Synthesis and evaluation of [125I]-(S)-iodozacopride, a high affinity radioligand for 5HT3 receptors

Stereological investigation of 5-HT3 receptors in the substantia nigra and dorsal raphe nucleus in the rat

Serotonin (5-HT) is a common neurotransmitter in mammals, playing a central role in the regulation of various processes such as sleep, perception, cognitive and autonomic functions in the nervous system. Previous studies have demonstrated that 5-HT type 3 (5-HT₃) receptors are expressed in either or both the substantia nigra (SN) and the dorsal raphe nucleus (DRN) in humans, marmosets, rats and Syrian hamsters. Here, we quantify the distribution of 5-HT₃ receptors across these regions in the adult rat. Fluorescent immunohistochemistry was performed on sections of rat brain covering the entire rostro-caudal extent of the SN and DRN with antibodies specific to the 5-HT_3A receptor subunit, as well as others targeting the monoaminergic markers tyrosine hydroxylase (TH) and the 5-HT transporter (SERT). The number of 5-HT_3A receptor-positive, TH-positive (n = 28,428 ± 888, Gundersen's m = 1 coefficient of error [CE] = 0.05) and SERT-positive (n = 12,852 ± 462, CE = 0.06) cells were estimated in both the SN and the DRN using stereology. We found that 5-HT_3A receptor-positive cells are present in the SNr (n = 1250 ± 64, CE = 0.24), but they did not co-localise with TH-positive cells, nor were they present in the SNc. In contrast, no 5-HT_3A receptor-positive cells were found in the DRN. These results support the presence of 5-HT₃ receptors in the SN, but not in the DRN, and do not support their expression on monoaminergic cells within these two brain areas.

Building a 5-HT3A Receptor Expression Map in the Mouse Brain

Of the many serotonin receptors, the type 3 receptors (5-HT3R) are the only ionotropic ones, playing a key role in fast synaptic transmission and cognitive and emotional brain function through controlled neuronal excitation. To better understand the various functions of 5-HT3Rs, it is very important to know their expression pattern in the central nervous system (CNS). To date, many distributional studies have shown localized 5-HT3R expression in the brain and spinal cord. However, an accurate pattern of 5-HT3R expression in the CNS remains to be elucidated. To investigate the distribution of 5-HT3R in the mouse brain in detail, we performed immunofluorescent staining using 5-HT3AR-GFP transgenic mice. We found strong 5-HT3AR expression in the olfactory bulb, cerebral cortex, hippocampus, and amygdala; and partial expression in the pons, medulla, and spinal cord. Meanwhile, the thalamus, hypothalamus, and midbrain exhibited a few 5-HT3AR-expressing cells, and no expression was detected in the cerebellum. Further, double-immunostaining using neural markers confirmed that 5-HT3AR is expressed in GABAergic interneurons containing somatostatin or calretinin. In the present study, we built a 5-HT3AR expression map in the mouse brain. Our findings make significant contributions in elucidating the novel functions of 5-HT3R in the CNS.

Neurochemical Markers in the Mammalian Brain: Structure, Roles in Synaptic Communication, and Pharmacological Relevance

BACKGROUND

Knowledge of molecular marker (typically protein or mRNA) expression in neural systems can provide insight to the chemical blueprint of signal processing and transmission, assist in tracking developmental or pathological progressions, and yield key information regarding potential medicinal targets. These markers are particularly relevant in the mammalian brain in the light of its unsurpassed cellular diversity. Accordingly, molecular expression profiling is rapidly becoming a major approach to classify neuron types. Despite a profusion of research, however, the biological functions of molecular markers commonly used to distinguish neuron types remain incompletely understood. Furthermore, most molecular markers of mammalian neuron types are also present in other organs, therefore complicating considerations of their potential pharmacological interactions.

OBJECTIVE

Here, we survey 15 prominent neurochemical markers from five categories, namely membrane transporters, calcium-binding proteins, neuropeptides, receptors, and extracellular matrix proteins, explaining their relation and relevance to synaptic communication.

METHOD

For each marker, we summarize fundamental structural features, cellular functionality, distributions within and outside the brain, as well as known drug effectors and mechanisms of action.

CONCLUSION

This essential primer thus links together the cellular complexity of the brain, the chemical properties of key molecular players in neurotransmission, and possible biomedical opportunities.

Interaction between the 5-HT system and the basal ganglia: functional implication and therapeutic perspective in Parkinson's disease

The neurotransmitter serotonin (5-HT) has a multifaceted function in the modulation of information processing through the activation of multiple receptor families, including G-protein-coupled receptor subtypes (5-HT1, 5-HT2, 5-HT4-7) and ligand-gated ion channels (5-HT3). The largest population of serotonergic neurons is located in the midbrain, specifically in the raphe nuclei. Although the medial and dorsal raphe nucleus (DRN) share common projecting areas, in the basal ganglia (BG) nuclei serotonergic innervations come mainly from the DRN. The BG are a highly organized network of subcortical nuclei composed of the striatum (caudate and putamen), subthalamic nucleus (STN), internal and external globus pallidus (or entopeduncular nucleus in rodents, GPi/EP and GPe) and substantia nigra (pars compacta, SNc, and pars reticulata, SNr). The BG are part of the cortico-BG-thalamic circuits, which play a role in many functions like motor control, emotion, and cognition and are critically involved in diseases such as Parkinson's disease (PD). This review provides an overview of serotonergic modulation of the BG at the functional level and a discussion of how this interaction may be relevant to treating PD and the motor complications induced by chronic treatment with L-DOPA.

Central 5-HT3 receptor-induced hypothermia is associated with reduced metabolic rate and increased heat loss

Activation of central 5-HT(3) receptors by the selective agonist m-CPBG (1-(3-chlorophenyl)biguanide hydrochloride, 40 nM i.c.v.) produced stronger hypothermic effect in mice than activation of 5-HT(1A) receptors by their agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propilamino)tetralin) injected by the same route at an equimolar dose. The hypothermic effect of m-CPBG was realized by influence on both the heat production and the heat loss: oxygen consumption and CO(2) expiration were decreased; heat dissipation determined by the tail skin temperature was increased. The heat loss effect of 5-HT(3) receptors was significantly shorter than the decrease in metabolism indicating the prevalent role of heat production decrease in 5-HT(3) receptor-induced deep and long-lasing hypothermia. In addition, the decrease in the respiratory exchange ratio (RER) was shown suggesting that the activation of the 5-HT(3) receptors switched metabolism to prevalent use of lipids as the main energetic substrate. 5-HT(1A) receptor agonist 8-OH-DPAT (40 nM i.c.v.) produced less depressing effect on general metabolism: a decrease in oxygen consumption and CO(2) excretion began later and was not so deep as after m-CPBG administration. Heat-loss effect of 5-HT(1A) receptors activation was not observed. In contrast to m-CPBG effect, RER after 5-HT(1A) receptors activation raised immediately after injection and then gradually decreased to the values observed in m-CPBG-treated mice. Obtained results show that activation of central 5-HT(3) receptors are more effective in hypothermia induction due to marked decrease in thermogenesis and increase in heat loss.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Receptor occupancy theory-based analysis of interindividual differences in antiemetic effects of 5-HT3 receptor antagonists

BACKGROUND

The aim of this study was to estimate interindividual differences in the antiemetic effects of 5-HT(3) receptor antagonists by evaluating the influence of pharmacokinetics on 5-HT(3) receptor occupancies, based on receptor occupancy theory.

METHODS

We analyzed interindividual differences of 5-HT(3) receptor occupancies and antiemetic effects after the oral and/or intravenous administration of standard doses of the following 5-HT(3) receptor antagonists: azasetron, granisetron, indisetron, ondansetron, ramosetron, and tropisetron.

RESULTS

The interindividual difference between maximum and minimum 5-HT(3) receptor occupancies after oral administration ranged from 0.6% to 64.0%, and that difference after intravenous administration ranged from 0.6% to 29.6%. Following oral administration, the interindividual difference between maximum and minimum complete vomiting inhibition rates ranged from 0.2% to 16.1%. After intravenous administration, that difference ranged from 0.8% to 52.5%.

CONCLUSION

Interindividual differences in the clinical effects of 5-HT(3) receptor antagonists could be evaluated based on receptor occupancy theory, and the differences varied among drugs. Drug selection considering these individual variations might be useful for the patients who experienced vomiting associated with chemotherapy.

Serotonin type 3 receptors modulate the aggression-stimulating effects of adolescent cocaine exposure in Syrian hamsters (Mesocricetus auratus)

Repeated cocaine (0.5 mg/kg) exposure throughout adolescence stimulates offensive aggression in hamsters. These studies examined whether the cocaine-induced aggressive response was regulated by serotonin Type 3 (5-HT(3)) receptor activity and correlated with altered 5-HT(3) receptor expression. Cocaine-treated Syrian hamsters (Mesocricetus auratus) were tested for aggression after the administration of either the 5-HT(3) antagonist 3-tropanylindole-3-carboxylate methiodide (tropisetron; 0.01-1.20 mg/kg) or the 5-HT(3) agonist l-(m-chlorophenyl)-biguanide hydrochloride (mCPBG; 5.0-15.0 mg/kg), alone or in combination. Tropisetron alone dose dependently reduced cocaine-induced aggression, with a significant reduction at 0.3 mg/kg, whereas mCPBG was ineffective. mCPBG administered prior to tropisetron required a higher dose (1.2 mg/kg) of antagonist to block aggression, indicating a selective 5-HT(3) effect. Cocaine-treated hamsters showed altered 5-HT-sub-3 immunoreactivity in several brain areas implicated in aggression control. These data support a role for 5-HT(3) receptors in adolescent cocaine-induced aggression.

Immunolabeling of the rat central nervous system with antibodies partially selective of the short form of the 5-HT3 receptor

Polyclonal antibodies were raised against a synthetic hexadecapeptide corresponding to the portion of the second intracytoplasmic loop of the short form of the mouse 5-hydroxytryptamine-3A receptor subunit (5-HT3A-S), which differs from the long form (5-HT3A-L) by the removal of six amino acids. Antibodies were detected by enzyme-linked immunosorbent assay as soon as two months after the first injection to rabbits of the peptide coupled to keyhole limpet hemocyanin. Immunoblot detection of fusion proteins comprising glutathione-S-transferase and the second intracellular loop of 5-HT3A-S or 5-HT3A-L, and immunoprecipitation of cloned receptors showed that antibodies exhibited some selectivity for the short variant. Affinity chromatography allowed the purification of selective anti-5-HT3A-S antibodies which yielded a strong positive labeling of plasma membrane, reticulum and Golgi apparatus of COS-7 cells expressing murine 5-HT3A-S. In contrast, COS-7 cells expressing similar levels of 5-HT3A-L exhibited only a very weak labeling. Selectivity was also observed on immunoblots of cloned receptors transiently expressed in COS-7 cells, or stably expressed in CHO cells, both systems showing an immunolabeled component at 53,000-54,000 mol. wt. Immunoautoradiographic labeling of central nervous system sections showed that 5-HT3A-S-like immunoreactivity was found mostly within the nucleus of the solitary tract, the nucleus of the spinal tract of the trigeminal nerve, and the dorsal horn of the the spinal cord in the rat. After unilateral ablation of the nodose ganglion, 5-HT3A-S-like immunoreactivity decreased markedly in the ipsilateral part of the nucleus of the solitary tract, as expected of the presynaptic localization of 5-HT3 receptors. Finally, immunohistochemistry at the light and electron microscope levels revealed that 5-HT3A-S-like immunoreactivity was associated essentially with terminals and axonal profiles. All these results demonstrate that the immunolabeling exhibited by these antibodies is consistent with a specific and partially selective recognition of the short isoform of the 5-HT3A subunit. Because the pattern of immunoautoradiographic labeling matches the distribution previously established with selective radioligands, it can be inferred that these antibodies probably recognized the same fully assembled form of the 5-HT3A-S receptor subunit.

[125I]S(-)-zacopride labels a novel 5-hydroxytryptamine sensitive recognition site in rat duodenum and ileum

Autoradiographic binding studies using [125I]S(-)-zacopride (0.1 nM) identified non-5-HT3 specific binding sites (defined by 5-hydroxytryptamine (5-HT), 1.0 microM) in the rat duodenum and ileum and some other peripheral tissues (adrenal gland, liver, stomach, kidney and spleen). In the rat duodenum and ileum, saturation studies with [125I]S(-)-zacopride indicated that the specific binding was saturable and of high affinity to an apparently homogenous population of binding sites (duodenum Bmax = 1.88 fmol/mg, Kd = 0.078 nM; ileum Bmax = 1.60 fmol/mg, Kd = 0.071 nM). Competition studies with slices of either duodenum or ileum indicated that the pharmacology of the [125I]S(-)-zacopride recognition site in both tissues was comparable but differed from all 5-HT receptors and uptake sites reported to date. However, the [125I]S(-)-zacopride recognition site displayed some pharmacological and regional similarity to the 5-HT1P recognition site: The sensitivity of the [125I]S(-)-zacopride binding in the duodenum and ileum to GTP indicates that the radiolabelled recognition site may represent a functional G-protein coupled receptor.