In vitro activity of LY393558, an inhibitor of the 5-hydroxytryptamine transporter with 5-HT(1B/1D/2) receptor antagonist properties

1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-1(2H)-pyridinyl]ethyl]-3-isopropyl-6-(methylsulphonyl)-3,4-dihydro-1H-2,1,3-benzothiadiazine-2,2-dioxide (LY393558) is a potent inhibitor of [3H]5-hydroxytryptamine ([3H]5-HT) uptake into rat cortical synaptosomes (pIC(50)=8.48+/-0.12). It produces a dextral shift of the 5-HT dose-response curves for the binding of GTPgamma[35S] to human 5-HT(1B) (pK(b)=9.05+/-0.14) and 5-HT(1D) (pK(b)=8.98+/-0.07) receptors and inhibits the contractile response of the rabbit saphenous vein to the 5-HT(1B/D) receptor agonist, sumatriptan (pK(b)=8.4+/-0.2). In addition, it is an antagonist at the 5-HT(2A) (pK(i)=7.29+/-0.19) and 5-HT(2B) (pK(i)=7.35+/-0.11) receptors. Presynaptic autoreceptor antagonist activity was demonstrated by its ability to potentiate the K(+)-induced outflow of [3H]5-HT from guinea pig cortical slices (pEC(50)=7.74+/-0.05 nM) in which the 5-HT transporter had been inhibited by a maximally effective concentration of paroxetine. It is concluded that LY393558 should be an effective antidepressant with the potential to produce an earlier onset of efficacy than selective serotonin uptake inhibitors.

5-Hydroxytryptamine(1A) receptor activation enhances norepinephrine release from nerves in the rabbit saphenous vein

Although serotonergic receptor agonists are known to modulate release of central serotonin, less is known about the ability of serotonin to alter neurotransmission in peripheral adrenergic nerves. The present study used field stimulation (40V, 0.7 ms duration, 1-16 Hz) to contract the rabbit saphenous vein, an effect that was abolished in the presence of tetrodotoxin and prazosin (10(-6) M), consistent with stimulation of neuronal norepinephrine release. Furthermore, the field-stimulated contraction was not altered by the 5-hydroxytryptamine (5-HT)(1B/1D) receptor antagonist GR127935 (10(-6) M), but was markedly inhibited by the 5-HT(1A) receptor antagonist WAY 100635 (10(-6) M). GR127935 (10(-8) M) inhibited contraction to sumatriptan, documenting that the concentration used was sufficient to block 5-HT(1B/1D-like) vascular receptors in this tissue. Likewise, WAY 100635 (10(-6) M) inhibited contraction to the 5-HT(1A) receptor agonists (+/-)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT) and LY238729, without altering contraction to norepinephrine or sumatriptan. Furthermore, both 8-OH-DPAT and LY228729 enhanced the contractile response to field stimulation (1. 0-8.0 Hz) and activated norepinephrine release in the absence of field stimulation. Contractile responses of the rabbit saphenous vein to both 5-HT(1A) receptor agonists were markedly inhibited by prazosin and dextrally shifted by WAY 100635, supporting the idea that the 5-HT(1A) receptor agonists were activating presynaptic 5-HT(1A) receptors to enhance norepinephrine release even in the absence of field stimulation. Thus, in the rabbit saphenous vein, 5-HT(1A) but not 5-HT(1B/1D) receptors enhanced neurotransmitter release from adrenergic nerves. These observations suggested that serotonergic nerves or other cell types in the saphenous vein are activated by field stimulation to release serotonin, which in turn activates presynaptic 5-HT(1A) receptors on adrenergic neurons to effect norepinephrine release. To support this hypothesis, serotonin levels were measured in the saphenous vein and were increased after pargyline pretreatment (30 mg/kg s.c.), decreased after dl-p-chlorophenylalanine methyl ester pretreatment (300 mg/kg s.c.), and unaltered after pretreatment with 6-hydroxydopamine hydrobromide (100 mg/kg s.c.). Thus, we provide strong evidence for the 1) presence of serotonin and its direct synthesis independent of adrenergic nerves and 2) a novel excitatory effect of presynaptic 5-HT(1A) receptor activation on adrenergic nerves in a peripheral blood vessel.

Migraine therapy: relationship between serotonergic contractile receptors in canine and rabbit saphenous veins to human cerebral and coronary arteries

Canine and rabbit vascular contractile responses to serotonergic agonists have been used to predict antimigraine efficacy for several antimigraine agents, including sumatriptan. The purpose of the present study was to establish the assumed predictive value of contractile responses in canine and rabbit saphenous veins to contractile efficacy for a series of agonists in human cerebral and coronary arteries and to understand better the receptors mediating such responses. The canine and rabbit saphenous veins contracted similarly (both qualitatively and quantitatively) to a series of structurally diverse serotonergic agonists, suggesting that the receptors mediating serotonin-induced contractility in these tissues were similar. In addition, the contractile potency (estimated as EC50 values) for these structurally diverse serotonergic agonists in either the rabbit or canine saphenous vein significantly correlated with contractile potency for these agonists in human cerebral arteries. Thus, to the extent that contractile responsiveness of human cerebral arteries may predict antimigraine agents, contractile responses of the rabbit and/or canine saphenous vein may be useful surrogates for antimigraine efficacy. In addition, the contractile potency for this series of serotonergic agonists in the rabbit or canine saphenous vein significantly correlated with contractile potency of these agonists in human coronary arteries. These data suggest that the use of the saphenous vein to identify potent vasoconstrictors will also reveal agents capable of contracting human coronary arteries, a liability for using this approach to evaluate promising antimigraine therapies.

Potent, selective tetrahydro-beta-carboline antagonists of the serotonin 2B (5HT2B) contractile receptor in the rat stomach fundus

A series of potent, selective 5HT2B receptor antagonists has been identified based upon yohimbine, with SAR studies resulting in a 1000-fold increase in 5HT2B receptor affinity relative to the starting structure (-log KBS > 10.0 have been obtained). These high-affinity tetrahydro-beta-carboline antagonists are able to discriminate among the 5HT2 family of serotonin receptors, with members of the series showing selectivities of more than 100-fold versus both the 5HT2A and 5HT2C receptors based upon radioligand binding and functional assays. As the first compounds reported with such selectivity and enhanced receptor affinity, these tetrahydro-beta-carboline antagonists are useful tools for elucidating the role of serotonin acting at the 5HT2B receptor in normal and disease physiology.

Is the "atypical" beta-receptor in the rat stomach fundus the rat beta 3 receptor?

The rat gastric fundus is known to possess an "atypical" beta-adrenergic receptor that mediates relaxation to isoproterenol. The purpose of this study was to characterize the relationship between this "atypical" beta receptor in the rat stomach and the cloned rat beta 3 receptor by taking advantage of highly selective pharmacological and molecular biological probes of the beta 3 receptor. Nuclease protection analysis of RNA from the rat gastric fundus identified beta 3 receptor mRNA whose levels in the stomach were exceeded only by those in adipose tissue. Pharmacological analysis of the recombinant rat beta 3 receptor expressed in Chinese hamster ovary cells indicated low affinity of propranolol with a Ki value of 2.3 microM. Therefore, 0.3 microM propranolol was chosen as a concentration that would completely block beta 1 and beta 2 receptors (Ki = 1-5 nM) but would leave beta 3 receptors largely intact in the rat stomach fundus. In the presence of propranolol, several beta-adrenergic receptor agonists relaxed the rat stomach fundus with a rank potency order of (R,R)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]-propyl]1,3- benzodioxole-2,2-dicarboxylate (CL316,243) > isoproterenol > norepinephrine = epinephrine = dl-4-3[(1,1-dimethylethyl)amino]-2- hydroxylproproy]1,3 dihydro-2H-benzimidazol-2-one hydrochloride (CGP12177) > clenbuterol > terbutaline > pindolol. Isoproterenol, norepinephrine and epinephrine were full agonists, whereas (R,R)-5-[2-[[2-(3-chlorophenyl)-2- hydroxyethyl]-amino]-propyl]1,3-benzodioxole-2,2-dicarboxylate was only a partial agonist with 66% intrinsic activity relative to isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective desensitization of 5-hydroxytryptamine 2A receptor mediated contraction in guinea pig trachea

5-Hydroxytryptamine (serotonin, 5-HT) contracts the guinea pig trachea through stimulation of 5-HT2A receptors, an effect previously reported to rapidly desensitize. The present studies were designed to examine further the putative desensitization to serotonin. In vitro studies investigating functional desensitization of the guinea pig tracheal 5-HT2A receptor documented that 5-HT (3 x 10(-7) M) significantly (50%) but incompletely reduced subsequent tracheal maximal contraction to 5-HT. In contrast, an equieffective concentration of carbamylcholine (3 x 10(-8) M) did not reduce guinea pig tracheal contraction to 5-HT. Furthermore, 5-HT (3 x 10(-7) M) did not diminish tracheal contraction to carbamylcholine. These data indicate that 5-HT can selectively desensitize guinea pig tracheal contraction to 5-HT. In addition, 5-HT-induced contraction but not carbamylcholine-induced contraction in guinea pig trachea declined over time, an effect that was more pronounced at high concentrations of 5-HT (1 x 10(-6) and 1 x 10(-5) M). Inhibitors of mechanisms that oppose contractility to 5-HT (5-HT-induced relaxation, uptake of 5-HT, or metabolism of 5-HT) did not reverse the decline in contraction to 5-HT (1 x 10(-5) M). The decline in 5-HT-induced contraction was most rapid in the guinea pig trachea and less so in the rat jugular vein and rat aorta, two preparations in which 5-HT induced contraction also occurred via activation of 5-HT2A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

DOI and alpha-methylserotonin: comparative vascular and nonvascular smooth muscle effects and central 5-hydroxytryptamine2 receptor affinities

Both alpha-methylserotonin and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) are agonists at 5-hydroxytryptamine2 (5-HT2) receptors. The present study compared these agonists for their binding affinities at the high- and low-affinity states of the 5-HT2 receptor and for their contractile activities in certain smooth muscle preparations. Both agonists contracted the rat aorta and rat jugular vein, tissues possessing 5-HT2 receptors, and contraction was blocked by ketanserin. However, alpha-methylserotonin produced greater maximal response (80-90% maximum response to serotonin) than DOI. In the rat jugular vein, the calculated dissociation constant of DOI (-log Kb = 7.7) corresponded well with its affinity for [3H]ketanserin- (pKi = 7.5) but not [125I]DOI- (pKi = 8.6) radiolabeled sites. This might suggest that binding to the agonist low-affinity state of the 5-HT2 receptor is more relevant to vascular agonist activity. alpha-Methylserotonin was slightly more potent than serotonin in contracting the rat aorta but not the jugular vein, whereas DOI was more potent in the jugular vein than in the aorta. In the jugular vein but not the aorta, the relative potency of these agents corresponded well with their relative affinities for either the [3H]ketanserin- or the [125I]DOI-labeled 5-HT2 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characteristics of the newly cloned rat 5-hydroxytryptamine2F receptor

The rat 5-hydroxytryptamine (5-HT)2F (serotonin2F) receptor is a newly cloned member of the 5-HT2/1C receptor family. The pharmacology of the 5-HT2F receptor was explored using a variety of structurally different compounds in a radioligand binding assay. In addition, the 5-HT2F receptor was shown to stimulate production of inositol 1,4,5-trisphosphate in the transformed cells. Based on the affinities of the compounds tested, their known affinities for certain of the other 5-HT receptors, and the fact that activation of the cloned 5-HT2F receptor stimulates inositol 1,4,5-trisphosphate production, the 5-HT2F receptor was determined to be a novel receptor and a member of the 5-HT2/1C receptor family. In addition, several agonists and partial agonists were evaluated for contractile activity in the rat stomach fundus, and these activities were correlated with their binding affinities at the 5-HT2F receptor. A highly significant correlation was found, providing additional evidence that is consistent with the 5-HT2F receptor being the stomach fundal contractile receptor. [3H]5-HT had high affinity for this receptor both at 37 degrees and at 0 degree (Kd = 7.87 +/- 0.55 and 0.12 +/- 0.02 nM, respectively). The difference in affinity for [3H]5-HT at the two temperatures prompted an investigation of potential temperature-dependent differences in the binding affinities of agonists versus antagonists. Agonists such as 5-HT, 5-methoxytryptamine, etc., showed higher affinity for the 5-HT2F receptor at 0 degree than at 37 degrees, whereas antagonists such as methysergide, 1-naphthylpiperazine, etc., showed no difference in affinity for this receptor at the two different temperatures. Therefore, the affinity of a compound for the 5-HT2F receptor at 37 degrees versus 0 degree was shown to be useful for predicting agonist or antagonist activity. Additionally, information is provided about some of the structural requirements for the affinity of certain tryptamines at the 5-HT2F receptor.

Selective down regulation of vascular beta 1 adrenergic receptors after prolonged isoproterenol infusion

Prolonged isoproterenol infusion (400 micrograms/kg/h for 4 days) in rats was previously shown to produce a reduction in the sensitivity of both cardiac and vascular beta-adrenergic receptors without affecting responsiveness to alpha 1 agonists or phosphodiesterase inhibitors in either vascular or cardiac muscle. The present study was designed to determine if the loss in beta receptor responsiveness was similar for both beta 1 and beta 2 vascular receptors. The rat jugular vein was previously shown to relax in response to both norepinephrine and isoproterenol with norepinephrine-induced relaxation being mediated by interaction with beta 1 adrenergic receptors and isoproterenol-induced relaxation being mediated by its interaction with beta 2 vascular receptors. Using this preparation, tissues from isoproterenol-infused rats were approximately threefold less responsive to isoproterenol when compared to responses in tissues from saline-treated rats. Relaxation to norepinephrine in jugular veins from isoproterenol-infused rats was virtually abolished relative to the response in saline-treated animals. These data suggest that beta 1-adrenergic receptors in blood vessels are considerably more susceptible to down regulation than are beta 2-adrenergic receptors. This observation may have importance in both the therapy of congestive heart failure, where down regulation of beta-adrenergic receptors has been observed, and in our understanding of the mechanism for the inotropic effects of beta receptor agonists.

In vitro vascular relaxation by new inotropic agents: relationship to phosphodiesterase inhibition and cyclic nucleotides

Several novel cardiotonic vasodilators including bipyridines (amrinone and milrinone), imidazolones (enoximone and piroximone), dihydropyridazinones (Cl-914, Cl-930 and LY195115) and an imidazopyridine (isomazole) relaxed rat aortic strips contracted previously with 30 microM serotonin. LY195115 and Cl-930 were the most potent vasorelaxant agonists (ED50 approximately 10(-7) M), whereas piroximone and amrinone were the least potent (ED50 approximately 10(-5) M). In addition to these positive inotropic agents, vascular relaxation was examined further for a series of novel dihydropyridazinones, and relaxant potencies correlated directly with the ability of these agents to inhibit an isozyme of cyclic nucleotide phosphodiesterase (PDE) located in the sarcoplasmic reticulum of cardiac muscle (SR-PDE) (r = 0.87, P less than .01). This excellent correlation suggests that vascular relaxation produced by these agents is related to their ability to inhibit a vascular enzyme similar or identical to SR-PDE. Furthermore, LY195115, milrinone and isomazole (10(-4) M) produced significant increases in both aortic cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Time courses for these changes were consistent with a role for cyclic nucleotides in relaxation; however, differences between the relative increases in cAMP or cGMP produced by these drugs were evident. Removal of the aortic endothelium had no effect upon relaxation produced by milrione and only a modest (approximately 2-fold decrease in potency) effect on relaxation produced by LY195115 and isomazole, indicating that the relaxant effect of these cardiotonics is primarily an endothelium-independent event.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of 5-hydroxykynurenamine, L-kynurenine and kynuramine with multiple serotonin receptors in smooth muscle

The enzymatic formation of kynurenine derivatives from tryptophan and the regulation of this metabolic pathway by both tryptophan concentrations and plasma cortisol concentrations have raised the possibility that the kynurenine derivatives, L-kynurenine, kynuramine and 5-hydroxykynurenamine (5-OH-kynurenamine) may be important as endogenous agonists or antagonists at serotonin (5-HT) receptors in smooth muscle. In fact, 5-OH-kynurenamine was an agonist at 5-HT2 receptors in the rat jugular vein and aorta, at 5-HT3 (neuronal "M") receptors in the guinea pig ileum and at 5-HT receptors in the rat stomach fundus. Maximal contractile responses to 5-OH-kynurenamine in these three smooth muscle preparations were similar to those produced by 5-HT, although 5-OH-kynurenamine was approximately 10- to 100-fold less potent than 5-HT as a contractile agonist. Using appropriate antagonists, LY53857 as a selective antagonist at 5-HT2 receptors, ICS 205-930 as a selective antagonist at 5-HT3 receptors and 1-(1-napthyl) piperazine as a potent antagonist in the rat stomach fundus, we documented further that the contractile responses to 5-OH-kynurenamine resulted from its interaction with 5-HT receptors in these tissues. Kynuramine did not contract either the rat jugular vein, aorta or the guinea pig ileum, although a contractile response did occur in the rat stomach fundus (maximal response approximately 70% of the maximal contractile response to 5-HT). High concentrations of kynuramine (10(-4) M) produced modest inhibition and reduction in maximal responses to 5-HT in the jugular vein, aorta and guinea pig ileum.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro receptor specificity of the 5HT1A selective phenylpiperazine, LY165163

LY165163, a ligand reported to be selective for the 5HT1A subtype of serotonin receptor, was examined for its ability to interact with 5HT2 receptors in the rat jugular vein and alpha-receptors in the rat aorta. In these smooth muscle preparations, no agonist activity of LY165163 occurred in concentrations up to 10(-5) M. However, LY165163 was an antagonist of serotonin-induced contractions in the jugular vein and of norepinephrine-induced contractions in the rat aorta. The dissociation constant calculated for LY165163 at 5HT2 receptors in the rat jugular vein was 10(-8) M and at alpha-receptors in the rat aorta was 2 X 10(-7) M. Thus, LY165163 is a relatively potent antagonist at vascular 5HT2 sites and possesses appreciable affinity at alpha-receptors. Based on these data, the multiple receptor interactions of LY165163 must be taken into consideration when utilizing this agent as a probe for the 5HT1A subtype of serotonin receptor.

2,3-Dialkyl(dimethylamino)indoles: interaction with 5HT1, 5HT2, and rat stomach fundal serotonin receptors

2,3-Dialkyl(dimethylamino)indoles, synthesized via the Fisher indole synthesis, were found to weakly bind to 5HT1 and 5HT2 sites in brain cortical membranes (IC50 greater than 1 microM at both sites for all compounds). These (dimethylamino)indoles were relatively potent antagonists of the serotonin receptor in the rat stomach fundus. At higher concentrations, several of the compounds were weak agonists at this receptor. For direct comparison with data obtained in the isolated rat fundus, antagonism of serotonin-induced contractions at 5HT2 receptors in the rat jugular vein was also examined. Several of the compounds showed good selectivity for the fundus receptor relative to the 5HT2 receptor; together with minimal affinity for 5HT1 and 5HT2 binding sites in brain cortical membranes, these results support the idea that the serotonin receptor in the stomach fundus is distinct from 5HT1 and 5HT2 binding sites.

Effects of pinacidil on serotonin-induced contractions and cyclic nucleotide levels in isolated rat aortae: comparison with nitroglycerin, minoxidil, and hydralazine

Pinacidil is a novel, clinically effective vasodilator used for the treatment of hypertension whose mechanism of action has not been precisely defined. In vitro, pinacidil (ED50 = 0.3 microM) was approximately 30-fold less potent than nitroglycerin and 700-fold more potent than minoxidil or hydralazine in relaxing rat aortic strip preparations. Aortic relaxations produced by nitroglycerin and acetylcholine were dramatically antagonized by methylene blue (10(-5) M), an inhibitor of soluble guanylate cyclase. In contrast, relaxation to hydralazine or minoxidil was unaffected and relaxation to pinacidil was only modestly inhibited (approximately threefold) by methylene blue (10(-5) M). Furthermore, aortic relaxation to pinacidil was similar in preparations with and without an intact endothelium. Relaxation induced by pinacidil (10(-7)-10(-4) M) was not associated with any elevation in either cyclic AMP (cAMP) or cyclic GMP (cGMP) levels in vitro, although nitroglycerin (10(-6) M) but not minoxidil (10(-3) M) or hydralazine (10(-3) M) significantly elevated cGMP levels. Thus, pinacidil was a potent relaxant agonist in vitro, in contrast to minoxidil and hydralazine, which were considerably weaker in this regard. Vascular relaxation produced by pinacidil was independent of an intact endothelium and was not associated with elevations in either cAMP or cGMP. These data are consistent with the proposal that the antihypertensive activity of pinacidil is due to nonspecific arterial vasodilation.

Role of 5-HT2 receptors in serotonin-induced contractions of nonvascular smooth muscle

Vascular receptors responsible for serotonin-induced contractions are of the 5-HT2 subtype (site in brain cortical membranes that is preferentially radiolabeled by [3H]spiperone) whereas serotonin receptors mediating contraction in nonvascular smooth muscle have not been extensively studied. The present in vitro studies using the 5-HT2 receptor antagonists ketanserin, LY53857 and 1-(1-naphthyl)piperazine show that serotonin-induced contractions in the rat uterus and guinea-pig trachea are also mediated by interaction with 5-HT2 receptors. Prazosin, but not the serotonin receptor antagonists, blocked serotonin-induced contractions in the rat vas deferens, indicating that alpha adrenergic and not 5-HT1 or 5-HT2 receptors mediate the contractile response to serotonin in this tissue. Because selective 5-HT2 receptor antagonists did not block contractions to serotonin in the rat fundus or guinea-pig ileum, receptors in these gastrointestinal tissues are clearly not 5-HT2. However, contractions to serotonin in the fundus but not in the ileum were blocked by certain antagonists [metergoline and 1-(1-naphthyl)piperazine] demonstrating that the receptors involved in serotonin-induced contractions in the fundus are different from the ileum. Other differences between the fundus and ileum in serotonin-induced contractions include: 1) the potency of serotonin is greater in the fundus than in the ileum; and 2) the primary action of serotonin in the fundus is activation of a postsynaptic receptor on the smooth muscle whereas, in the ileum, serotonin exerts an indirect neuronal action to effect acetylcholine release.(ABSTRACT TRUNCATED AT 250 WORDS)