Binding profile of the novel 5-HT1B/1D receptor antagonist, [3H]GR 125,743, in guinea-pig brain: a comparison with [3H]5-carboxamidotryptamine

Measuring endogenous 5-HT release by emission tomography: promises and pitfalls

Molecular in vivo neuroimaging techniques can be used to measure regional changes in endogenous neurotransmitters, evoked by challenges that alter synaptic neurotransmitter concentration. This technique has most successfully been applied to the study of endogenous dopamine release using positron emission tomography, but has not yet been adequately extended to other neurotransmitter systems. This review focuses on how the technique has been applied to the study of the 5-hydroxytryptamine (5-HT) system. The principles behind visualising fluctuations in neurotransmitters are introduced, with reference to the dopaminergic system. Studies that aim to image acute, endogenous 5-HT release or depletion at 5-HT receptor targets are summarised, with particular attention to studies in humans. Radiotracers targeting the 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors and the serotonin reuptake transporter have been explored for their sensitivity to 5-HT fluctuations, but with mixed outcomes; tracers for these targets cannot reliably image endogenous 5-HT in humans. Shortcomings in our basic knowledge of the mechanisms underlying changes in binding potential are addressed, and suggestions are made as to how the selection of targets, radiotracers, challenge paradigms, and experimental design might be optimised to improve our chances of successfully imaging endogenous neurotransmitters in the future.

Chiral aryloxyalkylamines: Selective 5-HT(1B/1D) activation and analgesic activity

A series of chiral 2,3-dichlorophenoxy and 1-naphthyloxy alkylamines were synthesized, and their binding affinities towards 5-HT(1D) and h5-HT(1B) receptors were evaluated. In the naphthyloxy series, the (R)-prolinol derivative was the most selective 5-HT(1D) ligand, while (S)-N-methyl-2-(1-naphthyloxy)propan-1-amine showed the highest selectivity for h5-HT(1B). Both compounds performed as 5-HT(1D) agonists in the isolated guinea pig assay and showed higher analgesic activity than both sumatriptan and the achiral analogue 20 b in the mouse hot-plate test. Neither ligand displayed any affinity for nicotinic ACh receptors present in mouse brain membranes, thus indicating that their analgesic activity does not arise through interaction with these receptors.

[11C]AZ10419369: a selective 5-HT1B receptor radioligand suitable for positron emission tomography (PET). Characterization in the primate brain

The 5-HT1B receptor has been implicated in several psychiatric disorders and is a potential pharmacological target in the treatment of depression. Here we report the synthesis of a novel PET radioligand, [11C]AZ10419369 (5-methyl-8-(4-methyl-piperazin-1-yl)-4-oxo-4H-chromene-2-carboxylic acid (4-morpholin-4-yl-phenyl)-amide), for in vivo visualization of 5-HT1B receptors in the brains of macaques and humans subjects. [11C]AZ10419369 was prepared by N-methylation of (8-(1-piperazinyl)-5-methylchrom-2-en-4-one-2-(4-morpholinophenyl) carboxamide, using carbon-11 methyl triflate. Regional brain uptake patterns of [11C]AZ10419369 were characterized by PET measurements in two macaques and a preliminary study in two human subjects. In addition, AZ10419369 was prepared in tritium labeled form for in vitro autoradiography studies in macaque brain tissue sections. The radiochemical purity of [11C]AZ10419369 was >99% and specific radioactivity was >3600 Ci/mmol. After iv injection of [11C]AZ10419369, 3-4% was in brain after 7.5 min. The regional brain distribution of radioactivity was similar in humans and macaques showing the highest uptake of radioactivity in the occipital cortex and the basal ganglia, in accord with autoradiographic studies performed using [3H]AZ10419369. Uptake was moderate in the temporal and frontal cortical regions, lower in the thalamus and lowest in the cerebellum. In macaques pre-treated with the selective 5-HT1B receptor antagonist, AR-A000002, binding was reduced in a dose-dependent manner, consistent with specific binding to 5-HT1B receptors. These data support [11C]AZ10419369 as a suitable radioligand for labeling 5-HT1B receptors in the primate brain. This radioligand may be useful in future studies evaluating drug-induced receptor occupancy and measurement of brain 5-HT1B receptor levels in patients with psychiatric disorders.

In vitro characterization of AR-A000002, a novel 5-hydroxytryptamine(1B) autoreceptor antagonist

The in vitro pharmacological properties of AR-A000002 ((R)-N-[5-methyl-8-(4-methylpiperazin-1-yl)-1,2,3,4-tetrahydro-2-naphthyl]-4-morpholinobenzamide), a novel 5-hydroxytryptamine (5-HT)(1B) receptor antagonist, were studied. AR-A000002 bound with high affinity to guinea pig cortex and recombinant guinea pig 5-HT(1B) receptors (Ki=0.24 and 0.47 nM) and with 10-fold lower affinity to 5-HT(1D) receptors. The compound displayed weak or no affinity for 63 other binding sites tested. In [35S]GTPgammaS assays AR-A000002 showed 50% efficacy and inhibited 5-HT stimulation with 66% and a pA2 value of 8.9. In slices of guinea pig cortex, AR-A000002 enhanced the outflow of [3H]5-HT upon electrical stimulation. The compound blocked sumatriptan-evoked contraction of rabbit saphenous veins without inducing any contraction itself. Thus, in these two systems AR-A000002 behaved as a 5-HT(1B) receptor antagonist. It is concluded that AR-A000002 is a selective high affinity 5HT(1B) receptor ligand that shows partial agonist activity in recombinant systems. In native tissues AR-A000002 behaves as a 5-HT(1B) receptor antagonist.

A 5-HT2C agonist elicits hyperactivity and oral dyskinesia with hypophagia in rabbits

Serotonergic 5-HT2C and 5-HT1B receptors mediate inhibitory controls of eating. Questions have arisen about potential behavioral and neurological toxicity of drugs that stimulate the 2C site. We evaluated eating and other motor responses in male Dutch-belted rabbits after administration of m-chlorophenylpiperazine (mCPP). Studies conducted in vitro and in vivo assessed the pharmacological specificity of the ingestive actions of this agent. mCPP (0.15-10 micromol/kg sc) reduced consumption of chow and 20% sucrose solution with equal potencies (ED50 approximately equal 0.6 micromol/kg). In radioligand binding to rabbit cortex, mCPP displayed 15-fold higher affinity for 5-HT2C than for 5-HT1B receptors. The serotonin antagonist mesulergine (7000-fold selective for 5-HT2C) reversed the hypophagic action of mCPP, but the 5-HT1B/1D antagonist GR127,935 did not. GR127,935 (0.5 micromol/kg) did prevent hypophagia produced by the highly selective 5-HT1B/1D agonist GR46,611. Observational methods demonstrated that mCPP decreased the frequency of eating chow but increased other motor activities. When rabbits consumed sucrose, videoanalysis revealed that mCPP reduced total time licking and the duration of individual bouts, but not bout frequency or the actual rate of consumption. mCPP increased locomotor and other activities, and greatly increased vacuous oromotor stereotypies and tongue protrusions. Nonetheless, rabbits licked accurately at the spout for sucrose. When sucrose was infused intraorally through a cheek catheter, mCPP actually increased the peak amplitude and overall magnitude of jaw movements. We conclude that mCPP stimulates 5-HT2C receptors to reduce food intake in rabbits. This hypophagia involves disruption of appetitive components of eating and is accompanied by adverse motor actions. This profile raises questions about the use of the 5-HT2C receptor as a target for novel therapeutic agents for obesity.

Withdrawal from chronic cocaine up-regulates 5-HT1B receptors in the rat brain

In the present study we examined the effect of prolonged treatment with cocaine (a sensitization and discrimination paradigm) on the expression of serotonin (5-HT)(1B) receptors in rat brain structures using a quantitative autoradiographic analysis. To estimate the distribution of 5-HT(1B) receptors in several brain coronal sections, we used [N-methyl-(3)H]GR 125743, a 5-HT(1B/1D) receptor antagonist, in the presence of ketanserin (a drug used to block 5-HT(1D) receptors). The binding of [N-methyl-(3)H]GR 125743 in the areas containing dopamine cell bodies (the ventral tegmental area, the substantia nigra) and terminals (the nucleus accumbens shell and core, but not in the caudate-putamen) and in the subiculum of the hippocampus was increased after withdrawal from repeated cocaine in both the discrimination and the sensitization paradigms, either being effective as confirmed by behavioral experiments. Neither acute cocaine injection nor the psychostimulant challenge following its repeated administration affected the binding of [N-methyl-(3)H]GR 125743 in the above brain areas. Our results indicate that withdrawal from chronic cocaine induces up-regulation of 5-HT(1B) receptors in a number of rat brain structures.

Specific labelling of serotonin 5-HT(1B) receptors in rat frontal cortex with the novel, phenylpiperazine derivative, [3H]GR125,743. A pharmacological characterization

Although several tritiated agonists have been used for radiolabelling serotonin (5-hydroxytryptamine, 5-HT)(1B) receptors in rats, data with a selective, radiolabelled antagonist have not been presented. Inasmuch as [3H]GR125,743 specifically labels cloned, human and native guinea pig 5-HT(1B) receptors and has been employed for characterization of cerebral 5-HT(1B) receptor in the latter species [Eur. J. Pharmacol. 327 (1997) 247.], the present study evaluated its utility for characterization of native, cerebral 5-HT(1B) sites in the rat. In homogenates of frontal cortex, [3H]GR125,743 (0.8 nM) showed rapid association (t(1/2)=3.4 min), >90% specific binding and high affinity (K(d)=0.6 nM) for a homogeneous population of receptors with a density (B(max)) of 160 fmol/mg protein. In competition binding studies, affinities were determined for 15 chemically diverse 5-HT(1B) agonists, including 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indole-3-yl]ethylamine (L694,247; pK(i), 10.4), 5-carboxamidotryptamine (5-CT; 9.7), 3-[3-(2-dimethylamino-ethyl)-1H-indol-6-yl]-N-(4-methoxybenzyl)acrylamide (GR46,611; 9.6), 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinyl)-1H-indole (RU24,969; 9.5), dihydroergotamine (DHE; 8.6), 5-H-pyrrolo[3,2-b]pyridin-5-one,1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl (CP93,129; 8.4), anpirtoline (7.9), sumatriptan (7.4), 1-[2-(3-fluorophenyl)ethyl]-4-[3-[5-(1,2,4-triazol-4-yl)-1H-indol-3-yl]propyl]piperazine (L775,606; 6.4) and (minus sign)-1(S)-[2-[4-(4-methoxyphenyl)piperazin-1-yl]ethyl]-N-methyl-3,4-dihydro-1H-2-benzopyran-6-carboxamide (PNU109,291; <5.0). Similarly, affinities were established for 13 chemically diverse antagonists, including N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide (GR125,743; pK(i), 9.1), (-)cyanopindolol (9.0), (-)-tertatolol (8.2), N-(4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiozol-3-yl)biphenyl-4-carboxamide (GR127,935; 8.2), N-[3-(1,4-benzodioxan-5-yl)piperidin-4-yl]N-(indan-2yl)amine (S18127; 7.9), metergoline (7.8), (-)-pindolol (7.6), 1'-methyl-5-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-biphenyl-4-ylcarbonyl]-2,3,6,7-tetrahydro-5H-spiro[furo[2,3-f]indole-3,4'-piperidine] (SB224,289; 7.5) and ketanserin (<5.0). These rank orders of affinity correspond to the binding profile of 5-HT(1B) rather than 5-HT(1D) receptors. The low affinities of L775,066 and PNU109,291 versus L694,247 should be noted, as well as the low affinity of ketanserin as compared to SB224,289. Finally, in line with species differences, the affinities of several ligands including CP93,129, RU24,969, (-)-pindolol and (-)-propanolol in rat 5-HT(1B) sites were markedly different to guinea pig 5-HT(1B) sites labelled with [3H]GR125,743. In conclusion, [3H]GR125,743 is an appropriate tool for the radiolabelling of native, rat 5-HT(1B) receptors and permitted determination of the affinities of an extensive series of ligands at these sites.

Autoradiographic mapping of 5-HT(1B) and 5-HT(1D) receptors in the post mortem human brain using [(3)H]GR 125743

The distribution of 5-HT(1B) and 5-HT(1D) receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [(3)H]GR 125743. [(3)H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [(3)H]GR 125743 binding was observed, reflecting low densities of 5-HT(1B/1D) receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT(1D) receptors and the second using SB 224289 to inhibit 5-HT(1B) receptor binding. The autoradiograms indicated that in the human brain, the 5-HT(1B) receptor is much more abundant than the 5-HT(1D) receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [(3)H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT(1B) receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT(1D) receptor in the human brain.

Constitutive activity at serotonin 5-HT(1D) receptors: detection by homologous GTPgammaS versus [(35)S]-GTPgammaS binding isotherms

Although many G-protein-coupled receptors (GPCRs) may display constitutive activity, their detection has, to date, depended on the use of inverse agonists. The present study exploited a novel procedure to investigate constitutive activity at recombinant human (h) serotonin (5-HT) 5-HT(1D) receptors stably expressed in Chinese hamster ovary (CHO) cells. 5-HT modestly stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]-GTPgammaS) binding to CHO-h5-HT(1D) membranes whereas methiothepin and the 5-HT(1B/1D)-selective ligand, SB224,289, exerted robust inhibition of basal [(35)S]-GTPgammaS binding (inverse agonism). These actions were specific inasmuch as they were reversed by the novel, selective 5-HT(1B/1D) ligand, S18127. Constitutive activity was investigated by homologous inhibition of [(35)S]-GTPgammaS binding to CHO-h5-HT(1D) membranes with unlabelled GTPgammaS. Under 'basal' conditions (absence of receptor ligand), biphasic isotherms were observed. Most (80%) [(35)S]-GTPgammaS binding sites were in the high affinity (HA) versus low affinity (LA) component of the isotherms. HA binding was augmented by 5-HT (to 155%; relative to basal values=100%), but decreased by methiothepin (to 23%) and by SB224,289 (to 67%). In contrast, LA binding was not altered. Further, membranes of untransfected CHO cells exhibited only LA binding sites, indicating that the latter are not related to h5-HT(1D) receptor-G-protein coupling. Thus, at 5-HT(1D) receptors expressed in this CHO cell line, HA binding detected in homologous inhibition experiments (GTPgammaS versus [(35)S]-GTPgammaS) under basal conditions provides a measure of constitutive G-protein activation. Thus, it is suggested that for h5-HT(1D) receptors and, possibly, other GPCRs, inverse agonists will be detectable by [(35)S]-GTPgammaS binding if a HA component is present under basal conditions.

Pharmacological analysis of contractile effects of eletriptan and sumatriptan on human isolated blood vessels

Eletriptan, a second-generation triptan with high affinity for 5-HT(1B/1D) receptors, is highly effective in migraine, with or without aura. We compared the effects of eletriptan and sumatriptan on the human isolated middle meningeal and coronary arteries and saphenous vein, used as models for therapeutic efficacy and potential side effects, and have investigated the role of 5-HT(1B/1D) receptors in contractions induced by these triptans. Concentration-response curves to eletriptan and sumatriptan were constructed in the absence or presence of a selective 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-py rid yl) benzamide (GR125743). All three blood vessels constricted in response to eletriptan and sumatriptan, but the middle meningeal artery relaxed following the highest concentration (100 microM) of eletriptan. In the middle meningeal artery, GR125743 antagonised the contractions induced by both eletriptan (pEC(50): 7.34+/-0.13) and sumatriptan (pEC(50): 6.91+/-0.17) to a similar degree (pA(2): 8. 81+/-0.17 and 8.64+/-0.21, respectively). In the human coronary artery and saphenous vein, sumatriptan-induced contractions (pEC(50): 6.24+/-0.14 and 6.19+/-0.12, respectively) were also potently antagonised by GR125743 (pA(2): 8.18+/-0.27 and 8.34+/-0.12, respectively). The eletriptan-induced contractions of the human saphenous vein (pEC(50): 6.09+/-0.13) were antagonised less effectively by GR125743 (pK(B): 7.73+/-0.18), and those of the human coronary artery (pEC(50): 5.54+/-0.22) remained unaffected by GR125743 up to a concentration of 100 nM. These results suggest that (i) based on the differences in pEC(50) values, the cranioselectivity of eletriptan (63-fold) is higher than that of sumatriptan (5-fold) in coronary artery, (ii) the contractile effects of sumatriptan and eletriptan (lower concentrations) in the three blood vessels are mediated via the 5-HT(1B) receptor, and (iii) additional mechanisms seem to be involved in coronary artery and saphenous vein contractions and middle meningeal artery relaxation following high concentrations of eletriptan.

Humanization of mouse 5-hydroxytryptamine1B receptor gene by homologous recombination: in vitro and in vivo characterization

We replaced the coding region of the murine 5-hydroxytryptamine (5-HT)1B receptor by the human 5-HT1B receptor using homologous recombination in embryonic stem cells and generated and characterized homozygous transgenic mice that express only the human (h) 5-HT1B receptor. The distribution patterns of h5-HT1B and murine (m) 5-HT1B receptor mRNA and binding sites in brain sections of transgenic and wild-type mice were identical as measured by in situ hybridization histochemistry and radioligand receptor autoradiography. When measured in parallel under identical conditions, the h5-HT1B receptor expressed in mouse brain had the same pharmacological characteristics as that in human brain. Stimulation by 5-HT1B agonists of [35S]guanosine-5'-O-(3-thio)triphosphate binding in brain sections demonstrated the functional coupling of the h5-HT1B receptor to G proteins in mouse brain. In tissue slices from various brain regions, electrically stimulated [3H]5-HT release was not modified by 5-HT1B agonists in tissue from either transgenic and wild-type mice; a 5-HT1B antagonist enhanced electrically stimulated [3H]5-HT release in wild-type mouse brain, but was ineffective in the transgenics. The centrally active 5-HT1A/5-HT1B agonist RU24969 induced hypothermia but did not increase locomotor activity in the transgenic mice. The ineffectiveness of RU24969 in the transgenic mice could be due to the lower affinity of the compound for the h5-HT1B receptor compared with the m5-HT1B receptor. The present study demonstrates a complete replacement of the mouse receptor by its human receptor homolog and a functional coupling to G proteins. However, modulation of [3H]5-HT release could not be shown. Furthermore, behavioral effects were not clearly observed, which may be due to a lack of appropriate tools.

Serotonin-induced inhibition of locomotor rhythm of the rat isolated spinal cord is mediated by the 5-HT1 receptor class

The neurotransmitter serotonin (5-HT) induces rhythmic motor patterns (fictive locomotion) of the neonatal rat spinal cord in vitro; this is a useful experimental model to study the generation of a motor programme at exclusively spinal level. Nevertheless, 5-HT slows down the fictive locomotion typically elicited by activation of NMDA glutamate receptors, suggesting a complex action of this monoamine. By means of electrophysiological recordings from multiple ventral roots we demonstrated that the decrease caused by 5-HT in NMDA-induced periodicity was dose-dependent, enhanced after pharmacological blocking of 5-HT2 excitatory receptors, and imitated by pharmacological agonists of the 5-HT1 receptor family. Selective blockers of the 5-HT1A or 5-HT1B/D receptor classes, either alone or in combination, largely (but not completely) attenuated this inhibitory action of 5-HT. It is concluded that the principal inhibitory action of 5-HT on the spinal locomotor network was mediated by certain subtypes of the 5-HT1 receptor class, which tends to oppose the 5-HT2 receptor-mediated excitation of the same network.