Biochemical assessment of the central 5-HT agonist activity of RU 24969 (a piperidinyl indole)

Characterization of the 5-HT1B recognition site in rat brain: binding studies with (-)[125I]iodocyanopindolol

(-)[125I]Iodocyanopindolol ([125I]CYP) labels rat brain membrane sites which display high affinity for several serotonergic and beta-adrenergic compounds. The binding of [125I]CYP to these serotonergic recognition sites was evaluated in the presence of 30 microM (-)isoprenaline in order to suppress binding to beta-adrenoceptors. [125I]CYP binds in rat cortex membranes rapidly, reversibly and stereoselectively to a finite number of recognition sites: Bmax = 180 fmol/mg, KD = 230 pM. Similar affinity values of [125I]CYP were obtained in membranes from rat hippocampus and striatum. Kinetic, saturation and competition experiments suggest that under these conditions [125I]CYP binds to a single serotonergic recognition site named 5-HT1B. The pharmacological profile of 5-HT1B sites is characteristic of a 5-HT1 binding site and shows the following rank order of affinity for agonists: RU 24969, (5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]1H-indole) greater than 5-CT, (5-carboxamidotryptamine) greater than 5-HT, (5-hydroxytryptamine, serotonin) greater than 5-OCH3-T, (5-methoxytryptamine) much greater than 2-CH3-5-HT, (2-methylserotonin) greater than 8-OH-DPAT, (8-hydroxy-2-(di-n-pro-pylamino)-tetralin). The rank order of affinity for antagonists is: (+/-)ICYP, ((+/- )-3-I-cyano-pindolol) greater than (-)21-009, (4-[3-ter-butyl-amino-2-hydroxy-propoxy]-indol-2-carbonic acid isopropyl ester) greater than (+)21-009 greater than (-)propranolol greater than metitepin greater than (-)pindolol much greater than ketanserin greater than spiroperidol greater than mesulergine. 5-HT1B recognition sites display low affinity for selective beta 1- and beta 2-adrenoceptor antagonists, e.g. atenolol, betaxolol, ICI 89-406 and ICI 118-551. The low affinity of 5-HT1B recognition sites for some 5-HT1A, 5-HT1C and 5-HT2 selective compounds (e.g. 8-OH-DPAT, mesulergine, ketanserin) suggests that 5-HT1B recognition sites are pharmacologically different from 5-HT1A, 5-HT1C and 5-HT2 recognition sites.

Molecular pharmacology of 5-HT1 and 5-HT2 recognition sites in rat and pig brain membranes: radioligand binding studies with [3H]5-HT, [3H]8-OH-DPAT, (-)[125I]iodocyanopindolol, [3H]mesulergine and [3H]ketanserin

The pharmacological characteristics of the binding of [3H]8-OH-DPAT ([3H]8-hydroxy-2(di-n-propylamino)tetralin, [125I]CYP ((-)[125I]iodocyanopindolol) (in the presence of 30 microM (-)isoprenaline) and [3H]mesulergine to 5-HT1 recognition sites were studied in rat and pig brain membranes. [3H]8-OH-DPAT bound in rat and pig cortex to the 5-HT1A recognition site characterized by high affinity for 5-CT (5-carboxamido-tryptamine), 8-OH-DPAT, 5-HT (5-hydroxytryptamine, serotonin) and low affinity for pirenperone, ketanserin and mesulergine. [125I]CYP bound in rat but not in pig cortex to the 5-HT1B site which shows high affinity for (-)21-009 (4[3-ter-butyl-amino-2-hydroxy-propoxy]indol-2-carbonic acid isopropyl ester), (+/-)ICYP (3-I-cyanopindolol), 5-HT, RU 24969 (5-methoxy-3-[1,2,3,6-tetrahydropyridon-4-yl]1H-indole) and low affinity for 8-OH-DPAT, mesulergine and pirenperone. [3H]Mesulergine bound in pig choroid plexus and in rat cortex (besides binding to 5-HT2 sites in rat cortex) to the 5-HT1C recognition site characterized by high affinity for metergoline, mesulergine, 5-HT and methergine and low affinity for (-)21-009, ICYP, 8-OH-DPAT and spiroperidol. The pharmacological profile of 5-HT1A sites in rat and pig cortex appears to be identical; 5-HT1C sites in pig choroid plexus and rat cortex show no differences. In contrast, it was not possible to label 5-HT1B sites with [125I]CYP in pig brain membranes indicating that like 5-HT2 receptors, 5-HT1 recognition sites show species differences. The pharmacological profiles of the three 5-HT1 recognition sites are clearly different from one another. Furthermore, the pharmacological profile of each individual 5-HT1 recognition site is also different from that of the 5-HT2 receptors labelled with [3H]ketanserin in rat cortex membranes although some similarities exist between 5-HT2 and 5-HT1C sites. Finally, the beta-adrenoceptor antagonist (-)21-009 which has different affinities for 5-HT1A, 5-HT1B and 5-HT1C recognition sites, yielded triphasic competition curves for [3H]5-HT binding in rat cortex membranes providing evidence that [3H]5-HT labels three distinct 5-HT1 sites in these membranes.

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors

The distribution of serotonin-1 (5-HT1) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with [3H]serotonin (5-[3H]HT), 8-hydroxy-2-[N-dipropylamino-3H]tetralin (8-OH- [3H]DPAT), [3H]LSD and [3H]mesulergine, and the densities quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Competition experiments for 5-[3H]HT binding by several serotonin-1 agonists led to the identification of brain areas enriched in each one of the three subtypes of 5-HT1 recognition sites already described (5-HT1A, 5-HT1B, 5-HT1C). The existence of these 'selective' areas allowed a detailed pharmacological characterization of these sites to be made in a more precise manner than has been attained in membrane-binding studies. While 5-[3H]HT labeled with nanomolar affinity all the 5-HT1 subtypes, the other 3H-labeled ligands labeled selectively 5-HT1A (8-OH-[3H]DPAT), 5-HT1C ([3H]mesulergine) and both of them ([3H]LSD). Very high concentrations of 5-HT1 receptors were localized in the choroid plexus, lateroseptal nucleus, globus pallidus and ventral pallidum, dentate gyrus, dorsal subiculum, olivary pretectal nucleus, substantia nigra, reticular and external layer of the entorhinal cortex. The different fields of the hippocampus (CA1-CA4), some nuclei of the amygdaloid complex, the hypothalamic nuclei and the dorsal raphé, among others, also presented high concentrations of sites. Areas containing intermediate densities of 5-HT1 receptors included the claustrum, olfactory tubercle, accumbens, central grey and lateral cerebellar nucleus. The nucleus caudate-putamen and the cortex, at the different levels studied, presented receptor densities ranging from intermediate to low. Finally, in other brain areas--pons, medulla, spinal cord--only low or very low concentrations of 5-HT1 receptors were found. From the areas strongly enriched in 5-HT1 sites, dentate gyrus and septal nucleus contained 5-HT1A sites, while globus pallidus, dorsal subiculum, substantia nigra and olivary pretectal nucleus were enriched in 5-HT1B. The sites in the choroid plexus, which presented the highest density of receptors in the rat brain, were of the 5-HT1C subtype. The distribution of 5-HT1 receptors reported here is discussed in correlation with the distribution of serotoninergic neurons and fibers, the related anatomical pathways and the effects which appear to be mediated by these sites.

beta-Adrenoceptor blocking agents recognize a subpopulation of serotonin receptors in brain

The interaction of beta-adrenoceptor blocking agents with serotonin receptors in rat brain was analyzed by quantitative light microscopic autoradiography using (-) [125I]cyanopindolol (ICYP) as a ligand. In some brain areas such as the globus pallidus and dorsal subiculum, binding of ICYP was blocked with high affinity by some serotoninergic ligands, RU-24969 and serotonin, but not by lysergic acid diethylamide and 8-hydroxy-2-(N,N-di-n-propylamino)tetralin. Some beta-blockers also showed high affinity for these sites. These results supply direct evidence for the labeling of a subclass of serotonin receptors, the 5-HT-1B class, providing anatomical and pharmacological basis for some known serotonin-related effects of beta-adrenoceptor drugs.

The 5-HT1 receptor agonist RU-24969 decreases 5-hydroxytryptamine (5-HT) release and metabolism in the rat frontal cortex in vitro and in vivo

K+-stimulated release of [3H]-5-hydroxytryptamine ( [3H]-5-HT) from rat frontal cortex slices was decreased by the 5-HT receptor agonists 5-methoxy-n1N-dimethyltryptamine and 5-methoxy-3(1,2,3,6,-tetrahydro-4-pyrindinyl)-1H-indole (RU-24969) (1 X 10(-5)M). RU-24969 (10 mg kg-1, i.p.) decreased extracellular 5-HT and its metabolite 5-hydroxyindoleacetic acid measured in vivo by use of intracerebral dialysis combined with high performance liquid chromatography and electrochemical detection. The decrease in extracellular 5-hydroxyindoleacetic acid in vivo after RU-24969 (10 mg kg-1, i.p.) was also observed by in vivo voltammetry. The non-selective 5-HT antagonist metergoline prevented the RU-24969-induced decrease in 5-HT release and metabolism in vivo while the 5-HT2 receptor antagonist R-55669 (ritanserin) did not. The results support the view that RU-24969 stimulates a 5-HT1 receptor that is involved in the autoregulation of 5-HT release and metabolism.

Differential selectivities of RU 24969 and 8-OH-DPAT for the purported 5-HT1A and 5-HT1B binding sites. Correlation between 5-HT1A affinity and hypotensive activity

RU 24969 and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) inhibited the specific binding of [3H]5-HT (2 nM) to rat brain membranes with shallow displacement curves. The displacement data were best fitted with a model of two independent, high and low affinity binding sites. Following addition of spiperone (1 microM) as a selective ligand for the putative 5-HT1A recognition site of [3H]5-HT, the displacement curve of RU 24969 underwent a leftward shift, whereas spiperone induced a shift to the right for the displacement curve of 8-OH-DPAT. In contrast to spiperone, pindolol (1 microM) shifted the displacement curve of RU 24969 to the right. These results suggest that RU 24969 possesses preference for the purported 5-HT1B subtype of central 5-HT1 recognition site. The reported significant linear correlation between hypotensive activity following intravenous (i.v.) administration to anesthetized rats and affinity for the central 5-HT1 binding site could only be maintained by incorporation of the affinity of RU 24969 for its low and 8-OH-DPAT for its high affinity binding site. Based on the proposal that the 5-HT1A site corresponds to the high affinity site of 8-OH-DPAT and the low affinity site of RU 24969, it is hypothesized that the late depressor phase of 5-HT agonists in rats is mediated by activation of peripheral (vascular) 5-HT receptors which have similarities with the 5-HT1A subtype of central 5-HT1 recognition site.

The putative 5-HT1 receptor agonist, RU 24969, inhibits the efflux of 5-hydroxytryptamine from rat frontal cortex slices by stimulation of the 5-HT autoreceptor

The putative central 5-HT receptor agonist, 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole succinate (RU 24969), was found to be a potent inhibitor of the continuous K+ evoked efflux of [3H]5-HT from superfused rat frontal cortex slices (pD2 7.45). The effects of RU 24969 were attenuated by the putative 5-HT autoreceptor antagonists, methiothepin, quipazine and (-)-propranolol but not by the alpha 2-adrenoceptor antagonist, idazoxan. It is concluded that RU 24969 inhibits K+ evoked efflux of [3H]5-HT from rat frontal cortex slices by stimulation of the 5-HT autoreceptor. Moreover, since RU 24969 potently displaced ligand binding to the 5-HT1 and 5-HT1B recognition sites but was only weakly active at the 5-HT2 receptor, the results lend support to the claim for a pharmacological resemblance between the 5-HT autoreceptor and the 5-HT1 recognition site and in particular the low affinity 5-HT1B subtype.

[3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to pre- and postsynaptic 5-hydroxytryptamine sites in various regions of the rat brain

The specific binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([ 3H]8-OH-DPAT) to 5-hydroxytryptamine (5-HT)-related sites was investigated in several regions of the rat brain. Marked differences were observed in the characteristics of binding to membranes from hippocampus, striatum, and cerebral cortex. Hippocampal sites exhibited the highest affinity (KD approximately 2 nM) followed by the cerebral cortex (KD approximately 6 nM) and the striatum (KD approximately 10 nM). Ascorbic acid inhibited specific [3H]8-OH-DPAT binding in all three regions but millimolar concentrations of Ca2+, Mg2+, and Mn2+ enhanced specific binding to hippocampal membranes, whereas only Mn2+ increased it in the cerebral cortex and all three cations inhibited specific binding to striatal membranes. Guanine nucleotides (0.1 mM GDP, GTP) inhibited binding to hippocampal and cortical membranes only. As intracerebral 5,7-dihydroxytryptamine markedly decreased [3H]8-OH-DPAT binding sites in the striatum, but not in the hippocampus, the striatal sites appear to be on serotoninergic afferent fibers. In contrast, in the hippocampus the sites appear to be on postsynaptic 5-HT target cells, as local injection of kainic acid decreased their density. Both types of sites appear to be present in the cerebral cortex. The postsynaptic hippocampal [3H]8-OH-DPAT binding sites are probably identical to the 5-HT1A subsites, but the relationship between the presynaptic binding sites and the presynaptic autoreceptors controlling 5-HT release deserves further investigation.

Progesterone stimulates sexual behaviour in female rats by increasing 5-HT activity on 5-HT2 receptors

We have investigated the possibility that there is a correlation between female sexual receptivity and CNS serotonergic activity. In ovariectomized rats primed with a submaximal steroid regime of 2 micrograms oestradiol benzoate (OB) plus 0.2 mg progesterone (P), so that only a proportion were receptive, the 5-hydroxyindoleacetic acid:5-hydroxytryptamine (5-HT) ratio in the hypothalamus was greater in the receptive compared to the non-receptive rats. When rats were primed with 10 micrograms OB alone, there was no difference in 5-HT activity in the receptive and non-receptive animals. Depletion of hypothalamic 5-HT levels by p-chlorophenylalanine methyl ester (PCPA) inhibited behaviour normally induced by OB plus P and this could be reversed by 5-hydroxytryptophan. PCPA had no effect in animals made receptive by OB alone. This indicates that P, but not OB, exerts its stimulatory effect on sexual behaviour via increasing 5-HT activity. Administration of 5-HT into the lateral, but not 3rd ventricle stimulated sexual behaviour and systemic injection of the selective 5-HT1 and 5-HT2 agonists, Ru 24969 and MK 212, inhibited and stimulated behaviour, respectively. It is suggested that 5-HT has a dual effect on female sexual receptivity acting via different systems, the inhibitory tract acting on 5-HT1 and the stimulatory tract on 5-HT2 receptors.

A behavioural and biochemical study in mice and rats of putative selective agonists and antagonists for 5-HT1 and 5-HT2 receptors

Radioligand binding techniques have demonstrated the existence of 5-hydroxytryptamine (5-HT) binding subtypes: 5-HT2, 5-HT1A and 5-HT1B. These techniques have also indicated that certain drugs appear to show sub-type specificity: 8-hydroxy-2-(di-n-propylamino)tetralin(8-OH-DPAT), a 5-HT1A agonist; 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)1-H indole (RU 24969), a 5-HT1B agonist; and ritanserin, a 5-HT2 antagonist. (-)-Propranolol is a 5-HT1 antagonist of uncertain sub-type specificity. An examination has been made in mice and rats of the behavioural and biochemical effects of these drugs to determine whether the binding sites have physiological functions and further characterise the behavioural models. Administration of carbidopa (25 mg kg-1) plus 5-hydroxytryptophan (100 mg kg-1) produced head-twitch behaviour in mice which was antagonized by ritanserin (ED50 = 65 micrograms kg-1) but not (-)-propranolol (20 mg kg-1). 8-OH-DPAT (1-10 mg kg-1 s.c.) and RU 24949 (5 mg kg-1 i.p.) did not produce head-twitch behaviour. 8-OH-DPAT decreased 5-HTP- but not 5-methoxy-N-N-dimethyltryptamine (5 mg kg-1)-induced head-twitch by a (-)-propranolol-insensitive mechanism. Locomotor activity produced in mice by RU 24969 (3 mg kg-1) was antagonized by (-)-propranolol (20 mg kg-1) but not the (+)-isomer. (-)-Propranolol did not antagonize the behaviour induced in rats. In mice, both 8-OH-DPAT and RU 24969 markedly inhibited whole brain 5-HT synthesis and this effect was not antagonized by (-)-propranolol. In rats, 8-OH-DPAT (3 mg kg-1 s.c.) produced all the behavioural changes seen after quipazine (25 mg kg-1). (-)-Propranolol inhibited the behaviour changes produced by both agonists, while ritanserin antagonized the behaviour produced by quipazine but not 8-OH-DPAT. It is concluded, therefore, that the 5-HT1A receptor exists between the 5-HT2 receptor and the behavioural effectors. 8-OH-DPAT (at 20 degrees C ambient temperature) rapidly decreased rat body temperature, an effect antagonized by (-)-propranolol but not ritanserin. Quipazine (at 27 degrees C ambient temperature, but not 20 degrees C) increased body temperature but the effect was not blocked by either antagonist. Ritanserin does not antagonize apomorphine-induced locomotion in either species. 9 We suggest that 5-HT-induced head-twitch behaviour in mice is a useful 5-HT2 receptor model and the temperature change following 8-OH-DPAT injection in rats may be a 5-HT,A model. While (-)- propranolol antagonizes 8-OH-DPAT effects in rat, it does not inhibit 8-OH-DPAT effects in mice, and instead antagonizes RU 24969-induced locomotion. Its status as a 5-HT, antagonist remains illdefined.

Mesulergine, a selective serotonin-2 ligand in the rat cortex, does not label these receptors in porcine and human cortex: evidence for species differences in brain serotonin-2 receptors

The kinetic and pharmacological characteristics of the binding of [3H]ketanserin and [3H]mesulergine to frontal cortical brain membranes from rat, pig and human were studied. In the 3 species [3H]ketanserin labeled sites with the characteristics of the 5-HT2 receptors previously described in the rat. In contrast, [3H]mesulergine labeled 5-HT2 receptors in rat, but not in pig and human cortices. The characteristics of the sites labeled by [3H]mesulergine in pig cortex were similar to those of sites in the choroid plexus of rats, pigs and humans. While several reputed 5-HT2 ligands presented a similar affinity for the [3H]ketanserin binding sites in the 3 species, other such ligands, e.g. mesulergine, methysergide, cinanserin and LSD which displaced these sites with high affinity in rat brain, had lower affinities in pig and human brain. These results indicate that 5-HT2 receptors show different pharmacological profiles in different species. Caution should thus be exerted in extrapolating data from laboratory animals to humans.

The binding of serotonergic ligands to the porcine choroid plexus: characterization of a new type of serotonin recognition site

The kinetic and pharmacological characteristics of the binding of [3H]5-HT (serotonin), [3H]8-OH-DPAT (8-OH-2-di-n-propylaminotetraline), [3H]LSD, [3H]ketanserin and [3H]mesulergine to membranes from frontal cortex, hippocampus and choroid plexus of pig brain were studied. The binding of these ligands to frontal cortex and hippocampus demonstrated the presence of 5-HT1 and 5-HT2 sites in both tissues, although hippocampus was richer in 5-HT1 (subtype 5-HT1A) sites. [3H]5-HT, [3H]mesulergine and [3H]LSD labeled the pig choroid plexus with high affinity. The pharmacological profiles of [3H]5-HT and [3H]mesulergine binding to this tissue were closely comparable. Ligands reported as selective for 5-HT1A, 5-HT1B or 5-HT2 subtypes did not show high affinity for these binding sites. Therefore, these 5-HT binding sites in pig choroid plexus could be named 5-HT1C. Other drugs with a high affinity for these sites were methysergide and mianserine. In pig frontal cortex, [3H]5-HT labeled the different subtypes of 5-HT1 sites. In contrast, [3H]mesulergine bound in pig frontal cortex to a small population of sites with pharmacological properties similar to those of the choroid plexus 5-HT1C sites. Possible physiological functions in which these sites might be involved are discussed.

Discriminative stimulus properties of the serotonin agonist 1-(3-trifluoromethylphenyl)piperazine (TFMPP)

Using a standard two-lever drug discrimination procedure, twelve rats were trained to discriminate 1.0 mg/kg of the serotonin (5-HT) agonist TFMPP from saline. Once trained, the animals displayed a dose-related decrease in discriminative performance upon administration of lower doses of TFMPP. Tests of stimulus generalization were performed using the purported 5-HT agonist RU-24, 969 and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM). While TFMPP produced stimulus effects similar to those of RU-24,969, these effects seem to be dissimilar to those of DOM. The results of the present study suggest that the discriminative stimulus effects of TFMPP may involve a 5-HT1-related mechanism.

Calcium-dependency of the pressor responses to selective 5-hydroxytryptamine receptor agonists in pithed rats

The pressor responses to six selective 5-HT receptor agonists, quipazine, TMFPP, Org 10155, Ru 24969, 5-methoxytryptamine and 5-methoxy-N,N-dimethyltryptamine, were quantified and tested for calcium dependency following i.v. administration to pithed normotensive rats. The pressor responses induced by 5-methoxytryptamine and 5-methoxy-N,N-dimethyltryptamine were not influenced by calcium entry blockade by means of nifedipine. The piperazine derivatives quipazine, TMFPP and Org 10155 were full agonists, but the pressor responses to these agonists were sensitive to calcium entry blockade. 5-Methoxy-N,N-dimethyltryptamine and Ru 24969 were found to be partial agonists. It is submitted as a hypothesis that within the present series of 5-HT receptor agonists, a restricted side chain flexibility will enhance the dependency of their pressor responses to extracellular calcium.

Antagonism of vascular serotonin receptors by m-chlorophenylpiperazine and m-trifluoromethylphenylpiperazine

m-Chlorophenylpiperazine and m-trifluoromethylphenylpiperazine, two compounds that act as agonists at central serotonin receptors mediating certain neuroendocrine, behavioral and serotonin turnover effects, lacked appreciable agonist activity at serotonin receptors mediating contraction of the rat jugular vein. Instead, these compounds were potent antagonists of serotonin-induced contraction of the jugular vein. Apparently these non-indole compounds affect serotonin receptors in various tissues differently, being agonists in brain but mainly antagonists at some peripheral serotonin receptor sites.