Autoradiography of [3H]quipazine in rodent brain

Imaging Technologies for Cerebral Pharmacokinetic Studies: Progress and Perspectives

Pharmacokinetic assessment of drug disposition processes in vivo is critical in predicting pharmacodynamics and toxicology to reduce the risk of inappropriate drug development. The blood–brain barrier (BBB), a special physiological structure in brain tissue, hinders the entry of targeted drugs into the central nervous system (CNS), making the drug concentrations in target tissue correlate poorly with the blood drug concentrations. Additionally, once non-CNS drugs act directly on the fragile and important brain tissue, they may produce extra-therapeutic effects that may impair CNS function. Thus, an intracerebral pharmacokinetic study was developed to reflect the disposition and course of action of drugs following intracerebral absorption. Through an increasing understanding of the fine structure in the brain and the rapid development of analytical techniques, cerebral pharmacokinetic techniques have developed into non-invasive imaging techniques. Through non-invasive imaging techniques, molecules can be tracked and visualized in the entire BBB, visualizing how they enter the BBB, allowing quantitative tools to be combined with the imaging system to derive reliable pharmacokinetic profiles. The advent of imaging-based pharmacokinetic techniques in the brain has made the field of intracerebral pharmacokinetics more complete and reliable, paving the way for elucidating the dynamics of drug action in the brain and predicting its course. The paper reviews the development and application of imaging technologies for cerebral pharmacokinetic study, represented by optical imaging, radiographic autoradiography, radionuclide imaging and mass spectrometry imaging, and objectively evaluates the advantages and limitations of these methods for predicting the pharmacodynamic and toxic effects of drugs in brain tissues.

Neuronal signaling repertoire in the mammalian sperm functionality

The common embryonic origin has been a recurrent explanation to understand the presence of "neural receptors" in sperm. However, this designation has conditioned a bias marked by the classical neurotransmission model, dismissing the possibility that neurotransmitters can play specific roles in the sperm function by themselves. For instance, the launching of acrosome reaction, a fundamental sperm function, includes several steps that recall the process of presynaptic secretion. Unlike of postsynaptic neuron, whose activation is mediated by molecular interaction between neurotransmitter and postsynaptic receptors, the oocyte activation is not mediated by receptors, but by cytosolic translocation of sperm phospholipase (PLCζ). Thus, the sperm has a cellular design to access and activate the oocyte and restore the ploidy of the species by an "allogenic pronuclear fusion." At subcellular level, the events controlling sperm function, particularly the capacitation process, are activated by chemical signals that trigger ion fluxes, sterol oxidation, synthesis of cyclic adenosine monophosphate, protein kinase A activation, tyrosine phosphorylations and calcium signaling, which correspond to second messengers similar to those associated with exocytosis and growth cone guidance in neurons. Classically, the sperm function associated with neural signals has been analyzed as a unidimensional approach (single ligand-receptor effect). However, the in vivo sperm are exposed to multidimensional signaling context, for example, the GABAergic, monoaminergic, purinergic, cholinergic, and melatoninergic, to name a few. The aim of this review is to present an overview of sperm functionality associated with "neuronal signaling" and possible cellular and molecular mechanisms involved in their regulation.

Presynaptic control of serotonin on striatal dopamine function

RATIONALE

The influences of the serotonergic system on dopamine (DA) neuron activity have received considerable attention during the last three decades due to the real opportunity to improve disorders related to central DA neuron dysfunctions such as Parkinson's disease, schizophrenia, or drug abuse with serotonergic drugs. Numerous biochemical and behavioral data indicate that serotonin (5-HT) affects dopaminergic terminal function in the striatum.

OBJECTIVE

The authors propose a thorough examination of data showing controversial effects induced by striatal 5-HT on dopaminergic activity.

RESULTS

Inhibitory and excitatory effects of exogenous 5-HT have been reported on DA release and synthesis, involving various striatal 5-HT receptors. 5-HT also promotes an efflux of DA through reversal of the direction of DA transport. By analogy with the mechanism of action described for amphetamine, the consequences of 5-HT entering DA terminals might explain both the excitatory and inhibitory effects of 5-HT on presynaptic DA terminal activity, but the physiological relevance of this mechanism is far from clear. The recent data suggest that the endogenous 5-HT system affects striatal DA release in a state-dependent manner associated with the conditional involvement of various 5-HT receptors such as 5-HT(2A), 5-HT(2C), 5-HT(3), and 5-HT(4) receptors.

CONCLUSION

Methodological and pharmacological issues have prevented a comprehensive overview of the influence of 5-HT on striatal DA activity. The distribution of striatal 5-HT receptors and their restricted influence on DA neuron activity suggest that the endogenous 5-HT system exerts multiple and subtle influences on DA-mediated behaviors.

The auspicious role of the 5-HT3 receptor in depression: a probable neuronal target?

The serotonergic mechanisms have been successfully utilized by the majority of antidepressant drug discovery programmes, while the search for newer targets remains persistent. The present review focused on the serotonin type-3 receptor, the only ion channel subtype in the serotonin family. Behavioural, neurochemical, electrophysiological and molecular analyses, including the results from our laboratory, provided substantial evidence that rationalizes the correlation between serotonin type-3 receptor modulation and rodent depressive-like behaviour. Nevertheless, the reports on polymorphism of serotonin type-3 receptor genes and data from clinical studies (on serotonin type-3 receptor antagonists) were insufficient to corroborate the involvement of this receptor in the neurobiology of depression. The preclinical and clinical studies that have contradicted the antidepressant-like effects of serotonin type-3 receptor antagonists and the reasons underlying such disagreement were discussed. Finally, this critical review commended the serotonin type-3 receptor as a candidate neuronal antidepressant drug target.

Long-term administration of monoamine oxidase inhibitors alters the firing rate and pattern of dopamine neurons in the ventral tegmental area

Monoamine oxidase inhibitors (MAOIs) exert their antidepressant action by increasing the function of the serotonin (5-HT), norepinephrine and dopamine (DA) systems. There is, however, limited electrophysiological data on the effects of MAOIs on DA neurons. The effects of 2-d and 21-d administration of three MAOIs were investigated (clorgyline, selective MAOI-A; deprenyl, selective MAOI-B; phenelzine, non-selective MAOI) on the firing activity of DA neurons in the ventral tegmental area using in-vivo electrophysiology in rats. Short-term clorgyline (1 mg/kg) and phenelzine (2.5 mg/kg) was devoid of effect on DA neurons, whereas prolonged administration significantly decreased their firing rate (by 30% and 20%, respectively), number of bursts (by 80% and 45%, respectively), and percentage of spikes occurring in bursts only in clorgyline-treated rats (70%). Deprenyl (0.25 mg/kg) was without effects. DA firing was restored in clorgyline-treated rats by inhibiting 5-HT synthesis using para-chlorophenylalanine (p-CPA; 300 mg/kg. d for three consecutive days). The 5-HT3 antagonist ondansetron (0.5 mg/kg) was devoid of effect in control rats, but completely reversed the alterations of DA neuronal activity in clorgyline-treated rats. An attenuation of DA neuronal activity was thus produced by prolonged blockade of MAOA activity. The absence of effect of MAOA inhibition after subacute administration suggested an indirect mechanism. This was confirmed by the observation that p-CPA antagonized the effects of clorgyline. Since ondansetron completely reversed the effects of clorgyline on DA neuronal activity, the effects of MAOA inhibition appeared to be mediated by 5-HT3 receptors.

The 5-HT3 receptor antagonist, ondansetron, blocks the development and expression of ethanol-induced locomotor sensitization in mice

Manipulation of the serotonergic system has been shown to alter ethanol sensitization. Ondansetron is a 5-HT3 receptor antagonist, reported to attenuate cocaine and methamphetamine-induced behavioral sensitization, but no reports are available on its role in ethanol-induced behavioral sensitization. Therefore, an attempt has been made to assess this issue by using an earlier used animal model of ethanol-induced locomotor sensitization. Results indicated that ondansetron (0.25-1.0 mg/kg, subcutaneously) given before the challenge dose of ethanol (2.4 g/kg, intraperitoneally) injection, significantly and dose dependently attenuated the expression of sensitization. In addition, ondansetron (1.0 mg/kg, subcutaneously) given before ethanol injection on days 1, 4, 7, and 10 significantly blocked the development (days 1, 4, 7, and 10), and expression (day 15) of sensitization to the locomotor stimulant effect of ethanol injection. Ondansetron had no effect per se on locomotor activity and did not affect blood ethanol levels. Therefore, the results raise the possibility that ondansetron blocked the development and expression of ethanol-induced locomotor sensitization by acting on 5-HT3 receptors.

A Potent Serotonin-Modulating Compound AP-267 Attenuates Morphine Withdrawal-Induced Blood-Brain Barrier Dysfunction in Rats

The possibility that a serotonin 5-HT2c receptor-modulating compound, AP-267, will influence spontaneous morphine withdrawal symptoms and the alterations in the brain fluid microenvironment was examined in a rat model. Daily administration of morphine (10 mg/kg, i.p.) for 10 days resulted in dependence of rats as seen by loss of analgesic response. On the 11th day, no morphine administration was given. This resulted in profound withdrawal symptoms 24 h after morphine withdrawal. The magnitude and severity of these symptoms were increased further 48 h after withdrawal. Measurement of the blood-brain barrier (BBB) permeability, a measure of perturbed brain fluid microenvironment showed leakage of Evans blue and radioiodine tracers in several parts of the brain in rats showing withdrawal symptoms. Whereas, rats treated with AP-267 either on the 1st day or 2nd day morphine withdrawal showed much less symptoms and leakage of the BBB. Taken together, these observations suggest that (a) stress associated with the withdrawal symptoms are sufficient enough to induce breakdown of the BBB function, and (b) modulation of serotonin 5-HT2c receptors may have some protective influence on the stress symptoms and the BBB disruption.

Regionally and functionally distinct serotonin3 receptors control in vivo dopamine outflow in the rat nucleus accumbens

Central serotonin(3) (5-HT(3)) receptors control the mesoaccumbens dopamine (DA) pathway. This control is thought to be conditional and might involve regionally distinct subpopulations of 5-HT(3) receptors. Here, using in vivo microdialysis in rats, we assessed the relative contribution of nucleus accumbens (Nacc) 5-HT(3) receptors to the overall influence exerted by 5-HT(3) receptors on accumbal DA release induced by different drugs or treatments. In freely moving rats, pre-treatment with 5-HT(3) antagonists (0.1 mg/kg ondansetron and/or 0.03 mg/kg MDL 72222, s.c.) reduced DA efflux enhanced by morphine (1-10 mg/kg, s.c.) and haloperidol (0.01 mg/kg, s.c.), but not amphetamine (1-2.5 mg/kg, i.p.) or cocaine (10-20 mg/kg, i.p.), the latter two drugs do not trigger depolarization-stimulated DA exocytosis. Intra-Nacc administration of ondansetron (1 microm) in freely moving rats reduced the DA effects elicited by 10 mg/kg morphine, but not 1 mg/kg morphine or haloperidol. The 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg, s.c.), known to decrease central 5-HT tone, reduced 10 but not 1 mg/kg morphine-stimulated DA outflow in freely moving rats. In halothane-anaesthetized rats, intra-Nacc ondansetron (1 microm) application reduced dorsal raphe nucleus electrical stimulation (20Hz)-induced DA outflow. Our results show that regionally distinct populations of 5-HT(3) receptors control the depolarization-dependent exocytosis of DA and suggest that the involvement of Nacc 5-HT(3) receptors occurs only when central DA and 5-HT tones are concomitantly increased.

The sperm, a neuron with a tail: 'neuronal' receptors in mammalian sperm

A number of plasma membrane receptor types originally thought to be specific to neurons have been found in other somatic cells. More surprisingly, the mammalian sperm and neuron appear to share many of these 'neuronal' receptors. The morphology, chromosome number, genomic activity, and functions of those two cell types are as unlike as any two cells in the body, but they both achieve their highly disparate goals with the aid of a number of the same receptors. Exocytosis in neurons and sperm is essential to the functions of these cells and is strongly influenced by similar receptors. 'Neuronal' receptor types in sperm may also play a role in the control of sperm motility (a function of course not shared by neurons). This review will consider the evidence for the presence of sperm plasma membrane 'neuronal' receptors and for their significance to mammalian sperm function. The persuasiveness of the evidence varies depending on the receptor being considered, but there is strong experimental support for the presence and importance of a number of 'neuronal' receptors in sperm.

Role of the serotonergic system in the neurobiology of alcoholism: implications for treatment

Preclinical studies have contributed greatly to our understanding of the neurochemical pathways associated with the development and maintenance of alcohol-seeking behaviour. These studies have demonstrated the important role of serotonin pathways, particularly as they relate to dopaminergic function, which mediates alcohol-induced reward associated with its abuse liability. Naturally, this has led to the study of serotonergic agents as treatments for alcoholism.SSRIs do not appear to be effective treatment for a heterogeneous alcoholic group. However, they may be useful as treatment for late-onset alcoholics, or alcoholism complicated by comorbid major depression. Buspirone, a serotonin 5-HT1A partial agonist, does not appear to be an effective treatment for alcoholics without comorbid disease. Buspirone may, however, have some utility for treating alcoholics with comorbid anxiety disorder. The 5-HT2 antagonist ritanserin, at pharmacologically relevant clinical doses, does not appear to be an effective treatment for alcoholism. Ondansetron, a 5-HT3 antagonist, is an efficacious and promising medication for the treatment of early-onset alcoholism. Preliminary evidence suggests that combining the mu antagonist naltrexone with the 5-HT3 antagonist ondansetron promises to be more effective for treating alcoholism than either alone. The differential treatment effect of SSRIs and ondansetron among various subtypes of alcoholic is intriguing. Future research is needed to understand more clearly the molecular genetic differences and the interactions of such differences with the environment that typify a particular alcoholic subtype. Such an understanding could enable us to make comfortable predictions as to which alcoholic subtype might respond best to a particular serotonergic agent, which could then be provided.

Distribution of [3H]GR65630 binding in human brain postmortem

We investigated the distribution of serotonin (5-HT) receptors of type 3 (5-HT3) in human brain areas, by means of the the specific binding of [3H]GR65630. The brains were obtained during autoptic sessions from 6 subjects. Human brain membranes and the binding of [3H]GR65630 were carried out according to standardized methods. The highest density (Bmax +/- SD, fmol/mg protein) of [3H]GR65630 binding sites was found in area postrema (13.1+/-9.7), followed at a statistically lower level, by nucleus tractus solitarius (6.7+/-3.4), nervus vagus (5.5+/-2.1), striatum (4.8+/-2.4) with a progressive decrease in amygdala, olivar nuclei, hippocampus, olfactory bulbus and prefrontal cortex, and then by the other cortical areas and the cerebellum, where no binding was detected. These observations extend previous findings on the distribution of 5-HT3 receptors and confirm interspecies variations that might explain the heterogeneous properties of 5-HT3 receptors in different animals.

Roles of 5-HT3 and opioid receptors in the ethanol-induced place preference in rats exposed to conditioned fear stress

The effect of the selective 5-HT3 receptor antagonist ondansetron on the ethanol-induced place preference in rats exposed to conditioned fear stress, which stimulates the release of endogenous opioid peptides (beta-endorphin and enkephalins), was investigated using the conditioned place preference paradigm. In addition, we also examined the effect of ondansetron on the ethanol-induced place preference enhanced by the administration of mu- and delta-opioid receptor agonists (exogenous opioids). The administration of ethanol (300 mg/kg, i.p.) induced a significant place preference in rats exposed to conditioned fear stress. Pretreatment with ondansetron (0.01 and 0.1 mg/kg, s.c.) effectively attenuated this ethanol-induced place preference. When the mu-opioid receptor agonist morphine (0.1 mg/kg, s.c.) or the selective delta-opioid receptor agonist 2-methyl-4a(alpha)-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a(alpha)-octah ydroquinolino [2,3,3-g] isoquinoline (TAN-67; 20 mg/kg, s.c.) was administered in combination with 75 mg/kg ethanol (which tended to produce a place preference), the ethanol-induced place preference was significantly enhanced. The selective mu-opioid receptor antagonist beta-funaltrexamine at a dose of 10 mg/kg significantly attenuated the enhancement of the ethanol-induced place preference produced by morphine. Ondansetron (0.1 mg/kg, s.c.) also significantly attenuated the enhancement of the ethanol-induced place preference produced by morphine. Furthermore, the selective delta-opioid receptor antagonist naltrindole at a dose of 3 mg/kg significantly attenuated the enhancement of the ethanol-induced place preference produced by TAN-67. Ondansetron (0.1 mg/kg, s.c.) slightly, but significantly, attenuated the enhancement of the ethanol-induced place preference produced by TAN-67. These results suggest that 5-HT3 receptors may be involved in the rewarding mechanism of ethanol under psychological stress, and may play an important role in the rewarding effect of ethanol through the activation of mu- and delta-opioid receptors.

Opposite change of in vivo dopamine release in the rat nucleus accumbens and striatum that follows electrical stimulation of dorsal raphe nucleus: role of 5-HT3 receptors

In the present study we investigate, using in vivo microdialysis, the involvement of central 5-HT3 receptors in the effect of dorsal raphe nucleus (DRN) electrical stimulation on dopamine (DA), 3, 4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindole-3-acetic acid (5-HIAA) extracellular levels monitored in the nucleus accumbens and the striatum of halothane-anesthetized rats. DRN stimulation (300 microA, 1 msec at 3, 5, 10, and 20 Hz for 15 min) induced a frequency-dependent increase of accumbal DA release and a concomitant reduction of DA release in the ipsilateral striatum at 20 Hz. In both structures DOPAC and 5-HIAA dialysate contents were enhanced in a frequency-dependent manner. Central serotonin (5-HT) depletion, induced by intra-raphe injections of 5, 7-dihydroxytryptamine neurotoxin, abolished the effect of 20 Hz DRN stimulation on DA, DOPAC, and 5-HIAA extracellular levels in both regions. The 5-HT synthesis inhibitor para-chlorophenylalanine (3 x 400 mg/kg, i.p., for 3 d), although preventing the effect on DA release, failed to modify significantly the effect of 20 Hz DRN stimulation on DOPAC and 5-HIAA outflow in both structures. Ondansetron (0.1 and 1 mg/kg) and (S)-zacopride (0.1 mg/kg), two 5-HT3 antagonists, significantly impaired the increase of accumbal DA release induced by 20 Hz DRN stimulation but did not affect either the decrease of striatal DA release or the increase in DOPAC outflow in both structures. These results indicate that an enhancement of central 5-HT transmission induced by DRN stimulation differentially affects striatal and accumbal DA release and that endogenous 5-HT, via its action on 5-HT3 receptors, exerts a facilitatory control restricted to the mesoaccumbal DA pathway.

Autoradiographic distribution of [3H]-(S)-zacopride-labelled 5-HT3 receptors in human brain

Autoradiographic binding studies using the 5-HT3 (5-hydroxytryptamine3) receptor radioligand, [3H]-(S)-zacopride (0.5 nM), identified a heterogeneous distribution of specific binding sites (defined by granisetron, 1 microM) throughout the human brain. Highest radiolabelled 5-HT3 receptor densities were detected in discrete nuclei within the brainstem (nucleus tractus solitarius, area postrema, spinal trigeminal nerve nucleus; 50-200 fmol/mg tissue equivalent) with more modest levels of expression in the forebrain (e.g. hippocampus, nucleus accumbens, putamen, caudate; 4-17 fmol/mg tissue equivalent). Within the hippocampal formation, radiolabelled 5-HT3 receptors were differentially distributed with highest levels in the granule cell layer of the dentate gyrus. Saturation studies with [3H]-(S)-zacopride (0.05-16 nM; non-specific binding defined by granisetron, 10 microM) binding to homogenates of human putamen indicated that [3H]-(S)-zacopride (0.05-16 nM; non-specific binding defined by granisetron, 10 microM) binding to homogenates of human putamen indicated that [3H]-(S)-zacopride labelled an apparently homogenous population of binding sites (Bmax = 72 + 7 fmol mg-1 protein, pKd = 8.69 +/- 0.09, Hill coefficient = 0.99 +/- 0.06, mean +/- SEM, n = 4). The pharmacological profile of [3H]-(S)-zacopride binding to homogenates of putamen indicated the selective labelling of the human variant of the 5-HT3 receptor. The marked differences, however, in the pharmacology (e.g. low affinity for D-tubocurarine) and relative distribution (e.g. presence of 5-HT3 receptors in the human extrapyramidal system) of 5-HT3 receptors in the human forebrain when compared with other species further necessitates caution in predicting clinical responses based on data generated in animal models of disease.

Effects of 5-HT3 receptor-selective agents on locomotor activity in rats following injection into the nucleus accumbens and the ventral tegmental area

5-Hydroxytryptamine (5-HT) is involved in the modulation of dopaminergic activity in the mesolimbic system, but its sites of action and the receptors involved are not well understood. Locomotor activity responses in rats were monitored in Animex automated activity boxes following injection of 5-HT3 receptor-selective agents directly into two mesolimbic nuclei, the nucleus accumbens and the ventral tegmental area, via stereotactically implanted injection guide cannulae. Neither spontaneous nor dexamphetamine-stimulated locomotor activity was changed by bilateral intra-nucleus accumbens injection of the selective agonist 2-methyl-5-HT or the selective antagonists ondansetron or granisetron. In contrast, intra-ventral tegmental area injection of 2-methyl-5-HT produced significant long-lasting (approximately 240 min) increases in locomotor activity; intra-ventral tegmental area injection of ondansetron elicited an initial inhibition of spontaneous and dexamphetamine-stimulated locomotor activity (for the 0-30 min period), but granisetron had no effect. The hyperlocomotor response to intra-ventral tegmental area 2-methyl-5-HT was abolished by pretreatment with the catecholamine synthesis inhibitor alpha-methyl-p-tyrosine, or by pretreatment with ondansetron. Methiothepin pretreatment had no effect on the hyperlocomotor response to 2-methyl-5-HT, although methiothepin itself produced an initial increase in spontaneous locomotor activity (for the 60-120 min period). Intra-ventral tegmental area injection of 5-carboxamidotryptamine, alpha-methyl-5-HT or renzapride produced no changes in spontaneous locomotor activity. In some of the ventral tegmental area experiments, other behaviours were also monitored. 2-Methyl-5-HT produced forward locomotion, rearing, and increased wakefulness, but did not appreciably alter circling, grooming or sniffing. Ondansetron alone had no effect on any of these behaviours, but it opposed the 2-methyl-5-HT-induced changes. Methiothepin alone increased forward locomotion and wakefulness but did not alter the other behaviours; it had no effect on the responses to 2-methyl-5-HT. These observations show that 5-HT3 receptors may mediate increased locomotor activity by modulating firing of mesolimbic dopaminergic cell bodies in the ventral tegmental area rather than terminals in the nucleus accumbens.

Effects of granisetron, a 5-HT3 receptor antagonist, on morphine-induced potentiation of brain stimulation reward

Using the curve-shift method, we studied the effects of four doses (0.003, 0.03, 0.3 and 3 mg/kg, s.c.) of granisetron (endo-N-(9-methyl-9-azabicyclo[3.3.1]non-3-yl)-1-methyl-1H-indazole-3- carboxamide hydrochloride), a selective 5-hydroxytryptamine3 (5-HT3) receptor antagonist, on the potentiation of brain stimulation reward by microinjection of 2.5 micrograms/0.25 microliters of morphine sulphate (7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol sulphate) into the ventral tegmental area. As previously reported, morphine produced a significant reduction in reward threshold without altering maximal rates of responding. Granisetron attenuated the potentiating effect of morphine at the highest dose and failed to alter reward threshold or maximal rates of responding when given alone, except at the lowest dose where a small and statistically significant increase in threshold was found. These results provide additional evidence that 5-HT3 receptor antagonists may reduce the rewarding effect of opiates and do not impair the ability to produce operant responses. The weak attenuation observed with granisetron alone suggests that 5-HT3 receptors are unlikely to constitute an important influence on the directly stimulated reward-relevant pathway(s).

Ventral tegmental area 5-HT receptors: mesolimbic dopamine release and behavioural studies

Serotoninergic neurones originating in the dorsal and median raphe nuclei project to mesolimbic structures, including the nucleus accumbens (NAcb) and the ventral tegmental area (VTA), where they may have an important regulatory role. Some evidence from dopamine release, behavioural and binding studies directly implicates involvement of 5-HT3 and 5-HT4 receptors in the NAcb. Other in vivo dopamine release and behavioural experiments in rats have provided evidence for 5-HT3 receptor-mediated enhancement of mesolimbic dopaminergic activity, although the location of the 5-HT3 receptors involved is unknown because the selective 5-HT3 receptor agents used were administered systemically or intracerebroventricularly. This raises the possibility of a VTA site of action; as yet, however, relatively little is specifically known about 5-HT receptors and function in the VTA. Mesolimbic dopamine release in rats, measured in-vivo with microdialysis probes in the NAcb, can be inhibited by tropisetron administered directly into the VTA. In our laboratory, behavioural studies in rats have shown that a sustained hyperlocomotion is produced by 2-methyl-5-HT administered into the VTA via stereotactically implanted guide cannulae. Mesolimbic dopaminergic activation is involved, because pretreatment with the catecholamine synthesis inhibitor alpha-methyl-p-tyrosine abolishes 2-methyl-5-HT-induced hyperlocomotion. The 2-methyl-5-HT hyperlocomotor response is blocked by prior intra-VTA injection of ondansetron but is not affected by methiothepin, and intra-VTA 5-carboxamidotryptamine, alpha-methyl-5-HT or renzapride were without effect, thus a 5-HT3 receptor in the VTA mediates the 2-methyl-5-HT response. These in vivo dopamine release and behavioural studies therefore confirm that 5-HT3 receptors in the VTA can mediate increased locomotor activity, by modulating the firing of mesolimbic dopaminergic cell bodies.

c-fos expression in specific rat brain nuclei after intestinal anaphylaxis: involvement of 5-HT3 receptors and vagal afferent fibers

The c-fos immediate-early gene is acutely induced in brain after various stimuli from the digestive tract. 5-HT3 receptors and vagal afferents have been found involved in intestinal motor disturbances induced by intestinal anaphylaxis. Our aim was to determine whether intestinal anaphylaxis activates brain structures, using c-fos expression, and to evaluate the modulation of c-fos induction by 5-HT3 receptors and vagal afferents. The effects of antigen challenge on intestinal motility were evaluated in ovalbumin-sensitized Hooded Lister rats chronically fitted with NiCr electrodes in the jejunal wall. Intestinal motility was assessed in conscious rats pretreated or not by perivagal capsaicin or a 5-HT3 antagonist (ondansetron). In sensitized rats, ovalbumin disrupted for 62.4 +/- 9.5 min the jejunal migrating motor complexes (MMC) and an important c-fos expression was detected in the nucleus tractus solitarius (NTS), lateral parabrachial nucleus (LPB) and paraventricular nucleus of the hypothalamus (PVN). Intraperitoneal administration of ondansetron or perivagal capsaicin treatment significantly reduced the duration of MMC disruption and attenuated markedly c-fos staining in the 3 brain sites. In contrast, intracerebroventricular administration of ondansetron significantly reduced jejunal motor alterations but did not diminish the c-fos expression, suggesting a role of central 5-HT3 receptors in the efferent control of the intestinal disturbances. Blockade of both c-fos expression and MMC disruption by systemic ondansetron and by perivagal capsaicin indicates that some brainstem nuclei are involved in digestive disturbances after intestinal anaphylaxis, and reflects an involvement of peripheral 5-HT3 receptors on vagal afferents. The reduction of c-fos staining in NTS as well as in LPB and PVN after perivagal capsaicin suggests that the NTS is the primary relay in the activation of the central nervous system during intestinal allergic challenge.

The interaction of RS 25259-197, a potent and selective antagonist, with 5-HT3 receptors, in vitro

1. A series of isoquinolines have been identified as 5-HT3 receptor antagonists. One of these, RS 25259-197 [(3aS)-2-[(S)-1-azabicyclo[2.2.2]oct-3-yl]-2,3,3a,4,5,6-hexahydro- 1- oxo-1H-benzo[de]isoquinoline-hydrochloride], has two chiral centres. The remaining three enantiomers are denoted as RS 25259-198 (R,R), RS 25233-197 (S,R) and RS 25233-198 (R,S). 2. At 5-HT3 receptors mediating contraction of guinea-pig isolated ileum, RS 25259-197 antagonized contractile responses to 5-HT in an unsurmountable fashion and the apparent affinity (pKB), estimated at 10 nM, was 8.8 +/- 0.2. In this tissue, the -log KB values for the other three enantiomers were 6.7 +/- 0.3 (R,R), 6.7 +/- 0.1 (S,R) and 7.4 +/- 0.1 (R,S), respectively. The apparent affinities of RS 25259-197 and RS 25259-198, RS 25233-197 and RS 25233-198 at 5-HT3 receptors in membranes from NG-108-15 cells were evaluated by a [3H]-quipazine binding assay. The -log Ki values were 10.5 +/- 0.2, 8.4 +/- 0.1, 8.6 +/- 0.1 and 9.5 +/- 0.1, respectively, with Hill coefficients not significantly different from unity. Thus, at these 5-HT3 receptors, the rank order of apparent affinities was (S,S) > (R,S) > (S,R) = (R,R). 3. RS 25259-197 displaced the binding of the selective 5-HT3 receptor ligand, [3H]-RS 42358-197, in membranes from NG-108-15 cells, rat cerebral cortex, rabbit ileal myenteric plexus and guinea-pig ileal myenteric plexus, with affinity (pKi) values of 10.1 +/- 0.1, 10.2 +/- 0.1, 10.1 +/- 0.1 and 8.3 +/- 0.2, respectively. In contrast, it exhibited low affinity (pKi <6.0) at 28 other receptors in binding assays, including adrenoceptors (alpha1A, alpha 1B, alpha2A, alpha 2B ,beta1, beta2), muscarinic (M1-M4), dopamine (D1, D2), opioid and other 5-HT(5-HTlA, 5-HTlD, 5-HT2C, 5-HT4) receptors.4. RS 25259-197 was tritium labelled (specific activity: 70 Ci mmol-1) and evaluated in pharmacological studies. Saturation studies with [3H]-RS 25259-197 in membranes from NG-108-15 and cloned homomeric a subunits of the 5-HT3 receptor from N1E-1 15 cells expressed in human kidney 293E1 cells,revealed an equilibrium dissociation constant (Kd) of 0.05 +/- 0.02 and 0.07 +/- 0.01 nM, and Bmax of610 +/- 60 and 1068 +/- 88 fmol mg-1, respectively. Competition studies in NG-108-15 cells indicated a pharmacological specificity entirely consistent with labelling a 5-HT3 receptor, i.e. RS 25259-197> granisetron> (S)-zacopride> tropisetron> (R)-zacopride> ondansetron> MDL 72222.5. In contrast to the majority of radioligands available to label 5-HT3 receptors, [3H]-RS 25259-197 labelled a high affinity site in hippocampus from human post-mortem tissue with an equilibrium dissociation constant (Kd) of 0.15 +/- 0.07 nM and density (BmaX) of 6.8 +/- 2.4 fmol mg-1 protein. Competition studies in this tissue indicated a pharmacological specificity consistent with labelling of a 5-HT3receptor.6. Quantitative autoradiographic studies in rat brain indicated a differential distribution of 5-HT3receptor sites by [3H]-RS 25259-197. High densities of sites were seen in nuclear tractus solitaris and area postrema, a medium density in spinal trigeminal tract, ventral dentate gyrus and basal medial amygdala,and a low density of sites in hippocampal CAl, parietal cortex, medium raphe and cerebellum.7 In conclusion, the functional, binding and distribution studies undertaken with the radiolabelled and non-radiolabelled RS 25259-197 (S,S enantiomer) established the profile of a highly potent and selective5-HT3 receptor antagonist.

Serotonin receptor gene expression in the rat suprachiasmatic nuclei

Serotonin (5HT) is thought to reset the biological clock in the suprachiasmatic nuclei (SCN) in vitro through a postsynaptic 5HT-1a receptor. Thus we examined 5HT receptor gene expression in the SCN by in situ hybridization. On film autoradiograms, 5HT-1c receptor mRNA showed intense SCN hybridization, while 5HT-1b receptor mRNA displayed a weaker signal. Emulsion autoradiograms additionally revealed expression of 5HT-1a and 5HT-2 receptor mRNAs by a few scattered SCN cells. 5HT-3 receptor mRNA was not detected in the SCN, although the transcript was detected elsewhere in the brain. 5HT-1d and -1e receptor mRNAs were not detected in the SCN or elsewhere in brain within the sections examined. The results do not support a major role for postsynaptic 5HT-1a receptors in resetting SCN rhythms.

An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit

The results of a previous experiment suggest that the cell bodies of many jaw closing muscle spindle afferents in the trigeminal mesencephalic nucleus of the rabbit are phasically inhibited during fictive mastication. The aim of this study was to investigate one possible neurotransmitter system that could be involved in this modulation, serotonin, by use of receptor autoradiography techniques and immunofluorescence combined with retrograde labelling of masseteric spindle afferents and motoneurons. A second objective was to compare the serotonin innervation of neurons in the trigeminal mesencephalic nucleus with that of masseteric motoneurons. Serotoninergic fibres were seen surrounding labelled masseteric spindle afferents, as well as unlabelled neurons, in the trigeminal mesencephalic nucleus. These fibres were close to the cell bodies and sometimes to the axon hillocks of the neurons. Although it has been reported that many neurons of the trigeminal nucleus are multipolar in some species, none of the labelled spindle afferent in this study had more than one process. Throughout the motor trigeminal nucleus, serotonin fibres were found in close proximity with cell bodies and with the proximal portions of axons and dendrites of labelled and unlabelled motoneurons. Serotonin fibres were also seen adjacent to cell bodies and processes of efferent neurons in cell group k. Autoradiography with several tritiated ligands was used to reveal the presence of receptors for serotonin as well as its uptake sites. Only serotonin2 receptors were found to be abundant in the trigeminal mesencephalic nucleus. The motor nucleus and cell group k contained serotonin2 and serotonin3 receptors, as well as serotonin uptake sites. Serotonin1A receptors appear to be absent from both nuclei. The findings suggest that release of serotonin from fibres in close proximity to trigeminal primary afferent somata could modify the transmission of action potentials from muscle spindle receptors during mastication through an action on serotonin2 receptors. In the motor nucleus and cell group k, serotonin may alter neuronal properties through actions on at least two receptor subtypes (serotonin2 and serotonin3).

Time- and dose-dependent effects of the serotonergic agent quipazine on regional cerebral metabolism in rats

The time course and the relation to dose of regional cerebral metabolic rates for glucose (rCMRglc) were measured in awake male adult Fischer-344 rats after administration of quipazine, a serotonin 5-HT2-3 receptor agonist. rCMRglc was determined, using the quantitative autoradiographic [14C]deoxyglucose technique, in 92 brain regions at 30, 60, 90 and 120 min after quipazine 20 mg/kg i.p. and at 60 min after quipazine 5 mg/kg i.p. Peak metabolic effects were observed 60 min after quipazine 20 mg/kg i.p. when rCMRglc was significantly elevated in 27 (29%) brain regions (mean rise 17%). Quipazine increased rCMRglc in brain regions with high densities of 5-HT3 receptors (area postrema, olfactory tubercle, amygdala), in dopaminergic nuclei (substantia nigra pars compacta and pars reticulata) and terminal fields of their projections (zona incerta, subthalamic nucleus, preoptic magnocellular area, nucleus of facial nerve). The topographic distribution and direction of rCMRglc changes induced by quipazine are different from those produced by the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and, consistent with the pharmacological and binding properties of quipazine, suggest a preferential activation of 5-HT3 receptors.

5-HT3 receptor antagonists inhibit morphine-induced stimulation of mesolimbic dopamine release and function in the rat

The effects of three different 5HT3 receptor antagonists, granisetron, ICS 205-930 and ondansetron (0.01, 0.1 and 1 mg/kg, s.c.) were tested on changes in mesolimbic dopamine function produced by 1 mg/kg of morphine in the rat. Increases of in vivo dopamine release and stimulation of behavioural activity (grooming, locomotion, rearing and sniffing) were monitored. Morphine (0.5 and 1 mg/kg, s.c.) increased dose-dependently the concentration of dopamine in dialysates obtained from the nucleus accumbens. This action of morphine was inhibited by the opiate antagonist naloxone (1 mg/kg, s.c.). Morphine (0.5 and 1 mg/kg) stimulated behavioural activity, which in the early part of the time course corresponded closely with the increase of dopamine in the nucleus accumbens. Pretreatment with 1 mg/kg (s.c.) of granisetron resulted in moderate inhibition (28%) of the morphine-induced stimulation of the extracellular dopamine levels, while doses of 0.01 and 0.1 mg/kg (s.c.) had no effect. The highest dose of granisetron (1 mg/kg, s.c.) also significantly reduced the morphine-induced enhancement of behavioural activity. The fact that granisetron attenuated morphine-induced effects on mesolimbic DA only at the highest dose tested (1 mg/kg, s.c.) was also true for ICS 205-930 and ondansetron. It is concluded that 5HT3 receptor antagonists partially inhibit, with low potency, the morphine-induced stimulation of dopamine release in the nucleus accumbens and the corresponding behavioural activation.

Interactions between 5-HT3 receptors and cerebral dopamine function: implications for the treatment of schizophrenia and psychoactive substance abuse

This article reviews current knowledge on the interaction between 5-hydroxytryptamine (5-HT), acting at 5-HT3 receptors in the CNS, and cerebral dopamine systems. Since 1987, a growing body of behavioural, neurochemical and electrophysiological evidence from animal studies has demonstrated a clear role for 5-HT3 receptors in the modulation of activity of mesolimbic and mesocortical dopamine neurones. This evidence has led to the suggestion that 5-HT3 receptor antagonists have potential as novel antipsychotic agents and may also find use in the treatment of psychoactive substance abuse. Data emerging from clinical studies generally support this hypothesis and suggest that 5-HT3 antagonists may prove to be among the first agents available to treat schizophrenia which are not dopamine D2 antagonists and hence lack their side-effect problems.

Effect of naftidrofuryl oxalate on 5-HT2 receptors in mouse brain: evaluation based on quantitative autoradiography and head-twitch response

The effects of naftidrofuryl oxalate (LS-121) on 5-HT2 receptors in the brain were assessed in mice on the basis of quantitative autoradiography and head-twitch responses. LS-121 inhibited [3H]ketanserin (2 nM) binding in all brain areas assayed in which there were 5-HT2 receptors, such as the frontal cortex, cingulate cortex, parietal cortex, occipital cortex, temporal cortex, nucleus accumbens, caudate-putamen, olfactory tubercle and hippocampus. In the frontal cortex, which has the highest density of 5-HT2 receptors, the Ki value of LS-121 was 6.08 x 10(-8) M. The inhibitory potencies of methysergide and ritanserin for 5-HT2 receptors were about 16- and 60-fold stronger, respectively, than that of LS-121. Moreover, in behavioral studies, LS-121 (12.5-50 mg/kg i.p.) produced dose-dependent and significant inhibitory effects on head twitches induced by 5-hydroxytryptophan (5-HTP) plus pargyline, which is a 5-HT2 receptor-dependent behavior in mice. These results suggest that LS-121 inhibits 5-HTP plus pargyline-induced head twitches by blocking 5-HT2 receptors.

Ligands and tracers for PET studies of the 5-HT system--current status

The status of the radiochemical development and biological evaluation of radioligands and tracers for PET studies of the serotonergic system is reviewed, indicating those agents with present value and those with future potential. Practical recommendations are given for the preparation of two useful radioligands for PET studies of central 5-HT2 receptors, namely [18F]setoperone and [18F]altanserin. Though, it has not proved possible to recommend tracers or radioligands for the study of other aspects of serotonergic system, prospects for future radiochemical development are indicated, especially for developing radioligands for the 5-HT re-uptake site, and for the 5-HT1 and 5-HT3 receptors.

Synthesis and regional rat brain distribution of [11C]MDL 72222: a 5HT3 receptor antagonist

MDL 72222, an antagonist of 5HT3 receptors, was labeled with a specific radioactivity of 340-400 mCi/mumol by alkylation of the nor-precursor with [11C]CH3I. The yield of the synthesis, starting from [11C]methyliodide to the purified product and corrected for decay, was good approximately 70-75%. After i.v. injection, [11C]MDL 72222 diffuses readily in the central nervous system but is not detected as metabolites in brain and blood, during 1 h study carried out in rats. The time course and distribution of [11C]MDL 72222 was assessed in various organs (liver, lung, kidney, heart, whole brain) and in blood; the organ uptake was rapid and large; the highest accumulation was found in the lung. The regional brain distribution shows initial uptake and subsequent retention of tracer in favor of the cerebral cortex. The level of brain radioactivity was not reduced by pretreatment with a 1000-fold excess of unlabeled MDL 72222. These results suggest that [11]MDL 72222 is of limited interest for 5HT3 receptor binding studies in brain in vivo, presumably mainly because of large non-specific binding.

Characteristics of 5-HT3 binding sites in NG108-15, NCB-20 neuroblastoma cells and rat cerebral cortex using [3H]-quipazine and [3H]-GR65630 binding

1. The biochemical and pharmacological properties of 5-HT3 receptors in homogenates of NG108-15 and NCB-20 neuroblastoma cells and rat cerebral cortex have been ascertained by the use of [3H]-quipazine and [3H]-GR65630 binding. 2. In NG108-15 and NCB-20 cell homogenates, [3H]-quipazine bound to a single class of high affinity (NG108-15: Kd = 6.2 +/- 1.1 nM, n = 4; NCB-20: Kd = 3.0 +/- 0.9 nM, n = 4; means +/- s.e.means) saturable (NG108-15: Bmax = 1340 +/- 220 fmol mg-1 protein; NCB-20: Bmax = 2300 +/- 200 fmol mg-1 protein) binding sites. In rat cortical homogenates, [3H]-quipazine bound to two populations of binding sites in the absence of the 5-hydroxytryptamine (5-HT) uptake inhibitor, paroxetine (Kd1 = 1.6 +/- 0.5 nM, Bmax1 = 75 +/- 14 fmol mg-1 protein; Kd2 = 500 +/- 300 nM, Bmax2 = 1840 +/- 1040 fmol mg-1 protein, n = 3), and to a single class of high affinity binding sites (Kd = 2.0 +/- 0.5 nM, n = 3; Bmax = 73 +/- 6 fmol mg-1 protein) in the presence of paroxetine. The high affinity (nanomolar) component probably represented 5-HT3 binding sites and the low affinity component represented 5-HT uptake sites. 3. [3H]-paroxetine bound with high affinity (Kd = 0.02 +/- 0.003 nM, n = 3) to a site in rat cortical homogenates in a saturable (Bmax = 323 +/- 45 fmol mg-1 protein, n = 3) and reversible manner. Binding to this site was potently inhibited by 5-HT uptake blockers such as paroxetine and fluoxetine (pKi s = 8.6-9.9), while 5-HT3 receptor ligands exhibited only low affinity (pK; < 7). No detectable specific [3H]-paroxetine binding was observed in NG108-15 or NCB-20 cell homogenates. 4. [3H]-quipazine binding to homogenates of NG108-15, NCB-20 cells and rat cortex (in the presence of 0.1 microM paroxetine) exhibited similar pharmacological characteristics. 5-HT3 receptor antagonists competed for [3H]-quipazine binding with high nanomolar affinities in the three preparations and the rank order of affinity was: (S)-zacopride > quarternized ICS 205-930 2 granisetron > ondansetron > ICS 205-209 (R)-zacopride > quipazine > renzapride > MDL-72222 > butanopride > metoclopramide. 5. [3H]-GR65630 labelled a site in NCB-20 cell homogenates with an affinity (Kd = 0.7 + 0.1 nms n = 4) and density (B__ = 1800 + 1000 fmol mg- protein) comparable to that observed with [3H]-quipazine. Competition studies also indicated a good correlation between the pharmacology of 5-HT3 binding sites when [3H]-GR65630 and [3H]-quipazine were used in these cells. 6. In conclusion, [3H]-quipazine labelled 5-HT3 receptor sites in homogenates of NG108-15 cells, NCB-20 cells and rat cerebral cortex. In rat cortical homogenates, [3H]-quipazine also bound to 5-HT uptake sites, which could be blocked by 0.1 microM paroxetine. The pharmacological specificity of the 5-HT3 receptor labelled by [3H]-quipazine was similar in the neuroblastoma cells and rat cortex and was substantiated in NCB-20 cells by the binding profile of the selective 5-HT3 receptor antagonist, [3H]-GR65630.