Günter Engel

Identity of inhibitory presynaptic 5-hydroxytryptamine (5-HT) autoreceptors in the rat brain cortex with 5-HT1B binding sites

Summary1.In rat brain cortex slices preincubated with [3H]5-HT, the potencies of 17 5-HT receptor agonists to inhibit the electrically evoked3H overflow and the affinities of 13 antagonists (including several β-adrenoceptor blocking agents) to antagonize competitively the inhibitory effect of unlabelled 5-HT on evoked3H overflow were determined.2.The affinities of the compounds for 5-HT1B and 5-HT2 binding sites in rat brain cortex membranes (labelled by [125I]cyanopindolol = [125I]-CYP in the presence of 30 μmol/l isoprenaline and [3H]ketanserin, respectively), for 5-HT1A binding sites in pig and rat brain cortex membranes (labelled by [3H]8-hydroxy-2-(di-n-propylamino)tetralin = [3H]8-OH-DPAT) and for 5-HT1C binding sites in pig choroid plexus membranes (labelled by [3H]mesulergine) were also determined. The affinities of the drugs for the various 5-HT recognition sites ranged over 4–5 log units (the functional experiments revealed the same range of differences between the drugs).3.There were no significant correlations between the affinities of the drugs at 5-HT1C and 5-HT2 binding sites and their potencies or affinities, determined for the 5-HT autoreceptors. In contrast, significant correlations were found between the potencies or affinities of the drugs for the autoreceptors and their affinities at 5-HT1A or 5-HT1B binding sites; the best correlations were obtained with the 5-HT1B binding site.4.Some of the drugs investigated were not included in the correlation since their agonistic or antagonistic effects on the autoreceptors were weak and pEC30 or apparent pA2 values could not be determined (<5.5). Among these drugs, 8-OH-DPAT, TVX Q 7821 (2-(4-(4-(2-pyrimidin-yl)-1-piperazinyl)-butyl)-1,2-benzisothiazol-3(2H)one-1,1-dioxide) and spiperone showed a very low affinity for 5-HT1B binding sites (pKD<5.3), but a high affinity for 5-HT1A binding sites (pKD>7.2).5.In conclusion, the evidence indicates that the presynaptic 5-HT autoreceptor belongs to the 5-HT1B receptor subtype.

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.

(±)[125Iodo]cyanopindolol, a new ligand for β-adrenoceptors: Identification and quantitation of subclasses of β-adrenoceptors in guinea pig

Summary(±)[125Iodo]cyanopindolol (ICYP) is a radioligand which binds with an extraordinarily high affinity and specificity to β-adrenoceptors. In contrast to (±)[125Iodo]-hydroxybenzylpindolol (IHYP), the new ligand has neither affinity to α- nor to 5-HT-receptors. The dissociation constants of ICYP for β-adrenoceptors in various tissues range from 27 to 40 pM, thereby exceeding the affinity of IHYP by a factor of ∼ 3.ICYP does not discriminate between β1− and β2−. Therefore, the densities of the two receptor subtypes can be determined from competition curves of ICYP by drugs previously found to show in vitro selectivity for β1−adrenoceptors.The guinea pig left ventricle contains only β1− adrenoceptors, whereas in the lung tissue, the ratio of β1− to β2−adrenoceptors is 1 to 4. The calculated affinities of five β1− selective antagonists for β1−adrenoceptors were nearly identical in the ventricle and the lung.Kinetic studies of ICYP binding to guinea pig lung membranes indicated that the dissociation reaction consists of two components, a fast process (t1/2=9 min) and a slower process (t1/2=8.8 h). A mathematical treatment revealed two possibilities of interpretation: 1. Two forms of the receptor exist which are interconvertible. 2. The (+)- and (−)-enantiomers of ICYP dissociate with different rate constants.The low dissociation constant of ICYP in combination with its high specific radioactivity (2175 Ci mmole−1) allows binding studies to be carried out with small protein and ligand concentrations, e.g. 3 μg protein per assay in guinea pig lung membranes.

Characterization of the 5-HTIB 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.

5-HT1A-receptors mediate stimulation of adenylate cyclase in rat hippocampus.

Summary1.Serotonin (5-HT) stimulated adenylate cyclase activity in homogenates of rat hippocampus. This effect was pharmacologically characterised with a series of agonists and antagonists of various structural classes.2.These compounds where also tested in radioligand binding studies using selective ligands for the various subtypes of 5-HT1 and 5-HT2 receptors. 5-HT1A, 5-HT1B and 5-HT1C recognition sites were labelled with [3H]8-OH-DPAT ([3H]8-hydroxy-2-(di-n-propylamino)-tetralin) in pig cortex membranes, [125I]CYP([125I]iodocyanopindolol) in rat cortex and [3H]mesulergine in pig choroid plexus membranes, respectively.3.The rank order of potency of 13 agonists stimulating adenylate cyclase activity in homogenates of rat hippocampus was in good agreement with the rank order of affinity of these agonists for the 5-HT1A binding site: N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT)>5-carboxamidotryptamine (5-CT)>8-OH-DPAT>5-HT> 5-methoxytryptamine (5-OCH3T)>d-LSD>5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969)>α-methylserotonin (α-CH3-5-HT)>dopamine>2-methylserotonin (2-CH3-5-HT). The correlation between the respective potencies and affinities of these agonists was r=0.934, P<0.001.4.There was no correlation between stimulation of adenylate cyclase activity by these agonists and their affinity for 5-HT1B, 5-HT1C or 5-HT2 binding sites. r=0.381–0.108, P<0.20–0.73.5.Potent antagonists at D-1 receptors (SCH 23390), 5-HTM receptors (ICS 205-930), 5-HT2-receptors (ketanserin) and 5-HT1C-receptors (mesulergine) antagonised the 5-HT stimulated adenylate cyclase activity only at very high concentrations. In contrast, spiperone and metitepin were potent antagonists of the effect of 5-CT and 5-HT on adenylate cyclase. The use of these selective antagonists allowed to exclude the possibility that 5-HT stimulates adenylate cyclase activity in rat hippocampus through D-1, 5-HTM, 5-HT2 or 5-HT1C receptors.6.These data support the concept that 5-HT stimulated adenylate cyclase activity in rat hippocampus is mediated by a 5-HT1A receptor.

Evidence for common pharmacological properties of [3H]5-hydroxytryptamine binding sites, presynaptic 5-hydroxytryptamine autoreceptors in CNS and inhibitory presynaptic 5-hydroxytryptamine receptors on sympathetic nerves

SummaryThe affinities of 16 5-hydroxytryptamine (5-HT) receptor agonists (indole derivatives) and 7 5-HT receptor antagonists for [3H]5-hydroxytryptamine ([3H]5-HT) binding sites in rat cerebral cortex membranes were determined. In addition, the potencies of the agonists for inhibiting the electrically induced tritium overflow from rat brain cortex slices preincubated with [3H]5-HT and from canine saphenous veins preincubated with [3H]noradrenaline were measured. Furthermore, the potencies of the indole derivatives for inducing contractile responses of canine saphenous veins were recorded. In addition, the interaction of the antagonists with unlabelled 5-HT at the 5-HT autoreceptor was studied in rat brain cortex slices.There was a good correlation between the binding affinities of the indole derivatives for the [3H]5-HT sites of rat brain cortex membranes and their potencies for inhibiting the evoked tritium overflow from both rat brain cortex slices and strips of canine saphenous vein. Comparison of the inhibition constants derived from the overflow experiments in both tissues again revealed a high correlation coefficient while there was only weak correlation between the binding affinities in rat brain cortex and the contractile potencies of the drugs in canine saphenous vein strips.When 5-HT receptor antagonists were investigated, metitepin and metergoline showed moderate affinities for the 5-HT autoreceptors in rat brain cortex slices, whereas quipazine had only weak affinity, and ketanserin, metoclopramide, cinanserin and cyproheptadine exhibited no antagonistic property. In binding experiments, the competition curves of most 5-HT receptor antagonists were biphasic, suggesting that the [3H]5-HT binding sites are heterogeneous. The affinities of the antagonists to the low affinity binding sites were roughly in accordance with their affinities for the 5-HT autoreceptors determined in release experiments.It is concluded that [3H]5-HT binding sites, presynaptic 5-HT autoreceptors in the rat brain cortex and inhibitory presynaptic 5-HT receptors on sympathetic nerve endings in the canine saphenous vein possess common pharmacological properties. In the rat brain cortex, the 5-HT1 sites are not homogeneous. Part of the [3H]5-HT binding sites (low affinity sites rather than high affinity sites) may be localized on the serotoninergic neurones and, hence, be identical with the serotonin autoreceptors. The results are also compatible with the suggestion that there may exist even more than two subtypes of [3H]5-HT1 binding sites in the rat brain cortex.

Binding characteristics of (+)-, (±)- and (-)-[125Iodo] cyanopindolol to guinea-pig left ventricle membranes

Summary[125Iodo]cyanopindolol [(±)-ICYP], a potent and selective ligand for β-adrenoceptors, exhibited complex biphasic dissociation kinetics. Consequently, in receptor binding studies, the pure (+)- and (-)-enantiomers of ICYP were synthesised and their equilibrium and kinetic binding characteristics were investigated on a membrane preparation of guinea pig left ventricle containing almost only β1-adrenoceptors.All three ligands, i. e. (+)-, (-)- and (±)-ICYP, bind to β-adrenoceptors as assessed by competition experiments with different β-blocking agents; irrespective of the ligand used, the same dissociation constant was found for the competitor. In a first series of saturation binding experiments performed in a low concentration range of free ligand (0–250 pM), ICYP showed the following dissociation constants: KD=93, 9 and 23 pM, and number of binding sites: Bmax=40,128 and 124 fmoles/mg protein for (+)-, (-)-, and (±)-ICYP, respectively. Asexpected, (±)-ICYP showed the same Bmax as (-)-ICYP, whereas its KD was approximately two times higher than that of (-)-ICYP. Surprisingly, the Bmax of (+)-ICYP represented only ∼ 30% of the Bmax of (-)-ICYP. All three ligands bound apparently to a single class of binding sites.In dissociation experiments, the enantiomers of ICYP showed biphasic dissociation curves as observed for the racemic ligand. (+)-, (-)- and (±)-ICYP showed a rapidly dissociating (k-1=0.488, 0.047 and 0.049 min−1) and a slowly dissociating component (k-2=0.0205, 0.0033 and 0.0025 min−1). The ratio slow dissociating/fast dissociating component represented respectively for (+)-, (-)- and (±)-ICYP 40/60, 90/10 and 90/10. For all three ligands, the association rate constants were of the same order of magnitude (ca. 109 M−1 min−1), typical for a diffusion controlled reaction.In contrast to equilibrium binding studies, the existence of multiple receptor affinity sites was evident from the biphasic dissociation behaviour observed especially with the nonracemic ligands (+)-ICYP and (-)-ICYP.Simulation of theoretical saturation curves performed with the ratios of high versus low affinity sites and the KD-values suggested by kinetic analysis, indicated that the delineation into two affinity states might be visible in saturation experiments, under certain conditions.Therefore, equilibrium binding studies were repeated with an increased number of ligand concentrations covering a large concentration range of 0–800 pM. Simultaneous analysis of saturation curves from the same experiment using three different ligands, provided more accurate estimates of the ratio of high and low affinity sites, as well as the affinity constants of the ligand for each receptor affinity state, in good agreement with the results from kinetic analysis.The contribution of the (+)enantiomer in the binding of the racemic ligand under low receptor concentrations could be neglected since dissociation characteristics of (±)- and (-)-ICYP were identical. A model that explains the biphasic dissociation of (±)-ICYP by differential binding of both enantiomers could be rejected. Kinetic and equilibrium binding characteristics of the three radioligands were not influenced by the guanylnucleotide Gpp(NH)p (10−4 M).The antagonist ICYP binds to β-adrenoceptors in a high and low affinity state which are probably interconvertible.

2-[125Iodo]LSD, a new ligand for the characterisation and localisation of 5-HT2 receptors.

Summary1.LSD was iodinated with Na125I and chloramine T, to get the radioligand [125I]LSD (125IOL) and with N-I-succinimide to obtain the nonradioactive compound 2-I-LSD (IOL) for comparative pharmacological studies.2.The introduction of iodine in position 2 of LSD leads to an increase in selectivity for 5HT2 receptors. In rat cortex membranes, 125IOL possesses a KD=0.9±0.1 nmol/l, Bmax=240±20 fmoles/mg, and a nonspecific binding of 30–40% in presence of 100 nmol/l ketanserin.3.In competition experiments, 5HT antagonists showed monophasic displacement curves. Their KI-values correlate well with pD′2-values for inhibition of 5HT-induced contraction of canine basilar artery. It can be concluded that the sites labelled by 125IOL have pharmacological properties in common with central 5HT2 receptors, which are identical with vascular postjunctional 5HT receptors.4.The high specific radioactivity of 125IOL permits detection of even small 5HT2 receptor densities which exist in the guinea pig ileum. These 125IOL binding sites are pharmacologically different to those found in the brain or on the vessels and might be a special subpopulation of 5HT2 sites. For example, ketanserin has a high affinity to the sites labelled by 125IOL in the brain and a 100 times lower affinity to the sites labelled in the ileum.5.In a routine binding screen with various ligands, the inhibition constants of IOL for α1, α2, β, histamine and muscarinic receptors are >100 nmol/l with the exception for dopamine receptors, 40 nmol/l.6.125IOL was employed for the autoradiographic localisation of its binding sites after in vitro labelling of microtome rat brain sections. 125IOL labelled 5HT2 sites in the cortex and dopamine receptors in the nucleus caudatus. The exposure times required were very short, compared to those of other 5HT2 ligands available.

Evidence for mediation by 5-HT2 receptors of 5-hydroxytryptamine-induced contraction of canine basilar artery

SummaryThe agonist potencies of 8 indole derivatives and the potencies of 19 recognized antagonists to inhibit constrictor responses to 5-hydroxytryptamine (5-HT) of canine basilar artery were established. In addition the affinities of the indole derivatives for [3H]5-hydroxytryptamine ([3H]5-HT) binding sites and the affinities of the antagonists for [125Iodo]LSD ([125I]LSD) binding sites in rat brain cortex membranes were determined. Comparison was also made between the potencies of the antagonists on canine basilar artery and the KD values published for displacement of [3H]ketanserin binding (Leysen et al. 1982).There was a good correlation between the affinities of the antagonists for 5-HT2 binding sites labelled by both [125I]LSD and [3H]ketanserin and the affinity parameters calculated for inhibition of constrictor responses to 5-HT of canine basilar artery. No correlation could be found between the affinities of the indole derivatives for 5-HT1 binding sites labelled by [3H]5-HT and their potencies to constrict canine basilar artery.It is concluded that constrictor responses to 5-HT of canine basilar artery are mediated by 5-HT2-like receptors.

Identification of 5HT2-Receptors on Longitudinal Muscle of the Guinea Pig Ileum

In binding experiments with the radioligands [3H]Ketanserin (HKet) and [125I]LSD (ILSD) 21 compounds were investigated using rat brain cortex membranes. The pKD-values of the compounds were virtually independent of the radioligand used and their rank order was consistent with classification of the binding sites as being of the 5-HT2-type. In contrast, in the longitudinal muscle of the guinea pig ileum in the presence of 0.3 microM cinanserin, ILSD labelled sites which were quite different to those in the cortex. In a functional test antagonism of the 5HT induced contraction of the guinea-pig ileum was measured in the presence of 1 microM atropine. The pharmacological inhibition constants (IC50-values) of 8 compounds correlated well with the dissociation constants for HKet binding in the cortex and did not correlate with the data from ILSD binding in the guinea pig ileum. It is concluded that the ileum contains postjunctional 5HT2-receptors which mediate contraction. The nature of the ILSD binding sites in the ileum remains to be elucidated.