In vivo binding of [123I]4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)- p-iodobenzamido-]ethyl-piperazine, p-MPPI, to 5-HT1A receptors in rat brain

Role of tandospirone, a 5-HT1A receptor partial agonist, in the treatment of central nervous system disorders and the underlying mechanisms

5-hydroxytryptamine (5-HT, serotonin) is an important neurotransmitter in the modulation of the cognitive, behavioral and psychological functions in animals and humans. Among the fourteen subtypes of 5-HT receptor, 5-HT1A receptor has been extensively studied. Tandospirone, an azapirone derivative with strong and selective agonist effect on 5-HT1A receptor, has been used for the treatment of anxiety disorders especially generalized anxiety disorder for decades. Recently, tandospirone showed the efficacy in relieving the syndromes of social anxiety disorder and post-traumatic stress disorder as well as in potentiating the effect of antidepressants in the treatment of depression in both preclinical and clinical studies. More impressively, the beneficial effect of tandospirone has been revealed on improvement of motor dysfunction of Parkinson's disease and cognitive deficits of schizophrenia either in monotherapy or in combination with other drugs. This review discusses the superiority of tandospirone in the treatment of the disorders and associated mechanisms in central nervous system from the literature.

Systemic modulation of serotonergic synapses via reuptake blockade or 5HT1A receptor antagonism does not alter perithreshold taste sensitivity in rats

Systemic blockade of serotonin (5HT) reuptake with paroxetine has been shown to increase sensitivity to sucrose and quinine in humans. Here, using a 2-response operant taste detection task, we measured the effect of paroxetine and the 5HT1A receptor antagonist WAY100635 on the ability of rats to discriminate sucrose, NaCl, and citric acid from water. After establishing individual psychometric functions, 5 concentrations of each taste stimulus were chosen to represent the dynamic portion of the concentration-response curve, and the performance of the rats to these stimuli was assessed after vehicle, paroxetine (7mg/kg intraperitoneally), and WAY100635 (0.3mg/kg subcutaneously; 1mg/kg intravenously) administration. Although, at times, overall performance across concentrations dropped, at most, 5% from vehicle to drug conditions, no differences relative to vehicle were seen on the parameters of the psychometric function (asymptote, slope, or EC50) after drug administration. In contrast to findings in humans, our results suggest that modulation of 5HT activity has little impact on sucrose detectability at perithreshold concentrations in rats, at least at the doses used in this task. In the rat model, the purported paracrine/neurocrine action of serotonin in the taste bud may work in a manner that does not impact overt taste detection behavior.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

Translational aspects of pharmacological research into anxiety disorders: the stress-induced hyperthermia (SIH) paradigm

In anxiety research, the search for models with sufficient clinical predictive validity to support the translation of animal studies on anxiolytic drugs to clinical research is often challenging. This review describes the stress-induced hyperthermia (SIH) paradigm, a model that studies the activation of the autonomic nervous system in response to stress by measuring body temperature. The reproducible and robust SIH response, combined with ease of testing, make the SIH paradigm very suitable for drug screening. We will review the current knowledge on the neurobiology of the SIH response, discuss the role of GABA(A) and serotonin (5-HT) pharmacology, as well as how the SIH response relates to infectious fever. Furthermore, we will present novel data on the SIH response variance across different mice and their sensitivity to anxiolytic drugs. The SIH response is an autonomic stress response that can be successfully studied at the level of its physiology, pharmacology, neurobiology and genetics and possesses excellent animal-to-human translational properties.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

[18F]MPPF as a tool for the in vivo imaging of 5-HT1A receptors in animal and human brain

Serotonin (5-hydroxytryptamine, 5-HT) and its various receptors are involved in numerous CNS functions and psychiatric disorders. 5-HT(1A), the best-characterized subtype of currently known 5-HT receptors, is tightly implicated in the pathogenesis of depression, anxiety, epilepsy and eating disorders. It thus represents an important target for drug therapy. Specific radioligands and positron emission tomography (PET) allow for a quantitative imaging of brain 5-HT(1A) receptor distribution in living animals and humans. Recently, the selective 5-HT(1A) receptor antagonist, MPPF, has been successfully labeled with [(18)F]fluorine ([(18)F]MPPF), and an increasing number of academic and industry centres have used this radiotracer in preclinical and clinical studies. After a brief account of some of the structural, distributional and electrophysiological characteristics of brain 5-HT(1A) receptors, this review focuses on studies conducted with [(18)F]MPPF, with emphasis on preclinical results illustrating the actual and potential value of this PET radioligand for clinical research and drug development.

Respiratory abnormalities resulting from midcervical spinal cord injury and their reversal by serotonin 1A agonists in conscious rats

Respiratory dysfunction after cervical spinal cord injury (SCI) has not been examined experimentally using conscious animals, although clinical SCI most frequently occurs in midcervical segments. Here, we report a C5 hemicontusion SCI model in rats with abnormalities that emulate human post-SCI pathophysiology, including spontaneous recovery processes. Post-C5 SCI rats demonstrated deficits in minute ventilation (Ve) responses to a 7% CO2 challenge that correlated significantly with lesion severities (no injury or 12.5, 25, or 50 mm x 10 g weight drop; New York University impactor; p < 0.001) and ipsilateral motor neuron loss (p = 0.016). Importantly, C5 SCI resulted in at least 4 weeks of respiratory abnormalities that ultimately recovered afterward. Because serotonin is involved in respiration-related neuroplasticity, we investigated the impact of activating 5-HT1A receptors on post-C5 SCI respiratory dysfunction. Treatment with the 5-HT1A agonist 8-hydroxy-2-(di-n-propylmino)tetralin (8-OH DPAT) (250 microg/kg, i.p.) restored hypercapnic Ve at 2 and 4 weeks after injury (i.e., approximately 39.2% increase vs post-SCI baseline; p < or = 0.033). Improvements in hypercapnic Ve response after single administration of 8-OH DPAT were dose dependent and lasted for approximately 4 h(p < or = 0.038 and p < or = 0.024, respectively). Treatment with another 5-HT1A receptor agonist, buspirone (1.5 mg/kg, i.p.), replicated the results, whereas pretreatment with a 5-HT1A-specific antagonist, 4-iodo-N-[2-[4(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (3 mg/kg, i.p.) given 20 min before 8-OH DPAT negated the effect of 8-OH DPAT. These results imply a potential clinical use of 5-HT1A agonists for post-SCI respiratory disorders.

Synthesis and in vivo biodistribution of F-18 labeled 3-cis-, 3-trans-, 4-cis-, and 4-trans-fluorocyclohexane derivatives of WAY 100635

Radioligands that are specific for the serotonin 5-HT(1A) receptor will be useful in characterizing the physiological action of this receptor subtype. With radioligands of varying pharmacokinetic properties, investigators can measure not only receptor density, but also the effect of endogenous serotonin concentration. To this end, three additional fluorinated analogs of WAY 100635 were prepared and evaluated as 5-HT(1A) receptor ligands of varying pharmacokinetic properties based on our previous studies. These four compounds are cis-4-fluoro-, trans-4-fluoro-, cis-3-fluoro-, and trans-3-fluoro-N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-N-(pyridin-2-yl)cyclohexanecarboxamides (FCWAYs). All four compounds were characterized and radiolabeled with fluorine-18, a 109.7 min half-life radionuclide used in positron emission tomography. We then determined in vitro inhibition constants at the 5-HT(1A) receptor; in vitro metabolic profile, using rat hepatocytes and liquid chromatography/mass spectroscopy (LC/MS); and the rate of defluorination and hippocampus to cerebellum ratio ex vivo. This led to the conclusion that high affinity 4-trans-F-18 FCWAY had the best properties for measuring receptor density given its high hippocampus to cerebellum ratio and 3-cis-F-18 FCWAY had the best properties for measuring dynamic change in receptors, with lower affinity and faster pharmacokinetics.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

Modulation of antagonist binding to serotonin1A receptors from bovine hippocampus by metal ions

1. The serotonin1A (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins. They appear to be involved in various behavioral and cognitive functions. Although specific 5-HT1A agonists have been discovered more than a decade back, the development of selective 5-HT1A antagonists has been achieved only recently. 2. We have examined the modulation of the specific antagonist [3H]p-MPPF binding to 5-HT1A receptors from bovine hippocampal membranes by monovalent and divalent metal ions. Our results show that the antagonist binding to 5-HT1A receptors is inhibited by both monovalent and divalent cations in a concentration-dependent manner. This is accompanied by a concomitant reduction in binding affinity. 3. Our results also show that the specific antagonist p-MPPF binds to all available receptors in the bovine hippocampal membrane irrespective of their state of G-protein coupling and other serotonergic ligands such as 5-HT and OH-DPAT effectively compete with the specific antagonist [3H]p-MPPF. 4. These results are relevant to ongoing analyses of the overall modulation of ligand binding in G-protein-coupled seven transmembrane domain receptors.

The selective 5-HT(1A) receptor antagonist p-MPPI antagonizes sleep--waking and behavioural effects of 8-OH-DPAT in rats

Systemic administration of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT) increases waking and reduces slow wave sleep (SWS) and rapid eye movement (REM) sleep in the freely moving rat. The selective 5-HT(1A) antagonist 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethyl-piperazine (p-MPPI) induces a dose-related decrease in REM sleep. The present study examined p-MPPI's potential as an antagonist of the sleep and waking responses elicited by 8-OH-DPAT. Also, the experiments explored the ability of p-MPPI to block behavioural reactions of the 5-HT syndrome induced by 8-OH-DPAT, and whether p-MPPI induced any behavioural effects of its own. This study demonstrated that pre-treatment with p-MPPI (5 mg/kg intraperitoneal (i.p.)) 30 min before 8-OH-DPAT (0.375 mg/kg subcutaneously (s.c.)) reduced the effect of 8-OH-DPAT on waking and REM sleep. Also, p-MPPI (5 and 10 mg/kg i.p.) reduced the effect of 8-OH-DPAT on locomotion and partially or completely antagonized hindlimb abduction and flat body posture. No overt behavioural change was produced by p-MPPI alone. Thus, p-MPPI behaved as a true 5-HT(1A) antagonist.

Serotonergic neuromodulation in the cerebellar cortex: cellular, synaptic, and molecular basis

The cerebellum, like most sensorimotor areas of the brain, receives a serotonergic innervation from neurons of the reticular formation. It is well established that local application of serotonin modulates the firing rate of cerebellar Purkinje cells in vivo and in vitro, but the mechanisms by which serotonin affects the cerebellar function are still poorly understood. Whereas interactions between serotonin, glutamate, and GABA have been reported to increase or decrease the firing frequency of Purkinje cells, there is little evidence for a modulation of excitatory and inhibitory synapses by serotonin in the cerebellar cortex. Changes in the intrinsic electrical properties of Purkinje cells upon application of serotonin have also been reported, but their impact on Purkinje cell firing is unclear. The recent finding that serotonin specifically modulates the activity of Lugaro cells, a class of inhibitory interneurons of the cerebellar cortex, offers new insights on the action of this neuromodulator. The peculiar axonal projection and specific interneuronal targets of the Lugaro cells suggest that the action of serotonin might occur upstream of Purkinje cells through a resetting of the computational properties of the cerebellar cortex. Understanding the mechanisms of the serotonergic modulation of the cerebellar cortex is of clinical relevance, as abnormal serotonin metabolism has been observed in animal models and pathological cases of motor disorders involving the cerebellum, and as chronic intravenous administration of L-5-hydroxytryptophan (5-HTP), a precursor of serotonin, was the first treatment shown to improve significantly cerebellar symptoms.

Sleep and waking following microdialysis perfusion of the selective 5-HT1A receptor antagonist p-MPPI into the dorsal raphe nucleus in the freely moving rat

The aim of this study was to examine the involvement of the dorsal raphe nucleus (DRN) presynaptic serotonergic 5-HT1A autoreceptors on sleep and waking parameters, in particular rapid eye movement (REM) sleep. In a previous study, the systemic administration of the selective 5-HT1A receptor antagonist p-MPPI reduced REM sleep in a dose-dependent manner suggesting a blockade of the 5-HT1A autoreceptors. In the present study, a blockade by microdialysis perfusion of 10 microM and 100 microM of p-MPPI for 7 h into the DRN in freely behaving rats influenced vigilance state only to a small extent. The administration of 10 microM of p-MPPI induced a reduction of total REM sleep mainly due to a suppression of REM sleep during the third 2 h period of the recording of sleep and waking. Perfusion of 100 microM of p-MPPI decreased total transition type sleep (TRANS) but the effect on REM sleep did not reach significance. There was no change in waking or slow wave sleep (SWS) following any of the doses. The data suggest that 5-HT1A receptor-mediated mechanisms in the DRN may be only moderately important in the serotonergic modulation of REM sleep.

Role of disulfides and sulfhydryl groups in agonist and antagonist binding in serotonin1A receptors from bovine hippocampus

I. The serotonin1A (5-HT1A) receptors are members of a superfamily of seven-transmembrane-domain receptors that couple to G-proteins. They appear to be involved in various behavioral and cognitive functions. Mutagenesis and modeling studies point out that the ligand-binding sites in serotonin receptors are located in the transmembrane domain. However, these binding sites are not very well characterized. Since disulfide bonds and sulfhydryl groups have been shown to play vital roles in the assembly, organization, and function of various G-protein-coupled receptors, we report here the effect of disulfide and sulfhydryl group modifications on the agonist and antagonist binding activity of 5-HT1A receptors from bovine hippocampus. 2. DTT or NEM treatment caused a concentration-dependent reduction in specific binding of the agonist and antagonist in 5-HT1A receptors from bovine hippocampal native and solubilized membranes. This is supported by a concomitant reduction in binding affinity. 3. Pretreatment of the receptor with unlabeled ligands prior to chemical modifications indicate that the majority of disulfides or sulfhydryl groups that undergo modification giving rise to inhibition in binding activity could be at the vicinity of the ligand-binding sites. 4. In addition, ligand-binding studies in presence of GTP-gamma-S, a nonhydrolyzable analogue of GTP, indicate that sulfhydryl groups (and disulfide bonds to a lesser extent) are vital for efficient coupling between the 5-HT1A receptor and the G-protein. 5. Our results point out that disulfide bonds and sulfhydryl groups could play an important role in ligand binding in 5-HT1A receptors.

Stimulation of 5-HT(1A) receptors reduces apoptosis after transient forebrain ischemia in the rat

It has recently been shown that 5-HT(1A) receptor stimulation reduced the infarct volume after occlusion of the middle cerebral artery in rats. Since there is increasing evidence that apoptosis is involved in neurodegenerative diseases and stroke, we investigated whether the 5-HT(1A) agonist Bay x 3702 could protect neurons against apoptotic damage in a rat model of transient forebrain cerebral ischemia. Bay x 3702 (4 microg/kg i.v.) caused a 10% reduction of neuronal damage in the hippocampal CA1 subfield. Higher doses of Bay x 3702 (40 and 12 microg/kg i.v.) did not cause any neuroprotective effect, most likely because of the strong reduction of mean arterial blood pressure during the period of Bay x 3702 infusion. Bay x 3702 (4 microg/kg i.v.) diminished DNA laddering in the hippocampus and striatum 4 days after 10 min forebrain ischemia. These results were confirmed by TUNEL-staining. The anti-apoptotic effect was abolished by additional treatment with the 5-HT(1A) receptor antagonist WAY 100635 (1 mg/kg). Taken together, the results suggest that Bay x 3702 can rescue hippocampal as well as striatal neurons from apoptotic cell death in vivo via stimulation of 5-HT(1A) receptors.

Sleep-wake effects following the selective 5-HT(1A) receptor antagonist p-MPPI in the freely moving rat

The 5-HT(1A) receptors appear to play an important role in the serotonergic modulation of sleep and waking. Both presynaptic somatodendritic 5-HT(1A) autoreceptors and postsynaptic 5-HT(1A) heteroreceptors may be involved. The present study addressed the question of whether the selective 5-HT(1A) receptor antagonist 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethy l-p iperazine (p-MPPI) affected sleep and waking and whether such an effect would be dose-related. Polygraphic recording of sleep and waking in freely moving rats was employed following control injection and three doses of p-MPPI (1, 5 and 10 mg/kg i.p. in a balanced order design. Waking was increased and deep slow wave sleep decreased, while rapid eye movement (REM) sleep was suppressed over the first 6 h following injection, compared to after control injection. REM sleep was also suppressed following 10 mg/kg i.p. of p-MPPI as compared to following 1 mg/kg i.p. of p-MPPI. The interpretation of the effects is complex and the effects are not easily compatible with a simple model for serotonergic sleep-waking modulation. However, the REM sleep reduction probably reflects p-MPPIs ability to block the presynaptic 5-HT(1A) autoreceptors, increasing the firing activity in the serotonergic neurones and possibly inhibiting serotonin sensitive REM sleep active neurones.

Effect of alcohols on G-protein coupling of serotonin(1A) receptors from bovine hippocampus

The serotonin(1A) (5-hydroxytryptamine [5-HT](1A)) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins. Serotonergic signalling has been shown to play an important role in alcohol intake, preference and dependence. G-protein coupling of the 5-HT(1A) receptor serves as an important determinant for serotonergic signalling. We have studied the effect of alcohols on G-protein coupling of bovine hippocampal 5-HT(1A) receptors in native membranes. This was done by monitoring the modulation of ligand (agonist and antagonist) binding in presence of alcohols by guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), a non-hydrolyzable analogue of GTP. Our results show that alcohols inhibit the specific binding of the agonist 8-hydroxy-2-(di-N-propylamino)tetralin (except in case of ethanol) and the antagonist 4-(2'-methoxy)-phenyl-1-[2'-(N-2"-pyridinyl)-p-fluorobenzamido]eth yl- piperazine to 5-HT(1A) receptors in a concentration-dependent manner. Further, we show here that the action of alcohols on the bovine hippocampal 5-HT(1A) receptors could be modulated by guanine nucleotides and that the mode of action of ethanol on the 5-HT(1A) receptor may be quite different than that of other alcohols. The effect of GTP-gamma-S on the agonist and the antagonist binding is found to be markedly different. Our results could be significant in the overall context of the role of G-protein coupling in serotonergic neurotransmission and its role in alcohol tolerance and dependence.

Central 5-HT(1A) receptors: regional distribution and functional characteristics

Among the multiple receptors for serotonin identified to date, the 5-hydroxytryptamine (5-HT)(1A) type is among the best known because selective ligands have been available for more than 15 years. Radioactive derivatives allowed the demonstration of the presence of 5-HT(1A) binding sites mainly in the limbic areas and the raphe nuclei in the brain, where they correspond to postsynaptic receptors and "presynaptic" autoreceptors, respectively. This review article summarizes key data on the molecular, pharmacological, and differential functional properties of pre- versus postsynaptic 5-HT(1A) receptors.

Fluoro analogs of WAY-100635 with varying pharmacokinetics properties

Radiolabeled derivatives of WAY-100635 have been shown to be important for imaging in vivo because of their antagonist properties and their specificity for the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor. Our goal is to prepare a series of radiofluorinated derivatives of WAY-100635 that, in the rat, range in pharmacokinetic properties from nearly irreversible to reversible in their behavior. It appears that derivatives containing a cyclohexanecarboxylic acid (e.g., FCWAY) with its high affinity and high target to nontarget contrast, has properties suited to measure receptor concentration. Derivatives based on phenylcarboxamide (e.g., FBWAY and MeFBAWAY) have properties more suited to the measurement of changes in endogenous serotonin. The compound containing the pyrimidine moiety in place of the pyridine moeity in FBWAY (FBWAY 1,3N) appears to have intermediate properties.

Radioligands for the study of brain 5-HT(1A) receptors in vivo--development of some new analogues of way

[Carbonyl-(11)C]WAY-100635 (WAY) has proved to be a very useful radioligand for the imaging of brain 5-HT(1A) receptors in human brain in vivo with positron emission tomography (PET). WAY is now being applied widely for clinical research and drug development. However, WAY is rapidly cleared from plasma and is also rapidly metabolised. A comparable radioligand, with a higher and more sustained delivery to brain, is desirable since these properties might lead to better biomathematical modelling of acquired PET data. There are also needs for other types of 5-HT(1A) receptor radioligands, for example, ligands sensitive to elevated serotonin levels, ligands labelled with longer-lived fluorine-18 for distribution to "satellite" PET centres, and ligands labelled with iodine-123 for single photon emission computerised tomography (SPECT) imaging. Here we describe our progress toward these aims through the exploration of WAY analogues, including the development of [carbonyl-(11)C]desmethyl-WAY (DWAY) as a promising, more brain-penetrant radioligand for PET imaging of human 5-HT(1A) receptors, and (pyridinyl-6-halo)-analogues as promising leads for the development of radiohalogenated ligands.

Differential discrimination of G-protein coupling of serotonin(1A) receptors from bovine hippocampus by an agonist and an antagonist

We have studied the effect of guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), a non-hydrolyzable analogue of GTP, on agonist and antagonist binding to bovine hippocampal 5-hydroxytryptamine (5-HT)(1A) receptor in native membranes. Our results show that the specific binding of the agonist is inhibited with increasing concentrations of GTP-gamma-S along with a reduction in binding affinity. In sharp contrast to this, antagonist binding to 5-HT(1A) receptor shows no significant reduction and remains invariant over a large range of GTP-gamma-S concentrations. The binding affinity of the antagonist also remains unaltered. This shows that the agonist and the antagonist differentially discriminate G-protein coupling of 5-HT(1A) receptors from bovine hippocampus.

Serotonin1B receptor stimulation enhances cocaine reinforcement

The effects of serotonin1B [5-hydroxytryptamine1B (5-HT1B)] receptor activation on cocaine reinforcement were investigated using intravenous cocaine self-administration by rats. The 5-HT1B receptor agonists 5-methoxy-3-1,2,3,6-tetrahydro-4-pyridinyl-1H-indole (RU 24969) (0.3-3 mg/kg), 3-(1,2,5, 6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94,253) (0.3-3 mg/kg), and 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3, 2-b]pyridine (CP 93,129) (3 and 10 micrograms, i.c.v.) each dose-dependently reduced the self-administration of a cocaine dose on the descending limb of the fixed-ratio 5 (FR-5) cocaine dose-effect function, in a manner similar to the effect produced by increasing the unit dose of cocaine. In addition, each of these 5-HT1B agonists lowered the threshold dose of cocaine that supported self-administration. These results are consistent with a 5-HT1B agonist-induced potentiation of cocaine reinforcement. On a progressive ratio schedule of reinforcement, RU 24969 and CP 94,253 dose-dependently (0.3-3 mg/kg) increased the highest completed ratio for cocaine self-administration, again by producing behavioral alterations similar to those induced by increasing the unit dose of cocaine. The effect of CP 94,253 was dose-dependently blocked by the 5-HT1B/1D receptor partial agonist 2'-methyl-4'-(5-methyl[1,2, 4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid[4-methodoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127, 935) (0.3-10 mg/kg) but was unaffected by the 5-HT1A receptor antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl- benzamide (p-MPPI; 1-10 mg/kg). Self-administration behavior was not maintained when either RU 24969 or CP 94,253 was substituted for cocaine, indicating that these 5-HT1B agonists do not produce significant reinforcing effects alone. Together, these findings indicate that 5-HT1B receptor stimulation facilitates the reinforcing properties of cocaine. These results are in opposition to recent findings with 5-HT1B receptor knock-out mice and may have important ontogenic implications in the area of drug abuse research.

Derivatives of WAY 100635 as potential imaging agents for 5-HT1A receptors: syntheses, radiosyntheses, and in vitro and in vivo evaluation

Analogues of the potent and selective 5-HT1A ligand, WAY 100635, were synthesized and examined as potential candidates for imaging 5-HT1A receptors by positron emission tomography (PET). Several of the analogues displayed nanomolar affinity for the 5-HT1A receptor, comparable to WAY 100635. Three of these were examined in a model of human liver metabolism vis-à-vis WAY 100635. All showed a markedly lower propensity for amide hydrolysis than WAY 100635. Radiolabelling of these three potential PET radiotracers with carbon-11 was readily achieved from [11C]-iodomethane, and the newly synthesized radioligands were tested in vivo in rats for binding to 5-HT1A receptors. Whereas two of the ligands failed to bind to 5-HT1A receptors in vivo, one was successful. The latter, [11C]-7 [4-(2'-methoxyphenyl)-1-[2'-[N-(2'-pyridinyl)-2-bicyclo[2.2.2]octanec arboxamido]ethyl]-piperazine], showed good brain penetration, hippocampal:cerebellar ratios of 10:1 at 45 min postinjection. Blocking studies with a variety of drugs demonstrated that the binding of [11C]-7 in vivo was selective for 5-HT1A receptors. [11C]-7 is a promising candidate as a ligand for imaging 5-HT1A receptors by PET.

Modulation of agonist and antagonist interactions in serotonin 1A receptors by alcohols

The serotonin type IA (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to GTP binding regulatory proteins (G-proteins). Serotonergic signalling has been shown to play an important role in alcohol tolerance and dependence. We have studied the effects of alcohols on ligand (agonist and antagonist) binding to bovine hippocampal 5-HT1A receptor in native as well as solubilized membranes. Our results show that alcohols inhibit the specific binding of the agonist OH-DPAT and the antagonist p-MPPF to 5-HT1A receptors in a concentration-dependent manner.

High-yield radiosynthesis and preliminary in vivo evaluation of p-[18F]MPPF, a fluoro analog of WAY-100635

No-carrier-added 4-[18F]fluoro-N-[2-[1-(2-methoxyphenyl)-1 piperazinyl]ethyl-N-2-pyridinyl-benzamide (p-[18F]MPPF) was synthesized by nucleophilic substitution of the corresponding nitro compound in the presence of Kryptofix 222 and K2CO3 by microwave heating (3 min, 500 W) using a remotely controlled radiosynthesis. Baseline separation of p-[18F]MPPF from the nitro derivative was performed on a semipreparative HPLC C18 column. After Sep-Pak formulation, the radiopharmaceutical was obtained with a radiochemical yield of 25% (EOS) in about 70 min. Specific radioactivity averaged between 1-5 Ci/micromol EOS. Labelling of the ortho and meta derivatives was also attempted. Brain uptake of p-[18F]MPPF was studied with PET on fluothane-anesthetized cats. Following intravenous injection of p-[18F]MPPF, high accumulation of radioactivity was observed in the hippocampus and cerebral cortex. Low levels of radioactivity were observed in cerebellum. At 30 min, the mean hippocampus/cerebellum and cortex/cerebellum ratios were 5 and 3.8, respectively. The accumulation of the tracer was blocked by prior administration of reference WAY-100635, demonstrating the specificity of the ligand.

Atypical in vitro and in vivo binding of [3H]S-14506 to brain 5-HT1A receptors

The tritiated derivative of the potent 5-HT1A receptor agonist S-14506 ¿1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)pipera zine¿ was tested for its capacity to selectively label the serotonin 5-HT1A receptors both in vitro in the rat and the mouse brain, and in vivo in the mouse. In vitro studies showed that the pharmacological profile and the distribution of [3H]S-14506 specific binding sites (Kd = 0.15 nM) in different brain regions matched perfectly those of the prototypical 5-HT1A receptor ligand [3H]8-OH-DPAT. However, in the three regions examined (hippocampus, septum, cerebral cortex), the density of [3H]S-14506 specific binding sites was significantly higher (+66-90%) than that found with [3H]8-OH-DPAT. Whereas the specific binding of [3H]8-OH-DPAT was markedly reduced by GTP and Gpp(NH)p and increased by Mn2+, that of [3H]S-14506 was essentially unaffected by these compounds. In addition, the alkylating agent N-ethylmaleimide was much less potent to inhibit the specific binding of [3H]S-14506 than that of [3H]8-OH-DPAT. Measurement of in vivo accumulation of tritium one hour after i.v. injection of [3H]S-14506 to mice revealed marked regional differences, with about 2.5 times more radioactivity in the hippocampus than in the cerebellum. Pretreatment with 5-HT1A receptor ligands prevented tritium accumulation in the hippocampus but not in the cerebellum. Autoradiograms from brain sections of injected mice confirmed the specific in vivo labeling of 5-HT1A receptors by [3H]S-14506, therefore suggesting further developments with derivatives of this molecule for positron emission tomography in vivo in man.

5-HT7 receptors mediate serotonergic effects on light-sensitive suprachiasmatic nucleus neurons

Serotonin (5-HT) has been shown to phase shift circadian rhythms in mammals and to affect responses of the circadian system to light, but it is not clear which receptors are involved in these actions. We found that drugs which act as 5-HT1A receptor agonists suppressed photic responses of hamster SCN cells, but these drugs also exhibit high affinity for the recently cloned 5-HT7 receptor. We therefore studied the effects of 5-HT agonists and antagonists with differential affinities for 5-HT7 and 5-HT1A receptors on responses of hamster SCN cells to retinal illumination. We confirmed that the 5-HT receptor agonists 5-HT, 8-OH-DPAT and 5-CT, dose-dependently reduced photic activation of SCN cells. These effects could be blocked by co-application of antagonists with high affinities for 5-HT7 receptors: ritanserin or clozapine. The 5-HT1A/B/D antagonist, cyanopindolol, which is inactive at 5-HT7 receptors, did not antagonize the actions of 8-OH-DPAT. Selective 5-HT1A antagonists, WAY100635 and p-MPPI, had weak or no antagonist effects on the responses to 8-OH-DPAT in the SCN, but they effectively antagonized the actions of 8-OH-DPAT in the hippocampus. In the cerebellar cortex where few 5-HT7 receptors are present, ritanserin failed to antagonize the effects of 8-OH-DPAT. Our results indicate that the 5-HT7 receptor subtype plays a major role in mediating the effects of 5-HT on photic responses of SCN cells in the hamster.

[18F]fluoroalkyl analogues of the potent 5-HT1A antagonist WAY 100635: radiosyntheses and in vivo evaluation

Two analogues of the potent 5-HT1A antagonist WAY 100635 have been synthesized and radiolabelled with 18F, namely N-[2-[4-(2-2'-[18F] fluoroethoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xan e carboxamide ([18F]FEC) and N-[2-[4-(2-3'-[18F] fluoropropoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cycloh exa ne carboxamide ([18F]FPC). Biodistribution studies in rats showed selective uptake of both radiotracers in regions known to be rich in 5-HT1A receptors following i.v. injection. The ratio of radioactivity in hippocampus to that in the cerebellum was 5.5 (for [18F]FEC) and 7.5 (for [18F]FPC) at 60 min postinjection. Regional brain heterogeneity of radioactivity could be abolished by pretreatment with WAY 100635 and FPC but was unaffected by pretreatment with a variety of drugs including ketanserin, sulpiride, and SCH 23390. These results are compared vis-a-vis with those obtained using [11C]WAY 100635 to evaluate [18F]FEC and [18F]FPC as potential radiotracers for imaging 5-HT1A receptors by positron emission tomography.