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), reduced the evoked EPSC amplitude in both wild-type and 5-HT1B receptor knockout mice. This effect of 8-OH-DPAT was minimally attenuated by the selective 5-HT1A receptor antagonist WAY 100635 but was reversibly and significantly reduced in the presence of ritanserin, a mixed 5-HT(2/7) receptor antagonist. Taken together with the authors' previous ultrastructural studies of 5-HT1B receptors in the mouse SCN, these results indicate that in the mouse, 5-HT reduces RHT input to the SCN by acting at 5-HT1B receptors located on RHT terminals. Moreover, activation of 5-HT7 receptors in the mouse SCN, but not 5-HT1A receptors, also results in a reduction in the amplitude of the optic nerve-evoked EPSC. The findings indicate that 5-HT may modulate RHT glutamatergic input to the SCN through 2 or more 5-HT receptors. The likely mechanism of altered RHT glutamatergic input to SCN neurons is an alteration of photic effects on the SCN circadian oscillator.

5-HT1B receptor knockout, but not 5-HT1A receptor knockout mice, show reduced startle reactivity and footshock-induced sensitization, as measured with the acoustic startle response

To investigate whether the hyperreactivity to mild environmental and novel stimuli in 5-HT1B receptor knockout (1BKO) mice, as suggested by measures of exploratory, aggressive, and impulsive behaviors, can be extended to phasic stimuli, 1BKO and wildtype mice were tested in acoustic startle reactivity and plasticity paradigms, including habituation, prepulse inhibition, and footshock-induced sensitization of the startle response. Furthermore, we compared 5-HT1A receptor knockout (1AKO) and 1BKO mice to further test the suggested opposite behavioral profiles in these two genotypes. Results show that startle reactivity and footshock-induced sensitization was reduced in 1BKO mice, with no changes in habituation or PPI. In contrast, 1AKO mice did not differ from WT mice in any of the measures. These results indicate that an absence of 5-HT1B receptors, but not of 5-HT1A receptors, affects the modulation of startle reactivity and footshock-induced sensitization, without influencing startle plasticity. Moreover, this study suggests that 1AKO mice display a distinct, but not opposite behavioral profile from 1BKO mice. Furthermore, it is concluded that the hyperreactivity in 1BKO mice cannot be generalized to all stimuli, including the startling stimuli used in this study, but is probably restricted to mild environmental stimuli only.

Mapping of 5-HT2A receptors and their mRNA in monkey brain: [3H]MDL100,907 autoradiography and in situ hybridization studies

The anatomic distribution of serotonin 5-HT2A receptors visualized with [3H]MDL100,907 and of their mRNA detected by in situ hybridization were studied in monkey brain. Both autoradiographic patterns of signal showed heterogeneous distributions and were in general in good agreement in the majority of brain regions. In most neocortical areas, [3H]MDL100,907 presented a four-banded pattern with layers I and III-IV more intensely labeled and layers II and V-VI showing weaker labeling. 5-HT2A receptor mRNA was detected in layers III and IV, and in some cases also in layers II and V. In intra- and extra-calcarine areas of striate cortex a five-banded pattern was distinguished, with layers III and IVc-V showing the highest densities of [3H]MDL100,907 labeling. These two areas showed the highest neocortical hybridization signal. An unexpected finding was the presence of low densities of [3H]MDL100,907 labeling and 5-HT2A receptor mRNA in choroid plexus. Comparison of the distribution of [3H]MDL100,907 and [3H]ketanserin binding sites in monkey brain regions with high nonspecific [3H]ketanserin binding (caudate, putamen, substantia nigra, inferior olive) revealed specific binding of [3H]MDL100,907 with very low nonspecific binding. Some differences were noted between the distribution of [3H]MDL100,907-labeled 5-HT2A receptors in monkey brain and the previously reported distribution of these receptors in human brain: absence of striosome labeling in monkey striatum and different patterns of neocortical labeling. The present results provide the first detailed comparison of 5-HT2A receptor and mRNA distribution in primate brain. The observed species differences in 5-HT2A receptor distribution should be considered when extrapolating results among different species.

Effects of the 5-HT(6) receptor antagonist Ro 04-6790 on learning consolidation

The 5-HT(6) receptor antagonist Ro-04-6790 or 8-OH-DPAT injection improved learning consolidation on an autoshaping task, while mCPP, scopolamine and dizocilpine decreased the performance. The effect induced by scopolamine, but not that induced by mCPP, was reversed completely by Ro-04-6790, while dizocilpine effect was antagonized partially. Nevertheless, ritanserin or WAY 100635, but not Ro 04-6790, antagonized the 8-OH-DPAT facilitatory effects on learning consolidation. As WAY 100635 did not modify the Ro 04-6790 facilitatory effect, hence 5-HT(1A), and/or 5-HT(7), but not 5-HT(6), receptors might mediate the 8-OH-DPAT facilitatory effect on learning consolidation. Since, the Ro 04-6790 facilitatory effect was unaffected by 5-HT(1A), 5-HT(2A)/(2B)/(2C), 5-HT(3) or 5-HT(4) receptor blockade, thereby, the facilitatory effect induced by Ro 04-6790 involved specifically 5-HT(6) receptors. Indeed, the present data provide further support to the notion that, 5-HT(6) receptors play a significant part in the learning consolidation under normal and dysfunctional memory conditions.

Inhibition of evoked glutamate release by the neuroprotective 5-HT(1A) receptor agonist BAY x 3702 in vitro and in vivo

Brain ischemia provoked by stroke or traumatic brain injury induces a massive increase in neurotransmitter release, in particular of the excitotoxin glutamate. Glutamate triggers a cascade of events finally leading to widespread neuronal cell damage and death. The aminomethylchroman derivative BAY x 3702 is a novel neuroprotectant which shows pronounced beneficial effects in various animal models of ischemic brain injury. As shown previously BAY x 3702 binds to 5-HT(1A) receptors of different species in subnanomolar range and is characterized as a full receptor agonist. In this study we investigated the influence of BAY x 3702 on potassium-evoked glutamate release in vitro and ischemia-induced glutamate release in vivo. In rat hippocampal slices BAY x 3702 inhibited evoked glutamate release in a dose-dependent manner (IC(50)=1 microM). This effect was blocked by the selective 5-HT(1A) receptor antagonist WAY 100635, indicating that BAY x 3702 specifically acts via 5-HT(1A) receptors. In vivo, release of endogenous aspartate and glutamate was measured in the cortex of rats by microdialysis before and after onset of permanent middle cerebral artery occlusion. Single dose administration of BAY x 3702 (1 microg/kg or 10 microg/kg i.v.) immediately after occlusion reduced the increase and total release of extracellular glutamate by about 50% compared to non-treated animals, whereas the extracellular aspartate levels were not significantly affected. Inhibition of glutamate release may therefore contribute to the pronounced neuroprotective efficacy of BAY x 3702 in various animal models of ischemic brain damage.

Identified serotonergic neurons in the Tritonia swim CPG activate both ionotropic and metabotropic receptors

Although G-protein-coupled (metabotropic) receptors are known to modulate the production of motor patterns, evidence from the escape swim central pattern generator (CPG) of the nudibranch mollusk, Tritonia diomedea, suggests that they might also participate in the generation of the motor pattern itself. The dorsal swim interneurons (DSIs), identified serotonergic neurons intrinsic to the Tritonia swim CPG, evoke dual component synaptic potentials onto other CPG neurons and premotor interneurons. Both the fast and slow components were previously shown to be due to serotonin (5-HT) acting at distinct postsynaptic receptors. We find that blocking or facilitating metabotropic receptors in a postsynaptic premotor interneuron differentially affects the fast and slow synaptic responses to DSI stimulation. Blocking G-protein activation by iontophoretically injecting the GDP-analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) did not significantly affect the DSI-evoked fast excitatory postsynaptic potential (EPSP) but decreased the amplitude of the slow component more than 50%. Injection of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate, to prolong G-protein activation, had mixed effects on the fast component but increased the amplitude and duration of the slow component of the DSI-evoked response and, with repeated DSI stimulation, led to a persistent depolarization. These results indicate that the fast component of the biphasic synaptic potential evoked by a serotonergic CPG neuron onto premotor interneurons is mediated by ionotropic receptors (5-HT-gated ion channels), whereas the slow component is mediated by G-protein-coupled receptors. A similar synaptic activation of metabotropic receptors might also be found within the CPG itself, where it could exert a direct influence onto motor pattern generation.

Studies on the involvement of the dopaminergic system in the 5-HT2 agonist (DOI)-induced premature responding in a five-choice serial reaction time task

The present experiments investigated whether the enhanced premature (impulsive) responding induced by DOI, [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], a 5-HT2A/2C receptor agonist, is mediated by activation of the dopaminergic system and if this effect of DOI occurs in the nucleus accumbens. Therefore, the effects of a dopamine (D1/2) receptor antagonist given alone or combined with DOI were examined on the performance of rats in a five-choice serial reaction time (5-CSRT) task. Secondly, the effects of DOI in nucleus accumbens core and shell were studied, in order to find the target brain area for DOI-induced premature responding. The results indicate that DOI (0.1 mg/kg, subcutaneously) increases the number of premature responses, as found previously. alpha-Flupenthixol (0.03 mg/kg), a D1/2 dopamine receptor antagonist, and raclopride (0.015 mg/kg), a D2 receptor antagonist, attenuated the DOI-induced enhancement in premature responding. SCH 23390 (0.005 mg/kg), a selective D1 receptor antagonist with little affinity to 5-HT2 receptors totally blocked the effect of DOI. Those doses of DA antagonists did not significantly decrease premature responding when given alone. On the other hand, higher doses of all of these dopamine antagonists increased the number of omissions and decreased the number of ITI hole responses. In contrast to subcutaneous administration, direct injections of DOI (1, 3, and 10 microg bilaterally) to the nucleus accumbens shell or core had no effect on premature responding. These results suggest that the activation of the dopamine system mediates, at least in part, the effect of a 5-HT2 agonist on premature responding, but the nucleus accumbens is not the primary site for this action.

Localization of 5-HT(7) receptors in rat brain by immunocytochemistry, in situ hybridization, and agonist stimulated cFos expression

5-HT(7) receptors are recently identified members of the serotonin receptor family that have moderate to high affinity for several important psychotropic drugs. However, the lack of selective ligands has impeded the study of the brain distribution of these receptors. In this report, we describe the localization of 5-HT(7) receptor in rat forebrain by immunocytochemistry, in situ hybridization of 5-HT(7) mRNA, and functional stimulation of cFOS expression by 5-HT(7) receptor activation. The anatomical localization of 5-HT(7) mRNA in situ hybridization signal. Prominent immunostaining was apparent in numerous sites within the cerebral cortex, hippocampal formation, tenia tecta, thalamus and hypothalamus. 5-HT(7) receptors were detected in suprachiasmatic nucleus by both immunocytochemistry and in situ hybridization. At a microscopic level, both cell bodies and proximal fibers were strongly stained in these regions, suggesting a somatodendritic subcellular distribution. 5-HT(7) receptor-like immunoreactivity was further compared with 5-HT(7) mediated biological function by administering 8-OH-DPAT intracerebroventricular injection (icv)with WAY 100135 (to block 5-HT(1A) receptors) followed by double immunostaining localization of cFos activation and 5-HT(7) receptors. In all regions examined, cFos stimulation and 5-HT(7)-like immunoreactivity colocalized to the same neurons. Furthermore, cFos activation by 8-OH-DPAT was blocked by pimozide--a 5-HT(7) antagonist. Therefore, by using multiple strategies, we were able to localize 5-HT(7) receptors in rat brain unequivocally. The distribution of these receptors is consistent with their involvement in the control of circadian activity and the action of anti-depressants and atypical neuroleptics.

5-HT4 receptors do not mediate the antidepressant-like behavioral effects of fluoxetine in a modified forced swim test

The receptors responsible for mediating the antidepressant effects of selective serotonin reuptake inhibitors are largely unknown. The role of the 5-HT4 receptor in mediating the antidepressant-like effects of fluoxetine in a modified rat forced swim test was examined. Fluoxetine (20 mg/kg) decreased immobility and increased swimming, a pattern shown to represent its actions on the serotonergic system. The selective 5-HT4 receptor antagonist, SB 204070A [8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1, 4-dioxan-5-carboxylate hydrochloride] (0.1-3 mg/kg), failed to change any of the active behaviors in the test compared with saline-treated animals. Upon combination, SB 204070A (3 mg/kg) failed to alter the effects of fluoxetine effects in the test. These data therefore suggest that activation of postsynaptic 5-HT4 receptors, subsequent to reuptake inhibition by fluoxetine, is not necessary for its antidepressant-like behavioral effects in this test.

Estimation of apparent pA2 values for WAY 100635 at 5-HT1A receptors regulating 5-hydroxytryptamine release in anaesthetised rats

5-HT1A receptor agonists decrease 5-hydroxytryptamine (5-HT) terminal release by activating somatodendritic 5-HT1A autoreceptors. The selective 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide (WAY 100635) inhibits these effects of 5-HT1A receptor agonists. The present study was aimed at estimating apparent pA2 values for WAY 100635 to antagonise 5-HT1A receptor agonist-induced decrease in 5-HT release in rat hippocampus. Extracellular concentrations of 5-HT were measured in microdialysis samples after administration of cumulative doses of 5-HT1A receptor agonists with different intrinsic activity, alone or in the presence of increasing doses of WAY 100635. Administration of cumulative doses of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.01-40 mg/kg), 1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)pipe razine (S 14506) (0.00063-2.5 mg/kg), or buspirone (0.16-40 mg/kg), dose-dependently decreased the extracellular concentrations of 5-HT in the ventral hippocampus. Pre-treatment with WAY 100635 (0.01-0.63 mg/kg) shifted the dose-response curve of each agonist to the right in a dose-dependent manner. WAY 100635 antagonised the effects of all three compounds in a competitive manner, with an estimated apparent in vivo pA2 value of 7.95 (95% confidence limits: 7.66-8.24). Taken together, the results are evidence that buspirone, S 14506 and 8-OH-DPAT, administered in cumulative doses, decreased 5-HT release by activating similar 5-HT1A receptors, because a common apparent pA2 value was obtained for WAY 100635. The results also show that orderly microdialysis data can be obtained using cumulative dosing, which enables one to collect dose-response data rapidly, with fewer animals.

Regulation of central synaptic transmission by 5-HT(1B) auto- and heteroreceptors

Although 5-HT(1B) receptors are believed to be expressed on nerve terminals, their precise mode of action is not fully understood because of the lack of selective antagonists. The 5-HT(1B) receptor knockout mouse was used in the present investigation to assess the function of 5-HT(1B) receptors in the modulation of synaptic transmission in three areas of the central nervous system: the dorsal raphe, the ventral midbrain, and the nucleus accumbens. N-(3-Trifluoromethylphenyl)piperazine, a 5-HT(1B) receptor agonist, potently inhibited 5-HT(1A) receptor-mediated slow inhibitory postsynaptic potentials (IPSPs) in the dorsal raphe of wild-type but not knockout mice. Both synaptically released 5-HT and exogenous 5-HT caused a presynaptic inhibition that outlasted the postsynaptic hyperpolarization only in wild-type mice. In the ventral midbrain, 5-HT(1B) receptor-dependent inhibition of gamma-aminobutyric acid(B) IPSPs in dopamine neurons was present in wild-type animals and absent in knockout animals. Similar results were obtained in the nucleus accumbens measuring glutamate-mediated excitatory postsynaptic currents in medium spiny neurons. Finally, cocaine, which blocks 5-HT uptake, inhibited IPSPs in the dorsal raphe and the ventral midbrain of wild-type but not knockout mice, whereas cocaine produced comparable inhibition of excitatory postsynaptic currents in the nucleus accumbens of both types of animals. These results indicate that 5-HT(1B) receptors function as autoreceptors and heteroreceptors to exert presynaptic inhibition of transmitter release in the central nervous system. Furthermore, this study underscores the role played by presynaptic 5-HT(1B) receptors in mediating the effects of cocaine on synaptic transmission.

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.

Serotonin inhibits voltage-gated sodium current by cyclic adenosine monophosphate-dependent mechanism in bullfrog taste receptor cells

We have investigated the effect of 5-hydroxytryptamine (serotonin) (5-HT) on the membrane properties of bullfrog taste receptor cells (TRCs) using patch-clamp technique. External application of 5-HT reversibly suppressed the voltage-gated Na(+) current (I(Na)) in about half of the TRCs sampled. The magnitude of suppression of peak I(Na) was dependent on the holding potential of the cell. Forskolin and cyclic adenosine monophosphate (cAMP) mimicked the suppressive effect of 5-HT on I(Na), but an internal protein kinase A-inhibitor potentiated I(Na). These results suggest that 5-HT suppresses I(Na) of bullfrog TRCs via protein kinase A-dependent phosphorylation, resulting in suppression of the excitability of bullfrog TRCs.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

5-HT(1B) receptor-mediated regulation of serotonin clearance in rat hippocampus in vivo

The 5-hydroxytryptamine (5-HT; serotonin) transporter (5-HTT) is important in terminating serotonergic neurotransmission and is a primary target for many psychotherapeutic drugs. Study of the regulation of 5-HTT activity is therefore important in understanding the control of serotonergic neurotransmission. Using high-speed chronoamperometry, we have demonstrated that local application of 5-HT(1B) antagonists into the CA3 region of the hippocampus prolongs the clearance of 5-HT from extracellular fluid (ECF). In the present study, we demonstrate that the 5-HT(1B) antagonist cyanopindolol does not produce this effect by increasing release of endogenous 5-HT or by directly binding to the 5-HTT. Dose-response studies showed that the potency of cyanopindolol to inhibit clearance of 5-HT was equivalent to that of the selective 5-HT reuptake inhibitor fluvoxamine. Local application of the 5-HT(1A) antagonist WAY 100635 did not alter 5-HT clearance, suggesting that the effect of cyanopindolol to prolong clearance is not via a mechanism involving 5-HT(1A) receptors. Finally, the effect of low doses of cyanopindolol and fluvoxamine to inhibit clearance of 5-HT from ECF was additive. These data are consistent with the hypothesis that activation of terminal 5-HT(1B) autoreceptors increases 5-HTT activity.

Mouse models of serotonin receptor function: toward a genetic dissection of serotonin systems

Central serotonin (5-hydroxytryptamine, 5-HT) systems regulate a wide variety of complex behaviors, and are targeted by drugs used in the treatment of diverse neuropsychiatric disorders. The actions of 5-HT are mediated by a large and heterogeneous family of 5-HT receptor subtypes. Studies of the functional significance of individual subtypes have been complicated by the limited availability of selective receptor agonist and antagonist drugs. Molecular genetic techniques offer complementary approaches for studying the behavioral roles of individual 5-HT receptor subtypes through the generation of gene-targeted and transgenic lines of mice with altered expression of 5-HT receptor genes. This review will examine insights into the serotonergic regulation of behavior that have been produced by the study of these lines, as well as discuss important caveats to the interpretation of these studies.

Effects of the serotonin 5-HT(2) antagonist, ritanserin, and the serotonin 5-HT(1A) antagonist, WAY 100635, on cocaine-seeking in rats

Manipulations of serotonergic systems have been shown to modify many of the behavioral effects of cocaine. It was recently demonstrated that serotonin (5-HT) depletions produced by inhibition of tryptophan hydroxylase reduced cocaine-seeking in an animal model. The present study was designed to determine whether pretreatment with specific 5-HT antagonists might also decrease cocaine-seeking. The effect of pretreatment with the 5-HT(2) antagonist, ritanserin (0.0, 1.0, or 10.0 mg/kg), or the 5-HT(1A) antagonist, WAY 100635 (0. 0, 0.1, 0.3, or 1.0 mg/kg), on cocaine (5.0, 10.0, or 20.0 mg/kg)-produced reinstatement of extinguished drug-taking behavior was measured. Although ritanserin was ineffective, WAY 100635 attenuated cocaine-produced reinstatement in a dose-dependent manner. These effects of WAY 100635 appeared to be specific since responding maintained by saccharin self-administration remained high following pretreatment with 0.3 or 1.0 mg/kg WAY 100635. These data suggest a role of 5-HT(1A), but not 5-HT(2), receptors in cocaine-seeking.

Nicotinic receptors co-localize with 5-HT(3) serotonin receptors on striatal nerve terminals

Nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors are present on presynaptic nerve terminals in the striatum, where they have been shown to be involved in the regulation of dopamine release. Here, we explored the possibility that both receptor systems function on the same individual nerve terminals in the striatum, as assessed by confocal imaging of synaptosomes. On performing sequential stimulation, nicotine (500 nM) induced changes in [Ca(2+)](i) in most of the synaptosomes ( approximately 80%) that had previously responded to stimulation with the 5-HT(3) receptor agonist m-chlorophenylbiguanide (mCPBG; 100 nM), whereas mCPBG induced [Ca(2+)](i) responses in approximately half of the synaptosomes that showed responses on nicotinic stimulation. The 5-HT(3) receptor-specific antagonist tropisetron blocked only the mCPBG-induced responses, but not the nicotinic responses on the same synaptosomes. Immunocytochemical staining revealed extensive co-localization of the 5-HT(3) receptor with the alpha4 nicotinic receptor subunit on the same synaptosomes, but not with the alpha3 and/or alpha5 subunits. Immunoprecipitation studies indicate that the 5-HT(3) receptor and the alpha4 nicotinic receptor subunit do not interact on the nerve terminals. The presence of nicotinic and 5-HT(3) receptors on the same presynaptic striatal nerve terminal indicates a convergence of cholinergic and serotonergic systems in the striatum.

Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality?

The serotonergic cells of the dorsal raphé nucleus innervate much of the forebrain and are thought to be involved in the mechanism of action of antidepressants. Dysfunction of these cells might be involved in the neural mechanisms underlying depression and suicide. The traffic in pathways emanating from the dorsal raphé nucleus is controlled by 5-HT(1) autoreceptors. Until recently it was thought that the autoreceptors in the dorsal raphé nucleus were solely of the 5-HT(1A) subtype. In this article, we discuss evidence that the situation is more complex and that multiple 5-HT(1) subtypes govern different aspects of 5-HT function in the dorsal raphé nucleus presenting new therapeutic opportunities.