Paul Van Gompel

Activity of serotonin (5-HT) receptor agonists, partial agonists and antagonists at cloned human 5-HT1a receptors that are negatively coupled to adenylate cyclase in permanently transfected hela cells

The activity of serotonin (5-HT) receptor agonists, partial agonists and antagonists, and various other neurotransmitter receptor antagonists at human 5-HT1A receptors that are negatively coupled to adenylate cyclase in permanently transfected HeLa cells was investigated. 5-HT1A receptor-mediated inhibition of adenylate cyclase was studied by measuring inhibition of cAMP accumulation, induced by forskolin. At 100 microM forskolin produced a 100-fold increase in cAMP formation: 5-HT concentration dependently inhibited the cAMP formation; maximal inhibition was attained at 1 microM 5-HT and represented 90% of the stimulated cAMP formation. Full inhibition was observed with 5-HT1A receptor agonists: N,N-dipropyl-8-hydroxy-2-aminotetralin (8-OH-DPAT) and flesinoxan, and non-selective 5-HT receptor agonists: d-lysergic acid diethylamide (d-LSD), RU 24,969, bufotenine, methysergide and tryptamine. The rank order of potency of the compounds for inhibiting the cAMP formation corresponded to the rank order of the binding affinities of the drugs for the 5-HT1A receptor. Partial inhibition was obtained with submicromolar concentrations of buspirone, spiroxatrine and ipsapirone. A slight inhibition was observed with 1 microM 5-HT receptor agonist CP 93129 and 1 microM 5-HT receptor antagonists mesulergine and BW-501. No inhibition was found with: the 5-HT receptor agonists quipazine, sumatriptan and 1-(2,5-dimethoxy-4-methylphenyl)-2- aminopropane (DOM); the 5-HT receptor antagonist ICS-205,930; and other neurotransmitter receptor antagonists such as pindolol, CGP 20712-A, prazosin, sulpiride and pyrilamine. Spiperone and pindolol fully antagonized the agonist-mediated inhibition of forskolin-stimulated cAMP formation. Partial inhibition of the agonist-mediated inhibition of forskolin-stimulated cAMP formation was apparent with 1 microM ocaperidone and 1 microM ipsapirone. It can be concluded that HeLa cells, permanently expressing human 5-HT1A receptors, are a valid cellular system for studying the negative coupling of 5-HT1A receptors to adenylate cyclase and the action of compounds thereupon.

Agonistic properties of alniditan, sumatriptan and dihydroergotamine on human 5-HT1B and 5-HT1D receptors expressed in various mammalian cell lines

Alniditan, a novel migraine abortive agent, is a potent 5‐HT1B/5‐HT1D receptor agonist of nM affinity. We compared the agonistic properties of alniditan, sumatriptan and dihydroergotamine on the cloned human 5‐HT1B receptor expressed at 200 fmol mg−1 protein (Bmax) in non‐induced L929sA cells, at 740 fmol mg−1 protein in HEK 293 and at 2300 fmol mg−1 protein in mIFNβ‐induced L929sA cells, and on the human cloned 5‐HT1D receptor expressed in C6 glioma cells (Bmax 780 fmol mg−1 protein). Sodium butyrate treatment increased the expression level of human (h)5‐HT1B receptors in HEK 293 cells and h5‐HT1D receptors in C6 glioma cells approximately 3 fold, the binding affinities of [3H]‐5‐HT and [3H]‐alniditan were unaffected. Agonistic properties were evaluated based on inhibition of cyclic AMP accumulation in the cells after stimulation of adenylyl cyclase by forskolin or isoproterenol. Alniditan, sumatriptan and dihydroergotamine were full agonists at the h5‐HT1B receptor (IC50 values were 1.7, 20 and 2 nM, respectively in HEK 293 cells) and h5‐HT1D receptors (IC50 values of 1.3, 2.6 and 2.2 nM, respectively). At the h5‐HT1B receptor the agonist potency of the compounds slightly increased with higher receptor density. The opposite was seen for antagonists (ocaperidone, risperidone and ritanserin). This comparative study demonstrated that alniditan was 10 times more potent than sumatriptan at the h5‐HT1B receptor, and twice as potent at the h5‐HT1D receptor. Dihydroergotamine was more potent an agonist at the h5‐HT1B receptor when expressed at high and low level in L929sA cells (but not in HEK 293 cells), and was less potent at the h5‐HT1D receptor.

Rapid desensitization and resensitization of 5-HT2 receptor mediated phosphatidyl inositol hydrolysis by serotonin agonists in quiescent calf aortic smooth muscle cells

Agonist regulation of 5-hydroxytryptamine2 (5-HT2) receptors was studied in calf aortic smooth muscle cultures incubated in a quiescent, defined synthetic medium that does not stimulate cell proliferation, but that provides cells with supplements that maintain cell viability. In these cells, 5-hydroxytryptamine (5-HT)-induced [3H]inositol phosphates accumulation showed the characteristics of a 5-HT2 receptor coupled transducing system according to the inhibition of the response by 5-HT2 antagonists at nanomolar concentrations. The 5-HT2 receptor coupled response became rapidly desensitized during continued incubation with 5-HT and 1-(2,5-dimethoxy-4-methylphenyl)-2- aminopropane (DOM); nearly full desensitization was obtained in two hours with 10 microM 5-HT and DOM pretreatment. The recovery of the response had a half-live of 5 hours after 2 hours pretreatment and of 9.5 to 12.5 hours after 24 to 96 hours agonist pretreatment. The DOM-induced desensitization of the 5-HT2 receptor coupled response was fully blocked by 0.1 microM cinanserin. Cinanserin alone did not induce desensitization or up-regulation of the 5-HT2 receptor coupled response at 0.1 microM. It may be that the down-regulation of central 5-HT2 receptors by antagonists in vivo is a heterologous process due to mediators which are triggered by 5-HT2 antagonistic action.

Distinction between adrenergic and serotonergic receptor subtypes: specificity of drugs and absence of cooperative interactions between adrenergic and serotonergic receptor binding sites.

In light of observed amplificatory interactions between serotonergic and adrenergic stimuli in functional studies on vascular tissue and platelets, we investigated the distinction and possible interactions between alpha 1-, alpha 2-, beta 1-, and beta 2-adrenergic and 5-HT1A-, 5-HT1B-, and 5-HT2-serotonergic receptor binding sites. Therefore, the binding affinities of archetypes of adrenergic and serotonergic agonists and antagonists for the various receptors were measured. Only the alpha 1-blocker prazosin revealed great specificity for alpha 1-adrenergic receptors; the other investigated antagonists and agonists showed cross-reactivity with adrenergic and serotonergic receptors in various combinations. Using [3H]ketanserin binding to rat frontal cortex and [3H]prazosin binding to rat cortex tissue as models for 5-HT2-serotonergic and alpha 1-adrenergic receptors, respectively, we did not find cooperative effects of epinephrine on the binding of 5-hydroxytryptamine or ketanserin to 5-HT2 receptors nor of 5-hydroxytryptamine on the binding of epinephrine or prazosin to alpha 1-adrenergic receptors. It was concluded that the various adrenergic and serotonergic receptor subtypes (a) have distinct drug binding properties; (b) occur on various central and peripheral tissues; (c) co-occur on some tissues; (d) each subtype mediates several distinct functions; (e) distinct receptors may mediate similar functions; (f) the drug binding properties of a particular receptor remains the same in different tissues, but purported alpha 2-like receptors on platelets reveal some differences from alpha 2-receptors in the brain and other peripheral tissues; (g) the various receptor subtypes appear to be distinct molecular entities; and (h) in brain tissue there is no evidence for the occurrence of direct adrenergic-serotonergic receptor-receptor interactions at the level of the binding sites.

Agonist-induced desensitization of 5-HT2 receptors on cultured calf aortic smooth muscle cells

[(32)P]Phosphatidic acid (PA)-formation was quantified in calf aortic smooth muscle cultures for measuring the activation of the signal transducing system coupled to the 5-hydroxytryptamine(2)-(5-HT(2)) receptor. [(32)P]PA-formation was increased upon stimulation of smooth muscle cells with serotonin (5-HT) and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), but not with the 5-HT(1) agonists N,N-dipropyl-8-hydroxy-2-aminotetralin and RU 24969. The potency of drugs to inhibit the 5-HT induced [(32)P]PA-formation closely corresponded to their binding affinity for 5-HT(2) receptors. 24-Hour treatment of smooth muscle cultures with 5-HT or DOM resulted in a substantial decrease of 5-HT induced [(32)P]PA-formation. In contrast to the anomalous 5-HT(2) receptor regulation in vivo, 5-HT(2) receptors on smooth muscle cells appeared to be desensitized by agonist treatment.

The Dissociation Rate of Unlabelled Drugs from Receptor Sites: A Poorly Investigated, Yet Important Aspect in Receptor Research. Studies on the Serotonin-S2 Receptor

The labelling by 3H-spiperone of serotonin-S2 receptors in rat frontal cortex tissue adsorbed to glass fibre filters was investigated. For 12 unlabelled serotonin antagonists the dissociation time from serotonin-S2 receptors was measured using rat frontal cortex tissue preparations adsorbed to glass fibre filters. The dissociation half-time varied from 4.8 min for pipamperone to 160 min for ritanserin. The drug-receptor dissociation time was not related to a particular class of chemical structure, or to the lipophilicity or the acid dissociation constant of the drugs. The essential requirement of experimental determination of the drug-receptor dissociation time for each drug individually is illustrated. The possible applications of the knowledge of the drug-receptor dissociation time in in vitro and in vivo receptor studies, in pharmacological and pharmacokinetic studies and in drug design and receptor modelling is discussed. For various serotonin-S2 antagonists, the type of inhibition produced by the drug on 3H-ketanserin binding to serotonin-S2 receptors was determined using suspensions of rat frontal cortical tissue. The observed patterns of inhibition were clearly related to the drug-receptor dissociation times: rapidly dissociating drugs produced competitive inhibition, drugs with dissociation half-times between 15-30 min produced mixed type inhibition, and the very slowly dissociating ritanserin produced non-competitive inhibition.

In vivo formation of epinine in adrenal medulla

SummaryEndogenous epinine was isolated from pig and bovine adrenal medulla and identified by means of thin layer chromatography and gas chromatography-mass fragmentography. By using labelled precursors, radioactive epinine was found to be formed in adrenal tissue slices and in whole perfused adrenal glands. These results suggest a possible and more logical pathway in the formation of adrenaline.

Effects of proteolytic enzymes and monovalent ions demonstrate proteasesensitive and protease-insensitive stereospecific binding sites on dopaminergic receptors in rat striatum☆

Rat striatal membranes from different subcellular fractions were treated with various proteolytic and other enzymes and the binding of a dopamine agonist ([3H]NPA) and of an antagonist ([3H]haloperidol) was assayed in several conditions. In membranes of striatal microsomal and mitochondrial fractions, stereospecific binding of both [3H]NPA and [3H]haloperidol assayed in a monovalent ion-poor buffer was potently and rapidly inhibited by trypsin and certain related proteases. The enzymes did not affect the binding of the ligands when assayed in a buffer containing monovalent ions (greater than or equal to 40 mM NaCl or KCl or a physiological mixture of electrolytes). The inhibition, seen in the monovalent ion-poor buffer, was dependent on the enzyme concentration. The endoproteases (trypsin, alpha-chymotrypsin, papain, ficin) showed nanomolar IC50-values for inhibition of both [3H]NPA and [3H]haloperidol binding. The inhibition occurred very rapidly at 0 degree and was different from the slow proteolytic inactivation seen by prolonged incubation at 37 degrees. It was demonstrated that monovalent ions did not themselves interfere with the interaction between the proteases and the membranes. The observations provide evidence for two different types of stereospecific dopaminergic binding sites which are differentially exposed for ligand binding depending on the concentration of monovalent ions. There sites are protease-sensitive sites, labelled in monovalent ion-poor media and protease-insensitive sites, labelled in media with higher concentrations of monovalent ions. Both types of binding sites bind dopamine agonists and dopamine antagonists with high affinity, but some differences were noted in the binding properties and the drug binding selectivity of the sites. It is argued that both sites form part of the same dopamine receptor macromolecular complex. The findings corroborate the hypothesis that dopamine receptors are composed of different sub-unit binding sites, but these are not distinct agonist and antagonist specific sites. The mechanism by which the protease-sensitive sites are rapidly inactivated by particular proteases, is probably a complexation between the enzymes and certain essential peptide moieties of the receptor sites involved.