Localization of 5-HT4 receptor mRNA in rat brain by in situ hybridization histochemistry

Investigation of serotonin type 4 receptor expression in human and non-human primate gastrointestinal samples

BACKGROUND

The serotonin type 4 (5-HT4) receptor has been associated with functions of the gastrointestinal tract such as modulation of the peristaltic reflex, smooth muscle tone, intestinal secretion and visceral sensitivity. The activation of peripheral 5-HT4 receptors with agonists such as tegaserod has been shown to accelerate gastric emptying and improve symptoms of constipation in animals and humans. However, detailed data on the expression profile and on the localization of this receptor subtype are lacking so far.

OBJECTIVE

To study the pattern and expression levels of 5-HT4 receptor messenger RNA expression in the gut.

METHOD

Normal tissue samples were collected from the whole gastrointestinal tract of patients undergoing abdominal surgery and, in addition, of monkeys. We performed a comprehensive analysis of 5-HT4 receptor expression by quantitative reverse transcription-polymerase chain reaction, using human and non-human primate tissues from the oesophagus to the rectum. In addition, the brain and heart of non-human primates were analysed.

RESULTS

Significantly higher levels of 5-HT4 receptor mRNA were measured in the human stomach, duodenum, jejunum, ileum and caecum and also in the corresponding non-human primate gut segments, ranging from 2- to 12-fold compared with the liver. No differences were found between females and males of both human and non-human primates.

CONCLUSIONS

These results show 5-HT4 receptor mRNA expression throughout the gastrointestinal tract in humans and primates, and also support the preclinical and clinical findings of 5-HT4 receptors ligands exhibiting multiple effects throughout the gastrointestinal tract.

Ontogenetic distribution of 5-HT2C, 5-HT5A, and 5-HT7 receptors in the rat hippocampus

It is known that serotonin exerts its different nociceptive and motor functions by interacting with distinct receptors subtypes, which could be either G-protein coupled or ionotropic. Previous reports demonstrated the early activation of serotonin receptor transcripts during rat development, suggesting a potential role of the serotoninergic system during ontogeny. In this study we have compared the cellular distribution of three serotonin receptor subtypes: 5-HT2C, 5-HT5A, and 5-HT7. Immunocytochemical methods were used in slices of rat hippocampus obtained during the postnatal development. 5-HT2c immunoreactivity was strong at all developmental stages in the CA1 region, whereas differences were observed between P0 and P5 in the CA3 region. The 5-HT5A receptor immunosignal in CA1 and CA3 was strong at P0, decreased at P11, and then increased in the adult. The immunoreactivity to 5-HT7 receptors was high in all regions at P0 and then decreased progressively during postnatal development; the signal was stronger for 5-HT2c than for 5-HT5A and 5-HT7 receptors. Changes in the expression level of each receptor may result in differences in functional and pharmacological properties of the cells expressing them as well as in the hippocampal neuronal network. The distribution of the three serotonin receptor subtypes studied varied during the ontogeny, which supports their potential role during development and will help to understand their mechanisms.

Neuronal expression of cAMP-specific phosphodiesterase 7B mRNA in the rat brain

cAMP plays an important role as second messenger molecule controlling multiple cellular processes in the brain. cAMP levels depend critically on the phosphodiesterases (PDE) activity, enzymes responsible for the clearance of intracellular cAMP. We have examined the regional distribution and cellular localization of mRNA coding for the cAMP-specific phosphodiesterase 7B (PDE7B) in rat brain by in situ hybridization histochemistry. PDE7B mRNA is specifically distributed in rat brain, preferentially in neuronal cell populations. The highest levels of hybridization are observed in olfactory tubercle, islands of Calleja, dentate gyrus, caudate-putamen and some thalamic nuclei. Positive hybridization signals are also detected in other areas, such as cerebral cortex, Purkinje cells of the cerebellum and area postrema. By double in situ hybridization histochemistry, we found that 74% and 79% of the cells expressing PDE7B mRNA in striatum and olfactory tubercle, respectively, were GABAergic cells (expressing glutamic acid decarboxylase mRNA), in contrast with the lack of expression in the few cholinergic cells (expressing choline acetyltransferase mRNA) present in those two areas (around 0.4% in olfactory tubercle). In the thalamic nuclei, a majority of cells containing PDE7B mRNA also expresses a glutamatergic marker (76.7% express vesicular glutamate transporter vGluT1 and 76% express vGluT2 mRNAs). Almost all PDE7B expressing cells in dentate gyrus (93%) were glutamatergic. These results offer a neuroanatomical and neurochemical base that will support the search for specific functions for cAMP dependent PDEs and for the development of specific PDE7 inhibitors.

Modulation of spontaneous firing in rat subthalamic neurons by 5-HT receptor subtypes

The subthalamic nucleus (STN) is considered to be one of the driving forces in the basal ganglia circuit. The STN is innervated by serotonergic afferents from the raphe nucleus and expresses a variety of 5-HT receptor subtypes. We investigated the effects of 5-HT and 5-HT receptor subtype agonists and antagonists on the firing properties of STN neurons in rat brain slices. We used cell-attached, perforated-patch, and whole cell recording techniques to detect changes in firing frequency and pattern and electrical membrane properties. Due to the depolarization of membrane potential caused by reduced potassium conductance, 5-HT (10 microM) increased the firing frequency of STN neurons without changing their firing pattern. Cadmium failed to occlude the effect of 5-HT on firing frequency. 5-HT had no effect on afterhyperpolarization current. These results indicated that the 5-HT action was not mediated by high-voltage-activated calcium channel currents and calcium-dependent potassium currents. 5-HT had no effect on hyperpolarization-activated cation current (I(H)) amplitude and voltage-dependence of I(H) activation, suggesting that I(H) was not involved in 5-HT-induced excitation. The increased firing by 5-HT was mimicked by 5-HT(2/4) receptor agonist alpha-methyl-5-HT and was partially mimicked by 5-HT2 receptor agonist DOI or 5-HT4 receptor agonist cisapride. The 5-HT action was partially reversed by 5-HT4 receptor antagonist SB 23597-190, 5-HT2 receptor antagonist ketanserin, and 5-HT2C receptor antagonist RS 102221. Our data indicate that 5-HT has significant ability to modulate membrane excitability in STN neurons; modulation is accomplished by decreasing potassium conductance by activating 5-HT4 and 5-HT2C receptors.

Frontocortical 5-HT4 receptors exert positive feedback on serotonergic activity: viral transfections, subacute and chronic treatments with 5-HT4 agonists

BACKGROUND

We recently identified a facilitory control exerted by serotonin4 (5-HT4) receptors on the in vivo firing activity of dorsal raphe nucleus (DRN) serotonergic (5-HT) neurons. However, these findings were based on acute administrations of 5-HT4 receptor agonists and antagonists, which were active only in a subpopulation of 5-HT neurons. We had no evidence that this influence was significant when considering the entire DRN, nor if it was persistent after chronic treatments. In addition, the poor distribution of 5-HT4 receptors within the DRN raised the question of the neuroanatomical bases underlying this control.

METHODS AND RESULTS

Here we show that the subacute intraperitoneal (IP) injection of the 5-HT4 receptor agonists prucalopride (2.5 mg/kg) and RS 67333 (1.5 mg/kg) 30 minutes before the beginning of recordings augment the mean firing rate of DRN neurons by 40% and 66%, respectively. These increases remain stable when the compounds are administered continuously during 3 and 21 days; the effects of the 3-day treatment are blocked by the 5-HT4 receptor antagonist GR 125487 (1000 microg/kg, intravenous [i.v.]). In addition, stereotaxic microinjections of herpes simplex viruses, transformed to overexpress 5-HT4 receptors, increase DRN 5-HT neuronal mean activity when performed in the medial prefrontal cortex (mPFC) but not in the striatum or in the hippocampus.

CONCLUSIONS

This finding suggests the existence of a 5-HT(4)-dependent activation of DRN that may involve the mPFC, unveiling the 5-HT4 receptor as a putative player in the physiopathology of several disorders related to central 5-HT dysfunction.

Serotonin 5-HT4receptors and their mRNAs in rat and guinea pig brain: Distribution and effects of neurotoxic lesions

Serotonin 5-HT4 receptors are widely distributed in the periphery and in brain, where they modulate the release of various neurotransmitters and have been implicated in learning and memory. Nine C-terminal splice variants of this receptor have been cloned in mammalian species. In the rat, three such variants have been described: 5-HT4(a), 5-HT4(b), and 5-HT4(e). In the present study, we have examined several aspects of the distribution of these receptors in brain. First, we provide, in rat and guinea pig, a detailed comparison of the distribution of 5-HT4 receptors labeled by the antagonist [125I]-SB 207710 with the distribution of their encoding mRNA visualized by in situ hybridization histochemistry (ISHH). The results suggest that, in several projection systems (striato-nigral and striato-pallidal pathways, projection from dentate granule cells to field CA3, habenulo-interpeduncular pathway), 5-HT4 receptors are located both somatodendritically and axonally. Second, we have analyzed the distribution of mRNA for the three known rat splice variants by reverse transcription-polymerase chain reaction (RT-PCR) and by ISHH. RT-PCR indicates that all three variants are widely distributed, with 5-HT4(b) mRNA being present in all regions examined (olfactory tubercle, striatum, hippocampus, inferior colliculus, substantia nigra, parietal cortex) and 5-HT4(a) and 5-HT4(e) showing a somewhat more restricted distribution. In other regions (periaqueductal gray, reticular formation, medial septum, diagonal band), faint ISHH signals are observed for 5-HT4(a)+4(e) mRNAs, whereas 5-HT4(b) mRNA signals are almost undetectable. Finally, neurotoxic lesions of basal ganglia components in guinea pig also indicate a location of these receptors on terminals of striatal projection neurons.

Modulation of hippocampal excitability by 5-HT4 receptor agonists persists in a transgenic model of Alzheimer's disease

5-HT(4) receptors are widely distributed in both peripheral and central nervous systems where they couple, via a G-protein, to the activation of adenylate cyclase. In the brain, the highest 5-HT(4) receptor densities are found in the limbic system, including the hippocampus and frontal cortex. It has been suggested that activation of these receptors may be of therapeutic benefit in diseases that produce cognitive deficits such as Alzheimer's disease (AD). Previous electrophysiological studies have shown that the 5-HT(4) agonist, Zacopride, can increase population spike amplitude recorded in region CA1 of rat hippocampal slices in a cyclic AMP (cAMP)/cAMP-dependent protein kinase A-dependent manner. We report here that the 5-HT(4) agonist, Prucalopride, and the 5-HT(4) partial agonist, SL65.0155, produce a similar effect in rat hippocampal slices and that the specific 5-HT(4) antagonist, GR113808, blocks these effects. To investigate the potential use of 5-HT(4) agonists in the treatment of AD, Prucalopride was applied to hippocampal slices from a transgenic mouse line that overexpresses the Abeta peptide. Despite the deficit in synaptic transmission present in these mice, the percentage increase of the CA1 population spike induced by Prucalopride was the same as that observed in wild-type mice. These data support 5-HT(4) receptors as a target for cognitive enhancement and suggest that a partial agonist would be sufficient to produce benefits, while reducing potential peripheral side effects. In addition, we show that 5-HT(4) receptors remain functional in the presence of excess Abeta peptide and may therefore be a useful target in AD.

The 5-Hydroxytryptamine4 Receptor Exhibits Frequency-dependent Properties in Synaptic Plasticity and Behavioural Metaplasticity in the Hippocampal CA1 Region In vivo

Long-term plasticity, in the forms of long-term depression (LTD) and long-term potentiation (LTP), of synaptic transmission are thought to underlie memory. Biogenic amino acids modulate the expression of LTD and LTP. The serotonergic 5-hydroxytryptamine4 (5-HT4) receptor has been shown to influence learning and memory. However, little is known about the role of this receptor in synaptic plasticity. Here we show that although induction of LTP is unaffected by either pharmacological activation or inhibition of 5-HT4, application of the 5-HT4 receptor agonist, RS67333, completely blocks learning-induced depotentiation of LTP in the hippocampal CA1 region of freely moving rats, suggesting a role for 5-HT4 receptors in behavioural metaplasticity. In addition, the 5-HT4 antagonist RS39604 enhances the intermediate phase of LTD and converts short-term depression into persistent LTD (>24 h), suggesting a significant role for 5-HT4 receptors in the expression of LTD in CA1. Stimulation at 10 Hz causes transient synaptic depression. However, 5-HT4 antagonist application prior to 10 Hz stimulation leads to LTD, whereas agonist application leads to LTP expression. 5-HT4 receptors thus shift the frequency-response relationship for induction of plasticity. Together, these findings suggest a key role for 5-HT4 receptors in the regulation of synaptic plasticity and the determination of the particular properties of stored synaptic information.

Opposing Electrophysiological Actions of 5-HT on Noncholinergic and Cholinergic Neurons in the Rat Ventral Pallidum In Vitro

The ventral pallidum in rat is a basal forebrain structure that contains neurons that project in the limbic striatopallidal circuitry and magnocellular cholinergic corticopetal neurons. Because 5-hydroxytryptamine (5-HT) terminals on dorsal raphe projections form close appositions with these neurons, we made patch-clamp recordings in immature rat brain slices to determine whether they are modulated by postsynaptic 5-HT receptors. Inward currents were predominantly induced by 5-HT in noncholinergic neurons, which were distinguished from cholinergic neurons by immunohistochemical and electrophysiological criteria. The inward current induced by 5-HT was mimicked and occluded when adenylyl cyclase was stimulated with forskolin, and was almost abolished when h-currents in noncholinergic neurons were blocked with cesium. Consistent with 5-HT7 receptor activation of h-curents by cAMP in other brain regions, we found inward currents were mimicked by the mixed 5-HT1/5-HT7 agonists 5-methoxytryptamine, and by 5-carboxamidotryptamine (5-CT), which was more potent than 5-HT. In contrast, 5-HT1 preferring 8-OH-DPAT was a weak partial agonist, and the 5-HT1–selective antagonist pindolol had no effect. However, despite this profile, antagonists that bind at the 5-HT7 receptor only partly reduced the agonist inward current (SB-269970 and clozapine), or had no effect (mianserin and pimozide). We found in cholinergic neurons that 5-HT predominantly induced hyperpolarizing currents, which were carried by potassium channels, and were smaller than currents induced by 8-OH-DPAT and 5-CT. We conclude from this study that ascending 5-HT projections from the dorsal raphe could have direct and opposite effects on the activities of neurons within the limbic striatopallidal and cholinergic corticopetal circuitry in the ventral pallidum.

Serotonin receptors represent highly favorable molecular targets for cognitive enhancement in schizophrenia and other disorders

RATIONALE

Current treatments for schizophrenia adequately treat the positive symptoms of schizophrenia but only modestly improve cognitive deficits. This review provides evidence for and against the use of selective 5-HT receptor drugs as cognition enhancing agents for schizophrenia and other disorders.

METHODS

Pre-clinical and clinical literature concerned with the role of the serotonergic system in cognition and memory as it relates to schizophrenia is reviewed. Individual 5-HT receptor subtypes for which selective drugs are available that are likely to improve cognition are reviewed. Recommendations for clinical testing are proposed.

RESULTS AND CONCLUSIONS

Four 5-HT receptor systems (5-HT(1A), 5-HT(2A), 5-HT(4), 5-HT(6)) are highlighted as suitable targets for enhancing cognition and memory. Because many clinically available antipsychotic drugs already target 5-HT(1A), 5-HT(2A) and 5-HT(6) receptors, design of clinical trials will need to take into account the serotonergic pharmacology of concurrently administered antipsychotic medications. 5-HT(1A) partial agonists and 5-HT(2A) antagonists have shown modest effectiveness in improving cognition in schizophrenia. 5-HT(6)-selective compounds for cognition enhancement are in late-stage clinical trials, while 5-HT(4) compounds have not yet been tested in humans for cognition enhancement.

RECOMMENDATIONS

For stand-alone therapy for enhancing cognition, 5-HT(1A) partial agonists, 5-HT(2A) antagonists, 5-HT(4) partial agonists and 5-HT(6) antagonists are all likely to induce at least modest improvement in cognition in schizophrenia. If "add-on therapy" is contemplated, antipsychotic drugs with weak affinities for serotonin receptors should be used to avoid confounds. It is likely that serotonergic drugs will soon be available as cognition enhancing medications for disorders other than schizophrenia (e.g. dementia).

Serotonergic regulation of the GABAergic transmission in the rat basal ganglia

The GABAergic neurons represent a major neuronal population in the basal ganglia. Although alterations in serotonin (5-HT) transmission are associated with neurodegenerative diseases involving these regions, the influence exerted by 5-HT afferents on GABAergic populations remains poorly understood. Here, we examined the consequences of 5,7-dihydroxytryptamine-induced lesion of 5-HT neurons on glutamic acid decarboxylase (GAD) activity, mRNA expression of the two isoforms of the enzyme, GAD65 and GAD67, GABA uptake, and extracellular GABA levels in the striatum. The 5-HT depletion produced an increase in GAD activity without modifying GAD65 and GAD67 mRNA levels, suggesting that 5-HT acts at the posttranscriptional level to regulate striatal GABA synthesis. No change in GAD activity was measured in the main striatal target structures, the globus pallidus and substantia nigra. Striatal GABA uptake and extracellular levels of GABA measured under basal conditions in freely moving rats were maintained in a normal range following 5-HT deprivation. By contrast, depolarization-induced increases in extracellular levels of GABA were larger in the striatum of 5-HT-deprived rats than in controls, which may be accounted for by an increase in a releasable pool of GABA due to increased synthesis rate. Together, these results suggest that 5-HT afferents may exert a phasic inhibitory control on striatal GABA transmission. Therefore, a decrease in striatal 5-HT transmission in disease states, such as Parkinson's disease, may contribute to pathological changes in striatal GABA neuron activity by increasing their reactivity to depolarizing stimuli.

Effects of chronic antidepressants and electroconvulsive shock on serotonergic neurotransmission in the rat hippocampus

The hippocampus may play a critical role in the pathophysiology and treatment of depression. There are two main lines of evidence for this: firstly, many of its functions correspond to those altered in depression, and secondly, many hippocampal functions are regulated by the serotonergic (5-HT) system, which is a common target of antidepressant treatments. Chronic effects of antidepressants and electroconvulsive shock (ECS) have been studied by various methods using electrophysiology, in vivo microdialysis or ex vivo neurochemical measurements. The aim of the current review is to point out possible correlations between these studies based on different methods and to suggest neurochemical mechanisms that result in the observed changes in hippocampal physiology and neurogenesis. These changes in hippocampal neurochemistry are reviewed and compared with the abnormalities associated with stress, corticosterone or depression.

Distribution of 5-HT4 receptors in the postmortem human brain--an autoradiographic study using [125I]SB 207710

The autoradiographic distribution of the 5-HT4 receptor was described using human postmortem brain sections and the selective radioligand [125I]SB 207710 [(1-n-butyl-4-piperidinyl)methyl-8-amino-7-[125I]iodo-1,4-benzodioxane-5-carboxylate]. The specific binding was highest in regions of the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus and substantia nigra) and the hippocampal formation (CA1 and subiculum). In the neocortex, the binding showed a distinct lamination pattern with high levels in superficial layers and a band displaying lower levels in deep cortical layers. The results confirm previous studies on the distribution of 5-HT4 receptors in the human brain in vitro and provide high-resolution correlates for in vivo imaging studies using the radioligand recently developed for single photon emission tomography (SPET), [123I]SB 207710.

5-HT4(a) receptors avert opioid-induced breathing depression without loss of analgesia

Opiates are widely used analgesics in anesthesiology, but they have serious adverse effects such as depression of breathing. This is caused by direct inhibition of rhythm-generating respiratory neurons in the Pre-Boetzinger complex (PBC) of the brainstem. We report that serotonin 4(a) [5-HT4(a)] receptors are strongly expressed in respiratory PBC neurons and that their selective activation protects spontaneous respiratory activity. Treatment of rats with a 5-HT4 receptor-specific agonist overcame fentanyl-induced respiratory depression and reestablished stable respiratory rhythm without loss of fentanyl's analgesic effect. These findings imply the prospect of a fine-tuned recovery from opioid-induced respiratory depression, through adjustment of intracellular adenosine 3',5'-monophosphate levels through the convergent signaling pathways in neurons.

Roles of substance P and NK(1) receptor in the brainstem in the development of emesis

The emetic response is primarily a protective reflex occurring in a wide variety of vertebrates in response to the ingestion of toxic compounds. The role of the nuclei in the brainstem, including the area postrema, nucleus tractus solitarius, the dorsal motor nucleus of the vagus, and the central pattern generator for vomiting, as well as the involvement of the abdominal visceral innervation relevant to the emetic reflex, have all been discussed by many researchers. The introduction of serotonin 5-HT(3)-receptor antagonists into clinical practice allowed for a dramatic improvement in the management of vomiting. However, vomiting still remains a significant problem. The mechanism of the emetic response is even more complicated than was first thought. This review attempts to bring together some of the evidence suggesting the roles of substance P and its receptor, neurokinin NK(1) receptor, in the brainstem nuclei in the development of emesis. Accordingly, NK(1)-receptor antagonists might represent novel drugs for the management of major types of emesis.

Elevated expression of 5-HT1B receptors in nucleus accumbens efferents sensitizes animals to cocaine

Although the effects of psychostimulants on brain dopamine systems are well recognized, the direct actions of cocaine on serotonin systems also appear to be important to its addictive properties. For example, serotonin actions at 5-HT1B receptors in the ventral tegmental area (VTA) modulate cocaine-induced dopamine release in the nucleus accumbens (NAcc) and alter the rewarding and stimulant properties of cocaine. However, the mechanisms of these effects have been unclear, because several neuron types in VTA express 5-HT1B receptors. One possibility is that 5-HT1B receptors on the terminals of GABAergic projections from NAcc to VTA inhibit local GABA release, thereby disinhibiting VTA neurons. We tested this hypothesis directly by using viral-mediated gene transfer to overexpress 5-HT1B receptors in NAcc projections to VTA. A viral vector containing either epitope hemagglutinin-tagged 5-HT1B and green fluorescent protein (HA1B-GFP) cassettes or green fluorescent protein cassette alone (GFP-only) was injected into the NAcc shell, which sends projections to the VTA. HA1B-GFP injection induced elevated expression of 5-HT1B receptors in neuronal fibers in VTA and increased cocaine-induced locomotor hyperactivity without affecting baseline locomotion. Overexpression of 5-HT1B receptors also shifted the dose-response curve for cocaine-conditioned place preference to the left, indicating alterations in the rewarding effects of cocaine. Thus, increased expression of 5-HT1B receptors in NAcc efferents, probably in the terminals of medium spiny neurons projecting to the VTA, may contribute to psychomotor sensitization and offer an important target for regulating the addictive effects of cocaine.

5-HT4 receptors in the hippocampus modulate rat locomotor activity

In the present study, the ability of 5-hydroxytryptamine-4 (5-HT4) receptors in the hippocampus to enhance locomotor activity in rats was investigated by local infusion via microdialysis probes. The local infusion of 5-HT bilaterally into the striatum did not alter rat motor activity. The local infusion of 1.0 mM 5-HT into the bilateral hippocampus, but not lower doses, significantly increased motor activity as compared with the baseline values or the control rats. During the day hours (0700-1900, light on), the local infusion of either 5-HT4 agonist, 5-MeOT (100 microM) or mosapride (10 microM), but not in their lower concentrations, into the bilateral hippocampus significantly increased motor activity as compared with the baseline values or the control rats. Almost all increased motor activity was normal forward locomotion. This 5-MeOT-induced hyperlocomotion was completely reversed by the combined infusion of a 5-HT4 antagonist, either GR125487D (100 microM), SB204070 (100 microM) or RS23597-190 (100 microM). During the night hours (1900-0700, light off), the local infusion of either SB204070 (100 microM) or RS23597-190 (100 microM), but not in their lower concentrations, into the bilateral hippocampus significantly decreased rat motor activity and inhibited rat nocturnal hyperactivity. These hypoactivities during the night hours induced by 5-HT4 antagonist were reversed by the combined infusion of a 5-HT4 agonist, 5-MeOT (100 microM). The present study demonstrates that the serotonergic neurons projecting to the hippocampus, but not to the striatum, modulate rat locomotor activity by stimulating 5-HT4 receptors in the hippocampus.

The 5-HT(4) receptor-induced depolarization in rat hippocampal neurons is mediated by cAMP but is independent of I(h)

Intracellular recordings were used to examine the mechanisms underlying the 5-hydroxytryptamine (5-HT)4 receptor-mediated depolarization seen in CA1 region pyramidal cells in in vitro hippocampal brain slices. This depolarization was mimicked and occluded by administration of the membrane permeable cyclic adenosine monophosphate (cAMP) analog 8-bromo-cAMP but was unaffected by blockade of protein kinase A (PKA). These results suggest that 5-HT4 receptors signal this depolarization through a cAMP-dependent but PKA-independent mechanism. In many cell types, 5-HT elicits a depolarization via cAMP by facilitating Ih, a hyperpolarization-activated cation current. In contrast, we find no evidence for the involvement of Ih in this response. Rather, this depolarization may involve a cyclic nucleotide gated channel.

Modulation by serotonin 5-HT(4) receptors of long-term potentiation and depotentiation in the dentate gyrus of freely moving rats

Tetanization-induced long-term potentiation (LTP) in the hippocampus can be depotentiated by low-frequency stimulation. 5-HT(4) receptors are expressed in the hippocampus and are suggested to be involved in hippocampus-dependent cognitive processes. Since the role of these receptors in the dentate gyrus has yet not been characterized, this study investigated the effects of 5-HT(4) receptors on basal synaptic transmission, LTP and depotentiation in the dentate gyrus of freely moving rats. Male Wistar rats were chronically implanted with a recording electrode in the dentate gyrus granule cell layer, a stimulation electrode in the medial perforant path and a cannula for drug administration in the ipsilateral ventricle. The 5-HT(4) agonist methoxytryptamine dose-dependently inhibited basal synaptic transmission and LTP. Priming of receptors by a dose of this agonist which elicited no significant change of basal synaptic transmission inhibited depotentiation. These effects could be prevented by the 5-HT(4) antagonist RS 39604, which did not produce independent effects on synaptic transmission, LTP or depotentiation. The effects of methoxytryptamine were confirmed with the highly selective 5-HT(4) agonist, RS 67333. These results strongly support a role for 5-HT(4) receptors in hippocampal synaptic plasticity and provide an important link to findings with regard to the involvement of 5-HT in processes related to learning and memory.

Differences in signal transduction of two 5-HT4 receptor splice variants: compound specificity and dual coupling with Galphas- and Galphai/o-proteins

This study documents differences in ligand binding and signal transduction properties between the human (h) 5-hydroxytryptamine (5-HT)4a and h5-HT4b receptor splice variants stably expressed in human embryonic kidney 293 cells. The fraction of the [3H]5-HT high-affinity site relative to the whole receptor population measured with [3H]GR113808 was higher for the h5-HT4a isoform (around 0.4) than for the 5-HT4b isoform (around 0.2) and was independent of the level of expression. The potency and efficacy of reference compounds tested for the cAMP response differed slightly but significantly between both variants. Most remarkably, 5-methoxytryptamine and prucalopride were found more potent on the 5-HT4b variant, whereas SDZ-HTF 919 and SB204070 were more potent on the 5-HT(4a) variant. Guanosine-5'-O-(3-[35S]thio)triphosphate binding on membranes and cAMP assays in whole cells revealed that only the h5-HT4b isoform coupled to Galphai/o-proteins in addition to its well-documented Galphas coupling. In contrast, the h5-HT4a receptor coupled only to Galphas-proteins, however, was able to trigger an increase in the intracellular calcium concentration ([Ca(2+)]i). The observed [Ca(2+)]i increase did not occur through inositol phosphate formation and was not sensitive to Bordetella pertussis toxin, forskolin, or 3-isobutyl-1-methylxanthine (pre)treatment but was due to Ca(2+) influx from the extracellular environment. Interestingly, the Ca(2+) pathway was dependent on high receptor expression levels and was compound-specific, because benzamide-like compounds triggered two to three times higher responses than indoleamines. Taken together, these data provide the first evidence for fine functional differences between C-terminal splice variants of the h5-HT4 receptor, which may contribute to a better understanding of the functional diversity of this receptor class.

Serotonin 5- HT (2C) receptor knockout mice: autoradiographic analysis of multiple serotonin receptors

Quantitative receptor autoradiography was used to study possible alterations of the densities of multiple serotonin (5-HT) receptor subtypes and of serotonin transporter in the brain of 5-HT(2C) receptor knockout mice. The radioligands employed were [(3)H]citalopram, [(3)H]WAY100,635, [(3)H]8-OH-DPAT, [(3)H]GR125743, [(3)H]sumatriptan, [(3)H]MDL100,907, [(125)I](+/-)DOI, [(3)H]mesulergine, [(3)H]5-HT, [(3)H]GR113808, and [(3)H]5-CT. As expected, radioligands that label 5-HT(2C) receptors showed a complete absence of labeling in mutant mice choroid plexus and significantly reduced densities in other brain regions expressing 5-HT(2C) receptors. With the rest of the radioligands, no significant alterations in the densities of labeled sites were found in any brain region. In situ hybridization showed no changes in 5-HT(2A) receptor and serotonin transporter mRNA levels, whereas 5-HT(2C) receptor mRNA levels were reduced in certain brain regions. The present results indicate that the mouse serotonergic system does not exhibit compensatory up- or down-regulation of the majority of its components (serotonin transporter and most 5-HT receptor subtypes) in response to the absence of 5-HT(2C) receptors.

Cloning and characterization of a novel human 5-HT4 receptor variant that lacks the alternatively spliced carboxy terminal exon. RT-PCR distribution in human brain and periphery of multiple 5-HT4 receptor variants

We have cloned a novel C-terminal splice variant of serotonin 5-HT4 receptors from human hippocampus. The deduced protein extends only one aminoacid past the splicing point. We propose to call the novel variant h5-HT4(n) since it contains none of the C-terminal exons alternatively spliced in other variants. The pharmacological profile of h5-HT4(n) stably expressed in HeLa cells is in agreement with other reported variants. Stably transfected cells showed increased basal levels of intracellular cAMP in absence of agonist, indicating constitutive activity of the expressed receptors. 5-HT induced robust increases of intracellular cAMP. The 5-HT4 receptor antagonist GR 113808 blocked the effects of 5-HT and brought intracellular cAMP below basal constitutive levels, indicating inverse agonism of this compound in this system. The RT-PCR distribution of all known human C-terminal splice variants in human brain regions and periphery showed complex patterns of variant expression, with the novel variant h5-HT4(n) being widely and abundantly expressed.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Effect of the intracerebroventricular administration of GR 113808, a selective 5-HT4 antagonist, on water intake during hyperosmolarity and hypovolemia

We demonstrate here that acute third ventricle injections of GR 113808, a highly selective 5-HT4 antagonist, decrease water intake induced by a previous salt load while potentiating drinking elicited by hypovolemia induced by previous subcutaneous administration of polyethylene glycol in male Wistar rats (200 +/- 20 g). At the dose of 160 nmol/rat, third ventricle injections of GR 113808 induced a significant reduction of water intake in salt-loaded animals after 120 min as compared to salt-loaded animals receiving third ventricle injections of saline (salt load + GR = 3.44 +/- 0.41 ml, N = 12; salt load + saline = 5.74 +/- 0.40 ml, N = 9). At the dose of 80 nmol/rat, GR 113808 significantly enhanced water intake in hypovolemic animals after 120 min as compared to hypovolemic animals receiving third ventricle injections of saline (hypovol + GR = 4.01 +/- 0.27 ml, N = 8; hypovol + saline = 2.41 +/- 0.23 ml, N = 12). We suggest that central 5-HT4 receptors may exert a positive drive on water intake due to hyperosmolarity and a negative input on drinking provoked by hypovolemia.

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.

Local modulation of the 5-HT release in the dorsal striatum of the rat: an in vivo microdialysis study

Using in vivo microdialysis in freely moving rats, we examined the involvement of major striatal transmitters on the local modulation of the 5-HT release. Tetrodotoxin reduced the striatal 5-HT output to 15-20% of baseline. The selective 5-HT(1B) receptor agonist CP 93129 (50 microM) reduced (50%) and the 5-HT(2A/2C) receptor agonist DOI (1-100 microM) increased (220%) the 5-HT output. Neither GABA nor baclofen (100 nM-100 microM) altered the 5-HT output. The glutamate reuptake inhibitor L-trans-PDC (1-4 mM) raised 5-HT to 280% of baseline. This effect was not antagonized by the NMDA receptor antagonist MK-801 (0.5 mg/kg i.p.). Local MK-801 (10-100 microM) did not significantly alter the 5-HT output. Finally, neither carbachol (10-100 microM) nor quipirole (10 microM-1 mM) affected 5-HT. These data suggest that the striatal 5-HT release is influenced by local serotonergic and glutamatergic (but not GABAergic) inputs.

Central 5-HT(4) receptors and drinking behavior

The aim of the present study was to investigate the effect of acute third ventricle injections of two different 5-HT(4) receptor antagonists, GR 113808 and SB 204070, on water intake in different situations. Injections of 80 nmol/rat of both GR 113808 and SB 204070 were unable to modify water intake in normohydrated rats. Pretreatment with GR 113808 (40 and 80 nmol/rat) and SB 204070 (80 and 160 nmol/rat) blunted water intake after third ventricle injections of angiotensin II (9.6 pmol/rat) compared to saline-pretreated controls. Pretreatment with 80 nmol/rat of both antagonists potentiated drinking induced by third ventricle injections of carbachol (11.0 nmol/rat) compared to saline-pretreated control. In all doses employed, none of the compounds was able to modify water intake in dehydrated rats. A separate control test using one-bottle taste aversion paradigm indicated that the reduction in water intake observed in some of the present experiments could not be attributed to a drug-induced malaise. It is suggested that central 5-HT(4) receptors exert a dualistic role on the control of water intake potentiating angiotensin II-induced drinking and inhibiting thirst induced by central cholinergic activation

Mapping of serotonin 5-HT(4) receptor mRNA and ligand binding sites in the post-mortem human brain

The anatomical localization of 5-HT(4) receptor mRNA and 5-HT(4) receptor protein was examined in sections of post-mortem human brain by in situ hybridization histochemistry and radioligand receptor autoradiography. In the in situ hybridization study, the highest levels of 5-HT(4) receptor mRNA were found in caudate nucleus, putamen, nucleus accumbens, and in the hippocampal formation. No 5-HT(4) receptor mRNA was detected in globus pallidus and substantia nigra. For receptor autoradiography, two new and highly selective radioligands were compared: [(3)H]prucalopride, which preferentially labels the G-protein coupled fraction of receptors, and [(3)H]R116712, which labels the entire receptor population at subnanomolar concentrations. [(3)H]Prucalopride and [(3)H]R116712 binding was performed on human brain hemisphere sections. The highest densities for both radioligands were found in the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus, substantia nigra). Moderate to low densities were detected in the hippocampal formation and in the cortical mantle. Mismatches between 5-HT(4) receptor mRNA and binding sites in the globus pallidus and the substantia nigra suggested that the binding sites may be localized on axonal projections originating from the striatum. To compare densities of binding sites, concentration binding curves with [(3)H]prucalopride, [(3)H]R116712 and [(3)H]GR113808 were performed on membranes from homogenates of several human brain regions. Comparison of B(max)-values obtained with [(3)H]prucalopride and [(3)H]R116712 indicated that the G-protein coupled fraction of 5-HT(4) receptors in the substantia nigra was exceptionally high (54%) in comparison with percentages (16-27%) found in the frontal cortex, the striatum and the hippocampus. Such a high percentage (40%) of [(3)H]prucalopride vs. [(3)H]R116712 binding was also observed in the substantia nigra in the receptor autoradiography experiments. The [(3)H]prucalopride binding was GppNHp-sensitive, whereas [(3)H]R116712 and [(3)H]GR113808 was not. These data indicate that in the substantia nigra 5-HT(4) receptors are more strongly coupled to their signal transduction pathway than in other brain regions.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

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

Structure of the human serotonin 5-HT4 receptor gene and cloning of a novel 5-HT4 splice variant

Several variants of the serotonin 5-HT4 receptor are known to be produced by alternative splicing. To survey the existence and usage of exons in humans, we cloned the human 5-HT4 gene. Based on sequence analysis seven C-terminal variants (a-g) and one internal splice variant (h) were found. We concentrated in this study on the functional characterization of the novel splice variant h, which leads to the insertion of 14 amino acids into the second extracellular loop of the receptor. The h variant was cloned as a splice combination with the C-terminal b variant; therefore, we call this receptor 5-HT4(hb). This novel receptor variant was expressed transiently in COS-7 cells, and its pharmacological profile was compared with those of the previously cloned 5-HT4(a) and 5-HT4(b) isoforms, with the latter being the primary reference for the h variant. In competition binding experiments using reference 5-HT4 ligands, no significant differences were detected. However, the broadly used 5-HT4 antagonist GR113808 discriminated functionally among the receptor variants investigated. As expected, it was an antagonist on the 5-HT4(a) and 5-HT4(b) variant but showed partial agonistic activity on the 5-HT4(hb) variant. These data emphasize the importance of variations introduced by splicing for receptor pharmacology and may help in the understanding of conflicting results seen with 5-HT4 ligands in different model systems.

A review of central 5-HT receptors and their function

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

Expression of muscarinic m2 receptor mRNA in dorsal root ganglia of neonatal rat

The expression of mRNA coding for m2 subtype of muscarinic cholinergic receptors was assessed in dorsal root ganglia (DRG) of 15-day post-natal rats. Northern blot analysis on total RNA using a mixture of two different oligonucleotide probes, indicated the presence of a single prominent band of approximately 6.5 kb in rat DRG; a band of the same size was observed both in brainstem and cortex taken as positive controls. Analysis by RT-PCR of the mRNA coding for a region of the third cytoplasmic loop of m2 receptor showed a single signal both in rat DRG and hippocampus. In situ hybridization was then used to identify the neuronal subpopulations expressing the mRNA for M2. The transcripts were preferentially localized in medium-small neurons of the ganglion as well as in satellite cells surrounding the neuron cell body. Large neurons were usually negative. Finally, competition binding experiments, performed in the presence of [3H]-quinuclidinyl benzilate (QNB) and methoctramine (a selective competitor for M2 receptors), demonstrated the presence of M2 receptor protein (Ki=100 nM), as previously observed in chick DRG. The preferential localization of M2 in medium-small neurons of the ganglion suggests the involvement of this receptor subtype in the transduction of nociceptive stimuli.

Identification of a human 5-HT6 receptor variant produced by alternative splicing

The complexity of the 5-hydroxytryptamine (5-HT) (serotonin) receptor family has been increased by the findings that isoforms or splice variants exist for subtypes such as the 5-HT2B, 5-HT2C, 5-HT4 and 5-HT7 subtypes. Further molecular biological studies in our laboratory have demonstrated that a splice variant of the 5-HT6 receptor exists in the human brain. Experiments performed using a degenerate PCR approach from human caudate cDNA revealed a 5-HT6 receptor clone with a 289 bp deletion of the region coding for transmembrane IV through the third intracellular loop. This deletion produces a frameshift creating a downstream stop codon which results in a truncated protein containing 10 unique amino acids at its carboxyl end. The variant transcript occurs as a result of alternative splicing using an upstream donor site and the acceptor site from the first intron in the 5-HT6 receptor gene. The splicing pattern seen for this transcript was not detected in rat or mouse whole brain cDNA by PCR due to the lack of a consensus 5' donor site. Coexpression of the variant 5-HT6 transcript and the full length 5-HT6 transcript was observed in caudate and substantia nigra but not in hippocampus, cortex, cerebellum and thalamus. Transient transfection of a 5-HT6 variant construct into Cos-7 cells demonstrated that a truncated receptor was translocated to the membrane but appeared nonfunctional.

5-HT4 receptors: gene, transduction and effects on olfactory memory

In this paper we discuss 1) the primary structures, pharmacology, and brain distribution of cloned 5-HT4 receptors; 2) the chromosomal localization of the h5-HT4 receptor; 3) whether benzamides are full or partial agonists because of a species or a coupling difference; 4) the intrinsic activity of 5-HT4 receptors and inverse agonism of GR125487 in COS-7 cells but not in colliculi neurons; 5) the modulation of 5-HT4 receptor binding and activity; and 6) the long-term blockade of K+ channels by 5-HT4 agonists and its effect on olfactory memory. We conclude that 1) the cloning of 5-HT4 receptors in different species using RT-PCR from different tissues reveals the presence of several splice variants for 5-HT4 receptors differing in the C-terminal part, downstream from the amino acid L358; 2) the pharmacological properties of 5-HT4 receptors are dependent on the cellular context in which they are expressed; and 3) 5-HT4 agonists can be added to the list of compounds having pro-cognitive properties.

Regulation of membrane excitability in the central nervous system by serotonin receptor subtypes

Serotonin exerts multiple electrophysiological effects on neurons of the central nervous system. It is now known that this diversity reflects at least in part the existence of multiple serotonin receptor subtypes. An example of this occurs in the CA1 region of the hippocampus where as many as ten different serotonin receptor subtypes appear to be expressed. Recent electrophysiological studies have been able to assign specific functional roles to at least 5 of these receptors. These receptors are differentially expressed in the two different cell types present in this region, pyramidal cells and GABAergic interneurons, and mediate different effects on membrane excitability. This distribution is consistent with the different functional roles played by these cells in hippocampus. Thus the differential expression of serotonin receptor subtypes in the CA1 region allows serotonin to modify the function of hippocampal neuronal networks in a manner that is both selective and precise.

Gs protein-coupled serotonin receptors: receptor isoforms and functional differences

Three distinct mammalian Gs coupled serotonin receptor genes have been identified, 5-HT4, 5-ht6, and 5-HT7, which produce at least seven different functional receptors through alternative splicing. One of the chief questions facing workers in this area mirrors that confronting the serotonin receptor field as a whole: why so many subtypes? The answer to this question is made more elusive at present by two further considerations. First, there may well be additional Gs coupled receptor subtypes yet to be described. Secondly, although the various isoforms of 5-HT4 and 5-HT7 have been shown to be functional in in vitro assays, it remains to be shown that all isoforms have biological significance. This paper will summarize some of the differences at the molecular and cellular level that are becoming apparent among the 5-HT4, 5-ht6 and 5-HT7 receptor subtypes and their various isoforms. As an example, it will focus on the 5-HT7 system, and describe recent developments in ascribing particular functions to differences due to alternative splicing.

5-HT4 receptors: cloning and expression of new splice variants

On the basis of differences in the potencies and intrinsic activity of 5-HT4 receptor agonists in different biological models it has been suggested that there is heterogeneity among 5-HT4 receptors. Here, we report the molecular cloning of several 5-HT4 receptor splice variants in mouse, rat, and human brain. Our data suggest that the differences in efficacy of 5-HT4 ligands on 5-HT4 receptor-mediated responses in several tissues is due to differences in coupling efficiency rather than to the presence of different 5-HT4 receptor isoforms.

Function and distribution of three rat 5-hydroxytryptamine7 (5-HT7) receptor isoforms produced by alternative splicing

Serotonin (5-HT7) receptor pre-mRNA is alternatively spliced in rat tissue to produce three isoforms, 5-HT(7a), 5-HT(7b) and 5-HT(7c), which differ in the amino acid sequences of their carboxyl terminal tails. Substantial species differences in structure and expression patterns exist for 5-HT7 isoforms. We have now compared some of the functional characteristics and level of expression for the three rat 5-HT7 receptor isoforms. Recombinant receptor isoforms were expressed in COS-7 cells for examination of [3H]5-HT binding characteristics and in JEG-3 cells to ascertain their ability to stimulate cAMP production. These studies showed that all three isoforms are functionally active and have similar agonist binding characteristics. Distribution of expression of the three rat receptor isoforms were examined in several brain regions and peripheral tissues using RT-PCR and in situ hybridization. The relative proportions of total 5-HT7 receptor message lent by each isoform varied little between these areas. In contrast to what has been observed in human tissue, the 5-HT(7a) isoform predominated in all regions examined, while the 5-HT(7c) isoform revealed a low level of expression (3% of total transcript). In situ hybridization was used to determine if the overall low level of expression of the 5-HT(7c) isoform by RT-PCR could be attributed to a small localized subpopulation of cells expressing high levels 5-HT(7c) message. In situ hybridization results indicate a generalized low level of expression of the 5-HT(7c) isoform throughout the CNS. These data suggest that while all three known 5-HT7 receptor isoforms in the rat are functionally competent, any functionally important differences between the three isoforms are not likely to involve differences in ligand binding or gross differences in adenylate cyclase coupling. However, differences in receptor phosphorylation, regulation or coupling to other effectors or cell trafficking could still exist.

The 5-HT1A and 5-HT2A/2C receptor antagonists WAY-100635 and ritanserin do not attenuate D-fenfluramine-induced fos expression in the brain

D-Fenfluramine is a serotonin (5-hydroxytryptamine, 5-HT) releaser and reuptake inhibitor. It is used to study the neurochemical control of feeding and has been used to treat obesity. It has also been employed as a pharmacological tool to study changes in serotonergic function in psychiatric patients. Brain sites activated by D-fenfluramine via the release of 5-HT have been mapped via the expression of the immediate early gene c-fos. Studies in our laboratory have indicated that D-fenfluramine induces Fos in the hypothalamus and cortex through 5-HT release. The present study investigated whether 5-HT released by D-fenfluramine induces Fos expression in the brain by activating 5-HT1A or 5-HT2A/2C receptors. Rats were pretreated either with WAY-100635, a 5-HT1A antagonist, or ritanserin, a 5-HT2A/2C antagonist, prior to d-fenfluramine injection. Blockade of either 5-HT1A or 5-HT2A/2C receptors was not sufficient to suppress the Fos response to D-fenfluramine in any region of the brain examined, including the cingulate cortex, frontal cortex, caudate-putamen, paraventricular nucleus of the hypothalamus, amygdala, and brainstem. These results indicate that Fos response elicited by D-fenfluramine may be mediated by other receptors, in addition to the 5-HT1A or 5-HT2A/2C receptors.

5-HT receptors in mammalian brain: receptor autoradiography and in situ hybridization studies of new ligands and newly identified receptors

In recent years the family of mammalian serotonin receptors has grown to 14 different subtypes, characterized by pharmacological or molecular biological techniques. In parallel, new ligand molecules have been developed for their study. However, selective ligands are not yet available to study every one of them. In addition the degree of selectivity of ligands, hitherto regarded as specific for a particular receptor subtype has been called in question by their affinities for newly discovered receptors. Consequently, a re-evaluation of past ligand receptor autoradiography work is necessary in view of the redefined receptor profiles of these ligands, and the introduction of newly developed ligands. A further difficulty for the characterization of these receptors is the absence of selective antagonist ligands which, for some of the subtypes, have become available only recently. In an attempt to overcome these difficulties we have combined in situ hybridization histochemistry and receptor ligand autoradiography to study the regional and cellular localization of several serotonin receptors in the rodent brain. In addition, for some receptors, we have expanded these studies to primates, including humans. We have found that the distribution of 5-HT1A receptors in monkey brain, labelled with the agonist 3H-8-OH-DPAT and the antagonist 3H-WAY 100635 was very similar at the levels examined, and corresponded well with that observed for the cells containing mRNA coding for this receptor, confirming the somatodendritic localization of 5-HT1A receptors in monkey brain. The labelling conditions to visualize 5-HT1F receptors in guinea pig brain, namely 3H-sumatriptan in the presence of 10(-8) M 5-CT to block 5-HT1D receptors, are suitable for visualizing this receptor, since the results agreed with those observed by in situ hybridization. By using 3H-ketanserin and 3H-mesulergine in parallel with in situ hybridization using the corresponding oligonucleotides, we were able to show that these ligands label respectively 5-HT2A and 5-HT2C binding sites in monkey brain. 5-HT4 receptors were localized in the brain of several species including humans by using 125I-SB 207710. In situ hybridization experiments performed in guinea pig confirmed that 5-HT4 receptors are localized on the terminals of the striatopallidal and striatonigral projections. 5-HT7 binding sites were labelled in rat and guinea pig brains by incubating with 3H-5-CT in the presence of 100 microM WAY 100135 and 250 microM GR 127935; the distribution obtained in both species agreed, in general, with that of the corresponding mRNA coding for them. These results are an illustration of the understanding of our current knowledge of the chemical neuroanatomy of the mammalian 5-HT system.